M Vlieghe

C Frances

C Coufort-Saudejaud

A Line

Morphological Properties of Flocs Under Turbulent Break-Up and Restructuring Processes

Keywords: Taylor-Couette reactor, flocculation, aggregation, break-up, morphology

Bentonite flocculation was performed in a Taylor-Couette reactor coupled with an in situ method of image acquisition and analysis. A hydrodynamic sequencing is imposed to perform successive cycles of flocculation and breakage. Depending on the shear rate applied during the breakage step, one or two cycles are needed after the first flocculation step to recover a full reversibility on both size and shape factors. The breakup step produces flocculi that are the build-ing blocks for the next. The re-flocculation steps produce smaller sizes and more regular shapes than the initial growth step. The floc size is calibrated by the turbulence as the radius of gyration is close to the Kolmogorov microscale whereas the floc structure is determined by flocculi aggregates. An analysis of the change of the flocs morphology, despite of their diversity, can also be achieved thanks to some relevant moments of the distributions.

Introduction

A wide variety of particulate processes deal with suspended particles in fluids (precipitation or synthesis processes, wet grinding, flocculation, solid-liquid mixing or product formulation, . . .). The population of suspended material, composed of particles, aggregates or flocs, is characterized by a distribution usually defined with respect to the mass, the volume, a characteristic dimension of the particle or the number of primary entities inside the aggregate. The morphological and structural properties also play an important role on the end-use properties (catalytic, shape, or texture properties), or their flowing behavior (viscosity, sedimentation). For these reasons, it is often important to analyze the effect of hydrodynamic conditions on the whole size and shape properties; this last one being described at several scales, from the local one (structure, porosity) to a more global one (silhouette, aspect ratio, circularity, . . .).

The structural properties of aggregates or flocs are currently described by a fractal dimension representative of the whole population. [START_REF] Tang | Fractal morphology and breakage of DLCA and RLCA aggregates[END_REF][START_REF] Sorensen | Light scattering by fractal aggregates: a review[END_REF][START_REF] Berka | Relation between aggregation kinetics and the structure of kaolinite aggregates[END_REF] Many other morphological properties may be extracted from image analysis, allowing characterizing the global shape of the aggregates, such as the aspect ratio [START_REF] Ehrl | Dependence of aggregate strength, structure, and light scattering properties on primary particle size under turbulent conditions in stirred tank[END_REF][START_REF] Soos | Effect of shear rate on aggregate size and morphology investigated under turbulent conditions in stirred tank[END_REF][START_REF] Frappier | Floc cohesive force in reversible aggregation: A couette laminar flow investigation[END_REF] or the elongation. [START_REF] Chakraborti | Characterization of alum floc by image analysis[END_REF] The presence of concavities (solidity), [START_REF] Amaral | Activated sludge monitoring of a wastewater treatment plant using image analysis and partial least squares regression[END_REF][START_REF] Mesquita | Monitoring of activated sludge settling ability through image analysis: validation on full-scale wastewater treatment plants[END_REF] the surface roughness (convexity) [START_REF] Amaral | Activated sludge monitoring of a wastewater treatment plant using image analysis and partial least squares regression[END_REF][START_REF] De Temmerman | Quantitative characterization of agglomerates and aggregates of pyrogenic and precipitated amorphous silica nanomaterials by transmission electron microscopy[END_REF] or the circularity which is sensitive to both the global and local shapes [START_REF] Allen | Particle Size Measurement[END_REF][START_REF] Gorczyca | Image analysis of alum coagulated mineral suspensions[END_REF] may also be calculated from image analysis as well as a 2D fractal dimension linking the floc area to the perimeter or to a floc size. [START_REF] Chakraborti | Characterization of alum floc by image analysis[END_REF][START_REF] Rahmani | Fractal structure of asphaltene aggregates[END_REF] A focus is done in this article on the flocculation process which is commonly used for example in drinkable water production. During this process, the aggregation of fine particles suspended in water is enhanced by the addition of a coagulant; particle aggregation allows their further separation by filtration or decantation. This operation is usually done under turbulent hydrodynamic conditions to promote the mixing and the collision of particles which encounter to form flocs. But the hydrodynamic conditions also induce floc breakage or compaction, limiting the growth of aggregates. During the first period of the flocculation process, the size of flocs increases rapidly but then the breakage phenomena take place which counteract the aggregation process and a stationary state is reached. [START_REF] Spicer | Shear-induced flocculation: the evolution of floc structure and the shape of the size distribution at steady state[END_REF][START_REF] Biggs | Activated sludge flocculation: on-line determination of floc size and the effect of shear[END_REF][START_REF] Serra | Effect of the shear and volume fraction on the aggregation and breakup of particles[END_REF] The floc sizes obtained at steady state are highly correlated with the hydrodynamic conditions usually expressed by the mean shear rate G (s 21 ). Higher is G, smaller are the flocs. Many experimental works of the literature [START_REF] Jarvis | A review of floc strength and breakage[END_REF] indicate a proportionality between the size and G 21=2 or directly with the Kolmogorov microscale [START_REF] Ehrl | Dependence of aggregate strength, structure, and light scattering properties on primary particle size under turbulent conditions in stirred tank[END_REF][START_REF] Biggs | Activated sludge flocculation: on-line determination of floc size and the effect of shear[END_REF][START_REF] Coufort | Flocculation related to local hydrodynamics in a Taylor-Couette reactor and in a jar[END_REF][START_REF] Bouyer | Experimental analysis of hydrodynamics in a jar-test[END_REF] g, both expressions being equivalent. Sometimes a decrease of the size is observed before reaching the steady state suggesting the existence of breakage or restructuring phenomena [START_REF] Spicer | Effect of shear schedule on particle size, density, and structure during flocculation in stirred tanks[END_REF][START_REF] Bouyer | Experimental analysis of floc size distribution and hydrodynamics in a jar-test[END_REF][START_REF] Selomulya | Evidence of shear rate dependence on restructuring and breakup of latex aggregates[END_REF][START_REF] Hopkins | Characterizing flocculation under heterogeneous turbulence[END_REF][START_REF] Bubakova | Effect of shear rate on aggregate size and structure in the process of aggregation and at steady state[END_REF] .

A sequence of the hydrodynamic conditions, applying alternative steps at low and high shear rates is performed to characterize the effect of hydrodynamics on the floc properties taking into account their history. Indeed, performing a cycle sequence comprising three phases (flocculation/breakage/reflocculation), Franc ¸ois [START_REF] Franc | Strength of aluminium hydroxide flocs[END_REF] put in evidence the non-reversibility of the process. The sizes of floc produced by re-flocculation were smaller than the ones produced during the first flocculation phase. The flocs obtained during the breakage step were also smaller if the shear rate applied is higher. Such a behavior was explained by the multi-scale structure of flocs: the primary particles bond each other to form small aggregates (flocculi) which themselves stick to each other to produce larger aggregates and flocs. Other papers report the same behavior concerning bentonite, [START_REF] Bouyer | Experimental analysis of floc size distributions in a 1-l jar under different hydrodynamics and physicochemical conditions[END_REF][START_REF] Coufort | Analysis of floc size distributions in a mixing tank[END_REF][START_REF] Barbot | Coagulation of bentonite suspension by polyelectrolytes or ferric chloride: floc breakage and reformation[END_REF] kaolin, [START_REF] Li | The strength and fractal dimension characteristics of alum-kaolin flocs[END_REF] or humic acid. [START_REF] Wang | Characterization of floc size, strength and structure in various aluminum coagulants treatment[END_REF][START_REF] Xiao | PIV characterisation of flocculation dynamics and floc structure in water treatment[END_REF] Moreover, concerning the structure of flocs, several authors have observed a change of the fractal dimension during the growth phase of a flocculation process, indicating the formation of open and irregular structures poorly compacted. The value of the fractal dimension itself, regardless of the definition used, varies with the nature of the material and the flocculation mechanism. [START_REF] Barbot | Coagulation of bentonite suspension by polyelectrolytes or ferric chloride: floc breakage and reformation[END_REF][START_REF] He | Characteristic analysis on temporal evolution of floc size and structure in low-shear flow[END_REF] When a shear rate is applied, the fractal dimension may change depending on the floc strength and the breakage mechanism. [START_REF] Bubakova | Effect of shear rate on aggregate size and structure in the process of aggregation and at steady state[END_REF] The change of the dispersed medium thus results from a competition between the aggregation, breakage and restructuring phenomena which all depend on the solid properties, the physico-chemical conditions and the hydrodynamic conditions. In this context, the in situ simultaneous analysis of size and shape properties of mineral flocs under controlled hydrodynamic conditions could bring a better understanding of the flocculation process. In this study the physico-chemical properties will be fixed, a focus will be made on the effect of the hydrodynamic conditions on the morphological properties of flocs.

This article presents the results of an experimental work on the flocculation of bentonite performed under turbulent conditions in a Taylor-Couette reactor. A sequence of the hydrodynamic conditions is applied to analyze the change of the morphological properties and their reversibility over successive cycles.

Experimental Set-Up and Validation Analysis Materials and methods

A natural clay (bentonite), supplied by CECA Chemicals was used in this study as a model material due to its properties similar to those of the materials usually encountered in drinkable water resources (rivers). The particles of bentonite dispersed in water are structured as small aggregates of a few microns in size constituted by platelets of a ten of nanometers, resulting themselves from the pile of primary sheets having a thickness of about one nanometer. [START_REF] Hofmann | Kristallstruktur und Quellung von Montmorillonit (Das Tonmineral der Bentonittone[END_REF] The experiments were done at a fixed bentonite mass concentration of 30 mg L 21 suspended in demineralized water. To avoid any disturbance due to the swelling ability of bentonite, the suspension was prepared long before the experiments. Aluminum sulfate hydrate with the formula Al

2 SO 4 ð Þ 3 Á 14H 2 O À Á
was chosen as a coagulant, and prepared at an appropriate concentration to get a fixed concentration of 3.5 3 10 25 mol L 21 after dilution of 5 mL of the aluminum sulfate solution into the bentonite suspension. The pH of the resulting suspension was equal to 4.5 6 0.1, ensuring the main presence of Al 31 ions and allowing a coagulation mechanism by charge neutralization. [START_REF] Duan | Coagulation by hydrolysing metal salts[END_REF] The experiments were performed in a Taylor-Couette reactor constituted by two concentric cylinders of a height of 20 cm. The outer cylinder of a radius R ext 511:5 cm is at rest while the inner cylinder of a radius R int 510 cm rotates at a chosen angular velocity X. The volume of the separation gap is equal to 2 L.

In the Taylor-Couette geometry, the dimensionless number which defines the flow type is the Taylor number which compares centrifugal inertia forces and viscous forces:

Ta5 R int X 2 R ext 2R int ð Þ 3 m 2 (1)
where X (rad s 21 ) is the angular velocity of the inner cylinder and m the suspension kinematic viscosity (m 2 s 21 ). Moreover, the mean velocity gradient is defined by:

hGi5 ffiffiffiffiffiffiffiffiffiffi ffi P qm V s (2)
where V is the volume of the reactor (m 3 ), q the suspension density (kg m 23 ), and P (Watt) the global power dissipated in the reactor function of the torque and the angular velocity, given by Wendt. [START_REF] Wendt | Turbulente Str€ omungen zwischen zwei rotierenden konaxialen Zylindern[END_REF] The dissipated power is also related to the mean dissipation rate of the turbulent kinetic energy hei (m 2 s 23 ) by:

hei5 P qV (3)
And thus:

hGi5 ffiffiffiffiffiffi hei m r (4)
In the case of isotropic turbulence, the mean Kolmogorov microscale, which has a great importance in flocculation analysis as indicated above, can be estimated from the mean dissipation rate by:

hgi5 m 3 hei 1=4 (5)
Floc characteristics were acquired through image processing.

It consists in the capture of images using a CDD camera (FlowSense EO 16M), synchronized with a laser sheet (Nd : Yag 532 nm, 30 mJ) illuminating a tangential plane with a thickness of about one millimeter at the middle distance in the gap between the cylinders. Grayscale images with a spatial resolution 4872 3 3248 pixels 2 were taken at a frequency of 4 Hz. To reduce refraction effect through the outer cylinder walls, the Taylor-Couette reactor was enclosed within a cubic box made of Plexiglas and filled with glycerol, both having close refractive indexes. Special attention was paid on particle image treatment to isolate individual flocs and get high quality binarized images applying a gray level threshold. Details of the image processing can be found in Vlieghe et al. [START_REF] Vlieghe | In situ characterization of floc morphology by image analysis in a turbulent Taylor-Couette reactor[END_REF] Binarized images were then analyzed using the Matlab function named "regionprops." Several size and shape properties can be directly determined as the perimeter, area, circle equivalent diameter, . . . etc. and other morphological parameters, such as circularity, convexity, solidity, . . . can also be derived from the previous ones. However many descriptors give redundant information and some of them are more relevant than the others. The radius of gyration and the circularity were chosen here since they were shown, respectively, as relevant size and shape properties to characterize bentonite floc size and shape. [START_REF] Vlieghe | In situ characterization of floc morphology by image analysis in a turbulent Taylor-Couette reactor[END_REF] Let's remember that the radius of gyration is a size property calculated by:

R 2 g 5 1 A p X Ap i51 x i 2x c ð Þ 2 1 y i 2y c ð Þ 2 h i (6)
where A p is the number of pixels on the image of the aggregate, (x i , y i ) the pixel coordinates, and (x c , y c ) the centroid coordinates.

The circularity is defined as the ratio of the circle equivalent perimeter to the actual perimeter:

C i 5 ffiffiffiffiffiffiffiffi 4pA p P (7)
The circularity may vary between 0 and 1; it is an indicator of the overall shape as well as the boundary roughness of the floc.

Precisions on experimental protocol and analysis

Sequenced flocculation runs were performed to analyze the aggregation, break-up and re-structuring processes. During the experiments, the Taylor-Couette reactor is first filled with the bentonite suspension and the inner cylinder rotation is switched on at 100 rpm. The flocculent is then added with a 5 mL calibrated pipette in the separation gap halfway-up the reactor. A 3-min step at a high rotating velocity (N5100 rpm) is then applied to promote a rapid mixing of the suspension with the solution of coagulant and to allow the collision of single particles the ones to the others leading to the formation of the first aggregates.

Then alternative steps at low and high shear rates are imposed as illustrated on Figure 1 allowing the aggregation and the breakage of flocs. The growth of flocs is thus promoted when a low shear rate is applied. During these steps, named "aggregation steps" (steps S1, S3, S5) in the following sections, the shear rate was fixed at 75 s 21 (which corresponds to an angular velocity N530 rpm) and maintained at this rate during two hours. The breakage of flocs is then promoted increasing the shear rate, even if aggregates can still be formed during this period. The duration of these steps, named "breakage steps" (steps S2, S4) was fixed at one hour and the shear rate applied was maintained constant at 280 s 21 (N590 rpm) for the Run 1 or 190 s 21 (N570 rpm) for the Run 2. The flow characteristics as defined by Eqs. 1-5 corresponding to the global hydrodynamic conditions imposed during the experiments are reported on Table 1. Since the experiments were conducted in diluted conditions, the suspension properties were assumed to be identical to those of water. Whatever the conditions imposed during the sequenced flocculation process, the hydrodynamic regime corresponds to turbulent Taylor vortex flow.

Several experiments were conducted under the same conditions to evaluate the reproducibility of the results. It was fairly good both on the number-based and surface-based distributions of any size or shape parameters obtained at the end of each cycle sequence. For example the error (in a logscale) of the modes of the area distribution calculated on images of flocs both on a surface or a number base was less than 2.5%. The errors on the modes of the perimeter distribution never exceed 3.5%. As an illustration, the surfacebased and the number-based perimeter distributions obtained at steady state after the first cycle sequence of breakage (S2) are, respectively, reported on Supporting Information (see Figures 10a,b).

However, to ensure a relevant morphological characterization, only floc images composed of 10 pixels or more were taken into account, thus the smallest aggregates analyzed have a circle equivalent diameter of 26 lm and the minimum radius of gyration considered is 9 lm. This treatment mainly affects the number of flocs considered for the analysis. Depending on measurements, between 50 and 65% of the detected areas are eliminated. They correspond to isolated pixels related to the imprecision of the background as well as to small flocs. Despite of this treatment, the number of flocs analyzed is always greater than 60,000.

Due to this procedure, some small flocs, mainly produced during the breakage steps (or not destabilized) and having a size less than 10 pixels are not taken into account in the results. The mass of the sample analyzed from one step to the other is thus not strictly conserved. However, since the eliminated areas are less than 10 pixels, the cumulated area is only a little bit affected during this treatment (less than 5%). Meanwhile, the cumulated perimeter is reduced of 15%. But the trends on the cumulated parameters, such as the number, the area or the perimeter calculated on the raw data or on the modified ones neglecting areas less than 10 pixels, are very similar (see Supporting Information Figures 11a-c). So, this image analysis procedure can adequately characterize the morphological properties of flocs and be used for comparison purposes. An increase of the number of flocs results from the preponderance of the breakage mechanism (steps S2 and S4) over the aggregation one (steps S1, S3 and S5). Inversely, a decrease of the number of flocs expresses the preponderance of aggregation. Similarly, an increase of the cumulated perimeter corresponds to the breakage of flocs, while a decrease indicates a preponderance of the aggregation mechanism or a significant restructuring phenomenon. Finally, an increase (resp. a decrease) of the cumulated apparent area, which can be seen in a 3D-view as an apparent volume, results from an increase of the porosity (resp. a floc compaction) as far as the mass is conserved.

Results and Discussion

Surface-based mean floc characteristics over the different flocculation steps

The results are first analyzed with respect to the mean floc characteristics. The changes of the surface-based mean gyration radius and the surface-based mean circularity vs. time are, respectively, reported on Figures 2a,b. The corresponding relative standard deviations (ratio of the standard deviation on the mean property) for the radius of gyration and the circularity are reported on the paper on-line version (see Supporting Information Figures 12a,b).

Opposite trends are observed on Figure 2a compared to Figure 2b all along the different phases. Moreover, the change corresponding to Run 1 (hGi5280 s 21 ) and Run 2 (hGi5190 s 21 ) are similar but the amplitude is more important for the Run 1, for which the gap between the shear stress for two consecutive steps if higher.

During the first phase (S1), the mean radius of gyration sharply increases during the first minutes, as flocs are formed, and then slowly decreases meanwhile a re-structuration of flocs occurs (cf. Figure 2a). When the flocs are formed by aggregation, the change of the standard deviation over time (see Figure 12a) reveals that the distribution of flocs rapidly spreads over time first and then it decreases indicating a homogenization of the population. Indeed it was pointed out on previously that during this change the floc number and the cumulative perimeter increase whereas the apparent total area slightly decreases; all of this being coherent with a breakage mechanism and finally a floc compaction vs. time. Symmetrically, the floc circularity strongly decreases first and then slowly increases during the first phase (Figure 2b). The decrease of the relative standard deviation (see Figure 12b) during the first aggregation step is also consistent with a restructuration of the system.

On the base of the evolution of the floc size and shape distributions (not reported here), it was concluded that a steady state was reached after about two hours. The stress conditions imposed during the first phase are the same for the two runs. As expected, the mean radii of gyration are quite similar. However it can be observed on Figure 2b that the mean circularity differs significantly. This difference can only be attributed to the poor reproducibility of the results during this first phase as it was reported before.

During the second phase (S2) and the fourth as well (S4), corresponding mainly to breakage due to an increase of the shear stress, the mean radius of gyration (Figure 2a) strongly decreases whereas the circularity increases (Figure 2b). The breakage steps S2 and S4 also lead to a decrease of the relative standard deviation (see Figures 12a,b in the Supporting Information) showing that the population is more homogeneous in size and shape. The changes vs. time during the two steps are rather similar. However the steady state is not reached after one hour during S2, whereas the mean values of the two characteristic parameters do not evolve any more after 1 h during the step S4. The process time-scale can be evaluated fitting the mean parameter vs. time with an exponential decreasing law. Thus, the breakage kinetics is more rapid for the step S4 compared to the step S2 with a reduction of the breakage timescale from approximately 3 to 2 min for the Run 1 (hGi5280 s 21 ) and from 11 to 7 minutes for the Run 2 (hGi5190 s 21 ). The structuration induced by the hydrodynamic conditions during S4 leads to more compact flocs, having a mean size slightly smaller (and a circularity slightly higher) than those produced at the end of S2. Moreover, the breakage steps imposed during the Run 1 produce flocs having a mean size clearly smaller (30%) than those produced during the Run 2, but the floc shape is nearly the same.

The third and the fifth phase (S3 and S5) mainly correspond to an aggregation mechanism due to a low shear stress as for the first phase. However, the initial population is not the same; it corresponds to the population created during the previous breakage step for S3 and S5 whereas it was composed by initial aggregates for the phase S1. As a consequence, the growth is very rapid during the phases S3 and S5 with a reduction of the aggregation time-scale from 1.5 to 0.9 min for the Run 1 (hGi5280 s 21 ) between the two steps and about 2 min for both steps for the Run 2 (hGi5190 s [START_REF] Bouyer | Experimental analysis of floc size distribution and hydrodynamics in a jar-test[END_REF] ). The mean radius of gyration of flocs produced during S3 (around 103 lm for Run1 and 96 lm for Run 2) and S5 (around 103 lm for Run1 and 86 lm for Run2) is smaller than during S1(119 lm for both runs). Concerning the shape change, it can be observed on Figure 2b that the mean circularity rapidly decreases during the steps S3 and S5 and then tends to a stationary and identical value. The re-flocculation steps S3 and S5 generate a variety of shapes. Thus, the population of flocs is more heterogeneous and the relative standard deviation gets higher values. This effect is more pronounced for the Run 1. Moreover, the flocs produced during the re-flocculation steps (S3 and S5) are larger for the Run 1 (hGi5280 s 21 ) than for the Run 2 (hGi5190 s [START_REF] Bouyer | Experimental analysis of floc size distribution and hydrodynamics in a jar-test[END_REF] ). The initial aggregates produce during the corresponding breakage steps (S2 and S4) are smaller for the Run 1 (higher shear stress) and this seems to promote aggregation. During the runs, a slight re-structuration can be observed. It can above all be observed during the step S5 for the Run 1. Indeed, the growth first and then the breakage and the re-structuration can only occur if the initial flocs are small enough. For the Run 1, the final state reached during S5 seems to be identical to the one observed at the end of the step S3. On the opposite, for the Run 2, the aggregates formed during S5 are more rounded (having a smaller size and a higher circularity) than those formed during S3.

To get more quantified information on the reversibility of the process over the hydrodynamic sequencing, several ratios based on the mean properties may be used. For example, the strength factor (SF) and the recovery factor (RF) as defined by Franc ¸ois 25 may be applied both on the breakage and reflocculation phases. SF is a measure of the floc strength property and RF quantifies the reversibility of the size change. These factors have been calculated on the base of the mean radius of gyration obtained at the end of each phase by the following expressions:

SF5 Rg B Rg A1 (8)
RF5 Rg A2 2Rg B Rg A1 2Rg B ( 9
)
where Rg B is the mean radius of gyration of flocs at the end of a breakage step (S2 or S4), and Rg A1 (respectively, Rg A2 Þ; the mean radius of gyration after a first flocculation step (S1 or S3) (respectively, a second flocculation step S3 or S5). Similar analysis can be done considering the mean circularity values and thus defining the circularity factor (CF) which characterizes the floc compaction and the recovery shape factor (RSF) which expresses the reversibility of the shape property as:

CF5 Ci B Ci A1 (10)
RSF5 Ci A2 2Ci B Ci A1 2Ci B ( 11
)
where Cig B is the mean circularity of flocs at the end of a breakage step (S2 or S4), and Ci A1 (respectively, Ci A2 Þ; the mean radius of gyration after a first flocculation step (S1 or S3) (respectively, a second flocculation step S3 or S5). The numerical values of the mean properties at the end of each phase, as well as the different factors calculated by Eqs. 8-11 are reported on Table 2.

Between the first and the second breakage cycle, the strength factor increases and the floc compaction is slightly reduced. The strength factor is also obviously higher and the compaction lower for the Run 2 (hGi5190 s 21 ) compared to Run 1 (hGi5280 s 21 ). The ability of breakage is higher when the stress conditions are more severe and the aggregates are thus more rounded. Moreover a full reproducibility on both size and shape factors is observed during the second cycle on Run 1 (RF(S4) and RSF(S4) 5 1). A longer phase duration or another breakage/re-flocculation cycle would have been necessary to recover perfect reversibility when soft stress conditions (as for Run 2) are applied.

Steady state distributions of floc properties

Here are analyzed the size distributions obtained after each phase under different stress conditions. The surfaced-based size distributions (with respect to the radius of gyration) and the number-based size distributions are plotted on Figures 3a,b for the Run 1 (hGi5280 s 21 ) and Figures 4a,b for the Run 2 (hGi5190 s 21 ).

It is clear on Figures 3a, b and4a, b that the surface-based distributions are monomodal whereas the number-based distributions are multimodal, characterized by an important population of very small particles and one or even two other populations of larger flocs. The fine sub-population does not evolve during the different steps, indicating that this population, mainly constituted by the initial particles or very small aggregates, is not destabilized by salt addition. Let's now compare the distributions obtained for these two runs. A clear distinction between the aggregation and breakage phases can be done for the Run 1 (hGi5280 s 21 ). This difference is less pronounced for the second run; the flocs produced during the aggregation phases being smaller and those formed during the breakage steps being larger. For both runs, the size distributions at the end of the breakage steps (S2 and S4) are close to each other but not perfectly superimposed. The surface-based distributions slightly move toward smaller sizes in agreement with the observations done previously on the mean values. Looking at the number-based distributions, it can be see that the mode of the main population is the same (around 39 lm for the Run 1 and 44 lm for the Run 2) for the steps S2 and S4, only the maximum size and the proportion of larger flocs are different for these two breakage steps. It may be assumed that the flocs are calibrated by the hydrodynamics conditions and this process initiated during S2 continues during S4. Stationary identical states could probably be achieved with a longer breakage phase duration.

Concerning the re-flocculation steps, the size distributions recovered at the end of the steps S3 and S5 are well superimposed (both on surface-based and number-based distributions) for the Run 1 (hGi5280 s 21 ) which can thus be assumed as fully reproducible, whereas the aggregation mechanism during the Run 2 (hGi5190 s 21 ) is only partially reversible. Moreover, the number-based distributions allow observing the presence on the curves associated with the re-flocculation steps S3 and S5 of a sub-population corresponding to the primary mode of flocs produced during the previous breakage steps (around 39 lm for the Run 1 and 44lm for the Run 2). This is more obvious for the Run 2, for which trimodal distributions are obtained, constituted by passive primary aggregates (mode around 12 lm), small flocs resulting of the breakage step (mode around 44 lm) and large flocs obtained by aggregation (mode around 75 lm).

As depicted in the introduction, the effect of the hydrodynamic conditions on the floc size can be interpreted on the base of the viscous dissipation of the kinetic energy occurring at the Kolmogorov micro-scale. Here, the mean Kolmogorov calculated for the different shear rates imposed during the cycle sequence: hGi575 s 21 ð ) for the aggregation steps, (hGi5280 s 21 -Run 1) on one hand and (hGi5190 s 21 -Run 2) on a second hand for the breakage steps has been compared on Figure 5 is slightly reduced between the two cycle sequences. The characteristic size corresponding to the flocs produced during the first aggregation step (step S1) is higher than the mean Kolmogorov microscale. But it decreases then for the following reflocculation steps (S3 and S5), and becomes lower than the mean microscale, especially for the Run 2. Let's remember that the hydrodynamics in the Taylor-Couette is heterogeneous showing more or less stressing conditions depending on the zones of the reactor; the more intense being is in the wall regions and the less one in the center of the toroidal vortices. So large flocs may be formed in the regions where the stress conditions are the softest and be broken when they pass through the zones where the stress conditions are harder.

We consider now the circularity distributions obtained at steady state for the different phases and runs. The surface-based and number-based circularity distributions are, respectively, reported on Figures 6a,b Contrary to the size distributions, both the surface-based and the number-based circularity distributions are monomodal. For both experiments, the distributions corresponding to the breakage steps (S2 and S4) are close, the circularities being slightly higher at the end of the step S4. Moreover the number and surface distributions are very similar. The effect of size in correlation with the type of representation is not significant (the surface distribution putting in evidence the property of the coarser fraction compared to the number distribution). The similarity between the two types of representation suggests that all the sub-populations, distinguishable by their size, present the same variety of shape. On the contrary the flocs produced during the aggregation steps, especially during the first step (S1) have different distributions depending on the type of representation. The effect of size correlated with the type of representation is now significant. Although the initial population or the flocs produced during the breakage steps have similar size and shape, the large flocs appeared during the aggregation phases lead to a variety of shapes. The range of variation of the circularity is increased on the left hand side of the curves (less circular flocs are created). This effect is clearly seen on the surface distributions, but also on the number distributions. For the Run 1, the circularity distributions after the three aggregation phases are rather similar. This is not surprising since the hydrodynamic conditions which produced the initial flocs were close (100 rpm during the coagulation phase before the step S1, compared to 90 rpm for the breakage steps before the reflocculation phases). On the contrary for the Run 2, the initial flocs for the re-flocculation steps S3 and S5 (produced at 75 rpm) are different to those forming the initial population for the step S1. Finally, if soft conditions are applied during the breakage steps, the initial flocs for the aggregation phase are larger and they give birth to smaller flocs, less complex (more rounded flocs having a higher circularity).

Quantitative Analysis of Populations During the Flocculation Steps

In this section, attention is paid on the shape of the distributions. It is often convenient to compare the distribution of a population with some classical ones, as the normal or the lognormal distributions. This can be easily done calculating selected parameters derived from some specific moments of the distribution. The first raw moment m 1 ð Þ taking into Since the distributions of the radius of gyration are plotted on a log-scale, it is more relevant to compare them to a lognormal distribution. For that purpose, the moments of the steady state distributions of Rg in log-scale were calculated allowing the determination of the two characteristic parameters l Rg and r Rg of the log-normal law (see Table 3a). The theoretical log-normal distributions were then compared to the experimental distributions of the radius of gyration. An illustration is given in the Supporting Information (Figure 13a) for the Run 1. Similar results were obtained for the Run 2 (not reported here). It is obvious that log-normal laws cannot properly fit the experimental size distributions. The general trend of the curves is well simulated but the theoretical laws underestimate the amount of small aggregates on the left part of the curves. The modes of the distributions are lower than the experimental modes and the maximum surface percentages are also underestimated.

The same analysis was done for the circularity distributions, excepted that they were compared to normal laws. The corresponding values of the characteristic parameters l Ci and r Ci of the normal law are reported on Table 3b. A comparison with the experimental distributions is reported on the Supporting Information (Figure 13b) for Run 1. The same observations were done for the Run 2. The experimental circularity distributions are better predicted by normal laws than previously but the curves are not perfectly fitted.

Additionally, some other moments of the distributions can be used to refine the analysis. Indeed, the skewness (s), which indicates if a distribution is symmetric or has a tail on the left or the right side, is the normalized third central moment and the kurtosis (or excess kurtosis) (j 2 3) which is a measure of the heaviness of the tail of the distribution, compared to a normal distribution is the normalized fourth central moment minus 3. By this way, a normal distribution is characterized by s 5 0 and j 2 3 5 0 and a log-normal distribution by s log 5 0 and j log 2 3 5 0. The corresponding values are reported on Figures 8a,b.

The skewness, calculated on a surface-based and characterizing here the distribution of log Rg ð Þ takes always negative values (Figure 8a), indicating a tail of the distributions on the left. This is the signature of the presence of the initial particles or small aggregates that are not destabilized and stay unchanged over time. Moreover during the aggregation phases, the absolute value of s log increases because the distributions move toward larger sizes due to the floc growth but the tail characterizing the small aggregates still remain. Inversely, during the breakage steps, the modulus of s log decreases, the distribution moves toward the left and becomes more symmetric. Simultaneously, the excess kurtosis has positive values (Figure 8b) revealing the heaviness of the tail of the distribution, once again due to the presence of the small passive sub-population.

Considering the circularity distributions, the skewness and the excess kurtosis, calculated on a surface basis have been reported on Figures 9a,b. The skewness (Figure 9a) which has initially a positive value decreases over time. It expresses the presence of a tail on the right side corresponding to the primary particles or small flocs which have a high circularity. Their proportion decreases due to the aggregation mechanism and hence the skewness decreases. The excess kurtosis is always negative (Figure 9b), characterizing rather large and flat distributions. During the breakage steps S2 and S4, the skewness takes negative values during the breakage steps, indicating a tail on the left of the circularity distributions. The absolute value of the excess kurtosis decreases so the circularity distributions become less flat. The variety of shapes is more important during the re-flocculation steps S3 and S5. While the excess kurtosis is quite similar, the skewness differs significantly between the two runs. For the Run 1, the skewness gets positive value, indicating a tail on the right part of the distribution; the small flocs, produced in a high proportion, having a higher circularity. For the Run 2, the absolute value of the skewness is lower than during the breakage steps, showing that the distributions becomes more symmetric but the skewness still remains negative. Larger are the flocs, less circular they are and their presence is detected on the queue at the left of the distributions. So, the soft conditions imposed during the breakage steps of the Run 2 (hGi5190 s 21 ) induce a better homogeneity of shapes during the re-flocculation steps than for the Run 1 (hGi5280 s [START_REF] Bouyer | Experimental analysis of floc size distribution and hydrodynamics in a jar-test[END_REF] ).

The analysis of variables such as the relative standard deviation, the skewness and the excess kurtosis gives an interesting complementary information to the one of the full distributions. Indeed, it has been seen here that nearly all the information on the change under hydrodynamic conditions of the full distributions over time is contained within these three variables. The main advantage of this analysis is that the knowledge of the first moments of the distribution, and not the distribution itself, is needed to interpret the effect of any parameter on the flocculation mechanism. The interest of this approach is in the modeling of the flocculation process based on bidimensional population balance and solving it using a method of moments.

However a limitation of this analysis is when the population distribution is multimodal, as with the number-based size distribution reported above. The interpretation of the specific variables, such as the relative standard deviation, the skewness and the kurtosis is much more complex.

Conclusion

This article reports experimental results of bentonite flocculation performed under controlled turbulent conditions applying successive cycles of aggregation and breakage. The different cycle sequences corresponding to preponderant aggregation or breakage mechanisms can easily be distinguished on the change of the cumulated floc number or the cumulated perimeter. The total area does not change a lot over the sequence. It can express a slight change of the internal porosity of flocs.

The change over the cycle sequence of the circularity and the radius of gyration have been analyzed both on the numberbased and surface-based distributions which give complementary information. Indeed, unlike the surface-based distributions which put more in evidence the biggest aggregates, the number-based distributions are more representative of the whole population of flocs, but they may include a measurement bias since the particles having a size less than 10 pixels are not taken into account.

The number-based distributions of the radius of gyration are bimodal; they involve the signature of an inert population of primary particles and a second population of flocs (or flocculi) whose mode changes based on the cycle sequence performed at a low or high shear rate. On the contrary, the surface-based distributions are monomodal, characteristic of the population of flocs or flocculi mode changing according to the shear rate applied. Both the number-based and surface-based distributions only have one mode; the different populations of flocs distinguishable by their size present a wide variety of shapes.

The breakage steps produce flocculi whose size is calibrated by the hydrodynamics. These small flocs, more dense than the initial aggregates formed, serve then as bricks to form larger flocs during the next aggregation step. For this reason, the restructuring phenomena usually observed maintaining a constant shear rate during a flocculation process is no more present after the first cycle during a sequenced run. So, after the first breakage step, a stronger correlation appears then between the size and the shape of flocs over the following cycle sequence, whatever the step (breakage or aggregation) at steady state and during the transitory state as well. Finally a full reversibility on both floc size and shape can be reached after one or two cycles depending on the shear rate applied. Moreover, the change over time of the size and shape of flocs can also be expressed looking at some relevant moments, especially those used for the calculation of the mean, the standard deviation, the skewness and the excess kurtosis. This approach may be particularly useful for modeling purposes based on population balance models. Indeed, once more than one property has to be considered, such as the radius of gyration and the circularity in the present case, or even more relevant floc properties, such as the area and the perimeter, the moment based methods to solve the population balance equations may be fruitful.

Literature Cited

Figure 1 .

 1 Figure 1. Experimental protocol.

Figure 2 .

 2 Figure 2. (a) Change of the surface-based mean radius of gyration vs. time over the different steps. (b) Change of the surface-based mean circularity vs. time over the different steps.

  to the mode of the related floc size distributions. All along the flocculation sequence, the flocs mainly have a size close to the mean Kolmogorov micro-scale, indicating that the flocs produced under a shear rate are calibrated by the Kolmogorov microscale in agreement with previous results of the literature. Concerning the breakage steps (S2 and S4), the floc characteristic diameter (mode of the Rg distributions) remains close to the mean Kolmogorov microscale although it

Figure 3 .

 3 Figure 3. (a) Surface-based gyration radius distributions for the different steps-Run 1(hGi5280 s 21 ). (b) Numberbased gyration radius distributions for the different steps-Run 1 hGi5280 s 21 À ).

Figure 4 .

 4 Figure 4. (a) Surface-based gyration radius distributions for the different steps-Run 2 (hGi5190 s 21 ). (b) Numberbased gyration radius distributions for the different steps-Run 2 (hGi5190 s 21 ).

  for the Run 1(hGi5280 s 21 ) and Figures 7a, b for the Run 2 (hGi5190 s 21 ).

Figure 5 .

 5 Figure 5. Comparison of the floc size with the Kolmogorov micro-scale.

Figure 6 .

 6 Figure 6. (a) Surface-based circularity distributions for the different steps (hGi5280 s 21 -Run 1). (b) Number-based circularity distributions for the different steps (hGi5280 s 21 -Run 1).

Figure 7 .

 7 Figure 7. (a) Surface-based circularity distributions for the different steps (hGi5190 s 21 -Run 2). (b) Number-based circularity distributions for the different steps (hGi5190 s 21 2Run 2).

Figure 8 .

 8 Figure 8. (a) Skewness of Rg distributions over time. (b) Excess kurtosis of Rg distributions over time.

Figure 9 .

 9 Figure 9. (a) Skewness of Ci distributions over time. (b) Excess kurtosis of Ci distributions over time.

Table 1 .

 1 Global Hydrodynamic Characteristics of the Taylor-Couette Reactor

	N (rpm)	30	70	90	100
	Ta (-) e m 2 s 23	3.3 3 10 6	1.8 3 10 7	3.0 3 10 7	3.7 3 10 7
	G s 21 ð Þ	75	190	280	325
	g lm ð Þ	116	72	60

ð Þ 5.4 3 10 23 37 3 10 23 77 3 10 23 106 3 10 23

Table 2 .

 2 Reported Values of the Mean Properties of Flocs at the End of Each Step: (a) Radius of Gyration; (b) Circularity

		Run 1	Run 2
	G5280 s 21	G5190 s 21	Gap %
	(a) Rg mean (mm) at the end of the cycle
	S1	118.9	119.0	0.1
	S2	53.7	69.6	29.6
	S3	103.2	95.8	7.7
	S4	50.1	63.9	27.5
	S5	102.9	85.5	20.3
	Characteristic ratios
	SF(S2)	0.45	0.58
	SF(S4)	0.49	0.67
	RF(S2)	0.76	0.53
	RF(S4)	0.99	0.68
	(b) Ci mean (-) at the end of the cycle
	S1	0.57	0.59	3.3
	S2	0.70	0.68	2.8
	S3	0.59	0.62	5.4
	S4	0.71	0.69	2.2
	S5	0.59	0.64	9.2
	Characteristic ratios
	CF(S2)	1.22	1.15
	CF(S4)	1.20	1.12
	RSF(S2)	0.87	0.66
	RSF(S4)	1.00	0.71

Table 3

 3

		. Fitting of the Experimental Distributions:
	(a) Log-Normal Distributions of the Radius of Gyration;
	(b) Normal Distributions of the Circularity
			0		2	1
	(a) dF dlog Rg 5	1 log rRg ð Þ p exp 2 ffiffiffiffi 2p @	log Rg = l Rg 2log rRg ð Þ	2	A
			Run 1		Run 2
			G5280 s 21		G5190 s 21
		l Rg		r Rg	l Rg	r Rg
	S1	108.85		1.52	109.95	1.49
	S2		49.40		1.49	64.07	1.51
	S3		92.76		1.58	87.35	1.52
	S4		46.06		1.49	59.14	1.49
	S5		92.76		1.58	78.26	1.51
	(b) dF dCi 5 1 r Ci ffiffiffi 2p p	ð exp 2 Ci2l Ci 2r 2 Ci	Þ 2
			Run 1		Run 2
			G5280 s 21		G5190 s 21
			l Ci		r Ci	l Ci	r Ci
	S1		0.57		0.12	0.59	0.11
	S2		0.70		0.10	0.68	0.10
	S3		0.59		0.12	0.62	0.11
	S4		0.71		0.09	0.69	0.10
	S5		0.59		0.12	0.64	0.11


Richard Bernhart Owen

Veronica Muiruri

Tim Lowenstein

Robin Renaut

Nathan Rabideaux

Shangde Luo

Alan Deino

Mark Sier

Guillaume Dupont-Nivet

Emma Mcnulty

Kennie Leet

Andrew Cohen

Christopher Campisano

Daniel Deocampo

Chuan-Chou Shen

Anne Billingsley

Anthony Mbuthia

Progressive aridification in East Africa over the last half million years and implications for human evolution

Keywords: Quaternary, paleoclimate, paleolimnology, hominins, Lake Magadi

Evidence for Quaternary climate change in East Africa has been derived from outcrops on land, lake cores and from marine dust, leaf wax and pollen records. These data have previously been used to evaluate the impact of climate change on hominin evolution, but correlations have proven to be difficult given poor data continuity and the great distances between marine cores and terrestrial basins where fossil evidence is located. Here we present the first continental coring evidence for progressive aridification since about 575 thousand years ago (ka), based on Lake Magadi (Kenya) sediments. This long-term drying trend was interrupted by many wet-dry cycles, with the greatest variability developing during times of high eccentricity-modulated precession. Intense aridification apparent in the Magadi record took place between 525-400 ka, with relatively persistent arid conditions after 350 ka and through to the present. Arid conditions in the Magadi Basin coincide with the Mid-Brunhes Event and overlap with mammalian extinctions in the South Kenya Rift between 500-400 ka. The 525-400 ka arid phase developed in the South Kenya Rift between the period when the last Acheulean tools are reported at about 500 ka and prior to the appearance of Middle Stone Age artefacts by about 320 ka. Our data suggest that increasing mid-late Pleistocene aridification and environmental variability may have been drivers in the physical and cultural evolution of H. sapiensin East Africa.

Several hypotheses have attempted to explain human evolution and its possible relationship with environmental change [START_REF] Potts | Evolution and climatic variability[END_REF][START_REF] Potts | Alternating high and low climate variability: The context of natural selection and speciation in Plio-Pleistocene hominin evolution[END_REF]. The savannah hypothesis suggested that bipedalism resulted from hominins moving from forests to grassy savannas [START_REF] Vrba | An hypothesis of heterochrony in response to climatic cooling and its relevance to early hominid evolution[END_REF]. Other theories emphasized climate as an evolution driver, including the aridity, turnover pulse, variability selection and accumulated plasticity hypotheses [START_REF] Potts | Hominin evolution in settings of strong environmental variability[END_REF][START_REF] Maslin | East African climate pulses and early human evolution[END_REF][START_REF] Grove | Evolution and dispersal under climatic instability: a simple evolutionary algorithm[END_REF][START_REF]Plio-Pleistocene African climate[END_REF]. Evaluation of these ideas has been hindered by a lack of basin-scale records that can provide a high-resolution environmental context. The Hominin Sites and Paleolakes Drilling Project (HSPDP) has attempted to fill this gap by providing continental sedimentary records that can be linked to nearby hominin fossils and artefacts in Ethiopia and Kenya [START_REF] Campisano | The Hominin Sites and Paleolakes Drilling Project: High-Resolution paleoclimate records from the East African Rift System and their implications for understanding the environmental context of hominin evolution[END_REF].

Here we present evidence from the southernmost HSPDP site, at Lake Magadi (Fig. 1), that is relevant to debates about the climatic context of human evolution, from an area close to some of the most important records of hominin prehistory. The Lake Magadi record spans the past one million years (Fig. 2; see SI Appendix text, Figs. S1-3 and Tables S1-7 for details of dating methods) and can be compared with a 1.2 million-year (Ma) sequence at Olorgesailie, 25 km to the northeast [START_REF] Owen | Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley[END_REF][START_REF] Behrensmeyer | Olorgesailie, Kenya: a million Submission PDF years in the life of a rift basin[END_REF][START_REF] Behrensmeyer | The Oltulelei Formation of the southern Kenyan Rift Valley: A chronicle of rapid landscape transformation over the last 500 k.y[END_REF]. The Olorgesailie deposits and archeological record document a transition from Acheulean to Middle Stone Age (MSA) toolkits [START_REF] Brooks | Long-distance stone transport and pigment use in the earliest Middle Stone Age[END_REF][START_REF] Deino | Chronology of the Acheulean to Middle Stone Age transition in eastern Africa[END_REF][START_REF] Potts | Environmental dynamics during the onset of the Middle Stone Age in eastern Africa[END_REF], with the Magadi core, and cores from the neighboring Koora Basin drilled by the Olorgesailie Drilling Project [START_REF] Campisano | The Hominin Sites and Paleolakes Drilling Project: High-Resolution paleoclimate records from the East African Rift System and their implications for understanding the environmental context of hominin evolution[END_REF][START_REF] Deino | Chronology of the Olorgesailie Drilling Project Core 1A, Koora Graben, southern Kenya Rift[END_REF], covering the period when H. sapiens emerged in Africa [START_REF] Hublin | New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens[END_REF]. The Magadi and Olorgesailie records also span a turnover in large mammals before 320 ka [START_REF] Potts | Environmental dynamics during the onset of the Middle Stone Age in eastern Africa[END_REF], which has also been documented at Lainyamok between 500-400 ka, 15 km west of Magadi [START_REF] Faith | New perspectives on middle Pleistocene change in the large mammal faunas of East Africa: Damaliscus hypsodon sp. nov. (Mammalia, Artiodactyla) from Lainyamok, Kenya[END_REF]. Thus, Lake Magadi is located in a region containing archeological and paleontological Middle Pleistocene sites that provide critical information about the relationships between climate dynamics and human prehistory.

Lake Magadi is a seasonally flooded saline alkaline pan about 606 m above sea level in the South Kenya Rift (Fig. 1a) surrounded by poorly correlated cherts, silts and evaporites [START_REF] Eugster | Lake Magadi, Kenya: a model for rift valley hydrochemistry and sedimentation?[END_REF][START_REF] Behr | Magadiite and magadi chert: a critical analysis of the silica sediments in the Lake Magadi Basin, Kenya[END_REF]. Core HSPDP-MAG14-2A (hereafter MAG14-2A, Fig. 1b) includes trona, zeolitic mud, chert, tuff, and carbonate grainstone deposited in a regional tectonic sump that has been occupied by a lake since eruption of the underlying lavas (1.08 Ma). This study combines geochemical, mineralogical, diatom and pollen analyses that indicate a trend towards a more saline, alkaline lake and a more arid climate from about 575 ka to the present. This progressive change was interrupted by wetter episodes, but was directional in overall character towards increasing aridity.

Results

Progressive changes in geochemistry and mineralogy. Loss on Ignition (LOI) at 1,000°C (Fig. 2a; SI Appendix, Table S7) indicates combustible carbonates (calcite, trona) and organic matter. High LOI values before 950 ka, or 187 meters below surface (mbs), reflect shallow water carbonate grainstones. LOI (550°C) is low (<3%) between 950-800 ka (187-178 mbs) with higher val-

Significance

Previous research hypotheses have related hominin evolution to climate change. However, most theories lack basin-scale evidence for a link between environment and hominin evolution. This study documents continental, core-based, evidence for a progressive increase in aridity since about 575 ka in the Magadi Basin with a significant change from the Mid-Brunhes Event (∼ 430 ka). Intense aridification in the Magadi Basin corresponds with faunal extinctions and changes in toolkits in the nearby Olorgesailie Basin. Our data are consistent with climate variability as an important driver in hominin evolution, but also suggest that intensifying aridity may have had a significant influence on the origins of modern H. sapiens and the onset of the Middle Stone Age. ues (5-20%) in younger sediments suggesting greater lake floor anoxia. Major increases in LOI in sediments <111 ka (65 mbs) reflect increases in organics and trona, which accumulated in highly saline alkaline, anoxic waters. Na/Ca ratios increased with time (Fig. 2a) as a result of a shift from calcium-rich (calcite, Mgcalcite) to Na-and K-rich (erionite, trona) chemical sediments. The upward decline in Ca probably reflects early precipitation of CaCO 3 near shorelines where streams entered an alkaline paleolake and relative increases in groundwater contributions, as drier conditions developed, which would have favored lower Ca through subsurface precipitation.

Reserved for Publication Footnotes

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Mineralogical data also document progressive changes (Fig. 2a). Authigenic minerals are dominated by calcite and Mg-calcite in sediments >385 ka (103 mbs), indicating fresh to mildly saline groundwater. Analcime occurs throughout the core with other zeolites accumulating since 375 ka, indicating a shift towards more saline alkaline conditions. The zeolites formed from Na-Al-Si alkaline spring gels washed into the basin or by alteration of aluminosilicate minerals or volcanic glass [START_REF] Hay | Occurrence of zeolites in sedimentary rocks: An Overview, in: Natural Zeolites: Occurrence, Properties, Applications[END_REF]. Trona was deposited after 111 ka (65 mbs) in highly saline, alkaline water.

Diatom and pollen stratigraphy. Diatoms are absent in MAG14-2A sediments >545 ka (132 mbs), but are present in basal limestones in a second core, MAG14-1A (Fig. 1a), where benthic and epiphytic taxa indicate freshwater swamps. A few cherts contain Anomoeoneis sphaerophora, a moderate to high salinity taxon present in shallow springs today [START_REF] Barker | Diatoms as paleolimnological indicators: a reconstruction of Late Quaternary environments in two East African salt lakes[END_REF]. The occurrence of diatoms only in well-cemented chert and limestone suggests they may have dissolved from unlithified deposits. The dominant taxa in the diatomaceous interval (about 545-16 ka; 132-38 mbs) include mixed planktonic freshwater (Aulacoseira granulata, A. agassizi) and saline species (Cyclotella meneghiniana, Thalassiosira faurii; details in SI Appendix, Fig. S4 and Dataset S1), suggesting a deep meromictic lake with permanent saline waters that were periodically overtopped by fresh fluvial inputs. The evidence for flooding is supported by intermittent freshwater benthic taxa [START_REF] Barker | Taphonomy and diagenesis in diatom assemblages; a Late Pleistocene palaeoecological study from Lake Magadi, Kenya[END_REF] such as Cocconeis placentula, Encyonema muelleri and Epithemia spp.

Mean transfer functions for all diatoms indicate pH of about 7.4-11.4 and conductivities of 300-40,000 μS cm -1 (Fig. 3). However, given the evidence for episodic meromixis, there is a need to recalculate the data separately for surface freshwater and deeper saline water taxa (Fig. 3), which suggests that the pH of freshwater inputs ranged between 7.3-8.5 with conductivities of 200-2,000 μS cm -1 . The saline floras document increased pH after about 385 ka (103 mbs), from 9.2-10 to 9.4-11.5. An absence of diatoms after about 18 ka suggests that pH and conductivity exceeded tolerance limits or resulted in dissolution.

Grasses (Poaceae) dominated with a smaller sedge (Cyperaceae) component before about 900 ka (184 mbs), with pollen not preserved between 900-735 ka (184-168 mbs) (Fig. 4; detailed floras in SI Appendix, Figs. S9-S10 and Dataset S2), possibly due to oxygenated conditions. Cyperaceae increased relative to Poaceae between about 735-520 ka (168-127 mbs) and dominated between 605-568 ka (143-136 mbs), with other minor aquatics (Typha, Potamogeton), suggesting shallow fresh waters [START_REF] Barker | Diatoms as paleolimnological indicators: a reconstruction of Late Quaternary environments in two East African salt lakes[END_REF]. There were also significant increases in Podocarpus (735-520 ka) and Olea (698-635 ka; 160-147 mbs) with Juniperus appearing after 735 ka. Podocarpus is common in modern upland forests in Kenya and, where abundant, has been used to infer expansion of Afromontane forests or changes in fluviallytransported regional pollen [START_REF] Bonnefille | Implications of pollen assemblage from the Koobi Fora Formation, East Rudolf, Kenya[END_REF][START_REF] Bonnefille | Cenozoic vegetation, climate changes and hominid evolution in tropical Africa[END_REF]. The parallel trends for Podocarpus, Cyperaceae and the other aquatic indicators imply a climatic control. Although broadly the wettest interval in the last million years, climate varied with a drier episode at about 662-625 ka (153-146 mbs) marked by increased Amaranthaceae, with the wettest conditions at 575 ka (137 mbs) (Fig. 4).

Cyperaceae and Podocarpus declined between about 520-400 ka (127-105 mbs), suggesting greater aridity at a time when diatoms indicate a meromictic lake and/or alternating saline and freshwater lakes. A recovery in Podocarpus and Cyperaceae between 400-275 ka (105-94 mbs) suggests a wetter interval that was followed by a decline in these taxa. During the last 275 ka (94 mbs) a variety of taxa expanded and contracted, reflecting wetter and drier settings, but with an overall trend towards greater aridity. Olea, for example, is derived from wet and dry upland evergreen forests and varies in abundance between about 275-5 ka (94-7 mbs) when it disappears. Commiphora and Acacia increase after 205 ka (87 mbs) suggesting dry semi-deciduous dense bushland, with drought-related Amaranthaceae and Juniperus, associated with drier upland forests [START_REF] Beentje | Kenya trees, shrubs and lianas[END_REF], also common. Increases in Cyperaceae along with herbaceous pollen such as Hydrocotyle between about 12-8 ka (17-12 mbs) suggest fresher waters. Afromontane and woodland species were replaced by herbaceous pollen and Poaceae through the last 8 ka with Poaceae forming nearly 100% of the flora after 4 ka (6 mbs). Na/Ca ratios, PCA data for all pollen, and grass to aquatic pollen ratios indicate an overall progressive change during the last half million years (Fig. 5). Prior to 575 ka the basin had trended towards wetter conditions, but then there was an overarching shift towards greater aridity superimposed on multiple wet-dry cycles. Independent terrestrial and aquatic datasets that varied in unison indicate that this change was not simply due to lake hydrology and local tectonics, but was driven by a directional climate shift. Intermittent positive spikes in diatom PCA data between 350-70 ka reflect increases in shallow, freshwater, diatoms. The strong contrast in habitat preferences between the dominant mixed saline and fresh, deep-water planktonic taxa (Thalassiosira spp., Aulacoseira granulata and variety valida) and episodic shallow, freshwater, lake/wetland (A. agassizi, A. granulata v. angustissima) and benthic floras (Fig. 3) suggest that the latter may have been transported intermittently by floods from nearby swampy and/or fluvial settings to the core site [START_REF] Barker | Taphonomy and diagenesis in diatom assemblages; a Late Pleistocene palaeoecological study from Lake Magadi, Kenya[END_REF]. Many of the younger spikes also match pollen evidence for wetter periods (195, 170, 125, 95, 80 ka) and interglacial episodes. The amplitude of the spikes decrease with time, as does their temporal spacing. A lack of diatoms after about 16 ka reflects the formation of an ephemeral hypersaline playa.

Discussion

Climate change. On a global scale a major inflection point in Pleistocene climate, the Mid-Brunhes Event (MBE), close to the boundary between Marine Oxygen Isotope stages 12 to 11, took place about 430 ka. Subsequently, there was increased climate variability with the development of colder glacial periods and warmer interglacial episodes [START_REF]African climate change and faunal evolution during the Pliocene-Pleistocene[END_REF][START_REF] Jouzel | Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years[END_REF], although it has been suggested that the MBE is regionally inconsistent. Terrestrial data from the UK (28), for example, have been used to infer no significant change across the MBE, whereas continental evidence from Spain (29) supports a climate transition.

The continental pollen record from equatorial Lake Magadi provides strong support for a climate transition at the MBE (Fig. 5), suggesting a potential link to global CO 2 /glacial cyclicity, with a major change from wetter conditions to greater aridity after about 430 ka. The overall trend towards dryer conditions was initiated about 575 ka, with particularly intense aridity developing between 525-400 ka, which partially overlaps with Marine Oxygen Isotope Stage 11 (424-374 ka), the warmest interglacial of the last 500 ka [START_REF] Howard | Warm future in the past[END_REF]. Subsequently, many wetter and drier cycles were superimposed on progressive aridification, with diatomaceous parts of the core documenting a tendency towards increased flood inputs of benthic taxa (increased PCA values) during interglacial episodes (Fig. 5). This directional increase in aridity since ∼575 ka has not previously been documented in continuous continental cores from East Africa, although there is support from pedogenic carbonate carbon isotopes in outcrops [START_REF] Levin | Environment and climate of early human evolution[END_REF] and eolian dust records from the northwest Indian Ocean (7, 26) (Fig. 5), which suggest a similar pattern of increasing aridity and intermittently wetter intervals through the last half million years. Limited pedogenic carbonate carbon isotope data from Olorgesailie indicate an overall increase in C4 grasslands during the last 800 ka [START_REF] Potts | Environmental dynamics during the onset of the Middle Stone Age in eastern Africa[END_REF]. Oxygen and carbon isotopes from several sites within a 990 ka Olorgesailie Formation paleosol, for example, suggest an abundance of wooded grassland in a cooler and moister environment at that time compared with the modern grassy semi-arid basin [START_REF] Sikes | Early Pleistocene habitat in Member 1 Olorgesailie based on paleosol stable isotopes[END_REF].

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There are also clear correlations for specific intervals in the Magadi record with other African regions. For example, the deposition of trona and intermittent severe reductions in aquatic pollen between about 110-80 ka indicate a series of very dry phases that alternated with wetter intervals. The termination of this drought period lies close to a transition from megadroughts to wetter conditions at Lake Malawi and more widely across tropical Africa [START_REF] Scholz | East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins[END_REF][START_REF] Ivory | Environmental change explains cichlid adaptive radiation at Lake Malawi over the past 1.2 million years[END_REF]. However, in contrast, the overall drying trend at Magadi is inconsistent with an inferred shift towards wetter conditions noted at Lake Malawi (35) indicating regional African contrasts in vegetation and climate patterns. However, pollen data [START_REF] Ivory | Waxing and waning of forests: Late Quaternary biogeography of southeast Africa[END_REF] show some similarities with high percentages of Podocarpus between 455-325 ka at Malawi coinciding with increased Podocarpus at Magadi after 455 Ka.

Climate and hominin evolution. The nearby Olorgesailie

Basin provides detailed information on hominin evolution for the last million years with evidence for a major transition in stone technologies (Fig. 5). The Olorgesailie Formation (∼1,200-500 ka) includes Acheulean tools [START_REF] Potts | Environmental dynamics during the onset of the Middle Stone Age in eastern Africa[END_REF] whereas MSA artefacts [START_REF] Brooks | Long-distance stone transport and pigment use in the earliest Middle Stone Age[END_REF] are present in the Oltululei Formation (∼320-36 ka) [START_REF] Behrensmeyer | The Oltulelei Formation of the southern Kenyan Rift Valley: A chronicle of rapid landscape transformation over the last 500 k.y[END_REF], with the transition between these toolkits taking place during a period of erosion at Olorgesailie that has been related to faulting and base-level change [START_REF] Behrensmeyer | Olorgesailie, Kenya: a million Submission PDF years in the life of a rift basin[END_REF]. New environmental data from Magadi show increasing aridity during the period of hiatus at Olorgesailie with intense desiccation between 525-400 ka. This suggests that erosion at Olorgesailie might partly reflect climatic conditions, with aridity lowering lake and base levels and changing/reducing the vegetation cover, which would, in turn, tend to enhance erosion of more exposed land surfaces [START_REF] Acosta | Effect of vegetation cover on millennial-scale landscape denudation rates in East Africa[END_REF]. Magadi pollen data (SI Appendix, Fig. S5), for example, indicate an expansion of grasslands and reduction in aquatic pollen after about 525 ka. The arid interval also closely overlaps with a major overturn in mammal faunas with local extinction of large-bodied specialized grazing mammals reported from both Olorgesailie [START_REF] Potts | Environmental dynamics during the onset of the Middle Stone Age in eastern Africa[END_REF] and Lainyamok [START_REF] Faith | New perspectives on middle Pleistocene change in the large mammal faunas of East Africa: Damaliscus hypsodon sp. nov. (Mammalia, Artiodactyla) from Lainyamok, Kenya[END_REF].

The 525-400 ka dry phase and environmental variability would likely have had a significant impact on contemporary hominin populations regionally. It has been hypothesised [START_REF] Potts | Environmental dynamics during the onset of the Middle Stone Age in eastern Africa[END_REF]

Submission PDF

example, that environmental pressures and variability can lead to an uneven distribution of resources that could drive hominins to travel more widely and to interact increasingly with other groups for both raw materials and information. In turn, this would help to drive technological change and its dissemination, resulting in increased foraging success rates and ability to survive.

The Magadi terrestrial pollen record suggests that the interval with greatest climate variability took place between about 650-350 ka with moister periods tending to be linked to highamplitude insolation variability and with drier episodes developing at times of low-amplitude insolation (e.g., 655-620, 560-510, 455-410, <75 ka; Fig. 5). Similarly, changes in toolkits overlap with the 650-350 ka period, with modest reductions in Olorgesailie Acheulean stone tool sizes reported between 615 and 499 ka [START_REF] Potts | Environmental dynamics during the onset of the Middle Stone Age in eastern Africa[END_REF] and with the smaller toolkits of the MSA developing by about 320 ka [START_REF] Brooks | Long-distance stone transport and pigment use in the earliest Middle Stone Age[END_REF][START_REF] Deino | Chronology of the Acheulean to Middle Stone Age transition in eastern Africa[END_REF][START_REF] Potts | Environmental dynamics during the onset of the Middle Stone Age in eastern Africa[END_REF]. This increased environmental variability, and the intense period of aridity, also overlap with a major turnover in mammal faunas with several large-bodied specialized grazing mammals becoming locally extinct and being replaced by related species with smaller body sizes [START_REF] Potts | Environmental dynamics during the onset of the Middle Stone Age in eastern Africa[END_REF][START_REF] Faith | New perspectives on middle Pleistocene change in the large mammal faunas of East Africa: Damaliscus hypsodon sp. nov. (Mammalia, Artiodactyla) from Lainyamok, Kenya[END_REF]. As the earlier fauna was already arid-adapted, the progressive increase in aridity was unlikely to have led to any turnover. However, a change in variability, that very specialized grazers couldn't adapt to, may have led to the turnover. It is also possible that increased aridity, or more variable environmental conditions in the context of increasing aridity may have impacted hominin populations during this transitional period by selecting for cognitive abilities to, for example transport increasingly diversified toolkits over greater distances, as is evidenced in the nearby Olorgesailie archeological record [START_REF] Brooks | Long-distance stone transport and pigment use in the earliest Middle Stone Age[END_REF].

Major steps in Quaternary hominin evolution have also been linked to eccentricity-modulated high-amplitude insolation cycles specifically associated with extreme climate variability during moist intervals, rather than low-amplitude periods when monsoons are weakened and climate becomes drier [START_REF] Trauth | Trends, rhythms and events in Plio-Pleistocene African climate[END_REF]. However, the possible overlap between intense aridity, major changes in toolkits, and mammal extinctions in the Magadi-Olorgesailie region argue against this version of a climate-evolution linkage.

The period between 350-50 ka represents the longest episode of eccentricity-modulated high amplitude insolation variability in the Middle to Late Pleistocene (Fig. 5). This coincides with significant environmental change when MSA tools emerged, symbolic cultures developed, H. sapiens appeared and the Late Stone Age commenced [START_REF] Potts | Alternating high and low climate variability: The context of natural selection and speciation in Plio-Pleistocene hominin evolution[END_REF]. Early anatomically modern human fossils from Asia indicate that they dispersed from Africa between 120-50 ka [START_REF] Liu | The earliest unequivocally modern humans in southern China[END_REF][START_REF] Grun | U-series and ESR analyses of bones and teeth relating to the human burials from Skhul[END_REF] with genetic data suggesting that ancestral modern non-African populations originated from Africans that dispersed between 75-50 ka [START_REF] Nielsen | Tracing the peopling of the world through genomics[END_REF]. Gulf of Aden leaf wax isotopes, close to a possible southern migration route, indicate multiple wet-dry cycles set against an overall drying trend [START_REF] Tierney | Climatic context for the out-of-Africa migration[END_REF]. Our continental record indicates arid climates that were punctuated by moist episodes, which may have supported a greener Sahara, opening the possibility of northern routes.

Recent hominin studies have noted that H. sapiens and the cultural materials that they produced may have a polycentric African origin with reproductively semi-isolated populations adapting to local environments alongside genetic drift [START_REF] Scerri | Did our species evolve in subdivided populations across Africa, and why does it matter?[END_REF]. In the South Kenya Rift, the 300-ka period of high-amplitude insolation variability was characterized by major environmental and hominin changes, providing support for hypotheses such as variability selection, which advocates adaptive evolutionary change during periods of increased environmental variability [START_REF] Potts | Hominin evolution in settings of strong environmental variability[END_REF][START_REF] Trauth | Trends, rhythms and events in Plio-Pleistocene African climate[END_REF][START_REF] Potts | Variability selection in hominid evolution[END_REF][START_REF] Potts | Environmental hypotheses of Pliocene human evolution, in Hominin environments in the East African Pliocene: an assessment of the faunal evidence[END_REF][START_REF] Maslin | Plio-Pleistocene East African pulsed climate variability and its influence on early human evolution, in The first humans -origin and early evolution of the genus Homo[END_REF]. However, it is important to note that this variability was superimposed on a strongly directional long-term trend towards increased aridity, especially during the critical 525-400 ka interval of drying documented here, which coincides with major technological and evolutionary events in the regional human prehistory. The Magadi record thus suggests that the species and technological changes in the South Kenya Rift were occurring against a backdrop of both increased aridity and enhanced variability, both of which could have acted as strong selective agents during the transition from the Early to Middle Stone Age and in the evolution of anatomically modern humans.

Methods

Details of the drilling (June 2014) are presented in the SI Appendix text. Three holes were drilled at Site 1 with one (MAG14-2A) recovered from Site 2 (Fig. 1a). Core recovery for MAG14-2A was 65% with drilling terminated in trachyte at ∼194mbs.

The chronology model (Fig. 2b) made use of one radiocarbon date from humate fractions of bulk organic matter in the upper core, nine replicate 40 Ar/ 39 Ar dates from mid core tephra, one 40 Ar/ 39 Ar date on a basal trachyte lava and seven U-series dates from chert. An 40 Ar/ 39 Ar date for a single feldspar grain from ash at 151 mbs was treated as supplementary and not included in the model, but plots within the 0.95 probability range for this depth (see SI Appendix for dating techniques). One 14 C date did not follow a monotonic sequence and was excluded from the model, but is shown in Fig. 2b. In addition, the model includes the Brunhes-Matuyama boundary in the lower core.

Samples for geochemical (n=343) and LOI (n=332) analyses were collected at about 32 cm intervals and where distinctive lithologies were present (see SI Appendix for techniques). Diatom samples (n=355) were collected every 32 cm with additional sampling at 10-15 cm intervals for diatomaceous sections, yielding 113 samples and 62 diatom taxa between 43-132 m (18-472 ka in SI Appendix, Fig. S4; Datasets S1, S2). A minimum of 400 diatoms were counted per slide, except where diatoms were rare, in which case all diatoms were included. Environmental reconstructions are based on the "Combined Salinity Dataset" in the European Diatom Database (EDDI) (http://craticula.ncl.ac.uk/Eddi/jsp/index.jsp) with taxa matched to the EDDI classification system. Pollen taxa (n=105) were identified in 354 samples with common pollen shown in SI Appendix, Figs. S5 and S6 and Datasets S3 and S4. Pollen and spores were mounted on slides and counted at 400x magnification. Pollen identification was made to the lowest possible taxonomic level, although some pollen types could only be identified to family level. The total count per sample generally ranged between 250-500 grains, except in a few samples where preservation was poor. Major hominin/faunal events in eastern Africa (except where noted) to left [START_REF] Campisano | The Hominin Sites and Paleolakes Drilling Project: High-Resolution paleoclimate records from the East African Rift System and their implications for understanding the environmental context of hominin evolution[END_REF]. First appearance of H. sapiens in Africa from Morocco data [START_REF] Hublin | New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens[END_REF]. H. sapiens dispersal: 1 = genetic data (41), 2 = fossil data [START_REF] Liu | The earliest unequivocally modern humans in southern China[END_REF][START_REF] Grun | U-series and ESR analyses of bones and teeth relating to the human burials from Skhul[END_REF]. FAD = First Appearance Datum; LAD = Last Appearance Datum. Thick dashed line shows the Mid-Brunhes Event (MBE). a, Na/Ca ratios. Decline reflects change from fresh to saline alkaline lake. b, Pollen PCA

Fig. 1 .

 1 Fig. 1. Core location and lithology. a, Location of MAG14-2A and 1A and geology. b,Sediment log for MAG14-2A; c=clay, m = mud, si = silt, s = sand, e = evaporites.

Fig. 2 .

 2 Fig. 2. Core chronology, geochemistry and mineralogy. a, Geochemistry and mineralogy. LOI (550/1000) = Loss on Ignition and ignition temperatures (centigrade). Profile of Na/Ca ratios. Major authigenic minerals (excluding clays, quartz, feldspar) determined by X-ray diffraction. Minerals ordered to reflect generally higher salinities to the right. b,Bayesian chronological model. 1 = Excluded 14 C date that fails to follow a monotonic ordering. 2 = Excluded 40 Ar/ 39 Ar date from a single crystal. Two chert dates from samples later found to contain secondary chalcedony were also excluded from the model. See SI for details.

Fig. 3 .

 3 Fig. 3. Diatom-based environmental data. Diatoms accumulated in a meromictic lake so separate conductivity and pH transfer functions are shown for mixed, saline and fresh water taxa. Habitats indicated separately for saline and fresh taxa.

Fig. 4 .

 4 Fig. 4. Pollen stratigraphy.Selected taxa shown. Pollen PCA 1 summarizes all pollen data and shows a long-term reduction in PCA values from ∼575 ka that reflects increased aridity. Poaceae increase upwards with Cyperaceae and Podocarpusdeclining. Other taxa suggest increasing aridity during the last half million years.

Fig. 5 .

 5 Fig. 5. Temporal environmental change in the Magadi basin and regional comparisons.Major hominin/faunal events in eastern Africa (except where noted) to left (8). First appearance of H. sapiens in Africafrom Morocco data (16). H. sapiensdispersal: 1 = genetic data (41), 2 = fossil data (39, 40).FAD = First Appearance Datum; LAD = Last Appearance Datum. Thick dashed line shows the Mid-Brunhes Event (MBE). a,Na/Ca ratios. Decline reflects change from fresh to saline alkaline lake. b,Pollen PCA 1. Low values reflect increased aridity. c,Poaceae to aquatic pollen ratios (P-A/P+A). High values indicate wetter episodes. d,DiatomPrincipal Component Analysis. High values reflect benthic taxa introduced to a meromictic lake by flooding. Note that many of these flood events broadly correlate (diagonal shading) with interglacial episodes in "g". e,Insolation (1°S) from AnalySeries 2.0. High variability periods shaded. f,Northwest Indian Ocean dust record[START_REF]African climate change and faunal evolution during the Pliocene-Pleistocene[END_REF]. Note long-term increasing aridity through the last 400 ka. g,Change in deuterium in Vostok ice core, East Antarctica[START_REF] Jouzel | Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years[END_REF]. Note increased variability in glacial-interglacial cycles after the MBE. h,Diatom-inferred environmental change, Olorgesailie Basin[START_REF] Owen | Quaternary geochemical stratigraphy of the Kedong-Olorgesailie section of the southern Kenya Rift[END_REF].

Figures Fig. 1 .Fig. 2 .Fig. 3 .Fig. 4 .

 1234 Core location and lithology. a, Location of MAG14-2A and 1A and geology. b, Sediment log for MAG14-2A; c=clay, m = mud, si = silt, s = sand, e = evaporites. Core chronology, geochemistry and mineralogy. a, Geochemistry and mineralogy. LOI (550/1000) = Loss on Ignition and ignition temperatures (centigrade). Profile of Na/Ca ratios. Major authigenic minerals (excluding clays, quartz, feldspar) determined by X-ray diffraction. Minerals ordered to reflect generally higher salinities to the right. b, Bayesian chronological model. 1 = Excluded 14 C date that fails to follow a monotonic ordering. 2 = Excluded40 Ar/39 Ar date from a single crystal. Two chert dates from samples later found to contain secondary chalcedony were also excluded from the model. See SI for details. Diatom-based environmental data. Diatoms accumulated in a meromictic lake so separate conductivity and pH transfer functions are shown for mixed, saline and fresh water taxa. Habitats indicated separately for saline and fresh taxa. Pollen stratigraphy. Selected taxa shown. Pollen PCA 1 summarizes all pollen data and shows a long-term reduction in PCA values from ∼575 ka that reflects increased aridity. Poaceae increase upwards with Cyperaceae and Podocarpus declining. Other taxa suggest increasing aridity during the last half million years.

Figure 5 .

 5 Temporal environmental change in the Magadi basin and regional comparisons.

  [START_REF] Potts | Evolution and climatic variability[END_REF]. Low values reflect increased aridity. c, Poaceae to aquatic pollen ratios (P-A/P+A). High values indicate wetter episodes. d, Diatom Principal Component Analysis. High values reflect benthic taxa introduced to a meromictic lake by flooding. Note that many of these flood events broadly correlate (diagonal shading) with interglacial episodes in "g". e, Insolation (1°S) from AnalySeries 2.0. High variability periods shaded. f, Northwest Indian Ocean dust record[START_REF]African climate change and faunal evolution during the Pliocene-Pleistocene[END_REF]. Note long-term increasing aridity through the last 400 ka. g, Change in deuterium in Vostok ice core, East Antarctica[START_REF] Jouzel | Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years[END_REF]. Note increased variability in glacial-interglacial cycles after the MBE. h, Diatom-inferred environmental change, Olorgesailie Basin[START_REF] Owen | Quaternary geochemical stratigraphy of the Kedong-Olorgesailie section of the southern Kenya Rift[END_REF].

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www.pnas.org ------Footline Author

www.pnas.org ------Footline Author

www.pnas.org ------Footline Author

Acknowledgments:

Funding was provided by the Hong Kong Research Grants Council (HKBU201912), the International Continental Drilling Program (ICDP), the U.S. National Science Foundation (EAR-1338553), and the Ministry of Science and Technology of Taiwan ROC (107L901001, MOST107-2119-M-002-051). We thank the National Museums of Kenya for support, the National Council for Science and Technology and the National Environmental Management Authority of Kenya for permits and Julia Richter for LOI analyses. DOSECC Exploration Services provided drilling supervision. The Operational Support Group of ICDP provided downhole logging services. Support was also provided by LacCore, the National Oil Corporation of Kenya, Tata Chemicals, the County Government of Kajiado. This is publication number #15 of the Hominin Sites and Paleolakes Drilling Project.


Olivier Commowick

Christian Barillot

Fli / Ofsep

Workshop Fli-Sfr

Laurent B Leray

Forbes F Knight

Santos Mm

Santos

MSSEG Miccai 2016 Challenge: Objective Evaluation of Multiple Sclerosis Lesion Segmentation using a Data Management and Processing Infrastructure

Team 12 0.06 1 Team 13 0.07 4 Team 1 Team 2 Team 3 Team 4 Team 5 Team 6 Consensus FLAIR Team 7 Team 8 Team 9

Team 11 Team 12 Team 13 Consensus Team 10

Visual results for center 03 (not in the training phase)

15 Team 1 Team 2 Team 3 Team 4 Team 5 Team 6 Consensus FLAIR Team 7 Team 8 Team 9

Team 11 Team 12 Team 13 Team 10

Consensus

Groups of methods : Comparison to Experts

•France

  Leray, E. et al., 2010. Evidence for a two-stage disability progression in multiple sclerosis.Brain, 133 (7), 1900Brain, 133 (7), -1913.Lesion segmentation in MS3 • Lesion load and lesion count crucial in MS • Part of diagnosis (McDonald criteria) • Evaluation of drug effectiveness • Delineation of lesion tedious • Manual à time consuming • Subject to intra-/ inter-individual variability è Automatic segmentation is key Thompson, A. et al., 2017. Diagnosis of multiple sclerosis: 2017 revisions of the McDonald criteria. The Lancet, 17 (2), 162 -173. number of automatic segmentation methods • Tissue classification & outlier detection • Machine learning (random forests, deep, etc.) • Many others • Large variety of modalities used • T1, T2, FLAIR, PD… • Large variety of implementations • GPU, Matlab, Python, C++ … • Evaluation complicated • Each method evaluated on a specific set • No comparison possible • The challenge concept • Have all methods evaluated on a common dataset • Examples: MICCAI 2008, IEEE-ISBI 2015 • Main drawbacks • Possibility to adapt parameters to each patient • Ground truth not well defined Styner et al., 2008. 3D Segmentation in the Clinic: A Grand Challenge II: MS lesion segmentation. Insight journal. Carass et al., 2017. Longitudinal multiple sclerosis lesion segmentation: Resource and challenge. Neuroimage, 148, 77-102. An OFSEP and FLI challenge @ MICCAI 6 • Evaluation objectives • Evaluate algorithms developed in the community • In a well defined computational framework (FLI) • Same set of parameters for all images • With respect to a solid ground truth • Additional objectives (OFSEP) • Evaluate lesion segmentation algorithms for MS • Fully automatic, on standardized images • Standardized but different centers http://www.ofsep.org Cotton, F., Kremer, S., Hannoun, S., Vukusic, S., Dousset, V., 2015. OFSEP, a nation-wide cohort of people with multiple sclerosis: Consensus minimal MRI protocol. Journal of Neuroradiology 42 (3), 133 -140. Black box (docker) including their optimal parameters • Parameters chosen or optimized on training set • Pipelines started automatically on testing set • On France Life Imaging (FLI-IAM) computing platform • By FLI-IAM project engineers • Ensures a uniform set of parameters on the whole testing database https://portal.fli-iam.irisa.fr/msseg-challenge/overview


Christian Barillot

Michel Dojat

Michael Kain

6 physical nodes +1 transversal node for Information Analysis and Management Noeud 'Information Analysis and Management' (IAM)

FLI-IAM: équipes impliquées

-•-----FLI-o•-

  CICIT, Nancy, Inserm -CATI, Paris/Neurospin, CEA -CREATIS, Lyon, CNRS-Inserm -GIN, Bordeaux, CNRS -GIN, Grenoble, Inserm -ICUBE, Strasbourg, CNRS -LATIM, Brest, Inserm -VISAGES, Rennes, Inria-Inserm-CNRS • Groupes de Travail: -I3S, Sophia, CNRS -ICM, Paris, Inserm-CNRS -CRMBM, Marseille, CNRS -CERMEP, Lyon -MIRCEN, Fontenay-aux-Roses, CEA Information Analysis and Management -IAM "l'infrastructure numérique de France Life Imaging" Objectifs: Proposer et mettre en place une infrastructure matérielle et logicielle pour la gestion et l'analyse des images acquises in vivo • Elaborer des scenarios d'usage de cette infrastructure • Proposer un opérateur de l'infrastructure, le soutenir dans l'organisation et l'extension de l'offre de services basée sur l'infrastructure logicielle de l'opérateur, l'ambition étant qu'au terme de FLI, l'opérateur parvienne à l'autonomie financière Cible pour ces services : • Des chercheurs et des médecins pour leurs recherches clinique et préclinique • Les Pharma et les CRO • La communauté de la recherche médicale développant des nouvelles technologies / Méthodologies d'imagerie in vivo Major task: integration of image processing from LATIM into FLI-IAM architecture Major task: integration of image processings from VISAGES into FLI-IAM architecture Major task: Shanoir extension for small animals and integration into FLI-IAM architecture Major task: extensions on/integration of MediBase/MediPy into FLI-IAM architecture Major task: Adaptation of FLI-IAM architecture to selected use cases (e.g. Miccai Challenge) -Pascal GIRARD (CNRS, Lyon): VIP Platform -Julien LOUIS (Inria, Rennes): Shanoir Platform -Baptiste LAURENT (Inserm, Brest): Challenges Integration H2020 Interopérabilité des solutions nationales existantes : CATI, Shanoir, ArchiMed et internationale (XNAT) -Liens avec la plateforme VIP (Creatis, Lyon) pour exécution de pipelines -Liens avec des plateformes d'exécution locales (BrainVisa, MedInria) -Définition d'un portail pour d'accès aux données et outils -Réalisation de deux challenges internationaux sur le traitement de données IRM en SEP et de TEP en oncologie avec la plateforme FLI-IAM (MICCAI 2016) -Réalisation d'un environnement pour le stockage et le traitement de données du petit animal -Participation au projet H2020 OpenAIRE-connect pour une ouverture sur les plateformes internationales de sciences ouvertes -Etude de marché pour préparer la phase II MEASURING the VOICE-OF-CUSTOMER in Un système délocalisé et virtualisé de stockage et d'archivage o Partagé au niveau national permettant à tous les centres ou acteurs (« exécutants » et utilisateurs finaux) de transférer ou d'accéder aux fichiers aisément o Regroupant de nombreuses données cliniques sur les patients (de manière anonyme) o Certifié par les autorités réglementaires (e.g. CNIL): -L'ANSM est en effet en charge de garantir la sécurité des produits de santé y compris les logiciels médicaux -La CNIL apportera « la caution » sur la sécurisation des données issues des essais cliniques o Les futurs clients de FLI-IAM demandent des garanties fortes sur : -La sécurisation des données -La stabilité de la structure proposant le service -L'indépendance de la structure vis-à-vis des intérêts économiques des contributeurs en solutions de traitement et d'analyses -Le projet peut être porté par une institution publique ou privée à partir du moment où l'état en est garant Test de concept La solution idéale de stockage, de transfert et d'analyse d'imagerie préclinique et de développement clinique est imaginé o Les utilisateurs académiques et universitaires sont la cible prioritaire (selon l'avis de la majorité des répondants) -L'utilisation de plateforme par les acteurs académiques et universitaires peut aussi servir de démonstrateur pour les autres segments (notamment l'industrie) -L'imagerie préclinique apparait également la plus « prête » pour la mise en commun de données d'imagerie et le partage de solutions logicielles o Le projet FLI-IAM peut permettre à la recherche institutionnelle de : -Faciliter la centralisation d'images médicales issues des essais cliniques multicentriques -Partager les images précliniques réalisées par les « exécutants » de plateformes académiques d'imagerie précliniques avec les « utilisateurs finaux » -Créer des bases de données d'imagerie médicales précliniques et cliniques pour : Le développement de logiciels par les universitaires • Des études épidémiologiques et statistiques -Les acteurs interrogés s'accorde sur le catalogue d'applications comme valeur ajoutée la plus importante du projet o L'imagerie préclinique (petit animal), puis la neurologie et la cancérologie sont estimées être les domaines les plus intéressants de par le volume d'images générées.Aspects opérationnelsLa recherche institutionnelle apparait comme l'utilisateur le plus pertinent au démarrage du projet FLI-IAMo Les futurs clients de FLI-IAM demandent des garanties fortes sur :-La sécurisation des données • Le cryptage des données d'imagerie provenant d'opérateurs privés (laboratoires pharmaceutiques) est quasiment indispensable -La stabilité de la structure proposant le service • Il est nécessaire pour le client de savoir que ses données pourront être accessibles à long terme -sans risque de voir le projet « mettre la clé sous la porte ».-L'indépendance de la structure vis-à-vis des intérêts économiques des contributeurs en solutions de traitement et d'analyses -L'offre doit être certifiée « hébergeur de données de santé » o Le projet peut être porté par une institution publique ou privée à partir du moment où l'état en est garant -Une certification par l'état serait un atout considérable pour crédibiliser le projet. -Certaines structures ne peuvent cependant pas prétendre à la commercialisation du projet : • Un laboratoire pharmaceutique pour cause de conflit d'intérêt avec des utilisateurs potentiels (autres laboratoires pharmaceutiques) • Une unité/service CNRS ou l'INSERM à cause d'une perception trop académique et déconnectée des problématiques économiques • Une « start-up » ne serait pas considérée comme suffisamment durable o Le projet doit être porté/crédité/valorisé par des leaders d'opinions o ➜ ➜Travail avec le CERF Agit « au nom » de FLI -« Opérateur » non membre du consortium FLI, de droit privé (règlement financier ANR) -Apporte la garantie de l'état (« trustee ») • Rôles -Interlocuteur de FLI -Guichet unique pour les usagers -Garant des règlementations -Garant des solutions proposées (entrepôts, logiciels, qualités, …) -Garant de la pérennité (infrastructures, données, outils,…) -Etablit un modèle de coût -Sélectionne les sous-contractants et garantie leurs services -Démarche les « clients » et fait émerger des nouveaux marchés -Plateforme d'accueil de solutions innovantes à la recherche de marché (e.g. start-ups)

Mai

-TMTG FLI-IAM -Rapport Final V1

Mai 2017 -TMTG FLI-IAM -Rapport Final V1

La crédibilité scientifique du projet FLI-IAM se construira au cours du tempsMai 2017 -TMTG FLI-IAM -Rapport Final V1