H Q Do

X Mercier

L-S Tran

L Gasnot

A El Bakali

Experimental investigation of hydrogen impact on atmospheric sooting premixed methane flames

. Various stable species (C1-C6) were measured by gas chromatography while polycyclic aromatic hydrocarbons (PAHs) such as naphthalene and pyrene were measured by jet-cooled laser induced fluorescence (JCLIF). To keep the C/O ratio and temperature values constants in the sooting flame region, all H2flames have been studied by replacing or adding the equivalent of 1.8%N2. The Pt/Rh thermocouple measurements confirmed similar temperature values in the post flame region of methane/O2/N2 and methane/H2/O2/N2flames. Moreover, additional experiments carried out in similar conditions with helium allowed to exclude the diluent affect. The new experimental results demonstrated that hydrogen can enhance significantly soot precursors (benzene, naphthalene, pyrene) even it is added at low amounts (lower than 2%). However, these aromatic compounds peak mole fraction can significantly be reduced if the experiments proceed via a low N2-amount substitution(1.8%).The soot volume mole fraction measurements and the modeling work which are in progress should confirm the importance of H atoms in the chemical growth process of PAHs and soot formation.

1.Introduction

Nucleation flames, which have been recently discovered in our laboratory, allow to efficiently investigate the nucleation process of soot particles from gaseous species formed in flames [3,4] because of the simplification of the soot formation mechanism they provide. Indeed, in these flames, nascent soot particles (NSP) are only generated without or very poorly undergoing surface growth all along the flame height.

Polycyclic aromatic hydrocarbons (PAHs), which are considered to be the main precursors of soot particles, have been highlighted to form and grow in size preferentially according to the HACA mechanism (Habstraction -C2H2 addition) [START_REF] Frenklach | On the driving force of PAH production[END_REF][6][7]. In the present study, we report some experimental results highlighting the hydrogen impact on the formation of PAHs and their crucial precursors in a nucleation methane flame stabilized at atmospheric pressure. Hydrogen addition to the nucleation flame is expected to affect the formation of active oxidative species such as O and OH leading to the enhancement of the PAH oxidation or to the increase of the formation of H atoms leading to the increase of the PAH formation.

Different experimental techniques have been used to carry out this work. The experimental conditions required to generate the nucleation flame have been determined by Laser Induced Incandescence (LII) while the impact of hydrogen on the formation soot precursors has been investigated by Gas Chromatography (GC) for the aliphatic species and benzene and jet-coolded laser induced fluorescence (JC-LIF) for PAHs (naphthalene and pyrene).

2.Experiment setup

Flame apparatus and conditions

The flames were stabilized on a McKenna burner (6 cm diameter) already described previously [8]. The conditions of all studied flames are summarized in Table 1.

Flame Ф- 1.82 Ф- 1.82_H2-S Ф- 1.82_H2-A Ф- 1.82_He

Table 1: Flame conditions

There are currently several approaches to investigate the impact of hydrogen in flames. In this work, we propose two different approaches to introduce hydrogen without affected the temperature profile of the reference flame (Ф-1.82). The first approach is characterized by a hydrogen substitution, flame Ф-1.82_H2-S. In this case, a small amount of the nitrogen dilution, equal to 10% of the methane flow rate, is subtracted from the reference flame, and hydrogen is added into flame in order to keep constant the total flow rate. The second approach correspond to a hydrogen addition, flame Ф-1.82_H2-A. This time, the small amount of hydrogen, still equal to 10% of the methane flow rate, is added, therefore slightly increasing the total flow rate of flame (less than 2%) compared to the reference flame.

To investigate the dilution effect of the substituted hydrogen to the nucleation flame, a similar experiment were also carried out by replacing hydrogen by helium in the flame Ф-1.82_He-S.

2.2.LII setup

A schematic of the used LII setup is reported in Fig 1.

2.3.Gas Chromatography (GC) setup

A schematic of the used GC setup is presented in Fig 2.

Figure 2: Experimental GC setup

Gas samples were extracted from the flames by a quartz sampling microprobe and then directly sent into a lowpressure injection system before injecting into the GC CP-3800 Varian. In this experiment, the micro-probe was fixed and the burner height adjustable along the vertical axis allowing to sampling of species from different heights above the burner. The alumina capillary and tamis molecular columns were located inside a furnace of GC and the two FID and TCD detectors located at the exit of alumina capillary and tamis column respectively allowing to detect species sampled from flame.

We used the gaseous commercial cylinder calibration mixture (Air Product) with uncertainty of mole fraction ± 0.5 -2% to calibrate the GC.

2.4.JC-LIF setup

A schematic of the used JC-LIF setup is shown in Fig 3 .  JC-LIF was also carried out after the sampling of the species from the flame thanks to the same micro-probe used for the GC experiments. Then species samples were directly cooled down in an expanded free jet generated inside the analysis chamber. LIF measurements were done directly in the free jet in this low-pressure chamber. In these very low temperature conditions provided by the free jet expansion (around 100 K), the spectra of the sampling PAHs simplify and highlight specific spectral structures enabling their selective measurements by LIF. In the absence of collision inside the free jet, LIF signal can be calibrated by studying the LIF signal issued from pure compounds with known concentrations sent and cooled down inside the free jet. The laser system we used consisted of a Quantel Nd:YAG laser, pumping a dye laser (TDL70 Quantel) with the 2 nd harmonic at 532 nm. Tunable wavelengths covering different spectral ranges could be generated by using different dyes to match the spectral excitation range of naphthalene and pyrene. The laser beam was spatially reduced to a diameter of approximately 1 mm with a pinhole and sent into the analysis chamber. The energy was adjusted around 0.048 J/cm 2 to be linear regime of fluorescence. Fluorescence emission spectra were recorded via spectrometer iHR 320 (300 mm focal length -gratings with 300 gr/mm) which can be coupled either to a 16 bit intensified charge coupled device camera (Roper Pimax II) or to a photomultiplier (Photonis XP2020Q). The conditions of excitation and collection wavelengths for the measurement of naphthalene and pyrene are reported in

3.Results and discussion

Nucleation condition identification

The equivalence ratio enabling the generation of the nucleation flame has been obtained by following the evolution of the LII temporal decays along the flame height. The relationship between the LII temporal decay and the soot size distribution is rather complex [9,10], and requires the use of LII modeling to interpret the signals in terms of primary particle diameter. However, it is established that this temporal decay is correlated to the diameter of the soot particles. The nucleation flame conditions can therefore be obtained by adjusting the flow rate of the reactants so that we do not observe any growth of the temporal LII decay rate all along the flame height as reported in Fig 4 . By this way, we defined the equivalence ratio conditions equal to Ф-1.82 to generate a methane nucleation flame at atmospheric pressure.

Impact of hydrogen on formation of soot precursors

Fig 5 reports the effect of hydrogen substitution on the formation of acetylene, propyne, benzene, naphthalene, and pyrene in the nucleation flame. The mole fraction profiles of all species shift about 2 mm toward the burner surface comparing to the reference flame because the hydrogen substitution enhances the laminar flame speed [11]. The above unsaturated aliphatic species play a significant role in first aromatic ring and PAHs formation. Acetylene (C2H2) can produce benzene by its reaction with butadienyl radicals and contributes significantly to the PAHs and soot growth processes [START_REF] Frenklach | On the driving force of PAH production[END_REF]12]. Propyne (P-C3H4) is a main source propargyl (C3H3) radicals which self-recombination yields benzene [13].

Figure 5: Impact of hydrogen substitution on formation of acetylene C2H2, propyne P-C3H4, benzene, naphthalene and pyrene in the nucleation flame -Hydrogen Substitution

: Impact of hydrogen addition on formation of acetylene (C2H2), propyne (P-C3H4), benzene, naphthalene and pyrene in the nucleation flame -Hydrogen Addition

According to Figs. 5 and6, hydrogen strongly affects benzene, naphthalene and pyrene formation while it only slightly influences acetylene and propyne formation. The observed hydrogen effect depends strongly on the operating flame conditions. The results of Fig. 6 exhibits that hydrogen can enhance significantly soot precursors (benzene, naphthalene, pyrene) even it is added at low amounts (lower than 2%). However as shown in fig. 5, these aromatic compounds peak mole fraction can significantly be reduced if the experiments proceed via a low N2amount substitution (1.8%).

As can be seen from Fig. 7, helium substitution/addition does not affect the formation of acetylene, propyne and benzene. Therefore, aromatic compounds peak mole fraction differences observed in methane and methane-H2 flames is probably due to the chemical effect. Note that the Helium and Hydrogen effect are very close [14] .

Figure 7: Impact of helium substitution on the formation of acetylene (C2H2), propyne (P-C3H4), and benzene in the nucleation flame -Helium Substitution

The modelling of these new experimental results using our recent mechanisms is in progress [1,15]. It should provide more precise information on H2 effects. As discussed above, our experimental results show clearly that substitution hydrogen enhances the overall reactivity indicating an increase of the laminar flame speed. The substitution should then enhance the branching reaction converting H atoms to oxidative active species OH and O. However, the addition of 1.8% H2 does not seem to disturb the reaction flame zone indicating that the laminar flame speed remains probably unchanged. It also means that the branching reaction proceeds with the same efficiency under these conditions. Accordingly, hydrogen atoms preferentially used for H-transfer reactions and termination reactions involving aromatic radicals.

Conclusion

In this work, we have studied the impact of hydrogen on the formation of important soot precursors (e.g. benzene, naphthalene, and pyrene) in a nucleation flame of methane at atmospheric pressure. Two approaches of hydrogen introduction to the nucleation flame have been examined. In these two approaches, hydrogen strongly influences on the formation of benzene, naphthalene and pyrene whereas it slightly impacts the formation of aliphatic species.

Hydrogen substitution enhances the flame speed, hence favoring the formation of oxidation active species. Thus, it strongly inhibits the formation of benzene, naphthalene, and pyrene.

Hydrogen addition does not modify the mole fraction profiles of all the measured species. It notably strongly enhances the formation of benzene, naphthalene and pyrene before their peak mole fraction while it only slightly increases the formation of acetylene and propyne in post combustion region. Thus, hydrogen addition favors the formation of active species which takes an important role in the PAHs growth mechanims.

In the two approaches, the most important impacts of hydrogen was observed for the pyrene formation. Therefore, hydrogen appears as an excellent additive in the studied nucleation flame to investigate the transition step leading to the primary soot particles in flame.

The effects of hydrogen on the formation of PAHs in the nucleation flames are very different according to the different investigated approaches. Therefore, we have to be careful when studying the impact of additives in sooting flames.

Figure 1 :

 1 Figure 1: Experimental LII setupLII experiments have been carried out by using a 1064 nm laser excitation wavelength generated by a Nd: YAG (Quantel) at 10Hz to heat soot particles. 1064 nm laser excitation wavelength was used to avoid the excitation of PAHs also formed in the flames. Firstly, the laser beam was expanded into a horizontal plane using two cylindrical lenses (f1 = -50mm, f2= 200mm). Then the laser beam passed through a rectangular slit (5.96mm (vertical) 0.61mm (horizontal)) and a converging lens (f3=200mm) in order to provide a top-

Figure 3 :

 3 Figure 3: Experimental JC-LIF setup

Figure 4 :

 4 Figure 4 :LII temporal decays measured at different HAB for the flame Ф-1.82. Peak LII signals have been normalised to 1. Laser fluence set at 0.41J/cm

Figure 6

 6 Figure 6: Impact of hydrogen addition on formation of acetylene (C2H2), propyne (P-C3H4), benzene, naphthalene and pyrene in the nucleation flame -Hydrogen Addition

Table 2 .

 2

		Naphthalene	Pyrene
	Transition	S1 ← S0	S2 ← S0
	Excitation	λex= 308 mm	λex= 321 mm
	Collection	320<λcollec<340	370<λcollec<390

Table 2 : Spectroscopic data

 2

Acknowledgments

This work was supported by the Agence Nationale de la Recherche through the LABEX CAPPA (ANR-11-LABX-0005), the Région Hauts-de-France, the Ministère de l'Enseignement Supérieur et de la Recherche (CPER Climibio) and the European Fund for Regional Economic Development.


Frédérique Battin-Leclerc

Jérémy Bourgalais

Zied Gouid

Olivier Herbinet

Gustavo Garcia

Philippe Arnoux

Wang Zhandong

Luc-Sy Tran

Guillaume Vanhove

Laurent Nahon

Zhandong Wang

Majdi Hochlaf

Chemistry deriving from OOQOOH radicals in alkane low-temperature oxidation: A first combined theoretical and electron-ion coincidence mass spectrometry study

Keywords: Jet-stirred reactor, electron/ion coincidence mass spectrometry, n-pentane, ketohydroperoxides, photo-ionization fragmentation, Ab initio computations

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l'Atmosphère, F-59000 Lille, France While there is consensus on the fact that OOQOOH radicals, produced by two oxygen additions from alkyl radicals, are the heart of the low-temperature oxidation of alkanes, the determination of the isomeric distribution and the quantification of their derived products (ketohydroperoxides and diones) are still a challenge. For the first time, heavy oxygenated products produced during alkane oxidation have been investigated using electron/ion coincidence mass spectrometry. The investigated prototype reaction is n-pentane oxidation carried out in a jet-stirred reactor (temperatures from 585 to 665 K, pressure of 1.1 bar, lean mixture). Identification attempts were made for m/z 100 and 118 species using coincident mass-tagged Slow PhotoElectron Spectra obtained by electron-ion coincidence mass spectrometry combined with first principle computations, consisting in the determination of their adiabatic ionization energies and the Franck-Condon envelope of the photoionization spectra. 4hydroperoxypentan-2-one has been confirmed as the dominant obtained ketohydroperoxide, as predicted by up-to-date kinetic models. However, difficulties due to fragmentation has made INTRODUCTION Hydroperoxides, especially ketohydroperoxides (KHPs), play a key role during the low-temperature (500-800 K) gas-phase oxidation of alkanes [START_REF] Curran | Proc. Developing detailed chemical kinetic mechanisms for fuel combustion[END_REF] [START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF] (see a summary of the up-to-date chemistry understanding related to ketohydroperoxides in Fig. S1 in Supplementary Materials (SM)). The importance of ketohydroperoxides in triggering autoignition is due to the well-known branching reaction initiated by O-OH bond breaking. However, competing non-branching reactions was recently discovered, such as the formation of carboxylic acids or of species including two ketone functions, called diones.

Photoionization mass spectrometry (PIMS) recently succeeded in detecting a wide range of alkyl, alkenyl, and ketohydroperoxides [START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF]. The first repeatable identification of ketohydroperoxide (KHP) was obtained using tunable synchrotron photoionization and molecular beam sampling [START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF]. However, the determination of the isomeric distribution of these elusive species has not been achieved yet. In addition, while the attempts of quantification of alkyl hydroperoxides lead to values consistent with modeling, astonishing deviations (e.g. above a factor 100 in [3]) are obtained for ketohydroperoxides, likely because the fragmentation of molecular ions is not taken into account [START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF]. In all these previous studies, diones were systematically found to be produced together with ketohydroperoxides.

Previous work in flames [START_REF] Felsmann | Electron ionization, photoionization and photoelectron/photoion coincidence spectroscopy in mass-spectrometric investigations of a lowpressure ethylene/oxygen flame[END_REF] and a Jet-Stirred Reactor (JSR) [START_REF] Bourgalais | Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane[END_REF] evidenced an interesting potential of PIMS using the PhotoElectron PhotoIon COincidence (PEPICO) technique for identifying isomers through the energy analysis of the correlated photoelectron. Therefore, the first purpose of this paper is to analyze the coincident mass-tagged Threshold PhotoElectron Spectra (TPES [6][7]) of ketohydroperoxides and their related products measured during an investigation of n-pentane oxidation (lean mixtures with dilution in helium) in JSR. n-Pentane is a representative fuel of gasoline components, which was thoroughly studied recently through JSR experiments with gas chromatography [11] and PIMS [3] measurements leading to very satisfactory modeling [11].

While their formation pathway is still not fully understood, diones are currently considered as directly formed from KHP or as products of chemically activated Q ̇OOH + O2 reaction in parallel with the formation of KHP [START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF] [START_REF] Eskola | Time-resolved measurements of product formation in the low-temperature (550-675 K) oxidation of neopentane: a probe to investigate chain-branching mechanism[END_REF]. Recent JSR works investigating the oxidation of oxygenated molecules (dimethyl ether [START_REF] Eskola | Time-resolved measurements of product formation in the low-temperature (550-675 K) oxidation of neopentane: a probe to investigate chain-branching mechanism[END_REF], tetrahydrofuran [START_REF] Hansen | Isomer-Selective Detection of Keto-Hydroperoxides in the Low-Temperature Oxidation of Tetrahydrofuran[END_REF]) have documented the possibility of better quantifying KHPs based on their fragmentation. The second purpose of this paper is shedding light on the first steps of the unimolecular decomposition pathways undertaken by the C5KHPs derived from n-pentane. Finally, we show also how these C5KHPs end up with the production of carboxylic acids such as acetic acid and formic acids.

II. EXPERIMENTAL FACILITY

The used set-up, based upon the undulator-based DESIRS VUV beamline [START_REF] Nahon | DESIRS: A state-of-the-art VUV beamline featuring high resolution and variable polarization for spectroscopy and dichroism at SOLEIL[END_REF] of synchrotron SOLEIL(see Fig. S2 (SM)), was described in [START_REF] Bourgalais | Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane[END_REF]). The SAPHIRS end-station [START_REF] Tang | Vacuum upgrade and enhanced performances of the double imaging electron/ion coincidence end-station at the vacuum ultraviolet beamline DESIRS[END_REF] is equipped with the double-imaging photoelectron/photoion (i 2 PEPICO) spectrometer DELICIOUS III [START_REF] Garcia | DELICIOUS III: A multipurpose double imaging particle coincidence spectrometer for gas phase vacuum ultraviolet photodynamics studies[END_REF]. The JSR mounted inside SAPHIRS is similar to that used in [11] (same geometry, heating devices and temperature control, and gas feeding). The reactor sphere includes a quartz nozzle with a 100-μm hole in order to create the molecular beam and is connected to two concentric quartz tubes mounted into a flange carrying all the gas inlets and outlets, which is itself mounted on a 3-axes manipulator [START_REF] Bourgalais | Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane[END_REF].

n-Pentane is provided by Sigma-Aldrich (purity ≥ 99%) and the gases by Air Liquide.

After leaving the reactor, the molecular beam traverses two consecutive skimmers expanding further (10 -6 mbar) towards the ionization chamber where it intersects the ionizing focused VUV beam of DESIRS [START_REF] Nahon | DESIRS: A state-of-the-art VUV beamline featuring high resolution and variable polarization for spectroscopy and dichroism at SOLEIL[END_REF]. The resulting electrons and cations are accelerated in opposite direction towards respectively a velocity map imaging and ion Time-Of-Flight (TOF) and imaging ion spectrometer [START_REF] Garcia | DELICIOUS III: A multipurpose double imaging particle coincidence spectrometer for gas phase vacuum ultraviolet photodynamics studies[END_REF].

The coincidence detection scheme enables mass tagging of the electron images, and the selection, via the correlated ion images, of the events corresponding to molecules having a net velocity along the molecular beam, discarding the residual background from the chamber. After Abel inversion of the electron images, one obtains threshold photoelectron spectra-by selecting photoelectrons with near zero kinetic energy while scanning the photon energy-on mass-tagged compounds originating from the reactor, with a typical mass resolution m/m of the order of 1700, and a typical electron energy resolution of about 20 meV.

III. CALCULATION METHODS

The main aim of the present computations is to elucidate the structure of the molecular species formed in the first steps after oxidation of n-pentane and their subsequent VUV photoionization. Thus, we carried out geometry optimizations, followed by frequency computations on neutral and ionic species of C5KHPs of m/z = 118, the major C5H8O2 species at m/z = 100 and formic acid. These geometry optimizations were done at the PBE0/aug-cc-pVDZ level as implemented in GAUSSIAN 09 [START_REF] Frisch | Gaussian 09, Revision A02[END_REF].

Subsequently, we performed single point (SP) computations using the costly (R)CCSD(T)-F12/augcc-pVTZ method as implemented in MOLPRO 2015 [START_REF] Ma | Explicitly correlated local coupled-cluster methods using pair natural orbitals[END_REF]. While PBE0/aug-cc-pVDZ(opt) level leads to AIEs off by more than 0.2 eV for medium-sized molecules, the composite scheme PBE0/aug-cc-pVDZ(opt)//(R)CCSD(T)-F12/aug-cc-pVTZ(SP) allows accurate derivation of these AIEs (to within a maximum of ± 0.02 eV) as discussed previously [START_REF] Hochlaf | Advances in spectroscopy and dynamics of small and medium sized molecules and clusters[END_REF] (see a comparison of the values obtained with both methods in Table S1 (SM)). Afterwards, we simulated the vibrationally-resolved Franck-Condon photoelectron spectra (FCPS), for the ion ground state of the species listed in Table 1, as well as some of their conformers that most closely mirror their corresponding neutral (see conformers list and all calculated FC PESs (SM)) using the Time-Independent Adiabatic Hessian Franck-Condon (TI-AH|FC) model as implemented in GAUSSIAN, as described in more details in [START_REF] Bourgalais | Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane[END_REF].

Anharmonicity was not considered. Indeed, its effect on the simulated spectra would not be appreciable due to the limited experimental resolution (20 meV) and the congestion of these spectra. Note that the simulated spectra treat only direct photoionization. While the contribution of autoionizing resonances, especially close to the AIEs, cannot be fully ignored, Cooper minima and shape resonances effects can be safely ruled out since they are associated with the ejection of fast electrons discriminated by the threshold techniques employed in this work. Nevertheless, it is commonly admitted that autoionization equally affects the TPES bands for medium-sized molecules (as those treated here) allowing hence direct comparison of the simulated and measured spectra.

In a second step, the origin bands were shifted in energy to coincide with the respective PBE0/aug-cc-pVDZ(opt)//(R)CCSD(T)-F12/aug-cc-pVTZ(SP) AIEs. Finally, the Franck-Condon factors are convoluted with a 20 meV bandwidth Gaussian profile in order to facilitate the comparison between the computed envelopes (PhotoElectron Spectra (PES)) and the measured TPES based on the experimental resolution. This approach already gave good results for understanding the structure of biological molecules up to m/z 136 [START_REF] Zhao | Unveiling the complex vibronic structure of the canonical adenine cation[END_REF]. In [START_REF] Bourgalais | Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane[END_REF], the validity of this approach not considering anharmonic calculations was demonstrated for organic species up to m/z 88. Concerning hydroperoxides, our calculations neglecting the torsional effect around the O-O bond lead to good agreement between experimental [START_REF] Bourgalais | Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane[END_REF] and simulated PES as demonstrated by figure S8 (SM) for methyl hydroperoxide (m/z 48).

IV. RESULTS AND DISCUSSION

The apparatus described above was used to study the oxidation of n-pentane between 585 to 665 K, under quasi-atmospheric pressure, at a mean residence time of 3.00 ± 0.02 s, for a fuel initial mole fraction of 0.01 and equivalence ratios () of 1/3 and 0.5.

Mass spectrum

Typical TOF Mass Spectra (TOF-MS) at fixed photon energies of 10.0 and 11.5 eV obtained for the oxidation of n-pentane at 606 K are shown in Fig. 1. The corresponding two-dimensional TOF-MS plotted as a function of m/z and photon energy is displayed in Fig. S3 (SM). When compared with the TOF-MS recorded in [3] for a similar temperature (600 K) at = 1, all the m/z peaks between 50 and 90 were present in both spectra. In addition, the TOF-MS of Fig. 1 contains peaks at m/z 100 and 118 as that of [3]. The main difference between both TOF-MSs is that in the present TOF-MS, the fragments exhibit broader peak shapes while the parent ions appear as narrow peaks. This is due to the lower extraction field used in the present work which renders the experiment more sensitive to the Doppler broadening caused by the kinetic energy release in dissociative ionization. From the TOF spectrum depicted in Fig. 1, clearly m/z 43, 57 and 85 can be identified as daughter species and as such will be discussed further in the text.

Attempts of determination of the isomeric distribution of ketohydroperoxides and diones

As is shown in [START_REF] Bourgalais | Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane[END_REF], the species indicated in the mass spectrum of Fig. 1 at m/z < 100 can easily be identified according to their recorded PES (examples for acetaldehyde and acetic acid are given in Fig.

S4 (SM)

. As already demonstrated [3] by comparison between ionization energies measured by PIMS at the Hefei synchrotron and theoretical calculations, m/z 100 and 118 correspond to C5 diones and C5 ketohydroperoxides, respectively. In the following, an isomer identification attempt is done for m/z 118 corresponding to KHPs and 100 corresponding to diones. The calculated theoretical spectra of Figure 2 were obtained from spectra derived from the Franck-Condon calculations given in SM, which were summed and weighted with the appropriate factors to adjust the experimental spectrum in the relevant photon energy range.

2.a/ m/z 118

Table 1 presents the 10 possible C5KHP isomers corresponding to m/z 118, as well as the mole fractions calculated at 600 K using the detailed kinetic model of [11]. A cyclic ether hydroperoxide has also be considered (1-methyl,2-hydroperoxyfurane). Table 1 also gives the optimized structures of the 10 KHP isomers and their AIEs as computed at the composite scheme PBE0(opt)/aug-cc-pVDZ//(R)CCSD(T)-F12/aug-cc-pVTZ(SP) level. As established [START_REF] Hochlaf | Advances in spectroscopy and dynamics of small and medium sized molecules and clusters[END_REF], the composite scheme is more reliable and we use its results for the assignment of the experimental spectra. Table 1 shows that all 10

KHPs have AIEs in the range 9.2 -9.7 eV-the range of the bands observed in the TPES spectrum associated with m/z 118. Franck-Condon simulations from the optimized geometries were performed for these 10 KHPs and their previously mentioned conformers (see SM) to reproduce their PES.

The Fig. 2a. displays the experimental TPES measured for m/z 118 and characterized by a slow rise after 8.8 eV, which is due to some geometry changes between the neutral and the cation leading to unfavorable FC factors. A significant signal drop after 9.4 eV is also observed, indicating that fragmentation starts to occur from this photon energy in agreement with the energy dependence of the signal at m/z 118 displayed in Fig. 3a. Therefore, the analysis of the TPES may not be possible from a certain photon energy and only the signal below 9.8 eV is considered in this work. Additionally, the simulated PES of 2,4KHP (4-hydroperoxypentan-2-one) displays a second intense peak between 9.8 and 10 eV that is not present in the experimental PES (cf. Fig. 2a). Unfortunately, most of the KHPs present simulated PES above 9.8 eV (see figures in SM), which would mean that the corresponding cation would be unstable. Thus, their formation could not be verified in this work.

5 KHPs (1,2KHP, 2,1KHP, 2,3KHP, 2,4KHP and 3,2KHP) and 1-methyl,2-hydroperoxyfurane have their PES below 9.8 eV (see SM) and have been considered in the analysis of the measured m/z 118 TPES. A signal convolution, made between 9.4 and 9.8 eV and presented in Fig. 2a, indicates that at least three hydroperoxides could be present, 1-methyl,2-hydroperoxyfurane, 1,2KHP and 2,4KHP, with a weight of 0.3, 0.1 and 1 respectively. Due to a somewhat low signal to noise ratio and an overlap between the simulated PES, the contribution of the three other KHPs could not be ruled out. However, the convoluted spectrum agrees well with the experimental TPES and 2,4KHP is the KHP formed in largest amounts amongst the 10 isomers, as predicted (see Table 1). In addition, this work suggests a more notable presence of 1,2KHP and 1-methyl,2-hydroperoxyfurane than predicted.

Note that the signal below 9.3 eV could not be attributed to any calculated Frank-Condon PES, as shown in Fig. S9 (SM). This could be due to the excitation of more internal modes when reaching relatively high masses, as well as to the existence of conformers leading to spectroscopic congestion, which results in a lack of structure explaining the large difference between the observed TPES and simulations. However, the conformers have too high AIEs to support this assumption. Thus, the lower part of the measured TPES is probably due to the increasing internal modes at this mass especially low frequencies that may be populated leading to hot bands which are not considered in the simulated PES since they are calculated at 0 K. Indeed, such a situation with an initial slow rise for the signal in the lower part of the spectrum, due to some geometry variation between the neutral and the cation, makes the PES more sensitive to hot band contributions.

Table 1: Adiabatic ionization energies (AIE ((R)CCSD(T)-F12/aug-cc-pVTZ method), eV) including PBE0/aug-cc-pVDZ zero point vibrational energy corrections, and simulated mole fractions (MF at

600 K using the model of [11] 1 predicted in highest amounts are displayed in Fig. 4. 1); possible minor pathways towards acids or diones are not presented. Products in blue (with their m/z) were identified in [3] or [START_REF] Bourgalais | Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane[END_REF].

All the oxygenated products, for which a formation channel is indicated in Fig. 4, have been quantified [3] or identified (formaldehyde, acetaldehyde, acetone, ketene, methylethylketone, methylvynilketone) [START_REF] Bourgalais | Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane[END_REF], except C3-C4 diones. Formaldehyde was not discussed in [START_REF] Bourgalais | Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane[END_REF], but an intense signal can be seen in Fig. S3 (SM) at m/z 30 from an energy of 10.88 eV, the ionization energy of this C1 aldehyde [21].

2.b/ m/z 100

Table 1 presents the structure of the five most likely isomers at m/z 100 (C5H8O2 species, previously identified as being diones) arising from the same OOQOOH radical as 2,4KHP, the main identified ketohydroperoxides. We only considered the C5 species, in which the two oxygenated functions are separated by a carbon atom. For m/z 100 TPES spectrum, Fig. 2b shows that it is mainly composed by bands that can be assigned to the contribution of 2,4-pentadione:3-oxopentanal:2-ene-3-hydroxypentanal:3-penten-2-one-4-hydroxy, since both the AIEs and the theoretical simulated spectra confirm this attribution, with the best fit obtained for a signal branching ratio of 0.6:1:0.6:0.04 with a reasonable uncertainty (uncertainty below 30% for the three dominant species). While it was not considered when starting this work since their formation has never been reported, the contribution of enols had to be taken into account to reproduce the spectrum shape below 9.5 eV. Their contribution in the signal indicates a non-negligible formation. Considering the possible formation of C5H8O2 products from the decomposition of the C5-OOQOOH radicals, the formation of enols is favored by their further stabilization in respect to their corresponding ketones because of the mesomeric effects. Despite the satisfactory overall agreement between experimental and simulated PES in fig. 2b, the deviation between ~9.2-9.4 eV could indicate a missing identified species such as those containing a cyclic ether, which were not considered in the present calculations.

Attempts of quantification of ketohydroperoxides

As discussed by [START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF] and confirmed in the present work, an important fragmentation contribution needs to be considered when quantifying KHPs by PIMS. In light of the mass spectrum of Fig. 1 and of the identification of the major KHP, fragmentation patterns can be derived and used to refine KHP quantification.

3.a/ Identification of KHP fragmentation patterns

The mass spectrum of Fig. 1 clearly shows the presence of three fragments that could be attributed to the fragmentation of C5KHP: m/z 43, 57 and 85. Fig. 3 coincides with the fragmentation onset of acetone [START_REF] Rennie | A Photoelectron and TPEPICO Investigation of the Acetone Radical Cation[END_REF]. In addition, the temperature profiles shown in Fig. 3b clearly show a similar temperature dependence of m/z 118, 85 and 57, further supporting the assumption that m/z 85 and 57 are fragments from dissociative ionization of m/z 118. The fragment at m/z 85 may correspond to the loss of HOO from 2,4 KHP+, 1,3 KHP+ as evidenced in Refs. [START_REF] Eskola | Time-resolved measurements of product formation in the low-temperature (550-675 K) oxidation of neopentane: a probe to investigate chain-branching mechanism[END_REF] and [START_REF] Hansen | Isomer-Selective Detection of Keto-Hydroperoxides in the Low-Temperature Oxidation of Tetrahydrofuran[END_REF]. While Hansen et al. [START_REF] Hansen | Isomer-Selective Detection of Keto-Hydroperoxides in the Low-Temperature Oxidation of Tetrahydrofuran[END_REF] only found this fragmentation, Moshammer et al. [START_REF] Eskola | Time-resolved measurements of product formation in the low-temperature (550-675 K) oxidation of neopentane: a probe to investigate chain-branching mechanism[END_REF] also established the possibility of losing CO and an H atom. In the present study, the mass spectra, such as that of Fig. 1, do not present any peak at m/z 117 (H-loss) and only a very tiny one at m/z 90 (CO loss).

Concerning the m/z 43 signal, Fig. 4b shows that the position of the maximum is shifted by 10 K, but it should be considered that all the three isomers of KHP displayed in Table 1 could fragment to contribute to the signal observed at m/z 43. In addition, it can be attributed to the contribution of acetone to this channel, since the signals of Fig. 3b were recorded at hv=10.5 eV, above the fragmentation onset of acetone.

Assuming the production of fragments at m/z 85, 57 and 43, the 2,4KHP + fragmentation patterns displayed in Fig. 5 can occur via the formation of an unstable fragment at m/z 100, the same m/z as diones.

3.b/ Refining KHP quantification based on their fragmentation

the KHP ionization energies). This value is certainly too high when compared to values recently published for similar KHPs:  6.56 Mb for hydroperoxymethyl formate at 11.5 eV, an energy 1.5 eV above its ionization energy [START_REF] Eskola | Time-resolved measurements of product formation in the low-temperature (550-675 K) oxidation of neopentane: a probe to investigate chain-branching mechanism[END_REF],  between 3.3 and 8.3 Mb for the different KHP isomers issued from tetrahydrofuran at an energy 1.5 eV above their ionization energies [START_REF] Hansen | Isomer-Selective Detection of Keto-Hydroperoxides in the Low-Temperature Oxidation of Tetrahydrofuran[END_REF].

Note that in the present case at least three isomers with potentially different cross-sections have to be taken into account. This means that as much as an error of a factor of three could be considered in the cross-section estimation. Considering both the error due to fragmentation and that to cross-section estimation, the remaining discrepancy between experiments and kinetic modeling could be around only a factor of three. Even if the kinetic pathways leading to ketohydroperoxides are quite trustable, the kinetics involved in the decomposition of these elusive intermediate is not so well determined [START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF].

Especially the rate constants for their major decomposition involving the O-OH bond breaking rely on a single 1982 experimental measurement reviewed as shown by [START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF].

Identification of carboxylic acid

A last information about the chemistry of C5 KHP is given by the mass spectrum obtained at an energy of 11.5 eV (Fig. 1), where notable peaks at m/z 46 and 60 can be seen. As it was already shown (see fig. S3 (SM)), m/z 60 corresponds to acetic acid and was previously detected by gas chromatography [11] with a selectivity above 5% in fuel-lean mixtures under the same temperature conditions as in Fig. 1. M/z 46 can be attributed to formic acid and was already observed during the oxidation of oxygenated fuel (e.g. dimethyl ether) and neo-pentane, but not yet from linear alkanes [START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF], while its formation from KHP together with acetic acid can occur from the Korcek mechanism as proposed in [START_REF] Jalan | New Pathways for Formation of Acids and Carbonyl Products in Low-Temperature Oxidation: The Korcek Decomposition of γ-Ketohydroperoxides[END_REF]. Indeed, Table 1 shows that the presently measured and computed AIE for m/z = 46 species are close to the established AIE for formic acid. In addition, the computed PES for HCOOH (See SM) is similar to the He I PES spectrum of [START_REF] Leach | He I photoelectron spectroscopy of four isotopologues of formic acid: HCOOH, HCOOD, DCOOH and DCOOD[END_REF].

In order to see if the kinetic data calculated for propane [START_REF] Goldsmith | Effect of non-thermal product energy distributions on ketohydroperoxide decomposition kinetics[END_REF] can be used for acid prediction in the case of n-pentane, Fig. 6 presents the ratios between the measured mole fraction of formic and acetic acids in the present study compared to the ratios calculated using the kinetic data calculated by [START_REF] Goldsmith | Effect of non-thermal product energy distributions on ketohydroperoxide decomposition kinetics[END_REF]. This figure shows that, when n-pentane is used as fuel, the formation of acetic acid compared that of formic acid is about five times higher than in what could be computed using the kinetic data related to propane.

The structure of the fuel has a significant influence on the prediction of acid formation. Therefore, fuel specific kinetic data need to be calculated if an accurate modeling of acid formation is expected. ): experimental (using cross sections from [START_REF] Leach | VUV absorption spectrum of acetic acid between 6 and 20 eV[END_REF] [START_REF] Cool | Photoionization cross sections for reaction intermediates in hydrocarbon combustion[END_REF], respectively) and computed based on the reaction branching ratio of [START_REF] Goldsmith | Effect of non-thermal product energy distributions on ketohydroperoxide decomposition kinetics[END_REF] (values divided by 5) values.

V. CONCLUSIONS

Electron-ion coincidence mass spectrometry has been used to revisit the JSR oxidation of n-pentane, confirming that 2,4KHP is the C5ketohydroperoxide produced in highest amounts. An unexpected interesting results was the notable formation of C5 compounds with an enol and a ketone function together with that of diones. However, the important fragmentation (for the high binding energy part of the PES) has prevented more progresses in identifying ketohydroperoxides. In parallel, it was also demonstrated that considering their fragmentation may reconciliate model predictions and experimental measurements for these ketohydroperoxides. Although the assumptions involved prevent us from making a categorical statement on the precise mole fractions, we do demonstrate that the decrease of KHP concentration when compared to models, is compatible with the presence of dissociative ionization processes. Finally, the measurement of formic and acetic acids demonstrates the need to consider the fuel structure in attempts of predicting acid formation.

Figure 1 :

 1 Figure 1: Mass spectra obtained during n-pentane oxidation at 606 K and  = 1/3 for photon

  have been obtained from detailed kinetic model simulations, b To be compared to the value from the present experimental work as deduced from the PEPICO spectrum (AIE=11.29 eV) or that of [20] (AEI=11.31 eV).

Figure 2 :

 2 Figure 2: Comparison between the measured (under the conditions of Fig. 1) TPES spectra for m/z 118

  .

Figure 3 :

 3 Figure 3: M/z 118, 85, 57 and 43 (under the conditions of Fig. 1): (a) TPES and (b) ion signal at 10.5 eV as a function on temperature (numbers in the graphs indicate factors by which the signal are multiplied/divided). KHP identification may also be attempted according to the main detected oxygenated products. The

Figure 4 :

 4 Figure 4: Main expected low-temperature reaction pathways from the major predicted KHPs

  displays the energy and temperature dependence of the signals at m/z 118, 85, 57 and 43.

Fig 3 (

 3 Fig 3(a) shows the TPES of the possible fragments and that of the m/z 118 parent. The fragment curves

Figure 5 :

 5 Figure 5: Fragmentation pathways proposed for 2,4-KHP + issued from the major KHP formed

Figure 6 :

 6 Figure 6: Ratio between the mole fraction of formic and acetic acids (under the conditions of Fig.

1

 1

ACKNOWLEDGEMENT

We are grateful to the whole SOLEIL staff for smoothly running the facility under project 20180021.

We warmly thank J.-F. Gil for his technical help around the SAPHIRS experiment. M.H. acknowledge the use of the computing center MésoLUM of the LUMAT research federation (FR LUMAT 2764). J.B. thanks the ERC Starting Grant PRIMCHEM, grant agreement n°636829 for financial support.

Z. Wang was supported by the National Natural Science Foundation of China (51976208).

SUPPLEMENTARY MATERIAL:

The SM directory contains:

1 supplementary description file including: 1/ Summary of the mechanism related to ketohydroperoxides (KHP) according to [START_REF] Wang | Exploring hydroperoxides in combustion: History, recent advances and perspectives[END_REF] 2/ Photograph and scheme of the experimental apparatus with obtained photoelectron and photoion images 3/ Typical Time-of-flight mass spectrum plotted as a function of m/z and photon energy 4/ Example of lighter species identification according to [START_REF] Bourgalais | Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane[END_REF] 5/ Comparison between AIEs calculated by at the PBE0/aug-cc-pVDZ level and with the (R)CCSD(T)-F12/aug-cc-pVTZ method 5/ Conformers considered for KHPs 6/ Calculated Franck-Condon photoelectron spectra 7/ Comparison between experimental [START_REF] Bourgalais | Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane[END_REF] and newly simulated PES related to methylhydroperoxide (m/z 48) 8) PES of the 10 C5 KHPs 9/ TPES and PIE of formic carboxylic acid


Davide Mencaroni
email: mencaroni@icm.csic.es

Jaume Llopart

Roger Urgeles

Sara Lafuerza

Eulàlia Gràcia

Anne Le Friant

Morelia Urlaub

From gravity cores to overpressure history: the importance of measured sediment physical properties in hydrogeological models

The development of overpressure in continental margins is typically evaluated with hydrogeological models. Such approaches are used to both identify fluid flow patterns and to evaluate the development of high pore pressures within layers with particular physical properties that may promote slope instability. In some instances, these models are defined with sediment properties based on facies characterization and proxy values of porosity, permeability or compressibility are derived from the existing literature as direct measurements are rarely available.

This study uses finite-element models to quantify the differences in computed overpressure generated by fine-grained hemipelagic sediments from Gulf of Cadiz, offshore Martinique and Gulf of Mexico, and their consequences in terms of submarine slope stability. By comparing our simulation results with in-situ pore pressure data measured in the Gulf of Mexico, we demonstrated that physical properties measured on volcanic-influenced hemipelagic sediments underestimate the computed stability of a submarine slope. Physical properties measured on sediments from the study

From gravity cores to overpressure history: the importance of measured sediment physical properties in hydrogeological models

Introduction A C C E P T E D M A N U S C R I P T
Overpressure development in marine sediments is a major pre-conditioning factor that favours slope instability by decreasing the amount of gravitational driving stresses required to mobilize sediments [START_REF] Dugan | Offshore sediment overpressures of passive margins: Mechanisms, measurement, and models[END_REF]. If the pressure of the water contained in the pore space exceeds the hydrostatic condition, failures can be initiated in low angle submarine slopes by a relatively minor trigger such as a small magnitude earthquake [START_REF] Urlaub | What causes large submarine landslides on low gradient sediment accumulation[END_REF]. Generation of overpressure is commonly associated with rapid sedimentation [START_REF] Dickinson | Geological aspects of abnormal reservoir pressures in Gulf Coast Louisiana[END_REF][START_REF] Gibson | The Progress of Consolidation in a Clay Layer Increasing in Thickness with Time[END_REF]) and with the physical properties of the sediments, such as porosity, hydraulic conductivity and compressibility. The occurrence of overpressure within sediments can be studied directly through in-situ measurements with piezocone tests [START_REF] Lunne | Cone Penetration Testing in Geotechnical Practice[END_REF]) and pore pressure penetrometers [START_REF] Flemings | Pore pressure penetrometers document high overpressure near the seafloor where multiple submarine landslides have occurred on the continental slope, offshore Louisiana, Gulf of Mexico[END_REF], or indirectly from borehole logging measurements, such as sonic, resistivity and density logs [START_REF] Mouchet | Abnormal Pressures While Drilling -Origins, Prediction, Detection, Evaluation[END_REF], and from velocity analysis derived from seismic reflection data. Finally, overpressure development and evolution can be evaluated using hydrogeological models. The application at the basin scale of those models, in some cases, aims to provide a broad understanding of the conditions that may have generated previous slope instability [START_REF] Gutierrez | Modeling of compaction and overpressuring in sedimentary basins[END_REF]Bellwald et al. 2019;[START_REF] Llopart | Fluid flow and pore pressure development throughout the evolution of a trough mouth fan, western Barents Sea[END_REF]. In many cases, however, sediment samples from the study area are not available for geotechnical testing. In such instances, porosity, permeability and compressibility values are typically derived from the literature based on their expected lithologies [START_REF] Urlaub | What causes large submarine landslides on low gradient sediment accumulation[END_REF], depositional environment (Bellwald et al. 2019) or geographic area. On the other hand, measuring sediments physical properties implies a series of inevitable errors and uncertainties, which affect the reliability of models, such as presence of test errors, insufficient number of tests and the inherent variability of soil properties [START_REF] Phoon | Characterization of geotechnical variability[END_REF][START_REF] Zhang | Characterising geotechnical model uncertainty by hybrid Markov Chain Monte Carlo simulation[END_REF].

This study aims to evaluate the suitability of hydrogeological models based on literature derived physical properties of sediments collected from similar depositional environments and with comparable grain size distributions. To do this, we built a series of finite-element models that simulate the development of overpressure on a hemipelagic unit rapidly buried by a series of mass wasting events. Using the same depositional scenario, we defined the hemipelagic layer with initial porosity, hydraulic conductivity and specific storage measured on comparable sediments with similar grain size distribution, but collected in three different depositional settings: Alentejo basin (Gulf of Cadiz, SW Iberian margin), Grenada basin (offshore Martinique, Lesser Antilles) and Ursa basin (Gulf of Mexico). We further used the actual sedimentary history at Ursa Basin, where overpressure is groundtruthed with piezometer measurements, and tested the influence of using actual physical properties from the area versus physical properties taken from other depositional environment of similar grain size.

Depositional settings

We based this study on mechanical properties from 18 hemipelagic sediment samples collected from three different oceanic regions, characterized by different depositional settings, and all affected by well-documented subaqueous slope instabilities (Fig. 1a). The Alentejo basin is located in the northern sector of the Gulf of Cadiz, SW offshore Portugal (Fig. 1b). The area is characterized by an extensive contourite depositional system [START_REF] Hernández-Molina | Evolution of the gulf of Cadiz margin and southwest Portugal contourite depositional system: Tectonic, sedimentary and paleoceanographic implications from IODP expedition 339[END_REF], generated by the Mediterranean Outflow Water, which distributes sediments entering the gulf mainly through the Guadalquivir river [START_REF] Mulder | The Gulf of Cadiz: An unstable giant contouritic levee[END_REF]. The Grenada basin (Fig. 1c), offshore Martinique, is mainly composed by volcanic ashes, hemipelagic sediments and volcanoclastic turbidite deposits [START_REF] Lafuerza | Geomechanical Characterization of Submarine Volcano-Flank Sediments, Martinique, Lesser Antilles Arc BT -Submarine Mass Movements and Their Consequences: 6th International Symposium[END_REF] due to the activity of the surrounding volcanoes and absence of important sediment inputs from rivers. The sediments in the deep part of the basin accumulate at sedimentation rates up to 20 cm/ky (Le [START_REF] Friant | record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF]. The third area is the Ursa basin (Fig. 1d), in the Gulf of Mexico.

The region is known for the very high terrigenous sedimentation, entering the basin from the Mississippi River, taking place upon a salt substrate [START_REF] Worrall | Evolution of the northern Gulf of Mexico, with emphasis on Cenozoic growth faulting and the role of salt[END_REF]. The slope is characterized by well-documented overpressure due to sedimentation rates exceeding 25 m/ky during some intervals in the Pleistocene [START_REF] Flemings | Expedition 308 Scientists[END_REF][START_REF] Sawyer | Seismic geomorphology, lithology, and evolution of the late Pleistocene Mars-Ursa Turbidite region, Mississippi Canyon Area, Northern Gulf of Mexico[END_REF]. IODP expedition 308 performed in-situ pore pressure measurements using the DVTPP and T2P piezoprobes (see [START_REF] Flemings | Pore pressure penetrometers document high overpressure near the seafloor where multiple submarine landslides have occurred on the continental slope, offshore Louisiana, Gulf of Mexico[END_REF][START_REF] Long | Data report: consolidation characteristics of sediments from IODP Expedition 308, Ursa Basin[END_REF] for further details on the measurements), and documented

A C C E P T E D M A N U S C R I P T
high overpressure associated mainly with the high sedimentation rates in the area (Expedition 308 Scientists 2005). Previous studies [START_REF] Urgeles | Recursive Failure Of The Gulf Of Mexico Continental Slope: Timing And Causes BT -Submarine Mass Movements and Their Consequences: 3 International Symposium[END_REF][START_REF] Urgeles | History of Pore Pressure Build Up and Slope Instability in Mud-Dominated Sediments of Ursa Basin, Gulf of Mexico Continental Slope BT -Submarine Mass Movements and Their Consequences[END_REF] modelled the stratigraphic evolution of the margin considering the physical properties of the different units in the Ursa Basin, with the aim of understanding the present-day pore pressure condition revealed by the in-situ measurements.

Despite the significant scatter observed in the direct overpressure measurement, especially within the uppermost hemipelagic drape, 1D simulations reproduced the general vertical overpressure distribution at site U1324 (Fig. 2) (for more information about previous overpressure modelling see [START_REF] Urgeles | History of Pore Pressure Build Up and Slope Instability in Mud-Dominated Sediments of Ursa Basin, Gulf of Mexico Continental Slope BT -Submarine Mass Movements and Their Consequences[END_REF].

Methods

Sediments from the Alentejo basin (6 samples) have been collected using a gravity corer during the INSIGHT_Leg1 (2018) cruise in the Gulf of Cadiz. Initial void ratio (e 0 ), compression index (C c ) and initial hydraulic conductivity (e 0 ) in these samples have been derived with stepped loading (Rowe and Barden type of cell) or Constant Rate of Strain (CRS) oedometer tests (Fig 3a andTable 1).

Samples from Grenada basin (3 samples) and from the Ursa basin (9 samples) were recovered from wells drilled during two separate IODP expeditions. The samples from the Grenada basin used in this study have been drilled at well U1399 during the IODP expedition 340 [START_REF] Friant | record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340[END_REF] and their compressibility/permeability properties measured with 1D consolidation tests (Fig. 3b) [START_REF] Lafuerza | Geomechanical Characterization of Submarine Volcano-Flank Sediments, Martinique, Lesser Antilles Arc BT -Submarine Mass Movements and Their Consequences: 6th International Symposium[END_REF]. Sediments from the Ursa basin have been taken from wells U1322 and U1324, drilled during the IODP expedition 308 (Expedition 308 Scientists 2005;[START_REF] Flemings | Expedition 308 Scientists[END_REF]. Also in this case, their physical properties have been analysed using a combination of incremental loading and CRS oedometer tests (Fig. 3c) [START_REF] Urgeles | Recursive Failure Of The Gulf Of Mexico Continental Slope: Timing And Causes BT -Submarine Mass Movements and Their Consequences: 3 International Symposium[END_REF][START_REF] Urgeles | History of Pore Pressure Build Up and Slope Instability in Mud-Dominated Sediments of Ursa Basin, Gulf of Mexico Continental Slope BT -Submarine Mass Movements and Their Consequences[END_REF][START_REF] Long | Data report: consolidation characteristics of sediments from IODP Expedition 308, Ursa Basin[END_REF][START_REF] Stigall | Overpressure and earthquake initiated slope failure in the Ursa region, northern Gulf of Mexico[END_REF].

In our basin-modelling approach, we input the physical properties of the sediments at their initial depositional conditions, to simulate the pre-failure overpressure behaviour of the sediment composing the stratigraphic column. Therefore, input data such as porosity, hydraulic conductivity and specific storage are extrapolated to 1 kPa along the virgin consolidation line (e.g. Llopart et al. 2019). All of the samples were composed by hemipelagic sediments, collected outside mass transport deposit areas: for this reason, we assumed they did not undergo heavy deformation or mixing processes and the extrapolation of their physical properties to 1 kPa can be considered as a fair reproduction of their behaviour at depositional conditions.

A C C E P T E D M A N U S C R I P T
Grain size distribution for all samples has been measured using laser diffractometers. We adopted the grain size classification proposed by [START_REF] Wentworth | A Scale of Grade and Class Terms for Clastic Sediments[END_REF] to derive the percentages of sand (2 mm -62.5 μm), silt (62.5 μm -4 μm) and clay (< 4 μm) measured in the sediments. The threecomponents textural classification for muddy sediments proposed by Flemming (2000) has been used to subdivide the analysed sediments in four classes based on their sand, silt and clay content: D-II (very silty slightly sandy mud), D-III (silty slightly sandy mud), E-II (slightly clayey silt) and E-III (clayey silt) (Fig. 4).

Overpressure development simulations have been performed using the finite element software BASIN [START_REF] Bitzer | Modeling Consolidation Sedimentary and Fluid Basins Flow[END_REF][START_REF] Bitzer | Two-dimensional simulation of clastic and carbonate sedimentation, consolidation, subsidence, fluid flow, heat flow and solute transport during the formation of sedimentary basins[END_REF]. Modelling the stratigraphic and hydrodynamic evolution of a 2D section, BASIN allows the calculation of non-equilibrium compaction and overpressure generation on sediments by coupling compaction and 2D fluid flow. The consolidation model incorporates porositydependent sediment compressibility through the equation [START_REF] Bitzer | Modeling Consolidation Sedimentary and Fluid Basins Flow[END_REF]:

( 𝜕 𝜕𝑥 𝑘 𝑥(Ø) 𝜕𝑝 𝜕𝑥 ) + ( 𝜕 𝜕𝑥 𝑘 𝑧(Ø) 𝜕𝑝 𝜕𝑧 ) = (1 -Ø) 𝜌𝑔𝛼 (Ø) 𝜕𝑝

𝜕𝑡

where k x(ϕ) is the porosity-dependent hydraulic conductivity in the x-direction, α (ϕ) is the porositydependent sediment compressibility, p the fluid pressure, ϕ the porosity, ρ is the sediment bulk density and g is the gravity constant. Sediment compressibility in BASIN is calculated from the specific storage (S s ), which is defined as the volume of water removed from a unit volume of a confined aquifer with an increase in the vertical stress. It relates with the more commonly used compression index by the relation [START_REF] Jorgensen | Relationships betwa Basic Soils-Engineering Equations and Basic Ground-Water Flow Equations[END_REF]):

A C C E P T E D M A N U S C R I P T 𝑠 𝑠 = 0.434 C c γ w σ' v (1 + 𝑒 0 )
where C c is the compression index, γ w is the specific weight of water, σ' v is the effective vertical stress and e 0 is the void ratio at depositional conditions (1kPa).

In first instance, we built a series of 1D models to simulate the deposition of a 500 m thick hemipelagic unit with a sedimentation rate of 30 cm/ky, followed by a series of turbidite events that deposit 250 m of sand-rich material during 1000 years. Initial thickness of the units, sedimentation rates and boundary conditions, as well as physical properties of the turbidite unit, are kept constant through the different scenarios. Since no samples associated with turbidites were available in our database, we defined its physical properties with values taken from the literature [START_REF] Reed | Porometric properties of siliciclastic marine sand: A comparison of traditional laboratory measurements with image analysis and effective medium modeling[END_REF].

The only variables that change in between the different models are initial porosity, initial hydraulic conductivity and initial specific storage of the hemipelagic layer. In the first set of analysis (Scenario A), we defined the properties of the hemipelagic unit by averaging arithmetically the physical properties values for each study area. In the second series of tests (Scenario B), we assigned physical properties to each hemipelagic layer based on the average specific storage, porosity and hydraulic conductivity of the samples from each grain size class. The BASIN input parameters used for the simulation are provided in Table 2.

Secondly, to evaluate the reliability of our results and their significance in terms of submarine slope stability, we applied our approach on a case study where in-situ overpressure measurements have been collected. To do so, we considered the results of site U1324 drilled in the Ursa Basin during the IODP expedition 308.

Using drape deposited during the last ~20ky [START_REF] Behrmann | Rapid sedimentation, overpressure, and focused fluid flow, gulf of mexico continental margin[END_REF][START_REF] Flemings | Pore pressure penetrometers document high overpressure near the seafloor where multiple submarine landslides have occurred on the continental slope, offshore Louisiana, Gulf of Mexico[END_REF]. The overpressure evolution of the basin had been modelled and is modelled in this study since deposition of the sand-rich Blue unit (~ 100 ky ago), followed by the silt lithologies associated with the filling of the Ursa Canyon, and by the mud-dominated sediments from the channel levee assemblage and hemipelagic sediments (Fig. 2) [START_REF] Urgeles | Recursive Failure Of The Gulf Of Mexico Continental Slope: Timing And Causes BT -Submarine Mass Movements and Their Consequences: 3 International Symposium[END_REF][START_REF] Urgeles | History of Pore Pressure Build Up and Slope Instability in Mud-Dominated Sediments of Ursa Basin, Gulf of Mexico Continental Slope BT -Submarine Mass Movements and Their Consequences[END_REF].

This study also refers to pore pressure in terms of overpressure ratio (λ) [START_REF] Flemings | Pore pressure penetrometers document high overpressure near the seafloor where multiple submarine landslides have occurred on the continental slope, offshore Louisiana, Gulf of Mexico[END_REF], defined as:

λ = (𝑝 -𝑝 ℎ ) (σ 𝑣 -𝑝 ℎ )
where p is pore pressure, p h is the hydrostatic pressure and σ v is the lithostatic or total vertical stress.

A λ value of 0 means that pore pressure is hydrostatic, while a value of 1 means that the overburden is fully sustained by the pore water.

Results

Physical Properties

The grain size distribution of sediments collected in each study area are relatively homogeneous although each location has a distinctly different composition. Whilst all samples analysed are silt dominated, the percentages vary significantly between 57.36% and 83.1%. Clay contents also ranged between 11.1% and 42.2%, and sand sized grains comprised the lowest fraction at each site (0% to

A C C E P T E D M A N U S C R I P T
10.5%) (Fig. 4). The Ursa Basin has the lowest sand content, with 8 out of 9 samples corresponding to the "E" class of Flemming's classification (sand < 5%). Within the Grenada basin 2 out of 3 samples tested show a sand content greater than 5% and a clay content less than 25%. Sediments from the Alentejo basin are similar to the Ursa basin: sand content is less than 5% for 5 out of 6 samples, with clay values between 25% and 35%. Therefore, group D-II in Flemming's classification is only represented by two samples from the Ursa basin.

We used ternary contour diagrams to visualize different textural composition of sediment in relation with their compression index (Fig. 5a), initial porosity (Fig. 5b) and initial hydraulic conductivity (Fig. 5c). Results show a broadly heterogeneous relationship between grain size and physical properties.

Compression index (Fig. 5a) ranges between 0.22 and 0.92. The highest C c values are related to an increase in silt percentage and tend to decrease in sediments with some sand content. Samples from the Grenada basin, with higher silt content, show a compression index ranging between 0.67 and 0.75, while the lowest values are found in sediments from the Ursa basin, where most of the samples show C c values below 0.65. An exception is represented by sample 308-1322B_H08, which has higher clay content (42.2%) and compression index (0.92).

A similar trend is illustrated in the initial porosity diagram (Fig. 5b), with porosity values ranging between 0.52 and 0.79. The higher values are reached in sediments with higher silt content while the lower values are linked to higher sand content. However, sample SdG_02_102 differs from this trend showing a high initial porosity (0.78) despite a relatively low silt content (61%).

Values of hydraulic conductivity at deposition (1kPa) vary within two orders of magnitude (1E-8 to 1E-10 m/s). The Alentejo basin samples produce the highest values while the lowest values correspond to samples from the Grenada basin (Fig. 5c).

Our first modelling stage (Scenario A) (Fig. 6a) consisted in three separate models, where physical properties of the hemipelagic layer described above have been averaged for each study area (Tab. The second modelling scenario (Scenario B) consisted on defining the hemipelagic units with physical properties averaged from their grain size distribution classes (Fig. 6b). The minimum peak value (~0.06) is observed in the model defined with properties from the group E-II (sand<5%, clay < 25%).

Models from groups E-III and D-III show comparable maximum overpressure ratios (0.24 and 0.19),

while the maximum values (~0.53) are reached by the model defined with the properties averaged from the samples in the group D-II (Sand > 5%, Clay < 25%).

Fig. 7 shows the present-day overpressure resulting at site U1324 from our 1D simulations. Changes in the physical properties of the hemipelagic unit, at the top of the stratigraphy, did not allow any of our scenarios to fit perfectly neither the reference 1D model [START_REF] Urgeles | History of Pore Pressure Build Up and Slope Instability in Mud-Dominated Sediments of Ursa Basin, Gulf of Mexico Continental Slope BT -Submarine Mass Movements and Their Consequences[END_REF] nor the in-situ overpressure measurements [START_REF] Flemings | Pore pressure penetrometers document high overpressure near the seafloor where multiple submarine landslides have occurred on the continental slope, offshore Louisiana, Gulf of Mexico[END_REF]. The model defined with hydraulic conductivity, specific storage and initial porosity from the Alentejo basin underestimates the overpressure within the hemipelagic layer, mainly because of its higher permeability compared with the hemipelagic sediment from the Ursa basin (Tab. 2). Finally, the model defined with physical properties from the Grenada basin provide a very different output compared with the other models (Fig. 7). Even if it reproduces better the measured in-situ overpressures from the hemipelagic layer, the hydrodynamics of the model is split in two separate systems: in the lower part of the stratigraphy the overpressure increases constantly from the bottom to the top, reaching a maximum overpressure value of 1 at the base of the muddy sediments. Above this point, the overpressure drops, keeping a constant value of 0.7 up to the seafloor.

Discussion

A C C E P T E D M A N U S C R I P T
In the first part of our study, two different scenarios have been used to analyse the influence of variable grain size composition of hemipelagic sediments in overpressure development. In scenario (A), the hemipelagic layer was defined using physical properties collected from specific study areas and resulted in much higher overpressure generation for sediments of the Grenada Basin compared to the other two areas considered. In Scenario (B), the samples with relatively high silt and low clay content and sand content higher than 5% (sediment class D-II) generated the highest overpressure.

This sediment class includes only two samples, both collected in the Grenada Basin. In this case, the results obviously coincide with those of samples for the Grenada Basin in scenario A. Our results therefore seem to show that overpressure generation is rather related with the sediment composition and depositional setting instead of grain size distribution. This is in agreement with previous studies showing that there is no direct correlation between grain size distribution and sediment physical properties [START_REF] Lafuerza | Geomechanical Characterization of Submarine Volcano-Flank Sediments, Martinique, Lesser Antilles Arc BT -Submarine Mass Movements and Their Consequences: 6th International Symposium[END_REF] in marine muddy sediments.

However, when considering each depositional environment (e.g. river influenced), the grain size distribution has a major influence on overpressure development. Thus, sediments with low clay content (E-II) and samples with similar clay content but higher sand content (D-III) generated lower overpressure (Fig. 4 and6a). Nonetheless, factors not considered in this study such as sorting and particles rounding might have played a significant role that should be investigated in future works.

Comparing the overpressure results with the physical properties used for the simulations indicates that the development of overpressure is associated with low initial hydraulic conductivities and specific storages (Table 2). Such differences in physical properties between the Grenada Basin and the other two areas could reflect substantial diversity in sediment composition and mineralogy. This Antilles volcanic arc [START_REF] Wiemer | Altered marine tephra deposits as potential slope failure planes?[END_REF].

Comparing the 1D consolidation behaviour for volcanically derived hemipelagic sediments to the river-fed samples, it is possible to make assumptions about the reason of such notable differences in physical properties (Fig. 3). The Grenada basin sediments show lower compressibility at low consolidation stress and a transition to high compressibility at higher vertical stress (Fig. 3b). Similar behaviour known as structuration has been documented especially for fossiliferous, cemented or highly bioturbated sediments [START_REF] Locat | Natural soils: geotechnical behavior and geological knowledge[END_REF][START_REF] Spinelli | Diagenesis, sediment strength, and pore collapse in sediment approaching the Nankai Trough subduction zone[END_REF]: in these instances, sediments are capable of holding water in rigid structures, preventing compaction during the early stages of consolidation or, in case of high degrees of cementation, also at high burial depths. Physical properties of the sediments from the Grenada basin, with higher compression index and lower specific storage and hydraulic conductivity, suggest that a similar mechanism could occur in the volcanic ash-rich hemipelagic sediments from the Lesser Antilles volcanic arc. Volcanic ashes, in fact, may generate the same type of structures preventing pore water to be released in the system at low consolidation stresses.

Structuration in the Grenada basin sediments retains the water in the pore space, reducing the specific storage (which is, by definition, the volume of water removed from a unit volume of a confined aquifer) and decreasing the permeability, since the pore water gets trapped into rigid structures. Those physical properties represent the ideal conditions for the development of overpressure in marine sediments, and this could explain why the Grenada basin sediments (scenario A) develop higher overpressure (near lithostatic), which leads to a lower vertical displacement in the model.

It must be also mentioned that defining hydrogeological models with marine sediments that show structuration might question the hydrogeological model results. "Delayed consolidation" [START_REF] Locat | Natural soils: geotechnical behavior and geological knowledge[END_REF], as it is visible for the Grenada basin hemipelagic sediments (Fig. 3b), implies two different

A C C E P T E D M A N U S C R I P T
consolidation responses within the same sediment sample. Considering that the compression index is usually input as one representative value for each lithology, modelling such variation can represent a challenge. This is the reason why it is important to recognize structuration phenomena in the analysed samples and to consider its consequences while evaluating simulation results.

This behavior is well exemplified when modelling the Ursa basin sedimentation history with physical properties from the Grenada Basin (Fig. 7). The lower hydraulic conductivity and specific storage of the sediments from the Grenada basin, mainly due to the structuring phenomena previously discussed, makes the hemipelagic unit act as a perfectly impermeable seal in case of the very high sedimentation rates of the Ursa Basin. The pore water from the lower formations, characterized by coarser grain size and higher permeability, flow upwards and accumulates at the base of the seal, reaching extreme overpressure values up to 1 (Fig. 7). Nonetheless, this trend does not find correspondence in the in-situ measurement, where overpressure reaches its maximum within the hemipelagic layer and decrease constantly with depth, indicating a continuity between the pore water flow in the lower part of the stratigraphy and in the uppermost hemipelagic layer.

Significance for submarine slope stability

The development of overpressure in a submarine slope affects the magnitude of the events that can trigger a failure (e.g. earthquakes), or, in case of very high values and/or steep slope, can lead to slope instability with no external trigger [START_REF] Stigall | Overpressure and earthquake initiated slope failure in the Ursa region, northern Gulf of Mexico[END_REF][START_REF] Urlaub | What causes large submarine landslides on low gradient sediment accumulation[END_REF]. A preliminary quantification of the consequences from the different overpressure resulting from our 1D models at site U1324 can be made by calculating the factor of safety (FoS) on an hypothetical 2º submarine slope using the equation [START_REF] Flemings | Pore pressure penetrometers document high overpressure near the seafloor where multiple submarine landslides have occurred on the continental slope, offshore Louisiana, Gulf of Mexico[END_REF]:

𝐹𝑜𝑆 = tan Ø 𝑓 sin 𝜃 cos 𝜃 (cos 2 𝜃 -𝜆) A C C E P T E D M A N U S C R I P T
where 𝜃 is the surface slope, 𝜆 the overpressure ratio and θ f the friction angle of 28º as it is was measured on the hemipelagic sediments in the Ursa Basin [START_REF] Urgeles | Recursive Failure Of The Gulf Of Mexico Continental Slope: Timing And Causes BT -Submarine Mass Movements and Their Consequences: 3 International Symposium[END_REF]. FoS greater than 1 indicates stable slope, while values lower than 1 indicate slope failure.

The FoS has been calculated on the sediment deposited starting from 40000 years ago, corresponding to the beginning of the mud dominated channel levee assemblage, until the present day (Fig. 8). As the FoS decreases with increasing overpressure, the model generated defining the hemipelagic units with the physical properties from the Alentejo and the original Ursa basin reference model, show a decrease in the factor of safety until 20 ka. At this point, the hemipelagic sediment starts depositing and pore water migrates upwards with overpressure dissipating towards the seafloor (Fig. 8). The FoS for these two models reaches a minimum of around 3, increasing up to five at the present day. On the contrary, the model where the hemipelagic layer is defined with the physical properties from the Grenada basin shows an increase in overpressure values also after 20ky.

This is because the deposition and compaction of the hemipelagic sediments creates a seal which prevent the water from the lower part of the stratigraphy to flow upwards. In this case, the FoS reaches values of around 2 which propagate almost constantly from 20 ka until the present-day (Fig.

8).

At the end of the simulations, none of the models results in factor of safety as low to induce the slope to fail (<1) in the considered stratigraphic level. Nevertheless, 1D sedimentation models at site U1324 show a completely different overpressure development history when the model is defined with the hemipelagic sediments from the Grenada basin, resulting in a considerable underestimation of the factor of safety at the base of the first deposited fine sediment unit.

Our modelling results show that physical properties derived from only the lithological characterization or the grain size distribution available in the literature may produce inaccurate overpressure models, with consequent misleading slope stability estimations. On the other hand, models based on physical properties derived from similar grain size, depositional environment and

Conclusions

Using hydrogeological modelling we show how the sediment characteristics in some depositional environments have a significant influence on the amount of overpressure generated in marine hemipelagic sediments. We demonstrate how fine-grained marine sediments collected from different sedimentary environments (volcanic-influenced vs. river-dominated) can result in differences in overpressure in excess of one order of magnitude, and the consequences of those discrepancies for submarine slope stability assessment. Phenomena such as structuration of marine sediments needs to be carefully considered when carrying out basin hydrogeological models. Our results support the idea that, to achieve accurate results, besides accurate stratigraphic architecture and age of the units, hydrogeological models should be defined with physical properties measured from samples collected in the study area. In instances where such an approach is not feasible, assigning initial void ratio, hydraulic conductivity and specific storage extracted from the literature for samples with similar expected lithologies, composition and depositional settings are likely to provide the most accurate estimates. Given that this data is generally difficult to find within the scientific literature, a global open database for published physical properties is paramount to increase the accuracy of basin hydrogeological models in the future.

Acknowledgments

A C C E P T E D M A N U S C R I P T A C C E P T E D M A N U S C R I P T A C C E P T E D M A N U S C R I P T A C C E P T E D M A N U S C R I P T A C C E P T E D M A N U S C

Fig. 4

Grain size distribution of the sediments analyzed for this study. The internal subdivision refers to the classification for muddy marine sediments proposed by (Flemming 2000) and adopted in this study.

  BASIN we reproduced the stratigraphic architecture, sedimentation rate, boundary conditions and lithologies of a 1D overpressure model at site U1324 from Urgeles et al. (Fig. 2) (2010). Then, we substituted the original physical properties associated with the top hemipelagic drape with the ones measured from the sediments collected in the Alentejo and Grenada basins (Tab. 2). By doing this, we aim to evaluate the differences in overpressure generated by defining hydrogeological models with physical properties measured from hemipelagic sediments collected in different areas, and compare them with the reference calibrated results from Urgeles et al. (2010). At site U1324, 612 m of sedimentary sequence deposited between the Pleistocene and the Holocene (Expedition 308 Scientists 2005; Flemings et al. 2008) were drilled. From bottom to top, the main units composing the Ursa Basin consist of: (1) the sand-dominated lower Mississippi Canyon Blue Unit (Late Pleistocene) (Sawyer et al. 2007), (2) a mud dominated channel levee assemblage and (3) a mud-

  Fine-grained sediments from the Alentejo basin and from the Ursa basin show a comparable range of overpressure development, reaching a maximum overpressure ratio (λ) of 0.18 at 343 mbsf for the Alentejo basin model and 0.25 at 341 mbsf for the Ursa basin (i.e., immediately below the turbidite unit). The computed overpressure ratio from the Grenada basin model shows higher values with λ up to 0.56 at 329 mbsf.

  may in turn may be directly linked to the depositional environments: a) the hemipelagic sediments in the Alentejo and Ursa basins are largely fluvially derived from the Guadalquivir and Mississippi rivers, respectively; b) volcanically derived sediments from the Grenada basin where these by volcanic ashes released by the extensive series of volcanos in the Lesser

  of sediments, such as the ones from the Alentejo basin and the Ursa basin, provided similar overpressure development results. This indicates that, in the absence of direct measurements, literature derived physical properties based on grain size characteristics and depositional setting are expected to produce more accurate overpressure development for stability analysis of submarine slopes. However, care should be taken in settings where presence of microfossils such as diatoms, initial cementation or volcanic glass particles could provide sediment structuration.

  initial porosity; C c : compression index; k 0 : initial hydraulic conductivity (m/s). #:(Flemming 2000)

Fig. 1 :

 1 Fig. 1:

Fig. 2

 2 Fig. 2

Fig. 3 :

 3 Fig. 3:

Fig. 5 :

 5 Fig. 5:

Fig. 6

 6 Fig.6

Fig. 7

 7 Fig.7

Fig. 8

 8 Fig.8

Table 1 :

 1 Compiled physical and grain size data from Gulf of Cadiz, Martinique and Gulf of Mexico

	R I P T

Table 2 :

 2 Parameters used for the simulations in BASIN. The physical properties associated with Turbidity Layer have been used in all the simulations, while the Hemipelagic changed.

	Turbidite Layer	Samples (Urgeles et al. 2010) 0.5 Initial Porosity U Initial spec. storage (m-1) 0.001 S C R I P T Initial hydr. cond. (m/s) 1 × 10¯⁶
	Hemipelagic Scenario A -Areas	Alentejo basin Grenada basin		0.74 0.78 A N	0.025 0.005	4.28 x 10¯⁸ 1.43 x 10¯⁹
		Ursa basin	0.64 M	0.023	2.71 x 10¯8
	Hemipelagic Scenario B -Classes C C E P T E D D -II D -III E -II E -III	0.77 0.69 0.70 0.69	0.003 0.014 0.017 0.027	1.48 x 10¯⁹ 2.90 x 10 ¯8 1.04 x 10 ¯7 3.09 x 10 ¯8
	A

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This research used samples and/or data provided by the International Ocean Discovery Program (IODP) and has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 721403. The Spanish "Ministerio de Economia y Competitividad" and the European Regional Development Fund through grant CTM2015-70155-R (project INSIGHT) are acknowledged. We also thank Brandon Dugan and another reviewer for their comments that significantly improved our manuscript.