Xu ( Yvon

) Zhang

Jérôme Gaillardet

Laurie Barrier

Julien Bouchez

Li and Si isotopes reveal authigenic clay formation in a palaeo-delta

Keywords: Reverse weathering, Li isotopes, Si isotopes, authigenic clays, delta, marine chemistry

Marine authigenic clay formation has long been postulated as a major process to explain the mass budgets of some elements in seawater, and might act a reverse reaction for the neutralization of atmospheric acidity by soil forming reactions on land.

Nevertheless, to date, a handful of studies have directly investigated the effect of reverse weathering due to the challenges associated with sampling complexity. Deltas are thought to be one of the possible environments where reverse weathering reactions may occur because of the abundant influx of weathering derived materials and dynamic activities. In this study, we use a unique combination of three isotope systems (Si, Li, and Nd) as evidence of authigenic clay formation using sediments collected from a ~40 Ma-old delta complex (Ainsa Basin, Spain). Sediments were collected along the land to sea depositional continuum, from alluvial and coastal plains to marine environments. Direct comparison between alluvial, coastal, and marine sediments allows for observing the potential effect of reverse weathering. Systematic differences in Si and Li isotope composition exist between marine and continental sediments, revealing the formation of an authigenic phase sequestrating light Li and Si as associated with the diagenesis of Fe in reducing deltaic environments. Finally, this study proposes a geochemical tool for helping distinguishing marine and continental origins of sedimentary rocks when lithofacies and biofacies prove to be ineffective.

Introduction

The changes in chemical composition of seawater over geological time witness the co-evolution of input fluxes to the ocean by rivers or seafloor hydrothermal activities and of output fluxes such as secondary mineral precipitations, i.e. as carbonates or authigenic clay formations [START_REF] Holland | Sea level, sediments and the composition of seawater[END_REF]. The pioneering work of [START_REF] Mackenzie | Chemical mass balance between rivers and oceans[END_REF] postulated the existence of so-called "reverse weathering" processes in the ocean, acting as back reactions of silicate weathering on terrestrial surfaces [START_REF] Michalopoulos | Rapid Clay Mineral Formation in Amazon Delta Sediments: Reverse Weathering and Oceanic Elemental Cycles[END_REF]. During reverse weathering in the marine environment, authigenic clay formation not only takes up cations, but it also consumes alkalinity, releases CO2 to the ocean-atmosphere system, modifies the ocean pH, thereby impacting the geological carbon cycle (e.g. [START_REF] Isson | Evolution of the Global Carbon Cycle and Climate Regulation on Earth[END_REF]Isson and Planavsky, 2018;[START_REF] Sillen | The ocean as a chemical system[END_REF]. This reverse weathering hypothesis is however challenging to test as the products of reverse weathering are difficult to identify in contemporary marine sediments which are overwhelmed by complex mixtures of terrigenous and biogenic particles. As a consequence, in contrast with silicate weathering fluxes occurring on terrestrial surfaces, cations fluxes involved in reverse weathering reactions at the global scale are still poorly known. In addition, it is still unclear where does reverse weathering occur (coastal margin or in deep-sea sediments) and how it is affected by different geological settings.

Experimental studies using major elements and microscopy techniques have shown that coastal environments (deltas and estuaries) are viable candidates as sites for reactions scavenging terrestrially-derived cations through early diagenesis in the sediment column (Michalopoulos andAller, 2004, 1995). Recently new approaches using stable and cosmogenic isotopes have identified reverse weathering reactions and offer the promise to better constrain the fluxes associated with reverse weathering reactions (e.g. [START_REF] Andrews | Exploring the importance of authigenic clay formation in the global Li cycle[END_REF][START_REF] Bernhardt | 10Be/9Be ratios reveal marine authigenic clay formation[END_REF][START_REF] Rahman | Cosmogenic 32Si as a tracer of biogenic silica burial and diagenesis: Major deltaic sinks in the silica cycle[END_REF][START_REF] Santiago Ramos | Lowtemperature oceanic crust alteration and the isotopic budgets of potassium and magnesium in seawater[END_REF]Zhang et al., 2021).

Among the non-traditional stable isotopes, Li isotopes have proven good proxies of reverse weathering reactions due to the affinity of Li for secondary minerals and because the two isotopes of Li, 6 Li and 7 Li, are discriminated during their coprecipitations in clays. Studies conducted in modern estuaries and pore fluids of marine sediments [START_REF] Andrews | Exploring the importance of authigenic clay formation in the global Li cycle[END_REF][START_REF] James | Marine geochemical cycles of the alkali elements and boron: the role of sediments[END_REF][START_REF] Pogge Von Strandmann | Lithium, magnesium and uranium isotope behaviour in the estuarine environment of basaltic islands[END_REF] have reported significant isotopic variations. Constraints on the global Li cycle suggest an outflux of Li from the ocean balancing the influx from weathering and hydrothermal processes. The uptake of seawater Li by the formation of authigenic clays during reverse weathering is a strong candidate for such a flux [START_REF] Li | Evolution of Cenozoic seawater lithium isotopes: Coupling of global denudation regime and shifting seawater sinks[END_REF][START_REF] Misra | Lithium Isotope History of Cenozoic Seawater: Changes in Silicate Weathering and Reverse Weathering[END_REF]. In the critical zone (soil, rivers, groundwaters), the Li isotope signature appeared to be a powerful tracer for the formation of secondary minerals in soils or floodplains (e.g. [START_REF] Dellinger | Tracing weathering regimes using the lithium isotope composition of detrital sediments[END_REF]Dellinger et al., , 2014;;[START_REF] Golla | The evolution of lithium isotope signatures in fluids draining actively weathering hillslopes[END_REF][START_REF] Huh | Lithium and its isotopes in major world rivers: implications for weathering and the oceanic budget[END_REF], and a good proxy for distinguishing modern products of chemical weathering (pedogenic) from ancient marine sedimentary rocks (Dellinger et al., 2014).

Reverse weathering processes could enrich Li and shift the isotopic composition of continental derived solid weathering products toward a 7 Li enrichment [START_REF] Teng | Lithium isotopic composition and concentration of the upper continental crust[END_REF]. River study in the Amazon and Mackenzie basins have revealed that river sediments derived from the weathering of a marine sedimentary reservoir clearly appear to be enriched in Li and isotopically lighter than the modern solid weathering products derived from granitic rocks (Dellinger et al., 2014). This modification of Li geochemistry in marine sediments compared to their continental counterparts can occur during early or burial diagenesis processes, and in a variety of environments from estuarine and deltaic zones to the open ocean and within the sedimentary pile.

Although studying modern coastal depositional systems to quantify reverse weathering is the simplest approach to understand early diagenesis processes, it suffers from several issues, chiefly (1) the difficulty of isolating the sediment from seawater or porewater without altering the sediment composition, and (2) limitations encountered when trying to explore the long-term effect of a process studied over relatively short timescales.

Here as an alternative, we turn to a geological analogue to circumvent these difficulties. We investigate the Tertiary Ainsa palaeo-delta system in Spain (Fig. 1), a sedimentological system that has been the object of a breadth of studies aiming at understanding the links between active tectonics and deltaic sedimentary facies and architecture (e.g. [START_REF] Caja | Provenance of siliciclastic and hybrid turbiditic arenites of the Eocene Hecho Group, Spanish Pyrenees: implications for the tectonic evolution of a foreland basin[END_REF][START_REF] Dreyer | Architecture of the tectonically influenced Sobrarbe deltaic complex in the Ainsa Basin, northern Spain[END_REF][START_REF] Pickering | Deconvolving tectono-climatic signals in deepmarine siliciclastics, Eocene Ainsa basin, Spanish Pyrenees: Seesaw tectonics versus eustasy[END_REF][START_REF] Pickering | Mass-Transport Complexes (MTCs) and Tectonic Control on Basin-Floor Submarine Fans, Middle Eocene, South Spanish Pyrenees[END_REF]. This foreland delta was formed during the Eocene, 43 Ma, by the accumulation of erosion products from the then-forming Pyrenees. Refined sequence stratigraphy works have characterized the alluvial, coastal, and marine components from the geometrical and petrological points of view [START_REF] Dreyer | Architecture of the tectonically influenced Sobrarbe deltaic complex in the Ainsa Basin, northern Spain[END_REF].

The advantage of focusing on a palaeo-delta sedimentary series is that it offers a spatially (~1 km) and temporally (~1 Myr) integrated view of the effects of deltaic processes. Although such an approach is associated with potential challenges including sediment preservation and uncertainties about palaeo-seawater chemistry, it allows for probing authigenic effects directly by comparing the marine and continental sediments deposited within a relatively limited time window. To our knowledge, this study is the first investigation of reverse weathering reactions in a palaeo-deltaic system. Our investigation provides significant evidence for authigenic clay formation in seawatercontacted (marine and coastal) environments.

Methods

Field description and sampling strategy

Samples were collected in the palaeo-deltaic system of the Ainsa Basin (San Vincente, Sobrarbre, and Escanilla formations; [START_REF] Dreyer | Architecture of the tectonically influenced Sobrarbe deltaic complex in the Ainsa Basin, northern Spain[END_REF], which is part of the Southern Pyrenees foreland basin system (Fig. 1A). This well-studied geological object, formed during the Eocene (from middle Lutetian to early Priabonian), is a natural laboratory to compare sedimentary rocks of roughly the same age (albeit deposited within several Myr), mechanically sorted by sediment transport, and deposited along a land-to-sea (hence salinity) continuum. Palaeogeographic reconstructions indicate that the Ainsa Basin developed between the two N-S Mediano and Boltaña anticlines (Fig. 1B). To the south, it is bordered by the Sierras Marginales thrust front. Source material was derived from the then-forming Pyrenees mountain which supplied terrigenous silicates and carbonates to the deltaic system [START_REF] Caja | Provenance of siliciclastic and hybrid turbiditic arenites of the Eocene Hecho Group, Spanish Pyrenees: implications for the tectonic evolution of a foreland basin[END_REF][START_REF] Dreyer | Architecture of the tectonically influenced Sobrarbe deltaic complex in the Ainsa Basin, northern Spain[END_REF].

Palaeoclimate reconstructions suggest a tropical to subtropical climate with high rainfalls [START_REF] Pickering | Mass-Transport Complexes (MTCs) and Tectonic Control on Basin-Floor Submarine Fans, Middle Eocene, South Spanish Pyrenees[END_REF]. The relative seawater level varied during basin filling due to local tectonics, as well as glacio-eustatic seawater fluctuations [START_REF] Dreyer | Architecture of the tectonically influenced Sobrarbe deltaic complex in the Ainsa Basin, northern Spain[END_REF][START_REF] Pickering | Deconvolving tectono-climatic signals in deepmarine siliciclastics, Eocene Ainsa basin, Spanish Pyrenees: Seesaw tectonics versus eustasy[END_REF].

Based on a field analysis of the lithology and sedimentary structures, one or several sampling sites were chosen (Fig. 1B and1C) for each of the main sedimentary environments composing the palaeo-deltaic system of the Ainsa Basin (facies as defined in [START_REF] Bhattacharya | Deltas, Facies Models Revisited[END_REF]: the alluvial plain (for the upstream alluvial plain and upper delta plain, samples with prefix SA), the coastal plain (for the lower delta plain, samples with prefix SC), the delta front (samples with prefix SD), and the prodelta (samples with prefix ST). At these sites, deposits with different grain sizes (from microconglomerates to claystones) were sampled in order to access the whole spectrum of terrigenous sediments. Examples of sedimentological sections representative of the deposits of the different environments, and of how they were sampled, are shown in Fig. S1. At each site, 4 to 15 unweathered rocks of different grain size were collected, generally along road cuts. During the sampling, the outermost part of each sample was discarded to eliminate any potential effects of present-day weathering. In the field, the colours of the different outcrops were observed to be distinct: alluvial deposits generally exhibit a red colour, typical of oxidized environments, whereas marine deposits show a greyish colour, typical of sediments formed in reducing environments.

Detailed information on each sample is listed in Table S1.

Sample preparation, concentrations of major and trace elements in bulk siliciclastic sediments, and isotope measurements

Thin sections of selected samples were prepared for optical inspection. Then all the collected samples were dried at 50ºC and crushed using an agate mortar. Major elements were measured at SARM (Service d'Analyse des Roches et des Minéraux, Vandoeuvre-les-Nancy, France; details of sample preparation analysing methods are provided at http://helium.crpg.cnrs-nancy.fr/SARM/pages/roches.html). For trace elements, approximately 0.1 g of the crushed sample powders were dissolved using an HF-HNO3 method (Dellinger et al., 2014). Trace element concentrations were measured at the Institut de Physique du Globe de Paris (IPGP), using Inductively Coupled Plasma Quadrupole Mass Spectrometry (ICP-Q-MS).

Different powder aliquots were used for measuring Si, Li, and Nd isotopes on Thermo Fisher Neptune Plus MC-ICP-MS (See supplementary materials S1.1-S1.3 for the methods used for isotope measurements). Sequential extractions of Li pools were performed on selected samples to better constrain the controls on the bulk Li isotope composition of the samples. In the "rinsing" experiments, each sample was rinsed with Milli-Q water (MQ) by repeating the same procedure three times: (1) First, 5 ml MQ water were added to ~1 g sample which was then shaken vigorously for 24 hours; (2) Then, the sample mixture was centrifuged at 4400 rpm and the top 4 ml solution were pipetted out and filtered at 0.2 µm; (3) Finally, another 4 ml MQ were added into the residual. Additionally, in a separate set of "leaching" experiments, the same previous protocol was applied, but with 0.5 N HCl instead of MQ. The rinsing and leaching solutions were analysed for Li concentration and its isotope composition. We emphasize that the water rinsing and HCl-leaching experiments were carried out on separate aliquots of sediment samples. Therefore, the first HCl leaches likely include solutes that could have been released during the water rinsing experiment. Due to the challenges associated with separate Si between biological and non-biological Si pools and to detection limits of Si measurements, no extraction experiment was conducted for Si.

Results

Texture

We selected samples from alluvial, coastal, and marine deposits of various grain sizes (from coarse sandstones to fine siltstones) for mineralogical observations on thin sections by optical microscopy. Representative pictures are shown in Fig. S2 and the associated observations are summarized in Table 1. Overall, the thin sections show that source rocks of the sandstones and siltstones deposited in the Ainsa Basin are a mixture of igneous (among which metamorphic) and sedimentary rocks. These deposits are mainly composed of subangular detrital quartz and carbonates (from crystalline, bioclastic, and micritic limestones), together with minor detrital feldspars, micas, recycled sandstones, and siltstones. In addition, detrital oxides or hydroxides are also common in many samples of the alluvial plain, coastal plain, and prodelta deposits. On the other hand, carbonated bioclasts such as benthic foraminiferas and echinoderm remains are also present in the coarse-grained sediments of the delta front. Between those particles, a calcite cement is visible in the coarse-and medium-grained sandstones of all environments, while a clayey matrix can be observed in the fine-grained sandstones and siltstones. Eventually, micrite can also be observed in the finer-grained samples of the delta front and prodelta deposits.

Major and trace elements

The major and trace element data are reported in Table S2 andS3, respectively. Grain size exerts a major control on the chemistry of sediments as revealed by the chemical composition of river sediments (e.g. Bouchez et al., 2011a). Al/Si ratio can be used as a surrogate for sediment grain size (Bouchez et al., 2011b;Dellinger et al., 2014). The Al/Si ratios of the Ainsa samples range from 0.10 (quartz-rich samples) to 0.44 (clay-rich samples), similar to those observed in modern suspended river sediments (Dellinger et al., 2014). In order to correct the grain size effects (such as the dilution effect of quartz, carbonates, and other potential components in the bulk sample; Dellinger et al., 2014), elemental ratios normalized to Al (X/Al) are reported for major elements (Fig. 2A ranked by increasing X/Al ratios from Mn to Ca) and a number of trace elements (Fig. 2B, ranked by increasing X/Al ratios from Cs to Sr) for different sample groups. Within the variability of each sample groups, major and trace elements do not show significant differences between alluvial, coastal and marine environments.

Among major elements, only Fe and Na tend to show slight enrichments in the marine and coastal sediments compared to their alluvial counterparts. It is also interesting to note that K, which has been shown to be sensitive to reverse weathering or diagenetic reactions (e.g. [START_REF] Fedo | Unraveling the effects of potassium metasomatism in sedimentary rocks and paleosols, with implications for paleoweathering conditions and provenance[END_REF][START_REF] Michalopoulos | Rapid Clay Mineral Formation in Amazon Delta Sediments: Reverse Weathering and Oceanic Elemental Cycles[END_REF][START_REF] Santiago Ramos | Lowtemperature oceanic crust alteration and the isotopic budgets of potassium and magnesium in seawater[END_REF], is not enriched in the marine deposits at Ainsa. Regarding trace elements, Li is slightly enriched in marine deposits (Fig. 2B). Finally, it is notable that the Li content of sampled sediments is closely correlated to the combined abundance of Al and Fe therein (Fig. S3).

Li, Si and Nd isotopes and sediment grain size (Al/Si)

 30 Si,  7 Li, and εNd were analysed for bulk samples, and their results are reported in Table S1. At Ainsa, grain size displays a strong influence on  30 Si and  7 Li of sediments, whereas little effect on εNd (Fig. 3). εNd values range from -10.0 to -11.9 and are of typical Western Europe upper crustal values [START_REF] Nägler | Evolution of the Western European continental crust: implications from Nd and Pb isotopes in Iberian sediments[END_REF].  30 Si shows different values among samples ranging from -0.7‰ to -0.1‰. The  30 Si decreases with increasing Al/Si ratio, thus with decreasing grain size, particularly when Al/Si > 0.25. The same relationship is observed in river sediments and in shales [START_REF] Bayon | The silicon isotopic composition of fine-grained river sediments and its relation to climate and lithology[END_REF]Savage et al., 2013). The difference in  30 Si among different environments is hardly visible, but seawater-contacted sediments are slightly lighter than the alluvial ones for the higher Al/Si ratios. For example, sediments with  30 Si values lower than -0.4‰ can only be found in coastal and marine environments.

Li isotope compositions show greater variations from -2.4‰ to 6.9‰ and reveal first that δ 7 Li is strongly dependent upon grain size as observed in modern-day river sediments (Dellinger et al., 2014), and second that the different sample groups show significant differences in  7 Li with marine sediments having lower  7 Li values ( 7 Li=1.0 ± 4.4‰, 2σ) than alluvial ones ( 7 Li=3.9 ± 3.4‰, 2σ). The coastal plain samples lie in between ( 7 Li=2.6 ± 3.8‰, 2σ). The negative relationship between  7 Li and Al/Si in alluvial sediments is similar (although shifted toward higher values) to what is observed in modern-day river sediments (Dellinger et al., 2014).

An enrichment of Li in marine sediments accompanied by a fractionation which favours light Li in these sediments is observed when we compare the  7 Li and Li/Al ratio among different deposits (Fig. 4). The probability densities of Li/Al and  7 Li were estimated using Kernel Density Estimations with programs provided by [START_REF] Pedregosa | Scikit-learn: Machine Learning in {P}ython[END_REF] and are respectively displayed in Fig. 4. A slight enrichment of Li in the marine sediments can be observed: marine samples have higher Li/Al (Li/Al=1.17±0.27×10 -3 ) values compared to alluvial samples (Li/Al=1.05±0.26×10 -3 ).

The different sediment groups display significantly different  7 Li fingerprints with marine sediments have lighter  7 Li signatures than alluvial sediments. This is first shown by statistical analysis with p<0.05 (t-test, two tails) between any two environments and is visually displayed by a gradual decrease of  7 Li values corresponding to the peaks of probability densities from alluvial to marine phase sediments. Namely the probability densities are maximal at  7 Li equals to 2.9 ‰ for alluvial sediments, 1.9 ‰ for costal sediments, and 0.4 ‰ for marine sediments. The difference of  7 Li between marine and alluvial sediments is approximately -3 ‰.

Sequential extractions of Li pools

Initially, two Li pools (palaeo-seawater and carbonate) from the sediment samples were targeted and expected to be extracted. To access these pools, we have opted for a simple and convenient extraction method, which can provide first-order understanding of the Li pools presenting in the palaeo-delta sediments. We note here that any leachate should rather be envisaged as operationally defined as no chemical extraction protocol is perfect.

The first Li pool was extracted from rinsing the sample powder with MQ. This water-rinseable Li pool represents the loosely-bound and possibly surface layer sites Li fractions, presumably containing Li from seawater inclusion and adsorbed Li during clay-seawater interaction in clay minerals (e.g. Zhang et al., 2021). Overall, as shown in Table 2, 0.32-0.87% of total Li was rinsed from seawater-contacted samples (marine and coastal), whereas this "rinseable" fraction of Li was 0.17-0.43% in alluvial samples.

A first-order observation is thus that the seawater-contacted samples contain more rinseable Li compared to alluvial sediments. This small difference in the rinseable Li fraction between seawater-contacted and alluvial sediments may be attributed to a small seawater influence on the sediments. Except for the alluvial samples that have a toolow amount of rinseable Li, the isotope signature of this rinsed fraction was measured for marine sediments and whose values range from 1.91‰ to 12.84‰.

The second Li pool was expected to be Li contained in carbonates. Unlike the water-rinsing experiment, significant amounts of cations were extracted during the HCl-leaching experiments (Fig. S4). In these leachates, the dissolved substances can correspond to cations released from the exchangeable sites (also leached to some extent by MQ), carbonates, oxides, clays, and probably primary minerals. Our approach was thus to use a stepwise HCl leaching procedure, in order to roughly separate the Li from all those pools. In all samples, Ca, which is indicative of carbonates, was significantly extracted during the 3 steps of the leaching procedure, in a consistently decreasing fashion, such that at the end of the leaching experiment, a significant fraction of bulk Ca had been extracted for all samples (~55% to ~100% of total Ca, Fig. S4 and Table S4). In the samples where Ca was significantly extracted (>90%), other metals such as Li, Al, or Fe started to be extracted in the second or third step of the procedure as indicated by the Al/Ca and Fe/Ca ratios, which can be quantitively indicative of different Li pools [START_REF] Bastian | Lithium Isotope Composition of Marine Biogenic Carbonates and Related Reference Materials[END_REF], in the successive leachates (Table 3). Generally, in the first leachates, the Al/Ca and Fe/Ca ratios are low (~10 -6 -10 -5 ) indicating that Li in the first leachates is sourced from a rinseable Li pool and from a carbonate Li pool.

In the last leachates, the Al/Ca and Fe/Ca become significantly higher (~10 -2 -10 -1 ), suggesting that carbonate was dissolved during the previous leaching steps, and that the Li in the last leachates is sourced from the Al-Fe-associated Li pool.

Only the δ 7 Li compositions of the first (rinseable Li and carbonate-bound Li) and third (Al-Fe-associated Li) leachate solutions were measured (Table 3). The δ 7 Li compositions in the first leachates were characterized by values close to or heavier than the bulk δ 7 Li, whereas the δ 7 Li of the last leachates were characterized by values lighter than the bulk δ 7 Li (Fig. S5). The leaching results of sample SD7 seem to be opposite to observations from other samples, which we attribute to the low carbonate content in this sample. In sample SD7, the Al-Fe-associated Li was most likely released "earlier" (i.e. during the first leaching step) than in other samples, resulting in a Li isotope composition consistent with what is expected for the Al-Fe-associated Li pool.

Among these different Li pools, rinseable and carbonate Li have a minor effect (<1.5% of total Li, Fig. S4) on the δ 7 Li signatures of Ainsa sediments but the Al-Feassociated Li plays an important role on the bulk Li signature: sequential extractions show that a significant portion (4% to 42% of bulk Li, Fig. S4) of light Li ( 7 Li=-6.0‰ to -2.8‰) is present as an (Al-Fe)-associated Li pool in seawater-contacted (marine and coastal) sediments, whereas this portion of Li is relatively smaller (5% to 16% of bulk Li) and heavier ( 7 Li=-2.3‰ to 0.1‰) in alluvial sediments. Although the enrichment of 6 Li in the Al-Fe component is observed for both seawater-contacted and alluvial sediments, the δ 7 Li difference between this Li pool and bulk Li is generally greater in the marine deposits than in the alluvial ones (Fig. S5).

Discussion

Overall, our results show that Li (and to a lesser extent Si) isotopes are very sensitive proxies distinguishing between marine and continental depositional environmentsmuch more than concentrations of Li or other major/trace elements.

River studies have shown that one major cause of  7 Li and  30 Si variability in modernday river sediments is grain size [START_REF] Bayon | The silicon isotopic composition of fine-grained river sediments and its relation to climate and lithology[END_REF]Dellinger et al., 2014). In this study, grain size effects were taken into account by comparing the isotopic signature of different sedimentary bodies as a function of Al/Si. As indicated by Fig. 3, isotopic shifts between alluvial and marine samples occur for all range of Al/Si values and are therefore not due to changes in grain size. In the following, we thus discuss the possible causes for the isotopic variabilities in Si and Li that characterize the Ainsa palaeo-delta sediments.

Ruling out the effect of temporal variations of rock source material and weathering regime

As pointed out above, the Ainsa palaeo-delta was formed within a couple of million years. Because it was impossible to sample rocks of exactly the same age in the field, a possible explanation of the variations in  7 Li and  30 Si of the different sedimentary units could be temporal changes in the parent material or in the weathering processes in the upstream watersheds.

As shown by a large number of studies, continental silicate rocks are rather homogeneous in terms of  7 Li and  30 Si, with the exception of shales, which are enriched in Li and have lower  7 Li and  30 Si than granitic rocks (Dellinger et al., 2014;Savage et al., 2013). If the Li isotopic variations reported in this study were essentially due to mixing between different rock sources, Fig. 4 should be seen as a mixing diagram between at least three end-members whose contribution changed with time. The continental samples define a potential mixing line between a granitic end-member and a shale end-member, close (but not exactly same) to what Dellinger et al. (2014) found in the modern sediments transported by the Amazon and Mackenzie rivers ( 7 Li =5±1‰, Li/Al =0.5±0.1×10 -3 for the granitic end-member and  7 Li =1±1‰, Li/Al = 1±0.1×10 -3 for the shale end-member). However, the marine and coastal samples would necessitate a third endmember, enriched in Li and more depleted in 7 Li, to be explained (Fig. 4). Such an end-member has not been found in modern-day weathering products.

The fact that palaeogeographic reconstructions indicate no changes of provenance during the construction of the Ainsa delta [START_REF] Caja | Provenance of siliciclastic and hybrid turbiditic arenites of the Eocene Hecho Group, Spanish Pyrenees: implications for the tectonic evolution of a foreland basin[END_REF][START_REF] Dreyer | Architecture of the tectonically influenced Sobrarbe deltaic complex in the Ainsa Basin, northern Spain[END_REF], and that the Nd isotopes (not fractionated by weathering processes, [START_REF] Mclennan | Rare earth elements in sedimentary rocks; influence of provenance and sedimentary processes[END_REF] show invariant εNd values (Fig. 3), also suggest a negligible variation of source rocks of all samples measured in this study, which does not support the idea that the Li and Si isotopic variability can be explained by a mixture of different parent materials changing through time.

Alternatively, the observed isotopic changes between the different sediment formations of the Ainsa palaeo-delta could be due to temporal variations in weathering regimes within the sampled time window. We know from modern river systems that Li isotope ratios of secondary solid products of chemical weathering vary as a function of the weathering regime [START_REF] Dellinger | Tracing weathering regimes using the lithium isotope composition of detrital sediments[END_REF]. Intensive weathering, i.e. high W/(W+E) ratio (where W is the chemical erosion flux and E the physical erosion, [START_REF] Bouchez | How accurate are rivers as gauges of chemical denudation of the Earth surface?[END_REF], results in lower river sediment  7 Li. Using this principle, the difference in  7 Li between marine and alluvial sediments could be interpreted as resulting from different weathering regimes on the continent: a higher weathering intensity reflected in the marine sediments compared to the alluvial ones. This scenario would however imply a stronger Li-depletion in marine sediments, i.e. a stronger solubilization of Li which produces lower Li/Al ratios in the solids (Dellinger et al., 2014). We observe the opposite in Ainsa sediments (Fig. 4). Therefore, the observed isotopic shifts between marine and alluvial sediments in Ainsa are unlikely to be related to changes in provenance or weathering regime with time, but rather to processes that operated in the palaeo-delta such as the scavenging of dissolved Li into sediments through authigenic mineral formations.

Evidence for the presence of an authigenic phase incorporating light Li and Si.

We can interpret Fig. 4 as reflecting the existence of an authigenic component contained in the marine sediments (and coastal sediments to a lesser extent), enriched in Li (high Li/Al) and with  7 Li lower than -3‰. Such a low value agrees with that found in the leached phase in the HCl experiments.

The MQ rinsing experiments and the sequential extraction experiments using

HCl show that the only significant pool of extractable Li is associated with Al and Fe, thus probably to Fe-bearing clay minerals, and that this Li pool displays a low δ 7 Li. The absence of this Li fraction in the MQ rinsing solution suggests that this extractable Li pool does not correspond to Li adsorbed or Li hosted in seawater inclusions. It is important to note that all sediments groups from alluvial plains, coastal, and marine environments show this extractable Li component even if it appears to be more important in the seawater-contacted sediments. We interpret these results as indicating the presence of an authigenic phase characterized by high contents of Al and Fe, mostly present in the marine sediments of the Ainsa palaeo-delta. We emphasize that this Li pool is a key to interrogate reverse weathering processes and it was often overlooked in previous studies based on Li isotopes in sedimentary records (e.g. [START_REF] Yang | Clay Li and Nd isotopes response to hydroclimate changes in the Changjiang (Yangtze) basin over the past 14,000 years[END_REF].

From an isotopic point of view, it is now well admitted that the incorporation of Li in secondary phases at low temperature favours the light Li isotope (e.g. [START_REF] Andrews | Exploring the importance of authigenic clay formation in the global Li cycle[END_REF][START_REF] Hindshaw | Experimental constraints on Li isotope fractionation during clay formation[END_REF][START_REF] Vigier | Quantifying Li isotope fractionation during smectite formation and implications for the Li cycle[END_REF]Zhang et al., 2021). Leached solutions from Ainsa sediments are enriched in 6 Li, and are therefore consistent with the presence of an authigenic component. Authigenic clay formation has been suggested to take place in the water column of an Icelandic estuary by [START_REF] Pogge Von Strandmann | Lithium, magnesium and uranium isotope behaviour in the estuarine environment of basaltic islands[END_REF]. In this study, the authors interpreted the δ 7 Li of suspended sediments as indicating the addition to basalt-derived detrital material of an authigenic component having a δ 7 Li 19‰ lower than seawater, consistent to the fractionation factor (~ -20‰) obtained from experimental studies (e.g. Zhang et al., 2021). Based on the available seawater  7 Li reconstructions, Eocene seawater  7 Li should be close to ~24‰ [START_REF] Misra | Lithium Isotope History of Cenozoic Seawater: Changes in Silicate Weathering and Reverse Weathering[END_REF] and thus inferred  7 Li value of the Ainsa authigenic component should be close to 4-5‰ assuming the same apparent fractionation factor. This value is higher that the inferred authigenic component in this study (< -3‰), suggesting that the authigenic phase in Ainsa may not have precipitated directly from Eocene seawater. To explain the < -3‰  7 Li signature inferred from Fig. 4, applying a Li isotope fractionation factor of ~ -20‰, the source of Li to authigenic phases should have a δ 7 Li signature lower than 17‰. A viable candidate for such a signature would be the sediment porewater. Although it is impossible to know the local δ 7 Li of the porewater when Ainsa sedimentary rocks formed, we note that the modern pore water recovered from coastal environments typically has δ 7 Li values lower than that of seawater [START_REF] James | Marine geochemical cycles of the alkali elements and boron: the role of sediments[END_REF].

The effect of reverse weathering on oceanic Si isotope composition has been long speculated (Frings et al., 2016 and references therein), and yet only limited information is available from a handful of previous studies, which in particular showed that authigenic clay formation is linked to the diagenesis of biogenic Si (BSi; e.g. [START_REF] Ehlert | Stable silicon isotope signatures of marine pore waters -Biogenic opal dissolution versus authigenic clay mineral formation[END_REF][START_REF] Geilert | Impact of ambient conditions on the Si isotope fractionation in marine pore fluids during early diagenesis[END_REF][START_REF] Michalopoulos | Early diagenesis of biogenic silica in the Amazon delta: alteration, authigenic clay formation, and storage[END_REF][START_REF] Rahman | The Missing Silica Sink: Revisiting the Marine Sedimentary Si Cycle Using Cosmogenic 32Si[END_REF][START_REF] Rahman | Cosmogenic 32Si as a tracer of biogenic silica burial and diagenesis: Major deltaic sinks in the silica cycle[END_REF]. The Si isotope signature of the BSi source at Ainsa is unknown, which in conjunction with the lack on constraints on Si isotope fractionation during BSi diagenesis, hampers interpretation of our results beyond a first-order glimpse on the potential role of authigenic clay formation on Si isotopes. However, the slight difference in δ 30 Si between the alluvial and seawater-contacted samples suggests that the newly formed authigenic phase preferentially takes up light Si, in agreement with previous findings from the precipitation of Si-bearing phases (e.g. [START_REF] Ehlert | Stable silicon isotope signatures of marine pore waters -Biogenic opal dissolution versus authigenic clay mineral formation[END_REF][START_REF] Geilert | Impact of ambient conditions on the Si isotope fractionation in marine pore fluids during early diagenesis[END_REF][START_REF] Oelze | The effect of Al on Si isotope fractionation investigated by silica precipitation experiments[END_REF].

Importantly, the lower  30 Si signature of seawater-contacted sediments is observed for samples with a high Al/Si ratio. In the context of previous studies on river sediments (e.g. Bouchez et al., 2011), this can be interpreted as the fine end-member of detrital sediment grain size undergoing the largest extent of authigenic clay formation, provided that this latter reaction does not shift significantly the sediment Al/Si ratio towards lower values through the incorporation of BSi. We argue that the Al/Si ratio is relatively stable during these reactions. First, Al is an immobile element and the source of Al for authigenic clay formation is thought to be the alteration of terrigenous Aloxides and aluminosilicates [START_REF] Michalopoulos | Rapid Clay Mineral Formation in Amazon Delta Sediments: Reverse Weathering and Oceanic Elemental Cycles[END_REF]. Therefore, the bulk Al content should be the same between continental and marine sediments. Regarding Si, although we are still lacking detailed information on authigenic clay formation, this Si flux to the sediment due to reverse weathering process is considered to be relatively small [START_REF] Frings | The continental Si cycle and its impact on the ocean Si isotope budget[END_REF] when compare to detrital Si content delivery to the oceans [START_REF] Milliman | River discharge to the coastal ocean: a global synthesis[END_REF]. As both bulk sediment Al and Si concentrations are not significantly modified by authigenic clay formation, the Al/Si ratio remains relatively unchanged during diagenesis. As a consequence, the lower  30 Si in high-Al/Si marine sediments is indeed evidence for finer detrital fractions being more prone to authigenic clay formation.

A possible reverse weathering pathway coupling Fe cycle in marine environment

In the Ainsa palaeo-delta, the presence of authigenic clays is suggested by the enrichment of light Li isotopes in seawater-contacted sediments. Moreover, the Li isotope geochemistry is closely linked to Fe cycling in marine environments. First, as indicated by Fig. 5, in bulk samples correlations exist between δ 7 Li and Li/Fe or Li/(Fe+Al), particularly for marine sediments. Second, the leaching experiment shows that light Li isotopes, which is indicative of the authigenic clays, is associated with an Al-Fe phase (Figs. S4 andS5). It is important to note that the range of isotopic values found in this Al-Fe-associated Li pool is compatible with that inferred (< -3‰) from the mixing diagram of Figs. 3 and4. Beyond our study in Ainsa, the association between δ 7 Li and Fe content was also found in other marine sedimentary rocks [START_REF] Qiu | Li and δ7Li in mudrocks from the British Caledonides: metamorphism and source influences[END_REF]. Therefore, our study provides evidence that reverse weathering, in certain geological context such as deltas, is coupled with the marine Fe cycle. Indeed, river deltas are known to act as a major sink of riverine dissolved Fe through colloidal flocculation and particle adsorption (e.g. [START_REF] Sholkovitz | Flocculation of dissolved organic and inorganic matter during the mixing of river water and seawater[END_REF]. The rapid formation of Fe-bearing products could then lead to efficient light Li sorption in the water column [START_REF] Pistiner | Lithium-isotope fractionation during continental weathering processes[END_REF]. Once deposited in the sediment column, early diagenesis in reducing marine environments would then allow for the formation of Fe 2+ -rich phases, such as oxides and Fe-rich aluminosilicates. Such processes can be possibly achieved by following reaction:

H4SiO4+Fe 2+ +2HCO - 3 →FeO-SiO2 (Fe-rich silicates) +3H2O+2CO2 (1)
The Fe-rich silicates minerals could incorporate Li, as Li + and Fe 2+ share similar ionic radii [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]. Iron-rich compositions are a common feature of authigenic minerals such as greenalite and odinite (e.g. Isson and Planavsky, 2018), which form relatively rapidly in the sediment column (Michalopoulos andAller, 2004, 1995). This Fe-rich phase is likely driven by the co-precipitation of Fe and Si (e.g. [START_REF] Wu | Stable iron isotope fractionation between aqueous Fe(II) and model Archean ocean Fe-Si coprecipitates and implications for iron isotope variations in the ancient rock record[END_REF].

The association of Li with an Al-Fe pool in Ainsa sediments can therefore be interpreted as the formation of authigenic clays in a reducing environment in the delta, for example in the sedimentary column.

A recent study of the authigenic formation effect on Li geochemistry in a carbonate-rich unit from deep-sea environment [START_REF] Andrews | Exploring the importance of authigenic clay formation in the global Li cycle[END_REF] suggests that significant clay neoformation occurs in the sedimentary column at depth further away from the seawater-sediment interface. In contrast to results from an estuary in Iceland [START_REF] Pogge Von Strandmann | Lithium, magnesium and uranium isotope behaviour in the estuarine environment of basaltic islands[END_REF] and experimental constraints on authigenic formation (Zhang et al., 2021), where rapid early diagenesis predominate, slow burial diagenesis controls the uptake of Li into authigenic phase at geological time scales.

Although our limited dataset does not permit us to fully explore the roles of both early and burial diagenesis, we speculate that our results are driven by early diagenesis processes as the authigenic phase is closely coupled to rapid Fe cycle (e.g. [START_REF] Sholkovitz | Flocculation of dissolved organic and inorganic matter during the mixing of river water and seawater[END_REF] and the deltaic environments, where detrital materials are abundant and these materials can be further dissolved during the interaction with seawater, allows for early diagenesis processes (e.g. [START_REF] Michalopoulos | Rapid Clay Mineral Formation in Amazon Delta Sediments: Reverse Weathering and Oceanic Elemental Cycles[END_REF]. Furthermore, the δ 7 Li fingerprint used in this study to trace authigenic formation shows a gradual decrease along the land-to-sea continuum, and the authigenic phase is not only observed in marine sediments but also in coastal settings where the burial effect is minor. This suggests the authigenic processes occurred in the Ainsa delta was rather impacted by early diagenesis effects.

At the Ainsa palaeo-delta, we observe that these reactions tied to Fe act as a sink for Li into the sediments and causes a fractionation resulting in a heavier δ 7 Li in water.

This deltaic process identified in this study therefore contributes to enrich marine shales in Li (Dellinger et al., 2014) even if the Li/Al difference between marine and nonmarine material is tight (Fig. 4). The contradictory observation between Ainsa marine sediments where light Li is preferred and shales having heavy δ 7 Li values [START_REF] Teng | Lithium isotopic composition and concentration of the upper continental crust[END_REF] indicates other mechanisms enriching Li in marine sedimentary rocks, such as the precipitation of other Li-bearing phases in other geological contexts or more probably during burial diagenesis [START_REF] Andrews | Exploring the importance of authigenic clay formation in the global Li cycle[END_REF]. The enrichment of Na in the seawater-contacted sediments observed at Ainsa compared to alluvial sediments (Fig. 5) is noteworthy. As the Na abundance of the HCl-leach extracts is similar among different environments, the enrichment of Na in the coastal and marine sediments can be attributed to its incorporation into authigenic phases (e.g. phyllosilicates), rather than to seawater inclusions. Nevertheless, the enrichment of the heavy alkali elements K, Rb, and Cs is less significant in the case of Ainsa (Table S5). Although K has been shown to be taken up by reverse weathering reactions (e.g. [START_REF] Michalopoulos | Rapid Clay Mineral Formation in Amazon Delta Sediments: Reverse Weathering and Oceanic Elemental Cycles[END_REF], our results suggest the diagenetic phases that formed in the Ainsa delta do not play a major role of seawater K sequestration. This difference in terms of K uptake between a palaeo-delta archive (this study) and modern delta systems [START_REF] Michalopoulos | Rapid Clay Mineral Formation in Amazon Delta Sediments: Reverse Weathering and Oceanic Elemental Cycles[END_REF] requires further work to understand the mechanism controlling K sinks in the ocean [START_REF] Holland | Sea level, sediments and the composition of seawater[END_REF].

Conclusion

The fact that at Ainsa, the Si and Li isotope compositions in marine and coastal plain sediments are lighter than in alluvial ones, together with the associated enrichment in the soluble cation Li and Na, suggest that formation of Fe-rich authigenic clays in deltaic environments is an important process for ocean elemental and isotope budgets.

Our results show that light Li is incorporated into seawater-contacted sediments, particularly the marine sediments in deltaic environments, resulting in a lower δ 7 Li signature than in alluvial ones. This observation is in contrast to that made on ancient, fine-grained marine sedimentary rocks (Dellinger et al., 2014;[START_REF] Teng | Lithium isotopic composition and concentration of the upper continental crust[END_REF]. Similar to continental weathering processes which can not be ascribed to a single reaction, reverse weathering reactions are likely associated with complex diagenesis processes and different yet-unidentified reactions, and here we have one which couples Li, Si, and Fe cycles in the deltas. Furthermore, our study supports a strong coupling among Fe, Li, and Si in deltaic environments (coastal and marine) through the neoformation of authigenic phyllosilicates in reducing conditions. This process is typical of a reverse weathering reaction as proposed initially, in that it consumes alkalinity and produces CO2 in the atmosphere-ocean system [START_REF] Isson | Evolution of the Global Carbon Cycle and Climate Regulation on Earth[END_REF][START_REF] Mackenzie | Chemical mass balance between rivers and oceans[END_REF][START_REF] Sillen | The ocean as a chemical system[END_REF]. By consuming ocean alkalinity, it acts as a counterbalancing reaction to the neutralization of atmospheric CO2 through silicate weathering reactions transforming rocks into soils in the terrestrial Critical Zone. Finally, our investigation clearly demonstrates that reverse weathering reactions existed at the time of deposition, and Li isotopes can be useful to constrain the marine or continental origin of sedimentary rocks. Such a tool is important in paleo-environmental studies including reconstruction of past living environments or the formation of mountain ranges when arguments based on sedimentary facies or fossil organisms are not conclusive. Seawater contacted sediments (coastal and marine) SD 3 -0.55±0.09 3.11×10 -5 7.51×10 -5 -6.01 ± 0.14 6.26×10 -2 1.00×10 -1 SD 7 -6.32±0.08 1.49×10 -2 2.72×10 -2 -3.53 ± 0.27 3.10×10 -1 3.94×10 -1 ST 5 2.88±0.22 2.29×10 -5 8.51×10 -5 -4.67 ± 0.37 7.80×10 -2 1.95×10 -1 SC 21 6.16±0.37 1.20×10 -5 6.60×10 -5 -2.83 ± 0.30 3.43×10 -2 2.38×10 -2

S1.1 Lithium isotope measurements

A double-step separation protocol using cation exchange resin (Biorad, AG50W-X12, 200-400 mesh) was employed in order to separate Li from the sample matrix (Zhang et al., 2021). During each separation session, five reference materials (BHVO-2, JB-2, NASS-6, SRM-2709a, and TILL-1) were routinely processed together with samples to evaluate the quality of this protocol. The Li isotope composition of the purified samples was measured using a Thermo Fisher Neptune Plus MC-ICP-MS. The pure Li standard solutions IRMM-016 and SPEC were routinely used to monitor the instrumental stability, while the L-SVEC solution was used as a bracketing standard to correct for instrumental mass fractionation. Accurate and reproducible Li isotope compositions for IRMM-016 (δ 7 Li=0.13±0.17‰, 2 SD, n=36), SPEC (δ 7 Li=94.32±0.19 ‰, 2 SD, n=26), NASS-6 (δ 7 Li=30.95±0.51 ‰, 2 SD, n=12), BHVO-2 (δ 7 Li=4.66±0.78 ‰, 2 SD, n=11), SRM-2709a (δ 7 Li=-0.30±0.58 ‰, 2 SD, n=13), TILL-1 (δ 7 Li=6.69±0.68 ‰, 2 SD, n=14), and JB-2 (δ 7 Li=4.72±0.80 ‰, 2 SD, n=15) were obtained.

All the results are in good agreement with values reported in previous studies [START_REF] Brand | Assessment of international reference materials for isotope-ratio analysis[END_REF]Dellinger et al., 2014;[START_REF] Weynell | Lithium isotopes and implications on chemical weathering in the catchment of Lake Donggi Cona, northeastern Tibetan Plateau[END_REF][START_REF] Kuessner | Automated Analyte separation by Ion Chromatography using a Cobot Applied to Geological Reference Materials for Li Isotope Composition: Geostandards and Geoanalytical Research[END_REF].

S1.2 Silicon isotope measurements

In order to analyse the Si isotope composition, an aliquot of each crushed sample was first digested using an alkali (NaOH) fusion method [START_REF] Georg | New sample preparation techniques for the determination of Si isotopic compositions using MC-ICPMS[END_REF], and the Si concentration of digested sample solution was analysed using ICP-Q-MS. Then, a widelyadopted Si separation protocol was used [START_REF] Georg | New sample preparation techniques for the determination of Si isotopic compositions using MC-ICPMS[END_REF][START_REF] Pringle | Silicon isotopes reveal recycled altered oceanic crust in the mantle sources of Ocean Island Basalts[END_REF] using AG50W-X12 (200-400 mesh) resin. Si isotope composition was measured on a Thermo Fisher Neptune Plus MC-ICP-MS within 24 hours after the Si separation. The well-characterized reference material BHVO-2 was routinely analysed as an external standard to check the quality of the separation protocol and of the isotope measurements. The reference material National Bureau of Standards 28 Sand Quartz (NBS-28, or also called NIST RM-8546) was processed through the same digestion and separation methods and was used as the bracketing standard to correct for instrumental mass fractionation. The Si isotope composition is expressed in δnotation in the unit of ‰ related to NBS-28: δ x Si= [( x Si/ 28 Si)Sample/( x Si/ 28 Si)NBS-28-1]×1000, where x=29 or 30. Mass-dependent fractionation of Si isotopes across all samples is indicated by the linear relationship observed between δ 30 Si and δ 29 Si across all samples, with a slope value of 0.5153, which is between the value of kinetic fractionation (0.5092; [START_REF] Young | Kinetic and equilibrium mass-dependent isotope fractionation laws in nature and their geochemical and cosmochemical significance[END_REF] and that of equilibrium fractionation (0.5178; [START_REF] Young | Kinetic and equilibrium mass-dependent isotope fractionation laws in nature and their geochemical and cosmochemical significance[END_REF], undistinguishable from the two fractionations. The δ 30 Si value of BHVO-2 measured in this study (-0.29±0.05‰, 2 SD, n=21) is in agreement with previously reported values [START_REF] Zambardi | Precise Determination of Silicon Isotopes in Silicate Rock Reference Materials by MC-ICP-MS: Geostandards and Geoanalytical Research[END_REF]Savage et al., 2013).

S1.3 Neodymium isotope measurements

To separate Nd from the matrix, we have employed a double-column method [START_REF] Caro | High-precision 142 Nd/144 Nd measurements in terrestrial rocks: constraints on the early differentiation of the Earth's mantle[END_REF]. Sample solution was first allowed to pass a column packed with the TRU-spec resin (Eichrom) to separate rare earth elements from the sample matrix, then to pass a second column packed with the Ln-spec resin (Eichrom) to further purify Nd. Finally, Nd isotope composition was measured on a Neptune MC-ICP-MS, using a 142 Nd/ 144 Nd ratio of 0.7218 for internal correction of instrumental mass fractionation. In this study, the Nd isotope composition is expressed in ε-notation: εNd=[( 143 Nd/ 144 Nd)Sample/( 143 Nd/ 144 Nd)CHUR-1]×10000 where CHUR stands for chondritic uniform reservoir ( 143 Nd/ 144 Nd = 0.512638). The reported results are within an analytical uncertainty estimated at ~0.2 εNd units from repeated runs of standard solutions.

  Figure captions

Figure 2 .

 2 Figure 2. "Box plots" of (A) major and (B) trace element concentrations

Figure 3 .

 3 Figure 3. Grain size (indexed by the Al/Si ratio) distribution of lithium (top),

Figure 4 .

 4 Figure 4. Relationship between δ 7 Li values and Li/Al ratios of the alluvial plain,

Figure 5 .

 5 Figure 5. Relationships between δ 7 Li values and Li/(Al+Fe), Na/(Al+Fe) and

  Figure 1 34

Table captions Table 1 .

 captions1 Summary of the thin section observations.

Table 2 .

 2 Amount of Li extracted by the water rinsing experiments (in % of bulk content).

Table 3 .

 3 Li isotope composition of the first and the last leach solution obtained from the HCl-leaching experiments.

Table 1 .

 1 Summary of the thin section observations.

	Depositional environment	Coarse fraction	Fine fraction
		Coarse-to medium-grained	Medium-grained sandtsones to
		sandstones mainly composed of	siltstones mainly composed of
		angular to subangular detrital quartz	subangular detrital quartz and
		and carbonates (crystalline,	carbonates (crystalline and micritic
		bioclastic, and micritic limestones),	limestones), together with common
		together with common detrital	detrital oxides or hydroxides and a
		oxides or hydroxides and a few	few unweathered grains of detrital
	Alluvial	unweathered grains of detrital	feldspars, micas, pyroxene,
		feldspars, micas, pyroxene,	amphiboles, and siltstones,
		amphiboles, sandstones, and	surrounded by a clay matrix and/or a
		siltstones, surrounded by a calcite	calcite cement
		cement
		Example:	Example:
		sample SA 13 (Fig. S2A)	sample SA 15 (Fig. S2B)
		Coarse-to medium-grained	Medium-grained sandtsones to
		sandstones mainly composed of	siltstones mainly composed of
		angular to subangular detrital quartz	subangular detrital quartz and
		and carbonates (crystalline,	carbonates (crystalline and micritic
		bioclastic, and micritic limestones),	limestones), together with common
		together with common detrital	detrital oxides or hydroxides and a
		oxides or hydroxides and a few	few unweathered grains of detrital
	Coastal	unweathered grains of detrital	feldspars, micas, pyroxene,
		feldspars, micas, pyroxene,	amphiboles, and siltstones,
		amphiboles, sandstones, and	surrounded by a clay matrix and/or a
		siltstones, surrounded by a calcite	calcite cement
		cement
		Example:	Example:
		sample SC 6 (Fig. S2C)	sample SC 8 (Fig. S2D)
		Coarse-to medium-grained	Coarse-to medium-grained
		sandstones mainly composed of	sandstones mainly composed of
		angular to subangular detrital quartz	subangular detrital quartz and
		and carbonates (crystalline,	carbonates (crystalline, bioclastic,
		bioclastic, and micritic limestones),	and micritic limestones), together
		together with bioclasts (benthic	with bioclasts (benthic foraminiferas
		foraminiferas and echinoderm	and echinoderm remains) and a few
	Marine	remains) and a few unweathered	unweathered grains of detrital
		grains of detrital feldspars, micas,	feldspars, micas, pyroxene,
		pyroxene, amphiboles, sandstones,	amphiboles, siltstones, and oxides or
		siltstones, and oxides or hydroxides,	hydroxides, surrounded by a clay
		surrounded by a calcite cement	and/or micrite matrix
		Example:	Example:
		sample SD 23 (Fig. S2E)	sample ST 5 (Fig. S2F)

Table 2 .

 2 Amount of Li extracted by the water rinsing experiments (in % of bulk content).

	Sample name	1 st rinse 2 nd rinse 3 rd rinse	Total rinsed Li	δ 7 Li	2 s.d.
	Alluvial sediments
	SA3	0.10%	0.04%	0.03%	0.17%
	SA4	0.17%	0.06%	0.04%	0.28%
	SA6	0.23%	0.10%	0.10%	0.43%
	SA9	0.13%	0.05%	0.05%	0.23%
	SA11	0.16%	0.06%	0.05%	0.27%
	SA15	0.11%	0.05%	0.04%	0.20%
	SA22	0.12%	0.06%	0.04%	0.22%
	Seawater contacted sediments (coastal and marine)
	SC4	0.24%	0.07%	0.04%	0.35%	10.57	0.02
	SC11	0.09%	0.04%	0.02%	0.15%	12.34	0.10
	SC21	0.15%	0.04%	0.03%	0.22%	12.84	0.13
	SD3	0.68%	0.12%	0.07%	0.87%	2.15	0.07
	SD7	0.39%	0.08%	0.05%	0.52%	1.91	0.17
	SD21	0.23%	0.06%	0.03%	0.32%	2.71	0.11
	SD23	0.33%	0.10%	0.05%	0.48%	7.50	0.13
	ST5	0.29%	0.17%	0.11%	0.57%	12.14	0.21

Table 3 .

 3 Li isotope composition of the first and the last leach solution obtained from the HCl-leaching experiments.

			First leach			Third leach
	Sample	δ 7 Li	Al/Ca	Fe/Ca	δ 7 Li	Al/Ca	Fe/Ca
	Alluvial sediments
	SA 4	3.68±0.24	3.91×10 -6	6.97×10 -5 -1.73 ± 0.20	9.67×10 -2	5.08×10 -2
	SA 9	5.52±0.13	1.61×10 -5	8.43×10 -5 -2.28 ± 0.21	3.94×10 -2	4.30×10 -2
	SA 11	2.60±0.07	5.29×10 -6	6.67×10 -5 0.13 ± 0.24	2.85×10 -2	2.27×10 -2
	SA 15	4.61±0.10	7.30×10 -6	6.93×10 -5 -0.45 ± 0.26	3.45×10 -2	2.56×10 -2

Acknowledgements

We sincerely thank those helped in this project, particularly Amandine Laborde for sampling assistance in the field, Marc Quintin for thin section preparation, and Laëticia Faure for Nd isotope analysis. This research is fully funded by the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/ under REA grant agreement [608069, ISONOSE]. Part of this work was supported by the IPGP multidisciplinary program PARI and the Region île-de-France SESAME Grant no. 12015908.

Supplementary tables

Table S1. Locations, depositional environments, δ 7 Li and δ 30 Si values (in ‰), εNd, and Al/Si ratios of the samples collected in the Ainsa Basin.

Table S2. Major element concentrations (expressed in weight percent oxides) of the samples collected in the Ainsa Basin. Table S3. Trace elements concentrations (in ppm) of the samples collected in the Ainsa Basin. Table S4. HCl extracted cations (Ca, Li, Al, and Fe) in proportion to bulk sample (%). Table S5. Average alkali elements concentration (normalized to Al) in sediments deposited in different environments.

Supplementary figures