Evolution of the alpine Critical Zone since the Last Glacial Period using Li isotopes from lake sediments

Comprehending and predicting the way humans affect the Earth ’ s Critical Zone remains a challenge. An understanding of the past changes resulting from human and non-human influences in the dynamics of the Critical Zone is crucial. Here, we use a retrospective approach to address this question based on a new lithium (Li) isotope record from the Late Glacial Period to the present from a pre-Alpine lake sediment sequence (Lake La Thuile, France). Coupled with the lake sediment archive, the investigation of present-day soils in the lake catchment suggests that lake sediments are not necessarily recording the erosoin of topsoil in the catchment. Our findings indicate that soil particles can be sorted during transportation to the lake, with finer particles being preferentially mobilized, highlighting the influence of fine particle transport on the Li isotope signature of soils and lake sediments. Characterized by low Li isotope signatures, changes in weathering signatures in lake sediments can be amplified by the combined effect of soil development and selective transport. In the La Thuile catchment, soil development was limited during the Late Glacial Period, whereas it became a dominant process during the Holocene climatic optimum together with enhanced selective transport of fine particles. Human activities since 3,000 – 4,000 yr cal BP induced a strong perturbation hindering both soil formation and selective transport by reinforcing erosion rates. After a period of topsoil destruction caused by intense deforestation and agriculture, lake Li isotopes record the evolution of soil profiles associated with changes in agricultural practices.


Introduction
The spread of agriculture over the Holocene has resulted in deforestation, shifts in vegetation types, and enhanced physical erosion (Ruddiman, 2013;Stephens et al., 2019).These factors threaten the sustainability of Earth's Critical Zone (CZ, the thin layer of the planet spanning from bedrock to land surface encompassing the hydrosphere and the biosphere), and hence of civilizations (e.g., Gaillardet et al., 2018;Montgomery, 2007).The CZ evolution over time is influenced by various factors with human activities progressively becoming the most powerful one, which has led scientists to coin the term "Anthropocene" designating a new geological epoch characterized by the rise of human activities as a global factor of the Earth's surface evolution (Gibbard et al., 2022;Steffen et al., 2018).To comprehend and forecast the response of the CZ to the pressures of climate change and escalating human impacts, it is crucial to gain insight into these aspects and to understand how each of these influencing factors affects the CZ.Unravelling these aspects and their impacts on the CZ, and thus the sustainability of human societies in the recent past of the Earth, is a scientific challenge of paramount importance.
Lake sediments are watershed-integrated archives of environmental conditions and provide unique insight into past changes in soil dynamics (Bajard et al., 2017b;Jenny et al., 2019).Lake sediment geochemical records are valuable tools for reconstructing CZ evolution and for exploring the complex interplays between human activities, geodynamics, and climate over centennial to millennial timescales (Arnaud and Sabatier, 2022;Nantke et al., 2021;Rapuc et al., 2022).Lithium (Li) isotope composition is an innovative tracer for reconstructing past weathering processes.Li is a trace element primarily carried by silicate minerals (e.g., Pogge von Strandmann et al., 2020;Rudnick et al., 2004;Song et al., 2021).During silicate weathering, the lighter Li isotope ( 6 Li) is preferentially incorporated into soil-forming phases such as clays, leaving soil-and river-solutions enriched in the heavier Li isotope ( 7 Li) (Huh et al., 1998).The Li isotope composition (δ 7 Li) of terrigenous sediments therefore reflects the extent of secondary mineral formation and thus pedogenetic processes (Dellinger et al., 2017;Dosseto et al., 2015).Furthermore, past local variations in soil formation rates and weathering processes can be recorded in the δ 7 Li of the detrital material accumulated in lakes (Rothacker et al., 2018).
Here we study pre-Alpine soils and associated lake sediments that have been previously well characterized for several Holocene paleoenvironmental proxies including pollen records, mineral and organic geochemistry, and sedimentary DNA (Bajard et al., 2017a(Bajard et al., , 2016;;Giguet-Covex et al., 2019).Previous studies suggest that the rates of soil processes were disrupted by the settlement of humans in the catchment.
Here we attempt to extract new information from the soil and sediment Li isotope data, focusing on historical variations in soil development and soil erosion.
Properly interpreting detrital archives of soil processes relies on understanding the soil material contributing to the sediment and its transport mechanisms.Recent studies have highlighted variations in sediment geochemistry under different hydrological conditions, favouring the transportation of finer particles with geochemical characteristics similar to clay minerals (e.g., Kim et al., 2018), leading to their accumulation in sediment deposits (Ma et al., 2015).While past changes in soil formation can impact the sediment record (Rothacker et al., 2018), it remains uncertain whether variations in sediment transport and associated transformations would play a significant role in setting the composition of detrital sediments.
The preferential mobilization of fine particles during soil erosion is well-documented (Morgan, 2009 and references therein).Lessivage, the downward migration of particles in soil profiles involves the loss of fine particles (eluviation) from the upper soil horizons and their accumulation in deeper soil horizons (illuviation) (Quénard et al., 2011).Such translocation of fine particles is part of soil formation processes, particularly in humid climates (Buol et al., 2011).Nevertheless, the potential impact of this transport on the geochemical signatures in lake sediments, remains a knowledge gap.
Recent studies have extensively examined the selective transport of fine particles within and from soils (Bern et al., 2011;Jin et al., 2010;Noireaux et al., 2021;Steinhoefel et al., 2021), emphasising the importance of the preferential export of fine particles (<1 µm), which have distinct compositions compared to bulk soil.This selective export results in the loss of elements typically considered immobile during weathering (e.g., Al, Ti, Zr; Jin et al., 2010;Kim et al., 2018).Regarding Li isotopes, the fine particle fraction, characterized by low δ 7 Li values, is preferentially exported from soil profiles (Steinhoefel et al., 2021).Consequently, detrital sediments in lakes may reflect at least in part selective transport, with finer particles being preferentially mobilised and enriched in the lake deposit (Dietze et al., 2014;McLaren and Bowles, 1985).This suggests that variations in lake sediment geochemistry are not solely influenced by temporal changes in the composition of the bulk (or surface) soils, but also by how selective export and sedimentation of grain size fractions varies as a function of climate and human forcing.If selective transport were significant, any increase in its magnitude within a catchment could result in (1) an elevated δ 7 Li signature in the remaining soil material (Steinhoefel et al., 2021) and (2) a depleted δ 7 Li signature in the lake deposit, possibly biasing the interpretation of the sedimentary record.
This potential complexity calls for new studies aiming at understanding how the evolution of soil erosion and weathering processes are recorded in detrital sediments by integrating lake δ 7 Li records and examining soils within the corresponding drainage basin.This study presents δ 7 Li values and elemental geochemical data from pre-Alpine soils and associated lake archives.The δ 7 Li signature in detrital sediments is influenced by both grain size sorting and chemical weathering reactions (Dellinger et al., 2014;Zhang et al., 2022).Consequently, it can serve as a tool to examine the impact of either selective transport or changes in soil development, providing that the other effect is accounted for.We first validate the use of Li isotopes in sediments for tracking past changes and then examine the CZ evolution of the lake La Thuile watershed since the Last Glacial Period.To support our interpretations, we employ additional geochemical tools such as element ratios and microscopic imaging to disentangle the impacts of selective transport and weathering on the geochemical signatures in soils and sediments.
Ultimately, we establish that variation in δ 7 Li within the lake sediment record is a powerful proxy to investigate the evolution of the CZ at the catchment scale.

Study site and sample material
Lake La Thuile is a small (0.06 km 2 ) glacial lake in the Northern French Alps (Fig. 1A).The catchment is primarily underlain by calcareous formations associated with Cretaceous marls, shales, and limestones, with sparse Quaternary glacial deposits in the northwest (Fig. 1B).This lake has a maximum depth of 8 m with an intermittent narrow outflux in the north and is fed by two ephemeral streams during snowmelts and long rainfall episodes.The present catchment is dominated by forests, except for lakeside grasslands (Fig. 1C).In April 2010, an 18-metre-long sediment sequence was collected from the deepest part (8 m) of the lake (45 • 31′48.8″N, 6 • 03′23.6″E) using a Uwitec platform and coring devices.The details of sampling and the analyses for geochemistry, chronology, and palynology were previously reported for the upper 6 m corresponding to the Holocene by Bajard et al. (2016) and 12 m for the Late Glacial Period by Banjan et al. (2023).The sediment sequence (THU10, IGSN:IEFRA00BB, www.geosamples.org)consists of two overlapping cores (THU 10-1 and THU 10-2), with a chronology established through nineteen 14 C AMS measurements of terrestrial macroremains, paleomagnetic data, and short-lived radionuclides (Fig. S1).Notably, no plant macroremains were collected between 11.62 m and the bottom of the sequence ca.18 m.The deepest radiocarbon date ranges between 16,160 and 16,600 yr cal BP.The base of the sedimentary sequence, located 650 cm below this level, contains crystalline material of glacial origin.The earliest laminated lacustrine sediments were deposited immediately after glacial retreat, according to cosmogenic dating (Roattino et al., 2022;Wirsig et al., 2016).While the ages of the deeper sediment core section obtained from extrapolation may appear too old compared to the age derived from cosmogenic dating, these samples are not discussed in detail in this article.
Two soil profiles (THU01 and THU02) were sampled in the northern part of the catchment under a spruce forest.THU01 (5 horizons, 45 • 31′55.290″N,6 • 4′1.464″E) was sampled down to 70 cm, and THU02 (4 horizons, 45 • 31′53.124″N,6 • 4′1.608″E) was sampled down to 80 cm (Figs. 1 and 2).Both profiles are cambisols and have been previously described by Bajard et al. (2017a).THU01 is a clay-loam soil with a blocky structure having an acidic pH ranging from 4.5 to 5.5.Charcoals are abundant between depths of 20 to 70 cm, and rust patterns are visible in the deepest horizons, with bedrock not reached at 70 cm depth.THU02 is developed on carbonate rocks, although its upper horizons are carbonate-free.Its pH ranges from acidic to neutral, except in the C horizons where the matrix is carbonated.The structure is blocky, and the texture is silt loam.Both profiles are in close proximity and can be considered to have developed over the same parent material.A bedrock sample was collected from the outcrop underlying profile THU02, corresponding to a fine-grained carbonate formation associated with shale, assumed to be representative of the marine sedimentary rocks making up the catchment bedrock.

Analytical methods
The grain size of Lake La Thuile detrital material during the Holocene period was previously reported by Bajard et al. (2016).In this study, additional grain size analyses on soil samples have been performed to complement the current lacustrine dataset.Fifty-milligram aliquots of soil samples were dispersed following ultrasonication in a sodium hexametaphosphate solution.The grain size distribution of the resulting suspension was then measured using a laser granulometer LS Coulter 320 at École Normale Supérieure (Paris, France).The grain size distributions of the two soil profiles are illustrated in Fig. S2.
Imaging analyses were performed for selected sediment and soil samples at the Electron Microscopy Centre at Utrecht University (Utrecht, Netherlands).Samples were attached to carbon stickers and coated with 5 nm platinum-palladium prior to analyses using a Zeiss Gemini 450 scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDX) for elemental analysis.
Lithium isotopic analyses were carried out at the PARI platform of the Institut de Physique du Globe de Paris.Soil and lake sediment samples were gently ground in an agate mortar and air dried at ~50 • C before being digested using a mixture of HF-HNO 3 (Kuessner et al., 2020) in Teflon beakers.The resulting solution was then repeatedly evaporated and redissolved in concentrated HCl and HNO 3 solutions until it was clear.Silicon content was measured using alkali fusion with NaOH (Georg et al., 2006).Major and trace elements were analysed using ICP-Q-MS, and the analytical uncertainties were controlled to be below 5% for major elements and 10% for trace elements by analysing certified reference materials (BHVO-2, JB-2, and Till-1), which were treated following the same digestion and analysing protocols.Procedure blanks yielded signals below the instrumental detection limits of ICP-Q-MS.

Results
The geochemical results, lake chronology, and inferred lake

Table. 1
Major and trace element concentrations, δ 7 Li values, and Al/Ti ratios of soil, bedrock, and lake sediment samples of the La Thuile catchment as well as 14 C ages and inferred sedimentation rate for lake sediments.
Sample name Depth (m) sedimentation rates are summarized in Tables 1 and S1.The two soil profiles display noteworthy δ 7 Li variations ranging from − 3.97 to − 1.95 ‰ with similar trends of higher δ 7 Li values toward the soil surface (Fig. 2).Conversely, the Al/Ti ratio (g/g) exhibits an opposing pattern, increasing with depth in both soil profiles.Compared to deep soil material, the bedrock sample collected at THU02 has a lower Al/Ti (within the margin of error) and an elevated δ 7 Li (Fig. 2).Lake sedimentation rates varied from moderate (~1 mm/yr) during X.(Y.Zhang et al. the Late Glacial Period to low values (~0.1 mm/yr) in the Holocene (Fig. 3A).An abrupt increase in sedimentation rates (~11 mm/yr) occurred around the Middle Age (~1000 yr cal BP), followed by a period of reduced albeit still high rates (~3 mm/yr) in recent history.The corresponding δ 7 Li varies accordingly (Fig. 3C), i.e., remained stable from the Late Glacial Period to the Early Holocene at ~− 4 ‰ and gradually decreased to reach its minimum value over the record (δ 7 Li = − 7.1 ‰) at ~3200 yr cal BP.Afterward, a slow increase was observed followed by a more significant rise in δ 7 Li at approximately 1300 yr cal BP up to − 2.2 ‰, a value close to the modern surface soil horizon (Fig. 2).Later, the δ 7 Li in lake sediments gradually decreased until it reached the value of recently deposited sediments (− 4.0 ‰).The Al/Ti ratios in the lake sediments display a temporal variation opposite to that of δ 7 Li (Fig. 3D).The concentration ratios between individual REEs and Ti are similar between the soil profiles and lake sediments (Fig. S3).

Deep enrichment of secondary phases within the La Thuile soils
The Li isotope distribution in the soil profiles of the present-day La Thuile catchment highlights the influence of the formation of fine, clay- X.(Y.Zhang et al. rich particles during pedogenesis, implying their potential role in shaping the lake sediment composition over time.In these soil profiles, δ 7 Li decreases with increasing depth and reaches values well below that of rock at the base of the soil profile.During chemical weathering, secondary minerals such as clays preferentially incorporate 6 Li (Hindshaw et al., 2019;Vigier et al., 2008).Consequently, δ 7 Li in the soil solids or river sediments serves as a proxy for the extent of secondary mineral precipitation, with secondary minerals having lower δ 7 Li compared to their source bedrock (Dellinger et al., 2014(Dellinger et al., , 2017)).Therefore, the low δ 7 Li values at the base of the La Thuile soil profiles suggest that these profiles are enriched in secondary minerals at depth relative to topsoil horizons.
This enrichment of clay-rich particles in the present-day La Thuile deep soil horizons can be further investigated using the mass transfer coefficient value (τ), defined as: where i and j represent immobile and mobile elements, respectively, and w and p denote the element concentrations in the sample of interest and in the parent material (Brimhall and Dietrich, 1987;Nesbitt, 1979).
Negative τ values indicate a loss of the mobile element, and positive values suggest a gain.In this study, Ti was selected as the immobile reference element, as it is one of the most immobile elements during weathering processes (Nesbitt, 1979).The downward increase in τ Ti,Al values suggests a depletion of Al-rich phases at the soil surface and a likely enrichment of Al-rich phases in the deeper horizons of La Thuile soils (Fig. 2).In particularly, profile THU02 exhibits positive τ Ti,Al values in the deepest soil horizons, which could suggest a net gain of Al in depth.Considering both δ 7 Li and τ Ti,Al (Fig. 2), it can be inferred that clay minerals are transported from shallower horizons and accumulated in deeper layers of La Thuile soils.In addition to the geochemical proxies, the presence of the clusters of fine particles is visually confirmed through imaging analysis of THU02 soil profile (Fig. 4).Scrutiny under SEM revealed only few secondary particles observed in the shallow soils.Nevertheless, in deeper horizons, fine secondary phases were identified based on mineral morphology and EDX analysis (Fig. 4A-4D).These sub-micron particles (Fig. 4B and 4D) are enriched in Al and Fe (Fig. S4).
Notably, these secondary particles (<1 µm) appear to be coated/attached to larger-sized grains (>200 µm, Fig. 4C) rather than being present as individual grains.The fact that in the La Thuile soils these secondary phases are present as a "coating" on larger soil particles is further supported by leaching experiments (see leaching experiment section in supplementary material, Fig. S5).Such coatings were also observed in other studies (e.g.Kim et al., 2018).This aggregation is possibly caused by the fact that mineral surfaces control the growth/crystallization of new phases (King et al., 2010;Ruiz-Agudo et al., 2016).The downward movement of fine particles revealed by the geochemical and imaging results is not reflected in the grain size data (Fig. S2).This apparent discrepancy can first be explained by the challenges associated with the isolation of fine particles due to the coating effect.Second, while fine clay fractions can represent a significant fraction of a soil sample, direct quantification by laser diffraction method is difficult (Dur et al., 2004;Taubner et al., 2009).This observed discrepancy between grain-size results and geochemical data highlights the importance of differentiating between clay-sized particles and clay minerals.Clay-sized particles are often isolated from soil and sediment samples to study weathering through sieving, settling, and/or chemical treatment.However, our findings suggest that such a procedure could introduce artefacts if larger grains, which may carry a significant portion of weathering products, are discarded, or if chemical treatments, such as using HCl to remove carbonate minerals (e.g., Yang et al., 2021), are applied, which can also effectively dissolve a fraction of the poorly crystallized neoformed secondary phases (see leaching experiment section in supplementary material, Figure S5 and S7).Therefore, we recommend that researchers cautiously consider the interchangeable use of clay-sized particles and clay minerals and carefully test sieving/leaching methods to achieve a more comprehensive understanding of soil and sediment particles.
Variations in Li geochemistry in La Thuile soils could be attributed to the vertical transfer of clay minerals as a chemical (dissolution-reprecipitation of clay minerals) and/or physical process (downward mobilization of clay particles).The dissolution-reprecipitation of clay minerals as suggested in the soils from the Strengbach Critical Zone Observatory (France; Lemarchand et al., 2010) could be a valid contender to explain the Li isotope and clay mineral mobilization in La Thuile.This mechanism entails clay minerals dissolution in the surficial horizons and reprecipitation at depth following downward percolation of solute-rich soil solutions.Such reprecipitation could lead to an accumulation of fine particles in deeper horizons (Brantley and White, 2009).Clay dissolution at the surface would release low δ 7 Li signatures in the solution (Lemarchand et al., 2010;Zhang et al., 2021), resulting in the remaining solids having δ 7 Li values close to that of bedrock.Conversely, the reprecipitation of secondary minerals from Al-rich and 6 Li-rich soil solutions would produce a low δ 7 Li signal and high Al content at the bottom of the soil profile -both features being observed in the sample soils from La Thuile.
To examine further the roles of chemical and physical mobilization of Li within the La Thuile soils, we use a combination of different mass transfer coefficients.In CZ waters, nominally dissolved elements can be transported as ions ("truly dissolved") or carried by fine particles such as colloids or nanoparticles ("apparently dissolved").Previous studies (Hasenmueller et al., 2017;Steinhoefel et al., 2021;Sullivan et al., 2016) have suggested that τ Ti,Li could indicate the total Li mass transfer in both forms of Al-rich fine particles and solutes (both truly and apparently dissolved fractions), whereas τ Al,Li strictly traces the loss of Li through solute export (truly dissolved only).Consequently, the difference between these two coefficients (τ Ti,Li -τ Al,Li ) should reflect the Li loss by the export of particles (Steinhoefel et al., 2021).However, we note that a small fraction of Al may dissolve if clay dissolution is effective (Zhang et al., 2021), in which case the τ Al,Li value provides a minimum estimate for the fraction of Li lost through chemical transfer in the dissolved phase.In the La Thuile soil profiles, Li is depleted in the soils, as most samples have negative τ Ti,Li values (Fig. 2).Negative τ Ti,Li -τ Al,Li values in the soil profiles, particularly in the upper horizons, imply that this export is occurring in the form of fine particles.At depth, the τ Ti,Li -τ Al,Li values increase to values close to 0, indicating no loss and even an accumulation of fine particles there.Similar geochemical features are observed for other elements (Na, K, Mg, and Fe) in these two soil profiles (Fig. S8).Notably, the mass transfer coefficient proxy matches the microscope imaging results (Fig. 4).For samples with the calculated τ Ti,Liτ Al,Li significantly below 0, few secondary particles are observed, whereas when τ Ti,Li -τ Al,Li is close to or greater than 0, secondary fine particles are clearly visible at the microscopic level.
The imaging results obtained from profile THU02 reveal that the presence of fine particles is limited to deep soil layers, and they exhibit better crystallization in the deepest horizon (Fig. 4D and S5B) compared to those found at shallower depths (Fig. 4B).Fine particles observed in deeper horizons have better defined morphology and are typically present as longer rods than in upper horizons.Here, we propose a scenario coupling the two mechanisms discussed above (dissolution-reprecipitation of clays and lessivage) to explain the La Thuile system (Fig. S9).At the soil surface dilute meteoric water dissolves minerals from marine sedimentary rocks, leading to the enrichment of dissolved clay-making solute such as Al in the fluid.As this solution percolates downward, permeability decreases, pH increases (Bajard et al., 2017a), and the formation of secondary phases is favoured, most likely first as amorphous material.Further downward fluid percolation results in the transport of these poorly-crystallized secondary phases.With later growth of these particles along the flow path (e.g., Caraballo et al., 2019;Gebauer et al., 2018), the accumulation of this secondary material might occur in lower-porosity, deep horizons.In this scenario, both chemical transformation and physical transport of fine, secondary material contribute to the formation of a pool of fine particles that are readily mobilizable via water percolation, and which accumulate in deep soil horizon, leading to the observed δ 7 Li decreases with depth.
To summarize, the present-day soil data in La Thuile show that soil Li concentrations and isotope profiles are not exclusively controlled by the formation of secondary minerals from parent materials in the topsoil, as in "classical" soil profiles (Golla et al., 2021;Rothacker et al., 2018).Instead, the Li profiles are better explained by the translocation of clay-rich fine particles from topsoils to deeper horizons (Figs. 4 and S9).Therefore, the Li isotope signature of topsoils is influenced not only by the extent of modern clay formation but also by the particle movement within the soil.Although illuviation is a well-known process in soil science (e.g., Egli et al., 2006), the associated metal stable isotope effects are poorly understood.Addressing this knowledge gap can enhance our understanding of soil evolution in a wider variety of CZ environments.In the next section, we explore the potential impact of these soil processes on the interpretation of the geochemical signals from lake sediments.

The relative role of fine particle transport in determining the lake sediment composition
Variations in lake sediment δ 7 Li have been previously attributed to the change in catchment soil processes, with the newly formed clay minerals carrying low δ 7 Li signatures enriched within topsoils (Rothacker et al., 2018).However, it has also been recognized that the δ 7 Li signature in lake sediments can be influenced by grain size (Weynell et al., 2017).The novel soil δ 7 Li data presented in this study suggest that the formation of easily mobilizable, clay-rich, fine particles may impact the interpretation of the paleo-environmental signal preserved in the sediments.Consequently, a re-evaluation of the message conveyed by lake sediments becomes necessary.
When comparing La Thuile sediments and soils, it is noteworthy that the youngest sediment exhibits higher Al/Ti ratio and lower δ 7 Li value than present-day surface soils (Table 1).This geochemical offset between deposited lake sediments and modern catchment soils may reflect (1) poor representation of the lake source materials in the studied soil profiles; (2) influence of lacustrine diagenetic processes on the sediment after deposition; (3) hydrological sorting of particles within the lake i.e., preferential removal of small particles through the lake outflux; or (4) selective export of soil particles with distinct geochemical compositions (relative to bulk soil) to the lake.
A sediment source different from the studied soil profiles could result in the observed geochemical offset but is unlikely.First, the soil sample sites sit on steeper slopes and have greater erosion potentials than other parts of the catchment, making them likely major sources of sediment to the lake (Fig. 1).Second, dust inputs into Lake La Thuile are negligible (Giguet-Covex et al., 2019).Accordingly, similarities between lake sediments and soils in their ratios of immobile elements (REEs/Ti), which are not fractionated by weathering and sorting processes (Goldstein and Jacobsen, 1988;Nesbitt, 1979), suggest a common rock source (Fig. S3).
Diagenetic processes could modify the sedimentary geochemical signals including δ 7 Li deposited in deltaic environments (Zhang et al., 2022).However, these processes typically occur in marine environments with Li-(and other cations)-rich saline water.Lake La Thuile is a proglacial lake whose aqueous chemistry and short water residence time are unlikely to experience such processes.As shown by Bajard et al. (2016), La Thuile sediments contain some proportions of authigenic components and organic matter, essentially carbonates and biogenic silica (diatoms).As these phases likely carry very low Li concentrations (Chan et al., 2006;Dellinger et al., 2018), their influence on Li isotopes can be neglected.However, we note that diagenetic processes do affect lake sediment grain size at La Thuile.Lake sediments from the mid-Holocene with clay-like geochemical signals have relatively large grain sizes (Bajard et al., 2016), which is opposite to previous observations in river sediments (Dellinger et al., 2014).SEM examination of these lake sediments reveal that they are characterized by abundant biological materials, possibly cemented with authigenic carbonate (Fig. S6).This observation suggests in-lake interactions between biological and detrital materials decouples sediment geochemical signatures and grain size.Moreover, this observation implies that the "bulk" sedimentation rates from this period (Holocene climatic optimum) could lead to an overestimation of the erosion rates (Francke et al., 2022(Francke et al., , 2020)).
In-lake hydrodynamic sorting may affect lake sediment geochemistry as finer particles have greater potential to be transported by the lake outflux than coarser particles.At La Thuile, the ephemeral outflux may transport some sediments when the epilimnion flow is rapid/turbid, likely associated with episodes of high sedimentation rates during which hydrodynamic sorting is weakened.During low sedimentation periods, fine particles are likely to be retained by settlings and/or interaction with the lake biomass (Jones et al., 2014), which is abundant in La Thuile during such periods (Fig. 4E and 4F).Finally, the preferential removal of fine particles through the lake outflow would result in remaining sediments deposited at the lake bottom having elevated δ 7 Li and reduced Al/Ti values compared to soils, which contrasts with our observations.Therefore, we can eliminate causes (1), (2), and (3) cited above and infer that the differences between modern soils and recent lake sediments in the La Thuile catchment are likely due to the selective export of particles from soils, together with the possible selective transport thereof to the site of deposition.
Direct examination of selective sediment transport is difficult (Bern and Yesavage, 2018;Kim et al., 2018;Walling et al., 2002), and fluvial transport of fine particles is a complex process (Dietze et al., 2014;Juergen et al., 2007).Nonetheless, the export of clay-rich fine particles by subsurface flow has been demonstrated to be a significant process at the Shale Hills Critical Zone Observatory, a catchment underlain by shale rock (e.g., Jin et al., 2010).Sedimentary column taken from deposits along the stream within the catchment have revealed an accumulation of fine particles resulting from catchment erosion (Ma et al., 2015).The selective transport of fine particles has also been observed during surface erosion driven by rainwater (Shi et al., 2012).During soil erosion and sorting in streams, this preferential transport effect could lead to an export of sediment dominated by fine particles rich in secondary phases and thus enriched in 6 Li (Steinhoefel et al., 2021;Weynell et al., 2017).In the following section, we discuss the variations in the geochemical record from the La Thuile sediment sequence in light of the possibility for selective export of fine particles as a significant soil and catchment process.

Refining the interpretation of the Lake La Thuile paleo-record using lithium isotopes
The importance of soil formation and selective transport of fine particles in the La Thuile catchment highlights the need to consider these processes when interpreting the sediment sequence in terms of local soil evolution and the corresponding weathering history (Fig. 3).As discussed above, Li isotope geochemistry in the La Thuile catchment is most likely associated with both secondary mineral formation during pedogenesis and selective transport of fine particles from soils.It is worth noting that these two processes are likely closely coupled, as both are sensitive to local modification in CZ functioning.
Broadly speaking, the relationships between δ 7 Li, Al/Ti, and τ Ti,Liτ Al,Li (Fig. 5A and 5B) encompassing modern soils and lake sediments from different epochs (Late Glacial Period, Holocene-Middle Age, post-Middle Age) allow us to draw the main features of the past CZ evolution at La Thuile.The most notable observation is that the sediments with the lowest δ 7 Li values (<− 5‰), highest Al/Ti ratio, and highest τ Ti, Li -τ Al,Li values are exclusively associated with low sedimentation rates (~0.1 mm/yr; Fig. S10).Such low sedimentation rates at La Thuile have been interpreted by previous studies as reflecting low catchment erosion X.(Y.Zhang et al. rates concomitant with dense forest cover (Bajard et al., 2017a(Bajard et al., , 2016)).
We can construct a historical scenario for Lake La Thuile using the δ 7 Li record as a proxy of secondary minerals, and by examining the possible effects of selective transport with the τ Ti,Li -τ Al,Li record.From the late Last Glacial Period to the onset of the Holocene (phase 1 in Fig. 3), the δ 7 Li record is relatively constant and slightly enriched in 6 Li compared to the bedrock.This suggests the presence of pre-Holocene soils in the catchment, associated with moderate sedimentation rates and bedrock-like Al/Ti signatures.The τ Ti,Li -τ Al,Li values of these sediments are close to 0, indicating no enrichment of fine particles.In essence, during this period unsorted soil surface material was transported to the lake.These data suggest a scenario of limited soil development and sustained erosion, resulting in lake sediments having geochemical characteristics close to that of the bedrock.
Lake La Thuile sediments deposited between approximately 8000 and 3000 yr cal BP have lower δ 7 Li values, down to ~ − 7.1 ‰, Al/Ti ratios higher than that of bedrock, and positive τ Ti,Li -τ Al,Li values, up to 0.75.SEM analysis revealed the influence of lake aquatic biological activity during this period, as indicated by the presence of abundant bioskeleton such as diatoms (Fig. 4E, 4F, and S6).However, there is no evidence suggesting that biological activity affected the Li-Al-Ti geochemistry as these elements are not nutrients.Therefore, we X.(Y.Zhang et al. propose that abiotic processes, such as clay formation and transport, were the primary drivers of the lake sediment signatures.The geochemical data indicate secondary formation (low δ 7 Li values) and suggest the role of selective transport of fine particles (positive τ Ti,Li -τ Al, Li values).The decrease in δ 7 Li values (phase 2 in Fig. 3) is consistent with the stabilization of slopes and soil development across the catchment.This was likely due to a climate transition from the Late Glacial to the Holocene climatic optimum and drastic vegetation changes (Bajard et al., 2017a(Bajard et al., , 2016;;Giguet-Covex et al., 2019).Pollen records (Bajard et al., 2016) illustrate denser vegetation in the catchment (Fig. 3E).Both warm climate and forest densification may have driven the observed reduced sedimentation rates at La Thuile, a feature also observed regionally in the Alps (Arnaud et al., 2012).This epoch marked a period of active soil horizon formation with diminished erosion rates (Fig. 3E).We note that these already-low erosion rates represent an upper-bound estimate due to the presence of authigenic (biological and carbonate) materials (Fig. S7), implying little detrital material was actually exported from the catchment.These conditions favoured both soil development -hence secondary mineral formation -and selective export of fine particles from soils, as witnessed by the sediment geochemical signatures.Essentially, the erosion flux associated with subsurface hydrological paths (spring, seeps, return flow, groundwater, etc.), where fine particles are preferentially transported (Morgan, 2009), played a more important role in the total erosion flux to the lake when surface soil erosion was restrained.
From 3000 to ~1600 yr cal BP (phase 3 in Fig. 3), we observe rising δ 7 Li values and sedimentation rates and concomitant decreases in Al/Ti and τ Ti,Li -τ Al,Li values.A previous investigation (Bajard et al., 2016) revealed the early human occupation of the watershed during this period, as shown by the increase in herbaceous pollen and the first appearance of Cerealia pollens.Our new dataset is consistent with the interpretation of increased erosion rates induced by human activities such as deforestation and agricultural practices at this time (Bajard et al., 2017a(Bajard et al., , 2016)), suggesting early human activities modulated soil erosion mechanisms, including the role of selective export from soils and the transport of fine particles.
From 1600 yr cal BP to the beginning of the Medieval Warm Period (phase 4 in Fig. 3), a notable rise in sedimentation rate coincides with a sharp increase in δ 7 Li values and reduced Al/Ti ratios.Negative τ Ti,Li -τ Al, Li values suggest that selective transport of fine particles was not in operation at this time.Lake sediment geochemical signatures imply that material transported to the lake during this period closely resembled present-day topsoils.This observation can be explained by an increase in erosion rates with more pronounced human-induced agricultural activities.Therefore, the Li data from phase 4 imply that physical erosion was the major process shaping soil evolution in the catchment, supporting previous inferences (Bajard et al., 2017b).Little weathering signal can be observed from these sediments (i.e., δ 7 Li values from this period are close to bedrock), offering a milestone marking that humans have become the dominant control on CZ trajectories in the La Thuile catchment.During this period, lake sediments deposited can be regarded as a direct record for surface materials in the catchment area, encompassing both soils and exposed bedrock.
In the last millennium (phase 5), physical erosion rates first reached unprecedented values, signifying a paroxysmal period of soil destruction by human activities (Bajard et al., 2017a).Lake sediments from this period essentially represent unsorted eroded soils and rocks.The slight increase in τ Ti,Li -τ Al,Li values (although still negative) and decrease in δ 7 Li values observed in the lake sediments suggest an increasing contribution from relatively deep soil layers, likely associated with soil thinning due to enhanced agricultural activities.In other words, lower soil horizons formed during the Holocene pedologic optimum became exposed and were subject to erosion.Finally, after the erosion maximum of the Middle Ages, erosion rates decreased and agricultural activities reduced, allowing for soil redevelopment in the catchment (Bajard et al., 2017a(Bajard et al., , 2016)).We can neglect the potential impact of industrial fertilizers on the recent δ 7 Li record, as are typically highly soluble and exhibit heavy δ 7 Li values (Négrel et al., 2010).As discussed earlier, the most recent lake sediments have a positive τ Ti,Li -τ Al,Li value and a δ 7 Li signature lower than that of present-day soils, suggesting that weathering and selective transport become active again in the catchment.

Conclusion and perspectives
In this article, we present soil and lake sediment Li isotopes from a pre-Alpine catchment underlain by calcareous rocks associated with shales.Our primary findings are that lake detrital sediment Li isotopes record distinct variations throughout the Late Glacial/Holocene periods and that humans have significantly altered the function of La Thuile CZ over the Holocene especially during the Middle Ages.We attributed these variations to changes in soil dynamics, erosion, and selective transport of fine particles across the catchment.Lake sediments have the potential to record the history of soil formation, but we show that these records are also dependent on several soil and catchment processes that might complicate their interpretation.In particular, the geochemical signature of weathering (characterized by low δ 7 Li and high Al/Ti in sediments) is modulated in lake sediments as a result of both soil development and selective transport of fine particles.
This study illustrates the complex interplay of lithology, ecosystem development, climate, and human activities in shaping the CZ trajectory over time.In the La Thuile catchment underlain by sedimentary rocks, Li isotopes capture the multi-millennial formation of soils by chemical weathering and the importance of two contrasting physical processes: selective particle transport under low erosion rates and unsorted soil erosion under enhanced erosion periods, both intimately linked to landscape modifications driven by climate and human forcings.Together with previous studies on the fine particle flux transported by waters (Jin et al., 2010;Kim et al., 2018), our results of soil and sediment samples highlight the importance of differentiating between conflated terms such as "mobility" vs. "solubility" and "clay-sized particles" vs. "clay minerals (or more precisely, secondary phases, as amorphous materials do not qualify as clays)".These distinctions are critical for accurately recording and interpreting geochemical signatures.
To go beyond the horizon of our current understanding of CZ systems, particularly regarding the selective transport of fine particles, future studies are needed to focus on (1) better constraining sediment budgets; (2) gauging sediment fluxes associated with subsurface waters and outflows; (3) characterizing the geochemistry of detrital sediments as a function of grain size; (4) investigating the circumstances permitting for selective sediment transport in different geomorphic environments; and (5) characterizing the contribution of fine particles to the total detrital flux.It is undoubtedly challenging to separate fine particles from parent materials and to collect enough material for analysis.However, these challenges also present a valuable opportunity to investigate the properties and behaviours of these particles in nature, ultimately advancing our understanding of CZ processes.

Fig 1 .
Fig 1.The La Thuile catchment: (A) map of the location of Lake La Thuile; (B) simplified geological map of the La Thuile catchment and the sample locations; and (C) a view of the lake from the south shore.Figure modified from Bajard et al. (2016, 2017b).

Fig 2 .
Fig 2. The two La Thuile soil profiles investigated in this study: pedogenic information (photo, colour, texture) and geochemical data, including δ 7 Li, Al/Ti ratio, and mass transfer coefficients.Soil profiles are from Bajard et al. (2017b).

Fig 3 .
Fig 3. Temporal variations in sedimentation rates (A), selective transport of fine particles indexed by τ Ti,Li -τ Al,Li (B), sediment δ 7 Li (C), and Al/Ti (D) in Lake La Thuile since the Late Glacial Period; (E) Sketch representing the history of soil development in the La Thuile catchment over the Late Glacial and Holocene periods.Fig. 3E modified from Bajard et al. (2017b) and Arnaud and Sabatier (2022).

Fig 4 .
Fig 4. SEM images of (A) Soil sample THU02-3, 15-30 cm depth; (B) Magnified secondary mineral phases observed in (A); (C) Soil sample THU02-4, 30-80 cm depth; (D) Magnified secondary minerals attached to surface of large grain from (C); (E) Holocene lake sediment (THU10-02C1a-79-80) with grain surfaces dominated by bio-skeletons; and (F) Example of bio-skeleton.The white boxes denote the regions magnified.The black dashed boxes highlight the regions with the presence of fine-sized secondary phases.

Fig 5 .
Fig 5. La Thuile soil and sediment Li isotope composition as a function of (A) Al/Ti and (B) τ Ti,Li -τ Al,Li .