# Glacially Enhanced Silicate Weathering Revealed by Holocene Lake Records

Yi Hou, J. Jotautas Baronas, Preston Cosslett Kemeny, Julien Bouchez, Áslaug Geirsdóttir, Gifford Miller, Mark Torres

► To cite this version:

Yi Hou, J. Jotautas Baronas, Preston Cosslett Kemeny, Julien Bouchez, Áslaug Geirsdóttir, et al. Glacially Enhanced Silicate Weathering Revealed by Holocene Lake Records. Geophysical Research Letters, 2025, 52 (18), pp.119649. 10.1029/2025GL115324. http://dx.doi.org/10.1029/2025GL115324

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# Geophysical Research Letters®

## RESEARCH LETTER

10.1029/2025GL115324

**Key Points:**

* A novel multi-proxy model to constrain absolute catchment-scale paleo chemical weathering flux with Ge/Si and Si isotopes was developed

* Chemical weathering fluxes of two Icelandic catchments with distinct and dynamic glacial histories in the Holocene were reconstructed

* Synchronous increase in weathering flux with glacier expansion indicates a partially positive glaciation-weathering-climate feedback

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

Y. Hou,
yihou.012559@gmail.com

**Citation:**

Hou, Y., Baronas, J. J., Kemeny, P. C., Bouchez, J., Geirsdóttir, Á., Miller, G. H., & Torres, M. A. (2025). Glacially enhanced silicate weathering revealed by Holocene lake records. *Geophysical Research Letters*, *52*, e2025GL115324.
https://doi.org/10.1029/2025GL115324

Received 11 FEB 2025

Accepted 8 AUG 2025

## Glacially Enhanced Silicate Weathering Revealed by Holocene Lake Records

<figure><img src="image_2.png" /></figure>

Yi Hou¹,² ID, J. Jotautas Baronas³,⁴ ID, Preston Cosslett Kemeny⁵ ID, Julien Bouchez³ ID, Áslaug Geirsdóttir⁶ ID, Gifford H. Miller⁷ ID, and Mark A. Torres¹ ID

¹Department of Earth, Environmental, and Planetary Sciences, Rice University, Houston, TX, USA, ²Now at Department of Earth and Planetary Sciences, ETH Zürich, Zürich, Switzerland, ³Institut de Physique du Globe de Paris (IPGP) - CNRS - Université Paris Cité, Paris, France, ⁴Department of Earth Sciences, Durham University, Durham, UK, ⁵Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, USA, ⁶Faculty of Earth Sciences, University of Iceland, Reykjavík, Iceland, ⁷Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO, USA

**Abstract** How glaciation affects CO₂ drawdown by chemical weathering influences the weathering-climate feedback strength, which controls the exogenic carbon cycle and planetary habitability. However, the role of glaciers remains elusive as glaciation alters multiple factors controlling weathering, the net effect of which is ambiguous even in directionality. To isolate and quantify the effect of glaciers, we developed a novel multi-proxy system for constraining catchment-scale weathering fluxes in the past. This approach utilizes the correlation between Ge/Si and Si isotope ratios in modern rivers and the preservation of these signals in lacustrine sediments. Reconstructed weathering fluxes in two Icelandic catchments with different glacial histories during the past 10,000 years show that chemical weathering fluxes are roughly 10 times higher when a catchment is glaciated versus ice-free. The synchronous variations in weathering fluxes with the expansion and contraction of glaciers indicate that glaciation may rapidly amplify climatic variations via a positive feedback.

**Plain Language Summary** Exactly how chemical breakdown, or weathering, of silicate rocks responds to glaciation remains extensively debated. Past work has either compared modern weathering fluxes between currently glaciated and ice-free catchments or relied on marine sedimentary records. To provide new insight, we developed an approach using lake sediment that estimates paleo-weathering fluxes at the same spatial scale as modern rivers. We find that fluxes are higher when a catchment is glaciated, which supports the hypothesis that enhanced weathering is in phase with glacial activity over millennia. This also suggests that some land areas do not follow the global pattern of weathering slowing down as the climate cools, which may be important in triggering or sustaining Ice Ages over Earth's history.

## 1. Introduction

The chemical weathering of silicate minerals is a major geologic source of alkalinity to the ocean-atmosphere system. The climatic dependence of silicate weathering reactions allows for a negative feedback thought to help regulate Earth's climate and habitability over geologic timescales (Eiriksdóttir et al., 2013; Kump et al., 2000). However, additional controls on silicate weathering exist (Bluth & Kump, 1994; Gudbrandsson et al., 2008; Maher & Chamberlain, 2014; West, 2012), and understanding these factors is required to accurately assess the weathering-climate feedback. In particular, large uncertainties exist regarding the effects of glaciation on chemical weathering. At temperatures sufficiently low to initiate and sustain glaciation, chemical weathering is expected to be attenuated due to the sluggish kinetics. Conversely, glaciers enhance supply of fresh mineral surfaces available for reaction (Anderson et al., 1997) by efficiently eroding bedrock (Herman et al., 2013; Louvat et al., 2008).

Today, catchment-scale weathering fluxes are often inferred from river solute chemistry (Cole et al., 2022; Dessert et al., 2003). However, observations from modern glacial catchments capture only snapshots in time and, essentially, reflect the behavior of glacial systems during the current interglacial period. Moreover, river chemistry in currently non-glaciated catchments can be heavily influenced by landscapes that are still recovering from past glaciation (Kemeny et al., 2021), therefore biasing the estimates of non-glacial weathering fluxes. By altering the topography and the supply of weatherable substrates for prolonged periods, the impact of glaciers on chemical weathering may last well beyond deglaciation (Anderson et al., 1997; Vance et al., 2009). As such,

<footnote>© 2025. The Author(s).
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
isolating the effect of glaciation on chemical weathering requires accounting for the effects of landscape evolution through time.

Marine sedimentary archives have been used to infer secular changes in continental weathering including during glacial/interglacial periods (Foster & Vance, 2006; Froelich et al., 1992; Hammond et al., 2004; Henderson et al., 1994). These inferences, however, cannot be directly compared to modern catchment-scale estimates of weathering fluxes from river chemistry as marine records derived from biogenic/inorganic precipitates integrate chemical weathering processes over sub-continental to global scales. Further, because the land surface is heterogeneous, global integration can blur the signal of weathering response to glaciation alone. Besides issues with different scales of spatial and temporal integration, available weathering proxies largely record flux ratios (e.g., weathering intensity) and/or utilize trace elements that can be decoupled from the release of alkalinity during silicate weathering (Dellinger et al., 2015; Li et al., 2021; Suhrhoff et al., 2019). To address these issues, we turn to lake sedimentary records instead, to infer paleo-weathering fluxes at the same spatial scale as modern riverine observations. We present the first application of a novel multi-proxy model that constrains absolute changes in chemical weathering fluxes (in mol Si/km² yr) back in time in response to (de)glaciation. Leveraging the sensitivity of river dissolved chemistry, that is germanium to silicon ratios (Ge/Si) and Si isotope ratios (δ³⁰Si), to silicate weathering processes, our approach utilizes these chemical signatures recorded in biogenic silica (bSi) preserved in lake sediments to estimate chemical weathering fluxes at the catchment scale.

## 2. Inferring Weathering Flux From bSi

River and lake water samples and sediment cores were collected from two lakes and the surrounding regions in western and central Iceland (Figure 1a, Text S1 in Supporting Information S1). The catchment of Haukadalsvatn (HAK) deglaciated ~12 ka and has been ice free throughout the Holocene (Geirsdóttir et al., 2013; Geirsdóttir, Miller, Thordarson, & Ólafsdóttir, 2009) (Figure 1b, Text S1 in Supporting Information S1). In contrast, Lake Hvítárvatn (HVT) deglaciated 11–10 ka but now drains the Langjökull glacier (Figure 1c, Text S1 in Supporting Information S1), which only started to grow in the recent past (i.e., between 5.5–5 ka). The region was ice-free during the Holocene Thermal Maximum between 8 and 6 ka in the mid-Holocene (Larsen et al., 2012). The distinct (de)glaciation histories of the two catchments spanning the past 10 ka, the same time period over which sedimentary archives are available (core dating and age models taken from Geirsdóttir, Miller, Axford, & Ólafsdóttir, 2009; Larsen et al., 2012), provide a unique opportunity to evaluate the effect of glaciation on chemical weathering in an internally consistent manner.

A significant negative correlation (Pearson correlation coefficient $ρ = -0.84$, $p = 2.1 \times 10^{-8}$) was observed between Ge/Si and δ³⁰Si of the dissolved loads of multiple rivers in the two lake catchments (Figure 2a). High δ³⁰Si (0.67–1.47‰) and low Ge/Si (0.4–1.5 pM/µM) values are observed at the non-glacial catchment of HAK, whereas the glacial rivers at HVT are characterized by lower δ³⁰Si (–0.09–0.27‰) and higher Ge/Si (2.1–2.8 pM/µM). The covariation between Ge/Si and δ³⁰Si primarily tracks the proportion of dissolved Si partitioned into secondary phases (Baronas et al., 2018; Frings et al., 2021) as empirical evidence suggests preferential uptake of Ge and the lighter Si isotopes associated with the formation of secondary phases (Anders et al., 2003). Other factors that may alter the riverine Ge/Si and δ³⁰Si compositions (Cornelis et al., 2011; Evans & Derry, 2002) can be ruled out–Iceland is virtually monolithologic (volcanic bedrock), sparsely vegetated, and δ¹⁸O and δD measurements of river water from our study sites indicate negligible hydrothermal inputs (Text S1, Figure S1 in Supporting Information S1). Therefore, it appears that the degree of Si uptake into secondary phases, manifested in the co-evolution of solute Ge/Si and δ³⁰Si signatures, is strongly modulated by glaciation (Text S5 in Supporting Information S1). This is also consistent with previous observations that δ³⁰Si of dissolved loads tends to be lower in catchments with high degrees of glacial cover (Georg et al., 2007; Opfergelt et al., 2013) though there can be some overlap in the ranges of δ³⁰Si between glacial and non-glacial rivers (Figure S8 in Supporting Information S1) due to, for example, complexity in the types of secondary phases formed in glacial rivers (Pryer et al., 2020). Utilizing this empirical correlation between riverine Ge/Si and δ³⁰Si and the expected decoupling between these two tracers during incorporation in biogenic silica, we can infer chemical weathering fluxes in the past from the compositions of diatoms preserved in lake sediments (Figure 2b).

Lake Ge/Si and δ³⁰Si compositions should represent a spatially and seasonally integrated signal of chemical weathering upstream and fall on the regression line defined by the river data within uncertainties (Figure 2b),
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Sampling sites. (a) Iceland with indicated locations of the lakes in this study: Haukadalsvatn (HAK) and Hvítárvatn (HVT). Black lines delineate modern ice cover. Data were sourced from Arctic Spatial Data Infrastructure. (b) HAK area with indicated sampling locations of river water (white diamonds), lake water (white square), and sediment core (red circle). (c) HVT area with indicated sampling locations. Black dashed lines indicate the extent of the Langjökull and the Eiríksjökull glacier. Elevation data were sourced from Polar Geospatial Center (ArcticDEM) in EPSG 3413 projection.</figcaption></figure>

which we observe to be true (Figure 2a). The lake chemistry is in turn preserved in the opaline frustules of lacustrine diatoms—these siliceous algae are considered to faithfully record the Ge/Si ratios of the water they grow in (Bareille et al., 1998; Filippelli et al., 2000; Sutton et al., 2010) and express a Si isotope fractionation according to the degree of nutrient (dissolved Si) utilization (Hendry & Brzezinski, 2014; Opfergelt et al., 2011; Zahajská et al., 2021). As the initial lake δ³⁰Si at the time of diatom growth can be determined from the Ge/Si ratio recorded in bSi and the empirical correlation between Ge/Si and δ³⁰Si in rivers, the degree of nutrient utilization can be calculated using empirically constrained fractionation factors (Alleman et al., 2005; de La Rocha et al., 1997) and the offset between δ³⁰Si of the lake and of preserved diatoms (Equation 1). The total supply of dissolved Si to the lake, which is effectively the chemical weathering flux in terms of Si, can therefore be computed from the bSi burial flux and the degree of nutrient utilization (Equation 2). Details on the implementation of this proxy model, assumptions and caveats, and the propagation of uncertainties are described in Text S1 in Supporting Information S1.

$$ \underbrace{\delta^{30}\text{Si}_{\text{bSi}}}_{\text{measured diatom}} = \underbrace{\delta^{30}\text{Si}_{\text{initial}}}_{\text{inferred from Ge/Si - \delta^{30}Si correlation}} + \underbrace{\varepsilon}_{\text{fractionation factor}} \cdot \underbrace{f}_{\text{fraction of Si remaining}} \quad (1) $$

$$ f = \frac{\text{weathering flux} - \text{bSi burial flux}}{\text{weathering flux}} \quad (2) $$
<figure><img src="image_4.png" /><figcaption>**Figure 2.** (a) Ge/Si and Si isotope ratios of river water (diamonds), lake water (squares), and biogenic silica (bSi, circles) samples at Haukadalsvatn (HAK) (nonglacial, green) and Hvítárvatn (HVT) (glacial, purple), numbers indicate the relative ages of bSi samples with 1 corresponding to the youngest, which is 41 and 51 years before present (2000 AD) for HAK and HVT, respectively. $\delta^{30}\text{Si}$ decreases with increasing Ge/Si in river and lake dissolved load. Offset between $\delta^{30}\text{Si}$ of bSi and lake water indicate the biological fractionation corresponding to the degree of nutrient utilization by diatoms. (b) Schematic of the proxy model.</figcaption></figure>

Central to our approach is the assumption that the correlation between riverine Ge/Si and $\delta^{30}\text{Si}$ compositions remained relatively the same in the past 10 ka, i.e. the apparent sensitivity of riverine Ge/Si and $\delta^{30}\text{Si}$ to the balance between primary mineral dissolution and secondary mineral formation did not vary. While it is currently not possible to validate this assumption with direct observation, we argue that it is a realistic approximation as the same dominant controls on the different styles of chemical weathering operative in Iceland (i.e., climatic and tectonic conditions) remained roughly constant throughout this time period (Geirsdóttir et al., 2020). Another premise of our proxy model regards the absence of post-depositional alteration of Ge/Si and $\delta^{30}\text{Si}$ of bSi and minimal bSi dissolution. This is corroborated by calculations of diffusive Si fluxes with reactive transport modeling based on the porewater profiles of surface cores (Text S3, Figure S4 in Supporting Information S1). Further, previous work supports the lack of meaningful Si isotope fractionation during diatom dissolution (Panizzo et al., 2015).

At HAK, little variation was observed in Ge/Si (0.5–1.0 pM/µM) and $\delta^{30}\text{Si}$ (–0.86––0.39‰) of bSi over time, contrary to the wider range of values (Ge/Si: 0.2–1.6 pM/µM; $\delta^{30}\text{Si}$: –1.28––0.40‰) measured in bSi at HVT (Figures 2a, 3a, and 3b). An unexpected offset between the Ge/Si of core-top bSi and the lake water at HVT is noted (Figures 2a and 3b). We attributed this unexpected biological fractionation of Ge/Si observed at HVT but not at HAK to the difference in lake diatom assemblages (i.e., dominated by planktic vs. benthic species due to different lake turbidity (Black, 2008)). This fractionation was corrected for in the chemical weathering flux calculations according to the diatom assemblages previously characterized through the core (Black, 2008) (Text S1, Figures S5 and S10 in Supporting Information S1). We performed additional calculations with different assumptions on Ge fractionation during diatom growth as sensitivity tests (Text S7 in Supporting Information S1). Under these alternative scenarios, which include assuming constant Ge/Si fractionation through time and assuming constant lake $\delta^{30}\text{Si}$, the results remain the same, corroborating the robustness of our conclusions. However, it is also worth noting that the authors are not aware of any existing work that has systematically examined the Ge uptake behavior of freshwater diatoms.

### 3. Silicate Weathering Promoted by Glaciers

The inferred chemical weathering fluxes (i.e., chemical denudation rates of Si) are presented in Figure 3d and compared against bSi burial fluxes (Figure 3c), which are calculated from previously published sedimentary bSi
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Temporal changes in (a) δ³⁰Si of bSi samples; (b) Ge/Si of bSi samples; Note the offset between coretop biogenic Si and modern lake water at HVT; (c) bSi burial flux, calculated with previously published sedimentation rate and bSi content with a bulk sediment density of 2 g/cm³ (Geirsdóttir et al., 2013); (d) Reconstructed chemical weathering fluxes; (e) Reconstructed glacial discharge and ice area of Langjökull (Flowers et al., 2008). Age models of the sediment cores have been previously constrained (Geirsdóttir, Miller, Thordarson, & Ólafsdóttir, 2009; Larsen et al., 2012) and the completeness of the sedimentary records is discussed in Text S2 in Supporting Information S1.</figcaption></figure>

content and sedimentation rates (Geirsdóttir et al., 2013) (Text S6 in Supporting Information S1). Our lake-derived estimates agree within uncertainties with the weathering fluxes estimated for other watersheds (n = 85) in Iceland from compiled river chemistry (Cole et al., 2022) (Figure 4a). Our estimates do appear to be on the higher end of the weathering flux distribution derived from river chemistry, which may reflect substantial groundwater contribution in addition to surface runoff (Schopka & Derry, 2012). Groundwater discharge can be important in the Si budget/water mass balance of lakes (Zahajská et al., 2021) and is implicitly accounted for in our approach.

At core top, the inferred chemical weathering flux, representative of chemical weathering flux at present, is almost an order of magnitude higher in the glacial catchment than the non-glacial catchment (Figures 3d and 4a). In addition, from the weathering fluxes estimated from compiled Icelandic river chemistry, we found the average of the non-glacial rivers (3.5 × 10⁵ mol Si/km²/yr, n = 38) to be lower than, or at least comparable to, given the potential uncertainties, the average of glacial rivers (7.3 × 10⁵ mol Si/km²/yr, n = 9) excluding the multi-source rivers (Figure 4a). These results are contrary to the previous global observation that “silica denudation rates are distinctly lower in glacier-covered catchments than in their nonglacial counterparts” (Anderson et al., 1997). One possible explanation for this discrepancy is that existing global compilations primarily include rivers that drain metamorphic bedrocks whereas volcanic/basaltic lithologies are underrepresented (Anderson et al., 1997). If true, this points to the potentially unique role of glaciation in basalt weathering. Despite its rare occurrence at Earth's surface today (Hartmann & Moosdorf, 2012), basalt plays a disproportional role in atmospheric CO₂ sequestration due to fast dissolution kinetics (Dessert et al., 2003) and its response to glaciation may therefore bear important implications for the carbon cycle.

The first-order changes in chemical weathering fluxes over time share similar patterns with the bSi burial fluxes (Figures 3c and 3d) due to the lack of drastic variation in the degree of dissolved Si utilization through time (Figure S2 in Supporting Information S1). At HAK (non-glacial), chemical weathering fluxes remained relatively constant over the past 10 ka (Figure 3d), and a Mann–Kendall statistical test (non-parametric trend detection) cannot reject the null hypothesis that no trend in weathering fluxes exists (Text S1.6 in Supporting Information S1). At HVT (glacial), greater variation in chemical weathering fluxes is observed and changes appear non-monotonic in time (Figure 3d). Specifically, the extent of glaciers exerts a strong control on the chemical
<figure><img src="image_6.png" /><figcaption>**Figure 4.** (a) Reconstructed chemical weathering fluxes from bSi are comparable with fluxes estimated from compiled Icelandic river data (Cole et al., 2022). (b) Positive correlation between chemical weathering flux at Hvítárvatn (HVT) and reconstructed ice extent of Langjökull (Flowers et al., 2008). (c) Positive correlation between chemical weathering flux at HVT and reconstructed glacial discharge into the lake (Flowers et al., 2008). The single data point of weathering flux at Haukadalsvatn plotted on Panels (b), (c) is the median of the average fluxes over time calculated from resampling of individual inferences (N = 10⁵). The error bar corresponds to the 10th–90th percentile. Dashed lines and shadings represent linear regression best fits of glacial extent and the logarithm of weathering flux and uncertainties as visual guides. However, it is worth noting that the exact functional forms of the correlation between chemical weathering flux and glacial extent/discharge is unknown and not necessarily exponential.</figcaption></figure>

weathering flux at HVT (Flowers et al., 2008) (Figures 3d and 3e), where higher flux corresponds to larger extents of glaciation (Figure 4b) and more glacial discharge into the lake (Figure 4c). In addition, we evaluated the completeness of the lake sedimentary records with the previously published age models and concluded that the observed (lack of) trends were robust and did not arise due to sediment preservation bias (i.e., the Sadler Effect (Sadler, 1981), Text S2, Figure S3 in Supporting Information S1).

Among currently glaciated watersheds, a slight decrease in the total chemical denudation rates with the increase in the percentage of glacial cover (up to 35%) was previously observed in southwest Iceland (Gislason et al., 1996). However, this negative correlation is inconclusive as superimposed effects from the difference in rock ages may be present (Gislason et al., 1996; Louvat et al., 2008). Alternatively, our approach affords an interpretation of the variations in chemical weathering fluxes over time primarily in terms of glacial influence and under invariance of other factors, as the evolution of a single catchment is monitored through time. Our results are therefore more representative of the true sensitivity of chemical weathering to the growth and retreat of glaciers.

As mentioned earlier, glaciation may promote chemical weathering through elevated erosion rates. Mechanical erosion rates of 16–55.8 t/km²/yr (~0.06–0.2 mm/yr) have been estimated for watersheds adjacent to HAK
(Vigier et al., 2006). Interpolating from the observed correlation between physical erosion and the percentage of glacial cover from the same study, up to two orders of magnitude of increase in erosion rates at HVT between ice-free conditions and the current state may be expected. Indeed, at HVT, the estimated average erosion rates integrated over the Holocene (low average glacial cover) from lake sediment deposits (0.01–0.05 mm/yr) (Black et al., 2004) are much slower than modern (high glacial cover) denudation rates estimated from river suspended load (0.49 mm/yr) (Louvat et al., 2008).

## 4. Implications for the Carbon Cycle

The observed response of chemical weathering to glaciation (Figures 3d, 3e and 4b) contrasts with the hypothesis that the weathering effect temporally lags peak glaciation due to the continued weathering of reactive glacial sediment (Vance et al., 2009). Our observation that absolute weathering fluxes are higher in the glaciated catchment further suggest that active glacial processes are more effective at elevating weathering rates compared to the reaction of relic glacial deposits (Figures 3d, 3e and 4). We would like to note that we do not observe a decay in silicate weathering fluxes at HAK (non-glacial) as predicted by age-dependent weathering rates (Vance et al., 2009), although the catchment retains deposits of glacial sediment from recent deglaciation (~10 ka) (Torres et al., 2020). It is also possible that a decay of a small magnitude exists but is unresolvable with the uncertainties associated with our estimates.

The positive correlation between silicate weathering and glaciation we observe (Figure 4) suggests that portions of land surface do not necessarily follow global predictions of a negative climate-weathering feedback. This also implies the effects of erosion and runoff as controls for chemical weathering (Maher & Chamberlain, 2014; West, 2012), both of which are increased for temperate glaciers (Anderson et al., 1997; Hallet et al., 1996; Torres et al., 2017), can outweigh the temperature dependence (Walker et al., 1981). Though the integrated response might be reflective of a negative feedback, individual catchments may be independent and display a wider range of behaviors, in some cases acting to amplify climatic variations. Heterogeneity in the response to climate change may be important in generating multiple steady states in the climate system (i.e., greenhouse and icehouse conditions) and the carbon cycle. While the type of behavior we observe in these two small catchments is not necessarily direct evidence, it represents a plausible mechanism giving rise to bi-stability, which could be further tested via additional study.

Here we presented a new framework that allows the inference of catchment-scale chemical weathering fluxes from terrestrial sedimentary archives, which is powerful because it returns absolute weathering fluxes for Si—one of the major elements involved in silicate weathering—in an analogous manner to observations from modern river systems. As siliceous organisms are widespread in lake systems (Opfergelt et al., 2011), our approach may be widely adopted to assess and quantify the effects of different environmental controls on chemical weathering.

## Data Availability Statement

All data generated as part of this study and code for flux calculations are available at (Hou, 2025).

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## Acknowledgments

M.A.T. acknowledges support from NSF-EAR 2017106. Y.H. acknowledges support from a Wagoner Foreign Study Scholarship from Rice University. J.J.B acknowledges support by European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant agreement No. 891464. P.C.K. acknowledges support through the T.C. Chamberlin Fellowship at the University of Chicago and through an NSF-EAR postdoctoral fellowship (2204376). We thank Trevor Cole, Will Larsen, Miriam Gammerman, Matthias Walder, and Sam Scott for assistance with field work; Wendy Roth for facilitating sediment core sampling at INSTAAR; Pierre Burckel, Tu-Han Luu, Dimitri Rigoussen, and Sofia López-Urzúa for help with chemical and isotopic analyses at IPGP; Evan J. Ramos for discussions. The work benefited from the IPGP multidisciplinary program PARI and the Region Île-de-France SESAME Grant 12015908. Dr. Robinson Fulweiler is thanked for editorial handling. Open access publishing facilitated by Eidgenössische Technische Hochschule Zurich, as part of the Wiley - Eidgenössische Technische Hochschule Zurich agreement via the Consortium Of Swiss Academic Libraries.
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