# Reconciling chemical weathering rates across scales: Application of uranium-series isotope systematics in volcanic weathering clasts from Basse-Terre Island (French Guadeloupe)

Jiye Guo, Lin Ma, Jerome Gaillardet, Peter B. Sak, Yvette Pereyra, Jacqueline Engel

<figure><img src="image_1.png" /></figure>

## To cite this version:

Jiye Guo, Lin Ma, Jerome Gaillardet, Peter B. Sak, Yvette Pereyra, et al. Reconciling chemical weathering rates across scales: Application of uranium-series isotope systematics in volcanic weathering clasts from Basse-Terre Island (French Guadeloupe). Earth and Planetary Science Letters, 2020, 530, p. 566-580. 10.1016/j.epsl.2019.115874. insu-03584817

HAL Id: insu-03584817

https://insu.hal.science/insu-03584817v1

Submitted on 5 Aug 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

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

Distributed under a Creative Commons Attribution 4.0 International License
1 Reconciling chemical weathering rates across scales: Application of uranium-series isotope

2 systematics in volcanic weathering clasts from Basse-Terre Island (French Guadeloupe)

3

4 Jiye Guo<sup>a*</sup>, Lin Ma<sup>a*</sup>, Jerome Gaillardet<sup>b</sup>, Peter B. Sak<sup>c</sup>, Yvette Pereyra<sup>a</sup>, and Jacqueline Engel<sup>a</sup>

5

6 <sup>a</sup> Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968, USA

7 <sup>b</sup> Université de Paris, Institut de physique du globe de Paris, CNRS, F-75005 Paris, France

8 <sup>c</sup> Department of Earth Sciences, Dickinson College, Carlisle, PA 17013, USA

9 *Corresponding authors (jguo2@miners.utep.edu; lma@utep.edu)

10

11 Abstract

12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
To elucidate the factors that contribute to the orders of magnitude difference generally
observed in apparent weathering rates across scales, we utilized a U-series isotopic technique to
directly determine the duration and rates of chemical weathering recorded in weathering clasts. In
this study, we systematically compiled 15 individual volcanic weathering clasts collected from
eight andesitic watersheds along a gradient of mean annual precipitation ranging from ~1600 mm
to 6400 mm on the tropical volcanic Basse-Terre Island of French Guadeloupe. We measured U-
series isotope compositions to quantify weathering advance rates along the core-rind transects in
these weathering clasts. Total rind formation ages, the length of time it took for these weathering
clasts to form from andesitic rock fragments or volcanic clasts in the soil zones, range from 60
kyr to 300 kyr. As expected for each individual core-rind weathering transect, the rind ages
generally increase almost linearly with distance away from the core-rind boundary. The derived
clast weathering rates range from 0.08 ± 0.04 to 0.34 ± 0.01 mm kyr⁻¹, and the rates exhibit a
strong positive correlation (R² = 0.74) with the annual runoff values of these watersheds. This
correlation documents the first direct evidence that weathering over geological timescales on the
island is controlled by the amount of precipitation and corroborates the relationship that was
previously reported for the riverine weathering fluxes at Basse-Terre Island in Guadeloupe, but
measured with much shorter time scales. The clast-scale rates (inferred for the characteristic
length scale of mm) are compared to the watershed-scale rates of Basse-Terre Island (length scale
of km): constantly higher rates are observed at the watershed scales than the clast scale and the
discrepancy equals ~ 1800 ± 400 times, despite changes in watershed characteristics such as
watershed size, relief, and runoff values. The discrepancy is attributed to the fractal nature of the
roughness and surface area across scales. For example, the presence of undulations and fractures
(e.g. roughness) at the bedrock-saprolite contact of the watershed, which is generally not assessed
in the watershed surface area estimates, is most likely the main contributor to the constant rate
difference of ~1800 times.

37

## 1. Introduction

Chemical weathering of volcanic rocks, i.e. the neutralization and release of base cations
by carbonic and sulfuric acids, is one of the most fundamental and important process at Earth's
surface, impacting Earth systems by consuming atmospheric CO₂ and creating important solute
fluxes to the ocean and ecosystems (e.g., Gaillardet et al., 1999; Vitousek et al., 1999; Dessert et
al., 2003). Understanding the mechanistic controls of volcanic rock weathering is thus a
fundamental research question in Earth System Science. Accurate measurement of the weathering
rates, i.e., the reaction kinetics between water and minerals in the Critical Zone (e.g., Brantley et
al., 2006), is necessary to investigate the potential feedbacks among weathering and erosion of
volcanic rocks, landscape, climate, hydrology, ecosystem, and tectonic regime (Brantley et al.,
2008; Oelkers and Schott, 2009). A number of methods and approaches have been used to
determine weathering rates. For example, weathering rates can be quantified using river
chemistry as "integrators" of chemical fluxes from watersheds, chemical and isotopic
compositions of solid weathering products such as sediments, soil and regolith profiles,
laboratory dissolution experiments, and reactive transport models (Brantley et al., 2008; Oelkers
and Schott, 2009). However, weathering rates that are measured across a range of spatial and temporal scales generally differ by 2 to 5 orders of magnitude (White and Brantley, 2003; Navarre-Sitchler and Brantley 2007). This discrepancy has been attributed to the fact that many of those rates were measured under intrinsically different conditions including surface roughness, reactive surface areas, erosional regimes, parent lithology, climate, conditions of fluid flows and residence times, and biological controls (Bluth and Kump, 1994; Drever and Stillings, 1997; Maher et al., 2004; 2010; Dupre et al., 2003; Anderson, 2005; West et al., 2005), hampering a direct comparison of weathering rates across scales. Independent measurements of weathering rates over a range of scales from the same study areas, e.g., from weathering clasts to soil profiles and to watersheds, are rarely reported and compared. Hence, very few studies have attempted to reconcile and elucidate the controls on the different weathering rates across scales (e.g., Navarre-Sitchler and Brantley, 2007; Jung and Navarre-Sitchler, 2018).

To investigate the factors contributing to the large difference in weathering rates across scales, we utilize a U-series isotopic technique for weathering clasts (Pelt et al., 2008; Ma et al., 2012) that facilitates the direct determination of the duration and rates of chemical weathering in solid weathering products. U-series isotopes in weathering products have shown great potential for quantifying rates of chemical weathering, erosion, and transport with timescales less than ~1.25 Ma (e.g., Sarin et al., 1990; Vigier et al., 2001; Dequincey et al., 2002; Maher et al., 2004; Krishnaswami et al., 2004; DePaolo et al., 2006; Dosseto et al., 2008; Pelt et al., 2008; Bourdon et al., 2009; Ma et al., 2010; 2012). The technique relies on U-series isotope disequilibrium that results from U-series fractionation due to different decay half-lives and geochemical properties (e.g., Ivanovich and Harmon, 1992; Bourdon et al., 2003). Mobility behavior of U-series isotopes (e.g., $^{238}$U, $^{234}$U, $^{230}$Th, and $^{232}$Th) during low temperature water-rock interactions at weathering interfaces, soil profiles, or watersheds have been studied extensively (e.g., Chabaux et al., 2003; Dosseto et al., 2008), enabling the use of U-series isotopes in weathering clasts as a geochronometer (Pelt et al., 2008; Ma et al., 2012).
In this study, we focus on 15 individual volcanic weathering clasts collected from 8 andesitic watersheds along a gradient of mean annual precipitation ranging from ~1600 mm to 6400 mm on the tropical volcanic Basse-Terre Island of French Guadeloupe. Basse-Terre Island is a suitable field location to study chemical weathering due to its well-defined environmental gradients including climate, relief, and bedrock ages. In the tropical and thick volcanic soil profiles of Basse-Terre Island, partially weathered rock fragments or volcanic clasts are commonly present. These weathering clasts develop a rind of permeable and altered weathering products that envelops an unweathered core. Weathering clasts provide valuable samples to study the initiation and duration of chemical weathering in field systems (e.g., Cernohouz and Solc, 1966; Porter, 1975; Colman and Pierce, 1981; Sak et al., 2010), due to the presence of an easily identified core-rind boundary and the apparent absence of physical erosion during rind formation. We measured U-series isotope compositions to quantify weathering advance rates of the core-rind boundary in these 15 weathering clasts, which represent weathering rates measured at the clast scale (with a characteristic length scale of mm). In contrast to previous studies of weathering clasts in which only one or two clasts from a single watershed were characterized, this study for the first time investigated systematically a large number of weathering clasts from multiple watersheds along a precipitation gradient. The compilation of rind weathering rates allows for a direct comparison to the river chemical fluxes of Basse-Terre Island (length scale of km; Gaillardet et al., 2011). The two sets of independently determined weathering rates at two different field spatial and temporal scales offer insights to reconcile the rate difference observed in the multiple watersheds of the island.

## 2. Background

### 2.1. Basse-Terre Island

Basse-Terre Island is a part of the Guadeloupe archipelago, Lesser Antilles, with a surface area of 850 km² (Fig. 1a). This island encompasses many small (<~10 km²) watersheds
developed on similar andesitic volcanic rocks. Superimposed on this compositionally similar bedrock are strong gradients in bedrock ages, relief, and precipitation (Fig. 1).

To study the impacts of steep environmental gradients on the dynamics of surface processes including landslides, floods, river fluxes, and sediment transport, Basse-Terre Island was designated and managed as the Observatory of Erosion in Antilles by the Institut de Physique du Globe de Paris (IPGP). The observatory is a part of the international Critical Zone Exploration Network (CZEN) and the French CZO network (OZCAR; Gaillardet et al., 2018) and has been extensively studied for understanding chemical weathering and Earth surface processes, in addition to the potential natural hazards by active volcanism (Dessert et al., 2003; 2015; Chatanantavet et al., 2010; Sak et al., 2010; 2018; Gaillardet et al., 2011; Lloret et al., 2011; Ma et al., 2012; 2018; Allemand et al., 2014; Engel et al., 2016). In particular, two of the watersheds investigated in this study (Bras David and Capesterre; Fig. 1d) have been extensively monitored and investigated by the OBSERA Critical Zone Observatory (PI E. Lajeunesse), as a part of the French CZO network (Gaillardet et al., 2018).

Basse-Terre Island is comprised of relatively homogenous andesitic to basaltic-andesitic volcanic materials such as lava flows, lava domes and pyroclastic deposits (e.g., Jordan, 1975; Hawkesworth and Powell, 1980; Samper et al., 2007). Bedrock ages decrease systematically from north to south (2.8 Ma to present) along the long axis of the island (Samper et al., 2007; Fig. 1b). The topography of the island reflects the patterns of bedrock ages, with high relief and steep slopes in the young and volcanic active southern area and lower relief and gentler slopes in the older and more stable northern part (Fig. 1c). Basse-Terre Island is characterized by a tropical and humid climate (Dessert et al., 2015). The topographic divide generates a strong pattern of orographic precipitation (MAP: ~1000 mm to >6000 mm) on Basse-Terre Island. Notably, the east and south coast receive high precipitation and the leeward west and north coast generally receives less precipitation (Fig. 1c). The variations of mean air temperatures are more limited (16-25 °C); only the high elevation areas in southern central island experience MAT <20 °C (Fig. 1c).
## 2.2. Short-term chemical weathering fluxes at watershed scale

The rivers on Basse-Terre Island transport a substantial load of solutes and sediments, among the highest fluxes found on Earth (Lloret et al., 2011; Gaillardet et al., 2011; Dessert et al., 2015). Chemical weathering fluxes of 28 major rivers on the island (40-430 t/km² watershed/yr), calculated based on the concentrations of rock-derived solutes multiplied by runoff, show a strong correlation with annual watershed runoff values (Gaillardet et al., 2011), indicating that the solute concentrations in rivers are not purely diluted by the addition of water during high runoffs, but rather that solute concentrations remain relatively constant across the Island for a suite of watersheds with highly variable magnitudes of runoff. The relative constancy of concentrations in rivers despite of large temporal runoff variations is classically observed as the chemostatic effect (e.g., Godsey et al., 2009; Gislason et al., 2009). In the Lesser Antilles, Gaillardet et al., (2011) reported that the total dissolved solute of rivers (TDS: mg/L) varies as a function of runoff (R: mm/yr): TDS = 590 R⁻⁰.⁴ where the exponent of -0.4 (> -1) indicates increased chemical weathering at high runoff compensates for water dilution. Therefore, the watershed-based fluxes (calculated by TDS×R) increase positively with runoff (R).

As orographic effect exerts a first order control on runoff across the island of Basse-Terre, Gaillardet et al. (2011) proposed that volcanic activity on Basse-Terre could exert a negative feedback on atmospheric CO₂ levels. For example, volcanic activity creates relief and promotes high amounts of orographic precipitation and relatively permeable infiltration regimes; these factors then result in elevated chemical weathering fluxes and atmospheric CO₂ consumption. The accelerated weathering of silicate rocks and associated CO₂ drawdown triggered by volcanic uplift could provide an important mechanism to regulate atmospheric CO₂ concentration, in addition to the well-established temperature-related feedback mechanism (e.g., Walker et al., 1981; Berner et al., 1983). This hypothesis highlights the importance of the hydrologic cycle in controlling chemical weathering and CO₂ consumption on volcanic islands
156 (Godsey et al., 2009; Gaillardet et al., 2011) and the necessity to accurately constrain the
157 relationship between solute concentration and runoff at the regional scale.

158 The chemical weathering fluxes calculated based on river chemistry are however
159 contemporary with modern climate conditions. They do not necessarily reflect the establishment
160 of the precipitation and relief patterns, for example at Basse-Terre Island, over geological
161 timescales. Furthermore, the calculation of riverine weathering fluxes, as shown above, uses
162 directly river runoff values and might create a mathematic complication to interpret the
163 relationship observed between weathering fluxes and river runoff. One way to circumvent some
164 of these complications is to measure chemical weathering rates in solid-state weathering products,
165 not only as an independent comparison to riverine fluxes, but also as an alternative approach to
166 document the controls on chemical weathering over geologic timescales at a different spatial
167 scale.

168 **2.3. Weathering clasts and U-series systematics**

169 Rock fragments in regolith commonly form alteration rinds. Compared to the
170 unweathered parent core, the rind is enriched in relatively immobile elements (e.g. Ti, Fe, Th, and
171 Al) and depleted in mobile elements (e.g. Mg, Ca, Na, and K) (e.g. Colman and Pierce, 1981; Sak
172 et al., 2010). The presence of thick weathering rinds, i.e. rind material remaining in place around
173 the core, suggests no significant physical erosion occurred to the weathering clasts. The easy-to-
174 identify core-rind boundary marks the initial location for weathering to occur. The simplicity of a
175 weathering rind system has led to several significant advances in the use of U-series systematics
176 in weathering rinds to quantify rates and duration of chemical weathering. Specifically, rind
177 formation rates have been successfully measured on a number of clasts from Costa Rica and
178 Guadeloupe (Pelt et al., 2008; Ma et al., 2012; Engel et al., 2016; Sak et al., 2018; Ma et al.,
179 2019). In those efforts, it was demonstrated that the behaviors of U-series isotopes during rind
180 formation are well constrained in individual clasts and that the isotope mass balance approach
181 (e.g. Dequincey et al., 2002; Ma et al., 2012; Dosseto et al., 2008) can be utilized as a reliable
dating tool.

183

184 **3. Materials and Methods**

185 We compiled 15 weathering clasts collected from 8 watersheds on Basse-Terre Island
186 along well-defined environmental gradients of bedrock ages, relief, and precipitation (Table 1).
187 New analyses and characterization were conducted for 11 of the 15 samples in this study and
188 results from previously published studies were compiled for the other 4 samples (Sak et al., 2010;
189 2018; Ma et al., 2012; 2019; Engel et al., 2016). The clast collection, preparation, and analytical
190 methods were described in detail in the supplement materials (Appendix 1; Table A1). Major
191 element concentrations and U-series isotope compositions of the weathering clast samples were
192 measured on ICP-OES and MC-ICPMS at Penn State University and University of Texas at El
193 Paso (UTEP), respectively.

194

195 **4. Results**

196 **4.1. Major element chemistry**

197 All but one of the newly analyzed 11 clast samples in this study have unweathered cores
198 in the center, similar to the 4 previously analyzed samples (Sak et al., 2010; 2018; Ma et al.,
199 2012; 2018; Engel et al., 2016). Only sample AN-14-19.4 has been completely weathered with no
200 core present (Fig. A1). Core compositions are all primarily andesitic (Ma, 2018). The dominant
201 minerals in these cores are pyroxene, plagioclase, both as small phenocrysts (<1000 µm, much
202 smaller than the diameter of the drilled bit, 3.2mm) and in fine groundmass matrix, volcanic glass
203 materials, and minor phases such as ilmenite. These weathering clasts were developed from rock
204 fragments or volcanic clasts within pyroclastic flows with major element compositions similar to
205 other andesitic volcanic materials, e.g., lava flows or bedrock, on Basse-Terre Island (Samper et
206 al., 2007).
The rind material is readily distinguishable from both the encasing soils and unweathered cores on the basis of color (Fig. A1). The core-rind boundary is sharp and abrupt with an increase in porosity, near the starting location for initial stage of core weathering. Relative to the core, the rind is significantly depleted in CaO, Na₂O, MgO, K₂O, and relatively enriched in SiO₂, Al₂O₃, and Fe₂O₃ (Ma, 2018). Such chemical changes across the core-rind boundary are consistent with the previously observed weathering reactions in weathering clasts from Basse-Terre Island: weathering of glass matrix, groundmass (fine crystals of plagioclase, pyroxene), and primary minerals (plagioclase, pyroxene) from the cores to produce Fe oxy-hydroxides, gibbsite and minor kaolinite in the rinds (Sak et al., 2010).

## 4.2. U-series systematics

The unweathered cores of all the weathering clasts from Basse-Terre Island show average (²³⁴U/²³⁸U) = 1.018 ± 0.031, (²³⁰Th/²³⁸U) = 0.972 ± 0.042, and (²³⁰Th/²³²Th) = 1.002 ± 0.048 (1SD, N=14; Table A2) and are generally in U-series equilibrium (i.e. U-series activity ratios =1), as expected with their emplacement ages older than 1 Ma. The U and Th elemental contents of these core samples show a relatively large range of values: 0.312 to 1.157 mg/Kg for U and 0.860 to 3.433 mg/Kg for Th. However, the core U/Th elemental ratios show limited variability, e.g. average (²³⁸U/²³²Th) ratio = 1.032 ± 0.064 (1SD, N=14), consistent with the homogenous U-series isotope signatures of the drilled core samples.

The rind materials of all of the weathering clasts show a clear signature of U series disequilibrium (Table A2) as evidenced by (²³⁴U/²³⁸U) ratios (ranging from 0.964 to 1.071) and (²³⁰Th/²³⁸U) ratios (ranging from 0.422 to 1.070), indicating that rind formation has disturbed the U-series equilibrium signatures. The U-series disequilibrium is also documented by variable and elevated (²³⁸U/²³²Th) ratios in rind materials, ranging from 0.841 to 4.442, as compared to the average (²³⁸U/²³²Th) ratio of 1.032 in the cores. Similarly, the rind materials are characterized by elevated (²³⁰Th/²³²Th) ratios, ranging from 0.865 to 2.950, as compared to the average (²³⁰Th/²³²Th) ratio of 1.002 in the cores.
For individual clast weathering transects, ($^{238}$U/$^{232}$Th) and ($^{230}$Th/$^{232}$Th) ratios of the rind materials show three different groups of general patterns (Fig. 2; transects are marked in Fig. A1).

Group 1 includes 6 weathering clasts that show general linear increases of both ($^{238}$U/$^{232}$Th) and ($^{230}$Th/$^{232}$Th) ratios with distance away from the core-rind boundary (AN-14-1.6, AN-14-19.6, AN-14-13.1, AN-14-13.2, AN-14-15.1, and AN-16-40.1; Fig. 2), similar to the patterns previously observed in weathering clasts from Ma et al. (2012), Engel et al. (2016), and Sak et al. (2018). In those studies, the linear increase was interpreted as the result of a continuous addition of uranium into the rind (relative to immobile Th) during rind formation; the excess $^{234}$U in rind decays and produces $^{230}$Th over time, leading to an increase of ($^{230}$Th/$^{232}$Th) ratios with distance into rind with time. Group 2 includes 2 samples, AN-14-40.2 and AN-14-19.4 that show elevated ($^{238}$U/$^{232}$Th) ratios near the core-rind boundary but the ratios remain constant in the rest of the rind (Fig. 2 and Fig. A2). The ($^{230}$Th/$^{232}$Th) ratios show a continuous increase from the core into the rind. Such a pattern was first observed in a weathering clast from Costa Rica (Pelt et al., 2008) and was attributed to a process that added uranium instantaneously in the rind near the core-rind boundary, but the rest of the rind remained as a closed system with no further uranium addition.

Group 3 includes samples AN-14-3.5, AN-14-17.2 and AN-14-25.3 that do not show any systematic changes of ($^{238}$U/$^{232}$Th) and ($^{230}$Th/$^{232}$Th) ratios with distance (Fig. A2; Appendix 2).

The U-series systematics in Group 3 clasts may have experienced complicated condition changes of conditions in rind that could possibly lead to later loss of or further addition of U. The mechanistic interpretations of the three different U-series patterns in clasts are further discussed in Section 5.2 below.

### 4.3. Determination of weathering advance rates in weathering clasts

The U-series disequilibrium generated during rind formation can be used to derive time information related to duration and intensity of chemical weathering (e.g., Chabaux et al., 2008; Dosseto et al., 2008). Recent successes of using U-series isotopes to quantify Earth surface processes have been highlighted by their applications in weathering clasts, soil profiles,
sediments, and streams and rivers from small and large watersheds (Chabaux et al., 2008). In particular, the determination of weathering advance rates in weathering rinds relies on the use of U-series mass balance models that account for chemical leaching and addition of U-series isotopes during rind formation. Based on the general patterns observed in U-series profiles, two types of U-series mass balance models have been used (Dequincey et al., 2002; Pelt et al., 2008; Dosseto et al., 2008; Ma et al., 2012). The first type assumes a dual process of continuous U-series isotope leaching and addition in an open system (Dequincey et al., 2002), i.e., as the continuous U addition pattern observed in the Group 1 clasts above. The second type relies on an instantaneous addition of U-series isotopes during the initial stage of chemical weathering near the core-rind boundary, and the system remains closed after uranium addition (Pelt et al., 2008), as observed in the Group 2 clasts above. In this study, we follow the mobility behavior of U-series isotopes during rind formation and apply the continuous U addition model and the instantaneous addition model to Group 1 and Group 2 clasts, respectively, to calculate rind ages and rind formation rates (Fig. 3; model calculations are presented in Appendix 3). Group 3 clasts cannot be used to calculate rind ages or rates due the lack of systematic U-series patterns (see Section 5.2 for further discussion). Of the 8 clasts used in model calculations, weathering transects from 7 clasts successfully yielded formation ages with the U-series mass balance models (Fig. 3) and clast AN-14-19.4 did not yield any formation ages (Appendix 3). The total rind formation ages for all rind transects vary from ~60 kyr to ~300 kyr (Table 2). Along these weathering rind transects, the rind ages generally increase with distance away from the core-rind boundary (Table 2; Fig. 4), and the average weathering advance rates range from 0.12 mm/kyr to 0.34 mm/kyr (Table 3). Model-derived k238U values from the six weathering clasts vary from 1.5 × 10⁻⁵ to 4.8 × 10⁻⁵ yr⁻¹ (Table 3), within the range of ²³⁸U rate constants derived from weathering profiles or river sediments with similar weathering time scales (Dosseto et al., 2008; Ma et al., 2010; 2013; Engel et al., 2016). Similarly, model-derived k234/k238 ratios are all greater than one, varying from 1.1 to 1.5 (Table 3), consistent with the fact that ²³⁴U isotope is preferentially lost to
285 the weathering fluids compared to <sup>238</sup>U (e.g., Fleischer, 1980). Such a range of values agrees well
286 with many field and experimental studies of U isotopic fractionation during water-rock
287 interaction (e.g., Vigier et al., 2001; Dequincey et al., 2002; Dosseto et al., 2008; Andersen et al.,
288 2009). Model results also indicate that the rind experienced addition of U with f values ranging
289 from 2.3 × 10<sup>-5</sup> to 8.7 × 10<sup>-5</sup> yr<sup>-1</sup>. The f values are all greater than the k values for the six
290 weathering clasts (Table 3), indicating net addition of U to the rind due to the dual processes of U
291 mobility. The model derived f<sub>234</sub>/f<sub>238</sub> ratios are also greater than one (1.02 to 1.46), consistent with
292 U addition from the infiltrating soil water that is generally characterized by (<sup>234</sup>U/<sup>238</sup>U) >1. For
293 each profile, the derived f<sub>234</sub>/f<sub>238</sub> value is similar to the derived k<sub>234</sub>/k<sub>238</sub> value, implying that the
294 dual processes of U mobility could lead to either increases of (<sup>234</sup>U/<sup>238</sup>U) (f<sub>234</sub>/f<sub>238</sub> > k<sub>234</sub>/k<sub>238</sub>) or
295 decrease of (<sup>234</sup>U/<sup>238</sup>U) (f<sub>234</sub>/f<sub>238</sub> < k<sub>234</sub>/k<sub>238</sub>) with time (Dequincey et al., 2002), consistent with the
296 observed non-linear changes of (<sup>234</sup>U/<sup>238</sup>U) ratios in the rinds (Fig. 2).
297

298

## 5. Discussion

299

### 5.1. Mobility behavior of U-series isotopes during rind formation

300

Uranium is soluble and highly mobile during water-rock interactions under oxidizing
301 conditions. The minor isotope of U, <sup>234</sup>U, is preferentially released to solution compared to <sup>238</sup>U,
302 due to the alpha recoil effect of <sup>238</sup>U decay that either ejects <sup>234</sup>U particles directly to fluids, or
303 damages the crystal lattice and releases <sup>234</sup>U easily during subsequent weathering (e.g., Fleischer,
304 1980). Water phases are thus characterized by (<sup>234</sup>U/<sup>238</sup>U) activity ratios > 1 while solid residual
305 weathering products with (<sup>234</sup>U/<sup>238</sup>U) activity ratios < 1 (e.g., Rosholt et al., 1966; Vigier et al.,
306 2001; Chabaux et al., 2003; 2008; Dosseto et al., 2008; Anderssen et al., 2009). However, the rind
307 (<sup>234</sup>U/<sup>238</sup>U) values in this study, as in solid residual weathering products, show disequilibrium
308 signatures both below and above one (0.964 to 1.071; Table A2). Such an observation reveals a
309 dual process of U mobility during rind formation: (1) chemical leaching that generates (<sup>234</sup>U/<sup>238</sup>U)
310 ratios < 1 in the rind; and (2) addition of U with (<sup>234</sup>U/<sup>238</sup>U) ratios > 1, such as precipitate or

300

301

302

303

304

305

306

307

308

309

310
311 sorbate to the rind from infiltrating soil pore water that generally carries ($^{234}$U/$^{238}$U) ratios >1. The
312 dual processes of U mobility during weathering could lead to either increases of ($^{234}$U/$^{238}$U) or
313 decrease of ($^{234}$U/$^{238}$U) in rinds with time, depending on the relative intensity of these two
314 processes.

315 Rind ($^{238}$U/$^{232}$Th) activity ratios of these clasts are higher than the average core
316 ($^{238}$U/$^{232}$Th) ratios (Fig. 2). The elevated ($^{238}$U/$^{232}$Th) activity ratios show evidence of U addition
317 (or U immobilization) during rind formation, as thorium is generally immobile during water-rock
318 interaction due to its extremely low solubility in water (e.g., Rosholt et al., 1966; Chabaux et al.,
319 2003). The assumption of Th immobility in weathering rinds has been justified by comparing Th
320 profiles with other immobile elements such as Ti and Zr (e.g., Ma et al., 2012; Engel et al., 2016;
321 Ma, 2018; Ma et al., 2019). U immobilization could occur when the environmental conditions
322 change from oxidizing to reducing conditions, under which the solubility of U-containing phases
323 is much lower (Chabaux et al., 2003; 2008 and references therein), or due to co-precipitation or
324 sorption of U onto newly formed secondary minerals such as Fe-hydroxides (Ames et al., 1983;
325 Duff et al., 2002; Chabaux et al., 2003; 2008 and references therein). In a weathering clast
326 system, the rind material is generally dominated with highly oxidized minerals, e.g. Fe oxy-
327 hydroxides, and the unweathered core contains reduced minerals such as pyroxene and ilmenite.
328 It is possible that the rind-core boundary marks a redox transition zone with oxidizing conditions
329 in outer rinds to reducing conditions near the core-rind boundary. The reducing conditions in the
330 innermost rind could lead to the first precipitation of the dissolved U from infiltration soil pore
331 water. The fate of this added U could depend on later processes in the rind, possibly controlled by
332 the mineralogy transformation of Fe oxy-hydroxides. For example, it has been shown that newly
333 formed secondary Fe-hydroxides have large and highly reactive surface areas (e.g., Pett-Ridge et
334 al., 2007) that could promote further co-precipitation or sorption of U from soil water to rind.
335 Such a process is consistent with the observation that the ($^{238}$U/$^{232}$Th) ratio increases continuously
336 with distance away from the core-rind boundary (e.g., in Group 1 clasts, Fig. 2). If the U addition
to the rind were more sensitive to the redox conditions at the core-rind boundary than the adsorption onto Fe-hydroxides, then no subsequent addition of U would occur in the rind. This would lead to an instantaneous U addition profile in rinds (e.g., in Group 2 clasts, Fig. 2). Any further changes of conditions in rind could also possibly lead to later loss or addition of U in the rind that could lead to complicated U profiles (such as in Group 3 clasts). Indeed, the affinity of U to Fe-hydroxides is sensitive to their morphology and crystalline states. U is strongly associated with amorphous and poorly crystalline Fe-oxides. Any later transformation into more organized and crystalline Fe-oxides could lead to a weak association with U (or other particle reactive elements such as P) or U loss (Pett-Ridge et al., 2007; Thompson et al., 2011; Hall et al., 2015). Group 3 clasts could be affected by this modification process and show no systematic U-series patterns (Fig. A2). Some of the outmost rinds in Group 1 clasts also show possible evidence of U loss (Fig. 2). To summarize, redox conditions and mineralogy and morphology states of the Fe-oxides in rinds play an important role in controlling U mobility history and generate the possible three different patterns of U profiles in these rind, with significant implications on how to use U-series isotopes to determine rind formation ages and rates (Appendix 2).

## 5.2. Long-term chemical weathering rates on Basse-Terre Island

Rind formation rates, or weathering advance rates (the rate at which the core-rind boundary migrates into the core) were calculated based on the observed U-series disequilibria, following the continuous U addition model or the instantaneous U addition model (Fig. 4). In combination with three previous studies (Ma et al., 2012; Engel et al., 2016; Sak et al., 2018), weathering advance rates in seven watersheds (in one watershed the clast rates were not available) on Basse-Terre Island are compiled from a large number of weathering clasts for the first time (Table 3). The seven watersheds of the island are characterized with strong environmental gradients (Table 1; Fig. 1). For example, the watershed relief decreases from southeast to northwest, and the mean annual precipitation (MAP) and watershed runoff values also decrease systematically from southeast to northwest. The mean annual air temperatures
363 (MAT) of the clast sampling sites in these watersheds have a limited range (23 to 25°C).

364 The rind advance rates in these watersheds range from 0.08 to 0.34 mm kyr⁻¹, with values 365 increasing systematically from northwest to southeast (Table 3). These rates were all determined 366 from the rind transects perpendicular to the core-rind boundary with low curvature values (e.g., 367 flat core-rind boundaries), hence by similar curvature control of core-rind boundary (Sak et al., 368 2010; Ma et al., 2012; Lebedeva et al., 2015). The rind weathering rates do not appear to correlate 369 with the clast scale parameters such as total rind thickness, clast size, or shape (Table A1), but 370 show a strong and positive correlation with the water availability parameters at the watershed 371 scale, such as the mean runoff values and MAP for individual watersheds (Fig. 5). The reported 372 annual runoff values for the individual watersheds are constrained by ~10 years of field 373 measurements and monitoring programs for the main rivers on Basse-Terre Island (Gaillardet et 374 al., 2011) and show the most significant correlation (R²=0.74) with rind weathering rates (Fig. 5). 375 Relatively weak but still significant and positive correlations were observed between MAP and 376 rind weathering rates (R²=0.30 to <0.10, depending on the MAP data sources; Fig. A3).

377 In a weathering clast, the weathering advance rate is determined not only by curvature, 378 initial mineralogical content, and solute transport characteristics, but also by solute concentrations 379 (i.e., C in weathering fluids such as infiltrating soil water) (e.g., Eqn. 30 in Lebedeva et al., 2015). 380 For example, the advance rate (W) should be a direct function of (C^e-C^R): C^e is the equilibrium 381 concentration at the temperature of interest, and C^R is the concentration of this component in the 382 pore fluid at the outermost boundary of the weathering clast, e.g., in infiltrating soil water. 383 Weathering at the same temperature but under higher rates of precipitation and infiltration are 384 expected to have more dilute soil water and therefore higher values of (C^e - C^R). If (C^e - C^R) 385 varies linearly with annual watershed runoff or MAP, then the Lebedeva et al. (2015) model is 386 consistent with the observed positive linear correlation between weathering advance rates and 387 annual watershed runoff values (Fig. 5). Remarkably, this observation not only confirms the 388 observation of modern river chemical fluxes at Basse-Terre Island, but also highlights the
important control of orographic precipitation on chemical weathering at a geological time scale for Basse-Terre Island. Indeed, river chemistry data of Basse-Terre Island has shown a strong correlation between a catchment-wide chemical weathering rate (46 to 181 t/km²/yr) and the annual watershed runoff (1000 to 5200 mm/yr) for the studied seven watersheds (Table 1; Gaillardet et al., 2011). As mentioned above, at a watershed scale, for an increase in runoff, hydrological buffering mechanisms maintain the concentration of solutes despite the effect of dilution, leading to the observed correlation between riverine weathering fluxes and annual watershed runoff (i.e. “the chemostatic effect”; e.g. Godsey et al., 2009; Gislason et al., 2009; Maher, 2010). The rind weathering advance rates were determined at a time scale of tens to hundreds of thousands years, i.e. the rates were averaged over the total weathering duration of the rinds between 60 to 300 kyrs (Table 2), much longer than the contemporary chemical weathering fluxes measured in river chemistry. The weathering advance rates were also determined in rinds independently without relying on any existing river runoff values in the calculations. Hence, the correlation between weathering rates and watershed runoff values does not involve the same mathematical complication as the calculations of the riverine weathering fluxes. This weathering rind-based approach confirms the important observations from the riverine studies by providing independent and long term weathering rates.

### 5.3. Reconciling difference in weathering rates between clast and watershed scales

The study of U-series systematics in weathering rinds expands our field toolbox for studying chemical weathering rates across scales. The weathering advance rates in rinds were reported as rock to rind conversion rate mm (rock)/kyr, or the volumetric rock to rind conversion rate, mm³ (rock) /mm² (surface area)/kyr. Here the surface area refers to the geometric area on the clast that is perpendicular to the weathering advance direction. With a parent rock density of 2600 kg/m³ and the mass fraction of total cations (Na+K+Mg+Ca) in unweathered andesitic rock of 0.1 (Ma et al., 2012), the weathering advance rates of 0.08 to 0.34 mm³/mm²/kyr in this study correspond to 0.022 to 0.089 tons/km²(clast area)/yr for the loss of total cations during andesite
415 weathering at the clast scale. These rind rates are three orders of magnitude slower (by ~1000 to
416 3000 times) than the river weathering fluxes derived from the corresponding individual
417 watersheds (46 to 181 tons/km²/yr; Fig 5). The rate discrepancy (clast vs. watershed) does not
418 appear to vary with watershed characteristics from all the watersheds such as annual watershed
419 runoff, relief, bedrock age, or watershed size (Fig. A4), but remains relatively constant, with an
420 average of ~1800 ± 400 times.

421 Such a comparison is consistent with the previous observations that chemical weathering
422 rates generally do not agree across scales and can differ by orders of magnitude. The discrepancy
423 has been attributed to differences in measurements of weathering rates such as duration of
424 weathering (White and Brantley, 2003; Maher et al., 2004), hydrologic regime and flow path
425 factors (Velbel, 1993; Pacheco and Alencoao, 2006), factors related to clay precipitation (Maher,
426 2010), and methods used to measure surface area or the presence of surface coatings (Nugent et
427 al., 1998; Maher et al., 2006; Navarre-Sitchler and Brantley, 2007), the influence of biota or
428 organic acids (Wasklewicz, 1994; Drever and Stillings, 1997), ongoing removal of weathered
429 materials by physical erosion (i.e., Bluth and Kump, 1994; Dupre et al., 2003; Anderson, 2005;
430 West et al., 2005), and a scale effect of time-dependent mineral dissolution (Jung and Navarre-
431 Sitchler, 2018). In this study, the weathering clasts were collected from the watersheds where the
432 river weathering fluxes were measured with the same climate conditions (mean air temperature,
433 precipitation, and river runoff) and general topographic features (average relief). The weathering
434 materials in the two different systems of this study, i.e. the core materials of the weathering clasts
435 and the bedrock materials in watersheds, have similar chemical and mineralogy compositions as
436 observed from local outcrops and quarries (Sak et al., 2010; Ma, 2018). One possible exception is
437 that weathering materials in watersheds could include porous and more readily weathered
438 pyroclastic flows. However, the contribution from weathering of pyroclastic flows may not be
439 constant across individual watersheds and hence cannot explain the relatively constant difference
440 in weathering rates reported in the two weathering systems (clast vs. watershed). The rate
441 difference thus cannot be attributed to different field conditions or parent materials.

442 In this study, we follow the notation that the surface areas for weathering clasts vs.

443 watershed are measured and reported by using different “rulers” at different spatial scales
444 (Navarre-Sitchler and Brantley, 2007). The rind rates were measured at the weathering interface
445 of core-rind boundary with a length scale at the level of mm while the river weathering fluxes
446 were measured in rivers that collect weathering products from watersheds at the length scale of
447 km. To reconcile the rate discrepancy, Navarre-Sitchler and Brantley (2007) proposed that the
448 rate of weathering of basalt was constant when compared across scales, from the laboratory to
449 clasts to soil profiles to watersheds, only when normalized by the “real” basalt surface area that
450 actually experienced weathering ($A_{\text{weathering}}$). However, because the value of $A_{\text{weathering}}$ is not
451 known at each scale, the weathering rate was instead reported as a rate normalized by the surface
452 area assessed at that spatial scale ($A_{\text{measured}}$), not the actual $A_{\text{weathering}}$. For example, the geographic
453 area of a watershed ($A_{\text{watershed}}$) is not comparable in magnitude with the surface area estimated
454 from the geometry of a clast ($A_{\text{clast}}$), but they are fractal in that they vary with the scale of
455 measurement. In addition, Navarre-Sitchler and Brantley (2007) pointed out that the surface area
456 of a weathering front varies by two types of fractal dimension, $d_r$ and $d_s$. The $d_r$ value represents
457 the undulation of the surface area and the $d_s$ value represents the presence of more particles
458 within the thickness of the reaction front. For a simple comparison, Navarre-Sitchler and Brantley
459 (2007) introduced the ratio of $A_{\text{weathering}}/A_{\text{watershed}}$, or, $A_{\text{weathering}}/A_{\text{clast}}$ as the roughness ($\lambda$) value at
460 the watershed or clast scale. With this approach, the basalt weathering rates compiled from a
461 global dataset across scales were predicted with the equation (Navarre-Sitchler and Brantley,
462 2007):

$$463 \qquad W_D^\beta = k_0 \left(\frac{\beta}{a}\right)^{d_r-2} e^{-E_a/RT} \qquad \text{Eqn. 6.}$$

464 Here $W_D$ is the weathering rate (mm³ mm⁻² yr⁻¹) measured at the scale of lab-BET, lab-geometry,
465 clast, soil, or watershed, respectively; $\beta$ indicates the length scale (mm) for each scale; $k_0$ is the
pre-exponential factor, $E_a$ is the activation energy, $a$ is a spatial constant related to the BET length scale ($a = 10^{-7}$ mm), R is gas constant and T is the temperature; $d_r$ is the fractal dimension that relates surface areas across scales (e.g., $d_r \sim 2.3$ for basalt weathering; Navarre-Sitchler and Brantley, 2007); and $(\frac{\beta}{a})^{d_r-2}$ is the roughness term ($\lambda$). If we focus on comparing the rate difference across scales for Basse-Terre Island, Equation 6 can be rearranged to highlight the rate difference across scales:

$$ \frac{W_D^\beta}{(\frac{\beta}{a})^{d_r-2}} = k_0 e^{-\frac{E_a}{RT}} = \text{constant}, $$

$$ \frac{W_{BET}^{\beta}}{(\frac{\beta_{BET}}{a})^{d_r-2}} = \frac{W_{GEO}^{\beta}}{(\frac{\beta_{GEO}}{a})^{d_r-2}} = \frac{W_{Clast}^{\beta}}{(\frac{\beta_{Clast}}{a})^{d_r-2}} = \frac{W_{Soil}^{\beta}}{(\frac{\beta_{Soil}}{a})^{d_r-2}} = \frac{W_{Watershed}^{\beta}}{(\frac{\beta_{Watershed}}{a})^{d_r-2}} \qquad \text{Eqn. 7} $$

here $\beta$ is assumed to be $10^{-7}$mm (BET), $10^{-3}$mm (GEO), 1 mm (clast), 10 mm (or 1cm, soil), and $10^6$ mm (or 1 km, watershed); and a is $10^{-7}$mm (BET) as defined by Navarre-Sitchler and Brantley (2007).

If we assume that the rate difference observed between the clast and the watershed scale ($W_{Watershed} / W_{Clast} = 1800 \pm 400$) is attributed to the fractal control on the roughness difference across scales, then the fractal dimension $d_r$ is $\sim 2.54 \pm 0.02$ for andesite weathering at Basse-Terre by solving Eqn. 7 (Fig. 6). This value is similar to the $d_r$ value for the global basalt weathering study (2.3; Navarre-Sitchler and Brantley, 2007). The $d_r$ value (>2) represents the undulations in the bedrock-saprolite contact, or the deep weathering front that is not easily accessible at the watershed scale. The weathering front roughness for the weathering clast is visible from the sectioned weathering clasts (e.g., Fig. A1) while it is hard to measure the roughness for the bedrock-saprolite contact in a watershed. It can be reasonably assumed that the bedrock-saprolite contact at the watershed scale has more pronounced undulation features than the core-rind boundary at the clast scale. Hence, the surface area available for weathering at the bedrock-saprolite contact is larger than the core-rind boundary, with a higher weathering rate controlled by the increase of scale length in the fractional dimension (Eqn. 7). The increase of weathering rates
across scales could also be due to the increase of particle numbers available for weathering to occur within the weathering front (e.g., at the fractal scale, the fractal dimension is controlled by the factor $d_s$). For example, widely spaced fracture systems are known to exist at the bedrock-saprolite contact at watershed scales and could supply additional particles (and surfaces) for weathering to occur, as compared to the weathering clast system in which the core-rind boundary is sharp and with no visible networks of fractures in cores. Hence, following the interpretation of Navarre-Sitcher and Brantley (2007), the presence of the undulations and fractures (e.g. roughness with fractal dimensions) at the bedrock-saprolite contact at the watershed scale is most likely the main factor contributing to the ~1800 times apparent faster weathering rates observed at the watershed scale relative to that observed at the clast scale.

Other processes could also contribute to the different weathering rates observed between the watershed and the clast scales for the volcanic island of Basse-Terre, such as weathering of pyroclastic flows and ash layers, impacts of hydrothermal sources, different regolith thickness with deep vs. shallow weathering fronts, and shallow vs. deep groundwater flow paths. However, most of the above processes cannot offer a reasonable controlling mechanism that leads to a relative constant difference between the two sets of weathering rates. The rate difference is best reconciled by using the fractal dimension in surface roughness across scales.

## 6. Conclusions

Our study systematically investigated U-series disequilibria and derived rind formation ages and weathering rates in a large number of weathering clasts collected from eight watersheds on Basse-Terre Island. Total rind formation ages in these rinds range from 60 kyr to 300 kyr, indicating these weathering clasts formed from andesitic rock fragments or volcanic clasts in soil zones over long time scales. For each individual transect, the rind ages generally increase with distance away from the core-rind boundary. The weathering advance rates from these rinds range from 0.08 to 0.34 mm kyr$^{-1}$, with a strong positive correlation with the watershed annual runoff
516 values, confirming the previous observation in riverine studies, but now determined
517 independently. Both the rind weathering rates and river weathering fluxes show similar positive
518 correlation with annual watershed runoff values, suggesting that the control of water cycle on
519 chemical weathering operates similarly at both short- and long-time scales for tropical volcanic
520 Basse-Terre Island. Furthermore, these rind weathering rates are slower than the river weathering
521 fluxes derived from the corresponding individual watersheds, by a constant difference of ~1800
522 ± 400 times despite changes in watershed characteristics. We reconcile the rate difference at
523 Basse-Terre Island by attributing to the fractal dimension control on the roughness and surface
524 area across scales (Navarre-Sitchler and Brantley, 2007). The presence of undulations and
525 fractures (e.g. roughness) at the bedrock-saprolite contact at the watershed scale is not assessed at
526 the watershed scale and is most likely the main fractal dimension control that contribute to ~1800
527 times rate discrepancy observed between the watershed scale and the clast scale. U-series isotope
528 systematics in weathering rinds provides a new useful tool to quantify how weathering interface
529 advances in weathering clasts and to reconcile rates from difference spatial scales.
530

531 Acknowledgements

532 We would like to thank two anomynous reviewers for their insightful comments that imrpoved
533 the original munuscript and Editor-in-Chief Dr. Itay Halevy for his effective editorial handling of
534 the submisions. This research was funded by the National Science Foundation grants
535 EAR1251952, EAR1251969, and EAR1251875. We thank the Parc National de Guadeloupe for
536 authorizing us sampling in the National Park. We thank Celine Dessert (IPGP) and
537 L'Observatoire Volcanologique et Sismologique de Guadeloupe (IPGP) and the Critical Zone
538 Observatory ObErA (PI. E. Lajeunesse) for providing logistical support. Jieye Guo thank CZO
539 SAVI Summer Interns Program for a scholarship to visit IPGP. LM also acknowledged Dr. Mark
540 Engle (USGS) for support on statistical analysis and Monte Carlo simulations. Insightful and
541 fruitful discussion with S. Brantley at Penn State University, J. Bouchez and E. Lajeunesse at
542 IPGP are appreciated.

543

544 References

545 Allemand, P., C. Delacourt, E. Lajeunesse, O. Devauchelle, F. Beauducel (2014), Erosive effects of the
546 storm Helena (1963) on Basse Terre Island (Guadeloupe — Lesser Antilles Arc), Geomorphology
547 206,79-86.

548 Ames L. L., McGarrah J. E. and Walker B. A. (1983) Sorption of trace constituents from aqueous solutions
549 onto secondary minerals. I. Uranium. Clay Clay Mineral. 31, 321-334.
550 Andersen, M.B., Erel, Y., and Bourdon, B., 2009. Experimental evidence for 234U-238U fractionation during granite weathering with implications for 234U/238U in natural waters. *Geochimica Cosmochimica Acta* 73, 4124-4141.

553 Anderson, S.P., 2005. Glaciers show direct linkage between erosion rate and chemical weathering fluxes. *Geomorphology* 67, 147-157.

555 Berner, R. A., Lasaga, A., and Garrels, R. M., 1983. The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years. *American Journal of Science* 283, 641-683.

558 Bluth, G.J.S., Kump, L.R., 1994. Lithologic and climatologic controls of river chemistry. *Geochim. Cosmochim. Acta* 58, 2341-2359.

560 Bourdon, B., Bureau, S., Andersen, M.B., Pili, E., and Hubert, A., 2009. Weathering rates from top to bottom in a carbonate environment. *Chemical Geology* 258, 275-287.

562 Bourdon, B., Henderson, G.M., Lundstrom, C.C., and Turner, S.P., 2003. Uranium-series Geochemistry, *Mineralogical Society of America Reviews in Mineralogy and Geochemistry* 52, pp. 656.

564 Brantley, S., Kubicki, J., White, A. (2008). Kinetics of Water-Rock Interaction, Springer, 10.1007/978-0-387-73563-4.

566 Brantley, S.L., White, T.S., White, A.F., Sparks, D., Richter, D., Pregizer, K., Derry, L., Chorover, J., Chadwick, O., April, R., Anderson, S., Amundson, R., 2006, Frontiers in Exploration of the Critical Zone: Report of a workshop sponsored by the National Science Foundation (NSF), October 24-26, 2005, Newark, DE, 30p.

570 Cernohouz, J., and Solc, I., 1966. Use of sandstone wanes and weathered basaltic crust in absolute chronology. *Nature* 212, 806-807.

572 Chabaux, F., Bourdon, B., and Riotte, J., 2008. U-series geochemistry in weathering profiles, river waters and lakes. *Radioactivity in the Environment* 13, 49-104.

574 Chabaux, F., Riotte, J., and Dequincey, O., 2003. U-Th-Ra fractionation during weathering and river transport. *Reviews in Mineralogy and Geochemistry* 52, 533-576.

576 Chatanantavet P., E. Lajeunesse, L. Malverti, P. Meunier and G. Parker, 2010. Physically-based model of downstream fining in bedrock streams with side input. Water Resources Research 46, W02518, doi:10.1029/2008WR007208.

579 Colman, S.M., and Pierce, K.L., 1981. Weathering rinds on andesitic and basaltic stones as a Quaternary age indicator. Western United States. *U.S. Geological Survey Professional Paper*. 41p.

581 DePaolo, D.J., Maher, K., Christensen, J.N., and McManus, J., 2006. Sediment transport time measured with U-series isotopes: Results from ODP North Atlantic drift site 984. *Earth and Planetary Science Letters* 248, 394-410.

584 Dequincey, O., Chabaux, F., Clauer, N., Sigmarsson, O., Liewig, N., and Leprun J.-C., 2002. Chemical mobilizations in laterites: Evidence from trace elements and 238U-234U-230Th disequilibria. *Geochim Cosmochim Acta* 66, 1197-1210.

587 Dessert, C., Lajeunesse, E., Lloret E., Clergue C., Crispi O., Gorge C. and Quidelleur X. (2015) Controls on chemical weathering on a mountainous volcanic tropical island: Guadeloupe (French West Indies). *Geochim. Cosmochim. Acta* 171, 216-237.

590 Dessert, C., Dupré, B., Gaillardet, J., Francois, L. and Allègre, C.J., 2003. Basalt weathering laws and the impact of basalt weathering on the global carbon cycle. *Chem. Geol.* 202, 257-273.

592 Dosseto, A., Bourdon, B., and Turner, S.P., 2008a. Uranium-series isotopes in river materials: insights into the timescales of erosion and sediment transport. *Earth and Planetary Science Letters* 265, 1-17.

594 Drever, J.I., Stillings, L., 1997. The role of organic acids in mineral weathering. *Colloids Surf.*, A 120, 167-181.
596 Duff M. C., Coughlin J. U. and Hunter D. B. (2002) Uranium coprecipitation with iron oxide minerals. Geochim. Cosmochim. Acta 66, 3533-3547.

598 Dupre, B., Dessert, C., Oliva, P., Godderis, Y., Viers, J., Francois, L., Millot, R., Gaillardet, J., 2003. Rivers, chemical weathering and Earth's climate. *C. R. Geosc.* 335, 1141-1160.

600 Engel, J. M., Ma, L., Sak, P. B., Gaillardet, J., Ren, M., Engle, M. A., & Brantley, S. L. (2016). Quantifying chemical weathering rates along a precipitation gradient on Basse-Terre Island, French Guadeloupe: New insight from U-series isotopes in weathering rinds. Geochimica et Cosmochimica Acta, 195, 29-67.

604 Fleischer, R.L., 1980. Isotopic disequilibrium of uranium: alpha-recoil damage and preferential solution effects. *Science* 207, 979-981.

606 Gaillardet, J., Dupré, B., Louvat, P., and Allègre, C. J., 1999. Global silicate weathering and CO₂ consumption rates deduced from the chemistry of large rivers. *Chemical Geology* 159: 3-30.

608 Gaillardet, J., I. Braud, F. Hankard, et al. (2018). OZCAR: The French Network of Critical Zone Observatories. Vadose Zone J. 17:180067. doi:10.2136/vzj2018.04.0067

610 Gaillardet, J., Rad, S., Rive, K., Louvat, P., Gorge, C., Allegre, C.J., and Lajeunesse, R., 2011. Orography-driven chemical denudation in the Lesser Antilles: Evidence for a new feed-back mechanism stabilizing atmospheric CO₂. *American Journal of Science*, 311, 851-894.

613 Gislason, S. R., Oelkers, E. H., Eiriksdottir, E. S., Kardjilov, M. I., Gisladottir, G., Sigfusson, B., Snorrason, A., Elefsen, S. O., Hardardottir, J., Torssander, P., and Oskarsson, N., 2009, Direct evidence of the feedback between climate and weathering: Earth and Planetary Science Letters, v. 277, n. 1-2, p. 213-222.

617 Godsey S.E., Kirchner J.W., Clow D.W., 2009. Concentration-discharge relationships reflect chemostatic characteristics of US catchments. *Hydrol. Proc.* 23, 1844-1864.

619 Hall, S. J., and W. L. Silver. 2015. Reducing conditions, reactive metals, and their interactions can explain spatial patterns of surface soil carbon in a humid tropical forest. Biogeochemistry 125:149-165.

621 Hawkesworth, C.J., Powell, M., 1980. Magma genesis in the lesser Antilles island arc. Earth and Planetary Science Letters 51, 297-308.

623 Ivanovich, M., and Harmon, R.S., 1992. Uranium-series Disequilibrium: Applications to Earth, Marine, and Environmental Sciences. Clarendon Press, Oxford.

625 Jordan, T.H., 1975. The present-day motion of the Caribbean plate. Journal of Geophysical Research 80, 4433-4439.

627 Jung, H. and Navarre-Sitchler, A. (2018) Scale effect on the time-dependence of mineral dissolution rates in physically heterogeneous porous media. Geochimica et Cosmochimica Acta. v. 231, p. 70-83. https://doi.org/10.1016/j.gca.2018.05.009.

630 Krishnaswavi, S., Williams, G.A., Graustein, W.C., and Turekian, K.K., 2004. The effect of weathering regime on uranium decay series and osmium in two soil profiles. *Geochemical Journal* 38, 651-660.

633 Lebedeva M. I., Sak P. B., Ma L. and Brantley S. L. (2015) Using a mathematical model of a weathering clast to explore the effects of curvature on weathering. Chem. Geol. 404, 88-99.

635 Lloret E., Dessert C., Gaillardet J., Alberic P., Crispi O., Chaduteau C. and Benedetti M. F. (2011) Comparison of dissolved inorganic and organic carbon export in the rivers of tropical volcanic island; example from Guadeloupe, French West Indies. Chem. Geol. 280, 65-78.

638 Ma, L., (2018), Major element concentrations in weathering rinds and andesitic cores of Basse-Terre Island (French Guadeloupe), Earth Chem Library, DOI: 10.1594/IEDA/111220.
640 Ma, L., Chabaux, F., Pelt, E., Blaes, E., Jin, L., and Brantley, S.L., 2010. Regolith production rates calculated with uranium-series isotopes at Susquehanna/Shale Hills Critical Zone Observatory. *Earth and Planetary Science Letters* 297, 211-225.

643 Ma, L., Chabaux, F., Pelt, E., Granet, M., Sak, P.B., Gaillardet, J., Lebedeva, M., and Brantley, S.L., 2012. The effect of curvature on weathering rind formation: evidence from Uranium-series isotopes in basaltic andesite weathering clasts in Guadeloupe. *Geochimica et Cosmochimica Acta* 80, 92-107.

646 Ma, L., Dosseto, A., Gaillardet, J., Sak, P., and Brantley, S.L. (2018), Quantifying weathering rind formation rates by in situ measurements of U-series disequilibria with laser ablation (LA) MC-ICPMS. *Geochimica et Cosmochimica Acta*, doi.org/10.1016/j.gca.2018.12.020.

649 Maher, K., 2010. The dependence of chemical weathering rates on fluid residence time. *Earth and Planetary Science Letters*.

651 Maher, K., DePaolo, D.J., and Lin, J.C.F., 2004. Rates of silicate dissolution in deep-sea sediment: In situ measurement using U-234/U-238 of pore fluids. *Geochimica et Cosmochimica Acta* 68, 4629-4648.

654 Maher, K., Steefel, C.I., DePaolo, D.J., Viani, B.E., 2006. The mineral dissolution rate conundrum: insights from reactive transport modeling of U isotopes and pore fluid chemistry in marine sediments. *Geochim. Cosmochim. Acta* 70, 337-363.

657 Navarre-Sitchler, A. and Brantley, S., 2007. Basalt weathering across scales. *Earth and Planetary Science Letters* 261, 321-334.

659 Nugent, M.A., Brantley, S.L., Pantano, C.G., Maurice, P.A., 1998. The influence of natural mineral coatings on feldspar weathering. *Nature* 395, 588-591.

661 Oelkers, E., Schott, J. (2009). Thermodynamics and kinetics of water-rock interaction. *Rev. Min. Geochem.*, 70.

663 Pacheco, F.A.L., Alencoao, A.M.P., 2006. Role of fractures in weathering of solid rocks: narrowing the gap between laboratory and field weathering rates. *J. Hydrol.* 316, 248-265.

665 Pelt, E., Chabaux, F., Innocent, C., Navarre-Sitchler, A.K., Sak, P.B., and Brantley, S.L., 2008. Uranium-thorium chronometry of weathering rinds: rock alteration rate and paleo-isotopic record of weathering fluids. *Earth and Planetary Science Letters* 276, 98-105.

668 Pett-Ridge, J & M. Monastra, Valerie & A. Derry, Louis & Chadwick, Oliver. (2007). Importance of atmospheric inputs and Fe-oxides in controlling soil uranium budgets and behavior along a Hawaiian chronosequence. *Chemical Geology*, 244, 691-707. 10.1016/j.chemgeo.2007.07.016.

671 Porter, S.C., 1975. Weathering rinds as a relative-age criterion: application to subdivision of glacial deposits in the Cascade Range. *Geology* 3, 101-104.

673 Rosholt, J., Doe, B., and Tatsumoto, M., 1966. Evolution of the isotopic composition of uranium and thorium in soil profiles. *Geological Society of American Bulletin* 77, 987-1004.

675 Sak P. B., Murphy M., Ma L., Gaillardet J., Herdon E. M., Daniel, C. and Brantley S. L. (2018) From unweathered core to regolith: rates and trends of in situ chemical weathering on tropical volcanic island (Bass Terre Island, French Guadeloupe). *Chem. Geol.* 498, 17-30.

678 Sak P. B., Navarre-Sitchler A. K., Miller C. E., Daniel C. C., Gaillardet J., Buss H. L., Lebedeva M. I. and Brantley S. L. (2010) Controls on rind thickness on basaltic andesite clasts weathering in Guadeloupe. *Chem. Geol.* 276, 129-143.

681 Samper A., Quidelleur X., Lahitte P. and Mollex D. (2007) Timing of effusive volcanism and collapse events within an oceanic arc island: Basse-Terre, Guadeloupe archipelago (Lesser Antilles Arc). *Earth Planet. Sci. Lett.* 258, 175-191.

684 Sarin, M.M., Krishnaswami, S., Somayajulu, B.L.K., and Moore, W.S., 1990. Chemistry of uranium, thorium, and radium isotopes in the Ganga-Brahmaputra river system - weathering processes and fluxes to the Bay of Bengal. *Geochimica et Cosmochimica Acta* 54, 1387-1396.
<table>
  <tr>
    <td>687</td>
    <td>Thompson, A., D. Rancourt, O. Chadwick, and J. Chorover. 2011. Iron solid-phase differentiation along a redox gradient in basaltic soils. Geochimica et Cosmochimica Acta 75:119–133.</td>
  </tr>
  <tr>
    <td>688</td>
    <td></td>
  </tr>
  <tr>
    <td>689</td>
    <td>Velbel, M.A., 1993. Constancy of silicate–mineral weathering-rate ratios between natural and experimental weathering: implications for hydrologic control of differences in absolute rates. Chem. Geol. 105, 89–99.</td>
  </tr>
  <tr>
    <td>690</td>
    <td></td>
  </tr>
  <tr>
    <td>691</td>
    <td></td>
  </tr>
  <tr>
    <td>692</td>
    <td>Vigier, N., Bourdon B., Turner S., and Allegre, C.J., 2001. Erosion timescales derived from U-decay series measurements in rivers. Earth and Planetary Science Letters 193, 485–499.</td>
  </tr>
  <tr>
    <td>693</td>
    <td></td>
  </tr>
  <tr>
    <td>694</td>
    <td>Vitousek, M. P., Kennedy, M. A., Derry, L., Chadwick, O. (1999). Weathering versus atmospheric sources of strontium in ecosystems on young volcanic soils. Oecologia. 121. 255–259.</td>
  </tr>
  <tr>
    <td>695</td>
    <td></td>
  </tr>
  <tr>
    <td>696</td>
    <td>10.1007/s004420050927.</td>
  </tr>
  <tr>
    <td>697</td>
    <td>Walker, J., Hays, P., and Kasting, J., 1981. A negative feedback mechanism for the long-term stabilization of Earth's surface temperature. Journal of Geophysical Research 86, 9776–9782.</td>
  </tr>
  <tr>
    <td>698</td>
    <td></td>
  </tr>
  <tr>
    <td>699</td>
    <td>Wasklewicz, T.A., 1994. Importance of environment on the order of mineral weathering in olivine basalts, Hawaii. Earth Surf. Process. Land. 19, 715–734.</td>
  </tr>
  <tr>
    <td>700</td>
    <td></td>
  </tr>
  <tr>
    <td>701</td>
    <td>West, A.J., Galy, A., Bickle, M., 2005. Tectonic and climatic controls on silicate weathering. Earth Planet. Sci. Lett. 235, 211–228.</td>
  </tr>
  <tr>
    <td>702</td>
    <td></td>
  </tr>
  <tr>
    <td>703</td>
    <td>White, A.F., and Brantley, S.L., 2003. The effect of time on the weathering of silicate minerals: why do weathering rates differ in the laboratory and field? Chemical Geology 202, 479–506.</td>
  </tr>
  <tr>
    <td>704</td>
    <td></td>
  </tr>
  <tr>
    <td>705</td>
    <td></td>
  </tr>
  <tr>
    <td>706</td>
    <td></td>
  </tr>
  <tr>
    <td>707</td>
    <td></td>
  </tr>
  <tr>
    <td>708</td>
    <td><b>Figure Captions</b></td>
  </tr>
  <tr>
    <td>709</td>
    <td></td>
  </tr>
  <tr>
    <td>710</td>
    <td><b>Figure 1. a)</b> Location map of Basse-Terre Island (French Guadeloupe); Major geographic features in the Caribbean region are shown; <b>b)</b> Distribution of bedrock ages (Myr) on Basse-Terre Island (Samper et al., 2007); <b>c)</b> Contours of mean annual precipitation (MAP: mm yr<sup>-1</sup>; NASA TRMM data; https://pmm.nasa.gov/trmm) and color map of mean annual temperature (MAT: °C); Shaded topographic map of Basse-Terre Island is in the background; <b>d)</b> Major rivers and watersheds on Basse-Terre Island; Clast sampling locations in this study are shown as stars.</td>
  </tr>
  <tr>
    <td>711</td>
    <td></td>
  </tr>
  <tr>
    <td>712</td>
    <td></td>
  </tr>
  <tr>
    <td>713</td>
    <td></td>
  </tr>
  <tr>
    <td>714</td>
    <td></td>
  </tr>
  <tr>
    <td>715</td>
    <td></td>
  </tr>
  <tr>
    <td>716</td>
    <td></td>
  </tr>
  <tr>
    <td>717</td>
    <td><b>Figure 2.</b> Measured U-Th activity ratios in rind samples: (<sup>238</sup>U/<sup>232</sup>Th), (<sup>230</sup>Th/<sup>232</sup>Th), and (<sup>234</sup>U/<sup>238</sup>U) plotted as a function of position relative to the core-rind boundary (mm) along individual rind transects. Core samples plotted at position 0 mm. U-series analytical error bars are smaller than the symbol sizes. The locations of these 9 transects on individual clast samples are shown in clast images (Appendix Fig. A1). Clast samples are grouped into three categories based on their (<sup>238</sup>U/<sup>232</sup>Th) patterns (see main text for details). Group 1: continuous U addition; Group 2: instantaneous U addition; and Group 3: no systematic patterns. Results from Group 3 clasts are shown in Appendix Fig. A2.</td>
  </tr>
  <tr>
    <td>718</td>
    <td></td>
  </tr>
  <tr>
    <td>719</td>
    <td></td>
  </tr>
  <tr>
    <td>720</td>
    <td></td>
  </tr>
  <tr>
    <td>721</td>
    <td></td>
  </tr>
  <tr>
    <td>722</td>
    <td></td>
  </tr>
  <tr>
    <td>723</td>
    <td></td>
  </tr>
  <tr>
    <td>724</td>
    <td></td>
  </tr>
  <tr>
    <td>725</td>
    <td></td>
  </tr>
  <tr>
    <td>726</td>
    <td><b>Figure 3.</b> (<sup>230</sup>Th/<sup>232</sup>Th) vs. (<sup>238</sup>U/<sup>232</sup>Th) isochron diagrams for weathering clasts of Basse-Terre Island in this study. The analytical error bars are within the symbol size. Core samples are at secular equilibrium (1.0). The U–Th radioactive disequilibria of the weathering rind samples from these 9 clast transects are best explained by (Group 1) continuous input of U plus leaching of U starting at the core–rind boundary (the modeled (<sup>238</sup>U/<sup>232</sup>Th) and (<sup>230</sup>Th/<sup>232</sup>Th) ratios with model parameters in Table 2 and their corresponding rind formation ages are shown as the black line with gray dots for selected transects from Group 1 clasts). Group 2 clast (AN-16-40.2) is best modeled by instantaneous input of U at the core–rind boundary without additional loss of U, and</td>
  </tr>
  <tr>
    <td>727</td>
    <td></td>
  </tr>
  <tr>
    <td>728</td>
    <td></td>
  </tr>
  <tr>
    <td>729</td>
    <td></td>
  </tr>
  <tr>
    <td>730</td>
    <td></td>
  </tr>
  <tr>
    <td>731</td>
    <td></td>
  </tr>
  <tr>
    <td>732</td>
    <td></td>
  </tr>
  <tr>
    <td>733</td>
    <td></td>
  </tr>
</table>
734 subsequent production of <sup>230</sup>Th from U-series decay series in the rind (rind formation ages are shown in Table 2).

735
736
737 **Figure 4.** Rind formation age vs. distance relative to the core–rind boundary for the 9 rind transects in this study. Slope of each linear trend represents the weathering advance rate of the core-rind boundary (or rind weathering rate) for each transect (the slope values or the rates and R<sup>2</sup> values are provided in Table 3). Regressions were forced through the origin as we define position 0 (core-rind boundary) as the start point (t = 0) for chemical weathering.

742
743 **Figure 5. a)** Rind weathering rates (mm/kyr) plotted as a function of mean annual watershed runoff (mm/yr) for weathering clasts from Basse-Terre. The watershed runoff values are from Gaillardet et al. (2011). The dash line indicates a linear fit between these two variables (R<sup>2</sup> = 0.74). Rind weathering rates also show positive correlation with MAP (Appendix Fig. A3). **b)** River total weathering rates (TWR, ton/km<sup>2</sup>/yr) plotted as a function of mean annual watershed runoff (mm/yr) for the same watersheds of clast samples from Basse-Terre (Gaillardet et al., 2011). TWR were derived from river chemistry and annual runoff for each watershed on Basse-Terre Island and represent weathering rates at the watershed scale (Gaillardet et al., 2011). **c)** Rind weathering rates (converted to ton/km<sup>2</sup>/yr at the clast scale, values of multiple clasts from each watershed were averaged) vs. TWR (Total weathering rates for watershed, ton/km<sup>2</sup>/yr). Dashed line indicates the rate difference of these two sets of rates.

754
755 **Figure 6.** Weathering advance rates (mm³/mm²/yr) for Basse-Terre Island plotted as a function of the scale length at which the surface area is assessed (β, mm²; watersheds are assumed with 1km, or 10⁶ mm length scale and clasts are assumed with 1mm length scale.). Figure shows that advance rates vary with the length scale and can be reconciled with Equation 7, following the fractal dimension factor *d*<sub>r</sub> = 2.51. Clast scale rates are from this study and watershed scale rates are from Gaillardet et al. (2011). Basalt weathering rates across scales from Navarre-Sitchler and Brantley (2007) are shown for comparison.
<table><caption>**Table 1.** Information of the compiled 15 weathering clasts from the 8 watersheds of Basse-Terre Island, French Guadeloupe.</caption><thead><tr><th>Site ID</th><th>Clast Name</th><th>Watershed Name</th><th>Latitude DD MM MMM</th><th>Longitude DD MM MMM</th><th>Relief<sup>1</sup> (m)</th><th>MAT<sup>2</sup> (°C)</th><th>MAP<sup>2</sup> (mm/yr)</th><th>Runoff<sup>1</sup> (mm/yr)</th><th>TWR<sup>1</sup> (ton/km<sup>2</sup>/yr)</th></tr></thead><tbody><tr><td>AN-14-1</td><td>AN-14-1.6<br>AN-14-19.4<br>AN-14-19.6</td><td>Rose</td><td>16 07.845 N</td><td>61 35.861 W</td><td>1079</td><td>24.7</td><td>5280</td><td>2825</td><td>79</td></tr><tr><td>AN-14-3</td><td>AN-14-3.2*<br>AN-14-3.5<br>AN-14-17.2</td><td>Lezarde</td><td>16 11.787 N</td><td>61 36.326 W</td><td>749</td><td>24.9</td><td>4520</td><td>3750</td><td>101</td></tr><tr><td>AN-14-6</td><td>Bras David*</td><td>Bras David</td><td>16 10.508 N</td><td>61 41.746 W</td><td>972</td><td>23.3</td><td>3840</td><td>3000</td><td>92</td></tr><tr><td>AN-14-7</td><td>AN-14-7.5*<br>AN-14-7.6*</td><td>Deshaies</td><td>16 18.635 N</td><td>61 46.601 W</td><td>465</td><td>23.2</td><td>1640</td><td>1000</td><td>46</td></tr><tr><td>AN-14-13</td><td>AN-14-13.1<br>AN-14-13.2</td><td>Sainte Rose</td><td>16 19.442 N</td><td>61 42.832 W</td><td>587</td><td>24.5</td><td>3810</td><td>1850</td><td>71</td></tr><tr><td>AN-14-15</td><td>AN-14-15.1</td><td>Grande Riv. at Goyave</td><td>16 13.856 N</td><td>61 42.197 W</td><td></td><td>23.4</td><td>3150</td><td>2950</td><td>78</td></tr><tr><td>AN-14-25</td><td>AN-14-25.3</td><td>Grand Carbet</td><td>16 00.319 N</td><td>61 36.185 W</td><td>1220</td><td>20.0</td><td>4000</td><td>6000</td><td>427</td></tr><tr><td>AN-16-40</td><td>AN-16-40.1<br>AN-16-40.2</td><td>Capesterre</td><td>16 06.705 N</td><td>61 34.433 W</td><td>1132</td><td>24.8</td><td>6350</td><td>5200</td><td>181</td></tr></tbody></table>

1. Gaillardet et al. (2011); Relief refers to total watershed relief; TWR: total weathering rates from watersheds.

2. Tropical rainfall measuring mission (TRMM) data source: pmm.nasa.gov/trmm; MAT: mean annual temperature; MAP: mean annual precipitation;

* clast samples from previous studies (Ma et al., 2012; 2018; Engel et al., 2016; Sak et al., 2017).
<table><caption>Table 2. Rind formation ages and model parameters</caption><thead><tr><th>Clast Name</th><th>Sample Name</th><th>Distance from core (mm)</th><th>Weathering age (ky)</th><th>+/- (1SD, ky)</th><th>Model parameters</th><th>Median value</th><th>+/- (1SD)</th></tr></thead><tbody><tr><td rowspan="10">AN-16-1.6</td><td>1.6 Core</td><td>0</td><td></td><td></td><td>f<sup>238</sup>U</td><td>2.31E-05</td><td>9.09E-06</td></tr><tr><td>1.6 Ba</td><td>2</td><td>2</td><td>3</td><td>f<sup>234</sup>U/f<sup>238</sup>U</td><td>1.24E+00</td><td>1.60E-01</td></tr><tr><td>1.6 Bb</td><td>8</td><td>19</td><td>11</td><td>k<sup>238</sup>U</td><td>1.46E-05</td><td>5.96E-06</td></tr><tr><td>1.6 Bc</td><td>16</td><td>80</td><td>28</td><td>k<sup>234</sup>U/k<sup>238</sup>U</td><td>1.21E+00</td><td>1.70E-01</td></tr><tr><td>1.6 Bd</td><td>22</td><td>165</td><td>33</td><td>Goodness of fit (Chi<sup>2</sup>)</td><td>1.52E+01</td><td>1.50E+00</td></tr><tr><td>1.6 Be</td><td>28</td><td>300</td><td>66</td><td></td><td></td><td></td></tr><tr><td>1.6 Bf</td><td>34</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>1.6 Ta</td><td>2</td><td>3</td><td>3</td><td></td><td></td><td></td></tr><tr><td>1.6 Tb</td><td>9</td><td>31</td><td>14</td><td></td><td></td><td></td></tr><tr><td>1.6 Tc</td><td>18</td><td>142</td><td>37</td><td></td><td></td><td></td></tr><tr><td>1.6 Td</td><td>27</td><td>218</td><td>33</td><td></td><td></td><td></td></tr><tr><td>1.6 Te</td><td>38</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="10">AN-14-19.6</td><td>19.6 Core</td><td>0</td><td></td><td></td><td>f<sup>238</sup>U</td><td>5.51E-05</td><td>2.21E-05</td></tr><tr><td>19.6 a3</td><td>3</td><td>7</td><td>6</td><td>f<sup>234</sup>U/f<sup>238</sup>U</td><td>1.27E+00</td><td>1.64E-01</td></tr><tr><td>19.6 b1</td><td>8</td><td>2</td><td>3</td><td>k<sup>238</sup>U</td><td>3.89E-05</td><td>1.59E-05</td></tr><tr><td>19.6 c1</td><td>13</td><td>112</td><td>22</td><td>k<sup>234</sup>U/k<sup>238</sup>U</td><td>1.27E+00</td><td>1.77E-01</td></tr><tr><td>19.6 d1</td><td>18</td><td></td><td></td><td>Goodness of fit (Chi<sup>2</sup>)</td><td>6.39E+00</td><td>2.94E-01</td></tr><tr><td>19.6 e1</td><td>22</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>19.6 p1</td><td>3</td><td>27</td><td>15</td><td>f<sup>238</sup>U</td><td>5.50E-05</td><td>2.47E-05</td></tr><tr><td>19.6 q1</td><td>7</td><td>70</td><td>18</td><td>f<sup>234</sup>U/f<sup>238</sup>U</td><td>1.45E+00</td><td>2.40E-01</td></tr><tr><td>19.6 r1</td><td>12</td><td>116</td><td>21</td><td>k<sup>238</sup>U</td><td>3.98E-05</td><td>1.83E-05</td></tr><tr><td>19.6 s1</td><td>16</td><td></td><td></td><td>k<sup>234</sup>U/k<sup>238</sup>U</td><td>1.46E+00</td><td>2.52E-01</td></tr><tr><td>19.6 t1</td><td>20</td><td></td><td></td><td>Goodness of fit (Chi<sup>2</sup>)</td><td>4.77E+00</td><td>1.47E+00</td></tr><tr><td rowspan="6">AN-14-13.1</td><td>13.1 Core</td><td>0</td><td></td><td></td><td>f<sup>238</sup>U</td><td>5.50E-05</td><td>2.77E-05</td></tr><tr><td>13.1 a</td><td>3</td><td>14</td><td>11</td><td>f<sup>234</sup>U/f<sup>238</sup>U</td><td>1.35E+00</td><td>2.27E-01</td></tr><tr><td>13.1 b</td><td>7</td><td>30</td><td>20</td><td>k<sup>238</sup>U</td><td>4.12E-05</td><td>2.11E-05</td></tr><tr><td>13.1 c</td><td>13</td><td>61</td><td>27</td><td>k<sup>234</sup>U/k<sup>238</sup>U</td><td>1.37E+00</td><td>2.41E-01</td></tr><tr><td>13.1 d</td><td>19</td><td>128</td><td>34</td><td>Goodness of fit (Chi<sup>2</sup>)</td><td>4.08E+00</td><td>1.34E+00</td></tr><tr><td>13.1 e</td><td>22</td><td>179</td><td>50</td><td></td><td></td><td></td></tr><tr><td rowspan="6">AN-14-13.2</td><td>13.2 Core</td><td>0</td><td></td><td></td><td>f<sup>238</sup>U</td><td>6.34E-05</td><td>2.52E-05</td></tr><tr><td>13.2e</td><td>2</td><td>52</td><td>17</td><td>f<sup>234</sup>U/f<sup>238</sup>U</td><td>1.46E+00</td><td>2.66E-01</td></tr><tr><td>13.2f</td><td>7</td><td>66</td><td>18</td><td>k<sup>238</sup>U</td><td>4.84E-05</td><td>1.97E-05</td></tr><tr><td>13.2g</td><td>12</td><td>45</td><td>16</td><td>k<sup>234</sup>U/k<sup>238</sup>U</td><td>1.51E+00</td><td>2.77E-01</td></tr><tr><td>13.2h</td><td>17</td><td>101</td><td>21</td><td>Goodness of fit (Chi<sup>2</sup>)</td><td>5.69E+00</td><td>1.57E+00</td></tr><tr><td>13.2i</td><td>22</td><td>184</td><td>34</td><td></td><td></td><td></td></tr><tr><td rowspan="6">AN-14-15.1</td><td>15.1 Core</td><td>0</td><td></td><td></td><td>f<sup>238</sup>U</td><td>5.21E-05</td><td>2.57E-05</td></tr><tr><td>15.1 a1</td><td>3</td><td>10</td><td>4</td><td>f<sup>234</sup>U/f<sup>238</sup>U</td><td>1.18E+00</td><td>1.14E-01</td></tr><tr><td>15.1 b1</td><td>10</td><td>28</td><td>8</td><td>k<sup>238</sup>U</td><td>3.57E-05</td><td>1.92E-05</td></tr><tr><td>15.1 c1</td><td>16</td><td>60</td><td>8</td><td>k<sup>234</sup>U/k<sup>238</sup>U</td><td>1.18E+00</td><td>1.45E-01</td></tr><tr><td>15.1 d3</td><td>23</td><td></td><td></td><td>Goodness of fit (Chi<sup>2</sup>)</td><td>1.66E+00</td><td>5.16E-01</td></tr><tr><td>15.1 e1</td><td>29</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="6">AN-16-40.1</td><td>40.1 Core</td><td>0</td><td></td><td></td><td>f<sup>238</sup>U</td><td>8.71E-05</td><td>1.72E-05</td></tr><tr><td>40.1 0-2cm</td><td>10</td><td>40</td><td>9</td><td>f<sup>234</sup>U/f<sup>238</sup>U</td><td>1.02E+00</td><td>1.84E-02</td></tr><tr><td>40.1 2-3cm</td><td>25</td><td>56</td><td>11</td><td>k<sup>238</sup>U</td><td>1.87E-05</td><td>4.31E-06</td></tr><tr><td>40.1 3-4cm</td><td>35</td><td>124</td><td>14</td><td>k<sup>234</sup>U/k<sup>238</sup>U</td><td>1.03E+00</td><td>4.17E-02</td></tr><tr><td>40.1 4-5cm</td><td>45</td><td>144</td><td>14</td><td>Goodness of fit (Chi<sup>2</sup>)</td><td>2.19E+01</td><td>2.19E+01</td></tr><tr><td>40.1 5-6cm</td><td>55</td><td>137</td><td>14</td><td></td><td></td><td></td></tr><tr><td rowspan="6">AN-16-40.2</td><td>40.2 Core</td><td>0</td><td></td><td></td><td>f<sup>238</sup>U</td><td>8.71E-05</td><td>1.72E-05</td></tr><tr><td>40.2 0-2cm</td><td>10</td><td>8</td><td></td><td>f<sup>234</sup>U/f<sup>238</sup>U</td><td>1.02E+00</td><td>1.84E-02</td></tr><tr><td>40.2 2-3cm</td><td>25</td><td>45</td><td></td><td>k<sup>238</sup>U</td><td>1.87E-05</td><td>4.31E-06</td></tr><tr><td>40.2 3-4cm</td><td>35</td><td>82</td><td></td><td>k<sup>234</sup>U/k<sup>238</sup>U</td><td>1.03E+00</td><td>4.17E-02</td></tr><tr><td>40.2 4-5cm</td><td>45</td><td>115</td><td></td><td>Goodness of fit (Chi<sup>2</sup>)</td><td>2.19E+01</td><td>2.19E+01</td></tr><tr><td>40.2 5-6cm</td><td>55</td><td>191</td><td></td><td></td><td></td><td></td></tr><tr><td>40.2 6-7cm</td><td>65</td><td>192</td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><caption>Table 3. Weathering advance rates from weathering clasts of Basse-Terre Island</caption><thead><tr><th>Clast Name</th><th>Weathering rate (mm/kyr)</th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td>AN-14-1.6 line 1</td><td>0.109</td><td>0.70</td></tr><tr><td>AN-14-1.6 line 2</td><td>0.127</td><td>0.92</td></tr><tr><td>AN-14-19.6 line 1</td><td>0.119</td><td>0.29</td></tr><tr><td>AN-14-19.6 line 2</td><td>0.108</td><td>0.99</td></tr><tr><td><strong>Average rate</strong></td><td><strong>0.116 ± 0.008</strong></td><td></td></tr><tr><td>AN-14-13.1</td><td>0.139</td><td>0.85</td></tr><tr><td>AN-14-13.2</td><td>0.129</td><td>0.67</td></tr><tr><td><strong>Average rate</strong></td><td><strong>0.134 ± 0.007</strong></td><td></td></tr><tr><td><strong>AN-14-15.1</strong></td><td><strong>0.282 ± 0.010</strong></td><td>0.94</td></tr><tr><td>AN-16-40.1</td><td>0.338</td><td>0.85</td></tr><tr><td>AN-16-40.2</td><td>0.338</td><td>0.84</td></tr><tr><td><strong>Average rate</strong></td><td><strong>0.338 ± 0.010</strong></td><td></td></tr><tr><td><strong>AN-14-3<sup>1</sup></strong></td><td><strong>0.22 ± 0.15</strong></td><td>0.89</td></tr><tr><td><strong>AN-14-6<sup>2</sup></strong></td><td><strong>0.18 ± 0.07</strong></td><td>0.87</td></tr><tr><td><strong>AN-14-7<sup>3</sup></strong></td><td><strong>0.08 ± 0.04</strong></td><td>0.75</td></tr></tbody></table>

1. From Sak et al. (2018); Ma et al. (2018);

2. From Ma et al. (2012);

3. From Engel et al. (2016);
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>Figure 1</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 3</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 4</figcaption></figure>
<figure><img src="image_9.png" /></figure>

Mean annual watershed runoff (mm/yr)

<figure><img src="image_10.png" /></figure>

Mean annual watershed runoff (mm/yr)

<figure><img src="image_11.png" /></figure>

Rind weathering rates (ton/km²/yr)

<caption>Figure 5</caption>
Weathering advance rates
(mm³ mm⁻² yr⁻¹)

<figure><img src="image_12.png" /><figcaption>Figure 6</figcaption></figure>