# High rates of rock organic carbon oxidation sustained as Andean sediment transits the Amazon foreland-floodplain

Mathieu Dellinger, Robert Hilton, J. Jotautas Baronas, Mark Torres, Emily Burt, Kasey Clark, Valier Galy, Adan Julian Ccahuana Quispe, A. Joshua West

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## To cite this version:

Mathieu Dellinger, Robert Hilton, J. Jotautas Baronas, Mark Torres, Emily Burt, et al.. High rates of rock organic carbon oxidation sustained as Andean sediment transits the Amazon foreland-floodplain. Proceedings of the National Academy of Sciences of the United States of America, 2023, 120 (39), 10.1073/pnas.2306343120. hal-04240730

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https://hal.science/hal-04240730v1

Submitted on 22 Nov 2023

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High rates of rock organic carbon oxidation sustained as Andean sediment
transits the Amazon foreland-floodplain

3

4 Mathieu Dellinger¹,²*, Robert G. Hilton¹,³, J. Jotautas Baronas⁴, Mark A. Torres⁵, Emily I.

5 Burt<sup>6</sup>, Kasey E. Clark<sup>7</sup>, Valier Galy<sup>8</sup>, Adan Julian Ccahuana Quispe<sup>9</sup>, A. Joshua West<sup>6</sup>

6

7 ¹Department of Geography, Durham University, Durham, DH1 3LE, UK

8 ²EDYTEM-CNRS-University Savoie Mont Blanc (USMB), Chambéry, 73000, France

9 ³Department of Earth Sciences, University of Oxford, Oxford, OX1 3AN, UK

10 ⁴Department of Earth Sciences, Durham University, Durham, DH1 3LE, UK

11 ⁵Department of Earth, Environmental, and Planetary Sciences, Rice University, Houston, TX,
12 USA

13 ⁶Department of Earth Sciences, University of Southern California (USC), Los Angeles, CA,
14 90089, USA

15 ⁷Department of Geography & Planning, University of Liverpool, Roxby Building, Liverpool,
16 L69 7ZT, UK

17 ⁸Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution,
18 Woods Hole, MA, 02543, USA

19 ⁹Escuela Profesional de Biologia, Facultad de Ciencas, Universidad Nacional de San Antonio
20 Abad del Cusco, Cusco 08000, Peru

21
22 *Email: mathieu.dellinger@univ-smb.fr

23
24 Author Contributions:

25 MD, RGH and AJW designed the research and obtained funding. All authors contributed to
26 fieldwork and sample collection. MD undertook the geochemical analysis of materials for this
27 work. MD analyzed and interpreted the data with RGH. MD and RGH wrote the manuscript
28 with input from all authors.

29
30 Competing Interest Statement:

31 The authors declare no competing interests.

32
33 Classification:

34 Earth, Atmospheric, and Planetary Sciences

35
36 Keywords:

37 Carbon cycle; weathering; rivers; Amazon; Andes; Organic carbon

38

39

40
Abstract

The oxidation of organic carbon contained within sedimentary rocks ("petrogenic" carbon, or hereafter OCpetro) emits nearly as much CO2 as is released by volcanism, thereby playing a key role in the long-term global C budget. High erosion rates in mountains have been shown to increase OCpetro oxidation. However, these settings also export un-weathered material that may continue to react in downstream floodplains. The relative importance of OCpetro oxidation in mountains versus floodplains remains difficult to assess as disparate methods have been used in the different environments. Here, we investigate the sources and fluxes of rhenium (Re) in the Rio Madre de Dios to quantify OCpetro oxidation from the Andes to the Amazon floodplains using a common approach. Dissolved rhenium concentrations (n=131) range from 0.01 to 63 pmol.L-1 and vary depending on lithology and geomorphic setting. We find that >75% of the dissolved Re derives from OCpetro oxidation and that this proportion increases downstream. We estimate that in the Andes, OCpetro oxidation releases 11.2<sup>4.5</sup>/2.8 tC km<sup>-2</sup> yr<sup>-1</sup> of CO2, which corresponds to ~41% of the total OCpetro denudation (sum of oxidized and solid OCpetro). A Re mass balance across the Rio Madre de Dios shows that 46% of OCpetro oxidation takes place in the Andes, 14% in the foreland-lowlands, and 40% in the Andean-fed floodplains. This doubling of OCpetro oxidation flux downstream of the Andes demonstrates that, when present, floodplains can greatly increase OCpetro oxidation and CO2 release.

Significance Statement:

Erosion and weathering play key roles in Earth's carbon cycle, controlling climate over millions of years by transferring CO₂ to and from the atmosphere. Weathering of sedimentary rocks in mountains can release CO₂ when ancient organic carbon in rocks is chemically broken down. However, mountain erosion also moves rock organic carbon that has not been broken down and is transported by rivers to lowland floodplains to an unknown fate. Here we use rhenium in rivers to quantify rock organic carbon weathering in the Andes and adjacent floodplain. Erosion in the Andes leads to high rates of CO₂ release. However, CO₂ release doubles when including the floodplains. The presence or absence of floodplains next to mountains governs CO₂ release by weathering.

Main text:

Introduction

Substantial climatic changes have occurred throughout geologic time, with the evolution of the carbon cycle and the planet's habitability modulated by the interplay between tectonic, climatic, erosional and biological processes. Over long timescales (>10⁵ years), the abundance of CO₂ in the atmosphere is determined by the balance of the major carbon sources and sinks (1). It has been widely debated how the Earth system responds to transient imbalances in the long-term carbon (C) cycle, with much focus of prior research on the CO₂ sink associated with the chemical weathering of silicate minerals (1, 2). However, recent studies have highlighted the potential importance of changes in CO₂ fluxes linked to oxidative weathering (OW) reactions of sedimentary rocks on the continents (3–5). Although receiving less attention to date, OW of reduced phases in sedimentary rocks is central to both the C and O cycles (6, 7). Carbon dioxide release and O₂ consumption can result from both (i) oxidation of fossil (i.e. “rock-derived” or “petrogenic”) organic matter (OCpetro) and (ii) the OW of sulfides that produces sulfuric acid, which can be neutralized by carbonate minerals, or by the carbonate buffer of continental waters leading to net CO₂ release to the atmosphere (8, 9).

The export of particulate OCpetro by rivers in their sediment load has been documented in many places (10–16), but only a few field-based studies have estimated the rate of OW of OCpetro (3). The highest fluxes of OW of OCpetro (reported as a yield per catchment area, of 5 to
30 tC.km⁻².yr⁻¹) have been measured in small high standing islands composed of sedimentary rocks such as in Taiwan (17) and New Zealand (4), as well as in catchments from major mountain ranges (0.5 to 5 tC.km⁻².yr⁻¹) like the Yamuna (18), Swiss Alps (19) and in the Mackenzie River basin (20). This reflects the role of physical erosion rate in mountains that can continuously supply OCpetro to oxygenated surface waters and the atmosphere (17, 21). A consequence is that, in combination with carbonate weathering by sulfuric acid, CO₂ release by OCpetro oxidation can be larger than CO₂ consumption by silicate weathering in many erosive settings (3), challenging the idea that chemical weathering in mountain belts is a long-term carbon sink (22–24).

While physical erosion can enhance the supply of reduced phases to OW reactions and increase the weathering flux (8, 17, 25), the overall intensity of weathering can also decline (26), meaning that erosive catchments can also export very large amounts of ‘unweathered’ particulate OCpetro (7). These particulate fluxes can be very high, for instance reaching 250 tC.km⁻².yr⁻¹ (13, 26). The subsequent fate of this material is critical to governing how physical erosion and weathering impact the carbon cycle. It has been proposed (10, 14) that this OCpetro can be largely oxidized in floodplains when present (e.g. in the Amazon and Ganges floodplain), because of the long residence time of sediments and the warm and oxidative conditions that prevail in the floodplains relative to upstream mountainous area. Hence, the mountain catchments underlain by sedimentary rocks with river floodplains could have greater net CO₂ release from OW than mountain catchments without floodplains. Over 10⁵-10⁶ year timescales, the configuration of continental drainages can shift dramatically (27) and young, high standing mountain ranges potentially operate differently in terms of OW and CO₂ release relative to mountain ranges with floodplain drapes.

Despite the recognition of these geomorphological controls on OW (3), the relative contributions of mountain versus floodplain weathering to OCpetro oxidation and CO₂ release remains unknown. This knowledge gap arises in part because different methods have been used to assess OW of OCpetro in mountain catchments (4, 17) versus floodplains (10, 14). In mountain catchments, dissolved rhenium (Re) fluxes have been used for quantifying OCpetro oxidation fluxes (17), whereas studies on floodplains have used sediment fluxes and radiocarbon content of river sediments (Bouchez et al., 2010). Both methods have benefits and drawbacks. Dissolved Re fluxes are easier to measure because water discharge is usually known with more precision than sediment flux. On the other hand, there are large uncertainties on the source of dissolved Re and on the estimation of the Re/OCpetro ratio of the bedrock (20). Because water and sediments integrate over different spatial and temporal scales, their direct comparison can also be problematic. Overall, the influence of floodplains on the global OCpetro budget remains an open question (3, 28).

To resolve these issues, here we investigate the source and fluxes of dissolved and particulate rhenium (Re) in the Madre de Dios watershed, part of the larger Amazon Basin. We focus on an elevation transect ranging from the high Andes to the low-elevation floodplains and use Re to quantify the flux of CO₂ released by OCpetro oxidation. These catchments have been well-characterized by previous studies in terms of erosion rates and their major dissolved load (11, 23, 29), and benefit from time-series samples over a range of river discharges. Most importantly, this setting provides an opportunity to determine the rate of OCpetro oxidation in each of the different geomorphological settings (mountain, foreland, floodplain) within a single river basin.

## Study site

The Madre de Dios watershed (124,231 km² at the confluence with the Rio Beni in Bolivia) is one of the main headwaters of the Amazon basin (30), the largest river basin in the world. The precipitation in the Madre de Dios region is between <1000 and >6000 mm.yr⁻¹ and the mean
annual temperature varies with elevation from 4 to 25°C (30–32). The Rio Madre de Dios drains: i) the eastern flank of the Andes in Peru with elevations from 1300 m to 6300 m, ii) the mountain front with elevations from 500 m to 1300 m, iii) the foreland corresponding to elevations of less than 500 m, in which river basins are not Andean-fed but had undergone some recent uplift associated with the Fitzcarrald Arch, and iv) the Andean-fed floodplains, which are the low-relief environments adjacent to active river channels from rivers originating from the Andes and where sediments are deposited and exchanged with the main channel. The bedrock in the headwaters is primarily composed of Paleozoic and Mesozoic sedimentary and metasedimentary rocks (with [OC<sub>petro</sub>] ~ 0.4 to 0.6%) and some minor granitic intrusions in the Andes (23). Observations from the neighboring Beni catchment with similar lithologies to the Madre de Dios show that this OC<sub>petro</sub> is a mixture of disordered and graphitic carbon (10).

To characterize the spatial variability of dissolved Re concentrations ([Re]<sub>diss</sub>) across the Madre de Dios River basin (Fig. 1A), we studied a large number of small tributaries draining each geomorphological setting that have been subject to prior work on water, sediment and carbonate and silicate weathering fluxes (11, 23, 29, 33–35). To characterize the dissolved Re sources, we sampled stream, spring and lysimeter waters from small first-order catchments in each geomorphological setting (referred as Wayqecha Small Catchment “WAY-SC”, Villa Carmen Small Catchment “VC-SC” and CICRA Small Catchment “CICRA-SC”; Burt et al., 2022). To examine the downstream evolution of OC<sub>petro</sub> weathering processes across these geomorphic settings, we studied in detail four main nested sub-catchments that define a geomorphic gradient from the Andes to the Foreland-floodplain: the Rio Kosñipata at Wayqecha in the Andes (referred as “WAY”, altitude 2250 m); the Rio Kosñipata at San Pedro in the Andes (“SP”, altitude 1360 m); the Alto Madre de Dios at Manu Learning Center (“MLC”, altitude 479 m) at the transition between the Andes and the Foreland; and the Madre de Dios at CICRA-Los Amigos research station (“CICRA”, altitude 217 m). Time series [Re]<sub>diss</sub> and discharge data from the years 2010-2011 are used to characterize changes in Re concentration during the hydrological cycle. The measured annual runoff for the year 2010-2011 was 3065 mm.yr<sup>-1</sup> at the “WAY” site (Upper Andes) to 2796 mm.yr<sup>-1</sup> at SP. The suspended sediment flux (a proxy for erosion rate) at San Pedro (SP) measured during the same period is 3500 t.km<sup>-2</sup>.yr<sup>-1</sup> (11), similar to the estimated suspended sediment load from the Andean area of the entire Madre de Dios watershed (3208 t.km<sup>-2</sup>.yr<sup>-1</sup>; 35). Finally, to integrate weathering processes over large areas and establish a Re mass-budget, we studied several large tributaries with Andean-fed floodplains that drain all geomorphic settings (Rios Manù, Colorado, Chiribi, Inambari and Tambopata).

## Results

Rhenium concentrations were measured on water samples collected from several field campaigns, including (i) from the Madre de Dios mainstem and major tributaries sampled unders different flow conditions in 2012 and 2013 (23, 34, 38), and again at high and low flow in March and May 2019 (35), (ii) from time-series samples collected at four nested catchment sites (areas 50 to 27 830 km²) in 2010 and 2011 at various hydrological conditions (29, 33), and (iii) from another time series of samples collected from smaller first-order catchments (<1 km² area) across the elevation gradient (36). The time-series dataset is used to characterize the relationship between Re concentration and instantaneous runoff.

## Spatial variability of dissolved Re concentrations

Rhenium concentrations in dissolved samples (n=133) range from 0.01 to 63 pmol.L⁻¹, with spatial variability depending on the sample type, bedrock lithology and geomorphic setting (Fig. 1B; Dataset S1). Rivers have [Re]<sub>diss</sub> ranging from 0.1 to 9.5 pmol.L⁻¹. The rainwater sample collected in the Andes has [Re]<sub>diss</sub> below detection limit ([Re]<sub>diss</sub> < 0.05 pmol.L⁻¹) while [Re]<sub>diss</sub> in the throughfall sample from the same location is 0.36 pmol.L⁻¹. Lysimeter samples
191 from first-order small catchments in the Andes and in the Foreland have also very low [Re]diss,
192 ranging from 0.09 to 0.12 pmol.L-1 (n=4). In contrast, the two hot springs samples from Aguas
193 Caliente have very high [Re]diss (39 pmol.L-1 and 63 pmol.L-1). However, the [Re]diss of the Alto
194 Madre de Dios downstream of the hot springs is unchanged compared to [Re]diss upstream of
195 the hot springs, suggesting that their contribution to the Re budget is negligible.

196 In terms of lithology, small rivers draining mostly granitic rocks in the High Andes have
197 lower Re concentrations (0.56±0.44 pmol.L⁻¹, 1σ, n=6) than rivers draining sedimentary rocks
198 (2.90±2.06 pmol.L⁻¹, 1σ, n=25). Rivers draining the mountain front composed of mostly
199 granites also have lower [Re]diss (1.32 pmol.L⁻¹, n=2) than those draining sedimentary rocks
200 (3.88±1.12 pmol.L⁻¹, 1σ, n=10).

201 We observe a notable geomorphic control on [Re]diss for the tributaries and first-order
202 catchments draining specific settings, but not for the nested catchments (Fig. 1B). Tributaries
203 draining sedimentary rocks in the High Andes and Mountain front have significantly higher
204 [Re]diss (3.22±1.82 pmol.L⁻¹, 1σ, n=38) than those draining the foreland (0.70±0.53 pmol.L⁻¹,
205 1σ, n=9). For first-order catchments, [Re]diss is the highest in the High Andes (0.12 to 0.48
206 pmol.L⁻¹), as compared to <0.15 pmol.L⁻¹ in the Mountain front and Foreland. In contrast, no
207 significant change in [Re]diss downstream is observed for nested catchments, although the
208 average concentration at WAY (3.76±0.76 pmol.L⁻¹, 1σ, n=6) is slightly higher than at the SP
209 site (2.59±0.44 pmol.L⁻¹, 1σ, n=7). We note that in March 2019, [Re]diss was higher (4.22
210 pmol.L⁻¹) at SP than the average [Re]diss of time-series measurements from the 2010-2011
211 period, perhaps related to a large landslide in this catchment in the intervening period. At the
212 foreland-floodplain site (CICRA), [Re]diss = 3.69±0.98 pmol.L⁻¹ (1σ, n=9). For the main
213 tributaries of the Madre de Dios, we observe the highest [Re]diss in the Manù (1.5 to 9.5 pmol.L⁻¹),
214 lowest [Re]diss in the Chiribi and Colorado rivers (0.5 to 2.2 pmol.L⁻¹), and intermediate
215 [Re]diss in the Alto Madre de Dios (3.4 to 4.7 pmol.L⁻¹), Inambari (3.3 pmol.L⁻¹) and the
216 Tambopata (2.3 pmol.L⁻¹).

217

218 Temporal variability of dissolved Re concentrations

219 In the four nested-catchments, [Re]diss generally decreases with increasing instantaneous runoff,
220 Q, except for the high-altitude Andean site (WAY) where [Re]diss shows no clear relationship
221 with runoff (Fig. 1C). The relationship can be modeled using a power law [Re]diss = a × Qᵇ,
222 with a and b as the two fitted constants. For our samples, we observe a decrease of the b value
223 from the Andes (-0.09±0.12 in WAY, -0.15±0.08 in SP; 1σ) to the mountain front (-0.40±0.06
224 in MLC) and the foreland-floodplain (-0.46±0.08 in CICRA), indicating a more chemostatic
225 behavior of Re in the Andes, and more dilutional behavior in the Foreland-floodplain. This
226 decrease of the b value with elevation is similar to that observed for most major elements (29).
227

228

228 Re concentrations in solids

229 The Re concentration in bedloads ([Re]BM) from Andean rivers draining shales ranges from
230 0.12 to 0.50 ppb with an average of 0.28±0.06 ppb (±2SE, n=14; Fig. 2). These are higher than
231 the Re concentration in Andean soil samples (39) collected close the SP site 0.07±0.07 ppb
232 (±2SE, n=3) and in the Foreland-floodplain 0.04±0.01 ppb (±2SE, n=4). The [Re]BM is slightly
233 higher in WAY (0.31 to 0.36 ppb) compared to SP (0.23 to 0.26 ppb). Two rock samples were
234 analyzed: [Re] of the OCpetro and sulfur-rich sedimentary rock sample is almost 100 times higher
235 (4.61 ppb) than the igneous rock (0.06 ppb). The [Re]/[OC] ratio in bedloads and rocks vary
236 from 0.27×10⁻⁷ to 2.65×10⁻⁷ g.g⁻¹ with an average of 0.93±0.38×10⁻⁷ g.g⁻¹ (±2SE, n=16; Fig. 2).
237 The average [Re]/[OC] for the Kosñipata stream (WAY and SP sites) is 0.80±0.12×10⁻⁷ g.g⁻¹
238 (±2SE, n=5). This [Re]/[OC] ratio value is similar to bedload from New Zealand Southern Alps
239 (4) but lower than bedloads from Taiwan (17), the Yamuna River (18) and the Mackenzie River
240 (20).
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Our aim is to determine the rates and controls of OW of OCpetro across the transition from the Andes Mountains to Amazon floodplain in the Madre de Dios basin. To do so, we need to characterize the source, behavior, and fluxes of dissolved Re. Following previous studies (4, 17, 19, 20, 40), the OCpetro oxidation yield ($J_{OCpetro-ox}$, tC km$^{-2}$ yr$^{-1}$) can be estimated from:

$$J_{OCpetro-ox} = J_{Re} \times f_{C} \times ([OC]/[Re])_{solids} \times (1 - f_{graphite}) \quad [1]$$

Where “$J_{Re}$” is the dissolved Re yield (in g km$^{-2}$yr$^{-1}$), the product of the Re concentration and runoff ($J_{Re} = [Re]_{diss} \times Q$), while “([OC]/[Re])$_{solids}$” is the OC/Re ratio in sedimentary rocks being weathered (in g.g$^{-1}$), “$f_{C}$” is the fraction of dissolved Re deriving from the oxidation of OCpetro and “$f_{graphite}$” is the proportion of graphite in the rocks that may be resilient to OW (14).

## Dissolved Re source and yield

To determine the OCpetro oxidation flux using the Re proxy, we must first quantify the proportion of dissolved rhenium derived from OCpetro oxidation relative to other potential Re sources (rainwater, carbonates, sulfides, silicates, carbonates; ref.18). Following the approach developed in ref. (20), we use the [Re]/[SO$_4$*] and [Re]/[Na*] ratios to do so ([Na*] and [SO$_4$*] are concentrations corrected for atmospheric-derived contributions). In general sulfides have low Re/SO$_4$ (41) and silicates low Re/Na relative to OCpetro (20), and both Na and SO$_4$ are conservative soluble species in the Madre de Dios catchment (34, 35, 38). In analogy with studies on silicate and carbonate weathering, here we use local constraints on the end member compositions, using the combination of water samples from first-order catchments and tributaries (e.g. refs. 39, 40), combined with river sediments and rocks (23, 44) (Materials and Methods).

We find that small Upper Andes tributaries draining shales have a large range of [Re]/[SO$_4$*] and [Re]/[Na*] ratios that can be interpreted as a mixing trend between sulfides and OCpetro weathering, with limited Re contribution from silicate weathering (Fig. 3). In contrast, Andean tributaries draining granites, and rivers draining the foreland, have ratios that can be mostly explained by a mixture between silicates and OCpetro (Fig. 3). A mixing analysis (Materials and Methods) shows that the fraction of dissolved Re derived from OCpetro oxidation, $f_{C}$ (Equation 1), is >0.75 in the Andes (sites WAY and SP) and increases downstream to >0.90. Seasonal variability of the Re source is small but significant (less than 20% variability), with higher proportion of Re derived from OCpetro oxidation in the Andes at higher runoff. This could reflect the larger contribution of shallow groundwaters from the unsaturated zone above the sulfide OW front (19). The fraction of Re derived from sulfide OW decreases from ~0.15-0.25 in the Andes (WAY and SP) to <0.05 in the Foreland-floodplain (CICRA). The fraction of Re derived from silicate weathering is generally <0.05 but reaches ~0.2 for rivers draining only the foreland (e.g. Los Amigos River). Altogether, this shows that in the Madre de Dios basin, the majority of dissolved Re is derived from OCpetro oxidation, confirming observations in other catchments where sedimentary rocks dominate the geology (19, 20, 45).

Calculation of dissolved Re yield ($J_{Re}$) can be done in several ways, i.e. using average [Re]$_{diss}$ (4, 17, 18, 46), discharge-weighted average [Re]$_{diss}$ (41) or rating curve-derived [Re]$_{diss}$ (19) and annual or instantaneous water discharge estimates. Here, these methods return similar results (Materials and Methods; Dataset S3). Therefore, we take advantage of paired [Re]$_{diss}$ and discharge measurements (at the four nested catchment sites and at the main tributaries) and use discharge-weighted average [Re]$_{diss}$ and annual water discharge to derive dissolved Re yield. For the main tributaries, the highest $J_{Re}$ is in the Manu River catchment (2.0$^{+0.9}/$-0.7 g km$^{-2}$ yr$^{-1}$) and the lowest for the Las Piedras river (0.3$^{+0.1}$ g km$^{-2}$ yr$^{-1}$). For the nested catchments,
291 the Re yield is 2.1±1.0 g km⁻² yr⁻¹ and 1.3±0.2 g km⁻² yr⁻¹ for WAY and SP sites respectively,
292 and 1.6±0.4 g km⁻² yr⁻¹ for the Foreland-floodplain (CICRA) site. Although there is slight
decrease with elevation, the main result is that the specific Re yield of the Rio Alto Madre de
293 Dios does not change, within uncertainty, when the Rio Alto Madre de Dios (Andes) reaches
294 the Foreland-Floodplains.
295

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297 Rates and control of OC<sub>petro</sub> oxidative weathering in the Andes

298 We compute the OC<sub>petro</sub> oxidative weathering rate (Equation 1) using the parameter
299 values defined above (Dataset S3). As we have no constraint on f<sub>graphite</sub> in this setting, we do
300 not account for this term here. To estimate the uncertainties, we use Monte Carlo
301 simulations with 10 000 resolutions of the equation with distribution sampling of values within
302 the errors of each parameter (4). The estimated OC<sub>petro</sub> oxidative weathering fluxes based on
303 dissolved Re yield are 16.7<sup>+11.9</sup>/<sub>-8.8</sub> tC km<sup>-2</sup> yr<sup>-1</sup> for the WAY site and 11.2<sup>+4.5</sup>/<sub>-2.8</sub> tC km<sup>-2</sup> yr<sup>-1</sup>
304 for the SP site. The lower uncertainty on the J<sub>OCpetro-ox</sub> for SP is due to a higher precision on the
305 water flux at this site (33).
306

306 It has been proposed that physical erosion rate is a major control on the rate of OW (3,
307 8), with a potential role for landslide erosion supplying fresh mineral surfaces (24). The present-
308 day erosion rate of the Rio Kosñipata is high, with values between 1200 and 3500 t.km⁻².yr⁻¹
309 (11) and frequent landslides (47). The J<sub>OCpetro-ox</sub> for the Kosñipata are much higher than in the
310 Mackenzie (0.45 to 1.01 tC km⁻² yr⁻¹) and the Swiss Alps (3.6 to 5.7 tC km⁻² yr⁻¹) characterized
311 by lower erosion rates, and more similar to rivers in New Zealand and Taiwan (4 to 30 tC km⁻²
312 yr⁻¹) which have higher erosion rates (3). Results from this study thus confirm the general
313 control of erosion rate on the rate of OC<sub>petro</sub> oxidation (3), though other factors can also play an
314 important role (3, 48). The un-weathered solid OC<sub>petro</sub> export at the SP site has been determined
315 previously to be 16.1±1.4 tC.km⁻².yr⁻¹ using sediment samples collected across the same time
316 period (11). These fluxes imply that 41±10% of the total bedrock OC<sub>petro</sub> is oxidized in the
317 Andes, and the remainder is exported downstream. This OC<sub>petro</sub> weathering intensity value is
318 slightly lower than in the Mackenzie River basin (50%), similar to the Swiss Alps (19) and
319 higher than in Taiwanese rivers (<20%) (17).
320

320 Our estimate of the flux of OC<sub>petro</sub> oxidation in the Rio Kosñipata is the first attempt to
321 quantify the CO₂ emission from weathering of OC<sub>petro</sub> in the Andes. This estimate can also be
322 compared with other carbon fluxes relevant to the long-term carbon cycle in the Andes in the
323 Kosñipata catchment. In the SP catchment, the rate of biospheric organic carbon (OC<sub>bio</sub>) export
324 in river suspended sediments is 12.6±0.4 tC km<sup>-2</sup> yr<sup>-1</sup> (11), which is slightly higher than the
325 OC<sub>petro</sub> oxidative weathering yield. At the same site, the yield of carbon release associated with
326 carbonate weathering by sulfuric acid is ~ 6 tC km<sup>-2</sup> yr<sup>-1</sup> (23), 45% lower than the rate of CO<sub>2</sub>
327 release by OC<sub>petro</sub> oxidation. Together, the OW of reduced phases (OC<sub>petro</sub> and sulfides) in
328 Andean sedimentary rocks release about 17 tC km<sup>-2</sup> yr<sup>-1</sup> to the atmosphere, confirming that OW
329 of sedimentary rocks in mountain belts is a large source of CO<sub>2</sub> to the atmosphere.
330

331

331 Oxidative weathering in the foreland and in the Andean-fed floodplains

332 Once the OC<sub>petro</sub> that survived OW in the Andes has reached the foreland and the floodplains,
333 it can undergo further OW during transport and deposition in the floodplains (10). It has been
334 suggested that this process can be a large source of CO₂ to the atmosphere in the neighbouring
335 Beni catchment (10) and in the Ganges floodplain (14). Here we take several approaches to
336 isolate the weathering signal from the foreland (here defined as <500 m) versus the Andean-
337 fed floodplain (<500 m and low relief adjacent to active river channels where Andean-derived
338 sediments are exchanged). First, we assess the OC<sub>petro</sub> weathering flux in the foreland by using
339 data from the Las Piedras river catchment, since it drains the foreland exclusively with no
340 Andean contribution. The [Re]<sub>diss</sub> concentration at Las Piedras (1.24±0.35 pmol.L⁻¹, 2SE) is
low like the two other medium-size foreland rivers, the Blanco (0.36±0.06 pmol.L⁻¹) and the Los Amigos (0.50±0.16 pmol.L⁻¹). The [Re]/[OC] ratio of bedrock in Las Piedras can be reasonably assumed to be equivalent to the Andean-derived bedload from the Rio Kosñipata, given that the foreland-floodplain region is predominantly underlain by Andean-derived sediments (23). Using this, we calculate a $J_{OCpetro-ox}$ of 1.9$^{+1.2}/_{-0.8}$ tC.km⁻².yr⁻¹ which is about 6 times lower than in the Andes and similar to the rate measured in the Mackenzie River basin (20).

To estimate the OCpetro weathering flux in the floodplains, we attempt a mass-balance at the scale of the whole Madre de Dios catchment (118 459 km² at El Sena station near Riberalta, Ref. 36). Assuming the $J_{OCpetro-ox}$ from SP (Upper Andes) is representative of the Andean area (40 868 km²; defined as <500 m) of the Madre de Dios, we calculate a total Andean $J_{OCpetro-ox}$ of 0.46$^{+0.18}/_{-0.11}$ MtC.yr⁻¹. This assumption is justified as the SP catchment has similar erosion rates as the inferred Andean erosion rate for the whole Madre de Dios River (37). The contribution from foreland weathering (< 500m; 77 591 km²) can be estimated using the $J_{OCpetro-ox}$ from the Las Piedras River (19 630 km²) as 0.15$^{+0.09}/_{-0.06}$ MtC.yr⁻¹. The total $J_{OCpetro-ox}$ at the mouth of the Madre de Dios is 1.00$^{+0.43}/_{-0.28}$ MtC.yr⁻¹. Hence, the contribution of Andes + Foreland is lower than the total $J_{OCpetro-ox}$ of the Madre de Dios at its mouth by 0.39$^{+0.16}/_{-0.11}$ MtC.yr⁻¹, which we attribute to weathering in the floodplain. This mass-balance indicates that ~46% of the total OCpetro oxidation takes place in the Andes, ~40% in the floodplains, and ~14% in the foreland-lowlands (Fig. 4A).

The deposition and weathering of sediments in the floodplain constitutes a different mechanism compared to upland weathering (29, 49). Sediment and water exchange and storage during floodplain transit by rivers can result in long sediment residence times (50), and influence water flux, and redox state, which could all facilitate OW (7). In addition, the warmer climatic conditions in the floodplain relative to the Andes could be important in setting reaction rates (21), while the length of floodplains and their channel migration rates (51) could also be important controlling variables. Scaled laboratory experiments have been used to suggest that in-situ oxidation during within river transport is small (52) meaning reactions in sediment stores of floodplains are likely to be of most importance (53).

Our paired instantaneous [Re]diss and discharge measurements from the same sampling trips (in 2013 and 2019, both wet and dry seasons) covering both upstream and downstream floodplain sections constrain the location of OCpetro oxidation reactions in the floodplains between MLC and CICRA (Fig. 4B). The ~85 km long section of the river along the course of the Alto Madre de Dios floodplain, between the Mountain Front (MLC) and the confluence with the Rio Manu, is mostly braided and multi-channel, with mobile channel bars comprised of sand and fine gravel. Measurements show a significant increase of $J_{Re}$ (Re flux). Water isotopes indicate lack of significant water, and therefore likely Re, contribution from tributaries in this river reach (see Materials and Methods). Hence, we attribute this $J_{Re}$ increase (from 4 to 30%, Fig. 4B) to weathering of OCpetro from Andean sediments transiting in the floodplains (SI Appendix). The same calculation can be done using mean annual Re flux, but with a larger uncertainty. We calculate that the mean annual $J_{Re}$ increases by 31±20% which corresponds to a flux of CO₂ release through OCpetro oxidation in the floodplain reach between MLC (Mountain front) and AMdD of 0.03±0.01 MtC.yr⁻¹. In comparison, the CO₂ release through OCpetro oxidation in the Andean part of the Alto Madre de Dios catchment is 0.08±0.02 MtC.yr⁻¹. As the solid OCpetro load at MLC is 0.12±0.01 MtC.yr⁻¹ (Ref. 32, assuming similar solid OCpetro yield as at SP), this would suggest that ~25% of Andean-derived solid OCpetro is oxidized before reaching the confluence with the Rio Manu over a floodplain length of 85 km.

In contrast, in the section between AMdD and CICRA (~180 km long), where a larger, single channel meanders through the floodplain, we observe no significant Re flux increase in wet seasons (2013 and 2019) and dry season 2019. For the dry season in 2013, we even observe
a small decrease of Re yield along the section (Fig. 4B). Similarly, using mean annual $J_{Re}$, we calculate no significant increase in $OC_{petro}$ oxidation, within uncertainties (Fig. 4B, SI Appendix), in this section nor in the CICRA-PM section (~170 km long). Over the transit from MLC to CICRA, the isotopic composition of sulfate suggests minimal sulfate reduction (36), suggesting that oxygen is available for OW weathering along this river reach. In addition, the warm tropical climate could drive higher reaction rates and $OC_{petro}$ oxidation (21). Therefore, the apparent limited floodplain weathering could reflect an exhaustion of reactive $OC_{petro}$ supplied from Andean erosion upstream. Regardless of the mechanisms at play, the Rio Madre de Dios example shows that high rates of $OC_{petro}$ oxidation in the mountain headwaters can be matched by OW in the foreland (Fig. 4). A large proportion of floodplain weathering appears to happen over a relatively short-length scale (~<85 km) after exiting the Andes.

Finally, for the section furthest downstream, PM-Ribe (~560 km long), calculation using mean annual $J_{Re}$ show a 10% increase in Re flux. This may reflect OW of reactive $OC_{petro}$ supplied by the Inambari and/or the Tambopata, in combination with a longer time sediments spend in this long floodplain section (e.g. see ref. 50). Yet, this observation should be interpreted with caution due to the large uncertainty (±13%) associated with our calculation. Additional work on this part of the catchment is necessary to refine these calculations.

## Implications for the long-term carbon cycle

Previous studies on the Amazon River found significant $CO_2$ release from $OC_{petro}$ oxidation during the transit of fluvial sediments in the Madeira floodplain (10, 11). They found that the solid $OC_{petro}$ flux from the Madeira at the confluence with the Amazon is much lower than the solid $OC_{petro}$ supplied by the Beni at the mountain front and transported through the plain. They estimated a $CO_2$ emission of 0.50 MtC.yr⁻¹ across the Rio Beni floodplain. Our Re-based estimate of $OC_{petro}$ oxidation during the transit of Madre de Dios sediments in the floodplain (0.39$^{+0.16}/$-0.11 MtC.yr⁻¹) is of similar magnitude. The total $CO_2$ emission from $OC_{petro}$ oxidation in the whole Madre de Dios catchment (1.00$^{+0.43}/$-0.28 MtC.yr⁻¹) is more than twice the silicate weathering $CO_2$ drawdown (0.43$^{+0.22}/$-0.09 MtC.yr⁻¹; ref. 53). Considering that sulfide oxidation also contributes to the carbonate weathering flux (23), the net $CO_2$ balance during weathering in the Madre de Dios catchment appears to be tipped firmly toward being a $CO_2$ source (3).

Our findings from the Rio Madre de Dios catchment allow us to postulate a broader role for floodplain weathering in enhancing the $CO_2$ release by $OC_{petro}$ oxidation. Uplift and exhumation of sedimentary rocks in a mountain range can increase the supply of $OC_{petro}$ to an oxygenated weathering zone (17, 21) and increase the rates of $OC_{petro}$ oxidation and $CO_2$ release, as we observe in the Andes (Fig. 4). However, the overall weathering intensity can be low, with only ~20% to 50% of $OC_{petro}$ oxidized, meaning there is further potential for $CO_2$ release. On high standing mountain islands, floodplains are short. For instance, in Taiwan rivers have floodplains <40 km long, and many are shorter than 5 km, and there un-weathered $OC_{petro}$ is re-buried offshore (16). However, if the tectonic setting permits the growth of a larger continental floodplain, additional $OC_{petro}$ oxidation is very likely to occur even if just ~100 km in length based on our findings (Fig. 4). In this case, only extremely refractory $OC_{petro}$ escapes oxidation (10). While future work will need to establish how temperature and $O_2$-supply impact $OC_{petro}$ oxidation rates in floodplains (7, 21), our results suggest that the formation of floodplain adjacent to a mountain range allows more complete $OC_{petro}$ oxidation during sediment transit. Over multi-million year timescales, the growth and waning of riverine floodplains and continental sediment storage (27) could thus act as a powerful carbon cycle modifier throughout Earth's history.

## Materials and methods
The materials and methods are summarized here; further details are provided in SI Appendix. All data used in this study are reported in Datasets S1–S5.

**Sample collection and discharge measurements.** Major cation and anion concentration data of samples from 2010 to 2013 are from Torres et al., (2015, 2016). Detailed information on the sampling protocol can be found in those studies. In summary here, for the four main nested catchment sites (WAY, SP, MLC and CICRA, Fig. 1), time-series samples were collected between 2010 and 2011 using a clean PP bottle and filtered on site with 0.2 µm porosity nylon filter (29). At these sites, water discharge was measured at the same time as sampling by monitoring water levels manually and converting to discharge using a rating curve. At the SP site, river level was monitored with a water level logger that recorded river level measurements every 15 min (33). At other localities, samples were collected on fieldtrips in 2012, 2013, 2016 and 2019 (Dataset S1). Water samples were collected from the river surface using clean bucket and transferred to 10 or 20 L plastic bags before filtration. Samples were filtered within 24 to 48h of collection with 0.2µm porosity polyethersulfone (PES) filters. The discharge ("Q") at CICRA was measured during each sampling trip using an Acoustic Doppler Current Profiler (ADCP, RD1 Sentinel GED154 in March 2013 and SonTek M9 in August 2013 and in March and May 2019). In 2019, the discharge of the Alto Madre de Dios, Rio Manù, Chiribi and Colorado were also measured by ADCP. All discharge measurements are from Torres et al., (2017) and Burt et al., (2021)

**Re concentration measurements in water and sediments.** The dissolved Re concentrations were measured following the same protocol as described in ref. (19). Briefly, dissolved Re concentrations ([Re]diss) were measured by direct calibration against a set of seven standards with varying Re abundances and similar matrixes to river water, by quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS, Agilent Technologies 7900). Calibration standards and samples were doped with 0.025 mg/L concentration of internal standard Tb and Bi to correct for instrumental drift and matrix drift. Accuracy and precision of the measurements was assessed by repeated measurements of various riverine standard reference materials, in particular reference materials SLRS-5 and SLRS-6 at various dilutions. The standards confirmed better than 10% accuracy and precision. For sediment samples, the rhenium concentrations were determined using the method in ref. (55). A mass of 0.2 to 0.5g was digested using a mixture of 3 mL 27M HF and 3 mL 16M HNO3 for 24 to 48h at 120°C on a hot plate. Digested solutions were processed through AG1-X8 resin to separate Re from the rest of the matrix. Rhenium concentrations were then measured with a Neptune MC ICP-MS at Durham University.

**Dissolved Re yield calculations.** Several methods can be used to determine dissolved ion yields depending on the number of paired water discharge (Q) and concentration measurements, their frequency and the behavior of the element of interest in relation with discharge (56). Previous studies with large datasets of Re concentration and discharge (19, 20) have used a rating curve approach to quantify dissolved yields, by fitting power law functions to the trends in the data and using this relationship to predict predicted for each daily discharge value and annual fluxes. Other studies have used discharge-weighted average concentration (41) or average concentration of several measurements (4, 17, 18, 46) combined with annual water discharge estimates.

The four nested catchments in our dataset have between five and nine Q and [Re]diss data pairs for each catchment (Fig. 1C). For the large tributaries (Manu, Colorado, Chiribi, Alto Madre de Dios), our dataset includes 4 paired Q and [Re]diss measurements each. For all the sites, we use three different methods to calculate the Re yield and uncertainties: i) mean [Re]diss ±SE multiplied by annual discharge; ii) discharge-weighted mean [Re]diss ±SE multiplied by annual discharge; and iii) average of measured instantaneous Re fluxes, JRe ±SE. The advantage of the first method is that it includes more [Re]diss measurements (since the instantaneous discharge was not measured for all samples). The disadvantage is that there is a potential bias toward high concentration by not weighting to discharge. The third method is useful if annual discharge is not known, however it has the disadvantage of being less accurate because instantaneous fluxes vary more than instantaneous concentrations (56). Discharge values and uncertainties are reported in Dataset (S3). For the WAY and SP catchments, we use annual discharge values from Clark et al., (2014), determined over the year 2010. In addition, we use average annual discharge values dataset for the main tributaries of the Madre de Dios that were determined by water balance over the period 1968-1982 (57). They calculate a total discharge at the mouth of the Madre de
Dios (Riberalta) of 6369 m³.s⁻¹ which are very close (only 12% higher) to the discharge measured during the 2002–2011 period (5661 m³.s⁻¹; Vauchel et al., 2017). For CICRA, the discharge can be estimated by adding the annual discharge values of the Manu, Alto Madre de Dios, Colorado from Abastos Lara, (1987) and considering that the Rio Chiribi contributes 7.5% of the total discharge in CICRA (see Dataset S5). This gives a value of 2165 m³.s⁻¹. For instantaneous discharge of the main tributaries (Alto Madre de Dios, Manù, Chiribi and Colorado) that were measured or calculated during the 2013 and 2019 sampling trip, we use values from previous studies (34, 35).

Calculation of uncertainty is done by a Monte Carlo simulation (run 10 000 times for each watershed). As the calculated distributions follow skewed rather than normal distributions, we report the median value (50th percentile) with the uncertainty range defined by the 16th and 84th percentiles (equivalent to 68% of the entire population; SI Appendix). Comparison between the various methods shows good agreement between the different methods for the different sites within uncertainties (Dataset S3). The third method gives larger uncertainty compared to methods 1 and 2. This could reflect the lower number of samples used, the uncertainty on the discharge measurement or an imbalance between the discharge during the 1968-1982 period and the 2010-2020 period. When quantifying OCpetro oxidation fluxes from the dissolved Re flux for all sites, we use the second method (discharge-weighted [Re]diss) when possible and the first method (mean [Re]diss) otherwise.

**Source partitioning of dissolved rhenium.** To quantify the OCpetro oxidation flux using dissolved Re, we need to: i) correct for Re inputs from precipitation and/or atmospheric deposition; ii) quantify the Re input from non-OCpetro sources (i.e. sulfide, carbonate and silicate minerals). Solutes in precipitation can come from dissolution of sea salts, dust or biogenic particles. The [Re]/[Cl] ratio (~7.5×10⁻⁵ pmol.µmol⁻¹) of the ocean is very low (58) compared to the [Re]/[Cl] of rivers from the Madre de Dios. The [Cl]diss in rivers from this study are also low, indicating that the proportion of Re derived from sea salts is very small. While we have only one rainwater sample from the upper Andes, its [Re]diss was below detection limit of the measurement session (< 0.05 pmol.L⁻¹), supporting this conclusion. Among all the samples, the lowest measured [Re]diss and [Re]/[Cl] ratio correspond to a lysimeter sample in the Foreland (PER19-38) with values of 0.009 pmol.L⁻¹ and 1.4×10⁻³ pmol.µmol⁻¹ respectively. The [Re]/[Cl] ratio of this sample is ~20 times higher than the [Re]/[Cl] ratio of the ocean. The major element concentration for this sample is in the range of the average composition of rainwater from the Andes from Torres et al., (2015). Hence, we use this sample as representative of the maximum Re/Cl ratio of rainwater. For calculating the proportion of Re derived from each source we used the following mass-balance. For the contribution of the rain:

$$ [Re]_{rain} = \left( \frac{Re}{Cl} \right)_{rain} \times [Cl]_{cycl} \quad [1] $$

Where (Re/Cl)rain is the proposed elemental ratio between Re and the Cl in the rain (1.4×10⁻³ pmol.µmol⁻¹) and [Cl]cycl is the cyclic chlorine concentration. Since the marine evaporite contribution is small or negligible in the Madre de Dios catchment (23, 29), we consider that [Cl]cycl = [Cl]riv. The concentration of any element X corrected for rainwater and evaporite inputs is referred as “[X*]”. We find that the proportion of riverine Re deriving from rainfall is negligible, being generally less than 0.5%, with a maximum of 6%. For SO₄ rain concentration we use a (SO₄/Cl) value of 0.53 corresponding to the median value of precipitation data from Torres et al., (2015). This value is only slightly lower than the lowest (SO₄/Cl) measured in our dataset (sample PER19-38, value of 0.60).

We then move to quantify the proportion of dissolved rhenium derived from OCpetro oxidation relative to other potential Re sources (sulfides, silicates, carbonates). Previous work has suggested carbonates are not a major source of dissolved Re (18). If the Re/Ca of carbonates is ~5×10⁻⁵ pmol.mol⁻¹ (18), <1% of total dissolved Re in the studied rivers here can be accounted for by carbonate weathering (a maximum proportion using this Re/Ca ratio and assuming all dissolved Ca in the Madre de Dios catchments is derived from carbonates). We therefore follow the approach developed in Horan et al., (2019), that uses the (Re/SO₄)* and (Re/Na)* ratio to characterized Re input from sulfides, silicates and OCpetro.

Sulfides have low Re/S (Miller et al., 2011) and silicates low Re/Na relative to OCpetro (Horan et al., 2019) and both Na and SO₄ are conservative soluble species in the Madre de Dios catchment
(Baronas et al., 2017; Torres et al., 2017; Burt et al., 2021). Assuming that all the [SO₄²⁻] is derived from pyrite oxidation (i.e. no evaporite contribution, Torres et al., 2016), we estimate the Re concentration derived from sulfide oxidation:

$$ [Re]_{\text{diss.sulfides}} = \left(\frac{Re}{SO_4^{2-}}\right)_{\text{sulfides}} \times [SO_4^{2-}] \quad [2] $$

Where (Re/SO₄²⁻)sulfides is the sulfide composition. Then we can determine the concentration of Re deriving from silicate weathering ([Re]diss.sil) using sodium and assuming that all the [Na⁺] is derived from silicate weathering:

$$ [Re]_{\text{diss.sil}} = \left(\frac{Re}{Na}\right)_{\text{silicates}} \times [Na^+] \quad [3] $$

Where (Re/Na)sil is the silicate signature. Then we attribute the excess Re to the oxidation of OCpetro, calculated as:

$$ [Re]_{\text{diss.Oc}} = [Re^+] - [Re]_{\text{diss.sulfides}} - [Re]_{\text{diss.sil}} \quad [4] $$

In analogy with studies on silicate and carbonate weathering, we use small tributaries (e.g. Gaillardet et al., 1997; Galy and France-Lanord, 1999) and/or sediment composition (23, 44) to constrain local weathering end-members [Re]/[SO₄] and [Re]/[Na] ratio values (20). Small Andean tributaries draining sulfur-rich metasedimentary rocks display two orders of magnitude variability in [Re]/[SO₄²⁻] and [Re]/[Na⁺] ratios and are positively correlated. Several Andean rivers with low [Re]/[SO₄²⁻] values (Fig. 3) have similar composition to the median Re/S values of pyrite from the literature (1.8×4.2/-1.7 ×10⁻³ pmol.µmol⁻¹, Miller et al., 2011) with the lowest [Re]/[SO₄²⁻] ratio corresponding to a lysimeter sample (PER19-97) in the riparian area of a small catchment ([Re]*/[SO₄²⁻]*) = 2.2×10⁻⁴ pmol.µmol⁻¹) and a river sample (r2400) from another small catchment in the Andes ([Re]*/[SO₄²⁻]*) = 4.8×10⁻⁴ pmol.µmol⁻¹). They both have high SO₄ concentration indicating a high rate of sulfide oxidation. The [Re]/[SO₄²⁻] ratio of these samples is similar to the sulfide-oxidation rich sample in the Mackenzie basin (Horan et al., 2019). Hence, this suggests that the Re and SO₄ composition of these rivers is dominated by sulfide oxidation of pyrite and that they can be used as end-member values for (Re/SO₄)sulfides. The lowest (Re/SO₄) value (2.2×10⁻⁴ pmol.µmol⁻¹) measured here could represent the most “pure” sulfide oxidation end-member and higher [Re]/[SO₄²⁻] for other rivers would be explained by a small contribution of Re from rock organic carbon oxidation. Alternatively, it is possible that the Re/S from the local bedrock is variable and explains the range of [Re]/[SO₄²⁻] observed in rivers dominated by sulfide oxidation. A third possibility is that the very low [Re]/[SO₄²⁻] and [Re]/[Na⁺] of some samples is due to non-conservative behavior and removal of dissolved Re because some of these rivers have low pH values (as low as 3.5) and ReO₄⁻ is less stable under acidic conditions (60). However, this cannot explain the low [Re]/[SO₄²⁻] value of sample PER19-3 (3.5×10⁻³ pmol.µmol⁻¹) which has pH value of 8. Considering the above discussion we consider (Re/SO₄)sulfides ranging from 2×10⁻⁴ to 4×10⁻³ pmol.µmol⁻¹ that encompass the range of [Re]/[SO₄²⁻] of Andean rivers that we identified as typical of sulfide oxidation.

Rivers draining mostly granites (sulfide-poor and no OCpetro) have low Re/Na but an order of magnitude higher [Re]/[SO₄] ratios relative to Andean tributaries draining shales (Fig. 3). Interestingly, rivers and lysimeter samples from the foreland have similar composition as rivers draining granites. For Foreland rivers, especially at low elevations, deeply weathered soils and high weathering intensity probably leads to almost complete oxidation of pyrite due to its fast kinetics, and therefore we expect the chemical composition of those samples to be less influenced by OW of pyrite than Andean rivers. Altogether, rivers draining granite and the foreland have ratios that are inferred to reflect mostly mixing between weathering of silicates and weathering of OCrock. These observations are supported by river bed sediment samples. For Andean rivers, the [Re]/[Na] ratio of river bed sediments is similar to the low [Re]/[Na⁺] of small tributaries draining granites (lowest value is 2.2×10⁻³ pmol.µmol⁻¹ for sample PER19-26) and of samples from small first-order catchments draining the foreland (lowest value is 1.5×10⁻³ pmol.µmol⁻¹ for sample PER19-35). In addition, one rock sample, corresponding to an igneous
rock (a granophyre), has a very low [Re]/[Na] ratio of 4.3×10⁻⁴ pmol·µmol⁻¹, similar to the crystalline
rock endmember [Re]/[Na] ratio value in the Himalaya (2×10⁻⁴ pmol·µmol⁻¹, Ref. 18). Hence, we
consider here a [Re]/[Na] ratio between 4×10⁻⁴ and 2×10⁻³ pmol·µmol⁻¹ for silicates, which spans the
range of lowest [Re]/[Na] values from local solid and river samples.

Using Eq. (1-4) and the range of above defined (Re/SO₄)₃Sulfides and (Re/Na)₃Silicates values for our
study area, we can calculate [Re]ₐd (**diss.O**C for each sample. The calculation of [Re]ₐd(**diss.O**C is done by a Monte
Carlo simulation (run 10 000 times for each river) assuming random distribution of (Re/SO₄)₃Sulfides values
between 2×10⁻⁴ and 4×10⁻³ pmol.mol⁻¹ and (Re/Na)₃Silicates values between 4×10⁻⁴ and 2×10⁻³ pmol.µmol⁻¹
(1 (SI Appendix; Dataset S4). Fractions of dissolved Re derived from OCₚetro OW (fₑ=[Re]ₐd.OC/[Re]ₐd) are reported in Dataset S3.

Floodplain mass-balance of dissolved Re. The section of the Alto Madre de Dios river between MLC (mountain front) and AMdD (confluence with Manu River) has relatively minimal tributary input. Over three sampling trips, we observe an increase in the Re concentration between MLC and AMdD by 3% (March 2013), 26% (March 2019) and 12% (May 2019). There is also a systematic increase in Re/Na and Re/SO₄ ratio. The Re concentration of sub-Andean tributaries is about 0.8 ppt, which is higher than the Re concentration at MLC and could explain part of the observed increase. The contribution of water and weathering inputs from sub-Andean-foreland tributaries can be assessed using δD and δ¹⁸O since tributaries (PER19-53, Rio Carbon, Rio Pini Pini) have higher δD (-54.9 to -70.2‰) and δ¹⁸O values (-8.2 to -10.4 ‰) compared to the Alto Madre de Dios at MLC (δD ~ -72.5‰ and δ¹⁸O ~ -10.6‰). The δD and δ¹⁸O of the Alto Madre de Dios does not change significantly which indicates that any Re contribution from these tributaries is too small to explain the observed 26% increase in Re concentration in 2019. Therefore, we conclude that this increase is due to ongoing OCₚetro oxidation of sediment during transit and/or within the floodplain between MLC and AMdD.

627

Acknowledgments

This research was funded by an ERC Starting Grant – ROC-CO2 (678779) to R. G. Hilton, by
a European Union COFUND/Durham Junior Research Fellowship (267209) to M. Dellinger
and by a National Science Foundation award EAR-1455352 to A.J. West. We thank Amazon
Journeys Nature Lodges and Amazon Conservation Association (Peru) for field support. We
thank Martin West for laboratory support in Durham and we thank Julien Bouchez for providing
the two Madre de dios samples collected at Riberalta. We thank William Santini for discussions
on existing discharge data. We thank Kim Genuite for his help in designing figure 4.

636

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<figure><img src="image_3.png" /><figcaption>**Figure 1.** (A) Map of the Madre de Dios River basin with the location of the samples from this study. (B) Inset showing the location of the samples in the High Andes. (C) Re concentration (in pmol.L⁻¹) in the various geomorphic settings for all river samples from this study. The [Re]diss from the two hot springs samples are not shown. (D) [Re]diss versus instantaneous runoff W (water discharge normalized by catchment area, mm day⁻¹) at the time of sampling for the main nested catchments of this study (WAY, SP, MLC and CICRA).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** (A) Rhenium concentration in solids ([$Re_{solids}$]: rock samples (red stars), bedloads (red box) and soils (yellow box). The middle line in boxes is the average value, upper and lower box limits correspond to the ±2SE values, the upper and higher bars represent extreme data points. (B) Average ± 2SE and extreme values of the [$Re$]/[$OC$] ratio in bedloads and rocks.</figcaption></figure>

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<figure><img src="image_5.png" /><figcaption>**Figure 3.** Rhenium to sodium, [Re]/[Na*], and rhenium to sulfate, [Re]/[SO₄*], ratios (pmol µmol⁻¹) for river waters from this study (High Andes = triangles; Andean Mountain front = square; Foreland = diamond; circles = Foreland-Floodplains). For the four nested-catchments, only the average values are represented. The red crosses correspond to the river sediments (bedloads) and bedrocks from the Kosnipata River basin. The shaded ovals show the ranges of elemental ratios associated with the rock weathering end-members (sulfides, silicates and OCpetro). The lines correspond to the mixing proportions between the rock weathering end-members, with the average proportion of Re derived from OCpetro weathering shown (in %).</figcaption></figure>

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<figure><img src="image_6.png" /><figcaption>**Figure 4.** (A) Estimated net contribution (vertical arrows) of each geomorphic zones to the total flux of OC<sub>petro</sub> oxidation and associated CO<sub>2</sub> release at the scale of the whole Madre de Dios River basin. MLC=Mountain Front, AMdD=Alto Madre de Dios, PM=Madre de Dios at Puerto Maldonado, Ribe=Madre de Dios at Riberalta. The red squiggly line corresponds to the 500m elevation. (B) Ratio between the measured dissolved rhenium flux derived from OC<sub>petro</sub> oxidation (J<sub>Re-OC meas</sub>) and the predicted flux (J<sub>Re-OC calc</sub>) using the tributary mixing model (SI Appendix) as a function of the channel distance from MLC. A value >1 can be interpreted as OC<sub>petro</sub> oxidation in the floodplain. Vertical bars represent the uncertainties. This studied floodplain transect correspond to the channel highlighted in light blue on panel A.</figcaption></figure>

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