The acid and alkalinity budgets of weathering in the Andes–Amazon system: Insights into the erosional control of global biogeochemical cycles

Torres, Mark A.; West, A. Joshua; Clark, K. E.; Paris, Guillaume; Bouchez, Julien; Ponton, C.; Feakins, Sarah J.; Galy, Valier; Adkins, Jess F.

doi:10.1016/j.epsl.2016.06.012

Cited by 131 publications

(232 citation statements)

References 43 publications

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“…Some model solutions include seemingly unlikely but not altogether impossible combinations, for example, the majority of cations sourced from precipitation and evaporites simultaneous with the majority of SO 4 sourced from sulfides. Further work partitioning solute sources rigorously by individual catchment, for example, including constraints from S isotopes (17,24), will be important to more thoroughly test the probabilistic inferences made here. Nonetheless, our data suggest that presentday glacial systems are characterized by more sulfide oxidation and carbonate weathering than nonglacial systems, consistent with prevailing interpretations (13).…”

Section: Resultsmentioning

confidence: 99%

“…The Alk:DIC ratio allows us to evaluate the net effect of weathering on atmospheric pCO 2 over different timescales (17). The pCO 2 of the atmosphere and ocean are coupled by gas exchange and, through carbonate equilibria, are set by the ratio of dissolved alkalinity to DIC concentrations in the oceans (Fig.…”

Section: Resultsmentioning

confidence: 99%

“…We determine the Alk:DIC of weathering in each catchment on the basis of the equivalents of cations released for combinations of reactions, coupling acid-generating reactions (sulfide and CO 2 dissolution) with acid-consuming reactions (silicate and carbonate dissolution) (17) (Fig. 3A).…”

Section: Resultsmentioning

confidence: 99%

“…Thus, even a small proportion of sulfide or carbonate present in rocks can dominate solute production (22, 23). Indeed, weathering of these trace phases is predominantly "supply limited" in the present day, so fluxes increase with erosion that supplies more minerals for reaction (17,24). In contrast, silicate weathering is not always limited by supply of weatherable minerals (25).…”

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confidence: 99%

“…Studies of glacial hydrochemistry have pointed to the importance of sulfide oxidation and carbonate dissolution as major solute sources (13)(14)(15). Because the residence time of sulfate in the oceans is long relative to Ca, sulfide-carbonate weathering can act to release CO 2 to the ocean-atmosphere system over timescales of <100 My (16,17). Sulfide oxidation also influences global budgets of redox-sensitive elements including sulfur, iron, and oxygen (16,18,19).…”

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Glacial weathering, sulfide oxidation, and global carbon cycle feedbacks

Torres

Moosdorf

Hartmann

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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Connections between glaciation, chemical weathering, and the global carbon cycle could steer the evolution of global climate over geologic time, but even the directionality of feedbacks in this system remain to be resolved. Here, we assemble a compilation of hydrochemical data from glacierized catchments, use this data to evaluate the dominant chemical reactions associated with glacial weathering, and explore the implications for long-term geochemical cycles. Weathering yields from catchments in our compilation are higher than the global average, which results, in part, from higher runoff in glaciated catchments. Our analysis supports the theory that glacial weathering is characterized predominantly by weathering of trace sulfide and carbonate minerals. To evaluate the effects of glacial weathering on atmospheric pCO 2 , we use a solute mixing model to predict the ratio of alkalinity to dissolved inorganic carbon (DIC) generated by weathering reactions. Compared with nonglacial weathering, glacial weathering is more likely to yield alkalinity/DIC ratios less than 1, suggesting that enhanced sulfide oxidation as a result of glaciation may act as a source of CO 2 to the atmosphere. Back-of-the-envelope calculations indicate that oxidative fluxes could change ocean-atmosphere CO 2 equilibrium by 25 ppm or more over 10 ky. Over longer timescales, CO 2 release could act as a negative feedback, limiting progress of glaciation, dependent on lithology and the concentration of atmospheric O 2 . Future work on glaciation-weathering-carbon cycle feedbacks should consider weathering of trace sulfide minerals in addition to silicate minerals.weathering | carbon cycle | glaciation | oxidation | sulfide E pisodes of glaciation represent some of the most dramatic changes in the history of Earth's climate. Basic questions remain unanswered about why the planet has occasionally but not always supported glaciers, about why some glaciations have been more intense than others, and about how glaciers have shaped Earth's landscapes, chemical cycles, and climate. Before the Phanerozoic (i.e., before ∼540 Mya), several glaciations are thought to have approached complete or near-complete planetary ice cover ("Snowball Earth" events; ref. 1). In contrast, more recent glaciations, including the Quaternary glaciations of the last million years, have been characterized by oscillating climate with ice extent limited to high latitudes even during glacial maxima (2). The formation of ice cover has long been known to fundamentally alter Earth's environment, changing albedo (3), influencing atmospheric and ocean circulation (4), and reshaping landscapes (5). These diverse effects may contribute to planetary-scale feedbacks that influence the transition to and the evolution of glaciation.Chemical weathering influences the composition of the atmosphere by releasing ionic species from minerals that alter the alkalinity and redox condition of the ocean-atmosphere system. Weathering of silicate minerals produces alkalinity, removing carbon from the at...

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Glacial weathering, sulfide oxidation, and global carbon cycle feedbacks

Torres

Moosdorf

Hartmann

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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157

171

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Erosion of organic carbon from the Andes and its effects on ecosystem carbon dioxide balance

et al. 2017

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Productive forests of the Andes are subject to high erosion rates that supply to the Amazon River sediment and carbon from both recently photosynthesized biomass and geological sources. Despite this recognition, the source and discharge of particulate organic carbon (POC) in Andean Rivers remain poorly constrained. We collected suspended sediments from the Kosñipata River, Peru, over 1 year at two river gauging stations. Carbon isotopes (14C, 13C, and 12C) and nitrogen to organic carbon ratios of the suspended sediments suggest a mixture of POC from sedimentary rocks (POCpetro) and from the terrestrial biosphere (POCbiosphere). The majority of the POCbiosphere has a composition similar to surface soil horizons, and we estimate that it is mostly younger than 850 14C years. The suspended sediment yield in 2010 was 3500 ± 210 t km−2 yr−1, >10 times the yield from the Amazon Basin. The POCbiosphere yield was 12.6 ± 0.4 t C km−2 yr−1 and the POCpetro yield was 16.1 ± 1.4 t C km−2 yr−1, mostly discharged in the wet season (December to March) during flood events. The river POCbiosphere discharge is large enough to play a role in determining whether Andean forests are a source or sink of carbon dioxide. The estimated erosional discharge of POCpetro from the Andes is much larger (~1 Mt C yr−1) than the POCpetro discharge by the Madeira River downstream in the Amazon Basin, suggesting that oxidation of POCpetro counters CO2 drawdown by silicate weathering. The flux and fate of Andean POCbiosphere and POCpetro need to be better constrained to fully understand the carbon budget of the Amazon River basin.

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Mixing as a driver of temporal variations in river hydrochemistry: 2. Major and trace element concentration dynamics in the Andes‐Amazon transition

Baronas

Torres

Clark

et al. 2017

Water Resources Research

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Variations in riverine solute chemistry with changing runoff are used to interrogate catchment hydrology and to investigate chemical reactions in Earth's critical zone. This approach requires some understanding of how spatial and temporal averaging of solute‐generating reactions affect the dissolved load of rivers and streams. In this study, we investigate the concentration‐runoff (C‐Q) dynamics of a suite of major (Na, Mg, Ca, Si, K, and SO4) and trace (Al, Ba, Cd, Co, Cr, Cu, Fe, Ge, Li, Mn, Mo, Nd, Ni, Rb, Sr, U, V, and Zn) elements in nested catchments of variable size, spanning the geomorphic gradient from the Andes Mountains to the Amazon Foreland‐floodplain. The major elements exhibit various degrees of dilution with increasing runoff at all sites, whereas the concentrations of most trace elements either increase or show no relationship with increasing runoff in the three larger catchments (160–28,000 km2 area). We show that the observed main stem C‐Q dynamics are influenced by variable mixing of tributaries with distinct C‐Q relationships. Trace element C‐Q relationships are more variable among tributaries relative to major elements, which could be the result of variations in geomorphology, lithology, and hydrology of the subcatchments. Certain trace metals are also lost from solution during in‐channel processes (possibly related to colloidal size‐partitioning), which may exert an additional control on C‐Q dynamics. Overall, we suggest that tributary aggregation effects should be assessed in heterogeneous catchments before C‐Q or ratio‐Q relationships can be interpreted as reflecting catchment‐wide solute generation processes and their relationship to hydrology.

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The acid and alkalinity budgets of weathering in the Andes–Amazon system: Insights into the erosional control of global biogeochemical cycles

Cited by 131 publications

References 43 publications

Glacial weathering, sulfide oxidation, and global carbon cycle feedbacks

Glacial weathering, sulfide oxidation, and global carbon cycle feedbacks

Erosion of organic carbon from the Andes and its effects on ecosystem carbon dioxide balance

Mixing as a driver of temporal variations in river hydrochemistry: 2. Major and trace element concentration dynamics in the Andes‐Amazon transition

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