Glacial Weathering, Sulfide Oxidation, and Global Carbon Cycle Feedbacks

Glacial weathering, sulfide oxidation, and global carbon cycle feedbacks

Mark A. Torresa,b,c,1, Nils Moosdorfa,d,e,1, Jens Hartmannd, Jess F. Adkinsb, and A. Joshua Westa,2

aDepartment of Earth Sciences, University of Southern California, Los Angeles, CA 90089; bDivision of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125; cDepartment of Earth, Environmental, and Planetary Sciences, Rice University, Houston, TX 77005; dInstitute for Geology, Center for Earth System Research and Sustainability (CEN), Universität Hamburg, 20146 Hamburg, Germany; and eLeibniz Center for Tropical Marine Research, 28359 Bremen, Germany

Edited by Thure E. Cerling, University of Utah, Salt Lake City, UT, and approved July 5, 2017 (received for review February 21, 2017) Connections between glaciation, chemical weathering, and the global erosion (5, 8, 9), which is thought to promote more rapid silicate carbon cycle could steer the evolution of global climate over geologic weathering (10, 11). If glaciation increases erosion rates, and if time, but even the directionality of feedbacks in this system remain to erosion enhances silicate weathering and associated production of be resolved. Here, we assemble a compilation of hydrochemical data alkalinity, then glacial weathering could act as a positive feedback from glacierized catchments, use this data to evaluate the dominant serving to promote further glaciation (5, 12). chemical reactions associated with glacial weathering, and explore the Glaciation may also affect rates of weathering of nonsilicate implications for long-term geochemical cycles. Weathering yields from minerals. Studies of glacial hydrochemistry have pointed to the im- catchments in our compilation are higher than the global average, portance of sulfide oxidation and carbonate dissolution as major which results, in part, from higher runoff in glaciated catchments. solute sources (13–15). Because the residence time of sulfate in the Our analysis supports the theory that glacial weathering is character- oceans is long relative to Ca, sulfide–carbonate weathering can act to ized predominantly by weathering of trace sulfide and carbonate min- release CO2 to the ocean–atmosphere system over timescales erals. To evaluate the effects of glacial weathering on atmospheric of <100 My (16, 17). Sulfide oxidation also influences global budgets pCO2, we use a solute mixing model to predict the ratio of alkalinity of redox-sensitive elements including sulfur, iron, and oxygen (16, 18, to dissolved inorganic carbon (DIC) generated by weathering reactions. 19). Although sulfide and carbonate minerals are usually present at Compared with nonglacial weathering, glacial weathering is more low abundance in most rocks, their rates of dissolution are typically likely to yield alkalinity/DIC ratios less than 1, suggesting that enhanced EARTH, ATMOSPHERIC,

orders of magnitude faster than for silicate minerals. For example, AND PLANETARY SCIENCES sulfide oxidation as a result of glaciation may act as a source of CO2 to log rates from laboratory experiments are −6to−10 for pyrite (20), the atmosphere. Back-of-the-envelope calculations indicate that oxida- vs. log rates of −10 to −16 for silicate minerals (21). Thus, even a tive fluxes could change ocean–atmosphere CO2 equilibrium by 25 ppm small proportion of sulfide or carbonate present in rocks can dom- or more over 10 ky. Over longer timescales, CO2 release could act as a inate solute production (22, 23). Indeed, weathering of these trace negative feedback, limiting progress of glaciation, dependent on lithol- phases is predominantly “supply limited” in the present day, so fluxes ogy and the concentration of atmospheric O2. Future work on glacia- increase with erosion that supplies more minerals for reaction (17, tion–weathering–carbon cycle feedbacks should consider weathering 24). In contrast, silicate weathering is not always limited by supply of of trace sulfide minerals in addition to silicate minerals. weatherable minerals (25). As a result, if glaciation enhances physical erosion rates, the increased exposure of fresh rock material weathering | carbon cycle | glaciation | oxidation | sulfide containing unweathered minerals should drive increased sulfide and carbonate weathering fluxes, but may not drive increased silicate pisodes of glaciation represent some of the most dramatic weathering fluxes in tandem. Thus, the net effect of glaciation could Echanges in the history of Earth’s climate. Basic questions be to supply CO2 to the atmosphere, rather than to remove it. remain unanswered about why the planet has occasionally but not always supported glaciers, about why some glaciations have been Significance more intense than others, and about how glaciers have shaped Earth’s landscapes, chemical cycles, and climate. Before the Phan- We compile data showing that, as hypothesized previously, waters erozoic (i.e., before ∼540 Mya), several glaciations are thought to draining glaciers have solute chemistry that is distinct from non- have approached complete or near-complete planetary ice cover glacial rivers and reflects different proportions of mineral weath- (“Snowball Earth” events; ref. 1). In contrast, more recent glacia- ering reactions. Elevated pyrite oxidation during glacial weathering tions, including the Quaternary glaciations of the last million years, could generate acidity, releasing carbon to the atmosphere. We have been characterized by oscillating climate with ice extent lim- show that this effect could contribute to changes in CO2 during ited to high latitudes even during glacial maxima (2). The formation glacial cycles of the past million years. Over the longer, multimillion- of ice cover has long been known to fundamentally alter Earth’s year timescales that Earth transitions into and out of glaciated environment, changing albedo (3), influencing atmospheric and states, sustained addition of pyrite-derived sulfate to the oceans ocean circulation (4), and reshaping landscapes (5). These diverse could shift the balance of the global carbon cycle toward increasing – effects may contribute to planetary-scale feedbacks that influence CO2 in the ocean atmosphere, thus providing a negative-feedback the transition to and the evolution of glaciation. mechanism preventing runaway glaciation. This mechanism de- Chemical weathering influences the composition of the atmo- pends on oxidation and thus sufficient O2. sphere by releasing ionic species from minerals that alter the al- – Author contributions: N.M., J.H., and A.J.W. designed research; M.A.T., N.M., and J.H. kalinity and redox condition of the ocean atmosphere system. performed research; M.A.T. and J.F.A. contributed new reagents/analytic tools; M.A.T., Weathering of silicate minerals produces alkalinity, removing N.M., J.H., J.F.A., and A.J.W. analyzed data; and M.A.T. and A.J.W. wrote the paper. carbon from the atmosphere in the process (6). If glaciers reduce The authors declare no conflict of interest. weathering fluxes from land area that was previously undergoing This article is a PNAS Direct Submission. silicate weathering, glaciation may decrease global alkalinity pro- 1M.A.T. and N.M. contributed equally to this work. duction, allow atmospheric CO2 levels to rise, and thus generate a 2To whom correspondence should be addressed. Email: [email protected]. negative feedback that ends glaciation (7). On the other hand, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. some evidence suggests that glaciation increases rates of physical 1073/pnas.1702953114/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1702953114 PNAS Early Edition | 1of6 Downloaded by guest on October 1, 2021 The directionality of long-term glaciation–weathering feed- proportion of glacial cover and the sampling distance from the backs depends on the extent to which glacial weathering is a net glacial snout (SI Appendix,TableS3). Subglacial weathering fluxes source or net sink of CO2 to the atmosphere, which in turn de- also depend on hydrology, for example, differing for warm- vs. cold- pends on the balance of sulfide vs. silicate mineral weathering. A based ice (35, 36) and along different flowpaths (37), and these growing number of studies have explored various aspects of differences may explain some of the variability within the dataset. glacial weathering by analyzing the chemical composition of glacial runoff, often focusing on specific case studies, and in Distinct Stoichiometry of Glacial Weathering. Our compilation provides some cases synthesizing global datasets (13, 26, 27). Here, we the opportunity to test the hypothesis that the solute stoichiometry assemble an expanded compilation of glacial weathering data of glaciated rivers is distinct from nonglaciated rivers, because the and use this compilation to identify characteristic features of large number of data points allows for statistical comparison be- present-day glacial weathering, including the relative roles of tween distributions. Although we do not directly evaluate temporal sulfide-carbonate vs. silicate mineral weathering compared with variability, dissolved major element concentrations in river waters rivers not dominated by glacial processes. We then consider the vary only slightly over time (38) relative to differences between possible implications for global-scale fluxes in the Quaternary catchments. Dissolved K:Na ratios, which reflect the stoichiometry carbon cycle and for planetary feedbacks over longer timescales. of silicate mineral dissolution, are shifted to higher values in the glacial dataset compared with either the world’s large rivers or the Materials and Methods GloRiCh dataset (distributions are significantly different at the P < We compiled a database containing stream chemistry data from glaciated 0.05 level via a Kolmogorov–Smirnov test), consistent with results of catchments, together with spatial data describing the contributing catch- previous studies and suggesting changes in the stoichiometry of ments (SI Appendix, Fig. S1, and Datasets S1 and S2). The dataset is based on weathering-derived solutes driven by glaciation (15, 39). solute concentrations for 7,700 samples, from 95 glaciated catchments Ca:Na and SO :Na ratios (Fig. 2) are also statistically different (at > 4 (glacial cover proportion 0.4%, and in most cases much higher; Fig. 1) with P < 0.05 level) when comparing our glacial database vs. either reported concentrations of Ca, Mg, Na, and K for at least one sample. nonglacial dataset. In contrast, when comparing between the two Measured discharge was available for 52 of these catchments. The contri- ’ bution from chemical weathering to the measured stream chemistry was nonglacial datasets (GloRiCh vs. the world s large rivers), Ca:Na determined following correction for precipitation inputs, as detailed in SI and SO4:Na ratios are not statistically different. Glacial solute Appendix. To provide a basis for comparing the glacial weathering data, we geochemistry is significantly distinct even after subsampling the refer to the chemical composition of the world’s large rivers (28), which GloRiCh database for rivers with the same range of catchment provide a spatially integrated view of weathering processes over continental areas as the glacial database (SI Appendix,Fig.S3A and B). Glacial areas and reflect the composition of solutes delivered to the oceans. For SO4:Na ratios are weakly correlated with the distance from the reference, we also consider data from the Global River Chemistry (GloRiCh) glacier snout (SI Appendix,Fig.S3C) and do not depend on database of solute chemistry from 1.27 million water samples from catchment size (SI Appendix,Fig.S3A) or the proportion of glacial ∼15,500 catchments around the world (29) (Dataset S3). cover (SI Appendix,Fig.S3D). So, although additional weathering Results may occur in the proglacial environment (Fig. 1), it does not appear SI Appendix General Characteristics of Glacial Weathering. Themeanofthesumof to be a major control on elemental ratios ( ,Fig.S3). cation concentrations (Ca plus Mg plus Na plus K) in the 95 glaci- Idealized versions of the reactions expected to generate dis- − − erized catchments is 530 μmol·L 1, decreasing to 453 μmol·L 1 after solved Ca, Na, SO4 are summarized as follows: correction for atmospheric inputs. The weathering-derived solutes + + ↔ 2+ + [1] are dominated by Ca (58.6 mol %, SI Appendix,TableS2). Mg and Carbonate dissolution: 2H CaCO3 Ca H2CO3, Na contribute 17.5% and 17.4%, respectively, followed by K + 2+ (6.5%). For the 54 glacial catchments with sufficient data to esti- Silicate dissolution: 4H + Ca2SiO3 ↔ 2Ca + H4SiO4, [2] mate dissolved pCO2 using PHREEQC (Dataset S1), 45 are su- persaturated relative to the atmosphere (median pCO2 for the + 2− Carbonic acid: CO ð Þ + H O ↔ H CO ↔ 2H + CO , [3] glacial catchments, 692 ppm; mean, 4,029 ppm). 2 diss 2 2 3 3 For the 52 catchments with available runoff data, mean runoff is −2 −1 1,730 L·m ·y , which is 5.8 times above the global average river Sulfuric acid from sulfide oxidation: FeS2 + 15O2 + 14H2O runoff (30). The mean cation weathering yield from the glacial + 2− − − ↔ 16H + 8SO + 4FeðOHÞ . catchments is 21.5 t·km 2·y 1. For comparison, the mean cation yield 4 3 − − from the world’s large rivers is 18 t·km 2·y 1, with a corresponding [4] − mean cation concentration of 2,433 μmol·L 1 (31). Thus, relative to global average river water, glacial catchments are characterized by Evaporites may also contribute Ca and SO4. Assuming glacia- below average cation concentrations but above average cation yields, tion is not associated with preferential evaporite weathering (an duetohighrunoff(26). Within the glacial dataset, precipitation-corrected cation concentrations are positively correlated with the proportion of AB carbonate-rich sediment in the catchments (r = 0.26, SI Appendix, ) Dominant lithology -1 100 (>70% catchment area) Table S3) and negatively with coverage of metamorphic rocks 2.0 Trendline: yr Trendline: Siliclastic sediment r = − r = − -2 Carbonate-rich sediment ( 0.24) and glacial cover ( 0.27). The cation yield of the r = -0.31; 75 r = -0.33; Metamorphic rock r = 1.5 p < 0.01 p = 0.02 Volcanic rock observed catchments is correlated with runoff ( 0.28), but this Plutonic rock correlation is weaker than reported for nonglaciated catchments 1.0 50 No dominant lithology (32, 33). Correlations between lithology classes and cation yield are Corrected cation 0.5 25

generally insignificant, although some significant correlations can be concentration (mmol/L)

SI Appendix 0 Weathering rate (t km 0 seen for individual elements ( ,TableS3). Additional 0 2550751000 255075100 weathering of glacially derived sediment is thought to take place in Glacial cover (%) Glacial cover (%) proglacial environments (34). Consistent with this hypothesis, cation Fig. 1. Scatter plots of (A) cation concentration corrected for precipitation yields in our compilation show considerable scatter but indicate a input (*), and (B) total cation weathering flux, both plotted against glacial slight increase with the proportion of glacial cover (Fig. 1). Of all cover. Fluxes do not decrease to zero as glacial cover approaches complete analyzed variables, cation yield is most highly correlated with the cover, indicating significant weathering under ice.

2of6 | www.pnas.org/cgi/doi/10.1073/pnas.1702953114 Torres et al. Downloaded by guest on October 1, 2021 log (Ca/Na) that would not act as CO2 sources. We have no a priori reason to 10 -2 -1 0 1 2 expect that such effects dominate globally. Nonetheless, fully A understanding the net effect of glaciation on the carbon cycle 2 depends on quantifying (i) the proportion of the dissolved sulfate derived from sulfide mineral oxidation, and (ii) the extent to 4 1 which this oxidative flux of protons (Eq. ) is balanced by alkalinity generation from silicate and carbonate weathering (Eqs. 1 and 2). To approach these questions quantitatively, we use a 0 / Na) mixing model to estimate the relative contribution of different 4 weathering reactions to the solute load for each catchment in our

(SO -1 compilation. The model uses Na-normalized elemental ratios 10 (Cl:Na, Ca:Na, and Mg:Na) to partition the dissolved load be- SO / Na log -2 1:1 tween silicate, carbonate, evaporite, and precipitation sources Mean / Median SI Appendix GloRiCh (e.g., ref. 40 and details in ). We apply our model to 0.9 / 0.4 ’ -3 Glacial all samples from the world s large river database (28) and a subset 3.7 / 1.0 n = 0.5 / 0.4 World of samples from the glacial ( 84) database due to the data requirements. We do not consider the GloRiCh data because the 2 nonuniform sampling distribution in this database (e.g., multiple B Carbonate samples from many small catchments in one region) compli- cates interpretation of results from our probabilistic approach, 1 described below. We do not precisely know the end-member ratios appropriate for each catchment within the large river and glacial catchment datasets, and these ratios may be modified by secondary pro- 0 Silicate cesses. Thus, we cannot calculate a single model result for each (Mg/Na) catchment. Instead, we randomly sample across a uniform dis- 10 tribution of possible end-member values (e.g., Fig. 2B), repeating log -1 for 10,000 combinations. For each combination, we use the frac- Mg / Na EARTH, ATMOSPHERIC, tional contribution from each solute source to calculate a ratio of AND PLANETARY SCIENCES Mean / Median Evaporite alkalinity to dissolved inorganic carbon equivalents (Alk:DIC) 1.2 / 0.5 -2 produced by weathering reactions. 2.6 / 0.8 The Alk:DIC ratio allows us to evaluate the net effect of 0.8 / 0.7 weathering on atmospheric pCO2 over different timescales (17). Ca / Na The pCO2 of the atmosphere and ocean are coupled by gas ex- Mean / Median change and, through carbonate equilibria, are set by the ratio of 2.9 / 1.1 dissolved alkalinity to DIC concentrations in the oceans (Fig. 3). 9.3 / 4.3 The ocean ratio is, in turn, determined by the supply of alkalinity 0.8 / 0.7 and DIC from weathering reactions and their removal through the burial of carbonate minerals and organic carbon. Carbonate Fig. 2. Stoichiometry of dissolved ions in the glacial data compilation presented 1 in this study (blue points, and blue dash-dot curves in histograms), the world’slarge removal is fixed at an Alk:DIC of 1:0.5 (Eq. ; 1:0.5 line in Fig. rivers (red points, and red dashed curves; data from ref. 28), and the GloRiCh 3). The Alk:DIC of weathering varies, from 0 to infinity. Over the 4 database of 17,000 nonglaciated catchments (gray points, and solid gray lines; ref. timescales characteristic of carbonate burial (greater than ∼10 y),

29). (A)SO4/Na vs. Ca/Na molar ratios, with population histograms (normalized to weathering will change the Alk:DIC ratio of the oceans (and thus number of catchments) of both ratios shown along each axis (reported mean and pCO2) if it supplies alkalinity and DIC in a ratio that deviates median values are not log-transformed). (B) Ca/Na vs. Mg/Na ratios, with the range from the 1:0.5 associated with carbonate burial (Fig. 3). Thus, of each lithological end-member illustrated on the figure and the values listed in SI silicate weathering by carbonic acid (supplying Alk:DIC at a Appendix,TableS4. The GloRiCh data illustrate that our choices of end-members 1:0 ratio, higher than 1:0.5) will consume CO , whereas car- bracket most river compositions. 2 bonate weathering by carbonic acid (1:0.5) will have no net effect (6, 41). Carbonate weathering by sulfuric acid (0:0.5 ratio) will release CO2 over tens of million years until sulfate is reduced in assumption we test below), elevated SO4:Na ratios during glacial marine sediments, returning alkalinity (reverse of Eq. 4; ref. 16). weathering suggest relatively more sulfide mineral oxidation com- On timescales shorter than required for the output flux via car- pared with silicate weathering in glacial catchments, and elevated bonate precipitation to respond to changes in input fluxes (103 to 4 Ca:Na ratios suggest more carbonate weathering than silicate 10 y), pCO2 will increase if weathering inputs have an Alk:DIC weathering. These observations are consistent with previous studies lower than the ocean (Fig. 3), which is presently approximately emphasizing the role of sulfide and carbonate mineral weathering in equal to 1, and vice versa. We thus evaluate model results with glacial catchments, even when these phases are minor constituents respect to Alk:DIC thresholds of 1 (characteristic of short – of the bedrock (e.g., refs. 13 15 and 26). timescales) and 2 (long timescales). CO2 is released if weathering occurs with a ratio below these thresholds; CO2 is consumed Glacial Weathering as Net CO2 Source. Higher dissolved ion yields and above these thresholds. higher SO4:Na and Ca:Na ratios together suggest that sulfide and We determine the Alk:DIC of weathering in each catchment on carbonate dissolution rates are enhanced in association with glacial the basis of the equivalents of cations released for combinations of weathering. To first order, glaciation thus appears to act as a CO2 reactions, coupling acid-generating reactions (sulfide and CO2 source, by maintaining total solute fluxes while shifting weathering dissolution) with acid-consuming reactions (silicate and carbonate toward reactions that release rather than consume CO2. dissolution) (17) (Fig. 3A). We use the proportions of solutes In principle, glacially enhanced Ca and SO4 fluxes could de- from carbonate, silicate, and sulfide weathering, as inferred from rive from preferential dissolution of sulfate-bearing evaporites the end-member mixing analysis, to tabulate the equivalents of or weathering of Ca-bearing silicates by sulfuric acid, reactions each reaction required to produce the solute chemistry. Protons

Torres et al. PNAS Early Edition | 3of6 Downloaded by guest on October 1, 2021 Contours = log( pCO ) ( atm) 1:1 Discussion A B 2.5 2 3.80 3.55 3.30 Why Increased Oxidation Fluxes from Glacial Weathering. The dis- 3.05 ) 2.4 tinct geochemistry of glacial weathering might be coincidental;

Reaction (Alk:DIC) -1 i. Silicate + Carbonic (1:0) 2.80 glaciers in the present day are typically found in steep, rapidly 2.3 iv ii. Silicate + Sulfuric (0:0) iii 2.55 eroding mountainous regions (8), where fluxes from sulfide oxida- iii. Carbonate + Carbonic (1:0.5) 2.2 i tion are relatively high independent of glaciation (16). Although our iv. Carbonate + Sulfuric (0:0.5) 2.31 global analysis cannot conclusively distinguish causality, we think it

v. Carbonate Burial (-1:-0.5) DIC (mmol kg v 2.1 is more likely that high sulfide weathering fluxes are the result of

2 glaciation, driven by glacial enhancement of erosion. Several studies 2 2.1 2.2 2.3 2.4 2.5 have proposed that glaciation increases erosion rates (5). Because -1 Alkalinity (mEq kg ) sulfide weathering is typically supply limited, increases in erosion rate as expected from glaciation should increase oxidation fluxes Fig. 3. Alk:DIC framework for evaluating effect of weathering on pCO2. (A) Tabulation of Alk:DIC associated with different reaction combinations, (17, 24). It is also possible that sediment production and commi- after normalization by the charge equivalents of cations released. (B) Con- nution during glacial retreat (8, 9) exposes fresh sulfides and facil-

tours of oceanic pCO2 (equivalent to atmospheric pCO2 over timescales of itates their oxidation, even if longer-term rates of erosion (and thus gas exchange) as a function of alkalinity and DIC, calculated using CO2sys of sulfide oxidation) are not elevated as a result of glaciation. In this (67). Dashed black line shows 1:1 addition of Alk:DIC (stoichiometry of latter case, high sulfide oxidation rates in the present day may re- constant pCO2 over timescales shorter than the response time of the ocean flect a transient postglacial increase but would not be sustained over carbonate system). Vectors correspond to combinations of reactions in A. timescales >105 y. High present-day sediment fluxes from glacierized catchments (8) and the oscillations in the Os isotope com- – not derived from sulfide oxidation (Eq. 4) are assumed to be position of seawater over Quaternary glacial interglacial cycles (42) from carbonic acid (Eq. 3). For each catchment, based on the both support a causative rather than coincidental relationship between glaciers, erosion, and sulfide weathering fluxes. However, it end-member mixing results across 10,000 different end-member remains unclear whether this is a transient effect of deglaciation or a ratios, we determine the proportion of the model simulations, long-term sustained increase in erosion and oxidation. F(x), that yield Alk:DIC < 1 and the proportion that yield Alk: < F x Low temperatures associated with glacial catchments may also DIC 2. A higher value of ( ) indicates greater likelihood of contribute to the high sulfide relative to silicate weathering CO2 release over either short or long timescales, respectively, with fluxes, and resulting low Alk:DIC ratios. If low temperatures F x > ( ) 0.9 reflecting that 90% of the model solutions for a given inhibit silicate mineral weathering (11), whereas high rates of river predict CO2 release rather than consumption. sulfide weathering are sustained by microbially mediated reac- To compare between the glacial and nonglacial datasets, we plot tions (43), we would expect that colder, glaciated catchments the cumulative distribution of the F(x) values for the catchments in would have higher ratios of solutes derived from sulfide vs. sili- each data compilation (Fig. 4). Calculated proportions of each cate mineral sources. Additional weathering of glacially derived weathering reaction are sensitive to the Ca:Na ratio of evaporite silicate material during fluvial transport may shift the overall inputs, which is not well known and which may vary from site to site. weathering balance toward CO2 consumption (17). However, To illustrate the sensitivity to this parameter, we plot multiple significant amounts of glacial debris are deposited without such curves for each dataset, assuming different maximum values of (Ca: reworking, and we do not observe systematic trends between SI Appendix Na)evaporite [the F(x) curves are not sensitive to choosing different Na:SO4 and catchment area ( ,Fig.S3), suggesting that minimum values]. Independent of the assumed value for (Ca: additional weathering of glacial detritus does not necessarily ne- gate enhanced sulfide oxidation. Na)evaporite, more catchments from the glacial database have higher F(x) than the signature of average continental weathering reflected Oxidative CO Release over Quaternary Glacial Cycles. The model in large rivers (Fig. 4). We thus infer that glacial catchments are 2 results indicate that glacierized catchments are more likely than more likely than nonglaciated catchments to yield low Alk:DIC nonglacierized catchments to generate weathering fluxes with ratios, as expected for glacial catchments characterized by more Alk:DIC < 1 (Fig. 4), the threshold for causing pCO2 changes sulfide oxidation relative to silicate weathering. For example, for over millennial timescales. Could the magnitude of changes in ∼20% of the catchments in the glacial database, more than 90% of glacial weathering be significant as a CO2 source over timescales model results yield Alk:DIC < 2, whereas none of the world rivers yield model results where more than 90% have Alk:DIC < 2. This result is not strongly biased by lithological differences in end- AB member composition; as shown in SI Appendix,Fig.S5, glacierized 1 Worlds' Largest Rivers catchments spanning the range of mapped lithology types yield high 0.8 F(x), that is, high likelihood of Alk:DIC < 1 or 2. Our probabilistic F(X) approach, necessitated by weak constraints on catchment-specific ≤ 0.6 Glacial Rivers end-member values, can at most be interpreted to reflect a greater 0.4 Color shading = upper bound on Ca:Na of likelihood of glacial weathering to act as a CO2 source, relative to of dataset 0.2 evaporite end-member

nonglacial weathering. Some model solutions include seemingly Cumulative proportion (light = 0.2, dark = 2) 0 unlikely but not altogether impossible combinations, for example, 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 the majority of cations sourced from precipitation and evaporites F(X) or fraction of model results for an F(X) for Alk/DIC ≤ 2 individual river with Alk/DIC ≤ 1 simultaneous with the majority of SO4 sourced from sulfides. Further work partitioning solute sources rigorously by individual Fig. 4. Effect of glacial vs. nonglacial weathering on the carbon cycle. catchment, for example, including constraints from S isotopes (17, (A) Proportion of model simulations, F(x), that yield Alk:DIC < 1 for glacial 24), will be important to more thoroughly test the probabilistic catchments (blue lines) and the world’s large rivers (28), red lines. (B)AsinA, but < inferences made here. Nonetheless, our data suggest that present- proportion of model simulations that yield Alk:DIC 2. The Monte Carlo simulations used to calculate F(x) propagate uncertainties on solute chemistry and day glacial systems are characterized by more sulfide oxidation end-member compositions (Fig. 2 and SI Appendix). Regardless of assumed and carbonate weathering than nonglacial systems, consistent with maximum Ca:Na ratio of evaporite weathering, more glacial catchments are

prevailing interpretations (13). likely to be characterized by CO2 release compared with the world’slargerivers.

4of6 | www.pnas.org/cgi/doi/10.1073/pnas.1702953114 Torres et al. Downloaded by guest on October 1, 2021 of Quaternary glacial–interglacial cycles? Simplistically taking the glaciation began producing enough CO2 during each glacial cycle difference in median glacial vs. nonglacial SO4:Na ratios at face to prevent further declines in atmospheric concentrations—even- value (Fig. 2), glacial weathering might increase sulfide oxidation tually reaching a stable steady state as seen for the past 1 My. This yields on average by ∼2.5× relative to nonglacial weathering. As- hypothesis could potentially be tested with high-resolution pCO2 suming present-day global sulfide oxidation fluxes of ∼1.5 × 1012 mol/y records during Pleistocene cooling between 3 and 1 Ma. S (44), Last Glacial Maximum (LGM) glacial weathering over Climatic stability also followed the rapid cooling and onset of 10% of global land area (14.5 × 106 km2; ref. 2) could generate an Antarctic glaciation at the Eocene–Oligocene transition, 34– ∼ × 11 × 11 additional 2.25 10 mol/y S, releasing up to 4.5 10 mol 33 Ma. We hypothesize that CO2 release from oxidative glacial CO2/y if sulfide oxidation produces CO2:SO4 in a 2:1 ratio (the weathering could have contributed to the transition to stable maximum possible based on reaction stoichiometry; ref. 16). Over Oligocene climate, in addition to possible silicate weathering 10 ky, for example during glacial onset or deglaciation, the feedbacks (59). Nd and Pb isotope records from the southern oxidation-related flux would then be on the order of ∼4.5 × 1015 Ocean suggest a brief pulse of increased Antarctic weathering, 206 204 mol CO2,equivalenttoa∼25 ppm increase in atmospheric con- ∼33.9–33.5 Ma (60, 61). Longer-lived changes in Pb/ Pb are centration (for an atmosphere of 1.8 × 1020 mol). In the more proposed to reflect increased carbonate weathering (61), and extreme case of doubling the entire global sulfide oxidation flux, as increases in 187Os/188Os, a potential proxy for weathering or suggested from modeling of Os isotope variations (42), the asso- organic- and sulfide-rich rocks (42), also continue after 33 Ma 11 ciated increase in SO4 flux of 7.5 × 10 mol/y could potentially (62). Although aridity limits present-day Antarctic denudation 16 amount to a much larger CO2 release of 1.5 × 10 mol, equivalent and solute fluxes (63), sulfide oxidation associated with glacia- to a ∼80 ppm increase in CO2. tion should at least be considered in evaluating the early Oli- An increased oxidation flux of 2.5–7.5 × 1011 mol S/y associated gocene carbon cycle. with glaciation would require glacially facilitated erosion of 16–48 × 1014 g sediment/y, assuming average pyrite S content in eroded Glacial Weathering and Carbon Cycle Feedbacks over Earth’s History. rocks of 0.5 wt% S, as is characteristic of sedimentary rocks (45). To Over geologic timescales, instead of glacial erosion driving a positive produce this material, sediment yield from an LGM glaciated area feedback via silicate weathering and CO2 drawdown(5,12),ourdata − − of 14.5 × 106 km2 wouldhavetobe∼110–330 t·km 2·y 1, well within suggest that C release dominates in glacial settings, such that glacial the range of rates of glacially enhanced erosion rates observed weathering could act as a negative feedback preventing the Earth during the present deglaciation (8). Resulting changes in concen- system from descending into colder conditions. A feedback via glacial trations of sulfate in the oceans or O2 in the atmosphere would be sulfide oxidation depends on elevated oxidation rates being sustained EARTH, ATMOSPHERIC,

very small and likely difficult to detect relative to the much larger over millions of years. Persistent increases in glacial erosion over AND PLANETARY SCIENCES reservoirs. Driving all oxidation by atmospheric O2 would consume these timescales, as suggested for the Plio-Pleistocene (5), could 11 15 5–14 × 10 mol O2/y, or 5–14 × 10 mol O2 over 10 ky, less than maintain increased oxidative weathering fluxes, providing a mecha- 19 0.01–0.04% of the atmospheric reservoir of ∼4 × 10 mol O2 and nism for weathering to limit the magnitude of global cooling. How- within the range of observed declines in pO2 over the past 1 My ever, the evidence for glacially enhanced erosion rates continuing (46). Similarly, proxies or models of total weathering fluxes to the over >1 My remains controversial (e.g., refs. 8 and 64). Alternatively oceans (36, 47, 48) may not capture modest increases in these or in addition, low temperatures could nudge global weathering away sulfate fluxes because oxidative weathering is a small portion of the from silicate weathering and thus toward lower Alk:DIC ratios. total global weathering flux. Either way, our analysis points to sulfide oxidation as a plausible The scenarios explored here are highly simplistic, based on ap- mechanism by which glaciation might be self-limiting, shifting the proximate estimates of 20–100% changes in global sulfide oxida- balance of the global carbon cycle by increasing CO2 supply to the – tion fluxes resulting from glaciation, and release of CO2 in a atmosphere ocean system (Fig. 5). The O2 concentration in the 2:1 proportion to SO4. Moreover, the timing of enhanced oxidative atmosphere places a further constraint on the extent to which such a weathering during glacial–interglacial cycles is not well known. In mechanism may be effective (19). The O2-rich atmosphere of the addition, these calculations do not include other biogeochemical Cenozoic holds a sufficiently large reservoir to accommodate processes that affect the carbon cycle, some of which may also be changes in sulfide oxidation without changing atmospheric O2 influenced by glacial erosion, such as organic matter cycling (e.g., concentrations outside of known bounds (16). At times of very low refs. 49 and 50) and sulfur redox transformation (e.g., refs. 43 and or effectively no atmospheric O2 concentration such as in the Ar- 51). Nonetheless, our calculations serve to illustrate that oscillation chaean and Paleoproterozoic (65), glacially driven CO2 release by of weathering between more and less sulfide oxidation over Qua- oxidative weathering may have been less active (e.g., ref. 66). This ternary timescales, expected based on the chemistry of present-day contrast raises the intriguing possibility of whether glaciations early glacial weathering, could play a quantitatively meaningful role in in Earth’s history, when atmospheric O2 levels were lower, were thecarboncycleoverglacial–interglacial cycles. prone to being more globally pervasive compared with glaciations in the Phanerozoic. Similarly, the effectiveness of an oxidative Glacial Weathering and Stable pCO2 for the Last 1 Myr. Our analysis weathering feedback depends on the sulfide content of rocks un- suggests that present-day glacial weathering causes net CO2 re- dergoing weathering, hinting at the possibility that growth of a lease, but measurements from ice cores show no long-term change sulfide reservoir in Earth’s crust over time could have helped to in mean atmospheric pCO2 averaged over multiple glacial cycles stabilize climate. during the past 1 My (52, 53). Highly responsive global silicate weathering feedbacks could provide one mechanism explaining constant pCO (54). Another explanation, consistent with our 2 Negative feedback via oxidative weathering In absence of oxidative weathering, observation of glacially enhanced CO2 release in the present day, preventing runaway glaciation? positive silicate weathering feedback? is that the extent of glaciation has adjusted over time such that the + Sulfide + + Silicate – Glaciation CO Glaciation CO amount of CO2 released via glacial weathering compensates for Oxidation 2 Weathering 2 the external driver that initially forced decreases in pCO2 and global cooling before 1 Ma. This forcing may have been via de- – – creased fluxes from solid Earth degassing (6) or greater weath- – Fig. 5. Earth system feedbacks via glacial weathering. Sulfide oxidation erability of continental silicate minerals (55 57). As pCO2 seen in present-day glacial weathering may inhibit runaway glaciation. Such

dropped and climate cooled between 3 and 1 Ma (58), glacial a stabilizing feedback may not operate at lower pO2, for example, early in advance would have stimulated greater release of CO2 until Earth’s history or on other planets.

Torres et al. PNAS Early Edition | 5of6 Downloaded by guest on October 1, 2021 ACKNOWLEDGMENTS. We thank three reviewers for constructive com- and the Cluster of Excellence “CliSAP,” EXC177, Universität Hamburg) and ments. M.A.T. was supported by a University of Southern California Bundesministerium für Bildung und Forschung Project PALMOD (Ref College Merit Fellowship and a Caltech Texaco Postdoctoral Fellowship, 01LP1506C), and A.J.W. by National Science Foundation Grant EAR-1455352. N.M. by a Deutscher Akademischer Austauschdienst exchange fellow- Requests for access to the GloRiCh database should be addressed to J. Hartmann ship, J.H. by the German Science Foundation (DFG-project HA4472/6-1 at [email protected].

1. Hoffman PF, Kaufman AJ, Halverson GP, Schrag DP (1998) A neoproterozoic snowball 36. Tranter M, et al. (2002) Direct effect of ice sheets on terrestrial bicarbonate, sulphate earth. Science 281:1342–1346. and base cation fluxes during the last glacial cycle: Minimal impact on atmospheric — 2. Ehlers J, Gibbard PL, Hughes PD (2011) Quaternary Glaciations Extent and CO2 concentrations. Chem Geol 190:33–44. Chronology : A Closer Look (Elsevier, Amsterdam). 37. Hindshaw RS, et al. (2011) Hydrological control of stream water chemistry in a glacial 3. Budyko MI (1969) The effect of solar radiation variations on the climate of the Earth. catchment (Damma Glacier, Switzerland). Chem Geol 285:215–230. Tellus 21:611–619. 38. Godsey SE, Kirchner JW, Clow DW (2009) Concentration–discharge relationships re- 4. Kennett JP (1977) Cenozoic evolution of Antarctic glaciation, the circum-Antarctic flect chemostatic characteristics of US catchments. Hydrol Process 23:1844–1864. Ocean, and their impact on global paleoceanography. J Geophys Res 82:3843–3860. 39. Anderson SP (2007) Biogeochemistry of glacial landscape systems. Annu Rev Earth 5. Herman F, et al. (2013) Worldwide acceleration of mountain erosion under a cooling Planet Sci 35:375–399. climate. Nature 504:423–426. 40. Négrel P, Allègre CJ, Dupré B, Lewin E (1993) Erosion sources determined by inversion 6. Berner RA (2004) The Phanerozoic Carbon Cycle: CO2 and O2 (Oxford Univ Press, of major and trace element ratios and strontium isotopic ratios in river water: The Oxford). Congo Basin case. Earth Planet Sci Lett 120:59–76. 7. Kump LR, et al. (1999) A weathering hypothesis for glaciation at high atmospheric pCO2 41. Urey HC (1952) On the early chemical history of the Earth and the origin of life. Proc during the Late Ordovician. Palaeogeogr Palaeoclimatol Palaeoecol 152:173–187. Natl Acad Sci USA 38:351–363. 8. Koppes MN, Montgomery DR (2009) The relative efficacy of fluvial and glacial erosion 42. Georg RB, West AJ, Vance D, Newman K, Halliday AN (2013) Is the marine osmium isotope over modern to orogenic timescales. Nat Geosci 2:644–647. record a probe for CO2 release from sedimentary rocks? Earth Planet Sci Lett 367:28–38. 9. Hallet B, Hunter L, Bogen J (1996) Rates of erosion and sediment evacuation by glaciers: 43. Boyd ES, Hamilton TL, Havig JR, Skidmore ML, Shock EL (2014) Chemolithotrophic – A review of field data and their implications. Glob Planet Change 12:213 235. primary production in a subglacial ecosystem. Appl Environ Microbiol 80:6146–6153. 10. Riebe CS, Kirchner JW, Finkel RC (2004) Erosional and climatic effects on long-term 44. Berner EK, Berner RA (1996) Global Environment: Water, Air, and Geochemical Cycles chemical weathering rates in granitic landscapes spanning diverse climate regimes. (Prentice-Hall, Upper Saddle River, NJ). – Earth Planet Sci Lett 224:547 562. 45. Nielsen H, et al. (1991) Lithospheric sources of sulfur. SCOPE 43: Stable Isotopes: 11. West AJ, Galy A, Bickle M (2005) Tectonic and climatic controls on silicate weathering. Natural and Anthropogenic Sulphur in the Environment, SCOPE (Wiley, New York). – Earth Planet Sci Lett 235:211 228. 46. Stolper DA, Bender ML, Dreyfus GB, Yan Y, Higgins JA (2016) A Pleistocene ice core 12. Foster GL, Vance D (2006) Negligible glacial-interglacial variation in continental record of atmospheric O concentrations. Science 353:1427–1430. – 2 chemical weathering rates. Nature 444:918 921. 47. Jones IW, Munhoven G, Tranter M, Huybrechts P, Sharp MJ (2002) Modelled glacial 13. Tranter M (2003) Geochemical weathering in glacial and proglacial environments. − and non-glacial HCO3 , Si and Ge fluxes since the LGM: Little potential for impact on Treatise of Geochemistry, Surface and Ground Water, Weathering, and Soils, eds atmospheric CO2 concentrations and a potential proxy of continental chemical ero- Holland HD, Turekian KK (Elsevier, Oxford), pp 189–205. sion, the marine Ge/Si ratio. Global Planet Change 33:139–153. 14. Raiswell R (1984) Chemical models of solute acquisition in glacial melt waters. 48. von Blanckenburg F, Bouchez J, Ibarra DE, Maher K (2015) Stable runoff and weathering J Glaciol 30:49–57. fluxes into the oceans over Quaternary climate cycles. Nat Geosci 8:538–542. 15. Tranter M, Brown G, Raiswell R, Sharp M, Gurnell A (1993) A conceptual model of 49. Cartapanis O, Bianchi D, Jaccard SL, Galbraith ED (2016) Global pulses of organic solute acquisition by Alpine glacial meltwaters. J Glaciol 39:573–581. carbon burial in deep-sea sediments during glacial maxima. Nat Commun 7:10796. 16. Torres MA, West AJ, Li G (2014) Sulphide oxidation and carbonate dissolution as a 50. Horan K, Hilton RG, Burton KW, Selby DS, Ottley CJ (2015) Enhanced oxidative source of CO over geological timescales. Nature 507:346–349. 2 weathering in glaciated mountain catchments: A stabilising feedback on atmospheric 17. Torres MA, et al. (2016) The acid and alkalinity budgets of weathering in the Andes– carbon dioxide? Proceedings of the American Geophysical Union, Fall Meeting 2015 Amazon system: Insights into the erosional control of global biogeochemical cycles. (American Geophysical Union, Washington, DC), Abstract EP21E-02. Earth Planet Sci Lett 450:381–391. 51. Pasquier V, et al. (2017) Pyrite sulfur isotopes reveal glacial-interglacial environmental 18. Berner RA, Canfield DE (1989) A new model for atmospheric oxygen over Phanerozoic changes. Proc Natl Acad Sci USA 114:5941–5945. time. Am J Sci 289:333–361. 52. Petit JR, et al. (1999) Climate and atmospheric history of the past 420,000 years from 19. Bachan A, Kump LR (2015) The rise of oxygen and siderite oxidation during the Lo- the Vostok ice core, Antarctica. Nature 399:429–436. magundi Event. Proc Natl Acad Sci USA 112:6562–6567. 53. Higgins JA, et al. (2015) Atmospheric composition 1 million years ago from blue ice in 20. Williamson MA, Rimstidt JD (1994) The kinetics and electrochemical rate-determining the Allan Hills, Antarctica. Proc Natl Acad Sci USA 112:6887–6891. step of aqueous pyrite oxidation. Geochim Cosmochim Acta 58:5443–5454. 54. Zeebe RE, Caldeira K (2008) Close mass balance of long-term carbon fluxes from ice- 21. White A, Brantley SL (2003) The effect of time on the weathering of silicate minerals: core CO and ocean chemistry records. Nat Geosci 1:312–315. Why do weathering rates differ in the laboratory and field? Chem Geol 202:479–506. 2 55. Raymo ME, Ruddiman WF, Froelich PN (1988) Influence of late Cenozoic mountain 22. White AF, Bullen TD, Vivit DV, Schulz MS, Clow DW (1999) The role of disseminated calcite building on ocean geochemical cycles. Geology 16:649–653. in the chemical weathering of granitoid rocks. Geochim Cosmochim Acta 63:1939–1953. 56. Kump LR, Arthur MA (1997) Global chemical erosion during the Cenozoic: Weath- 23. Hercod DJ, Brady PV, Gregory RT (1998) Catchment-scale coupling between pyrite oxidation and calcite weathering. Chem Geol 151:259–276. erability balances the budgets. Tectonic Uplift and Climate Change, ed Ruddiman WF – 24. Calmels D, Gaillardet J, Brenot A, France-Lanord C (2007) Sustained sulfide oxidation (Springer, Boston), pp 399 426. by physical erosion processes in the Mackenzie River basin: Climatic perspectives. 57. Caves JK, Jost AB, Lau KV, Maher K (2016) Cenozoic carbon cycle imbalances and a – Geology 35:1003–1006. variable weathering feedback. Earth Planet Sci Lett 450:152 163. 25. West AJ (2012) Thickness of the chemical weathering zone and implications for erosional 58. Zachos J, Pagani M, Sloan L, Thomas E, Billups K (2001) Trends, rhythms, and aber- – and climatic drivers of weathering and for carbon-cycle feedbacks. Geology 40:811–814. rations in global climate 65 Ma to present. Science 292:686 693. 26. Anderson SP, Drever JI, Humphrey NF (1997) Chemical weathering in glacial envi- 59. Zachos J, Kump L (2005) Carbon cycle feedbacks and the initiation of Antarctic gla- – ronments. Geology 25:399–402. ciation in the earliest Oligocene. Global Planet Change 47:51 66. 27. Anderson SP (2005) Glaciers show direct linkage between erosion rate and chemical 60. Scher HD, Bohaty SM, Zachos JC, Delaney ML (2011) Two-stepping into the icehouse: weathering fluxes. Geomorphology 67:147–157. East Antarctic weathering during progressive ice-sheet expansion at the Eocene- Oligocene transition. Geology 39:383–386. 28. Gaillardet J, Dupré B, Louvat P, Allègre C (1999) Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers. Chem Geol 159:3–30. 61. Basak C, Martin EE (2013) Antarctic weathering and carbonate compensation at the 29. Hartmann J, Lauerwald R, Moosdorf N (2014) A brief overview of the GLObal RIver Eocene-Oligocene transition. Nat Geosci 6:121–124. 187 188 Chemistry database, GLORICH. Procedia Earth Planet Sci 10:23–27. 62. Ravizza G, Peucker-Ehrenbrink B (2003) The marine Os/ Os record of the Eocene–Oligo- 30. Fekete BM, Vorosmarty CJ, Grabs W (2002) High-resolution fields of global runoff cene transition: The interplay of weathering and glaciation. Earth Planet Sci Lett 210:151–165. combining observed river discharge and simulated water balances. Global Biogeochem 63. Nezat CA, Lyons WB, Welch KA (2001) Chemical weathering in streams of a polar Cycles 16:15-1–15-10. desert (Taylor Valley, Antarctica). Geol Soc Am Bull 113:1401–1408. 31. Moon S, Chamberlain CP, Hilley GE (2014) New estimates of silicate weathering rates 64. Willenbring JK, Jerolmack DJ (2016) The null hypothesis: Globally steady rates of and their uncertainties in global rivers. Geochim Cosmochim Acta 134:257–274. erosion, weathering fluxes and shelf sediment accumulation during Late Cenozoic 32. Bluth GJS, Kump LR (1994) Lithologic and climatologic controls of river chemistry. mountain uplift and glaciation. Terra Nova 28:11–18. Geochim Cosmochim Acta 58:2341–2359. 65. Lyons TW, Reinhard CT, Planavsky NJ (2014) The rise of oxygen in Earth’s early ocean 33. Maher K (2010) The dependence of chemical weathering rates on fluid residence and atmosphere. Nature 506:307–315. time. Earth Planet Sci Lett 294:101–110. 66. Holland HD (2002) Volcanic gases, black smokers, and the great oxidation event. 34. Anderson SP, Drever JI, Frost CD, Holden P (2000) Chemical weathering in the fore- Geochim Cosmochim Acta 66:3811–3826. land of a retreating glacier. Geochim Cosmochim Acta 64:1173–1189. 67. van Heuven S, Pierrot D, Rae JWB, Lewis E, Wallace DWR (2011) MATLAB Program De-

35. Hodgkins R, Tranter M, Dowdeswell JA (1997) Solute provenance, transport and de- veloped for CO2 System Calculations, ORNLCDIAC-105b (Carbon Dioxide Information Anal- nudation in a High Arctic glacierized catchment. Hydrol Process 11:1813–1832. ysis Center, Oak Ridge National Laboratory, US Department of Energy, Oak Ridge, TN).

6of6 | www.pnas.org/cgi/doi/10.1073/pnas.1702953114 Torres et al. Downloaded by guest on October 1, 2021