Research Article

Glacial weathering, sulfide oxidation, and global carbon cycle feedbacks

Mark A. Torres, Nils Moosdorf, Jens Hartmann, Jess F. Adkins, and A. Joshua West
PNAS August 15, 2017 114 (33) 8716-8721; first published July 31, 2017 https://doi.org/10.1073/pnas.1702953114

  1. Edited by Thure E. Cerling, University of Utah, Salt Lake City, UT, and approved July 5, 2017 (received for review February 21, 2017)

Significance

We compile data showing that, as hypothesized previously, waters draining glaciers have solute chemistry that is distinct from nonglacial rivers and reflects different proportions of mineral weathering reactions. Elevated pyrite oxidation during glacial weathering could generate acidity, releasing carbon to the atmosphere. We show that this effect could contribute to changes in CO2 during glacial cycles of the past million years. Over the longer, multimillion-year timescales that Earth transitions into and out of glaciated states, sustained addition of pyrite-derived sulfate to the oceans could shift the balance of the global carbon cycle toward increasing CO2 in the ocean–atmosphere, thus providing a negative-feedback mechanism preventing runaway glaciation. This mechanism depends on oxidation and thus sufficient O2.

Abstract

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 pCO2, 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 CO2 to the atmosphere. Back-of-the-envelope calculations indicate that oxidative fluxes could change ocean–atmosphere CO2 equilibrium by 25 ppm or more over 10 ky. Over longer timescales, CO2 release could act as a negative feedback, limiting progress of glaciation, dependent on lithology and the concentration of atmospheric O2. Future work on glaciation–weathering–carbon cycle feedbacks should consider weathering of trace sulfide minerals in addition to silicate minerals.

Episodes 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 atmosphere in the process (6). If glaciers reduce weathering fluxes from land area that was previously undergoing silicate weathering, glaciation may decrease global alkalinity production, allow atmospheric CO2 levels to rise, and thus generate a negative feedback that ends glaciation (7). On the other hand, some evidence suggests that glaciation increases rates of physical erosion (5, 8, 9), which is thought to promote more rapid silicate weathering (10, 11). If glaciation increases erosion rates, and if erosion enhances silicate weathering and associated production of alkalinity, then glacial weathering could act as a positive feedback serving to promote further glaciation (5, 12).

Glaciation may also affect rates of weathering of nonsilicate minerals. Studies of glacial hydrochemistry have pointed to the importance of sulfide oxidation and carbonate dissolution as major solute sources (13⇓–15). Because the residence time of sulfate in the oceans is long relative to Ca, sulfide–carbonate weathering can act to release CO2 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). Although sulfide and carbonate minerals are usually present at low abundance in most rocks, their rates of dissolution are typically orders of magnitude faster than for silicate minerals. For example, log rates from laboratory experiments are −6 to −10 for pyrite (20), vs. log rates of −10 to −16 for silicate minerals (21). 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). As a result, if glaciation enhances physical erosion rates, the increased exposure of fresh rock material containing unweathered minerals should drive increased sulfide and carbonate weathering fluxes, but may not drive increased silicate weathering fluxes in tandem. Thus, the net effect of glaciation could be to supply CO2 to the atmosphere, rather than to remove it.

The directionality of long-term glaciation–weathering feedbacks depends on the extent to which glacial weathering is a net source or net sink of CO2 to the atmosphere, which in turn depends on the balance of sulfide vs. silicate mineral weathering. A growing number of studies have explored various aspects of glacial weathering by analyzing the chemical composition of glacial runoff, often focusing on specific case studies, and in some cases synthesizing global datasets (13, 26, 27). Here, we assemble an expanded compilation of glacial weathering data and use this compilation to identify characteristic features of present-day glacial weathering, including the relative roles of sulfide-carbonate vs. silicate mineral weathering compared with rivers not dominated by glacial processes. We then consider the possible implications for global-scale fluxes in the Quaternary carbon cycle and for planetary feedbacks over longer timescales.

Materials and Methods

We compiled a database containing stream chemistry data from glaciated catchments, together with spatial data describing the contributing catchments (SI Appendix, Fig. S1, and Datasets S1 and S2). The dataset is based on solute concentrations for 7,700 samples, from 95 glaciated catchments (glacial cover proportion >0.4%, and in most cases much higher; Fig. 1) with reported concentrations of Ca, Mg, Na, and K for at least one sample. Measured discharge was available for 52 of these catchments. The contribution from chemical weathering to the measured stream chemistry was determined following correction for precipitation inputs, as detailed in SI Appendix. To provide a basis for comparing the glacial weathering data, we refer to the chemical composition of the world’s large rivers (28), which provide a spatially integrated view of weathering processes over continental areas and reflect the composition of solutes delivered to the oceans. For reference, we also consider data from the Global River Chemistry (GloRiCh) database of solute chemistry from 1.27 million water samples from ∼15,500 catchments around the world (29) (Dataset S3).

Fig. 1.
Fig. 1.

Scatter plots of (A) cation concentration corrected for precipitation input (*), and (B) total cation weathering flux, both plotted against glacial cover. Fluxes do not decrease to zero as glacial cover approaches complete cover, indicating significant weathering under ice.

Results

General Characteristics of Glacial Weathering.

The mean of the sum of cation concentrations (Ca plus Mg plus Na plus K) in the 95 glacierized catchments is 530 µmol⋅L−1, decreasing to 453 µmol⋅L−1 after correction for atmospheric inputs. The weathering-derived solutes are dominated by Ca (58.6 mol %, SI Appendix, Table S2). Mg and Na contribute 17.5% and 17.4%, respectively, followed by K (6.5%). For the 54 glacial catchments with sufficient data to estimate dissolved pCO2 using PHREEQC (Dataset S1), 45 are supersaturated relative to the atmosphere (median pCO2 for the glacial catchments, 692 ppm; mean, 4,029 ppm).

For the 52 catchments with available runoff data, mean runoff is 1,730 L⋅m−2⋅y−1, which is 5.8 times above the global average river runoff (30). The mean cation weathering yield from the glacial catchments is 21.5 t⋅km−2⋅y−1. For comparison, the mean cation yield from the world’s large rivers is 18 t⋅km−2⋅y−1, with a corresponding mean cation concentration of 2,433 µmol⋅L−1 (31). Thus, relative to global average river water, glacial catchments are characterized by below average cation concentrations but above average cation yields, due to high runoff (26).

Within the glacial dataset, precipitation-corrected cation concentrations are positively correlated with the proportion of carbonate-rich sediment in the catchments (r = 0.26, SI Appendix, Table S3) and negatively with coverage of metamorphic rocks (r = −0.24) and glacial cover (r = −0.27). The cation yield of the observed catchments is correlated with runoff (r = 0.28), but this correlation is weaker than reported for nonglaciated catchments (32, 33). Correlations between lithology classes and cation yield are generally insignificant, although some significant correlations can be seen for individual elements (SI Appendix, Table S3). Additional weathering of glacially derived sediment is thought to take place in proglacial environments (34). Consistent with this hypothesis, cation yields in our compilation show considerable scatter but indicate a slight increase with the proportion of glacial cover (Fig. 1). Of all analyzed variables, cation yield is most highly correlated with the proportion of glacial cover and the sampling distance from the glacial snout (SI Appendix, Table S3). Subglacial weathering fluxes also depend on hydrology, for example, differing for warm- vs. cold-based ice (35, 36) and along different flowpaths (37), and these differences may explain some of the variability within the dataset.

Distinct Stoichiometry of Glacial Weathering.

Our compilation provides the opportunity to test the hypothesis that the solute stoichiometry of glaciated rivers is distinct from nonglaciated rivers, because the large number of data points allows for statistical comparison between distributions. Although we do not directly evaluate temporal variability, dissolved major element concentrations in river waters vary only slightly over time (38) relative to differences between catchments. Dissolved K:Na ratios, which reflect the stoichiometry of silicate mineral dissolution, are shifted to higher values in the glacial dataset compared with either the world’s large rivers or the GloRiCh dataset (distributions are significantly different at the P < 0.05 level via a Kolmogorov–Smirnov test), consistent with results of previous studies and suggesting changes in the stoichiometry of weathering-derived solutes driven by glaciation (15, 39).

Ca:Na and SO4:Na ratios (Fig. 2) are also statistically different (at P < 0.05 level) when comparing our glacial database vs. either nonglacial dataset. In contrast, when comparing between the two nonglacial datasets (GloRiCh vs. the world’s large rivers), Ca:Na and SO4:Na ratios are not statistically different. Glacial solute geochemistry is significantly distinct even after subsampling the GloRiCh database for rivers with the same range of catchment areas as the glacial database (SI Appendix, Fig. S3 A and B). Glacial SO4:Na ratios are weakly correlated with the distance from the glacier snout (SI Appendix, Fig. S3C) and do not depend on catchment size (SI Appendix, Fig. S3A) or the proportion of glacial cover (SI Appendix, Fig. S3D). So, although additional weathering may occur in the proglacial environment (Fig. 1), it does not appear to be a major control on elemental ratios (SI Appendix, Fig. S3).

Fig. 2.
Fig. 2.

Stoichiometry of dissolved ions in the glacial data compilation presented in this study (blue points, and blue dash-dot curves in histograms), the world’s large rivers (red points, and red dashed curves; data from ref. 28), and the GloRiCh database of 17,000 nonglaciated catchments (gray points, and solid gray lines; ref. 29). (A) SO4/Na vs. Ca/Na molar ratios, with population histograms (normalized to number of catchments) of both ratios shown along each axis (reported mean and median values are not log-transformed). (B) Ca/Na vs. Mg/Na ratios, with the range of each lithological end-member illustrated on the figure and the values listed in SI Appendix, Table S4. The GloRiCh data illustrate that our choices of end-members bracket most river compositions.

Idealized versions of the reactions expected to generate dissolved Ca, Na, SO4 are summarized as follows:Carbonate dissolution: 2H++CaCO3↔Ca2++H2CO3,[1]Silicate dissolution: 4H++Ca2SiO3↔2Ca2++H4SiO4,[2]Carbonic acid: CO2(diss)+H2O↔H2CO3↔2H++CO32−,[3]Sulfuric acid from sulfide oxidation: FeS2+15O2+14H2O↔16H++8SO42−+4Fe(OH)3.[4]

Evaporites may also contribute Ca and SO4. Assuming glaciation is not associated with preferential evaporite weathering (an assumption we test below), elevated SO4:Na ratios during glacial weathering suggest relatively more sulfide mineral oxidation compared with silicate weathering in glacial catchments, and elevated Ca:Na ratios suggest more carbonate weathering than silicate weathering. These observations are consistent with previous studies emphasizing the role of sulfide and carbonate mineral weathering in glacial catchments, even when these phases are minor constituents of the bedrock (e.g., refs. 13⇓–15 and 26).

Glacial Weathering as Net CO2 Source.

Higher dissolved ion yields and higher SO4:Na and Ca:Na ratios together suggest that sulfide and carbonate dissolution rates are enhanced in association with glacial weathering. To first order, glaciation thus appears to act as a CO2 source, by maintaining total solute fluxes while shifting weathering toward reactions that release rather than consume CO2.

In principle, glacially enhanced Ca and SO4 fluxes could derive from preferential dissolution of sulfate-bearing evaporites or weathering of Ca-bearing silicates by sulfuric acid, reactions that would not act as CO2 sources. We have no a priori reason to expect that such effects dominate globally. Nonetheless, fully understanding the net effect of glaciation on the carbon cycle depends on quantifying (i) the proportion of the dissolved sulfate derived from sulfide mineral oxidation, and (ii) the extent to which this oxidative flux of protons (Eq. 4) is balanced by alkalinity generation from silicate and carbonate weathering (Eqs. 1 and 2). To approach these questions quantitatively, we use a mixing model to estimate the relative contribution of different weathering reactions to the solute load for each catchment in our compilation. The model uses Na-normalized elemental ratios (Cl:Na, Ca:Na, and Mg:Na) to partition the dissolved load between silicate, carbonate, evaporite, and precipitation sources (e.g., ref. 40 and details in SI Appendix). We apply our model to all samples from the world’s large river database (28) and a subset of samples from the glacial (n = 84) database due to the data requirements. We do not consider the GloRiCh data because the nonuniform sampling distribution in this database (e.g., multiple samples from many small catchments in one region) complicates interpretation of results from our probabilistic approach, 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 processes. Thus, we cannot calculate a single model result for each catchment. Instead, we randomly sample across a uniform distribution of possible end-member values (e.g., Fig. 2B), repeating for 10,000 combinations. For each combination, we use the fractional contribution from each solute source to calculate a ratio of alkalinity to dissolved inorganic carbon equivalents (Alk:DIC) produced by weathering reactions.

The Alk:DIC ratio allows us to evaluate the net effect of weathering on atmospheric pCO2 over different timescales (17). The pCO2 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. 3). The ocean ratio is, in turn, determined by the supply of alkalinity and DIC from weathering reactions and their removal through the burial of carbonate minerals and organic carbon. Carbonate removal is fixed at an Alk:DIC of 1:0.5 (Eq. 1; 1:0.5 line in Fig. 3). The Alk:DIC of weathering varies, from 0 to infinity. Over the timescales characteristic of carbonate burial (greater than ∼104 y), weathering will change the Alk:DIC ratio of the oceans (and thus pCO2) if it supplies alkalinity and DIC in a ratio that deviates from the 1:0.5 associated with carbonate burial (Fig. 3). Thus, silicate weathering by carbonic acid (supplying Alk:DIC at a 1:0 ratio, higher than 1:0.5) will consume CO2, whereas carbonate 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 marine sediments, returning alkalinity (reverse of Eq. 4; ref. 16). On timescales shorter than required for the output flux via carbonate precipitation to respond to changes in input fluxes (103 to 104 y), pCO2 will increase if weathering inputs have an Alk:DIC lower than the ocean (Fig. 3), which is presently approximately equal to 1, and vice versa. We thus evaluate model results with respect to Alk:DIC thresholds of 1 (characteristic of short timescales) and 2 (long timescales). CO2 is released if weathering occurs with a ratio below these thresholds; CO2 is consumed above these thresholds.

Fig. 3.
Fig. 3.

Alk:DIC framework for evaluating effect of weathering on pCO2. (A) Tabulation of Alk:DIC associated with different reaction combinations, after normalization by the charge equivalents of cations released. (B) Contours of oceanic pCO2 (equivalent to atmospheric pCO2 over timescales of gas exchange) as a function of alkalinity and DIC, calculated using CO2sys (67). Dashed black line shows 1:1 addition of Alk:DIC (stoichiometry of constant pCO2 over timescales shorter than the response time of the ocean carbonate system). Vectors correspond to combinations of reactions in A.

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 CO2 dissolution) with acid-consuming reactions (silicate and carbonate dissolution) (17) (Fig. 3A). We use the proportions of solutes from carbonate, silicate, and sulfide weathering, as inferred from the end-member mixing analysis, to tabulate the equivalents of each reaction required to produce the solute chemistry. Protons not derived from sulfide oxidation (Eq. 4) are assumed to be from carbonic acid (Eq. 3). For each catchment, based on the end-member mixing results across 10,000 different end-member ratios, we determine the proportion of the model simulations, F(x), that yield Alk:DIC < 1 and the proportion that yield Alk:DIC < 2. A higher value of F(x) indicates greater likelihood of CO2 release over either short or long timescales, respectively, with F(x) > 0.9 reflecting that 90% of the model solutions for a given river predict CO2 release rather than consumption.

To compare between the glacial and nonglacial datasets, we plot the cumulative distribution of the F(x) values for the catchments in each data compilation (Fig. 4). Calculated proportions of each weathering reaction are sensitive to the Ca:Na ratio of evaporite inputs, which is not well known and which may vary from site to site. To illustrate the sensitivity to this parameter, we plot multiple curves for each dataset, assuming different maximum values of (Ca:Na)evaporite [the F(x) curves are not sensitive to choosing different minimum values]. Independent of the assumed value for (Ca:Na)evaporite, more catchments from the glacial database have higher F(x) than the signature of average continental weathering reflected in large rivers (Fig. 4). We thus infer that glacial catchments are more likely than nonglaciated catchments to yield low Alk:DIC ratios, as expected for glacial catchments characterized by more sulfide oxidation relative to silicate weathering. For example, for ∼20% of the catchments in the glacial database, more than 90% of 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-member composition; as shown in SI Appendix, Fig. S5, glacierized catchments spanning the range of mapped lithology types yield high F(x), that is, high likelihood of Alk:DIC < 1 or 2. Our probabilistic approach, necessitated by weak constraints on catchment-specific end-member values, can at most be interpreted to reflect a greater likelihood of glacial weathering to act as a CO2 source, relative to nonglacial weathering. 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 SO4 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 present-day glacial systems are characterized by more sulfide oxidation and carbonate weathering than nonglacial systems, consistent with prevailing interpretations (13).

Fig. 4.
Fig. 4.

Effect of glacial vs. nonglacial weathering on the carbon cycle. (A) Proportion of model simulations, F(x), that yield Alk:DIC < 1 for glacial catchments (blue lines) and the world’s large rivers (28), red lines. (B) As in A, but proportion of model simulations that yield Alk:DIC < 2. The Monte Carlo simulations used to calculate F(x) propagate uncertainties on solute chemistry and end-member compositions (Fig. 2 and SI Appendix). Regardless of assumed maximum Ca:Na ratio of evaporite weathering, more glacial catchments are likely to be characterized by CO2 release compared with the world’s large rivers.

Discussion

Why Increased Oxidation Fluxes from Glacial Weathering.

The distinct geochemistry of glacial weathering might be coincidental; glaciers in the present day are typically found in steep, rapidly eroding mountainous regions (8), where fluxes from sulfide oxidation are relatively high independent of glaciation (16). Although our global analysis cannot conclusively distinguish causality, we think it is more likely that high sulfide weathering fluxes are the result of glaciation, driven by glacial enhancement of erosion. Several studies have proposed that glaciation increases erosion rates (5). Because sulfide weathering is typically supply limited, increases in erosion rate as expected from glaciation should increase oxidation fluxes (17, 24). It is also possible that sediment production and comminution during glacial retreat (8, 9) exposes fresh sulfides and facilitates their oxidation, even if longer-term rates of erosion (and thus of sulfide oxidation) are not elevated as a result of glaciation. In this latter case, high sulfide oxidation rates in the present day may reflect a transient postglacial increase but would not be sustained over timescales >105 y. High present-day sediment fluxes from glacierized catchments (8) and the oscillations in the Os isotope composition of seawater over Quaternary glacial–interglacial cycles (42) both support a causative rather than coincidental relationship between glaciers, erosion, and sulfide weathering fluxes. However, it remains unclear whether this is a transient effect of deglaciation or a long-term sustained increase in erosion and oxidation.

Low temperatures associated with glacial catchments may also contribute to the high sulfide relative to silicate weathering fluxes, and resulting low Alk:DIC ratios. If low temperatures inhibit silicate mineral weathering (11), whereas high rates of sulfide weathering are sustained by microbially mediated reactions (43), we would expect that colder, glaciated catchments would have higher ratios of solutes derived from sulfide vs. silicate mineral sources. Additional weathering of glacially derived silicate material during fluvial transport may shift the overall weathering balance toward CO2 consumption (17). However, significant amounts of glacial debris are deposited without such reworking, and we do not observe systematic trends between Na:SO4 and catchment area (SI Appendix, Fig. S3), suggesting that additional weathering of glacial detritus does not necessarily negate enhanced sulfide oxidation.

Oxidative CO2 Release over Quaternary Glacial Cycles.

The model results indicate that glacierized catchments are more likely than nonglacierized catchments to generate weathering fluxes with Alk:DIC < 1 (Fig. 4), the threshold for causing pCO2 changes over millennial timescales. Could the magnitude of changes in glacial weathering be significant as a CO2 source over timescales of Quaternary glacial–interglacial cycles? Simplistically taking the difference in median glacial vs. nonglacial SO4:Na ratios at face value (Fig. 2), glacial weathering might increase sulfide oxidation yields on average by ∼2.5× relative to nonglacial weathering. Assuming present-day global sulfide oxidation fluxes of ∼1.5 × 1012 mol/y S (44), Last Glacial Maximum (LGM) glacial weathering over 10% of global land area (14.5 × 106 km2; ref. 2) could generate an additional ∼2.25 × 1011 mol/y S, releasing up to 4.5 × 1011 mol CO2/y if sulfide oxidation produces CO2:SO4 in a 2:1 ratio (the maximum possible based on reaction stoichiometry; ref. 16). Over 10 ky, for example during glacial onset or deglaciation, the oxidation-related flux would then be on the order of ∼4.5 × 1015 mol CO2, equivalent to a ∼25 ppm increase in atmospheric concentration (for an atmosphere of 1.8 × 1020 mol). In the more extreme case of doubling the entire global sulfide oxidation flux, as suggested from modeling of Os isotope variations (42), the associated increase in SO4 flux of 7.5 × 1011 mol/y could potentially amount to a much larger CO2 release of 1.5 × 1016 mol, equivalent to a ∼80 ppm increase in CO2.

An increased oxidation flux of 2.5–7.5 × 1011 mol S/y associated with glaciation would require glacially facilitated erosion of 16–48 × 1014 g sediment/y, assuming average pyrite S content in eroded rocks of 0.5 wt% S, as is characteristic of sedimentary rocks (45). To produce this material, sediment yield from an LGM glaciated area of 14.5 × 106 km2 would have to be ∼110–330 t⋅km−2⋅y−1, well within the range of rates of glacially enhanced erosion rates observed during the present deglaciation (8). Resulting changes in concentrations of sulfate in the oceans or O2 in the atmosphere would be very small and likely difficult to detect relative to the much larger reservoirs. Driving all oxidation by atmospheric O2 would consume 5–14 × 1011 mol O2/y, or 5–14 × 1015 mol O2 over 10 ky, less than 0.01–0.04% of the atmospheric reservoir of ∼4 × 1019 mol O2 and within the range of observed declines in pO2 over the past 1 My (46). Similarly, proxies or models of total weathering fluxes to the oceans (36, 47, 48) may not capture modest increases in these sulfate fluxes because oxidative weathering is a small portion of the total global weathering flux.

The scenarios explored here are highly simplistic, based on approximate estimates of 20–100% changes in global sulfide oxidation fluxes resulting from glaciation, and release of CO2 in a 2:1 proportion to SO4. Moreover, the timing of enhanced oxidative weathering during glacial–interglacial cycles is not well known. In addition, these calculations do not include other biogeochemical processes that affect the carbon cycle, some of which may also be influenced by glacial erosion, such as organic matter cycling (e.g., refs. 49 and 50) and sulfur redox transformation (e.g., refs. 43 and 51). Nonetheless, our calculations serve to illustrate that oscillation of weathering between more and less sulfide oxidation over Quaternary timescales, expected based on the chemistry of present-day glacial weathering, could play a quantitatively meaningful role in the carbon cycle over glacial–interglacial cycles.

Glacial Weathering and Stable pCO2 for the Last 1 Myr.

Our analysis suggests that present-day glacial weathering causes net CO2 release, but measurements from ice cores show no long-term change in mean atmospheric pCO2 averaged over multiple glacial cycles during the past 1 My (52, 53). Highly responsive global silicate weathering feedbacks could provide one mechanism explaining constant pCO2 (54). Another explanation, consistent with our observation of glacially enhanced CO2 release in the present day, is that the extent of glaciation has adjusted over time such that the amount of CO2 released via glacial weathering compensates for the external driver that initially forced decreases in pCO2 and global cooling before 1 Ma. This forcing may have been via decreased fluxes from solid Earth degassing (6) or greater weatherability of continental silicate minerals (55⇓–57). As pCO2 dropped and climate cooled between 3 and 1 Ma (58), glacial advance would have stimulated greater release of CO2 until glaciation began producing enough CO2 during each glacial cycle to prevent further declines in atmospheric concentrations—eventually reaching a stable steady state as seen for the past 1 My. This hypothesis could potentially be tested with high-resolution pCO2 records during Pleistocene cooling between 3 and 1 Ma.

Climatic stability also followed the rapid cooling and onset of Antarctic glaciation at the Eocene–Oligocene transition, 34–33 Ma. We hypothesize that CO2 release from oxidative glacial weathering could have contributed to the transition to stable Oligocene climate, in addition to possible silicate weathering feedbacks (59). Nd and Pb isotope records from the southern Ocean suggest a brief pulse of increased Antarctic weathering, ∼33.9–33.5 Ma (60, 61). Longer-lived changes in 206Pb/204Pb are proposed to reflect increased carbonate weathering (61), and increases in 187Os/188Os, a potential proxy for weathering or organic- and sulfide-rich rocks (42), also continue after 33 Ma (62). Although aridity limits present-day Antarctic denudation and solute fluxes (63), sulfide oxidation associated with glaciation should at least be considered in evaluating the early Oligocene carbon cycle.

Glacial Weathering and Carbon Cycle Feedbacks over Earth’s History.

Over geologic timescales, instead of glacial erosion driving a positive feedback via silicate weathering and CO2 drawdown (5, 12), our data suggest that C release dominates in glacial settings, such that glacial weathering could act as a negative feedback preventing the Earth system from descending into colder conditions. A feedback via glacial sulfide oxidation depends on elevated oxidation rates being sustained over millions of years. Persistent increases in glacial erosion over these timescales, as suggested for the Plio-Pleistocene (5), could maintain increased oxidative weathering fluxes, providing a mechanism for weathering to limit the magnitude of global cooling. However, the evidence for glacially enhanced erosion rates continuing over >1 My remains controversial (e.g., refs. 8 and 64). Alternatively or in addition, low temperatures could nudge global weathering away from silicate weathering and thus toward lower Alk:DIC ratios.

Either way, our analysis points to sulfide oxidation as a plausible mechanism by which glaciation might be self-limiting, shifting the balance of the global carbon cycle by increasing CO2 supply to the atmosphere–ocean system (Fig. 5). The O2 concentration in the atmosphere places a further constraint on the extent to which such a mechanism may be effective (19). The O2-rich atmosphere of the Cenozoic holds a sufficiently large reservoir to accommodate changes in sulfide oxidation without changing atmospheric O2 concentrations outside of known bounds (16). At times of very low or effectively no atmospheric O2 concentration such as in the Archaean and Paleoproterozoic (65), glacially driven CO2 release by oxidative weathering may have been less active (e.g., ref. 66). This contrast raises the intriguing possibility of whether glaciations early in Earth’s history, when atmospheric O2 levels were lower, were prone to being more globally pervasive compared with glaciations in the Phanerozoic. Similarly, the effectiveness of an oxidative weathering feedback depends on the sulfide content of rocks undergoing weathering, hinting at the possibility that growth of a sulfide reservoir in Earth’s crust over time could have helped to stabilize climate.

Fig. 5.
Fig. 5.

Earth system feedbacks via glacial weathering. Sulfide oxidation seen in present-day glacial weathering may inhibit runaway glaciation. Such a stabilizing feedback may not operate at lower pO2, for example, early in Earth’s history or on other planets.

Acknowledgments

We thank three reviewers for constructive comments. M.A.T. was supported by a University of Southern California College Merit Fellowship and a Caltech Texaco Postdoctoral Fellowship, N.M. by a Deutscher Akademischer Austauschdienst exchange fellowship, J.H. by the German Science Foundation (DFG-project HA4472/6-1 and the Cluster of Excellence “CliSAP,” EXC177, Universität Hamburg) and Bundesministerium für Bildung und Forschung Project PALMOD (Ref 01LP1506C), and A.J.W. by National Science Foundation Grant EAR-1455352. Requests for access to the GloRiCh database should be addressed to J. Hartmann at geo@hattes.de.

Footnotes

  • ↵1M.A.T. and N.M. contributed equally to this work.

  • ↵2To whom correspondence should be addressed. Email: joshwest{at}usc.edu.

  • Author contributions: N.M., J.H., and A.J.W. designed research; M.A.T., N.M., and J.H. performed research; M.A.T. and J.F.A. contributed new reagents/analytic tools; M.A.T., N.M., J.H., J.F.A., and A.J.W. analyzed data; and M.A.T. and A.J.W. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1702953114/-/DCSupplemental.

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Glaciers, sulfide oxidation, and the carbon cycle
Mark A. Torres, Nils Moosdorf, Jens Hartmann, Jess F. Adkins, A. Joshua West
Proceedings of the National Academy of Sciences Aug 2017, 114 (33) 8716-8721; DOI: 10.1073/pnas.1702953114
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