Andrew D. Jacobson

Carbonate versus silicate weathering in the Raikhot watershed within the High Himalayan Crystalline Series

The major element and Sr isotope geochemistry of surface waters, bedrock, and river sands was investigated in the Raikhot watershed within the High Himalayan Crystalline Series (HHCS) in northern Pakistan. Mass-balance calculations of mineral-weathering contributions to the dissolved flux of ions from the watershed indicate that 82% of the HCO 3 − flux is derived from the weathering of carbonate minerals and only 18% is derived from silicate weathering, even though the bedrock is predominantly quartzofeldspathic gneiss and granite with only ∼1% carbonate in the watershed. This study demonstrates the importance of trace amounts of bedrock carbonate in controlling the water chemistry of glacial watersheds. We suggest that the flux of Sr with a high 87 Sr/ 86 Sr ratio in the major Himalayan rivers may be derived in large part from weathering of trace amounts of calcite within the largely silicate HHCS. Models that use the flux of radiogenic Sr from the Himalayas as a proxy for silicate weathering rates may, therefore, overestimate the amount of CO 2 consumption due to silicate weathering in the Himalaya.

Climatic and tectonic controls on chemical weathering in the New Zealand Southern Alps

Abstract Climatic and tectonic controls on the relative abundance of solutes in streams draining the New Zealand Southern Alps were investigated by analyzing the elemental and Sr isotope geochemistry of stream waters, bedload sediment, and hydrothermal calcite veins. The average relative molar abundance of major cations and Si in all stream waters follows the order Ca2+ (50%) > Si (22%) > Na+ (17%) > Mg2+ (6%) > K+ (5%). For major anions, the relative molar abundance is HCO3− (89%) > SO42− (7%) > Cl− (4%). Weathering reactions involving plagioclase and volumetrically small amounts of hydrothermal calcite define the ionic chemistry of stream waters, but nearly all streams have a carbonate-dominated Ca2+ and HCO3− mass-balance. Stream water Ca/Sr and 87Sr/86Sr ratios vary from 0.173 to 0.439 μmol/nmol and from 0.7078 to 0.7114, respectively. Consistent with the ionic budget, these ratios lie solely within the range of values measured for bedload carbonate (Ca/Sr = 0.178 to 0.886 μmol/nmol; 87Sr/86Sr = 0.7081 to 0.7118) and hydrothermal calcite veins (Ca/Sr = 0.491 to 3.33 μmol/nmol; 87Sr/86Sr = 0.7076 to 0.7097). Streams draining regions in the Southern Alps with high rates of physical erosion induced by rapid tectonic uplift and an extremely wet climate contain ∼10% more Ca2+ and ∼30% more Sr2+ from carbonate weathering compared to streams draining regions in drier, more stable landscapes. Similarly, streams draining glaciated watersheds contain ∼25% more Sr2+ from carbonate weathering compared to streams draining non-glaciated watersheds. The highest abundance of carbonate-derived solutes in the most physically active regions of the Southern Alps is attributed to the tectonic exhumation and mechanical denudation of metamorphic bedrock, which contains trace amounts of calcite estimated to weather ∼350 times faster than plagioclase in this environment. In contrast, regions in the Southern Alps experiencing lower rates of uplift and erosion have a greater abundance of silicate- versus carbonate-derived cations. These findings highlight a strong coupling between physical controls on landscape development and sources of solutes to stream waters. Using the Southern Alps as a model for assessing the role of active tectonics in geochemical cycles, this study suggests that rapid mountain uplift results in an enhanced influence of carbonate weathering on the dissolved ion composition delivered to seawater.

Reconciling the elemental and Sr isotope composition of Himalayan weathering fluxes: insights from the carbonate geochemistry of stream waters

Determining the relative proportions of silicate vs. carbonate weathering in the Himalaya is important for understanding atmospheric CO2 consumption rates and the temporal evolution of seawater Sr. However, recent studies have shown that major element mass-balance equations attribute less CO2 consumption to silicate weathering than methods utilizing Ca/Sr and 87Sr/86Sr mixing equations. To investigate this problem, we compiled literature data providing elemental and 87Sr/86Sr analyses for stream waters and bedrock from tributary watersheds throughout the Himalaya Mountains. In addition, carbonate system parameters (PCO2, mineral saturation states) were evaluated for a selected suite of stream waters. The apparent discrepancy between the dominant weathering source of dissolved major elements vs. Sr can be reconciled in terms of carbonate mineral equilibria. Himalayan streams are predominantly Ca2+-Mg2+-HCO3− waters derived from calcite and dolomite dissolution, and mass-balance calculations demonstrate that carbonate weathering contributes ∼87% and ∼76% of the dissolved Ca2+ and Sr2+, respectively. However, calculated Ca/Sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock, suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and PCO2 gradients in the Himalaya. The state of calcite and dolomite saturation was evaluated across these gradients, and the data show that upon descending through the Himalaya, ∼50% of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas CO2. Stream water Ca/Mg and Ca/Sr ratios decrease as the degree of supersaturation with respect to calcite increases, and Mg2+, Ca2+, and HCO3− mass balances support interpretations of preferential Ca2+ removal by calcite precipitation. On the basis of patterns of saturation state and PCO2 changes, calcite precipitation was estimated to remove up to ∼70% of the Ca2+ originally derived from carbonate weathering. Accounting for the nonconservative behavior of Ca2+ during riverine transport brings the Ca/Sr and 87Sr/86Sr composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock, thereby permitting consistency between elemental and Sr isotope approaches to partitioning stream water solute sources. These results resolve the dissolved Sr2+ budget and suggest that the conventional application of two-component Ca/Sr and 87Sr/86Sr mixing equations has overestimated silicate-derived Sr2+ and HCO3− fluxes from the Himalaya. In addition, these findings demonstrate that integrating stream water carbonate mineral equilibria, divalent cation compositional trends, and Sr isotope inventories provides a powerful approach for examining weathering fluxes.

Ca/Sr and 87Sr/86Sr geochemistry of disseminated calcite in Himalayan silicate rocks from Nanga Parbat: Influence on river-water chemistry

Trace amounts of disseminated calcite were identified in gneiss, schist, and granite bedrock sampled from the Raikhot watershed and other locations within the Nanga Parbat massif of northern Pakistan. The calcite grains occur interstitially within individual silicate minerals, at grain boundaries, and as fracture fillings that transect the mineralogic fabric of the rock. Disseminated calcite composes is ≤ 0.29 wt% of silicate rocks sampled in the Raikhot watershed and has Ca/Sr (µmol/nmol) and 87Sr/86Sr ratios that range from 0.878 to 5.33 and from 0.794 039 to 0.930 619, respectively. Elsewhere in the Nanga Parbat region, disseminated calcite composes ≤ 3.6 wt% of the silicate rock samples and has Ca/Sr (µmol/nmol) and 87Sr/86Sr ratios that range from 0.535 to 3.33 and from 0.715 757 to 0.771 244, respectively. For all samples, the 87Sr/86Sr ratios of disseminated calcite are similar to the 87Sr/86Sr ratios measured in the silicate host rock. Within the partially glaciated Raikhot watershed, the rapid weathering of disseminated calcite with high Ca/Sr and 87Sr/86Sr ratios has a strong influence on the chemical composition of stream water and exceeds contributions from silicate mineral dissolution. Comparisons of disseminated calcite compositions with source-water chemistry throughout the Himalaya suggest that disseminated calcite may be a more important component of Himalayan silicate rocks than previously recognized. Therefore, calculations relating the Sr isotope geochemistry of Himalayan rivers to atmospheric CO2 consumption should consider this widespread and compositionally variable carbonate end member.

Ca/Sr and Sr isotope systematics of a Himalayan glacial chronosequence: Carbonate versus silicate weathering rates as a function of landscape surface age

We explored changes in the relative importance of carbonate vs. silicate weathering as a function of landscape surface age by examining the Ca/Sr and Sr isotope systematics of a glacial soil chronosequence located in the Raikhot watershed within the Himalaya of northern Pakistan. Bedrock in the Raikhot watershed primarily consists of silicate rock (Ca/Sr 0.20 mol/nmol, 87 Sr/ 86 Sr 0.77 to 1.2) with minor amounts of disseminated calcite (Ca/Sr 0.98 to 5.3 mol/nmol, 87 Sr/ 86 Sr 0.79 to 0.93) and metasedimentary carbonate (Ca/Sr 1.0 to 2.8 mol/nmol, 87 Sr/ 86 Sr 0.72 to 0.82). Analysis of the exchangeable, carbonate, and silicate fractions of seven soil profiles ranging in age from 0.5 to 55 kyr revealed that carbonate dissolution provides more than 90% of the weathering-derived Ca and Sr for at least 55 kyr after the exposure of rock surfaces, even though carbonate represents only 1.0 wt% of fresh glacial till. The accumulation of carbonate-bearing dust deposited on the surfaces of older landforms partly sustains the longevity of the carbonate weathering flux. As the average landscape surface age in the Raikhot watershed increases, the Ca/Sr and 87 Sr/ 86 Sr ratios released by carbonate weathering decrease from 3.6 to 0.20 mol/nmol and 0.84 to 0.72, respectively. The transition from high to low Ca/Sr ratios during weathering appears to reflect the greater solubility of high Ca/Sr ratio carbonate relative to low Ca/Sr ratio carbonate. These findings suggest that carbonate weathering controls the dissolved flux of Sr emanating from stable Himalayan landforms comprising mixed silicate and carbonate rock for tens of thousands of years after the mechanical exposure of rock surfaces to the weathering environment. Copyright

Relationship between mechanical erosion and atmospheric CO2 consumption in the New Zealand Southern Alps

To examine the influence of mountain uplift on the long-term carbon cycle, we used geochemical, hydrologic, and suspended-load data for 12 streams draining the New Zealand Southern Alps to quantify rates of erosion, weathering, and atmospheric CO 2 consumption. Rapid uplift in the western Southern Alps elevates mechanical erosion rates by a factor of ∼13 relative to those on the tectonically stable eastern side [125 × 10 8 vs. 9.4 × 10 8 g/(km 2 ·yr), respectively]. Similarly, the average chemical weathering rate is ∼5 times higher on the western compared to eastern side of the mountain range [9.8 × 10 7 vs. 2.0 × 10 7 g/(km 2 ·yr), respectively]. However, because the proportion of stream-water Ca 2+ and Mg 2+ from carbonate weathering increases as the rate of mechanical erosion increases, the long-term atmospheric CO 2 consumption rate on the western side is ∼2 times higher than that on the eastern side [14 × 10 4 vs. 6.9 × 10 4 mol/(km 2 ·yr), respectively] and only ∼1.5 times higher than the global mean value [∼9 × 10 4 mol/(km 2 ·yr)]. Data for major world rivers (including Himalayan rivers) provide a consistent interpretation regarding the relationship between mechanical erosion intensity and the ratio of silicate to carbonate weathering. Thus, we conclude that mountain building increases atmospheric CO 2 consumption rates by only a factor of ∼2, which is much lower than previous estimates.

Watershed reconstruction of a Paleocene–Eocene lake basin using Sr isotopes in carbonate rocks

Provenance studies have used Sr isotopes ( 87 Sr/ 86 Sr) of silicate source rocks as a link to their eroded basinal equivalents. This technique, however, cannot identify the proportional inputs from different watersheds or define more precisely sedimentation events due to tectonic or climatic change. Erosion of carbonate rocks dominates the Sr input within basin drainage and potentially can be used through 87 Sr/ 86 Sr ratios to reconstruct paleohydrology of the entire basin and trace watershed inputs and depositional patterns in continental basins. The Sr isotopic ratios from waters of the source area, allowing for the mixing of shallow groundwater and surface water along the transport path, are homogenized in the basinal carbonate sediments. Mineralogy and diagenesis of carbonate rocks generally do not affect the Sr isotopic signal in a near-surface system lacking external influence by volcanism, eolian dust, or deep geothermal waters. The 87 Sr/ 86 Sr ratios from the source area are directly comparable to those in the receiving continental basin. The Sr isotopic signal of the Paleocene–Eocene Flagstaff Formation (central Utah), a carbonate lake deposit in a foreland basin, is compared to that of projected source waters draining its thrust front, the Sevier fold-thrust belt. Freshwater carbonates compose a large portion of the lowermost Ferron Mountain and uppermost Musinia Peak Members of the formation, whereas gypsum and carbonates predominate in the middle Cove Mountain Member. Previous research had attributed gypsum deposition to the deposition of the middle Cove Mountain Member to either climatic change or unroofing of diapirs of Jurassic gypsiferous carbonates. To examine more closely the influence of climate versus tectonics on Flagstaff sedimentation as well as the efficacy of provenance studies using carbonates, we collected rock samples from the three members of the formation on the Wasatch Plateau of central Utah in addition to sampling stream water associated with Pennsylvanian–Permian and Jurassic carbonate terrains from the nearby thrust front. The 87 Sr/ 86 Sr ratios in carbonates and gyprock belonging to the Flagstaff Formation remained unchanged during deposition, the average Sr isotope composition of the Flagstaff rocks being identical to that of sampled waters draining the projected provenance area. There was little change in source rock weathering as the thrust front evolved. Deposition of gypsum occurred in the basinal lake only during the deposition of the middle Cove Mountain Member, despite its constant exposure in the drainage area, suggesting a changing balance of tectonic and climatic controls during lake sedimentation. The 87 Sr/ 86 Sr isotopic studies targeting carbonate rocks and their presumed source waters are a simple but accurate method for reconstructing the paleohydrology of lake basins.

Optimization of a 48Ca–43Ca double-spike MC-TIMS method for measuring Ca isotope ratios (δ44/40Ca and δ44/42Ca): limitations from filament reservoir mixing

We used a Monte Carlo error model to optimize a 48Ca–43Ca double-spike technique for measuring Ca isotope ratios (δ44/40Ca and δ44/42Ca) by Multi-Collector Thermal Ionization Mass Spectrometry (MC-TIMS). The model considers errors for counting statistics and Johnson noise, as well as changes in collector cup efficiency (drift). For a 20 V 40Ca ion-beam implemented in a three-hop, dynamic multi-collection routine, the model predicts that a wide range of 48Ca/43Ca and spike/sample ratios should yield internal precisions (2σSEM) of 0.015–0.020‰ for δ44/40Ca and 0.025–0.030‰ for δ44/42Ca. Using a Thermo Fisher MC-TIMS (Triton), we tested 48Ca/43Ca = 1.5 [43Ca/(48Ca + 43Ca) = 0.40 mol mol−1] and spike/sample = 0.66 [Cadsp/(Cadsp + Casmp) = 0.40 mol mol−1] by repeatedly analyzing OSIL Atlantic seawater, NIST SRM 915a, NIST SRM 915b, USGS BHVO-1, and CaF2 over 4 sessions spanning 1 month. While the measured internal precisions generally agreed with model predictions, external reproducibility (2σSD) was much worse than expected. For the 81 measurements made, the average external reproducibility was 0.223‰ for δ44/40Ca and 0.126‰ for δ44/42Ca. After processing raw data through the double-spike equations, nearly all fractionation-corrected ratios showed remnant fractionation patterns. Such patterns reflect deviation from ideal exponential mass-fractionation due to mixing of multiple, independently fractionating reservoirs on the filament. Additional model simulations, as well as comparison against δ44/40Ca values determined with a 43Ca–42Ca double-spike, support the concept of an “average mass rule”, which states that inaccuracies in fractionation-corrected data are greater for isotope ratios having an average mass further away from the average mass of the normalizing ratio. Until advancements are made to eliminate filament reservoir effects, 43Ca–42Ca and 46Ca–43Ca double-spikes should yield the most precise δ44/40Ca and δ44/42Ca values, respectively, when using MC-TIMS. Within the limits of the 48Ca–43Ca double-spike technique, we observed no evidence for 40Ca enrichments among the standards analyzed. Finally, we found that sample matrix effects do not influence the quality of Ca isotope measurements by MC-TIMS, and we tentatively propose that the external reproducibility determined from the repeated analysis of standards can represent the uncertainty of a single sample analysis.

Technical Note: Calcium and carbon stable isotope ratios as paleodietary indicators.

Calcium stable isotope ratios are hypothesized to vary as a function of trophic level. This premise raises the possibility of using calcium stable isotope ratios to study the dietary behaviors of fossil taxa and to test competing hypotheses on the adaptive origins of euprimates. To explore this concept, we measured the stable isotope composition of contemporary mammals in northern Borneo and northwestern Costa Rica, two communities with functional or phylogenetic relevance to primate origins. We found that bone collagen δ(13) C and δ(15) N values could differentiate trophic levels in each assemblage, a result that justifies the use of these systems to test the predicted inverse relationship between bioapatite δ(13) C and δ(44) Ca values. As expected, taxonomic carnivores (felids) showed a combination of high δ(13) C and low δ(44) Ca values; however, the δ(44) Ca values of other faunivores were indistinguishable from those of primary consumers. We suggest that the trophic insensitivity of most bioapatite δ(44) Ca values is attributable to the negligible calcium content of arthropod prey. Although the present results are inconclusive, the tandem analysis of δ(44) Ca and δ(13) C values in fossils continues to hold promise for informing paleodietary studies and we highlight this potential by drawing attention to the stable isotope composition of the Early Eocene primate Cantius.

On the geochemical heterogeneity of rivers draining into the straits and channels of the Canadian Arctic Archipelago

Ten rivers across northern Canada and the Canadian Arctic Archipelago (CAA) were sampled during spring 2014 and summer 2015 to investigate their geochemical heterogeneity for comparison against larger North American (i.e., Mackenzie and Yukon Rivers) and Siberian rivers. In general, rivers draining the western and/or northern regions of the study area have higher solute concentrations and lower 87Sr/86Sr ratios compared to rivers draining the eastern and/or southern regions. The inorganic geochemical signatures largely reflect the bedrock geology, which is predominately carbonate in the western and/or northern regions and silicate in the eastern and/or southern regions. Riverine δ18O values primarily correlate with latitude, with only a few exceptions. Measurements of total alkalinity (TA) were combined with a regional analysis of bedrock geology and extrapolated to produce a range for the mean characteristic TA of rivers draining into the straits and channels of the CAA (628–819 µeq kg−1). Combining this estimate with contributions from the Mackenzie River yields a revised North American river runoff TA of 935–1182 µeq kg−1, which is much lower than that of the Mackenzie River (1540 µeq kg−1). This lower concentration suggests that TA may not be used to distinguish between North American and Siberian river contributions in regions such as Davis Strait.