The radiogenic and stable Sr isotope geochemistry of basalt weathering in Iceland: Role of hydrothermal calcite and implications for long-term climate regulation

Several studies have examined the geochemistry of Icelandic rivers to quantify the relationship between basalt weathering and long-term climate regulation. Recent research has suggested that the chemical weathering of hydrothermal and metamorphic calcite contributes significant quantities of HCO₃⁻ to the Icelandic riverine flux (Jacobson et al., 2015). Because the HCO₃⁻ derives from volcanic CO₂ that was sequestered in mineral form prior to atmospheric injection, the strength of the basalt weathering feedback occurring in Iceland may be lower than previously realized. To test these hypotheses, we analyzed the radiogenic and stable Sr isotope composition (⁸⁷Sr/⁸⁶Sr and δ^88/86Sr) of the same suite of water, rock, and mineral samples as examined in Jacobson et al. (2015), and we developed a simple model of the long-term C cycle that considers the transformation of volcanic CO₂ to HCO₃⁻ during subsurface silicate weathering, which is a precursor to hydrothermal calcite formation. Interpretations based on ⁸⁷Sr/⁸⁶Sr and Ca/Sr ratios suggest that conservative, three-component mixing between basalt, calcite, and atmospheric deposition adequately explains river geochemistry. On average, the δ^88/86Sr values of glacial and non-glacial rivers (0.414‰ and 0.388‰, respectively) are generally higher than those for basalt (0.276‰); however, calcite δ^88/86Sr values (0.347‰) are also higher than those for basalt and span the range of riverine values. Thus, riverine δ^88/86Sr values are also consistent three-component mixing between basalt, calcite, and atmospheric deposition. Isotopic fractionation is not required to explain riverine trends. Finally, model equations for the long-term C cycle demonstrate that subsurface silicate weathering reduces the magnitude of the volcanic CO₂ degassing flux, which in turn causes the atmosphere to stabilize at lower pCO₂ values compared to the case where no subsurface silicate weathering occurs. However, the proportion of the net volcanic C flux introduced to the atmosphere-ocean system as HCO₃⁻ after subsurface silicate weathering does not regulate long-term climate. Because hydrothermal calcite simply sequesters some of this HCO₃⁻ and delays its transmission to the atmosphere-ocean system until it dissolves at the surface later in time, it can be concluded the weathering of hydrothermal calcite bearing non-atmospheric C also has no effect on long-term climate regulation. Icelandic riverine HCO₃⁻ fluxes should be corrected for the hydrothermal calcite weathering contribution prior to quantifying atmospheric CO₂ consumption rates by basalt weathering at the Earth's surface.