Tightly coupled records of Ca and C isotope changes during the Hirnantian glaciation event in an epeiric sea setting

A delta Ca-44/40 excursion of -0.5 parts per thousand is recorded during the Hirnantian glaciation event in carbonate rocks of the Monitor Range section in Nevada. The timeframe for the glaciation is 1.03 +/- 0.2 Ma. The delta Ca-44/40 changes are synchronous with lithofacies and biofacies indicators of sea-level change, and co-vary negatively with a large positive delta C-13 excursion of 7 parts per thousand that was previously interpreted to reflect local C-cycling effects in circulation-restricted waters of the (epeiric) Martin Ridge Basin. The synchronousness of the Ca and C isotope excursions is inconsistent with what we know of Ca and C geochemical cycles in the modern ocean because the residence time of Ca is longer than that of C. A box model is used to illustrate that the negative delta Ca-44/40 excursion would take longer to recover than the positive delta C-13 excursion if the relative timescales of the Hirnantian Ca and C cycles were scaled similarly to those in the ocean today. One way to speed up the timescale of the Ca-isotope response is to decrease the size of the Ca-reservoir. This is what is accomplished when seawater circulation is restricted between oceans and epeiric seas. Approximately 30% of the -0.5 parts per thousand shift in Hirnantian delta Ca-44/40 values may be attributed to lower sea surface temperatures in the tropics during glaciation. The remainder is attributed to local Ca-cycling effects in the Martin Ridge Basin, which include an increase in isotopically light Ca-inputs by submarine groundwater discharge (SGD) and a larger kinetic isotope fractionation effect due to increased carbonate precipitation rate. The hypothesized increase in carbonate precipitation rate may have been caused by high photosynthesis rates, maintained by high nutrient fluxes from SGD, which increased the local carbonate-saturation state of the waters. Additionally, the conditions in this setting may have fostered a suite of organisms employing a calcification mechanism that involved microenvironments (bacterial or algal surfaces, biofilms, or sedimentary pore fluids) where biological activity could effectively alter the carbonate chemistry in very small C-reservoirs. A similar scenario was presented by LaPorte et al. (2009) to explain the bulk of the anomalously large 7 parts per thousand positive delta C-13 excursion in the same section. The strong negative coupling between the stratigraphic records of Ca and C isotope changes in the Martin Ridge Basin may serve as a tool to identify other epeiric sea settings in the rock record where similar physical and ecological attributes developed during periods of restricted circulation brought on by low sea-level. (C) 2012 Elsevier Ltd. All rights reserved.