Quantifying the effect of diagenetic recrystallization on the Mg isotopic composition of marine carbonates

The Mg and Sr isotopic compositions (delta Mg-26 and Sr-87/Sr-86) of pore fluids and bulk carbonates from Ocean Drilling Project Site 1171 (South Tasman Rise; 2148.2 m water depth) are reported, in order to evaluate the potential of diagenesis to alter carbonate-based geochemical proxies in an open marine system. Given the trace amounts of Mg in marine carbonates relative to coexisting pore fluids, diagenesis can alter carbonate delta Mg-26, a promising proxy for seawater delta Mg-26 that may help elucidate long-term changes in the global Mg cycle. Constraints on the effect of diagenetic recrystallization on carbonate delta Mg-26 are therefore critical for accurate proxy interpretations. This study provides context for assessing the fidelity of geochemical proxy-reconstructions using the primary components (i.e., foraminiferal tests and nannofossils) of bulk carbonate sediments. We find that pore fluid delta Mg-26 values (on the DSM3 scale) at Site 1171 increase systematically with depth (from -0.72 parts per thousand to -0.39 parts per thousand in the upper similar to 260 m), while the delta Mg-26 of bulk carbonates decrease systematically with depth (from -2.23 parts per thousand to -5.00 parts per thousand in the upper similar to 260 m). This variability is ascribed primarily to carbonate recrystallization, with a small proportion of the variability due to down-hole changes in nannofossil and foraminiferal species composition. The inferred effect of diagenesis on bulk carbonate delta Mg-26 correlates with down-core changes in Mg/Ca, Sr/Ca, Na/Ca, and Sr-87/Sr-86. A depositional reactive-transport model is employed to validate the hypothesis that calcite recrystallization in this system can generate sizeable shifts in carbonate delta Mg-26. Model fits to the data suggest a fractionation factor and a partition coefficient that are consistent with previous work, assuming calcite recrystallization rates of <= 7%/Ma constrained by Sr geochemistry. In addition, either partial dissolution or a distinctly different previous diagenetic regime must be invoked in order to explain aspects of the elemental chemistry and Sr-87/Sr-86 of relatively deep sediments from Holes A and C. This study indicates that the dynamics of a given sedimentary system can significantly alter bulk carbonate geochemistry, and presents a framework for considering the potential impact of such alteration on picked archives such as foraminiferal tests and nannofossils. Ultimately, this study contributes to the development of delta Mg-26 as a proxy for seawater delta Mg-26 by quantifying the susceptibility of carbonate delta Mg-26 to diagenetic alteration, particularly in sediments in open marine systems. This study suggests that because of the sensitivity of carbonate delta Mg-26 to diagenetic recrystallization, it can, in certain systems, be used to quantify the impact of diagenesis on carbonate-based geochemical proxies. (C) 2017 Elsevier Ltd. All rights reserved.