|کد مقاله||کد نشریه||سال انتشار||مقاله انگلیسی||ترجمه فارسی||نسخه تمام متن|
|5783547||1637953||2017||21 صفحه PDF||سفارش دهید||دانلود رایگان|
The Mg and Sr isotopic compositions (Î´26Mg and 87Sr/86Sr) of pore fluids and bulk carbonates from Ocean Drilling Project Site 1171 (South Tasman Rise; 2148.2Â 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 Î´26Mg, a promising proxy for seawater Î´26Mg that may help elucidate long-term changes in the global Mg cycle. Constraints on the effect of diagenetic recrystallization on carbonate Î´26Mg 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 Î´26Mg values (on the DSM3 scale) at Site 1171 increase systematically with depth (from â0.72â° to â0.39â° in the upper â¼260Â m), while the Î´26Mg of bulk carbonates decrease systematically with depth (from â2.23â° to â5.00â° in the upper â¼260Â 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 Î´26Mg correlates with down-core changes in Mg/Ca, Sr/Ca, Na/Ca, and 87Sr/86Sr. A depositional reactive-transport model is employed to validate the hypothesis that calcite recrystallization in this system can generate sizeable shifts in carbonate Î´26Mg. Model fits to the data suggest a fractionation factor and a partition coefficient that are consistent with previous work, assuming calcite recrystallization rates of â©½7%/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 87Sr/86Sr 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 Î´26Mg as a proxy for seawater Î´26Mg by quantifying the susceptibility of carbonate Î´26Mg to diagenetic alteration, particularly in sediments in open marine systems. This study suggests that because of the sensitivity of carbonate Î´26Mg to diagenetic recrystallization, it can, in certain systems, be used to quantify the impact of diagenesis on carbonate-based geochemical proxies.
Journal: Geochimica et Cosmochimica Acta - Volume 204, 1 May 2017, Pages 219-239