Geochemistry and Mg-isotopes of carbonate sediments and porefluids of ODP Site 130-807

Deep-sea pore fluids are potential archives of ancient seawater chemistry. However, the primary signal recorded in pore fluids is often overprinted by diagenetic processes. Recent studies have suggested that depth profiles of Mg concentration in deep-sea carbonate pore fluids are best explained by a rapid rise in seawater Mg over the last 10-20 Myr. To explore this possibility we measured the Mg isotopic composition of pore fluids and carbonate sediments from Ocean Drilling Program (ODP) site 807. Whereas the concentration of Mg in the pore fluid declines with depth, the isotopic composition of Mg in the pore fluid increases from -0.78 per mil near the sediment–water interface to -0.15 per mil at 778 mbsf. The Mg isotopic composition of the sediment, with few important exceptions, does not change with depth and has an average d26Mg value of -4.72 per mil. We reproduce the observed changes in sediment and pore-fluid Mg isotope values using a numerical model that incorporates Mg, Ca and Sr cycling and satisfies existing pore-fluid Ca isotope and Sr data. Our model shows that the observed trends in magnesium concentrations and isotopes are best explained as a combination of two processes: a secular rise in the seawater Mg over the Neogene and the recrystallization of low-Mg biogenic carbonate to a higher-Mg diagenetic calcite. These results indicate that burial recrystallization will add Mg to pelagic carbonate sediments, leading to an overestimation of paleo-temperatures from measured Mg/Ca ratios. The Mg isotopic composition of foraminiferal calcite appears to be only slightly altered by recrystallization making it possible to reconstruct the Mg isotopic composition of seawater through time.

深海孔隙流体是记录古海水化学组成的潜在信息储库。然而，孔隙流体所记录的原始信号往往会受到成岩作用的叠加改造。近期研究表明，深海碳酸盐孔隙流体中镁浓度的深度剖面，最适合用过去1000万至2000万年间海水镁含量的快速升高来解释。为探究这一可能性，我们对大洋钻探计划（Ocean Drilling Program, ODP）807站位的孔隙流体与碳酸盐沉积物的镁同位素组成开展了测定。尽管孔隙流体中的镁浓度随深度增加而降低，但其镁同位素组成却从沉积物-水界面附近的-0.78‰升高至778米海底以下（meters below sea floor, mbsf）处的-0.15‰。沉积物的镁同位素组成几乎不随深度变化，其平均δ²⁶Mg值为-4.72‰，仅存在少数重要例外。我们采用整合了镁、钙、锶循环过程的数值模型，重现了沉积物与孔隙流体中镁同位素值的观测变化，且该模型符合已有的孔隙流体钙同位素与锶同位素观测数据。我们的模型显示，观测到的镁浓度与同位素变化趋势，最适合通过两种过程的共同作用来解释：新近纪时期海水镁含量的长期持续升高，以及低镁生物碳酸盐向高镁成岩方解石的重结晶作用。这些结果表明，埋藏重结晶作用会向远洋碳酸盐沉积物中引入镁，导致通过实测镁钙比估算古温度时出现高估。有孔虫方解石的镁同位素组成似乎仅会因重结晶作用发生轻微改变，这使得通过地质记录逐时重建海水镁同位素组成成为可能。