Calcium isotope ratios of pore fluids and solid phase of organic rich sediments

Pore fluid calcium isotope, calcium concentration and strontium concentration data are used to measure the rates of diagenetic dissolution and precipitation of calcite in deep-sea sediments containing abundant clay and organic material. This type of study of deep-sea sediment diagenesis provides unique information about the ultra-slow chemical reactions that occur in natural marine sediments that affect global geochemical cycles and the preservation of paleo-environmental information in carbonate fossils. For this study, calcium isotope ratios (d44/40Ca) of pore fluid calcium from Ocean Drilling Program (ODP) Sites 984 (North Atlantic) and 1082 (off the coast of West Africa) were measured to augment available pore fluid measurements of calcium and strontium concentration. Both study sites have high sedimentation rates and support quantitative sulfate reduction, methanogenesis and anaerobic methane oxidation. The pattern of change of d44/40Ca of pore fluid calcium versus depth at Sites 984 and 1082 differs markedly from that of previously studied deep-sea Sites like 590B and 807, which are composed of nearly pure carbonate sediment. In the 984 and 1082 pore fluids, d44/40Ca remains elevated near seawater values deep in the sediments, rather than shifting rapidly toward the d44/40Ca of carbonate solids. This observation indicates that the rate of calcite dissolution is far lower than at previously studied carbonate-rich sites. The data are fit using a numerical model, as well as more approximate analytical models, to estimate the rates of carbonate dissolution and precipitation and the relationship of these rates to the abundance of clay and organic material. Our models give mutually consistent results and indicate that calcite dissolution rates at Sites 984 and 1082 are roughly two orders of magnitude lower than at previously studied carbonate-rich sites, and the rate correlates with the abundance of clay. Our calculated rates are conservative for these sites (the actual rates could be significantly slower) because other processes that impact the calcium isotope composition of sedimentary pore fluid have not been included. The results provide direct geochemical evidence for the anecdotal observation that the best-preserved carbonate fossils are often found in clay or organic-rich sedimentary horizons. The results also suggest that the presence of clay minerals has a strong passivating effect on the surfaces of biogenic carbonate minerals, slowing dissolution dramatically even in relation to the already-slow rates typical of carbonate-rich sediments.孔隙流体钙同位素、钙浓度与锶浓度数据，被用于测定富含黏土与有机质的深海沉积物中方解石的成岩溶蚀与沉淀速率。此类深海沉积物成岩作用研究，可获取天然海洋沉积物中发生的超慢速化学反应的独有信息——这类反应会影响全球地球化学循环，以及碳酸盐化石中古环境信息的保存状态。本研究针对大洋钻探计划（Ocean Drilling Program, ODP）984站位（北大西洋）与1082站位（西非近海）的孔隙流体钙同位素比值（δ⁴⁴/⁴⁰Ca）进行测定，以补充现有的钙、锶浓度孔隙流体测量数据。两个研究站位均具有较高的沉积速率，且发育完整的硫酸盐还原、甲烷生成与厌氧甲烷氧化过程。984与1082站位的孔隙流体钙δ⁴⁴/⁴⁰Ca随深度的变化模式，与此前研究的近乎纯碳酸盐沉积的深海站位（如590B、807站位）存在显著差异。在984与1082站位的孔隙流体中，δ⁴⁴/⁴⁰Ca在沉积物深部仍维持在接近海水的高值，并未快速向碳酸盐固相的δ⁴⁴/⁴⁰Ca偏移。该观测结果表明，本研究站位的方解石溶蚀速率远低于此前研究的富碳酸盐沉积站位。本研究通过数值模型与更简化的解析模型对数据进行拟合，以估算碳酸盐溶蚀与沉淀速率，以及该速率与黏土、有机质丰度的关联关系。本研究构建的模型结果相互一致，显示984与1082站位的方解石溶蚀速率约比此前研究的富碳酸盐沉积站位低两个数量级，且溶蚀速率与黏土丰度呈显著相关。由于未纳入其他会影响沉积孔隙流体钙同位素组成的过程，本研究计算得到的速率对于该站位而言属于保守估计（实际速率可能显著更低）。本研究结果为一则经验性观测提供了直接地球化学佐证：保存最为完好的碳酸盐化石通常产自黏土或富有机质沉积层位。此外，研究结果还表明，黏土矿物对生物成因碳酸盐矿物的表面具有极强的钝化作用，即便相较于富碳酸盐沉积本已偏低的溶蚀速率，该作用仍可大幅减缓溶蚀过程。