Calcium isotope fractionation in sedimentary pore fluids from ODP Leg 175: Resolving carbonate recrystallization
Calcium isotope fractionation in sedimentary pore fluids from ODP Leg 175: Resolving carbonate recrystallization
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DOI：
10.1016/j.gca.2018.01.040
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发表时间：
2018-09
影响因子：
5
通讯作者：
H. Bradbury;A. Turchyn
中科院分区：
文献类型：
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作者：
H. Bradbury;A. Turchyn
我们目前的钙同位素数据孔隙流体和固体大洋钻探计划腿175：网站1081和1086（西非海岸）。这些地点在地理位置、沉积物年龄（从现代到800万年）和水深（800米）方面相似，但1081号地点碳酸盐贫乏，而1086号地点碳酸盐丰富。因此，这些地点适合于探索沉积物类型对碳酸盐溶解、沉淀和重结晶的影响。我们使用两种数值模拟方法来探索碳酸盐溶解和沉淀在沉积柱的速率。第一种是标准扩散反应方法，使用孔隙流体内的锶浓度来确定碳酸盐的溶解速率，然后将其应用于孔隙流体内的钙同位素的第二模型以计算沉淀速率。考虑到高沉降速率，我们还应用对流反应模型（Huber等人，2017年），这导致碳酸盐沉淀的深度分布相同，但总体速率显着更高，对此进行了讨论。在孔隙流体钙的钙同位素比增加的区域，我们的模型预测碳酸盐沉淀，并接近同位素平衡与固体的区域，我们的模型预测的溶解和沉淀的等效速率，类似于以前的研究结果。与以前在海洋沉积物中的发现相反，我们的模型需要碳酸盐沉淀的钙同位素分馏来拟合数据，因为流体和固体的δ 44 Ca之间存在偏移。使用从模型和先前公布的溶解无机碳的碳同位素剖面从这些网站确定的碳酸盐沉淀带，我们认为，自生碳酸盐的δ 13 C是均匀低于生物碳酸盐，但由一个更广泛的范围比以前建议。
We present calcium isotope data from pore fluids and solids from Ocean Drilling Program Leg 175: Sites 1081 and 1086 (off the coast of West Africa). These sites are similar with respect to geographic location, sediment age (from modern to 8 Myr), and water depth (800 m), but Site 1081 is carbonate-poor, whereas Site 1086 is carbonate-rich. Therefore, these sites are suited for the exploration of the influence of sediment type on carbonate dissolution, precipitation, and recrystallization. We use two numerical modelling approaches to explore the rates of carbonate dissolution and precipitation in the sediment column. The first is the standard diffusion-reaction approach, using the strontium concentration within the pore fluid to ascertain a dissolution rate for the carbonate, which is then applied to a second model of calcium isotopes within the pore fluid to calculate precipitation rates. Given the high sedimentation rates we also apply an advection-reaction model (Huber et al., 2017) which results in the same depth distribution of carbonate precipitation but significantly higher overall rates, which is discussed. Calcium isotope ratios in pore fluid calcium increase in zones where our model predicts carbonate precipitation, and approach isotopic equilibrium with the solid in zones where our model predicts equivalent rates of dissolution and precipitation, similar to previous findings. Contrary to previous findings in marine sediments, our model requires a calcium isotope fractionation on carbonate precipitation to fit the data, as there is an offset between the δ44Ca of the fluid and the solid. Using the zones of carbonate precipitation determined from the models and previously published carbon isotope profiles of the dissolved inorganic carbon from these sites, we suggest that the δ13C of the authigenic carbonate is uniformly lower than biogenic carbonate but by a wider range than was previously suggested.