Calcium isotopes in deep time: Potential and limitations
Calcium isotopes in deep time: Potential and limitations
复制标题
DOI：
10.1016/j.chemgeo.2020.119601
复制
发表时间：
2020-06
期刊：
影响因子：
3.9
通讯作者：
N. Gussone;A. Ahm;K. Lau;H. Bradbury
中科院分区：
文献类型：
--
作者：
N. Gussone;A. Ahm;K. Lau;H. Bradbury
钙是调节地球长期气候状态的生物地球化学循环中的一种基本元素。二氧化碳从海洋-大气系统中的去除受到碳酸盐沉积物(CaCO3)埋藏的控制，最终将全球的钙和碳循环联系在一起。这一基本联系推动了稳定钙同位素替代品的发展，并将其应用于古代骨骼和非骨骼块状碳酸盐沉积物。钙同位素比率(δ44/40Ca)被用来跟踪海水化学的长期变化(例如，文石与方解石海)，并阐明与大规模灭绝事件有关的短期气候扰动。然而，钙同位素替代物的发展表明，碳酸盐矿物中的δ44/40Ca值对沉积速率、矿物学和成岩作用的变化也很敏感，从而使该替代物在全球旋回重建中的应用复杂化。首先，无机碳酸盐沉淀实验表明，碳酸盐δ44/40Ca值对沉淀速率很敏感，速率越高，分馏程度越大。其次，δ44/40Ca值对碳酸盐矿物学十分敏感，无机文石和方解石较同期海水平均亏损约1.5‰和0~0.9‰。碳酸盐矿物学和沉淀速率的变化对原生矿物和次生矿物都有影响，但在碳酸盐成岩过程中尤为明显，此时重结晶和新形态的速率相对较慢，可导致整体沉积物δ44/40Ca值发生显著变化。第三，骨骼化石档案中表达的动物群组成的变化可以导致碳酸盐δ44/40Ca值的巨大变化，而这些值与全球旋回的变化是脱钩的。然而，当这些因素被适当考虑时，钙同位素在古代碳酸盐沉积物中的应用成为了解许多尺度上的生物地球化学过程的有力工具；从沉积物孔隙空间的成岩变化，到古碳酸盐台地的区域变化，以及随着时间的推移海水化学的全球变化。重要的是，导致碳酸盐δ44/40Ca值变化的过程可能会影响其他与碳酸盐结合的代用品，突显了钙同位素作为更好地了解其他同位素体系变化的工具的潜力。
Calcium is an essential element in the biogeochemical cycles that regulate the long-term climate state of Earth. The removal of CO2from the ocean-atmosphere system is controlled by the burial of carbonate sediments (CaCO3), ultimately linking the global calcium and carbon cycles. This fundamental link has driven the development of the stable calcium isotope proxy with application to both ancient skeletal and non-skeletal bulk carbonate sediments. Calcium isotope ratios (δ44/40Ca) have been used to track long-term changes in seawater chemistry (e.g., aragonite vs. calcite seas) and to elucidate short-term climatic perturbations associated with mass extinction events. However, developments in the calcium isotope proxy have shown that δ44/40Ca values in carbonate minerals also are sensitive to changes in precipitation rates, mineralogy and diagenesis, thereby complicating the application of the proxy to the reconstruction of global cycles. First, inorganic carbonate precipitation experiments have demonstrated that carbonate δ44/40Ca values are sensitive to precipitation rates with higher rates generally leading to larger fractionation. Second, δ44/40Ca values are sensitive to carbonate mineralogy with inorganic aragonite and calcite being on average ~ 1.5‰ and ~ 0.9‰ depleted relative to contemporaneous seawater, respectively. The effects of both changes in carbonate mineralogy and precipitation rates affect primary and secondary minerals, but are particularly pronounced during carbonate diagenesis where relatively slow rates of recrystallization and neomorphism can lead to significant changes in bulk sediment δ44/40Ca values. Third, changes in faunal composition expressed in skeletal fossil archives can lead to large changes in carbonate δ44/40Ca values that are decoupled from changes in global cycles. Nevertheless, when these factors are appropriately considered the application of calcium isotopes in ancient carbonate sediments becomes a powerful tool for understanding biogeochemical processes that operate over many scales; from diagenetic changes within the sediment pore-space, to regional changes across ancient carbonate platforms, and to global changes in seawater chemistry through time. Importantly, the processes that contribute to variability in carbonate δ44/40Ca values are likely to impact other carbonate-bound proxies, highlighting the potential for calcium isotopes as a tool to better understand the variability of other isotope systems.