Calcium isotopes in the global biogeochemical Ca cycle: Implications for development of a Ca isotope proxy
Calcium isotopes in the global biogeochemical Ca cycle: Implications for development of a Ca isotope proxy
复制标题
DOI：
10.1016/j.earscirev.2013.10.004
复制
发表时间：
2014-02-01
影响因子：
12.1
通讯作者：
Tipper, Edward T.
中科院分区：
文献类型：
--
作者：
Fantle, Matthew S.;Tipper, Edward T.
在地球表面，钙（Ca）是各种尺度的关键元素：它可溶于天然沃茨，是生物营养素，也是海洋中碳的主要矿物质汇（CaCO 3）的主要成分。地球上各种钙储层的钙同位素组成（44 CarmCa表示为Oita）存在4%的变化，表明钙同位素是一种很有前途的钙循环示踪剂。15年来高精度的Ca同位素测量揭示了许多关于Ca同位素在地球表面环境中的行为，但关于如何使用Ca同位素来阐明海洋和陆地Ca循环仍然存在一些基本问题。目前的工作提出了70多个已发表的Ca同位素研究的数据汇编，在一个共同的三角洲尺度上总共提出了2600多个测量值，其中包括河流和地下水，灰尘，土壤和土壤孔隙流体，植被，雨水，硅酸盐矿物/岩石和自生海洋矿物的数据（碳酸盐、硫酸盐和磷酸盐，包括现代和古代）。资料汇编表明：（1）碳酸盐和硫酸盐之间存在着显著的差异。（2）河流沉积的644 Ca（0.88%0）不能简单地反映碳酸盐沉积的644 Ca;（3）陆生植被的Ca同位素组成范围最大，为-3.5%0。我们在全球Ca循环的背景下讨论这些观测结果，探索海水644 Ca变化的可行程度以及如何在地质时间尺度上实现海水644 Ca的准确重建。目前的研究提出了简单的质量平衡模型，该模型量化了输入的杠杆作用，以改变海洋的Ca同位素组成，因为这直接影响了解释Ca同位素的方式。虽然钙分馏同位素在现代系统在大陆循环，644 Ca的河流输入到海洋的范围是相当小的变化比观察到假定的海水代理，如超微化石软泥和海洋重晶石。在陆地领域，植物表现出广泛的644 Ca的范围，有证据表明，通过生物质降解钙通量是显着的集水区尺度。因此，我们评估大陆生物圈的能力，影响河流，因此海水，544钙。一个稳定状态的生物圈几乎没有影响力，以改变河流644钙，除了在情况下，644钙的再循环通量是同位素不同的644钙的吸收通量。一个非稳态的生物圈可以大大影响土壤和河流644钙，驱动交换钙或重或轻，这取决于回收通量相对于吸收通量的大小。根据对全球生物圈大小的估计（-1.5 10' mol Ca），我们认为，在时间尺度上，一个衰退的生物圈有可能对河流644 Ca产生十分之一的影响
At the Earth's surface, calcium (Ca) is a critical element at a variety of scales: it is soluble in natural waters, a biological nutrient, and a major constituent of the dominant mineral sink for carbon in the ocean (CaCO3). There is a 4%0 variation in the Ca isotopic composition (44CarmCa expressed as Oita) of various Ca reservoirs on Earth, suggesting Ca isotopes as a promising tracer of Ca cycling in both the present and the past. Fifteen years of high precision Ca isotope measurements has revealed much about the behavior of Ca isotopes in the Earth surface environment, but there remain fundamental questions concerning how Ca isotopes are used to elucidate the marine and terrestrial Ca cycles. The current work presents a data compilation of over 70 published Ca isotope studies, totaling over 2600 measurements presented on a common delta scale, that includes data on rivers and groundwater, dust, soils and soil pore fluids, vegetation, rainwater, silicate minerals/rocks, and authigenic marine minerals (carbonates, sulfates, and phosphates, both modem and ancient).The data compilation suggests that: (1) there is a significant difference between carbonate (0.60%0) and silicate 644Ca (0.94%0); (2) riverine 644Ca (0.88%0) does not simply reflect the compiled carbonate 644Ca; and (3) terrestrial vegetation exhibits the largest range of Ca isotopic compositions -3.5%0 in the terrestrial setting. We discuss these observations in the context of the global Ca cycle, exploring the extent to which seawater 644Ca variability is feasible and how we can achieve accurate reconstructions of seawater 644Ca over geologic time scales.The current study presents simple mass balance models that quantify the leverage of inputs to change the Ca isotopic composition of the ocean, as this directly impacts the manner in which Ca isotopes are interpreted. Although Ca fractionates isotopically in the modem system during continental cycling, the 644Ca range of riverine inputs to the ocean is considerably smaller than the variability observed in putative seawater proxies such as nannofossil ooze and marine barite. In the terrestrial realm, plants exhibit a wide 644Ca range and there is evidence that Ca fluxes via biomass degradation are significant at the catchment scale. We therefore assess the ability of the continental biosphere to influence riverine, and consequently seawater, 544Ca. A steady state biosphere has little leverage to alter riverine 644Ca, except in cases where the 644Ca of the recycling flux is isotopically distinct from the 644Ca of the uptake flux. A non-steady state biosphere can substantially impact both soil and riverine 644Ca, driving exchangeable Ca either heavier or lighter depending on the magnitude of the recycling flux relative to the uptake flux. Based on estimates of the size of the global biosphere (-1.5 10' mol Ca), we suggest a decaying biosphere has the potential to impact riverine 644Ca by tenths of a perrnil over time scales