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首页> 外文期刊>Geochimica et Cosmochimica Acta: Journal of the Geochemical Society and the Meteoritical Society >The Ca isotopic composition of dust-producing regions: Measurements of surface sediments in the Black Rock Desert, Nevada
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The Ca isotopic composition of dust-producing regions: Measurements of surface sediments in the Black Rock Desert, Nevada

机译：产尘区的钙同位素组成：内华达州黑岩沙漠地表沉积物的测量

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摘要

Dust is a relatively unconstrained flux in the geochemical cycle of Ca. The isotopic composition of dust-derived Ca has not been studied, though it is an important part of any attempt at deciphering Ca isotope-based proxy records. Accordingly, this study reports the elemental and calcium isotope geochemistry of 30 surface (upper ~0.5cm) sediments from an arid dust producer, the Black Rock Desert in northwestern Nevada. Geochemical data for sequential water and 0.5N HCl leaches, meant to sample mobile Ca, and selected leached residues are presented, along with X-ray diffraction (XRD) determinations of major mineralogy. Bulk playa sediments have Ca concentrations between 0.28 and 40wt.% (median: 6.8wt.%) and calcite concentrations of 2-32%. Isotopically, Ca sampled by water leaches (<1% of total Ca) are, on average, +0.33±0.16‰ (1SD) heavier than acid leaches (>60% of total Ca), though the degree of fractionation (Δ _(w-a)) varies between 0‰ and 0.6‰. Acid leaches, which are the primary component of mobile Ca in the sediments, have δ ~(44)Ca _(SRM-915a) values of 0.78±0.08‰, similar to the δ ~(44)Ca of modern nannofossil ooze and modern rivers. This means that dust produced in closed continental basins likely has little isotopic leverage to change the ocean's isotopic composition, and suggests that the Ca isotopic composition of dust is tied to rivers/weathering. In addition, while the Ca concentration data in the water leach suggest that evaporative evolution controls the amount of Ca in this reservoir, the isotope data are inconsistent with this conclusion. Instead, we hypothesize that adsorption of Ca on clays controls the Ca isotope systematics in the water leach. This hypothesis requires that there is no significant isotopic fractionation during evaporite mineral precipitation and may suggest that sorption at rates appropriate for natural systems might fractionate differently than sorption at laboratory rates.
机译：在Ca的地球化学循环中，粉尘是相对不受约束的通量。尽管粉尘来源的Ca的同位素组成是解密基于钙同位素的代理记录的任何尝试的重要组成部分，但尚未对其进行研究。因此，本研究报告了来自内华达州西北部的一个干旱粉尘产生者黑岩沙漠的30个表面（约0.5厘米以上）沉积物的元素和钙同位素地球化学。给出了连续水和0.5N HCl淋滤的地球化学数据，旨在采样流动性Ca，以及选定的淋滤残渣，以及主要矿物学的X射线衍射（XRD）测定。大块的普拉亚沉积物的Ca浓度在0.28至40wt。％（中位数：6.8wt。％）之间，方解石的浓度为2-32％。在同位素上，尽管分馏程度为（Δ_（（_ Ca（ Ca的60％）重+ 0.33±0.16‰（1SD）。 wa））在0‰和0.6‰之间变化。酸浸液是沉积物中移动性Ca的主要成分，其δ〜（44）Ca _（SRM-915a）值为0.78±0.08‰，与现代纳米化石软泥和现代纳米泥化石的δ〜（44）Ca相似。河流。这意味着在封闭的大陆盆地中产生的粉尘可能几乎没有同位素杠杆作用来改变海洋的同位素组成，并且表明粉尘的Ca同位素组成与河流/风化有关。此外，虽然浸出水中的钙浓度数据表明蒸发演化控制了该储层中的钙含量，但同位素数据与该结论不一致。相反，我们假设钙在粘土上的吸附控制了水浸过程中的钙同位素系统。该假设要求在蒸发矿物质沉淀过程中没有明显的同位素分馏，并且可能表明以适合自然系统的速率进行的吸附可能与以实验室速率进行的吸附不同。

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