Stable isotope evidence for an atmospheric origin of desert nitrate deposits in northern Chile and southern California, USA
Stable isotope evidence for an atmospheric origin of desert nitrate deposits in northern Chile and southern California, USA
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DOI：
10.1016/s0009-2541(96)00124-6
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发表时间：
1997-03-27
期刊：
影响因子：
3.9
通讯作者：
Revesz, K
中科院分区：
文献类型：
--
作者：
Bohlke, JK;Ericksen, GE;Revesz, K
在智利北部的阿塔卡马沙漠和加利福尼亚州南部莫哈韦沙漠的死亡谷地区，富含硝酸盐的盐的自然地表积累是众所周知的，但尽管进行了许多地质和地球化学研究，硝酸盐的起源仍然存在争议。测量了硝酸盐中的N和O同位素，以及共存的可溶性硫酸盐中的S同位素，以确定是否支持或拒绝某些提出的N来源，并确定这些天然沉积物的同位素特征是否可以用于区分它们与沙漠地下水中可能发现的各种人为硝酸盐污染。这两个地区的高品位钙型硝酸盐矿床的δ (15)N值范围为-5至+5‰，但大多数接近0‰。δ (15)N在0 / 1000附近的值与大量大气氮沉降或微生物固氮是沉积物中氮的主要来源相一致。δ (15)N接近0 ppm的沙漠硝酸盐δ (18)O值在+31 ~ +50 ppm （V-SMOW）之间，显著高于大气O-2 （+23.5 ppm）。如此高的δ (18)O值被认为不太可能完全是由还原氮的硝化作用造成的，而与其他一些地区降水中现代大气硝酸盐的值很相似。假设有限的现代大气同位素数据适用于沉积物，并考虑到共沉积铵的硝化作用，估计沉积物中可由大气N沉积同位素占硝酸盐的比例可能至少为20%，甚至可能高达100%。δ (34)S值的诊断性较差，但也可以与沉积物中一些可溶性硫酸盐的大气成分相一致。稳定同位素数据支持这样一种假设，即在地球上一些极度干旱的沙漠中，一些富含高钙型硝酸盐的盐沉积物代表了大气沉积的长期积累(死亡谷地区可能为10（4)年，阿塔卡马沙漠为10(7)年），而土壤淋溶或生物循环相对缺乏。这些矿床中硝酸盐的N、O组合同位素特征与许多其他自然和人为来源的硝酸盐有显著不同。
Natural surficial accumulations of nitrate-rich salts in the Atacama Desert, northern Chile, and in the Death Valley region of the Mojave Desert, southern California, are well known, but despite many geologic and geochemical studies, the origins of the nitrates have remained controversial. N and O isotopes in nitrate, and S isotopes in coexisting soluble sulfate, were measured to determine if some proposed N sources could be supported or rejected, and to determine if the isotopic signature of these natural deposits could be used to distinguish them from various types of anthropogenic nitrate contamination that might be found in desert groundwaters.High-grade caliche-type nitrate deposits from both localities have delta(15)N values that range from -5 to +5 parts per thousand, but are mostly near 0 parts per thousand. Values of delta(15)N near 0 parts per thousand are consistent with either bulk atmospheric N deposition or microbial N fixation as major sources of the N in the deposits. delta(18)O values of those desert nitrates with delta(15)N near 0 parts per thousand range from about +31 to +50 parts per thousand (V-SMOW), significantly higher than that of atmospheric O-2 (+23.5 parts per thousand). Such high values of delta(18)O are considered unlikely to result entirely from nitrification of reduced N, but rather resemble those of modern atmospheric nitrate in precipitation from some other localities. Assuming that limited modern atmospheric isotope data are applicable to the deposits, and allowing for nitrification of co-deposited ammonium, it is estimated that the fraction of the nitrate in the deposits that could be accounted for isotopically by atmospheric N deposition may be at least 20% and possibly as much as 100%. delta(34)S values are less diagnostic but could also be consistent with atmospheric components in some of the soluble sulfates associated with the deposits. The stable isotope data support the hypothesis that some high-grade caliche-type nitrate-rich salt deposits in some of the Earth's hyperarid deserts represent long-term accumulations of atmospheric deposition (possibly in the order of 10(4) yr for the Death Valley region, 10(7) yr for the Atacama Desert) in the relative absence of soil leaching or biologic cycling. The combined N and O isotope signature of the nitrate in these deposits is significantly different from those of many other natural and anthropogenic sources of nitrate.