Controls on seawater 231Pa, 230Th and 232Th concentrations along the flow paths of deep waters in the Southwest Atlantic
Controls on seawater 231Pa, 230Th and 232Th concentrations along the flow paths of deep waters in the Southwest Atlantic
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DOI：
10.1016/j.epsl.2013.12.038
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发表时间：
2014-03-15
影响因子：
5.3
通讯作者：
Henderson, Gideon M.
中科院分区：
文献类型：
--
作者：
Deng, Feifei;Thomas, Alex L.;Henderson, Gideon M.
在GEOTRACES GA 02 S巡航期间，对沿西南大西洋剖面沿着的12个全深度剖面进行了溶解Th-230、Pa-231和Th-232的测量。采样捕获所有主要的大西洋深水质量沿着他们的纬向流路径，并允许洞察Th和Pa的控制在设置中，沃茨在相反的方向流动，直接相关的了解使用Pa-231/Th-230作为海洋环流代理。水柱Th-230随深度线性增加，符合预期的可逆清除模型。Pa-231从表面增加到1200-1500米，但随着深度的增加而不变或减少，偏离了可逆清除的预期行为。溶解的Pa-231/Th-230比值在1000-2000米处显示出中层水柱最大值，这与绕极上层深水大致一致。低于2000米，核素分布和比例表现出不依赖于水团，也没有任何迹象表明水团内的渐进变化，挑战使用Pa-231/Th-230作为过去的循环示踪剂在南大西洋。通过海洋环流计算Th-230和Pa-231的水平输送表明，该海洋产生的19%的Pa-231和3%的Th-230向南净输出到大西洋之外。这种迁移全部来自北大西洋，而在南大西洋，迁移到沉积物等于生产。简单的一维建模可以模拟Th-230剖面，但不能模拟Pa-231剖面中观察到的中层水柱最大值，这表明Pa-231还有一个来源（可能是从边缘横向迁移），或由于海底清除作用而在深度处被清除。近海底浓度最低值表明Th-230和（231）Pa的海底清除作用，在云状层的存在下，特别是231 Pa的清除作用增强。这种额外的清除分馏Th-230和Pa-231，并在霞状层的存在下，可能会导致增加沉积Pa-231/Th-230的比例。Th-232浓度与Th-230衍生的停留时间配对在水柱的上部250米，以测试Th作为粉尘沉积的示踪剂的应用。Th-232中的最大值表明来自非洲大陆和可能的南美大陆的高尘埃输入。(C)2014爱思唯尔有限公司版权所有。
Measurements of dissolved Th-230, Pa-231 and Th-232 were made for twelve full-depth profiles along a Southwest Atlantic section during GEOTRACES cruise GA02S. Sampling captures all the main Atlantic deep water masses along their meridional flow paths and allows insight into the control on Th and Pa in a setting where waters are flowing in opposing directions, with direct relevance to understanding the use of Pa-231/Th-230 as an ocean-circulation proxy. Water-column Th-230 increases linearly with depth, in line with expected reversible scavenging models. Pa-231 increases from the surface to similar to 1200-1500 m, but is invariant or decreases with greater depth, deviating from the behavior expected for reversible scavenging. Dissolved Pa-231/Th-230 ratios display a mid-water-column maximum at similar to 1000-2000 m which is broadly coincident with Upper Circumpolar Deep Water. Below 2000 m, nuclide distributions and ratios exhibit no dependence on water mass, nor any indication of progressive change within a water mass, challenging the use of Pa-231/Th-230 as a past circulation tracer in the South Atlantic. Calculation of horizontal transport of Th-230 and Pa-231 by ocean circulation indicates a net southward export out of the Atlantic of 19% of the Pa-231 and 3% of the Th-230 produced in that ocean. This removal is all from the North Atlantic while, in the South Atlantic, removal to sediment equals production. Simple one-dimensional modeling can simulate Th-230 profiles but not the mid-water-column maximum observed in Pa-231 profiles, suggesting an additional source of Pa-231 (perhaps lateral transport from the margin) or removal at depth due to bottom scavenging. Near seafloor minima in concentrations indicates bottom scavenging of Th-230 and (231)pa, which is enhanced in the presence of nepheloid layers, particularly for 231Pa. This additional scavenging fractionates Th-230 and Pa-231 and, in the presence of nepheloid layers, may lead to an increase in sedimentary Pa-231/Th-230 ratios. Th-232 concentrations were paired with Th-230-derived residence times in the upper 250 m of the water column to test the application of Th as a tracer of dust deposition. Maxima in Th-232 indicate high dust input from the African and possibly South American continents. (C) 2014 Elsevier B.V. All rights reserved.