U and Th concentrations and isotope ratios in modern carbonates and waters from the Bahamas
U and Th concentrations and isotope ratios in modern carbonates and waters from the Bahamas
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DOI：
10.1016/j.gca.2003.10.005
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发表时间：
2004-04
影响因子：
5
通讯作者：
L. Robinson;N. Belshaw;G. Henderson
中科院分区：
文献类型：
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作者：
L. Robinson;N. Belshaw;G. Henderson
Th和Pa在海水中的短停留时间使它们对海洋环境的变化非常敏感。我们使用一种新的多离子计数技术，使Th和Pa同位素测量海水中的近岸环境中，海洋化学示踪剂不淹没陆地输入（巴哈马）。巴哈马群岛的一个不寻常的特点是岸边的水很浅。这些沃茨的232 Th/230 Th（10,000）明显低于紧邻河岸的水体（24000 - 31000），a（231 Pa/230 Th）接近于产率（100.1）。232 Th/230 Th和231 Pa/230 Th的变化主要是由于Th和Pa被清除，并被下伏碳酸盐的230 Th和231 Pa α-反冲以及尘埃溶解的Th所替代。对分隔大巴哈马银行和小巴哈马银行的海洋之舌的水剖面进行分析，使我们能够追踪银行顶部水的深度运动。232 Th/230 Th（13，000）和（231 Pa/230 Th）（0.5）的明显最小值均出现在430 m处，并被解释为反映了岸边水与夹带的碳酸盐沉积物的密度级联。这些结果表明，Th和Pa可以作为水团示踪剂在近岸环境。对同一沃茨的铀浓度测量表明，在盐度为2 psu的范围内，铀浓度为3.33 ppb（盐度为35）。还通过分析来自巴哈马沃茨同一地点的碳酸盐样品，评估了U和Th同位素在海洋碳酸盐中的掺入情况。这种结合对U-Th年代学至关重要。U同位素分析表明，海水δ 234 U平均值为146.6，变化不超过2.5%，碳酸盐岩捕获了该值。对现代碳酸盐岩的高精度测量（± 1%）证实了所有海洋都有相同的δ 234 U。现代海洋碳酸盐的232 Th/230 Th比值反映了它们形成的当地海水。
The short residence times of Th and Pa in seawater make them very responsive to changes in the ocean environment. We use a new multi-ion-counting technique to make Th and Pa isotope measurements in seawaters from a near-shore environment in which oceanic chemical tracers are not overwhelmed by terrestrial inputs (the Bahamas). An unusual feature of the Bahamas setting is the shallow depth of water residing on the bank tops. These waters have significantly lower232Th/230Th (∼10,000) than those immediately adjacent to the banks (24,000–31,000) and a (231Pa/230Th) near the production ratio (∼0.1). The change in232Th/230Th and (231Pa/230Th) on the bank tops is explained by almost quantitative removal of Th and Pa by scavenging, and their replacement with a mixture of230Th and231Pa alpha-recoiled from the underlying carbonates, together with Th from dust dissolution. Analysis of a water profile in the Tongue of the Ocean, which separates the Great and Little Bahama Banks, allows us to trace the movement of bank-top water to depth. A distinct minimum in both232Th/230Th (∼13,000) and (231Pa/230Th) (∼0.5) is observed at ∼430 m and is interpreted to reflect density cascading of bank-top water with entrained carbonate sediment. These results suggest that Th and Pa can be used as water-mass tracers in near-shore environments. Uranium concentration measurements on the same waters demonstrate that U is conservative across a range in salinity of 2 psu, with a concentration of 3.33 ppb (at a salinity of 35). The incorporation of U and Th isotopes into marine carbonates has also been assessed by analyzing carbonate samples from the same location as these Bahamas waters. Such incorporation is critical for U-Th geochronology. U isotope analyses demonstrate that seawater δ234U averages 146.6 and does not vary by more than 2.5%o, and that carbonates capture this value. Additional high precision measurements (≈±1%o) on modern carbonates confirm that all oceans have identical δ234U. Modern marine carbonates are shown to have232Th/230Th ratios that reflect the local seawater in which they formed.