High-precision radiogenic strontium isotope measurements of the modern and glacial ocean: Limits on glacial-interglacial variations in continental weathering
High-precision radiogenic strontium isotope measurements of the modern and glacial ocean: Limits on glacial-interglacial variations in continental weathering
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DOI：
10.1016/j.epsl.2015.01.036
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发表时间：
2015-04-01
影响因子：
5.3
通讯作者：
Burton, Kevin W.
中科院分区：
文献类型：
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作者：
Mokadem, Fatima;Parkinson, Ian J.;Burton, Kevin W.
现有的锶放射性同位素（Sr-87/Sr-86）测量有孔虫在第四纪冰川-间冰期气候循环提供的证据，在+/- 9-13 ppm的精度水平的海水同位素组成的变化。然而，建模表明，即使在这种不确定性的水平显着（高达30%）的变化，在大陆的化学风化是允许的，占较长期的上升Sr-87/Sr-86在第四纪，和明显的不平衡的Sr在海洋中在今天。这项研究提供了非常高精度的Sr-87/Sr-86同位素数据的现代海水从每个主要的海洋，和冰川间冰期的海水记录保存大洋钻探计划（ODP）站点758在东北印度洋的南极有孔虫。在本研究中获得的精确度水平（+/- 4.9 ppm 2 sigma）下，大西洋、太平洋和印度洋现代海水的锶同位素Sr-87/Sr-86测量结果彼此无法区分（Sr-87/Sr-86 = 0.7091792 +/- 0.0000021，n = 17）。这一观察结果与锶在海水中的长停留时间一致，并支持了这种元素在高精度同位素地层学中的应用。由南极有孔虫保存的Sr-87/Sr-86海水记录显示没有可分辨的冰期-间冰期变化（Sr-87/Sr-86 = 0.7091784 +/- 0.0000035，n = 10），并将海水对化学风化通量和/或成分变化的响应限制在+/- 4.9 ppm或更低。计算表明，稳定状态下的风化通量约为12%，可以通过这里获得的不确定度来调节。新的数据不能容纳短期的风化脉冲在冰川消退，虽然更弥漫的风化脉冲伴随着长期的冰退缩是允许的。然而，这些结果仍然表明，现代风化通量可能高于第四纪的平均水平，通过冰川循环的这种变化也可以解释Sr-87/Sr-86在这段时间内的长期上升。在这项研究中，对海洋Sr-87/Sr-86记录进行了非常高精度的测量，对冰川-间冰期循环期间化学风化通量变化的幅度和时间进行了明确的限制。此外，必须从更高精度的测量或在海洋中停留时间较短的元素（如锇）中寻找限制因素，因为这些元素有能力对输入的短期变化作出反应。皇冠版权所有（C）2015由Elsevier B. V.出版。保留所有权利。
Existing strontium radiogenic isotope (Sr-87/Sr-86) measurements for foraminifera over Quaternary glacial-interglacial climate cycles provide no evidence for variations in the isotope composition of seawater at the +/- 9-13 ppm level of precision. However, modelling suggests that even within this level of uncertainty significant (up to 30%) variations in chemical weathering of the continents are permitted, accounting for the longer-term rise in Sr-87/Sr-86 over the Quaternary, and the apparent imbalance of Sr in the oceans at the present-day. This study presents very high-precision Sr-87/Sr-86 isotope data for modern seawater from each of the major oceans, and a glacial-interglacial seawater record preserved by planktic foraminifera from Ocean Drilling Program (ODP) Site 758 in the north-east Indian ocean. Strontium isotope Sr-87/Sr-86 measurements for modern seawater from the Atlantic, Pacific and Indian Oceans are indistinguishable from one another (Sr-87/Sr-86 = 0.7091792 +/- 0.0000021, n = 17) at the level of precision obtained in this study (+/- 4.9 ppm 2 sigma). This observation is consistent with the very long residence time of Sr in seawater, and underpins the utility of this element for high precision isotope stratigraphy. The Sr-87/Sr-86 seawater record preserved by planktic foraminifera shows no resolvable glacial-interglacial variation (Sr-87/Sr-86 = 0.7091784 +/- 0.0000035, n = 10), and limits the response of seawater to variations in the chemical weathering flux and/or composition to +/- 4.9 ppm or less. Calculations suggest that a variation of +/- 12% around the steady-state weathering flux can be accommodated by the uncertainties obtained here. The new data cannot accommodate a short-term weathering pulse during de-glaciation, although a more a diffuse weathering pulse accompanying protracted ice retreat is permissible. However, these results still indicate that modern weathering fluxes are potentially higher than average over the Quaternary, and such variations through glacial cycles can also account for the longer-term rise in Sr-87/Sr-86 over this time interval. The very high-precision measurements made for the marine Sr-87/Sr-86 record in this study place clear limits on the magnitude and timing of changes in the chemical weathering flux during glacial-interglacial cycles. Further, constraints must be sought from even higher precision measurement or elements with shorter residence times in the ocean, such as osmium (Os), that have the capacity to respond to short-term variations in input. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.