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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Boron and calcium isotope composition in Neoproterozoic carbonate rocks from Namibia: evidence for extreme environmental change
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Boron and calcium isotope composition in Neoproterozoic carbonate rocks from Namibia: evidence for extreme environmental change

机译：纳米比亚新元古代碳酸盐岩中的硼和钙同位素组成：极端环境变化的证据

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摘要

The level and evolution of atmospheric carbon dioxide throughout Earth's history are key issues for palaeoclimate reconstructions, especially during times of extreme climate change such as those that marked the Neoproterozoic. The carbon isotope ratios of marine carbonates are crucial in the correlation and identification of Neoproterozoic glacial deposits, and they are also used as a record for biogeochemical cycling and potential proxy for atmospheric pCO2. Likewise, the boron and calcium isotope compositions of marine carbonates are potential proxies for palaeo-seawater pH and the ratio of calcium fluxes into and out of seawater, respectively, and together they may be used to estimate atmospheric carbon dioxide. Here we use B and Ca isotopes to estimate palaeoenvironmental conditions in the aftermath of a major Neoproterozoic glaciation in Namibia. The validity of the B and Ca isotope variation in the ancient marine carbonates is evaluated using the oxygen isotope composition of the carbonates and its correlation to the carbon isotope variation. A negative (2.7 to ?6.2‰) δ11B excursion occurs in the postglacial carbonates and is interpreted to reflect a temporary decrease in seawater pH. Associated variations in δ44Ca values (ranging between 0.35 and 1.14‰) are linearly coupled with the carbon isotope ratios and imply enhanced postglacial weathering rates. The reconstructed seawater pH and weathering profiles indicates that high atmospheric CO2 concentrations were likely during the melt back of Neoproterozoic glaciations and precipitation of cap carbonates. However, the B isotope trend suggests that these concentrations rapidly ameliorated and they do not co-vary with δ~(13)C. Thus models attempting to link long-lived negative δ~(13)C excursions to elevated pCO_2 need to be reconsidered.
机译：整个地球历史上大气二氧化碳的水平和演变是古气候重建的关键问题，特别是在极端气候变化时期，例如标志着新元古代的那些。海洋碳酸盐的碳同位素比在新元古代冰川沉积的相关性和识别中至关重要，它们还被用作生物地球化学循环的记录和大气pCO2的潜在替代物。同样，海洋碳酸盐的硼和钙同位素组成分别是古海水pH和进出海水的钙通量之比的潜在代表，并且它们可一起用于估算大气中的二氧化碳。在这里，我们使用B和Ca同位素来估计纳米比亚发生的主要新元古代冰川消融之后的古环境条件。使用碳酸盐的氧同位素组成及其与碳同位素变化的相关性，评估了古代海洋碳酸盐中B和Ca同位素变化的有效性。冰川后碳酸盐中的δ11B偏移为负值（2.7至？6.2‰），并被解释为反映了海水pH值的暂时降低。 δ44Ca值的相关变化（在0.35和1.14‰之间）与碳同位素比线性相关，并暗示冰川后的风化速率增加。重建的海水pH值和风化剖面表明，在新元古代冰川融化和帽状碳酸盐沉淀期间，大气中的二氧化碳浓度可能很高。但是，B同位素趋势表明这些浓度会迅速改善，并且不会与δ〜（13）C共同变化。因此，需要重新考虑尝试将长期存在的负δ〜（13）C偏移与升高的pCO_2相关联的模型。

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