Constraining the evolution of Neogene ocean carbonate chemistry using the boron isotope pH proxy
Constraining the evolution of Neogene ocean carbonate chemistry using the boron isotope pH proxy
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DOI：
10.1016/j.epsl.2018.06.017
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发表时间：
2018-09
影响因子：
5.3
通讯作者：
S. Sosdian;R. Greenop;M. Hain;G. Foster;P. Pearson;C. Lear
中科院分区：
文献类型：
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作者：
S. Sosdian;R. Greenop;M. Hain;G. Foster;P. Pearson;C. Lear
在新第三纪的过程中，地球经历了深刻的气候变化，从中新世中期的持续温暖到上新世-更新世冰川-间冰期循环的发展。全球碳循环的重大扰动伴随着这些变化而发生，然而，缺乏长期的碳酸盐系统重建目前限制了我们对这段时间内二氧化碳变化，碳循环和气候之间联系的理解。本文利用三裂叶有孔虫（Trilobatustrilobus）的硼同位素数据重建了连续的表层海洋pH、CO2和表层海洋文石饱和状态，并对计算中的关键变量（如δ 11 B sw、[Ca] sw、CCD）进行了敏感性分析。结果表明，δ 11 B sw的选择对海水pH和CO2均有影响，而[Ca] sw仅对CO2有显著影响。在过去的2200万年里，最低的pH值水平发生在中新世中期气候最佳时期（MMCO; 1700 - 1400万年前），在我们所有的情景中达到107.6 ±0.1个单位。MMCO的延长温暖分别对应于470-630 ppm和2.7-3.5的平均CO2和文石饱和状态水平。尽管我们的CO2记录和气候之间的一般对应关系，所有的CO2情景显示，在2009 Ma的峰值不匹配的气候重建的相应变化。这可能意味着在晚中新世（11.6-8.5 Ma）的有限时间间隔内（即显著的CO2变化而没有明显的气候响应）的解耦，尽管我们对海水硼同位素组成的时间演变的理解的进一步细化是必要的，以充分评估CO2和气候之间关系的性质。尽管如此，从我们的长期观点来看，很明显，在过去的1400万年（66% CI）中，低纬度开阔洋海洋生态系统不太可能经历持续的表面pH值和饱和度分别低于7.7和1.7。
Over the course of the Neogene, the Earth underwent profound climatic shifts from the sustained warmth of the middle Miocene to the development of Plio-Pleistocene glacial–interglacial cycles. Major perturbations in the global carbon cycle have occurred alongside these shifts, however the lack of long-term carbonate system reconstructions currently limits our understanding of the link between changes in CO 2, carbon cycling, and climate over this time interval. Here we reconstruct continuous surface ocean pH, CO 2, and surface ocean aragonite saturation state using boron isotopes from the planktonic foraminifer Trilobatus trilobus and we perform a sensitivity analysis of the key variables in our calculations (eg δ 11 B sw,[Ca] sw, CCD). We show that the choice of δ 11 B sw influences both seawater pH and CO 2 while [Ca] sw reconstructed dissolved inorganic carbon exerts a significant influence only on CO 2. Over the last 22 Myr, the lowest pH levels occurred in the Middle Miocene Climate Optimum (MMCO; 17–14 Myr ago) reaching∼ 7.6±0.1 units in all our scenarios. The extended warmth of the MMCO corresponds to mean CO 2 and aragonite saturation state levels of 470–630 ppm and 2.7–3.5, respectively. Despite a general correspondence between our CO 2 record and climate, all CO 2 scenarios show a peak at∼ 9 Ma not matched by corresponding changes in climate reconstructions. This may suggest decoupling (ie significant CO 2 change without a discernible climate response) for a limited interval in the Late Miocene (11.6–8.5 Ma), although further refinement of our understanding of the temporal evolution of the boron isotopic composition of seawater is necessary to fully evaluate the nature of the relationship between CO 2 and climate. Nonetheless, from our long-term view it is clear that low-latitude open ocean marine ecosystems are unlikely to have experienced sustained surface pH and saturation levels below 7.7 and 1.7, respectively, during the past 14 million years (66% CI).