Early Cenozoic Decoupling of Climate and Carbonate Compensation Depth Trends
Early Cenozoic Decoupling of Climate and Carbonate Compensation Depth Trends
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DOI：
10.1029/2019pa003601
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发表时间：
2019-06-01
影响因子：
3.5
通讯作者：
Hoogakker, A. A.
中科院分区：
文献类型：
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作者：
Greene, S. E.;Ridgwell, A.;Hoogakker, A. A.
我们对地球系统长期演化的理解是基于这样的假设：陆地风化率应该对大气二氧化碳和气候做出反应，从而有助于调节。陆地风化作用的增加需要海洋沉积物中碳酸盐积累的增加，这反过来预计将导致碳酸盐补偿深度（CCD）的长期加深。在这里，我们批判性地评估气候与碳循环之间的长期关系。我们生成了从选定的古新世晚期到始新世早期时间切片的海洋深海碳酸盐丰度记录，以重建 CCD 的位置。尽管我们的数据集允许适度的 CCD 加深，但尽管全球变暖超过 3 摄氏度，但我们发现 CCD 没有统计上显着的变化，这突出表明需要对碳酸盐积累进行额外的深海限制。使用地球系统模型，我们表明变暖和风化加剧对 CCD 的影响可能会被海洋环流模式和有机物沉积呼吸的相反影响所掩盖。根据我们的数据合成和建模，我们认为对变暖、δ C-13 下降和相对稳定的 CCD 的观测可以通过古近纪中期火山 CO2 释气和风化的增加来广泛再现。然而，剩余的数据模型差异暗示我们的模型中缺少过程，最有可能涉及有机碳的保存和埋藏。我们发现 CCD 与全球海洋碳酸盐埋藏率之间存在脱钩，这意味着在尝试使用 CCD 直接测量全球碳酸盐埋藏率以及风化率时需要相当小心。 通俗语言摘要 风化，即地球表面岩石的分解，被广泛认为充当地球的恒温器，调节大气二氧化碳 (CO2) 的浓度和全球温度。这是因为（a）风化消耗了大气中的二氧化碳，（b）风化速率被认为在较高温度下更大。重建全球风化作用的一种广泛使用的代理是深海中碳酸钙（白垩）矿物的保存。从古新世晚期到始新世早期，地球变暖了3摄氏度以上，这是我们有详细记录的最大的长期全球变暖趋势。目前还不确定全球风化是否对这种气温上升做出了反应。我们重建了这次变暖过程中深海碳酸钙的保存情况，发现几乎没有明显的变化。我们还使用计算机建模来表明全球风化与深海碳酸钙保存之间的关系非常微妙。总之，我们可以将古新世晚期到始新世早期的变暖归因于火山二氧化碳排放量的增加。我们认为，由此产生的气温升高实际上确实导致了全球风化的加剧，但从我们的深海碳酸钙指标来看，这一点并不明显。
Our understanding of the long-term evolution of the Earth system is based on the assumption that terrestrial weathering rates should respond to, and hence help regulate, atmospheric CO2 and climate. Increased terrestrial weathering requires increased carbonate accumulation in marine sediments, which in turn is expected to result in a long-term deepening of the carbonate compensation depth (CCD). Here, we critically assess this long-term relationship between climate and carbon cycling. We generate a record of marine deep-sea carbonate abundance from selected late Paleocene through early Eocene time slices to reconstruct the position of the CCD. Although our data set allows for a modest CCD deepening, we find no statistically significant change in the CCD despite >3 degrees C global warming, highlighting the need for additional deep-sea constraints on carbonate accumulation. Using an Earth system model, we show that the impact of warming and increased weathering on the CCD can be obscured by the opposing influences of ocean circulation patterns and sedimentary respiration of organic matter. From our data synthesis and modeling, we suggest that observations of warming, declining delta C-13 and a relatively stable CCD can be broadly reproduced by mid-Paleogene increases in volcanic CO2 outgassing and weathering. However, remaining data-model discrepancies hint at missing processes in our model, most likely involving the preservation and burial of organic carbon. Our finding of a decoupling between the CCD and global marine carbonate burial rates means that considerable care is needed in attempting to use the CCD to directly gauge global carbonate burial rates and hence weathering rates.Plain Language Summary Weathering, the breakdown of rocks at the Earth's surface, is widely assumed to act as Earth's thermostat, regulating the concentration of atmospheric carbon dioxide (CO2) and global temperatures. This is because (a) weathering consumes CO2 from the atmosphere and (b) weathering rates are thought to be greater at higher temperatures. One widely used proxy for reconstructing global weathering is the preservation of calcium carbonate (chalk) minerals in the deep ocean. From the late Paleocene to early Eocene, the planet warmed by more 3 degrees C, the biggest long-term global warming trend for which we have detailed records. It is still uncertain whether global weathering responded to this temperature increase. We reconstruct the preservation of deep-sea calcium carbonate across this warming and find little apparent change. We also use computer modeling to show that the relationship between global weathering and preservation of deep-sea calcium carbonate is highly nuanced. In sum, we are able to attribute the late Paleocene to early Eocene warming to an increase in CO2 emissions from volcanoes. We argue the resulting elevated temperatures did, in fact, cause an increase in global weathering, but that this is not readily apparent from our deep-sea calcium carbonate proxy.