Carbon Dioxide and Polar Cooling in the Miocene: The Monterey Hypothesis
Carbon Dioxide and Polar Cooling in the Miocene: The Monterey Hypothesis
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DOI：
10.1029/gm032p0455
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发表时间：
2013-03
期刊：
影响因子：
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通讯作者：
E. Vincent;W. Berger
中科院分区：
文献类型：
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作者：
E. Vincent;W. Berger
早中新世晚期，有孔虫的δ13C发生了明显的变化。我们在热带印度洋的数据显示δ13C信号向更重的值偏移，持续了大约400万年。对于底栖有孔虫、深栖和浅栖浮游生物物种，这种偏移都有记录。δ13c的初始位移发生在磁年轮16内，约为17.5 Ma。它代表了地表和底水δ13C值向更重的方向变化约10/00。偏移终止于大约13.5毫安。从δ18O信号中可以看出，Chron 16碳的移动与中新世早期变暖趋势的停止是一致的。中新世中冷却阶段（可能与南极冰的形成有关，大约在15ma）以碳同位素偏移为中心。我们认为最初的碳转移是由海洋-大气系统中有机碳的快速提取引起的。随后，海洋边缘沉积物中有机碳的持续提取维持了向重值的偏移。从早中新世末期开始，富含有机质的细粒硅藻质沉积物沉积在北太平洋的边缘。在加州，这些沉积物被称为蒙特利地层。这种形成是沿海上升流的结果，这是由于强大的纬向风和强大的永久温跃层的发展而产生的。纬向风和温跃层的演化反过来又依赖于高纬度和低纬度之间不断增加的温差。我们假设建立了一个反馈循环，这样，行星温度梯度的初始增加开始了温跃层的发展，导致海洋边缘的有机碳提取，导致大气二氧化碳浓度下降。随之而来的降温（反向温室效应）加强了温跃层的发展，导致进一步降温。当可用的养分耗尽时，这个循环就被打破了。根据这一假设，在蒙特雷碳同位素漂移期间，过量碳积累的总量在40到80个大气碳质量之间。这个量相当于海洋中存在的碳量，即一个海洋碳质量。
A pronounced shift in the δ13C of foraminifera in the latest early Miocene has been proposed by various authors. Our data in the tropical Indian Ocean show an excursion of δ13C signals toward heavier values, lasting for about 4 million years. The excursion is documented for benthic foraminifera as well as for deep‐living and for shallow‐dwelling planktonic species. The initial δ13c shift occurs within Magnetic Chron 16, at about 17.5 Ma. It represents a change toward heavier δ13C values by about 10/00 in surface and bottom waters. The excursion terminates at approximately 13.5 Ma. The Chron 16 Carbon shift coincides with the cessation of an early Miocene warming trend, seen in the δ18O signals. The mid‐Miocene cooling step (presumably associated with Antarctic ice buildup, near 15 Ma) is centered on the carbon isotope excursion. We propose that the initial carbon shift was caused by rapid extraction of organic carbon from the ocean‐atmosphere system. Subsequently, the excursion toward heavy values was maintained by continued extraction of organic carbon, into ocean‐margin deposits. Beginning at the end of the early Miocene, fine‐grained diatomaceous sediments rich in organic matter were deposited all around the margins of the northern Pacific. In California, these sediments are known as the Monterey Formation. This formation is the result of coastal upwelling, which arose because of the development of strong zonal winds and a strong permanent thermocline.Zonal winds and thermocline evolution, in turn, depended on increasing temperature contrast between high and low latitudes. We hypothesize that a feedback loop was established, such that an initial increase in the planetary temperature gradient started thermocline development which led to organic carbon extraction at the ocean margins which resulted in a drop in atmospheric carbon dioxide concentration. Concomitant cooling (reverse greenhouse effect) strengthened thermocline development, leading to further cooling. The loop was broken when available nutrients were used up. The total amount of excess carbon buildup, according to the hypothesis, is between 40 and 80 atmospheric carbon masses for the duration of the Monterey carbon isotope excursion. This amount corresponds to that present in the ocean, that is, one ocean carbon mass.