The Paleocene-Eocene thermal maximum (PETM) stable isotopic values of planktonic and benthic foraminifera at Millville, NJ

We present new d13C and d18O records of surface (Morozovella and Acarinina) and thermocline dwelling (Subbotina) planktonic foraminifera and benthic foraminifera (Gavelinella, Cibicidoides, and Anomalinoides) during the Paleocene-Eocene Thermal Maximum (PETM) from Millville, New Jersey, and compare them with three other sites located along a paleoshelf transect from the U.S. mid-Atlantic coastal plain. Our analyses show different isotopic responses during the PETM in surface versus thermocline and benthic species. Whereas all taxa record a 3.6-4.0 per mil d13C decrease associated with the carbon isotope excursion, thermocline dwellers and benthic foraminifera show larger d18O decreases compared to surface dwellers. We consider two scenarios that can explain the observed isotopic records: (1) a change in the water column structure and (2) a change in habitat or calcification season of the surface dwellers due to environmental stress (e.g., warming, ocean acidification, surface freshening, and/or eutrophication). In the first scenario, persistent warming during the PETM would have propagated heat into deeper layers and created a more homogenous water column with a thicker warm mixed layer and deeper, more gradual thermocline. We attribute the hydrographic change to decreased meridional thermal gradients, consistent with models that predict polar amplification. The second scenario assumes that environmental change was greater in the mixed layer forcing surface dwellers to descend into thermocline waters as a refuge or restrict their calcification to the colder seasons. Although both scenarios are plausible, similar d13C responses recorded in surface, thermocline, and benthic foraminifera challenge mixed layer taxa migration.

本研究报道了来自新泽西州米尔维尔（Millville, New Jersey）古新世-始新世极热事件（Paleocene-Eocene Thermal Maximum, PETM）期间的表层浮游有孔虫（planktonic foraminifera）、温跃层栖息浮游有孔虫及底栖有孔虫（benthic foraminifera）的全新δ¹³C与δ¹⁸O同位素记录：其中表层浮游类群涵盖球室虫属（Morozovella）与阿克林虫属（Acarinina），温跃层栖息类群为苏博特虫属（Subbotina）；底栖类群则包括加夫林虫属（Gavelinella）、希贝虫属（Cibicidoides）以及异常虫属（Anomalinoides）。随后将该数据集与美国中大西洋海岸平原古陆架断面沿线的另外3处遗址的同位素记录开展对比分析。研究结果显示，古新世-始新世极热事件期间，表层、温跃层栖息类群与底栖有孔虫呈现出截然不同的同位素响应特征。尽管所有类群均记录了与碳同位素漂移事件（carbon isotope excursion）相关的3.6‰~4.0‰的δ¹³C降幅，但相较于表层栖息类群，温跃层类群与底栖有孔虫的δ¹⁸O下降幅度更为显著。本研究提出两种可解释上述同位素观测记录的假说：其一为水体垂向结构发生改变；其二为受环境胁迫（如升温、海洋酸化、表层海水淡化及/或富营养化）影响，表层栖息类群的栖息环境或钙化季节发生变化。在第一种假说框架下，古新世-始新世极热事件期间的持续升温会将热量传递至更深水层，使得水体垂向结构更趋均一：暖混合层厚度增加，温跃层加深且温度梯度变缓。本次研究将该水文变化归因于经向温度梯度降低，这与预测极地放大效应的数值模型结果相一致。第二种假说则认为，混合层内的环境变化更为剧烈，迫使表层栖息类群下移至温跃层水域作为避难所，或是将钙化过程限制在温度更低的季节进行。尽管两种假说均具备合理性，但表层、温跃层及底栖有孔虫所记录的δ¹³C响应特征相似，这对混合层类群的垂向迁移假说提出了挑战。