Reconciling substrate specific differences in the carbon isotope excursion marking the Paleocene-Eocene thermal maximum.

对未来气候变化的模型预测和长期预测部分基于对过去全球变暖事件的地质记录的研究。一个这样的全球变暖事件发生在大约5580万年前，被称为古新世-始新世热最大值（PETM）。对世界各地地质记录的研究表明，地表温度上升了5-8°C，生物化学过程发生了改变，全球生物圈在这段时间内受到了深刻的干扰。然而，PETM最显著的标志是陆地和海洋含碳物质的稳定碳同位素（13C/12C）比值急剧下降。这种碳同位素偏移（CIE）在全球许多地方都有记录，它的普遍存在标志着大量同位素轻碳（12C）快速释放到海洋-大气系统中。这种碳的来源仍有争议，但最简洁的解释是大陆斜坡上大量沉积甲烷水合物的分离。这一古老的全球变暖事件与全球碳循环的重大扰动密切相关，这一事实独特地使PETM成为由当前社会实践驱动的未来气候变化的模拟物。估计PETM期间碳排放量的一个关键证据是CIE的大小，但人们早就认识到不同的含碳基质产生不同的CIE大小。例如，从陆地记录（土壤结核和陆地植物残骸）中报告的CIE大约是海洋沉积物测量结果的两倍。这种差异是令人烦恼的，因为气体很容易在海气界面交换，所以表面海洋的物理化学状态应该在时间尺度上与大气平衡，按地质标准几乎是瞬间的。因此，在陆地和海洋表面的记录中，CIE的大小应该是相似的，但这并没有被公开的记录所证实。绝大多数海洋PETM记录来自浮游有孔虫生长的微小（1毫米）方解石壳的稳定同位素组成，浮游有孔虫是一种现存的变形虫原生生物，具有丰富的化石记录。深海沉积物中浮游有孔虫壳的优势使它们成为使用地球化学分析来重建过去海洋/气候变化的研究的首选基质。然而，困扰这些记录的一个问题是，海底沉积后的过程会改变有孔虫壳的原始化学成分，从而使用于重建过去气候条件的稳定同位素记录产生偏差，就像PETMAt期间流行的那样。威斯康星次级离子质谱仪（wissims）实验室已经开发出新的原位技术，用于测量方解石样品中微小（~10微米）目标的稳定同位素比率。这种方法允许识别和分析受化学变化影响较小的有孔虫壳内的子域，从而提供更准确的过去气候条件记录。目前，这些稳定同位素的测量无法通过任何其他技术进行。初步研究结果表明，在这些保存完好的浮游有孔虫壳亚域中记录的CIE震级实际上是先前报道的两倍，因此与陆地记录的CIE震级高度一致。这个项目的重点是获得更准确的CIE量级的测量。确定CIE的真实幅度很重要，因为它将(1)更好地限制碳排放量，(2)为评估拟议碳输入源的可行性提供一个更现实的试验场，(3)提高我们衡量气候对温室气体强迫敏感性的能力，(4)有助于确定气候系统内放大或减弱全球变暖的反馈机制。(5)使更好地校准生物（陆地和海洋）对快速碳输入驱动的环境变化的响应成为可能。该项目将有利于一名年轻女科学家的研究生教育，并为正在开发变革性尖端分析技术的WiscSIMS实验室提供基础设施支持。

Model predictions and long-range forecasts for future climate change are partly based on studies of geologic records of past global warming events. One such global warming event occurred ~55.8 million years ago and is referred to as the Paleocene-Eocene thermal maximum (PETM). Study of geologic records around the world has shown that surface temperatures warmed 5-8°C, biochemical processes were altered, and the global biosphere was profoundly perturbed during this time. However, the most distinctive hallmark of the PETM is a precipitous decrease in the stable carbon isotope (13C/12C) ratios of terrestrial and marine carbon-bearing materials. This carbon isotope excursion (CIE) has been documented at numerous locations across the globe, and its ubiquity signals the rapid release of massive quantities of isotopically light carbon (12C) into the ocean-atmosphere system. The source of this carbon is still debated, but the most parsimonious explanation invokes the dissociation of massive quantities of sedimentary methane hydrate along continental slopes. The fact that this ancient global warming event is closely linked to a major perturbation to the global carbon cycle uniquely qualifies the PETM as an analogue for future climate change driven by current societal practices.A key piece of evidence for estimating the amount of carbon emitted during the PETM is the magnitude of the CIE, yet it has long been recognized that different carbon-bearing substrates yield different CIE magnitudes. For example, the CIE reported from terrestrial records (soil nodules and land plant remains) is about twice as large as indicated by measurements of marine sediments. This discrepancy is vexing because gases are readily exchanged across the air-sea interface so that the physiochemical state of the surface ocean should equilibrate with the atmosphere on time scales virtually instantaneous by geologic standards. Hence, the magnitude of the CIE in terrestrial and surface-ocean records should be similar, yet this is not borne out by published records. The vast majority of marine PETM records are derived from the stable isotopic compositions of tiny (1 mm) calcite shells grown by planktic foraminifera, an extant group of amoeboid protists with a rich fossil record. The preponderance of planktic foraminiferal shells in deep-sea sediments has made them a preferred substrate for studies using geochemical analyses to reconstruct past ocean/climate change. However, a problem plaguing such records is that post-depositional processes on the seafloor can alter the original chemical compositions of foraminiferal shells, thereby biasing stable isotope records used to reconstruct past climate conditions like those that prevailed during the PETMAt the Wisconsin Secondary Ion Mass Spectrometer (WiscSIMS) laboratory, novel in situ techniques have been developed for measuring stable isotope ratios in tiny (~10 micrometers) targets in calcite samples. This approach permits the identification and analysis of subdomains inside foraminiferal shells that are less affected by chemical alteration and therefore provide a more accurate record of past climatic conditions. Currently, these stable isotope measurements cannot be performed by any other technique. First results from a pilot study indicate that the CIE magnitude recorded within these well-preserved subdomains in planktic foraminiferal shells is actually twice as high as previously reported and thus highly congruous with the CIE magnitude in terrestrial records. This project focuses on obtaining a more accurate measurement of the CIE magnitude. Determining the true magnitude of the CIE is important because it will (1) better constrain the amount of carbon emitted, (2) provide a more realistic testing ground for assessing the viability of proposed sources of the carbon input, (3) improve our ability to gauge climate sensitivity to greenhouse gas forcing, (4) help identify feedback mechanisms within the climate system that either amplify or attenuate global warming, and (5) make it possible to better calibrate biotic (terrestrial and marine) responses to environmental change driven by rapid carbon input. This project will benefit the graduate education of a young female scientist, and provide infrastructure support for the WiscSIMS laboratory where transformative, cutting-edge analytical technologies are being developed.