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

协调标志着古新世-始新世热最大值的碳同位素偏移中基质的特定差异。

• 批准号: 1405224
• 负责人: D Kelly
• 金额: $ 31.37万
• 依托单位: University of Wisconsin-Madison
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-08-01 至 2017-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1405224&HistoricalAwards=false
• 关键词: Reconciling; substrate; specific; differences; carbon

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.

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。其中一个全球变暖事件发生在大约5580万年前，被称为古新世-始新世热最大期（PETM）。对世界各地地质记录的研究表明，地表温度升高了5-8°C，生物化学过程发生了变化，全球生物圈在此期间受到了深刻的干扰。然而，PETM最显著的特征是陆地和海洋含碳物质的稳定碳同位素（13 C/12 C）比值急剧下降。这种碳同位素漂移（CIE）在地球仪的许多地方都有记录，它的普遍存在表明大量同位素轻碳（12 C）迅速释放到海洋-大气系统中。这些碳的来源仍然存在争议，但最简单的解释是沿着大陆斜坡沿着大量沉积的甲烷水合物的分解。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，这一事实使PETM成为当前社会实践驱动的未来气候变化的唯一模拟物。估计PETM期间碳排放量的一个关键证据是CIE的量值，但长期以来人们一直认为不同的含碳基质产生不同的CIE量值。例如，从陆地记录（土壤结核和陆地植物遗骸）报告的CIE大约是海洋沉积物测量值的两倍。这种差异令人烦恼，因为气体很容易在气-海界面交换，因此海洋表面的物理化学状态应该与大气在时间尺度上平衡，根据地质标准，这几乎是瞬时的。因此，CIE在陆地和海洋表面记录中的量级应该是相似的，但这并没有被公布的记录所证实。绝大多数的海洋PETM记录来自微小的（1毫米）方解石壳的稳定同位素组成的生长的浮游有孔虫，一个现存的组的变形虫原生生物与丰富的化石记录。深海沉积物中浮游有孔虫壳的优势使它们成为利用地球化学分析重建过去海洋/气候变化的研究的首选基底。然而，保存这些记录的一个问题是，海底沉积后的过程可能会改变有孔虫外壳的原始化学成分，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，就像威斯康星州二次离子质谱仪上的PETMA一样。（WiscSIMS）实验室，已经开发了用于测量方解石样品中微小（~10微米）目标的稳定同位素比的新型原位技术。这种方法可以识别和分析有孔虫壳内受化学蚀变影响较小的子域，因此可以更准确地记录过去的气候条件。目前，这些稳定同位素测量无法通过任何其他技术进行。初步研究的结果表明，在这些保存完好的子域记录在南极有孔虫壳的CIE震级实际上是以前报道的两倍，因此高度一致的CIE震级在陆地记录。该项目的重点是获得更准确的CIE星等测量。确定CIE的真实量级很重要，因为它将（1）更好地限制碳排放量，（2）为评估碳输入的拟议来源的可行性提供更现实的测试基础，（3）提高我们衡量气候对温室气体强迫的敏感性的能力，（4）帮助确定气候系统内放大或减弱全球变暖的反馈机制，以及（5）使得有可能更好地校准生物（陆地和海洋）对由快速碳输入驱动的环境变化的响应。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。