Collaborative Research: Understanding Carbon Isotope Excursions in the Paleozoic: An Integrated Chemostratigraphic and Modeling Approach

显生界大气中O2和CO2含量的变化量仍然是一个基本但尚未解决的问题。海洋碳酸盐(D13Ccarb)的碳同位素组成表明，有机碳的全球埋藏率是模拟大气组成的一个关键参数。虽然以前的努力主要集中在d13Ccarb的长期、渐进趋势(107年)的氧化还原意义及其与d34S的关系上，但包括许多短期事件或持续数百万年或更短时间的同位素漂移的更不稳定的记录正在变得明显。在古生代地层中可以找到一些最好的证据来证明d13Ccarb的大规模漂移及其与地球表面环境的扰动(例如，通过大规模灭绝所揭示的)的关系。拟议的研究旨在建立两个古生代时间片的详细的d13Corg、d34S硫酸盐和87Sr/86Sr，这两个古生代时间片的特点是异常大的和有充分记录的d13Ccarb漂移。这些晚寒武世和密西西比早期的事件都被假设为反映了全球有机碳埋藏率的增加。该项目将使用C、S和锶的地球化学循环的耦合箱模式进行数值实验，以检验多个工作假设，并为C和S循环的变化以及对大气二氧化碳和O2含量的影响开发内部一致的假设。我们对高分辨率数据集的关注将使我们能够批判性地检查一些拟议的因果反馈循环，这些反馈循环稳定了地球化学碳循环，从而稳定了大气中的二氧化碳和氧气成分。例如，未被黄铁矿氧化平衡的有机质的埋藏应增加大气O2水平(从d13Ccarb、d34S硫酸盐的耦合和相变推断)，这对有机碳埋藏提供了负反馈，通过去除与铁氧化物相关的海洋磷。此外，有机质的埋藏将降低二氧化碳分压(降低d13Ccarb和d13Corg之间的净差)，使气候变冷，最终导致大陆风化速率的降低(降低87Sr/86Sr)。对这两个目标古生代时间片的更全面的了解也将提供对地球气候系统在关键地质生物学间隔期间的独特洞察，期间(1)晚寒武世全球三叶虫的大规模灭绝和(2)密西西比早期维管陆地植物的扩散和随后冈德瓦冰川的生长。更广泛的影响：这项研究将支持俄亥俄州立大学、密苏里州和宾夕法尼亚州立大学的研究生和本科生论文，包括那些来自代表不足的群体的论文。学生将在野外和实验室环境中接受广泛的培训，使用各种地球化学、地层学和建模技术。俄亥俄州立大学和密苏里州的学生将与肯普一起在宾夕法尼亚州立大学与盒子模特一起工作，并加入萨尔茨曼的实地工作，从而促进学生团体之间的交流，并接触到不同的教师和设施。研究人员还将参与在会议和同行审查出版物上传播科学信息以外的外联活动。中学生将与一位七年级的科学老师一起协调实地考察和课堂教学。汇编的化学地层学数据库将为评估关键时期的碳循环提供关键的参考节。它将促进更好地了解地质时间尺度上的碳循环及其与过去气候和生态系统的关系，这将在人类引起的气候变化的规模和影响的背景下提供一个重要的社会视角。

ABSTRACTThe amount of variation in the O2 and CO2 contents of the Phanerozoic atmosphere remain fundamental but unresolved questions. The global burial rate of organic carbon, as indicated by the carbon isotopic compositions of marine carbonates (d13Ccarb), is a critical parameter in modeling atmospheric compositions. While previous efforts have been largely focused on the redox significance of long-term, gradual trends (107 yr) in d13Ccarb and their relationship with d34S, a more volatile record that includes numerous short-term events, or isotopic excursions lasting a few million years or less, is becoming apparent. Some of the best evidence for large d13Ccarb excursions and their relationships to disturbances in the Earth's surface environment (as revealed, for example, by mass extinctions) can be found in Paleozoic strata.The proposed research is aimed at the construction of detailed d13Corg, d34Ssulfate, and 87Sr/ 86Sr for two Paleozoic time slices that are characterized by exceptionally large and well-documented d13Ccarb excursions. These Late Cambrian and Early Mississippian events have both been hypothesized to reflect global increases in the rate of organic carbon burial. This project will carry out numerical experiments using coupled box models of the geochemical cycles of C, S, and Sr to test multiple working hypotheses and to develop internally consistent scenarios for changes in the C and S cycles and the implications for atmospheric CO2 and O2 contents. Our focus on high-resolution data sets will allow us to critically examine some of the proposed cause-and effect feedback loops that stabilize the geochemical carbon cycle and thus the CO2 and O2 composition of the atmosphere. For example, burial of organic matter that is not balanced by oxidation of pyrite should increase atmospheric O2 levels (inferred from coupling and phasing ofd13Ccarb, d34Ssulfate), which provides a negative feedback on organic carbon burial through removal of oceanic phosphorus associated with ferric oxides. In addition, the burial of organic matter should lower pCO2 (lowering the net difference between d13Ccarb and d13Corg), cool the climate, and result ultimately in a decrease in continental weathering rates (reducing 87Sr/ 86Sr). A more complete understanding of these two targeted Paleozoic time slices will also provide unique insight into the Earth's climate system at critical geobiological intervals during (1) a worldwide mass extinction of trilobites in the Late Cambrian and (2) the Early Mississippian spread of vascular land plants and subsequent growth of glaciers on Gondwana.Broader Impacts:The research will support graduate and undergraduate student theses at Ohio State, Missouri and Penn State, including those from underrepresented groups. Students will receive broad training in field and laboratory settings using a variety of geochemical, stratigraphic and modeling techniques. Students at Ohio State and Missouri will spend time with Kump working with box models at Penn State and join Saltzman in the field, thus facilitating exchange among student groups and exposure to diverse faculty and facilities. The researchers will also be involved in outreach activities beyond the standard dissemination of scientific information at meetings and in peer review publications. Middle-school students will be involved by coordination of fieldwork and classes with a 7th grade science teacher. The chemostratigraphic database assembled will provide key reference sections for the assessment of carbon cycling during the critical time periods. It will promote a better understanding of carbon cycling on geological timescales and its relationship with past climates and ecosystems, which will provide an important societal perspective in the context of the magnitude and impact of human-induced climate change.