Collaborative Research: Comparative Geochemistry of the Late Cambrian (SPICE) and Toarcian (Jurassic) Positive Carbon Isotope Excursions: New Perspectives on Paleozoic OAEs

古生代记录被多次、大的正碳同位素偏移打断，这些偏移即使不是全球性的，也是区域性的。在较年轻的记录中，类似的短途旅行通常通过直接观察与海洋尺度上有机碳埋藏增强的事件联系在一起——通常是在特征明显的缺氧条件下。在较老的层序中，由于缺乏保存完好的深海海底，海洋氧化还原的性质只能通过使用地球化学代用物来推测，而这些代用物通常是在较年轻的环境中得到证实的。特别是，我们和其他研究人员的工作已经阐明了钼同位素代理在量化全球深海氧化还原方面的巨大潜力，当仔细考虑当地氧气条件和盆地Mo库存时。在这项研究中，我们扩展了海洋缺氧事件的概念——最初定义为中生代普遍存在的深海和陆表黑色页岩沉积事件——以解决古生代海洋尺度氧化还原的本质。根据陆架碳酸盐岩的C、S同位素初步数据和地球化学盒模型，我们重点研究了晚寒武纪SPICE事件（阶梯正碳同位素偏移），该事件可能是早古生代OAE的主要候选事件。瑞典的明矾黑色页岩在有机C组分中显示出明显的SPICE偏移迹象，并且在事件发生之前，期间和之后至少存在局部氧化锆的证据-使其成为Mo同位素分析的理想候选者。因此，一个中心目标是将OAE概念外推到古生代，古生代有许多全球表达变化的正C同位素偏移，但很少有深海条件的物理证据。我们将具体测试SPICE是否为OAE。C和S同位素行为、黑色页岩地球化学和生物灭绝的强烈相似模式已经从更为人所知的Toarcian事件中得到了描述。因此，我们试图比较这两个层段的页岩地球化学和支持的碳酸盐岩海水成分代用指标，以提出以下问题：1)瑞典上寒武统明矾页岩的Mo同位素记录是否表明，从美国的高分辨率delta13C和delta34S碳酸盐岩数据推断出的全球范围内的缺氧？2) SPICE和Toarcian C同位素漂移是否表现出类似的Mo同位素和C- s - fe痕量金属行为，从而为寒武纪事件的性质提供重要的新线索？3)与SPICE相关的有机生物标志物模式是否与Toarcian OAE观察到的有机生物标志物模式相匹配，表明随着面积的增加，euxinia变浅了？4)通过类比，我们能否推断出Toarcian的模型来理解古生代早期无脊椎动物的灭绝模式，包括有问题的三叶虫的灭绝？5)以SPICE综合数据为模板，古生代明显的三角洲- 13c正沉积和陆表黑色页岩沉积是否类似于中生代的oae ？所有这些问题都将通过与每个时间间隔的专家合作，并通过使用地球化学代理来解决，这些代理是我们联合研究小组的主要内容——特别是铁形态，微量金属化学（强调Mo），以及S， Fe和Mo的新同位素方法。为了将这项研究的影响扩展到里昂/安巴尔研究小组之外，我们设计了一种双管齐下的方法，以推广到更大的河滨-洛杉矶地区。首先，Lyons/Anbar/Gill将在项目的每一年参与UCR的指导暑期研究实习计划，来自历史上在科学（特别是地球科学）中代表性不足的少数民族的学生将在两个实验室获得实践经验。其次，Lyons/Anbar/Gill将与加利福尼亚州长滩的太平洋水族馆合作，设计一个以全球人为沿海缺氧流行病为中心的展览。令人高兴的是，近年来洛杉矶地区的沿海水质有所改善。基于这一成功和过去的经验教训——包括非常深刻的过去——我们希望以一种翔实、乐观、希望有趣的方式向公众宣传沿海环境的健康。考虑到水族馆每年有130万游客，我们确信我们的信息会广泛传播。

The Paleozoic record is punctuated by multiple, large positive carbon isotope excursions that are expressed regionally if not globally. In younger records, similar excursions are often linked through direct observation to ocean-scale events of enhanced organic carbon burial--typically under well-characterized anoxic conditions. In older sequences, lacking preserved deep seafloor, the nature of oceanic redox on the broad scale can only be surmised through use of geochemical proxies that are typically ground-truthed in younger settings. In particular, our work and that of other researchers has illuminated the great potential of the molybdenum isotope proxy for quantifying global deep-ocean redox, when applied with careful consideration to local oxygen conditions and the basinal Mo inventory. In this study we expand the concept of an Oceanic Anoxic Event--first defined for episodes of pervasive deep-ocean and epicontinental black shale deposition during the Mesozoic--to address the nature of ocean-scale redox in the Paleozoic. We focus on the Late Cambrian SPICE event (Steptoean Positive Carbon Isotope Excursion), suggested by our preliminary C and S isotope data from shelf carbonates and our geochemical box model to be a prime candidate for an early Paleozoic OAE. The Alum black shale in Sweden shows clear signs of the SPICE excursion in the organic C fraction and evidence for at least local euxinia before, during, and after the event--making it an ideal candidate for Mo isotope analysis. A central goal, then, is to extrapolate the OAE concept to the Paleozoic, which has numerous positive C isotope excursions of varying global expression but little physical evidence for conditions in the deep ocean. We will specifically test whether the SPICE is an OAE. Strongly analogous patterns of C and S isotope behavior, black shale geochemistry, and biotic extinction have been described from the better-known Toarcian event. We therefore seek to compare the shale geochemistry and supporting carbonate proxies for seawater composition for these two intervals to ask: 1) Do Mo isotope records from the Upper Cambrian Alum Shale in Sweden point to the globally expanded anoxia inferred from our high-resolution delta13C and delta34S carbonate data from the United States? 2) Do the SPICE and Toarcian C isotope excursions show analogous Mo isotope and C-S-Fe-trace metal behavior, such that significant new light is shed on the nature of the Cambrian event? 3) Do organic biomarker patterns associated with the SPICE match those observed from the Toarcian OAE, suggesting that euxinia shallowed as its areal extent increased? 4)By analogy, can we extrapolate models for the Toarcian to understand invertebrate extinction patterns in the early Paleozoic, including problematic trilobite extinctions? 5)Using our integrated data for the SPICE as a template, are Paleozoic intervals of pronounced positive delta13C and epicontinental black shale deposition analogous to the OAEs of the Mesozoic? All of these questions will be addressed through collaboration with experts on each time interval and through use of the geochemical proxies that are staples in our combined research groups--specifically Fe speciation, trace metal chemistry (emphasizing Mo), and novel isotope approaches for S, Fe, and Mo.Broader Impacts. In a sincere effort to extend the impact of this study beyond the Lyons/Anbar research groups, we have designed a two-pronged approach for outreach in the greater Riverside-LA area. First, Lyons/Anbar/Gill will participate during each of the project years in the UCR Mentoring Summer Research Internship Program, whereby students from ethnic groups historically underrepresented in the sciences (particularly the earth sciences) will receive hands-on experience in the two labs. Second, Lyons/Anbar/Gill will work with the Aquarium of the Pacific in Long Beach, CA, to design an exhibit centered on the global epidemic of anthropogenic coastal hypoxia. Happily, the LA area has seen an improvement in its coastal water quality in recent years. Building from this success and lessons learned from the past--including the very deep past--we expect to reach out to the public about the health of coastal environments in an informative, optimistic, and hopefully entertaining way. Given the 1.3 million annual visitors to the aquarium, we are certain our message will spread widely.