COLLABORATIVE RESEARCH: Geochemical and Isotopic Constraints on Mesproterozoic Ocean Chemistry Working Toward a Global Perspective

9725397从真核生物多样化(诺尔，1991年)、地球大气和海洋氧化状态的根本转变(Des Marais等人，1992年；Canfield和Teske，1996年)以及全球构造重组(Dalziel，1991年)的角度来看，弗兰克中元古代Eon(1600-1000 Ma)代表了地球历史上的一个关键时期。然而，尽管有这种明显的意义，中元古代仍然是地球历史上最鲜为人知、记录最少的时期之一。例如，根据中元古代海洋碳同位素记录的明显稳定性，1600 Ma至1000 Ma之间的时代被报告为“地球历史上最迟钝的时期”(Buick等人，1995年)。然而，最近在西伯利亚、加拿大北极、格陵兰和美国的中元古代晚期(1300 Ma)序列中发现了海相碳酸盐(13C值)的大正移(3-4%)。这种转变似乎代表了一种长期而根本的变化，可能代表着地球全球碳循环的一次重大转变，而不是一次短暂的漂移。这种观察到的转变发生在~1300 Ma和~1150 Ma之间的时间特别有趣，因为它发生在罗德尼超大陆的早期组装和据称的大气氧浓度上升的同时。拟议研究的目标是产生这一区间的碳、硫和锶同位素数据，从而记录并开始从机械上解决中元古代海洋化学中最基本的(一级)变化。这些目标将通过将沉积、成岩和地球化学信息整合到一个单一的综合框架中的多学科研究来实现。尽管没有一个单一的地层序列能够完全覆盖所观察到的碳同位素位移的区间，但可以有效地拼接两个不同的序列(巴芬岛北部的社会悬崖组；西北地区令人沮丧的L岩群，铜矿同斜)，以覆盖所观察到的过渡区间。这些层序中的每一个都是根据地层单位的可及性和暴露程度、存在适合进行地球化学分析的各种相和岩性以及来自不同沉积环境的各种异物和海相胶结物的丰度来选择的。这些先决条件结合在一起，确保了对沉积和成岩趋势的认识，否则就会阻碍对地球化学和同位素数据的解释。这种综合的方法将为理解元古代生物圈的复杂演化提供最清晰的途径。

9725397 Frank The Mesoproterozoic Eon (1600-1000 Ma) represents a critical period in Earth history from the standpoint of eukaryotic diversification (Knoll, 1991), fundamental transitions in the oxidation state of the Earth's atmosphere and oceans (Des Marais et al., 1992; Canfield and Teske, 1996), and global tectonic reorganization (Dalziel, 1991). Yet despite this obvious significance, the Mesoproterozoic remains one of the most poorly understood and poorly documented intervals in Earth history. For example, in reference to an apparent stability in the Mesoproterozoic marine carbon isotopic record, the era between 1600 Ma and 1000 Ma has been reported as 'the dullest time in Earth's history' (Buick et al., 1995). However, a large positive shift (3-4%) in marine carbonate (13C values has recently been recognized in late Mesoproterozoic (1300 Ma) successions from Siberia, Arctic Canada, Greenland, and the United States. Rather than a short-lived excursion, this shift appears to represent a prolonged and fundamental change that may represent a significant transition in the Earth's global carbon cycle. The timing of this observed transition, between ~1300 and ~1150 Ma, is particularly intriguing in that it occurs in conjunction with the early assembly of the Rodinian supercontinent and a purported rise in the concentration of atmospheric oxygen. The goal of the proposed research is to generate carbon, sulfur and strontium isotopic data for this interval and, thereby, document and begin to address mechanistically the most fundamental (first-order) shifts in the chemistry of the Mesoproterozoic ocean. These objectives will be achieved through a multidisciplinary study that integrates depositional, diagenetic, and geochemical information into a single comprehensive framework. Although no single stratigraphic succession adequately encompasses the interval surrounding the observed carbon isotopic shift, two distinct successions (Society Cliffs Formation, northern Baffin Island; Dismal L akes Group, Coppermine homocline, Northwest Territories) can be effectively spliced to provide coverage across the interval of the observed transition. Each of these successions has been chosen on the basis of accessibility and exposure of stratigraphic units, the presence of a wide variety of facies and lithologies suitable for geochemical analysis, and the abundance of a variety of allochems and marine cements from different depositional environments. Combined, these prerequisites assure recognition of depositional and diagenetic trends that would otherwise hamper the interpretation of geochemical and isotopic data. Such an integrated approach will provide the clearest path toward understanding the complex evolution of the Proterozoic biosphere.