Oxygenation of the Earth's atmosphere-ocean system: A review of physical and chemical sedimentologic responses

摘要

The Great Oxidation Event (GOE) is one of the most significant changes in seawater and atmospheric chemistry in Earth history. This rise in oxygen occurred between ca. 2.4 and 2.3 Ga and set the stage for oxidative chemical weathering, wholesale changes in ocean chemistry, and the evolution of multicelluar life. Most of what is known about this important event and the subsequent oxygenation history of the Precambrian Earth is based on either geochemistry or "data mining" published literature to understand the temporal abundance of bioelemental sediments. Bioelemental sediments include iron formation, chert, and phosphorite, which are precipitates of the nutrient elements Fe, Si, and P, respectively. Because biological processes leading to their accumulation often produce organic-rich sediment, black shale can also be included in the bioelemental spectrum. Thus, chemistry of bioelemental sediments potentially holds clues to the oxygenation of the Earth because they are not simply recorders of geologic processes, but intimately involved in Earth system evolution. Chemical proxies such as redox-sensitive trace elements (Cu, Cr, V, Cd, Mo, U, Y, Zn, and REE's) and the ratio of stable isotopes (δ~(56)Fe, δ~(53)Cr, δ~(97/95)Mo, δ~(98/95)Mo, δ~(34)S, Δ~(33)S) in bioelemental sediments are now routinely used to infer the oxygenation history of paleo-seawater. The most robust of these is the mass-independent fractionation of sulfur isotopes (MIF), which is thought to have persisted under essentially anoxic conditions until the onset of the GOE at ca. 2.4 Ga. Since most of these proxies are derived from authigenic minerals reflecting pore water composition, extrapolating the chemistry of seawater from synsedimentary precipitates must be done cautiously. Paleoenvironmental context is critical to understanding whether geochemical trends during Earth's oxygenation represent truly global, or merely local environmental conditions. To make this determination it is important to appreciate chemical data are primarily from authigenic minerals that are diage-netically altered and often metamorphosed. Because relatively few studies consider alteration in detail, our ability to measure geochemical anomalies through the GOE now surpasses our capacity to adequately understand them. In this review we highlight the need for careful consideration of the role sedimentology, stratigraphy, alteration, and basin geology play in controlling the geochemistry of bioelemental sediments. Such an approach will fine-tune what is known about the GOE because it permits the systematic evaluation of basin type and oceanography on geochemistry. This technique also provides information on how basin hydrology and post-depositional fluid movement alters bioelemental sediments. Thus, a primary aim of any investigation focused on prominent intervals of Earth history should be the integration of geochemistry with sedimentology and basin evolution to provide a more robust explanation of geochemical proxies and ocean-atmosphere evolution.