Early oceans(>520 Ma) were characterized by widespread water-column anoxia, stratification, and limited oxidant availability, which are comparable to the chemical characteristics of modern marine sedimentary pore-waters in productive continental margins. Based on this similarity and our current understanding of the formation mechanism of early Earth ocean chemistry, we propose an idealized chemical zonation model for early oceans that includes the following redox zones(from shallow nearshore to deep offshore regions): oxic, nitrogenous(NO3?-NO2?-enriched), manganous-ferruginous(Mn2+ or Fe2+-enriched), sulfidic(H2S-enriched), methanic(CH4-enriched), and ferruginous(Fe2+-enriched). These zones were dynamically maintained by a combination of processes including surface-water oxygenation by atmospheric free oxygen, nitrate reduction beneath the chemocline, nearshore manganese-iron reduction, sulfate reduction, methanogenesis, and hydrothermal Fe2+ inputs from the deep ocean. Our modified "euxinic wedge" model expands on previous versions of this model, providing a more complete theoretical framework for the chemical zonation of early Earth oceans that helps to explain observations of unusual Mo-S-C isotope patterns. This model may provide a useful foundation for future studies of ocean chemistry evolution and elemental biogeochemical cycles in early Earth history.
Phytoplankton biomarkers were analyzed using suspended particles collected from the northern South China Sea(SCS)during the summer cruise of2008,with the goal of understanding the algal community structure and biomass distribution pattern in the summer season.The results indicated that the distribution of algal biomarkers in surface water of SCS was impacted and constrained by the local hydrological settings:the high biological community generally appeared in Pearl River estuaries,southwestern off Taiwan island and southeastern off Hainan Island,while the relatively lower biomass was found in the deep basin and strait areas.Diatoms were the dominant phytoplankton species,which were followed by dinoflagellates.Coccolithophorid biomass gradually increased toward the open ocean.The present work indicated that the algal biomarkers effectively documented the variability of the phytoplankton biomass and community structure as well as their linkage with the oceanic dynamics in SCS during summer 2008.This research provided not only the foundation for the application of algal lipids in the modern ocean ecosystem,but also the basis for the reconstruction of the past oceanic algal community structures.
The two key mechanisms for biologically driven carbon sequestration in oceans are the biological pump(BP) and the microbial carbon pump(MCP); the latter is scarcely simulated and quantified in the China seas. In this study, we developed a coupled physical-ecosystem model with major MCP processes in the South China Sea(SCS). The model estimated a SCSaveraged MCP rate of 1.55 mg C m^(-2) d^(-1), with an MCP-to-BP ratio of 1:6.08 when considering the BP at a depth of 1000 m.Moreover, the ecosystem responses were projected in two representative global warming scenarios where the sea surface temperature increased by 2 and 4°C. The projection suggested a declined productivity associated with the increased near-surface stratification and decreased nutrient supply, which leads to a reduction in diatom biomass and consequently the suppression of the BP. However, the relative ratio of picophytoplankton increased, inducing a higher microbial activity and a nonlinear response of MCP to the increase in temperature. On average, the ratio of MCP-to-BP at a 1000-m depth increased to 1:5.95 with surface warming of 4°C, indicating the higher impact of MCP in future ocean carbon sequestration.
The Ediacaran Doushantuo Formation(ca. 635–551 Ma) deposited immediately after the last Neoproterozoic glaciations and recorded the most prominent negative excursions of carbonate carbon isotopic composition(δ^(13)C_(carb)). These excursions have been interpreted as a result of widespread remineralization of a large dissolved organic carbon(DOC) reservoir in the Ediacaran deep oceans. However, there is no direct evidence so far found in rocks for the proposed DOC reservoir, which devalues such an interpretation. Here, we conducted a detailed study on the glow-curves characteristics and signal origins of spurious thermoluminescence(TL) of the Doushantuo Formation at Jiulongwan in Yangtze Gorges area, South China, through sequential tests under CO_2, N_2 and air. Spurious TL intensities for test samples before and after removing soluble organic matter via accelerated solvent extraction(ASE) are nearly identical. Further, significant positive correlation between the spurious TL intensity and total inorganic carbon(TIC) content(R^2=0.7) indicate that the Doushantuo spurious TL with the characteristic peak at 393.5 °C from the sequential test is chemiluminescence(CL) which is derived from the oxidation of a type of non-volatile organic matter strongly associated with carbonate mineral lattice(termed as "X-OM"). A most likely explanation is that the X-OM is a type of dissolved organic matter which co-precipitated with carbonate minerals into sediments in the Ediacaran Doushantuo Ocean. Furthermore, a significant exponential negative correlation(R^2=0.55) is observed between the CL data and the isotopic difference between carbonate and coexisting bulk organic matter(i.e., Δ^(13)C_(carb-org), a proxy for remineralization degree of DOC reservoir in proposed DOC hypothesis), suggesting that the X-OM was derived from the oxidation of the DOC reservoir in the Ediacaran Ocean. We thus propose that the X-OM and its CL detected in our study may have recorded the evolution of the possible DOC reservoir in the Ediacaran Dous
Molybdenum(Mo) proxies, including bulk concentration and isotopic composition, have been increasingly used to reconstruct ancient ocean redox states. This study systematically reviews Mo cycles and their accompanying isotopic fractionations in modern ocean as well as their application in paleo-ocean redox reconstruction. Our review indicates that Mo enrichment in sediments mainly records the adsorption of Fe-Mn oxides/hydroxides and chemical bonding of H2 S. Thus, Mo enrichment in anoxic sediments generally reflects the presence of H2 S in the water column or pore waters. In addition to the effect of euxinia, sedimentary Mo enrichment is related to the size of the oceanic Mo reservoir. Given these primary mechanisms for oceanic Mo cycling, Mo abundance data and Mo/TOC ratios acquired from euxinic sediments in geological times show that fluctuations of the oceanic Mo reservoir are well correlated with oxygenation of the atmosphere and oceans and suggest that oxygenation occurred in phases. Mo proxies suggest that Mo isotopes in strongly euxinic sediments reflect the contemporaneous Mo isotopic composition of seawater, but other processes such as iron-manganese(Fe-Mn) adsorption and weak euxinia can result in different fractionations. Diagenesis may complicate Mo enrichment and its isotopic fractionation in sediments. With appropriate constraints on the Mo isotopic composition of seawater and various outputs, a Mo isotope mass-balance model can quantitatively reconstruct global redox conditions over geological history. In summary, Mo proxies can be effectively used to reconstruct oceanic redox conditions on various timescales due to their sensitivity to both local and global marine redox conditions. However, given the complexity of geochemical processes, particularly the effects of diagenesis, further work is required to apply Mo proxies to ancient oceans.
