Cu And Zn Isotope Fractionation During Continental Weathering And The Application Of Cu And Zn Isotopes In Paleoceanography

Recent studies show that zinc isotopes could provide valuable clues to environmental change and biogeochemical cycle of the past oceans.The present doctoral dissertation makes new progress summarized as follows:?1?This study reports a modified procedure for leaching the carbonate fractions in sedimentary rocks,a thorough evaluation of diagenetic effects,and systematic variations of Zn and Sr isotopic ratios in lower part of the Ediacaran stratigraphic unit?Doushantuo Formation?deposited in the aftermath of the Marinoan glaciation in South China.The influence of post-depositional diagenesis on Zn isotope compositions of the studied samples is assessed by comparing?⁶⁶Zn with other geochemical indexes(⁸⁷Sr/⁸⁶Sr,?¹³C,?¹⁸O and Mn/Sr ratios).The five studied cap carbonate sections?Member I of the Doushantuo Formation?were modified by post-depositional diagenesis which could lower carbonate?⁶⁶Zn values.The lightest?⁶⁶Zn value shows that Zn isotopic ratios of these‘overprinted'cap dolostones could have been reset by hydrothermal fluids with relatively high Zn concentration and extremely low?⁶⁶Zn values.Samples from the Member II of the Doushantuo Formation above the cap dolostones are pristine and have low ⁸⁷Sr/⁸⁶Sr ratios identical to the proposed early Ediacaran seawater values.Chemical and isotopic variations in these samples are expected to reflect primary signals that record paleoenvironmental changes of the early Ediacaran ocean.A rapid increase of Zn isotopic values?⁰.3 to 1.1 per mil?occurs in the middle part of Member II,accompanying by relatively invariant ⁸⁷Sr/⁸⁶Sr ratios that imply no significant changes in input from continental weathering.Considering no significant changes atmospheric oxygen during this time,the rapid?⁶⁶Zn increase indicates an increase in buried organic matters,which is consistent with the coupled positive shift of?¹³C_carb,as well as the fossil records found in the same strata.These results provide insights into Zn cycling in the post-Marinoan oceans and facilitate the application of Zn isotopes in carbonates as a proxy for the fate of marine organic matter.?2?Black shales represent one of the main reservoirs of metals released to hydrosphere via chemical weathering and play an important role in geochemical cycling of metals in the ocean.To investigate the behaviors of Cu and Zn isotopes during metal enrichment of black shales and the migration during weathering,in this study we reported Cu and Zn concentration and isotope data for unweathered and weathered metalliferous shales and siliceous interbeds from the Maokou Formation in central China.The unweathered shales and cherts have moderately enriched Cu and Zn concentrations with silicate-like?⁶⁵Cu but heavy?⁶⁶Zn.The elevated?⁶⁶Zn values reflect an important contribution from seawater via sulfide precipitation and/or organic matter adsorption.The Zn isotopic compositions of the metalliferous shales are different from those of the‘normal'shales,highlighting the potential of Zn isotopes as a tracer for metal enrichment in natural systems.The weathered shales and cherts have an extreme?⁶⁵Cu range from-6.42‰to+19.73‰and a modest?⁶⁶Zn range of+0.25‰to+0.78‰.The leaching of Cu-and Zn-rich sulfides in shallow depths and their downward transport and refixation by Fe-sulfide account for the Cu and Zn isotope fractionation.In general,?⁶⁶Zn values of the weathered shales shift towards light values compared to the unweathered protoliths,suggesting that shale weathering releases Zn which is isotopically heavier than igneous rocks and the global riverine average?+0.33‰?.Our results therefore indicate that Cu isotopes can be extremely fractionated during weathering of Cu-rich shales and both heavy Cu and Zn isotopes are preferentially released into fluids during shale weathering.These results should be considered when evaluating geochemical cycling of Cu and Zn in the modern or past oceans.?3?In modern oceans,burial of organic/sulfide-rich sediments could increase zinc isotope ratios of seawater,which is mass-balanced by sinks of oxic sediments enriched in isotopically heavy Zn.Zinc isotopes may thus be utilized to trace changes of redox condition and organic carbon burial in the past oceans.Here we studied black shales spanning Ordovician–Silurian?O-S?transition in South China to provide new constraints on the Hirnantian glaciation which is causally linked to the Late Ordovician mass extinction.The black-shales deposited before and after the Hirnantian glaciation have high and similar degrees of enrichments of redox-sensitive elements?RSEs?,indicating a reducing condition.The authigenic fractions in black-shales have chance to capture the Zn isotope composition of coeval seawater.A?⁶⁶Zn elevation of⁰.2‰in the early Rhuddanian after the Hirnantian glaciation compared to that in Katian before glaciation could be generated by removal of Zn in organic or sulfide phase,which thus indicates an anoxia event in the early Rhuddanian.Our Zn isotope data support the assumption of the early Rhuddanian anoxia related to post-Hirnantian global marine transgression.The higher isotope compositions of authigenic Zn during the Hirnantian glaciation than Katian could be explained by increased organic matter burial,rather than increased sulfide precipitation which runs counter to other evidences of Hirnantian oxygenation.The larger organic matter reservoir during the Hirnantian glaciation draw down the atmosphere CO₂,and leads to climate cooling.The Zn concentration fluctuations provide new insights into the change of primary productivity in the beginning of glaciation when mass extinction occurred.Zn isotope compositions in the black shales are the proxies potential for the study of sulphur and carbon cycling in the past oceans.