Evolution Of Marine Chemistry In The Late Paleoproterozoic

The Paleoproterozoic Great Oxidation Event(GOE)has brought unprecedented changes to the Earth surface system.Accompanying with the rise of atmospheric oxygen,the oceanic dissolved inorganic carbon has experienced the longest-lasting positive?¹³C excursion in earth's history,known as the Lomagundi-Jatuli Event(LJE).The increased net burial of organic matter is thought to be the key factor of this unusual?¹³C excursion and have release massive free oxygen.The continuously rise of atmosphere-ocean oxygen level during the LJE is further supported by the expansion of oceanic sulfate reservoir and the explosive evolution.In traditional views,these unprecedented perturbation in seawater carbon isotope composition ended by the return of?¹³C values to near 0‰.However,researches on the?2.0 Ga carbonates in Russian and Gabon reported the rapid negative excursions of?¹³C_carb,which are thought to the result of re-oxidation of the organic matter buried in the preceding LJE.During which the oxidation of large amounts organic matter might have consumed massive oxygen in atmosphere.Since then,more and more views suggest that the atmosphere-ocean system underwent a significant deoxygenation event following the LJE.At the same time,there are still several more recent researches questioned the existence of the post-LJE deoxygenation event.The Paleoproterozoic Hutuo Supergroup on the North China Craton is a volcanic-sedimentary sequence that has only undergone low-green schist phase of metamorphism.The deposition age of the Hutuo Supergroup is constrained between 2.14 Ga and 1.9 Ga according to the post-Huronian Glaciation glaciogenic diamictite from the Sijizhuang Formation(the bottom of the Hutuo Supergroup)and the available geochronological data.Previous carbon isotope analyses on the Hutuo Supergroup carbonates show slightly positive excursions,which was explained to represent the termination of the LJE.Also,a negative?¹³C_carbexcursion is suspected in the Huaiyincun Formation(Dongye Group),similar to those post-LJE records found in Russia and Gabon at the end of the LJE.Therefore,the Hutuo Supergroup should have preserved key evidence for the atmosphere-ocean redox status after the Paleoproterozoic GOE.This paper carried out multiproxy researches on the Huaiyincun Formation which preserved the negative?¹³C_carbexcursion.Based on detailed reconstruction of the depositional environment through sedimentological and petrographic work,we used the carbon and sulfur isotope,rare earth element etc.integrating with previously published data from other continent to better discussed the surface nutrient cycling,evolution of the oceanic carbon pools,sulfur pools and redox status of the atmosphere-ocean system after the Paleoproterozoic GOE.The main results,findings and conclusions from this study are shown as following:1.The light pink dolostone in the lower Huaiyincun Formation(below 91.6 m)is enriched in large grained,rounded quartz,feldspar,apatite,zircon and hematite compared to the light grey dolostone in the upper Huaiyincun Formation(above 91.6m),while the latter is relatively enriched in organic matter.The decreasing abundance of detrital minerals and iron oxides suggests that carbonates in the Huaiyincun Formation were first deposited in a relatively oxidized,shallow environment and then changed into a deeper,less oxidized environment accompanied with a transgression.The evolution of stromatolite morphology(from conical to columnar to wave to meter-scale domal bioherms back and forth)in the light pink dolostone unit indicates that sea level underwent at least two declining and then rising processes prior to the overall transgression in the Huaiyincun Formation.2.The frequent occurrence of storm-related structures in the Huaiyincun Formation suggests that the carbonates in the Huaiyincun Formation experienced a number of storm events during their deposition.In detail,intraclasts layers are only centimeter-scale and occasionally interbedded with bedded dolostones in the lower Huaiyincun Formation,while the dolostones in the upper Huaiyincun Formation is characterized by massive decimeter-to meter-scale storm-related structures such as intraclasts,hummocky cross stratification,graded bedding etc.These distinct differences in abundance and size of storm-related structures indicate that only small-scale storm events occurred intermittently during the deposition of the lower Huaiyincun Formation,and that the storms gradually intensified with the rise of sea-level.3.The evolution of the?¹³C_carbvalues in the Huaiyincun Formation is inconsistent with the regional sea-level changes,also barely affected by the post-depositional process.Therefore,they have preserved information about the shallow seawater at the end of the LJE.The negative?¹³C_carb excursion from-0.1‰to-3.7‰in the Huaiyincun dolostones was therefore resembling with those in Gabon and Russia,as part of a multi-basinal phenomenon that may have been global.Such a widespread negative?¹³C excursion would require oxidation of large quantities of organic matter and hence a drawdown of the atmospheric and oceanic oxygen level at the end of the LJE.4.The high variability of?³⁴S_CAS values ranging from 33.1%to-1.2‰and the low CAS concentrations of the carbonates in the Huaiyincun Formation reveal an extremely low seawater sulfate level after the LJE.Modelling based on the values of(?)?³⁴S_CAS/(?)t and?³⁴S_CAS-Py further suggests the[SO4^2-]_sw might be<0.8 Mm,suggesting that the marine sulphate reservoir experienced a dramatic contraction at the end of the LJE,which may be the result of a possible global deoxygenation event at the end of the LJE.The decrease in?³⁴S_CAS and the increase of CAS concentration in the upper Huaiyincun Formation,however,probably represent a slight expansion of the seawater sulfate reservoir related to elevated sulfate flux by oxidative continental weathering.5.The occurrence of aragonite crystal fan pseudomorphs in the Huaiyincun Formation indicates that seawater dissolved oxygen concentration was low during its formation.Compilation of the existing reports reveals a clear absence of the reliable record of the ACFs since the initial of the GOE which corresponds to an increase in oceanic oxygen content during the same period.This might be attributed to the rise of free oxygen and decrease in dissolved inorganic carbon concentration in the paleo-ocean.Aragonite crystal fans preserved in the Hutuo Supergroup,Belcher Supergroup,Bear Creek Group and Libby Greek Group suggest the worldwide reappearance of ACFs in?2.0 Ga successions which is consistent with the presence of a large DIC reservoir in the post–LJE ocean.Further,these aragonite crystal fans were precipitated on seafloor in an anoxic upper subtidal to intertidal environment.This oxygen limited shallow seafloor might be the result of a deoxygenation event after the LJE.6.The REE data of the pure carbonate minerals of the Huaiyincun dolostones might be affected by local weathering input as their?REE values are strongly correlated to the change of mineral assemblage.However,the light grey dolostones in the upper Huaiyincun Formation was less affected since they were precipitated in relatively deep water.The PAAS normalized results of the upper grey part of the Huaiyincun Formation feature without significant Ce anomalies,suggesting that the shallow seawater might have returned to a sub-oxidation state at the end of the LJE.Comparison of all the results indicates that the influence of terrestrial inputs persists in all extraction methods and can not be excluded as it's the real feature of regional seawater.Therefore,using carbonate REE value as a proxy to trace the redox state of global seawater needs to be carefully considered and must be based on a thorough comparison of regional stratigraphic and lithological features.