| Emerging evidence from several early Mesoproterozoic localities increasingly supports a relatively high abundance and diversity of eukaryotic organisms by this time.Moreover,the discovery of the decimeter-scale,multicellular eukaryote fossils in early Mesoproterozoic(?1560 Ma)shelf sediments from the Gaoyuzhuang Formation of the Yanliao Basin,North China provides the strongest evidence yet for the evolution of complex multicellular eukaryotes as early as the Mesoproterozoic.However,the lack of firm constraints on Earth surface oxygen levels has fueled debate on whether early eukaryote diversification was directly linked to environmental changes.Here we present rare earth element(REE),Fe-speciaiton and inorganic carbon isotope data for marine carbonate rocks from the 1600?1400Ma Jixian Section,Yanliao Basin,to investigate ocean redox conditions in the basin where the Gaoyuzhuang Fossils were discovered,which could also provides a direct assessment of potential links between the environmental changes and early eukaryote evolution.When using REEs for palaeoenvironmental studies,it is crucial to ensure that the information extracted from bulk carbonate rocks is not affected by diagenetic alteration or contaminated by non-carbonate materials.To avoid such contamination,we firstly optimized the dissolution method for REE studies of bulk carbonate rocks through detailed stepwise dissolution experiments.The experiment results show that secondary carbonate phases and adsorbates on exchangeable sites are likely to be dissolved or released at the beginning of the dissolution process but this doesn't increase REE concentration though the REE pattern may be altered e.g.lessening of negative Ce anomaly,altering of Eu anomaly,Y/Ho ratio and relative fractionation between light REE and heavy REE.Non-carbonate minerals e.g.terrestrial particulate matter,Fe-Mn oxyhydroxides and phosphates are likely to be dissolved towards the end of the total dissolution,especially when excess acid is added.This should increase REE concentration and alter REE pattern significantly.In contrast,solutions from intermediate steps are less contaminated.Further,compared with using 10%v/v acetic acid,the solutions obtained using 5%v/v acetic acid return more pristine REE information from the carbonate.Therefore,we suggest an initial dissolution of 30%?40%followed by the sampling dissolution of 30%?40%of total carbonate using 5%v/v acetic acid to produce REE information considered to best represent that of the carbonate source water.Our REE,Fe-speciation and inorganic carbon isotope data from the Gaoyuzhuang Formation and Tieling Formation in Jixian Section,Yanliao Basin demonstrate that oxygenation of the Mesoproterozoic environment was far more dynamic and intense than previously envisaged.In the earliest Mesoproterozoic,Yanliao Sea was anoxic and ferruginous with a very shallow chemocline.Carbonates of this stage show marine REE patterns that are not typical of oxic seawater,and the corresponding Fe-speciation data show FeHR/FeT>0.38 and Fepy/FeHR<0.7.Significantly,however,we find evidence for a progressive oxygenation event starting at?1570 Ma.Negative Ce anomaly occurred,and there was an increase in the magnitude of negative Ce anomalies.The chemocline deepened,which resulted in extensive precipitation of Fe(oxyhydr)oxide minerals at the chemocline as ferruginous deeper waters became oxygenated,and negative?13Ccarb excursion due to oxidation of a ?13C depleted pool of dissolved organic carbon and/or methane at the redoxcline.Then until at least?1400Ma,Yanliao Sea continued progressive oxygenation.Carbonates of this stage show relatively stable or more negative Ce anomalies,and a gentle long-term increase of ?13Ccarb record.In addition,the onset of the progressive oxygenation event is immediately prior to the occurrence of complex multicellular eukaryotes in the Gaoyuzhuang Formation,providing strong support for the fundamental role of rising oxygen levels in early eukaryote evolution. |
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