FERRUGINOUS SHALLOW AND MID-DEPTH SEAWATERS IN AN EARLY TONIAN MARINE BASIN

As a bridge between the “Boring Billion” and Cryogenian global glaciations, the Tonian Period witnessed a critical transition in Earth history. Recent paleobiological data have revealed a significant increase in eukaryotic biodiversity and innovation in the Tonian Period. It has been proposed that during the Tonian, ferruginous conditions became well-developed in mid-depths of the oceans, replacing toxic sulfidic waters, and creating more favorable environmental conditions for the diversification of eukaryotes. However, more geochemical data are needed to test this hypothesis. In our study, we analyzed rare earth elements and iron speciation data of carbonates from the Tonian Huaibei Group in the North China in order to reconstruct the local marine redox conditions. These carbonates contain stromatolites and molar tooth structures and are interpreted to have been deposited in shallow-water environments on a carbonate platform. Most samples from this succession display a Ce anomaly that ranges between 0.80 and 1.33 with an average of 0.97, indicating an anoxic depositional environment. Building upon Clarkson et al.’s (2014) validation of the Fe_HR/Fe_T proxy for carbonate-rich samples, we assembled a dataset of modern and ancient samples with known iron speciation data and redox conditions to test the applicability of the Fe_py/Fe_HR proxy on carbonate-rich samples with Fe_T > 0.5. We confirm that samples with Fe_py/Fe_HR lower than 0.4 and Fe_HR/Fe_T greater than 0.38 were most likely deposited under ferruginous conditions regardless of the carbonate content of the sample. Fe_HR/Fe_T values of Huaibei samples vary from 0.42 to 0.81, whereas Fe_py/Fe_HR values range between 0.01 and 0.28. Together our proxy data suggest an anoxic and ferruginous water column was present during the deposition of the shallower carbonate facies of the Huaibei Group. Our new data are also consistent with previously published Fe speciation data from correlative deeper siliciclastic facies elsewhere the basin. Taken together, all these data point to the presence of locally ferruginous redox conditions in shallow and mid water depths in the ocean during the Tonian Period when major eukaryotic innovation and diversification occurred.