Constraints on bio-essential elements in the Paleoproterozoic seawater: Estimation from in situ analyses of the carbonate rocks, Francevillian Group, Gabon

Tracing secular change in seawater bio-essential element concentration is important for testing the widely endorsed concept that life and seawater composition coevolved. The earliest eukaryotic fossils have been discovered in the Paleoproterozoic Francevillian Group; thus, the carbonate rocks within the sedimentary rock offer key insights for the availability of bio-essential elements in Paleoproterozoic seawater. However, the Francevillian dolostones experienced complex diagenetic processes, complicating the estimation of the original paleoseawater composition. We measured the concentrations of Co, Ni, Cu, and Zn in the upper Francevillian B (FB) Formation dolostone with LA-ICP-MS/MS. Cobalt concentration in the dolostones shows a positive correlation with Fe/Ca and Mn/Ca values, indicating that the primary host phase of Co was Fe-Mn oxides. Compared to modern equivalents, the lower Co/Mn value in the dolostones suggests a faster growth rate of Fe-Mn oxides. The values of Cu/Ca and Zn/Ca in the FB dolostones are most closely correlated with the S/Ca value, which can be explained by the presence of fine sulfide inclusions. For dolostones deposited below the chemocline, the Cu and Zn concentrations in the carbonate end-member are estimated to be 0.82-2.70 and 3.97-5.40 μg/g, respectively, comparable to those of Phanerozoic carbonate rocks. Meanwhile, data from dolostones deposited above the chemocline indicate that marine Cu and Zn were nearly depleted in the Lastoursville basin. Given the behavior of Cu and Zn under relatively oxic seawater of the Lastoursville basin at the time, these depletions in the Lastoursville basin may signify Cu and Zn utilization by eukaryotes.