Abstract:
We report here high-resolution Mn/Ca ratios of planktonic foraminifer, Globigerinoides sacculifer from ODP Site 1144 in the northern South China Sea (SCS). The variation pattern of the Mn/Ca ratios of G. sacculifer from the top 170 mcd is very similar to those of the generally used paleoclimate records, such as delta (super 18) O of Globigerinoides ruber and SSTs derived from Mg/Ca ratios of G. sacculifer. Robust Milankovitch periodicities are presented in the power spectrum of the Mn/Ca record, as well as in those of the other paleoclimate records, suggesting orbital forcing for the variation of the Mn/Ca ratios. Further spectral analysis indicates that the variations of the Mn/Ca ratios lead those of the delta (super 18) O of G. ruber at all the orbital periodicities, and are roughly in pace with or slightly lead those of the SSTs at eccentricity and obliquity periodicities, but lag behind that of the SSTs at precession periodicity. This suggests that the driven forcing for the variation of the Mn/Ca ratios mainly associate with those for the SSTs. Comparisons between the Mn/Ca ratios and other geochemical proxies for redox condition in sediments show that higher Mn/Ca ratios of G. sacculifer generally correspond to higher authigenic Mn concentrations and lower contents of total sulfur (TS) and total organic carbon (TOC) Thus, the Mn coatings in the G. sacculifer at ODP Site 1144 are mainly associated with Mn oxides, and the variation of the Mn/Ca ratios of G. sacculifer appears to indicate changes of redox condition in sediments. Exception is observed at the top 10 mcd, in which the Mn/Ca ratios show an upward decreasing trend and low core-top values, while other proxies suggest oxic environment in this section. The low Mn/Ca ratios agree with the general low Mn/Ca ratios in foraminifer tests from core-top sediments in the SCS, and seem to attribute to the fact that authigenic Mn precipitation are mainly generated within sediment column rather than at sediment surface and the oxidation states for them may change from Mn(III) oxyhydroxides to Mn(IV) oxides after burial. However, details are not well known yet. Despite of such uncertainties, the response of the variation of the Mn/Ca ratios of G. sacculifer to paleoclimate changes is significant, indicating that it is an informative paleoclimate indicator.