Sr/Ca ratios as indicators of varying modes of pelagic carbonate diagenesis in the ooze, chalk and limestone realms

摘要

There has been an interest in the application of Sr/Ca ratios in pelagic carbonates to reconstruct paleoceanographic conditions such as changes in the oceanic Ca and Sr budgets or calcareous nannoplankton productivity, but the effect of burial diagenesis on primary Sr/Ca records has not been sufficiently understood. In order to clearly document Sr/Ca records of pelagic carbonates relative to original paleoceanography or secondary diagenetic processes, Sr/Ca analyses were performed on bulk sediments from an 822.5 m-thick ooze-chalk-limestone sequence at the Mid-Pacific Mountains, Deep Sea Drilling Project Site 463. The Sr/Ca ratios exponentially decrease from ～2.3 to ～1.1 mmol/mol with increasing burial depth in the ooze-chalk interval (0-452.0 m below seafloor (mbsf)). The Sr/Ca ratios further decline to as low as ～0.4 mmol/mol in the limestone interval (> 452.0 mbsf), with relatively high values of > 1.1 mmol/mol at carbonate-poor lithology around 620 mbsf. The stratigraphic Sr/Ca trends appear to be consistent with porosity profile, and similar Sr/Ca—porosity covariation in association with diagnetic changes in lithologies is also common in Ontong Java Plateau sections. By adopting a recent diagenetic model from backscattered electron image analysis of Ontong Java Plateau sections, it is postulated for Site 463 that exponential Sr/Ca decrease in the ooze-chalk interval is due to the release of Sr into interstitial water via recrystallization (i.e., purification effect), and that further decrease in Sr/Ca ratios in the limestone interval is most likely due to acquisition of Sr-depleted calcite cement at the 'chalk-limestone transition' (newly proposed as the dilution effect). Specifically, Sr/Ca ratios of deeply buried pelagic limestones are sensitive to the amount of calcite cement that is acquired in proportion to CaCO_3 content; i.e., carbonate-rich lithology facilitates precipitation of secondary calcite at the chalk-limestone transition. Therefore, paleoceanographic interpretation by means of Sr/Ca ratios in pelagic carbonates will become valid only when these diagenetic effects are appropriately constrained.