ALBERT

Description: In order to elucidate early Aptian marine paleotemperature evolution across the period of enhanced organic carbon (Corg)-burial [Oceanic Anoxic Event (OAE) 1a], stable isotope analyses were performed on pelagic limestones at Deep Sea Drilling Project Site 463, central Pacific Ocean. The delta18O data exhibit a distinct anomaly by ~-2 per mil spanning the OAE 1a interval (i.e., a ~6 m-thick, phytoplanktonic Corg-rich unit constrained by magneto-, bio- and delta13C stratigraphy). Elucidation of paleotemperature significance of the delta18O shift is made by taking account of recent Sr/Ca evidence at the same section, which revealed that geochemical signals in carbonate-poor lithologies are relatively unaltered against burial diagenesis. By discriminating delta18O values from carbonate-poor samples (CaCO3 contents=5-30 wt.%), it appears that an abrupt rise in seasurface temperatures (SSTs) by 8 °C (=-1.7 per mil shift in delta18O) occurred immediately before OAE 1a, whereas a cooling mode likely prevailed during the peak Corg-burial. In terms of its stratigraphic relationship as to the Corg-rich interval and to a pronounced negative delta13C excursion, as well as its timescale, the observed SST rise resembles those associated with the Paleocene-Eocene thermal maximum and, more strikingly, Jurassic Toarcian OAE. This observation is consistent with the hypothesis that these paleoenvironmental events were driven by a common causal mechanism, which was likely initiated by the greenhouse effect via massive release of CH4 or CO2 from the isotopically-light carbon reservoir and terminated by a negative productivity feedback.

Description: An integrated chemostratigraphic (87Sr/86Sr, d13C and 18O) study of benthic foraminifera is presented for a 210 m-thick, intermediate depth (upper/middle bathyal transition), Miocene nannofossil ooze section of Ocean Drilling Program Site 1120, Campbell Plateau off New Zealand. Our results indicate that new 87Sr/86Sr, d13C and d18O profiles are wholly consistent with their respective Miocene reference curves. These observations facilitate identification of a total of five reliable chemostratigraphic datums, which are based on the fundamental structural changes in the 87Sr/86Sr curve and paired simultaneous d13C and d18O events. The resultant age-depth relationship clearly shows that the Miocene (20-5 Ma) biopelagic sedimentation on the Campbell Plateau was essentially continuous at a moderate to high, linear sedimentation rate (17.5 m/m.y. with an exception of the uppermost 13 m). Our findings do not support the shipboard biostratigraphic age model, which assumes that the critical early-middle Miocene transition was interrupted by a major hiatus (〈~3 m.y.). Because of its unique bathymetric setting at a paleowater-depth of ~ 600 m, which is among the shallowest of the coeval isotopically studied deep-sea sections in the South Pacific/Southern Ocean, Site 1120 will serve as a reference section for surveying the evolution of intermediate-water paleoceanography in the Southern Hemisphere across the middle Miocene climatic transition.

Description: 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 CaCO3 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.