Ocean Drilling Program (ODP) Leg 115 post-cruise research was focused on two Maldives sites, more precisely on the top 108 m of Hole 716B (water depth, 540 m), equivalent to the past 3.5 m.y., and the top 19.5 m of Hole 714A (water depth, 2195 m), equivalent to the past 0.55 m.y. These sediments consist of mostly unaltered and undisturbed, turbidite-free, periplatform ooze. Results of our research are compared with existing data on Hole 633A (water depth, 1681 m), drilled in the Bahamas during ODP Leg 101, using age/depth models built on the basis of oxygen isotope, nannofossil, and magnetic stratigraphies. Climate-induced, long-term (roughly 0.5 m.y.) aragonite cycles, superposed on short-term (roughly 0.04 and 0.1 m.y.) aragonite cycles, have been established at least during the past 2.0 m.y., in the Maldives and the Bahamas. Our most interesting result is the clear correlation among the aragonite long-term cycles in the Maldives and the Bahamas and the carbonate-preservation, long-term cycles from the open Pacific, Indian, and North Atlantic oceans. The mid-Brunhes dissolution interval, corresponding to the youngest preservation minima of the carbonate-preservation, longterm cycles, is clearly defined by fine aragonite minimum values in the deep periplatform sites, and by maximum fragmentation of pteropod tests in the shallow sites. Aragonite and planktonic 5 18 O records, usually in phase during the late Pleistocene, display, further back in time, discreet intervals where the two records do not match with one another. Major mismatches between both records occur synchronously in the Maldives and Bahamas periplatform sites and seem to correspond to extreme events of either carbonate-preservation or dissolution in the deep pelagic carbonate sites of the equatorial Pacific Ocean. Based on our findings, short-and long-term aragonite cycles can no longer be explained only by variations of aragonite input from the nearby shallow carbonate banks, in response to their alternate flooding and exposure through cyclic sea-level fluctuations. The aragonite long-term cycles in the periplatform environments are interpreted as carbonatepreservation cycles at intermediate-water depths. Their occurrence shows, therefore, that the carbonate chemistry of the entire water column has been influenced by long-term (0.5 m.y.) cyclic variations during the past 2.0 m.y. These major changes of the water-column carbonate chemistry are linked to the climate-induced carbon cycling among the different atmospheric, oceanic, and sedimentary carbon reservoirs.
At Site 716, drilled during Ocean Drilling Program Leg 115, a long continuous carbonate-rich sequence was recovered from a shallow (533 m) basin within the Maldives region of the Chagos-Laccadive Ridge. The oxygen isotope stratigraphy, along with four internally consistent biostratigraphic datums, indicates a late Quaternary record with a sedimentation rate of 3 cm/1000 yr, a rate rarely found in the equatorial Indian Ocean. Thus, the upper 18.4 m of Hole 716B provides us with an excellent opportunity to conduct high-resolution studies of changes in foraminifer faunas and aragonite preservation within a time-stratigraphic framework established by oxygen isotope stratigraphy. Analyses of two indicators of aragonite preservation (°7o whole pteropods, % pteropods) reveal significant variability over the past 600,000 yr. A long interval of poor aragonite preservation occurs during isotope stages 9-13 (-300-500 k.y.). Significant smaller amplitude and higher frequency (1/f on the order of 10 4 yr) aragonite preservation fluctuations occur throughout the last 600,000 yr. Analyses of the indicators of calcite preservation (°?o whole foraminifers and % whole Globorotalia menardii complex) reveal low variability and high levels of calcite preservation throughout the section. Excellent preservation enables us to evaluate downcore changes in the relative abundance of foraminifer species in terms of changes in environmental conditions of equatorial Indian Ocean surface waters and without regard to dissolution. The seven most abundant foraminifer species exhibit considerable systematic downcore variation, with ranges of variability approaching 15%. All seven species exhibit statistically independent high-frequency (1/f on the order of 10 4 yr) variability, with the two most abundant species (Globorotalia ruber and Globorotalia dutertrei) exhibiting a series of low-frequency (1/f on the order of 10 5 yr) maxima and minima. A set of similar, well-preserved, northern Indian Ocean analogs was found for all Hole 716B samples. The Modern Analog Technique was used to calculate sea-surface temperature (SST) changes at Site 716 over the past 600,000 yr for the warm and cold seasons. Warm SSTs show little variation and are similar to modern values (near 28°C). Cold SSTs are consistently cooler than the modern values that dip below 26°C during glacial isotope stages 6 and 8. The absence of large SST changes and the presence of considerable species variation indicate that foraminifer faunas are responding to non-SST-related environmental changes in surface waters, such as changes in salinity and/or nutrient levels.
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