Late Quaternary Variations in Planktonic Foraminifer Faunas and Pteropod Preservation in the Equatorial Indian Ocean

Abstract

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 104 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 104 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 105 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.