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 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.

Planktonic Foraminifer Faunal Variations in the Northeastern Indian Ocean: A High-Resolution Record of the Past 800,000 Years from Site 758

“…The dissolution trend is the "Brunhes Dissolution Cycle" first described in sedimentary records from the equatorial Pacific (Adelseck, 1977). This cycle has been observed in other Indian Ocean records in water depths ranging from 533 m (Cullen and Droxler, 1990;Droxler et al, 1990) to over 4000 m (Peterson and Prell, 1985b). The cause of the cycle is unknown but has been related to forces both internal (Pisias and Rea, 1988) and external (Jansen et al, 1986) to the climate system.…”

Variations in planktonic foraminifer faunas and carbonate preservation at Site 927: evidence for changing surface water conditions in the western tropical Atlantic Ocean during the middle Pleistocene

“…Dissolution RSP are identified with an asterisk in Table 1 and are similar to those used in Berger (1975) and Cullen (1981). These indicators have been widely used to assess the preservational state of calcite in a sample and to assess the extent to which the differential dissolution of foraminifer species has modified the original faunal composition of a sample (e.g., Berger et al, 1982;Cullen, 1985;Cullen and Prell, 1984;Cullen and Droxler, 1990;Chen and Farrell, 1991;Le and Shackleton, 1992). In addition, we calculated the percent coarse fraction (>63 µm) of each sample and measured the percent calcium carbonate content of each sample using the technique described in Curry and Cullen (this volume).…”

“…We chose to analyze Site 927 faunal data further by comparing downcore faunas to present-day faunas by means of the Modern Analog Technique (MAT). This technique has been used in the study of Quaternary palynology (e.g., Prentice, 1980;Overpeck et al, 1985) and was first applied to planktonic foraminifer data by Hutson (1979) and more recently by Prell (1985), Cullen and Droxler (1990), Howard and Prell (1992), Cullen et al (1994), and Curry and Oppo (1997).…”

“…A similar conclusion was reached while studying a 600 k.y. record from the tropical Indian Ocean (Cullen and Droxler, 1990).…”

Quaternary CaCO3 Input and Preservation within Antarctic Intermediate Water Mineralogic and Isotopic Results from Holes 818B and 817A, Townsville Trough (Northeast Australian Margin)

“…This study showed that, during the past 0.5 m.y., pteropods were more fragmented and often more dissolved (as determined by Scanning Electron Microscopy) during glacial intervals than during interglacial intervals at water depths as shallow as 665 m within the Northwest Providence Channel, Bahamas. Further work on periplatform records from the Maldives has shown that pteropod fragmentation is a much more sensitive monitor of aragonite preservation, near the aragonite saturation depth, than is fine aragonite content Cullen and Droxler, 1990). However, at periplatform water depths considerably below the aragonite saturation depth, where pteropods are rare or absent, the input signal of metastable carbonate can be strongly overprinted by dissolution .…”