Late Oligocene through early Pleistocene calcareous nannofossils of ODP Leg 115 holes

The chronostratigraphy, the calcareous nannofossil biochronology, and the biostratigraphy of the Miocene and Pliocene sediments retrieved during Leg 115 in the equatorial western Indian Ocean are presented and discussed. Most of the zonal boundaries of the standard 1971 zonation of Martini and the 1973 zonation of Bukry are easily recognized in these low-latitude sediments. We also comment on the secondary events that are proposed in the literature to improve the biostratigraphic resolution provided by the standard zonations. The study of calcareous nannofossil biostratigraphy and taphonomy of sequences from the Northern Mascarene Plateau area, which was drilled to investigate the Neogene history of carbonate flux and dissolution, indicate that the accumulation of carbonates in this area results from a complex interplay among carbonate bioproductivity, carbonate removal by chemical dissolution and mechanical erosion, and carbonate addition by mass and current transport. In spite of these drawbacks, major changes and trends in carbonate accumulation can be recognized, most of which, if not all, correlate with major steps in the evolution of the Neogene climatic system.

Revised Cenozoic hiatus database

The differential effects of climate change, sea level, and water mass circulation on deposition/erosion of marine sediments can be constrained from the distribution of unconformities in the world's oceans. I identified temporal and depth patterns of hiatuses ("hiatus events") from a large and chronologically well constrained stratigraphic database of deep-sea sediments. The Paleogene is characterized by few, several million year long hiatuses. The most significant Cenozoic hiatus event spans most of the Paleocene. The Neogene is characterized by short, frequent hiatus events nearly synchronous in shallow and deep water sediments. Epoch boundaries are characterized by peaks in deep water hiatuses possibly caused by an increased circulation of corrosive bottom water and sediment dissolution. The Plio-Pleistocene is characterized by a gradual decrease in the frequency of hiatuses. Future studies will focus on the regional significance of the hiatus events and their possible causes.

Chemical composition of basalts from ODP Leg 115

Leg 115 of the Ocean Drilling Program recovered basalts from four locations along the hotspot track that leads from the Deccan flood basalts in India to Reunion Island in the western Indian Ocean (Sites 706, 707, 713, and 715). The drilled basalts range in age from 35 Ma (Site 706) to 64 Ma (Site 707), and including the Deccan basalts (66 to 68 Ma), Mauritius Island (0.2 to 8 Ma), and Reunion Island (0 to 2 Ma), seven sites are provided for sampling the volcanic record of the 5000-km-long hotspot track. Chemical and age comparisons indicate that Site 707 lavas correlate with basalt units near the top of the Deccan flood basalt sequence. The lavas of Site 715 (55 to 60 Ma) are most similar to the islands of Mauritius and Reunion. Site 713 basalts (48 Ma) are similar to the earliest lavas of the Deccan province, and Site 706 basalts are intermediate in chemistry between those of central Indian spreading-ridge basalts and Reunion. Differences in lava compositions along the hotspot track can be related to variable mixing of plume and asthenospheric mantle, depending on the changing position of spreading-ridge segments and the hotspot during the opening of the Indian Ocean. Alternatively, time-dependent changes in the composition of hotspot melts may be due to a decrease in partial melting of a heterogeneous plume or to intrinsic changes in the composition of material supplied by the plume.

Calcium carbonate global LGM Burial rate data

Global databases of calcium carbonate concentrations and mass accumulation rates in Holocene and last glacial maximum sediments were used to estimate the deep-sea sedimentary calcium carbonate burial rate during these two time intervals. Sparse calcite mass accumulation rate data were extrapolated across regions of varying calcium carbonate concentration using a gridded map of calcium carbonate concentrations and the assumption that accumulation of noncarbonate material is uncorrelated with calcite concentration within some geographical region. Mean noncarbonate accumulation rates were estimated within each of nine regions, determined by the distribution and nature of the accumulation rate data. For core-top sediments the regions of reasonable data coverage encompass 67% of the high-calcite (>75%) sediments globally, and within these regions we estimate an accumulation rate of 55.9 ± 3.6 x 10**11 mol/yr. The same regions cover 48% of glacial high-CaCO3 sediments (the smaller fraction is due to a shift of calcite deposition to the poorly sampled South Pacific) and total 44.1 ± 6.0 x 10**11 mol/yr. Projecting both estimates to 100 % coverage yields accumulation estimates of 8.3 x 10**12 mol/yr today and 9.2 x 10**12 mol/yr during glacial time. This is little better than a guess given the incomplete data coverage, but it suggests that glacial deep sea calcite burial rate was probably not considerably faster than today in spite of a presumed decrease in shallow water burial during glacial time.

(Appendix B) Benthic foraminifera in Indian Ocean surface sediments

Multivariate analysis was performed on percentages of 46 species of unstained deep-sea benthic foraminifera from 131 core-top to near-core-top samples (322-5013 m) from across the Indian Ocean. Faunal data are combined with GEOSECS geochemical data to investigate any relationship between benthic foraminifera (assemblages and species) and deep-sea properties. In general, benthic foraminifera show a good correlation to surface productivity, organic carbon flux to the sea floor, deep-sea oxygenation and, to a lesser extent, to bottom temperature, without correlation with the water depths. The foraminiferal census data combined with geochemical data has enabled the division of the Indian Ocean into two faunal provinces. Province A occupies the northwestern Indian Ocean (Arabian Sea region) where surface primary production has a major maximum during the summer monsoon season and a secondary maximum during winter monsoon season that leads to high organic flux to the seafloor, making the deep-sea one of the most oxygen-deficient regions in the world ocean, with a pronounced oxygen minimum zone (OMZ). This province is dominated by benthic foraminifera characteristic of low oxygen and high organic food flux including Uvigerina peregrina, Robulus nicobarensis, Bolivinita pseudopunctata, Bolivinita sp., Bulimina aculeata, Bulimina alazanensis, Ehrenbergina carinata and Cassidulina carinata. Province B covers southern, southeastern and eastern parts of the Indian Ocean and is dominated by Nuttallides umbonifera, Epistominella exigua, Globocassidulina subglobosa, Uvigerina proboscidea, Cibicides wuellerstorfi, Cassidulina laevigata, Pullenia bulloides, Pullenia osloensis, Pyrgo murrhina, Oridorsalis umbonatus, Gyroidinoides (= Gyroidina) soldanii and Gyroidinoides cf. gemma suggesting well-oxygenated, cold deep water with low (oligotrophic) and pulsed food supply.