Lithologic correlation of ODP Leg 115 holes

Whole-core (WC) measurements of low-field magnetic susceptibility (MS) provide an extremely simple, rapid, and nondestructive technique for high-resolution core logging and lithostratigraphic correlation between subsidiary holes at Ocean Drilling Program (ODP) sites. This is particularly useful for reconstructing composite, stratigraphically continuous sequences for individual ODP sites by splicing the uninterrupted records obtained from subsections of offset cores recovered from adjacent holes. Correlation between the WCMS profiles of holes drilled at different sites is also possible in some instances, especially when lithologic variations at each site are controlled by regional paleoceanographic or global (i.e., orbitally forced) paleoclimatic changes. In such circumstances, WCMS may also be used as a proxy paleoclimatic indicator, duly assisting climatostratigraphic zonation of the recovered sequence by more conventional microfossil and isotopic techniques. High-resolution WCMS profiles are also useful in detecting intervals of the recovered sequence affected by drilling disturbance, in the form of contamination by pipe rust or similar metallic artifacts as well as discontinuities related to repenetration of the corer or loss of material between successively cored intervals. Stratigraphic intervals that have been affected by early (suboxic) diagenesis resulting from a high initial organic matter content of the sediment are also readily identified by WCMS logging. The MS signal of horizons affected by suboxic diagensis is typically degraded in proportion to the duration and intensity (related to initial Corg concentration) of organic matter remineralization. The lowering of MS values during suboxic diagenesis results from "dissolution" (bacterially mediated ionic dissociation) of magnetic iron and manganese oxides and oxyhydroxides in the sediment. It is to be hoped that, on future ODP (or similar) cruises, WCMS logging will cease to be regarded as a mere adjunct to paleomagnetic measurements, but rather as a simple, yet powerful, lithostratigraphic tool, directly analogous to downhole geophysical logging tools, and complimentary to shipboard techniques for whole-core measurements of physical properties (e.g., P-wave logging, GRAPE, etc.).

Late Cenozoic carbonate accumulation rates of ODP Leg 115 samples

The principal paleoceanographic objective of Ocean Drilling Program Leg 115 was to collect a suite of materials that would allow reconstruction of the dynamic features of the late Cenozoic carbonate system in the equatorial Indian Ocean. This goal was achieved with the recovery of sediments from a closely spaced depth transect (1541-4428 m) of five sites (Sites 707 through 711) from on and around the Mascarene Plateau that record the last 50 m.y. of pelagic deposition. More than 2200 measurements of carbonate content are combined here with a highly resolved bio- and magnetostratigraphy to produce the first detailed compilation of bulk, carbonate, and noncarbonate mass accumulation rates (MARs) from the Indian Ocean. These results allow us to recognize three major depositional intervals, each characterized by a distinct depth-dependent pattern of carbonate accumulation: (1) the Paleogene, a time of moderate accumulation rates (0.4-0.7 g/cm**2/1000 yr) and reduced between-site accumulation differences; (2) the early and middle Miocene, a period characterized by greatly reduced carbonate MARs (typically <0.2 g/cm**2/1000 yr) at all sites and a shallow carbonate compensation depth; and (3) the late Miocene to Holocene, a time span marked by the highest bulk and carbonate accumulation rates of the last 50 Ma (1.6-1.8 g/cm**2/1000 yr), and the first appearance of substantial contrasts in carbonate accumulation as a function of the water depth of the drill site. The fundamentally different character of the carbonate system during each of these intervals must represent a regional response to the complex evolution of late Cenozoic oceans and climate.