(Table 1) 40Ar/39Ar plateau and isochron ages for basalts from the Réunion hotspot track

Concordant plateau and isochron ages were calculated from 40Ar/39Ar incremental heating experiments on volcanic rocks recovered by drilling at four Leg 115 sites and two industry wells along the volcanic lineament connecting Reunion Island to the Deccan flood basalts, western Indian Ocean. The new ages provide unequivocal evidence that volcanic activity migrated southward along this sequence of linear ridges. The geometry and age distribution of volcanism are most compatible with origin above a stationary hotspot centered beneath Reunion. The hotspot became active with rapid eruption of the Deccan flood basalts, western India, and subsequent volcanic products record the northward motion of the Indian and African plates over the hotspot through Tertiary time. The radiometric ages are in general accord with basal biostratigraphic age estimates, although some adjustments in current magnetobiostratigraphic time scales may be required.

Concentrations and isotopic ratios of thorium and uranium in pelagic sediments

The predictable in situ production of 230Th from the decay of uranium in seawater, and its subsequent removal by scavenging onto falling particles, provides a valuable tool for normalizing fluxes to the seafloor. We describe a new application, determination of the 232Th that dissolves in the water column and is removed to the seafloor. 232Th is supplied to the ocean in continental minerals, dissolution of which leads to a measurable standing stock in the water column. Sedimentary adsorbed 232Th/230Th ratios have the potential to provide a proxy for estimating the amount of dissolved material that enters the ocean, both today and in the past. Ten core top samples were treated with up to eight different leaching techniques in order to determine the best method for the separating adsorbed from lattice bound thorium. In addition, separate components of the sediments were analyzed to test whether clay dissolution was an important contribution to the final measurement. There was no systematic correlation between the strength of acid used in the leach and the measured 232Th/230Th ratios. In all cases clean foraminifera produced the same ratio as leaches on bulk sediment. In three out of five samples leaches performed on non-carbonate detritus in the <63 µm size fraction were also identical. Without additional water column data it is not yet clear whether there is a simple one to one correlation between the expected deep-water 232Th/230Th and that produced by leaching, especially in carbonate-rich sediments. However, higher ratios, and associated high 232Th adsorbed fluxes, were observed in areas with high expected detrital inputs. The adsorbed fraction was ~35-50% of the total 232Th in seven out of ten samples. Our 230Th normalized 232Th fluxes are reasonable by comparison to global estimates of detrital inputs to the ocean. In nine cases out of ten, the total 230Th-normalized 232Th flux is greater than predicted from the annual dust fall at each specific location, but lower than the average global detrital input from all sources.

Pore-water chemistry of ODP Leg 115 samples

Analyses of the Sr2+ concentrations of interstitial fluids obtained from sediments squeezed during Leg 115 were used to estimate the rates and total amount of recrystallization of biogenic carbonates. The total amount of recrystallization calculated using this method varies from less than 1 % in sediments at Site 706 to more than 40% at Site 709 in sediments of 47 Ma. Five of the sites drilled during Leg 115 (Sites 707 through 711) were drilled in a depth transect within a restricted geographic area so that theoretically they received similar amounts of sediment input. Of these, the maximum rate of recrystallization occurred in the upper 50 m of Site 710 (3812 m). The amount of recrystallization decreased with increasing water depth at Sites 708 (4096 m) and 711 (4428 m), presumably as a result of the fact that most of the reactive calcium carbonate was dissolved before burial. We also observed significant alkalinity deficits at many of these sites, a condition which most likely resulted from the precipitation of calcium carbonate either in the sedimentary column, or during retrieval of the core. Precipitation of CaCO3 as a result of pressure changes during core retrieval was confirmed by the comparison of Ca2+ and alkalinity from water samples obtained using the in-situ sampler and squeezed from the sediments. At Sites 707 and 716, the shallowest sites, no calcium or alkalinity deficits were present. In spite of our estimations of as much as 45% recrystallization at Site 709, all the carbonate sites exhibited what would be previously considered conservative Ca2+/Mg2+ profiles, which varied from -1 to -0.5. By virtue of the position of these sites relative to known basaltic basement or through the actual penetration of basalt (i.e., Sites 706, 707 and 712), these sites are all known to be underlain by basalt. Our results suggest, therefore, that more positive Ca2 + /Mg2+ gradients cannot necessarily be used as indicators of the nature of basement material.