Geochemistry of basement carbonates from ODP Site 801 in the western Pacific Ocean

Many studies argue, based partly on Pb isotopic evidence, that recycled, subducted slabs reside in the mantle source of ocean island basalts (OIB) (Hofmann and White, 1982, doi:10.1016/0012-821X(82)90161-3; Weaver, 1991 doi:10.1016/0012-821X(91)90217-6; Lassiter, and Hauri, 1998, doi:10.1016/S0012-821X(98)00240-4). Such models, however, have remained largely untested against actual subduction zone inputs, due to the scarcity of comprehensive measurements of both radioactive parents (Th and U) and radiogenic daughter (Pb) in altered oceanic crust (AOC). Here, we discuss new, comprehensive measurements of U, Th, and Pb concentrations in the oldest AOC, ODP Site 801, and consider the effect of subducting this crust on the long-term Pb isotope evolution of the mantle. The upper 500 m of AOC at Site 801 shows >4-fold enrichment in U over pristine glass during seafloor alteration, but no net change to Pb or Th. Without subduction zone processing, ancient AOC would evolve to low 208Pb/206Pb compositions unobserved in the modern mantle (Hart and Staudigel, 1989 [Isotopic characterization and identification of recycled components, in: Crust/Mantle Recycling at Convergence Zones, Eds. S.R. Hart, L. Gqlen, NATO ASI Series. Series C: Mathematical and Physical Sciences 258, pp. 15-28, D. Reidel Publishing Company, Dordrecht-Boston, 1989]). Subduction, however, drives U-Th-Pb fractionation as AOC dehydrates in the earth's interior. Pacific arcs define mixing trends requiring 8-fold enrichment in Pb over U in AOC-derived fluid. A mass balance across the Mariana subduction zone shows that 44-75% of Pb but <10% of U is lost from AOC to the arc, and a further 10-23% of Pb and 19-40% of U is lost to the back-arc. Pb is lost shallow and U deep from subducted AOC, which may be a consequence of the stability of phases binding these elements during seafloor alteration: U in carbonate and Pb in sulfides. The upper end of these recycling estimates, which reflect maximum arc and back-arc growth rates, remove enough Pb and U from the slab to enable it to evolve rapidly (<<0.5 Ga) to sources suitable to explain the 208Pb/206Pb isotopic array of OIB, although these conditions fail to simultaneously satisfy the 207Pb/206Pb system. Lower growth rates would require additional U loss (29%) at depths beyond the zones of arc and back-arc magmagenesis, which would decrease upper mantle kappa (232Th/238U) over time, consistent with one solution to the "kappa conundrum" (Elliott et al., 1999, doi:10.1016/S0012-821X(99)00077-1). The net effects of alteration (doubling of l [238U/204Pb]) and subduction (doubling of omega [232Th/204Pb]) are sufficient to create the Pb isotopic signatures of oceanic basalts.

Minerals of bulk samples and the fine-grained (<2-µm) fractions from DSDP Leg 67 Holes

Leg 67 sample sediments are mainly composed of biogenic carbonate and biogenic siliceous materials. Apart from calcite, crystallized minerals are scarce; however, they are better represented in sites near the continent and in all Quaternary sediments. These minerals are: quartz, feldspars, smectite, and, rarely, chlorite, dolomite, and zeolite.

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.

Geochemistry of ODP Site 123-765 sediments

Drilling at Site 765 in the Argo Abyssal Plain sampled sediments and oceanic crust adjacent to the Australian margin. Some day, this site will be consumed in the Java Trench. An intensive analytical program was conducted to establish this site as a geochemical reference section forcrustal recycling calculations. About 150 sediment samples from Site 765 were analyzed for major and trace elements. Downhole trends in the sediment analyses agree well with trends in sediment mineralogy, as well as in Al and K logs. The primary signal in the geochemical variability is dilution of a detrital component by both biogenic silica and calcium carbonate. Although significant variations in the nonbiogenic component occur through time, its overall character is similar to nearby Canning Basin shales, which are typical of average post-Archean Australian shales (PAAS). The bulk composition of the hole is calculated using core descriptions to weight the analyses appropriately. However, a remarkably accurate estimate of the bulk composition of the hole can be made simply from PAAS and the average calcium carbonate and aluminum contents of the hole. Most elements can be estimated within 30% in this way. This means that estimating the bulk composition of other sections dominated by detrital and biogenic components may require little analytical effort: calcium carbonate contents, average Al contents, and average shale values can be taken from core descriptions, geochemical logs, and the literature, respectively. Some of the geochemical systematics developed at Site 765 can be extrapolated along the entire Sunda Trench. However, results are general, and Site 765 should serve as a useful reference for estimating the compositions of other continental margin sections approaching trenches around the world (e.g., outboard of the Lesser Antilles, Aegean, and Eolian arcs).

Geochemistry and accumulation rates of the early Paleogene in the Weddel Sea, Maud Rise and Kerguelen Plateau

Marine biological productivity has been invoked as a possible climate driver during the early Paleogene through its potential influence on atmospheric carbon dioxide concentrations. However, the relationship of export productivity (the flux of organic carbon (C) from the surface ocean to the deep ocean) to organic C burial flux (the flux of organic C from the deep ocean that is buried in marine sediments) is not well understood. We examine the various components involved with atmosphere-to-ocean C transfer by reconstructing early Paleogene carbonate and silica production (using carbonate and silica mass accumulation rates (MARs)); export productivity (using biogenic barium (bio-Ba) MARs); organic C burial flux (using reactive phosphorus (P) MARs); redox conditions (using uranium and manganese contents); and the fraction of organic C buried relative to export productivity (using reactive P to bio-Ba ratios). Our investigations concentrate on Paleocene/Eocene sections of Sites 689/690 from Maud Rise and Site 738 from Kerguelen Plateau. In both regions, export productivity, organic C burial flux, and the fraction of organic C buried relative to export productivity decreased from the Paleocene/early Eocene to the middle Eocene. A shift is indicated from an early Paleogene two-gyre circulation in which nutrients were not efficiently recycled to the surface via upwelling in these regions, to a circulation more like the present day with efficient recycling of nutrients to the surface ocean. Export productivity was enhanced for Kerguelen Plateau relative to Maud Rise throughout the early Paleogene, possibly due to internal waves generated by the plateau regardless of gyre circulation.

Late Quaternary siliciclastic/carbonate sedimentation in the Cabricorn Channel

A model is presented for hemipelagic siliciclastic and carbonate sedimentation during the last glacial-interglacial cycle in the Capricorn Channel, southern Great Barrier Reef (GBR). Stable isotope ratios, grainsize, carbonate content and mineralogy were analysed for seven cores in a depth transect from 166 to 2892 m below sea level (mbsl). Results show variations in the flux of terrigenous, neritic and pelagic sediments to the continental slope over the last sea level cycle. During the glacial lowstand terrigenous sediment influenced all the cores down to 2000 mbsl. The percentages of quartz and feldspar in the cores decreased with water depth, while the percentage of clay increased. X-ray diffraction analysis of the glacial lowstand clay mineralogy suggests that the siliciclastic sediment was primarily sourced from the Fitzroy River, which debouched directly into the northwest sector of the Capricorn Channel at this time. The cores also show a decrease in pelagic calcite and an increase in aragonite and high magnesium calcite (HMC) during the glacial. The influx of HMC and aragonite is most likely from reworking of coral reefs exposed on the continental shelf during the glacial, and also from HMC ooids precipitated at the head of the Capricorn Channel at this time. Mass accumulation rates (MARs) are high (13.5 g/cm**/kyr) during the glacial and peak at ~20 g/cm** 3/kyr in the early transgression (16-14 ka BP). MARs then decline with further sea level rise as the Fitzroy River mouth retreats from the edge of the continental shelf after 13.5 ka BP. MARs remain low (4 g/cm**3/kyr) throughout the Holocene highstand. Data for the Holocene highstand indicate there is a reduction in siliciclastic influx to the Capricorn Channel with little quartz and feldspar below 350 mbsl. However, fine-grained fluvial sediments, presumably from the Fitzroy River, were still accumulating on the mid slope down to 2000 mbsl. The proportion of pelagic calcite in the core tops increases with water depth, while HMC decreases, and is present only in trace amounts in cores below 1500 mbsl. The difference in the percentage of HMC in the deeper cores between the glacial and Holocene may reflect differences in supply or deepening of the HMC lysocline during the glacial. Sediment accumulation rates also vary between cores in the Capricorn Channel and do not show the expected exponential decrease with depth. This may be due to intermediate or deep water currents reworking the sediments. It is also possible that present bathymetry data are too sparse to detect the potential role that submarine channels may play in the distribution and accumulation of sediments. Comparison of the Capricorn Channel MARs with those for other mixed carbonate/siliciclastic provinces from the northeast margin of Australia indicates that peak MARs in the early transgression in the Capricorn Channel precede those from the central GBR and south of Fraser Island. The difference in the timing of the carbonate and siliciclastic MAR peaks along the northeast margin is primarily related to differences in the physiography and climate of the provinces. The only common trend in the MARs from the northeast margin of Australia is the near synchronicity of the carbonate and siliciclastic MAR peaks in individual sediment cores, which supports a coeval sedimentation model.

Permeabilities, grain size and carbon composition of sediments from ODP Hole 170-1040C and two Leg 205 holes

Constant-pressure difference and constant-flow permeability tests were conducted on core samples from Ocean Drilling Program Legs 170 and 205 from the Costa Rica subduction zone representing pelagic carbonate and hemipelagic mud lithologies. Seven whole-round core samples from Sites 1040, 1253, and 1255 were tested for vertical permeabilities. The permeabilities of the pelagic carbonate sediments range from ~4 x 10**-16 to ~1 x 10**-15 m**2. The permeabilities of the hemipelagic mud sediments vary from ~2 x 10**-18 to ~4 x 10**-18 m**2. To further characterize the sediments, grain size, total carbon, and total inorganic carbon analyses were conducted.