Total organic carbon content and maceral composition of sediment at DSDP Holes 72-515B and 72-516F

The amount, type, and thermal maturation of organic matter in sediments from two DSDP holes in the South Atlantic (Leg 72) were investigated. Isolated kerogens were studied by microscopy, and nonaromatic hydrocarbons were characterized by capillary gas chromatography. Organic carbon values are low in all samples and range between 0.05 and 0.21% in Hole 515B (Brazil Basin) and only between 0.02 and 0.10% in Hole 516F (Rio Grande Rise). The organic matter is predominantly terrigenous, mixed with some unicellular marine algae; it is severely oxidized in most samples. N-alkane distributions are usually dominated by long-chain wax alkanes with odd-over-even carbon number predominance; when the marine organic matter is relatively more abundant, however, significant amounts of n-alkanes are centered upon n-C17. The organic matter is not mature enough in any sample to generate appreciable amounts of hydrocarbons.

Paleomagnetic of ODP Leg 101 sites

Paleomagnetic measurements were made on 913 samples from 11 holes (626B, 626C, 627B, 628A, 630A, 631A, 632A, 632B, 633A, 634A, and 635B) drilled in and around the Bahamas carbonate bank during Ocean Drilling Program Leg 101. These samples displayed a wide range of magnetic intensities (from about 1.0 A/m to 1.6 * 10**- 6 A/m) and magnetic behavior. Most samples were weakly magnetized and had low mean destructive fields; however, sediments from sections of several holes were strongly magnetic with stable magnetizations. Magnetic-polarity interpretations were made on a Campanian unit from Hole 627B, a mid-Oligocene unit from Hole 628A, and a Plio-Pleistocene section from Hole 633A. Sediments in the upper parts of Holes 627B, 632A, and 633A have high magnetic intensities that decay 2 to 3 orders of magnitude over depths of 5 to 18 mbsf. The pattern of decline of the magnetism and the change in mean destructive fields and geochemical conditions in these holes are consistent with diagenetic dissolution of the magnetic minerals in a suboxic or anoxic-sulfidic environment. Paleolatitudes were calculated from samples from 16 time units in 7 holes and compared to the apparent polar wander path of the North American plate.

Helium isotope concentrations and ratios in sediments from ODP Hole 130-806B

We have determined the helium abundance and isotopic composition of seafloor carbonate sediments from the flanks of the Ontong Java Plateau, western equatorial Pacific Ocean (ODP Site 806). These results provide a two million year record of the burial flux of extraterrestrial 3He, which we believe is a proxy for the terrestrial accretion rate of interplanetary dust particles. The 3He burial flux prior to ~700 ka was relatively low, ~0.5 pcc/cm**2/kyr, but from 700 ka to the present, the burial flux gradually increased to a value of ~1.0 pcc/cm**2/kyr. 100 kyr periodicity in the 3He burial flux is apparent over the last 700 kyr and correlates with the oxygen isotope record of global climate, with high 3He burial fluxes associated with interglacial periods. This periodicity and phase are consistent with previous 3He measurements in North Atlantic sediments. Although 100 kyr periodicity in 3He burial flux is in agreement with recent predictions of the accretion rate of interplanetary dust based on a model of the orbital evolution of asteroidal debris, the measurements and predictions differ by one half cycle in phase. Nevertheless, our observations suggest the terrestrial accretion rate of interplanetary dust is controlled by orbital eccentricity and/or inclination relative to the solar-system invariable plane. Such control is a necessary but not sufficient condition for the hypothesis of that variations in extraterrestrial dust accretion modulates terrestrial climate with a 100 kyr period. We also identify several brief (<25 kyr) intervals of strongly enhanced 3He burial, possibly related to random and transient fluctuations in the accretion rate of asteroidal or cometary dust particles.

Paleotemperature reconstructions from high Late Jurassic paleolatitudes

Oxygen and carbon isotopes have been determined from Late Jurassic (Oxfordian-Tithonian) belemnites and inoceramid bivalves from two Deep Sea Drilling Project (DSDP) sites located on the Falkland Plateau. Mean belemnite delta18O values, derived from well preserved skeletal material, were -1.29? from DSDP site 330 and -1.45? from DSDP site 511. Assuming a seawater SMOW value of -1.0?, mean palaeotemperatures calculated from the oxygen isotopic composition are 17.2°C and 17.9°C, respectively. The inoceramid bivalves yielded much lighter delta18O values (mean -3.58?). Petrographic and geochemical evidence points to the inoceramid bivalves being altered by diagenesis which accordingly accounts for the observed differences in isotopic values. "Vital effects" or the importation of belemnites or inocerarnids from another area, are considered not to account for the observed isotopic trends. The palaeotemperatures interpreted from the belemnites are significantly warmer than other recent estimates of Late Jurassic temperature (from oxygen isotope studies and climate model predictions) from similar southern palaeolatitudes. We suspect our apparent warmer temperatures are because of a combination of increased freshwater runoff depleting surface waters with respect to delta18O and related to the semi-enclosed nature of the depositional basin retaining warmth, relative to the open ocean of similar latitudes.

Geochemistry of neritic carbonates of ODP Hole 134-831B

Drilling at Bougainville Guyot (Ocean Drilling Program Site 831), New Hebrides Island Arc during Leg 134 revealed that 727.5 m of carbonate overlies an andesite basement. The carbonate cap at Site 831 consists of 20 m of pelagic carbonate overlying 707.5 m of neritic carbonates. The neritic section consists of ~230 m of largely unaltered aragonite sediment that overlies ~497 m of totally calcitized limestone. The deeper portion of the calcitized interval has been pervasively altered by diagenesis. Prior to this study the age distribution of sediments at Bougainville Guyot was poorly known because age diagnostic fossils are sparsely and discontinuously distributed in the sequence. We have used Sr isotopes to provide temporal constraints on the deposition of carbonates at Site 831; these constraints are critical in reconstructing the vertical movement of Bougainville Guyot before its collision with the New Hebrides Island Arc. Overall, the chronostratigraphy of Bougainville Guyot can be subdivided into three intervals: (1) a Pleistocene interval (102.4 to 391.11 meters below sea floor [mbsf]); (2) a Miocene interval (410.31 to 669.53 mbsf); and (3) an Oligocene interval (678.83 to 727.50 mbsf). Strontium isotopic ages of samples increase with increasing depth in the carbonate sequence, except near the bottom of the sequence, where several samples exhibit a consistent reversed age vs. depth trend. Such age reversals are most likely the product of post-depositional rock-water interaction. Preliminary stable isotope data are consistent with diagenetic alteration in the marine and meteoric environments. Several abrupt decreases in d87Sr, and hence age, of sediments are recognized in the carbonate cap at Bougainville Guyot. These disconformities are most likely the product of subaerial exposure in response to relative sea-level fall. Indeed, Sr-isotope ages indicate that 2 to 9 m.y. of sediment deposition is missing across these d87Sr disconformities.

Oxygen-isotope composition, temperature and depth of formation of chert at DSDP Leg 62 Holes

We measured oxygen-isotope compositions of 16 siliceous rocks from Deep Sea Drilling Project Sites 463, 464, 465, and 466 (Leg 62). Samples are from deposits that range in age from about 40 to 103 m.y. and that occur at sub-bottom depths of 9 to 461 meters. Mean d18O values range from 28.4 to 36.8 per mil and 36.0 ± 0.3 per mil for quartz-rich and opal-CTrich rocks, respectively. d18O values in chert decrease with increasing sub-bottom depth; the slope of the d18O/depth curve is less steep for Site 464 than for the other sites which indicates that chert at Site 464 formed at higher temperatures than chert at Sites 463, 465, and 466. Temperatures of formation of cherts were 7 to 42°C, using the silica-water fractionation factor of Knauth and Epstein (1976), or 19 to 56°C, using the equation of Clayton et al. (1972). Temperatures in the sediment where the cherts now occur are lower than their isotopically determined temperatures of formation, which means that the cherts record an earlier history when temperatures in the sediment section were greater. Estimated sediment temperatures when the cherts formed are comparable to, but generally slightly lower than, those calculated from Knauth and Epstein's equation. The isotopic composition of cherts is more closely related to environment of formation (diagenetic environment) or paleogeothermal gradients, than to paleoclimates (bottom-water temperatures). Opal-CT-rich rocks may better record paleo-bottom-water temperature. In Leg 62 cherts, better crystallinity of quartz corresponds to lower d18O values; this implies progressively higher temperatures of equilibration between quartz and water during maturation of quartz. The interrelationship of d18O and crystallinity is noted also in continental-margin deposits such as the Monterey Formation - but for higher temperatures. The apparent temperature difference between open-ocean and continental-margin deposits can be explained by the dominant control of temperature on silica transformation in the rapidly deposited continental-margin deposits, whereas time, as well as temperature, has a strong influence on the transformations in open-ocean deposits. Comparisons between the chemistry and d18O values of cherts reveal two apparent trends: both boron and SiO2 increase as d18O increases. However, the correspondence between SiO2 and d18O is only apparent, because the two cherts lowest in SiO2 are also the most deeply buried, so the trend actually reflects depth of burial. The correspondence between boron and d18O supports the conclusion that boron is incorporated in the quartz crystal structure during precipitation

Concentrations and accumulation rates of platinum-group elements and rhenium in Pacific and Atlantic sediments, DSDP and ODP data

The nature of Re-platinum-group element (PGE; Pt, Pd, Ir, Os, Ru) transport in the marine environment was investigated by means of marine sediments at and across the Cretaceous-Tertiary boundary (KTB) at two hemipelagic sites in Europe and two pelagic sites in the North and South Pacific. A traverse across the KTB in the South Pacific pelagic clay core found elevated levels of Re, Pt, Ir, Os, and Ru, each of which is approximately symmetrically distributed over a distance of ~1.8 m across the KTB. The Re-PGE abundance patterns are fractionated from chondritic relative abundances: Ru, Pt, Pd, and Re contents are slightly subchondritic relative to Ir, and Os is depleted by ~95% relative to chondritic Ir proportions. A similar depletion in Os (~90%) was found in a sample of the pelagic KTB in the North Pacific, but it is enriched in Ru, Pt, Pd, and Re relative to Ir. The two hemipelagic KTB clays have near-chondritic abundance patterns. The ~1.8-m-wide Re-PGE peak in the pelagic South Pacific section cannot be reconciled with the fallout of a single impactor, indicating that postdepositional redistribution has occurred. The elemental profiles appear to fit diffusion profiles, although bioturbation could have also played a role. If diffusion had occurred over ~65 Ma, the effective diffusivities are ~10**?13 cm**2/s, much smaller than that of soluble cations in pore waters (~10**?6 cm**2/s). The coupling of Re and the PGEs during redistribution indicates that postdepositional processes did not significantly fractionate their relative abundances. If redistribution was caused by diffusion, then the effective diffusivities are the same. Fractionation of Os from Ir during the KTB interval must therefore have occurred during aqueous transport in the marine environment. Distinctly subchondritic Os/Ir ratios throughout the Cenozoic in the South Pacific core further suggest that fractionation of Os from Ir in the marine environment is a general process throughout geologic time because most of the inputs of Os and Ir into the ocean have Os/Ir ratios >/=1. Mass balance calculations show that Os and Re burial fluxes in pelagic sediments account for only a small fraction of the riverine Os (<10%) and Re (<0.1%) inputs into the oceans. In contrast, burial of Ir in pelagic sediments is similar to the riverine Ir input, indicating that pelagic sediments are a much larger repository for Ir than for Os and Re. If all of the missing Os and Re is assumed to reside in anoxic sediments in oceanic margins, the calculated burial fluxes in anoxic sediments are similar to observed burial fluxes. However, putting all of the missing Os and Re into estuarine sediments would require high concentrations to balance the riverine input and would also fail to explain the depletion of Os at pelagic KTB sites, where at most ~25% of the K-T impactor's Os could have passed through estuaries. If Os is preferentially sequestered in anoxic marine environments, it follows that the Os/Ir ratio of pelagic sediments should be sensitive to changes in the rates of anoxic sediment deposition. There is thus a clear fractionation of Os and Re from Ir in precipitation out of sea water in pelagic sections. Accordingly, it is inferred here that Re and Os are removed from sea water in anoxic marine depositional regimes.

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.

Geochemistry of basal dolomitic and Fe-Mn sediments from the Tyrrhenian Sea

In the Tyrrhenian Sea (Western Mediterranean), unusual reddish, soft to lithified, dolomitic sediments up to 45 m thick overlie igneous crust at the base of thick Pliocene-Quaternary deep-sea sediment successions in the Marsili (Site 650) and Vavilov (Site 651) basins. These sediments also overlie the Gortani Ridge, a basaltic Seamount near the base of the Sardinian continental margin (Site 655). At both basinal sites (650, 651), the lowest sediments are dolomitic, with manganese oxide (MnO) segregations. Whole-rock X-ray diffraction indicates abundant dolomite and quartz, with subordinate calcite, illite (authigenic), feldspar and minor kaolinite, chlorite, and anhydrite. Chemical analyses show strong enrichment in magnesium oxide (MgO) and MnO relative to shale or deep-sea clay. Mg and Mn correlate positively and exhibit decreasing concentrations up the succession in the Marsili Basin (Site 650). The following scenario is proposed: peridotites were exposed on the seafloor in the Vavilov Basin (Site 651) and then eroded, depositing talc in local fine-grained dolomitic sediments within the igneous basement. After local magmatism ended, the igneous basement at each site subsided rapidly (about 800 m/m.y.) and was blanketed with calcareous and clay-rich oozes. During early diagenesis (from isotopic evidence; McKenzie et al., this volume) tepid fluids, of modified seawater composition, reacted with and dolomitized the overlying deep-sea sediments. At Site 651 additional Mg may have been extracted from asthenosphere peridotite cored at shallow depths (about 100 m). One can hypothesize that fluids rich in Mg and Mn were flushed from the igneous basement, triggered by extensional faulting and local tilting during subsidence of the basement, and that these fluids then dolomitized the base of the overlying sediment succession. Late tectonic movements in the Vavilov Basin (Site 651) fractured already lithified dolomitic sediments and more reducing (? hydrothermal) fluids locally remobilized Fe and Mn and corroded dolomite crystals.

Chemical composition of sediments from DSDP Hole 149

The diagenesis and geochemical evolution of deep-sea sediments are controlled by the interaction between sediments and their associated pore waters. With increasing depth, the pore water of Hole 149 (DSDP) exhibits a strong depletion in Mg and a corresponding enrichment in Ca, while the alkalinity remains relatively constant. Dissolved SiO2 is nearly constant in the upper 100 m of sediment, but is highly enriched in the deepest pore waters. The pore waters exhibit a depletion in K with increasing depth, and O18/O16 pore water ratios also decrease. The sediment section has three zones of sedimentary regimes with increasing depth in the drill hole: an upper 100 m section of detrital clays, a middle section enriched in calc-akalic volcanics which have undergone submarine weathering to a smectite phase, and a lower section of siliceous ooze which still has a diagenetic smectite phase. The quartz-feldspar ratios and O18/O16 composition of the silicate phases are in agreement with these interpretations. The submarine weathering of volcanics to a smectite can account for the observed pore water gradients. Volcanics release Ca and Mg to the pore waters causing the alkalinity values to increase. Smectite is formed, depletes the pore waters in Mg and O18 and causes the alkalinity to decrease. The net reaction allows for the observed relationship between pore water Ca and Mg gradients with little net change in alkalinity. Given the abundance of volcanics in many deep-sea sediments, especially in lower sections which often form near ridge crests, the submarine formation of smectite may be an additional oceanic Mg sink which has not yet been fully considered.

Porosity, permeability, trace elements, and carbon-oxygen isotopes at DSDP Hole 77-536

Talus deposits recovered from Site 536 show evidence of aragonite dissolution, secondary porosity development, and calcite cementation. Although freshwater diagenesis could account for the petrographic features of the altered talus deposits, it does not uniquely account for isotopic or trace-element characteristics. Also, the hydrologic setting required for freshwater alteration is not easily demonstrated for the Campeche Bank. A mixing-zone model does not account for the available trace-element data, but does require somewhat less drastic assumptions about the size of the freshwater lens. Although a seawater (bottom-water) alteration model requires no hydrologic difficulties, unusual circumstances are required to account for the geochemical characteristics of the talus deposits using this model.

Frequency distribution of graded turbidite cycles in DSDP sediments from the Coral Sea Basin and the Sea of Japan

At sites 390 and 392 (Deep Sea Drilling Project, Leg 44) on the Blake nose, thoroughly lithified Lower Cretaceous limestone more than 250 m thick is abruptly overlain by a condensed sequence of Barremian to Eocene pelagic carbonate ooze. The Lower Cretaceous sediments consist of three units: limestone with moldic porosity (base), oolitic limestone, and fenestral limestone. Subaerial diagenesis of the limestone section is recorded by (1) caverns with vertical dimensions of up to 10 m, (2) stalactitic intergranular cement, and (3) meniscus sediment (or cement). Compatible with these subaerial features are mud cracks, fenestral fabrics, intraclasts, and cryptalgal structures. Inasmuch as these shallow-water and tidal-flat deposits are now beneath 2,607 m of sea water (plus 99 m of younger sediments), they serve to dramatize the apparent degree of Barremian and later subsidence of this part of the Atlantic outer continental shelf. Porosity and permeability are high in vuggy samples, which are common in the skelmoldic limestone. Cementation has destroyed most of the extensive primary porosity of the two younger units.

Ba distribution in surface sediments of the Southern Ocean

We present excess Ba (Baxs) data (i.e., total Ba corrected for lithogenic Ba) for surface sediments from a north-south transect between the Polar Front Zone and the northern Weddell Gyre in the Atlantic sector and between the Polar Front Zone and the Antarctic continent in the Indian sector. Focus is on two different processes that affect excess Ba accumulation in the sediments: sediment redistribution and excess Ba dissolution. The effect of these processes needs to be corrected for in order to convert accumulation rate into vertical rain rate, the flux component that can be linked to export production. In the Southern Ocean a major process affecting Ba accumulation rate is sediment focusing, which is corrected for using excess 230Th. This correction, however, may not always be straightforward because of boundary scavenging effects. A further major process affecting excess Ba accumulation is barite dissolution during exposure at the sediment-water column interface. Export production estimates derived from excess 230Th and barite dissolution corrected Baxs accumulation rates (i.e., excess Ba vertical rain rates) are of the same magnitude but generally larger than export production estimates based on water column proxies (234Th-deficit in the upper water column; particulate excess Ba enrichment in the mesopelagic water column). We believe export production values based on excess Ba vertical rain rate might be overestimated due to inaccurate assessment of the Baxs preservation rate. Barite dissolution has, in general, been taken into account by relating it to exposure time before burial depending on the rate of sediment accumulation. However, the observed decrease of excess Ba content with increasing water column depth (or increasing hydrostatic pressure) illustrates the dependence of barite preservation on degree of saturation in the deep water column in accordance with available thermodynamic data. Therefore correction for barite dissolution would not be appropriate by considering only exposure time of the barite to some uniformly undersaturated deep water but requires also that regional differences in degree of undersatuation be taken into account.

Sediment and pore water C and N composition of ODP Holes 201-1227A and 201-1230A

This study addresses the problem of diagenetic fractionation of d15N in sedimentary organic matter by constructing isotopic mass balances for the sedimentary nitrogen and pore water ammonium at two Ocean Drilling Program (ODP) sites, 1227 and 1230. At Site 1230, ammonium production flux integrated through the sedimentary column indicates that >60% of organic matter is lost to decomposition. The d15N of pore water ammonium is <0.7 per mil different from that of the sedimentary organic matter, which implies that very little isotopic fractionation is associated with degradation of organic matter at this site. The constant d15N of the solid-phase sedimentary nitrogen through the whole profile supports this conclusion. Atomic C/N ratios (9-12) indicate that organic matter at this site is primarily of marine origin. At Site 1227, the sedimentary organic matter appears to be a mixture of terrestrial and marine components. Ammonium is ~4 heavier than the organic matter. The observed isotopic enrichment of pore water ammonium relative to the sedimentary nitrogen might indicate either the preferential decomposition of isotopically heavier marine fraction of the organic matter, or possibly, a nonsteady-state condition of the ammonium concentration and d15N profiles. Interpretation of the results at Site 1227 is further complicated by the contribution of ammonium with d15N of ~4 per mil that is diffusing upward from Miocene brines.