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Recent studies suggest that as much as half of the organic carbon (OC) undergoing burial in the sediments of tectonically active continental margins may be the product of fossil shale weathering. These estimates rely on the assumption that vascular plant detritus spends little time sequestered in intermediate reservoirs such as soils, freshwater sediments, and river deltas, and thus only minimally contributes to the extraneously old [sup.14]C ages of total organic matter often observed on adjacent shelves. Here we test this paradigm by measuring the [DELTA][sup.14]C and [delta][sup.13]C values of individual higher plant wax fatty acids as well as the [delta][sup.13]C values of extractable alkanes isolated from the Eel River margin (California). The isotopic signatures of the long chain fatty acids indicate that vascular plant material has been sequestered for several thousand years before deposition. A coupled molecular isotope mass balance used to reassess the sedimentary carbon budget indicates that the fossil component is less abundant than previously estimated, with pre-aged terrestrial material instead composing a considerable proportion of all organic matter. If these findings are characteristic of other continental margins proximal to small mountainous rivers, then the importance of petrogenic OC burial in marine sediments may need to be reevaluated.

Diatomaceous deposits in the Miocene--Pliocene Pisco Formation contain abundant whales preserved in pristine condition (bones articulated or at least closely associated), in some cases including preserved baleen. The well-preserved whales indicate rapid burial. The 346 whales within ~1.5 [km.sup.2] of surveyed surface were not buried as an event, but were distributed uninterrupted through an 80-m-thick sedimentary section. The diatomaceous sediment lacks repeating primary laminations, but instead is mostly massive, with irregular laminations and speckles. There is no evidence for bioturbation by invertebrates in the whale-bearing sediment. Current depositional models do not account for the volume of diatomaceous sediments or the taphonomic features of the whales. These taphonomic and sedimentary features suggest that rapid burial due to high diatom accumulation, in part by lateral advection into protected, shallow embayments, is responsible for the superb preservation of these whales, leading to a higher upper limit on phytoplankton accumulation rates than previously documented. Keywords: taphonomy, diatoms, Pisco Formation, Miocene, Pliocene, Cetacea, sedimentation rates.

Submarine-flank deposits of Hawaiian volcanoes are widely recognized to have formed largely by gravitationally driven volcano spreading and associated landsliding. Observations from submersibles show that prominent benches at middepths on flanks of Mauna Loa and Kilauea consist of volcaniclastic debris derived by landsliding from nearby shallow submarine and subaerial flanks of the same edifice. Massive slide breccias from the mature subaerial tholeiitic shield of Mauna Loa underlie the frontal scarp of its South Kona bench. In contrast, coarse volcaniclastic sediments derived largely from submarine-erupted preshield alkalic and transitional basalts of ancestral Kilauea underlie its Hilina bench. Both midslope benches record the same general processes of slope failure, followed by modest compression during continued volcano spreading, even though they record development during different stages of edifice growth. The dive results suggest that volcaniclastic rocks at the north end of the Kona bench, interpreted by others as distal sediments from older volcanoes that were offscraped, uplifted, and accreted to the island by far-traveled thrusts, alternatively are a largely coherent stratigraphic assemblage deposited in a basin behind the South Kona bench. Keywords: volcanic processes, volcano spreading, landslides, Mauna Loa, Hawaii.

The O, S, and Sr isotope compositions were determined for 17 samples of gypsum that replaced anhydrite in the sheeted-dike complex of the Macquarie Island ophiolite. Elevated [delta][sup.34]S (26.2 [per thousands]-29.0 [per thousands]) and [delta][sup.18]O values (12.5 [per thousands]-14.4 [per thousands],) of gypsum compared to those of seawater sulfate are the result of microbial sulfate reduction. Low organic carbon contents and little sulfate reduction in sediments, plus a large basaltic Sr component in the gypsum ([sup.87]Sr/[sup.86]Sr = 0.70446-0.70524), indicate that the sulfate source was not pore waters in the overlying sediment. Low [delta][sup.34]S values of sulfide in basalt lavas are consistent with microbial reduction of seawater sulfate within the volcanic rocks. Tectonic activity at the slow-spreading ridge allowed evolved formation waters to enter hot sheeted-dike complex basement, resulting in heating and precipitation of anhydrite. Results show that microbes can leave geochemical tracers of their activity in oceanic basement and that anhydrite can be preserved in oceanic crust and may be of significance for the oceanic sulfur budget. Keywords: sulfates, hydrothermal processes, bacteria, biosphere, mid-ocean ridges.

The Nankai hydrate field, Japan, is an example of gas-hydrate deposits associated with an active subduction zone. In order to determine the origin of gas hydrates in this area, [sup.129]I/I ratios together with halogen concentrations were measured in a set of pore-water samples collected from two boreholes in the Nankai hydrate field. Iodine concentrations are between 100 and 230 [micro]M, i.e., strongly enriched compared to seawater, while CI concentrations were found to he close to that of seawater. Except for one sample, [sup.129]I/1 ratios are between 180 and 520 x [10.sup.-15], giving minimum ages between 24 and 48 Ma. Because these ages are considerably older than present host sediments ( Keywords: gas hydrates, I-129 dating, Nankai Trough, pore waters, marine sediments.

A new morphosedimentary map of the Gulf of Cadiz is presented, showing the contourite depositional system on the gulf's middle slope. This map is constructed from a broad database provided by the Spanish Research Council and the U.S. Naval Research Laboratory. Our map shows that this contourite depositional system comprises five morphosedimentary sectors: (1) proximal scour and sand ribbons; (2) overflow sedimentary lobe; (3) channels and ridges; (4) contourite deposition; and (5) submarine canyons. The Gulf of Cadiz contourite depositional system stems directly from the interaction between Mediterranean Outflow Water and the seafloor; its morphosedimentary sectors are clearly related to the systematic deceleration of the Mediterranean Outflow Waters westward branches, bathymetric stress on the margin, and the Coriolis force. The slope's depositional system can be considered as a mixed contourite and turbidite system, i.e., a detached combined drift and fan. Keywords: contourite depositional system, middle slope, drifts, channels, diapirs, Gulf of Cadiz.

Sediment cores as long as 20 m, dated by [sup.14]C, [sup.14]Pb, and [sup.137]Cs methods and pollen stratigraphy, provide a history of diatom productivity and sediment-accumulation rates in Chesapeake Bay. We calculated the flux of biogenic silica and total sediment for the past 1500 yr for two high-sedimentation-rate sites in the mesohaline section of the bay. The data show that biogenic silica flux to sediments, an index of diatom productivity in the bay, as well as its variability, were relatively low before European settlement of the Chesapeake Bay watershed. In the succeeding 300-400 yr, the flux of biogenic silica has increased by a factor of 4 to 5. Biogenic silica fluxes still appear to be increasing, despite recent nutrient-reduction efforts. The increase in diatom-produced biogenic silica has been partly masked (in concentration terms) by a similar increase in total sediment flux. This history suggests the magnitude of anthropogenic disturbance of the estuary and indicates that significant changes had occurred long before the twentieth century. Keywords: Chesapeake Bay, biogenic silica, sediments, sedimentation, dating, nutrients.

The Indian monsoon system, as recorded by ocean-floor biota (benthic foraminifera) at Ocean Drilling Program Site 758 in the eastern equatorial Indian Ocean, has varied dramatically over the past 5.5 m.y., long after the onset of the monsoons at 10-8 Ma. Benthic foraminifera that thrive with high productivity year-round were common before the formation of Northern Hemisphere continental ice sheets ca. 3.1-2.5 Ma, indicating that the summer (southwest) monsoon had high intensity and long seasonal duration. Ca. 2.8 Ma benthic faunas became dominated by taxa that flourish with a seasonally strongly fluctuating food supply, indicating that the northeast (winter) monsoon, during which primary productivity is relatively low, increased in duration and strength to form a system similar to that of today. The change occurred coeval with the initiation of the Northern Hemisphere glaciation, documenting a close link between the development of the Indian monsoon and Northern Hemisphere glaciation. Keywords: monsoons, Indian Ocean, productivity, benthic foraminifera, Pliocene, Ocean Drilling Program.

Volcanic strata on southwest Isla Tiburon define the age of interstratified marine rocks and, through revision of existing correlations, the age of the proto-Gulf of California marine incursion. A 5.7 [+ or -] 0.2 Ma ash flow was emplaced at the base of the marine section. A rhyodacite dike and its related lava flow, dated as 11.2 [+ or -] 1.3 Ma, 3.7 [+ or -] 0.9 Ma, and 4.2 [+ or -] 1.8 Ma, intrudes and overlies, respectively, the marine rocks. The 11.2 Ma age, which was the core datum for a middle Miocene proto-Gulf of California origin for the underlying rocks, is discordant with all other isotopic and microfossil ages. An alternative interpretation, utilizing all available geologic and geochronologic data except this discordant age, is that marine strata on Isla Tiburon are latest Miocene to early Pliocene age. Reinterpretation of these strata supports a simplified history of marine incursion into the Gulf of California. Marine rocks as old as 8.2 Ma in the southern Gulf of California indicate an early marine incursion, perhaps flooding a region of more intense proto-Gulf of California continental extension. Flooding of the entire basin by 6.5-6.3 Ma correlates to the sudden onset of significant Pacific-North American plate-boundary motion within the Gulf of California. Keywords: Gulf of California, Salton Trough, Mexico, marine sediments, rifting.

Isotope fractionations during sulfate reduction by natural bacterial populations were measured in seven different marine sediments and compared with the isotopic composition of solid-phase sulfides in the same sediments. The measured fractionations during sulfate reduction could explain only between 41% and 85% of the [sup.34]S depletion in the sedimentary sulfides. This result directly demonstrates that the depletion of [sup.34]S in solid sulfides is an expression of the combined activity of sulfate-reducing organisms with additional fractionations accumulated during the oxidative part of the sulfur cycle. The only known process to significantly augment the fractionations created during sulfate reduction is the microbial disproportionation of the intermediate sulfur compounds: elemental sulfur, thiosulfate, and sulfite. In a simple model, we show how important each of these disproportionation processes could be in generating the isotopic composition of sedimentary sulfides. The sulfate reduction rates in the sediments studied varied by a factor of 1000 and did not correlate with the fractionation during sulfate reduction. By contrast, a correlation was observed between sulfate-reduction rate and the extent of [sup.34]S depletion into sedimentary sulfides, the most [sup.34]S-depleted sulfides found in sediments supporting the lowest rates of sulfate reduction. Thus, the fractionations imposed during the disproportionating processes are better expressed in sediments with low sulfate-reduction rates. The direct phototrophic oxidation of sulfide to sulfate, with minimal fractionation, may have been important in the sediments that had a high sulfate-reduction rate. Keywords: marine sediments, microorganisms, sulfides, sulfates, rates, oxidation.

Marine strata of Aptian age in the Pacific Basin include two distinct levels that represent episodes during which sediments rich in organic carbon were deposited. Both episodes lasted less than 1 m.y., as revealed in strata deposited atop submerged topographic highs. One unusually widespread episode of early Aptian age ( 117.5 Ma) correlates with coeval units in Europe and thus is analogous to the Cenomanian-Turonian oceanic anoxic event in its short duration and wide geographic extent. The second episode of late Aptian age ( 116.5 Ma) is restricted to allochtonous pelagic deposits in the Franciscan Complex of California. These results support the concept of widespread and narrowly synchronous anoxic events. Further, they show that organic carbon deposition in the Pacific Basin took place in intermediate water oxygen-minimum zones and thus differed in the mode of deposition, and hence paleoceanography, from that in other middle Cretaceous ocean basins.

Near-surface rates of microbiological sulfate reduction in most sediments often exceed rates of iron release, which consequently limit the precipitation of early diagenetic iron sulfides. Excess hydrogen sulfide produced by sulfate reduction generally escapes from the sediment before it can be fixed as iron sulfide; however, the efficiency of hydrogen sulfide fixation may be significantly increased by the presence of metastable, easily dissolved iron-bearing phases, such as volcanic glass. Sediments that efficiently trap hydrogen sulfide may have lower ratios of carbon to sulfur than those typically found in normal marine environments. The abundance of volcanic ash in sediments related to the closing of the proto-Atlantic may account for the low ratios of carbon to pyritic sulfur in lower Paleozoic sediments from Europe and the United States compared to equivalent sediments from Australia. The increase in the efficiency of trapping of hydrogen sulfide does not affect the amount of organic matter oxidized or sulfate reduced. It does, however, decrease the amount of sulfide that escapes the sediment and is reoxidized to sulfate. The net effect is a decrease in oceanic sulfate and an increase in atmospheric oxygen.