Peru Shelf Research Articles

Multiple oxidation state trace elements in suboxic waters off Peru: In situ redox processes and advective/diffusive horizontal transport

Abstract Oxygen minimum zones (OMZs), where the concentrations of oxygen are at analytical detection limits, but sulfate reduction is not occurring, are termed suboxic and typically nitrate serves as the terminal electron acceptor for heterotrophic respiration. Such respiration is highest near the top of OMZs where maxima in nitrite and other redox active species are observed. Within OMZs, the availability of free electrons for reduction reactions (pe) can range from − 2 to 10 and at thermodynamic equilibrium could control the oxidation states of essential trace elements like iron and toxic ones like arsenic. In turn, this oxidation/reduction speciation affects trace element solubility and bioavailability , thus controlling their reactivity and transport. Field observations and mesoscale models demonstrate the increasing areal extent of oxygen minimum zones, but whether this in turn affects trace element cycling and fluxes remains to be verified. From current observations, it is difficult to determine the relative importance of in situ processes and lateral transport from boundaries to trace elements' redox cycling, which are typically highly redox- and biologically-active in these systems. Such processes could lead to the co-occurrence of redox species that are not controlled by a unique pe. In order to evaluate the effects of in situ redox processes versus advective/diffusive transport on trace elements, the cycling of nitrate/nitrite, iodate/iodide, Fe(II), As(III)/As(V), and hydrogen sulfide was examined in the low oxygen waters off the Peru shelf to the open South Pacific Ocean during the US GEOTRACES GP16 transect in 2013. Nitrite, Fe(II), and iodide were observed from the shelf to 95° W, while reduced forms of sulfur and arsenic were absent. Maxima in nitrite, Fe(II) and iodide were coincident, indicating the utilization of Fe(III), iodate and nitrate as terminal electron acceptors was possible. For iron, this finding was unexpected with regards to thermodynamics. However, closer inspection combined with advective/diffusive modeling of water column data and 228Ra-based flux calculations, showed that benthic redox processes, coupled with westward isopycnal transport, influenced the chemical composition and redox speciation within the upper OMZ well offshore. This horizontal transport contributed to the coexistence of Fe(II) with nitrate, indicating that Fe redox chemistry is under kinetic, rather than thermodynamic, control.

Organic complexation of iron in the eastern tropical South Pacific: Results from US GEOTRACES Eastern Pacific Zonal Transect (GEOTRACES cruise GP16)

Abstract Dissolved iron, organic iron-binding ligands, and organic carbon were determined in full water column depth profiles across the US GEOTRACES Eastern Pacific Zonal Transect (GEOTRACES cruise GP16) in late 2013. Dissolved iron concentrations exhibited subsurface maxima associated with the remineralization of organic matter at the Peru shelf and with hydrothermal inputs from the East Pacific Rise. Iron-binding organic ligands are described as ligand classes based on defined ranges in conditional stability constants. The stronger L1-type ligands were measured in large excesses in surface and intermediate waters, and these excesses were negatively correlated with Si*, a biogeochemical proxy for iron limited diatom growth. These data suggest sources of strong iron-binding ligands from iron limitation of diatom communities, both locally and in waters originating from the Southern Ocean. Benthic sources of strong ligands were associated with new iron inputs from hydrothermal activity at the East Pacific Rise and from bottom sediments. In contrast to most studies in the Atlantic basin but consistent with previous datasets from the Pacific, stronger L1 ligands in this dataset were generally restricted to the upper water column and did not show large excesses through the water column. At depth, iron-binding ligands on GP16 were instead best described as L2 and L3 ligands. Concomitant decreases in excess L1, excess total ligands and dissolved organic carbon suggest similar degradation pathways of these pools below the surface.

Intraseasonal variability of nearshore productivity in the Northern Humboldt Current System: The role of coastal trapped waves

The impact of intraseasonal coastal-trapped waves on the nearshore Peru ecosystem is investigated using observations and a regional eddy-resolving physical-ecosystem coupled model. Model results show that intraseasonal variability over the period 2000–2006 represents about one fourth of the total surface chlorophyll variance and one third of the carbon export variance on the Peruvian shelf. Evidence is presented that subsurface nutrient and chlorophyll intraseasonal variability are mainly forced by the coastally trapped waves triggered by intraseasonal equatorial Kelvin waves reaching the South American coast, and propagate poleward along the Peru shore at a speed close to that of high order coastal trapped waves modes. The currents associated with the coastal waves induce an input of nutrients that triggers a subsequent phytoplankton bloom and carbon export. The impact of the local wind-forced intraseasonal variability on the ecosystem is of a similar order of magnitude to that remotely forced in the northern part of the Peru shelf on [50–90] day time scales and dominates over the entire shelf on [20–30] day time scales.

Bacterial production of anomalously high dissolved sulfate concentrations in Peru slope sediments: steady-state sulfur oxidation, or transient response to end of El Niño?

Concentrations of dissolved sulfate and sulfur isotopic ratios of dissolved sulfide in surface sediments of the Peru shelf and upper slope indicate that the sediments can be divided into two depth intervals based on the dominant biogeochemical reactions. Although rates of bacterial sulfate reduction are high throughout Peru surface sediments, chemistry of the upper interval (<10–20 cm) is dominated by chemoautotrophic oxidation of dissolved sulfide and elemental sulfur, while the lower interval (>10–20 cm) is dominated by dissimilatory sulfate reduction. In three of the four cores examined here, pore water concentrations of sulfate in the top 10 cm of the sediment are significantly higher than those of the overlying seawater. Peak sulfate concentrations in pore water (37–53 mmol/l) are ∼1.3–1.9 times that of seawater sulfate and are located 1–6 cm below the sediment/water interface (SWI). The excess sulfate is most likely produced by oxidation of elemental sulfur coupled to reduction of nitrate, a reaction mediated by a facultative chemoautotrophic sulfide-oxidizing bacterium, Thioploca spp. Numerical simulations demonstrate that the anomalously high concentrations of dissolved sulfate can be produced by steady-state or non-steady-state processes involving high rates of bacterial oxidation of elemental sulfur. If bacterial sulfur oxidation is a transient phenomenon, then it is probably triggered by seasonal or El Niño-induced changes in water-column chemistry of the Peru undercurrent.

Sulfur diagenesis and partitioning in Holocene Peru shelf and upper slope sediments

Sediments of the Peru margin were sampled in two transects across the shelf and upper slope (75 to 1000 m) in October of 1992. This region is characterized by high surface-water productivity and consequent high organic carbon fluxes, low terrigenous inputs, as well as an intense oxygen-minimum zone (OMZ) impinging on the slope at depths between 75 and 550 m. Sediments analyzed in this study were collected from one ship-deployed box core and five submersible-deployed push cores. Organic carbon (TOC) concentrations in surface sediments range from 1 to 18% on a dry weight basis. Formation of pyrite begins at the sediment/water interface (SWI). Maximum concentrations of pyrite sulfur in the cores from this study range from 0.5 to 1.8 wt.%. Concentrations of pyrite are influenced by availability of reactive iron and an oxidant capable of converting AVS to pyrite. Highly reactive iron is stripped from terrigenous detritus in the core of the OMZ. Some of this iron may contribute to glauconite formation in sediments near the base of the OMZ, where it is concentrated into a glauconitic green sand as a result of reworking and winnowing. In a shallow-water site (109 m) at the upper edge of the OMZ, high concentrations of pyrite, total iron and iron/aluminum ratios indicate that easily reduced iron species are available for sulfidation at the SWI. Pyrite formation at this site is nearly complete in the upper 4 cm of the sediment. In contrast, pyrite concentrations are slightly lower at the surface of two of the deeper-water sites (at 309 m, in the center of the OMZ, and to a lesser extent at 671 m, at the bottom edge of the OMZ), and appear to be influenced, in part, by availability of reactive iron. Concentrations of acid-volatile sulfides (AVS) are low in all sediments (≤ 0.04 wt.%), but are slightly elevated in sediments from the center of the OMZ. The ratio of organic sulfur (S-org)to organic carbon (TOC)at the sediment surface of the cores examined here is remarkably constant (0.012 ± 0.002) and within the range observed in marine biota (0.01 to 0.02), suggesting that organic sulfur at the SWI is dominantly biogenic. Although S-org/TOC ratios increase downcore, there is no clear correlation between concentrations of organic sulfur and organic carbon below the SWI. There is also no clear relation between concentrations of organic sulfur and total iron. Organic sulfur concentrations vary from site to site (0.06 to 0.33 wt.%), but are relatively constant downcore at any given site. Maximum concentrations are in cores from the center of the OMZ.

Water depth and diagenetic constraints on the use of barium as a palaeoproductivity indicator

Abstract Sediments underneath the high productivity areas of the coastal upwelling and equatorial divergence zones in the Pacific and Indian oceans are enriched in barium, and on this basis the barium content of the sediments has long been suggested as a potential palaeoproductivity indicator. Analyses of sediments from the Peru margin corroborate the pattern of enhanced barium accumulation in areas of high primary productivity in the deep ocean, but also show that this pattern cannot be extended to shallow-water deposits. Sediments from the Peru shelf lack any barium enrichment, whereas this element is significantly enriched in slope and basinal deposits in water columns deeper than 2000 m. This depth effect is further illustrated at Site 682 located in the rapidly subsiding Lima Basin. The Quaternary sequence deposited at water depths &gt;3000 m is enriched in organic carbon, opal and biogenic barium. The Miocene sediments, although deposited under highly productive waters associated with a coastal upwelling centre, do not show an enrichment in the barium record corresponding to the very high organic carbon and opal levels; a pattern that is consistent with deposition in shallow basins. In addition to the effect of water depth on the barium distribution, overprinting of primary signals can be observed in sediments from upwelling areas undergoing strong anoxic diagenesis. Microcrystalline barite is partly dissolved in intervals depleted in interstitial sulphate as shown for sediments from the Peru margin. Dissolution of barite in sulphate depleted intervals of deep water sections leads to high barium concentrations near the termination of the sulphate reducing zone, where downward sulphate and upward barium diffusion foster local barite precipitation in diagenetic fronts. The barium distribution in sedimentary oxic and suboxic environments at deep water depositional sites has a high potential as a palaeoproductivity indicator; however, barium accumulation as a proxy for ocean fertility should be used with caution in areas of varying water depths and in anoxic diagenetic environments where sulphate depletion undersaturates the interstitial waters with respect to barite.

Carbon/sulphur/iron relationships in upwelling sediments

Abstract Carbon/sulphur/iron relationships in sediments from beneath three upwelling areas (Benguela, Peru and Oman) were investigated using DSDP and ODP cores. The three areas displayed markedly different C/S/Fe characteristics. Metabolizable organic matter appears to limit production of reduced sulphur in the Benguela upwelling area sediments, as evidenced by incomplete reduction of dissolved sulphate. On the Peru shelf infusion of a sulphate-containing brine at depth has a major influence on diagenesis. Weight ratios of organic carbon to reduced inorganic sulphur are generally higher in sediments from upwelling areas than in normal marine sediments. C/S ratios generally increase with increasing organic carbon concentration in upwelling sediments. This does not appear to be the result of limited availability of reactive iron. Both carbon and reduced sulphur accumulation rates are well correlated with sediment accumulation rates, but are higher than in non-upwelling sediments having similar accumulation rates.

Sedimentary and geochemical expressions of oxic and anoxic conditions on the Peru Shelf

Abstract Sediments from drill hole transects across the Peru shelf and upper slope show that the extent and intensity of the oxygen minimum zone (OMZ) has varied considerably from the early Pleistocene to the present. Primary sedimentary features, such as laminations and bioturbation, and the hydrogen index of sedimentary organic matter, appear to covary with the presence and intensity of the OMZ. Sediments deposited under dysoxic to anoxic bottom-water conditions are significantly more hydrogen-rich relative to bulk TOC. The molecular composition of pyrolytic hydrocarbons and heterocompounds, however, is surprisingly uniform, and is indicative of neither oxic nor anoxic degradation at either the sediment-water interface or in the water column. The concept of better preservation of organic matter under anoxic conditions in the bottom water is reflected in the bulk parameter of pyrolyzable organic matter content relative to TOC, but not in individual compound classes incorporated into kerogen. The most important control on the hydrogen-richness of organic matter appears to be sediment reworking and redeposition.

Sulfate reduction and carbon oxidation rates in continental shelf sediments, an examination of offshelf carbon transport

Sediments from 12 sites in the Gulf of Maine were sampled for sulfate reduction rates and interstitial water solute profiles. The profiles of redox sensitive solutes confirmed that sulfate reduction was probably the major mechanism of organic matter oxidation below the upper 2 cm of sediments. Sulfate reduction rates were assessed using 35S-sulfate tracer conversion to reduced sulfur in both the acid volatile sulfide fraction and the chromate reducible sulfur fraction. Rates, integrated over the upper 30 cm, decreased exponentially with water column depth from 20,000 to 1800 pmol sulfate cm −2 h −1 between 50 and 300 m. For the entire Gulf, the areal mean rate of carbon oxidation by sulfate reduction was 20.4 g C m −2 y −1. Based on the relationship from other regions of sediment trap carbon flux to water depth and primary productivity ( Betzer et al., 1984, Deep-Sea Research, 31, 1–11), the carbon flux to the sediments would be 47.0 g C m −2 y −1. Because the Gulf of Maine is a semi-enclosed sea, little export of organic carbon to the adjacent continental slope is expected. With estimated rates of oxygen consumption, denitrification, and carbon burial of 22.7, 4, and 5 g C m −2 y −1, the sediment respiration and burial rates, totaling 52.1 g C m −2 y −1, closely balanced the carbon input indicating no significant export of organic material. In contrast, a similar analysis of published data from the narrow exposed Washington state continental shelf showed that the carbon input (264 g C m −2 y −1) greatly exceeded the sediment respiration and burial rate (99.5 g C m −2 y −1), indicating between 22 and 50% of the primary production may have been exported. These and previous results from the New York Bight, the Bering Sea, and the Peru shelf suggest that shelf ecosystems may differ considerably in the proportion of shelf production which escapes seaward. Although only a fraction of the exported carbon may accumulate in slope sediments, shelf export may be important in redistributing organic carbon in particular geographic regions.

Uranium-series and AMS 14C studies of modern phosphatic pellets from Peru shelf muds

Recent sediments and separated phosphate pellets (∼ 125–500 μm in diameter) from the Peru shelf have been analyzed for uranium decay-series isotopes and 14C in order to determine age relationships and mineralization rates. Uranium-series ages of pellets separated from one box core are significantly higher than AMS radiocarbon ages determined for the same pellets. These differences appear to be a consequence of mixing of an older generation of pellets with ones which are more recently formed. Postdepositional adsorption of reactive elements such as thorium and protactinium onto pellet surfaces may also contribute to the observed discordancy with radiocarbon ages. Sediment radiocarbon and 210Pb sediment results, as well as some trends in the uranium-series data, suggest that high concentrations of phosphate pellets have accumulated in some Peru shelf sediments without extensive reworking. Individual pellets apparently form very quickly, on time scales of a few years. Estimated authigenic uptake rates of phosphorus into pellets ranges from 0.5 to 9.40 μmol-P cm − 2 yr − 1, somewhat higher than rates measured for nodules from the same area. This is consistent with observations that pelletal morphologies predominate over nodular forms within ancient phosphorite deposits.

Rare earth elements in the phosphatic-enriched sediment of the Peru shelf

Apatite-enriched materials from the Peru shelf have been analyzed for their major oxide and rare earth element (REE) concentrations. The samples consist of (1) the fine fraction of sediment, mostly clay material, (2) phosphatic pellets and fish debris, which are dispersed throughout the fine-grained sediment, (3) tabular-shaped phosphatic crusts, which occur within the uppermost few centimeters of sediment, and (4) phosphatic nodules, which occur on the seafloor. The bulk REE concentrations of the concretions suggest that these elements are partitioned between the enclosed detrital material and the apatite fraction. Analysis of the fine-grained sediment with which the samples are associated suggested that this detrital fraction in the concretions should have shale REE values; the analysis of the fish debris suggested that the apatite fraction might have seawater values. The seawater contribution of REE's is negligible in the nodules and crust, in which the apatite occurs as a fine-grained interstitial cement. That is, the concentration of REE's and the REE patterns are predominantly a function of the amount of enclosed fine-grained sediment. By contrast, the REE pattern of the pelletal apatite suggests a seawater source and the absolute REE concentrations are relatively high. The REE P 2 O 5 ratios of the apatite fraction of these samples thus vary from approximately zero (in the case of the crust and nodules) to as much as approximately 1.2 × 10 −3 (in the case of the pellets). The range of this ratio suggests that rather subtle variations in the depositional environment might cause a significant variation in the REE content of this authigenic fraction of the sediment. Pelletal glauconite was also recovered from one sediment core. Its REE concentrations closely resemble those of the fish debris.

Carbon isotopic composition and lattice-bound carbonate of Peru-Chile margin phosphorites

New light-stable carbonate-carbon isotope and lattice-bound CO 2 data from Quaternary Peru-Chile margin phosphatic nodules, crusts and pelletal grains, and from associated dolomicritic concretions, are presented, which provide constraints on the timing and mechanisms of growth of these phases in organic carbon-rich sediments. Comparison of δ 13C values from carbonate fluorapatite (CFA) nodules and pelletal grains (−4.8 to 0.0‰ and −2.9 to +1.0‰ PDB, respectively) with pore-water total dissolved δ 13C values from these sediments suggests early authigenic CFA precipitation from pore waters within a few centimeters of the sediment-water interface in association with suboxic to perhaps anoxic microbial degradation of organic matter. In contrast, the dolomicritic cores of nodules recovered from about 12°S display both strongly negative to positive δ 13C values (−10.8 to +6.1‰) characteristic of formation deeper in the sediments in association with methanogenic and perhaps sulfate reduction microbial processes. The amount of structural carbonate in CFA suggests that carbonate substitution generally increases as δ 13C in CFA decreases, a probable consequence of increasing carbonate and accompanying charge-balancing substitutions in the CFA lattice in response to increasing pore-water carbonate ion concentrations with depth below the sediment-water interface. In one buried upward-growing nodule, decreasing CFA δ 13C and increasing structural CO 2 also correspond to decreasing CFA growth rates. These data suggest that in addition to other constraints such as pore-water phosphorus and fluoride availability, the lower limit of CFA precipitation in suboxic to anoxic sediments may be controlled by lattice poisoning due to excessive dissolved carbonate ion concentrations. In organic-rich Peru-Chile margin sediments this depth threshold appears to be at approximately 5–10 cm below the sediment-water interface where maximum CFA CO 2 contents of about 6 Wt.% occur; in less organic-rich settings, greater depths of precipitation of CFA may be anticipated. Below this relatively shallow depth of CFA precipitation on the Peru shelf, high pore-water alkalinity and associated elevated total dissolved carbon and carbonate ion concentration apparently favor the precipitation of authigenic carbonates.

Accumulation and biological mixing of Peru margin sediments

Rates of sediment accumulation and biological particle mixing in eight cores from the Peru shelf have been estimated by application of uranium-series disequilibrium techniques and radiocarbon dating. Activities of 210Pb, 226Ra, 234Th and 238U have been determined in closely spaced intervals in each sediment core. Biological particle mixing coefficients were determined via a two-box advection-diffusion steady-state mixing model based on the distribution profiles of excess 210Pb and, in some cases, excess 234Th activities. The sedimentation rates estimated were in the order of a few millimeters/yr except for cores enriched in phosphatic components which displayed rates a few orders of magnitude slower. Bioturbation was significant in most cores studied with the exception of one core collected from within the middle of the oxygen-minimum zone. Estimated biological mixing coefficients were in the order of 10 0–10 1 cm 2 yr −1 for the surface mixed layer and 10 −1–10 0 cm 2 yr −1 for the layer below. Sediment slumping appears to have affected the 210Pb distribution of two or three of our cores.

Distribution, texture and composition of modern phosphate pellets in Peru shelf muds

Textural analysis, including estimates of concentrations of authigenic phosphate pellets, were made for eight sediment cores from the Peru continental margin. Phosphatic pellets separated from these modern organic-rich sediments are black, spherical-ovoidal in shape, and in thin section often display a concentric structure around a nucleus consisting of inorganic mineral grains. Some pellets have a gray-white exterior coating which appears to be secondary diagenetic calcite. Phosphatic pellets account for upwards of 80% of the sediment mass in some cores. Pellets concentrate in specific size classes, generally between 125 and 500 μm in diameter, and occur within a poorly sorted sediment.

Uranium-series growth history of a quaternary phosphatic crust from the peruvian continental margin

A 20-mm-thick oriented phosphatic crust recovered together with its overlying (14 cm) and underlying (4 cm) associated sediment from the Peruvian sea floor has been analyzed in detail for uranium-series radionuclides in an attempt to determine its rate and direction of growth. Growth curves based upon 226Ra and 230Th ages show that this crust grew upward toward the sediment-water interface. Calculated growth rates in the range of 12–13 mm ka −1 are slightly higher but comparable to values previously reported. Crystallographical analyses of this phosphatic crust show a trend of decreasing unit-cell dimension a with sample age. The upward growth of a buried crust toward the sediment-water interface is consistent with results from recent pore-water studies of fluoride and phosphate in Peru shelf sediments.

Dimethylsulfide in the water column and the sediment porewaters of the Peru upwelling area1

The concentration of dimethylsulfide (DMS) and supporting parameters were determined on longshore and cross‐shelf traverses in the Peru upwelling area. Vertical DMS profiles were measured at several stations in the marine water column and in the sedimentary porewaters. The distribution of DMS in surface waters is related to the patchiness of the upwelling process and of the resulting phytoplankton biomass. Substantial concentrations of DMS were found in the sedimentary porewaters, with a sharp maximum a few centimeters below the sediment‐water interface. The source of this DMS is most likely the decomposition of algal material which settles to the bottom beneath the upwelling areas. The rates of DMS production in the water column and its flux into the atmosphere across the air‐sea interface are only a small fraction of the rate of sulfate assimilation by the plankton community. The concentrations of DMS in the surface waters of the Peru shelf are similar to those found in other coastal areas; the fluxes of reduced sulfur gases to the atmosphere from this upwelling region do not require special consideration in the global atmospheric sulfur budget.

Vertical copepod aggregations and interactions with chlorophyll and production on the Peru shelf

Data consisting of high resolution profiles of in situ chlorophyll fluorescence and copepods have been measured with a towed Batfish on the Peru shelf during November 1977. The day to day vertical variability of chlorophyll fluorescence was highly complex; however, there were indications of common features consisting of a surface chlorophyll layer about 10 to 15 m thick and a subsurface chlorophyll maximum at about 25-m depth. Of the dominant copepods measured, Eucalanus inermis, Calanus chilensis, and Centropages brachiatus, the latter two were aggregated in layers at about 15 to 20 m directly within a depression located between the surface and subsurface chlorophyll layer. Primary production profiles estimated from light and chlorophyll data indicated that carbon production was generally restricted to the surface layer (≈15m). Estimates of integrated carbon production (m −2) within the surface layer ranged from 1.3 to 1.5 g C m −2 d −3. Copepod grazing estimates indicated that only 10% of daily carbon production was utilized.

Pore water fluoride in Peru continental margin sediments: Uptake from seawater

Fluoride analyses display downward decreasing pore water gradients in Peru shelf phosphatic muds that require diffusion from the overlying seawater into the sediment column and removal by reaction within the upper few tens of centimeters, presumably by incorporation into carbonate fluorapatite. The profiles can be modeled as first-order F-removal with rate constants of ~3 yr −1 and asymptotic F-concentrations deep in the cores of 35–45 μM, almost one-half the seawater value. The integrated flux of fluoride from seawater into organic-rich shelf sediments in coastal-upwelling zones (phosphatic muds) yields a contemporaneous global F-burial of 0.54 × 10 10 mol-F yr −1, about one-fifth the burial in other sinks (mostly carbonates and opal). The associated burial flux of phosphorus in shelf phosphorites is about 1.6 × 10 10 mol-P yr −1, comparable to P-burial in the deep sea with organic matter (~1.4 × 10 10 mol yr −1) and biogenic carbonates (~1.4 × 10 10 mol yr −1). Thus phosphorite formation on the Peru shelf is a significant contemporaneous process.

Volatile C 1C 8 organic compounds in sediments from the Peru upwelling region

Volatile C 1C 8 organic compounds were analyzed in seven cores recovered from a transect across the Peru Shelf Upwelling Region. Compounds detected in ng/g dry weight quantities included methane, ethane, propane and other C 4C 7 alkanes as well as functionalized compounds including alkenes, furans and aldehydes. Strong correlations were observed between some groups of compounds with similar structures. Maxima in two groups of compounds were observed: (1) C 1, C 2, C 3, furans, and butanals (plus sporadic alkanes) as nitrate levels decreased below about 2–4 μM in the subsurface, and (2) C 4C 6 alkenes together with predominantly branched and cycloalkanes at the sediment-water interface in cores recovered from oxygenated bottom waters. The levels of C 1C 7 alkanes in these sediments are comparable to levels found in DSDP cores buried to depths of less than 600 m (or not exposed to geothermal temperatures exceeding about 30°C).