D.E. Hammond

Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis

Abstract Pore water profiles of total-CO 2 , pH, PO 3− 4 , NO − 3 plus NO − 2 , SO 2− 4 , S 2− , Fe 2+ and Mn 2+ have been obtained in cores from pelagic sediments of the eastern equatorial Atlantic under waters of moderate to high productivity. These profiles reveal that oxidants are consumed in order of decreasing energy production per mole of organic carbon oxidized ( O 2 > manganese oxides ~ nitrate > iron oxides > sulfate). Total CO 2 concentrations reflect organic regeneration and calcite dissolution. Phosphate profiles are consistent with organic regeneration and with the effects of release and uptake during inorganic reactions. Nitrate profiles reflect organic regeneration and nitrate reduction, while dissolved iron and manganese profiles suggest reduction of the solid oxide phases, upward fluxes of dissolved metals and subsequent entrapment in the sediment column. Sulfate values are constant and sulfide is absent, reflecting the absence of strongly anoxic conditions.

Benthic fluxes and the cycling of biogenic silica and carbon in two southern California borderland basins

Benthic fluxes in two southern California borderland basins have been estimated by modeling water column property gradients, by modeling pore water gradients and by measuring changes in concentration in a benthic chamber. Results have been used to compare the different methods, to establish budgets for biogenic silica and carbon and to estimate rate constants for models of CaCO3 dissolution. In San Pedro Basin, a low oxygen, high sedimentation rate area, fluxes of radon-222 (86 ± 8 atoms m−2 s−1), SiO2 (0.7 ± 0.1 mmol m−2 d−1), alkalinity (1.7 ± 0.3 meq m−2 d−1), TCO2 (1.9 ± 0.3 mmol m−2 d−1) and nitrate (−0.8 ± 0.1 mmol m−2 d−1) measured in a benthic chamber agree within the measurement uncertainty with fluxes estimated from modeling profiles of nutrients and radon obtained in the water column. The diffusive fluxes of radon, SiO2 and TCO2 determined from modeling the sediment and pore water also agree with the other approaches. Approximately 33 ± 13% of the organic carbon and 37 ± 47% of the CaCO3 arriving at the sea floor are recycled. In San Nicolas Basin, which has larger oxygen concentrations and lower sedimentation rates than San Pedro, the fluxes of radon (490 ± 16 atoms m−2 s−1), SiO2 (0.7 ± 0.1 mmol m−2 d−1), alkalinity (1.7 ± 0.3 meq m−2 d−1), TCO2 (1.7 ± 0.2 mmol m−2 d−1), oxygen (−0.7 ± 0.1 mmol m−2 d−1) and nitrate (-0.4 ± 0.1 mmol m−2 d−1) determined from chamber measurements agree with the water column estimates given the uncertainty of the measurements and model estimates. Diffusion from the sediments matches the lander-measured SiO2 and PO43− (0.017 ± 0.002 mmol m−2 d−1) fluxes, but is not sufficient to supply the radon or TCO2 fluxes observed with the lander. In San Nicolas Basin 38 ± 9% of the organic carbon and 43 ± 22% of the CaCO3 are recycled. Approximately 90% of the biogenic silica arriving at the sea floor in each basin is recycled. The rates of CaCO3 dissolution determined from chamber flux measurements and material balances for protons and electrons are compared to those predicted by previously published models of CaCO3 dissolution and this comparison indicates that in situ rates are comparable to those observed in laboratory studies of bulk sediments, but orders of magnitude less than those observed in experiments done with suspended sediments.

Early diagenesis of organic material in equatorial Pacific sediments: stpichiometry and kinetics

Abstract Benthic incubation chambers and sediment pore water profiles were used to study early diagenesis of organic matter in equatorial Pacific sediments. Replicate measurements with a flux chamber covering 720 cm2 indicated that the spatial variability of oxygen, TCO2, alkalinity, nitrate and silica fluxes at a single station did not exceed 10–35%. In contrast, diffusive fluxes of oxygen from replicate cores covering 70 cm2 at a single station often showed greater variation. In January 1992, benthic oxygen consumption was fairly constant along the equator from 103°W to 140°W at 0.6-0.8 mmol m−2 day−1. In November 1992, consumption was roughly symmetrical across the equator along 140°W, with rates of 0.6-0.8 mmol m−2 day−1 between 2°S and 2°N, declining to rates of 0.1-0.2 mmol m−2 day−1 at 12°S and 9°N. Pore water oxygen profiles were fit with a reaction-diffusion model equation to evaluate reaction kinetics. Most profiles were adequately fit with a model that assumed reaction rates declined exponentially with depth, but at low latitudes better fits often were obtained with a model that assumed decomposing organic matter has two labile components and that each decays with first-order kinetics and decreases exponentially with depth. Results of both fits indicate that at least 70% of the organic matter degradation occurs within the upper 1–2 cm of sediment. At the low-latitude stations fit with the two-component model, 70–90% of the flux is attributable to the more labile component which has an average 1/e penetration depth of 0.4 ± 0.1 cm. The more refractory component at these stations has a penetration depth of 4.4 ± 0.4 cm. From estimates of sediment mixing rates, the mean life of all degrading organic matter at the higher latitude stations is 4–55 years, while at the stations fit with the two-component model, the lifetime of the more labile fraction is weeks to months, and the lifetime of the less labile component is 40–300 years. A third carbon fraction exists at all stations that is far more refractory. The O2:CO2 stoichiometry of remineralization is −1.45 ± 0.17, and the C:N ratio is 8 ± 1. Both ratios are in good agreement with those observed from sediment trap and hydrographic studies in the water column, and suggest that degrading organic matter has about 70% of its carbon in -CH2O-groups and 30% in -CH2-groups. The C:P atom ratios for benthic remineralization differ by a factor of 3 for the two cruises, showing substantial temporal variability and de-coupling from carbon, although the mean for the two cruises (170 ± 85) is not significantly different than remineralization ratios observed in the water column. The aerally-integrated benthic respiration rate for the equatorial Pacific upwelling region is at least 25% of the integrated respiration rate for the continental margin (slope + rise) areas of the Pacific, emphasizing the importance of the equatorial Pacific sediments as a major site of benthic carbon recycling. Benthic carbon remineralization rates determined during the past decade near the equator and 140°W have varied by a factor of 2, which is not surprising given the short lifetime of the majority of the carbon degrading. The temporal patterns of carbon remineralization rates resemble those of sea-surface temperature, suggesting that benthic carbon oxidation at this site may reflect water column productivity over relatively short timescales.

Benthic fluxes and pore water studies from sediments of the central equatorial north Pacific: Nutrient diagenesis

Benthic exchange rates of radon-222, oxygen, nitrate, ammonia, and silica were determined using an in situ benthic flux chamber and by modeling pore water profiles at three sites in the central equatorial north Pacific. A comparison of these results reveals several artifacts of pore water collection and processing. Whole-core squeezer (WCS) silica profiles are influenced by adsorption during squeezing and yield calculated fluxes that are too large. Pore water ammonia profiles show near-surface maxima that appear to be an artifact of core recovery. Near-surface nitrate measurements may also be suspect due to oxidation of the ammonia released, causing anomalously large nitrate gradients that yield overestimates of benthic exchange rates. Fluxes of radon, oxygen, and nitrate calculated from WCS profiles agree with chamber fluxes to better than 40% at all sites. Fluxes of silica and nitrate calculated from pore water data collected at coarser scales (> 1 cm intervals) agree within 50% with chamber measurements. Previous flux estimates from pore water and solid phase models established at two of these sites using data collected 6 years prior to this work differ from these chamber measurements, in some cases by up to a factor of 5 due to modeling uncertainties and temporal variabilities in the delivery of organic matter to a site. The benthic oxygen consumption rates measured at these sites are similar (they average 0.36 ± 0.03 mmol m−2 day−1) and are consistent with a trend of oxygen uptake vs. water depth previously established by others on a transect through the oligotrophic north Pacific gyre.

In situ measurements of calcium carbonate dissolution rates in deep-sea sediments

Benthic fluxes of alkalinity, carbon dioxide, and oxygen have been measured using an in situ incubation chamber at three sites (MANOP Sites C and S and PACFLUX Site SC) in the central equatorial north Pacific. At two carbonate-rich sites (C and SC), a budget for oxygen, alkalinity, and TCO2 fluxes indicate a net CaCO3 dissolution rate of approximately 0.4 mmol m−2 day−1. This rate is only 20% of previous estimates but is consistent with a dissolution rate constant predicted from laboratory experiments with deep-sea sediments and derived from in situ flux measurements in sediments of the southern California borderland. Organic matter oxidation in the sediment column provides the acid for 60–100% of the calcium carbonate dissolution occurring at these sites with the remainder derived from undersaturated bottom water. At a low-carbonate site (S), no carbonate dissolution in the sediment or on the sea floor is apparent, although sediment traps indicate a rain of CaCO3 through 3400 m. The rates of remineralization relative to resuspension or erosion at this site must differ for organic carbon and calcium carbonate, so that carbonate grains reaching the sea floor are physically removed by erosion or resuspension before they dissolve, while organic carbon is largely oxidized before it can be physically removed.

The calibration of a new free-vehicle benthic flux chamber for use in the deep sea

Abstract A new free-vehicle benthic lander has been developed, capable of measuring benthic fluxes of nutrients, alkalinity, oxygen and radon. Important features of the device are the capability to collect sequential samples of 200 ml or more from three flux chambers, the ease of deployment, and the low cost. It has been tested in two basins of the California Borderland. Radon fluxes measured in these basins are consistent with those obtained by integrating the standing crop in the water column. Measurements of oxygen and nitrate uptake by these sediments are not strongly dependent on the rate of stirring within the lander flux chambers.

Intermediate water ventilation on the Northeastern Pacific Margin during the Late Pleistocene inferred from benthic foraminiferal δ13C

Late Pleistocene intermediate water ventilation history in the northeastern Pacific has been inferred from benthic foraminiferal carbon isotopic data from seven California margin basins. Secular variations in oceanic δ13C recorded at North Pacific ODP Site 849 were subtracted from each basin record leaving a residual δ13C history that reflects variations in ventilation. During the previous interglacial intermediate waters above 2000m contained less oxygen than they do today or Pacific deep water at Site 849 was better ventilated. Intermediate water ventilation began to improve during oxygen isotope stage 4 and continued to improve throughout stages 3 and 2. This enhanced ventilation was not contemporaneous at all depths and appears to have progressed upwards through the water column. The diachronous nature of these changes suggest that there was not simply an “on”/”off” mechanism which resulted in higher or lower ventilation in the North Pacific during the last glacial cycle.

Benthic fluxes and nutrient budgets for sediments in the Northern Adriatic Sea: burial and recycling efficiencies

The Northern Adriatic Sea, between Italy and Yugoslavia, has been plagued by problems of eutrophication. Anthropogenic nutrient loading in rivers (the Po River being the largest) entering the northern Adriatic has increased nutrient input to this system and stimulated algal growth. This area is relatively shallow (maximum depth=60 m), but becomes stratified during the summer months, inhibiting oxygen transport to bottom waters

Radon fluxes measured with the MANOP Bottom Lander

Abstract At five Pacific Ocean sites, radon fluxes were determined from water samples collected by the MANOP Lander, from measurements of 222 Rn and 226 Ra concentrations in Lander-collected box core sediments, and from measurements of excess radon in the water column. At MANOP sites H and M, fluxes (all in atoms m −2 s −1 ) determined with Lander water samples (2200 and 1540 ± 480) agree within the measurement uncertainty with water column standing crop measurements (2220 ± 450, 2040 ± 470). At MANOP site C, the diffusive flux calculated from measurements of 226 Ra in box core sediments (550 ± 20), the integrated deficiency of 222 Rn in the sediments (720 ± 90), and the water column standing crop (500 ± 160) are in agreement, but all are about twice as large as the single Lander water measurement of the radon flux (330). At MANOP site S radon fluxes from measurements of Lander water (3000 ± 260) are in agreement with the predicted diffusive flux from site S sediments (2880), and both fluxes are close to the lower end of the range of water column standing crop measurements (3000–5170). In San Clemente Basin, California, the Lander water flux measurements at four different sites vary by a factor of 3 due to variability in the sediment radium distribution, but the average (1030 ± 190) is close to the water column standing crop value (780 ± 230). Because there is excellent agreement between the fluxes measured with Lander water samples and the predicted diffusive fluxes in most cases, diffusion must be the primary process controlling benthic exchange of radon at the sites studied. The agreement between the Lander water flux estimates and the water column standing crop estimates indicates that the MANOP Lander functions as an accurate benthic flux chamber in water depths ranging from 1900 to 4900 m. In San Clemente Basin, surficial sediments are enriched in manganese and radium, due to manganese cycling near the sediment-water interface. Molecular diffusion of radon from this radium-rich surficial layer is sufficient to obscure any effect of macrofaunal irrigation on the standing crop of radon in this basin.

Germanium geochemistry in the Southern California Borderlands

Abstract Concentrations of Ge(OH) 4 (denoted as Ge i , or inorganic germanium) and Si(OH) 4 were measured in water column and pore water samples from San Pedro and San Nicolas Basins in the Southern California Borderlands. There is a characteristic linear relationship between Ge i and Si in the water column which implies that the distributions of germanium and silica are controlled by similar processes. In contrast, the pore water chemistry of germanium appears to differ from that of silica, particularly in the reducing, organic-rich sediments of San Pedro Basin. Silica concentrations in San Pedro Basin pore water asymptotically increase to 480 μM at 35–40 cm depth. Ge i concentrations in the topmost 2 cm of pore water from San Pedro Basin are an order of magnitude greater than bottom water concentrations. Below 2 cm, Ge i concentrations decrease exponentially with a half-distance of about 8 cm. Cycling of ferric oxyhydroxides at a redox boundary is the most likely cause of the Ge i enrichment in San Pedro Basin pore water, while authigenic pyrite formation with the concomitant inclusion of germanium is a probable, but undocumented, cause of the downcore decrease in Ge i . Pore water Ge i and silica concentrations in sediments of San Nicolas Basin are generally greater than bottom water concentrations but vary irregularly with depth, probably due to irrigation.