Richard A. Jahnke

The global ocean flux of particulate organic carbon: Areal distribution and magnitude

The magnitude and distribution of the particulate organic carbon (POC) rain rate to the seafloor in the Atlantic, Pacific and Indian Ocean basins between 61°N and 61°S has been estimated from benthic oxygen flux estimates (for water depths ≥ 1000 m only). The calculation utilizes the extensive data sets of sedimentary organic carbon, CaCO3, and accumulation rate to extrapolate between individual benthic flux measurement sites using an empirically-derived correlation between the seafloor oxygen flux and these parameters. The POC flux through the 1000 m depth horizon was then estimated from published correlations between sediment trap-determined fluxes and water depth. Total oxygen utilization in the deep ocean is estimated to be 1.2×1014 mol O2 yr−1, a value that agrees well with previous estimates which were based on surface water primary productivity, sediment trap, and depth relationships and with deep water respiration rates estimated from apparant oxygen utilization (AOU)-14C relationships. On the basis of the derived global ocean flux distribution, it is concluded that (1) dissolved organic carbon (DOC) inputs are not required to account for estimated deep water respiration rates; (2) the majority of the POC input to the deep ocean occurs within 30° of the equator; (3) the proportion of primary production that reaches the deep sea does not vary greatly with latitude; (4) gyre and continental margin regions contribute roughly equally to the deep POC flux with a relatively minor contribution from the equatorial divergence region; (5) of the estimated 7.2×1013 mol C yr−1 of POC that sinks below the 1000 m depth horizon, 45% (3.3×1013 mol C yr−1) reaches the seafloor where it is oxidized; (6) when normalized to basin area, average deep flux rates in the Atlantic and Pacific are similar while highest rates are observed in the Indian Ocean; and (7) the results can be fully reconciled only if the benthic flux of DOC is significantly less than the benthic O2 flux.

EVIDENCE FOR ENHANCED PHOSPHORUS REGENERATION FROM MARINE SEDIMENTS OVERLAIN BY OXYGEN DEPLETED WATERS

Phosphorus regeneration and burial fluxes determined from in situ benthic flux chamber and solid phase measurements at sites on the Californian continental margin, Peruvian continental slope, North Carolina continental slope, and from the Santa Monica Basin, California are reported. Comparison of these sites indicates that O2-depleted bottomwaters enhance P regeneration from sediments, diminishing overall phosphorus burial efficiency. Based on these observations, a positive feedback linking ocean anoxia, enhanced benthic phosphorus regeneration, and marine productivity is proposed. On shorter timescales, these results also suggest that O2 depletion in coastal regions caused by eutrophication may enhance P regeneration from sediments, thereby providing additional P necessary for increased biological productivity.

Early diagenesis of organic matter in Peru continental margin sediments: Phosphorite precipitation

Abstract Pore water chemistry (total dissolved CO 2 , NH 4 , NO 3 , NO 2 , PO 4 , Si(OH) 4 , Ca, Mg, Fe, Mn, SO 4 , H 2 S and F, and titration alkalinity), solid phase chemistry (C org , P org , C TOT , N TOT , F, Si OPAL and S II ), and sediment characteristics (porosity, dry bulk density and formation factors) were determined on a centimeter-scale spacing in the upper 20–40 cm of sediments under intense upwelling areas on the Peru continental shelf. These data demonstrate that carbonate fluorapatite (CFA) is precipitating from pore waters in the upper few centimeters of a gelatinous mud with high organic carbon content (up to 20% C org ), very high porosity (> 0.96 ml cm −3 ) and very low dry bulk density ( −3 ). Dissolved phosphate concentrations at the sediment-water interface range from 20 to 100 μ M , orders of magnitude higher than bottom-water concentrations, and much higher than predicted from regeneration of organic matter. The mechanism of this interfacial phosphate release is unclear, but is apparently uncoupled from carbon and nitrogen metabolism and thus may be linked either to dissolution of fish debris or to the presence of a microbial mat in surficial sediments. Fluoride is incorporated into CFA by diffusion from the overlying seawater, and carbonate ions are provided from pore-water alkalinity. Magnesium concentrations in this reaction zone are not significantly different from those of seawater, suggesting that magnesium depletion is not a necessary prerequisite for CFA precipitation. The environment of precipitation is interface-linked rather than driven by organic diagenesis of phosphorus deeper in the sediment. Most of the cores display a wide range of diagenetic characteristics below the immediate interfacial region, but almost all show the precipitation signature near the interface. This interface-linked early diagenetic porewater environment for the precipitation of CFA explains many of the geochemical characteristics of phosphorites and provides a “testable” model to compare the modern phosphogenic analog with ancient phosphorite deposits. Two of the cores display very high solid phase phosphorus and fluoride contents reflecting the presence of apparently modern pelletal apatites.

Carbon fluxes and burial rates over the continental slope and rise off Central California with implications for the global carbon cycle

The present invention relates to brown cigarette paper having reduced gas phase constituents during pyrolysis wherein the paper which has been stained with humic acid or salts thereof is further treated by washing with water in an amount effective to reduce the amount of water-soluble alkali metal salts present in the paper.

Influence of water-column anoxia on the elemental fractionation of carbon and phosphorus during sediment diagenesis

Abstract Organic carbon oxidation and benthic total phosphorus flux measurements from sites covering a range of bottom-water oxygen concentrations have been compiled. The data show that P regeneration relative to carbon oxidation is much more extensive in sediments overlain by oxygen-depleted waters. These results are consistent with studies showing that low oxygen bottom-water concentrations correlate with reduced total phosphorus burial efficiencies and enhanced organic carbon burial efficiencies. Based on these observations a positive feedback is proposed between water-column anoxia, enhanced benthic phosphorus regeneration, and marine productivity. This feedback may help explain the widespread accumulation of organic-rich marine sediments from anoxic waters observed in the geologic record.

Benthic flux of biogenic elements on the Southeastern US continental shelf: influence of pore water advective transport and benthic microalgae

In situ, paired light and dark benthic #ux chamber incubations were used to estimate the exchange of nutrients, oxygen and inorganic carbon across the sediment } water interface of the South Atlantic Bight (SAB) continental shelf. The results indicate that physically forced non-di!usive pore water transport and benthic primary production (BPP) by sea #oor microalgae exert a major in#uence on benthic exchange rates on the mid- and outer-continental shelf (depths of 14}40 m). Light #uxes to the sea #oor and sediment photosynthetic pigment distributions determined on two, widely spaced cross-shelf transects suggest that BPP may occur over 84% of the SAB continental shelf area. Microalgal gross BPP rates at all study sites averaged 400

The influence of organic matter diagenesis on CaCO3 dissolution at the deep-sea floor

260 mg C m~2 d~1 between May and September 1996 while water column primary productivity averaged 682

BENTHIC FLUXES AND POREWATER CONCENTRATION PROFILES OF DISSOLVED ORGANIC CARBON IN SEDIMENTS FROM THE NORTH CAROLINA CONTINENTAL SLOPE

176 mg C m~2 d~1, implying a total primary productivity for this region of approximately 1100 mg C m~2 d~1 (1.6 times the water column productivity alone). The results are also consistent with the advective transport of pore waters. Benthic #ux chambers appear to retard this exchange, a!ecting the accuracy of derived net #uxes. Given our inability to relate pore water gradients to #uxes in non-di!usive regimes and to mimic natural advective transport in intact core incubations, traditional techniques such as pore water gradient di!usion calculations or shipboard core incubations also may not provide accurate #ux estimates. Because of these limitations, fundamental questions remain concerning the

CaCO3 dissolution in California continental margin sediments: The influence of organic matter remineralization

Abstract In situ benthic flux chamber measurements were performed at seafloor locations in the Atlantic and Pacific oceans where the bottomwaters were supersaturated, approximately saturated, and undersaturated with respect to calcite. There was no evidence of significant dissolution at or above the saturation horizon. Below the saturation horizon, however, significant benthic fluxes of titration alkalinity and calcium out of the sediments were observed indicating appreciable dissolution. Where observed, the rate of dissolution is consistent with a rate expression of reaction order 4.5 and a rate constant in the range of 0.05 to 0.5% per day. A numerical simulation was constructed to represent organic matter and CaCO3 diagenesis at the seafloor. At the supersaturated and near-saturation sites, the model over-predicts the influence of metabolically produced CO2 on the extent of CaCO3 dissolution. It is possible that these results can be reconciled by a better understanding and parameterization of organic matter mineralization and porewater transport processes in surface sediments. The primary factors to be considered include ( 1 ) the oxidation state of the carbon being oxidized, (2) the vertical distribution of oxidation rates including the role of DOC in sediment remineralization, (3) the influence of nondiffusive transport of near-surface porewaters, and (4) the influence of episodic depositional events on organic and inorganic carbon cycling.

Benthic recycling of biogenic debris in the eastern tropical Atlantic Ocean

Abstract Numerous studies of marine environments show that dissolved organic carbon (DOC) concentrations in sediments are typically tenfold higher than in the overlying water. Large concentration gradients near the sediment–water interface suggest that there may be a significant flux of organic carbon from sediments to the water column. Furthermore, accumulation of DOC in the porewater may influence the burial and preservation of organic matter by promoting geopolymerization and/or adsorption reactions. We measured DOC concentration profiles (for porewater collected by centrifugation and “sipping”) and benthic fluxes (with in situ and shipboard chambers) at two sites on the North Carolina continental slope to better understand the controls on porewater DOC concentrations and quantify sediment–water exchange rates. We also measured a suite of sediment properties (e.g., sediment accumulation and bioturbation rates, organic carbon content, and mineral surface area) that allow us to examine the relationship between porewater DOC concentrations and organic carbon preservation. Sediment depth-distributions of DOC from a downslope transect (300–1000 m water depth) follow a trend consistent with other porewater constituents (ΣCO2 and SO42−) and a tracer of modern, fine-grained sediment (fallout Pu), suggesting that DOC levels are regulated by organic matter remineralization. However, remineralization rates appear to be relatively uniform across the sediment transect. A simple diagenetic model illustrates that variations in DOC profiles at this site may be due to differences in the depth of the active remineralization zone, which in turn is largely controlled by the intensity of bioturbation. Comparison of porewater DOC concentrations, organic carbon burial efficiency, and organic matter sorption suggest that DOC levels are not a major factor in promoting organic matter preservation or loading on grain surfaces. The DOC benthic fluxes are difficult to detect, but suggest that only 2% of the dissolved organic carbon escapes remineralization in the sediments by transport across the sediment-water interface.