David J. Burdige

The biogeochemistry of manganese and iron reduction in marine sediments

Manganese and iron reduction in marine sediments are known to play important roles in the biogeochemical cycles of many elements, including carbon, sulfur, phosphorus and several trace elements. These reduction reactions affect these cycles on a variety of time scales, ranging from those as short as seasonal time scales (e.g., nutrient cycling in coastal ecosystems), to those as long as thousands to tens of thousands of years (e.g., glacial-interglacial transformations in deep sea sediments). In this review article I will briefly summarize the results of laboratory studies on the types of manganese and iron reduction that are known to occur in marine sediments, and then discuss the occurrence of these processes in different sedimentary environments. Particular efforts will be given to examining the rates and mechanisms of sedimentary manganese and iron reduction in relationship to other biogeochemical processes in sediments.

Geochemistry of barium in marine sediments : Implications for its use as a paleoproxy

Abstract Variations in the accumulation rate of barium in marine sediments are thought to be indicative of variations in marine biological productivity through time. However, the use of Ba as a proxy for paleoproductivity is partly dependent upon its being preserved in the sediment record in a predictable or consistent fashion. Arguments in favor of high Ba preservation are partly based on the assumption that sediment porewaters are generally at saturation with respect to pure barite. The idea is that because nondetrital sedimentary Ba predominantly exists as barite, porewater saturation would promote burial. We present sediment porewater, sediment solid phase, and benthic incubation chamber data suggesting that solid-phase Ba preservation may be compromised in some geochemical settings. We propose that under suboxic diagenetic conditions, characterized by low bottom water oxygen and high organic carbon respiration rates, Ba preservation may be reduced. Independent of the mechanism, if this assertion is true, then it becomes important to know when the Ba record is unreliable. We present evidence demonstrating that the sedimentary accumulation of authigenic U may serve as a proxy for when the Ba record is unreliable. We then provide an example from the Southern Ocean during the last glacial period where high authigenic U concentrations coincide with high Pa:Th ratios and high accumulation rates of biogenic opal, but we find low accumulation rates of sedimentary Ba. Thus, for the study sites presented here during the last glacial, we conclude that Ba is an unreliable productivity proxy.

Chemical and microbiological studies of sulfide‐mediated manganese reduction 1

Abstract Laboratory studies of manganese reduction by naturally occurring reduced inorganic compounds were undertaken, both to study further possible in situ mechanisms of manganese reduction and to examine how manganese redox reactions might be coupled to other biogeochemical processes. Chemical manganese reduction by sulfide (in the presence of excess manganese oxide) was found to be rapid and complete, with all sulfide being oxidized within 5–10 min. The reduction of δMnO2 by sulfide involves a two‐electron transfer, with S° the predominant oxidized sulfur product. Using a marine sulfate‐reducing bacterium (Desulfovibrio sp.), the kinetics of sulfide‐dependent, bacterially mediated manganese reduction were studied; the rate‐limiting step was bacterial sulfide production. These findings suggest that in stratified marine environments (such as the Black Sea, Saanich Inlet, or certain coastal sediments) manganese reduction should occur just below the oxic‐anoxic (O2/H2S) interface or redox boundary as a re...

Biogenic matter diagenesis on the sea floor: A comparison between two continental margin transects

Benthic chamber measurements of the reactants and products involved with biogenic matter diagenesis (oxygen, ammonium, nitrate, silicate, phosphate, TCOP, alkalinity) were used to define fluxes of these solutes into and out of the sediments off southern and central California. Onshore to offshore transects indicate many similarities in benthic fluxes between these regions. The pattern of benthic organic carbon oxidation as a function of water depth, combined with published sediment trap records, suggest that the supply of organic carbon from vertical rain can just meet the sedimentary carbon oxidation + burial demand for the central California region between the depths 100-3500 m. However, there is not enough organic carbon raining through the upper water column to support its oxidation and burial in the basins off southern California. Lateral transport and focusing of refractory carbon within these basins is proposed to account for the carbon buried. The organic carbon burial efficiency is greater off southern California (40-60%) compared to central California (2-20%), even though carbon rain rates are comparable. Oxygen uptake rates are not sensitive to bottom water oxygen concentrations nor to the bulk wt. % organic carbon in surficial sediments. Nitrate uptake rates are well defined by the depth of oxygen penetration into the sediments and the overlying water column nitrate concentration. Nitrate uptake accounts for about 50% of the total denitrification taking place in shelf sediments and denitrification (0. l-l .O mmolN/m*d) occurs throughout the entire study region. The ratio of carbon oxidized to opal dissolved on the sea floor is constant (0.8 t 0.2) through a wide range of depths, supporting the hypothesis that opal dissolution kinetics may be dominated by a highly reactive phase. Sea floor carbonate dissolution is negligible within the oxygen minimum zone and reaches maximal rates

Fluxes of dissolved organic carbon from Chesapeake Bay sediments

Benthic fluxes of dissolved organic carbon (DOC) were measured over an annual cycle at two contrasting sites in Chesapeake Bay. At an organic-rich, sulfidic site in the mesohaline portion of the Bay (site M) DOC fluxes from the sediments ranged from 1.4 to 2.9 mmol/m2/d. Measured benthic DOC fluxes at site M corresponded to ~3–13% of the depth-integrated benthic C remineralization rates (ΣOCR), and agreed well with calculated diffusive DOC fluxes based on porewater DOC profiles. This agreement suggests that DOC fluxes from site M sediments were likely controlled by molecular diffusion. The second site that was studied is a heavily bioturbated site in the southern Bay (site S). The activity of macrobenthos did not appear to enhance DOC fluxes from these sediments, since measured benthic DOC fluxes (< 0.5 mmol/m2/d) were lower than those at site M. The ratios of benthic DOC fluxes to ΣOCR values at site S were also slightly smaller than those observed at site M. Benthic DOC fluxes from Chesapeake Bay sediments do not appear to significantly affect the transport of DOC through this estuary, although uncertainties in the reactivity of DOC in estuaries makes this conclusion somewhat tentative at this time. However, when these results are used to make a lower limit estimate of the globally integrated benthic DOC flux from marine sediments, a value similar to that previously calculated by Burdige et al. (1992) is obtained. This observation further supports suggestions in this paper about the importance of benthic DOC fluxes in the oceanic C cycle.

A time series of benthic flux measurements from Monterey Bay, CA

In situ incubation chamber measurements of benthic nutrient recycling rates were made on the Monterey Bay shelf at 100 m during various years and seasons. Variability in nutrient (Si, PO4 ,N H 3 ,N O 3 ) and trace metal (Mn, Fe (II), Cu) fluxes correlate with variability in the amount of organic carbon oxidized on the sea floor. Patterns of primary productivity show a mid-year maxima, consistent with the timing of increased rates of benthic Corg and opal recycling. High rates of Corg rain to the shelf promote nitrate consumption at a rate that equals or exceeds ammonia efflux. Low rates of Corg rain promote greater effluxof DIN; thus these margin sediments provide a negative feedback to local productivity cycles. The effluxof iron (II) from shelf sediments is sufficient to support >100% of new production, yet Fe fluxis positively correlated with C org recycling which lags the maximum in new production. On account of this time lag, diagenetically recycled Fe is not likely a micro-nutrient trigger of new production, but could serve as a positive feedback. Bio-irrigation rates are seasonally variable by 30% but maximal during the maximum productivity months. r 2003 Elsevier Science Ltd. All rights reserved.

Molecular weight distribution of dissolved organic carbon in marine sediment pore waters

Abstract The molecular weight distribution of dissolved organic carbon (DOC) in pore waters from estuarine and continental margin sediments was examined using ultrafiltration techniques. The majority of this pore water DOC (∼60–90%) had a molecular weight less than 3 kDa. This percentage appeared to vary systematically among the different sediments studied and showed very slight changes with depth (upper ∼30 cm). The absolute concentration of this low molecular weight DOC (LMW-DOC) increased, along with total DOC, with depth in the sediments. LMW-DOC therefore represents the vast majority of the DOC that accumulated with depth in these sediment pore waters. These results have been examined in the context of a model which assumes that remineralization processes exert the primary influence on the molecular weight distribution of DOC in the upper portions of the sediments. This model, in conjunction, with other recent studies of DOC in sediment pore waters and in the water column, suggests that there was preferential accumulation of refractory LMW-DOC in sediment pore waters. Abiotic condensation reactions (i.e., geopolymerization) appear to have secondary effects on the observed molecular weight distributions of pore water DOC, at least in the upper portions of the sediments examined here. Using this model to explain differences in the molecular weight distributions in these sediments suggests that organic matter remineralization in continental margin sediments may be controlled more by hydrolytic processes than it is in estuarine sediments, where fermentative or perhaps respiratory processes may exert a greater overall control on carbon remineralization. These observations provide further evidence that the extracellular hydrolysis of macromolecular (i.e., high molecular weight) organic matter may not always be the rate limiting step in organic matter degradation.

Fluxes of dissolved organic carbon from California continental margin sediments

Abstract Fluxes of dissolved organic carbon (DOC) from marine sediments represent a poorly constrained component of the oceanic carbon cycle that may affect the concentration and composition of DOC in the ocean. Here we report the first in situ measurements of DOC fluxes from continental margin sediments (water depths ranging from 95 to 3,700 m), and compare these fluxes with measured benthic fluxes from 20 other coastal and continental margin sediments. With this combined data set data we have estimated that benthic DOC fluxes are less than ∼10% of sediment carbon oxidation rates, and that the integrated DOC flux from sediments in water depths less than 2,000 m is ∼180 Tg C/yr. These fluxes are roughly equivalent to the riverine DOC flux, and the organic carbon burial rate in marine sediments. Benthic DOC fluxes therefore represent an important net source of DOC to the oceans. We also note that: (1) benthic DOC fluxes represent a loss of organic carbon from sediments; (2) in many sediments these fluxes appear to be controlled by molecular diffusion (i.e., by pore water concentration gradients); (3) pore water DOC may be an important intermediate in sediment carbon burial and preservation. These observations therefore suggest a linkage between benthic DOC fluxes and sediment carbon preservation that may be mediated by pore water DOC concentrations and cycling. The magnitude and fate of DOC effluxing from marine sediments is thus important to understanding carbon cycles and budgets in the marine environment.

The role of benthic fluxes of dissolved organic carbon in oceanic and sedimentary carbon cycling

Benthic fluxes (sediment-water exchange) of dissolved organic carbon (DOC) represent a poorly quantified component of sedimentary and oceanic carbon cycling. In this paper we use pore water DOC data and direct DOC benthic flux measurements to begin to quantitatively examine this problem. These results suggest that marine sediments represent a significant source of DOC to the oceans, as a lower limit of the globally-integrated benthic DOC flux is comparable in magnitude to riverine inputs of organic carbon to the oceans. Benthic fluxes of DOC also appear to be similar in magnitude to other sedimentary processes such as organic carbon oxidation (remineralization) in surface sediments and organic carbon burial with depth.

Effects of manganese oxide mineralogy on microbial and chemical manganese reduction

Abstract In this study we examine the effects of manganese oxide mineralogy and crystal structure on two types of manganese reduction: microbial manganese reduction coupled to the oxidation of organic matter, and chemical reduction of manganese oxides by Fe2+. Three synthetic manganese minerals were used: δ‐MnO2 (or vernadite), Mg‐bimessite, and pyrolusite. The δ‐MnO2 and Mg‐birnessite are relatively high‐surface‐area amorphous materials, while the pyrolusite is a low‐surface‐area crystalline solid. Studies of microbial manganese reduction were carried out using an enrichment culture isolated from sediments in the mesohaline region of the Chesapeake Bay, and with a pure culture of the organism Shewanella (formerly Alteromonas,) putrefaciens strain MR‐1. With the Chesapeake Bay enrichment culture, identical rates of manganese reduction were observed with equivalent initial suspension concentrations of all three manganese minerals. This occurred in spite of ∼ 200‐fold differences in the available surface ar...