David T. Heggie

Organic carbon dynamics and preservation in deep-sea sediments

Abstract Three methods of estimating the particulate organic carbon fluxes to the sediment-water interface of the deep Pacific Ocean agree to within the error of the measurements at MANOP sites M, H, and C. Sediment trap experiments, pore water results, and surface sediment organic carbon data suggest that a major fraction of the particulate organic carbon raining to abyssal depths at these locations is degraded within the surface sediments rather than at the sediment-water interface or in the nephloid layer. Organic carbon rain rates at the three sites are similar—within a factor of two; however, the preservation rate of organic carbon and the chemistry of sediment pore waters are very different. A model developed to describe the pore water oxygen and sedimentary carbon distributions indicates model developed to describe the pore water oxygen and sedimentary carbon distributions indicates that changes in the rate constant for organic matter degradation and the bioturbation rate may contribute significantly to the observed differences in character of both pore water and sediment chemistry at these locations. The implication with respect to interpreting the sedimentary record is that cycles of organic carbon and redox sensitive metals (i.e., manganese) are not simply related to particulate organic carbon flux or surface water primary productivity. The residence time of organic carbon with respect to degradation in the surface sediments is on the order of 15 to 150 y.

Fate of organic carbon reaching the deep sea floor: a status report☆

The rates of organic carbon oxidation by O2, NO3−, MnO2, Fe2O3 and SO4− have been calculated for five pelagic Pacific and Atlantic sites using simple diffusion-reaction models. O2 everywhere oxidizes > 90% of the raining Corg; the fraction oxidized by the secondary oxidants decreases as the rain rate of organic C to the seafloor decreases. A large fraction of the Corg escaping oxidation by O2 is oxidized by the secondary oxidants. Hence while these oxidants play a small role in remineralization, they are important in regulating the burial of organic matter and the consequent removal from the oceans of reduced carbon and nutrients.

Metal diagenesis in oxic marine sediments

The metal-nutrient relationships observed for nickel and cadmium in the deep ocean are continued at the interface between seawater and oxidizing pore water. This continuum results in pore water concentrations of these metals which are only slightly greater than near-bottom seawater levels. Manganese concentrations in these oxidizing pore waters are also extremely low, less than three times bottom water. In contrast, release in the boundary layer produces a maximum of dissolved copper which is 10–40 times ambient seawater. Assuming these pore waters are at steady state, flux estimates based on these measurements suggest that the manganese in todorokite-rich nodules of the central equatorial Pacific was not supplied by upward diffusion through pore waters below the interface. Most nodular nickel is precipitated with manganese while nodular copper is supplied by diffusion.

Pore waters of the central Pacific Ocean: Nutrient results

Interstitial waters from the MANOP siliceous and calcareous ooze sites in the central Pacific Ocean were recovered by shipboard squeezing and centrifuging techniques and an in situ harpoon sampler. The pore waters were analyzed for nutrients, carbonate system species, Mn 2+ , Ca 2+ , total alkaline earth metals and dissolved oxygen (siliceous ooze site only). In calcareous sediments, the ΣCO 2 , alkalinity, PO 4 3− , and Ca 2+ results obtained from shipboard techniques are significantly different than those recovered in situ. The other species measured as well as all of the data from the siliceous ooze site, reveal no significant dependence on the method used to sample the pore water. The relative changes in O 2 , NO 3 − , Mn 2+ , alkalinity, and ΣCO 2 indicate that respiration using O 2 , NO 3 − , and MnO 2 as electron acceptors is the dominant reaction affecting the pore water species. We develop a model which simulates the fate of pore water oxygen and nitrate and particulate organic carbon in sediments. This model includes the effects of pore water diffusion, oxygen and nitrate respiration, sediment accumulation, sediment mixing, and changes in sediment porosity. We observe that factor of 2 variations in the rates of sediment mixing and accumulation have only a minor effect on the pore water profiles. The major factor controlling the pore water profiles is the flux of organic carbon across the sediment-water interface. We conclude that the large intra-site variability observed in the interstitial water nitrate profiles is probably caused by small-scale variations in the flux of organic material.

Organic carbon oxidation and benthic nitrogen and silica dynamics in San Clemente Basin, a continental borderland site

Organic carbon oxidation rates in San Clemente Basin were determined by benthic chamber experiments using the Bottom Lander, along with studies of pore water chemistry. Non-steady-state diagenetic models are developed for interpreting concentration-time data from the benthic chamber experiments. O2, NO3−, and SO42− are all important oxidants for organic carbon at our study site. Regenerated fixed nitrogen was consumed by NO3− reduction. There is a flux of NO3− into the sediments, and the benthic flux of NH4+ is undetectable. The total rate at which fixed nitrogen is removed from the oceans at this site is about twice the flux of PON to the sea floor. SiO2 fluxes calculated from interfacial pore water gradients are in satisfactory agreement with those determined using the Lander. Most silica dissolution must therefore occur within the sediments, although interstitial profiles show that little dissolution occurs below 1 cm depth.

Manganese and copper fluxes from continental margin sediments

Total dissolvable Cu and Mn have been measured in seawaters collected from the continental shelf of the eastern Bering Sea. Copper concentrations of 4 nmole kg−1 inshore of a hydrographie front over the 100 m isobath. Manganese concentrations also were low over the shelf break, 10 nmole kg−1 inshore of the hydrographic front. Depth distributions of Mn at all continental shelf stations showed gradients into the sediments, with concentrations typically >20 nmole kg−1 in a bottom layer extending about 30 m off the bottom. Benthic Cu and Mn fluxes are indicated by cross-shelf pore water profiles that show interfacial concentrations more than an order of magnitude greater than in bottom water. These data and the results of a model of metal transport across the shelf suggest that Cu and Mn fluxes, estimated at 2 and 18 nmole cm−2y−1, respectively, from continental shelf sediments may be one “source” of these metals to the deep sea.

Organic carbon cycling and modern phosphorite formation on the East Australian continental margin: an overview

Abstract During 1987, the Australian Bureau of Mineral Resources conducted a multidisciplinary investigation of the modern phosphorites on the continental margin of southeastern Australia between 28 and 32°S. The objectives of the work were to examine the processes controlling the cycling of organic carbon and bioactive elements, nitrogen, phosphorus, sulphur and iron in the sediments, and to investigate the roles which these processes played in the formation of the modern phosphorites. Bacterial productivities, sulphate-reduction rates, sedimentary oxygen and pore-water concentrations of nitrate, ammonia, phosphate, iron, sulphate and fluoride were measured at sea. The highest rates of microbial productivity were found in the surficial (0–20 mm) sediments of the modern phosphorite zone in 350–460 m water depth. These rates were about double those in shallower shelf (<300 m) sediments and 3–4 fold those rates in mid-slope (600–1000 m) sediments. Aerobic and anaerobic oxidation rates of organic matter, calculated from sediment oxygen profiles and sulphate-reduction rates were highest in the surface sediments in the modern phosphorite zone. The recycling of sedimentary iron, via reductive dissolution of iron oxyhydroxides and reprecipitation at the oxic/anoxic boundary results in a near-surface sedimentary trap for iron in the phosphorite zone sediments. Phosphate released from organic matter in the interfacial sediments, and fluoride from seawater, are scavenged by iron oxyhydroxides in the top few centimetres of sediment. Phosphorus, in this way, is decoupled from organic carbon in the near-surface sediments and linked to the redox cycling of iron. Phosphate and fluoride scavenged onto iron oxyhydroxides, and concentrated in the surficial sediments, are subsequently released to pore waters in the anoxic sediments when iron oxyhydroxides are buried and dissolve. The recycling process releases phosphate and fluoride for incorporation into apatite; fluoride is depleted from pore waters at depths <18 cm, phosphorite nodules form within anoxic sediments at depths <18 cm and continue to accumulate iron and phosphorus while resident in the mixed layer. Combinations of rapid sediment mixing rates, a slow sedimentation rate and a mixed layer to about 18 cm result in an average particle residence time in the phosphorite zone sediments which is about ten-fold that of the mid-slope sediments. Long residence times and rapid mixing promote the oxidation of organic carbon and release of phosphate, while the continuous recycling of iron and phosphate concentrates the phosphorus for apatite precipitation and accumulation into phosphorite nodules. Phosphorite nodules are not found in mid-slope sediments probably because of combinations of relatively rapid sedimentation rates, ineffective iron, phosphorus and fluoride recycling and trapping mechanisms, plus dilution and dissemination of any incipient apatite.

Sedimentation dynamics and redox iron-cycling : controlling factors for the apatite-glauconite association on the East Australian continental margin.

Abstract Detailed sedimentological and geochemical studies of phosphorites and sediments from the East Australian continental margin have shown that both apatite and glauconite are forming at a transition zone between relict, iron oxyhydroxide-rich, organic-poor (TOC<0.3%) outer shelf (200–350 m) sediments and relatively rapidly accumulating, iron oxyhydroxide-deficient, organic-rich (TOC>0.8%) deep water (460–650 m) sediments. The interaction between sediment mixing and Fe-P cycling processes (between the pore waters and the solid phase) appear critical to the formation of modern phosphorites in this area. The phosphate nodules form within the anoxic zone in the sediments at depths of approximately 10–18 cm below the sediment-seawater interface. Nodules which remain in the sediment mixed layer after they form continue to accumulate both P and Fe for up to 60 ka; during this time their apatite and iron oxyhydroxide contents more than double and the nodules become denser and more lithified. Apatite and glauconite formation are favoured by periods of high sea-level and low current velocities, as these conditions allow a relatively high organic carbon input to the sediments and thereby the maintenance of anoxia at shallow depths within the sediments. During periods of low sea-level and high current velocities, the carbon flux into the sediments decreases and the sediments become oxic. Consequently the Fe-cycling processes cease and apatite and glauconite formation stops: the glauconite is progressively transformed to goethite, and phosphorite nodules are concentrated into lag deposits and ferruginized. Alternations of high and low sea-level cycles eventually result in the formation of the massive ferruginous Neogene phosphorites that mantle much of the outer shelf. The iron enrichment processes observed in the modern to Neogene phosphorites on the East Australian continental margin provide explanations for many of the features seen in ferruginous Neogene deposits in the world’s oceans.

Trace metals in metalliferous sediments, MANOP Site M: interfacial pore water profiles

We present data showing enrichments of metals Mn, Cu, V, Cr, Cd and Ni and dissolved organic matter (DOM) in surficial pore waters, depths < 1 cm, at a metalliferous sediment site in the Pacific. We also present a model of metal release during particulate organic carbon (POC) oxidation and opal dissolution at the sediment-seawater interface. A comparison of measured interfacial metal concentrations with those predicted from the model suggest that Cd, Ni and Cr are released during oxidation of POC at the sediment-seawater interface. However, for V, Cu and Mn additional ‘source(s)’ and/or processes are required to account for the measured interfacial concentrations. For Cu, recycling via interfacial remineralization and release to bottom waters, with subsequent scavenging out of bottom waters by particulates, probably provides the major additional ‘source’. For V, we suggest that, in addition to some contribution from a benthic flux-bottom water scavenging scenario, complexation with DOM at the interface serves as a trap to accumulate V released from particulates in the zone of oxygen reduction. For manganese, we suggest that most of the interfacial pore water content is produced from reduction of sedimentary Mn(IV) by reactions with DOM at the sediment-seawater interface. The calculated benthic flux of manganese as a result of this reaction may account for a large fraction of the particulate manganese rain that is apparently not preserved in the sediment at this site.

Copper in Sub-Arctic Waters of the Pacific Northwest

Copper concentration data from the north-central continental shelf-slope of the Bering Sea and nearshore regions of the western Gulf of Alaska are summarized. Copper concentrations which increase toward the sediments were found in a frontally confined continental shelf water mass of the Bering Sea, and also in deep waters of restricted circulation of a fjord on the north-western Gulf of Alaska coastline. These data are indicative of a benthic flux of copper to overlying waters. Mass balance estimates suggest that the benthic remineralization and flux to the overlying waters in the shallow shelf of the Bering Sea contributes significantly to the copper input to the deep Bering basin. However, in the fjord basin of the Gulf of Alaska, remineralized copper from inshore surface sediments was estimated at less than 20% of copper added to surface waters. Also, most copper input could be accounted for trapped in accumulating fjord sediments.