Ellery D. Ingall

Benthic phosphorus regeneration, net primary production, and ocean anoxia: A model of the coupled marine biogeochemical cycles of carbon and phosphorus

We examine the relationships between ocean ventilation, primary production, water column anoxia, and benthic regeneration of phosphorus using a mass balance model of the coupled marine biogeochemical cycles of carbon (C) and phosphorus (P). The elemental cycles are coupled via the Redfield C/P ratio of marine phytoplankton and the C/P ratio of organic matter preserved in marine sediments. The model assumes that on geologic timescales, net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone. The model incorporates the dependence on bottom water oxygenation of the regeneration of nutrient phosphorus from particulate matter deposited at the water-sediment interface. Evidence from marine and lacustrine settings, modern and ancient, demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia-dysoxia expands. Steady state simulations show that a reduction in the rate of thermohaline circulation, or a decrease of the oxygen content of downwelling water masses, intensifies water column anoxia-dysoxia and at the same time increases surface water productivity. The first effect reflects the declining supply of oxygen to the deeper parts of the ocean. The second effect is caused by the enhanced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides. Sedimentary burial of organic carbon and authigenic calcium phosphate mineral (francolite), on the other hand, is promoted by reduced ocean ventilation. According to the model, global-scale anoxia-dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system. Consequently, higher rates of primary production and organic carbon burial can be achieved, even when the continental supply of reactive phosphorus to the oceans remains unchanged.

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.

Influence of water column anoxia on the burial and preservation of carbon and phosphorus in marine shales

Abstract Organic P and organic C concentrations were measured in several well-characterized Phanerozoic marine shale sequences with the primary focus being the Camp Run Member of the Devonian-Mississippian New Albany Shale. Sequences were selected with close spatial association of bioturbated and laminated sediments which reflect deposition from oxic and anoxic waters, respectively. Average organic C/P mole ratios calculated from the data are 150 for bioturbated shales and 3900 for the laminated shales of the New Albany. Differences in the extent and mechanisms of early diagenesis related to the oxygenation of waters at the sediment-water interface can account for the C/P ratios of buried organic matter. Low C/P ratios of bioturbated sediments are attributed to 1. (1) the increased ability of bacteria to store P in well-oxygenated environments which leads to the production of low C/P ratio bacterial biomass and 2. (2) the extensive oxidation of sedimentary organic matter resulting in the formation of residual organic phases with low C/P ratios. High organic C/P ratios of laminated sediments are explained by a combination of 1. (1) the limited ability of bacteria to store P under anoxic conditions, 2. (2) extensive P regeneration from sedimentary organic matter, 3. (3) enhanced C preservation relative to the bioturbated shales. It is also shown that relative to organic C, laminated shales are not as effective a P sink as are bioturbated shales. This provides a mechanism in anoxic environments for burying large quantities of organic matter without simultaneously sequestering the P needed to sustain further productivity.

Redox Stabilization of the Atmosphere and Oceans by Phosphorus-Limited Marine Productivity

Data from modern and ancient marine sediments demonstrate that burial of the limiting nutrient phosphorus is less efficient when bottom waters are low in oxygen. Mass-balance calculations using a coupled model of the biogeochemical cycles of carbon, phosphorus, oxygen, and iron indicate that the redox dependence of phosphorus burial in the oceans provides a powerful forcing mechanism for balancing production and consumption of atmospheric oxygen over geologic time. The oxygen-phosphorus coupling further guards against runaway ocean anoxia. Phosphorus-mediated redox stabilization of the atmosphere and oceans may have been crucial to the radiation of higher life forms during the Phanerozoic.

Marine Phosphorus is Selectively Remineralized

Phosphorus is a vital nutrient of the worlds oceans,, where in vast regions it is associated with dissolved organic matter (DOM) in surface waters,. We have characterized the major compound classes of high-molecular-weight marine dissolved organic phosphorus, phosphorus esters and phosphonates, by using tangential-flow ultrafiltration and phosphorus-31 nuclear magnetic resonance (31P NMR). We find that the composition and abundance of organic phosphorus in DOM differ significantly from the values in fresh organic matter, indicating that dissolved organic phosphorus (DOP) is preferentially remineralized from DOM.

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.

Marine Polyphosphate: A Key Player in Geologic Phosphorus Sequestration

The in situ or authigenic formation of calcium phosphate minerals in marine sediments is a major sink for the vital nutrient phosphorus. However, because typical sediment chemistry is not kinetically conducive to the precipitation of these minerals, the mechanism behind their formation has remained a fundamental mystery. Here, we present evidence from high-sensitivity x-ray and electrodialysis techniques to describe a mechanism by which abundant diatom-derived polyphosphates play a critical role in the formation of calcium phosphate minerals in marine sediments. This mechanism can explain the puzzlingly dispersed distribution of calcium phosphate minerals observed in marine sediments worldwide.

Influence of water column anoxia and sediment supply on the burial and preservation of organic carbon in marine shales

Abstract Previous work has suggested that the laminated, organic-rich and bioturbated, organic-poor shales of the Camp Run Member of the Late Devonian-Early Mississippian New Albany Shale formed under anoxic and oxygenated bottomwater conditions, respectively, and that the interbedding of the two facies was due to the vertical oscillation of a water-column anoxic/oxic boundary where it impinged on the basin margin. We have extended this analysis by examining the chemical and mineralogical differences between the two shale facies in a single borehole core, by seeking evidence for deposition of the laminated shales under bottom-water oxia or anoxia, and by determining whether the laminated shales formed when the carbon supply to the sea floor was higher. The results of this study show that the laminated and bioturbated shales are mineralogically and chemically distinct; relative to Al, an index of the aluminosilicate content, Si, Ti, Fe, P, Na, Ba, Co, Cr, Cu, Mo, Ni, V, Zn, and Zr are all higher, whereas Mn, Ca, Mg, and Sr are lower in the laminated compared with the bioturbated shales. The differences are due to a higher quartz, feldspar, titanite/ilmenite, and zircon content in the laminated shales, probably indicating a coarser grain-size, and the greater abundance of manganoan calcite in the bioturbated shales. Dissolved oxygen was present in bottom waters during the deposition of some of the laminated shale intervals because of the presence of manganoan calcite, a phase that can only form in sediments with an oxic surface. In addition, the organic matter preserved in the two shale types is isotopically different; δ 13Corganic values are 1.9z.permil; lighter on average in the laminated compared with bioturbated intervals, possibly indicating a larger fraction of terrestrial organic matter in the latter. δ15N values are 1.9z.permil; lighter on average in laminated compared with bioturbated intervals, possibly indicating a larger fraction of terrestrial organic matter in the latter. δ15N values are 1.9z.permil; lighter on average in laminated compared with bioturbated intervals, suggesting that nutrient drawdown was less during the deposition of the organic-rich, laminated shales. The chemical, mineralogical, and isotopic contrasts between the two shale facies of the Camp Run Member indicate that the conditions of sedimentation were different during their deposition. The difference was possibly related to variations in sea level, which would have caused the Camp Run shoreline to move closer to and farther from the core site, causing, in turn, the deposition of coarser and finer grained sediments that contained different mixtures of marine and terrestrial organic matter. Bottomwater conditions were anoxic during deposition of most laminated intervals. Bottom-water anoxia or dysoxia led to decreased burial and preservation of the essential nutrient phosphorus in the laminated, organic-rich shales relative to the rocks deposited beneath better oxygenated bottomwaters. Increased availability of phosphorus in the water column on long timescales leads to increased productivity and a higher settling flux of organic matter, causing bottom-water oxygen levels to fall. This is consistent with the nitrogen isotope evidence suggesting that production was probably higher during the deposition of the organic-rich shales. Thus, production variations coupled with enhanced sedimentary regeneration of phosphorus from sediments related to low oxygen bottom-water concentrations provide a general mechanism for the formation of the alternating facies of this member of the New Albany Shale.

The Nature of Phosphorus Burial in Modern Marine Sediments

Phosphorus is a key element in biogeochemical cycles because of its role as an essential nutrient. Because of the ability of certain marine organisms, such as cyanobacteria, to fix nitrogen, it has been normally assumed that the long term limiting factor in global oceanic productivity is phosphorus (e.g. Holland, 1978). Thus, a knowledge of phosphorus chemistry in the ocean is a key to a better understanding of the cycling of carbon, nitrogen, sulfur, and other bio-elements. Over shorter time scales, days to millenia, the cycle of phosphorus in the ocean is controlled by a combination of processes involving oceanic circulation, biosynthesis, sinking of particles, and bacterial regeneration of the particles at depth or on the sea floor (e.g. Berger et al., 1989). Consequently, concentrations of dissolved phosphorus in seawater vary from place to place as a result of the interaction of these processes. In this context sediments are important only as their surficial portions release additional dissolved phosphate to the overlying water. By contrast, sediments become much more important on longer time scales of tens of thousands to millions of years, because they are the ultimate repository for the removal of phosphate from the oceans. The overall level of phosphorus in the ocean, and therefore global biological productivity, is controlled by the long-term balance between input via rivers and output via burial in sediments.

Polyphosphates as a source of enhanced P fluxes in marine sediments overlain by anoxic waters: Evidence from 31P NMR

Sedimentary phosphorus (P) composition was investigated in Effingham Inlet, a fjord located on the west coast of Vancouver Island in Barkley Sound. Solid-state 31P nuclear magnetic resonance (NMR) spectroscopy was applied to demineralized sediment samples from sites overlain by oxic and anoxic bottom waters. The two sites were similar in terms of key diagenetic parameters, including the mass accumulation rate, integrated sulfate reduction rate, and bulk sediment organic carbon content. In contrast, P benthic fluxes were much higher at the anoxic site. 31P NMR results show that P esters and phosphonates are the major organic P species present at the surface and at depth in sediments at both sites. Polyphosphates were only found in the surface sediment of the site overlain by oxic waters. The varying stability of polyphosphates in microorganisms under different redox conditions may, in part, explain their distribution as well as differences in P flux between the two sites.