Burial of Terrestrial Organic Matter in Marine Sediments: A Re-Assessment

Calculations based on recent observations indicate that approximately one third of the organic matter presently being buried in marine sediments may be of terrestrial origin, with the majority of this terrestrial organic matter (TOM) burial occurring in muddy, deltaic sediments. These calculations further suggest that the remineralization of terrestrial organic matter in the oceans is also much less efficient than that of marine organic matter. These two underappreciated observations have important implications in terms of our understanding of the controls on the global carbon cycle. From a paleoceanographic perspective, the results presented here also suggest that changes in TOM burial on glacial-interglacial timescales have the potential to impact the global carbon cycle (i.e., atmospheric CO2 levels)

The Kinetics of Organic Matter Mineralization in Anoxic Marine Sediments

The kinetics of sulfate reduction and inorganic nutrient production (ΣCO2, ammonium, and phosphate) were examined in the sediments at five sites in the southern Chesapeake Bay, using long term (\u3e 200 d) sediment decomposition experiments. Average first order rate constants for these processes (at 25oC) decreased from 8.2 to 3.7 yr-1 in the surface sediments (0-2 cm), to 2.1 to 0.2 yr-1 at 12-14 cm. The C/N and C/P ratios of the organic matter undergoing decomposition also increased with depth at these sites. Taken together, these results indicate that the reactivity of the organic matter undergoing mineralization decreases with depth in these sediments. A model based on the multiple-G model for organic matter decomposition (hereafter referred to as the mixture model) was developed to examine the observed kinetics of all of these processes. As in the multiple-G model, the mixture model is based on the mineralization of organic matter in sediments occurring from distinct fractions of organic matter with differing reactivities. However here, differences in reactivity are indicated by differences in both the intrinsic rate constants for decomposition as well as the C/N or C/P ratios of the organic matter in the fractions. The mixture model was useful in interpreting the results of these experiments, and provided explanations for differences in the reactivity of organic matter in the surface sediments at these sites. It also appeared to provide information on the nature of the organic matter undergoing remineralization in these sediments, based on the predicted C/N or C/P ratios of these apparent fractions. The data from this study was also examined using the recently presented power model of Middelburg (1989). This analysis indicated the importance of pre-depositional decomposition in affecting the reactivity of sedimentary organic matter. While all of these models provided insights into organic matter remineralization in marine sediments, they also all had mixed successes in describing (in a unified fashion) sulfate reduction and inorganic nutrient production in these southern Chesapeake Bay sediments. This observation indicates that care must be taken in interpreting information (e.g., rate constants, elemental ratios or apparent initial ages of the sedimentary organic matter) on organic matter in sediments, based on model derived parameters such as those that have been presented here. The ability to independently verify these model derived parameters of the sedimentary organic matter undergoing mineralization will likely be important in further refining these models and improving their usefulness in describing (and predicting) the factors controlling organic matter mineralization in marine sediments

Anaerobic Oxidation of Methane and the Stoichiometry of Remineralization Processes in Continental Margin Sediments

In many coastal and continental margin sediments, pore-water property-property plots yield values of rc:s, the stoichiometric ratio of dissolved inorganic carbon (DIC) produced to sulfate reduced, that are lower than the commonly assumed value of 2. Remineralization of organic matter more reduced than CH2O can cause such observations, as can DIC loss due to authigenic carbonate precipitation. However, through studies of Santa Monica Basin sediments, we have observed that these observations could also be related to the occurrence of anaerobic oxidation of methane (AOM) in sediments. Specifically, using a reactive transport sediment model, we have shown that AOM driven by an external methane source (e.g., an upward flux from ancient gas hydrates) can lead to values of rc:s \u3c 2. This contrasts with what is observed when AOM is driven by methane produced during in situ methanogenesis, resulting from the deposition of reactive organic matter to the sediment surface. This situation does not lead to deviations in the value of rc:s from that seen solely for the occurrence of organic matter remineralization by sulfate reduction. With real pore-water data, if carbonate precipitation is adequately accounted for, observed deviations in values of rc:s from predicted end-member values for organic matter remineralization and AOM can provide information about the occurrence of AOM and upward methane fluxes in sediments

The Biogeochemical Cycling of Dissolved Organic Nitrogen in Estuarine Sediments

Benthic fluxes and pore-water profiles of dissolved organic nitrogen and carbon (DON and DOC, respectively) were determined in seasonal studies at contrasting sites in Chesapeake Bay. Pore-water dissolved organic matter (DOM) concentrations were elevated over bottom-water values, generally increased with depth, and ranged from 15 to similar to 160 μM for DON and ~200-2000 μM for DOC. Pore-water DOM concentrations and the C:N ratio of this material showed spatial (depth) and temporal changes that varied among the sites studied. These trends appeared to be related to differences in the types of sediment organic matter (SOM) undergoing remineralization, as well as differences in the biogeochemical processes occurring in the sediments (e.g., the presence or absence of bioturbation and bioirrigation). Measured DON fluxes ranged from essentially zero to similar to 0.4 mmol m-2 d-1, and together with benthic DOC fluxes were coupled to seasonal trends in temperature and SOM remineralization rates. On an annual basis, benthic DON fluxes were a small fraction (similar to 3%) of benthic inorganic nitrogen fluxes. At an anoxic nonbioturbated site measured DON fluxes were essentially identical to calculated diffusive DON fluxes, whereas at a bioturbated and bioirrigated site, measured DON fluxes were much greater than calculated fluxes. The molar ratios of DOC to DON benthic fluxes ranged from ~2 to 6 and were lower than those of pore-water DOM, which were \u3e similar to 10. This implies that DOM accumulating in these sediment pore waters was carbon-rich compared with the DOM that was either remineralized or escaped the sediment as a benthic flux. These measured benthic DON fluxes and estimated DON fluxes from continental margin sediments combine to yield a lower limit for the integrated sediment DON flux to the oceans that is similar to a value estimated previously. These net DON inputs to the oceans are small compared with internal oceanic DON cycling rates, although sediment DON fluxes and riverine DON inputs are roughly of the same order. At the same time, our results also suggest that the DON escaping from these sediments may not be inherently refractory because of its observed low C:N ratio. This implies that estuarine sediments land perhaps marine sediment in general) may not be a major source of refractory DON to the oceans

Shallow Marine Carbonate Dissolution and Early Diagenesis-Implications from an Incubation Study

Surface carbonate sediments fro, sites on the Bahamas Bank with different seagrass densities were incubated across a range of O2 delivery rates, to study the controls on metabolic carbonate dissolution in these sediments. The results continued the 1:1 ratio between the rates of O2 consumption and carbonate dissolution, demonstrating that microbial respiration was the rate-limiting step in metabolic carbonate dissolution. Furthermore, the dissolution we observed was actually net dissolution resulting front Coupled dissolution and reprecipitation. This carbonate reprecipitation occurs on the time scale of days, and significantly alters the pore water dissolved inorganic carbon (DIC) stable isotopic composition. The carbonate reprecipitation/dissolution ratios observed here were similar to those reported in the literature for other sediments. Dissolution/reprecipitation appeared to involve preferential dissolution of high magnesium calcite and reprecipitation of a carbonate phase with a M content that was only slightly lower than that of the dissolving phase, This result agrees with conclusions in the literature that Ostwald ripening may be responsible for this reprecipitation

Using Ammonium Pore Water Profiles to Assess Stoichiometry of Deep Remineralization Processes in Methanogenic Continental Margin Sediments

In many continental margin Sediments, a deep reaction zone exists which is separated from remineralization processes near the sediment surface. Here, methane diffuses upward to a depth where it is oxidized by downwardly diffusing sulfate. However, the methane sources that drive this anaerobic oxidation of methane (AOM) in the sulfate-methane transition zone (SMT) may vary among sites. In particular, these sources can be thought of as either (i) internal sources from in situ methanogenesis (regardless of where it occurs in the sediment column) that are ultimately coupled to organic matter deposition and burial, or (ii) external sources such as hydrocarbon reservoirs derived from ancient source rocks, or deeply buried gas hydrates, both of which are decoupled from contemporaneous organic carbon deposition at the sediment surface. Using a modeling approach, we examine the relationship between different methane sources and pore water sulfate, methane, dissolved inorganic carbon (DIC), and ammonium profiles. We show that pore water ammonium profiles through the SMT represent an independent tracer or remineralization processes occurring in deep sediments that complement information obtained from profiles of solutes directly associated with AOM and carbonate precipitation, I.E., DIC, methane, and sulfate. Pore water DIC profiles also show an inflection point in the SMT based on the type of deep methane source and the presence/absence of accompanying upward DIC fluxes. With these results, we present a conceptual framework which illustrates how shallow pore water profiles from continental margin settings can be used to obtain important information about remineralization processes and methane sources in deep sediments

Elimination of Dissolved Sulfide Interference in the Flow-Injection Determination of Sigma-CO2 by Addition of Molybdate

A previously described flow injection method for the analysis of ΣCO2 included the addition of ZnCl2 to some samples before analysis in order to precipitate dissolved sulfide (which interferes with the method) as ZnS. However, the use of Zn2+ in samples with high concentrations of dissolved sulfide causes the coprecipitation of ZnCO3, and in our experience with this technique, ZnCO3 also precipitates even in the absence of dissolved sulfide. The addition of molybdate effectively complexes dissolved sulfide without interfering with the determination of ΣCO2 by this technique

A Tribute to Thomas M. Church: Exploring Chemical Oceanography in the Coastal Zone-The History and Future

( First paragraph) One can find different historical perspectives on the development of studying the chemistry of oceans as well as names for this study—marine chemistry, chemistry of the sea, marine aquatic chemistry, marine biogeochemistry, or chemical oceanography. It could be argued that chemical oceanography is the most inclusive for an earth science since oceanography itself is an integrated discipline that links the biology, chemistry, geology, and physics together. Regardless of the name, perhaps the first intensive, modern/post-nineteenth century study of the ocean’s chemistry was the GEOSECS Program from ca. 1970–1978. The significance of GEOSECS was that it examined the chemistry of the world’s oceans from nutrients to radionuclides, and even a few trace elements, but in a physical context of ocean circulation (e.g., Craig 1972). Thomas M. Church (Figs. 1 and 2) was ‘‘born’’ into the GEOSECS world, receiving his Ph.D. in 1970 from Scripps Institution of Oceanography in the laboratory of Edward Goldberg with the first examination of marine barite in the world’s oceans. GEOSECS was a ‘‘blue water’’ program, but Tom Church decided to take the road less travelled at the time to examine chemical processes in the coastal zone. The coastal zone has been described, both then and now and always somewhat facetiously, as the ‘‘brown ring around the bathtub,’’ but many would argue that this minimizes its importance since it is here where continental weathering products are primarily introduced to the ocean and where many of these same products are also removed. Primary productivity is at a maximum in coastal waters, and human populations and effects are also concentrated here

Microbial Manganese Reduction by Enrichment Cultures from Coastal Marine Sediments

Manganese reduction was catalyzed by enrichment cultures of anaerobic bacteria obtained from coastal marine sediments. In the absence of oxygen, these enrichment cultures reduced manganates when grown on either lactate, succinate, or acetate in both sulfate-free and sulfate-containing artificial seawaters. Sodium azide as well as oxygen completely inhibited microbial manganese reduction by these enrichment cultures, whereas molybdate had no effect on them. The addition of nitrate to the medium slightly decreased the rate of Mn2+ production by these enrichment cultures. These findings are consistent with the hypothesis that the manganese-reducing organisms in these enrichment cultures use manganates as terminal electron acceptors and couple manganese reduction in some way to the oxidation of organic matter

A Coupled, Non-Linear, Steady State Model for Early Diagenetic Processes in Pelagic Sediments

A steady state, coupled, non-linear model has been developed for early diagenetic processes in pelagic and hemi-pelagic marine sediments. Model results show that the occurrence of oxic and sub-oxic diagenetic processes is significantly affected by variations in parameters such as the sedimentation rate, bioturbation coefficient, sediment porosity, and organic matter flux to the sediments. Increases in the sedimentation rate or the bioturbation coefficient increase organic matter oxidation by sub-oxic processes, whereas an increase in sediment porosity decreases organic matter oxidation by sub-oxic processes. Sediment data from three contrasting MANOP sites are fit reasonably well with the model. The resulting best-fit organic carbon, oxygen, and nitrate fluxes at the sediment-water interface and depth-integrated organic carbon oxidation rates for these sites are also within the range of independent estimates of these quantities. Model results show that the internal redox cycling of manganese in sediments leads either to the formation of a Mn-peak near the sediment redox boundary or to surficial Mn-rich oxic sediments, depending on the depth zonation of manganese oxidation and bioturbation. In sediments with a shallow redox boundary (\u3c5 \u3ecm), upward diffusion of pore water manganese into the oxic sediments dominates over manganese oxidation near the redox boundary. The majority of the manganese oxidation therefore occurs in the surficial, bioturbated sediments, and as a result, manganese-rich oxic sediments are formed. In contrast, in sediments with a deeper redox boundary (\u3e10 cm), manganese oxidation near the sediment redox boundary dominates over pore water manganese diffusion into the overlying oxic sediments. Here, majority of the manganese oxidation occurs below the zone of active bioturbation (assumed to be the upper 8-10 cm of sediment), and in this case, a well developed Mn-peak forms near the sediment redox boundary. Previous models explained the occurrence of this Mn-peak by neglecting bioturbation or suggested that this peak could not occur in bioturbated sediments due to this sediment mixing