B51D-01 INVITED
Changes in Trace Metal Cycling Driven by Sediment Redox Conditions
The cycling and accumulation of trace metals in sediments is often strongly dependent on the redox conditions within the sediments. Understanding the relationship between redox chemistry, metal chemistry and other diagenetic processes, such as bioturbation, is essential in order to use trace metals as tracers of paleo-redox conditions and for predicting the long term fate of metal-contaminated marine sediments. In this study the behavior of the metals, silver, copper and lead, under seasonally varying redox conditions has been investigated at two contrasting sites in coastal Massachusetts. The first site is in Boston Harbor and has high rates of organic carbon oxidation and active sulfate reduction, with porewater sulfide detected below 5 cm. The second site is further offshore in Massachusetts Bay and also has an oxygen penetration depth of less than 1 cm, but no sulfide is detected in the porewaters down to a depth of 30 cm. The variations in redox conditions throughout the year have a strong impact on the metal behavior and the metal profiles in the sediments. At both sites the trace metals are scavenged by iron oxyhydroxides in the surface oxic zone and are released to the porewaters when these oxides are reduced. At the more reducing Boston Harbor site, as the rates of organic matter degradation increase over summer, the sulfide interface moves upwards and the trace metals are transferred from the oxide phases to sulfides phases. At the offshore site, due to the lack of sulfide, the metals are focused into the surface oxide layer, giving a solid phase enrichment that is not observed at the Harbor site. Silver porewater data from the coastal zone are further compared with porewater profiles from an oceanic transect heading offshore from the Oregon coast. The transect covers a range of organic carbon fluxes and bottom water oxygen levels, extending our investigation of metal behavior across a spectrum of sediment redox conditions, from strongly reducing coastal environments to oxic oceanic sites.
B51D-02
Controls of Submarine Groundwater Discharge on the Redox Conditions in the Outflow Face System of Indian River Lagoon
Fresh groundwater discharge to estuaries creates a subsurface zone where oxygenated surface water mixes with reduced pore water, forming redox boundaries in sediment of the outflow face (the subterranean estuary). This mixing has been widely cited to satisfy discrepancies of mass balance calculations for submarine groundwater discharge of terrestrially derived water (terrestrial SGD). Redox boundaries in the subsurface are clearly shown in distributions Fe and Mn in pore waters collected from Indian River Lagoon, Florida. Pore waters were collected from 8 sites in a 30 m transect offshore and at a single site 250 m offshore. Terrestrial SGD decreases offshore to the seaward end of the outflow face at 22.5 m. Both Fe and Mn show maxima in their depth profiles. Manganese maxima are restricted to the upper 40 cmbsf (centimeter below the surface) and always occur above the Fe maxima, which increase in depth offshore. Maximum Fe and Mn concentrations (1.05 and 0.28 μM, respectively) at the shoreline are of the same order of magnitude as maximum Fe and Mn concentrations (0.48 μM and 0.56 μM, respectively), at 250 m offshore. Maximum Fe and Mn concentrations increase to 261 μM, and 2.9 μM respectively, 22.5 and 30m offshore, a few meters seaward of the outflow face. Between 5 and 20 m offshore, the maximum Fe concentrations increase from 3.7 to 22.8 μM, but maximum Mn concentrations show little variability, averaging around 1.05 μM ± 20%. Sediments collected in three vibracores at 0, 20, and 30 m offshore are orange-yellow at the shoreline. A black layer, which occurs above these orange-yellow sediments, increases in thickness from 5 cm to 60 cm from the shoreline to 30 m offshore. The variation in color corresponds to changes in solute concentrations: S2- maxima occur within the black layer, Fe maxima occur in the orange-yellow layer, and Mn maxima mostly occur in the black layer. Pore water Fe and S2- are inversely correlated and no correlation exists between Mn and S2-. These distributions of solutes suggest the source of Fe and Mn are oxyhydroxides coating the orange-yellow sediments. Dissolved Fe accumulates in absence of S2-, and the depths of the Fe maxima increase as S2- concentrations increase offshore. The low concentrations of Fe and Mn within the outflow face may reflect discharge of water from the outflow face, while the elevated concentrations seaward of the outflow face reflects a zone of no flow. Farther offshore, Fe concentrations are low because of elevated S2- production there. The distributions of these redox sensitive elements suggest there is little landward flow of marine water that mixes with the terrestrial source of SGD.
B51D-03
Biogeochemical snapshot of an urban water system: The Anacostia River, Washington DC
Highly urbanized and contaminated with PAHs, heavy metals, and sewage, the Anacostia River flows through Maryland and Washington, DC into the tidal Potomac River. Efforts have been underway to assess the river's ecological integrity and to determine the extent of anthropogenic influences. This study examines the nutrients, bacterial biomarkers, organic material, and carbon, nitrogen and sulfur sources in the Anacostia. High biological oxygen demand and low nitrogen (0.33-0.56 mg /L)/phosphorus (0.014 - 0.021 mg/L) concentrations were observed in three areas of the river. Bacterial activity based on carbon source utilization was higher in sediment samples than in water column samples. While bacterial abundances were decreased in downstream areas of sediment; abundances increased in downstream areas in the water column. Downstream sites had higher nutrient concentrations and dissolved organic carbon (up to 13.7 mg/L). Odd-chain length and branched fatty acids (FAs) in the sediments indicated bacterial sources, but long chain FAs indicative of terrestrial primary production were also abundant in some sediments. Also dominant among methyl esters and ketones in some sediment and water column samples was methyl isobutyl ketone, a common industrial solvent and combustion by-product. Sediment carbon stable isotope analyses show a mix of autochthonous and allochthonous derived materials, but most carbon was derived from terrestrial sources (-23.3 to -31.7‰). Sediment nitrogen stable isotopes ranged from -5.4 to. 5.6, showing nitrate uptake by plants and also recycling of nitrogen within the river. Sulfur sources were generally between 3 and -5, reflecting local sulfate sources and anaerobic sulfate reduction.
B51D-04
Phosphorus forms and controls on phosphorus dynamics in sediments from Monterey Bay, California
Phosphorus is an essential element for all organisms. Phosphorus can be removed from the water column as sinking particulate matter, where it may either be buried or remineralized and released to the water column. The objective of our study was to determine P forms, dynamics and variability in sediment samples from Monterey Bay, CA. Six pushcore samples were randomly collected from a 2-m2 area, and were divided into 4 layers, each 1.5-2.5 cm thick. Samples were analyzed for total P, C and N, P forms by 31P NMR spectroscopy and SEDEX fractionation, phosphatase activity, and P retention. Total P, C and N did not change with depth, but there were significant changes with depth for P forms by NMR and SEDEX, and well as alkaline phosphatase and diesterase activity. However, P retention was strong in all layers and increased with depth, suggesting that remineralized P not taken up by organisms is sorbed onto sediment minerals rather than released to the surface water.
B51D-05
Effects of Deepwater Algae on C-N-P Cycling in Permeable Sediments
Extensive deepwater (50-100 m depth) meadows of native and introduced macroalgae occur offshore of the main Hawaiian Islands. Sediment porewater (to 25 cm sediment depth) and overlying seawater were collected during September and December 2004, and November and December 2006 from 13 vegetated and non-vegetated deepwater sites on sand and muddy-sand seafloors offshore from Oahu to Kaho'olawe. Porewater dissolved nutrient concentrations were comparable to those in nearshore permeable (sandy) sediments, with highly elevated concentrations at sediment depths available to algal holdfasts. Maximum observed concentrations were: 2.3 uM phosphate, 30 uM nitrate, 0.65 uM nitrite, 105 uM ammonium, and 88 uM silica. The sediment organic matter content was not correlated with the presence or absence of macroalgae. Vegetated sediments were consistently more oxidizing than non-vegetated sediments, judged from (nitrate + nitrite)/ammonium ratios. Halimeda-vegetated sediments had low DIN levels compared to other sites, suggesting algal uptake. These sands also exhibited dissolved inorganic carbon depletion at depth, perhaps also related to algal metabolism. In contrast, Udotea-vegetated sediments had high levels of DIN, indicating little or no algal DIN uptake and/or the presence of active N-fixation; these sands also had no DIC depletion at depth. Udotea-vegetated sediments also appeared highly oxidizing, evidenced by very high levels of dissolved nitrate and nitrite, along with low levels of ammonium. The calculated tracer N* was used to verify the conclusions regarding net DIN uptake; a similar calculated tracer, computed using Redfield-type N:Si ratios, was used to confirm the N* results. Unvegetated, nutrient- and organic-rich sandy mud along the flanks of Haleakala volcano showed near complete dissolved phosphate removal, perhaps due to adsorption onto freshly weathered iron oxy-hydroxides. Such phosphate removal was not seen at other sites, including a nearby non-vegetated non-muddy sand with elevated nutrient levels. Our results suggest that the presence of deepwater macroalgae may have significant effects on the geochemistry of permeable (sandy) sediments, in contrast to the widely held belief that macroalgae do not interact chemically with the sediment they are associated with.
B51D-06
Unexpected Microbial Diversity in Anaerobically Methane-oxidizing Mats of the Black Sea
Sediments of the Black Sea harbour consortia of anaerobically methane-oxidizing microorganisms in dense microbial mats, incrusted in large chimney structures consisting of carbonate precipitate. A number of convincing facts collected previously suggests that anaerobically methane-oxidizing Archaea (ANME) as well as delta- proteobacterial sulphate-reducing bacteria are the key players in anaerobic methane oxidation in Black Sea Mats: their presence has been shown by fluorescent-in-situ hybridization (FISH) with 16S rRNA-targeting probes, lipid biomarkers have typical, low delta13C ratios in archaeal and bacterial lipids, a methyl coenzyme M reductase-like protein was purified from the mat, and mat samples exhibit anaerobic methane oxidation. Here, we show that the diversity of Bacteria in both, pink and black mat samples, is larger than previously known. T-RFLP analysis of 16S rRNA and 16S rRNA genes and cloning and sequencing of randomly selected clones revealed the presence of taxa hitherto unknown to be present in anaerobically methane-oxidizing consortia. Besides the previously known delta-proteobacterial sulphate reducers, clones fell into 7 and 5 different phyla in pink and black coloured mats, respectively. Our findings suggest that the turnover of carbon in anaerobically methane-oxidizing communities might involve a larger diversity of microorganisms than was previously assumed.
B51D-07 INVITED
Tracing The Fate Of Methane-Derived Carbon: Insights Into The Ecological Physiology Of Anaerobic Methanotrophy Via Quantitative Molecular and Geochemical Approaches
The consumption of methane in anoxic marine sediments is a biogeochemical phenomenon mediated by archaea in varying associations with sulfate-reducing bacteria. These anaerobic methanotrophs have yet to be recovered in pure culture and consequently key aspects of their ecology and physiology remain poorly understood. To study the ecological physiology of anaerobic methanotrophy, we developed a high-pressure continuous flow bioreactor to maintain (and when desired, enrich) these communities in conditions that are representative of those in situ. The high pressure bioreactor incorporates a high-pressure membrane inlet mass spectrometer, as well as a porewater and sediment subsampling system, to allow real time isotopic tracer studies, rate measurements as well as frequent subsampling for molecular biological studies. In the experiments presented here, we examined the influence of pressure and key metabolite concentrations on the rates of anaerobic methanotrophy and methanogenesis by anaerobic methanotrophs, and coupled rate measurements to quantitative biological metrics, e.g. growth rates and yields via qPCR and cell sorting. Our data suggest that rates of archaeal methanotrophy and methanogenesis –as well as population growth rate and yield - are strongly influenced by key chemical factors in the geochemical milieu, and this in turn significantly governs the fate of methane-derived carbon in marine ecosystems.