Mechanisms of metal release from contaminated coastal sediments

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2005The fate of trace metals in contaminated coastal sediments is poorly understood, yet critical for effective coastal management. The aim of this thesis is to investigate and quantify the mechanisms leading to the release of silver, lead and copper across the sediment-water interface. Two contrasting sites were investigated, a heavily contaminated site in Boston Harbor and a less impacted, offshore site in Massachusetts Bay. High-resolution porewater and solid phase samples were collected in each season to determine the diagenetic cycles and chemistry controlling the fate of these metals. The trace metals are scavenged by iron oxyhydroxides and released to the porewaters when these oxides are reduced. At the strongly reducing site in Boston Harbor, there is seasonal transfer of trace metals from oxide phases in winter, to sulfides phase in summer. At the Massachusetts Bay site, due to the lack of sulfide, the metals are focused into the surface oxide layer, giving a solid phase enrichment. There is a diffusive flux of copper to the water column throughout the year, while silver is released only in winter. Lead is strongly scavenged and is rarely released to the overlying waters. Analysis of reduced sulfur compounds in the porewaters has shown that there is also a significant flux of these strong ligands to the overlying waters. Polysulfide species enhance the solubility of copper within the porewaters. Sediment resuspension fluxes were quantified using an erosion chamber. Sediment resuspension leads to enhanced release of dissolved metals and is especially important in redistributing contaminants as the first particles to be eroded are enriched in trace metals. The total release of dissolved metals from the sediments by diffusion and sediment resuspension is estimated to be 60% and 10% of the riverine flux for copper and lead respectively. With continued pollution control reducing the discharge of metals from other sources, the benthic release of metals will become increasingly important terms in the metal budget of Boston Harbor.Tills work is a result of research sponsored by the NOAA National Sea Grant College Program Office, Department of Commerce, under Grant. No. NA16RG2273, Woods Hole Oceanographic Institution Sea Grant Project Nos. 1-01-22227310 and 1-01-22227338. Additional funding was provided by the University of Western Australia Hackett Scholarship, the United States Geological Survey under Cooperative Agreement Number OOHQAGOOOI and the National Science Foundation under Grant OCE-0220892

Geochemical cycling of silver in marine sediments along an offshore transect

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Chemistry 110 (2008): 77-88, doi:10.1016/j.marchem.2008.02.008.Although there have been many surface water and water column silver (Ag) analyses in the ocean, the absence of high resolution pore water and solid phase Ag profiles has hampered our understanding of its oceanic geochemical cycling. This manuscript presents pore water and solid phase profiles of Ag along an offshore transect in the northeast Pacific off the coasts of Washington/Oregon states, U.S.A.. Pore water Ag concentrations are uniformly low (< 0.3 nmol kg-1) in profiles from sediments that have low bottom water oxygen concentrations, have shallow oxygen penetration depths (O2,pen < 1 cm) and underlie short water columns (< 500 m water depth). The solid phase Ag concentrations at these sites are also low (< 1 μmol kg-1). This is in contrast to sediments from intermediate water depths (~2000 m) that have similar oxygen penetration depths (O2,pen < 1 cm), but have elevated pore water Ag concentrations (0.7 nmol kg-1) at the sediment–water interface and higher solid phase Ag concentrations (4– 8 μmol kg-1). At sites from ~3000–4000 m water depth, where O2,pen > 1 cm, pore water Ag concentrations reach extremely high concentrations in the top 5 cm (8–24 nmol kg-1). High concentrations in pore waters provide evidence for a flux of Ag from ocean sediments, but the more oxidizing nature of these sediments precludes appreciable solid phase Ag accumulation in the top 30 cm (< 2 μmol kg-1). The accumulation of Ag in sediments is not simply dependent on redox conditions; more oxidizing sediments do not accumulate solid phase Ag, and neither do more reducing sediments from shallow water depths. Only a sufficiently long water column will result in additional delivery of Ag to sediments by scavenging onto settling particles, and result in Ag accumulation in sediments where O2,pen < 1 cm. Although upward Ag fluxes from sediments underlying shorter water columns are small (0.02–0.07 nmol cm-2 y-1), calculated fluxes increase for sediments underlying longer water columns and are largest for the more oxidizing sediments (2–5 nmol cm-2 y- 1). Calculated fluxes of pore water Ag to the solid phase at these more oxidizing stations are inconsistent with measured solid phase Ag concentrations and suggest that the pore water profiles represent non–steady state conditions. Clearly, the early diagenesis of Ag is a highly dynamic process and more research is required to fully understand Ag cycling in sediments in continental margin locations.Funding for this work was provided to JLM and F&M students by Research Corporation and the Hackman Summer Research Program at Franklin & Marshall College. Financial support to JLM and LHK was also provided by the National Science Foundation (OCE–0220892). LHK received additional support from a Hackett Scholarship from the University of Western Australia and the WHOI Academic Programs Office

Ecological distribution and population physiology defined by proteomics in a natural microbial community

Community proteomics applied to natural microbial biofilms resolves how the physiology of different populations from a model ecosystem change with measured environmental factors in situ.The initial colonists, Leptospirillum Group II bacteria, persist throughout ecological succession and dominate all communities, a pattern that resembles community assembly patterns in some macroecological systems.Interspecies interactions, and not abiotic environmental factors, demonstrate the strongest correlation to physiological changes of Leptospirillum Group II.Environmental niches of subdominant populations seem to be determined by combinations of specific sets of abiotic environmental factors

Insights on geochemical cycling of U, Re and Mo from seasonal sampling in Boston Harbor, Massachusetts, USA

Author Posting. © The Author(s), 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 71 (2007): 895-917, doi:10.1016/j.gca.2006.10.016.This study examined the removal of U, Mo, and Re from seawater by sedimentary processes at a shallow-water site with near-saturation bottom water O2 levels (240-380 μmol O2/L), very high organic matter oxidation rates (annually averaged rate is 870 μmol C/cm2/y), and shallow oxygen penetration depths (4 mm or less throughout the year). Under these conditions, U, Mo, and Re were removed rapidly to asymptotic pore water concentrations of 2.2–3.3 nmol/kg (U), 7–13 nmol/kg (Mo), and 11–14 pmol/kg (Re). The order in which the three metals were removed, determined by fitting a diffusion-reaction model to measured profiles, was Re < U < Mo. Model fits also suggest that the Mo profiles clearly showed the presence of a near-interface layer in which Mo was added to pore waters by remineralization of a solid phase. The importance of this solid phase source of pore water Mo increased from January to October as the organic matter oxidation rate increased, bottom water O2 decreased, and the O2 penetration depth decreased. Experiments with in situ benthic flux chambers generally showed fluxes of U and Mo into the sediments. However, when the overlying water O2 concentration in the chambers was allowed to drop to very low levels, Mn and Fe were released to the overlying water along with the simultaneous release of Mo and U. These experiments suggest that remineralization of Mn and/or Fe oxides may be a source of Mo and perhaps U to pore waters, and may complicate the accumulation of U and Mo in bioturbated sediments with high organic matter oxidation rates and shallow O2 penetration depths. Benthic chamber experiments including the nonreactive solute tracer, Br-, indicated that sediment irrigation was very important to solute exchange at the study site. The enhancement of sediment-seawater exchange due to irrigation was determined for the nonreactive tracer (Br-), TCO2, NH4 +, U and Mo. The comparisons between these solutes showed that reactions within and around the burrows were very important for modulating the Mo flux, but less important for U. The effect of these reactions on Mo exchange was highly variable, enhancing Mo (and, to a lesser extent, U) uptake at times of relatively modest irrigation, but inhibiting exchange when irrigation rates were faster. These results reinforce the observation that Mo can be released to and removed from pore waters via sedimentary reactions. The removal rate of U and Mo from seawater by sedimentary reactions was found to agree with the rate of accumulation of authigenic U and Mo in the solid phase. The fluxes of U and Mo determined by in situ benthic flux chamber measurements were the largest that have been measured to date. These results confirm that removal of redoxsensitive metals from continental margin sediments underlying oxic bottom water is important, and suggest that continental margin sediments play a key role in the marine budgets of these metals.We appreciate the financial support from the National Science Foundation (OCE-0220892). Funding for this work was also provided to JLM by the Postdoctoral Scholar Program at WHOI courtesy of the Cabot Marine Environmental Science Fund and the J. Seward Johnson Fund. Financial support to IMK was given by The Swedish Foundation for International Cooperation in Research and Higher Education

Sampling marine pore waters for Mn, Fe, U, Re and Mo: Modifications on diffusional equilibration thin film gel probes

Pore water metal profiles are important for identifying redox horizons and understanding trace metal geochemical cycling. The challenges of pore water sampling for trace metals are minimizing disturbance, especially at the sediment–water interface, and minimizing oxidation during sampling. We are investigating diffusional equilibration in thin films (DET) probes for obtaining pore water profiles. Our goal is to use probes for redox-sensitive trace metals U, Re and Mo, in addition to Mn and Fe, in coastal marine areas. Initial solution equilibration tests and laboratory core incubation experiments suggest that equilibration times for probes in sediments are approximately 24–48 h. Control tests suggest that the incubation does not alter the redox conditions in the pore waters. Pore water profiles from cores sampled by slicing, centrifuging and filtering (in a nitrogen atmosphere) and from probes are similar. Two modifications on the gel probe design were tested to determine their impact. (1) PVC wedges were attached to the backs of probes to increase the contact between sediments and the probe surface and to reduce the risk of forming channels along the probe surface, which might allow vertical pore water transport. Lower Fe concentrations were measured from probes without PVC wedges, but other metal profiles were similar. (2) A modified face frame was removed from the front of a probe, to reduce disturbance of the sediments during insertion and to increase the contact between the sediments and probe surface. Probes with modified face frames did not have increasing U and Mo concentrations with depth, whereas two of the three probes without face frames did have increasing concentrations. Increasing U and Mo concentrations at depth may be reflecting the influence of irrigating burrows and their supply of oxygen to reduced sediments, which could oxidize previously reduced metals. The distribution of burrows is heterogeneous and resulting profiles would also be expected to be heterogeneous in their response. Differences between probe profiles and sliced/centrifuged profiles are examined to gain insight into possible sampling artifacts. Peaks in the Re sliced/centrifuged profiles suggest a large Re flux to the overlying waters, which is neither calculated from probe profiles nor measured in benthic chamber samples. It is possible that heterogeneity at the sampling site in Buzzards Bay resulted in these differences; however, it is also possible that centrifugation releases Re from pore structures that would not be measured with less intrusive sampling methods, such as gel probes or benthic chambers

Proteogenomic basis for ecological divergence of closely related bacteria in natural acidophilic microbial communities

Bacterial species concepts are controversial. More widely accepted is the need to understand how differences in gene content and sequence lead to ecological divergence. To address this relationship in ecosystem context, we investigated links between genotype and ecology of two genotypic groups of Leptospirillum group II bacteria in comprehensively characterized, natural acidophilic biofilm communities. These groups share 99.7% 16S rRNA gene sequence identity and 95% average amino acid identity between their orthologs. One genotypic group predominates during early colonization, and the other group typically proliferates in later successional stages, forming distinct patches tens to hundreds of micrometers in diameter. Among early colonizing populations, we observed dominance of five genotypes that differed from each other by the extent of recombination with the late colonizing type. Our analyses suggest that the specific recombinant variant within the early colonizing group is selected for by environmental parameters such as temperature, consistent with recombination as a mechanism for ecological fine tuning. Evolutionary signatures, and strain-resolved expression patterns measured via mass spectrometry–based proteomics, indicate increased cobalamin biosynthesis, (de)methylation, and glycine cleavage in the late colonizer. This may suggest environmental changes within the biofilm during development, accompanied by redirection of compatible solutes from osmoprotectants toward metabolism. Across 27 communities, comparative proteogenomic analyses show that differential regulation of shared genes and expression of a small subset of the ∼15% of genes unique to each genotype are involved in niche partitioning. In summary, the results show how subtle genetic variations can lead to distinct ecological strategies