Graduation Year

2008

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Geology

Major Professor

Thomas Pichler, Ph.D.

Keywords

Arsenic, Biogeochemical cycling, Shallow-water, Marine, Hydrothermal

Abstract

The marine shallow-water hydrothermal vent system of Tutum Bay, Ambitle Island, PNG discharges hot, acidic, arsenic-rich, chemically reduced fluid into cool, alkaline, oxygenated seawater. Gradients in temperature, pH, total arsenic (TAs) and arsenic species, among others, are established as the two aqueous phases mix. Hydrous ferric oxides (HFO) are precipitated around focused venting, and coat the surrounding sediments visibly to 150 m away. HFO coatings, mechanical transport and weathering of volcanoclastic sediments, as well as dissolution of carbonate sediments nearer to venting, combine to alter sediment chemistry substantially. Tutum Bay surface sediments have a mean As concentration of 527 mg/kg. Arsenic at concentrations up to 50 mg/kg (mean = 19.7 mg/kg) was extracted from the easily extractable fraction of surface sediments.

Arsenic is elevated in surface seawaters (8 µg/L) directly over hydrothermal vents, and As(III) is substantially enriched in both surface and bottom seawater throughout Tutum Bay. Surprisingly, aqueous As(V) far exceeded aqueous As(III) at almost all distances and depths investigated for Tutum Bay pore waters. These data indicate that throughout Tutum Bay, chemical disequilibria among As species provides potential metabolic energy for arsenite oxidizing and arsenate reducing microorganisms, and that As is bioavailable from two major environments: 1) easily-exchangeable As from surface sediments, and 2) in surface seawaters, which may allow for biological uptake and trophic transfer through plankton. The soft coral Clavularia sp., the calcareous algae Halimeda sp., and the tunicate Polycarpa sp. were collected and analyzed to assess bioaccumulation and biotransformation patterns. All organisms collected from the hydrothermal area displayed higher (2 to 20 times) TAs.

Concentrations of arsenic species in their tissues were also elevated compared to the control site. Increased concentrations were observed near focused venting. Distinct arsenic speciation patterns in Clavularia and Polycarpa collected from near hydrothermal venting suggest rapid methylation/detoxification of arsenic, with enhanced bioaccumulation of dimethylarsenate and arsenobetaine as products of the organisms metabolic pathways. Elevated concentrations of As(III) in Halimeda suggest that this organism is not as efficient at methylating inorganic arsenic. The presence of arsenobetaine in Halimeda suggests the biomethylation pathway for calcareous algae is different from commonly studied seaweeds.

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