This thesis examines two important factors influencing POC export in the water column: mineral association and aggregation. Specifically, it focuses on (1) the occlusion of organic matter within biominerals, which may be the most permanent form of organic matter protection by minerals, and (2) the occlusion and transport of organic matter within aggregates, which may be the most quantitatively important form of organic-mineral association.
To assess the effects of organic matter occlusion within biominerals, scanning transmission X-ray microscopy (STXM) and carbon X-ray absorption near-edge structure (XANES) spectroscopy were used in combination to characterize the distribution and composition of organic matter in frustules of the diatom Cylindrotheca closterium and a biomimetic silica gel. Organic carbon, most likely protein, was distributed throughout the frustules and was not completely removed by extensive chemical treatment, suggesting that frustule-bound organic carbon is protected from decomposition until the frustule dissolves. The physical structure of the frustules appeared to be related to the chemical composition of this organic matter, with aromatic or unsaturated carbon concentrated in areas resembling the thin, perforated silica roofs that often cover pores in the frustule. A similar physical and chemical structure was observed in a biomimetic silica gel precipitated with poly-lysine. These results are consistent with the theory that organic constituents of diatom frustules direct silica precipitation and become incorporated within the silica matrix as it forms.
The fate of organic matter within water column aggregates was assessed by comparing the organic compositions of sinking (sediment trap) and suspended (in situ pump) particles collected in 2003 and 2005 in the northwest Mediterranean Sea. Sinking particles were enriched in fecal pellet and silicifying algae indicators, whereas suspended particles were enriched in fresh phytoplankton and calcifying algae indicators. Mass balance calculations indicate that the observed composition of suspended particles is best explained by extensive disaggregation of phytoplankton aggregates. Fecal pellets do not appear to undergo extensive disaggregation and exchange with suspended material, suggesting they are a more efficient mode of POC export to the deep sea than phytoplankton aggregates. In the summer of 2003, suspended and sinking particles were both enriched in microbial alteration products rather than algal or fecal pellet material, indicating greater exchange during periods of low flux when fecal pellets are not abundant.
To experimentally verify the field results, suspended and sinking particles were collected at two depths (20 m and 200 m) in the Mediterranean Sea in 2006. These particles were incubated independently in rotating tanks to assess exchange between sinking and suspended particles. Particles collected at 20 m were primarily composed of phytoplankton, and particles collected at 200 m were composed of both phytoplankton and fecal pellets. During the course of incubation, sinking phytoplankton aggregates underwent extensive exchange with suspended particles, as evidenced by repeated mass transfers and relative homogeneity in composition. Fecal pellets collected at 200 m by Net Trap underwent less exchange, with some transfer of suspended phytoplankton material into the sinking phase, but no apparent disaggregation. Unlike phytoplankton aggregates, fecal pellets appear to undergo little exchange with surrounding material and are more likely to remain intact during transit from the surface to deep ocean.
The results of this thesis highlight the importance of biogenic minerals and fecal pellets in contributing to the vertical flux of particulate organic carbon from surface waters to the deep sea. Better understanding of the factors controlling the formation and robustness of different particle types is critical for quantitative prediction of particle fluxes and the drawdown of carbon dioxide from the atmosphere.
|School:||State University of New York at Stony Brook|
|School Location:||United States -- New York|
|Source:||DAI-B 69/11, Dissertation Abstracts International|
|Subjects:||Biological oceanography, Biogeochemistry, Organic chemistry|
|Keywords:||Biominerals, Degradation, Fecal pellets, Particulate organic matter|
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