Publication Date:
2022-05-26
Description:
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 February 2017
Description:
Organic carbon (OC) preserved in marine sediments acts as a reduced carbon sink that balances
the global carbon cycle. Understanding the biogeochemical mechanisms underpinning the
balance between OC preservation and degradation is thus critical both to quantifying this carbon
reservoir and to estimating the extent of life in the deep subsurface biosphere. This work utilizes
bulk and spatially-resolved X-ray absorption spectroscopy to characterize the OC content and
composition of various environmental systems in order to identify the role of minerals and
surrounding geochemistry in organic carbon preservation in sediments. Biogenic manganese
(Mn) oxides formed either in pure cultures of Mn-oxidizing microorganisms, in incubations of
brackish estuarine waters, or as ferromanganese deposits in karstic cave systems rapidly
associate with OC following precipitation. This association is stable despite Mn oxide structural
ripening, suggesting that mineral-associated OC could persist during early diagenetic reactions.
OC associated with bacteriogenic Mn oxides is primarily proteinaceous, including intact proteins
involved in Mn oxidation and likely oxide nucleation and aggregation. Pelagic sediments from
16 sites underlying the South Pacific and North Atlantic gyres and spanning a gradient of
sediment age and redox state were analyzed in order to contrast the roles of oxygen exposure,
OC recalcitrance, and mineral-based protection of OC as preservation mechanisms. OC and
nitrogen concentrations measured at these sites are among the lowest globally (〈0.1%) and, to a
first order, scale with sediment oxygenation. In the deep subsurface, however, molecular
recalcitrance becomes more important than oxygen exposure time in protecting OC against
remineralization. Deep OC consists of primarily amide and carboxylic carbon in a scaffolding of
aliphatic and O-alkyl moieties, corroborating the extremely low C/N values observed. These
findings suggest that microbes in oxic pelagic sediments are carbon-limited and may
preferentially remove carbon relative to nitrogen from the organic matter pool. As a whole, this
work documents how interactions with mineral surfaces and exposure to oxygen generate a
reservoir of OC stabilized in sediments on at least 25-million year time scales.
Description:
This research was supported by the NSF graduate research fellowship 1122374, NSF EAR-
82279000, NASA Exobiology grant NNX15AM04G, WHOI Coastal Ocean Institute and Ocean
Ventures Fund grants, the NSF Center for Dark Energy Biosphere Investigations (C-DEBI,
OCE-0939564) graduate fellowship, and C-DEBI research grant CH20655.
Keywords:
Biosphere
;
Nitrogen
;
Microorganisms
;
Knorr (Ship : 1970-) Cruise KN223
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
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