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43282-AC2
Partitioning of Organic Inputs Among Organic and Mineral Phases During Early Diagenesis
In this project, three questions about earliest diagenesis are being addressed: Is organic matter preserved primarily in its “native state”, or is it subject to significant microbial repackaging? Is organic matter preferentially retained within specific organic and/or mineral matrices? How rapidly does organic matter move between nominally labile phases and more refractory phases?
There is evidence for preservation of microbial products in some studies (e.g., Grutters et al., 2002; Pedersen et al., 2001), but laboratory incubation studies (e.g., Zhang et al., 2001) have not shown evidence of retention of microbial products. The latter studies have been conducted as water column incubations. Competition of sorption with degradation in particle-rich sedimentary environments may favor the preservation of both detrital and microbial products. Encapsulation within both biological matrices, such as cell walls, or within mineral aggregates has been proposed as a preservation mechanism, but the time scales over which these reactions occur have been only vaguely defined.
To investigate these issues, a 13C labeling study of earliest (~ 1 year) diagenesis has been undertaken. The overall strategy is to introduce ~100% 13C labeled organic matter (as algae) to sediment microcosms and to trace the partitioning of labeled isotopomers among mineral and organic phases within the sedimentary matrix. Incubated sediment samples are separated into different density fractions by heavy-liquid centrifugation. The fractions are then subject to chemical extractions of increasing rigor in order to separate material that is loosely-bound vs. tightly-bound to the organic-mineral matrix. Fatty acids and amino acids are purified from the organic residue and then analyzed by GC-MS as their MTBSTFA derivatives. Bacterial transformations of labeled inputs are identified as either bacterial biomarkers (among the fatty acids) or a mixed (13C +12C) label amino acid or fatty acid molecules indicative of microbial resynthesis from labeled and unlabeled precursor compounds. To date, all of the prefatory analytical methods development for sediment separations and isotopomer measurements have been completed. However, the experiments themselves have been deferred because of extensive research cruise conflicts.
A no-cost extension has been requested to allow the experiments to proceed.
