There are four physical forms of organic matter (OM) in marine sediments; free of mineral association, bound within biogenic minerals, clustered with lithogenic minerals forming aggregates, and sorbed to mineral surfaces. Proportions of the types of OM change with depositional regime and oxygen exposure time (OET). In sediments underlying suboxic waters (OET < 1y), the predominant form of OM is as distinct debris (65% of the total OM). Along continental margins, where most OM is stored and where OET is more moderate (years to centuries), the predominant form of OM is within mineral aggregates (50%). Along the continental rise, where sediments are exposed to oxygen for longer periods (centuries to millennia), the little OM that is present is predominantly sorbed to lithogenic minerals (60%). OM in association with biogenic minerals is a consistently small (<10%) component of total OM except for the rise, where biogenic- protected OM approaches 20% of total OM.
In this project, researchers at the University of Washington will conduct laboratory experiments to evaluate the short-term effects of aggregation and OET on early diagenesis. These experiments will directly test whether it is oxygen itself that causes the OET effect, or whether fauna and mixing are more important contributors. Similarly, a field investigation of sediment dynamics in a closed fjord system (Clayoquot Sound, British Columbia) will address this question. To look at the longer-term effects of aggregation on diagenesis (and vice-versa), a set of sediment multicores recovered from the Washington and Vancouver margins will be split into different physical fractions and probed using a combination of .14C and d13C isotopic analyses (to determine age and source), amino acid analyses (to determine degradation state), elemental analyses, and X-ray photoelectron spectroscopy (XPS; to determine OM and mineral interactions and OM glue thicknesses). This multi-faceted approach will permit quantitatively estimation of both the short-term degradation rates of the different physical forms of OM (even if they have similar bulk OM compositions) and the longer-term influence of physical reworking and OET on aggregate abundance and OM preservation. This study will also help probe the processes that may be responsible for the loss of terrestrial OM and its replacement by marine OM during lithogenic mineral transport and burial.
Broader Impacts: An understanding of organic matter cycling and storage is important on a broad scale and is amenable to evaluation at a variety of levels of detail and experience. This project will provide quantified rate and fate information on carbon cycling and training for graduate students. In addition, it will leverage an undergraduate mentoring program, the Apprenticeship Learning Program, where undergraduate researchers are mentored by two faculty members and receive multidisciplinary research training
