MINERALOGICAL AND TEXTURAL CONTROLS ON THE ORGANIC COMPOSITION OF COASTAL MARINE-SEDIMENTS - HYDRODYNAMIC SEPARATION USING SPLITT-FRACTIONATION

SPLITT-fractionation was used to sort hydrodynamically surficial sediments from the Washington margin, USA, into sand- (>250, 63-250 mu m), silt-(35-63, 17-35, 8-17, 3-8 mu m), and clay-sized (1-3, 0.5-1, <0.5 mu m) fractions. Two types of organic matter were discerned; distinct organic debris and organic matter associated with inorganic sedimentary particles. Discrete organic detritus was a minor component (<10%) of the organic carbon in all the sediment fractions except in sand-sized fractions (>64 mu m) from the shelf, where terrestrially-derived vascular plant debris accounted for >95% of the organic matter. Organic matter that could not be separated from the inorganic sediment accounted for >90% of the total organic carbon in most fractions, and loadings of organic carbon increased as the surface area of the inorganic particles increased. For the sand- and silt-sized fractions, the observed relationship of 0.81 +/- 0.04 mg C m(-2) (r = 0.97) was consistent with the hypothesis that a monolayer of organic matter is sorbed to the mineral surfaces. Clay-sized particles had lower organic loadings (0.37 +/- 0.07 mg C m(-2), r = 0.85), probably because the large interlamellar area of expandable clays was inaccessible to most organic molecules. After correcting for interlamellar area, clay-sized particles have the same organic carbon: surface area relationship as sands and silts (0.78 +/- 0.08 mg C m(-2), r = 0.96). The relationship over all the particle sizes was 0.76 +/- 0.03 mg C m(-2), (r = 0.96). While total organic matter concentrations were largely controlled by sediment surface area, the elemental composition of the organic matter appears to be partially affected by sediment mineralogy, and shifted from carbon-rich material (atomic C:N similar to 18.0) in larger, quartz-dominated fractions to N-rich material (C:N similar to 9)in the smaller, clay mineral-dominated fractions. Nitrogen enrichment relative to carbon (atomic N:C) was correlated with the amount of total clay (r = 0.80), smectite (r = 0.79), and the iron content (r = 0.74) of the sediments. Measurements of stable carbon isotopes indicate that clay-sized particles preferentially transport sorbed soil organic matter to deep sites while sand-sized fractions contain terrestrial plant debris (discrete and sediment-associated) that is transported along the shelf. The concentrations of terrestrially-derived organic matter in organic matter from shelf and slope sediments was estimated to be 60-85% and 10-15%, respectively. The quantity, bulk chemical composition, and distribution of marine and terrestrially derived organic matter to Washington margin sediments are influenced by (1) the surface area of the sediment minerals, (2) the mineralogical composition of the sedimentary matrix, and (3) the natural hydrodynamic sorting of sedimentary materials along the continental margin. The major fraction of organic material in these sediments is sorbed to mineral grains. Interactions between organic material and mineral surfaces strongly influence the distribution and elemental composition of the organic material present in marine sediments.