TEM study of in situ organic matter on continental margins: occurrence and the “monolayer” hypothesis

TEM photomicrographs show that organic matter in the continental margin sediments that we have examined is primarily patchy in distribution and occurs as: (1) discrete, discontinuous blebs and micro-blebs of differentiated and undifferentiated protoplasm; (2) bacterial cells and associated muco-polysaccharide networks and (3) localized smears generally associated with clay minerals and domain junctions in clay-rich flocs, not as thin uniform grain coatings or as infillings of nicks or etch pits on sediment grains with high degrees of surface roughness. This occurrence contradicts recent assertions that most organic matter in continental margins occurs as monolayer or “monolayer equivalent” coatings on mineral grains and inorganic bioclastic particles. Our results are based on TEM photomicrographs of samples from the northern California continental slope that fall within the “monolayer equivalent” envelope defined by Mayer (1994), marine snow from the overlying nepheloid layer, and sediments from Eckernforde Bay in the Baltic Sea. These samples were prepared specifically to preserve and image in situ sediment fabric and organic constituents. Textural evidence suggests that low permeabilities caused by the interaction between the organic, inorganic and biologic components of the clay size fraction play a major role in determining organic matter preservation on continental margins.