The importance of ultrafine particles as a control on the distribution of organic carbon in Washington Margin and Cascadia Basin sediments

The combined effects of hydrodynamic particle sorting, sorptive protection and oxygen exposure time (OET) on the distribution of organic carbon in continental margin surface sediments was investigated in detail using the SPLITT fractionation (Split Flow Thin Cell) technique along a well-characterized seven-station transect from the inner Washington shelf to the central Cascadia Basin. The new data highlight the significance of virtually buoyant ultrafine particles (size < 38 μm and settling velocity < 1 m/d) for organic carbon (OC) deposition. For the entire surface sediment transect, this ultrafine particle fraction hosted 45–85% of the bulk OC, with an increasing contribution further away from the coast. In this fraction, OC to mineral Surface Area (OC:SA) ratios generally ranged from 0.5 to 1.1 mg OC m− 2, which is typical of most continental margin sediments, underlining the key role that ultrafine particles undertake in OC sorption. An exception was the lower OC:SA ratios in the < 38 μm fraction of Cascadia Basin sediments, which coincided with an order of magnitude increase in Mn oxyhydroxides abundance. An inverse relationship between OC:SA and opal was attributed to a diluting effect of opal on the SA and a lack of association between opal and OC. Finally, two distinct regimes with different preservation controls were revealed along the shelf–slope–basin transect. Finally, the shelf sediments do not follow the relationship between OET and OC:SA observed in the slope and basin sites, presumably due to rapid sedimentation closer to the coast.