Hydrodynamic sorting of Washington margin sediments using SPLITT fractionation

SPLITT fractionation: Sieving and SPLITT (split flow thin cell) fractionation were used to hydrodynamically sort surface sediments collected along an E-W transect from the mouth of the Columbia River, across the Washington continental margin, to the Cascadia Basin in the N.E. Pacific. Sediments were separated into various coarse (>250, 100-250, 63-100, 38-63 and <38 μm) and two fine (38->1m/d and 38-<1m/d) fractions. The sieve-SPLITT method was recently tested and validated by using a variety of natural sediments and results indicated that many biogeochemical processes governed by particle dynamics could be examined using this approach. Washington Margin results: Mass distribution of sieve fractions indicates that sediments from the Washington shelf are dominated by large particles (>250 μm) whereas fine particles (<38 μm) are predominant in slope and Cascadia Basin sediments. This is consistent with the natural grain size distribution occurring in Washington margin sediments. The mass distribution is shown. Total Organic Carbon (TOC) contents of the sediments generally follow the same distribution as the mass. With the exception of inner shelf stations, most TOC resides in the fine fraction and ranges around 1.5-3%. Stable carbon isotopic measurements on TOC indicate an increase in δ13C values from the shelf to the Cascadia Basin (from -34 to -20 per mille). Consistent with this, TOC/TON ratios decrease along the same transect (from 80 to 10), suggesting a transition from terrestrial to autochthonous marine sources. These results will be completed by those from the splittate fractions. Results of other analysis of the sieve-SPLITT fractions (inorganic elements, surface area, mineralogy and Sr isotopes) will also be presented