2.2. Measurement of biogenic opal, POC and lithogenic matter fluxes
Biogenic opal was determined by applying a modified version of the method described by Mortlock and Froelich (1989), includ- ing sequential treatment of the samples with 2 N HCl and H2O2 (2%) to remove carbonate and organic matter, and leaching of the residual material with 7% Na2CO3 at 85 1C for 5 h. Subsequently, the concentration of dissolved silica in the supernatant was measured photometrically, and the mean standard deviation of the triplicate analyses was 7 1%. Total particulate organic carbon (POC) was measured on a Carlo Erba NA 1500 Elemental Analyzer after removal of carbonate carbon by acidification of the sample with 1 N HCl. The precision of this method was 7 1%. An empirical factor of 1.8 was used to convert organic carbon to total organic matter (TOM) (Müller et al., 1986).