The biogenic opal data presented in this report were measured at the Lamont-Doherty Earth Observatory Marine Sediment Analysis Facility (MSAF) using the reduction colorimetric technique described in Mortlock and Froelich (1989). A sample between 25 and 200 mg (depending on Si and CaCO3 content) is freeze dried, powdered, weighed, and placed in 50-mL centrifuge tubes. The sample is oxidized using 10% H2O2 solution to remove organic carbon and then decarbonated using 5 mL of 1 N HCl. Twenty milliliters of deionized water is added to each tube, the sample is centrifuged at 4500 rpm, and the supernatant is discarded. A single-step extraction of Si is performed on the samples using 40 mL of 2 M Na2CO3 at 85°C for 5 hr. After centrifugation, 20 mL of the supernatant is transferred to a polyethelene vial. Dissolved silica is determined by amolybdate blue spectrometry. Absorbances are read at 812 nm using a Milton Roy Spectronic 501 spectrophotometer with a sipper flow-cell attachment.
Relative analytical precision, based upon hundreds of internal marine sediment standard analyses performed at the MSAF, is ±4%. Relative analytical precision is reduced to ±8% for samples with low opal content (<15 wt%).
Results in this data report are presented as percent biogenic opal as estimated from the equation