Abstract
From the complete downcore record of IODP Site 383-U1539, sampling was conducted at 1 m resolution. Quantitative diatom slides were prepared following the settling method of Warnock and Scherer (2015, doi:10.1016/j.csr.2015.04.012) using 15-25 μg of dry bulk sediment treated with 2 ml of 10 % hydrogen peroxide and two drops of saturated sodium hexametaphosphate. Sample solution was added to 2 L Elix water in B-KER2 chambers with prepared coverslips secured on a table above the height of the drainage valve. After settling and draining, coverslips were adhered to glass slides using Norland Optical Adhesive #61 (refractive index = 1.56). Slides were observed with a Leitz Dialux 20 light microscope, using a 100X oil immersion objective and 10X eyepieces for 1000X total magnification. A minimum of 400 valves were counted for each slide. Absolute diatom abundance and relative abundances of species are reported here.
Biogenic silica analysis was conducted at the University of Otago following a modified version of Mortlock and Froelich (1989, doi:10.1016/0198-0149(89)90092-7) for the same set of samples used for diatom analysis. For each analysis, approximately 7 mg of dry sediment was homogenized and dissolved in 0.1 M NaOH solution in an 85°C water bath. Sub-samples were collected after 3, 4, and 5 hours. Dissolved silicon content was measured using a Shimadzu UV-1800 spectrophotometer.