METHODS

Sediment samples were divided for size-fraction microscopic and geochemical analyses. Subsamples were prepared for microscopic analyses by washing through 40- and 63-µm sieves. The <40-µm fraction residue was retained for later clay-mineral analyses, and the >40-µm fraction was used for coarse-fraction analysis and eventual isotope studies. In the coarse-fraction analysis, the >63-µm fraction was sieved into 63- to 125-µm, 125- to 250-µm, 250- to 500-µm, and >500-µm fractions. In each fraction, 800 grains (if present) were counted and up to 25 biogenic, lithogenic, and authigenic components were distinguished. The relative composition of the sand (>63 µm) and coarse silt (40- to 63-µm) fractions were calculated by multiplying the percentage of each individual component in each fraction by the weight of the fraction. These data are the basis for calculations of the mass accumulation rates (MAR) of components using the following formula:

where MAR_comp and MAR_{bulk sed} are the mass accumulation rates of various sediment components and the bulk sediment, respectively. The stratigraphy used for the calculation of linear sedimentation rates and MARs is based on oxygen isotope data of benthic foraminifers, a continuous X-ray fluorescence iron (XRF-Fe) profile, and orbital tuning.

Subsamples intended for geochemical analyses were first freeze-dried and then analyzed for CaCO₃ using the carbonate bomb technique of Müller and Gastner (1971). Weighed samples were reacted with 3-N HCl, and the volume of CO₂ released from each sample was measured and compared to the volumes released from known amounts of pure CaCO₃ to determine the percentage in the sample. The carbonate-free residue remaining after acid treatment was collected, rinsed, and dried for elemental CHN determinations. Organic carbon and nitrogen concentrations in the carbonate-free residues were directly measured with a Carlo Erba 1108 CHNS analyzer. This procedure involves heating the sample at 1020°C and measuring the combustion products by gas chromatography (Verardo et al., 1990). Known amounts of sulfanilamide (C₆H₈N₂O₂S) are used to calibrate the instrument and to calculate the quantities of carbon and nitrogen released from each sample. Total organic carbon (TOC) concentrations were calculated on a whole-sediment basis, adjusting for the carbonate concentrations determined from the bomb technique. C/N ratios were calculated on an atomic basis.

Organic ¹³C values were determined from analyses done in the stable isotope laboratory at the University of Michigan. Carbonate-free sediment samples were combusted at 800ºC in sealed Vycor tubes in the presence of CuO and Cu. The ¹³C/¹²C ratios of organic carbon were determined with a Finnigan Delta-S mass spectrometer calibrated with the NBS-21 (graphite) standard. Data are corrected for ¹⁷O and are expressed in conventional ¹³C notation relative to the Peedee belemnite standard.