METHODS

All samples were freeze-dried overnight and subsequently ground. Concentrations of calcium carbonate were determined using the "carbonate bomb" technique (Müller and Gastner, 1971), which measures the volume of CO₂ released by treatment of the sediments with 3-N HCl. Inorganic carbon concentrations were calculated assuming that all inorganic carbon is present as CaCO₃.

The carbonate-free residues were washed and rinsed three times with deionized water, oven dried, and analyzed for organic carbon and total nitrogen (TN) using a Carlo Erba EA1108 CHNS-O analyzer. Relative precision for carbon is ±2% and ±3% for nitrogen. TOC and TN concentrations are expressed on a whole-sediment basis after adjusting for the removed carbonate fraction. C_org/N_tot ratios are calculated on an atomic basis.

The ¹⁵N and ¹³C_org values of bulk organic matter were determined on the carbonate-free sediments using a continuous-flow gas-ratio mass spectrometer (Finnigan Delta Plus XL) coupled to an elemental analyzer (Costech) at the Laboratory of Isotope Geochemistry, University of Arizona (USA). Data are expressed in the conventional ¹⁵N and ¹³C notations relative to air and Peedee belemnite standard, respectively. Standardization is based on International Atomic Energy Agency (IAEA)-N-1 and IAEA-N-2 for ¹⁵N, and National Bureau of Standards-22 and U.S. Geological Survey-24 for ¹³C. Precision is better than ±0.2 for ¹⁵N and ±0.06 for ¹³C, based on repeated internal standards.

Freeze-dried ground sediment samples were analyzed by programmed pyrolysis at the Institute of Sedimentary and Petroleum Geology of the Geological Survey of Canada (Calgary, Alberta) using a Delsi Nermag Rock-Eval 6 system. The Rock-Eval 6 yields T_max values that are systematically higher than those from the Rock-Eval 2 system that is the basis for T_max interpretations, so the values we report have been adjusted to the Rock-Eval 2 standard.

For determination of the major and minor element compositions, samples were prepared using a sodium peroxide digest technique modified from Knoop (2005). Approximately 0.1000 g of sample was well mixed with 1 g of sodium peroxide in a zirconium crucible and heated in a 500°C oven for 30 min. After cooling to room temperature, 5 mL of 5% HNO₃ was slowly added to the solid residue in the crucible and the sample was transferred to a 60-mL Nalgene bottle. Next, 15 mL of 5% HNO₃ was added to the bottle along with 5 drops of concentrated HCl and 20 drops of HF. The total sample volume was brought to ~50 mL with additional 5% HNO₃. Samples were shaken by hand for ~30 s, at which time no visible solid residue remained. Elemental analyses of Al, Ti, Fe, Mg, K, Ba, Cr, and Zn were carried out by inductively coupled plasma–optical emission spectrometer (ICP-OES) using a PerkinElmer Optima 3300DV. Analyses for Cd, Cu, Mo, Re, U, and V were carried out by ICP-mass spectroscopy (MS) on a Finnegan Element mass spectrometer.