Methods Carbonate, TOC and TN: Total carbon and total nitrogen were analyzed on a Carlo Erba 1500 elemental analyzer (Milan, Italy) with a precision of 0.2% and 0.005%, respectively. Total organic carbon (TOC) was measured with the same instrument after samples were treated with 1 M hydrochloric acid (HCl) to remove inorganic carbon. Analytical precision for organic carbon was 0.02%. Carbonate carbon was calculated as the difference between total carbon and organic carbon. Lithogenic matter: Lithogenic matter (%) was calculated by subtracting carbonate, organic matter and biogenic opal from 100%.