Global comparisons of organic matter in sediments across the Cretaceous/Tertiary boundary

A~tract--The Cretaceous/Tertiary (K/T) boundary is marked by extensive changes in the sedimentary fossil record of continental and marine life. Organic matter in sediments from above and below the K/T boundary has been studied to identify consequences of the biotic extinctions and of the changes in biological productivity which occurred at this time. Data from locations encompassing a variety of marine paleoenvironments were examined to assess the global extent and local expressions of these biological changes. Characterizations of organic matter included Rock-Eval pyrolysis, stable carbon isotope ratios, and distributions of extractable biomarkers. In general, organic carbon concentrations are a few tenths of a percent in the chalks and marls above and below the K/T boundary; in some boundary clay samples concentrations up to 5% are found. The elevated amounts near the boundary result from oxidation of marine organic matter, consequent dissolution of carbonates, and concentration of the surviving organic matter. At all locations, the organic matter has been microbially reworked and evidently was deposited in oxidizing environments. Carbon isotope shifts in open marine sediments suggest biological productivity was depressed following the K/T boundary. Repetition of these changes at different locations verifies the global extent of the boundary event, yet local variations in its expression are evident.