Nitrogen isotope ratios of Cretaceous Atlantic sedimentary sequences

While these lithologies are of marine origin, the black strata in particular have delta15N values that are significantly lower than those previously found in the marine sediment record and most contemporary marine nitrogen pools.

In contrast, black, organic-carbon-rich beds at a third site (DSDP Site 603) contain predominantly terrestrial organic matter and have C- and N-isotopic compositions similar to organic matter of modern terrestrial origin. The recurring 15N depletion in the marine-derived Cretaceous sequences prove that the nitrogen they contain is the end result of an episodic and atypical biogeochemistry. Existing isotopic and other data indicate that the low 15N relative abundance is the consequence of pelagic rather than post-depositional processes.