Fluctuations of the oxygen minimum zone at the end of Oceanic Anoxic Event 2 in the Gulf of Mexico and the response of ammonites

Abstract

The latest Cenomanian to basal middle Turonian (Late Cretaceous) section at Vallecillo, Mexico, contains a continuous and highly fossiliferous sedimentary record. It preserves a detailed insight into a pelagic ecosystem that recovered from Oceanic Anoxic Event (OAE) 2. The deposit formed during this period under the influence of an expanded, but fluctuating oxygen minimum zone, which also affected the fauna. In this section, the abundances of ammonites do not correlate with those of inoceramids; they not only rule out a preservational bias but also indicate that ammonites and their hatchlings populated shallower water depths than inoceramid larvae and planktic foraminifers, contrasting previous models. The most abundant species at Vallecillo, Pseudaspidoceras flexuosum, shows a response to these environmental changes. Pseudaspidoceras flexuosum clearly inhabited the open water column. It is among the first to reappear at the end of OAE 2 and may have been adapted to the layered water body. The improving conditions during the early Turonian led to a migration of P. flexuosum into other seas and oceans. Phases of a contracted OMZ are phases of immigration and changes in the ammonite faunas at Vallecillo, whereas phases of an expanded OMZ are phases of stagnation. Although the abundance of ammonites is independent of fluctuations of the OMZ, their dispersal and migration are related to them. This study is an example for the complexity of species-based analyses.