Fluctuations of the oxygen minimum zone at the end of Oceanic Anoxic Event 2 in the Gulf of Mexico and the response of a
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Fluctuations of the oxygen minimum zone at the end of Oceanic Anoxic Event 2 in the Gulf of Mexico and the response of ammonites Christina Ifrim1
Received: 25 February 2015 / Accepted: 11 June 2015 Akademie der Naturwissenschaften Schweiz (SCNAT) 2015
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.
& Christina Ifrim [email protected] 1
Institut fu¨r Geowissenschaften, Ruprecht-Karls-Universita¨t, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany
Keywords OMZ fluctuation Faunal change Ammonites Turonian Mexico Cretaceous
Introduction The Oceanic Anoxic Event (OAE) 2 was a palaeoceanographic event which caused a major global perturbation in the Cretaceous oceans ca. 95 Ma ago. It is related to one of the most extreme carbon cycle perturbations in Earth’s history. Increased rates of organic carbon were buried in marine sediments at the height of the mid-Cretaceous greenhouse (Voigt et al. 2008). The OAE 2 is characterized globally by a major positive carbon isotope excursion in carbonates and organic matter (Arthur et al. 1987; Gale et al. 1993; Jenkyns et al. 1994; Tsikos et al. 2004; Sageman et al. 2006). It was accompanied by a rapid and profound warming of surface and shallow waters (Huber et al. 1999; Voigt et al. 2004, 2008; Forster et al. 2007), and the global occurrence of sediments indicating oxygen deficiency in the oceans. It has been shown that this oxygen deficiency did neither start nor end contemporaneously (Lu¨ning et al. 2004; Tsikos et al. 2004; Voigt et al. 2008). This is why OAE 2 was defined to end with the cessation of the positive excursion
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