Friday, February 24, 2006

Mid-Cretaceous Cold Not Hot

Recognizing the Albian-Cenomanian (OAE1d) sequence boundary using plant carbon isotopes: Dakota Formation, Western Interior Basin, USA. 2006. Darren R. Gröcke Geology 34: 193–196

High-resolution stable-isotope analysis of fossil wood from a mid-Cretaceous (~100 m.y. ago) terrestrial section in Nebraska provides us with the ability to precisely correlate terrestrial with detailed oceanic records in a way that was not previously possible. Based on matching the shape of the oceanic and terrestrial curves it is evident that a portion was missing in the terrestrial curve, representing less than 500,000 yr. A global regression (sea-level fall) is known to have occurred during this time interval and would explain the missing part of the terrestrial curve. This short-lived regressive phase coincides with a breakdown in oceanic stratification and a marine extinction event. Gröcke et al. interpret this rapid change in sea level as a result of glacier formation; this is contrary to the current understanding of the mid-Cretaceous period, which is considered as a super-greenhouse period. Further investigation of terrestrial sequences will provide a greater understanding of the extent and duration of this sea-level event and whether in fact it was caused by glacial cycles in a greenhouse Earth.