Changes in rainfall accompanied and perhaps partly caused the development of oxygen-poor oceans (OAE2), about 94 million years ago. Since the seventies it is known that at that time, the age of the dinosaurs, ocean waters in many regions became poor in oxygen, which all animals require. OAE2 is, hence, often regarded as an analogue of today, as oxygen concentrations are currently decreasing in many coastal waters due to human impact. This new paper shows evidence for large changes in the water cycle during OAE2, based on the chemistry and microfossils preserved in sediments deposited in a shallow sea, the current New Jersey coastal plain. When widespread ocean-anoxia developed, marine algae show an increase in fresh water from rivers. Moreover, during a brief cooling, when oceans became slightly richer in oxygen, freshwater supply decreased and dust input increased. This indicates that climate, not only temperature but also precipitation, was strongly coupled to the availability of oxygen in the ocean. Potentially, the additional supply of freshwater to coastal waters regionally inhibited the mixing of oxygen-rich surface waters with deeper waters, causing a lack of oxygen. Analogously, modern global warming and associated intensification of seasonal rainfall might also result in near-shore oxygen depletion.
Trilobites in early Cambrian tidal flats and the landward expansion of the Cambrian explosion
M. Gabriela Mngano et al., Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada, . Posted online ahead of print on 13 Dec. 2013; http://dx.doi.org/10.1130/G34980.1.
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Geological Society of America