Letters to Nature

Nature 409, 325-327 (18 January 2001) | doi:10.1038/35053041; Received 10 July 2000; Accepted 8 November 2000

Increased thermohaline stratification as a possible cause for an ocean anoxic event in the Cretaceous period

Jochen Erbacher1, Brian T. Huber2, Richard D. Norris3 & Molly Markey2,4

  1. Bundesanstalt für Geowissenschaften und Rohstoffe, Referat Meeresgeologie, Stilleweg 2, 30655 Hannover, Germany
  2. Department of Paleobiology, NHB-121, National Museum of Natural History, Smithsonian Institution, Washington DC 20560, USA
  3. Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543-1541, USA
  4. Present address: Department of Invertebrate Paleontology, Museum of Comparative Zoology, 26 Oxford Street, Harvard University, Cambridge, Massachusetts 02138, USA.

Correspondence to: Jochen Erbacher1 Correspondence and requests for materials should be addressed to J.E.
(e-mail: Email: erbacher@bgr.de).

Ocean anoxic events were periods of high carbon burial that led to drawdown of atmospheric carbon dioxide, lowering of bottom-water oxygen concentrations and, in many cases, significant biological extinction1, 2, 3, 4, 5. Most ocean anoxic events are thought to be caused by high productivity and export of carbon from surface waters which is then preserved in organic-rich sediments, known as black shales. But the factors that triggered some of these events remain uncertain. Here we present stable isotope data from a mid-Cretaceous ocean anoxic event that occurred 112 Myr ago, and that point to increased thermohaline stratification as the probable cause. Ocean anoxic event 1b is associated with an increase in surface-water temperatures and runoff that led to decreased bottom-water formation and elevated carbon burial in the restricted basins of the western Tethys and North Atlantic. This event is in many ways similar to that which led to the more recent Plio-Pleistocene Mediterranean sapropels, but the greater geographical extent and longer duration (approx46 kyr) of ocean anoxic event 1b suggest that processes leading to such ocean anoxic events in the North Atlantic and western Tethys were able to act over a much larger region, and sequester far more carbon, than any of the Quaternary sapropels.