The Winter of Our Discontent: short-term cooling following the Chicxulub impact.

The K-T impact by Don Davis.

The Cretaceous–Paleogene extinction that followed the  Chicxulub impact was one of the five great Phanerozoic  mass extinctions. The impact released an estimated energy equivalent of 100 teratonnes of TNT and produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere. Model simulations suggest that the amount of sunlight that reached Earth’s surface was reduced by approximately 20%.This decrease of sunlight caused a drastic short-term global reduction in temperature. This phenomenon is called “impact winter”.

Cold and darkness lasted for a period of months to years.  Photosynthesis stopped and the food chain collapsed. This period of reduced solar radiation may only have lasted several months to decades. Three-quarters of the plant and animal species on Earth disappeared. Marine ecosystems lost about half of their species while freshwater environments shows low extinction rates, about 10% to 22% of genera.

A paleogeographic map of the Gulf of Mexico at the end of the Cretaceous (From Vellekoop, 2014)

Three factors can be associated with the impact winter in marine and fresh water environments. First, starvation caused by the stop of photosynthesis. Second, the loss of dissolved oxygen. Third, the low temperatures. Because the late Cretaceous climate was warm, a major challenge for aquatic organisms, especially in inland waters, may have been the persistence of low temperatures. Additionally, the vapour produced by the impact  could have led to global acid rain and a dramatic acidification of marine surface waters.

Fossil evidence for this impact winter was recovered in the Brazos River region of Texas.  The biostratigraphy of the section presents the Ir anomaly, and impact-related tsunami beds. The age of the outcrops was updated using  planktonic foraminifera and  dinocysts.

The “impact winter”  model is supported by a migration of cool, boreal dinoflagellate species into the subtropic Tethyan realm directly across the K–Pg boundary interval and the ingression of boreal benthic foraminifera into the deeper parts of the Tethys Ocean, interpreted to reflect millennial timescale changes in ocean circulation following the impact (Vellekoop, 2014).

References:

Johan Vellekoop, Appy Sluijs, Jan Smit, Stefan Schouten, Johan W. H. Weijers, Jaap S. Sinningh Damsté, and Henk Brinkhuis, Rapid short-term cooling following the Chicxulub impact at the Cretaceous–Paleogene boundary, PNAS (2014) doi: 10.1073/pnas.1319253111

Douglas S. Robertson, William M. Lewis, Peter M. Sheehan and Owen B. Toon, K-Pg extinction patterns in marine and freshwater environments: The impact winter model, Journal of Geophysical Research: Biogeosciences, JUL 2013, DOI: 10.1002/jgrg.20086.