Publication Date

Spring 2000

Abstract

A massive extinction of more than 50 percent of existing life forms on Earth occurred 65 million years (Ma) ago. This event is marked in the geological record by the Cretaceous-Tertiary (KT) boundary and corresponds to the Chicxulub meteorite impact in the Yucatan Peninsula. Since 1985, large quantities ofreduced elemental carbon in the form of characteristic spheroidal clusters of soot have been found in twelve KT boundary sites from across the globe. Because ofthe wide geographic distribution ofthese sites, the data was interpreted to indicate that deposition ofsoot was a global phenomenon. The source of this global soot layer is suspected to be eolian (airfall) deposition of fine-grained particles resulting from widespread wildfires. A global soot concentration of 2.2 ± 0.7 . mg/cm2 has been estimated from these studies.

As a systematic study of the correlation between meteorite impact and soot formation, five sedimentary sequences ofmeteorite impact-related samples were analyzed for the presence ofelemental carbon in the form of soot. Two KT boundary sedimentary sequences from the Berwind, Colorado, and Madrid, Colorado sites were analyzed as continuations of previous KT boundary studies. Soot concentrations of 3000 ± 0 parts per million (ppm) and 780 ± 90 ppm, respectively, were found in the KT boundary sediments ofthese sequences. These concentrations are similar to those found in previous KT boundary studies and further refine the KT boundary global soot concentration value.

Likewise, samples from Deep Sea Drilling Project (DSDP) Core 465-A were analyzed as a determination ofthe global nature ofthe soot layer. A soot concentration of 550 ± 750 ppm was found, again similar to those found in previous KT boundary studies. The mid-Pacific location of Core 465-A rules out the possibility ofsoot deposition from' groundwater runoff and indicates that eolian transport is the most probable mecharllsm for soot deposition. Therefore, the presence of soot in boundary sediments in DSDP Core 465-A supports the theory that the KT boundary soot layer is global.

In addition, samples related to two other impact events of differing size and age were analyzed for evidence ofwildfires. In the first study, samples were analyzed from the Sudbury, Ontario impact structure. This structure is the result of an impact similar in size to the Chicxulub event occurring 1850 Ma ago. These sediments were found to contain soot similar in concentrations to those found in KT boundary studies, ranging from 2300 ± 200 ppm soot to 3000 ± 300 ppm soot across the post-impact sedimentary sequence. The presence of soot in these sediments further strengthens the correlation between meteorite impact and soot formation.

In a second study, samples were analyzed from the Gardnos impact structure, Norway. This structure is the result ofan impact at least one order ofmagnitude smaller than the Chicxulub event occurring between 900 and 400 Ma ago~ No appreciable amounts of soot were found in sediments directly related to the impact event. The absence ofsoot in Gardnos samples suggests that an impact event ofthis size is below the impact threshold required for ignition of soot-producing wildfires.

Disciplines

Chemistry

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