The primary focus of my present research is on submarine impact craters and their contribution to climate change and megatsunamis. This research grew out of my work on the thermal history of the earth when Ann Isley and I discovered that mantle plumes had the same periodicity as impact cratering events (Isley and Abbott, 2002, Journal of Geology, 110, 141-158). We did a compilation of impact cratering events and found that the record was woefully undersampled (Abbott and Isley, 2002. Earth and Planetary Science Letters, 205, 53-62). As a result, I started to look for impact craters on the ocean floor. I found that the Holocene age impact crater candidates could be located using a combination of bathymetry derived from satellite altimetry and the directions to the source of chevron dunes. I am now part of a research group (the Holocene Impact Working Group) that is focusing on two goals: looking at the effect of submarine impacts on climate and determining if chevron dunes are megatsunami deposits. We have located a candidate crater set in the Gulf of Carpentaria, Australia with an inferred age of AD 572±86 (Abbott et al., 2007). The craters have produced impact spherules of magnetite, impact glass, and probable shocked quartz. The date of AD 572±86 is, within error, the same as the age of the climate downturn at AD 536. Ice core work is underway to see if the samples of the GISP2 ice core dating to 536 A.D. contain impact ejecta from the Carpentaria craters. We are also working on samples from chevron dunes in Madagascar to see if they contain impact ejecta.
Past Research Focus
My past research was on the thermal history of the earth, and the manner in which heat transport through the crust and upper mantle influenced geological processes, both ancient and present-day.
An important theme in my past research centers about my theory that plate tectonics - meaning sea floor spreading and subduction - has been active over nearly the entire span of earth history, that is, since the early Archean era 4 billion years ago. The key difference between ancient and present-day tectonics is due to the ever decreasing thickness of oceanic crust caused by falling upper-mantle temperature. The most important consequence of this decrease, which I investigated in my 1984 Abbott & Hoffman Tectonics paper, has been a change in the style of subduction, from predominantly shallow-dipping "buoyant" subduction in the Archean to predominantly steeply-dipping "Andean" subduction today. The rarity of Archean andesites and potassium-rich granites is thus explained. In my 1991 Geophys. Res. Lett. paper I propose that buoyant subduction is also responsible for the formation of the thick tectosphere beneath Archean cratons, because buoyant oceanic lithosphere is carried beneath and underplates originally thinner continental lithosphere. This underplating mechanism is able to explain the low degree of metamorphism of most cratons, because the mantle is thichened without significant horizontal compression of the crust.
Without independent information on the amount of cooling of the upper mantle, and hence the thickness of oceanic crust through geologic time, these ideas would be rather speculative. I have therefore concentrated my recent efforts on deriving an upper mantle cooling history from petrological data (my 1994 Abbott et al. J. Geophy. Res. paper). This work used measurements of the iron-magnesium ratio for a suite of "MORB-like" rocks to infer their temperature of formation. We are able to estimate both the distribution of mantle temperatures at any one time in earth history and a clear decrease of the mean temperature with time. These temperature data, together with a Mackenzie & Bickle - style model for crustal production, allow us to predict the time of transition between styles of subduction. The timing agrees very well with appearance of rock suites associated with Andean subduction.