This award provides funds, under the auspices of the EArly-concept Grants for Exploratory Research (EAGER) program, to investigate whether a bolide impact occurred at the beginning of the Younger Dryas (approximately 12,900-11,500 years ago) thereby bringing about dramatic climatic change. The research team plans to accomplish this science goal by measuring iridium concentrations in the Greenland Ice Sheet Project Two (GISP2) ice core across the Allerød-Younger Dryas boundary.
The researchers argue that, as with other impact studies, anomalously high iridium concentrations are an indicator of a major bolide impact and that the ice core analyses should provides a continuous, well-documented, and well-preserved stratigraphic sequence in which to search for an anomaly. They estimate that an impact of the currently hypothesized magnitude would have produced an iridium signal between 200 and 130,000 times the background iridium level due to the continuous accumulation of terrestrial and cosmic dust. As such, the researchers postulate that their analyses will be a definitive test of the impact.
The research could have broad impact on the wider science community by catalyzing new scientific thinking about the existing scientific theory that a bolide impact was a contributory cause of abrupt climatic change at the Younger Dryas boundary.
The planned activities fit well into the potentially transformative, high risk, and exploratory nature of the EAGER programs.
The Younger Dryas (YD), a millennium-long cooling period amidst post-glacial warming, is well documented in the Greenland ice cores but apparently missing in Antarctica. It is thought to result from an abrupt change in atmospheric and oceanic circulation, but the cause of such a change remains controversial due to the lack of compelling evidence. Among other causes, the Younger Dryas impact hypothesis is based on observations and is testable. It interprets the C-rich layer (referred to as YDB – YD boundary exposed in many sites in the Northern America and Europe) enriched in magnetic grains with Ir, magnetic microspherules, charcoal, soot, carbon spherules, glass-like carbon with nanodiamonds, and fullerenes with extraterrestrial He as evidence for impact or aerial blast at ~12,900 years ago. Subsequent studies questioned some petrographic evidence for an impact or found new ones. However, the invoked petrographic impact markers have never been supported by a geochemical signature of an impact such as a sharp increase in Ir or other PGE concentrations. We measured trace and major element concentrations in ice samples from the GISP2 ice core across the Bølling-Allerød/YD boundary (depth of 1709-1720 m, 12279-13064 years old) with a spatial resolution of ~12.5 cm (time resolution of 2.5-4.6 years). The samples were analyzed in two analytical sessions, first for Ir, Pt, Lu, and Hf and then for Mg, Al, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Zr, Ru, Rh, Pd, Ag, Ba, La, Ce, and Au using a GV Instruments Platform ICP-MS equipped with an Apex inlet system in order to increase sensitivity and reduce oxide interferences. Ratios of LuO/Lu (<0.01) and HfO/Hf (<0.05), measured in a Lu-Hf solution together with ice samples, were used to correct for LuO and HfO interferences on 191Ir and 194,195,196Pt, respectively. The corrections were typically <5%. For calibration we used a 10 ppb Pt-Ir solution made from 1000 ppm HPS single element standards. The BHVO-1 standard was used for major and trace elements. Our results will soon be submitted for publication and discussed in terms of the various proposed theories for the Younger Dryas cooling. The main conclusion of our study is the detection of an unusual catastrophic event during the Bølling-Allerød - YD transition period, but our results clearly rule out an impact or aerial explosion of a chondritic bolide.
