This award supports a project to make high-resolution logs of dust and ash in the Dome C borehole using an optical dust logger. The dust logger employs a temperature-regulated diode laser at 404 nm wavelength near the absorption minimum for glacial ice, a photon counting detector, onboard computer control and DSL telemetry to the surface. The logger shines light into the ice surrounding a borehole; photons scatter off of bubbles and impurities to be eventually absorbed; a small fraction return to be detected. Annular black nylon brushes sweep ice crystals and debris from the source beam, and intercept stray light to ensure that all detected photons have exited and re-entered the borehole. Depth resolution is determined mainly by the focus of the source and the logger probes into the ice of order one effective scattering length. In bubbly ice the logger signal responds strongly to absorption by thin volcanic ash layers; in clear deep ice the back-scattering signal is a tracer of both ash and continental mineral dust. Logging at 20-50 cm/sec, in a matter of hours, mm-scale depth resolution of dust concentration and volcanic ash layers over the entire 3270 m borehole back to ~800 ka can be obtained
The logger probes an area of order m2 of the horizon compared to the ~0.02 m2 core, greatly suppressing depositional noise and making the technique immune to core damage or loss. The method achieves unprecedented resolution of climate variations for matching or comparing ice core records, can detect particulate layers from explosive fallout which are invisible or missing in the core, and often reveals subtle trend changes which can elude standard core analyses. With the highly resolved dust record, it is expected to find new synchronous age markers between East Antarctica, West Antarctica and Greenland. The data could be instrumental in unifying global climate records, or resolving mysteries such as the transition from 41-kyr glacial cycles to apparent 100-kyr cycles. The project will extend previous finding, which make the most convincing case to date for a causal relationship between explosive volcanic events and abrupt climate change on millennial timescales. A search will also be made for evidence that some of the worldwide explosive fallout events that have been identified may have resulted from impacts by comets or asteroids. The investigators will evaluate the reliability of terrestrial impact crater records and the possibility that Earth impacts are considerably more frequent than is generally appreciated. Better understanding of the factors which force abrupt climate changes, the recurrence rate and triggering mechanisms of large volcanic eruptions, and the frequency of Gt to Tt-energy bolide impacts are of vital interest for civilization.