P31A-1880: Large Quantities of Melt-Quenched Impact Spherules in Late Pleistocene Alaskan and Yukon "Muck" Deposits
Authors: Jonathan T Hagstrum1, Richard B Firestone2, Allen West3, James C Weaver4, Ted E Bunch5, David R Kimbel6
Author Institutions: 1. U.S. Geological Survey, Menlo Park, CA, USA; 2. Lawrence Berkeley National Laboratory, Berkeley, CA, USA; 3. GeoScience Consulting, Dewey, AZ, USA; 4. Harvard University, Cambridge, MA, USA; 5. Northern Arizona University, Flagstaff, AZ, USA; 6. Kimstar Research, Fayetteville, NC, USA
The so-called “muck”ù deposits of Alaska and the upper Yukon are wind-transported, silt-dominated, organic-rich sediments, including paleosols, erosional unconformities, and buried forests with in situ stumps, that have accumulated in creek valleys over at least the last 2 million years. Underlying the frozen muck are gold-bearing gravels, and removal of Late Pleistocene muck layers in mining operations has uncovered a remarkable collection of usually broken and disarticulated megafaunal bones and carcasses. Previously, Fe-rich particles have been found embedded in a number of mammoth tusks and a bison skull, and those particles have high-Ni and low-Ti compositions indicative of an extraterrestrial origin. These fossils range in age from ~21 to 37 ka B.P. Additional fossil skulls (bison, rangifer, mammoth) and a mammoth tusk from Alaska and the Yukon Territory have been found in museum and government collections with embedded Fe-rich particles as well, and nine skulls also contain significant quantities of original host sediment within them. This associated sediment was removed and examined for the presence of spherules and other cosmic impact proxies. The additional megafaunal bones are estimated to date from between 13 to 40 ka, and radiocarbon dating of samples from these specimens is currently underway. Magnetic grains were extracted from aliquots of bulk sediment from each of the fossil skulls. The magnetic fractions ranged from ~5 to 44 g/kg, averaging 23.6 g/kg. We then examined each sample fraction for magnetic spherules. Two samples contained rounded detrital magnetite and no spherules, while the other seven samples contained numerous melt-quenched magnetic spherules ranging in abundance from ~1000 to 18,000/kg, averaging ~8000/kg. We performed SEM-EDS analyses on 49 selected spherules and identified two distinct compositional populations. One group from a mammoth skull is predominately aluminosilicate (Al2O3 = 30.7 wt.%, SiO2 = 34.4 wt.%, FeO = 23.4 wt.%, CaO = 2.9 wt.%, all other oxides <2.3 wt.%). The second group from all other skulls is typically iron-rich (FeO = 87.4 wt.%, Al2O3 = 2.0 wt.%, SiO2 = 2.3 wt.%, CaO = 4.0 wt.%, all other oxides <1.1 wt.%). Using ternary diagrams, we plotted various oxides of the 49 spherules against those for known populations of other spherule types. The results indicate that the 49 spherule compositions are consistent with those of known impact spherules; apparently they are not cosmic, anthropogenic, or volcanic in origin. These preliminary results suggest that large quantities of melt-quenched impact spherules were deposited across Alaska and western Canada (Beringia) within the last 40 kyr. We propose that they were most likely produced by hypervelocity impact/airburst events in the region during the Late Pleistocene. The presence of geochemically distinct populations indicates that there were at least two such impacts/airbursts into different source rocks.