Joanne P Ballard

Abstract: We analyzed stable nitrogen isotopes, total nitrogen, and macroscopic charcoal in sediments from three lakes in Alabama and Michigan to characterize temporal patterns in nitrogen cycling and explore links between nitrogen, climate, and fire across the late glacial in eastern North America. We used cores from Cahaba Pond, Alabama, recovered by Delcourt et al. (1983, Ecology), and matched our isotope and charcoal analyses to their pollen stratigraphy. Cores from Swift and Slack Lakes in Michigan were obtained in 2008. Thin-section analysis across a 20-cm section from Cahaba Pond that encompasses the Younger Dryas shows a transition from mineral-rich to organic-rich sediments, with loessal silt aggregates. All three lakes recorded roughly coeval nitrogen perturbations at the onset of the Younger Dryas, when a dramatic shift occurred in terrestrial and aquatic vegetation at Cahaba Pond. All three sites also registered fire events across the late glacial. We explore the possibi...

Abstract: A female mammoth tusk from Yakutia, Siberia, exhibits a remarkable dissolution pattern on the part of the tusk that was exposed to the atmosphere. This pitting took place while the animal was alive, as evidenced by subsequent polishing of the damaged ivory. The hypothesis we will test is that this dissolution was the result of a nitric acid rain event.
An acid of pH 2.0, the equivalent of lemon juice, can dissolve tusks, antlers and even exposed bones. One possible source of nitric acid rain is an extraterrestrial event. Thermal shock waves resulting from a bolide impacting earth’s atmosphere would cause dissociation of O3 and N2; chemical reactions between NOx and water then lead to production of nitric acid precipitation lasting a year or longer (Prinn and Fegley 1987). Nitric acid rain is documented for the Tunguska extraterrestrial event of 1908 (Kolesnikov et al. 1998, 2003).
Our research has two objectives: to replicate the dissolution pattern on the ivory, and to radiocarbon date this unusual tusk specimen. We predict that the age of the tusk is the Bolling-Allerod/Younger Dryas Boundary ca. 10,900 14C years BP, or ca. 12,800 cal yrs BP. If we are correct, then this mammoth most likely witnessed the extraterrestrial event at the beginning of the Younger Dryas as hypothesized by Firestone et al. (2007), and she survived the impact.

Authors:
Joanne P. Ballard, University of Tennessee, USA jballa13@utk.edu *
Dick Mol, Research Associate, Natuurhistorisch Museum, Rotterdam, The Netherlands
Andre Bijkerk, Independent Researcher, Zoetermeer, The Netherlands
Jelle Reumer, Director, Natuurhistorisch Museum, Rotterdam; Faculty,
University of Utrect

The late-glacial transition to the Holocene, 15,000–11,600 cal yr BP, is an enigmatic period of dynamic global changes and a major extinction event in North America. Fire is an agent of disturbance that transforms the environment physically and chemically, and affects plant community composition. To improve understanding of the linkages between fire, vegetation, and climate over the late glacial and Holocene in the eastern U.S., I analyzed lake-sediment cores for charcoal and indicators of wood ash, and compared results to existing pollen records. A new microscopic charcoal record from Anderson Pond, Tennessee revealed high fire activity from 23,000–15,000 cal yr BP when conifers dominated, and during the Mid-Holocene Warm Period (8000–5200 cal yr BP), when hardwoods dominated. Macroscopic charcoal analysis of sediments from Pigeon Marsh, Georgia showed high fire activity from 16,500–14,500 cal yr BP, below a major hiatus. Jackson Pond, Kentucky and Cahaba Pond, Alabama had low macroscopic charcoal concentrations during the late glacial; largest charcoal peaks occurred around 5000 cal yr BP at Jackson Pond, and from 1370–640 cal yr BP at Cahaba Pond.
Thin sections were prepared for cores from the four southeastern U.S. sites and from Swift and Slack Lakes, Michigan, and analyzed together with nitrogen isotopes and element data from XRF. Thin sections showed the presence of siliceous aggregates, a unique grain type, in sediments from five sites. These grains are rare, occurring in only three periods, around 19,250, 14,000 and 12,400 cal yr BP. In laboratory experiments, I produced siliceous aggregates from wood ash with simulated rain, and found their formation requires silt, but not high acidity. On the landscape, siliceous aggregates form after fires in wood ash by the action of water. The alkaline pH of the wet ash dissolves phytoliths, and amorphous silica nucleates around silt-sized quartz grains. Then aggregates are transported into lake sediments. My research demonstrates that siliceous aggregates are a new proxy for wildfires in paleoenvironmental records. The wildfire-derived siliceous aggregates in cores examined from the eastern U.S. are contemporaneous with combustion signals in Greenland ice cores, suggesting widespread late-glacial fire events.