PP42A-01
Deglaciation Chronology of the Southern Laurentide Ice Sheet and Implications for Meltwater Routing bracketing the Younger Dryas Event
During the last glacial hemicycle the intricate interplay of ice sheet margin geometry, isostatic rebound, and topography dictated the locations of meltwater storage and routing. Perhaps the least well understood of these is the ice margin chronology. Consequently, we have developed ice margin chronologies at three potential outlets of the large meltwater reservoir in the center of North America known as glacial Lake Agassiz. At the southern outlet of Lake Agassiz, several channels cut across the Big Stone Moraine. Radiocarbon dates from depressions and an intermediate level meltwater channels indicate initial lake formation after 13,680 cal yr. Coupled with regional data this implies a minimum ice margin retreat of ~270 m/yr at the beginning of Bölling/Alleröd warm interval. The chronology of subsequent events and the location of the ice margin at the beginning of the Younger Dryas is still being studied. However, in order for the ice margin to retreat sufficiently to produce a volume of Lake Agassiz suggested in prior reconstructions for the start of the Younger Dryas requires retreat rates comparable to those of present day catastrophic collapse of the Columbia Glacier (~570 m/yr). The northwest outlet is via the Clearwater spillway of northern Alberta. New geomorphic mapping confirmed fluvial activity within the spillway, but also revealed several unrecognized ice margin positions. According to our reconstructions retreat rates from 15,000 cal yr to 12,400 cal yr average some 25 m/yr then increase to ~200 to 300 m/yr as the ice margin calving into water. The ice margin position could not allow for northward drainage until about 11,000 cal yr, well after the start of the Younger Dryas. The eastern outlet is of interest as the suggested location of meltwater change (flood or simple diversion) from the Gulf of Mexico to the North Atlantic. However, the lack of a clear geomorphic meltwater route is problematic. About 14,000 cal yr the ice sheet retreat rate increased to ~63 m/yr and continued at that pace until 12,100 cal yr at which time the ice margin crossed the lowest topographic pathway out of the Lake Agassiz basin. In this reconstruction the lowest pathway would not be available for meltwater flow until after the start of the Younger Dryas.
PP42A-02
Exploring the Human Ecology of the Younger Dryas Extraterrestrial Impact Event
Several lines of evidence now exist for a major extraterrestrial impact event in North America at 12.9 ka (the YDB). This impact partially destabilized the Laurentide and Cordilleran ice sheets, triggered abrupt Younger Dryas cooling and extensive wildfires, and contributed to megafaunal extinction. This event also occurred soon after the well established colonization of the Americas by anatomically modern humans. Confirmation of this event would represent the first near-time extraterrestrial impact with significant effects on human populations. These likely included widespread, abrupt human mortality, population displacement, migration into less effected or newly established habitats, loss of cultural traditions, and resource diversification in the face of the massive megafaunal extinction and population reductions in surviving animal populations. Ultimately, these transformations established the context for the special character of plant and animal domestication and the emergence of agricultural economies in North America. We explore the Late Pleistocene archaeological record in North America within the context of documented major biotic changes associated with the YDB in North America and of the massive ecological affects hypothesized for this event.
PP42A-03
Before and After the Younger Dryas: Chronostratigraphic and Paleoenvironmental Research at Arlington Springs, Santa Rosa Island, California
To corroborate the age of 10,960 +/- 80 RCYBP (CAMS-16810) obtained on collagen from human bone from Arlington Springs, Santa Rosa Island, several seasons of fieldwork have been conducted at the site. This date, the oldest yet obtained for human skeletal remains in North America, indicates that Paleoamericans had reached the offshore Channel Islands by 13,000 CALYBP and confirms the use of watercraft along the Pacific coast of North America in Clovis times. The Arlington Springs field investigations have been undertaken as part of a research program to (1) identify the stratum from which the bones were originally discovered by Phil Orr in 1959, (2) establish the geological and chronostratigraphic context for the remains, (3) gather data for paleoenvironmental reconstructions, (4) assess geophysical techniques for detecting buried features at the site, and (5) obtain a series of deep sediment cores for further study. To date, twenty AMS radiocarbon dates from charcoal, humic acids, and marine shells in the various geological and cultural strata at the Arlington Springs locality have been obtained from samples collected during our fieldwork. These supplement six AMS dates that were derived from the human bone and associated materials in the soil matrix collected in 1960 by Orr. Our recently-collected samples corroborate that the age of the human bone dates no later than 13,000 CALYBP. An overlying, very dark soil layer, prominently visible in the side of Arlington Canyon, dates from the inception of the Younger Dryas at 12,900 CALYBP. Other, thinner, dark bands of sediment appear to be derived from fires and/or cienega/marsh deposits. In the stratum that contained the 13,000-year-old human bone, some small chert flakes have been recovered, indicating Paleoamerican cultural activities in the vicinity. In this cultural layer, as well as in the overlying dark soil of the Younger Dryas Period, there are abundant remains of an extinct Pleistocene deer mouse (Peromyscus nesodytes). Although our studies to date have not yielded well-preserved pollen in late Pleistocene soils at Arlington Springs, samples obtained from alluvial deposits at another locality upstream did yield abundant pollen indicating that a pine and cypress forest once existed on Santa Rosa Island at 10, 860 +/- 70 RCYBP (Beta- 161032). This date calibrates to 13,130 to 12,800 CALYBP or 12,270 to 12,650 CALYBP. The Arlington Canyon stratigraphy records major environmental and sedimentological changes that began after the inception of the Younger Dryas Period.
PP42A-04
Megatsunami of the World Ocean: Did They Occur in the Recent Past?
The comprehensive historical tsunami database collected at the Novosibirsk Tsunami Laboratory, contains data on more than 2250 historical tsunamis in the World Ocean from 1628 BC to present. Even if the historical data set is incomplete for many areas, especially for older times, the world catalog contains enough data to estimate average run-up heights for the largest seismically-induced tsunamis that caused wide-spread damage and many fatalities (1755 Lisbon, 1868 and 1877 Chile, 1952 Kamchatka, 1957 Aleutians, 1960 Chile, 1964 Alaska, 2005 Sumatra). This average run-up does not exceed 30-35 meters on the nearest coast with 10-12 meters at the distances of more than 5000 km. Somewhat larger waves (up to 40-45 m) can be generated by volcanic explosions followed by volcanic cone collapse (Santorini 1628 BC, Kuwae 1453, Unzen 1792, Tambora 1815, Krakatau 1883). Landslide-generated tsunamis have the largest recorded heights (up to 525 m) but normally these events are very local with a width of inundated area from hundreds of meters to several kilometers (1958, 1936, 1853 Lituya Bay, 1936 Norway, 2000 Greenland). Meanwhile, many parts of the World Ocean coastline contain prominent features of catastrophic impact of water currents and waves that came from the ocean. They are large boulders, weighing well above one hundred tons, lying on the top of vertical cliffs at the height up to 60 m and large vortexes cut-down in rather resistive coastal rocks. On a smaller scale, these features include sculptured bedrocks, grooves, canyons, cavettos and flutes, found in areas where hurricanes and severe tropical storms are not common. Sedimentary features of water impacts include mega-ripples found in the north-western Australia and so-called chevrons (parabolic and blade- like sand dunes) that are common along many parts of the Indian Ocean coast. In southern Madagascar, chevrons reach an altitude of 205 m with 30-45 km of in-land penetration. A high energy water flux of that scale could be generated by Storegga-class submarine landslides or Santorini-class volcanic explosions, but for this area does not have nearby active volcanoes or large sedimentary basins with the potential for large-volume submarine sliding. Not widely acknowledged presently, but still a real possibility is the creation of these coastal features by catastrophic oceanic waves generated by deep-water impacts of large comets or asteroids. In the Indian Ocean, several crater candidates (Burckle, Mahuika, Kukla, Christie) have been found recently by geomorphological analysis of detailed bathymetric maps. They are geologically young and analysis of nearby deep-sea cores shows the presence of some elements and minerals typical of oceanic impact structures. This paper discusses the consistency of these data with spatial and azimuthal distribution of the large-scale erosional and sedimentary features found at the Australian and Madagascar coast.
PP42A-05
Evidence for an Extraterrestrial Impact Origin of the Carolina Bays on the Atlantic Coast of North America
The Carolina Bays, one of the most conspicuous geomorphic features on the Atlantic Coastal Plain of the United States, are a group of about 500,000, oriented, crater-like, elliptical lakes, wetlands, and depressions, ranging from a few dozen meters to about 11 km in length. Although long proposed as impact structures (Melton and Schriever, 1933; Prouty, 1934), this origin for the Carolina Bays has remained controversial mainly because of an apparent absence of associated extraterrestrial materials. Analyses of Bay orientation showed that their long axes converge near the Great Lakes, suggesting that an impact or airburst over that region may have formed the Bays (Eyton and Parkhurst, 1975). However, Bays dates have been reported over a wide range, calling into question whether all Carolina Bays could have formed simultaneously, although this issue remains unresolved and controversial. Many Bay researchers, who subscribe to widely differing theories, agree that modern Carolina Bays have been subject to repeated modification and that they most likely evolved from some type of ancestral depressions. Now for the first time, we present conclusive geochemical and sedimentary evidence in support of an extraterrestrial connection for the Carolina Bays. Analyses of sediment from the rim sands and basins of fifteen Bays, widely distributed across North and South Carolina, reveal anomalously high abundances of microspherules, iridium, fullerenes with ET helium, carbon spherules, glass-like carbon, and other potential markers for extraterrestrial impact. No such markers were found in paleosols beneath the rim sands or basal sediments of the Bays examined. The assemblage of geochemical and sediment signatures of extraterrestrial impact found in Bay sediments are essentially the same as in the pan-North-American Younger Dryas impact boundary layer (the YDB), dated at 12.9 ka. We hypothesize that at least some Bays were formed by the YD impact during the last deglacial, and we present OSL and radiocarbon dating, along with stratigraphic profiling, in support of this age. Data from the Carolina Bays we have examined suggest that at least some modern Carolina Bays may have evolved from depressions which were excavated by primary ejecta, secondary ejecta, and/or the shock wave from the Younger Dryas impact event.
PP42A-06
Tests for causes and consequences of the Younger Dryas in the Laurentian Great Lakes
The cause of the Younger Dryas climate episode is being reexamined with increasing vigor. The conventional explanation has been a switching of meltwater runoff from a large sector of the Laurentide ice sheet from the Mississippi to the St. Lawrence River systems, with a concomitant freshening of the north Atlantic Ocean and a suppression of the Atlantic Meridional Overturning, which transports heat to high northern latitudes. Reexamination of the evidence for both the timing and the location of meltwater routing at the beginning of the Younger Dryas has lead to searches for other causes for this distinctive climate event, including possible extraterrestrial events. Changes in meltwater routing certainly occurred. Cessation of flow down the Mississippi and a major fall in the level of Lake Agassiz are well documented, and the water had to go somewhere. Timing and routing of the meltwater flows are now the questions. Several tests of timing and routing are potentially available from the Laurentian Great Lakes. The unique, but poorly dated Wilmette bed in Lake Michigan was deposited at about the beginning of the of the Younger Dryas. Further analysis of this bed are underway to determine whether it was deposited in association with flows from Lake Agassiz, or whether it contains material indicative of other potential causes of climate change at about that time. In addition, both the purported flood from Lake Agassiz at the beginning of the Younger Dryas and a younger uncontested flood from Lake Agassiz through the Nipigon Basin emptied into the Lake Superior basin. Seismic-reflection surveys are planned to determine if the younger, uncontested flows produced distinctive deposits in the Superior basin, and whether similar deposits of possible Younger Dryas age exist near Thunder Bay or other potential Agassiz outlets. Various explanations for the cause of the Younger Dryas are not necessarily exclusive or independent. For example, an initial event that was largely independent of meltwater routing may have caused changes in the configuration of the Laurentide Ice Sheet, leading to changes in meltwater routing. The changes in meltwater discharge, in turn, could have reinforced or retarded the initial climate forcing.