Matthew Boyd

Freshwater Outburst from Lake Superior as a Trigger for the Cold Event 9300 Years Ago

Down the Drain A pervasive cooling event affected much of the Northern Hemisphere approximately 9300 years ago. This event was accompanied by changes in ocean circulation in the North Atlantic, forced presumably by a large injection of fresh water produced by melting of the Laurentide Ice Sheet, but the source, magnitude, and routing of the meltwater remain unknown. Yu et al. (p. 1262, published online 29 April) present evidence that the trigger for this cooling episode was an outburst flood from Lake Superior. Reconstructing lake-level changes in the Superior basin suggests that a rapid fall of lake level of about 45 meters occurred 9300 years ago, possibly due to the sudden failure of a drift dam. Rapid drainage through the North Bay–Ottawa River–St. Lawrence River valleys into the North Atlantic should have been sufficient to disturb ocean circulation in line with the geologic record. The trigger for the dramatic North Atlantic cooling event 9300 years ago was an outburst flood from Lake Superior. Paleoclimate proxy records reveal a pervasive cooling event with a Northern Hemispheric extent ~9300 years ago. Coeval changes in the oceanic circulation of the North Atlantic imply freshwater forcing. However, the source, magnitude, and routing of meltwater have remained unknown. Located in central North America, Lake Superior is a key site for regulating the outflow of glacial meltwater to the oceans. Here, we show evidence for an ~45-meter rapid lake-level fall in this basin, centered on 9300 calibrated years before the present, due to the failure of a glacial drift dam on the southeast corner of the lake. We ascribe the widespread climate anomaly ~9300 years ago to this freshwater outburst delivered to the North Atlantic Ocean through the Lake Huron–North Bay–Ottawa River–St. Lawrence River valleys.

Absence of geochemical evidence for an impact event at the Bølling–Allerød/Younger Dryas transition

High concentrations of iridium have been reported in terrestrial sediments dated at 12.9 ka and are interpreted to support an extraterrestrial impact event as the cause of the observed extinction in the Rancholabrean fauna, changes in the Paleoindian cultures, and the onset of the Younger Dryas cooling [Firestone RB, et al. (2007) Proc Natl Acad Sci USA 104:16016–16021]. Here, we report platinum group element (PGE: Os, Ir, Ru, Rh, Pt, Pd), gold (Au) concentrations, and 187Os/188Os ratios in time-equivalent terrestrial, lacustrine, and marine sections to seek robust evidence of an extraterrestrial contribution. First, our results do not reproduce the previously reported elevated Ir concentrations. Second, 187Os/188Os isotopic ratios in the sediment layers investigated are similar to average crustal values, indicating the absence of a significant meteoritic Os contribution to these sediments. Third, no PGE anomalies distinct from crustal signatures are present in the marine record in either the Gulf of California (DSDP 480, Guaymas Basin) or the Cariaco Basin (ODP 1002C). Our data show no evidence of an extraterrestrial (ET)-PGE enrichment anomaly in any of the investigated depositional settings investigated across North America and in one section in Belgium. The lack of a clear ET-PGE signature in this sample suite is inconsistent with the impact of a large chondritic projectile at the Bølling–Allerød/Younger Dryas transition.

Identification of Anthropogenic Burning in the Paleoecological Record of the Northern Prairies: A New Approach

Despite recent interest in the North American fire record, paleoecological evidence for the deliberate burning of grassland by hunter–gatherers has not previously been sought. Through the analysis of grass phytoliths preserved in a sequence of buried soil horizons in the Lauder Sandhills, southwestern Manitoba, Canada, this article reconstructs a local grassland fire record for the past 5,000 years. I propose that an apparent peak in fire frequency shortly after 2,500 14C years b.p. corresponds to the deliberate burning of prairie by Sonota–Besant (Plains Woodland) hunter–gatherers, rather than climatic “forcing.” This practice, which is clearly documented in the historic record, may have functioned as a means of making bison–herd movements more predictable and may have enabled higher human carrying capacities in the Plains Woodland period. This hypothesis is meant to stimulate multidisciplinary discussion on a significant, but neglected, topic.

NORTHERNMOST PRECONTACT MAIZE IN NORTH AMERICA

Corn (Zea mays ssp. mays) was a key economic plant in the Americas, yet little information exists on the northern limit of maize consumption before European contact. Based on the analysis of carbonized food residue on pottery for plant microfossils (phytoliths and starch granules) from 58 precontact archaeological sites, we report the first evidence of maize consumption in the subarctic boreal forest. Recovery of maize microfossils from several widely distributed Laurel phase (Middle Woodland/Initial Shield Woodland) vessels indicates that the diffusion of corn into this region was surprisingly early (ca. A.D. 500) and may have initially spread through long-distance exchange networks linking temperate and boreal North America during the Woodland period. After A.D. 1000 maize was a widespread component of diet and was consumed by every major Late Woodland culture in the region. These results have profound implications both for the history of this cultigen and for small-scale human societies in northern North America.

Interdependence and Power: Complexity in Hunter-Gatherer/Farmer Exchanges

In recent years, socioeconomic relations between horticulturists and hunter-gatherers have received greater attention in the archaeological literature. These relations have been probed primarily from theoretical perspectives that emphasize adaptation and dietary optimization. For example, the exchange of bison meat for maize between hunters and farmers is seen as an efficient means of acquiring protein in otherwise carbohydrate-rich diets, and carbohydrates in protein-abundant diets. On the surface, such cases in the social world seem to mimic examples of interdependence in the natural world, and they appear to presume a degree of equality between participants. It is argued here that relations ofpower and conflict may develop against the background of interdependence and adaptive exchange. A brief examination of the historic bison-corn exchange network in the Northern Plains is presented. Available data on the early phase (ca. 1730s-1810) of Mandan-Hidatsa and Cree-Assiniboin exchanges suggest that this interdependent system was characterized by oscillating conflict over time. Moreover, conflict and inequality may be at least in part attributed to the use of subsistence and non-subsistence monopolies as tools for socioeconomic leverage. Archaeologists must be sensitive to the potential presence ofsimilar processes prehistorically if the complexity of culture-contact scenarios is to be appreciated.

Reconstructing a Prairie-Woodland Mosaic on the Northern Great Plains: Risk, Resilience, and Resource Management

Abstract Recent geological evidence of high-amplitude, short-term, climatic variability on the northern Plains in the late Holocene implies that significant fluctuations in resource availability may have regularly occurred on the scale of human generations. In this region, evidence of high mobility, low population density, and storage are generalized responses of hunter-gatherer populations to the effects of environmental variability on resourcepredictability. In order to achieve more sophisticated understanding of the relationship between risk, environment, and land-use for the last few thousand years, we suggest that multidisciplinary reconstruction of detailed landscape histories is necessary. This is so because landscape histories may encode: (1) spatial and temporal variability in habitat diversity (i.e., patchiness); (2) geographical differences in ecosystem resilience and resistance to short-term macro climatic variability; and (3) enhancement of resource predictability or diversity through lnanagement practices such as anthropogenic burning. Modern vegetation surveys, presettlement landcover reconstructions, and recent geomorphic and paleo vegetation data from the Oak Lake Sandhills, Manitoba, Canada, are assembled to illustrate these points.