Douglas J. Kennett

Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling

A carbon-rich black layer, dating to ≈12.9 ka, has been previously identified at ≈50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at ≅12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at ≈12.9 ka. This layer also extends throughout at least 15 Carolina Bays, which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain. We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.

Development and Disintegration of Maya Political Systems in Response to Climate Change

Maya and Climate Climate has affected the vitality of many different societies in the past, as shown by numerous records across the globe and throughout human history. One of the most obvious and spectacular examples of this is from the Classic Maya civilization, whose advanced culture left highly detailed records of all aspects of their existence between 300 and 1000 C.E. Kennett et al. (p. 788; see the cover) present a detailed climate record derived from a stalagmite collected from a cave in Belize, in the midst of the Classic Maya settlement. The fine resolution and precise dating of the record allows changes in precipitation to be related to the politics, war, and population fluctuations of the Mayans. A record of rainfall from a stalagmite in southern Belize provides a context for better understanding Maya civilization. The role of climate change in the development and demise of Classic Maya civilization (300 to 1000 C.E.) remains controversial because of the absence of well-dated climate and archaeological sequences. We present a precisely dated subannual climate record for the past 2000 years from Yok Balum Cave, Belize. From comparison of this record with historical events compiled from well-dated stone monuments, we propose that anomalously high rainfall favored unprecedented population expansion and the proliferation of political centers between 440 and 660 C.E. This was followed by a drying trend between 660 and 1000 C.E. that triggered the balkanization of polities, increased warfare, and the asynchronous disintegration of polities, followed by population collapse in the context of an extended drought between 1020 and 1100 C.E.

Competitive and Cooperative Responses to Climatic Instability in Coastal Southern California

Abstract Archaeological data indicates that socially and politically complex hunter-gatherer societies had become well established on the southern California coast by A.D. 1300. Major developmental changes in sociopolitical complexity are generally considered to have taken place rapidly between AD 1150 and 1300. Recently, two hypotheses have been proposed to account for this rapid cultural evolution, both invoking stressful climatic conditions as an important trigger for cultural change. One suggests that the sociopolitical development was stimulated, in part, by multiple marine and terrestrial subsistence stresses, particularly low marine productivity resulting from regional warming. The other suggests that these developments were largely driven by decreases in terrestrial productivity and water availability linked to drought. Resolution of this debate has been hampered by insufficient paleoclimatic and archaeological data. We present a well-dated, relatively high resolution (25-year intervals) oxygen isotopic marine climate record and new archaeological data from the Northern Channel Islands for the last 3,000 years. These data strongly suggest that changes in human behavior associated with increasing cultural complexity: 1) accelerated after A.D. 500 and became dominant by A.D. 1300, 2) occurred during one of the coldest and most unstable marine climatic intervals of the Holocene (A.D. 450-1300), and 3) coincided with cool, dry terrestrial conditions. Incipient cultural complexity emerged during an interval marked by inferred high marine productivity, reduced terrestrial food and water availability, and large, unpredictable variations in terrestrial resource availability. Our records suggest a strong relationship during this time between climatically induced changes in environmental conditions and social, political, and economic responses, including the emergence of more intensified fishing, and increased sedentism, violence, and trade.

Paleoindian Seafaring, Maritime Technologies, and Coastal Foraging on California’s Channel Islands

Archaeological sites reveal a variety of tools used to hunt marine birds, mammals, and fish 12,000 years ago. Three archaeological sites on California’s Channel Islands show that Paleoindians relied heavily on marine resources. The Paleocoastal sites, dated between ~12,200 and 11,200 years ago, contain numerous stemmed projectile points and crescents associated with a variety of marine and aquatic faunal remains. At site CA-SRI-512 on Santa Rosa Island, Paleocoastal peoples used such tools to capture geese, cormorants, and other birds, along with marine mammals and finfish. At Cardwell Bluffs on San Miguel Island, Paleocoastal peoples collected local chert cobbles, worked them into bifaces and projectile points, and discarded thousands of marine shells. With bifacial technologies similar to those seen in Western Pluvial Lakes Tradition assemblages of western North America, the sites provide evidence for seafaring and island colonization by Paleoindians with a diversified maritime economy.

Environmental imperatives reconsidered: demographic crises in western North America during the medieval climatic anomaly.

Review of late Holocene paleoenvironmental and cultural sequences from four regions of western North America shows striking correlations between drought and changes in subsistence, population, exchange, health, and interpersonal violence during the Medieval Climatic Anomaly (a.d. 800–1350). While ultimate causality is difficult to identify in the archaeological record, synchrony of the environmental and cultural changes and the negative character of many human responses—increased interpersonal violence, deterioration of long‐distance exchange relationships, and regional abandonments—suggest widespread demographic crises caused by decreased environmental productivity. The medieval droughts occurred at a unique juncture in the demographic history of western North America when unusually large populations of both hunter‐gatherers and agriculturalists had evolved highly intensified economies that put them in unprecedented ecological jeopardy. Long‐term patterns in the archaeological record are inconsistent with the predicted outcomes of simple adaptation or continuous economic intensification, suggesting that in this instance environmental dynamics played a major role in cultural transformations across a wide expanse of western North America among groups with diverse subsistence strategies. These events suggest that environment should not be overlooked as a potential cause of prehistoric culture change. The present paper was submitted 16 11 98 and accepted 5 vi 98; the final version reached the Editors office 6 vii 98.

An archaeological and paleontological chronology for Daisy Cave (CA-SMI-261), San Miguel Island, California.

We provide detailed contextual information on 25 14 C dates for unusually well-preserved archaeological and paleontological remains from Daisy Cave. Paleontological materials, including faunal and floral remains, have been recovered from deposits spanning roughly the past 16,000 yr, while archaeological materials date back to ca. 10,500 BP. Multidisciplinary investigations at the site provide a detailed record of environmental and cultural changes on San Miguel Island during this time period. This record includes evidence for the local or regional extinction of a number of animal species, as well as some of the earliest evidence for the human use of boats and other maritime activities in the Americas. Data from Daisy Cave contribute to a growing body of evidence that Paleoindians had adapted to a wide variety of New World environments prior to 10,000 PB. Analysis of shell-charcoal pairs, along with isotopic analysis of associated marine shells, supports the general validity of marine shell dating, but also provides evidence for temporal fluctuations in the reservoir effect within the Santa Barbara Channel region.

Nanodiamonds in the Younger Dryas Boundary Sediment Layer

We report abundant nanodiamonds in sediments dating to 12.9 ± 0.1 thousand calendar years before the present at multiple locations across North America. Selected area electron diffraction patterns reveal two diamond allotropes in this boundary layer but not above or below that interval. Cubic diamonds form under high temperature-pressure regimes, and n-diamonds also require extraordinary conditions, well outside the range of Earths typical surficial processes but common to cosmic impacts. N-diamond concentrations range from ≈10 to 3700 parts per billion by weight, comparable to amounts found in known impact layers. These diamonds provide strong evidence for Earths collision with a rare swarm of carbonaceous chondrites or comets at the onset of the Younger Dryas cool interval, producing multiple airbursts and possible surface impacts, with severe repercussions for plants, animals, and humans in North America.

Environmental Imperatives Reconsidered

Review of late Holocene paleoenvironmental and cultural sequences from four regions of western North America shows striking correlations between drought and changes in subsistence, population, exchange, health, and interpersonal violence during the Medieval Climatic Anomaly (a.d. 800–1350). While ultimate causality is difficult to identify in the archaeological record, synchrony of the environmental and cultural changes and the negative character of many human responses—increased interpersonal violence, deterioration of long‐distance exchange relationships, and regional abandonments—suggest widespread demographic crises caused by decreased environmental productivity. The medieval droughts occurred at a unique juncture in the demographic history of western North America when unusually large populations of both hunter‐gatherers and agriculturalists had evolved highly intensified economies that put them in unprecedented ecological jeopardy. Long‐term patterns in the archaeological record are inconsistent with the predicted outcomes of simple adaptation or continuous economic intensification, suggesting that in this instance environmental dynamics played a major role in cultural transformations across a wide expanse of western North America among groups with diverse subsistence strategies. These events suggest that environment should not be overlooked as a potential cause of prehistoric culture change. The present paper was submitted 16 11 98 and accepted 5 vi 98; the final version reached the Editors office 6 vii 98.

Intrashell Radiocarbon Variability in Marine Mollusks

We demonstrate variable radiocarbon content within 2 historic (AD 1936) and 2 prehistoric (about 8200 BP and 3500 BP) Mytilus californianus shells from the Santa Barbara Channel region, California, USA. Historic specimens from the mainland coast exhibit a greater range of intrashell variability (i.e. 180-240 14C yr) than archaeological specimens from Daisy Cave on San Miguel Island (i.e. 120 14C yr in both shells). δ13C and δ18O profiles are in general agreement with the upwelling of deep ocean water depleted in 14C as a determinant of local marine reservoir correction (∆R) in the San Miguel Island samples. Upwelling cycles are difficult to identify in the mainland specimens, where intrashell variations in 14C content may be a complex product of oceanic mixing and periodic seasonal inputs of 14C-depeleted terrestrial runoff. Though the mechanisms controlling ∆R at subannual to annual scales are not entirely clear, the fluctuations represent significant sources of random dating error in marine environments, particularly if a small section of shell is selected for accelerator mass spec- trometry (AMS) dating. For maximum precision and accuracy in AMS dating of marine shells, we recommend that archae- ologists, paleontologists, and 14C lab personnel average out these variations by sampling across multiple increments of growth.

Late Pleistocene Human Skeleton and mtDNA Link Paleoamericans and Modern Native Americans

American Beauty Modern Native American ancestry traces back to an East Asian migration across Beringia. However, some Native American skeletons from the late Pleistocene show phenotypic characteristics more similar to other, more geographically distant, human populations. Chatters et al. (p. 750) describe a skeleton with a Paleoamerican phenotype from the eastern Yucatan, dating to approximately 12 to 13 thousand years ago, with a relatively common extant Native American mitochondrial DNA haplotype. The Paleoamerican phenotype may thus have evolved independently among Native American populations. The differences between Paleoamericans and Native Americans likely resulted from local evolution. Because of differences in craniofacial morphology and dentition between the earliest American skeletons and modern Native Americans, separate origins have been postulated for them, despite genetic evidence to the contrary. We describe a near-complete human skeleton with an intact cranium and preserved DNA found with extinct fauna in a submerged cave on Mexico’s Yucatan Peninsula. This skeleton dates to between 13,000 and 12,000 calendar years ago and has Paleoamerican craniofacial characteristics and a Beringian-derived mitochondrial DNA (mtDNA) haplogroup (D1). Thus, the differences between Paleoamericans and Native Americans probably resulted from in situ evolution rather than separate ancestry.