Dorothy M. Peteet

Responses of an arctic landscape to Lateglacial and early Holocene climatic changes: the importance of moisture

Many of the physical and biological processes that characterize arctic ecosystems are unique to high latitudes, and their sensitivities to climate change are poorly understood. Stratigraphic records of land–surface processes and vegetation change in the Arctic Foothills of northern Alaska reveal how tundra landscapes responded to climatic changes between 13,000 and 8000 14 C yr BP. Peat deposition began and shrub vegetation became widespread ca. 12,500 14 C yr BP, probably in response to the advent of warmer and wetter climate. Increased slope erosion caused rapid alluviation in valleys, and Populus trees spread northward along braided floodplains before 11,000 14 C yr BP. Lake levels fell and streams incised their floodplains during the Younger Dryas (YD) (11,000– 10,000 14 C yr BP). A hiatus in records of Populus suggest that its geographic range contracted, and pollen records of other species suggest a cooler and drier climate during this interval. Basal peats dating to the YD are rare, suggesting that rates of paludification slowed. Immediately after 10,000 14 C yr BP, lake levels rose, streams aggraded rapidly again, intense solifluction occurred, and Populus re-invaded the area. Moist acidic tundra vegetation was widespread by 8500 14 C yr BP along with wet, organic-rich soils. Most of these landscape-scale effects of climatic change involved changes in moisture. Although low temperature is the most conspicuous feature of arctic climate, shifts in effective moisture may be the proximate cause for many of the impacts that climate change has in arctic regions. r 2002 Elsevier Science Ltd. All rights reserved.

Long-Term Arctic Peatland Dynamics, Vegetation and Climate History of the Pur-Taz Region, Western Siberia

Stratigraphic analyses of peat composition, LOI, pollen, spores, macrofossils, charcoal, and AMS ages are used to reconstruct the peatland, vegetation and climatic dynamics in the Pur-Taz region of western Siberia over 5000 years (9300 - 4500 BP). Section stratigraphy shows many changes from shallow lake sediment to different combinations of forested or open sedge, moss, and Equisetum fen and peatland environments. Macrofossil and pollen data indicate that Larix sibirica and Betula pubescens trees were first to arrive, followed by Picea obovata. The dominance of Picea macrofossils 6000-5000 BP in the Pur-Taz peatland along with regional Picea pollen maxima indicate warmer conditions and movement of the spruce treeline northward at this time. The decline of pollen and macrofossils from all of these tree species in uppermost peats suggests a change in the environment less favorable for their growth, perhaps cooler temperatures and/or less moisture. Of major significance is the evidence for old ages of the uppermost peats in this area of Siberia, suggesting a real lack of peat accumulation in recent millennia or recent oxidation of uppermost peat.

Late-glacial vegetational, tephra, and climatic history of southwestern Kodiak Island, Alaska

AbstractLate-glacial vegetational, tephra, and climatic history of western Kodiak Island is developed based upon detailed palynological, macrofossil, and AMS 14C stratigraphy. A series of distincti...

Oxygen isotopes in fresh water biogenic opal ‐ Northeastern US Alleröd‐Younger Dryas temperature shift

The first oxygen isotope analysis of biogenic opal from lake sediments, from the Allerod/Younger Dryas transition in a core from Linsley Pond, Connecticut, gives an average estimate of a 6°C drop in temperature during the Younger Dryas. This shift represents temperatures during the bloom season, and may be less than the winter temperature drop. The sharp transition itself, with a duration of about 200 years, suggests that the temperature decrease may have been as large as 12°C. Previous estimates of the Allerod/Younger Dryas temperature shifts are controversial, and range from 3–20°C, suggesting that further interdisciplinary research on the same samples is warranted.

The Mount Edgecumbe tephra deposits, a marker horizon in southeastern Alaska near the Pleistocene-Holocene boundary

Late Pleistocene tephra deposits found from Sitka to Juneau and Lituya Bay are assigned to a source at the Mount Edgecumbe volcanic field, based on similarity of glass compositions to nearvent deposits and on thinning away from Kruzof Island. The sequence of near-vent layers is basaltic andesite and andesite at the base, rhyolite, and mixed dacite and rhyolite on top. The only breaks in the tephra sequence are two 1-mm-thick silt partings in a lake-sediment core, indicating a depositional interval from basaltic andesite to dacite of no more than about a millennium. Tephra deposits at sites >30 km from the vent are solely dacite and rhyolite and are 10,600 to 11,400 14C yr old based on interpretation of 18 radiocarbon ages, including 5 by accelerator mass spectrometry (AMS). Basaltic andesite and andesite deposits nearer the vent are as much as 12,000 yr old. n nDiscrepancy among radiocarbon ages of upland tephra deposits provisionally correlated as the same grainfall is resolvable within ±2 σ of analytical uncertainty. Comparison of bulk and AMS ages in one sediment core indicates a systematic bias of +600 to +1100 yr for the bulk ages; correlation of tephra deposits among upland and lacustrine sites implies an additional discrepancy of 200–400 yr between upland (relatively too young) and lacustrine ages. In any case, the Mount Edgecumbe tephra deposits are a widespread, latest Pleistocene stratigraphic marker that serves to emphasize the uncertainty in dating biogenic material from southeastern Alaska.

Chapter 14 – Mid-Holocene culture and climate on the Northwest Coast of North America

Publisher Summary nThis chapter reviews the cultural dynamics as a result of climatic changes in the Northwest Coast of North America. It reviews the paleoclimatic and archaeological records of this region to establish possible causal relationships between them. It suggests that on the Northwest Coast of North America, the middle Holocene was a time of changing climate and culture. The mid-Holocene climate of the Northwest Coast was cooler and wetter than the early Holocene, but warmer and somewhat drier than today. 5800 cal yr BP (5000 14C yr BP) is viewed as a major turning point in the prehistory of the Northwest Coast. Compared to the early Holocene, during the mid-Holocene the number of archaeological sites increased, their average size was larger, and shell middens became common, preserving bone and antler technologies as well as abundant faunal remains. To explain this, the study presents a synthesis of mid-Holocene environmental history based on records of pollen, plant macrofossil, charcoal, limnologic, glacial, and marine sediments. It indicates that the environmental changes during the transition from early Holocene to mid-Holocene and then late Holocene were registered throughout the Northwest Coast, and superimposed on these long-term shifts, were climate variations that took place more locally on annual-to-centennial time scales. Finally, it suggests a collaboration and frequent data sharing between paleoecologists and archaeologists as an obvious first step in approaching the effects of climate change on culture.

A Climate Model Intercomparison for the Younger Dryas and its Implications for Paleoclimatic Data Collection

It has been suggested by a number of paleoclimatologists that formation of a low salinity meltwater layer in the North Atlantic ocean during the period(s) of most rapid volumetric glacial retreat following the Wisconsin glacial maximum could have led to an extension of North Atlantic sea ice sufficient to cause a temporary, regional (at least) reversal of the longer term deglacial warming trend. In particular, it is speculated that such an extension of North Atlantic sea ice may have been responsible for the Younger Dryas (10,800-10,000 yr. BP) cold interval, evident in European proxy climatic records. Others have suggested (in the context of ice age initiation) that one feedback mechanism whereby equatorward extension of North Atlantic sea ice could amplify the regional high latitude temperature response is through a reduction in the poleward transport of heat by ocean currents as sea ice extends equatorward. At the same time, there is evidence that the Younger Dryas interval was not merely a European event, but may have been worldwide in scope. Some of the above-mentioned hypotheses and feedback mechanisms are quantitatively investigated using several energy balance climate models as well as general circulation model experiments in which (NCAR model) all oceanic grid points in the North Atlantic Ocean north of 45° latitude will be considered ice covered and sea ice; or land and sea ice as well as orbital elements are imposed appropriate to 10,500 yr. BP (GISS GCM). All of these results are intercompared. All sets of results show that there are substantial seasonal and regional differences in the climatic response for these particular forcings. Because the climatic signals are so different from season to season and from place to place, these model results help to suggest a priority list indicating where paleoclimatologists might dig through the geologic record in order to uncover remains which might help to tell us the seasonal and regional distribution of past climates. As that field evidence becomes available, then not only will we have an opportunity for an explanation of a very intriguing, rapid and dramatic past climatic change, but we will have another opportunity to independently verify the fidelity of our climatic models.