|Abstracts on Global Climate Change|
Paleoclimate and faunal evolution in the Plio-Pleistocene of Africa and South America
Quinteros, RB Behrensmeyer, AK Ormazabal, GC
AMEGHINIANA 41:4 641-649
PALEOCLIMATE AND FAUNAL EVOLUTION IN THE PLIO-PLEISTOCENE OF AFRICA AND SOUTH AMERICA. Climatic change is often hypothesized to be a major variable in forcing evolutionary change. Recent work on Pliocene and Pleistocene fossil mammals from the Turkana Basin of Kenya and Ethiopia shows that climate may play an important role in the spread of savanna environments, and thus in the increasing abundance of mammals adapted to open and seasonally and conditions. If global climate change is behind some of these trends in East Africa, we hypothesize that similar patterns of faunal change may occur elsewhere. An analysis of fossil mammals from the Pampean region of Argentina shows an increase in the number of species adapted to open and seasonally and conditions through the Plio-Pleistocene. The South American pattern is not identical to the East African pattern, but both show a significant increase in open and and adapted mammals shortly after 2 million years ago, at the very beginning of the Pleistocene. Although global climate change may be invoked as a common cause of these intercontinental trends, local and regional geography and tectonics play a critical role in modulating the global signal.
Evolution of late glacial ice-marginal lakes on the northwestern Canadian Shield and their influence on the location of the Dubawnt Lake palaeo-ice stream
Stokes, CR Clark, CD
PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY 215:1-2 155-171
During deglaciation of the North American Laurentide Ice Sheet large proglacial lakes developed in positions where proglacial drainage was impeded by the ice margin. For some of these lakes, it is known that subsequent drainage had an abrupt and widespread impact on North Atlantic Ocean circulation and climate, but less is known about the impact that the lakes exerted on ice sheet dynamics. This paper reports palaeogeographic reconstructions of the evolution of proglacial lakes during deglaciation across the northwestern Canadian Shield, covering an area in excess of 1,000,000 km(2) as the ice sheet retreated some 600 km. The interactions between proglacial lakes and ice sheet flow are explored, with a particular emphasis on whether the disposition of lakes may have influenced the location of the Dubawnt Lake ice stream. This ice stream falls outside the existing paradigm for ice streams in the Laurentide Ice Sheet because it did not operate over fined-grained till or lie in a topographic trough. Ice margin positions and a digital elevation model are utilised to predict the geometry and depth of proglacial takes impounded at the margin at 30-km increments during deglaciation. Palaeogeographic reconstructions match well with previous independent estimates of lake coverage inferred from field evidence, and results suggest that the development of a deep lake in the Thelon drainage basin may have been influential in initiating the ice stream by inducing calving, drawing down ice and triggering fast ice flow. This is the only location alongside this sector of the ice sheet where large (>3000 km(2)), deep lakes (similar to120 m) are impounded for a significant length of time and exactly matches the location of the ice stream. It is speculated that the commencement of calving at the ice sheet margin may have taken the system beyond a threshold and was sufficient to trigger rapid motion but that once initiated, calving processes and losses were insignificant to the functioning of the ice stream. It is thus concluded that proglacial lakes are likely to have been an important control on ice sheet dynamics during deglaciation of the Laurentide Ice Sheet. (C) 2004 Elsevier B.V. All rights reserved.