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PaleolimnologyThe History and Evolution of Lake Systems$
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Andrew S. Cohen

Print publication date: 2003

Print ISBN-13: 9780195133530

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195133530.001.0001

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Sedimentological Archives in Lake Deposits

Sedimentological Archives in Lake Deposits

(p.162) 7 Sedimentological Archives in Lake Deposits

Andrew S. Cohen

Oxford University Press

Lake sediments are both repositories and sources of information about lake history. Depositional products tell us about the mechanisms of transport or accumulation of important geochemical and fossil archives, but important clues about that history are imbedded in the pattern of sedimentation itself. Geologists have recognized this fact since the earliest paleolimnological studies. Although he would certainly not have called himself a paleolimnologist, Charles Lyell’s (1830) classic studies and interpretation of the depositional environments of the Eocene Paris Basin set the tone for a time-honored approach to the study of ancient lake deposits. Lyell recognized that understanding the physical, chemical, and biological attributes of lakes that affect sedimentation, obtained through modern observation, must be applied to a four-dimensional (spatial plus time) analysis of sedimentary deposits and depositional history. However, not everything we need to know or every process we need to invoke will necessarily arise from our short-term observations of modern lakes. Events that are unlikely to occur in the course of a brief, several-year experiment or period of monitoring may become virtual certainties over the long history of some lakes and may leave a sedimentary archive of which we have little prior understanding from modern studies (Dott, 1983). Furthermore, the sedimentary response that we observe to some external forcing event may differ depending on the time scale over which we observe the response (Dearing, 1991). Consider a hill slope that is undergoing accelerated erosion, and that is producing an accumulation of sediment in a downstream channel as a result of land-clearing activities. Initially there may be no response in terms of sedimentation rate in the downstream lake; all of the sediment is being held in temporary storage. This process may occur over time scales of a few decades. At some later time a triggering event, perhaps a series of abnormally high rainfall and discharge years, causes this sediment to be released to the lake, now at an accelerated rate. This becomes a sedimentary response that the paleolimnologist can record. But, over geological time scales of millennia or longer, the original process may be modified, and new ones may gain in importance.

Keywords:   Alkaline pathway, Backshore, Calcite, Dawsonite, Eocene, Gamma ray attenuation, Half-graben, Ikaite

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