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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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PRINTED FROM OXFORD SCHOLARSHIP ONLINE (oxford.universitypressscholarship.com). (c) Copyright Oxford University Press, 2021. All Rights Reserved. An individual user may print out a PDF of a single chapter of a monograph in OSO for personal use. date: 17 June 2021

The Chemical Environment of Lakes

The Chemical Environment of Lakes

(p.69) 4 The Chemical Environment of Lakes

Andrew S. Cohen

Oxford University Press

Understanding lake chemistry is critical for correctly interpreting the geochemical archives of lake deposits. Elemental and isotopic distributions in lakes are closely linked to external climatic and watershed processes. Solute concentrations regulate the distribution of organisms, and the precipitation or dissolution of mineral phases. Both fossils and minerals leave sedimentary archives, and when we can interpret aspects of ancient water chemistry from these records we may be able to reconstruct paleoclimate or prior human activity around the lake. Interpretation of isotopic records likewise requires an initial understanding of their behavior in lakes and the links between this behavior and external factors such as rainfall or nutrient discharge. Oxygen (O2) is the second most abundant component of the atmosphere (∼ 21%) after nitrogen (∼ 78%): Paleolimnological indicators of oxygenation at the sediment–water interface may provide clues as to the nature and frequency of water-column mixing, water depth, the lake’s trophic condition, and possibly climate. Thus, it is important to understand how oxygen is generated and consumed in lakes to properly interpret the oxygenation archives and filters. Oxygen is dissolved in lakes directly from air, and as a byproduct of photosynthesis by autotrophic organisms (multicellular plants, algae, and photosynthetic bacteria), greatly simplified as: . . . 6CO2 + 6H2O + sunlight → C6H12O6 + 6O2 (4:1) . . . Photosynthesis is performed by both organisms on the lake floor (phytobenthos), and by floating algae and bacteria (phytoplankton). Molecular oxygen is lost from the water column through respiration, secondary oxidation of organic or inorganic compounds in the water column or sediments, or directly to the atmosphere following supersaturation. Oxygen concentrations in the water column are a function of productivity, respiration, temperature, and mixing, all of which vary both diurnally and seasonally within a lake, and because of climatic or morphometric differences between lakes. To understand this variation, it is useful to return to our examples at Junius Ponds #5 and #7. In dimictic Junius Pond #7, the water column just below the ice is well oxygenated during the winter, but is anoxic at the lake floor.

Keywords:   Acidification of lakes, Bedrock, Calcite, Deforestation, Epilimnion, Fossil fuels, Global change

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