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A paleoenvironmental reconstruction of the early late Paleocene North Sea from intrashell δ18O and δ13C profiles of mollusks

Chapter in book
Authors Birger Schmidtz
Elisabet I. Thompson
Lennart Bornmalm
Claus Heilmann-Clausen
Published in Causes and Consequences of Globally Warm Climates in the Early Paleogene; Editors: Scott L. Wing, Philip D. Gingerich, Birger Schmitz, and Ellen Thomas
Pages 265-274
ISBN 0813723698
Publisher The Geological Society of America, Special Paper 369
Place of publication Boulder, Colorado
Publication year 2003
Published at Department of Earth Sciences
Department of Environmental Science and Conservation
Pages 265-274
Language en
Keywords Early late Paleocene North Sea, Stable isotopes, Mollusks
Subject categories Geology, History of geology and palaeontology

Abstract

Stable oxygen and carbon isotopic analyses of early Selandian (ca. 60 Ma) mollusk shells from Denmark (southeast margin of the North Sea Basin, paleolatitude ∼47°N) were used to estimate intraannual temperature variations. We used shells from two water depths to constrain possible effects of freshwater influx. Assuming normal marine salinity we derived average temperatures for the winter of 16.9 °C and for the summer 22.5 °C at depths of 20–60 m, consistent with a subtropical climate. In contrast, Turritella spp., living at water depths of <20 m, record winter and summer temperatures of 23.4 °C and 28.5 °C, respectively, suggesting an unrealistically warm climate, especially in winter. The data indicate similar seasonal temperature ranges, 5–6 °C, at shallower as at greater depths. These estimates might reflect that Turritella did not calcify during the coldest months. Alternatively, they might indicate influx of local freshwater plumes. Positive δ13C values (2.7‰–3.8‰) of Turritella shells, however, suggest that conditions were close to or fully marine. The deeper-dwelling mollusks were probably not affected by local freshwater influx, but we cannot rule out that the entire southern North Sea had slightly reduced (1–3 psu) salinity. In that case, the estimated deeper water temperatures are too high by 1–3.5 °C. Even with these uncertainties, the isotopic data suggest that summer surface water temperatures were between 22 °C and 28 °C, and winter temperatures not colder than 13 °C. Denmark thus had a warm subtropical climate, as had other locations at these paleolatitudes in western Europe during most of the early Paleogene.

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