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Radiocarbon Age Offsets in Different-Sized Carbonate Components of Deep-Sea Sediments

Published online by Cambridge University Press:  18 July 2016

John Thomson ,
G. T. Cook ,
Robert Anderson ,
A. B. MacKenzie ,
D. D. Harkness  and
I.N. McCave
John Thomson
Affiliation:
Institute of Oceanographic Sciences Deacon Laboratory, Brook Road, Wormley, Godalming, Surrey, GU8 5UB, England
G. T. Cook
Affiliation:
Scottish Universities Research and Reactor Centre, Scottish Enterprise Technology Park, East Kilbride, G75 0QF, Scotland
Robert Anderson
Affiliation:
Scottish Universities Research and Reactor Centre, Scottish Enterprise Technology Park, East Kilbride, G75 0QF, Scotland
A. B. MacKenzie
Affiliation:
Scottish Universities Research and Reactor Centre, Scottish Enterprise Technology Park, East Kilbride, G75 0QF, Scotland
D. D. Harkness
Affiliation:
NERC Radiocarbon Laboratory, Scottish Enterprise Technology Park, East Kilbride, G75 0QF, Scotland
I.N. McCave
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, England
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Abstract

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We compared accelerator mass spectrometry (AMS) 14C ages of large (>150 μm) pelagic foraminifera with radiometric bulk carbonate 14C ages in two northeastern Atlantic cores. The foraminiferal ages are consistently older than those of the bulk sediment (by + 0.76 ka in Core 11881 and by + 1.1 ka in Core 11886), whereas corresponding fine (<5 μm) fraction ages are similar to those of the bulk sediment carbonate. We calculated near-identical sediment accumulation rates from both the foraminiferal and bulk sediment age/depth relations (3.0 cm ka−1 in Core 11881 and 5.9 cm ka−1 in Core 11886). Consideration of various factors that might produce such offsets leads us to believe that they are not artifacts, but were most probably caused by differential bioturbation of the different size-fractions in the sediment surface mixed layer. The importance of this finding is that many paleoceanographic records, such as the oxygen isotope record, also derive from analyses of large foraminifera, so that these records must be offset in time from the bulk of the sediments that they characterize.

Type
I. 14C in the Reconstruction of Past Environments
Information
Radiocarbon , Volume 37 , Issue 2 , 1995 , pp. 91 - 101
Copyright
Copyright © the Department of Geosciences, The University of Arizona 

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