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Radiocarbon Age Profiles and Size Dependency of Mixing in Northeast Atlantic Sediments

Published online by Cambridge University Press:  18 July 2016

Louise Brown ,
Gordon T Cook ,
Angus B MacKenzie  and
John Thomson
Louise Brown*
Affiliation:
Scottish Universities Research and Reactor Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride, G75 0QF, United Kingdom
Gordon T Cook
Affiliation:
Scottish Universities Research and Reactor Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride, G75 0QF, United Kingdom
Angus B MacKenzie
Affiliation:
Scottish Universities Research and Reactor Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride, G75 0QF, United Kingdom
John Thomson
Affiliation:
Southampton Oceanography Centre, Empress Dock, Southampton, SO14 7ZH, United Kingdom
*
Corresponding author. Email: [email protected].
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Abstract

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In recent years, the most common technique for radiocarbon dating of deep-ocean sediments has been accelerator mass spectrometry (AMS) analysis of hand-picked planktonic foraminifera (forams). Some studies have exposed age offsets between different sediment size fractions from the same depth within a core and this has important implications when establishing a chronological framework for palaeoceanographic records associated with a particular sediment component. The mechanisms generating the age offsets are not fully understood, a problem compounded by the fact that the fraction defined as “large” varies between different studies. To explore this problem, we dated samples of hand-picked forams from two Biogeochemical Ocean Flux Study (BOFS) cores, for which the presence of an offset between the bulk carbonate and >150 μm foraminiferal calcite had already been demonstrated. The presence of a constant age offset between bulk carbonate and coarse fraction material at the two BOFS sites has been confirmed, but the magnitude of the offset is dependent on whether a simple size-separation technique or hand-picking of well-preserved forams is applied. This may be explained if the selection of well preserved forams biases the sample towards those specimens that have spent least time in the surface mixed layer (SML) or have undergone less size selective mixing. Modeling of the 14C profiles demonstrates that SML depth and sediment accumulation rates are the same for both the bulk and coarse sediment fractions, which is consistent with the hypothesis that size-selective mixing is responsible for the age offset.

Type
II. Our ‘Wet’ Environment
Information
Radiocarbon , Volume 43 , Issue 2B: Proceedings of the 17th International Radiocarbon Conference (Part 2 of 3), Conference Editors: Israel Carmi, Elisabetta Boaretto , 2001 , pp. 929 - 937
Copyright
Copyright © 2001 by the Arizona Board of Regents on behalf of the University of Arizona 

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