Severe AMS- 14 C Dating Errors in Quaternary Deep-sea Sediments Caused by Zoophycos-Bioturbation and Foraminifer Abundance Variations
Abstract
AMS 14C-dating performed on planktonic foraminifers from the trace fossil Zoophycos and from directly surrounding deep-sea sediment from the Portuguese continental slope reveals not only large differences between trace and surrounding sediment but also between different species. That the combination of bioturbation and variations in the abundance of the foraminifers carrying the proxy information leads to a smearing of the stratigraphic record and may create false leads and lags between paleoclimatic proxies is well known and several mathematical techniques have been developed to account for this problem. However, the effect of single, discrete, deep-reaching traces such as Zoophycos defies this type of mathematical approach. Zoophycos is a highly complex trace common in all oceans at depths exceeding 1000 m. It is often difficult or even impossible to discern in fresh sediment and represents the downwards transport of considerable amounts of surface material more than 1 m into the sediment. At this depth the surface material is mixed with surrounding sediment and deposited into distinct layers. Our study shows differences of up to 3,320 years between the trace and surrounding sediment at sedimentation rates of 10-20 cm ka-1. However, there are also differences between the different foraminifer species. In surrounding sediment the differences amount to 75 to 380 years between G. bulloides and G. ruber which can be explained by small variations in abundance of the two species and the homogenization of the sediment in the mixed layer. In the Zoophycos material, the differences between the two species may reach 2,500 years. These large differences result from the mixing of surface material rich in one species and poor in the other with surrounding material where the conditions are reverse. Thus, the Zoophycos material will be relatively enriched in younger specimens of one type and depleted in concern of the other, resulting in large differences in age between the two species. This kind of errors is an obvious threat to the accuracy of paleoclimatic and paleoceanographic studies. Therefore, because Zoophycos burrows are often poorly or not at all visible in fresh cores, it is strongly recommended that X-ray radiographs should always be consulted prior to subsampling of the core material to avoid dating errors of this kind.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFMPP31A0490L
- Keywords:
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- 0932 Radioactivity methods;
- 1035 Geochronology;
- 1050 Marine geochemistry (4835;
- 4850);
- 3030 Micropaleontology;
- 4267 Paleoceanography