10
m lower than today.We
have performed U-Th isotope analyses on pure aragonite samples from the upper
sections of Leg 166 cores to assign each aragonite-rich sediment package to the
correct sea-level highstand. The uppermost sediment package from each of the
four sites investigated (Sites 1003, 1005, 1006, and 1007) yielded a Holocene U-Th
age. Sediment packages from deeper in the cores have suffered diagenesis. This
diagenesis consists of significant U loss (up to 40%)
in the site nearest the platform (Site 1005), slight U gain in sites further
from the platform, and continuous loss of pure 234U
caused by alpha recoil at all sites. The difference in diagenesis between the
sites can be explained by the different fluid-flow histories they have
experienced. Site 1005 is sufficiently close to the platform to have probably
experienced a change in flow direction whenever the banks have flooded or become
exposed. Other sites have probably experienced continuous flow into the
sediment. Although diagenesis prevents assignment of accurate ages, it is
sufficiently systematic that it can be corrected for and each aragonite-rich
package assigned to a unique highstand interval. Site 1005 has sediment packages
from highstands associated with marine isotope Stages 1, 5, 7, 9, and 11. Site
1006 is similar, except that the Stage 7 highstand is missing, at least in Hole
1006A. Site 1003 has sediment only from Stage 1 and 11 highstands within the U-Th
age range. And Site 1007 has sediment only from the stage 1 highstand. This
information will allow the construction of better age models for these sites. No
high-aragonite sediments are seen for Stage 3 or Substages 5a and 5c. Unless
rather unusual erosion has occurred, this indicates that the banks did not flood
during these periods. If true, this would require the sea level for Substages 5a
and 5c to have remained at least 10
m lower than today.
1
Swart, P.K., Eberli, G.P., Malone, M.J., and Sarg, J.F. (Eds.), 2000. Proc.
ODP, Sci. Results, 166 [Online]. Available from World Wide Web:
<http://www-odp.tamu.edu/publications/166_SR/166TOC.HTM>. [Cited YYYY-MM-DD]
2
Lamont-Doherty Earth Observatory of Columbia University, Route 9W, Palisades
NY 10964, USA. (Present address: Department of Earth Sciences, Oxford
University, Parks Road, Oxford, OX1 3PR, England. gideonh@earth.ox.ac.uk)
3 GEOMAR Research
Centre for Marine Geosciences, Christian-Albrechts-Universitat zu Kiel,
Wischhofstrasse 1-3, Gebaude 4, D-24148 Kiel, Federal Republic of Germany.
4 Department of
Oceanography, Texas A&M University, College Station TX 77843, USA.
Date of
initial receipt: 11 August 1998
Date of acceptance: 21 May 1999
Ms 166SR-106
