Variations in Atmospheric 14C content 40, 000 ago and Radiocarbon age of Heinrich Event 4.
by
Irka Hajdas
ETH Hönggerberg, 8093 Zurich, Switzerland
Coauthors: Georges Bonani (ETH Hönggerberg, 8093 Zurich, Switzerland), Jerry McManus (WHOI, Woods Hole, MA, 02543 USA), Millie Mendelson (LDEO, Columbia University, Palisades NY, 10964 USA), Sidney Hemming (LDEO, Columbia University, Palisades NY, 10964 USA)

The atmospheric content of ¹⁴C changes because of variations in production rate and the exchange between carbon reservoirs of ocean, atmosphere and biosphere. The radiocarbon calibration curve reconstructs the relationship between radiocarbon age and calendar age of the last 11, 600 calendar years. However, the application of the radiocarbon method in dating climatic records extends back to 40 kyr BP. Beyond the range of the calibration curve (>11, 600) and Cariaco chronology (>14, 500 cal BP), the INTCAL98 data set [1] is available but the resolution of this data is low. Therefore, fast changes in 14C content similar to the one observed during the Younger Dryas cannot be reproduced and taken into account in data analysis. An important aspect for correlation between records is chronology. Most deep-sea sediments are dated using AMS ¹⁴C dating of foraminifera shells. Although radiocarbon age of the Heinrich Events HE1 (14.3 kyr BP), HE2 (21 kyr BP), HE3 (27 kyr BP) and HE4 (ca. 35 kyr BP) are established [2], discrepancies in chronologies have been reported [3]. Our results of dating the HE4 layer in high sedimentation ODP 984 core, which was recovered south of Iceland, show that the age of this event can be difficult to assess for various reasons. First of all, records typically studied are deep-sea sediments with low sedimentation rates on an order of few cm/kyr. Because of that, it might be difficult to collect sufficient amount of material (foraminifera shells) needed for an AMS sample. Secondly, sediment layers of Heinrich events are known to be poor in foraminifera. This fact is making the dating even more difficult. The third complication is characteristic for HE4. The age of 35 kyr BP is close to the limit of the radiocarbon dating method. Therefore any contamination with modern or younger carbon (secondary calcite) can result in age which are too young. Moreover, dramatic changes in atmospheric ¹⁴C might add to those problems. Recently, an increased atmospheric ¹⁴C/¹²C ratio at ca. 40 kyr BP has been observed in U/Th dated spaleothems and deep-sea cores. The timing of this excursion is close to the HE4 and can influence the radiocarbon chronology of studied records. In order to reproduce the reported excursion, ostracode shells from the Wilson Creek exposure north of Mono Lake (CA) were dated. Both records show an increase in ¹⁴C content. Based on the North Atlantic record we can say that the excursion precedes HE4 and, as already discussed, might add to the problems of dating that event. [1]M. Stuiver et al., Radiocarbon 40, 1041-1083 (1998) [2]G. Bond et al., Nature 365, 143-147 (1993) [3] L. Vidal et al., Earth and Planet. Sci. Lett. 146, 13-27 (1997)

Date received: April 26, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahi-18.