HR: 1340h
AN: PP13B-1272    [Abstracts]
TI: Assessment of radiocarbon-based deep-ocean ventilation proxies using an Earth-system model of intermediate complexity
AU: Franke, J
EM: joerg.franke@palmod.uni-bremen.de
AF: Department of Geosciences, University of Bremen, PO Box 33 04 40, Bremen, 28334, Germany
AU: * Paul, A
EM: apau@palmod.uni-bremen.de
AF: Department of Geosciences, University of Bremen, PO Box 33 04 40, Bremen, 28334, Germany
AU: Schulz, M
EM: mschulz@uni-bremen.de
AF: Department of Geosciences, University of Bremen, PO Box 33 04 40, Bremen, 28334, Germany
AB: A common method to reconstruct past deep-water ventilation is the calculation of the radiocarbon age difference of planktonic and benthic foraminifera ("top-to-bottom age"). Adkins and Boyle (1997) developed an alternative method, the so-called "projection age" method, to eliminate the circulation-induced time lag, which originates from atmospheric Δ¹⁴C variations. So far, the uncertainties of both methods have not been quantified. We use the University of Victoria Earth System-Climate Model including carbon, radiocarbon and idealized ventilation-age tracers to assess which method is more accurate. Due to the large volume of the ocean and the mixing of water masses of different age, the sub-surface ocean Δ¹⁴C evolution over time is smoother than the atmospheric Δ¹⁴C evolution, with some local variations. We show that the assumption of closed system radiocarbon decay (i.e. without mixing) is nearly valid in the northern North Atlantic Ocean. As the deep water moves along the global conveyor belt, mixing becomes more important and reaches a maximum in the deep Pacific Ocean. The use of a constant reservoir age in the projection-age method appears as another source of error, as our model predicts reservoir-age changes by hundreds of years. The model results suggest that simple benthic-planktonic age differences are more reliable than projection ages in regions where the ventilation age is small (a few hundred years) and the reservoir age variation are comparably large (also a few hundred years). With increasing ventilation age, the correction of the time lag between the signals improves the results of the projection-age method, while errors due to enhanced damping and mixing increase uncertainties in both methods. This leads to errors of similar magnitude for regions such as the deep Pacific Ocean. We present a potential improvement of the projection- age method based on the use of a smoother marine Δ¹⁴C record instead of the atmospheric one and the possibility to employ reservoir-age variations in the source areas of deep-water masses.
DE: 1115 Radioisotope geochronology
DE: 4924 Geochemical tracers
DE: 4962 Thermohaline
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting