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6.1. RADIOMETRIC DATING OF WATER AND SEDIMENTS 197
6.1.7 U/Th dating of deep-water corals from the North Atlantic
Participating scientist Andrea Schröder-Ritzrau, A. Mangini, A. Freiwald (Tübingen), G. Hoffman
(Zürich)
Abstract Coupled uranium-series and radiocarbon measurements on deep-water corals are a proxy
to estimate intermediate water ages. 17 intermediate water ages were estimated on deep-water corals
from the eastern North Atlantic. These data give strong evidence for periodic reduced ventilation in
the North Atlantic during the LGM/Holocene transition.
600
400
� 14 C ‰
200
0
� C Cariaco Basin; Hughen et al. 2004
14
� 14
C model/atm; Laj et al. 2002
� 14
C atm INTCAL; Reimer et al. 2004
� 14
C marin INTCAL; Hughen et al. 2004
-200
0 4000 8000 12000 16000 20000 24000
age (years BP)
Figure 6.6: ∆ 14 Cintermediate water (red data points) compared to various ∆ 14 C records and models (see
references in the Fig.). Data are plotted without error bars for clarity - except for the deep-water coral
data.
Background Thermohaline circulation in the
world ocean has a major influence on climate.
Hence, changes of thermohaline circulation and
water mass distribution especially in the North
Atlantic during climate transitions are of major
interest for paleoceanographic and climatic reconstructions.
It is suggested that at terminations
the intermediate water has a larger component of
poorly ventilated SSW (Southern Source Water)
compared to interglacials and full glacials.
The 14 C/ 12 C ratio of dissolved inorganic carbon
is a powerful tracer for reconstructing the
pathways of deep- and intermediate water in the
ocean. Coupled AMS- 14 C- and Th/U- (TIMS)
measurements on coral carbonate, deliver a snapshot
of the 14 C-concentration of deep- or intermediate
water at the time and place the coral lived
[Mangini et al. 1998].
Funding DFG and EU
Methods and results Intermediate water ages
were calculated via the method of 14 C projection
ages [Adkins and Boyle, 1997]. Radiocarbon
dates (measured at the ETH-Zurich and at
the Leibniz-Laboratory in Kiel) and uranium series
ages (via TIMS in HD) were used to calculate
the ∆ 14 Cintermediate water of the former sur-
rounding intermediate water the coral lived in.
Assuming closed system radiocarbon decay and
no further exchange with atmospheric radiocarbon,
the calculated ∆ 14 Cintermediate water of the
coral (Fig. 6.6) can be backtracked to the crossover-point
with the (∆ 14 C past/atm) [Reimer et al.,
2004]. This ratio (∆ 14 C past/coral) is the ratio of
the water that had equilibrated with the past atmosphere.
This value was applied in the equation
for the ventilation age [Mangini et al., 1998] where
non-reservoir-corrected ventilation ages (intermediate
water ages) are determined.
A total of 69 deep-water corals were investigated
and 17 intermediate water ages were calculated.
Our data document short periods of reduced conveyor
circulation associated with increasing influence
of SSW at intermediate water depth at the
end of Heinrich I and the end of the Younger Dryas
event. These periods are associated with meltwater
discharge (MPIA and B). A similar event in
the early Holocene has to be verified.
Outlook/Future work none in this case, the
project is finished
Main publication Schröder-Ritzrau et al.
[2003]; Schröder-Ritzrau et al. [2005]