| Radiocarbon
Chronology In addition to possessing an accurate annual varve chronology, the Cariaco Basin
sediments are also well-suited for reliable 14C dating. The high sediment
deposition rate and concentration of foraminifera provide high-resolution 14C
sampling (10-15 varve years per 14C date, with no mixing artifacts from
bioturbation), at closely spaced intervals (one 14C date every ~100 varve
years). To test whether radiocarbon dates thus obtained could be reproduced, 14C
ages from cores PL07-56PC and -39PC were compared. Calendar ages for the two cores were
assigned using detailed cross-correlations with individual millimeter-scale
marker laminae and high-resolution grey scale records. The 14C
dates for both cores show close agreement, demonstrating that Cariaco Basin 14C
dates are reproducible and that radiocarbon and calendar ages from one core can be readily
applied to other cores from the basin.
The ocean reservoir stores vastly more carbon than the
atmosphere, particularly in deep waters. Depending on the rate of mixing from below, the
surface ocean typically has 14C ages 400-1600 years older than the atmosphere,
reflecting the marine reservoir age. In any radiocarbon-dated marine sediment record, the
magnitude and stability of reservoir age with time is an important issue. The present-day
Cariaco Basin reservoir age has been measured on two sediment samples of known recent age
and averages 420 years. This age is close to the open-ocean surface Atlantic value,
despite the fact that the basin experiences variable seasonal upwelling. The good
agreement with the open-ocean reservoir age is probably related to basin bathymetry.
Shallow sills surrounding the basin limit entry to waters less than 146 m deep that are
well-equilibrated with the atmosphere. Furthermore, tritium as well as d13C and
total CO2 profiles within the Cariaco Basin indicate the presence of continual
mid-depth ventilation with Caribbean Sea thermocline water and an estimated residence time
of approximately 100 years. Thus, only young water is ultimately available
within the basin to replace surface water advected offshore during Ekman drift-induced
upwelling.
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The reliability
of Cariaco Basin 14C dates also depends on the stability of the reservoir age
through time. Researchers have used the Vedde volcanic ash layer (~10.3 14C kyr
BP on land) to identify terrestrial and North Atlantic marine sediments deposited at the
same time, and showed that, on average, high-latitude 14C reservoir age
increased during the Younger Dryas relative to the present value by about 300-400 years.
This was due to reduced northward advection of young, well-equilibrated surface waters
into the high-latitude North Atlantic, together with increased sea ice, which isolated
surface waters from the atmosphere and allowed a greater proportion of upward mixing of
old, deep waters. However, this effect was probably limited to high-latitudes and would
not have affected the Cariaco Basin. At low latitudes, reservoir ages are less variable,
due to a well-ventilated thermocline and the lack of sea ice to act as a barrier to the
atmosphere.
Direct evidence for a stable Cariaco Basin reservoir age
through time is seen in the close match between tree-ring and Cariaco Basin 14C
ages from 10.0-11.8 cal kyr BP (Figure 3). The reservoir age remains the same, within
errors, during a period of almost 2000 years. More importantly, the reservoir age remains
constant across the large change in upwelling at the Younger Dryas termination. This
climate shift, representing one of the largest transitions in the Cariaco Basin record
between periods of intense and reduced upwelling, occurred in less than a decade. Cariaco
Basin 14C dates overlap with the tree-ring radiocarbon record immediately prior
to the Younger Dryas-Preboreal transition (Figure 3). If variable upwelling had influenced
reservoir age, we would expect to see it here. However, there is no discernible shift to
older 14C ages in Cariaco Basin dates during the Younger Dryas. In addition,
high-resolution series of 14C dates from terrestrial macrofossils have been
measured from Lake Krakenes, Norway and Lake Madtjärn, Sweden. The Lake Krakenes and Lake
Madtjärn records can be correlated to the Cariaco Basin at sediment transitions
bracketing the Younger Dryas, clearly discernible in both records. The Cariaco Basin 14C
ages show no offset from the terrestrial 14C dates throughout the Younger Dryas
and during the late Allerød period. The weight of evidence clearly supports the
conclusion that the Cariaco Basin reservoir age reflects open Atlantic values, and has not
changed significantly through time due to variations in local upwelling. |