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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.

 

 

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.