NSRL continues in collaborative efforts to exploit intermittently laminated sediments of the tropical North Atlantic Cariaco Basin in order to extend and in-fill calibration of the ¹⁴C timescale. Varve-calibrated ¹⁴C results provide approximately decadal resolution in the calibration for the deglacial interval 15,000 - 10,000 ¹⁴C years BP [see Hughen et al. 2000 at http://www.ngdc.noaa.gov/paleo/pubs/hughen2000/hughen2000.html].

Figure 1. Cariaco Basin varve-calibrated 14C results (dark blue) compared to other calibration data sets spanning the interval of the last deglaciation [after Hughen et al. 2000]. Cariaco Basin 14C results are for samples of foraminifera picked by Chanda Herring at NSRL. All Cariaco Basin 14C measurements for this study were performed at LLNL-CAMS by Dr. John Southon and colleagues.

Recently we have extended our analysis of Cariaco Basin sediments to ~50,000 ¹⁴C year BP in long Ocean Drilling Program cores (Site 165). In this study several hundred ¹⁴C determinations were performed at NSRL-NOSAMS and at LLNL-CAMS. Inter-lab agreement, evaluated by comparing Fm (fraction modern) of NSRL-NOSAMS results to interpolated Fm for bracketing LLNL-CAMS measurements was -0.3 ± 0.6 ‰, indicating the absence of any systematic offset between labs. The root-mean-square difference, representing the combined variances for preparation and correction at LLNL-CAMS vs. NSRL, measurement at LLNL-CAMS vs. NOSAMS, and the effects of sampling and interpolation, is 3.9 ‰.

Figure 2. Cariaco Basin 14C age vs. estimated calendar age [following Peterson et al., 2000], showing agreement between results from NSRL-NOSAMS (red and blue) and LLNL-CAMS (green). NSRL-NOSAMS results were prepared and blank corrected at NSRL and measured at NOSAMS.

These results will be compared to calibration results from Lake Suigetsu, Japan and Barbados (not shown) and to ¹⁴C production rates estimated from paleo magnetic time series (Figure 3) in Hughen et al., in preparation

Figure 3: Atmospheric D14C calculated from Cariaco Basin age pairs in Figures 1 and 2 (and from tree rings), compared to results of a simple carbon reservoir and exchange model (orange line, upper panel) forced by 14C production rates estimated from paleo magnetic data (lower panel). Timing of events is similar for data and model, but smaller carbon reservoir sizes are required to match observations for the glacial period prior to ~15,000 years ago (red line, upper panel).

References cited:

Hughen, K.A., Southon, J.R., Lehman, S.J., and J.T. Overpeck (2000). Synchronous radiocarbon and climate shifts during deglaciation. Science 290: 1951-54.

Hughen, K., Overpeck, J., Lehman, S.J., Kashgarian, M., Southon, J. and L.C. Peterson (1998). A new ¹⁴C calibration data set for the last deglaciation. Radiocarbon 40: 483-494.

Hughen, K., Overpeck, J., Lehman, S.J., Kashgarian, M., Peterson, L.C., Alley, R. and D.M. Sigman (1998). Deglacial changes in ocean circulation from an extended ¹⁴C calibration. Nature 391: 65-68.

Hughen, K.A., Lehman, S.J., Southon, J.R., Turnbull, J., Marchal, O. and J.T. Overpeck. (in prep.). ¹⁴C activity and carbon cycle changes over the past 50,000 years. (to be submitted to Science).