| Calibration
Spline The evaluation of both Cariaco
Basin varve and radiocarbon chronologies provides independent evidence that the
chronologies are accurate and can be used as an alternative data set for 14C
calibration during the period of deglaciation. In order to provide continuous calibration
coverage, an interpolation spline was fitted to the Cariaco Basin data set (Figures 3 and
4). This spline was chosen over other possibilities (for instance, cubic spline, linear
interpolation) for several reasons. First, the spline approximates a linear interpolation
between points, which is a simple interpretation and avoids the risk common to cubic
splines of creating structure beyond the resolution of the data. Second, the spline has
the advantage over a linear interpolation of providing a continuous curve which passes
smoothly through the data points. The interpolation spline shows excellent agreement with
the German pine data (r = 0.99) and preserves detailed century-scale variability, for
example during the Preboreal period, around 9.6 14C kyr BP (Figure 3).
The interpolation spline was used to create a curve through
the entire Cariaco Basin data set to extend continuous 14C calibration from 9.0
to 12.7 14C kr BP (Figure 4). This represents an objective treatment of the
calibration in that no Cariaco Basin data points are excluded, and the same interpolation
procedure used to match the curve to tree rings is used to extend the curve further back
in time. The full Cariaco Basin calibration spline is plotted together with paired 14C-U/Th
dates from Atlantic and Pacific corals, and agrees in general with corals throughout most
of the record (Figure 4). However, the data sets do show some large differences,
particularly during the Preboreal and Younger Dryas periods. The Cariaco Basin data show
less scatter around the tree-ring data than corals, suggesting greater reliability during
earlier periods as well. The Cariaco Basin data also provide greater resolution and bridge
the numerous gaps in the coral data. The new Cariaco curve resolves detailed changes prior
to the 9.6 14C kyr BP plateau, including the precise timing of the beginning
and end of a long sloping plateau with superimposed century-scale structure
during the Younger Dryas. In addition, there are plateaus at 11.7 and 11.4 14C
kyr BP, and the possibility of a brief plateau or reversal immediately preceding the
Younger Dryas at 11.1 14C kyr BP. |
Identifying
features such as plateaus in the radiocarbon calibration curve, and determining their
relationships to abrupt climatic events occurring at the same time, is crucial for a wide
range of paleoenvironmental studies. Calculating rates of change or sedimentation rates in
14C-dated sediment cores depends greatly on whether, and how, the 14C
dates are calibrated. For example, a sediment age-depth curve based on uncalibrated
radiocarbon dates will introduce error into rate-of-change and proxy flux calculations.
Using uncalibrated 14C ages to calculate sedimentation rates creates large
anomalies, artificially increasing values during plateaus when the radiocarbon
clock is running slower than calendar time, and decreasing values when
radiocarbon is running faster.Uncalibrated 14C chronologies can result in
substantial, abrupt changes in sedimentation rate (and rate of change) calculations that
coincide with, but do not necessarily relate to, paleoclimate change. Ironically, this
problem appears to be compounded by high-resolution sampling for 14C dating, as
a limited number of 14C dates will tend to smooth over the sharp bends in the
calibration curve and thus produce a smaller, smoothed anomaly during the Younger Dryas.
This new radiocarbon calibration data set from the Cariaco
Basin is available for use as a higher-resolution alternative to curves based on coral 14C-U/Th
dates during deglaciation (11.5-14.5 cal kyr BP), and as a complement to longer curves
based on corals that extend back to 30 cal kyr BP. The Cariaco Basin spline data are
provided at decade resolution for ease of calculation on the part of the user, because 14C
dates are typically reported to the nearest decade. However, it should be noted that the
resolution of the actual calibration data set is approximately one date per 100 years and
does not resolve decade-scale changes in D14C that may have occurred. |