Towards radiocarbon calibration beyond 28 ka using speleothems from the Bahamas

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Abstract

We present a new speleothem record of atmospheric Δ14C between 28 and 44 ka that offers considerable promise for resolving some of the uncertainty associated with existing radiocarbon calibration curves for this time period. The record is based on a comprehensive suite of AMS 14C ages, using new low-blank protocols, and U–Th ages using high precision MC-ICPMS procedures. Atmospheric Δ14C was calculated by correcting 14C ages with a constant dead carbon fraction (DCF) of 22.7 ± 5.9%, based on a comparison of stalagmite 14C ages with the IntCal04 (Reimer et al., 2004) calibration curve between 15 and 11 ka. The new Δ14C speleothem record shows similar structure and amplitude to that derived from Cariaco Basin foraminifera (Hughen et al., 2004, 2006), and the match is further improved if the latter is tied to the most recent Greenland ice core chronology (Svensson et al., 2008). These data are however in conflict with a previously published 14C data set for a stalagmite record from the Bahamas — GB-89-24-1 (Beck et al., 2001), which likely suffered from 14C analytical blank subtraction issues in the older part of the record. The new Bahamas speleothem ∆14C data do not show the extreme shifts between 44 and 40 ka reported in the previous study (Beck et al., 2001). Causes for the observed structure in derived atmospheric Δ14C variation based on the new speleothem data are investigated with a suite of simulations using an earth system model of intermediate complexity. Data-model comparison indicates that major fluctuations in atmospheric ∆14C during marine isotope stage 3 is primarily a function of changes in geomagnetic field intensity, although ocean–atmosphere system reorganisation also played a supporting role.

Introduction

Accurate records of past atmospheric radiocarbon concentration (Δ14C) are essential for the calibration of the widely-used radiocarbon timescale (Reimer et al., 2004, Stuiver, 1982). Accurate records are also necessary in order to validate models of 14C production rates in the upper atmosphere (Lal and Peters, 1967, Masarik and Beer, 1999), to constrain the flux of carbon transfer between earth system reservoirs (Broecker, 1963, Stuiver and Braziunas, 1993, Toggweiler et al., 1989) and to use 14C as tracer for changes in past ocean circulation (Sarnthein et al., 2007). For the Holocene period, continuous tree-ring records are generally thought to provide reliable archives of past atmospheric Δ14C variations (Stuiver, 1982). Major international efforts over the past 60 years have yielded continuous records from present day to 12.41 ka BP (Friedrich et al., 2004), but only sparse, floating tree-ring records are found for time periods earlier than this. This has made it necessary to draw upon archives from the marine realm such as corals (Bard et al., 1990, Burr et al., 1992, Burr et al., 2004, Cutler et al., 2004, Fairbanks et al., 2005) and foraminifera (Bard et al., 2004c, Hughen et al., 2006, Hughen et al., 1998) to infer Δ14C and provide a calibration curve to 26 ka cal BP (Reimer et al., 2004). However, for times earlier than 26 ka cal BP no consensus has yet been achieved because of substantial dispersion in the records (van der Plicht et al., 2004). Nonetheless, a 14C calibration is still needed and this has led to the development of stand-alone calibrations based on subsets of the available data (Fairbanks et al., 2005, Mellars, 2006, Weninger and Joris, 2008). Some of these are in broad use today, though the 14C community has yet to ratify any one of these for a variety of reasons (Reimer et al., 2006). Critically, the lack of consensus for the pre-26 ka cal BP period undermines efforts to establish the relative timing of important events in both the palaeoenvironmental and archaeological sciences.

Among the potential problems associated with using marine archives for atmospheric Δ14C reconstruction is the possibility that the marine reservoir age may have varied in the past. Hence, additional terrestrial records are highly desirable in order to determine the reliability of marine-based 14C records. Stalagmites have been previously proposed for 14C calibration purposes (Beck et al., 2001, Genty et al., 1999, Vogel, 1983, Vogel and Kronfeld, 1997), although these suffer from the possibility of variations in dead carbon fraction (DCF), the component of carbon derived from the host limestone that is devoid of 14C. This problem is analogous to the marine reservoir age which can vary up to 300 a in the Atlantic (Singarayer et al., 2008). Beck et al. (2001) published the first high-resolution attempt to derive past atmospheric radiocarbon concentrations from a U–Th dated stalagmite. They found a reasonably constant DCF when compared to overlapping parts (11–16 ka) of the IntCal98 calibration curve, but also surprisingly elevated (> 1000‰) and variable Δ14C levels for the period between 45 and 40 ka BP. While similar fluctuations of ∆14C had been reported in a foraminifera-based record from the North Atlantic (Voelker et al., 1998), they are inconsistent with estimates based on U–Th dated corals (Fairbanks et al., 2005) and sub-tropical Atlantic foraminifera from the Cariaco Basin (Hughen et al., 2004, Hughen et al., 2006). Hence, further work on Bahamas speleothem Δ14C records was needed.

Here, we present a new speleothem-based Δ14C dataset derived from a submerged Bahamas stalagmite (GB89-25-3), which we believe provides a robust high-resolution terrestrial record of past Δ14C. We show that the older part of the previous speleothem record (GB89-24-1) was adversely affected by a previously unrecognised 14C blank effect. New measurements on GB89-24-1 using low-blank measurement techniques much more closely resemble coral (Fairbanks et al., 2005), Iberian Margin sediment (Bard et al., 2004c) and Cariaco Basin sediment records (Hughen et al., 2004, Hughen et al., 2006). These new GB89-24-1 measurements are also in excellent agreement with the new U–Th dated high-resolution 14C record presented here.

Continuous records of past atmospheric 14C variation, such as that provided by the new speleothem record, enable us to test model results of transient 14C distribution under changing earth system scenarios. We adopt here the intermediate complexity model GENIE-1 (Grid ENabled Integrated Earth system model) (Lenton et al., 2006) to provide comparison model simulations for much of MIS3 and assess the relative influence of changes of geomagnetic field intensities and the ocean–atmosphere system.

Section snippets

Site and samples

Stalagmites GB89-25-3 and GB89-24-1 were collected in the shallow underwater cave Sagittarius Blue Hole on Grand Bahama, one of the four northern islands of the Bahamas Archipelago. GB89-24-1 was previously described and analysed for U–Th (TIMS) and AMS 14C in the study by Beck et al. (2001). GB89-25-3 is a candle-stick type stalagmite with a mean diameter of about 5.5 cm that formed at a depth of 15 m below present sea level. Six broken pieces with a total length of 960 mm were recovered, 4

Speleothem sampling

The six sections of speleothem GB89-25-3 were sliced in half along the growth axis and then polished, with one side of each section archived for future reference. Slabs (~ 5 mm width) were cut from each section either side of the growth axis as shown in Fig. 1 with one slab used for 14C and U–Th sub-sampling. The 14C samples were prepared with a diamond coated wire saw (Well Diamond Wire Saws, Inc.) using cuts of 2 mm depth following the growth layer with a spacing of 1 mm, producing sequential

U–Th chronology for GB89-25-3

Stalagmite GB89-25-3 is composed of dense non-porous polycrystalline calcite and is mostly free from detrital contamination. The 232Th concentration in GB89-25-3 is generally very low (< 0.5 ng/g). For 26 of 45 samples from the basal section and 21 of 35 samples from the upper section no 232Th was detectable. Samples with detectable 232Th all have 230Th/232Th activity ratios greater than 200. Nevertheless, isochron analyses were performed to determine initial 230Th/232Th ratio on one of the two

Bahamas speleothem 14C ages

Accuracy of past atmospheric Δ14C reconstruction based on speleothems critically depends on a reliable DCF correction. We therefore reassess and discuss our approach to establish a constant DCF, which is used to correct 14C results for sections older than 28 ka. We obtain the same DCF corrected 14C age results for the two stalagmites between 44 – 40 ka using the DCF individually established on the overlap with IntCal, indicating that this approach is robust for speleothems from this cave.

Acknowledgement

This research was supported by the NERC (NER/A/S/2001/00526), the Leverhulme Trust (F/00 182/AU) and the NSF (EAR0223311, EAR0446861 and EAR0622305). We thank J.S. Pigati and J. Quade for use of their new low-blank 14C preparation line. We also acknowledge C. Coath for technical assistance at BIG facilities and J. Tooby for preparing one of the figures. Andy Ridgwell is acknowledged for help with the GENIE model.

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