Edouard Bard

NotCal04; comparison/ calibration 14C records 26-50 cal kyr BP

The radiocarbon calibration curve IntCal04 extends back to 26 cal kyr BP. While several high-resolution records exist beyond this limit, these data sets exhibit discrepancies of up to several millennia. As a result, no calibration curve for the time range 26-50 cal kyr BP can be recommended as yet, but in this paper the IntCal04 working group compares the available data sets and offers a discussion of the information that they hold.

234U/238U mass spectrometry of corals: How accurate is the UTh age of the last interglacial period?

Direct measurement of U and Th by mass spectrometry recently provided a spectacular improvement in the precision of coral dating by the UTh disequilibrium method [1]. Mass spectrometric data also provide a much better resolution for the examination of subtle diagenetic effects revealed by small variations of the 234U/238U ratio. Such perturbations may strongly affect the accuracy of the UTh chronometer. n nA compilation of all the corals analyzed to date by mass spectrometry shows that most of the corals from terraces of the last interglacial have initial 234U/238U ratios higher than present-day seawater, in contrast to modern, Holocene and last Glacial corals. Some samples that have very high 234U/238U initial ratios, up to 1.2, and high U concentrations, up to 4 ppm, were probably contaminated by continental groundwaters. However, even apparently pristine samples have 234U/238U initial ratios which are still slightly higher than present-day seawater (mean value: 1.160, compared to 1.140–1.150 [2]), with little overlap between the two distributions. n nThis difference in the U initial ratio raises some uncertainty about the accuracy of the UTh age determinations of these corals. In spite of the fact that the 234U230Th ages cluster in a narrow range between 122 and 133 kyr, the data could also be interpreted as resulting from slight contamination of corals that are significantly older than 125 kyr. n nTwo possible explanations may explain these data: (1) All these samples may have been diagenetically altered, since they all come from surface outcrops which have been directly exposed to precipitations or soil waters for 125 kyr. In this case, the true age of these corals remains uncertain, depending on the timing of the alteration process (i.e., initial, late or continuous). (2) The second possibility is that some of the differences in 234U/238U ratios measured in ∼ 125 kyr cold corals, compared to modern seawater, may be due to a higher 234U/238U ratio in seawater 125 kyr ago. This could result from temporal variations in the weathering regimes of continental land masses.

Uranium-234 anomalies in corals older than 150,000 years

We present new precise U-Th ages of well-preserved coral specimens collected from the island of Barbados, West Indies, and the atoll of Mururoa, French Polynesia. Our new data confirm the ages attributed to oxygen isotope stage 7 in the framework of the Milankovitch theory (BERGER, 1978; Mar-TINSON et al., 1987). By using thermal ionization mass spectrometry (TIMS), it is also possible to quantify precisely the 234U238U ratios in corals. Samples older than 150 kyr B.P. are shown to be characterized by significant excesses of 234U relative to the uranium isotopic composition expected if the corals grew in present-day sea water. Assuming that the 230Th-ingrowth ages are accurate, these anomalies translate into high initial 234U238U ratios: about 1.2 at 200 kyr and up to 1.5 at about 450 kyr B.P. We propose that the anomalies result from both diagenetic addition and replacement of U and possibly from global changes in the 234U238U composition of sea water through time. The 234U anomalies cast doubt on the accuracy of the classical 230Th-ingrowth dating method in old corals, and in particular for the use of measured 234U238U ratios alone to date corals older than 150 kyr.

Sea-level estimates during the last deglaciation based on δ18O and accelerator mass spectrometry 14C ages measured in Globigerina bulloides

Coupled measurements of δ18O and accelerator mass spectrometry (AMS) 14C in a particular species of planktonic foraminifera may be used to calculate sea-level estimates for the last deglaciation. Of critical importance for this type of study is a knowledge of the seasonality of foraminiferal growth, which can be provided by δ18O measurements of modern shells (core tops, plankton tows). Isotopic (δ18O, AMS-14C dating) and faunal records (transfer function sea surface temperature) were obtained from two cores in the North Atlantic at about 37°N. The locations were chosen to obtain high sedimentation rate records removed from the major ice-melt discharge areas of the last deglaciation. Based upon Globigerina bulloides data, four δ18O-based sea-level estimates were calculated: −67 ± 7 m at 12,200 yr B.P. and −24 ± 8 m at about 8200 yr B.P. for core SU 81-18; −83 ± 10 m at 12,200 yr B.P. and −13 ± 11 m at about 8500 yr B.P. for core SU 81-14. Using a second working hypothesis concerning the seasonability of G. bulloides growth, it is suggested that the sea-level rose by about 40 m during the millennium which followed 14,500 yr B.P.

The Last Deglaciation in the Southern and Northern Hemispheres: A Comparison Based on Oxygen Isotope, Sea Surface Temperature Estimates, and Accelerator 14C Dating from Deep-Sea Sediments

The last deglaciation in two deep-sea sediment cores recovered from the Southern Indian Ocean is studied and compared with two records obtained from the North Atlantic. The chronology has been established by accelerator mass spectrometric (AMS) 14C dating of planktic foraminifers. Climatic changes are inferred based on δ 180 measurements in planktic foraminifers and on sea surface temperatures (SST) obtained by means of faunal and floral transfer functions. In the North Atlantic, the last deglaciation began at about 15 –14.5 ka, Holocene conditions were reached at about 12.5 – 12.0 ka and a cold interval occurred between 11.0 and 10.0 ka (Younger Dryas Event). In the Southern Ocean, the last deglaciation began between 16.5 and 13.0 ka and Holocene temperatures were reached at about 12.0 ka. Both Southern Ocean records present transitory oscillations: Core MD 84–551 (55°S) exhibits a temporary increase in δ 18O dated at about 10.5 ka (but it is still unresolved if this feature is due to SST changes or other regional causes) while Core MD 84–527 (44°S) is characterized by a cold event at about 11.6 ka (only in the SST records) and a prominent warm optimum between 10.5 and 8.0 ka (in the SST and δ 18O records). More data are needed to determine if there was a time lag between the last deglaciations in both hemispheres and if common transitory oscillations can be recognized.

AMS-14C ages measured in deep sea cores from the Southern Ocean: Implications for sedimentation rates during isotope stage 2

Abstract 14 C dates obtained by accelerator mass spectrometry (AMS) on monospecific foraminiferal samples from two deep-sea sediment cores raised in the Indian sector of the Southern Ocean have been corrected for the difference in 14 C composition between atmosphere and sea surface by using a reconstruction of the latitudinal 14 C gradient which existed in the Southern Ocean prior to 1962. The corrected AMS- 14 C data show a reduced sedimentation rate in core MD 84-527 between 25,000 and 10,000 yr BP. For core MD 84-551 the available data suggest that the sedimentation rate was higher during the Holocene than during the glacial period. These changes in sedimentation rates may be attributed to an increased opal dissolution during the last glacial maximum.

Glacial-to-Interglacial Changes in Ocean Circulation

Only a few years after its discovery by Anderson and Libby (1947), radiocarbon has revolutionized our understanding of the climatic history of the upper Quaternary. Flint and Rubin (1955) demonstrated that the moraines deposited over the northern United States by the last great ice sheets contained organic material with measurable 14C activity. Numerous measurements performed on moraines and glacial sediments from northern America and northern Europe (see a review in Denton and Hughes (1981)) showed that continental ice sheets reached their maximum extension only 18,000 years ago, and retreated slowly to disappear about 6500 years ago.