Supplementary material to “ESF EuroCLIMATE Workshop: Radiocarbon and Ice-Core Chronologies During Glacial and Deglacial Times”

Bernd Kromer, Heidelberg Academy of Sciences, Heidelberg, Germany, bernd.kromer@iup.uni-heidelberg.de; Barbara Wohlfarth, Stockholm University, Stockholm, Sweden, barbara@geo.su.se; Daniela Turk, European Science Foundation, Strasbourg, France, dturk@esf.org

Citation:
Kromer, B., B. Wohlfarth, and D. Turk (2007), ESF EuroCLIMATE workshop: Radiocarbon and Ice-Core Chronologies During Glacial and Deglacial Times, Eos Trans. AGU, 88(27), 278. [Full Article (pdf)]

Accurate time scales are the backbone for palaeoclimatic and palaeoenvironmental reconstructions. A detailed comparison between different land, ocean and ice archives and of their respective climate proxies requires high-resolution and accurate dating. For terrestrial and marine sequences radiocarbon dating provides a common time scale. However, calibration of radiocarbon ages between 12.4–26 k cal BP in IntCal04, i.e. beyond the tree-ring based interval, relies at present on marine ¹⁴C series from corals and foraminifera in marine sediments, with additional uncertainties of the calendar scale, and spatial and temporal variability in the marine reservoir age. For ages older than 26 k cal BP the IntCal04 working group did not recommend any radiocarbon calibration curve, in view of major unresolved time-scale discrepancies in various data sets before 26 k cal BP. Ice cores set templates for high latitude climate sequences (Dansgaard-Oeschger [D/O] and Heinrich events) during the last glacial, and provide an independent time scale through annual layer counting. Counting uncertainties caused by false identification of annual layers in ice cores are small during the Holocene (several decades), but may accumulate to up to several thousand years during Oxygen Isotope Stage (OIS) 3.

Several ongoing projects of the European Science Foundation’s (ESF) EuroCORES programme on EuroClimate ( http://www.esf.org/activities/eurocores/programmes/euroclimate.html) rely on a common time scale for glacial and deglacial intervals. During the past few years, important progress has been made regarding the age control of radiocarbon data sets older than 26 k cal BP. Also, the glacial ice core time scale has been improved by multi-tracer continuous flow analyses in the NGRIP ice core, and U/Th dated speleothems added a new dimension as terrestrial archives of climate proxies, such as ¹⁸O and trace elements.

To discuss recent progress in Glacial and Late Glacial chronologies, participants of four projects of the EuroClimate program and external guests met in Heidelberg, Germany, on 5–7 March, 2007, for a workshop entitled ‘Radiocarbon and ice-core chronologies during Glacial and deglacial times.’ The workshop started with presentations on the potential to obtain tree-ring based ¹⁴C data sets beyond 12.4 k cal BP. K.F. Kaiser (Univ. of Zurich, Switzerland) presented an extension of this absolutely dated range back to 12.590 cal BP. He and M. Friedrich (Univ. of Stuttgart-Hohenheim, Germany) discussed two independently established floating Late Glacial pine chronologies, from Swiss and German sites, respectively, which had recently been cross-matched into a 1600-year long chronology. This chronology covers the ¹⁴C age range between 12.500 and 10.600 ¹⁴C BP, hence early Bølling to early Younger Dryas. F. Guibal and C. Miramont (IMEP, Aix-en-Provence, France) further showed several pine chronologies from the Pre-Alps of Southern France in the same age range. Most importantly they may already have collected pine sections which cover the gap between the floating Late Glacial and the absolutely dated tree-ring chronologies. S. Talamo and B. Kromer (Heidelberg Academy of Sciences, Germany) discussed the status of ¹⁴C work on these tree-ring series, and provided evidence for a variable marine reservoir correction in the tropical Atlantic (Cariaco basin ¹⁴C series) during the Allerød interval. R. Muscheler used fluctuations in Δ¹⁴C calculated from the floating pine sections and 10Be from the GISP2 ice-core, which are considered coeval due to production changes, to suggest an absolute age for the floating pine chronology. These results depend crucially on the still unresolved question of the length of the Younger Dryas interval, which differs by more than a century among the Greenland ice cores, European lacustrine sediments and the Cariaco basin sediments. Exciting new ¹⁴C tree-ring based ¹⁴C sequences, obtained from new finds of Kauri trees at several locations in New Zealand, were presented by C. Turney (Univ. of Wollongong, Australia) and A. Hogg (Waikato Radiocarbon Dating Laboratory, New Zealand). Individual sections may have more than 1000 rings, and cover critical time intervals across the Younger Dryas, around 26 k BP and between 35 and 45 k BP. Special techniques were developed to arrive at high ¹⁴C precision by radiometric (LSC) and AMS dating techniques.

A. Svensson and S. Johnsen (Univ. of Copenhagen, Denmark) presented the new Greenland ice core time scale GICC05, which in the time range of interest for the workshop, profits much from the progress in multi-tracer, continuous flow analysis to detect annual layers. R. L. Edwards (Univ. of Minnesota, USA) demonstrated recent improvements in U/Th dating and new high-resolution ¹⁸O series from Dongge and Hulu cave (China) stalagmites, which exhibit strong Dansgaard-Oeschger-type fluctuations during the last Glacial. These signals were used by K. Hughen (WHOI, USA) to arrive at a new age model for the non-varved sediment sections of the Cariaco basin record (previously matched to GISP2), with notable improvements in the period between the Last Glacial Maximum and the Late Glacial, and in the >40 k year interval. The revised Cariaco ¹⁴C data set and ¹⁴C data from U/Th dated corals now converge well in the pre-26 k year range and prospects for consistent ¹⁴C calibration beyond 26 k years BP are good.

C. Spötl (Univ. of Innsbruck, Austria) and A. Mangini (Univ. of Heidelberg, Germany) presented complex response patterns of ¹⁸O in speleothems in the Austrian Alps, as compared to Northern Hemisphere climate proxies. B. Wohlfarth (Univ. of Stockholm, Sweden) and M. F. Sánchez-Goñi (EPHE-Univ. of Bordeaux 1, France) gave an overview of the RESOLuTION project, which addresses linking of high-resolution terrestrial, marine and ice-core archives during OIS-3 and exemplifies spatially different response patterns to D/O variability, relying in part on direct land-sea correlation through pollen analysis in North Atlantic cores. J.A.A. Bos (Vrije Universiteit of Amsterdam, The Netherlands) discussed the response of the terrestrial environment (based on three sites in Central and Northern Europe) to Greenland D/O interstadials, and highlighted the importance of high-resolution age control and the necessity of ¹⁴C calibration during OIS-3 to better correlate between different sites. These aspects were taken up in the presentations of M. Blaauw (Queen’s Univ Belfast, N-Ireland) who applied Bayesian techniques to test for synchronicity of events between archives. F. Preusser (Univ. of Berne, Switzerland) demonstrated the state of the art on OSL dating of lacustrine sediments, using two cores from Les Echets, France. A. Brauer (GFZ Potsdam, Germany) and U. v. Grafenstein presented new advances in chronology and stratigraphy of sediments records from five European lakes within the DecLakes project, based on detailed micro-facies and δ¹⁸O analyses. D. Verschuren (Ghent Univ., Belgium) showed first results of the CHALLACEA project, which aims at reconstructing climate variability from before LGM (25 k years BP) to present in equatorial East Africa, using a finely laminated sediment record from Lake Challa, Mt.Kilimanjaro.

J. Beer (EAWAG, Switzerland) addressed the question of common, production-induced signals in the fluctuations of ¹⁴C and ¹⁰Be records, as opposed to climate and carbon system effects, and presented new evidence for a dominant production signal. I. Hajdas (ETH Zurich, Switzerland) discussed ¹⁴C anomalies in several data sets during Heinrich event 4. W. Banks used Eco-Cultural Niche Modeling to evaluate the impact of climatic variability on human and animal populations during OIS 3-2 (before and during H4), based on a large database of 6200 ¹⁴C ages for 1112 sites.

Participants noted the strong synergies from a close collaboration between ‘producers’ of chronologies and the ‘user’ community, as offered by the ESF EuroCores concept, and exemplified during the workshop. Participants agreed that tree-ring based ¹⁴C calibration of terrestrial sequences is of prime importance, and therefore the Europe-wide network to locate Glacial and Late-Glacial sub-fossil wood and the initiative to extend the Kauri database was considered essential. The availability of an update of the radiocarbon calibration data set IntCal04 in the very near future was welcomed by the participants. New climate proxies, especially precipitation and temperature indicators derived from well-dated speleothems, will be an important addition to the hemispheric coverage of climate records, and will fill gaps at lower latitudes.

The workshop was funded by ESF and PAGES.