Theme 2- Chronological tools and refinements

A. Radiocarbon reservoir age corrections for freshwater and marine fish in medieval British Isles

Fish were an extremely important component of past diets in the British Isles. Freshwater species such as pike, trout and cyprinid species dominate the archaeological record until the end of the first millennium AD. At this time, technological advances allowed fishing to take place in more open and deeper waters, allowing more marine resources to be exploited. The extent to which old carbon contributes to marine reservoir effects has been studied using molluscs and corals around the world. However, relatively little information is available on freshwater reservoir effects which can have a considerable impact when radiocarbon ages are obtained from bone.

Fish bone is often not well preserved in an archaeological site and recovery biases, especially when sieving does not take place, may also contribute to the dearth of freshwater fishbone assemblages. Consequently, the age offsets seen in human bones may not be recognized nor appropriately attributed to effects of freshwater fish consumption. The aim of the project, undertaken by Evelyn Keaveney, is to develop a methodology which will allow freshwater reservoir effects to be predicted from measurements made on modern bone, especially in the absence of well preserved freshwater fish bone.

Measurements will be made on freshwater fish bone from various localities in the British Isles and compared to contemporaneous terrestrial archaeological material to calculate the reservoir offset. Measurements will also be made on modern material to calculate the freshwater offset from the modern atmosphere. Stable isotope analysis (δ¹³C and δ¹⁵N) will also be performed on both modern and archaeological material.

Samples have been obtained from a fishbone assemblage excavated in Coppergate in York and from an assemblage from Flixborough in Lincolnshire, with the co-operation of James Barrett of the University of York. Other samples have been provisionally located in Clonmacnoise in Kildare and from a Roman camp-ground assemblage in Cambridgeshire. Modern samples of the species analysed will also be obtained from the areas through cooperation with environmental agencies and university biology departments.

B. A Holocene tephrochronology from the Greenlandic ice

In August 2006, Dr Gill Plunkett was appointed as a CHRONO Postdoctoral Research Fellow on a new project to construct a Holocene tephrochronology for the Greenlandic ice cores. This project has been initiated by Prof Valerie Hall and Profs Emeritus Jonathan Pilcher and Mike Baillie, working with Prof Sigfus Johnsen and Dr J. P. Steffensen of the glaciology group of the University of Copenhagen. This group has produced outstanding high resolution records of temperature change from Greenland ice cores (DYE3, GRIP, NGRIP) which underpin much of modern palaeoclimatic research. These scientists are of the highest international standing and have been immensely supportive of tephrochronology research undertaken by CHRONO staff. Sample collection from the ice storage in Copenhagen is planned for the early autumn.

C. Radiocarbon calibration and marine reservoir corrections

Radiocarbon ages form the chronological basis for most Holocene and late Pleistocene palaeoclimatic records and paleaeoenvironmental reconstructions as well as archaeological studies. However, the basic radiocarbon age calculation assumption of constant atmospheric ¹⁴C content is not valid, due to solar and geomagnetic-induced changes in production rate and ocean circulation changes. In order to compare or combine radiocarbon ages with those derived from other means, such as ice core layer counts or U/Th dated sequences, it is necessary to calibrate radiocarbon dates against measurements of known age samples. Paula Reimer leads the international radiocarbon calibration working group which released the currently internationally accepted radiocarbon calibration curves in 2004 (Reimer et al. 2004; Hughen et al. 2004; McCormac et al. 2004) for Northern Hemisphere terrestrial, marine, and Southern Hemisphere terrestrial radiocarbon samples, respectively. Plans are underway for an update and extension to the calibration curves. Ron Reimer is responsible for the widely-used CALIB program which utilises the calibration curves to convert radiocarbon dates to calibrated ages.

There is also increasing use of atomic-weapons produced ¹⁴C as a tracer or dating tool in geosciences and forensics. Through comparison with past atmospheric ¹⁴C levels, a very precise date may be possible for recent samples of peat or bone. Ron Reimer and Paula Reimer provide a program for on-line calibration of post-bomb ¹⁴C levels which is linked to the ¹⁴CHRONO website under the Resources menu.

Marine samples require a correction for regional upwelling or mixing of water containing old carbon. The Marine Radiocarbon Reservoir Correction Database (MRRCD) also accessible at www.calib.org, was developed by Ron and Paula Reimer to provide this information in an easily accessible format and is regularly updated with measurements on known age mollusk or corals. The MRRCD has recently undergone a revision to fully searchable on-line database. Identification of the species feeding preferences, which can affect the reservoir offset, is being researched for inclusion by Emily Murray.