Radiocarbon dates of shells and stable isotope values of bulk sediment
The Limfjord is a sound in Northern Jutland, Denmark, connecting the North Sea with the Kattegatt. The complex interplay of eustatic sea level changes and isostatic land-rise caused the relative sea level of the region to fluctuate throughout the later part of the Holocene. Consequently, the region experienced periods mostly with freshwater/ brackish conditions and others with predominantly marine conditions. The changes in relative sea level resulted in a landscape which at periods were characterised as a fjord (only one connection to the sea), sometimes a sound (more than one connection to the sea). A multi-proxi approach has been applied on a sediment core from Kilen (a former fjord arm in the Limfjord, now a brackish lake) near the town of Struer to reveal the Limfjord’s development in more detail. In this paper, we concentrate on stable isotope measurements of bulk sediment and on radiocarbon dating of shells covering the period from c. 6000BC to c. 1000AD. An age model for the sediment core, based on radiocarbon dating of terrestrial material, is constructed, assigning an age to each centimetre of depth. d13C values and C/N ratios of bulk sediment are measured. The correlation between these values verifies the assumption that the isotope values are mainly controlled by the source of the organic matter, either marine, brackish, freshwater or terrestrial. With d13C and C/N from different depths we can thus follow changes in the Limfjord’s environment. Another aspect of this study is the determination of the radiocarbon reservoir effect. A reservoir effect occurs when a sample’s carbon derives from a reservoir with a different, typically lower, 14C concentration than the atmosphere. Samples that initially contain less 14C than contemporaneous samples in equilibrium with the atmosphere (“terrestrial samples”) yield too high radiocarbon ages. The age difference between such a terrestrial sample and one from a reservoir with lower 14C concentration is called the reservoir age. In the case of the Limfjord, two different reservoir effects have occurred through time, the marine reservoir effect and the hardwater effect. Upwelling of bottom water and mixing with surface water results in a marine reservoir age of c. 400 years in the seas around Denmark. The hardwater effect occurs in freshwater systems with a high content of dissolved minerals, with “hard water”. These contain considerable amounts of “14C-dead” carbon because the dissolved carbonates have infinite ages, compared to the 14C time scale. A hardwater effect of a few thousand years is possible. The total reservoir effect for the Limfjord is a combination of the hardwater and the marine reservoir effect. As the relative proportion of marine and fresh water varied with time, the reservoir age for the Limfjord is also expected to vary. Radiocarbon datings of shells, compared to the age model based on terrestrial samples, resulted in reservoir ages that differed from the model ocean by between -150 and +320 years. In addition to using this information as an indicator for the Limfjord’s environment, our results may furthermore shed light on the magnitude of reservoir age corrections to be applied when dating marine derived archaeological samples in this region. The stable isotope and radiocarbon data will briefly be compared to the results from ongoing multiproxy analyses, including lithology and physical parameters, diatoms, sedimentary pigments, macrofossils and foraminifera.
Main Research Area:
Socio-environmental dynamics over the last 12,000 years: the creation of landscapes II, 2010