Testing palaeomagnetic and 14C based geochronological methods in the Baltic Sea

Abstract: The Baltic Sea is a continental shelf sea that is influenced by both river runoff and marine water. The two water masses are vertically stratified in the Baltic Sea, resulting in a positive, estuarine circulation system, which has undergone changes in the past due to isostatic uplift of the Baltic basin. For researchers to understand past changes in the marine environment of the Baltic Sea, a sound geochronology is of utmost importance. Radiocarbon (14C) dating, a common age proxy method employed for Holocene marine sediments, is difficult to carry out in the Baltic Sea. There is a lack of suitable macrofossils for 14C dating, due to the prevalence of anoxic bottom waters in the deeper parts of the Baltic Sea, which, together with low alkalinity and salinity, precludes the necessary environmental conditions required for higher order organisms to thrive. Furthermore, the temporally dynamic estuarine circulation of the sea, combined with multiple input sources of marine and terrestrial carbon, means that there is uncertainty surrounding spatial and temporal trends in the 14C reservoir age, or R(t). In this thesis, an alternative geochronological method, namely palaeomagnetic secular variation (PSV), is carried out on Baltic Sea sediments, and its suitability for different Baltic Sea sediment types is considered. A PSV age model for the period 6.3 – 1.3 cal ka BP was successfully constructed for the Gotland Deep region of the Baltic Sea by matching PSV features shared with the Fennostack PSV master curve for Fennoscandia (Snowball et al., 2007). The PSV age model was augmented by Pb isochrones based on reconstructions of atmospheric Pb pollution. 14C determinations carried out on foraminifera, compared to the PSV age model, infer a decrease in reservoir age throughout the Holocene, possibly due to a reduction in 14Cmarine influence due to isostatically induced shallowing of the Baltic Sea. Spatial variation in R(t) was reconstructed by 14C dating museum collection shell samples from a salinity transect of the Baltic Sea. A hard-water effect was found to influence R(t) in coastal regions near land carbonate bearing bedrock. When these hard-water locations are disregarded, and the Macoma mollusc is exclusively considered, in order to exclude species-specific R(t) effects, statistically significant relationships are found for R(t) vs. salinity and for R(t) vs. δ18Oshell. These relationships suggest that R(t) in locations free of hard-water can be explained using a basic two end-member mixing model comprising of 14Cmarine and 14Crunoff. Lastly, this thesis explores PSV records from deglacial (14-10 cal ka BP) sediments from across Fennoscandia, including new data from the Baltic Sea. These PSV records are critically evaluated and geochronologies are updated where necessary. Three sites are subsequently selected and included in a deglacial PSV master curve, which will serve as a new PSV dating tool for Fennoscandian deglacial sediments. Additionally, when this deglacial PSV master curve is combined with the existing Holocene Fennostack PSV master curve, recurring periods of steep inclination in Fennoscandia for the past 14 ka are found, consistent with a 1350 yr period cyclical behaviour in geomagnetic pole movement previously hypothesised for the Holocene by Nilsson et al. (2011).