Coupling of palaeoceanographic shifts and changes in marine reservoir ages off North Iceland through the last millennium

Our study of spatial and temporal marine reservoir age changes on the North Icelandic shelf is based on two high-resolution 1000-year sedimentary records from IMAGES core sites MD99-2273 and MD99-2275, located at 650 m and 470 water depths, respectively. The sedimentary and fossil record is extremely sensitive to past oceanographic and climatic changes in this region. It has been demonstrated that the position of the Polar Front across the shelf has been very dynamic through the last millennium, and our results show that deviations in the marine reservoir ages can be used as a palaeoceanographic tool for tracing changes in the water masses. The presence of historically and terrestrially dated tephra markers from Icelandic volcanoes makes it possible to construct a reliable tephrochronological age-depth model for each of the marine sediment cores. A comparison of these age models with age models based on calibrated AMS radiocarbon age determinations of marine molluscs displays major deviations through the last 1000 years. The discrepancy between the two types of age models shows that there was an average reservoir age of about 650 years at MD99-2273 and 590 years at site MD99-2275, with considerable changes through time. This is suggested to be related to a generally high, but fluctuating, inflow of apparently old Polar waters to the area. A comparison with the distribution of two benthic foraminiferal indicator species in the cores, one for Polar and Arctic water masses (Islandiella norcrossi) and one for Atlantic Water (Cassidulina neoteretis), as well as with different proxies for sea-ice cover in the area, shows that there was an increase in the penetration of cold Polar and Arctic water masses after AD 1300, i.e. during the Little Ice Age, at both sites, resulting in an increase in the reservoir ages. Relatively high reservoir ages throughout the last millennium at the deepest core site (MD99-2273) can be attributed to a continuously high Polar Water influence at the sea floor. The location of this site to the west of the Kolbeinsey Ridge, which has acted as a submarine barrier throughout the Holocene, may have contributed to a relatively high influence of cold Denmark Strait overflow water in this area.