High-resolution multiproxy reconstruction of extratropical North Atlantic climate dynamics during the last millennium
Final Report Abstract
In this project, the environmental variability in the North Atlantic during the last millennium was reconstructed with annual resolution by means of bivalve sclerochronology. For this purpose, master chronologies were built using shells of the bivalve Arctica islandica from various different sites and water depths. Geochemical properties and changes in growth rate provided information on past water temperature, ocean productivity, pollution and wind stress. Main findings were as follows. (1) Amongst other century-long, absolutely dated master chronologies (NE Iceland, Gulf of Maine, Faroe Islands), a master chronology was constructed from 51 shells of the Greater North Sea covering the time interval of 1040 to 2010 AD. According to this chronology, temperature and primary productivity were controlled by the North Atlantic Oscillation (NAO) and coupled ocean-atmosphere dynamics in the subpolar North Atlantic (12-16 year cycles) as well as the Atlantic Meridional Overturning Circulation (AMOC). Higher frequency climate modes (NAO, 12-16 yr cycles), more variable temperature and food supply, stronger westerlies and more frequent winter blockings (= persistent high pressure system over Central Europe) dominated during times of major climate regime shifts, whereas lower frequencies (AMOC) occurred during climatically more stable times. (2) Major drivers of shell growth were temperature and food availability, each factor accounting for about 50% of the variance in growth. (3) A millennial scale pollution (Pb, Fe) history of the North Sea was provided. During the heyday of metallurgy in medieval times, shells recorded strong lead contamination, but barely any iron, whereas the opposite occurred at modern times. At first glimpse, this finding is counterintuitive. However, aside from respective metal fluxes to the ocean, the redox state at the sediment surface, i.e., where these bivalve collect food, is essential. Only due to eutrophication in modern times, iron became biologically available to the bivalves. Since A. islandica prefers fresh (Pb-depleted) organics, rather than (Pb-enriched) dead organic matter at the sea floor, lead content in modern shells is lower than during medieval times. (4) Although a general temperature rise was observed in subsurface waters (70-120 m depth) since the end of the Little Ice Age, no marine “hockeystick” was found. Oxygen isotope data and temperatures reconstructed thereof show signal consistency in large areas of the North Atlantic, including the North Sea, Norwegian Sea and NE/E Iceland. Through time, temperatures in these regions changed in concert. (5) Sr/Cashell of A. islandica is in fact negatively correlated to temperature and positive to δ18Oshell, not only when annual growth lines are formed, but also throughout the entire growing season. This new findings was achieved by ICP-OES instead of LA-ICP-MS analysis. However, since much larger sample quantities are needed for wet chemistry, only a few (fast-growing) portions of each shell can be sampled with sufficient temporal resolution. (6) Na/Cashell of A. islandica is strongly positively correlated to δ13Cshell and thus, to ocean productivity. Further research is need to confirm this finding and identify the underlying reason. (7) Ba/Cashell signals are strongly synchronous among contemporaneous A. islandica specimens from the same site and habitat. This ratio serves as a new cross-dating tool that will greatly facilitate future construction of long master chronologies. Furthermore, Ba records changes in ocean productivity. By using this proxy, we noticed significant changes in primary productivity during the last millennium. Notably, low productivity characterizes the medieval metallurgy peak.
Publications
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2013. Arctica islandica (Bivalvia): A unique paleoenvironmental archive of the northern North Atlantic Ocean. Global and Planetary Change 111, 199-225
Schöne BR
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2013. Climate signatures on decadal to interdecadal time scales as obtained from mollusk shells (Arctica islandica) from Iceland. Palaeogeography, Palaeoclimatology, Palaeoecology 373, 152-162
Lohmann G and Schöne BR
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2014. Decadal climate variability of the North Sea during the last millennium reconstructed from bivalve shells (Arctica islandica). The Holocene 24, 771-786
Holland HA, Schöne BR, Lipowski C and Esper J
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2014. History of bioavailable lead and iron in the Greater North Sea and Iceland during the last millennium a bivalve sclerochronological reconstruction. Marine Pollution Bulletin 87, 104-116
Holland HA, Schöne BR, Marali S and Jochum KP
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2015. Oceanographic control on shell growth of Arctica islandica (Bivalvia) in surface waters of Northeast Iceland - implications for paleoclimate reconstructions. Palaeogeography, Palaeoclimatology, Palaeoecology 420, 138-149
Marali S and Schöne BR
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Ba/Ca ratios in shells of Arctica islandica – Potential environmental proxy and crossdating tool. Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 465, Part B, 1 January 2017, Pages 347-361
Marali S, Schöne BR, Mertz-Kraus R, Griffin SM, Wanamaker Jr AD, Matras U and Butler PG
