The temperature variability in the North Atlantic is known to be a major influence on the regulation of Northern Hemisphere climate. The north-eastward transport of waters from the subtropics by the North Atlantic Current modulates the European climate. Understanding this climate variability over extended timescales informs climate and ecosystem models. In turn, these allow the forecast of future climatic conditions and the improv the management food sources. In this work, two annually resolved climatic reconstructions and one ecological reconstruction for the northern North Sea based on Arctica islandica (ocean quahog) sclerochronology are presented. Sclerochronology is the study of physical and chemical variations in the accretionary hard tissues of organisms, and the temporal context in which they formed. The bivalve A. islandica is an excellent organism on which to carry out such studies as it shows clear annual increments in its aragonitic shell which grows in close isotopic equilibrium with the ambient waters.
In the ecological study, an Atlantic herring (Clupea harengus) recruitment reconstruction for the 1551-2005 (Common Era, CE) interval based on carbon stable isotope geochemistry (δ13C) is presented. The results indicate a link between δ13C and recruitment mediated by the dissolved inorganic carbon and nutrition. The reconstruction suggests that there were five extended episodes of low recruitment levels before the 20th century. These results are supported by instrumental recruitment estimates and historical fish catch and export documentation.
In the first climatic study, a bottom water temperature (BWT) reconstruction for the 1551-2004 CE interval based on oxygen stable isotope geochemistry (δ18O) is presented. Sub-annual shell sampling suggests that the average A. islandica growing season in the northern North Sea goes from February to October. The reconstruction shows an average temperature of 7.20 °C for the past c. 500 years, with warming trends of 0.05 ± 0.02 °C decade-1 for the years 1640-1740 and 0.08 ± 0.02 °C decade-1 for the years 1880-2001, and a cooling trend of -0.11 ± 0.02 °C decade-1 for the years 1810-1910. These trends cannot be attributed to North Atlantic sea surface temperatures (SST). Between 9 and 54 % of variability in average Feb-Oct BWT can be explained by low frequency changes in the winter North Atlantic Oscillation (NAO). A BWT/SST regression analysis suggests that the thermal stratification in the northern North Sea is susceptible to local storm activity, while persisting under calmer conditions and storms passing through the central and southern North Sea.
In the second climatic study, a BWT and relative water stratification reconstruction during the 8.2 ka event (8,200 years before 1950 CE, cal BP) based on δ18O and δ13C is presented. The results indicate that a sudden sea level rise (SSLR) event-driven column stratification occurred between ages 8320-8220 cal BP. Thirty years later, cold conditions inhibited water column stratification but an eventual incursion of sub-Arctic waters into the North Sea re-established density-driven stratification. The water temperatures reached their minimum of ~3.8 °C 55 years after the SSLR. Intermittently-mixed conditions were later established when the sub-Arctic waters receded.
Through these studies it is demonstrated that molluscan sclerochronological records can contribute to climatic and ecological investigations in different timescales. These studies show how factors like sea level, water mass and storminess can all contribute to the strength of the seasonal water column stratification and how primary production affects the δ13C of the dissolved inorganic carbon and the herring recruitment.