Annually resolved North Atlantic marine climate over the last millennium.

Owing to the lack of absolutely dated oceanographic information before the modern instrumental period, there is currently significant debate as to the role played by North Atlantic Ocean dynamics in previous climate transitions (for example, Medieval Climate Anomaly-Little Ice Age, MCA-LIA). Here we present analyses of a millennial-length, annually resolved and absolutely dated marine δ18O archive. We interpret our record of oxygen isotope ratios from the shells of the long-lived marine bivalve Arctica islandica (δ18O-shell), from the North Icelandic shelf, in relation to seawater density variability and demonstrate that solar and volcanic forcing coupled with ocean circulation dynamics are key drivers of climate variability over the last millennium. During the pre-industrial period (AD 1000-1800) variability in the sub-polar North Atlantic leads changes in Northern Hemisphere surface air temperatures at multi-decadal timescales, indicating that North Atlantic Ocean dynamics played an active role in modulating the response of the atmosphere to solar and volcanic forcing.

Identification of growth increments in the shell of the bivalve mollusc Arctica islandica using backscattered electron imaging.

Annually resolved growth increments in the shell of the bivalve mollusc Arctica islandica have previously been used in combination with geochemical measurements to successfully construct high-resolution proxy records of past marine environmental conditions. However, to ensure the accuracy of these paleoenvironmental reconstructions it is essential that the annual growth series of increments within the examined shells are reliably identified, and can be distinguished from spurious lines caused by nonannual perturbations such as those resulting from storm disturbance. The current methods used for identifying the growth increment series are sometimes compromised because of ambiguity that results from the employed preparation methods. Here it is shown that backscattered electron imaging of polished shell cross sections may be used to clearly discriminate between the two compositionally and structurally distinct increments that comprise 1 year of outer shell growth. This method, involving minimal specimen preparation, is likely to be primarily useful as a validation technique of particular value in cases where increment identification using existing methods is difficult or ambiguous.