Integrated understanding of phasings within the climate system over the last glacial cycle, and at higher frequencies, is inhibited because no absolute timescale for the marine environment currently exists. This precludes identification of forcings and feedbacks, accurate temporal calibration of the marine radiocarbon reservoir effect, and the application of radiocarbon as a proxy of short-timescale ocean ventilation. This has prompted a search for annually banded marine proxies in the hope of establishing an accurate marine chronometer. We present annual growth band series from dead-collected specimens of the long-lived bivalve mollusc Arctica islandica from the northern North Sea and demonstrate their successful cross-matching, with the general timescale context independently verified by radiocarbon dating. Though at present limited to only a few statistically cross-matched series, this has already generated the longest Arctica chronology, and the first ‘floating’ chronology constructed entirely from marine fossils. The record covers the period from c. AD 1000 to 1400 and integrates a 267-yr series from the longest-lived Arctica specimen yet recorded from the North Sea. This breakthrough in cross-matching demonstrates that Arctica islandica can fulfill its potential as the ‘tree of the sea’ to provide an absolute timescale for the marine environment.
[1] The creation of networks of shell-based chronologies which can provide regionally extensive highresolution proxies for the marine environment depends on the spatial extent of the common environmental signal preserved in the shell banding and on the reliability of the dating model. Here Arctica islandica chronologies from five neighboring sites in the North Sea are compared, and the strength of the common environmental signal across distances up to 80 km is analyzed using statistical techniques derived from dendrochronology. The signal is found to be coherent across these distances. In a linked study, chronologies based on one of the same sites but constructed by two different research teams are compared. Methodological differences in increment interpretation are found to lead to slippage in the dating models. Systematic inclusion or exclusion of intermittently occurring increments results in the two chronologies becoming misaligned by 4 years over a 70-year period. Comparisons with neighboring chronologies indicate that such increments can generally be regarded as genuine annual increments even if they are not visible in all shells.
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