Climate records from the Faroe-Shetland Channel using Lophelia pertusa (Linnaeus, 1758)

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Abstract

The longest available time series on ocean currents indicates that the southward flow of water from the Greenland Sea is weakening, and that correlative large-amplitude changes have occurred in the rate of formation of intermediate Labrador Sea water. These have been linked to changes in regional climate which, if trends continue, could within 30 years alter the flow of the North Atlantic Drift and possibly interrupt the formation of Labrador Sea water, profoundly affecting regional climates, marine ecosystems and fisheries. We are attempting to use the carbonate skeletons of cold-water corals to find out how rapidly and how often the thermohaline circulation of the NE vs. NW Atlantic has changed in the past, just as tree rings and ice cores are used to investigate climate change on land.

We have focussed on the NE Atlantic for our preliminary work, notably the Faroe-Shetland Channel: a major gateway between the Atlantic Ocean and the Norwegian Sea. Warm North Atlantic Drift water passes north through this channel on the surface, warming northern Europe. Cold Norwegian Sea Overflow Water returns at depth, contributing to the formation of North Atlantic Deep Water. Existing records are too short to allow conclusions regarding recent temporal changes in this inflow, so proxies are sought. We have analysed live-collected Lophelia pertusa skeletons collected in October 2001 using a ring dredge from RV Scotia along the worlds longest-running hydrographic transect (from 1893). Corals were sectioned using a slow-speed Isomet saw, and sampled for isotopic analysis using a Merchantek micromill. Observation of sectioned corals revealed dense-less dense couplets, as in every coral studied to date from tropical to deep cold-water environments worldwide. We sampled circumferentially, in the centres of individual bands, so as to produce temperature estimates using the “lines” technique of Smith et al. (2000). The results were simultaneously encouraging and confusing.

Each of the coral samples generated lines from which temperatures could be estimated. Dense skeletal bands had lower temperatures than the less-dense bands, hence we conclude these were winter and summer bands, respectively. The mean annual temperature (MAT) range determined from one of the corals was 3.8°C. Pooling results from several corals yielded a lower estimate for MAT range: 2.3°C. The absolute temperatures from the corals, however, were somewhat lower than the instrumental record with “winter” records being more depleted than the “summer values”. This was unexpected and shows that determining detailed climate records from L. pertusa may be more difficult than hoped.