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North Atlantic diatom-based SST reconstructions

Diatom-based sea surface temperatures (SST) have been reconstructed for 3 Nordic Sea cores (MD95-2011, LO0914, MD99-2269) using different transfer function methods: Imbrie & Kipp regression (I&K); Maximum Likelihood (ML); and weighted averaging partial least square (WA-PLS). The surface calibration set is based on 52 species from 139 surface samples. The reconstructed SST time series have been analysed with SiZer (Significance of Zero Crossings of the Derivative) to find significant features, and with Wavelet power spectrum.

SST reconstructions from the Nordic Seas describe the general Holocene climatic development as a period of cooling in step with decreasing insolation at the northern hemisphere since 11.000 years BP. In contrast, the SSTs at the Reykjanes Ridge site (core LO09-14) exhibit highly variable temperature fluctuations throughout the Holocene, with a general trend showing a relatively cool and variable early Holocene period (11-7 kyr), a mid Holocene Climate Optimum (7-5 kyr), and a generally stable late Holocene period (5 kyr to present). Multi-centennial variability on the time-scales from 640, 950, 1000, 1500 and 2500 years is prominent in core LO09-14.

Superimposed on the general Holocene climate development there is a high-amplitude millennial to century scale variability in the surface waters of the subpolar North Atlantic Ocean. Even though climate oscillations with 2500, 1500, 1000, 950 and 640 yrs periodicities are evident in the LO09-14 SST records, there is also a time dependent dominance of different periodicities through the Holocene indicating a clear change of mode from about 5 kyr BP. Before 5 kyr BP a strong 2500 and 1500 yrs periodicity is present in the records. After 5 kyr BP the LO09-14 record shows lower amplitude variations with periods of 900 and 1000-1500 yrs.

Century scale variability in the LO09-14 SST record document several Holocene surface ocean cooling events (HCE). Three cooling events (HCE 6, 9 and 13) recorded in core LO09-14 show clear positive correlation to IRD events in core V29-191. The other HCEs show either negative correlation or none-conclusive relation. Whereas in core V28-14, HCE 1, 2, 3, 11 and 13 show positive correlation to the IRD events. We conclude that in cases where there is a positive correlation between the IRD events and HCE the advances of icebergs into the North Atlantic were accompanied by 2-3˚C SST cooling.

SST results from cores MD95-2011 and MD99-2269 have contrasting temperature anomalies on a multidecadal time scale and indicate phase-locked intervals with a North Atlantic Oscillation signature. The results also show a close match to solar irradiance. We therefore suggest that oceanic climate variability during the last 800 years is a result of atmosphere-ocean interactions, i.e. the North Atlantic Oscillation and the Atlantic Multidecadal Oscillation, responding to solar forcing.

Reported by

Norwegian Polar Institute
9296 Tromsø
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