Sea surface temperature and salinity variability at Bermuda during the end of the Little Ice Age

Abstract

[1] We use geochemical and isotope measurements on a 225-year old brain coral (Diploria labyrinthiformis) from the south shore of Bermuda (64°W, 32°N) to construct a record of decadal-to-centennial-scale climate variability. The coral was collected alive, and annual density bands visible in X radiographs delineate cold and warm seasons allowing for precise dating. Coral skeletons incorporate strontium (Sr) and calcium (Ca) in relative proportions inversely to the sea surface temperature (SST) in which the skeleton is secreted. Previous studies on this and other coral colonies from this region document the ability to reconstruct mean annual and wintertime SST using Sr/Ca measurements (Goodkin et al., , 2005. The coral-based records of SST for the past 2 centuries show abrupt shifts at both decadal and centennial timescales and suggest that SST at the end of the Little Ice Age (between 1840 and 1860) was 1.5°± 0.4°C colder than today (1990s). Coral-reconstructed SST has a greater magnitude change than does a gridded instrumental SST record from this region. This may result from several physical processes including high rates of mesoscale eddy propagation in this region. Oxygen isotope values (d 18 O) of the coral skeleton reflect changes in both temperature and the d 18 O of seawater (dO w ), where dO w is proportional to sea surface salinity (SSS). We show in this study that mean annual and wintertime d 18 O of the carbonate (dO c ) are correlated to both SST and SSS, but a robust, quantitative measure of SSS is not found with present calibration data. In combination, however, the Sr/Ca and dO c qualitatively reconstruct lower salinities at the end of the Little Ice Age relative to modern day. Temperature changes agree with other records from the Bermuda region. Radiative and atmospheric forcing may explain some of the SST variability, but the scales of implied changes in SST and SSS indicate large-scale ocean circulation impacts as well.

Geochemistry-based coral palaeoclimate studies and the potential of ‘non-traditional’ (non-massive Porites) corals: Recent developments and future progression

“…Sea surface temperature reconstructions have been successfully completed using calibrated colonies of Bermudian Diploria labyrinthiformis (Goodkin et al, 2005(Goodkin et al, , 2008, and D. strigosa geochemistry appears to correlate well with North Atlantic SST anomaly indices (Hetzinger et al, 2006). Colonies of Siderastrea radians and S. siderea also hold significant potential as new climate indicators with similar inter-species sensitivities of Sr/Ca (Moses et al, 2006a;Maupin et al, 2008), which will be useful for drill core based reconstructions where colony identification may be limited to the genus level.…”

“…Growth corrected Sr/Ca profiles from D. labyrinthiformis were used to determine that Bermudian SSTs during the Little Ice Age were approximately 2°C cooler than present day conditions (Goodkin et al, 2005(Goodkin et al, , 2008, suggesting that this coral species can be effectively used as a palaeoclimate proxy.…”

Two centuries of limited variability in subtropical North Atlantic thermocline ventilation

“…However, cross-correlation analysis between the entire record of the raw ∆ 14 C values presented here and the instrumental record of the NAO 10 shows no significant correlation at high or low frequencies. Low ∆ 14 C variability along the south shore is also seen during the end of the Little Ice Age from 1800 to 1870, when SSTs are low 22 and NAO variability is weak 21 . In summary, surface waters to the south of the Bermuda exhibit a significant atmospheric 14 C imprint and minimal changes through time in the mixing rate with older subsurface waters, despite significant changes to NAO behaviour 21 and mean hemispheric climate 16 (Fig.…”

Coral Reefs of the United Kingdom Overseas Territories

“…The growth of corals in Bermuda has also been examined in the context of biogeochemical signals of oceanographic and climate processes via the incorporation of elements in the skeletal matrix (Nozaki et al 1978 ;Druffel 1989 ;Draschba et al 2000 ;Cardinal et al 2001 ;Kuhnert et al 2002 ;Cohen et al 2004 ;Creuger et al 2006 ;Goodkin et al 2008 ) , the fi delity of proxy signals for seawater temperature (Fairbanks and Dodge 1979 ), and anthropogenic inputs (Dodge et al 1984c ;Shen and Boyle 1988 ;Kelly et al 2009 ). One study on growth records in the axial skeleton of a soft coral, Plexaurella dichotoma , was performed to assess suitability for temperature proxy signals (Bond et al 2005 (Lesser and Schick 1989 ) but zooxanthellae appear to have attenuated responses to factors that can induce coral bleaching in experimental conditions (Venn et al 2008 ) .…”