500 Years of Atlantic Sea Surface Temperature in Brain Coral Skeleton

Anne Cohen, Geology & Geophysics
Mike McCartney, Physical Oceanography

OCCI Project Funded: 2001

Proposed Research

Understanding the full extent of the earth's climate system depends in large measure on proxy reconstructions of past climate derived from natural climate recorders, including long-lived corals on tropical reefs. In this project we reconstruct 500 years of Atlantic climate history by quantifying temperature-sensitive skeletal density variations in a massive brain coral using a new technique called gamma densitometry. Although the potential for using coral skeletal density as an Sea Surface Temperature (SST) proxy has been largely overlooked, the encouraging results of our calibration study of corals on Bermuda provides strong impetus to focus on developing this technique as a means to provide multiple, multi-site records of Atlantic paleoSSTAs at a fraction of the cost and effort involved to produce geochemical data. We will use the Bermuda coral-based proxy record to examine long-term, low-frequency variability in the North Atlantic Oscillation (NAO). NAO is a mid-latitude atmospheric seesaw whose influence extends beyond the weather and climate of the eastern United States into the adjacent oceans, where it has devastated economically important fisheries. In particular we ask whether the unusual behaviour of NAO in the past 50 years is related to global warming trends or whether it is simply an expression of natural climate variability that occurs on longer timescales than is apparent to us in the instrumental record.

Final Report - 500 275 Years of Atlantic SST in Brain Coral Skeleton

This award supported the reconstruction of a seasonally resolved, 275 year record of wintertime sea surface temperature (SST) variability in the western sub-tropical Atlantic from the skeleton of a massive brain coral collected on Bermuda's south shore reef (Fig.1). The coral proxy record extends the instrumental record of SST for this region (Hydrographic Station S) by more than 200 years and provides data required to assess decadal-scale variability as well as secular trends.  SST variability on interannual, decadal and multidecadal timescales is attributed to three main forcings: global warming (especially evident in the last 30 years of the record), the North Atlantic Oscillation and volcanic activity. The record, based on temperature-sensitive changes in the density of the braincoral skeleton, shows that present-day SSTs at Bermuda are more than 1 ºC higher than they were during the coolest period of the Little Ice Age (~1745 AD). Highest SSTs in the record were obtained and maintained during the 1990's, coincident with the first episodes of coral bleaching observed on Bermuda. Superimposed upon the long-term warming trend are decadal-scale fluctuations that correspond with known variations in the atmospheric circulation associated with the North Atlantic Oscillation. An abrupt decade long cooling event that starts in 1815, coincident with the eruption of Mount Tambora, indicates that volcanic activity may be an additional forcing of SST variability in the subtropical western Atlantic.

 Cohen is currently working with R. Sohn (WHOI) using power and cross-spectral  analysis to resolve the dominant modes of variability in the record. ; These results were presented an invited paper at the Spring AGU  meeting in May 2002 (Session: Atlantic Decadal Variability chaired  by M.Visbeck and Y.Kushnir) and form the basis of a full proposal  submitted to NSF's ESH program in January 2003 (P.I.'s Cohen, Hughen, McCartney).

The results of this study were also used in Cohen's Congressional  Testimony presented to the U.S. House Of Representatives Resources  Subcommittee On Oceans, Fisheries Conservation And Wildlife in June 2002 and are featured the August 2003 issue of Woods Hole Currents  (Diving  into Climate History, Currents Volume 10, No. 1).

In addition, this award has supported Cohen's initiation of three  related projects which explore geochemical and alternative imaging  techniques to extract an SST record from Bermuda Braincoral. They  are:

N.Goodkin, K.Hughen, A.Cohen, C.Herring (2003) Reconstructing  the North Atlantic Oscillation at Bermuda from Monthly Resolved  Records of Sr/Ca in Diploria labyrinthiformis (brain coral).  Abstract submitted to the EuroConference on North Atlantic Climate  Variability: Achieving Climate Predictability using Paleoclimate  Data. (October 11-16th, San Feliu de Guixols, Spain).

R.Cleveland (Boston University), A Cohen (WHOI), R. Roy (BU), H.  Singh (WHOI), T. Szabo (BU) (2003). Imaging Coral III: Advances  in Imaging of Coral Skeletons using Ultrasound. Accepted for publication  in Journal of Subsurface Sensing Technologies and Applications. 

N. Nichols, H. Singh and A. Cohen. CAT Scanning of BrainCoral for  3-D Image Analysis. WHOI summer student fellow project co-advised by H. Singh (AOPE) and A. Cohen in July 2003.