Dr. Anne Cohen, Woods Hole Oceanographic Institution


	Polished, gold coated coral sample for ion microprobe analysis.

The Effect of Algal Symbionts on the Accuracy of Sr/Ca Paleotemperatures from Coral
Anne L. Cohen, Kathryn E. Owens, Graham D. Layne, Nobumichi Shimizu
Originally published in Science Express as 10.1126/science.109330 on March 7, 2002
Science, Vol. 296, Issue 5566, 331-333, April 12, 2002

The strontium-to-calcium ratio (Sr/Ca) of reef coral skeleton is commonly used as a paleothermometer to estimate sea surface temperatures (SSTs) at crucial times in Earth's climate history. However, these estimates are disputed, because uptake of Sr into coral skeleton is thought to be affected by algal symbionts (zooxanthellae) living in the host tissue. Here, we show that significant distortion of the Sr/Ca temperature record in coral skeleton occurs in the presence of algal symbionts. Seasonally resolved Sr/Ca in coral without symbionts reflects local SSTs with a temperature sensitivity equivalent to that of laboratory aragonite precipitated at equilibrium and the nighttime skeletal deposits of symbiotic reef corals. However, up to 65% of the Sr/Ca variability in symbiotic skeleton is related to symbiont activity and does not reflect water temperature.

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Kinetic Control of Skeletal Sr/Ca in a Symbiotic Coral: Implications for the Paleotemperature Proxy
Anne L. Cohen, Graham D. Layne, and Stanley R. Hart, Woods Hole Oceanographic Institution
Philip S. Lobel, Boston University Marine Program
Paleoceanography, Vol. 16, No. 1, Pages 20-26, February 2001

Modeling of past climates is critically dependent on estimates of past sea surface temperatures (SSTs) for which one of the principal techniques used is the measurement of Sr/Ca ratios in corals (Guilderson et al. 1994, McCulloch et al., 1999; Hughen et al., 1999). The link between coral Sr/Ca and SST is not well understood and there have beena number of descrepant observations (de Villiers et al., 1995; Alivert, 1998). Corals with symbiotic zooxanthellae are known to show large diurnal fluctuations in calcification rate associated with the photosynthetic activity of their symbionts. Using detailed measurements with the ion microprobe, we compared the Sr/Ca content of discrete daytime and nighttime skeletal structures in the massive hermatypic coral Porites lutea over the course of 1 year and a seasonal temperature range of 4æC. The Sr/Ca content of daytime skeleton is always lower thatn that of adjacent nighttime skeleton. While the slope of the nighttime Sr/Ca-SST correlation is close to that seen in inorganic aragonite precipitates, that of the daytime correlation is>4 times as steep. We attribute these differences to the role of photosynthesis in calcification and conclude that bulk Sr/Ca is related principally to daytime calcification rate rather than directly to SST. More reliable estimates of past SST may be arrived at through selective analysis of nighttime skeleton.

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	Porites skeleton in thin section.

Diurnal Changes in Microstructure and Microscale Chemistry of Reef Coral Skeleton
Anne L. Cohen, Graham D. Layne, Nobumichi Shimizu
Poster #1 presented at AGU in 2001

Calcification by corals with symbiotic zooxanthellae occurs 3 times faster in daylight than it does at night. We investigated the effects of light enhanced calcification on the microstructure and elemental chemistry of the aragonite skeleton over the diurnal cycle. The morphology of nighttime crystals accreted in the absence of photosynthesis imitates that of slow growing inorganic cements in a high CO2 environment. The morphology of daytime crystals accreted during the photosynthetic period imitates that of fast growing inorganic cements in a low CO2 environment. We used an Cameca IMS 3f ion microprobe to measure changes in the strontium-calcium content (Sr/Ca) of the growing skeleton of the tropical reef coral, Porites lutea, over the diurnal cycle. Sr/Ca in nighttime skeleton is close to equilibrium values but a large decrease in Sr/Ca is observed as the daytime crystals grow to fill the extracellular calcifying space in summer. The amplitude of change in skeletal Sr/Ca between night and day is as large as the annual cycle in Sr/Ca. During summer, at peak water temperature and symbiont photosynthesis, the amplitude of the diurnal Sr/Ca cycle is 3 times greater than that incurred only by the diurnal change in water temperature. During winter, the amplitude of the diurnal Sr/Ca cycle is equivalent to that incurred by temperature alone. Our data show that processes linked to symbiont photosynthesis exert significant influence on both skeletal microstructure and microchemistry, and that temperature is not the primary control of diurnal Sr/Ca variability in reef coral skeleton.

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	Astrangia skeleton.

The Biological Nature of Geochemical Proxies: Algal Symbionts Control Coral Chemistry
Kathryn E. Owens, Anne L. Cohen, Graham D. Layne, Nobumichi Shimizu
Poster #2 presented at AGU in 2001

The strontium/calcium ratio (Sr/Ca) of reef coral skeleton is used as a paleothermometer to estimate sea surface temperatures (SSTs) at crucial times in the earths’ climate history. However, these estimates are disputed because uptake of Sr into coral skeleton is thought to be affected by algal symbionts (zooxanthellae) living in the host tissue. Here we show for the first time that significant distortion of the Sr/Ca temperature record in coral skeleton occurs in the presence of algal symbionts. Microscale measurements of skeletal Sr/Ca in co-occuring symbiotic and asymbiotic colonies of Astrangia poculata were made using SIMS ion microprobe. Seasonally-resolved Sr/Ca in asymbiotic skeleton reflects local SSTs with a temperature sensitivity equivalent to laboratory aragonite precipitated at equilibrium, and the nighttime skeletal deposits of symbiotic reef corals. However, up to 65% of the Sr/Ca variability in symbiotic skeleton is related to the symbiosis and does not reflect water temperature. Our data indicate that Sr/Ca in asymbiotic species and nighttime accretions of tropical reef corals should be targeted for accurate estimates of past SSTs.

Full poster PDF version (Acrobat 5.0 - 1.3 MB)
(You will need Acrobat Reader 5.0 - a free version is available for download here)