Decadal-scale, high resolution records of sea surface temperature in the eastern Indian and south western Pacific Oceans from proxy records of the strontium/calcium ratio of massive porites corals

Our understanding of natural climate change is limited because the historical record of climate is restricted to the last 150 years or so. Corals, with their annual growth banding, ca~ provide one of the few high-resolution proxy records in the tropics that extend back trough time, and have the ability of determining changes in the f.w frequency and intensity of previous El Nifio events as well as other climatic phenomena. The measurement of the Sr/Ca ratio in corals using isotope dilution thermal ionisation mass spectrometry (ID-TIMS) has the potential for measuring past sea surface (~~--r_) temperatures at very high accuracy. However, the veracity of the technique has been ~ questioned on the basis that there is both a spatial and temporal variation in the Sr/Ca ratio of seawater, and that kinetic effects, such as the calcification rate, can affect the Sr/Ca ratio of corals. In the present study, a number of cores of the massive hermatypic scleractinian coral Porites, collected from Christmas Island and Cocos (Keeling) Atoll (bP.>R) in the eastern Indian Ocean and also from the central Great Barrier Reef have been " analysed for Sr/Ca at weekly to monthly resolution. The Sr/Ca results have been augmented by stable isotope analysis and LA-ICP-MS, as well as X-radiography, ydensitometry and UV fluorescence. Results frol! a 24 year record ( 1972 to 1996) from Christmas Island shows an overall temperature variation of 6.7°C with an average winter minimum of 25.8°C and I. average summer maximum of 29.2°C. In some years, SSTs are l.2°-l.8°C cooler than the average winter minimum. While some of the cool events appear to be related to local upwelling, the coolest winter, that of 1994, is considered to be a manifestation of the Indian Ocean dipole. On the other hand, above average summer SSTs coincide with · El Nifio events in the Pacific Ocean. This warming in the equatorial eastern Indian Ocean is thought to be caused by a relaxation of the Indian Ocean dipole, but it has also been attributed to an ENSO wave that propagates from west to east along the equatorial Indo-Pacific. The Sr/Ca and 80 records of cores from Cocos (Keeling) Atoll show an annual signal in places, but this does not agree with the instrumental SST record. This is thought to be related to stress induced by periodic algal blooms in the lagoon. ~[i_~S~, ~. it.CBR. A 24 year long record from Myrmidon Reef shows an overall variation from ( t 22.7°C to 30.4 °C. The record shows a warming/cooling trend with maximum warming centred on the 1986-87 summer. During the 1982-91 period bleaching occurred in 1982. Subsequent summer temperatures were much higher, but no bleaching was recorded. Comparison of the Myrmidon Reef Sr/Ca derived SST data with other results from Davies and Pandora Reefs suggests that there is a 3°C temperature gradient across the shelf. Deeper drilling at Myrmidon and Stanley Reefs recovered Porites sp. which range in age from 7600-8000 years and 6200-6350 years. Sr/Ca derived SSTs show that temperatures were similar to present at around 8000 years, but were some 2°C cooler than present at 6000 years. i-~ Various arguments have been put forward .to suggest that the Sr/Ca method can ~~~~ h produce inaccuracies of up to 2-4°C. However, while corals do not calcify in l,. equilibrium with seawater due to physiological control on the uptake of Sr into the lattice of coralline aragonite, it can be argued that, provided only a single genus such as Porites sp. is used, and that the coral is sampled along a major vertical growth axis, then the Sr/Ca ratio should vary uniformly with temperature. Similarly, objections based on the spatial and temporal variability of the Sr/Ca activity ratio of seawater can be countered on the basis that in most areas where coral reefs grow there is a uniformity in the Sr/Ca activity ratio, and there does not appear to be an increase in this ratio over time. Evidence from several corals in this study suggest that stress can be a major cause of the breakdown in the Sr/Ca-SST relationship. Thermal stress, resulting from either extremely warm or cool temperatures, can produce anomalously low Sr/Ca derived SSTs as a result of the breakdown of the biological control on Sr/Ca fractionation. It is considered that other stresses, such as increased nutrients and changes in light intensity, can also lead to a breakdown in the Sr/Ca-SST relationship. The two main issues affecting the reliability of the Sr/Ca method are the calibration of the Sr/Ca ratio with measured SST and the estimation of tropical last glacial maximum (LGM) palaeotemperatures. Instead of producing a constant calibration, just about every one published so far is different to the previous one. What is obvious is that for most calibrations while the slope of the calibration equation is similar, the intercepts are not. While the cause for this variation is still unknown, it would appear that corals from different localities around the world are responding to their own particular environment or that certain types of environments exert a control on the corals' physiology. Sr/Ca derived SST estimates of 5°-6°C cooler than present for the LGM are at odds with estimates of 2°-3°C cooling by other climate proxies. The ~ -u apparentlaek ot reef growth during the LGM suggests that SSTs were too cold in most parts of the tropics for reefs to develop. This would lend support to the idea that tropical SSTs were much cooler than what the CLIMAP data suggests. However, arguments suggesting that Sr/Ca derived SSTs for LGM corals are overestimated because of changes to the Sr/Ca ratio of seawater or as a result of thermal stress need to be evaluated. Holocene Sr/Ca records show SSTs rising from values of -6°C to modern values between 10,000 and 8000 years ago, after which temperatures fell to -2 to -4°C until ~ about 5000 years ago. They then rose sharply to modern values by 4000 years. There L appears to be a relation between the mid-Holocene cooling in the western Pacific and the change in frequency of El Nifio around about 5000 years ago. During the midHolocene the western Pacific was slightly cooler than today because of a more equal distribution of heat across the equatorial Pacific Ocean due to¢ relaxation of the Trade winds.