CLIMATE CHANGE RECORD IN MODERN POLAR AND TROPICAL CARBONATE ARCHIVES (MOLLUSCS, BRACHIOPODS)

Extensive emission of anthropogenic greenhouse gases in the last few decades has accelerated Earth’s warming trend. Seawater has consequently experienced a noticeable temperature rise in response to changes in global climate.

For assessing differences in sea surface temperature (SST) at low and high latitudes, we investigated the isotopic compositions of modern molluscs and brachiopods. Screening tests demonstrated that the archives used in this study incorporated their oxygen isotopic compositions in equilibrium with ambient seawater, and thus should reflect SST. Their skeletal growth increments are powerful archives for recording shifts in seawater temperature. The clam Tridacna maxima collected from the same locality in the Red Sea as the oyster Hyotissa hyotis of Titschack et al. (2010), was used to obtain a long-term record of seawater temperature variation in a sub-tropical region (1956-2012). For polar-subpolar changes in SST, the brachiopod Magellania venosa was recovered from the coastal area of southern Chile. It revealed a long-term (1961-2012) time-series of Antarctic-induced oceanographic change in the southern hemisphere. To document oceanographic changes in SST in the northern hemisphere, we adopted the δ¹⁸O results of the brachiopod Hemithiris psittacea from Hudson Bay of Brand et al. (2014).

The δ¹⁸O values of the sub-tropical molluscs display an increasingly negative trend since 1988, indicating an increase in tropical seawater temperature of ~0.8ºC. Conversely, the slopes of the δ¹⁸O values in the polar-subpolar brachiopods are more pronounced signifying a rise in temperature of ~1.5ºC in Hudson Bay seawater between 1991 and 2013, and of ~2.1ºC in southern Chilean seawater between 1988 and 2012.

Our findings are consistent with the observation that low latitudes are less impacted by global warming. Furthermore, our results show that the rate of seawater temperature rise in polar regions is about three times faster than that of tropical water. This represents a fundamental environmental hazard of great concern that significantly impacts polar ice cover, ocean stratification, coastal morphology, marine habitats, and all inhabitants of polar regions.