Coral Evidence for Abrupt Changes in Ocean-Atmosphere Dynamics in the SW Pacific since 1565 AD

A coral-based multi-tracer approach can give an overview of the whole tropical ocean-atmosphere system. Key indicators are sea surface temperature (SST), which sets climate boundary conditions, sea surface salinity (SSS), which provides a measure of energy transfer through the evaporation-precipitation balance, and river runoff, which can establish the strength and variability of precipitation. We present palaeoenvironmental records from eight massive { \it Porites} coral colonies, spanning 120 to 420 years of continuous growth, collected from the central Great Barrier Reef, Australia. Stable isotopes (\delta¹⁸O and \delta¹³C), Sr/Ca, U/Ca, and Ba/Ca ratios were measured in 5-year increments and a record of annual UV luminescence was developed. By replicating the measurements between colonies we demonstrate how faithfully corals record changes in their environment over decadal-to-centennial timescales, constructing composite records in a manner analogous to dendroclimatology and confidence intervals for each proxy. The competing environmental influences affecting a number of tracers can be distinguished by comparison between the SST-tracers (Sr/Ca, U/Ca, \delta¹⁸O), the freshwater flux tracers (\Delta\delta¹⁸O, Ba/Ca and luminescence) and tracers of water mass characteristics (\delta¹⁸O, \delta¹³C, and \Delta¹⁴C). The coral palaeothermometers Sr/Ca and U/Ca ratios, measured in tandem with \delta¹⁸O, allow the separation of SST changes from changes in seawater \delta¹⁸O, thereby resolving SSS. The composite Sr/Ca and U/Ca are in excellent agreement back to 1565, and capture the 20^th century warming trend, up to the 1980s when the cores were collected. The most remarkable feature of the 420-year record is that SSTs were consistently as warm as the second half of the 20^th century from the early 18^th and through most of the 19^th centuries. Changes in the evaporation-precipitation balance dominate the \delta¹⁸O record. A striking 0.2\permil\ shift from the 1850s to modern values in the 1870s, indicates an abrupt freshening, which is common to coral \delta¹⁸O records throughout the SW Pacific. Interdecadal variability of \Delta\delta¹⁸O, Ba/Ca and luminescence correspond strongly in response to the variable strength of the Australian summer monsoon circulation. We investigate the scenario that a strengthened latitudinal temperature gradient may have prevailed prior to the 1870s, with evidence for an intensified Hadley circulation impacting evaporation rates and the surface-ocean circulation in the SW Pacific, in addition to altered precipitation patterns. The late-19^th century demise of the extratropical 'Little Ice Age' in the Northern Hemisphere coincides with cooling and abrupt freshening of the SW Pacific.