The coral record of last interglacial sea levels and sea surface temperatures

Abstract

The rise and fall of the Last Interglacial (LI) sea levels and sea surface temperatures (SSTs) are evaluated using U-series dating combined with Sr/Ca ratios in corals from both stable and tectonically uplifted sites. Along the stable coastal margin of Western Australia, an extensive series of LI coral reefs occur at heights of 2–3 m above present-day sea level. These corals have a very tight cluster of 234U– 230Th ages ranging from 129±1 to 119±1 ka, as well as a narrow range of initial δ 234U values of 150±5, similar to modern seawater. Bahamas, which is also a stable site, has an essentially identical pattern of U-series ages from 130±1 to 120±1 ka. Barbados and Huon Peninsula are tectonically active sites where the LI terraces are found at elevations of >50 and >200 m, respectively. U-series ages from corals exposed in the lower footwall of these uplifted reefs, allow better constraints to be placed on the rate of sea level rise which initiated the LI. Corals from the Huon Peninsula constrain sea level at −80±10 m at 131±2 ka, and from Barbados, at −30±5 m at 129±1 ka. Combined with constraints from stable sites, these observations require an exceedingly rapid rise in sea level of 30–50 m per 1000 years at 130±1 ka. This indicates that large-scale catastrophic melting of the once massive continental ice sheets occurred in phase with the rapidly increasing northern hemisphere (NH) summer insolation, consistent with the orbital forcing being the main driver of glacial–interglacial climate change. There is also some evidence from Huon Peninsula, although still not conclusive, for a precursor oscillation in sea level during the penultimate deglaciation, that may have been within ∼−20 m of present-day levels at ∼135 ka. SSTs for the LI Porites corals from the Huon Peninsula and Western Australia have mean annual temperatures and seasonal ranges that are remarkably similar to present-day patterns. The tropical site of Huon Peninsula has SSTs of 29±1°C, which is indistinguishable from the SSTs given by modern corals. At Ningaloo Reef in Western Australia, similar mean annual (∼24°C) and summer maximum SSTs of 27–29°C are found in both LI and modern corals. The only significant difference is the ∼1°C cooler winter minimum SSTs of ∼21°C for the LI compared to present-day minimums of ∼22°C. LI SSTs from these southern hemisphere (SH) sites were thus very similar, or at most, only slightly cooler than today, despite sea levels being up to 4 m higher. This maybe indicative of asymmetric warming of the Earth, with the increased NH insolation during the LI period being responsible for the extensive melting of the mainly NH-based ice sheets, and hence, higher global sea levels. The observation of relatively high sea levels in the LI, together with the rapid pulses of sea level rise, indicates that the potential now exists for greenhouse warming to initiate increases in sea level of at least several metres on relatively short time-scales (10 2 years).