Rapid changes of sea level, ice sheets and deep-sea temperatures during the last glacial period

J. Chappell, T. Esat and Y. Yokoyama

All major components of the global climate system were in play during large, rapid climate changes that occurred in the last glacial cycle: continental ice sheets, ocean circulation, and the circulation and composition of the atmosphere. In order to understand the system it is necessary to know the timing of events for each component, so as to know which factors may be the triggers of other processes.

Ongoing research based on coral terraces at Huon Peninsula (HP), Papua New Guinea, has produced a new and more detailed analysis of the sea level, derived with the help of a computer model which simulates reef growth on uplifting terrain under varying sea level. The analysis uses the inverse method of searching for a sea level curve that leads to simulations closely matching observed topographic and stratigraphic sections, which also are controlled by precise U-series ages from the coral terraces. A restricted envelope of sea level curves was found that yield simulations that closely match the observed sequences at HP. The limit of resolution of sea level changes is a few metres and is limited by "noise" in the processes of uplift and reef growth at HP, arising from repeated metre-scale uplift events.

In addition to sea level cycles with the orbital precession period of ~20 ka, identified previously, the new sea level envelope shows shorter cycles within the later part of the last glacial cycle, with peaks at 33, 38, 44.5, 52 and 58-60 ka. The timing is fixed by precise U-series dates. Each cycle of 6-7000 years ended with a sea level rise of 10-15 m lasting at 1-2000 years, following a longer period of falling sea level. Except for the short cycle terminating at 33 ka, each sea level rise corresponds to a Heinrich episode marked by ice-rafted detritus (IRD) in the north Atlantic Ocean, signalling massive ice outbreak from the north American ice sheet. The sea level rise events may also have triggered ice breakouts from Antarctica. Furthermore, the sea level cycles terminating at 38, 44.5, 52 and 58-60 ka coincide with similar cycles in benthic oxygen isotope reported from north Atlantic sediment cores, indicating that the temperature in the north Atlantic deep ocean varied by 1-2 C, in phase with the sea level cycles.