High resolution coral proxy records of an abrupt shift in mid-Holocene climate in the Western Pacific Warm Pool

High resolution coral proxy records of an abrupt shift in mid-Holocene climate in the Western Pacific Warm Pool

H.V. McGregor, M.K. Gagan, M.T. McCulloch and J. Chappell

This study uses coral climate reconstructions from the Western Pacific Warm Pool (WPWP), a key player in the El Niño-Southern Oscillation (ENSO) system, to investigate WPWP-ENSO-monsoon interactions during the mid-Holocene. Modern and fossil Porites sp. corals were drilled from Koil and Muschu Islands, Papua New Guinea (PNG), in the WPWP core region. Here reductions in sea surface temperature (SST) and rainfall characterise El Niño events. Changes in salinity and SST are reflected in coral skeletal oxygen isotope residuals 18O) and Sr/Ca respectively, allowing reconstruction of mean climate conditions.

Bulk samples were analysed to examine mean climate conditions (Figure 18a,b). Fossil coral 18O from 7.6 to 6.1 ka show drier conditions compared to present, and coral Sr/Ca-SST indicate warming from 2.5(C below present to a SST peak similar to today. Between 6.1 and 5.4 ka Sr/Ca-SSTs and 18O values suggest an abrupt shift to cooler and less saline conditions. By 2ka, salinity is similar to present, though SSTs are slightly cooler. The PNG coral records show a similar abrupt shift to that at the end of the African Humid Period. This suggests a climate link between these regions, perhaps via

monsoons. The abrupt shift may represent the rapid establishment of a modern-like warm pool.

Bimonthly analyses were used to "zoom-in" on periods of interest in the mean records, before and after the abrupt shift (Figure 18c). Prior to the shift the ((18O records show:

  • Reduced rainfall
  • Increased dry season salinity and evaporation, therefore stronger winds across the western Pacific
  • 18O minimums do not fall on the Sr/Ca SST minimum line, suggesting a change in timing of salinity and SST minima.

Sediment core records from the east and west tropical Pacific suggest an increased mid-Holocene SST gradient, with stronger winds, Walker circulation and western Pacific SSTs. PNG coral records also suggest stronger winds, though in contrast PNG SSTs indicate cooler conditions.

One possible scenario to reconcile conflicting SSTs would be an enhanced La Niña state. For example, the La Niña of 1999 saw strong winds send a cold tongue of water across the equatorial Pacific. With stronger mid-Holocene Pacific winds, this cold tongue could extend to PNG giving cooler SSTs but warming the surrounding region.

Effect of early marine diagenesis on coral reconstructions of 20th century changes in surface-ocean carbonate saturation state

Figure 18: Relative changes in ((18O and Sr/Ca-SST between fossil and modern corals. (a) Changes in mean Sr/Ca-SST. (b) Changes in mean ((18O. (c) Changes in seasonal ((18O for selected fossil and modern corals.