Climate variation in the tropical Pacific: coral provides proof

The Younger Dryas period, about 12 000 years ago, was marked by a sharp cooling event in the Northern Hemisphere. Temperatures there fell by between 2 and 10°C. The East Antarctic in contrast experienced an episode of warming. Data have up to now been insufficient or too inconclusive to enable palaeoclimatologists to track this climatic event in the southern temperate regions and the tropics. An IRD researcher campaign took a 2 m drill core sample from the isle of Espiritu Santo, Vanuatu, found to contain a giant fossil coral of a single species, Diploastrea heliopora, well preserved in a condition of growth. The specimen age was estimated at between 12 449 and 11 719 calendar years, a span covering nearly the entire Younger Dryas. This unique fossil provides clear evidence of the spatial signature this major climatic cooling event left in the tropics.

Mineral skeleton growth of these corals is a steady few millimetres per year over many centuries, which offers a precise record of ancient environmental conditions. Fossil skeleton concentrations in chemical elements such as strontium or oxygen isotopes indicate the sea surface temperature (SST) on which they depended when the corals were alive. Corals of the genus Porites, which which grow by about 1 cm per year, are the type most used as paleothermometers, but the Diploastrea used in this study have the advantage of growing more slowly. Moreover, there is only one species of this marker, Diploastrea heliopora, which eliminates any inter-specific differences, always a source of uncertainty.

The IRD researchers, working with Australian and American colleagues (1), first compared SSTs, data obtained by way of Sr/Ca ratio analyses in the Diploastrea and modern Porites originating respectively from New Caledonia and Indonesia. Similarity of the figures obtained both validated the use of Diploastrea as a palaeothermometer and allowed calibration of the data acquired from this coral in modern times, before applying it as a palaeoclimatic marker on fossil forms from Vanuatu.

The temperature curves drawn from fossil Diploastrea data show that during the Younger Dryas period the STT around Vanuatu was on average 4.5°C lower than at present (2). The data furthermore indicate large interdecadal variations. The periods during which the SSTs were relatively warm coincides with annual amplitudes of about 3°C, similar to those currently observed in Vanuatu. However, cooler periods were marked by greater amplitudes, of about 5 to 6°C, like those observed at present in New Caledonia, 7 to 10° of latitude further South of Vanuatu. These data indicate an upward movement of the thermocline (3) and suggest that the Younger Dryas cooling resulted from compression of Pacific tropical waters towards the Equator.

In addition, coupled analysis of Sr/Ca and of 18O/16O isotope ratios at biannual and monthly time-scales provided information on the ocean-atmosphere exchanges operating during this period, and especially on the evaporation/precipitation ratio.
The 18O level in the corals depends on the SST and surface water salinity. It is an expression of seasonal variations linked with rainfall, poor in 18O compared with sea water. At present in Vanuatu, the sea surface 18O concentration and salinity declines as the temperature rises. This stems from the intense activity, during the Southern hemisphere summer, of the South Pacific trades convergence zone (SPTCZ) which brings strong precipitation. Conversely, in the South-West Pacific subtropical area, unaffected by rainfall coming from the SPTCZ, the SST and surface water salinity are positively correlated. This is why in the subtropical oceanic environments, where evaporation largely exceeds precipitation, the salinity and 18O concentration rise with the SST. This situation is similar to that observed in Vanuatu during the Younger Dryas period, but quite different to the present prevailing situation. The data drawn from Diploastrea samples therefore suggest strongly that the South Pacific Convergence Zone did not exist in the Younger Dryas. This climatic scenario is similar to conditions seen in the present during an El Niño event, during which the west Pacific warm pool contracts towards the Equator, the SPTCZ then moving towards the North to fuse with the intertropical convergence zone. These results should also lead to improvements in climatological modelling and to better understanding of ocean-atmosphere exchanges and the processes influencing the activity of convergence zones.

Mina Vilayleck – IRD

(1) IRD research unit UR055 “Palaeotropics”, Research School of Earth Sciences, Australian National University, Canberra, Australia and NSF Arizona AMS Facility, University of Arizona, Tucson, USA
(2) This deviation is similar to the one observed in earlier studies made on Porites core samples from Vanuatu, but greater than the cooling calculated for the tropical Pacific zones closer to the Equator.
(3) Thermocline: a zone of steep temperature gradient within the oceanic water layer which marks the transition between warm surface waters and the colder waters at deeper levels.