A 35-year window into the climate of one of the longest interglacial periods of the last million years has sounded a warning that El Niño events were half as frequent then as they are today.
A study into the earth’s ancient ocean climates by Bridget Ayling, a PhD student at the Australian National University and member of the ARC Centre of Excellence for Coral Reef Studies, used the skeletal remains of corals and giant clams up to 420 000 years old to view ‘snapshots’ of seasonal climate in warm interglacial periods in the Earth’s past.
Fossil clam sampling
“From fossil corals and clams, we can obtain fantastic high resolution proxy-records of the El Niño Southern Oscillation (ENSO) for short windows of time in the past that can be compared to those occurring today” says Ms. Ayling. “We can determine how strong and frequent past El Niño events were, which allows us to better assess whether today’s climate is unusual or a normal feature of warm interglacial periods”.
These windows into the earth’s past climate are achieved by looking at the chemical elements and isotopes locked in the skeletons of fossil Porites corals and Tridacna clams.
“We have analyzed a giant clam that was alive sometime between 380 000 and 420 000 years ago, during the Marine Isotope Stage 11 interglacial period. This clam lived for about 35 years, and during that time, El Niño events appear half as frequent as today,” says Ms. Ayling. “To the best of my knowledge, our detailed record of climate seasonality may be a global first for this time interval. That’s pretty exciting.”
However, it wasn’t an easy task to gain this groundbreaking insight into ocean climates; it relied on finding the perfect samples – and a bit of luck.
The team needed well-preserved fossils, ones which have changed very little over a few hundred thousand years – and began looking for them in a most unexpected place, 1200m above sea level.
The clams were collected from Huon Peninsula in Papua New Guinea, where today and in the past, living coral reefs are rapidly uplifted by geological movements to higher elevations and preserved above the ocean.
“There are actually quite a few [fossils] but we wouldn’t have known about them because at 1200m elevation, the fossil reefs are obscured by dense, tropical rainforest and a lot of mud,” says Ayling.
“Our team initially didn’t find any, and we tried to explain to the local villagers in basic pigeon English that we were looking for giant clams. One guy took us to a place where he had seen one. We were pretty excited, and paid him some kina, and that was that.
“We stayed in the village that night and the next day when we woke up, 12 clams had been marched into camp.”
The fossils found by the locals proved to be well preserved and Ms. Ayling began analysis of the isotopes and elements that are precipitated daily in the clam shells as they grow.
“In one fossil giant clam I looked at the seasonal cycle of oxygen isotopes, which are influenced by temperature and salinity and can be used as an indicator of ENSO climate variability at Huon Peninsula in the past,” says Ayling.
The 35 year ‘snapshot’ of climate seasonality is not only remarkable given the old age of the clam, but also valuable for observing the patterns of El Niño in times well before human influences, when atmospheric greenhouse gas concentrations were different, or the seasonal distribution of solar radiation was different.
However, the 35 year ‘snapshot’ is just a small part of the 40 000-year long interglacial period and further analyses of other fossil clams from this time are needed to build a better picture of climate during this warm interglacial.
The findings of Ms. Ayling’s research into ancient coral fossils from a more recent interglacial period around 330,000 years ago are soon to be published in the science journal ‘Earth and Planetary Science Letters’.
 Ayling, BF, McCulloch, MT, Gagan, MK, Stirling, CH, Andersen, MB and Blake, SG (2006). “Sr/Ca and δ18O seasonality in a Porites coral from the MIS 9 (339–303 ka) interglacial.” Earth and Planetary Science Letters 248(1-2): 462-475.
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Bridget Ayling, PhD graduate, CoECRS, Phone +61 2 6240 8929 or +61 0419 434 227
Malcolm McCulloch, Deputy Director CoECRS, +61 2 6125 9969
Jane O’Dwyer, Media Manager ANU, +61 2 6125 5001