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Coral cores tell different warming stories

Madagascar coral

Coral cores from Madagascar reveal important information about past and current climate change in the Indian Ocean(Source: Jens Zinke)

Climate biomarkers Growth rates of coral can override the climate information held in their skeletons, a study of giant corals has found.

The study, published in Geochemistry Geophysics Geosystems, indicates that scientists need to analyse multiple coral cores to build a picture of past climates.

Dr Jens Zinke from the University of Western Australia's Oceans Institute led an international group of researchers studying the growth patterns of giant corals in the Indian Ocean.

As coral grows, trace elements are incorporated into its calcium carbonate skeleton, with ratios of strontium to calcium (Sr/Ca) used like tree rings to determine yearly variations in ocean temperature.

Zinke's team took core samples from two Porites lutea coral colonies located 700 metres apart off Madagascar's north east coast.

However, when they compared Sr/Ca ratios and growth rates over a 43-year period with historical sea surface temperature data for the region, they were puzzled to find that while one coral seemed to show a strong warming trend, the other showed no obvious trend at all.

"Both cores showed similar seasonal variations in growth rate and Sr/Ca ratios, but on interannual time scales, the two corals growth rates had opposite responses to temperature," says Zinke.

"In one core, increasing sea surface temperature was accompanied by increasing growth rate and increasing Sr/Ca (normally associated with cooling), while in the other, over the same period rising temperatures led to a declining growth rate and declining Sr/Ca (normally associated with warming)," he explains.

Although a faster growth rate may seem positive it is actually a stress response when temperatures exceed a maximum threshold, he adds.

"Corals need time to form a dense skeleton; if this is diminished then the coral will be more susceptible to the influence of cyclones, storms and acidification."

While both corals were from the same area, Zinke says microscopic differences in local temperature, salinity and hydrodynamic regimes (waves and currents) create microclimates that affect growth.

"People speculate that we can reconstruct past climates from biological archives like tree rings and coral growth bands. But living things grow differently in different years depending on what is happening in the local area."

"We need to be careful when reconstructing climate based on coral, and need to be able to replicate results from a number of different samples to create an accurate picture of past climate."

The cost of testing coral core samples meant that usually only a single sample was tested from a location. While it would be ideal to test 10 or more samples from a location, Zinke argues testing three to four corals is more realistic. "Even doubling the amount of samples would improve precision."

300-year battle of the bands

In another study published in the journal Climate of the Past, the same group of scientists found that organic matter trapped in coral cores can also shed light on the impacts of decades long climate swings.

The team used a new technique known as spectral luminescence scanning to identify bands of organic material in 300-year-old coral core samples from the same region to build up a picture of annual changes in river runoff.

The cores identified long climate swings of 20-30 years and 50-70 years. They also revealed that climate swings in the Pacific Ocean, likely triggered by changes in the North Pacific sea surface temperature, have a significant impact on rainfall variation and ocean temperatures in the Western Indian Ocean around Madagascar.

"We found that the response of rainfall and river runoff in Queensland to the same climate swings in the Pacific is opposite to that of the western Indian Ocean," says Zinke.

"Currently the swing is in a phase that results in higher rainfall and more runoff in Queensland [and therefore less in Madagascar], probably intensified by warming oceans."

The results also revealed that deforestation caused by rice cultivation between 1950 and 1980, coupled with a return to subsistence agriculture following Madagascar's economic collapse in the 1990s, caused such significant topsoil erosion that it too overwrote the climate indicators for the period.

"Again this research underpins the importance of using bioarchives like coral cores in context of regional climate and environmental change," says Zinke.

"The corals can serve as natural monitoring stations that help us to understand what drove environmental change at times when human impact was low and how it evolved with increasing pressure in the 20th century."
