Environmental factors, not just climate change, can also affect isotopic composition of corals

28.09.2016 09:10
Kategorie: News

Important details found in calcium carbonate skeletons of corals

The calcium carbonate skeletons of corals retain important details about their environment throughout their lifespan. As such, scientists led by the GEOMAR Helmholtz Centre for Ocean Research Kiel have been looking to them as they reconstruct climate trends thousands of years in the past. However, extreme events like El Niño have greatly affected the growth and metabolism of corals, thereby distorting the results of such temperature reconstructions.

Gallery 1 here

They are among the largest master builders in the seas; they construct the habitat for countless species of marine life; and as breakwater, they protect the coastline from erosion. Indeed, corals serve very diverse functions in the oceans. For scientists, corals are important chroniclers of the environment. Their calcium carbonate skeletons retain detailed information about the conditions they encounter throughout their lifetimes. Such information includes the seawater’s temperature or salinity. Because of this, researchers can use dead coral to precisely reconstruct the climate thousands of years ago.

However, corals are very sensitive. They can be drastically affected by temperature fluctuations or the turbidity of the water. “After the prominent El Niño in 1997 to 1998, the resultant coral bleaching damaged about 16 percent of global stocks,” said paleo-oceanographer Dr Steffen Hetzinger from GEOMAR in German. Together with colleagues from the RWTH Aachen University, the University of Kiel and the Free University of Berlin, he studied the coral reefs at the Venezuelan-Caribbean coast and proved that such events could affect the metabolism and growth of corals so much that their temperature records could be misleading.

The results of this study have been published in the international Scientific Reports journal.

The effects of El Niño have severely hit the Caribbean again. Since the mid-1990s, coral bleaching has occurred several times,” explained Dr Hetzinger. Additional stress factors like unusual plankton blooms also polluted the corals there. He added that heavy rain in December 1999 had led to so much mud being deposited in the sea that the coastal waters were affected for years.

All these events have led to the corals along the coast to grow much slower or not at all. However, there are differences between the two scenarios. In the case of extreme events, the corals regained their normal growth rates several years after the event. In the case of climate-induced events, the lowered growth rates were permanent. “When we examined the coral samples on the basis of specific isotope ratios, significant shifts between the times before and after the environmental stress events were obvious,” said Dr Hetzinger.
This is important as the isotope ratios serve as indicators of specific environmental details like temperature and salinity. According to Dr Hetzinger, although extreme events like the El Niño could be deduced from examining the coral skeletons, their study demonstrated that one needed to consider the physiological changes in the corals as well, if they are to be used for climate reconstructions.

Link to study: www.nature.com/articles/srep32879