The first sea-surface temperature model based on Porites astreoides coral skeletons

The potential of the coral species Porites astreoides as a paleoclimate archive for the Tropical South Atlantic Ocean

N.S. Pereira, A. N. Sial, R. Frei, C. V. Ullmann, C. Korte, R. K. P. Kikuchi, V. P. Ferreira, K. H. Kilbourne

Summarized by Harry Janoff, who is currently a senior at Binghamton University and is majoring in Geology. After graduating from Binghamton University, Harry plans to find entry-level work in order to receive more field experience. He is unsure if he will return to grad school right now but he has definitely considered it and definitely does not want to go until he has more experience in the field. When he is not studying Geology, Harry loves to play chess and the violin.

What data were used: In their research, Pereira and their team use aragonitic coral skeletons to create records of strontium to calcium ratios and records of d18O and d13C in order to determine ancient environmental conditions of the ocean. They also used the coral’s skeletons to create the first ever d18O calibrations based on sea surface temperatures for the Porites astreoides coral species.

Methods: Pereira and their team collected Porites astreoides samples from the Rocas Atoll, located about 270 kilometers from the northeast coast of Brazil, because it is the only atoll present in Western region of the South Atlantic ocean. They collected series of sea surface temperature data for the region through the use of the NOAA pathfinder AVHRR between October 2012 and October 2013 and collected samples of P. astreoides coral from the atoll’s Cemiterio tide pool during July of 2013. The Porites samples were then cut into 5 mm thick samples and from those, carbonate samples were collected at .5 mm intervals.

The geochemical analysis of these coral samples was performed at the University of Copenhagen and involved the use of a Micromass IsoPrime mass spectrometer to measure the d18O and d13C isotopes. Coral Sr/Ca ratios were measured using a Perkin Elmer Optima 7000 DV ICP-OES and the samples were measured at a calcium concentration of ~10 mg/g.

Results: The researchers found that the two isotopes had abundance rates of 0.09‰ for d13C and 0.10‰ for d18O. They also discovered that the Sr/Ca ratios and isotope cycles displayed seasonality between 2001 and 2013. The Sr/Ca values for the coral varied between 8.86 to 9.15 mmol/mol and the mean value for the samples was 8.98 mmol/mol. d13C values for this research ranged from -1.38% to 1.05‰  with a mean value of 0.07‰ and the annual cycle for d13C contained the most positive d13C values in September and the most negative values in April or May. d18O values range from -4.26 to -3.69% and their mean value was -3.96‰. Unlike the d13C values, the d18O values were found to be  highly consistent overall, but there was a small increase in more-positive values during the years of 2006 and 2007.

Graphs of data that indicate sea surface temperatures and other surface ocean conditions as obtained from the Porites corals.
The data collected from 2001 to 2013 using the Porites coral was plotted into graphs and includes data on d13C concentrations, d18O concentrations, Sr/Ca values, sea surface temperatures, and rainfall in millimeters. The d18O, Sr/Ca and SST data are all cyclical and contain peaks towards the beginning of years and low points during the middle of years. d13C is the only outlier because its peaks are in the middle of years while its low points are closer to the beginning of years. The highest amount of rainfall was recorded around the middle of 2007 and this peak correlates to one of the largest sea surface temperature dips and a dip in Sr/Ca and d18O.

Why is this study important? This study is incredibly important because knowing the cycles in d13C values and consistency of the d18O values allows us to begin piecing together the conditions where these coral skeletons formed. Pereira and their team’s research was also the first to relate these Porites astreoides values to sea surface temperatures, which will allow us to learn even more about, and maybe fully understand, the climate when these corals were formed. This study also creates links between d13C concentrations and sea surface temperature through the coral skeletons. By understanding the conditions under which these coral formed, we are able to understand climate cycles that have occurred within the Southern Atlantic ocean. Using that information, we may be able to interpret what the climate of this region was like in the past and what it may become in the future. 

Citation: Pereira, N. S., Sial, A. N., Frei, R., Ullmann, C. V., Korte, C., Kikuchi, R. K. P., … & Kilbourne, K. H. (2017). The potential of the coral species Porites astreoides as a paleoclimate archive for the Tropical South Atlantic Ocean. Journal of South American Earth Sciences77, 276-285.


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