Corals are marine invertebrates that play a significant role in ocean ecosystems by building reef structures. These organisms secrete hard exoskeletons of calcium carbonate, forming the frameworks of coral reefs. These long-lived creatures act as natural archives, recording information about their environment within their skeletons. Scientists study these ancient records to gain insights into Earth’s past environmental conditions, complementing modern observations and aiding understanding of long-term climate variability and ocean changes.
Coral Growth and Skeletal Records
Corals grow by extracting calcium and carbonate ions from seawater to form aragonite, a calcium carbonate mineral that makes up their skeletons. This continuous deposition forms distinct growth bands, similar to tree rings. These bands are visible when coral cores are X-rayed, revealing a chronological record.
The density and composition of these growth bands are influenced by environmental factors such as water temperature, light availability, and nutrient levels. Denser growth occurs during warmer periods or specific seasons, with less dense growth in cooler times. By analyzing the characteristics of these bands, scientists can determine past growth rates and identify periods of environmental stress, such as temperature extremes or disease outbreaks. Annual banding allows precise dating of the skeletal material, enabling reconstruction of environmental conditions for specific years and seasons.
Chemical Signatures as Environmental Proxies
As corals grow, they incorporate various chemical elements and their isotopes into their calcium carbonate skeletons. The ratios and compositions of these elements reflect the ambient ocean conditions at the time the skeleton was formed. Scientists analyze these chemical signatures, which serve as “proxies” for past environmental variables.
The Strontium/Calcium (Sr/Ca) ratio in the coral skeleton is inversely related to sea surface temperature (SST). As temperature increases, less strontium is incorporated. This allows scientists to reconstruct historical SSTs with an accuracy of better than 0.5°C. Oxygen isotopes (δ18O) are another widely used proxy. The ratio of oxygen-18 to oxygen-16 in the coral skeleton is influenced by both SST and the isotopic composition of the surrounding seawater, which relates to salinity. Colder water and higher salinity lead to greater incorporation of the heavier oxygen-18 isotope.
Boron isotopes (δ11B) and Boron/Calcium (B/Ca) ratios in coral skeletons provide insights into ocean pH and acidification levels. Boron incorporation into the coral’s aragonite skeleton is sensitive to water pH. Lower pH values, indicating more acidic conditions, lead to a change in the boron isotopic signature. Trace metals like barium and manganese can also be incorporated into coral skeletons, serving as indicators of specific events. Elevated barium levels can signal increased river runoff, while manganese spikes suggest volcanic activity or pollution events.
Reconstructing Past Climates and Events
Analyzing chemical proxies within coral skeletons allows scientists to reconstruct various aspects of past climate and oceanography over centuries to millennia. These reconstructions reveal long-term trends in sea surface temperature and ocean pH, offering a baseline for understanding contemporary changes. For example, coral records show Pacific warming since the 20th century led to unusual interdecadal oscillations.
Coral records are valuable for understanding the past variability of major climate phenomena like the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Changes in δ18O and Sr/Ca ratios in tropical Pacific corals provide continuous, high-resolution data on ENSO events, extending records back over a thousand years. Sr/Ca-based temperature reconstructions from corals have also been used to study the Interdecadal Pacific Oscillation, revealing centennial-scale temperature trends.
Corals also record extreme events. Luminescent lines in coral skeletons, caused by terrestrial humic acids during flood events, serve as proxies for reconstructing past river discharge and rainfall variations. Records of ocean acidification, deciphered from boron isotopes in coral skeletons, highlight the historical impact of increasing carbon dioxide absorption by the oceans. These historical records help validate climate models and improve predictions for future environmental changes.