problem also arises in studies of the concentration of DIC in the upper ocean. The d13C signal in seawater DIC ( 115 ) and as recorded in sclerosponges remains as a promising method for reconstructing the time history of the anthropogenic CO2 signal into the surface ocean.
To decouple the natural and anthropogenic effects on the carbon cycle, a more intensive effort has to be made to define natural variability in the upper ocean. We must also identify other accurate integrators of the anthropogenic CO2 signal.
Future Research Directions. Reconstruction of past SST changes in the world’s oceans during the period of rising atmospheric pCO2 is a top priority. A lowering of d18O in numerous coral records over the past 150 years may be evidence of a general rise of SST throughout the temperate and tropical surface oceans ( 107 ). Measurements of high precision Sr/Ca ratios will help resolve this important issue. Also, intense sampling and analyses of coral along a transect known for its sensitive response to climate change will add significantly to our knowledge of the global ocean’s response to climate change. Such an area is a meridional section through the warm pool in the western tropical Pacific.
The reconstruction of multiple tracer records from the same corals are essential to fully glean all of the available information regarding past climate and circulation change in the oceans. Measurements of nutrient-like elements (i.e., Ba/Ca), 14C, and stable isotopes on monthly time scales will enhance pursuits of the timing of past changes in water transport and climate. This approach can be extended to the last deglaciation, where the question of the timing of glacial melting and the onset of deep water formation is still not well known. A recorder of salinity changes in ocean water contained within the skeletons of corals should be pursued with vigor.
One of the most exciting and potentially fruitful areas of coral geochemical research is the study of deep-sea species. The primary carbon source to several species of deep calcareous corals is from the DIC in the surrounding seawater ( 116 ). The life spans of single specimens have been shown to be several hundred years ( 62 , 117 ) to more than 1,000 years ( 118 ). It is now possible to reconstruct time histories of tracers in deep water masses, and thus extend our view to three dimensions within the world’s oceans. Records of deep water stable isotopes (d18O and d13C) ( 117 ), trace metals ( 119 ), and bomb fallout products ( 62 ) are starting to emerge in the literature. We need to understand the processes controlling isotopic fractionation of the CaCO3, as well as high-resolution age dating of the fine structure within the skeleton of deep species using the highly precise chronometer 230Th. Subsequently, we can reconstruct time histories of nutrient distributions and ventilation of the deep ocean basins ( 120 ).
Reconstruction of past dissolved organic carbon (DOC) concentrations in seawater is very important for understanding the changes in the carbon cycle over the past 100 years, as well as the past glacial–interglacial cycle. Whether DOC concentrations are recorded within corals remains to be demonstrated. Tritium, a bomb product used as a tracer of ocean circulation, could be reconstructed from the organic matter within corals. Accelerator mass spectrometry measurements of a variety of radionuclides in corals (e.g., 10Be and 7Be) could help to understand particle cycling in seawater. Individual organic compounds would be of interest as indicators of primary production or pollution in coastal locales. It is likely that these records, and many more, are hidden within the pages of the coral diary.
I dedicate this article to Roger Revelle. I fondly remember his comments and encouragement in that melodic, deep voice. I thank Jess Adkins and an anonymous reviewer for their helpful reviews of the manuscript, Sheila Griffin for assisting with drawings and preparation, Andrea Grottoli-Everett for advice, and Ed Urban and Karl Turekian for editorial treatment. This work was funded by the Chemical Oceanography Division of the National Science Foundation through Grant OCE-9314691.
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