Potential solution to the cool tropics paradox

Abstract

The “cool tropics paradox” has been a thorn in the side of paleoclimate researchers, who draw on diverse techniques to improve their estimates of ancient climates. For the early Paleogene, between approximately 56 million and 49 million years ago, atmospheric reconstructions hint at a world much warmer than present, yet estimates of tropical sea surface temperatures determined by looking at the shells of foraminifera— a type of marine plankton—skew low, falling below present values. Throughout their lives, foramanifera form a calcium carbonate shell that records chemical parameters of the ambient seawater, and analyzing the oxygen isotope ratios in preserved shells is one of the dominant methods of estimating ancient ocean temperatures. Recently, however, researchers have found that posthumous alteration can displace the original biological structures, changing the oxygen isotope ratio and presenting a potential explanation for the source of the cool tropics paradox.