Coral reefs are characterized by high biodiversity and are composed of foundation species, namely reef-building corals, which grow by forming calcium carbonate skeletons. As coral skeletons preserve records of environmental conditions during growth, such as seawater temperature, geochemical techniques can be used to reconstruct past marine environments with high temporal resolution. This special issue brings together the geochemical aspects of coral skeletons from a variety of spatial and temporal perspectives.

The warming rate of sea surface temperature (SST) due to the global warming varies by ocean region or location. But it is challenging to clarify long-term spatial differences in SST trends, due to the limited number of SST observation. The Sr/Ca ratio in coral skeleton have been known as a useful proxy for reconstructing SST, especially for the industrial era warming. In this review, previous studies reconstructing SST over hundred years by using coral Sr/Ca ratio were summarized. Differences in SST warming trends among ocean regions were discussed, particularly with respect to latitudinal differences.

The interaction of tropical and subtropical sea-surface waters with the atmosphere on seasonal, interannual, and decadal timescales is one of the sources of climate changes. Understanding the Quaternary history of climate and ocean variability requires knowledge of how the ocean-atmosphere system operated in the past. Hermatypic scleractinian corals, especially Porites sp., are one of the best archives for extending the short and rather sparse instrumental record of sea-surface observations at monthly resolution. Coupled determinations of Sr/Ca ratio and oxygen isotope composition in fossil corals can yield high-resolution time series of sea-surface temperature and salinity with precise chronology, reconstructing seasonal-to-decadal climatic variability associated with the monsoon, the El Niño/Southern Oscillation, and the Pacific Decadal Oscillation during the Quaternary. This paper is a review of coral paleoclimatology, highlighting selected studies on coral-based climate reconstructions of surface ocean conditions during the Holocene, the last deglaciation, and the last interglacial. Although potential biases in paleoclimate reconstructions caused by diagenetic alteration, inter-colony offset, the past seawater chemistry, and errors of thermometers should be handled carefully, future works on skeletal geochemistry in corals with advanced analytical and statistical methods will provide a paleo-observational constraint on climate model simulations of ocean-atmosphere variability for the past.

The rising concentration of carbon dioxide in the atmosphere is driving both global warming and ocean acidification. Understanding the history and mechanisms of past climate changes is essential for predicting future climate conditions. Reef-building corals form aragonite skeletons, and the trace element concentrations in their skeletons are known to archive the marine environments experienced by the corals, making them indispensable tools in paleoenvironmental reconstruction. However, these trace element concentrations are influenced not only by ambient marine conditions but also by coral physiology, a phenomenon referred to as the “vital effect.” Thus, improving the accuracy of paleoenvironmental reconstructions using coral skeletons necessitates a thorough understanding of how coral physiology and skeletal formation processes influence these geochemical proxies. Advances in biological and geochemical research have significantly improved our understanding of coral biomineralization mechanisms. Based on these new knowledge, new temperature proxies have been developed to minimize the impact of vital effect. This paper first reviews studies on the calcification fluid involved in coral biomineralization. The second part examines research on novel temperature proxies designed to mitigate the influence of biomineralization on trace element concentrations in coral skeletons.

The stable isotopic composition of the carbonate skeletons of reef corals has been used as a powerful paleoclimatic and paleoenvironmental proxy. Corals of the genus Porites have been mainly used for paleoenvironmental reconstructions in the Indo-Pacific Oceans because of their massive, long-lived, and dense skeletal structures. However, coral skeletons of different genera and species are used in the Atlantic Ocean, high latitudes, and mesophotic coral reef ecosystems where the long-lived Porites corals are not distributed. This paper aims to broaden the use of geochemical proxies of coral skeletons from various environments by understanding changes in oxygen and carbon stable isotope ratios resulting from skeletal microstructure, growth rate, and metabolic activity of corals of different genera and species, and the characteristics of nitrogen isotope ratios that reflect metabolic processes.

This review provides an overview of recent research into coral skeletogenesis and skeletal climatology, by introducing current topics such as coral mass-bleaching, ocean acidification, and an effort for ocean alkalinity enhancement as an option of ocean negative emission technology. The so-called McConnoughy model, which considers the counter-transport of calcium ions and proton by Ca²⁺-ATPase, was widely accepted as calcification model for corals, but recently new models have been proposed that carbonic anhydrase and bicarbonate anion transporter are also important in biological mineralization. Advances in coral calcification models have influence on the prediction of the effects of the global ocean acidification and on the evaluation of the climate proxies of coral skeleton. The kinetics isotope effects seen in the oxygen isotope ratios of coral skeleton are discussed in relation to disequilibrium of CO₂ species in the DIC (dissolved inorganic carbon)–H₂O–CaCO₃ system under the presence of carbonic anhydrase. Since the precipitation of calcium carbonate in seawater increases the partial pressure of CO₂ in seawater, it seems difficult to directly apply corals to ocean negative emission technology. There is, however, no doubt that corals will continue to attract attention as an iconic organism for a global social goal “Nature Positive by 2030.”

Marine organisms like corals, foraminifera and bivalves produce calcium carbonate in two crystalline forms: whether aragonite, calcite or both. The carbonate polymorph mainly depends on the taxonomy of organisms, suggesting genetic control over its formation. Throughout the Phanerozoic Eon (541 million years ago to present),the molar Mg/Ca ratio (mMg/Ca) in seawater has fluctuated, resulting in periods of “aragonite seas” (mMg/Ca>2) and “calcite seas” (mMg/Ca<2).While modern coral skeletons are primarily aragonite, low mMg/Ca levels can lead to calcite formation. Scleractinian corals, however, produce more aragonite than expected under low mMg/Ca conditions, demonstrating biological control over skeletal formation. Transcriptomic studies show significant expression of organic and extracellular matrix proteins during early skeletal development under low mMg/Ca conditions. Notably, fewer genes associated with skeletal organic matrices of aragonite coral are down-regulated compared to the number of up-regulated genes, underscoring corals’ ability to initiate aragonite formation under unfavorable conditions. However, coral growth rates decline significantly in such environments, limiting their dominance as reef builders during calcite sea periods. Future perspectives include the importance of exploring the genetic and molecular mechanisms underlying skeletal polymorph control and understanding the adaptive potential of corals in changing oceanic conditions.