PALEOCLIMATE STUDIES IN INDIA CURRENT SCIENCE, VOL. 119, NO. 2, 25 JULY 2020 273 *e-mail: supriyo@tropmet.res.in Potential of reef building corals to study the past Indian monsoon rainfall variability Supriyo Chakraborty* Indian Institute of Tropical Meteorology, Ministry of Earth Sciences, Pune 411 008, India and Department of Atmospheric and Space Sciences, Savitribai Phule Pune University, Pune 411 007, India Reef building corals are known to provide high resolu- tion records of ocean–atmospheric variables for the last several centuries. The most important parameter is the sea surface temperature, which is accurately recorded by the oxygen isotopic composition as well as by a few trace elemental ratios such as, Sr/Ca, Mg/Ca, U/Ca, etc. Determination of the sea surface tempera- ture has enabled the palaeocenographers to study a variety of oceanic processes, such as the El Niño and Southern Oscillation, ocean circulation, air–sea gas exchange, Indian Ocean dipole, etc. Monsoon is an important atmospheric process in which sea surface temperature plays an active role in governing the moisture production and modulation of the wind cir- culation. Apart from temperature reconstruction, the corals have also been used to study the monsoon processes by some investigators, viz. the estimation of rainfall. In this article, we review the available coral records, specifically for studying the Indian summer monsoon rainfall variability. The study reveals that the coral records only from a few specific regions show promise in this endeavour. Keywords: Arabian Sea, coral, ISM, Lakshadweep, SST, δ 18 O. Introduction REEF building corals are known to provide a wealth of information on the physical and chemical condition of the environment in which they live. The oxygen isotopic composition of the coral skeleton is a function of sea surface temperature (SST) and the oxygen isotopic values of the surface seawater 1–7 . Hence the isotopic analysis of long lived corals is potentially useful to derive these parameters for geological past 8–11 . During the carbonate precipitation a variety of metals, albeit in trace quantity, get incorporated into the lattice structure of the carbonate mineral. Some of these metals, namely Sr, Mg, U, etc. when get incorporated into the carbonate matrix, their proportion is determined by the ambient temperature. So the elemental ratios, i.e. Sr/Ca, Mg/Ca and U/Ca in the coral skeleton will be largely determined by the temperature dependent distribution coefficients of Sr/Ca, Mg/Ca and U/Ca between aragonite and sea water 12 . Since these ratios are independent of the oxygen isotopic composition of the sea water, analysis of trace metal con- centrations has been widely used to reconstruct the SST 13–23 . On the other hand, paired analysis of the oxy- gen isotopes and trace metal abundance provide a means to estimate the salinity of the ocean surface 12,24 . Since both the isotopic values and the sea surface salinity are governed by the evaporation and precipitation, the salini- ty could also be estimated. So the combined analysis of isotope and trace metal is useful to reconstruct the past history of the temperature and salinity of the surface ocean 12 . A large number of studies are available across the world’s oceans. In the absence of the elemental records, the isotopic analysis has also been used to study the long-term behaviour of the SST. Cole et al. 25 estab- lished a 194-year record of coral δ 18 O from the equatorial coastal region of Africa in the Indian Ocean. From this record, the authors demonstrated that the western equa- torial Indian Ocean experienced a warming trend ca. 1.3°C since the early 1800s, if the salinity change was considered to be minimal. Carbon isotopes have also been attempted by some investigators 26 . But the controlling mechanism, especially on the stable carbon isotopic com- position is numerous and the pathways are not well understood. Hence, the study of stable carbon isotopes in coral as environmental indicator got limited success. However, the radioactive carbon, basically the 14 C/ 12 C ratio in annual bands of corals has widely been used to study a variety of processes. For example, estimation of CO 2 exchange rate across the air–water boundary 27–29 , surface ocean circulation 30 , surface–subsurface mix- ing 31,32 , changes in thermocline depth 33 , major current shift 34 , to study the meridional transport in the Indian Ocean 35 , etc. Corals have also been used for high resolution pH re- construction. Boron isotopes (δ 11 B) in coral skeleton provide an excellent means in understanding the bleach- ing effect due to enhance acidity of surface water 36–42 . Unlike the Pacific and the Atlantic Ocean, coralline records of oceanic pH, to our knowledge are not available from the Indian Ocean. But model simulation studies show that the acidification trends, especially in the west- ern Arabian Sea have intensified in the recent decades 43 .