An evaluation of quantitative reconstruction of past precipitation records using coral skeletal Sr/Ca and δ18O data

Coupled records of δ18O and Sr/Ca in Porites coral have been used to derive hydrological conditions by removing the Sr/Ca-inferred temperature component from the δ18O signal. Nanwan, a semi-enclosed bay in southern Taiwan, provides an opportunity to demonstrate the feasibility of quantitatively reconstructing past precipitation history. Recurrence of seawater δ18O offsets between wet and dry seasons in the early 1990s is well correlated with the precipitation record. Even though the hydrological signal only accounts for 20% of the total annual coral δ18O variation of ca. 1‰, offsets can be found in the residual δ18O of modern corals after removing the thermal effect, which contributes to the other 80%. The observed timing and amplitude of the seasonal seawater δ18O offsets in Nanwan and their correlation with precipitation are reproduced by hydrological models. In the mid-Holocene, the seasonal anomaly of residual δ18O was twice that of the modern value based on the 9-yr Sr/Ca-δ18O data recorded in a 6.73-ka Porites coral. Hydrological models suggest an annual rainfall of 1800-3000 mm/yr at the window during mid-Holocene, 20% higher than that of the average of 30-yr modern instrumental records of 1500-2500 mm, consistent with the qualitative pollen record from lake sediments. The seasonal decrease of residual δ18O in 5 of 9 yr was earlier than the increase of the coral Sr/Ca-inferred temperature, which implies that these rainy seasons probably occurred from the early-mid-spring to mid-summer, earlier than that from late spring to late summer today. The driving force may be related to the changes of solar insolation and the East Asian monsoon. It is cautioned that the variability of hydrographic conditions imposes restrictions on a precise calculation of the amount of paleo-precipitation. The dynamic nature of local tectonics, monsoons and water circulation should be further addressed to precisely quantify precipitation over the past 10,000 yr from coral geochemical records.