Paleoclimate Reconstruction during the 17th to 18th Century Using Fossil Coral Tsunami Boulders from Ishigaki Island, the Ryukyus, Japan

Abstract:

Little Ice Age (LIA) which lasted during approximately 1450 to 1850 (e.g. Grove, 1988) is known as naturally occurring climate fluctuation, so knowing LIA in detail is necessary to improve climate models. Instrumental records can only extend back to about 100 years in many areas, thus paleoclimate records are reconstructed using proxies in tree rings, sediments and so force. However there are differences in reconstructed timings and magnitudes of LIA occurred in each area (Overpack et al., 1997), and most of the records are based mainly on terrestrial proxies such as tree ring records, whereas only limited numbers of marine records are available (Mann et al., 2008).

Coral skeletal climatology is a useful tool to reconstruct marine paleoclimate records in tropics and subtropics. Hermatypic corals, Porites spp. have aragonite skeletons and they have annual bands, which are suitable to reconstruct high-resolution paleoclimate in seasonal scale by measuring chemical compositions. Skeletal Sr/Ca ratio in Porites spp. has been widely used as a reliable proxy of SST (Sea Surface Temperature). However, corals grow for approximately several decades to 200 years, hence it is difficult to reconstruct LIA paleoclimate using living corals. Cobb et al. (2003) used fossil corals casted on shore by storms to reconstruct millennial scale paleoclimate. There are fossil coral boulders in the eastern coast of Ishigaki Island, Ryukyus, Japan. These fossil coral boulders were casted on shore by paleo Tsunami events, thus they are called “Tsunami boulders” (Araoka et al., 2010). Fossil corals used by Cobb et al. have 30-90-year records while some large Tsunami boulders have multi-centennial continuous records.

In this study, we reconstruct paleo SST using the Tsunami boulder from Ishigaki Island. The boulder has 185 years of annual banding. U-series dating shows the boulder was moved on shore at Meiwa earthquake in 1771. We measure Sr/Ca ratio using LA-HR-ICPMS (Laser Ablation High Resolution Inductively Coupled Plasma Mass Spectrometry) to reconstruct paleo SST during LIA (Kawakubo et al., 2014). LA-HR-ICPMS enables us to measure the long coral core rapidly.

Our result shows SST variation in 17th-18th century in this area and SST declined in around 1700. This result reveals the response of Little Ice Age in the northwestern Pacific.