Reconstruction Of Storm/tsunami Records Over The Last 4000 Years Using Transported Coral Blocks And Lagoon Sediments In The Southernsouth China Sea

摘要

Large transported coral blocks on reef flats and elevated sedimentation rates in atoll lagoons were demonstrated to be excellent proxies for past strong storms/tsunamis in the southern South China Sea. Here we introduce another important proxy-coarse-fraction contents of lagoon sediments. This proxy is based on the principle that coarse fractions of enclosed lagoon sediments are usually controlled by strong sedimentary dynamics on the surrounding reef flat, and thus increases in the weight percentages of coarse-grained fractions of the lagoon sediments should also reflect strong storm/tsunami events or increases in storminess, with the size of the content peak as the proxy for the degree of storminess. Bearing this in mind, we measured the contents of the ＞ 1 mm grain-size fractions (referred to as "coarse-fraction content") in an undisturbed lagoon core precisely dated to cover the last 4000 years. The decadal-resolution data show a systematic variation, with some peaks strongly correlated with in timing with strong storm/tsunami events dated at AD 1872±15, 1685±8～1680±6, 1443±9, 1336 ± 9, 1210 ± 5～1201±4 and 1064 ± 30 using wave-transported coral blocks. Using the mean coarse-fraction content (9.2%) for the entire core as a reference line, a total of 77 peaks were identified for the past 4000 years. Twenty of these peaks are higher than 23%, the mean value for peaks that are correlated in age with the six strong storm events identified using independently dated coral blocks. Among these ＞ 23% peaks, 13 occurred within the last 1000 years. The concentration of content peaks in the last millennium could be partially due to higher deposition rates and sampling resolution. Over the last ～4000 years there appears to be a weakly increasing trend in coarse-fraction contents towards present time, but this trend is not obvious within the last millennium. This long-term variation may be related to changes in both storminess and reef-lagoon morphology. However, the data do suggest three extremely stormy periods centring around AD ～1200, ～400 BC and ～1200 BC, respectively. Overall, this study demonstrates that combined use of transported coral blocks and coarse-fraction contents of lagoon sediments provides an excellent means for comprehensive reconstruction of past storm/tsunami activity.