High-resolution grain-size analysis and non-destructive hyperspectral imaging of sediments from the Gaoping canyon levee to establish past typhoon and monsoon activities affecting Taiwan during the late Holocene
Authors: Bertaz Joffrey , Jacq Kévin , Colin Christophe , Liu Zhifei , debret Maxime , Zhao Hongchao , Lin Andrew Tien-Shun 
Published: February 2023

Non-destructive and high-resolution hyperspectral analyses are widely used in planetary and environmental sciences and in mining exploration. In recent years, the scanning method was applied to lacustrine sediment cores in complement to XRF core scanning. However, this approach was rarely applied to marine sediments. The Gaoping canyon, located south of Taiwan island, is connected to the Gaoping River and is a very active canyon with large sediment transfer capacity. In particular, about 4 typhoon-driven hyperpycnal flows have been recorded by mooring systems in every recent year. Studying their frequency and intensity responding to past climate and environmental changes is a key to understand future tropical storm frequency and related climate variability. Core MD18-3574 was collected on the western levee of the Gaoping canyon and displays numerous fine laminations (millimetric to centimetric) recording the deposition of the gravity flows occurring in the canyon and on the slope. In this study, we combined non-destructive analyses such as XRF core scanning and hyperspectral imaging with high-resolution grain size and XRD bulk mineralogy analyses to understand the sedimentological and geochemical variations at the scale of the laminae. Core MD18-3574 sediments consist mainly of fine silt, presenting an alternance of fine-grained and coarse-grained laminations. The average mean grain size is 13.4 &#181;m ranging from 9 to 20.5 &#181;m. Thick coarser grained laminations are showing grain size distributions and asymmetric sorting of typical turbidite sequence. Grain size and bulk mineralogy display great visual and statistical correlation with XRF (Fe/Ca, Si/Al) and hyperspectral proxies (sediment darkness (Rmean), Clay_R2200). Principal component analyses (PCA) demonstrates that darker laminae are composed of coarser sediments with high Si/Al (quartz and feldspar-rich) and Clay_R2200 values and low Fe/Ca (calcite-rich) resulting from gravity flows.&#160; Inversely, lighter laminae consist of finer sediments with low Si/Al (muscovite and illite-rich), Clay_R2200 and high Fe/Ca resulting from hemipelagic deposition. Thus, such interpretation was extended to the core scale to identify gravity flows deposits layers. Moderate intensity tropical storm frequency is decreasing since the last 4 ka in response to the sea surface temperature (SST) decrease and enhanced East Asian winter monsoon since the middle Holocene. Tropical storm intensity increased after 2 ka in La Ni&#241;a like periods indicating that the surge of super-typhoons hitting Taiwan could be triggered by El Ni&#241;o Southern Oscillation (ENSO) state and variability. We can then assess that tropical storm activity is controlled by SST, monsoon system and ENSO conditions. This study brings new insights in the prediction of the ongoing climate change impacts on storms activity in the western Pacific Ocean.