GEO-01 Present and past ocean-atmosphere-climate interactions
Lunar nodal tide cycle triggered cross-slope transport of coarse sediments during the last glaciation in the South China Sea
Hongchao Zhao* , State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China
Zhifei Liu, State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China
Yulong Zhao, State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China
Jiawang Wu, State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China
Christophe Colin, GEOPS, CNRS, Université Paris-Saclay, Orsay 91405, France

Cross-slope transport of coarse sediments is thought to be dominated by gravity-flow or hyperpycnal-flow processes triggered by hydrodynamic events. However, whether long-term non-event hydrological processes can affect longitudinal transport is still unclear. In this study, high-resolution (1 mm) X-ray fluorescence core scanning combined with coarse-fraction observation has been conducted on a laminated sequence (dated as last glacial 14.5–20.7 cal ka BP) of Core MD05-2892 in the South China Sea. The laminae show interbedded millimeter-scale dark and light layers. Dark layers are distinguished by coarse-grained (sand-containing clayey silt) sediments rich in Ca, Zr, and Si, whereas light layers are characterized by fine-grained (silty clay) deposits rich in K, Ti, and Fe. Accordingly, Zr/K and Si/K ratios are used to indicate lamina occurrences, and the ratios are well correlated to grayness. Spectral analyses of the ratios and grayness all present a persistent 18.6-yr cycle, which is interpreted as the lunar nodal tidal cycle. This signal in laminae could be related to remobilization of coarse sediments via mean high-water-line variations driven by the lunar nodal tide during the last glacial sea-level lowstand. Coarse sediments were more likely transported from the shelf break via suspended-load-dominated hyperpycnal flow to the slope to form dark layers during periods of strengthened tidal activity, while fine sediments were dispersed via hemipelagic transport to form light layers during periods of weakened tidal activity. Our study highlights that the lunar nodal tide cycle can periodically trigger the transport of coarse sediments from the shelf to the deep sea.