Magnetite with anomalously high Cr₂O₃ as a fingerprint to trace upper Yangtze sediments to the sea

This paper examines geochemical properties of detrital magnetite, in order to link sediments in a Plio-Quaternary core taken in the delta area to their sources in the Yangtze River basin. A total of 40 sediment samples were collected from both the main river channel/tributaries and a sediment core from the Yangtze delta. The geochemical compositions of detrital magnetite in these sediments were analyzed by electron microprobe, including FeO, TiO₂, CoO, MgO, Cr₂O₃, MnO, ZnO, Al₂O₃ and V₂O₃. The results revealed that the detrital magnetite grains with anomalously high Cr₂O₃ occurred exclusively in the upper reaches of the Yangtze (upstream of the Three Gorges Dam), where the E'mei Basalt block is located. This type of magnetite could therefore be considered a unique sediment proxy of the upper river basin to help identify sediment source in the delta area. Our analysis found such magnetite grains with high Cr₂O₃ occurring throughout the core depth above 186.5 m, in contrast to the extremely low Cr₂O₃ below this depth. The boundary between high and low Cr₂O₃ in magnetite grains of the core sediments was dated by paleomagnetism at ca. ~1.2-1.0 Ma, signifying that the linkage between the Yangtze River course and the sea was before ~1.2-1.0 Ma. This demonstrates that the sediment provenance of the Yangtze delta has experienced a change from local to distal Yangtze River, which took place with the uplift of the Tibetan plateau and coastal subsidence during the Plio-Quaternary.