Magnetostratigraphy and ²³⁰Th dating of Pleistocene biogenic reefs in XK-1 borehole from Xisha Islands, South China Sea

Biogenic reefs and carbonate platforms are valuable natural resources, which play a role in modulating global climate and carbon cycle through biological processes. In this study, by combining paleomagnetic data and ²³⁰Th dating with lithostratigraphic characters for the XK-1 biogenic reef sequence from the Xisha Islands, northern South China Sea, new insights into the regional geochronology since the early Pleistocene are obtained. The main results are as follows: (1) Paleomagnetic findings suggest that the sequence records the Brunhes normal chron and the Matuyama reverse chron, including the Jaramillo and Olduvai normal subchrons. (2) The studied biogenic reef sequence named as the Ledong Formation has two subunits, i. e., upper and lower sections, which can be correlated to the middle and late Pleistocene, and the early Pleistocene, respectively. By comparing our results with those for previous long cores from the Xisha area, we conclude that the previous debate on the position and age of the Ledong Formation around the Xisha area mainly arose from inadequate absolute dating and use of different criteria for lithostratigraphic boundary determination. After determining the lower lithological boundary of the Ledong Formation, we set the age of the boundary as ~2.0 Ma.