Abstract:

Coral reefs can record past climate change. For long-term climate studies, the establishment of a high-precision stratigraphic chronological framework can provide accurate chronological constraints on the correlation between regional and global climate events. In this paper, coral-reef core samples from well CK-2, Xisha Islands, South China Sea are used in magnetic testing, and anhysteretic remanent magnetization (ARM) is selected as climate proxy to identify astronomical cycles in ancient reefs in the South China Sea since the Miocene. The results of ARM depth series analysis reveal well defined stable depositional cycles corresponding to a ~1.2 Ma obliquity modulation cycle and a ~405 ka long eccentricity cycle; these two cycles are used in orbital tuning of the ARM depth series between 0-878.21 m. The orbital tuning results set the depositional constraints on the coral-reef core at 0-19.2 Ma, with a resolution up to 100000 years, which can be mutually calibrated with Sr isotope dating. The ~1.2 Ma obliquity modulation cycle is most obvious in the reef strata of the South China Sea through the Miocene, and the ~405/~95 ka eccentricity cycles and the ~173 ka obliquity modulation short cycle are discontinuous. These results suggest that the millennium-scale orbital cycles may play a major role in regulating the evolution of coral reefs in the South China Sea. The change of the main control period reflected in the reef records may be an indicator of reef’s timely response to glaciation associated climate change.

Key words: coral reef, well CK-2, ARM, astronomical tuning, astronomical age scale