Frequent event deposits in IODP Hole U1433A (South China Sea) over the past 800 kyr: Implications of a revised methodology for chronology and paleoclimate reconstruction

Instantaneous event deposits (e.g., turbidites and slumping layers) are common and occur frequently in the abyssal plain of marginal seas. These instantaneous deposits represent seconds to days in duration, and may significantly impact age-depth models and interpretation of paleoclimate history based on sedimentary sequences recovered in deep cores. However, these event deposits have rarely been considered when investigating IODP/ODP deep cores from marginal seas. To resolve this problem, we take the South China Sea as a typical research example, which is one of the largest active marginal seas worldwide. We apply the methodology of high-resolution event sedimentology to Hole U1433A (189-0 m, 800-0 kyr) from the SW South China Sea deep basin. We identify centimeter-to-meter-scale turbidite layers (N=129) using high-resolution NGR, GRA, magnetic susceptibility data, and core images. These instantaneous event deposits account for ~16% of the total sediment thickness in Hole U1433A. We refine the preliminary age-depth model that is based on paleomagnetic and microfossil ages recovered from the hole by removing those instantaneous event deposits. We further test the effectiveness of our revised approach by comparing paleoclimate profiles that either include or omit those event layers. This test indicates that the event-free approach is effective and essential for a better reconstruction of the age-depth model and paleoclimate history based on the IODP deep core. Our innovative sedimentological methodology may also prove suitable for other marginal seas with frequent instantaneous event deposits elsewhere in the world.