Sediment mixing and accumulation rates in the Sulu and South China Seas: Implications for organic carbon preservation in deep-sea environments

Abstract

Recent rates of carbon burial in two deep-sea basins, the Sulu and South China Seas, are contrasted to determine factors influencing organic carbon preservation in deep-sea sediments. The Sulu Sea is an oxygen-deficient, warm-water basin, whereas the adjacent South China Sea is more typical of open ocean conditions. High-resolution sea-bed profiles of 14C reveal that accumulation rates for the Sulu Sea are 2–3 times higher than the South China Sea at any given water depth (e.g., 1.3 g cm −2 kyr −1 versus 4.3 g cm −1 kyr −1, respectively, at 3000 m). Higher accumulation rates for the Sulu Sea are attributed to higher terrigenous input and better carbonate preservation in the warmer bottom waters. Benthic mixing rates determined using 210Pb geochronology generally are lower in the Sulu Sea (0.004–0.254 cm 2 yr −1) as compared with the South China Sea (0.17–1.57 cm 2 yr −1), especially at the 3000 m and 4000 m sites in the Sulu Sea where benthic mixing is negligible. Despite the higher bottom-water oxygen levels in the South China Sea, organic carbon concentration profiles are strikingly similar for the two basins at any given water depth. With similar concentration profiles, higher accumulation rates in the Sulu Sea result in equivalently higher carbon preservation rates. As primary production is thought to be similar for both basins (if anything slightly higher for the South China Sea), the production rate can not explain the observed differences in carbon preservation. In addition, benthic mixing rates apparently do not play a major role in carbon preservation as extremely low rates at the deeper Sulu Sea would predict relatively low carbon preservation, contrary to our observations. We conclude that sediment accumulation rate is the dominant control on carbon preservation in this area of the western equatorial Pacific.