Abstract:
Organic carbon burial is an important part of the carbon cycle, but there are few studies on organic carbon in coral reef sediments, which affects the accurate assessment of coral reef carbon cycle. In this paper, the modern surface sediments from the outer slope of Yongle Atoll, Xisha Islands, South China Sea, were analyzed for their total organic carbon (TOC) content, total nitrogen (TN) content, organic carbon isotope (δ13C) value, as well as sediment grain size, chlorophyll content and other indicators. The ecological condition of the outer slope of Yongle Atoll reef is relatively good, and has the potential to represent the organic carbon characteristics in sediments of healthy coral reef ecosystems. The results showed that: 1) the TOC content in the sediments from the outer slope of Yongle Atoll reef ranged from 0.71 to 1.66 mg·g-1, with an average of 1.23±0.31 mg·g-1; the TN content ranged from 0.12 to 0.28 mg·g-1, with an average of 0.20±0.05 mg·g-1. 2) the C/N ratio ranged from 6.16 to 7.59, with an average of 6.75±0.34; δ13C values ranged from -17.49 to -15.85‰, with an average of -16.61±0.49‰, indicating that the organic carbon in the sediments was predominantly of marine autochthonous origin and mainly from benthic plants. 3) Organic carbon content was negatively correlated with water depth and positively correlated with chlorophyll a content and pheophytin content, indicating that primary productivity controlled by benthic plants was the main source of organic carbon content on the outer slope of the reef. Combined with the average deposition rate of surface sediments in this area over the past 3,500 years (1.27 mm·a-1), it is hypothesized that the organic carbon burial flux on the outer slope of Yongle Atoll ranges from 1.12 to 2.61 g·m-2·a-1, with an average of 1.93±0.49 g·m-2·a-1. The present study provides new information for evaluating the contribution of coral reefs in the South China Sea to the carbon cycle.
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