Old carbon entering the South China Sea from the carbonate-rich Pearl River Basin: Coupled action of carbonate weathering and aquatic photosynthesis

Generally, negative Delta C-14 values of riverine particulate organic carbon (POC) are interpreted as old carbon derived from the erosion of deep soils and sedimentary rocks. Here we present natural C-14 and C-13 data from the carbonate-rich Pearl River Basin that discharges into the South China Sea. We found that the Delta C-14 values of POC and DIC (dissolved inorganic carbon) transported by the carbonate-rich river are all negative. This, however, does not mean the POC is necessarily old but indicates control of carbonate weathering (producing old DIC with negative Delta C-14 values) coupled with contemporary aquatic photosynthesis (producing new autochthonous POC but with negative Delta C-14 values) through the dead carbon effect of carbonate rocks, which was further evidenced by particular seasonal change in Delta C-14 values of DIC and POC (both higher in the rainy season and lower in dry season), spatial variation (both getting higher downstream), and negative correlation between delta C-13 and age of POC. This finding indicates that previous studies suggesting that riverine POC depleted in C-14 is old may be problematic in carbonate-dominated river basins. The finding that river basins rich in carbonates can release old POC may have important implications for the interpretation of organic carbon age in rivers and coastal oceans affected by the runoff from this basin type. It also indicates that it is necessary to examine the concentrations of both DIC and autochthonous organic carbon in rivers to correctly assess the carbon sink produced by rock weathering. (C) 2016 Elsevier Ltd. All rights reserved.