THE FLUVIAL CARBON CYCLE IN TWO PENINSULAR MALAYSIAN BASINS: AN ISOTOPE RECONNAISSANCE STUDY

Two rivers in Peninsular Malaysia, the Langat and Kelantan, were surveyed for dissolved organic and inorganic carbon (DOC/DIC) concentrations, stable carbon isotopes, and concentrations of selected chemical constituents during 20 months of sampling. The objective of this study was to clarify carbon cycling processes in both rivers. Water samples were taken twice a month from three points along the Kelantan and Langat Rivers, and from adjacent groundwater reservoirs. The results suggest the dominance of geogenic and anthropogenic inputs on river chemistry, based on PCA analyses of the aggregate chemical dataset. These are decoupled from DOC and DIC, which show δ¹³C signatures that imply dominance of biologically-respired carbon sourced from C3-vegetation. An estimate of riverine pCO₂ values, derived from the DIC data, revealed an over-pressure for CO₂relative to atmospheric levels during most periods of sampling in both basins, sometimes approaching nearly 30 times the averaged global value of ~380 ppm.

The averaged δ¹³C_DIC signatures in river waters are ¹³C-enriched by about 10‰, relative to the C3 plant source. Given the general absence of carbonates in either watershed, coupled with the observed CO₂ overpressures, the probable cause of this enrichment is diffusive fractionation of DIC via CO₂ evasion. The average annual efflux of CO₂ in the upstream sub-catchment of the Langat and Kelantan rivers (20 x 10³ t C/yr and 125 x 10³ t C/yr, respectively) is comparable to the fluvial export of dissolved carbon (~18 x 10³ t C/yr and 84 x 10³ t C/yr, respectively) and particulate organic carbon (~4 x 10³ t C/yr and 42 x 10³ t C/yr, respectively). However, the combined gas efflux and fluvial export of carbon only amount to less than 2 % of total ecosystem uptake of CO₂ within the two basins. The remainder of this carbon is mostly respired back into the atmosphere, although a minor portion may be stored within the watersheds. These findings confirm the more complex role of rivers in the global carbon cycle, beyond that of a simple link between terrestrial and oceanic reservoirs.