Abstract
Stable carbon isotope ratios have been used to study the sources of particulate organic carbon (POC) in the Amazon River and its tributaries as well as to examine the transport of the riverine POC into the oceanic environment. POC in the upper reaches of the Amazon River has more positive δ 13C values (−24·5 to −28·0‰) than that in the middle and lower reaches (−27·9 to −30·1‰). The δ 13C content of POC from the tributaries is generally more negative than that observed in the Amazon main channel. This δ 13C evidence shows that the POC in the Amazon main channel is predominantly of terrestrial origin rather than a result of in situ production. A large range of δ 13C values (−17·5 to −28·4‰) is observed in the Amazon estuary and plume and is attributed to the mixing of riverine and marine POC. POC δ 13C measurements detect riverine organic detritus as far as 290 km offshore in a direction parallel to the river channel; the isotopic signal of riverine POC can be seen as far as 1100 km to the north-west of the river mouth in the Amazon River plume.
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