Sesquiterpenoids in Amazon shelf waters

Surface water samples were collected with Niskin bottles on the Amazon continental shelf during peak river discharge. A suite of sesquiterpenoid compounds, mainly cadinane types, are present in the hydrocarbon fractions in trace amounts (ppb level) as dominant neutral components with abundances much higher than other compounds (e.g., n-alkanes). In general, the sesquiterpenoids found can be divided into four major groups: hydroxylated compounds, a series of saturated and unsaturated bicyclanes, mono-, and di-aromatic hydrocarbons. In samples closer to the river mouth the co-occurrence of cadalene together with its possible precursors (e.g. cadinols and cadinenes) as well as intermediates (e.g. α-calacorene, calamenene) is more notable. As distance from the river mouth increases the co-occurrence becomes less notable, indicating the degradation of these biogenic compounds during their transport seaward. Moreover, the absence of the ring B mono-aromatic component (5,6,7,8-tetrahydrocadalene) and its analogs with one double bond in ring A provides evidence that the early degradation of sesquiterpenoids starts with aromatization of ring A.