Impact of seasonal hypoxia on diagenesis of phytol and its derivatives in Long Island Sound

Isoprenoid compound distributions were measured in sediment at two sites in Long Island Sound, one with oxygenated overlying water throughout the year, the other seasonally variable with hypoxia in summer. A comparison of these two sites showed that the initial rate and pathway of phytol degradation were strongly influenced by benthic faunal activity and environmental redox conditions. Solvent-extractable phytol (in detritus of phytoplankton origin) was rapidly converted into highly-bound phytol (in sediment matrix) after deposition. Paradoxically, dihydrophytol, a product of phytol reduction, was produced under oxic environmental conditions, apparently by macrofaunal digestion. In contrast, the 6,10,14-trimethylpentadecan-2-one and 4,8,12-trimethyltridecanoic acid, products of phytol oxidation, were formed under hypoxic environmental conditions during suboxic or sulfidic microbial metabolism. Incubation of planktonic material further revealed the difference in degradation pathway between oxic and anoxic conditions. Degradation processes of phytol and its derivatives were quantified based on a conceptual model and incubation data. Relatively greater preservation of isoprenoid compounds at the periodically low oxygen site may result from the higher input of planktonic detritus and slower degradation rate than at the continuously oxic site.