Sedimentary pigments on the Pakistan margin: Controlling factors and organic matter dynamics

Sedimentary pigments can provide information on various aspects of benthic processes and biogeochemistry. In this study, sediments from sites at depths of 140 to 1850 m spanning the Pakistan margin oxygen minimum zone (OMZ), and representing dramatic contrasts in depositional conditions and benthic communities, were analysed for pigment yields and compositions. This has allowed a rare consideration of how different factors, including oxygen concentration, organic matter (OM) supply and biological activity (e.g., degree of bioturbation) may influence sedimentary pigment distribution in a continental margin environment. It has also allowed one of the first efforts to study the impact of biological activity on pigment concentrations in the natural environment, rather than in a microcosm setting. Total extractable surface sediment pigment concentrations showed a range of ∼1.5–49 μg g−1 of dry sediment. The pigment suite was dominated by pheophytin and pheophorbide (which together constituted ∼75% of total pigments), with minor amounts of chlorophyll-a, alloxanthin, diatoxanthin, zeaxanthin and β-carotene. Low oxygen concentration appeared to influence pigment abundance by increasing the concentration of refractory pigments ultimately buried. In addition, maximal pigment abundances were associated with minimal amounts of sediment mixing by macrofauna, and at one site macrofaunal digestion may have altered the relative dominance of the different pheopigments. In contrast, the lack of a marked increase in pigment concentrations after the summer monsoon plus the lack of a correlation between pigment concentration and water depth suggest that OM supply plays a relatively small role in controlling sedimentary pigments. Pigment concentrations and suites, together with modelled decay half-lives (∼0.8–15 years), suggest that in general OM on the Pakistan margin, despite the low oxygen concentrations and monsoon-driven productivity, is surprisingly degraded compared to that on other continental margins with OM-rich sediments. As an exception to this general situation, pigment suites appeared to be relatively “fresh” (i.e., contained more labile components) compared to those found on the Oman margin of the Arabian Sea, but overall concentrations were lower. This is consistent with the higher oxygen concentrations, productivity and macrofaunal abundances found on the western margin of the Arabian Sea. Principle component analysis of pigment suites showed that they correlated with an amino-acid-based degradation index (DI). Thus, the pigment composition of marine sediments is related to, and may serve as an indicator of, the state of decay of sedimentary OM.