Steroid geochemistry in the oxygen minimum zone of the eastern tropical North Pacific Ocean

Abstract

Particulate matter was collected in the eastern tropical North Pacific Ocean, an area characterized by a shallow and intense oxygen minimum zone, in order to investigate the geochemistry of particulate organic matter in the ocean. Sinking, large particles and suspended, small particles were analyzed for sterols, 3-ketosteroids, 3-methoxy-steroids and sterenes. Vertical fluxes of steroid classes in sinking particles and concentrations of steroids in suspended particles decreased rapidly below the euphotic zone consistent with upper ocean sources and deep water decomposition. Significant compositional changes were observed with increasing depth and as a function of particle size. Sinking particles were enriched in C27-Sterols but deficient in C28 and C29-sterols compared to the suspended particles. Suspended particles, especially in the oxygen minimum zone, were enriched in steroidal ketones and sterenes compared to sinking particles. Steroid distributions suggest that the oxygen minimum zone is the site of active “diagenetic” transformations, most likely microbially-mediated, of stenols to steroidal ketones, stanols and sterenes. These transformations occur preferentially in suspended particles relative to sinking particles.