Deployment: OC1705A

Purpose: To study methane cycling in the Oxygen Minimum Zone.

Cruise Plan: We will sample in the Eastern Tropical North Pacific (ETNP) oxygen minimum zone (OMZ) at ~10 stations along two E-W transects starting from on-slope waters west of Manzanillo, Mexico and extending offshore (see proposed cruise track on page 3). At each station, general water column parameters will be assessed via CTD equipped with sensors for fluorometry, PAR, and dissolved oxygen. Seawater collections for microbial biomass and chemical measurements will be done via rosette casts to discrete depths spanning the oxycline and suboxic zones. More extensive sampling (2-3 days per station) will be done at ~3 process stations. At these sites, additional seawater samples will be collected for shipboard stable isotope and microcosm experiments to measure chemical (methane, nitrogen, sulfur, oxygen) transformation rates and community compositional and transcriptional responses to seawater amendments. These seawater collections will also be used as inocula for incubations to enrich for target microorganisms. We anticipate ~6-12 CTD/rosette casts per process station and 1-3 casts per non-process (survey) station, with most casts to depths of less than 800 m. At ~5 stations (likely including the 3 process stations), depending on time and operational capabilities, we will also collect sediment samples using either the MC400 or gravity corer.