Dataset: Pore water geochemistry from sediments collected on cruise OC1703A aboard R/V Oceanus and cruise AT36 aboard R/V Atlantis

Sediment cores were collected in the northwest Pacific Ocean off the coast of San Francisco, CA on the R/V Oceanus during cruise OC1703A in March 2017 and in the northeast Atlantic off the coast of Woods Hole, MA on the R/V Atlantis during cruise AT36 in July 2016. Sediment cores in the northwest Pacific were 15–30 cm long and collected at five sites (100–4475 water depths) using an MC-800 multicore (Ocean Instruments). Sediment cores in the northeast Atlantic were 15 cm long and collected at two sites (1252–1496 water depth) using an MC-400 multicore (Ocean Instruments). Both multicores were modified with Go-Pro cameras in custom pressure housing to provide real-time video feeds and guide sampling (MISO Facility, Woods Hole Oceanographic Institute). Cores were stored at 4ºC for ≤24 hours until they were sectioned on board in 2.5, 3 or 5 cm depth increments. At each site, sediment porewater was extracted from triplicate cores in the Pacific and single cores in the Atlantic using either a porewater pressing bench (two cores/site in the Pacific; KC Denmark Research Equipment, Silkeborg, Denmark) under a stream of argon gas or Rhizon samplers (one core/site in the Pacific, all cores in the Atlantic; Rhizosphere Research Products, Wageningen, The Netherlands). Extracted water was either preserved with zinc acetate or filtered via 0.2 μm filters and frozen at −20°C.

Concentrations of nitrate, nitrite, ammonium, sulfate, sulfide, phosphate, bromide, and acetate were measured in pore water extracted from two cores at each site. Ammonium concentrations were obtained using a colorimetric assay (Bower et al.,1980). Phosphate concentrations were obtained using a Malachite Green Phosphate Assay Kit (BioAssay Systems; Hayward, CA; Cat# POMG-25H), following the manufacturer’s instructions. The analysis of nitrite and nitrate was performed using a microphotometry assay (Schnetger and Lehners, 2014). Concentrations of sulfide were measured in pore water preserved with zinc acetate via the colorimetric Cline Assay (Cline 1969). Optical densities of ammonium, phosphate, nitrite, nitrate and sulfide were obtained with a Lab BioTek Synergy HT microplate reader (Winooski, VT). Check standards for each nutrient were run in duplicate or triplicate along with samples. Sulfate, bromide, acetate and formate concentrations were determined with a Dionex DX-500 Ion Chromatograph, AS11 column, 5-50% sodium hydroxide gradient (Dionex Corporation, Sunnyvale CA) in the Stanford Environmental Measurements Facility. Samples and standards were diluted with ultrapure water before analysis. Check standards were analyzed every 10–15 samples. Oxygen concentrations were measured directly in the sediment through pre-drilled holes in the core tubes using an optical oxygen meter, FirestingO2 (FSO2-4, PyroScience, Aachen, Germany), and the robust oxygen probe (OCROB10, PyroScience). A two-point calibration was performed (0 and 100% air saturation), and the measurement was compensated for temperature and ambient pressure using internal or external sensors, as well as seawater salinity (35 g/L).