Dataset: Nitrite, ammonium, nitrate, and nitrous oxide concentrations from R/V Roger Revelle cruise RR2311 in the Eastern Tropical South Pacific from November to December 2023

Collection

Water samples were collected as part of a 32-day cruise in the Eastern Tropical South Pacific, aiming to study nitrification in this oxygen minimum region of the ocean. High-resolution profiles were obtained at 5 process and 2 transect stations. In the Event Log for RR2311 (see Related Dataset section), transect stations are designated "T" and process stations are designated "PS". Nitrous oxide samples were collected on the first cast upon arrival at a process station, and in one case on the last day, of a 2-5 day station occupation.

Samples were collected in duplicate into 60-ml serum vials, preserved with HgCl2 and stored until analysis on the Delta V mass spectrometer (Frey et al. 2024). 

Analysis

N₂O: Duplicate samples for measurements of in situ N₂O concentration were collected from Niskin bottles into 60 mL serum bottles after overflowing three times the bottle’s volume. Samples were then immediately preserved with 100 µL saturated HgCl₂. N₂O concentrations were measured on an isotope ratio mass spectrometer (Delta V) based on the major ion (m/z = 44) peak area compared to the reference N₂O gas (Frey et al. 2024). 

Ammonium: Ammonium concentration was measured manually fluorometrically using standard methods (Holmes et al 1999). Water was collected using Niskin sampling bottles. Samples were measured immediately upon retrieval and were not filtered prior to analysis. Five ml volumes were analyzed. 

Nitrite: Nitrite concentration was measured manually colorimetrically using standard methods (Strickland and Parsons 1972). Water was collected using Niskin sampling bottles. Samples were measured immediately upon retrieval and were not filtered prior to analysis. Five ml volumes were analyzed.

Nitrate: Nitrite + Nitrate (NOx) concentration was measured using the chemiluminescence method (Garside 1982) on frozen samples in the laboratory after the cruise. Nitrate was determined as NOx - Nitrite (from the colorimetric determination).