Dataset: Water Column Nitrate d15N Cocos Ridge Upper 1000m
Deployment: SR2113

Water samples were collected directly from the CTD Niskin bottles and filtered through 0.2-micrometer (um) PVDF syringe filters. Samples were collected from approximately 24 depths throughout the water column ranging from 7 to 2900 meters (m), with the shallowest depths being sampled first to avoid nitrate contamination from deeper, higher nitrate samples. The samples were collected in pre-cleaned acid-washed 30-milliliter (mL) HDPE bottles. The bottles and caps were rinsed three times with the sample water. The syringe was rinsed three times before attaching the PVDF syringe filter& and, once attached to the syringe, 10 mL of sample water was used to rinse the filter before filling the sample bottles. Once filled, sample bottles were labeled and parafilmed. Samples were immediately frozen on the ship and continued to be frozen until analysis.

For isotopic analysis using the denitrifier method, bacteria are grown in aerobic environments starting the seed of the bacteria Pseudomonas aurofaciens. Once all nitrate is fully consumed, the bacteria are harvested from a growth media and placed in clean pre-combusted vials. The vials are then sealed and purged with N2 gas for three hours to completely remove the N2O background. Approximately 200-500 mL of nitrate samples are then injected into a purged vial. After overnight complete conversion of nitrate to N2O in the sealed vials, the δ15N of N2O is analyzed using a customized gas bench isotope ratio mass spectrometer.