Dataset: AT37-06 Porewater NO3+NO2 and Ammonia
Deployment: AT37-06

Sampling and Analytical Methodology: Freshly collected sediment cores were sliced on the ship into 3 cm layers; porewater was obtained by gently centrifuging freshly collected sediment in 50 ml conical Falcon tubes for ca. 5 to 10 minutes until the sediment had settled; one Falcon tube produced ca. 8 to 10 ml of porewater.  For porewater sulfate measurements, 1 ml subsamples of the overlying porewater were drawn into syringes and injected through 0.45 μm filters into screw cap Eppendorf vials, each acidified with 50 µl of 6N HCl, and then gently bubbled with nitrogen for 4 min to remove sulfide; the samples were then stored at 4ºC before shipping and analysis

Geochemical Analyses: Porewaters from Guaymas Basin sediments were analyzed for Nitrate+Nitrite and ammonia concentrations colorimetrically using a Flow Solutions IV segmented flow Auto Analyzer from O.I Analytical, College Station, TX. The samples were preserved in H2SO4 and were neutralized with NaOH before analysis. Nitrate+Nitrite was determined using the cadmium reduction method and ammonia was determined using the phenate method. Both nutrients were diluted to get their concentrations within the linear range of the auto analyzer. Quality control standards from certified stock standard purchased from Environmental Research Associates, were analyzed every 15-20 samples.  

Analysis Notes:
Samples with a NH4 or N+N  concentration of greater than 20um were repated at a higher dilution. 
Samples were run for NH4 first at a dilution of 40 X (0.1 ml sample to 3.9 ml DDI water).
All NH4 samples were repeated by making a 10x dilution from the 40x dilutions resulting in a 400x dilution.
NH4 and N+N were analyzed separately.
A NaOH solution was made to adjust the pH of the sample to between 5 and 9 upon sample dilution.
The N+N samples were run at a 2x dilution (0.75 ml sample to 0.75 ml NaOH solution).
Repeated 4869-21 (0-3) at 140x dilution; value confirmed.
Repated all remaining  N+N samples greater than 20um at 20x dilution.
EPA Method: NO2+NO3 = EPA 353.4; NH4 = EPA 350.1.

Data Processing: The porewater data of Guaymas Basin sand Sonora Margin piston cores were tabulated in Excel sheets.

Quality Control: Note that Sample_IDs "QCS" are internal quality control standards (10 uM) and Sample_IDs "ICV" are external quality control standards.