Dataset: Particle d15N values of amino acids
Deployment: SR1812

In situ pump filtration was performed using large volume pumps (WTS-LV; McLane Research Laboratories, Inc.) outfitted with mini-MULVFS (Multiple Unit Large Volume in-situ Filtration System) filter holders (Bishop et al. 2012). Pumps were loaded with filter stacks of either 3 filter pore sizes (51- and 5-um acid cleaned Nitex screens and 1 um pre-combusted quartz fiber filter) or 4 filter pore sizes (51- and 6-um acid cleaned Nitex screens, a double layer of 1 um pre-combusted quartz fiber filters, and a double layer of 0.3 um of pre-combusted glass fiber filters). Samples were frozen at -80 degrees Celsius until laboratory analysis. Sediment trap collection was performed using surface tethered sediment traps (STT) and neutrally buoyant sediment traps (NBST). Particles were collected in polycarbonate tubes with a collection area of 0.0113 m² and triggered to close at the end of the sampling period. Samples were collected in 500 mL of 70 ppt salinity, 0.1% formaldehyde-poisoned brine, buffered to pH 8.5 with borate and overlain with 1‑um filtered surface seawater. After collection and allowing particles to sink to the bottom of the tube on board the ship for an hour, brine was filtered through a 335‑um polyester screen and rotary wet split onto pre-combusted glass fiber filters with a pore size of 0.7 um. Samples were frozen at -80 degrees Celsius until laboratory analysis.

6 um and 51 um Nitex filters were washed down onto 0.7 um GFFs and inspected using a dissecting microscope, removing foreign (non-natural) material. Samples were freeze-dried and then stored at -20 degrees Celsius for the remainder of preparation. Samples were hydrolyzed using 6N HCl at 110 degrees Celsius for 20 hours, filtered, purified (to remove organic molecules other than amino acids and amino acid like molecules) using cation exchange chromatography. Amino acids in samples were derivatized to their isopropyl ester-trifluoroacetyl derivatives in a two-step derivatization: esterification with 4:1 isopropanol:acetyl chloride (110 degrees C, 60 minutes) and acetylation with 3:1 dichloromethane:trifluoroacetic anhydride (100 degrees C, 15 minutes), with the headspace exacuated with N₂ prior to each reaction. They were de-salted using phosphate buffer and chloroform and then re-acetylated. The solvent was exchanged for ethyl acetate immediately prior to analysis.

Analyses were carried out using a Thermo Fisher Scientific Trace 1310 Gas Chromatograph (GC) coupled to a MAT 253 Plus Isotope Ratio Mass Spectrometer (IRMS) through a Thermo Fisher GC Isolink II system with a combined oxidation/reduction reactor held at 1000 degrees C, a liquid nitrogen trap, and a Conflo IV open split interface.