Dataset: Compound-specific nitrogen stable isotopes of amino acids in planktic foraminifera from Santa Barbara Basin sediment traps from 2018 to 2021

Samples were collected using a McLane Parflux 78H sediment trap deployed at depths >400 meters (m) in the central Santa Barbara Basin (34.2450389, -120.0592917). The sampling interval was 10 to 14 days, and samples were preserved in borate-buffered formalin solution. A 1/16th split of the sediment trap sample was used. Samples were rinsed with tap water over a 125-micron sieve, and foraminifera tests were removed from other trap material using a fine paintbrush. Foraminifera tests were dried on a micropaleontology slide, then species were identified by test morphology and sorted. Tests of the three most abundant species were removed to separate micropaleontology slides. These tests were inspected under a dissecting microscope to ensure no organic particles were adhered to the exterior or interior of tests. Any organic particles were gently removed with a wet brush. The removed tests were counted and weighed on a microbalance. Tests from multiple sediment trap samples were combined to achieve a sample of 5-10 milligrams (mg). This combined sample was gently rinsed with methanol three times, then dried.

Compound-specific stable isotopes were measured at the UC Santa Cruz Stable Isotope Lab. Tests were demineralized by adding ~1 milliliter (mL) 1N HCl to dissolve carbonate, then stored at 4 degrees Celsius (°C) overnight to complete the demineralization reaction. The HCl was then evaporated under N₂. The remaining organic matter was then hydrolyzed with ~ 1mL 6N HCl at 110°C for 20 hours after the vial was purged with N₂ to remove oxygen. Samples were purified by cation-exchange chromatography with DOWEX 50WX8-400 resin. Amino acids were measured as trifluoroacetyl isopropyl ester derivatives following Silfer et al. (1991). After drying under N₂, samples were esterified with a 1:5 mixture of acetyl chloride:isopropanol at 110 °C for 60 minutes. Samples were dried again under N₂, then trifluoroacetylation was completed using a 1:3 mixture of trifluoroacetic anhydride (TFAA) and dichloromethane (DCM) at 110 °C for 15 minutes. Inorganic salts were removed from samples by liquid-liquid extraction of derivatized amino acids in chloroform and an aqueous phosphate buffer. Trifluoroacetylation was completed again after liquid-liquid extraction. Samples were dried and dissolved in ethyl acetate for gas chromatography-isotope ratio mass spectrometry (GC-IRMS).

Amino acid stable isotopes were measured on a Thermo Trace gas chromatograph coupled to a Finnegan Delta-Plus IRMS and GCC III (isoLink). Samples were analyzed alongside a set of amino acid standards of known δ¹³C and δ¹⁵N values. Amino acids included in analysis were: alanine (Ala), glycine (Gly), threonine (Thr), serine (Ser), valine (Val), leucine (Leu), isoleucine (Ile), proline (Pro), aspartic acid/asparagine (Asx), glutamic acid/glutamine (Glx), phenylalanine (Phe), and lysine (Lys).