Dataset: Depth profiles of the isotopic composition (δ30Si) of silicon within dissolved silicic acid from samples collected on Leg 1 of the US GEOTRACES Pacific Meridional Transect (PMT) cruise (GP15, RR1814) on R/V Roger Revelle from September to October 2018

Seawater samples for δ30Si measurements were collected using the Oceanographic Data Facilitiy's (ODF, Scripps Institution of Oceanography) CTD-rosette mounted with Niskin samplers and a Sea-Bird Electronics CTD (SBE9plus). Seawater was gravity-filtered into polypropylene containers through in-line Supor filter capsules (0.8/0.45-micrometer (µm)) attached directly to each Niskin bottle. Sample bottles were capped and stored without preservative in the dark.

Back in the laboratory, the silicon from seawater was precipitated off as trimethylamine silicomolybdate using a high-purity triethlyamine ammonium molybdate solution (TEA-Moly). The precipitate was isolated by filtration onto a polycarbonate filter and purified by high-temperature combustion to produce solid silicon dioxide. SiO2 was then converted to solid Cs2SiF6 by dissolution in HF and addition of CsCl. The Cs2SiF6 was rinsed with ethanol, dried, and dissolved in high purity (>18MΩ) water. BaSiF6 was precipitated by the addition of BaCl, rinsed with ethanol and dried.

Isotopic ratio analysis was performed using a Nu Perspective isotope ratio mass spectrometer equipped with a Nu Sil inlet system. Samples of BaSiF6 are loaded into sample vials that are placed in a sample carousel in the Nu Sil. Samples are sequentially heated to 590 degrees Celsius (°C) to evolve.

Isotope delta values are normalized against the international standard NBS28.

Methods are further described in the Supplemental File "Natural_Abundance_Protocol.pdf".