Location: Two primary stations within the Gulf of Maine's Wilkinson and Jordan basins with sampling depths ranging from surface (~5 meters (m)) to just above gulf bottom (~260 m). Coordinates are as follows: Station 1(42.618 N and 69.6357 W) and Station 2 (43.4868 N and 67.8267 W). The cruise campaign (EN665) was performed aboard the R/V Endeavor. The cruise spanned 6 days in 2021 (4/7 to& 4/12). Dr. Adam Subhas was the Chief Scientist.

General Sampling and storage: McLane pumps were deployed four at a time for a total pumping time of approximately four hours during each deployment. For carbon, nitrogen, and thorium-related measurements, pre-combusted 142-millimeters (mm) filters glass fiber GF/F were used to capture small particles (nominal pore size 0.7 microns (µm)), and acid-washed Nitex screens were used for the large particles (51 micron pore size). Immediately upon pump retrieval, pump filterheads were brought into a custom HEPA-laminar flow hood setup. The GF/F filters were sectioned using a light box and a ceramic plate (slices) or a custom stainless punch. Sample splits or punches were then stored according to best practices for those sample types. For small particle bulk carbon and nitrogen, samples were air-dried for several hours and then stored cool and in the dark until analysis. For large particle carbon, nitrogen, and thorium-234 a half section of the nitex filter was rinsed using filtered seawater onto a pre-combusted 25 mm GF/F, dried in a 50 degree Celsius (C) oven overnight, and then mounted for later beta counting. Small particle thorium-234 was measured using a 25 mm punch of the GF/F that was dried at 50 degrees C overnight and then mounted for beta counting. Photos were taken of all filters in the lab for general assessment of particle loading and color differences. Particulate dipped blanks were collected by attaching a plastic housing (for the dipped blank), containing a 51 µm filter and a 1 µm filter, to the side of the filtration apparatus. The plastic housing allowed for the free exchange of sample water.

Analyses: Small particle carbon and nitrogen filters were analyzed at the UC Davis Stable Isotope Facility following established protocols. Large particle carbon and nitrogen filters were analyzed at the Woods Hole Oceanographic Nutrient Facility following established protocols.

Particulate inorganic carbon samples were analyzed in the Subhas lab using a G-2121i Picarro Cavity Ringdown Spectrometer with an Automate-Liaison front end. The methods were modified from Subhas et al. (2019) for filter samples. The filter slice was placed in an Exetainer vial, put under vacuum, and acidified using 4 mL phosphoric acid (10%). Standardization was done using in-house Iceland Spar and the IAEA C2 standard material.

Size fractionated thorium-234 samples (25 mm GF/F) were mounted with mylar and two aluminum foil layers and beta counted within days of filtration in the Buesseler Radiochemistry Facility at Woods Hole Oceanographic. Second counts were conducted again at least 5 months after filtration. Uncertainties on particulate thorium-234 activities are derived from counting statistics and error propagation from sampling processing. Methods follow those in Black et al. (2018).

RA_P234 is the residual β activity of particulate thorium-234 and behaves in a similar manner to lithophile elements (Lin et al. 2016). The residual β activity is found by beta counting the large and small particle filters well after all the shorter-lived and unsupported isotopes have decayed away (e.g. unsupported thorium-234). These are the 'second count' activities. Data from this study has a method limit for RA_P234 equivalent to 2 times the s.d. of the dipped blank mean value of RA_P234