Sampling:

Size-fractionated particles were collected using dual-flow McLane Research in-situ pumps (WTS-LV) and 142-millimeter (mm) "mini-MULVFS" filter holders (Bishop et al., 2012; Lam et al., 2018; Lam et al., 2015a; Xiang and Lam, 2020). At most of the stations, two casts of 8 pumps each and two filter holders per pump were deployed to collect samples at 16 depths throughout the water column. At super stations, a 24-depth profile was obtained with three casts. The targeted depths of the wire-out were corrected using pressure readings from a self-recording Seabird 19plus CTD at the end of the line. One filter holder/flowpath was loaded with a Sefar polyester mesh prefilter (51 micrometers (μm) pore size, called the "Qp" filter) and paired Whatman QMA quartz fiber filters (1 μm pore size) in series ("QMA-side"). The other filter holder/flowpath was also loaded with a 51 μm prefilter (called the "Sp" filter), but it was followed by paired 0.8 μm Pall Supor800 polyethersulfone filters ("Supor-side"). A 150 μm Sefar polyester mesh was placed underneath all 51 μm prefilters and QMA filters as a support to facilitate filter handling but not analyzed. All filters and filter holders were acid-leached before use based on the recommended methods in the GEOTRACES sample and sample-handling protocols Cutter et al., 2010). QMA filters were pre-combusted at 450 degrees Celsius (°C) for 4 hours after acid leaching.

A special plate was manufactured for one of the pumps that could hold two additional mini-MULVFS filter holders that were loaded with full filter sets but not connected to plumbing. These 'dipped blank' filters included the full filter sets (51 µm prefilter on top of paired QMA or paired Supor filters) with a 0.2um Supor polyethersulfone filter on top of the 51 µm polyester prefilters to exclude all particles from the main filter set. These were processed identically to the regular filters and functioned as full process blanks A total of 49 dipped blank filters were used for blank subtraction, calculations of uncertainties, and determination of detection limits (Table 2).

In this dataset, data reported from the 51 μm prefilter are referred to with an "LPT" suffix to indicate large particulate total concentrations (>51 μm). Samples could come from the QMA side ("Qp" filter) or the Supor side ("Sp" filter) and are treated equivalently. Data reported from the main filters (QMA—1-51 μm —or Supor—0.8-51 μm) are from the top filter of the pair only, and are referred to with an "SPT" suffix to indicate the small particulate total concentrations.

Analytical Procedures:

Particulate organic carbon (POC) and particulate nitrogen (PN):
For SPT particles, a 25 mm punch of QMA filter was dried at 55°C at sea. LPT (>51 µm) particles were rinsed at sea from the Sp filter using 0.2 µm-filtered seawater onto a 25mm 0.8µm Ag filter (Sterlitech) and dried at 55°C at sea. These samples were beta counted for ²³⁴Th at sea by the CafeTh lab at WHOI, then sent to the Lam lab at UCSC for POC and PN analyses. POC and PN sample processing was similar to what was described in Xiang and Lam (2020). Filter samples were dismounted from the beta mounts. Typically, a 22 mm QMA subpunch or the entire 25 mm 0.8 µm Ag filter were fumed in a desiccator with concentrated HCl and dried in the oven at 60 °C overnight, and then pelletized with 30mm tin discs from EA Consumables. Tin disc encapsulated samples were measured using a CE Instruments NC 2500 model Carbon/Nitrogen Analyzer interfaced to a ThermoFinnigan Delta Plus XP isotope ratio mass spectrometer (IRMS) at the Stable Isotope Laboratory at University of California, Santa Cruz. Isotopic results obtained from the IRMS were calibrated using reference materials Acetanilide (C8H9NO). The effect of dissolved organic carbon sorption is corrected with isotopic values of dipped blanks. The isotopic data are expressed in the standard delta notation as per mil deviations (‰) with respect to international standards of Pee Dee Belemnite (PDB) and atmospheric nitrogen. The precision of the internal standard (Pugel) analyzed along with the samples in the run is 0.07‰ for d13C and 0.14‰ for d15N.

Particulate inorganic carbon (PIC):
A UIC Carbon dioxide coulometer was used for PIC measurement. Briefly, PIC on SPT QMA punches or 1/16 LPT QMA-side prefilter was converted to CO₂ by addition of 2 N sulfuric acid. CO₂ produced is carried by a gas stream into a coulometer cell where CO₂ is quantitatively absorbed by a cathode solution, reacted to form a titratable acid and measured based on the change in current.

Biogenic silica (bSi):
A 1-hour alkaline leach with 0.2 M NaOH at 85°C was used to leach bSi for both size fractions prior to the measurement on a Lachat QuikChem 8000 Flow Injection Analyzer at UCSC. A 4-hour time-series leaching approach to correct for the contribution from lithogenic Si was applied to a subset of stations and the deepest 3-4 bare bottom samples from all stations (cf., Barão et al., 2015; DeMaster, 1981; Lam et al., 2018). Lithogenic Si was only important for some near-bottom samples, so the intercept ("t0" value) was used for the near-bottom samples; the 1-hour timepoint was used for all other samples.

Particulate trace metals (pTM):
pTM total concentrations (SPT, LPT): The digestion method of pTM is based on a refluxing method (Cullen and Sherrell, 1999; Planquette and Sherrell, 2012) with light modifications similar to the "Piranha method" in Ohnemus et al. (2014). In brief, the Supor filter was adhered to the wall by surface tension in a 15-milliliter (mL) flat-bottom screw-cap Savillex vial to avoid immersion. After 4-hours of refluxing at 110 °C with an ultrapure (ARISTAR® or Optima grade) 50% HNO3/10% HF (v/v) mixture, digestion acids were transferred into secondary vials and heated to near dryness. The residue was heated in 50% HNO3/15% H2O2 (v/v) to dryness at 110 °C. The final residue was re-dissolved with 2 mL 5% HNO3 spiked with 1 ppb In. Two certified reference materials (BCR-414 and PACS-2) were digested routinely alongside the samples to assure the quality of each digestion.

pTM leachable concentrations (SPL): A 1/16ᵗʰ slice of Supor filter was leached using the "Berger Leach", a weak acid leach comprising 25% Q-grade acetic acid and 0.02M reagent grade hydroxylamine hydrochloride (Berger et al. 2008). Samples are submerged in the Berger Leach solution and heated in a 95°C water bath for 10 minutes, cooled at room temperature for about 1.3 hours, and then centrifuged for 30 minutes at 4100 rpm. Total leach time should be 2 hours. The supernatant is transferred to a 15 mL Savillex vial and dried for over an hour at 110°C. The residue is redissolved and dried in concentrated HNO3 twice and brought up in 5% HNO2 spiked with 1 ppb In. Two certified reference materials (BCR-414 and PACS-2) were leached routinely alongside the samples to assure the quality of each leach.

Analysis by ICP-MS: Sample solutions were analyzed using an Element XR high-resolution ICP-MS (Thermo Scientific) at the UCSC Plasma Analytical Facility. Elemental concentrations were standardized using multi-element, external standard curves prepared from NIST atomic absorption-standards in 5% HNO₃. Instrument drift and matrix effects were corrected using the internal 1ppb In standard and monitored using a mixed element run standard. Concentrations were determined using external standard curves of mixed trace elements standards. If a parameter was measured on the ICP-MS, the isotope and resolution (LR=Low Resolution; MR=Medium Resolution) used for the concentration measurement are indicated in Table 4.

Further details on instrumentation can be found in Table 3.