Dataset: Rare earth element adsorption onto Pacific marine sediments sampled by coring during Kilo Moana cruise(s) KM2012 and KMxxxx in October and November 2020.

Equatorial Pacific sediments were sampled using a ______corer during R/V Kilo Moana cruise KM2012 (was there another cruise?). Recovered cores with negligible sedimentary and sampling disturbance (i.e., sediment-water interface visibly intact) were sub-sampled immediately in the laboratory at 4°C on the ship. Cores were sectioned every 1 cm between 0 and 10 cm, and sectioned at 2 cm between 10 and 40 cm. Sectioned mud was placed in a cold van after sectioning and later analyzed for REY, εNd, other isotopic (41K; Li et al., 2022), trace metals, porewater and bulk geochemistry, and mineralogical (XANES, XRD; Li et al., 2022) analysis at Oregon State University, Bigelow Laboratory for Ocean Sciences, University of Tokyo, and University of Massachusetts Boston. Porewaters were preserved following the methods described by (Abbott et al., 2015b; 2019). Briefly, the cores were sectioned on board the ship into hydrochloric acid (HCl) cleaned centrifuge tubes in a glove bag filled with an inert (N2) atmosphere. After centrifugation, pore water was pulled off using HCl clean syringe, filtered with a 0.45 μm syringe filter, and acidified with ultrapure distilled HCl to a pH ≤ 2. Sediments were frozen and later freeze-dried prior to analysis.
Batch adsorption experiments to investigate REE sorption were conducted at UMass Boston.We employed the pH drift technique, which involves adjusting the pH of a solution stepwise and allowing the system to reach equilibrium at each step. This technique helps to identify the pH at which maximum adsorption occurs by gradually adjusting the pH and monitoring how the adsorption behavior changes. Solutions of 0.01M sodium nitrate (NaNO₃) were freshly prepared from stock solutions using ultrapure water (i.e., 18.2 MΩ·cm; MilliQ IQ 7000 with multipack 0.22 µm). Solutions were equilibrated for 24 hours before use to allow stabilization of pH, ionic strength, and dissolved species concentrations. To minimize changes in solution composition, they were used within one week of preparation, stored in airtight, acid-washed containers at room temperature, and monitored periodically for pH drift. These precautions ensured that adsorption experiments were conducted under consistent and reproducible geochemical conditions. 
YREE concentrations were quantified using high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS; Nu Instruments AttoM ES). Isotopes were monitored at various resolution modes—low (300), medium (4000), and high (10000). Precisely, 139La, 140Ce, 141Pr, 143Nd, 145Nd, 146Nd, 147Sm, 149Sm, 151Eu, 153Eu, 155Gd, 157Gd, 158Gd, 159Tb, 161Dy, 163Dy, 165Ho, 166Er, 167Er, 169Tm, 172Yb, 173Yb, and 175Lu were measured in low, medium, and high-resolution modes. The high-resolution mode was explicitly used to monitor 151Eu and 153Eu to resolve interferences from BaO⁺ species on the Eu isotopes, as well as LREEO⁺ and MREEO⁺ interferences on other heavy REEs. Before instrumental analysis, an internal standard (100 µL of 115In at ~871 nM) was added to each filtered and acidified sample to account for potential matrix effects and signal drift. Recoveries of the internal standard ranged between 92% and 100% for all YREE. Calibration was performed using a series of NIST traceable YREE standard solutions (Inorganic Ventures; CMS-1-125 mL in 5% HNO₃) with concentrations ranging from 0.01, 0.1, 1, 5, and 10 µg L⁻¹. All reported YREE concentrations were derived from five replicate analyses for which the analytical precision consistently remained better than 5% relative standard deviation.

Detailed operating parameters for the HR-ICP-MS, including resolution settings, plasma conditions, and calibration methods, are provided in the supplemental appendix.   (Not provided to BCO-DMO. Please provide the manuscript's expected citation!)