Dataset: Shell trace elemental data (geochemical fingerprints) from larval samples collected during cruises AT42-24, AT50-04, and TN391 in the Gulf of Mexico and Northwestern Atlantic in 2020, 2021, and 2022

366 larvae of Gigantidas childressi (urn:lsid:marinespecies.org:taxname:1346725) and Bathymodiolus heckerae (urn:lsid:marinespecies.org:taxname:420693) were collected from eight methane seep sites during the three research cruises using a suction sampler mounted to the Deep Submersible Vehicle (DSV) Alvin (AT- cruises) and the ROV Jason (TN391). Shells were sorted and stored at -20 celsius for 1 year prior to sample preparation. Larval shells were separated into two valves using fine paintbrushes under a dissecting microscope, soaked for 5 minutes in a solution of 15% H₂O₂ (Suprapur^®, Supelco) and 0.05 M NaOH (Suprapur^®, Supelco) diluted with Milli-Q^® water (EMD Millipore). Valves were then rinsed thoroughly with Milli-Q^® water, placed in a drop of 1% HNO₃ (Optima^® grade, Fisher Scientific) for 5 seconds, and again rinsed with Milli-Q^® to remove debris and organic material from valves. Valves were then affixed to microscope slides in random order using double-sided tape. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to determine larval shell trace elemental composition ("fingerprints") by means of a New Wave 193 nm FX laser ablation system attached to an Agilent 7500ce ICP-MS at the Jackson School of Geosciences at the University of Texas at Austin. Pre-ablation was conducted to remove any surface contaminants (line scan mode; spot size = 35 µm, scan rate = 50 µm/s, repetition rate = 10 Hz; laser fluence = ~1.35 J cm^-2). After pre-ablation, samples were ablated within the pre-ablation track (line scan mode; line scan mode; spot size 25 µm; scan rate = 5 µm s^-1; repetition rate = 20 Hz; laser fluence = ~1.47 J cm^-2) with a sampling depth of 6-7 µm. The concentrations of magnesium (²⁴Mg and ²⁵Mg), calcium (⁴⁴Ca), chromium (⁵³Cr), manganese (⁵⁵Mn), iron (⁵⁷Fe), cobalt (⁵⁹Co), nickel (⁶⁰Ni), copper (⁶³Cu), strontium (⁸⁸Sr), cadmium (¹¹¹Cd), tin (¹¹⁸Sn), barium (¹³⁸Ba), and lead (²⁰⁸Pb) were measured. Calcium was used as the internal standard, for which Ca⁴³ was measured and a concentration of 400432 ppm Ca in the shell was assumed (i.e., stoichiometrically pure CaCO₃). IOLITE (v.4) trace elemental reduction software was used to convert analyte counts-per-second (CPS) to elemental ratios of calcium (X:CaCO₃) and to compute the limits of detection. Samples were bracketed using standard reference materials ECRM-752-NP, NIST 612, and USGS GSE-1G to evaluate instrument precision.