Selected taxa and sample localities

The samples analyzed for [Hg] in this study include the same specimens analyzed for clumped isotope compositions in previous studies27,28. In addition to the aforementioned Antarctica and Alabama samples, we also analyzed fossils for [Hg], Δ 47 , δ18O, δ13C, and 87Sr/86Sr values from Late Cretaceous deposits at localities in Argentina, India, Egypt, Libya, Sweden, and the U.S. states of Alaska, California, and Washington (see Supplementary Note 1, Supplementary Data Files 1–3, and Supplementary Fig. 4). The Argentina42, Egypt43,44,45, Libya, and India44 sample regions range from middle to latest Maastrichtian and were selected as possible records for direct comparison to Seymour Island and Moscow Landing, Alabama27. The samples from Washington46 and Sweden47,48 are known to be of earliest Maastrichtian age from biostratigraphic constraints46,47,48,49 and were intended to serve as controls with respect to measured [Hg], because the host deposits predate eruptive windows of the Deccan Traps, and thus the samples were expected to have [Hg] values near background. The latitudinal range of sample localities spans from 70°N (Ocean Point, Alaska; 83–85°N paleolatitude) to 64°S (Seymour Island, Antarctica; 67°S paleolatitude), please see Supplementary Table 1 for detailed information.

Additional samples used in this study, included twenty-eight Cretaceous specimens from eight distinct regions, detailed as follows: six Glycymerita aleuta (Mt. Katmai region, Alaska), three Glycymeris sp. and two Gryphaea sp. (Merced County, California), one Cyrtodaria sp. (Ocean Point, Colville River, North Slope, Alaska), two Arca vancouverensis (San Juan Islands, Washington), four Exogyra overwegyi (Kharga Oasis, Egypt), two Agerostrea ungulata (Fezzan Region, Libya and Cauvery Basin, India), three Pycnodonte vesicularis (Neuquén basin, Argentina), two Belemnitella sp. (Scania, Sweden) and three unidentified bivalves (one from Scania, Sweden and two specimens from the Mt. Katmai and North Slope regions of Alaska, respectively). Specimens were either collected in the field by the authors and/or collaborators, or were loaned courtesy of the University of Michigan Museum of Paleontology (Ann Arbor, Michigan) and the University of California Museum of Paleontology (Berkeley, California). We also have collected modern bivalve specimens from: Sabin Point Park along the estuary of the Providence River in Providence, Rhode Island, with 4 specimens that included Crepidula fornicata (MOD-PRO-FORa), Crassostrea virginica (MOD-PRO-VIRa), Geukensia demissa (MOD-PRO-DEMa), and an unidentified bivalve (MOD-PRO-BIVa); Spectacle Island, Boston, Massachusetts, with four specimens of Crassostrea virginica (MOD-SPE-VIRa, MOD-SPE-VIRb, MOD-SPE-VIRc, MOD-SPE-VIRd); Lake Tahoe, California, with four specimens of Corbicula fluminea (MOD-TAH-CORa, MOD-TAH-CORc, MOD-TAH-CORd); and the South River, Virgina, with six specimens of Corbicula fluminea (MOD-SOU-CORa-1.1, MOD-SOU-CORb-1.1, MOD-SOU-CORa-6, MOD-SOU-CORb-6, MOD-SOU-CORa-14, MOD-SOU-CORb-14). Lastly, we analyzed 4 Pleistocene specimens collected from Rocky Bay, Bermuda, three of which were unidentified bivalves (PLE-RB-UNKa, PLE-RB-UNKb, PLE-RB-UNKc), and one specimen of Cittarium pica (PLE-RB-JAPa) collected in a previous study40.

Shell sampling and preservation assessment

All samples were subject to a visual assessment by optical microscopy to observe carbonate fabrics, eliminating samples with obvious recrystallization, and/or sampling away from recrystallized vugs or portions of any given shell specimen. We applied the same criteria for assessing alteration for Δ 47 from a previous study27. Roughly half the specimens are of the oyster genus Exogyra, and were sampled near the ventral margin of the shell over a large enough area to represent at least three distinct layers of ordered carbonate in the shell matrix, presumed to be annual growth bands. Carbonate fragments from the ventral margin were ground by hand using a mortar and pestle. Some smaller individual samples were crushed and used in their entirety (e.g., Anomia and Cyrtodaria).

Mercury concentration preparation and determination

With no established analytical standard for the determination of [Hg] in carbonate samples, we selected a variety of carbonate reference materials to use as in-house standards and to propose as community-wide standards. We compared NIST SRM-88b (dolomitic limestone quarried near Skokie, Illinois, USA), NBS-20 (Solnhofen limestone, Germany; exhausted in terms of commercial availability), USGS COQ-1 (carbonatite from the Oka complex, Lake of Two Mountains, Canada), IAEA-B-7 (limestone collected near Maiella, Abruzzo, Italy), Carrara marble (Italy), and in-house standard LV-3 (limestone from Lake Valley Formation, New Mexico) to two reference materials of known [Hg] values, NRC MESS-3 (Beaufort Sea marine sediment; 91 ± 9 ng/g, certified value) and USGS SGR-1 (Green River Shale; 0.3 μg/g, certified value). As additional points of comparison we also measured [Hg] in the following reference materials: ATHO (Icelandic rhyolite obsidian), USGS AGV-2 (andesite from Guano Valley, Oregon), USGS BCR-2 (basalt from the Bridal Veil Flow Quarry near the Columbia River, Oregon), and USGS BHVO-2 (surface pahoehoe lava from the Halemaumau crater, Hawaii). Initial comparisons of a sample unknown (MC-PRB-EXOb), reference materials, and proposed carbonate concentration standards were digested in a range of acid normalities from 4N to concentrated solutions of both HNO 3 and Lefort aqua regia (also referred to as “inverse aqua regia,” 3:1 HNO 3 :HCl) to ensure that Hg dissolution was consistent and to test for any loss by volatilization through the acid reaction (none was observed). Splits of SRM-88b, NBS-20, and MC-PRB-EXOb were measured in aliquots ranging from 10 to 300 mg, and were digested in 3 ml of 4N trace metal grade HNO 3 in capped 7 ml acid-cleaned polytetrafluoroethylene (PTFE) or perfluoroalkoxy alkane polymer (PFA) vials to assess any potential matrix effects (e.g., the influence of impure sample matrix to enhance or suppress analytical detection) in these materials. To minimize sample material consumption, carbonate sample unknowns were routinely dissolved in 3 ml 4N trace metal grade HNO 3 in 7 ml PTFE/PFA vials with sample loads between 50 and 150 mg, and all non-carbonate material was routinely digested in 2 ml of Lefort aqua regia with identical sample loads. We tested the scalability of larger sample loads in acid digestions for future isotopic analyses with up to 500 mg of NBS-20 and 1000 mg of MESS-3 in 16 ml of Lefort aqua regia in 180 ml PTFE/PFA vials, later diluted by 50% with 16 ml 18.2 MΩ deionized water prior to analysis. Digestions were conducted at 80 °C between 12 and 48 h, after which an aliquot of the acid digestate (50–200 μl) was diluted with 5 ml of a 1% BrCl and 40 μl (NH 3 OH)Cl solution, before being loaded onto a Nippon Instruments Inc. RA-3000FG + cold vapor atomic fluorescence spectroscopy (CV-AFS) analyzer for [Hg] determination in accordance with US EPA Method 163150 at the University of Michigan Biogeochemistry and Environmental Isotope Geochemistry Laboratory. Precision of the CV-AFS was determined from an average measured analytical blank value of Hg at 0.070 ± 0.466 pg (6σ, n = 117) and an average [Hg] of 0.014 ± 0.093 pg g−1 (6σ, n = 117), which would imply an analytical sensitivity for whole-sample [Hg] at effectively 0.1 pg g−1. However, sample reproducibility both within a given analytical session and between analytical sessions on the AFS varied on average by 11% (as determined from the 2σ uncertainty of 22 replicated samples) of the calculated [Hg] from the measured mass of Hg. All samples replicates are listed in the extended data files. Process blanks averaged 2.50 ± 0.19 pg g−1 (2σ, n = 12) as a definitive background, and for all determined [Hg] represents <0.5% of blank contribution to CV-AFS peak signal intensity, and in the majority of cases <0.025% blank contribution. For the purposes of clarity, all samples described as “replicates” can further be subdivided between samples from the same analytical session where the “date of acid digestion” of the solution permits comparison of aliquots from the same digestate solution, and “date of analysis” allows the comparison of aliquots run on different analytical sessions.

Reference material mercury concentrations

In addition to Carrara marble, we also analyzed [Hg] in other carbonate and non-carbonate reference materials in order to provide additional method development with respect to conducting sample digestions under varying acid strengths, temperatures for dissolution, and duration of acid reaction. We used the analysis of these reference materials as a ‘proof of concept’ approach to understanding Hg in high-temperature diagenesis and volcanism, which can inform the limits of preservation of Hg in carbonate fossils and the introduction of Hg to the environment volcanically.

NBS-20 (69.6 ± 7.1 ng g−1) and COQ-1 (37.4 ng g−1) yielded the highest measured [Hg]. NBS-20 (a sample of Solnhofen limestone) has been exhausted and is no longer commercially available, but COQ-1 can still be obtained from the USGS. COQ-1, a 120 Ma calcite-rich carbonatite from the Oka complex in Canada51, bears surprisingly high [Hg] compared to other lava samples (see Supplementary Data File 1 and Supplementary Fig. 5), and we would anticipate Hg0 and Hg(II)-bearing mineral phase thermal decomposition to occur at magmatic temperatures. The only modern active carbonatite eruptions occur at Oldoinyo Lengai (Tanzania), where alkaline natrocarbonatite (Na- and K-rich) is produced at eruptive temperatures ranging from ~490 to 545 °C52. These eruptions are several hundred degrees cooler than all comparative modern silicate lavas and have lower average viscosities than modern basalts51,52. The [Hg] of COQ-1 compares to the [Hg] of reference materials that are also flow units of eruptive lavas including: BCR-2 (1.8 ng g−1), BHVO-2 (3.0 ng g−1), AGV-2 (2.8 ng g−1), and ATHO (2.0 ng g−1). BHVO-2 is a modern Hawaiian basalt, BCR-2 (basalt) and AGV-2 (andesite) are Cenozoic volcanics from Oregon, and ATHO is a Cenozoic rhyolite obsidian from Iceland. With an order of magnitude lower [Hg] values than COQ-1, these lavas may exhibit a greater effectiveness at thermally decomposing Hg(0) and Hg(II) phases (and/or exhibit higher volatile loss before these phases can crystallize). Fundamental compositional differences and/or postdepositional alteration could explain the discrepancies in these materials, or the crystallization of a higher proportion of Hg-retentive mineral phases in the lower temperature carbonatite melt prior to eruption (possibly due to less volatile loss of Hg). We anticipate that Hg-retentive mineral phases will be uncommon in volcanic settings due to the high degree of Hg0 volatilization and likely thermal decomposition of Hg(II)-bearing mineral phases from the generation of melt. The presence of significant [Hg] in all measured reference materials, carbonate and non-carbonate, shows promise in the determination of [Hg] throughout the geological record across a broad range of preserved materials. The retention of relatively elevated [Hg] in COQ-1 would also reinforce the idea that Hg is likely not easily mobilized diagenetically at elevated temperatures.

Clumped isotope methodology

We have utilized the carbonate clumped isotope paleothermometer from measured Δ 47 values to determine the temperature of formation in which the fossil mollusk taxa from this study have precipitated their shells27,28. These formation temperatures are interpreted to represent coastal marine temperatures, where from the same sample aliquot we can obtain both [Hg] and temperatures directly reflecting the environment that these organisms resided in.

Aliquots of 3–5 mg per replicate of sampled biogenic carbonate powder were measured for δ18O, δ13C, and Δ 47 isotopic compositions in the University of Michigan Stable Isotope Laboratory using the same instrumentation and procedure as previous studies27,28, with a PorapakTM trap temperature held between −10 and −15 °C. Isotopic values were determined from measured voltage intensities and measured carbonate Δ 47 values were placed in the absolute reference frame using heated (1000 °C) and H 2 O-equilibrated (25 °C) standard gases, and converted to temperature values using acid fractionation factor of +0.067‰ and high-temperature composite calibration developed in the University of Michigan Stable Isotope Laboratory53. δ18O sw values are calculated from carbonate δ18O and Δ 47 -derived temperatures using the appropriate fractionation factors for calcite and aragonite27,28,53. All taxa were dominantly calcite with the exception of Anomia, Turritella, and Cyrtodaria which were aragonitic. Measured δ18O, δ13C, Δ 47 , and calculated paleotemperature and δ18O sw values for all samples are reported along with gas and carbonate standard data in the Supplementary Material.

Using the same approach as a previous study28, we present raw data calculated with both old and new 17O parameters in Supplementary Data File 3 for future recalculation of temperatures. Given that corrections to both measured unknowns and calibration samples within a given lab will likely be similar, we anticipate only small (<1–3 °C) variations between temperatures calculated using old and new 17O parameters. Analytical uncertainties on single samples are reported in terms of 1 S.E. (determined on a minimum of three replicates per sample), and locality or region average temperatures are determined for multiple specimens, and reported with 1σ uncertainties.

Strontium isotope analysis

Carbonate samples were also analyzed for strontium isotopic compositions following methods established in previous studies28. Briefly, a split from each homogenous carbonate powder was digested in 7.5 M HNO 3 and Sr was separated using column chromatography with Eichrom Sr resin (after a previous study28,54). The Sr elutions collected from column separations were loaded onto rhenium filaments and measured for 200 cycles on either a Finnigan MAT 262 or Thermo Scientific Triton PlusTM TIMS at the University of Michigan. Measurement sessions where 87Sr/86Sr values of the standard NIST SRM-987 deviated from the accepted value of 87Sr/86Sr = 0.710248 ± 0.000011 were normalized to that value (see Supplementary Data File 1). The long-term mean 87Sr/86Sr value for NIST SRM-987 was 0.710238 ± 0.000016 (1σ).

An age for each measured fossil was calculated by comparison to the most recent iteration of the LOWESS global seawater strontium isotope curve for the Late Cretaceous obtained and applied in a previous study28. Sample 87Sr/86Sr values were matched to the closest 87Sr/86Sr value for the Campanian/Maastrichtian portion of the mean LOWESS curve, and the analytical uncertainty of each strontium measurement was propagated through the uncertainty envelope of the LOWESS curve itself to provide the most conservative cumulative uncertainty on any given age (~0.45–1.5 Ma per sample).