The PAGES CoralHydro2k Seawater δ¹⁸O Database: a FAIR-aligned compilation of seawater δ¹⁸O data to uncover “hidden” insights from the global ocean

The stable isotope values of seawater (δ¹⁸O and δ²H) provide valuable information on the exchange of water between the ocean, atmosphere, and cryosphere and on ocean mixing processes. As such, observational seawater δ¹⁸O and δ²H data place powerful constraints on hydrologic changes in the modern ocean. Seawater δ¹⁸O data are also essential for calibrating paleoclimate proxies based on the δ¹⁸O of marine carbonates and are an increasingly critical diagnostic tool for assessing model performance and skill in isotope-enabled climate models. Despite their broad value, no centralized and actively-curated database for this type of data exists, even though a growing number of new seawater δ¹⁸O datasets have been generated over the last decade. As such, many seawater δ¹⁸O datasets remain “hidden”. To improve the accessibility of seawater δ¹⁸O data for the Earth Science research community, the Past Global Changes (PAGES) CoralHydro2k project has created a new, machine-readable, and metadata-rich database of observational seawater δ¹⁸O data, paired with seawater δ²H and salinity data, that is compliant with findability, accessibility, interoperability, and reusability (FAIR) standards for digital assets. The data has been collected from public databases and repositories, direct researcher data submissions, scientific papers, and student theses. In total, the PAGES CoralHydro2k Seawater δ¹⁸O Database contains over 18 600 data points with extensive metadata that makes the database suitable for a myriad of research applications. For hidden data, we searched for and included all datasets within the global ocean. For public data, our data collation efforts were focused on the upper 50 m from 35° N to 35° S (to aid in CoralHydro2k's seawater δ¹⁸O reconstruction studies using δ¹⁸O and $\mathrm{Sr}/\mathrm{Ca}$ in tropical-subtropical coral skeletons). We also provide a set of best practices to the community for reporting seawater isotope data in the future. The database is available on the NOAA NCEI World Data Service for Paleoclimatology landing page: https://www.ncei.noaa.gov/access/paleo-search/study/34575 (last access: 11 February 2026; https://doi.org/10.25921/ap7d-2k16, Atwood et al., 2026). A Seawater Oxygen Isotopes Community was also developed within the EarthChem Library (https://www.earthchem.org/communities/seawater-oxygen-isotopes/, last access: 20 February 2026) to help researchers submit new datasets and obtain a dataset DOI. This template is aligned with the CoralHydro2k Seawater δ¹⁸O Database to facilitate future updates to the database.

1 Introduction

1.1 Progress and challenges in the synthesis of seawater oxygen isotope data

The stable isotope composition of water (δ¹⁸O and δ²H) provides a powerful tracer of the global water cycle, tracking water as it continuously cycles between the ocean, atmosphere, and land. The isotopic composition of seawater is nearly conservative when no phase change occurs. As water molecules undergo phase changes however, the lighter, more abundant isotope (¹⁶O and ¹H) is preferentially vaporized during evaporation with respect to the heavier, less abundant isotope (¹⁸O and ²H), while the heavy isotope is preferentially condensed during precipitation (Dansgaard, 1964). This partitioning of isotopes based on mass allows the isotope values (where ${\mathit{\delta }}^{\mathrm{18}}\mathrm{O}=\left[\frac{\frac{{}^{\mathrm{18}}\mathrm{O}}{{}^{\mathrm{16}}\mathrm{O}}\mathrm{sample}}{\frac{{}^{\mathrm{18}}\mathrm{O}}{{}^{\mathrm{16}}\mathrm{O}}\mathrm{standard}}-\mathrm{1}\right]\cdot \mathrm{1000})$ of water to be used as a tracer of the hydrologic cycle (Dansgaard, 1954; Galewsky et al., 2016; Gat, 1996). In the ocean, the isotope values of seawater (δ¹⁸O_sw and δ²H_sw) can provide valuable information on an array of processes, including heat and mass exchange with the atmosphere (via precipitation and evaporation), large-scale ocean circulation, and freshwater input from rivers and ice sheets (Akhoudas et al., 2021; Benetti et al., 2016; Benway and Mix, 2004; Biddle et al., 2019; Craig and Gordon, 1965; Dee et al., 2023; Frew et al., 2000; Imbrie et al., 1984; Jacobs et al., 1985; Lisiecki and Raymo, 2005; Meredith et al., 1999; Strain and Tan, 1993). δ¹⁸O_sw and δ²H_sw values can also provide insight into other ocean tracers such as salinity, since they covary strongly due to the influence of evaporation and precipitation on each of these variables (Craig and Gordon, 1965; LeGrande and Schmidt, 2011). However, because key processes act differentially on salinity as compared to the stable isotope values, δ¹⁸O_sw and δ²H_sw provide additional constraints on ocean mixing and the local moisture budget.

Seawater isotope values also create a common unit that uniquely links paleoclimate reconstructions to modern climate observations and isotope-enabled model simulations. Modern δ¹⁸O_sw data are essential for the calibration of paleoclimate proxies of past ocean variability based on the δ¹⁸O of marine carbonates such as corals, foraminifera, molluscs, ostracods, and coralline algae. Recent paleoclimate data assimilation efforts such as the Last Millennium Reanalysis project (e.g., Tardif et al., 2019) would greatly benefit from a spatial network of δ¹⁸O_sw data to improve quantification of proxy uncertainty and for training the proxy system models that underlie those efforts. Modern δ²H_sw data are used in the calibration of paleoceanographic proxies based on the δ²H of alkenones and other lipid biomarkers in marine sediments (e.g., Eglinton and Eglinton, 2008). When used in tandem with δ¹⁸O data (i.e., to calculate d-excess in surface ocean and overlying water vapor), δ²H data can be used to constrain evaporation parameters (e.g., Benetti et al., 2014). As such, observational and reconstruction efforts based on seawater isotope values enable scientists to better understand the underlying physics that govern the water cycle, and to extend hydroclimate records back to the preindustrial era, thus contextualizing anthropogenic climate change and improving the skill of future climate projections. In modern environments, δ¹⁸O in marine biominerals and δ²H in lipids can be used to trace plankton and animal movement and provide provenance for ecology, conservation, archaeology, and food forensics studies (Doubleday et al., 2022). Given these wide-ranging applications, seawater isotope data are used in a wide range of fields, including paleo and modern oceanography, atmospheric science, geology, marine biology, archaeology, and geography.

Observational δ¹⁸O_sw data can also be used to provide boundary conditions in climate models and to assess model performance and skill. The increasing integration of oxygen isotopes of water in climate models – from models of intermediate complexity to fully coupled Earth System Models (e.g., Blossey et al., 2010; Bong et al., 2024; Bony et al., 2008; Brady et al., 2019; Cauquoin et al., 2019; Dee et al., 2015; Field et al., 2014; Fiorella et al., 2021; Kurita et al., 2011; Lee and Fung, 2008; Noone and Simmonds, 2002; Nusbaumer et al., 2017; Risi et al., 2010, 2020, 2021; Schmidt et al., 2007; Tada et al., 2021; Wei et al., 2018; Werner et al., 2011; Yoshimura et al., 2008) – bolsters the interpretation of modern and paleoclimate observations, while also providing opportunities to test model performance in resolving key features of the hydrologic cycle, e.g., the representation of moisture transport, circulation, and surface water fluxes.

Paralleling recent advances in the numerical simulation of water isotopes, new analytical capabilities have also developed in recent years, including new in situ atmospheric measurement techniques and strategies (Finkenbiner et al., 2022; Gupta et al., 2009; Henze et al., 2022), and the development of global atmospheric data products from a variety of remote sensors (e.g., Diekmann et al., 2021; Schneider et al., 2022; Worden et al., 2019). As a result, measurements of water isotopes have become increasingly incorporated in coordinated observing networks and monitoring studies of precipitation and atmospheric water vapor, including the Global Network of Isotopes in Precipitation (http://www.iaea.org/services/networks/gnip. last access: 20 February 2026) and the National Ecological Observatory Network (http://www.neonscience.org/, last access: 20 February 2026).

However, no such coordinated observing network for seawater δ¹⁸O currently exists. Unlike meteorological observations on land, observations of ocean hydrological properties (e.g., precipitation, evaporation, and salinity) are largely either limited to the past few decades (via satellite remote sensing, the ARGO (Wong et al., 2020) and GOOS (Dexter and Summerhayes, 2010) programs, the TAO/TRITON array, and other mooring, drifter, and ship-of-opportunity measurements), or are confined to selected coastal and island locations that have the necessary infrastructure to support sustained in situ measurements of ocean surface properties. Furthermore, these ocean observations rarely include δ¹⁸O_sw because there is currently no cost-effective, easily deployable instrumentation to measure seawater isotopes in situ. Thus, seawater samples must be taken back to a laboratory for isotopic analysis. Despite these structural challenges, a growing number of δ¹⁸O_sw datasets have been generated in recent decades due to the accelerated collection of δ¹⁸O_sw samples, new instrumentation based on cavity-ring down spectroscopy (CRDS) with reduced analytical costs, the capability to measure both δ¹⁸O and δ²H in parallel, and new sampling devices that enable long-term seawater sample collections (e.g., Jannasch et al., 2004; Khare et al., 2021).

In recognition of the broad value of δ¹⁸O_sw data to the Earth Sciences, a major effort to gather δ¹⁸O_sw data occurred in the 1990s (Bigg and Rohling, 2000; Schmidt, 1999; Schmidt et al., 1999) and resulted in the development of the NASA's Goddard Institute for Space Studies (GISS) Global Seawater Oxygen-18 database (https://data.giss.nasa.gov/o18data/, last access: 20 February 2026), which contains over 26 000 global measurements of δ¹⁸O_sw (and some δ²H data) from the 1950s to 2000s. In 2006, that database was used to construct a global gridded dataset of δ¹⁸O_sw and to characterize regional relationships between δ¹⁸O_sw and salinity (LeGrande and Schmidt, 2006) and it has subsequently been used in a broad range of studies involving δ¹⁸O_sw. However, the NASA GISS database is no longer actively updated, with the last δ¹⁸O_sw measurement added in 2011. As a result, a growing number of new δ¹⁸O_sw datasets published since 2011 remain without an active δ¹⁸O_sw-specific data repository in which to archive the data. Researchers have instead provided the δ¹⁸O_sw data in the supplemental tables of journal articles, or have archived the δ¹⁸O_sw data with other geochemical data (e.g., coral δ¹⁸O), in data repositories such as the National Centers for Environmental Information (NCEI) for Paleoclimatology (https://www.ncei.noaa.gov/products/paleoclimatology, last access: 20 February 2026) and PANGAEA (https://www.pangaea.de/, last access: 20 February 2026). Because these datasets can be difficult to find, non-machine-readable, and/or decentralized, they are not easily accessible to the wide range of research communities that would benefit from this data (see a related review by Chamberlain et al., 2021). Furthermore, many publishers and several funding agencies now require researchers to archive their data in FAIR and public repositories. For these reasons, a comprehensive database of δ¹⁸O_sw data that is publicly available and actively maintained is critically needed.

1.2 The PAGES CoralHydro2k Seawater δ¹⁸O Database

CoralHydro2k and its Seawater δ¹⁸O Database started in 2017 as a project in Phase 3 of the Past Global Changes (PAGES) 2k network, a long-running initiative to study past global changes over the last 2000 years and to compile paleoclimate data in publicly available, machine-readable databases (Konecky et al., 2023; PAGES 2k Network Coordinators, 2017; Tierney et al., 2015) The CoralHydro2k project was formed to investigate the variability of hydrology and temperature in the tropical surface ocean during the past 2000 years based on the combination of coral δ¹⁸O, which varies with temperature and δ¹⁸O_sw, and the strontium-to-calcium ratio ($\mathrm{Sr}/\mathrm{Ca}$) in corals, which is a temperature proxy. To aid in the calibration and interpretation of the paired coral δ¹⁸O and $\mathrm{Sr}/\mathrm{Ca}$ records in the database, and derive coral-based reconstructions of seawater δ¹⁸O, the CoralHydro2k project started to compile δ¹⁸O_sw data.

In recognition of the growing number of δ¹⁸O_sw datasets that have been generated during the last two decades, the CoralHydro2k Seawater δ¹⁸O Database project was launched in 2020 to recover “hidden” δ¹⁸O_sw data that were not easily findable. During the past five years, we have integrated these records, along with any associated δ²H, salinity, and temperature data, with data from public databases and repositories to create a new, centralized, machine-readable, and metadata-rich database that aligns with findability, accessibility, interoperability, and reusability (FAIR) standards (Wilkinson et al., 2016). Here we provide a detailed description of the PAGES CoralHydro2k Seawater δ¹⁸O Database, highlighting the opportunities and limitations of the current database. We also provide a data submission template with comprehensive metadata for guiding the reporting of seawater isotope data in the future.

2 Methods

2.1 Collaborative model

CoralHydro2k included team members from the Phase 1 PAGES Ocean2k working group (Tierney et al., 2015) and Phase 2 Iso2k working group (Konecky et al., 2020) and many new members, particularly from the coral paleoclimate community. CoralHydro2k continued into Phases 3 and 4 of PAGES 2k, focusing on reconstructing past changes in tropical ocean temperature and hydroclimate using paired $\mathrm{Sr}/\mathrm{Ca}$ and δ¹⁸O from coral archives over the last 2000 years (Hargreaves et al., 2020; Walter et al., 2023). Recurring calls went out within the international paleoclimate community for working group members, coral experts, and paleo data assimilation experts to join the effort with monthly teleconference meetings and one in-person meeting in 2019 (Hargreaves et al., 2020). As a result, the CoralHydro2k database was produced, a global, actively curated compilation of coral δ¹⁸O and $\mathrm{Sr}/\mathrm{Ca}$ proxy records of tropical ocean hydrology and temperature for the Common Era (Walter et al., 2022, 2023). A number of sub-projects were developed in conjunction with CoralHydro2k, including a project to develop new proxy system models (PSM) for coral δ¹⁸O. The group working on this sub-project realized that the spatial and temporal coverage of observational δ¹⁸O_sw data were too sparse to integrate into the PSM framework and that many new δ¹⁸O_sw datasets produced during the last few decades are not easily findable or accessible.

CoralHydro2k thus formed a new sub-project in 2020 to compile existing seawater δ¹⁸O data with rich metadata following FAIR standards (Atwood et al., 2024; DeLong et al., 2022). Researchers were invited to submit their data to the CoralHydro2k Seawater δ¹⁸O Database via a Qualtrics survey and accompanying YouTube video that provided instructions on how to submit data.

The workload for assembling the seawater data and metadata was performed by CoralHydro2k members and new members of the Seawater δ¹⁸O Database sub-project. The team was made of volunteer scientists from all academic levels, including undergraduate and graduate students, postdoctoral researchers, and early- to senior-level scientists from several international academic and research institutions. The work was completed remotely in synchronous working sessions and asynchronously across several virtual platforms (Google Suite, Slack, and Zoom). Data discovery, metadata protocols, and compilation were done collaboratively as the project progressed.

2.2 Data curation in the EarthChem Seawater Oxygen Isotopes Community

To facilitate future curation of seawater isotope data by the research community, we established the Seawater Oxygen Isotopes Community (https://www.earthchem.org/communities/seawater-oxygen-isotopes/, last access: 20 February 2026) in the EarthChem Library (ECL), a data repository that archives, publishes, and provides access to data in the geosciences. The ECL offers a suite of services for data preservation and access, including long-term archiving and data registration with a Digital Object Identifier (DOI). Through the new Seawater Oxygen Isotopes Community, new seawater δ¹⁸O (and δ²H) datasets can be submitted and assigned a DOI, which allows the datasets to be cited and tracked when used by other researchers. The CoralHydro2k members promoted this new database at international conferences in the United States and Europe, in the PAGES newsletter (Atwood et al., 2024), and in Eos, the monthly magazine of the American Geophysical Union (AGU) (DeLong et al., 2022).

2.3 Data aggregation and formatting

The CoralHydro2k Seawater δ¹⁸O Database was designed to be as inclusive and comprehensive as possible in its record-selection criteria to support the project goal of developing a FAIR database of global seawater δ¹⁸O measurements, paired with δ²H and salinity measurements, and to include as much “hidden” data as possible. Thus, the Seawater δ¹⁸O Database selection criteria were less restrictive than other PAGES 2k efforts, and the database includes data from peer-reviewed scientific literature, student theses and dissertations, public data repositories, and direct author submission.

For hidden data, we searched for and included datasets spanning all depths and all latitudes across the global ocean. For publicly available data, given the substantial time commitment involved in finding and adding the extensive metadata, we typically only included data from the upper 50 m between 35° N to 35° S (to aid in CoralHydro2k's seawater δ¹⁸O reconstruction studies using δ¹⁸O and $\mathrm{Sr}/\mathrm{Ca}$ in tropical-subtropical shallow-water corals). In subsequent versions of the database, we will target the inclusion of all publicly available datasets.

2.4 Metadata, quality control, and best practices for future data reporting

In alignment with FAIR data principles, the Seawater δ¹⁸O Database contains extensive metadata. 12 metadata fields are required, with an additional 38 optional metadata fields that provide important supporting information on the sampling site, sample collection and storage, isotope analysis method, evaporation and correction flags, and error information. Where available, paired seawater δ²H, salinity, and temperature data are also reported. The full set of required and optional metadata fields in the database are intended to establish a set of best practices for future reporting of seawater isotope data. The Scientific Committee on Oceanic Research (SCOR) working group 171 “Towards best practices for Measuring and Archiving Stable Isotopes in Seawater (MASIS)” further intends to build upon the set of best practices established for the CoralHydro2k Seawater δ¹⁸O Database. We note that many of the optional metadata fields in the database are essential for proper quality control, inter-comparison, and interpretability across datasets. However, this information was often not reported in the original datasets and publications, and thus this metadata could not be made mandatory without greatly restricting the number of datasets in the database. However, we strongly encourage the inclusion of all metadata fields in this database (required and optional) for future reporting of seawater isotope data. To assist researchers with this process, a blank data submission template (with examples) is provided with the CoralHydro2k Seawater δ¹⁸O Database.

The metadata fields are described in Tables 1–2. The CoralHydro2k Seawater δ¹⁸O Database team implemented several rounds of quality control measures for the data and metadata. Following the Iso2k database procedure (Konecky et al., 2020), each metadata field has an associated quality control certification “Level” from 1 to 5, described below and in Table 1. Level 1 and Level 2 metadata fields constitute “essential” metadata, and if a dataset lacked one of these fields, it was excluded from the database.

• Level 1 fields are required for inclusion in the database and they contain standardized vocabularies, according to Table 2. They are recommended as primary fields for filtering and querying records in the database. They were subject to the highest Quality Control (QC) standard. Examples of Level 1 metadata are: “Collection year”, “Collection month”, “Latitude”, “Longitude”, and “Depth”.

• Level 2 metadata fields are required for inclusion, but they are not generalizable enough to use standardized vocabularies. They were subject to the highest QC standard and the metadata were obtained from the original publication or data source. An example of Level 2 metadata is “Site name or geographic area”.

• Level 3 metadata fields add important supplementary information related to the seawater δ¹⁸O measurements. They contain standardized vocabularies and can be used as secondary fields for filtering and querying the database; however, they are generally not available for all records and thus not required for inclusion in the database. They were subject to the highest QC standard. Examples of Level 3 metadata are: “Collection day”, “δ¹⁸O error”, “δ¹⁸O analysis technique”, “Water isotope analysis date”, “δ²H value”, “Temperature value”, and “Salinity value”.

• Level 4 metadata fields also add important supplementary information related to the seawater δ¹⁸O data, but they are not generalizable enough to use standardized vocabularies. They are also generally not available for all records and thus not required for inclusion in the database. They were subject to the highest QC standard. Examples of Level 4 metadata are: “δ¹⁸O correction notes”, “δ¹⁸O error notes”, “Sample ID”, “Publication citation”, “Dataset citation”, “Cruise ID”, “δ¹⁸O analysis location”, “Sample collection, processing, and storage notes”, and “Water isotope analysis notes”.

• Level 5 metadata fields may be useful to some users of the database, but they are generally not available for all records and thus not required for inclusion in the database. In many cases, these fields contain freeform text with direct quotes from the original publications. During the QC certification process, these fields were checked against the original publication and a quote or summary of the relevant information was provided in the database, but the information provided may not be comprehensive. The Level 5 metadata are: “Location description”, “Location type”, and “Temperature/salinity notes”.

Table 1 Description of all metadata fields in the PAGES CoralHydro2k Seawater δ¹⁸O Database. Bold text indicates required fields in the database (Level 1 and 2).

^a There are four cases in which datasets obtained from the GISS database were not clearly associated with a publication, or the provided reference did not match the dataset. In those cases, the Schmidt et al. (1999) database citation is provided in the “Dataset citation” (Level 4) metadata field and the author letters “SC” are used in the unique CoralHydro2k ID to reference that citation (SC99AO0001, SC99PO0001, SC99IO0001, SC99GI0001). Additional details about the citations and data provenance appear in the “Data provenance notes” (Level 4) metadata field. ^b Corrections were applied to the δ¹⁸O data in several datasets in the NASA GISS δ¹⁸O_sw database to standardize the data based on deep water masses to correct for changes in standards, different analysis techniques, and other systematic errors. Corrections were also applied to the δ¹⁸O and δ²H data in the Reverdin et al. (2022) LOCEAN database to account for the effect of salt on the IRMS and CRDS analyses (Benetti et al., 2017b). According to Reverdin et al. (2022): “these corrections could present differences with datasets processed in other institutions without this proposed adjustment of up to 0.10 ‰ in δ¹⁸O and 0.20 ‰ in δ²H.” The adjusted LOCEAN CRDS δ¹⁸O data thus may be higher (more enriched in heavy isotopes) than other CRDS and IRMS data. Corrections were also applied to some of the samples in the LOCEAN database to adjust for minor evaporation biases, based on the deviation from the expected relationship between d-excess and salinity (see Sect. 2.5 in Reverdin et al. (2022) and Appendix B in Benetti et al. (2017a)). The correction values were reported in the GISS database, but not the LOCEAN database. In the current CoralHydro2k database, all corrections are noted by the δ¹⁸O and δ²H correction flags and, where available, the values of the corrections are reported in the “δ¹⁸O correction values” and “δ²H correction values” (Level 3) metadata fields so the user can remove the corrections if desired. Any accompanying information about how and why the correction was made is reported in the “δ¹⁸O correction notes” and δ²H correction notes” (Level 4) metadata fields.

Table 2 Standardized controlled vocabulary options for metadata fields in the database.

3 Key characteristics of the seawater δ¹⁸O data

3.1 Spatial and temporal coverage of the database

The CoralHydro2k Seawater δ¹⁸O Database contains 18 598 data points from 106 datasets (Fig. 1A, B). 53 % of the data (9862 data points) is categorized as “hidden” data (i.e., data not currently available in public databases or public repositories; Fig. 1C, D), and the remaining 47 % of the data is from public databases or public repositories (Table 3). 10,407 measurements (56 %) are from the sea surface (depth ≤ 5 m; Fig. 1A), 3693 (20 %) are from the mixed layer (between 5–50 m), and 4498 (24 %) are below 50 m. The time span of the database covers 1972 to 2021 (Fig. 2) and the depth range covers the surface to 5797 m below sea level. The earliest data point in the database was collected in September 1972 and the most recent data point was collected on 8 October 2021. 3480 data points (19 %) were collected before the year 2000, and 15 118 data points (81 %) were collected on or after the year 2000 (Fig. 2). Because the addition of public datasets focused on the region between 35° N and 35° S, 75 % of the measurements in the database are located within the tropical-subtropical region.

Figure 1 Locations and seawater δ¹⁸O values of data in the database: (A) all surface ocean data (upper 5 m of the water column), (B) all subsurface ocean data (below 5 m), (C) the hidden surface ocean data only (upper 5 m), (D) the hidden subsurface ocean data only (below 5 m).

Figure 2 Sample collection dates of the seawater δ¹⁸O data in the database from (A) the surface ocean (upper 5 m) and (B) the subsurface ocean (below 5 m).

In addition to δ¹⁸O measurements, the database also includes paired δ²H, salinity, and temperature measurements when available. 16 098 data points (87 %) have paired salinity values (Fig. 3A), 13 871 data points (75 %) have paired temperature values (Fig. 3B), and 9769 data points (53 %) have paired δ²H measurements (Fig. 3C). 185 measurements have an evaporation flag (Fig. 3D), which allows the user to filter out samples that may be influenced by post-collection evaporation from the database.

Figure 3 Locations and seawater δ¹⁸O values of the data in the database with (A) paired salinity measurements, (B) paired temperature measurements, (C) paired δ²H measurements, and (D) an evaporation flag.

Compared to the seawater δ¹⁸O database presented in LeGrande and Schmidt (2006), data coverage in the surface ocean (upper 5 m) is substantially improved in the tropics and subtropics, particularly in the northern Indian Ocean, the eastern Atlantic Ocean, the northeast coast of South America, the Mediterranean Sea, and the equatorial Pacific Ocean. However, poor data coverage still exists in the western Pacific Ocean and Maritime Continent region, the southeastern Indian Ocean, and the subtropical Pacific Ocean regions in both hemispheres. Below 5 m depth, the data coverage is even more limited (Fig. 1B). At all depths, regions with reasonable spatial coverage of δ¹⁸O_sw data contain limited temporal coverage (Fig. 4A). Typically, only a few years of regular measurements are available from the most highly sampled regions. For example, only 21 % of locations contain at least 12 measurements spanning two years within a 2° latitude × 2° longitude grid box and only 8 % of the grid boxes contain data that cover at least 50 % of the annual cycle (i.e., 6/12 months of the calendar year; Fig. 4B). While the coverage of seawater isotope data has been growing over the last decade, these measurements are still sparse in space and time, thus highlighting the need for globally coordinated sampling campaigns and archiving efforts.

Figure 4 Temporal distribution of near-surface (upper 5 m) δ¹⁸O_sw measurements in the database, aggregated in 2° × 2° (latitude × longitude) grid boxes. (A) Total number of δ¹⁸O_sw measurements in each grid box. (B) Fraction of calendar year with δ¹⁸O_sw measurements.

Outside of the tropics and subtropics, the coverage of δ¹⁸O data in the CoralHydro2k Seawater δ¹⁸O Database is sparser, since only hidden datasets were collected from all latitudes and all depths across the global ocean, while datasets from public repositories were only incorporated into the database if the measurements were made in the upper 50 m between 35° N and 35° S. Future database development efforts will include incorporating additional hidden and public datasets.

3.2 Data-model comparisons of the seawater δ¹⁸O data

To assess how the δ¹⁸O_sw data in the database compares with isotope-enabled climate model simulations and other products, we compare the climatological annual cycle in δ¹⁸O_sw at different island sites using four data products: two simulations of isotope-enabled General Circulation Models (the National Center for Atmospheric Research Community Earth System Model Last Millennium Ensemble (1000 years; Brady et al., 2019) and the NASA Goddard Institute for Space Studies E2-R last millennium simulation (ensemble member E4rhLMgTck; 255 years; Colose et al., 2016)), a regional ocean model of the Pacific called isoROMS (44 years; Stevenson et al., 2018), and a gridded dataset of global monthly mean δ¹⁸O_sw based on data assimilation with the MITgcm (Breitkreuz et al., 2018). The Breitkreuz dataset is based on a 400-year quasi-equilibrated simulation of a water isotope-enabled global ocean general circulation model constrained by global monthly δ¹⁸O_sw data collected from 1950 to 2011 and climatological salinity and temperature data collected from 1951 to 1980.

The characteristics of δ¹⁸O_sw variability at the four selected sites in the tropical Pacific and Atlantic Oceans vary widely across the different data products and the δ¹⁸O_sw observations from the CoralHydro2k Seawater δ¹⁸O Database, with large differences in both the amplitude and phase of the annual cycle of δ¹⁸O_sw (Fig. 5). These differences could be due to deficiencies in the models (associated with subgrid-scale parameterizations, treatment of atmospheric exchange and ocean mixing processes, and/or the limited spatial resolution of the models not capturing the local influence of evaporation, precipitation, runoff, and upwelling at the sampling sites), and/or uncertainties in the observational data given the low temporal resolution of the δ¹⁸O_sw measurements. Clearly, more observational data is needed to determine the source of the discrepancies, pointing to the need for more coordinated and sustained seawater isotope sampling programs. Seawater isotope sampling at multinational observing systems that are already in place, such as the Tropical Pacific Observing System (TPOS), Bermuda Atlantic Time-series Study (BATS), GO-SHIP, and GEOTRACES, could expand and complement existing observational programs. For example, incorporating new sampling devices such as long-term osmotically pumped fluid samplers (Jannasch et al., 2004; Khare et al., 2021) could provide a relatively straightforward way to add δ¹⁸O_sw measurements to existing programs. The development of sustained seawater isotope measurements at a network of observational hotspots around the global ocean would provide powerful new constraints on hydrologic changes in the modern ocean, generating data that could be used to test theoretical predictions, assess climate model performance and skill, and calibrate paleoclimate proxies for improved paleoclimate reconstruction.

Figure 5 Monthly climatology of δ¹⁸O_sw at four island locations: Kiritimati Atoll in the Central Pacific Ocean, Palau in the western Pacific Ocean, Puerto Rico in the Caribbean Sea, and Dry Tortugas in the Gulf of Mexico. Five data sources are shown: observed δ¹⁸O_sw (from this database; black dashed line), simulated δ¹⁸O_sw from iCESM, iGISS, and isoROMS, and δ¹⁸O_sw from the reanalysis product of Breitkreuz et al. (2018) (purple; monthly climatology constrained by observed monthly δ¹⁸O_sw data collected from 1950 to 2011 and climatological salinity and temperature data collected from 1951 to 1980). The three Earth system models are the National Center for Atmospheric Research Community Earth System Model Last Millennium Ensemble (1000 years; red) (Brady et al., 2019), the NASA Goddard Institute for Space Studies E2-R last millennium simulation (ensemble member E4rhLMgTck; 255 years; yellow) (Colose et al., 2016), and the isoROMs Pacific Ocean simulation (44 years; blue) (Stevenson et al., 2018).

4 Applications of the database

While the primary motivation of the CoralHydro2k Seawater δ¹⁸O Database was for coral paleoclimate research, this database was designed to be useful to researchers from a wide range of disciplines, including paleoceanography and paleoclimatology, oceanography, marine biology, Earth science, and climatology. Of relevance to paleoclimate applications, the pairing of seawater oxygen isotope data with salinity data can provide transfer equations for reconstructing past salinity variations (e.g., Gagan et al., 1998; Kilbourne et al., 2004; McCulloch et al., 1994; Ren et al., 2003). For example: coral $\mathrm{Sr}/\mathrm{Ca}$, a temperature proxy, paired with coral δ¹⁸O, a proxy for both temperature and δ¹⁸O_sw, can be used to remove the temperature component from the coral δ¹⁸O signal. The derived δ¹⁸O_sw can then be converted to salinity using the local δ¹⁸O_sw to salinity transfer equation (e.g., Kilbourne et al., 2004). The same method can be applied to foraminifera $\mathrm{Mg}/\mathrm{Ca}$ and δ¹⁸O records to reconstruct salinity variations and can also potentially be applied to bivalves, coralline algae, ostracods, and otoliths (e.g., Light et al., 2018; Schmidt and Lynch-Stieglitz, 2011; Stott et al., 2004; Trofimova et al., 2020; Warner et al., 2022). Many studies have used this paired approach to reconstruct δ¹⁸O_sw variations for a wide range of time scales (e.g., Brocas et al., 2019; Felis et al., 2009; Giry et al., 2013; Gorman et al., 2012; Hereid et al., 2013; Knebel et al., 2024; Wu et al., 2013); however, few studies have been able to validate their reconstructions with observed δ¹⁸O_sw records that span more than one year (Conroy et al., 2017; O'Connor et al., 2021). Instead, most studies use reanalysis products such as Simple Ocean Data Assimilation (SODA) (Carton et al., 2000, 2018), or satellite-derived sea surface salinity (SSS) products (e.g., NASA Aquarius, NASA SMOS) (Boutin et al., 2021) for validating the reconstructions (e.g., Cahyarini et al., 2008; Harbott et al., 2023; Hetzinger et al., 2006). Typically, these applications assume that the salinity versus δ¹⁸O_sw relationship is stable in time; however, this assumption has been shown to break down in many instances: e.g., with temporal variations observed between monsoon and non-monsoon seasons (Ghosh et al., 2013; McConnell et al., 2009), in regions affected by sea ice formation and melting (Strain and Tan, 1993), and under variations in ocean advection, upwelling, and ocean-atmosphere interactions (Conroy et al., 2017; Rohling and Bigg, 1998).

The CoralHydro2k Seawater δ¹⁸O database can be leveraged to explore the relationship between these two parameters by combining the seawater δ¹⁸O data with the paired salinity data in the database (e.g., Conroy et al., 2017; Durack et al., 2012; LeGrande and Schmidt, 2006; Wagner and Slowey, 2011). In this way, the Seawater δ¹⁸O Database also has important applications to modern oceanography. By allowing these relationships to be more comprehensively assessed across space and time, this database (alongside future improvements in data coverage) will have important applications to resolving past and present oceanographic variations and change.

Additionally, the CoralHydro2k Seawater δ¹⁸O Database can be used in proxy-system model development, paleo-data assimilation, and comparison studies between proxy reconstructions and climate model output (Dee et al., 2023; Evans et al., 2013; Reed et al., 2022; Sanchez et al., 2021; Smerdon, 2012; Stevenson et al., 2018; Thompson et al., 2011). Proxy-derived δ¹⁸O_sw data can be directly compared with simulations from isotope-enabled models as part of the validation process and to understand oxygen isotope fractionation processes within the hydrological cycle (Dee et al., 2015; Stevenson et al., 2015, 2023). Furthermore, the proxy-derived salinity reconstructions can be compared with reanalysis and other salinity data products, such as SODA, as a separate validation step (e.g., Cahyarini et al., 2008). Finally, the Seawater δ¹⁸O Database offers the opportunity for improved proxy system models with rigorous uncertainty quantification of proxy-derived estimates of salinity. With such estimates, long reconstructions of salinity would provide valuable insights into the low frequency variability of the hydrological cycle over the data sparse tropical oceans. This Seawater δ¹⁸O Database is the most comprehensive to date and will be updated as new datasets are published to support ongoing research (see Sect. 6).

5 Code and data availability

5.1 Accessing the database

The CoralHydro2k Seawater δ¹⁸O Database follows the FAIR data principles (Wilkinson et al., 2016) that strive to make scholarly data findable, accessible, interoperable, and reusable. The CoralHydro2k Seawater δ¹⁸O Database uses the Comma Separated Values (*.CSV) file format, a machine-readable format for archiving and describing seawater isotope data. Access to the database has been granted for reviewers and editors during the review phase. The data are also available upon request for members of the public that wish to participate in the review process by emailing the corresponding author. The database is available on the NOAA NCEI World Data Service for Paleoclimatology landing page: https://www.ncei.noaa.gov/access/paleo-search/study/34575 (last access: 11 February 2026) (https://doi.org/10.25921/ap7d-2k16, Atwood et al., 2026). We recommend accessing the database through the NOAA NCEI landing page to find the latest instructions on using the database.

5.2 Code availability

Example Jupyter notebooks and MATLAB scripts are available on the CoralHydro2k Seawater Database GitHub page: https://github.com/CoralHydro2k/ch2kSeawater_Database (last access: 11 February 2026, and Zenodo at https://doi.org/10.5281/zenodo.18828491, Hargreaves, 2026) to help users search, filter, and visualize the database. We encourage users of the database to share their scripts on GitHub as well for improved access.

5.3 Underlying data sources

The CoralHydro2k Seawater δ¹⁸O Database includes records (0–50 mbsl, 35° N to 35° S) from ten international databases, including GEOTRACES, NASA GISS, PANGAEA, CISE LOCEAN, GLODAPv2, WIDB, NOAA NCEI, and BODC (Table 3). Literature searches were also conducted to find hidden seawater δ¹⁸O data (from all depths and latitudes) published only in tables and supplemental data files of published papers, theses, and dissertations. Data was also sourced from author contributions sent directly to this project or the EarthChem community (2026): https://earthchem.org/communities/seawater-oxygen-isotopes (last access: 11 February 2026). Users of this database should adhere to the data use policies for the underlying data sources (see Table 3 and Supplement Sects. S1–S4).

Table 3 Databases included.

6 Submission of new datasets and versioning scheme

The CoralHydro2k Seawater δ¹⁸O project will accept data submissions for updates to the database. All seawater δ¹⁸O observations are welcomed regardless of location or water depth. To facilitate this process, a Seawater Oxygen Isotopes Community was developed within the EarthChem Library, an open-access repository for geochemical datasets (https://earthchem.org/communities/seawater-oxygen-isotopes, last access: 11 February 2026), where researchers can submit their seawater isotope data and obtain a dataset DOI. The Seawater Oxygen Isotopes Community contains a template that can be downloaded to help researchers submit their data. This template is aligned with the CoralHydro2k Seawater δ¹⁸O Database to facilitate future updates to the database. The template has a README tab in the Microsoft Excel file with details on the template and an example. We hope that the creation of this site helps researchers publish their seawater isotope datasets, thus minimizing the number of ”hidden” datasets.

The initial release of the CoralHydro2k Seawater δ¹⁸O Database is Version 1.0.0 for this publication. With new submissions, the database will grow as new datasets are added. Database users who find errors in the database can use the “Report an issue” option in the GitHub site. Datasets submitted to the Seawater Oxygen Isotopes Community within the EarthChem Library (https://earthchem.org/communities/seawater-oxygen-isotopes) can be updated through that site.

As the CoralHydro2k Seawater δ¹⁸O Database is updated, it will be versioned following the scheme used by other PAGES data collection projects (Ahmed et al., 2013; Emile-Geay et al., 2017; Kaufman et al., 2020; McKay and Kaufman, 2014; Walter et al., 2023). The version number has three counters in the following form: C₁.C₂.C₃, where C₁, C₂, and C₃ are incrementing integers. When C₁ increases, C₂ and C₃ reset to zero. When C₂ increases, C₃ resets to zero. C₁ represents the number of publications describing the database. C₂ increments each time the set of records in the database changes (addition or removal of a dataset). C₃ increments when the data or metadata within the dataset changes, but the set of records remains the same. Upon updates, extensions, or corrections to the database, rather than issuing errata to this publication, changes will be included in subsequent versions of the database and updated and described through the online data repository.

7 Citation

This CoralHydro2k Seawater δ¹⁸O Database descriptor publication should be cited when the database is used in whole or in part, including its metadata fields, for subsequent studies. We encourage end users of this database to also cite the original publications and/or data sources of the underlying primary data (Table 3). To facilitate this process, citation information for every data point is included in the metadata, including a full citation and DOI of the original publication, as well as a dataset citation and DOI for the original public archive of the data. Researchers should also adhere to the data use policies for the underlying data sources (see Supplement Sects. S1–S4).

8 Conclusions and anticipated applications of the Seawater δ¹⁸O Database

Observational seawater δ¹⁸O and δ²H data can place powerful constraints on the global water cycle, providing valuable information on the exchange of water between the ocean, atmosphere, and cryosphere, as well as on ocean-mixing processes. As such, these data provide an additional set of constraints for understanding the complex hydrologic system, beyond what standard oceanographic variables like temperature and salinity can offer. They also provide a “common currency” that links paleoclimate reconstructions, modern climate observations, and isotope-enabled model simulations, allowing hydrologic processes to be evaluated on a wide range of time and spatial scales. Given the broad value of this data, and the growing number of seawater δ¹⁸O and δ²H datasets that have been generated since 2011, the CoralHydro2k Seawater δ¹⁸O Database was developed to improve the accessibility of seawater isotope data for the Earth Science research community. This new, machine-readable, and metadata-rich database contains over 18 600 observational seawater δ¹⁸O data points, paired with seawater δ²H and salinity data and extensive metadata that makes the database suitable for a myriad of research applications. The metadata template also provides a set of best practices for reporting seawater isotope data in future studies.

The CoralHydro2k Seawater δ¹⁸O Database and its extensive metadata can provide insight into the multiple processes that impact seawater δ¹⁸O and δ²H. Furthermore, the database can be used to better constrain the relationship between δ¹⁸O_sw and salinity in the global ocean, and (in conjunction with future improvements in data coverage) provide insight into how this relationship varies in space and time. The database also provides updated seawater δ¹⁸O and δ²H data critical for the calibration and validation of paleoclimate reconstructions using δ¹⁸O and δ²H to reconstruct past ocean temperature and salinity variations. For example, recent paleoclimate data assimilation efforts would greatly benefit from a spatial network of observational δ¹⁸O_sw data for training the proxy system models that underlie those efforts. This database could also be used to construct a new gridded dataset of δ¹⁸O_sw to update that of LeGrande and Schmidt (2006), which has been widely used for providing climate model boundary conditions and to assess model performance and skill in resolving key features of the hydrologic cycle. In this way, the PAGES CoralHydro2k Seawater δ¹⁸O Database can be used in a wide variety of applications to bolster our understanding of the modern climate system, while also providing new insights into past and future climate variability and change.