A diver in scuba gear holds a coring device to collect a sample from a coral head on the seafloor in clear, light blue water.
A researcher collects a core from a coral head in Dry Tortugas National Park in the Gulf of Mexico in 2008. Credit: Kristine DeLong

In the global water cycle, water that evaporates from the ocean into the atmosphere is transported as water vapor. Some of this water returns directly to the ocean, while the rest eventually precipitates as rain, snow, or ice on land before much of it is ultimately recycled to the ocean via rivers and other sources. As water changes phase from liquid to vapor, the lighter, more abundant isotope of oxygen (16O) preferentially enters the vapor phase compared with the heavier, less abundant isotope (18O), and the reverse occurs when water vapor condenses to liquid water and ice. This variable partitioning of these stable oxygen isotopes by mass provides a means of tracing water as it moves through the water (hydrologic) cycle—a vital tool in studies of climate, meteorology, oceanography, and more.

A wide variety of research networks (e.g., the Global Network of Isotopes in Precipitation, Global Network of Isotopes in Rivers, and National Ecological Observatory Network) have measured—and maintain databases of—the oxygen isotope ratios (referred to as δ18O) of water on and above land (namely, in precipitation, rivers, and the atmosphere) to examine the cycling of water between the land and atmosphere. However, no such active observing network exists to document the oxygen isotope ratios of seawater (δ18Osw) needed to understand the cycling of water between the ocean and the atmosphere.

Measurements of the oxygen isotope ratios of seawater (δ18Osw) provide important information about the modern ocean and its relationship to the water cycle.

Measurements of δ18Osw provide important information about the modern ocean and its relationship to the water cycle. For example, δ18Osw can inform us about processes related to ocean circulation (upwelling and advection), riverine input into the oceans, ocean-atmosphere water exchange through precipitation and evaporation, and continental ice sheet volume on timescales spanning glacial–interglacial periods and longer.

In 2017, the PAGES (Past Global Changes) CoralHydro2k project was formed to investigate the variability of temperature and δ18Osw in the surface ocean during the past 2,000 years. Corals incorporate oxygen from seawater into their calcareous skeletons, thus preserving a record of the environment in which they live, and with the very limited coverage of modern δ18Osw measurements, corals offer vital information for oxygen isotopic variability in the ocean. The CoralHydro2k investigation combines measurements of δ18O in corals and of coral strontium-to-calcium ratios (Sr/Ca). Coral Sr/Ca is a seawater temperature proxy, whereas the δ18O preserved in coral skeletons varies depending on both the water temperature and the δ18Osw at the time the coral skeleton is formed. Therefore, δ18Osw can be calculated using the combined measurements of δ18O and Sr/Ca in corals, given the calibrations for the coral proxies (Sr/Ca and δ18O) to ocean conditions are known, yet verification of these δ18Osw reconstructions requires δ18Osw measurements.

To aid in this effort, we are creating an updated open-access seawater oxygen isotope database of modern δ18Osw measurements. Here we summarize our crowdsourcing efforts and describe the δ18Osw database to date.

A Host of Questions Await Isotopic Clues

Measurements of δ18Osw provide important information about the modern ocean, but they are sparse in space and time. Unlike meteorological observations on land, oceanic observations remained relatively limited and regionally focused until the past few decades. This is because most ocean observations are made by satellite remote sensing or by in situ measurements at coastal and island locations that have the infrastructure to support sustained observations of ocean surface properties.

These ocean observations generally include measurements of salinity; however, they rarely include δ18Osw because there is no cost-effective, easily deployable instrumentation to measure seawater isotopes in situ. Seawater samples must be taken back to a laboratory for isotopic analysis, and these data are rarely provided publicly in real time as other ocean observations are.

Modern δ18Osw data are also essential for calibrating proxies of past ocean variability in marine carbonates that are used in paleoclimate reconstructions.

Surface seawater δ18O covaries with salinity because precipitation and evaporation exert a similar influence on both variables. Lighter isotopes are preferentially evaporated—and heavier isotopes are preferentially precipitated—leaving the ocean isotopically heavier and saltier in regions dominated by evaporation (relative to precipitation) and isotopically lighter and fresher in regions dominated by precipitation. However, the strength of this relationship can vary across time and space, even within individual ocean basins, making salinity an imperfect proxy for δ18Osw [Conroy et al., 2017]. One reason for this is that δ18Osw is sensitive to changes in the source and transport pathway of atmospheric water vapor, whereas seawater salinity is not. Thus, measurements of δ18Osw taken independently of salinity measurements provide additional useful information for models of the ocean-climate system, yielding, for example, more accurate constraints on local moisture budgets and ocean mixing.

Modern δ18Osw data are also essential for calibrating proxies of past ocean variability in marine carbonates, such as corals, foraminifera, mollusks, ostracods, and coralline algae, that are used in paleoclimate reconstructions. 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 δ18Osw data for training the proxy system models that underlie those efforts. Such reconstruction and assimilation efforts enable scientists to extend climate records back into the preindustrial era, thereby contextualizing anthropogenic climate change and improving the skill of future climate projections.

Additionally, observational δ18Osw data are needed by climate model researchers running isotope-enabled Earth system models (e.g., NCAR iCESM, iHadCM3, ECHAM5-wiso); these data allow researchers to assess model performance and skill and provide model boundary conditions (in model configurations that include only active atmosphere and land surface). Given these wide-ranging applications, δ18Osw data are useful to research communities in oceanography, atmospheric science, geology, and geography alike. For these reasons, a comprehensive database of δ18Osw data that are publicly available and actively maintained is critically needed.

Seawater Isotope Data Enter the FAIR Era

A major effort to gather δ18Osw data was completed in the 1990s [Schmidt, 1999; Bigg and Rohling, 2000], resulting in the NASA Goddard Institute for Space Studies (GISS) Global Seawater Oxygen-18 Database, which includes more than 25,500 individual data points. That database was used to construct a global gridded data set of δ18Osw and to characterize regional relationships between δ18Osw and salinity [LeGrande and Schmidt, 2006]. It has subsequently been used in many studies. However, the support needed to maintain that database has been limited in recent years, and it is no longer being updated—the last δ18Osw measurements were added more than a decade ago in 2011.

During the past decade, a growing number of new δ18Osw data sets have been published, yet there is no active repository dedicated to archiving these δ18Osw data.

New isotope analyzers using cavity ring-down spectroscopy (an ultrasensitive laser-enabled form of spectroscopy) have reduced analytical costs and rejuvenated the collection and measurement of water δ18O. During the past decade, a growing number of new δ18Osw data sets, many collected using cavity ring-down spectroscopy, have been published, yet there is no active repository dedicated to archiving these δ18Osw data. As a result, authors often resort to providing their data in supplemental tables in journal articles or to merging their δ18Osw measurements with other geochemical data and submitting them to repositories dedicated to those other types of data. Researchers cannot easily find or access these “hidden” data sets, thus limiting their inclusion and usability for further research [Chamberlain et al., 2021]. Other δ18Osw data sets are publicly available and findable, but they are scattered across a myriad of repositories (e.g., GISS, PANGAEA, GEOTRACES, Waterisotopes.org, and EarthChem) and thus are not easily collated together for analysis.

CoralHydro2k is building upon previous PAGES 2k efforts, namely, Ocean2k [Tierney et al., 2015] and Iso2k [Konecky et al., 2020], which compiled published coral δ18O and other data into new machine-readable databases. To aid in the calibration and interpretation of these coral records and in recognition of the value of δ18Osw data to the broader Earth science community, the CoralHydro2k project is also collecting δ18Osw data.

We are collating these records in a new, machine-readable, and metadata-rich database consistent with findability, accessibility, interoperability, and reusability (FAIR) standards for digital assets. Funding agencies and many publishers are now requiring researchers to archive their data in FAIR-compliant, public repositories, providing yet another reason for the new database, as no such repository exists for seawater oxygen isotopes. Once the first version of the database is made public—slated for spring 2023—it will be accessible (here) via the NOAA World Data Service for Paleoclimatology. We are also working with EarthChem to set up a Seawater Oxygen Isotopes Community, whereby new δ18Osw data sets can be submitted and each data submission will be assigned a digital object identifier (DOI) so that the researchers who produced the δ18Osw data can be cited directly when their data are used by other researchers.

Gathering All the Data

We will continue adding to the new database as part of the ongoing CoralHydro2k project, with the goal of including as many δ18Osw data points as possible from the global ocean.

As of 1 May 2022, we have collected a total of 77 δ18Osw data sets and have added 5,664 measurements from 58 of those data sets to our database. Approximately 50% of these measurements are from hidden sources (e.g., journal supplemental tables), 35% are from public repositories (e.g., PANGAEA, Waterisotopes.org, EarthChem), and 15% are currently stored in the NASA Global Seawater Oxygen-18 Database. Nearly all of the measurements (94%) are from the surface ocean (upper 5 meters). We will continue adding to the new database for the coming year and beyond as part of the ongoing CoralHydro2k project, with the goal of including as many δ18Osw data points as possible from the global ocean. The first phase of the project has prioritized surface ocean data from the tropics; however, the database will ultimately include δ18Osw data from all depths and regions.

Our compilation thus far reveals the sparse distribution of surface δ18Osw observations in both space and time (Figure 1). There are vast ocean regions for which no δ18Osw measurements are available, including large swaths of the tropical oceans. In places where data exist, there are typically fewer than two measurements. Notable exceptions are Palau and the Galápagos Islands (marked with red circles in Figure 1), where researchers have collected some of the longest δ18Osw records with weekly sampling maintained for several years [Conroy et al., 2017].

Fig. 1. The locations of all instrumental δ18Osw in the NASA Global Seawater Oxygen-18 Database and select coral-derived δ18Osw observations from the CoralHydro2k δ18Osw Database, binned into 2° × 2° grid boxes, are shown here. Small gold circles indicate grid boxes for which two or fewer distinct months of δ18Osw observations are available. Larger solid circles denote locations for which more than two distinct months of observations are available; the color bar corresponds to the number of unique months observed. Blue stars denote the locations where the coral δ18O records selected for Figure 2 were collected. Click image for larger version.

Comparing δ18Osw outputs from several isotope-enabled models as well as coral-derived δ18Osw reconstructions from five coral study sites demonstrates the wide variability of δ18Osw among the different models and between models and reconstructions (Figure 2). The discrepancies among the models may be due to their structural differences (e.g., in their resolution, subgrid-scale parameterizations, or treatment of atmospheric exchange or ocean mixing processes). To reconcile these discrepancies, more direct measurements of δ18Osw data are needed to train the models and assess their skill.

Furthermore, at several locations (Little Cayman, Kiritimati, and Rarotonga), the coral-derived δ18Osw variability exceeds that of nearly all the model estimates (Figure 2). Large variability in coral-based δ18Osw has also been found relative to an isotope-enabled regional ocean model (isoROMS) [Stevenson et al., 2018]. More δ18Osw observations are needed to determine whether such model-reconstruction offsets are due to deficiencies in the models, uncertainties in the coral δ18Osw reconstructions associated with the calculation of δ18Osw from coral δ18O and Sr/Ca, or both.

Fig. 2. These plots compare coral-derived δ18Osw with simulated δ18Osw from isotope-enabled Earth system models and from a reanalysis product for the five island locations denoted by large blue stars in the map in Figure 1: Cocos (Keeling) in the eastern Indian Ocean, Rarotonga and Kiritimati in the central Pacific Ocean, Clipperton in the eastern Pacific Ocean, and Little Cayman in the Atlantic Ocean. Blue bars show the annual standard deviation of δ18Osw calculated from the monthly climatology of δ18Osw at each location in the coral archives. Orange bars show the National Center for Atmospheric Research Community Earth System Model Last Millennium Ensemble (1,000 years) [Brady et al., 2019]. Yellow bars show the NASA Goddard Institute for Space Studies E2-R last millennium simulation (ensemble member E4rhLMgTck; 255 years) [Colose et al., 2016]. Purple bars show the isoROMs Pacific Ocean simulation (44 years) [Stevenson et al., 2018]. Green bars show the 2018 Breitkreuz reanalysis (monthly climatology constrained by global monthly δ18Osw data collected from 1950 to 2011 and climatological salinity and temperature data collected from 1951 to 1980 [Breitkreuz et al., 2018a, 2018b]). The interannual standard deviation of δ18Osw was calculated from the 2- to 7-year bandpass-filtered time series of each data set, except for the Breitkreuz reanalysis data set (for which an interannual standard deviation cannot be calculated from the monthly climatology).

The CoralHydro2k Seawater δ18Osw Database Project is accepting data submissions as we continue populating the new database. All δ18Osw observations are welcomed (published and unpublished), regardless of the depth or location where the water samples were collected. We strongly encourage submissions with detailed metadata (analytical precision, standards and instrument used, etc.) as part of our commitment to generating a FAIR-aligned database. Researchers can submit their data to the CoralHydro2k δ18Osw database via our Qualtrics survey, where we also provide a YouTube video with instructions on how to submit your data. If you know of δ18Osw data that should be included in the database, please submit this information along with a DOI or citation via our Google Form. Following release of the first version of the database, it will be updated periodically with additional data.

We are confident that this new and growing seawater isotope database will become a vital tool for scientists as they work to paint a clearer picture of Earth’s dynamic water cycle and its relationship to the oceans and climate in the past, present, and future.

Acknowledgments

We thank the paleoclimate, paleoceanography, and oceanography communities for contributing their δ18Osw data. We thank the other members of the PAGES CoralHydro2k team for their efforts in building this δ18Osw database, especially the helpful comments and suggestions from Amy Wagner, Thomas Felis, Hali Kilbourne, and Emilie Dassié. Many thanks are owed to Erika Ornouski for her work in finding hidden δ18Osw data files. We are grateful to Kerstin Lehnert and the EarthChem team at Lamont Doherty Earth Observatory and Carrie Morrill and Bruce Bauer at NOAA Paleoclimatology for providing opportunities to host the new database. We also recognize the efforts of Gavin Schmidt, Eelco Rohling, Grant Bigg, and Allegra LeGrande in building and maintaining the first δ18Osw database.

References

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Author Information

Kristine DeLong (kdelong@lsu.edu), Department of Geography and Anthropology and Coastal Studies Institute, Louisiana State University, Baton Rouge; Alyssa Atwood and Andrea Moore, Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee; and Sara Sanchez, University of Colorado Boulder

Citation: DeLong, K. L., A. Atwood, A. Moore, and S. Sanchez (2022), Clues from the sea paint a picture of Earth’s water cycle, Eos, 103, https://doi.org/10.1029/2022EO220231. Published on 4 May 2022.
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