The goal of the present project is to use oxygen-18 to map water masses North of Iceland. Here we report on findings from field campaigns carried out over the period from 1988 to 2019. The samples were collected on Icelandic vessel cruises from waters that currents bring from the south and from the north and are found west and north of Iceland.  An emphasis was placed on low salinity waters that characterize the Polar Water of the East Greenland Current. The data cover all seasons and a salinity range from 29.1 to 35.3.  The observations generally also include biogeochemical constituents: dissolved oxygen and the nutrients phosphate, nitrate, and silicate (Olafsson, Olafsdottir et al. 2010).   

Oxygen isotope measurements were carried out at the Science Institute, University of Iceland. Prior to 2007 we used a Finngan MAT 251 Mass-spectrometer and extracted oxygen from the water by equilibrating degassed water with a small amount of CO₂ gas (Epstein and Mayeda, 1953). After 2007 the measurements were performed on a continuous flow Delta V Advantage mass-spectrometer, with a Gas bench device. The accuracy of the measurements is better than 0.05‰. 

We examine the δ¹⁸O -S relationships for variations with seasons and time. The surface waters of the observed regions are seasonally productive but with different winter nutrient concentrations to support phytoplankton spring blooms. We examine variations in the seasonal δ¹⁸O-nutrient relationships.

Epstein, S. and Mayeda, T.K. (1953). Variation in O18 content of waters from natural sources. Geochim Cosmochim. Acta 4:213-224. 

Olafsson, J., S. R. Olafsdottir, A. Benoit-Cattin and T. Takahashi (2010). "The Irminger Sea and the Iceland Sea time series measurements of sea water carbon and nutrient chemistry 1983–2008." Earth Syst. Sci. Data 2(1): 99-104.

How to cite: Sveinbjornsdottir, A., Olafsson, J., and Olafsdottir, S.: Observation of Oxygen-18 in Ocean Waters in the Vicinity of Iceland , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10953, https://doi.org/10.5194/egusphere-egu24-10953, 2024.

Since 2010 seawater samples for stable carbon and water isotope measurements were collected during various hydrographic cruises in the NE Atlantic between 29° and 60°N and 9° and 33°W, including pluriannual sampling in the Madeira basin, along the western Iberian margin and along the OVIDE/BOCATS (A25) transect between Portugal and Greenland. The samples cover the complete depth range at the respective station. For all samples δ¹⁸O was analyzed, whereas δ²H (δD) measurements were more limited and mainly focused on the wide range of water masses encountered along the OVIDE/BOCATS transect. Carbon isotope values for the dissolved inorganic carbon (δ¹³C-DIC) was monitored over several years at selected stations of the OVIDE/BOCATS transect and in the waters along the Portuguese margin. For some stations along the OVIDE/BOCATS transects, the results of intercomparison measurements between the laboratories at the GeoZentrum Nordbayern (Erlangen) and at LOCEAN (Paris) will also be presented.

The δ¹³C-DIC profiles show a clear signal of anthropogenic carbon entering the water column in the NE Atlantic and leading to lower isotopic values. Whereas data obtained for samples collected in 2010 more or less agree with the data from previous decades compiled in the GLODAP database, shifts to lower values became apparent in the subsurface waters already in 2012. The signal transfer is accelerated in the subsequent years with data from 2016 onwards showing penetration of anthropogenic carbon down to 2000 m and already all the way south to 31°N (Madeira basin). New data from a cruise to the southwestern Portuguese margin in 2022 indicate that the changes now already penetrate down to 2200 m.

Changes in the δ¹⁸O/ δ²H data are less obvious and mostly linked to the subpolar gyre and water mass changes associated with the “North Atlantic cold blob” between winter 2013-2014 and 2016. The presence of surface and subsurface waters with lower isotopic signals clearly tracks the eastward displacement of the subarctic front in 2014 and 2016. Likewise, the front’s subsequent retraction to the west is reflected in the data from 2018 and 2021. Low δ¹⁸O and  δ²H values in depths down to 300 m in the region between the Rockall Plateau and the Reykjanes ridge also clearly distinguish the subpolar mode water formed during the previous “cold blob” winters. On the other hand, and in agreement with the δ¹³C-DIC evidence, hardly any isotope signal changes are observed in the depths of the North Atlantic Deep Water (NADW) and the Northeast Atlantic Bottom Water (NEABW).

How to cite: Voelker, A. H. L., Salgueiro, E., Reverdin, G., Fontela, M., Abrantes, F., and van Geldern, R.: Monitoring hydrographic changes in the mid-latitudinal Northeast Atlantic Ocean using seawater stable isotope data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11848, https://doi.org/10.5194/egusphere-egu24-11848, 2024.

Water isotopes are one of the most widely used proxies in ocean climate research. However, there are still gaps in our understanding of the processes that control their composition.  Compared to other large ocean basins, the Mediterranean is ideally suited to improve our understanding of the processes influencing and driving oxygen isotopic variability, and to refine the current modelling approach. For the first time in a high-resolution Mediterranean dynamical model (NEMO-MED12), stable water isotopes (δ¹⁸O and δD) were successfully implemented and simulated in the whole basin. The well-known east-west gradient of δ¹⁸O in Mediterranean water masses is successfully simulated by the model. Results also show good agreement between simulated and observed δD. δD shows a strong linear relationship with δ¹⁸O (r² = 0.98) and salinity (r² = 0.94) for the entire Mediterranean basin. Furthermore, the modelled δ¹⁸O/salinity relationships are in good agreement with observations, with a weaker gradient simulated in the eastern basins than in the western basins. We investigate the relationship of the isotopic signature of the CaCO₃ shell (δ18Oc) with temperature and the influence of seasonality. Our results suggest a more quantitative use of δ¹⁸O records, combining reconstruction with modelling approaches. This opens up broad perspectives for paleoclimate-related applications.

How to cite: Ayache, M., Dutay, J.-C., Mouchet, A., Tachikawa, K., Risi, C., and Ramstein, G.: Implementing and simulating the water isotopes (δ18O and δD) distribution in the Mediterranean Sea using a high-resolution oceanic model , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17734, https://doi.org/10.5194/egusphere-egu24-17734, 2024.

It was determined the values of δ(D) and δ(¹⁸O) on 432 seawater samples from 18 depth profiles collected during the Geotraces West Atlantic cruise leg 3; JC057 on RRS James Cook, 03/2011-04/2011. The first sampling station was located north of Falkland Island and the last one near the Equator line. The samples were collected using a 24 bottles rosette and storage on 2 mL glass vials, kept at 3-4 ^oC until the measurement. The measurements were carried out applying a PICARRO water analyzer, using IAEA VSMOW2, SLAP2 and GISP reference materials as standards, each batch consisted of the three standards and six samples, with seven injections each and the sample result represent the mean value of the three last injections.

The delta values ranged from -0.91 (station 9, 1250 m) to 1.46 (station 7, 250 m) for δ(¹⁸O) and -4.5 (station 5, 5150 m) to 8.2 (station 13, 10 m).

The δ(¹⁸O)-salinity relationship varies with the depth range with a slope of 0.37 (R=0.845) for (10- 100 m), 0.39 (R=0.747) for (100-250 m), 0.53 (R=0.813) for (250-1000 m) but poor correlated (R = 0.506) for deeper samples (>1000 m). Similar figure was observed for the δ(D)-salinity relationship with a slope of 3.04 (R=0.938) for (10- 100 m), 3.02 (R=0.879) for (100-250 m), 4.21 (R=0.898) for (250-1000 m) and poor correlated (R = 0.610) for deeper samples (>1000 m).

The δ(¹⁸O)-temperature presented a similar figure as observed for the salinity although constant for the depths higher than 1000 m changing for deeper samples. The δ(¹⁸O)-temperature relationship had a slope of 0.053 (R=0.820) for (10- 100 m), 0.054 (R=0.739) for (100-250 m), 0.048 (R=0.716) for (250-1000 m) but poor correlated (R = 0.582) for deeper samples (>1000 m). The δ(D)-temperature relationship has also quite constant until 1000 m with a slope of 0.45 (R=0.933) for (10- 100 m), 0.43 (R=0.886) for (100-250 m) and 0.41 (R=0.850) for (250-1000 m) changing the slope to 0.82 (R=0.710) for deeper samples (>1000 m).

How to cite: Godoy, J. M., Gerringa, L., and Rijkenberg, M.: Hydrogen and Oxygen Isotopes as Mass Proxies Along the South Atlantic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10260, https://doi.org/10.5194/egusphere-egu24-10260, 2024.

The Southern Ocean is undergoing rapid transformations, marked by significant regional shifts in salinity that carry widespread and irreversible consequences. While the most noticeable changes are observed in the upper ocean, changes in deeper water masses have been identified and are expected to intensify over time. Changes in upper-ocean water mass salinity can be influenced by multiple drivers, and play a crucial role in changing ocean dynamics. However, the underlying causes of these characteristic changes remain poorly understood. In this study, we present a unique three-decade time-series focusing on salinity and oxygen isotopes in the upper 1200 m of the Indian sector of the Southern Ocean. Two regions emerge with pronounced surface ocean salinity trends: freshening of subpolar waters and salinification of subtropical waters. These robust changes in surface salinity are associated with an observed freshening of intermediate and winter waters in the subpolar sector of the Indian sector over the past three decades. Our findings reveal salinity changes of comparable magnitude to those reported in other regions of the upper-ocean water masses in the Southern Ocean. The oxygen isotope data allows for discriminating between different freshwater processes, showing that in the subpolar region, surface freshening is largely caused by the increase in net precipitation, while the decrease in sea ice melt is largely offset by the contribution of glacial meltwater at these latitudes. These changes strengthen the growing evidence of an acceleration of the hydrological cycle and a melting cryosphere resulting from human-induced climate change, which affect Southern Ocean water mass characteristics.

How to cite: Akhoudas, C., Sallée, J.-B., Auger, M., Reverdin, G., Haumann, A., Lo Monaco, C., Metzl, N., and Stranne, C.: Investigating upper ocean salinity changes over the past three decades in the Indian sector of the Southern Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19941, https://doi.org/10.5194/egusphere-egu24-19941, 2024.

The oxygen isotope ratio (δ18O) of seawater is a powerful tracer of 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, observational seawater δ18O data place powerful constraints on hydrologic changes in the modern ocean, are 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 global climate models. In recognition of the broad value of seawater δ18O data to the Earth science community and the growing number of new seawater δ18O data sets that have been generated over the last decade, we launched the PAGES CoralHydro2k Seawater δ18O Database Project in 2020 to recover ‘hidden’ seawater oxygen isotope data sets. We have collated these records and combined them with public data to create a new, machine-readable, and metadata-rich database that aligns with findability, accessibility, interoperability, and reusability (FAIR) standards for digital assets.

Here, we present a summary of our crowdsourcing efforts and description of the database to date, and report initial findings from the new database. The database consists of over 19,000 observations of seawater δ18O with more than 50 metadata fields. We compare seawater δ18O variability from the database to that simulated by a suite of isotope-enabled climate models and to seawater δ18O reconstructions derived from coral records and find substantial differences at annual to decadal timescales across different data sets. Lastly, we discuss the potential for future investments in water isotope observation networks to tackle 21st century science questions related to ocean changes in the past, present, and future.

How to cite: Atwood, A. R., Moore, A. L., Long, S., Pauly, R., Dassie, E., Hargreaves, J., DeLong, K. L., Morris, C., Sanchez, S. C., Wagner, A. J., and Felis, T.: Uncovering 'Hidden' Insights from the Ocean in the PAGES CoralHydro2k Seawater δ18O Database , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12179, https://doi.org/10.5194/egusphere-egu24-12179, 2024.

Precise and accurate measurements of the stable isotope composition of water with cavity ring down laser spectrometers require post-processing and calibration of raw analytical signals that involves a number of critical procedures to counteract instrumental drift, inter-sample memory effects, and the quantification of total uncertainty. Due to advent of off-the-shelf water isotope analyzers, there is an increased demand from a variety of new, often smaller labs to obtain guidance and assistance when performing such tasks.

Here we present a new, lightweight, free software tool for the post-processing and calibration of water isotope data files from Picarro-brand cavity ring-down spectrometers. It is developed at the Facility for Advanced Isotopic Research and Monitoring of Weather, Climate and Biogeochemical Cycling (FARLAB) at the University of Bergen. FLIIMP¹ (FARLAB liquid water isotope measurement processor) is written in MATLAB, but also exists as downloadable precompiled code and runs on Windows, MacOS and Linux. In its current version 2.1, FLIIMP facilitates sample processing by a graphical user interface that guides the user along the processing steps from corrections for memory effects, drift, and mixing ratio to calibration. FLIIMP provides detailed memory correction procedures, creates calibration reports and data files, and includes tools to monitor long-term measurement system behavior. Being an open-source software for the major operating systems, users can adapt FLIIMP to their laboratory environment, and the community can contribute to the software development. We hope that adoption of FLIIMP at other laboratories will lead to its further development into a mature set of calibration and correction routines for consistent, accurate, well-documented measurements of the stable isotope composition in liquid water samples.

¹ Sodemann, H, Mørkved, PT, and Wahl, S. (2023) FLIIMP - a community software for the processing, calibration, and reporting of liquid water isotope measurements on cavity-ring down spectrometers. Methods X 11:2023 DOI:https://doi.org/10.1016/j.mex.2023.102297

How to cite: Mørkved, P. T., Sodemann, H., and Wahl, S.: FLIIMP - a free, open source software for the processing, calibration and reporting of liquid water isotope measurements on cavity-ring down spectrometers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16217, https://doi.org/10.5194/egusphere-egu24-16217, 2024.

Measurements of dissolved inorganic carbon (DIC) concentration and its isotopic composition (δ¹³C_DIC) are essential to study chemical and biological processes involved in the ocean carbon cycle, including photosynthesis, respiration, and air-sea CO₂ fluxes. Anthropogenic CO₂ emissions from fossil fuel combustion have caused an increase in DIC accompanied by a decline in δ¹³C_DIC (called the Suess effect). δ¹³C_DIC is thus a useful tracer to assess the oceanic uptake of anthropogenic CO₂.

Annual assessments of the Global Carbon Budget (e.g. Friedlingstein et al., 2023) have revealed a growing deviation over the last 10 to 15 years between the estimates of the ocean carbon sink based on observations and models, with the growth of the observation-based ocean CO₂ sink being larger compared to the models. Discrepancies in the multi-decadal trend originate from all latitudes but are greatest in the Southern Ocean.

Here, we present DIC and δ¹³C_DIC measurements from surface and water column samples collected in the South-West Indian Ocean during repeated summer cruises over the last two decades (1998-2021) conducted on board the RV Marion Dufresne within the French monitoring program OISO (Océan Indien Service d’Observation). We compare these measurements with the DIC and δ¹³C_DIC simulated over the same period by the δ¹³C-enabled version of the NEMO-PISCES ocean-biogeochemical model.

We use different methods to separate the natural and anthropogenic signals over the last 20 years. Our analysis reveals some inconsistencies between simulated and observed DIC and δ¹³C_DIC, as well as between other simulated and observed biogeochemical parameters, whereas physical parameters are generally well reproduced by the model. Identifying the cause for this mismatch bears the potential to explain all or part of the divergence between the observation-based and model-based estimates of oceanic carbon uptake.

How to cite: Waelbroeck, C., Leseurre, C., Buchanan, P. J., Reverdin, G., Metzl, N., Racapé, V., Lo Monaco, C., Pierre, C., Demange, J., and Fin, J.: Investigation of the Suess effect in the South-West Indian Ocean over the last two decades – A model-data study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5864, https://doi.org/10.5194/egusphere-egu24-5864, 2024.

The Amazon shelf of South America is known to be highly contrasted in its surface carbon dioxide concentrations, from very high concentrations near the estuary, and very low concentrations downstream in the saltier Amazon plume, which results in a great contrast in carbon dioxide exchange with the atmosphere. During three cruises in 2020-2023 (Eurec4A-OA, Tara-Microbiomes legs 5, 6 and 7, Amaryllis), dissolved inorganic carbon (DIC) concentration, its isotopic composition (δ¹³C-DIC), the water isotopic composition (d¹⁸O-H₂O and d²H-H₂O), as well as inorganic nutrients and surface CO₂ partial pressure (pCO₂) were measured on the Amazon shelf of South America during three cruises in different seasons. These data are used to better understand mixing in the continuum between river water and open-ocean waters, and the biogeochemical processes taking place on the shelf close to the Amazon and Para river estuaries. The water isotopes are furthermore used to identify different freshwater origins.

The accuracy of the data is discussed as well as its representativeness. The data are then combined to first identify large variations of the river freshwater sources, compatible with 2021 being a year of very large discharge, and 2023 a year of exceptional low discharge. In addition, the data mostly from August and September 2021 identify a smaller influence of sources and sinks of dissolved inorganic carbon in the mixing shelf region than what had been earlier observed during the Amasseds cruise data in November-December 1991, a much lower river discharge period. This indicates that there might be a larger seasonal and/or interannual variability of these processes than what was earlier assessed. Measured pCO₂ data on the Amazon shelf in 2021 are then discussed in this context.

How to cite: Reverdin, G., Olivier, L., Boutin, J., Waelbroeck, C., Leseurre, C., Bretel, P., Demange, J., Fin, J., Pesant, S., Huber, P., Sarmento, H., Vandemark, D., Hunt, C., Iudicone, D., Govin, A., and Speich, S.: The Amazon plume in 2020-2023: its shelf carbon budget and water origin revisited , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10048, https://doi.org/10.5194/egusphere-egu24-10048, 2024.

Dissolved inorganic carbon (DIC) concentration and stable carbon isotope value (δ¹³C-DIC) are valuable for studying aquatic carbon cycles. These parameters reveal significant geochemical insights, such as the discernible effect of ocean anthropogenic CO₂ uptake, the primary control of surface δ¹³C-DIC distribution by photosynthesis and respiration against a meridionally variable air-sea equilibrium background, and the notable impact of terrestrial carbon inputs in estuarine environments. However, one cannot take full advantage of this coupled pair as only 15% or less of water samples during past ocean cruises and very few coastal ocean samples have been analyzed for δ¹³C-DIC as the traditional isotope analytical technology is labor-intensive and limited in shore-based laboratories. This study reports a rapid and cost-effective method based on Cavity Ring-Down Spectroscopy (CRDS) for automatically and simultaneously analyzing DIC concentration and δ¹³C-DIC on shipboard. Compared to traditional techniques, our analyzer is more portable and operational-friendly. We also prepared and preserved a set of stable in-house NaHCO₃ standards for seawater δ¹³C-DIC calibration during long cruises. This work represents the first effort to collect a large dataset of δ¹³C-DIC onboard on any oceanic transect; here along the North American eastern ocean margins in summer 2022. We efficiently processed 30 samples daily per analyzer over a 40-day expedition with excellent on-site uncertainty of ±1.1 μmol kg^-1 for the DIC concentration and ±0.03‰ for the δ¹³C-DIC value (1σ). The duplicates taken from varying depths demonstrated high consistency with average standard deviations of 1.6 μmol kg^-1 for DIC concentrations ranging between 1900 and 2300 μmol kg^-1 and 0.04‰ for δ¹³C-DIC from -0.5‰ to 1.8‰. The DIC concentration measurements of CRM displayed average discrepancies of 1.4±1.7 μmol kg^-1 for Batch #188 and 1.0±1.1 μmol kg^-1 for Batch #195 against certified values, indicating reliable accuracy. Our δ¹³C-DIC analysis of CRM from Batch #188 yielded an average of -0.20±0.04‰, closely matching the reference value of -0.19±0.02‰ obtained by Isotope Ratio Mass Spectrometry (IRMS). Consistent standard deviations for δ¹³C-DIC of CRM from Batch #188 (0.04‰, n = 36) and Batch #195 (0.03‰, n = 7) further affirmed the potential utility of CRM as a viable liquid standard for δ¹³C-DIC measurements in seawater. An interlaboratory comparison of DIC analysis with NOAA/AOML revealed an average offset of 2.0±3.8 μmol kg^-1 between onboard CRDS measurements and Coulometry results. Moreover, the cross-validation of δ¹³C-DIC against historical deep-ocean data exhibited a mean difference of only -0.04±0.06‰, emphasizing the high quality of our data.

How to cite: Sun, Z., Li, X., Ouyang, Z., Li, Q., Featherstone, C., Atekwana, E., Hussain, N., Pan, Y., and Cai, W.-J.: Simultaneous Onboard Analysis of Seawater Dissolved Inorganic Carbon (DIC) Concentration and Stable Isotope Ratio (δ13C-DIC), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21082, https://doi.org/10.5194/egusphere-egu24-21082, 2024.