Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
  • Problem Description
• Data Files
• Related Publications
• Related Datasets
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

These samples were during cruises onboard the R/V Neil Armstrong to recover and redeploy mooring infrastructure of the international Overturning in the Subpolar North Atlantic Program (OSNAP) in 2020 (AR45) and 2022 (AR69-03). The mooring infrastructure maintained on these cruises is located in the eastern Labrador Sea (referred to as the LS line) and western Irminger Sea (referred to as the CF line). Beginning in 2020, the Gases in the Overturning and Horizontal circulation of the Subpolar North Atlantic Program (GOHSNAP) has added moored oxygen sensors to these sections of the OSNAP mooring array. During these cruises, Conductivity Temperature Depth (CTD) casts are conducted to provide data necessary to calibrate the moored sensors (Miller et al., in review), as well as hydrographic data that provide a valuable dataset in and of themselves.

This dataset contains dissolved oxygen, dissolved inorganic carbon (DIC), and total alkalinity (TA) measurements from discrete water samples collected from Niskin bottles on the CTD rosette. Additional context about the cruises and all cruise operations can be found in the cruise reports for AR45 (2020) and AR69-03 (2022). We note that the 2020 cruise was conducted in the context of operational and cruise staffing limitations imposed by the COVID-19 pandemic, which is reflected in the lower number of discrete samples collected and in modifications to the procedure for dissolved oxygen sample analysis.

Dissolved Oxygen:

Samples were collected into volume-calibrated flasks and preserved for Winkler dissolved oxygen analysis following standard protocols (Langdon, 2010). For AR69-03, in 2022, all samples were titrated onboard the ship within 24-48 hours of collection using a custom-built Winkler titrator with automated potentiometric end point detection (control software available here: Nicholson et al., 2023). For AR45, in 2020, COVID-19 pandemic restrictions precluded the ability to conduct titrations onboard the ship and samples were instead preserved following the procedures of Zhang et al. 2002 and titrated on land at the end of the cruise.

Precision of the sample collection and analysis procedure is determined by agreement between replicate measurements from the same Niskin bottle. Due to the need to preserve samples for land-based analysis, all samples from AR45 were collected in triplicate. After removal of measurements where outlier data indicated that samples had not been successfully preserved (4 of 63 total samples), median agreement among replicates was 0.15% (0.4 μmol/kg). For AR69-03, all samples were collected either in duplicate or in triplicate and median agreement among replicates was 0.11% (0.3 μmol/kg). In some cases, larger discrepancies among replicates reflect errors in sample collecting and/or preservation; these results are reported for completeness but are flagged as questionable based on outlier analysis conducted in using these samples to calibrate the SBE43 oxygen sensor included in the ship’s sensor package (see Related Dataset: Fogaren and Palevsky, 2024).

Accuracy of sample measurements depends on standardization of the sodium thiosulfate titrant based on a reference standard. The sodium thiosulfate titrant used on each cruise was determined by standardization with a 0.01N potassium iodate reference solution from Ocean Scientific International Ltd (OSIL). Lab-prepared potassium iodate standards, measured routinely throughout AR69-03 and before, during, and after land-based analysis of the AR45 samples to verify titration accuracy and stability, were verified and adjusted by measurements against the OSIL standard.

Dissolved Inorganic Carbon and Total Alkalinity:

Samples were collected for dissolved inorganic carbon (DIC) and total alkalinity (TA) analysis following standard protocols (Dickson et al., 2007). Samples were collected into either 250 mL or 500 mL borosilicate glass bottles and preserved with saturated mercuric chloride (100 μL in 250 mL bottles, 200 μL in 500 mL bottles) for later analysis.

Samples were analyzed at the Boston College Marine Biogeochemistry Laboratory. DIC was analyzed using an Apollo SciTech AS-C6L DIC Analyzer and TA was analyzed using an Apollo SciTech AS-ALK2 TA Analyzer. Both DIC and TA were measured from each sample bottle. All DIC measurements were made on the day the bottle was opened for analysis, and TA measurements were made within the same week. DIC and TA instruments were calibrated daily and monitored throughout each analysis session by measuring Certified Reference Materials (Andrew Dickson, UCSD).

Analytical replicates were measured for all samples such that after analytical outliers (sigma) were removed, all samples for both DIC and TA retained at least two replicate measurements (median number of replicates for DIC = 3; median number of replicates for TA = 4). Analytical precision was determined for each sample as the standard deviation of analytical replicates. Mean analytical precision for all DIC samples in this dataset is 0.6 µmol/kg. Mean analytical precision for all TA samples in this dataset is 1.8 µmol/kg. Individual samples are flagged as questionable (QC flag = 3) if the analytical precision is >8 µmol/kg. Individual samples for DIC are also flagged as questionable (QC flag = 3) if CRMs run prior and subsequent to the sample in question differ by >8 µmol/kg.

* Merged data from cruise AR45 and AR69-03 into 1 dataset
* Added cruise id to dataset
* Added ISO date notation to dataset
* Converted missing value flag 9 and missing identifier -999 to blank

NSF Award Abstract:
Every winter, frigid winds blowing eastward from the North American continent cool the surface waters of the Labrador Sea, which is situated between Canada and Greenland. As the ocean cools, oxygen and carbon dioxide are mixed from the atmosphere into a thick layer of water that ultimately spreads southward to fill a large volume of the North Atlantic and beyond. The presence of this water mass prevents the North Atlantic anywhere from becoming completely devoid of oxygen. Vertical mixing in the Labrador Sea also redistributes carbon dioxide into the deep ocean, where it can remain for hundreds of years, preventing it from contributing to the greenhouse effect. Yet, the processes governing the uptake of gases by the ocean are not well understood or quantified. Given that, over the last century, the ocean has become steadily more depleted in oxygen while also absorbing a large fraction of anthropogenic carbon dioxide, observing gas exchange processes is essential for understanding and predicting the evolution of the ocean and climate system. The circulation of the Labrador Sea has been monitored since 2014 with an array of instrumented cables extending from the seafloor to nearly the ocean surface. This project adds gas sensors to this array to investigate the rates and processes governing gas exchange. Through this project, a student and postdoc will be trained in interdisciplinary oceanography with a rich network of international collaborators. Responding to the need to increase public ocean literacy, the project scientists will work with University of Rhode Island’s Inner Space Center to broadcast live, interactive science sessions to educators at partner high schools and will follow-up with in-person science cafés at three participating schools.

Given the unique role of the Labrador Sea in providing a pathway for oxygen (O2) and carbon dioxide (CO2) to enter the intermediate depths of the ocean, a quantification and mechanistic understanding of the gas uptake and transport in the basin is a leading scientific priority. Oxygenation of Labrador Sea water prevents large-scale hypoxia from developing anywhere in the Atlantic Ocean and anthropogenic CO2 storage in the basin is the highest in the global ocean. The assumption that, in the Atlantic Ocean, O2 and CO2 uptake and their variability are tied to the dynamics of heat loss and the overturning circulation pervades the literature but has never been evaluated on the basis of direct observations. Thus, GOHSNAP (Gases in the Overturning and Horizontal circulation of the Subpolar North Atlantic Program) addresses this gap and the urgent need to better understand interactions between gas uptake, transport, and the overturning circulation. Specifically, this program will provide a continuous 2-year record of the trans-basin, full water column transport of O2 across the southern boundary of the Labrador Sea, leveraging the mooring infrastructure of the US-lead, international Overturning in the Subpolar North Atlantic Program (OSNAP). The addition of O2 sensors at various depths on this array, supplemented by observations collected by autonomous platforms will allow for the quantification of O2 export from the Labrador Sea. The data will further be used to empirically model carbon concentrations and estimate carbon export. Proposed instruments will also measure the mixed layer O2 and pCO2 for two winters, from which air-sea gas exchange will be calculated and compared against analogous observations in the convective interior of the Labrador Sea.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.