DOI: 10.5281/zenodo.10276222
title: Surface-to-bottom data of total alkalinity, total inorganic carbon, pH and dissolved oxygen in the subpolar North Atlantic along the CLIVAR 59.5N hydrographic section during 2009-2019.

description: Contact: magdalena.santana@ulpgc.es; melchor.gonzalez@ulpgc.es; david.curbelo@ulpgc.es
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1. Introduction
The dataset comprises physical and carbonate system data collected during eight summer cruises (2009-2019) along the meridional hydrographic CLIVAR 59.5N section. This repeated section covered the longitudinal span of the subpolar North Atlantic at 59.5ºN between Scotland and Greenland (4.5-43.0ºW), encompassing the Irminger and Iceland basins, and the Rockall Trough. Sampling stations were equidistantly spaced every 20 n.m. apart (~1/3º longitude) in most cruises, with exceptions in 2016 where station spacing was decreased to 10 n.m. over Reykjanes Ridge slopes. Notably, the distance between stations over the east Greenland slope and shelf decreased from 10 n.m. to about 2 n.m. The dataset provided here is the result of an international collaboration between researchers from the P. P. Shirshov Institute of Oceanology at the Russian Academy of Science and the QUIMA-IOCAG group from the ULPGC. The cruise ID, dates, research vessels and chief scientist of each cruise (2009, 2010, 2011, 2012, 2013, 2014, 2016 and 2019) are summarized as follows:
Year     Cruise ID         Date                             Research Vessel (R/V)            Chief Scientist
2009     AI28                Aug 15-Sept 27            Akademik Ioffe                         A. Sokov
2010     AI31                Sep 2-Sep 27               Akademik Ioffe                         A. Sokov
2011     SV33                Sep 9-Sep 28              Akademik Sergey Vavilov        A. Sokov
2012     AI38                May 25-Jul 1                Akademik Ioffe                         S. Gladyshev
2013     AI41                Jun 26-Jul 23                Akademik Ioffe                         S. Gladyshev
2014     AI44                Jun 27-Jul 20                Akademik Ioffe                         S. Gladyshev
2016     AI51                Jun 3-Jul 13                  Akademik Ioffe                         S. Gladyshev
2019     AMK77            Aug 8-Sep 10               Akademik Mstislav Keldysh     S. Gladyshev
2. Data collection: measurements and determination methodologies
The surface-to-bottom sampling and in situ measurements were performed by using a SBE 911plus CTD with SBE32 Carousel containing 24 Niskin bottles (10 L) with additional sensors for pressure, temperature, salinity and dissolved oxygen (DO). The Chief Scientists (Alexey Sokov and Sergey Gladyshev, supported by FMWE-2023-0002) were responsible for the operational and maintenance procedures for the CTD and provided the physical variables (temperature, salinity, depth and bottom depth) for all the cruises and the sensor-measured DO for the cruise of 2019, all of them included in this dataset. The use of these data for scientific purposes is subject to request and granted only upon prior contact to A. Sokov and/or S. Gladyshev.
The dataset includes high-quality CO2 measurements obtained through a standardized analytical methodology applied across hydrographic cruises. The procedures adhere to the DOE method manual for CO2 analysis in seawater by Dickson et al., 2007. Seawater samples were onboard analysed for total alkalinity (AT), total inorganic carbon (CT), pH and dissolved oxygen (DO) determination. The QUIMA-IOCAG group from the ULPGC was responsible for the seawater sampling and chemical variables determination (CO2 system variables in all the cruises and WINKLER-measured dissolved oxygen from 2009 to 2016).  The use of these data for scientific purposes is subject to request and granted only upon prior contact to any of the dataset authors.
     2.1. Total Alkalinity (AT) and Total Inorganic Carbon (CT)
Total alkalinity (AT) and total inorganic carbon (CT) were determined onboard using a VINDTA 3C according to Mintrop et al., 2000. AT was analyzed via potentiometric titration with HCl, following the carbonic acid endpoint method (Millero et al., 1993; Dickson and Goyet, 1994), while CT was determined through coulometric titration (Johnson et al., 1993). In-situ calibration of the VINDTA 3C using Certified Reference Material (CRMs) by A. Dickson ensured accuracy of ±1.5 μmol kg-1 for AT and ±1.0 μmol kg-1 for CT.
     2.2. pH
Spectrophotometric pH measurements were conducted between 2009 and 2016 at a constant temperature of 15ºC (pH15). The measurements utilized a spectrophotometric pH sensor (SP101-SM) developed by the QUIMA-IOCAG group at the ULPGC in collaboration with SensorLab (González-Dávila, 2014; González-Dávila et al., 2016). The method employs 4-wavelength analysis for m-cresol purple, incorporates auto-cleaning steps, and performs a blank for pH calculation post-dye injection. In-situ testing with a TRIS seawater buffer confirmed an accuracy of ±0.002 units, and a correction of +0.0047 units was applied to experimental pH values based on DelValls and Dickson, 1998, which reported an uncertainty associated with TRIS calibration.
The pH at in situ temperature (pH) was computed by using the CO2SYS programme developed by Lewis and Wallace, (1998) and run with the MATLAB software (van Heuven et al., 2011; Orr et al., 2018; Sharp et al., 2023) from the measured AT and pH15. The pH at in situ temperature for the cruise of 2019, in which direct pH measurements were not performed, was computed from the measured AT and CT.
     2.3. Dissolved Oxygen (DO)
The WINKLER method, initially introduced by Winkler (1888) and subsequently optimized by Carpenter (1965) and Carrit and Carpenter (1966), was employed to analytically determine dissolved oxygen (DO) in seawater samples across all cruises from 2009 to 2016. During sample collection, seawater samples for DO determination were carefully collected in pre-calibrated glass wide-neck bottles to prevent bubble formation, and the water temperature was recorded at the time of sampling. Titration was performed using a Metrohm 888 Titrando and 794 Basic Titrino, operated with Tiamo software and a potentiometric electrode, as outlined by Culberson and Huang (1987). Thiosulfate standardization occurred every two days using a KIO3 0.01N solution. The reagents and solutions for DO determination were prepared following procedures by Dickson and Goyet (1994), with regular blank determinations every two days to control for possible impurities. As DO could not be analytically measured during the cruise of 2019 (due to limitations related with the oceanographic cruise plan), sensor-measured DO data were included in this dataset for this year.
3. Dataset content
The dataset includes the following variables: 

"cruise" (year of the cruise).
"cruise_ID" (ID of each cruise).
"date" (date of the day in which half of the cruise was completed).
"station" (ID of each sampling station).
"lon" (longitude in decimal degrees).
"lat" (latitude in decimal degrees).
"niskin" (number of each niskin bottle obtained from the bottle dataset).
"depth" (depth of each sample in meters, m).
"bottomdepth" (depth of the bottom in meters, m).
"temp" (temperature in ºC).
"sal" (salinity).
"pH15" (measured pH at a constant temperature of 15ºC, in total scale).
"pH" (pH at in situ temperature, in total scale).
"CT" (total inorganic carbon, in mmol m-3).
"AT" (total alkalinity, in mmol m-3).
"DO" (Dissolved Oxygen, in mmol m-3).
Acknowledgement
The participation on the cruises for the Spanish Team from the ULPGC was funded by the Science Spanish Ministry under the Complimentary Actions CTM2008-05255, CTM2010-09514-E and CTM2011-12984-E (years 2009-2011), the FP7 European project CARBOCHANGE under grant agreement no. 264879 and by the Spanish Innovation and Science Ministry through the Projects EACFe (CTM2014-52342-P) and ATOPFe (CTM2017-83476-P). The participation of DCH was funded by the PhD grant PIFULPGC-2020-2 ARTHUM-2. Special thanks go to the technician and researchers Adrian Castro Álamo (2 cruises), Anna Barrera Galderique (3 cruises), Rayco Alvarado Medina (2 cruises) and Pilar Aparicio Rizzo (1 cruise) who helped with in situ analysis. We also thanks technicians at the P. P. Shirshov Institute of Oceanology from the Russian Academy of Science for their onboard help with sampling and analysis works. We are deeply grateful to A. Sokov and S. Gladyshev from the P. P. Shirshov Institute of Oceanology from the Russian Academy of Science for invite the QUIMA-IOCAG group (ULPGC) to participate in the 8 cruises between 2009 and 2019 and provide CTD data.