Surface Marine Carbonate System Data (2019-2024) from Volunteer Observing Ship monitoring across the Western Mediterranean Sea

1.     Introduction

We present here a high-resolution dataset which spans five years (February 2019 - February 2024) and is based on weekly physicochemical observations of the surface waters along the western boundary of the Mediterranean Sea. Data were automatically collected by a Surface Ocean Observation Platform (SOOP) operating in underway mode aboard the Volunteer Observing Ship (VOS) MV JONA SOPHIE (formerly RENATE P until November 2021), a container ship managed by Nisa Maritima on the route between the Canary Islands and Barcelona. A total of 92 routes were completed in the Mediterranean Sea during the observation period.

The SOOP CanOA-VOS line, designed and maintained by the QUIMA research group at IOCAG-ULPGC, is part of Spain’s contribution to the Integrated Carbon Observation System (ICOS-ERIC) since 2021 and is recognized as an ICOS Class 1 Ocean Station, ensuring that the measurement equipment and data collection techniques meet ICOS-ERIC's high-quality standards and methodological recommendations. The data collected is also available at the ICOS Data Portal (https://www.icos-cp.eu/data-products/ocean-release).

2.     Data collection

The dataset includes continuous monitoring of CO2 levels in both surface ocean and low atmosphere, following protocols to ensure data comparability and quality given by Pierrot et al., (2009). A detailed description is provided by Curbelo-Hernández et al., 2021a, 2021b). An automated CO2 molar fraction (xCO2) measurement system, developed by Craig Nail and commercialized by General Oceanics™, was installed on the ship. This system integrates air and seawater equilibrators with a non-dispersive infrared analyzer by LICOR® for xCO2 detection. The analyzer is regularly calibrated using standard gases provided by the NOAA, traceable to the World Meteorological Organization (WMO). They were in the order of 0 ppm, 250 ppm, 400 ppm and 550 ppm until January 2021, when the gas bottles for standard 2 to 4 were changed for a new set with concentrations in the order of 300 ppm, 500 ppm and 800 ppm.

In addition to xCO2, sea surface temperature (SST) and sea surface salinity (SSS) were monitored using high-precision instruments. The SST was monitored by using a SBE38 thermometer placed at the main seawater intake in the engine room, with a reported error of ±0.01ºC. A SBE45 thermosalinograph and a Hart Scientific HT1523 Handheld Thermometer, with reported errors of ±0.01ºC, were used to monitor the temperature at the entrance of the wet box and inside the equilibrator, respectively. The SBE45 thermosalinograph measured the sea surface salinity (SSS) with an estimated error of ±0.005.

Discrete seawater samples were also collected during three round trips in February 2020, March 2021, and October 2023, for further analysis of total alkalinity and dissolved inorganic carbon. A total of 102 discrete samples has been collected in the Mediterranean Sea. Total alkalinity (AT) and total inorganic carbon (CT) were determined 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). The VINDTA 3C was calibrated using Certified Reference Material (CRMs) by A. Dickson, ensuring an accuracy of ±1.5 μmol kg-1 for AT and ±1.0 μmol kg-1 for CT.

The dataset contains some gaps, including a year-long gap from September 2021 to 2022 due to vessel maintenance and shorter gaps due to technical issues, which were addressed during routine maintenance. Some technical issues in 2020 were delayed due to COVID-19 constraints.

3. Dataset content

The dataset includes the following variables: 

·       “Date” (dd/mm/yyyy)

·       “Time” (hh:mm:ss)

·       “Latitude”

·       “Longitude”

·       “equTemp”: seawater temperature measured inside the equilibrator using a Hart Scientific HT1523 Handheld Thermometer.

·       “stdVal”: value of the standard gases used for automatically calibrations.

·       “xCO2”: measured CO2 molar fraction without performing any correlation (raw data).

·       “atmPress”: Atmospheric pressure (units: atm).

·       “equPress”: Diffeence in pressure between the atmosphere and the equilibrator (units: atm).

·       “SST_SBE38”: Sea surface temperature measured with a SBE38 thermometer at the main seawater intake of the vessel.

·       “Temp_SBE45”: Seawater temperature measured with a SBE45 thermosalinograph just before the water supply to the equilibrator.

·       “SSS_SBE45”: Sea surface salinity measured with a SBE45 thermosalinograph just before the water supply to the equilibrator.

·       “xCO2corr_sw”: CO2 molar fraction measured in the sea surface after correction by using standard gases.

·       “xCO2corr_atm”: CO2 molar fraction measured in low atmosphere after correction by using standard gases.

·       “pCO2sw”: Partial pressure of CO2 in the sea surface (units: µatm).

·       “pCO2atm”: Partial pressure of CO2 in low atmosphere (units: µatm).

·       “fCO2sw”: fugacity of CO2 in the sea surface (units: µatm).

·       “fCO2atm”: fugacity of CO2 in low atmosphere (units: µatm).

·       “AT”: Total Alkalinity (µmol kg-1)

·       “CT”: Total Inorganic Carbon (µmol kg-1)

·       “pH”: pH in surface seawater at in situ temperature computed in CO2sys using as input variables AT and fCO2sw.

·       “pH25”: pH in surface seawater at constant temperature of 25ºC computed in CO2sys using as input variables AT and fCO2sw.

Acknowledgement

This research was supported by the Canary Islands Government and the Loro Parque Foundation through the CanBIO project, CanOA subproject (2019–2024), and the CARBOCAN agreement (Consejería de Transición Ecológica y Energía, Gobierno de Canarias). We would like to thank the JONA SOPHIE ship owner, the NISA-Marítima company and the captains and crew members for the support during this collaboration. Special thanks to the technician Adrian Castro-Alamo for biweekly equipment maintenance and discrete sampling of total alkalinity aboard the ship. The SOOP CanOA-VOS line is part of the Spanish contribution to the Integrated Carbon Observation System (ICOS-ERIC; https://www.icos-cp.eu/) since 2021 and has been recognized as an ICOS Class 1 Ocean Station. The participation of D. C-H was funded by the PhD grant PIFULPGC-2020-2 ARTHUM-2