A uniform, quality controlled Surface Ocean CO2 Atlas (SOCAT)

A well documented, publicly available, global data set of surface ocean carbon dioxide (CO2) parameters has been called for by international groups for nearly two decades. The Surface Ocean CO2 Atlas (SOCAT) project was initiated by the international marine carbon science community in 2007 with the aim of providing a comprehensive, publicly available, regularly updated, global data set of marine surface CO2, which had been subject to quality control (QC). Many additional CO2 data, not yet made public via the Carbon Dioxide Information Analysis Center (CDIAC), were retrieved from data originators, public websites and other data centres. All data were put in a uniform format following a strict protocol. Quality control was carried out according to clearly defined criteria. Regional specialists performed the quality control, using state-of-the-art web-based tools, specially developed for accomplishing this global team effort. SOCAT version 1.5 was made public in September 2011 and holds 6.3 million quality controlled surface CO2 data points from the global oceans and coastal seas, spanning four decades (1968–2007). Three types of data products are available: individual cruise files, a merged complete data set and gridded products. With the rapid expansion of marine CO2 data collection and the importance of quantifying net global oceanic CO2 uptake and its changes, sustained data synthesis and data access are priorities

Earth System Science Data Social Geography A uniform, quality controlled Surface Ocean CO 2 Atlas (SOCAT)

Abstract. A well-documented, publicly available, global data set of surface ocean carbon dioxide (CO 2 ) parameters has been called for by international groups for nearly two decades. The Surface Ocean CO 2 Atlas (SOCAT) project was initiated by the international marine carbon science community in 2007 with the aim of providing a comprehensive, publicly available, regularly updated, global data set of marine surface CO 2 , which had been subject to quality control (QC). Many additional CO 2 data, not yet made public via the Carbon Dioxide Information Analysis Center (CDIAC), were retrieved from data originators, public websites and other data centres. All data were put in a uniform format following a strict protocol. Quality control was carried out according to clearly defined criteria. Regional specialists performed the quality control, using state-of-the-art web-based tools, specially developed for accomplishing this global team effort. SOCAT version 1.5 was made public in September 2011 and holds 6.3 million quality controlled surface CO 2 data points from the global oceans and coastal seas, spanning four decades . Three types of data products are available: individual cruise files, a merged complete data set and gridded products. With the rapid expansion of marine CO 2 data collection and the importance of quantifying net global oceanic CO 2 uptake and its changes, sustained data synthesis and data access are priorities. Data coverage Motivation The net absorption of CO 2 by the oceans, caused by rising atmospheric CO 2 concentrations since the industrial revolution, has been responsible for removing CO 2 equivalent to approximately 50 % of the fossil fuel and cement manufacturing emissions or about 30 % of the total anthropogenic emissions, including land use change (Sabine et al., 2004). Because of the availability of the carbonate ion, an important species of the dissolved inorganic carbon pool, and carbonate sediments, the oceans have a tremendous CO 2 uptake capacity and will, on timescales of ten to hundred thousand years, absorb all but a small fraction of the fossil CO 2 that has been and will be emitted These technological developments have led to a rapid increase in new surface ocean CO 2 data being collected each year. This is reflected in the number of data underlying the successive surface ocean pCO 2 (partial pressure of CO 2 ) climatologies of The first version of SOCAT (version 1.5) was made public on 14 September 2011 during "The ocean carbon cycle at a time of change: Synthesis and Vulnerabilities" meeting at the UNESCO (United Nations Educational, Scientific and Cultural Organization), Paris History and organisation of SOCAT History of SOCAT In the late 1990s attempts were made by the SCOR-IOC (Scientific Committee on Oceanic Research-Intergovernmental Oceanographic Commission) committee on ocean CO 2 , the forerunner of the IOCCP (International Ocean Carbon Coordination Project), to assemble a comprehensive, well documented, publicly available data set of surface ocean f CO 2 for the global oceans and coastal seas. Efforts for encouraging data submission to a central location, the Carbon Dioxide Information Analysis Center, were partly successful. In 2004 the marine carbon community agreed on recommendations for the reporting of surface water CO 2 data and metadata (IOCCP, 2004). However, most data gatherers did not strictly follow these. Only a subset of all global surface water CO 2 data were made publicly available via CDIAC, with many data only available via the investigators, institute websites and national or world data centres. Over the past decades several attempts have been made to establish a global surface ocean CO 2 database. In the late 1990s, Taro Takahashi from Lamont-Doherty Earth Observatory (LDEO) compiled an initial data set and updated this collection in 2002 and every year from 2007 onwards In 2001, Bakker began to assemble a surface ocean CO 2 data set by putting public data from CDIAC into a uniform format, as part of the European Union (EU) project ORFOIS (Origin and fate of biogenic particle Fluxes in the Ocean and their Interaction with the atmospheric CO 2 concentration as well as the marine Sediment). Pfeil and Olsen streamlined and expanded this effort within the EU project CarboOcean from 2005 onwards. They compiled public surface ocean CO 2 data held at CDIAC, PANGAEA -Data Publisher for Earth & Environmental Science (an International Council for Science (ICSU) World Data Center, formerly the World Data Center for Marine Environmental Sciences, WDC-MARE) and elsewhere into a common format f CO 2 database based on the recommended formats for data and metadata reporting (IOCCP, 2004 The Surface Ocean CO 2 Atlas was initiated at the Surface Ocean CO 2 Variability and Vulnerability (SOCOVV) meeting by the international ocean carbon research community -A gridded product consisting of monthly surface f CO 2 means (including number of data points and standard deviation) on a 1 • latitude by 1 • longitude grid with no interpolation. A gridded surface ocean f CO 2 product was deemed to be more useful than air-sea CO 2 flux estimates for modelling and other purposes (IOCCP, 2007). Regional groups and a global group for coordination were formed Coastal regions north of 30 • S were quality controlled by the coastal group, while coastal regions south of 30 • S were quality controlled by the Southern Ocean group. A series of meetings was held in which SOCAT gradually took shape and in which the regional groups coordinated their work The SOCAT community evaluated existing data compilations and selected the data collection by Pfeil and Olsen as the basis for SOCAT (IOCCP, 2008). The focus for SOCAT has been the assembly of publicly available data (including metadata), standardisation of the file formats, recalculation of consistent and uniform surface water f CO 2 data, and basic and secondary level quality control (Sects. 3, 4 and 5). SOCAT is independent from the LDEO database SOCAT groups Roughly 45 international, seagoing marine carbon scientists and data managers from 12 countries actively participated in the assembly and quality control of SOCAT version 1.5. These participants were organised into regional groups and a global group • S), the North Atlantic Ocean (north of 30 • N, including the Atlantic Arctic Ocean), the Tropical Atlantic Ocean (30 • N to 30 • S), the North Pacific Ocean (north of 30 • N, including the Pacific Arctic Ocean), the Tropical Pacific Ocean (30 • N to 30 • S), the Indian Ocean (north of 30 • S) and the Southern Ocean (south of 30 • S, including coastal waters). Coastal regions were initially defined by bathymetry (shallower than 200 m) for regions north of 30 • S (IOCCP, 2008). This definition was later replaced by a criterion of distance from a major land mass (less than 400 km) in order to better reflect the environmental significance of these regions as continental margins. SOCAT has been a large, complex undertaking and has involved activities focused on: data retrieval, assembling data in a uniform format, recalculating surface water f CO 2 using the same agreed-upon protocol, defining SOCAT QC criteria, developing the QC cookbook and Matlab QC code, making SOCAT available via the Live Access Server (LAS) for QC and public release, data QC, gridding SOCAT, making SOCAT documentation and products available via the web, designing the SOCAT logo, internal communication, organisation of SOCAT meetings, and liaising with the international marine carbon community. Numerous colleagues have played a role in these activities SOCAT data assembly Data sources and instrumentation SOCAT includes 6.3 million f CO 2 data points measured in all ocean areas from 1968 to 2007. Most of these data were gathered from the online sources at CDIAC (30 % of the cruises) and PANGAEA (10 %), as well as from institute and project websites (37 %). The remaining cruises (23 %) were obtained directly from the data originators. Almost half of the cruises (45.7 %) originated in the USA. Other significant contributors are based in Japan (20.1 %), Norway (9.6 %), the United Kingdom (7.4 %), Germany (5.8 %), France (4.5 %), Belgium (2.4 %), Canada (1.6 %), Spain (1.5 %), Australia (1.2 %) and the Netherlands (0.3 %). The data in SOCAT are a synthesis of 4 decades of seagoing fieldwork by numerous scientists from 12 countries. Various instruments have been used to obtain these data and only the basic principles will be summarised here. Further information is available in the metadata, which accompany individual cruise files at PANGAEA (doi:10.1594/PANGAEA.769638) (Sect. 5.2). The seawater f CO 2 values included in SOCAT have been measured according to one of the following two principles: (1) analysis of the CO 2 content in an air sample in equilibrium with a large volume of seawater or (2) calculation of the seawater f CO 2 from the colour response of an acidbase indicator dye (sulfonephtalein) in contact with seawater across a CO 2 permeable membrane. The analysis of the CO 2 content in an air sample in equilibrium with a large volume Earth Syst. Sci. Data, 5, 125-143, 2013 www.earth-syst-sci-data.net/5/125/2013/ B. Pfeil et al.: A uniform, quality controlled Surface Ocean CO 2 Atlas (SOCAT) 131 of seawater is recommended in the standard work by Data harmonisation and basic quality control All data files available for SOCAT were first converted to a common file structure. This also included discarding data not directly relevant for surface ocean CO 2 , e.g., meteorological parameters like wind speed and direction, whenever these were supplied in the file. Next, the unit of each parameter was checked and converted into the agreed standard unit, if required (e.g., conversion of atmospheric pressure from atmospheres to hPa, and of latitude and longitude to decimal degrees). For around 10 % of the cruises, different versions of the data had been obtained from various sources. In these cases only the most recent version was included in SOCAT in consultation with the data originator. Basic, primary quality control was carried out at this stage. Outliers and unrealistic values in date, time, position, intake temperature, salinity, atmospheric pressure and surface water CO 2 were identified. The criteria were that ship speeds calculated from position should be realistic, that atmospheric pressures should be within 800 hPa and 1100 hPa and that the dates should exist. Rapid changes in intake or equilibrator temperature of several degrees, in salinity of several units or in surface f CO 2 of several hundreds of microatmospheres were also questioned, except for data in coastal or ice-covered regions. Whenever several such data points were encountered, the data originator was contacted and this often resulted in resubmission of an updated (corrected) version. In some cases several iterations were required, making this a time-consuming task. In a few cases interaction with the data originator was not possible, and obviously bad data were removed from the data file. In version 2 of SOCAT this class of quality control will be used to assign quality flags to individual data points, using the conventions of the World Ocean Circulation Experiment (WOCE): flag 2 (good), flag 3 (questionable) or 4 (bad). Only a very small number of WOCE flags 3 and 4 are found in the version 1.5 data collection. f CO 2 (re-)calculations The final stage of the SOCAT data assembly was the (re-)calculation of f CO 2 values at sea surface (or intake) temperature in order to ensure a uniform representation of CO 2 concentration. The conversions from xCO 2 and pCO 2 were carried out using a single set of equations with a clear hierarchy for the preferred CO 2 input parameter (Table 4) ). We used the equations recommended by for the conversion of dry CO 2 mole fraction to partial pressure at 100 % humidity, where P equ is the pressure in the equilibrator. The water vapour pressure pH 2 O is calculated as pH 2 O = exp 24.4543 − 67.4509 100 T K eq