A 1998–1992 comparison of inorganic carbon and its transport across 24.5°N in the Atlantic

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Abstract

In January and February 1998, when an unprecedented fourth repetition of the zonal hydrographic transect at 24.5°N in the Atlantic was undertaken, carbon measurements were obtained for the second time in less than a decade. The field of total carbon along this section is compared to that provided by 1992 cruise which followed a similar path (albeit in a different season). Consistent with the increase in atmospheric carbon levels, an increase in anthropogenic carbon concentrations of 8±3μmolkg−1 was found in the surface layers. Using an inverse analysis to determine estimates of absolute velocity, the flux of inorganic carbon across 24.5° is estimated to be −0.74±0.91 and −1.31±0.99PgCyr−1 southward in 1998 and 1992, respectively. Estimates of total inorganic carbon flux depend strongly upon the estimated mass transport, particularly of the Deep Western Boundary Current. The 1998 estimate reduces the large regional divergence in the meridional carbon transport suggested by previous studies and brings into question the idea that the tropical Atlantic constantly outgasses carbon, while the subpolar Atlantic sequesters it. Uncertainty in the carbon transports themselves, dominated by the uncertainty in the total mass transport estimates, are a hindrance to determining the “true” picture.

The flux of anthropogenic carbon (CANTH) across the two transects is estimated as northward at 0.20±0.08 and 0.17±0.06PgCyr−1 for the 1998 and 1992 sections, respectively. The net transport of CANTH across 24.5°N is strongly affected by the difference in concentrations between the northward flowing shallow Florida Current and the mass balancing, interior return flow. The net northward transport of CANTH is opposite the net flow of total carbon and suggests, as has been found by others, that the pre-industrial southward transport of carbon within the Atlantic was stronger than it is today. Combining these flux results with estimates of atmospheric and riverine inorganic carbon input, it is determined that today's oceanic carbon system differs from the pre-industrial system in that today there is an uptake of anthropogenic carbon to the south that is advected northward and stored within the North Atlantic basin.

Introduction

Throughout the last decade approximately 6.2PgCyr−1 (1PgC=1GtC=1015g of carbon) have been released into the atmosphere through the burning of fossil fuels (IPCC, 2001; Battle et al., 2000). Meanwhile, the amount of carbon in the atmosphere has increased at a rate of only 2.8PgCyr−1 (1PgCyr−1=2642kmolCs−1). The increased partial pressure of atmospheric CO2 has caused, through air–sea exchange, the amount of carbon stored in the ocean to increase by about 2.0±0.6PgCyr−1 (IPCC, 2001; Battle et al., 2000; Schimel et al., 1996; Siegenthaler and Sarmiento, 1993). These estimates strongly suggest that the ocean and ocean circulation are significant players in the global carbon cycle and control, in part, the rate of the atmospheric increase in CO2.

Just where the oceanic uptake of anthropogenic CO2 is occurring and how circulation patterns affect the sources and sinks of carbon within the oceans are not well understood. Until recently direct oceanic observations have been sparse and numerical models, which have been extremely good at producing similar net ocean CO2 storage estimates, have failed to produce a quantitative consensus on where the uptake and storage of anthropogenic carbon are occurring (Wallace, 2001; Orr et al., 2001). This study seeks to understand better one piece of this puzzle, that is the uptake and transport of carbon within the North Atlantic. We focus on the carbon transport across 24.5°N.

Previous estimates of the carbon transport across this line of latitude have been made. The earliest, Brewer et al. (1989) was based on carbon measurements taken at just nine stations occupied in the late fall of 1988, combined with the transport results of the full hydrographic transect made in 1981 (Hall and Bryden, 1982). The increased effort to measure carbon dioxide concentrations in seawater during the World Ocean Circulation Experiment (WOCE), the Ocean–Atmosphere Carbon Exchange Studies (OACES) and the US Joint Global Ocean Flux Study (JGOFS) programs has now provided us with a far more detailed view of the ocean carbon field. Rosón et al. (2002) presented inorganic carbon and anthropogenic carbon transport estimates based on data from the summer 1992 occupation of the Atlantic 24.5°N transect. Their carbon transport estimates along with estimates from previous studies at other latitudes suggested that to the north of 24.5°N the subpolar ocean takes up carbon from the atmosphere, whereas to the south it is released to the atmosphere. We compare their data and results to those from the winter 1998 occupation of the 24.5°N transect, with particular emphasis on the carbon and anthropogenic carbon fields. Calculated meridional transport of inorganic and anthropogenic carbon allows us to provide an updated view of the uptake and storage within the Atlantic Basin.

Section snippets

The observations

In 1998, the R.V. Ronald Brown made a high density hydrographic transect across the North Atlantic at 24.5°N (Fig. 1 in black) sponsored by OACES. It was the second transect along this line of latitude to obtain measurements of carbon and carbon related species. The second leg of the cruise, which made these measurements left Las Palmas on January 23, 1998 and arrived in Charleston S.C. 1 month later on February 24. This leg included 130 CTD stations at which oxygen, nutrient and

Method

To obtain an estimate of the absolute meridional velocity field for each of the two 24.5°N transects a simple inverse box model has been used. It is based upon the same box inverse model method introduced to the physical oceanographic community by Wunsch (1978) and later described in much greater detail in Wunsch (1996). The inverse method as used here is simply a technique for estimating ocean circulation based on hydrographic observations and a set of simple physical constraints. The

Discussion

The net northward transport of CANTH is opposite the net flow of total carbon and suggests, as has been found by others (Holfort et al., 1998; Rosón et al., 2002; Wallace, 2001), that in spite of today's greater atmospheric carbon levels, the pre-industrial southward transport of carbon within the Atlantic was stronger than it is today.

Taking the average of the 1998 and 1992 estimates1 for the

Acknowledgements

This research was carried out in part under the auspices of the Cooperative Institute of Marine and Atmospheric Studies (CIMAS), a Joint Institute of the University of Miami and the National Oceanic and Atmospheric Administration, cooperative agreement #NA67RJ0149, and it is was funded by the Global Carbon Cycle Program of the National Oceanic and Atmospheric Administration Office of Global Programs and the Office of Oceanic and Atmospheric Research. We gratefully acknowledge the support of J.

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