To understand the role of the oceans as a sink for anthropogenic CO, it is important to determine the distribution of carbon species in the ocean interior and the processes affecting the transport and storage of CO taken up from the atmosphere. Figure 7 shows the typical north-south distribution of DIC in the Atlantic, Indian, and Pacific oceans prior to the introduction of anthropogenic CO. In general, DIC is about 10–15% higher in deep waters than at the surface. Concentrations are also generally lower in the Atlantic than the Indian ocean, with the highest concentrations found in the older deep waters of the North Pacific. The two basic mechanisms that control the distribution of carbon in the oceans are the solubility and biological pumps.
Figure 7. Zonal mean pre-industrial distributions of dissolved inorganic carbon (in units of µmol kg) along north-south transects in the Atlantic, Indian and Pacific oceans. The Pacific and Indian Ocean data are from the Global CO Survey (this study), and the Atlantic Ocean data are from Gruber (1998).
The solubility pump is driven by two interrelated factors. First, CO is more than twice as soluble in cold polar waters than in warm equatorial waters. As western surface boundary currents transport water from the tropics to the poles, the waters are cooled and absorb more CO from the atmosphere. Second, the high-latitude zones are also regions where intermediate and bottom waters are formed. As these waters cool, they become denser and sink into the ocean interior, taking with them the CO accumulated at the surface.
The primary production of marine phytoplankton transforms CO and nutrients from seawater into organic material. Although most of the CO taken up by phytoplankton is recycled near the surface, a substantial fraction, perhaps 30%, sinks into the deeper waters before being converted back into CO by marine bacteria. Only about 0.1% reaches the seafloor to be buried in the sediments. The CO that is recycled at depth is slowly transported over long distances by the largescale thermohaline circulation. DIC slowly accumulates in the deep waters as they travel from the Atlantic to the Indian and Pacific oceans. Using a 3-D global carbon model, Sarmiento et al. (1995) estimated that the natural solubility pump is responsible for about 20% of the vertical gradient in DIC; the remaining 80% originates from the biological pump.
The approaches for estimating anthropogenic CO in the oceans have taken many turns over the past decade. Siegenthaler and Sarmiento (1993) summarized early approaches for estimating the anthropogenic sink in the oceans, including ocean models of various complexity, atmospheric measurements and transport models used together with pCO measurements and estimates based on changes in oceanic C and oxygen mass balance. They noted the wide range of ocean uptake estimates (1.6–2.3 Pg C yr) and concluded that the larger uptake estimates from the models were the most reliable.
The first approaches for using measurements to isolate anthropogenic CO from the large, natural DIC signal were independently proposed by Brewer (1978) and Chen and Millero (1979). Both these approaches were based on the premise that the anthropogenic DIC concentration could be isolated from the measured DIC by subtracting the contributions of the biological pump and the physical processes, including the pre-industrial source water values and the solubility pump.
Gruber et al. (1996) improved the earlier approaches by developing the C* method. This method is based on the premise that the anthropogenic CO concentration (Cant) can be isolated from measured DIC values (Cm) by subtracting the contribution of the biological pumps (Cbio), the DIC the waters would have in equilibrium with a preindustrial atmospheric CO concentration of 280 ppm (Ceq280), and a term that corrects for the fact that surface waters are not always in equilibrium with the atmosphere (Cdiseq):
Cant = Cm – Cbio – Ceq280 – Cdiseq = C* – Cdiseq. (2)
The three terms to the right of the first equal sign make up C*, which can be explicitly calculated for each sample. The fact that C* is a quasi-conservative tracer helps remedy some of the mixing concerns arising from the earlier techniques (Sabine and Feely, 2001). The Cdiseq term is evaluated over small isopycnal intervals using a water-mass age tracer such as CFCs.
We have evaluated anthropogenic CO for the Atlantic, Indian, and Pacific oceans using the C* approach. Figure 8 shows representative sections of anthropogenic CO for each of the ocean basins. Surface values range from about 45 to 60 µmol kg. The deepest penetrations are observed in areas of deep water formation, such as the North Atlantic, and intermediate water formation, such as 4050°S. Integrated water column inventories of anthropogenic CO exceed 60 moles m in the North Atlantic (Figure 9). Areas where older waters are upwelled, like the high-latitude waters around Antarctica and Equatorial Pacific waters, show relatively shallow penetration. Consequently, anthropogenic CO inventories are all less than 40 moles m in these regions (Figure 9).
Figure 8. Zonal mean distributions of estimated anthropogenic CO concentrations (in units of µmol kg) along north-south transects in the Atlantic, Indian and Pacific oceans. The Pacific and Indian Ocean data are from the Global CO Survey (this study), and the Atlantic Ocean data are from Gruber (1998).
Figure 9. Zonal mean anthropogenic CO inventories (in units of moles m) in the Atlantic, Indian and Pacific oceans.
Data-based estimates indicate that the oceans have taken up approximately 105 ± 8 Pg C since the beginning of the industrial era. Current global carbon models generally agree with the total inventory estimates, but discrepancies still exist in the regional distribution of the anthropogenic inventories. Some of these discrepancies stem from deficiencies in the modeled circulation and water mass formation. There are also a number of assumptions in the data-based approaches regarding the use of constant stoichiometric ratios and time-invariant air-sea disequilibria that may be inadequate in some regions. These are all areas of current research. Anthropogenic estimates should continue to converge as both the models and the data-based approaches are improved with time.
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