The Upper Ocean

The distribution of CO2 and the associated carbonic acid system species in the upper ocean (here loosely defined as waters above the thermocline and generally only a few hundred meters in depth) is primarily controlled by the exchange of CO2 across the air-sea interface, biological activity, and circulation of the ocean, mainly through vertical mixing processes. Other factors, such as the temperature and salinity of the water, can also contribute to variations by influencing the solubility of CO2 in seawater and the equilibrium constants of the carbonic acid system. 

Revelle and Suess (1957) introduced a dimensionless factor (y), commonly referred to as the Revelle factor, to describe the relation between changes in Pc02 and XCO2 in seawater. The equation for this relation (based on the form of Takahashi et al., 1980a), where A = alkalinity, is  

The Revelle factor is about 10 for typical surface seawater. The details of the chemistry of this general relationship and its derivation have also been discussed by Sundquist et al. (1979), who called it the homogeneous buffer factor. Of interest is the fact that using the Revelle factor one can calculate for an instantaneous change in the Pco2 °f the atmosphere, the distribution of carbon between the atmosphere and seawater. 

It should be kept in mind that, in spite of these major variations in the CO2-carbonic acid system, virtually all surface seawater is supersaturated with respect to calcite and aragonite. However, variations in the composition of surface waters can have a major influence on the depth at which deep seawater becomes undersaturated with respect to these minerals. The CO2 content of the water is the primary factor controlling its initial saturation state. The productivity and temperature of surface seawater also play major roles, in determining the types and amounts of biogenic carbonates that are produced. Later it will be shown that there is a definite relation between the saturation state of deep seawater, the rain rate of biogenic material and the accumulation of calcium carbonate in deep sea sediments. 

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Other limitations on phytoplankton growth are chemical in nature. Nitrogen, in the form of nitrate, nitrite and ammonium ions, forms a basic building material of a plankton s cells. In some species silicon, as silicate, takes on this role. Phosphorus, in the form of phosphate, is in both cell walls and DNA. Iron, in the form of Fe(III) hydroxyl species, is an important trace element. Extensive areas of the mixed layer of the upper ocean have low nitrate and phosphate levels during... 

The quantity of primary production that is exported from the upper ocean is said to be equivalent to new production (18, 19) New primary production is that associated with allocthonous nutrients (i.e., those upwelled or mixed into the euphotic zone or input via rivers and rain). In order for steady state to be maintained, an equivalent flux out of the euphotic zone is required. Earlier studies (19) suggested that sediment-trap measurements of particulate organic carbon (POC) flux were equivalent to new primary production however, recently it has become clear that these measurements probably represent only a... 

Alt, J.C. and Honnorez, J. (1984) Alteration of the upper oceanic crust, DSDP site 417 mineralogy and chemistry. Contr. Mineral. Petrol, 87, 149-169. 

Cochran JK, Buesseler KO, Bacon MP, Livingston HD (1993). Thorium isotopes as indicators of particle dynamics in the upper ocean Results from the JGOFS North Atlantic Bloom Experiment. Deep-Sea... 

Moran SB, Buesseler KO (1992) Short residence time of colloids in the upper ocean estimated from 238u 234-rh disequilibria. Nature 359 221-223... 

Andrew Dickson (Chair) is an Associate Professor-in-Residence at the Scripps Institution of Oceanography. His research focuses on the analytical chemistry of carbon dioxide in sea water, biogeochemical cycles in the upper ocean, marine inorganic chemistry, and the thermodynamics of electrolyte solutions at high temperatures and pressures. His expertise lies in the quality control of oceanic carbon dioxide measurements and in the development of underway instrumentation for the study of upper ocean biogeochemistry. Dr. Dickson served on the NRC Committee on Oceanic Carbon. He is presently a member of the IOC C02 Advisory Panel and of the PICES Working Group 13 on C02 in the North Pacific. 

The concentration of 210Po in seawater is generally close to what would be expected from radiochemical equilibrium with its grandparent, 210Pb. Below 400 m, Po activities are very similar to those of Pb. At shallower depths, the concentrations of Po and Pb are somewhat decoupled. Po is more readily adsorbed by particles than is Pb in the surface ocean (Bernat and Church, 1989) and, due to the short residence times of particles in the upper ocean, the relative concentrations of Po and Pb are controlled, in part, by particle fluxes and differences in polonium and lead solution and surface chemistries. The chemistry of... 

Fluorine exists as F and MgF+ in seawater in approximately equal concentrations. The free ion fractions of Cl and Br- are essentially 100%. Iv in the form IO3 is the thermodynamically favoured (stable) form of iodine in oxygenated seawater. In the upper ocean, where thermodynamic disequilibria are common, a significant fraction of total iodine is present as iodide (I-). IO3, like I , is weakly interactive. 

Longhurst, A.R. and Harrison, W.G., The biological pump profiles of plankton production and consumption in the upper ocean, Prog. Oceanogr., 22, 47, 1989. 

One of the major environmental issues of our time is the impact of anthropogenically generated CO2 on the environment (see Chapter 9 for discussion). The major processes associated with fossil fuel CO2 in the oceans are the uptake of fossil fuel CO2 by the upper ocean, mixing and transport within the ocean, and reaction with calcium carbonate in sediments. In addition, biologic productivity may be influenced by increased Pc02 values- The two major reactions for uptake of CO2 by the oceans, beyond those that would occur for a chemically unreactive gas, are in a simple form ... 

It has been proposed that temperature gradients in the oceans could be used as a source of power. Assuming that water is at 4°C at lower levels, where it has maximum density, and at upper levels, it has a temperature of 25°C, typical of the tropical ocean, what is the maximum efficiency with which heat could be extracted from the upper ocean to produce work ... 

Bruland, K.W., and Coale, K.H. (1986) Surface water 234Th/238U disequilibria spatial and temporal variations of scavenging rates within the Pacific Ocean. In Dynamic Processes in the Chemistry of the Upper Ocean (Burton, J.D., ed), pp. 159-172, Plenum Publications, New York. 

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