Circulation of the ocean

The natural circulation of the oceans also exchanges waters between the deep and surface ocean reservoirs. Because biological uptake con-... 

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. 

A shutting down of the large-scale circulation of the oceans (the so-called thermohaline ocean current) would probably require a 3-degree Celsius increase during this century. The extent of the disruption that would result for societies and ecosystems is uncertain but it could, for instance, result in permanent loss of the Gulf Stream, which helps warm western Europe. 

The rate of global production of bicarbonate by weathering can be determined because we loiow approximately the flow of HCO J in the world s major rivers. This represents a drain of the CO2 of the atmosphere, which must be balanced by resupply to maintain a steady One is tempted to estimate the vulnerability of atmospheric/co to the imbalance between the atmospheric CO2 drain by weathering and resupply by CaCOs precipitation by focusing on the CO2 fluxes from and to the atmosphere. This, however, would be incorrect because the ocean and atmosphere carbon reservoirs are approximately in chemical equilibrium on time scales greater than the circulation of the ocean (see Chapter 11). In order to emphasize the severity of the HCOs imbalance estimated in Fig. 2.4 one should focus on the fluxes of DlC and alkalinity between the land and ocean. Because we have not yet discussed alkalinity and DIC relationships (Chapter 4) a simple approximation can be made by considering the fluxes of bicarbonate and calcium. 

The natural circulation of the oceans also exchanges waters between the deep and surface ocean reservoirs. Because biological uptake constantly strips P from the surface layers, the P concentration is much less in the surface reservoir than the deep reservoir. Thus, although the continuity of water demands equal volumes of water to be exchanged between the reservoirs, far more P is carried by the upwelling waters (Fes) than by the downwelling waters (Fje). 

As a consequence of the general deep circulation of the ocean (Box 3.2), a permanent thermocline is present in temperate and tropical oceans (at depths of c.300 and 100m, respectively).The permanent thermocline persists throughout the year at middle and low latitudes but is absent at latitudes above c.60° because the cooler climate and reduced insolation do not cause sufficient... 

CO2. Underlying all these is a model of the physical circulation of the ocean, usually arising from an ocean general circulation model. Three such estimates of the air-sea carbon flux are shown in Figure 1. The estimates are taken from the Ocean Carbon Model Intercomparison (OCMIP) (Orr, 1997). 

It is well known that deep circulation of the oceans is much more pronotmced during periods of ice house Earth (characterized by Polar ice sheets and a well-... 

Overall, during the past roughly four decades noble gases have significantly contributed to the field of oceanography. We anticipate that future studies will provide further extensive data sets that will form the foundation for improvement in our knowledge of a wide variety of topics. These topics reach from modem circulation of the ocean to understanding processes that shaped the oceanic environment over many millions of years. 

Perhaps we can invoke Uniformitarianism by pointing out that North America and northern Eurasia were in the grip of continental ice sheets only about 10,000-15,000 years ago and that the climate of these areas has warmed up and now supports abundant vegetation without requiring a major change in latitude because the circulation of the oceans and of the atmosphere distribute solar heat and thereby determine the climate in different parts of the Earth. 

While river discharge is the primary means of transferring water from the land to the oceans, its magnitude pales in comparison to circulation within the oceans themselves. The total average... 

The chemistry and biology of the ocean are superimposed on the ocean s circulation, thus it is important to review briefly the forces driving this circulation and give some estimates of the transport rates. There are many reasons why it is important to understand the basics of the circulation. Four examples are given as an illustration. 

In this section we briefly review what controls the density of seawater and the vertical density stratification of the ocean. Surface currents, abyssal circulation, and thermocline circulation are considered individually. 

It seems unlikely that all of the oceanic crust produced interacts with seawater (Holland, 1978). Accepting that 30% of oceanic crust interacts with circulating seawater, hydrothermal As flux is estimated to be (3.8-0.1) x lO g As/year. This flux, although... 

At lower temperatures, reducing conditions are present (CH4 is stable) this is typical for the oceanic crust. Most of the hydrothermal water circulates in the oceanic crust at a temperature of around 420 K, and the reducing conditions present there are mainly controlled by the PPM mineral mixture (Alt et al., 1989). 

Fig. 3-1. A simple representation of the general circulation of the global ocean, adapted from Broecker and Peng (1982, p. 382). The arrows denote fluxes of water expressed in Sverdrups (1 Sverdrup = 106 m3/sec). River water is added to the surface reservoirs, and an equal volume of water is removed by evaporation.

The results for 14C are plotted in Figure 6-3. Again, the response of the atmosphere is quite pronounced. The response of the shallow ocean is less marked, and the deep ocean shows no response at all on this time scale. Radiocarbon ratios are lower in the ocean than in the atmosphere because radioactive decay reduces the 14C ratio. The difference between the steady-state atmosphere and the steady-state values in the oceanic reservoirs is an indication of how much time has elapsed since these masses of water last equilibrated with the atmosphere. Measurements of radiocarbon are an important source of information on the circulation of the deep ocean, and the differences between 13C ratios in the different reservoirs have quite different causes The deep ocean is lighter than the surface ocean because... 

---