Circulation, oceanic, causes

The D-O events are far too rapid to be caused by insolation changes, and they most likely result from changes in ocean circulation. Their prominence and clarity in the Greenland cores relative to the Antarctic ones is due to the proximity of Greenland to the sites of deep-water formation in the North Atlantic and the tremendous amount of heat being delivered to them by the Gulf Stream (Broecker and Denton, 1989). 

The Water Cycle. The evaporation of water from land and water surfaces, the transpiration from plants, and the condensation and subsequent precipitation of rain cause a cycle of transportation and redistribution of water, a continuous circulation process known as the hydrologic cycle or water cycle (see Fig. 86). The sun evaporates fresh water from the seas and oceans, leaving impurities and dissolved solids behind when the water vapor cools down, it condenses to form clouds of small droplets that are carried across the surface of the earth as the clouds are moved inland by the wind and are further cooled, larger droplets are formed, and eventually the droplets fall as rain or snow. Some of the rainwater runs into natural underground water reservoirs, but most flows, in streams and rivers, back to the seas and oceans, evaporating as it travels. 

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... 

Thermohaline circulation Deep-water circulaUon caused by density differences created in the surface waters of polar regions. Cooling increases the density of the surface waters, which sink and then advect horizontally throughout the deep ocean. The water is returned to the sea surface by eddy diffusion. 

The density of seawater varies from a maximum of c = 29, observed in deep antarctic waters, to a minimum of c = 25 in subtropical oceanic thermoclinal waters. The high density of polar waters causes them to sink beneath subtropical waters and constitutes the driving force of deep oceanic circulation. 

NADW flows southward the ongoing oxidation of organic matter results in a progressive C-depletion down to less than 0.4%c in the Southern Ocean. Reductions in observed in many cores from the North-Atlantic (Samtheim et al. 2001 Elliot et al. 2002) have been interpreted as meltwater input to the surface ocean (Heinrich events), which caused changes in deep water circulation. 

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