Deep water age

Earlier in Ghapter 5 a map of the G age of DIG in the ocean s deep waters (Fig. 5.17) revealed that the age difference between the northern North Atlantic Deep Water and that in the Northeast Pacific is C.1700 y. This value compares the most recent and most ancient ventilation ages of the deep ocean, whereas the box model compares the mean deep water age of the entire ocean, c. - 160%o (c. 1480 y) with that of the surface ocean, c. - 50%o (400 y) (1480 - 400 = 1080 y). In some ways, the largest task of the two-layer-ocean calculation is determining representative A G values for the mean surface and deep ocean. More complicated models with more reservoirs (see, for... 

Fig. 10-12 The A C values of the cores of the North Atlantic, Pacific, and Indian Ocean deep waters. The oldest waters are encountered near 40°N in the Pacific Ocean. (Modified with permission from M. Stuiver et al. (1983). Abyssal water carbon-14 distribution and the age of the world oceans. Science 219 849-851, the AAAS.)...

Mangini A, Lomitschka M, Eichstadter R, Frank N, Vogler S, Bonani G, Hajdas I Patzold J (1998) Coral provides way to age deep water. Natme 392 347-348... 

Adkins JF, Boyle EA (1997) Changing atmospheric A " C and the record of deep water paleoventilation ages. Paleoceanography 12(3) 337-344... 

The higher the AOU, the greater the amount of O2 removed since the water mass was last at the sea smfece. Thus, AOU increases with increasing distance from the site at which the subsurfece water mass was formed. Since the AOU increases with the age of the water mass, the pathway of deep-water circulation can be traced from the distribution of AOU in the deep zone. As shown in Figure 8.2, the AOU at 4000 m is lowest in polar regions, indicating these areas are the sites of deep-water formation. 

As a water mass ages, the ratio of [CO3 ] to [HCO3] declines because the continuing generation of CO2 from the remineralization of POC pushes the equilibrium reaction in Eq. 15.18 further toward the products. Thus, as a deep water mass ages, it becomes increasingly more undersaturated with respect to biogenic calcium carbonate. 

North Atlantic to 500 m in the North Pacific. This reflects an increasing addition of CO2 to deep waters as meridional overturning circulation moves them from the Atlantic to the Indian and then to the Pacific Ocean. Thus, as a water mass ages, it becomes more corrosive to calcium carbonate. Since aragonite is more soluble than calcite, its saturation horizon lies at shallower depths, rising from 3000 m in the North Atlantic to 200 m in the North Pacific. 

Figure 7. Plots of Hg concentration versus 210Pb age for deep-water cores. Dates older than about 1800 are extrapolations based on mean dry-mass sediment accumulation rates.

The concentration of RDOM is simply equal to the measured concentration of DOM in deep waters (>1000 m), where its apparent radiocarbon age of 4000-6000 years is substantially greater than the timescale of thermohaline circulation in the earth s oceans (Druffel et al., 1992). Bioassay experiments have been used to verify the refractory nature of DOM in the deep sea (Barber, 1968). 

Example In the Milo Holdings 3 well, 1421 m deep, in the Jurassic rock sequence of the Great Artesian Basic, east Australia, the helium concentration of 1000 x 10 x cc/cc water was found (Mazor and Bosch, 1990). The following local parametric values were applied to calculate the water age H = 1 U = 1.7 ppm Th = 6.1 ppm d= 2.6 effective porosity of 0.2 or a rock water ratio of 4. Thus the age of this groundwater was found to be... 

The obtained age for the deep water in the artesian Milo Holdings 3 well is semiquantitative because of uncertainties in the parameters applied in the calculation. Yet the order of magnitude—millions of years—is of utmost importance the water resource is not renewed, but it is entirely shielded from the surface and, hence, is immune to anthropogenic pollution. 

Shackleton N. J., Duplessy J.-C., Arnold M., Maurice P., Hall M. A., and Cartlidge J. (1988) Radiocarbon age of the last glacial Pacific deep water. Nature 335, 708-711. 

Cresswell R. G., Bauld J., Jacobson G., Khadka M. S., Jha M. G., ShresthaM. P., and Regmi S. (2001) A hrst estimate of ground water ages for the deep aquifer of the Kathmandu Basin, Nepal, using the radioisotope chlorine-36. Ground Water 39, 449-457. 

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