Glacial ages estimation

Figure 17 Average shell weights for whole G. sacculifer shells of glacial age as a function water depth. The open circles are for cores from the Ontong-Java Plateau, and the closed circles are for cores from the equatorial Atlantic. The vertical dashed line is the initial G. sacculifer weight obtained from measurements on Holocene samples from the Ceara Rise. The solid vertical line is the estimated initial weight for glacial-age G. sacculifer based on the assumption that the carbonate ion concentration dependence of initial shell weight established by Barker and Elderfield (2002)...

Figure 4.8 shows an example of the estimation of paleotemperature based on noble gas measurements in the groundwater of some aquifers in Texas (State et al., 1992). Paleotemperatures estimated from the noble gas concentration shows a large drop (about 5.2°C) at the distance 30km away from the recharge area [Figure 4.8(c)], which corresponds to an age between 12,000 and 17,000 years, close to the last glacial maximum (about 18,000 years ago). 

With the crude age one can estimate the paleoenvironmental conditions in terms of glacial or interglacial period, sea level, distance of paleorecharge area from the seashore, season of major recharge, altitude of the former water table, former evapotranspiration intensity, or the type of plant community that prevailed at the studied paleorecharge site. These considerations should lead to the selection of the pertinent paleo-input functions. 

Since the age of agriculture is estimated to be about 10 ky, it follows that any discussion of natural inputs of minerals to agricultural soils from an external source will be limited to comparatively recent phenomena. Thus, rocks and minerals that have been transported in the distant geologic past by ice, wind, and water to form glacial, aeolian, alluvial, lacustrine, and marine deposits are outside the scope of this chapter, since these are generally parent materials from which modem soils were formed. Dense populations usually developed where the soils had been enriched by recent volcanic activity or by young alluvial deposits. 

The development of more sensitive ways to measure levels of radioactive substances has allowed scientists to take advantage of the decay of nuclides other than carbon-14. For example, chlorine-36 can be used to date ground water, marine sediments can be dated by measuring levels of beryllium-11 and aluminum-26, and krypton-81 has been used to estimate the age of glacial ice. 

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