Changes in the Recharge Season Possible Amplification of the Temperature Change Recorded by the Noble Gases

A relatively small drop in the average temperature during the Ice Age could cause a shift of the rainy season from summer rains to winter rains. Such a change in the recharge season would not affect the noble gas based paleo-temperatures in the case of groundwater recharged through a porous aerated zone of a thickness of greater than 15 m, that is, enough to average out the seasonal and annual temperature variations. However, the change in the recharge season would result in an amplification of the temperature change recorded by noble gases in the case of fast percolating water through a thin or conduit-dominated aerated zone. In such systems the noble gas concentrations reflect a temperature that is between that of the rainy season, or snowmelt, and the average annual temperature (Herzberg and Mazor, 1979).

Effects of this nature should be borne in mind when addressing an inconsistency between reconstructed oceanic and continental temperatures, as pointed out by Stute et al. (1995a). The authors reported a temperature decrease of about 5 °C during the last glacial maximum, as recorded by noble gases in groundwaters at several locations in temperate countries, whereas the corresponding temperature decrease of the ocean surface was

Fig. 15.1 Suggested formation of slightly saline groundwater, found in the Mediterranean coastal plane of Israel. (a) The filling up of the basin of the Mediterranean Sea as a result of the reopening of the Strait of Gibraltar: intensified sea spray of evaporitic nature prevailed, resulting in saltier rain and saline groundwater (open arrows). (b) The present Mediterranean Sea (open to the Atlantic Ocean), characterized by a higher sea level (bringing the ancient saline groundwater to below the terminal base of drainage, and thus making it stagnant), less sea spray, and formation of fresher groundwater (black arrows) overlying the ancient saline groundwater; a well producing fresh groundwater is marked. (c) Overpumping causes local lowering of the level of the fresh water table, resulting in lateral encroachment of remnants of the ancient saline groundwater, causing salinization of a coastal well.

Fig. 15.1 Suggested formation of slightly saline groundwater, found in the Mediterranean coastal plane of Israel. (a) The filling up of the basin of the Mediterranean Sea as a result of the reopening of the Strait of Gibraltar: intensified sea spray of evaporitic nature prevailed, resulting in saltier rain and saline groundwater (open arrows). (b) The present Mediterranean Sea (open to the Atlantic Ocean), characterized by a higher sea level (bringing the ancient saline groundwater to below the terminal base of drainage, and thus making it stagnant), less sea spray, and formation of fresher groundwater (black arrows) overlying the ancient saline groundwater; a well producing fresh groundwater is marked. (c) Overpumping causes local lowering of the level of the fresh water table, resulting in lateral encroachment of remnants of the ancient saline groundwater, causing salinization of a coastal well.

only about 2 °C, as indicated by oxygen isotope measurements on foraminifera and fauna abundances in deep sea sediments (section 13.8.3).

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