HR: 1330h
AN: PP42A-0872    [PDF]
TI: Noble Gas Thermometry and Hydrologic Ages: Evidence for Late Holocene Warming in Southwest Texas
AU: * Castro, M
EM: mccastro@umich.edu
AF: University of Michigan, Department of Geological Sciences C.C. Little Building 425 E. University Ave., Ann Arbor, MI 48109 United States
AU: Goblet, P
EM: patrick.goblet@ensmp.fr
AF: Ecole des Mines de Paris, Centre d\'{}Informatique G\'{e}ologique 35, rue Saint Honor\'{e}, Fontainebleau, 77305 France
AB: Paleoclimatic reconstruction through the use of noble gases dissolved in groundwater has been the object of numerous studies in recent years. Unlike many other continental temperature proxies, noble gases have the advantage of providing direct information on atmospheric temperatures at the time rainwater penetrated the ground and joined a particular groundwater reservoir. In recent years, new methods for determination of noble gas temperatures have been developed, which provide a high level of accuracy on such temperature estimations. The issue of paleoclimatic reconstruction through noble gases however, is not only one of accurate temperature determination, but also one of accurate water age estimation so that a correct correspondence between noble gas temperatures and groundwater age can be established and proper paleoclimatic reconstruction attempted. The typical approach to estimate groundwater ages has been based on computing water travel times along streamlines from the recharge to the observation point taking into account only advection. This approach is limited because, like any other tracer, the movement of water in porous media is also affected by cinematic dispersion and molecular diffusion. We have therefore undertaken the formulation of hydrologic models that yield significantly better constraints on groundwater ages in the Carrizo aquifer and surrounding formations of south Texas, where noble gas temperatures have already been determined. To account for groundwater mixing we treat age as one would treat a solute concentration. In order to simulate groundwater ages we used a finite element model of groundwater flow that has been validated by $^{4}$He and $^{3}$He. The finite model spans a 120.6 Km cross-section between altitudes of +220m and -2210 m, and comprises 58,968 elements and 31,949 nodes. Combination of these newly calculated water ages and previously reported noble gas temperatures reveals new aspects of late Pleistocene and Holocene climate in southwestern Texas, in particular, an abrupt late Holocene temperature increase previously unidentified through $^{14}$C dating. Temperature increased by up to $3.4\deg$C in the first half of the last millennium and by $1.5\deg$C between $\sim$5.6 and 3.7 kyrs BP. More important than the resolution of individual paleoclimate episodes is the identification of a slow cooling trend between $\sim$1,200 kyrs and $\sim$200 kyrs, a trend that accelerates during the late Pleistocene and early Holocene. This cooling trend gives way to an extremely rapid increase in temperature in the late Holocene. Such abrupt warming seems to have accelerated in the last millennium and seems to continue at present. This temperature increase is the most striking feature arising from the determination of new groundwater ages.
DE: 1620 Climate dynamics (3309)
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1833 Hydroclimatology
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting