87Sr/86Sr in Gypsic Relict Soils and Paleosols of Hyperarid Settings as a Paleoaltitude Proxy: Results for Northern Chile (19.5-21.7°S)

JORDAN, TERESA E.; COSENTINO, N.J.

San Francisco

Conferencia; AGU Fall Meeting 2014; 2014

AGU

We have developed a new altimeter proxy based on the 87Sr/86Sr ratio ofsurficial accumulations of salts in hyperarid settings. Under hyperaridconditions like in the Andean forearc in northern Chile at altitudesbelow ~3000 m.a.s.l., gypsum and other soluble salts form saline soils.The altimeter is based on the first-order topographic control on theextent of inland incursion of fog-transported marine aerosols derivedfrom the Pacific Ocean. Once this fog is evaporated it deposits calciumsulfates with a marine 87Sr/86Sr signal. At positions in the landscapenot reached by marine aerosols, calcium sulfates have no marine source,except by eolian reworking, and have a lower 87Sr/86Sr. 87Sr/86Sr valuesfor Holocene accumulations of salts show a bimodal distribution: highvalues between 225-1075 m.a.s.l. (0.70807 ± 0.00004) and lowvalues below and above those altitude thresholds (0.70746 ±0.00010). We sampled dated gypsic relict soils and Gypsisols to studypost-5 Ma surface height evolution using this method, after correctingfor changes in the altitudinal structure of the paleo-fog zone withtime. Locations spanning ~250 km strike-parallel distance within theforearc and at elevations between 450-1650 m.a.s.l. display moderateamounts of altitudinal change during the Pliocene, Pleistocene andQuaternary. Site results include two in the northern zone (19.5ºSand 19.65ºS) that experienced maximum net surface uplifts of ~350 mand ~600 m since 2.5 ± 0.5 Ma and ~2.6 Ma, respectively.Locations at ~21.4°S display an estimated minimum net decline inaltitude of ~250 m since 3.4 ± 1.1 Ma and ~200 m since 1.1± 0.4 Ma. These constraints will shed light on the geodynamicprocesses responsible for surface elevation change, by acting asboundary conditions to numerical models of the Andean forearc.