Insights on the snow provenance of central western argentina using stable isotopes.

MILANA, JUAN PABLO; DAPEÑA, CRISTINA

Bahia Blanca

Congreso; II Reunión Argentina de Geoquímica de la Superficie (II RAGSU).; 2012

Instituto Argentino de Oceanografia (IADO, CONICET-UNS)

The South American Diagonal is a belt over which the marginal effects of several climatic systems take place. In this sense, we are interested in the interaction of the climate systems within this arid region that are capable to produce snow storms, with the double objective of finding an isotopic signature for each system, and if that exist, use it to trace changes in the snow provision to local glaciers. It is well known that while in Bolivia, glaciers are fed by heavy summer snow-storms coming from the Amazon basin (the ?bolivian winter?), in subtropical latitudes of 32ºS, glaciers are mainly fed by winter polar-front storms entering from the Pacific. At the central Arid Andes of Argentina, the first system creates precipitation out of a recycled amazonian-pampean water source (NE winds), while the second brings water carried by polar fronts moving northeastward from the Southern Pacific across the Andes (Zonda wind). A third snow supplying system is formed by the south wind, which is a less intense and low altitude polar front that enters from Patagonia and sometimes from the South Atlantic. All these three systems create snow falls in the San Juan province. Although the samples are not enough to elaborate statistically correct assumptions, they illustrate about a significant differentiation of the isotopic signature according to the sources described above. The 2 samples from the first system showed d18O values between -7.7 ? and -7.3 ? and d2H between -34 ? and -33 ?, the three samples of the second system gave quite different results, with d18O between -10.5 ? and -9.8 ?and d2H between -54 ? and -51 ?. The third system with 3 samples shows higher dispersal with d18O between -7.5 ? and -5.1 ? and d2H between -39? and -21 ?. All samples have high deuterium excess (>25 ?). Although these values are not common, similar values were previously reported in the central dry Andes.   The preliminary hypothesis is that samples seem to fall in two main families that differentiate sources: one with more enriched values produced by recycled water from amazonian-pampean sinks which produce summer snow storms and snow coming from weak winter fronts; and the other snow produced out of pacific humidity groups quite different with depleted compositions. These preliminary results suggests that it might be possible to identify changes in snow supplied to subtropical glaciers, and therefore track displacements of the ITCZ over longer periods of time and also the contribution of the cold-warm ENSO and SPA anomalies. Additional information should be obtained from glaciers fed today by these two dominant systems (pacific and amazonian/pampean), in order to help identifying the related isotopic signatures.