Geological evidence for a drought episode in the western Pampas (Argentina, South America) during the early–mid 20th century

Abstract

Drought episodes during the early-mid 20th century were recognized and described in several places around the world, with extreme dry conditions and widespread landscape denudation, like during the famous 'Dust Bowl' in North America. However, there is scant documentation of droughts in southern South America, particularly from the Pampas, and none based on the geological record. In this article, we provide clear evidence of aeolian reactivation and sand deposition in some areas of La Pampa and San Luis provinces, western Pampas (Argentina), during early-mid 20th century in response to drier conditions, probably amplified, like historic droughts in North America, by anthropogenic factors (e.g. significant population increase and agriculture expansion into a fragile environment). Evidence includes widespread bare sand blowouts, extensive surfaces with active sand migration, steep dune lee slopes, and sharp crests covered by weak soil development (A/C profile), accompanied by historical documents. Optically stimulated luminescence (OSL) ages on aeolian beds confirm mobilization and sedimentation by wind processes c. 95-60 yr BP. Considering the dominant (over 70%) austral springsummer precipitation, it is possible the rainfall deficit in western Pampas was linked to positive sea surface temperature (SST) anomalies in the western subtropical South Atlantic Ocean (20-30°S and 30-50° W), according to significant canonical correlation between the precipitation field in subtropical South America and the Atlantic Ocean SST anomalies.

Eolian depositional phases during the past 50 ka and inferred climate variability for the Pampean Sand Sea, western Pampas, Argentina

“…These results, combined with prior observations (Tripaldi and Forman, 2007;Tripaldi et al, 2013;Forman et al, 2014), yield a basis for identifying six eolian depositional phases, formed by successive eolian strata interposed with paleosols and/or bounded by erosive surfaces. Age control was provided by optically stimulated luminescence (OSL) dating of small aliquots of quartz grains from eolian stratigraphic units and this approach has yielded previously a consistent chronology (Tripaldi and Forman, 2007;Tripaldi et al, 2013;Forman et al, 2014). This paper provides new insight into the nature and the timing of eolian depositional phases for the past ca.…”

“…Of the 61 OSL ages presented 15 ages are new, with the remaining 47 ages reported previously (Tripaldi and Forman, 2007;Tripaldi et al, 2013;Forman et al, 2014) (Table 2). The multiple aliquot regenerative (MAR) dose method (Jain et al, 2003) was applied to four out of 61 quartz separates (Tripaldi and Forman, 2007).…”

“…The datum year for all OSL ages is AD 2000 to be compatible with previous reported ages (cf. Tripaldi and Forman, 2007;Tripaldi et al, 2013;Forman et al, 2014).…”

“…These deposits, as indicated by OSL ages, span from~50 ka ago to the 20th century ( Table 2). The integration of previously presented sections (Tripaldi and Forman, 2007;Tripaldi et al, 2013;Forman et al, 2014), and new observations for the oldest dated stratigraphic units, provides a larger data base for inferring the extent, timing and paleoenvironmental significance of the eolian deposits in the western Pampas.…”

“…These episodes of dune reactivation are inferred to reflect an intrinsic response to periodic droughts. More recently, in the 1930s the western Pampas experienced a severe drought associated with 30e60% deficit in precipitation (Compagnucci et al, 2002) that was concomitant with agriculture landscape disturbance, which resulted in pervasive reactivation of eolian sand systems (Tripaldi et al, 2013).…”