Valeria Outes

Holocene climate variability and environmental history at the Patagonian forest/steppe ecotone: Lago Mosquito (42°29'37.89''S, 71°24'14.57''W) and Laguna del Cóndor (42°20'47.22”S, 71°17’07.62”W)

Along the eastern Andes, a sharp ecotone separates steppe from North Patagonian forest dominated by Nothofagus spp. and Austrocedrus chilensis. The longitudinal position of the ecotone is largely determined by effective moisture, which in turn is partly governed by the strength and latitudinal position of the Southern Westerlies. As a result, changes in the ecotone provide an opportunity to examine past climate variations. Holocene environmental history at two sites in close proximity is inferred from pollen and high-resolution charcoal data. Prior to 9000 cal. yr BP, vegetation resembled a steppe, in accordance with widespread aridity. Fires were infrequent, likely as a consequence of fuel discontinuity associated with low vegetation cover. At 9000 cal. yr BP, forest taxa expanded into steppe and fires became frequent, indicating that summers were arid enough to support fires but winter moisture was sufficient for Nothofagus spp. to expand. A two-step increase in effective moisture is inferred for the middle Holocene. The first step occurred at 8500 cal. yr BP, as interpreted from the increase in A. chilensis in the region, probably as a consequence of an eastward migration from glacial refugia. The second step at 5500 cal. yr BP is based on a Nothofagus spp. expansion into the steppe. Steppe readvances into the forest between 5250 and 3000 cal. yr BP indicate decreased temperatures and/or effective moisture. The last 3000 years are characterized by expansions of A. chilensis and an eastward shift of the ecotone, suggesting more humid conditions. European settlement is reflected in the establishment of non-native species and disturbance-adapted taxa.

Multiple melt bodies fed the AD 2011 eruption of Puyehue-Cordón Caulle, Chile

Within the volcanological community there is a growing awareness that many large- to small-scale, point-source eruptive events can be fed by multiple melt bodies rather than from a single magma reservoir. In this study, glass shard major- and trace-element compositions were determined from tephra systematically sampled from the outset of the Puyehue-Cordón Caulle (PCC) eruption (~1 km3) in southern Chile which commenced on June 4th, 2011. Three distinct but cogenetic magma bodies were simultaneously tapped during the paroxysmal phase of this eruption. These are readily identified by clear compositional gaps in CaO, and by Sr/Zr and Sr/Y ratios, resulting from dominantly plagioclase extraction at slightly different pressures, with incompatible elements controlled by zircon crystallisation. Our results clearly demonstrate the utility of glass shard major- and trace-element data in defining the contribution of multiple magma bodies to an explosive eruption. The complex spatial association of the PCC fissure zone with the Liquiñe-Ofqui Fault zone was likely an influential factor that impeded the ascent of the parent magma and allowed the formation of discrete melt bodies within the sub-volcanic system that continued to independently fractionate.

A past-millennium maximum in postglacial activity from Volcán Chaitén, southern Chile

Volcan Chaiten (southern Chile, ∼43°S) initiated an historically unprecedented eruption in A.D. 2008, surprising the local inhabitants, Chilean and Argentine authorities, and the geologic community. Available data at the time indicated an absence of explosive eruptions from this rhyolitic volcano since a large-magnitude eruptive event dated at ca. 10,500 yr B.P. We present lake-sediment data from Lago Teo, a small closed-basin lake located in the immediate vicinity of both Chaiten township and the volcano, that spans the past ∼10,000 yr and contains 26 pyroclastic fallout deposits. Glass-shard electron microprobe analyses revealed ten rhyolitic tephras indistinguishable in composition from the 2008 Volcan Chaiten eruption, and ten others potentially derived from the Michinmahuida volcanic complex. Among the rhyolites, we detected three closely spaced tephras deposited between ca. 9460 and 9680 yr B.P., followed by two thick tephras dated at ca. 7700 and ca. 5080 yr B.P. Three other closely spaced tephras occur between ca. 600 and 850 yr B.P., the most recent prehistoric event at ca. 420 yr B.P., and a 3-cm-thick tephra deposited during the 2008 event. We calculate a median recurrence of ∼310 yr between eruptive events from all sources over the past ∼10,000 yr, and ∼200 yr between Volcan Chaiten events over the past millennium. Our results not only challenge the notion of an ∼10,500-yr-long quiescence for Volcan Chaiten activity, but also suggest that the 2008 eruption was an overdue phenomenon in the context of its postglacial eruptive history, illustrating the advantage and absolute necessity of utilizing lake-sediment archives for developing continuous well-dated time series inventories of explosive volcanic events.

Impacts to agriculture and critical infrastructure in Argentina after ashfall from the 2011 eruption of the Cordón Caulle volcanic complex: an assessment of published damage and function thresholds

The 2011 Cordón Caulle (Chile) was a large silicic eruption that dispersed ashfall over 75,000 km2 of land in Central Argentina, affecting large parts of the Neuquén, Río Negro, and Chubut provinces, including the urban areas of Villa la Angostura, Bariloche and Jacobacci. These regions all received damage and disruption to critical infrastructure and agriculture due to the ashfall. We describe these impacts and classify them according to published damage/disruption states (DDS). DDS for infrastructure and agriculture were also assigned to each area using the tephra thickness thresholds suggested by previous studies reported in the volcanological literature. The objective of this study was to evaluate whether the impacts were as expected based on the DDS suggested thresholds, and to determine whether other factors, apart from ashfall thickness, played a part. DDS thresholds based on tephra thickness were a good predictor of the impacts that occurred in the semi-arid steppe area around Jacobacci. This was unexpected as the more severe impacts were related to the challenging environmental conditions (low precipitation levels, high levels of wind erosion) and the daily wind remobilisation of ash that occurred, rather than the ashfall thicknesses received. The temperate region, including Villa la Angostura and Bariloche, performed better than the DDS assigned by ashfall thickness suggested. Despite deposits as thick as 300 mm, full recovery occurred within months of the ashfall event. The DDS scales need to incorporate a wider range of system characteristics, and environmental and vulnerability factors, as we propose here.