Streamflow variations across the Andes (18°-55°S) during the instrumental era.

The rivers originating in the southern Andes (18°-55°S) support numerous ecosystems and a large number of human populations and socio-economic activities in the adjacent lowlands of Chile, Argentina and Bolivia. Here we show that ca. 75% of the total variance in the streamflow records from this extensive region can be explained by only eight spatially coherent patterns of variability. Five (three) of these Andean patterns exhibit extreme dry (wet) conditions in recent years, with strong interannual variations in northern Chile; long-term drying trends between 31° and 41°S; a transitional pattern in the central Patagonian Andes; and increasing trends in northwestern Argentina and southern Bolivia, the Fueguian Andes, and the eastern portion of the South Patagonian Icefield. Multivariate regression analyses show that large-scale indices of ENSO variability can predict 20% to 45% of annual runoff variability between 28° and 46°S. The influence of Antarctic and North Pacific indices becomes more relevant south of 43°S and in northwestern Argentina and southern Bolivia, respectively, but their overall skill as predictors of Andean streamflows is weak. The analyses provide relevant new information to improve understanding of the spatial coherence, the main temporal features, and the ocean-atmospheric forcings of surface runoff across the southern Andes.

Tree-ring correlations suggest links between moderate earthquakes and distant rockfalls in the Patagonian Cordillera.

Earthquakes with magnitudes M > 7 can trigger large landslides and rockfalls at epicenter distances of up to 400 km, whereas moderate shaking (M = 5-7) is generally thought to result in abundant co-seismic mass movements in the vicinity of the epicenter. Although one might anticipate that large magnitude earthquakes off the Chilean coast would result in abundant rockfall in the Patagonian Cordillera, only limited research has explored this hypothesis. Here, we use tree-ring records from 63 cross-sections of century-old (103.9 ± 40.1 yr) Nothofagus pumilio trees to develop a calendar-dated record of small rockfall events (101-102 m3) on a talus slope located next to Monte Fitz Roy (El Chaltén, Argentina; 49°4'S, 72°57'W). The resulting rockfall record is used to infer that subduction zone seismicity at the Triple Junction and intraplate shaking around Lago Argentino almost systematically caused rockfall activity at this site, even if seismicity occurred at large distances (up to 300 km away) and with moderate intensity (M = 5-7). About one third of the rockfalls are triggered by factors other than earthquakes, predominantly in spring when freeze-thaw cycles occur frequently at the site. Despite the fact that seismicity is not the only trigger of rockfall activity at Cerro Crestón, at the foot of Monte Vespignani, we conclude that, in regions where topographic amplification plays a role, small rockfalls can be triggered by earthquakes of moderate intensity at large distances from the epicenter.

Holocene timberline fluctuations in jasper national park, alberta.

Pollen, fossil logs, and macrofossils from three high-elevation sites in the Maligne Range, Jasper National Park, Alberta, provide the first detailed record of timberline fluctuations in the Canadian Rockies during the last 8700 years. Timberlines were much higher than at present between 8700 to 5200 years ago but oscillated significantly in elevation, with a major episode of timberline recession punctuating two periods of high timberline between about 6700 to 5900 and about 8700 to 7000 years ago. Since 5200 years ago, regional timberlines have generally receded with perhaps brief reversals, reaching their lowest recorded positions sometime after 500 years ago.