Nadja Insel

Onset of Convective Rainfall During Gradual Late Miocene Rise of the Central Andes

Separated About Lift The uplift history of the Andes of South America is a contentious issue, with the two main hypotheses polarizing from rapid growth between roughly 10 and 7 million years ago to more gradual elevation over most of the past 40 million years. The oxygen isotopic composition of soil carbonates has been used as a proxy for altitude and to measure the timing of uplift. Poulsen et al. (p. 490, published online 1 April) applied a global atmospheric general circulation model to show that the oxygen isotopic composition changes seen in carbonates formed in the late Miocene were driven more by changes in the amount of precipitation than by the altitude at which the precipitation forms. Consequently, it seems that oxygen isotopes are not a reliable paleoaltimeter, and Andean uplift may not have been as precipitate as thought. Increased precipitation, rather than rapid uplift, drove isotopic changes in soil carbonates of the Andes in the late Miocene. A decrease in the ratio of 18O to 16O (δ18O) of sedimentary carbonate from the Bolivian Altiplano has been interpreted to indicate rapid surface uplift of the late Miocene Andean plateau (AP). Here we report on paleoclimate simulations of Andean surface uplift with an atmospheric general circulation model (GCM) that tracks oxygen isotopes in vapor. The GCM predicts changes in atmospheric circulation and rainfall that influence AP isotopic source and amount effects. On eastern AP slopes, summer convective precipitation increases by up to 6 millimeters per day (>500%) for plateau elevations that are greater than about 2000 meters. High precipitation rates enhance the isotope amount effect, leading to a decrease in precipitation δ18O at high elevations and an increase in δ18O lapse rate. Our results indicate that late Miocene δ18O depletion reflects initiation and intensification of convective rainfall.

Linking orography, climate, and exhumation across the central Andes

Quantifying interactions between uplift, climate, deformation, and exhumation remains difficult, in many cases due to a paucity of data relevant to all processes. We synthesize new and existing data to understand the orogen-scale orographic changes across the central Andes, Bolivia. We use a regional climate model and geo-thermochronologic data to identify the correlations between changes in precipitation due to surface uplift and spatiotemporal patterns of deformation and erosional exhumation. Mean orographic rainfall patterns do not reach near present-day gradients and values until the topography grows to >75% modern elevations. New fission-track data near the orocline apex indicate that rapid exhumation moved eastward, beginning in the Eastern Cordillera ca. 50–15 Ma, the Interandean zone ca. 18–6 Ma, and in the Subandes ca. 7–3 Ma. Throughout Bolivia, exhumation is consistent with deformation until ca. 15–11 Ma, after which the pattern corresponds better with the increased rainfall toward modern values. These linked observations suggest that ca. 15–11 Ma, regional elevations reached threshold values (>75% modern) necessary to generate near present-day, enhanced rainfall gradients. These gradients have resulted in variable exhumation implied by the structural level of rocks exposed across the thrust belt and confirmed by fission tracks in apatite. The main insight is that the climate-induced Middle Miocene–recent exhumation varies over scales of a few hundred kilometers across Bolivia and implies that high mean rainfall (>∼3 m/yr) and long time scales (∼10 m.y.) may be necessary for climate to induce orographically driven exhumation patterns recorded by fission tracks.