Abstract
Determining how the elevation of the Northern Andes has evolved over time is of paramount importance for understanding the response of the Northern Andes to deformational and geodynamic processes and its role as an orographic barrier for atmospheric vapor transport over geologic time. However, a fundamental requirement when using stable isotope data for paleoaltimetry reconstructions is knowledge about the present-day changes of δ18O and δD with elevation (isotopic lapse rate). This study defines the present-day river isotopic lapse rate near the Equator (∼3°S) based on analysis of δ18O and δD of surface waters collected from streams across the Western Cordillera and the Inter-Andean depression in Southern Ecuador. The results for the two domains show a decrease of δ18O with elevation which fits a linear regression with a slope of −0.18‰/100 m (R2= 0.73,n= 83). However, we establish a present-day lapse rate of −0.15‰/100 m for δ18O (R2= 0.88,n= 19) and -1.4‰/100 m for δD (R2= 0.93,n= 19) from water samples collected along the west facing slopes of the Western Ecuadorian Cordillera which is mainly subject to moisture transport from the Pacific. We argue that this empirical relationship, consistent with those obtained in different tropical areas of the world, can inform stable isotope paleoaltimetry reconstructions in tropical latitudes.
Highlights
The long-term climatic evolution and atmospheric circulation patterns of the Earth are influenced on the first order by the topography of large mountain chains (e.g., Molnar and England, 1990; Boos and Kuang, 2010; Barnes et al, 2012).The Andes are a non-collisional subduction orogen and represent one of the main topographic features on Earth
1) Air masses from the Pacific Ocean that directly rise across the Western Cordillera (Puerto Inca transect) experience Rayleigh distillation on the windward side; δD and δ18O values define a linear relationship between δ18O and altitude of -0.15‰/100 m (Figure 3A). 2) The δD and δ18O values of precipitation in the Inter-Andean valley cannot be directly correlated with elevation; they display additional variability that can be explained by a combination of factors such as mixing of air masses from the Pacific, the Atlantic and the Amazon Basin, evaporation, and seasonality
Calculated from samples sourced at lower elevation, the lapse rate from the Pasaje transect provides a δ18O and δD vs. altitude relationship of -0.15‰/100 m (δ18O; R2 0.65; Figure 3A) and -1.4‰/100 m and confirms the altitude effect on the isotope composition of tropical rains observed at higher elevation. These lapse rates are lower than those compiled or modeled for mid-latitude settings (-0.28 ‰/100 m in δ18O or -2.0 ‰/100 m in δD) (e.g., Poage and Chamberlain, 2001; Quade et al, 2011). They are in good agreement with isotope-elevation relationships defined from the isotopic composition of tropical rains in Cameroon (−0.156‰/100 m in δ18O; Gonfiantini et al, 2001), in the Colombian Eastern Cordillera (-0.18‰/100 m in δ18O and −1.46‰/100 m in δD; Saylor et al, 2009) and in Ecuador (−0.17‰/100 m in δ18O; Garcia et al, 1998) using water samples from springs and International Atomic Energy Agency (IAEA) weather stations along two transects across the Western and Eastern Cordillera near 1 and 3°S latitude (Supplementary Table S1 and Supplementary Figure 2S)
Summary
The long-term climatic evolution and atmospheric circulation patterns of the Earth are influenced on the first order by the topography of large mountain chains (e.g., Molnar and England, 1990; Boos and Kuang, 2010; Barnes et al, 2012).The Andes are a non-collisional subduction orogen and represent one of the main topographic features on Earth. Most studies aiming at understanding the links among geodynamic processes, Andean topography, and climate have focused on the Southern and Central Andes; the role and overall long-term landscape evolution of the Northern Andes remains enigmatic. This negligence is partly due to the scarce paleoaltimetry data obtained in the Northern Andes, and especially in the Ecuadorian Andes, a region of intense debate about the timing and rate of topographic development (e.g., Steinmann et al, 1999; Gregory-Wodzicky, 2000; Hungerbuhler et al, 2002; Witt et al, 2017; Jackson et al, 2019).
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