Abstract

The tropical Andes of Peru and Bolivia are important for preserving geomorphic evidence of multiple glaciations, allowing for refinements of chronology to aid in understanding climate dynamics at a key location between hemispheres. This review focuses on the deglaciation from Late-Pleistocene maximum positions near the global Last Glacial Maximum (LGM). We synthesize the results of the most recent published glacial geologic studies from 12 mountain ranges or regions within Peru and Bolivia where glacial moraines and drift are dated with terrestrial cosmogenic nuclides (TCN), as well as maximum and minimum limiting ages based on radiocarbon in proximal sediments. Special consideration is given to document paleoglacier valley localities with topographic information given the strong vertical mass balance sensitivity of tropical glaciers. Specific valley localities show variable and heterogeneous sequences ages and extensions of paleoglaciers, but conform to a generally cogent regional sequence revealed by more continuous lake sedimentary records. There are clear distributions of stratigraphically older and younger moraine ages that we group and discuss chronologically. The timing of the local LGM based on average TCN ages of moraine groups is 25.1 ka, but there are large uncertainties (up to 7 ka) making the relative timing with the global LGM elusive. There are a significant number of post-LGM moraines that date to 18.9 (± 0.5) ka. During the Oldest Dryas (18.0 to 14.6 ka), moraine boulders date to 16.1 (± 1.1) ka, suggesting that glaciers either experienced stillstands or readvances during this interval. The Antarctic Cold Reversal (ACR; 14.6 to 12.6 ka) is another phase of stillstanding or readvancing glaciers with moraine groups dating to 13.7 (± 0.8) ka, followed by retreating ice margins through most of the Younger Dryas (YD; 12.9 to 11.8 ka). During the early Holocene, groups of moraines in multiple valleys date to 11.0 (± 0.4) ka, marking a period when glaciers either readvanced or paused from the overall trend of deglaciation. The pattern of glacial variability during the Late Glacial after ~14.6 ka appears to be more synchronous with periods of cooling in the southern high latitudes, and out-of-phase with the overall deglacial trend in the Northern Hemisphere. While insolation and CO2 forcing likely drove the general pattern of deglaciation in the southern tropical Andes, regional ocean-atmospheric and hypsometric controls must have contributed to the full pattern of glacial variability.

Highlights

  • The Andes of Peru and Bolivia comprise numerous glaciated ranges containing most of Earth’s extant tropical glaciers, along with multiple superimposed moraines that have provided glacial geologists appealing time-space markers to infer the extent and timing of late-Pleistocene to Holocene climate changes in the tropics (Clapperton, 1972; Hastenrath, 1985)

  • We focus on sites that have published data from specific valleys with radiometrically-dated glacial geomorphologic evidence to provide absolute chronology for deglaciation

  • Regardless of initiation, many terrestrial cosmogenic radio nuclide (TCN) dates on moraines and 14C dates from sediments fall within a range close to the last glacial maximum (LGM), suggesting that after the global LGM, glaciers either lingered at select locations for several millennia, or ice experienced a readvance

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Summary

Introduction

The Andes of Peru and Bolivia comprise numerous glaciated ranges containing most of Earth’s extant tropical glaciers, along with multiple superimposed moraines that have provided glacial geologists appealing time-space markers to infer the extent and timing of late-Pleistocene to Holocene climate changes in the tropics (Clapperton, 1972; Hastenrath, 1985). Initial geomorphologic observations extending from the first decades of the twentieth century documented multiple glacial stages based on the superposition and relative weathering of inset moraines. Because these features appeared in form and position (elevation, distance from headwall) in different regions, this was interpreted as evidence of synchronous advances along the Andes (Clapperton, 1983). A prominent twostage moraine sequence associated with the Last Glaciation was documented, and appeared well-correlated between different Peruvian ranges (Machare et al, 1990) These early reviews featured very few absolute ages. This region featured only a single maximum limiting age for late-glacial moraines until the 21st century (Mercer and Palacios, 1977)

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