Tropical Glaciation and Equilibrium Line Depression

Site map of glaciated areas in Central America.

1) Cordillera de Talamanca and Chirripó Park, Costa Rica. 2) Sierra Cuchumatanes, Guatemala

Our research in Central America involves fieldwork in remote glaciated areas to determine the paleoclimate of the region. The primary field area is Chirripó National Park in the Cordillera de Talamanca, which peaks at Cerro Chirripó at 3819 m a.s.l. (Map PDF). New research is underway on the glacial geology of the Sierra Cuchumatanes in western Guatemala with M.S. student Alex Roy. Both Costa Rican and Guatemalan glacial deposits were studied in the 1970's by Stefan Hastenrath, and our work is a continuation of this research in Central America. A large ice cap ~60 km² in diameter covered the high limestone plateau north of Todos Santos Cuchumatán during the late Quaternary, and deposited fantastic complexes of boulder-studded moraines and outwash plains. Research is underway to establish the chronology of glaciation in Guatemala to constrain late Quaternary temperatures in Central America using both cosmogenic and ¹⁴C isotopes. We are thankful to the kind people and government of La Ventosa to allow us access to this fantastic field area.

Preliminary Digital Elevation Model (DEM) map of the glaciated area of the Sierra Cuchumatanes, Guatemala. Created by Alex Roy.

Glaciation was centered around the highest peaks of the high limestone plateau above ~3500 m altitude.

View down the Llanos de San Miguel, a glacial outwash valley train, from a terminal moraine in the Sierra Cuchumatanes, Guatemala.

A striated and faceted limestone boulder in glacial till clearly demonstrates

the presence of a former ice cap in the Sierra Cuchumatanes.

Alex Roy sampling a boulder for ³⁶Cl cosmogenic nuclide analysis on a left lateral moraine crest in

Valle Ninguitz, Cuchumatanes. In the background are crests of treeless terminal moraines that dam dry lake basins.

Downvalley panorama of lateral and terminal moraines in the Valle Ninguitz de la Ventosa,

Sierra Cuchumatanes. Visible is a large left lateral moraine on the left side of the photograph, and terminal moraines in center

dam numerous closed dry lake depressions.

Reconnaissance level map of probable extent of the late Quaternary Cuchumatanes Ice cap, Guatemala. Ice limits based on work by Anderson (1969) and Hastenrath (1973). More detailed mapping is underway to further document the ice extent and glacial geology in the Sierra Cuchumatanes.

Our group maps the type, location, and extent of surficial glacial and periglacial deposits (Costa Rica: Map PDF), which are incorporated into a Geographic Information System. The positions and elevations of lateral and terminal moraines delineate paleoglacier extent (Costa Rica: Map PDF) at various stages. The glacial geologic map provides data needed to reconstruct former glacial extents, from which a hypsometry (glacial area vs elevation) is reconstructed. The hypsometry is then used to estimate former equilibrium line altitudes, which are closely linked to the 0°C isotherm in the tropics. The methods employed are the area-altitude ratio (AAR), toe-to-headwall-altitude ratio (THAR) and the area-altitude-balance-ratio (AABR) method. Since no modern glaciers exist in Costa Rica, the modern ELA is estimated using radiosonde data from San José (1997-1999), which places the 0°C isotherm at between 4800 and 5200 m a.s.l.

The results indicate that during the last local glacial maximum, ELAs in Chirripó National Park were located at ~3400 to 3600 m a.s.l, which suggests a depression up to 1500 m compared to today. If interpreted as reflecting temperature depression alone (a reasonable assumption in the humid tropics), this suggests a high-altitude cooling of 7 to 9°C. This result is both interesting and controversial, since marine temperature reconstructions for maximum late-Quaternary cooling using alkenones and foraminifera are typically 2 to 3°C, while trace metals in corals indicate ~5°C cooling. Lowland terrestrial pollen and groundwater temperature reconstructions also suggest a 5 to 8°C cooling, more in line with ELA estimates. Reconciling high-altitude ELA-based temperature depressions with marine and lowland estimates remains an important issue in tropical paleoclimatology. A partial resolution invokes changes in atmospheric lapse rates, which are relatively narrowly constrained between ~ 5 to 7°C/km in the humid tropics, or errors in one or more of the temperature reconstruction methods. No matter how ELAs are interpreted, it still isn't possible to put an 'ice cube in the oven', so glacial ELA estimates are probably the most robust of methods for estimating late Quaternary land surface paleotemperatures.

A portion of the Costa Rican research was published in the GSA Bulletin (2002), and two invited book chapters (see references). Coupling past glacial ELA temperature estimates with the speleothem rainfall reconstructions provides a unique opportunity in understanding of past tropical paleoclimate.

View of glacial moraines in Talari Valley, Chirripó National Park, Costa Rica.

Los Crestones in Chirripó National Park - Andesite tors stood above Quaternary Glaciers as a Nunatak.

Glacial lakes (tarns) in Duchi Valley, Chirripó National Park, formed in glacially scoured bedrock basins.