High Mountain Andes lakes chronicles written in diatom and ostracod records

How scientists study tropical climatic conditions from 5 million years ago?

One of the most important geologic and climatic events on Earth´s history was the uplift of the Andes mountains, in the South American continent. Nowadays, the tropical Andes are considered a hotspot of biodiversity for both macro (vegetation) and microorganisms. They are also a key source of freshwater to many South American countries. Once the Andes reached its present-day elevation over 4000 m above sea level, 5 million years ago, a variety of newly formed ecospaces developed. Among them, aquatic ecosystems such as lakes, streams and marshes, were the most characteristic. However, very little is known about how these ecosystems have evolved and their responses to past climate changes that could serve as modern analogues for the ongoing climate change impacts.

When the Andes uplifted, climate was warmer due to the higher concentrations of atmospheric CO₂ (450 ppm vs 400 ppm) and sea level was higher due to absence of ice caps in the Artic continent. Therefore, the implications of understanding past tropical climate behavior to predict future climate change impacts on Andean freshwater ecosystems are clear, partly because all such past climatic conditions are predicted to occur in the most extreme scenarios drawn by the Intergovernmental Panel of Climate Change reports (IPCC).

Using diatoms (unicellular microalgae) and ostracods (very small invertebrates), a multidisciplinary team of geologists, climatologists and biologists performed a study in the Andean Altiplano of Perú to better understand how climate operates at the South American tropics. Both diatoms and ostracods occupy a wide range of environmental conditions and have hard extracellular walls, made of silica (diatoms) or calcium carbonate (ostracods), which are preserved in the sediments when the organism die. By looking at these structures we can identify the species. So, if their remains are recovered, environmental and climatic conditions might be then inferred from the occurring species. In other words, diatoms and ostracods are indicators of both modern and past environmental conditions. The authors of this study collected sediment and biological samples at the Descanso formation, a geological outcrop - defined as an ancient rock exposed on the surface of the Earth - that was deposited between 5 and 3.9 million years ago, during the so called Pliocene geologic epoch.

The diatom Pseudostaurosira zeilleri, indicative of high saline-alkaline shallow waters    

The results of this study, recently published in Palaeoegeography, Palaeoclimatology, Palaeoecology journal, reveal that fluvial conditions prevailed just after the Andes uplift. Then, lakes and marshes were established as result of increased precipitation and direct underground water input due to tectonic activity. Basically, diatoms and ostracods record indicated that lakes were developed under a higher precipitation and seasonality regime compared to modern conditions. In this region of the central Andean Altiplano, Pliocene climatic conditions were characterized by enhanced precipitation during the rainy season, intensified under the El Niño-like conditions that are hypothesized to have prevailed at that time, between 5 and 3.9 million years ago, driven by a warmer atmosphere (~3ºC).

Example of a surveyed lake in the Peruvian Altiplano. Photo: Maria I Velez.

Recent climatic models indicate that high altitude tropical regions will be much more affected by a warmer atmosphere than low altitude temperate regions. If model predictions are correct, these changes will have important consequences for high altitude Andean freshwaters such as streams, lakes and even glaciers. As these freshwater ecosystems feed many reservoirs, more than 40 million people from Colombia, Ecuador, Peru and Venezuela that directly depend on these water resources, will be seriously affected. The present study contributes to a better understanding of the aquatic ecosystem response to climate change in highly sensitive ecologically regions like the tropical Andes, and highlight the importance of fossil records in addressing modern and future socio-environmental research questions.

You can read the summary in Spanish and Portuguese (thanks to Isabel Fernandes for the PT translation)

Velez, M.I., Jaramillo, C., Salazar, F., Benito, X., Fritz, S.C., Tapia, P.M., Lubiniecki, D., Kar, N. & Escobar, J. Aquatic ecosystems in a newly formed ecospace: Early Pliocene lakes in the Central Andean Altiplano. Palaeoegeography, Palaeoclimatology, Palaeoecology (in press).