Even the driest place on Earth, the Atacama Desert in Chile, still sees intermittent rainfall. In the past 215,000 years, these sporadic rainfall events may have coincided with elevated sea surface temperatures nearby that resemble El Niño conditions.
“The Atacama Desert experienced several interspersed episodes of ‘wetter,’ still arid, conditions,” Benedikt Ritter, a paleoclimatologist at the University of Cologne in Germany, told Eos. “We are exploring…the mutual evolutionary relationship between climate, geomorphology, and biological evolution.”
Ritter and his team published these results last month in Scientific Reports.
A Lack of Rain and Records
The hyperarid core of the Atacama Desert currently gets less than 2 millimeters of rainfall a year. Scientists don’t know when those conditions began or how often they were interrupted or for how long. The area’s sediment record for the most recent geologic period “appears like a white spot on the map,” Ritter said.
Water runoff from the Altiplano, or Andean Plateau, to the east confuses sediment records in the hyperarid region, making it difficult to isolate local precipitation records.
“The mostly barren landscape is almost undiscovered in terms of paleoclimate studies for the younger timescale,” Ritter said.
Ritter and his team focused on a basin in the coastal mountain range, the Coastal Cordillera, that cuts through the hyperarid region. The basin’s location separates it from the surrounding mountain drainage networks, and its clay pan bottom helps it retain water. Sediment cores collected from this endorheic basin, the researchers hypothesized, should track past precipitation near the hyperarid core of the desert.
Relatively Wet Periods
The team used percussion drilling to collect a sediment core from the top 6.2 meters of the clay pan. The rock record spans the past 215,000 years and is the first paleoclimate record of the middle and upper Pleistocene for this region.
The researchers looked at the size and composition of sediment grains as well as the abundance of fossilized microorganisms at different depths along the core. On the basis of these measures, they identified two significant wet times in the paleoclimate record: one around 200,000 years ago and a shorter period around 120,000 years ago.
“Wet” is relative in the most arid place on the planet, Ritter said. “What we can tell, based on the sedimentological data, is that there was enough water available to transport coarse-grained sediment from the catchment into this pan.”
Moreover, “the abundance of some planktonic diatoms further indicates the existence of an ephemeral water body,” meaning the basin may have periodically flooded to become a temporary lake.
Atlantic Versus Pacific
The researchers compared the timing of the basin’s wet periods with other nearby climate records and found something pretty surprising, Ritter said.
“It seems that ‘wetter’ episodes in the recent past in the Coastal Cordillera, between Antofagasta and Arica, line up with El Niño–like conditions,” specifically, higher sea surface temperatures along the Chilean and Peruvian coasts, he explained.
The wet periods are out of sync with similar events in the Altiplano, the team members found. They suspect this is because the Coastal Cordillera gets its moisture from the Pacific Ocean and the Altiplano gets its from the Atlantic.
“The pattern is totally inverse to the Andes,” said Marco Pfeiffer, a geoscientist at the Universidad de Chile in La Pintana who has studied the Atacama’s paleolakes and paleoclimate. “In this sense, [the study] is extremely novel and without a doubt a great contribution to the local paleoclimatology.” Pfeiffer was not involved with this research
Because Ritter’s team collected this sediment core from a basin near to, but not within, the hyperarid zone, “there is still the question [of whether] these results could be extrapolated to iconic sites of the hyperarid core such as Yungay,” Pfeiffer cautioned.
Drilling Down Deeper
“Our record covers only the first glacial-interglacial cycle,” Ritter said. “Whether this pattern is representative for all glacial-interglacial times has to be tested with longer paleoclimate records.”
The researchers are working to see whether the El Niño–like pattern extends further back. In 2017, they conducted a follow-up expedition to this region and drilled deeper into the clay pan. Their new cores reach 8 times deeper than their first, Ritter said.
“This new deep drilling sediment record extends the published reconstructed paleoclimate in this part of the Atacama Desert to even older times,” he said. The team plans to publish these records in the near future.
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer