It's an exciting time for Quaternary studies in South America. Paleoclimatic research has accelerated, in part due to mounting interest in El Niño and its conspicuous impacts on key environmental and socioeconomic phenomena. These include flooding on the Peruvian Coast, drought and biomass burning in the Amazon Basin, and glacier and lake level fluctuations throughout the Andes. Questions about the deep history of the Bolivian High, a monsoonal system similar to the Indian and Mexican monsoons, have also fueled a flurry of recent studies in the central Andes. Today, the Atacama lies at the downstream end of this monsoonal flow across the Amazon Basin, where it barely spills over the Andes.
During our outing in July, in the dead of the austral winter, the word "desert" took on a new meaning for us. Actually, a better description for the lower parts of the Atacama is "Absolute Desert", to distinguish it from the kind of lush tropical desert of our Sonoran Desert here. A closer cousin of the Atacama would be the surface of Mars, devoid not only of plants, animals or insects of any kind, but also of evidence of recent running water. It never rains in Calama (2450 m elevation, 100,000 inhabitants), the bustling mining town where we rented vehicles, bought groceries and eventually celebrated our success.
Like Mars, there is plenty of evidence that things haven't always been the same in at least some portions of the Atacama. At elevations below where it rains today (2700 m or almost 9000 ft), there are hillslopes carved by channels now buried by shifting sands. A particularly powerful record of climate change might be evidence of exactly when and where there was rain and runoff, as well as plant cover, in the Absolute Desert of the Atacama.
Our mission to reconstruct vegetation and hydrology in the Atacama was assisted by the incredible mummification that occurs in this hyperarid environment. Plant remains, textiles and even human burials (a local museum displays one under the label "Miss Chile") can persevere for millennia in the open, and even longer under shelter in rocky environments. Around Calama and San Pedro de Atacama, these rocky environments are characterized by Neogene tuffs and ignimbrites intercalated with fluvial sediments. The ignimbrites form cliffs, some of which have wasted down to boulder fields. The cliffs and boulder fields are ideal habitats for rodents, some of which produce plant-rich deposits akin to packrat middens in our own southwestern deserts.
Evidence of major climate switches in the region has been known to Chilean archaeologists for some time. For example, paleoindians appear suddenly in the region about 10,500 carbon-14 years ago (carbon-14 years are about 10% shorter than calendar years for this period), when the region was significantly wetter than today. This event is much in evidence at places like Tuina Cave, where we spent several days.
The bottom layers of the cave are sterile wind-blown dust and rock. Sooty black smudges mixed with thousands of fragments of bone denote the arrival of man and the many meals he made of the local llamas. Mixed with and overlying the human occupation levels are thick masses of fossil grass, so thick that at first they looked like bales of hay (see photos). Today, these grasses grow hundreds of meters higher in elevation, on the edge of the Andean Altiplano, where it is colder and wetter. The area around the cave today, devoid of all but a few desperate-looking shrubs, makes Death Valley look lush.
In fact, we spent much of this trip in pursuit of fossil plant material from the caves. Julio has obtained a suite of carbon-14 dates on a few of the deposits, and most cluster between about 12,000 and 8000 carbon-14 years ago. The fossils invariably included plants now found in wetter, higher elevation settings.
So by Atacaman standards, conditions were good when man first entered the region midway into this "wet" phase. But the good times ended with virtually complete abandonment of the lower areas of the Atacama desert between 8000 and 3200 carbon-14 yars ago. The Chilean archaeologists aptly called this period "El Silencio Archeologico". Looking at the Atacama now, it is hard to imagine that it was actually once drier than today, but the evidence clearly points to expansion of the Absolute Desert during that time. So far, Julio and Cladio have found no fossil plants from caves in the lower Atacama that date to that period.
The last 3500 years have been highly eventful both archaeologically and climatically. More rainfall began to fall during this period, which nourished springs and streams that flowed far down into the bleak interior of the desert. These stream courses have been the focus of intensive agriculture and village building, which in many ways reminded us of the ruins of the Anasazi here in the Southwest.
Some of these stream courses, or quebradas, are cut deeply into the huge ignimbrite sheets on the flanks of the Andean massif. The quebradas contained some real geological surprises that were to occupy our attention toward the end of the trip. Plastered high on the walls of the quebradas, we noticed patches of green, muddy sediment and hard, yellow travertine. Our first instinct was that these were the remains of irrigation ditches built by natives, which are common in some parts of the valleys. On closer examination we realized these deposits were natural and represented remnants of spring and creek deposits that once filled many of the quebradas.
In effect, the spring deposits are old bathtub rings that mark episodes when the water table stood much higher in all the valleys of the region. At this writing, we still do not have any dates on these strange deposits, but not for lack of datable organic remains. Beautifully preserved plant parts abound, including thick tap roots of mesquite that once shaded the water courses. We placed bets on the expected ages of these deposits, and all the guesses fell between 12,000 and 8000 carbon-14 years ago, the last time the region was much wetter than today. This semester, if you see Jason Rech staring into glass beakers in room 376, he is in search of the biggest and best charcoal fragments for radiocarbon dating. So stay tuned.
Personally, I have had few more productive or enjoyable field seasons than our July stay in Chile. Many bad Peruvian and Clinton jokes, and hours of fireside brainstorming about the geological story unfolding before us compensated for the long, Martian-cold evenings. I actually felt a bit wistful as I unpacked my bags back in the States and was greeted by clouds of Atacaman dust and the scent of burnt saltbush in all my field gear. But there is little time for regrets, as we and Camille Holmgren, a new graduate student in Geosciences, plan to return next summer and push our explorations of the Atacama north toward Peru.
