Pitt researchers find new links between drought and South American civilization dynamics
By: Will Peters
Throughout history, trends in climate greatly indicate patterns of precipitation and evaporation that effect different places on Earth. During the Holocene (11,700 years ago to present), there have been fluctuations in global temperatures and climate that last for centuries. One period, known as the Medieval Climate Anomaly (MCA) occurred between 950 CE and 1250 CE, where certain parts of the planet experienced warmer or cooler than average temperatures, or more or less precipitation than the surrounding centuries. In the Andean regions of South America, anomalous warmth, drought and glacier retreat characterized the MCA, where heavy monsoonal precipitation is normally present. Dr. Elliot Arnold, a geochemist here at the University of Pittsburgh, is currently conducting research regarding the collapse of the Tiwanaku Civilization (500 CE-1000 CE), and how it could be related to climate changes during the MCA. His research has encompassed analyzing water composition in Lake Orurillo, a small nearby lake to the population and how precipitation and evaporation rates recorded in data measured from the lake indicate periods of severe drought, and how they correlate to the collapse of Tiwanaku.
Global warming is a familiar topic today as atmospheric greenhouse gases continue to be emitted by human activity around the world. The concept of climate impacting society is not new however, as agriculturally based societies have been at risk over many millennia because of their reliance on consistent agricultural growth conditions. In some areas such as where the Tiwanaku Civilization was based, societies became powerful and dominant because of the heavy monsoonal precipitation, allowing their agricultural system to flourish. The South America Monsoon System (SASM), which dictates precipitation mostly along the Pacific coast of South America, emits large amounts of rainfall in the December, January, and February. Monsoon systems are created by a reversal in low-level winds during different seasons. The change in air circulation changes the thermal temperature of both land and water. This difference in thermal temperature changes as seasons progress, creating changes in air pressure, forming precipitation near oceanic regions, such as Peru, where the Tiwanaku Civilization settled.
First developed about 1500 years ago, the civilization settled at the basin of Lake Titicaca in Bolivia. Being located around this large lake basin and in an area which receives heavy monsoonal precipitation, allowed the Tiwanaku to utilize this heavy rain flow to their advantage. With their development of raised crop fields and complex irrigation systems, Tiwanaku became a centralized, dominant power within the region. Despite their flourishing political and agricultural system, the civilization collapsed shortly after around 1100 CE, In the midst of the MCA.
Dr. Arnold’s research is actually a revisit of former research from the 1990’s, when scholars hypothesized that the collapse of Tiwanaku was a cause of severe droughts by analyzing Lake Wiñaymarca, however they were heavily disputed (Ortoff and Kolata, 1993). Dr. Arnold revisited this theory using new environmental proxies measured from lake cores from Lake Orurillo, a lake adjacent to the civilization. Lake Orurillo was chosen because of its sensitivity to hydrological changes in precipitation and evaporation. The Lake is small and shallow with a small watershed, leaving the lake with little inflow from surrounding water sources. Because of this lack of hydrological movement, the lake relies on precipitation and evaporation to keep recharge and discharge even. However, when a dry and arid climate such as that experienced during the MCA occurs, increased evaporation will decrease the water level and change the water composition of the lake. This allowed for Dr. Arnold and his team to use Lake Orurillo to recreate the climate history of the lake, and of the surrounding area.
The main component of Dr. Arnold’s research is the study of two different isotope proxies of the lake water: hydrogen and oxygen isotopes. These isotopes, found in leaf waxes and lacustrine carbonates measured from the lake sediments, can help recreate the climate trends over thousands of years and constrain when certain periods experienced droughts and heavy rainfall. The hydrogen isotopes found in leaf waxes, have a greater concentration in more arid environments, and oxygen isotopes are more positive when there are large amounts of evaporation. This indicates that when the concentrations and values of these isotopes are greater, the climate during that time period was likely much more arid because of a lack of precipitation. The research team here at Pittsburgh was also able to determine the timeline of the lake back to almost 4,000 years ago through radiocarbon dating of the sediment cores.
Figure 1: The figure above from Dr. Arnold’s article represents both the hydrogen (circles) and oxygen (squares) isotopes and their concentrations overtime. The major spike in hydrogen isotope concentration around 1000 CE represents the drought period in South America that Dr. Arnold is analyzing (Arnold, et al, 2020).
This analysis portrays a drought and overall arid climate between 500 CE and 1250 CE, which directly overlaps with the time period of Tiwanaku’s settlement. Dr. Arnold’s analysis also indicated that this time period was the most intense warm and arid period of the 4,000-year archive of Lake Orurillo. This desiccated period overlapping with the height of the Tiwanaku civilization created multiple agricultural problems within the society. There was a major reduction in primary production in local bodies of water, like Lake Orurillo. Due to the decrease in precipitation, the raised crop fields and complex irrigation systems were consolidated because of the limited land amendable. These systems continued to produce viable food for a population of at least 10,000 people, however, the output of agricultural products decreased. As the lack of water source continued for centuries, the collapse of the Tiwanaku civilization was inevitable.
Dr. Arnold’s research provides a unique insight into the downfall of Tiwanaku and ties together the effects climate has on human history. His contributions to paleoclimatology and anthropology allow us to better analyze periods of history in which a changing climate effects the function of society and how centennial trends have an impact on a long-term scale. Dr. Arnold’s future research will consist of different methods of collecting isotopic proxies through human activity and processes. The arid and dry climate that led to the collapse of the Tiwanaku civilization was identified through an isotopic analysis provided by Dr. Arnold’s research, and will serve as a steppingstone for future research to analyze how the Earth’s past climates impact smaller systems such as the SASM, and where we might be seeing an increase in drought severity throughout today’s climate.
Posted: May 14, 2021