May 28, 2012
Seismic data as a record of past erosion
Sediment layers are a useful tool for reconstructing ancient land use. They act as a record of soil movement and deposition, from which human activities may be inferred. In a 2007 study of ancient Maya deforestation, Anselmetti et al. use seismic and sediment core data from Lake Salpetén to quantify changes in soil erosion rate, which then tell us about Maya deforestation.
Lake Salpetén is a closed-drain basin located in northern Guatemala. It has an area of 2.55 km2 and is surrounded by a catchment area of 3.81 km2. A catchment is where surface water from rain and melting snow converges at a single place, creating a sink. Because Lake Salpetén is a simple source-to-sink system with no permanent inflows or outflows, its sediment deposits are a good indicator of erosion and runoff into the basin over time. This study compares erosion processes with archaeological estimates of Maya population at various time intervals, to interesting results.
A unique feature of the study is its use of a seismic reflection survey to construct a geophysical map of sediment architecture throughout the entire basin. This is much more accurate than sediment cores, which can only provide a few small, localized samples that may not be indicative of overall conditions. Sediment cores were, however, used in conjunction with radiocarbon dating to match sediment layers with specified time intervals. They were also tested for total organic carbon (TOC), which indicates the concentration of organic matter within the sediment layers. It is assumed that organic matter originates in the lake while inorganic matter (such as carbonate and clays) is debris that has been washed into the lake, and this assumption has proven to be mostly true. Using differences in TOC values, sediments were divided into 4 lithologic units which display sharp contrasts in organic matter content.
L4, the lowermost unit is composed of lacustrine sediments (native to lake) even though organic content is low since it’s from the bottom of the lake. L3 and L1 feature dark-colored sediments, known as gyttja, that are rich in organic material and also native to the lake. Sandwiched between them is L2, a thick layer of mostly inorganic deposit known as Maya Clay. Maya Clay is composed of fine-grained clays that are linked to eroded soil and deforestation of the watershed by humans.
Seismic data displaying 7 units of layered sediment
Seismic data revealed a 10 meter profile of regularly layered sediments, which was divided into 7 seismic units and correlated to the lithographic units from the sediment core. L1 corresponds to S1, L2 corresponds to S2 through S5 (Maya Clay), and L3 corresponds to S6. The authors estimated the volume of each deposit according to thickness and seismic lines, and then calculated the dry volume using known values of grain density and average porosity for gyttja and clay. Finally, the dates determined from radiocarbon analysis were applied to units S1-S6, which made it possible to then calculate the average sedimentation rate per year for each unit and therefore the rate at which the watershed was eroding.
What they found:
- Before 2200 B.C., when pre-Maya gyttja was being deposited, erosion rates were low across the basin. After 2200 B.C., erosion rates consistently increased, with a period of intense erosion from the early Preclassic to the late Preclassic period.
- During the Classic period, when Maya population reached peak numbers, erosion rates drop but remain relatively high compared to pre-Maya rates—average erosion rate for the entire Maya Clay unit (a period of 3,100 years) is about 22 times the baseline rate from before Maya occupation.
- After A.D. 1000, by which the Maya collapse was mostly complete, soil erosion rates decrease dramatically.
These results show that erosion rates and population density did not peak simultaneously, but that the highest occurrence of erosion happened before the population reached its maximum. This lag suggests that even small disturbances have big impacts, and that by the time human disturbance reaches a noticeable scale it is likely that damage has already been done. About 71% of total soil loss occurred prior to the Classic period, and almost all the soil in the surrounding catchment area ended up in the lake by the end of the Classic period. After the Maya collapse, catchment soil recovered partially.
Soil erosion rates were also compared with pollen in sediment cores, an indication of changes in vegetation. High forest plants are considered to be natural to the environment, while weeds and grasses are categorized as disturbance taxa. Increases in disturbance taxa coincide with times of increased erosion rates—both peaking during the Late Preclassic period—which suggests a causal link between deforestation and soil loss. Even though erosion rates decrease in the Classic period, the amount of pollen from disturbance taxa remains high, showing that the Maya were still clearing land of forests. However, this too decreases after A.D. 1000.
Source: Anselmetti, Flavio S. et al. “Quantification of Soil Erosion Rates Related to Ancient Maya Deforestation.” Geology October (2007): 915-18. Print.
Michelle Lim is a rising senior from Queens, NY, currently double-majoring in Narrative Studies and Interdisciplinary Archaeology at USC. She is interested in the cultural systems, thoughts, and stories of the (near & distant) past, especially in the ways they inform and enrich our present. In the future, Michelle would like to pursue nonfiction writing on topics involving science, history and social commentary.