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.
May 23, 2012
Using proxies to measure rainfall
Three different sediment time series from the Cariaco Basin.
Dr. Gregory Haug and colleagues raised a question: “does climate make history?” in their 2003 paper they presented scientific evidence that supported the theory that drought due to climate change caused the collapse of the Maya civilization. Unexpectedly, everyone has their own unique answer to the open-ended question and I will attempt to explain my thought process and my final answer.
History can be defined as a study of past events and by this definition, anything, including climate can make history because it has the ability to influence individuals who can observe, examine and record the climate. Additionally, climate can make history because of its ability to leave physical evidence of its presence or change in a variety of ways. There is no question that climate has physically affected the earth is more ways than one; however, we as scientists, still struggle to find the most accurate and definitive method to represent climate in the past. Haug, et al. 2003 successfully draws a proxy, where they utilize a new method for the measuring bulk sediment chemistry; therefore, developing a substantial record of river-derived inputs to the Cariaco Basin.
The Cariaco Basin is located off northern Venezuela and the sediments of this basin are considered a superior proxy to other paleoclimate proxies. It provides an excellent comparison to the ancient Mayan climate and environment because the basin shares the same climate regime as the center of the Maya civilization. Additionally, the Caricao basin is anoxic, which preserves most of the sediment as it were thousands of years ago during the peak of Maya civilization. The anoxic environment also prevents any small organisms from burrowing in the sediment and disturbing the deposition pattern. The Cariaco basin is an ideal location because of its detailed resolution. Scientists are able to gather data at a bimonthly resolution, which makes analysis and comparison much more accurate as there is significantly more evidence to support their claims.
So how exactly does climate make history? Climate can leave physical evidence and data for an extended amount of time, allowing scientists to determine exactly what and how the climate was during that period. The primary method of data collection in the Cariaco basin is measuring for titanium content in the sediment. Haug explains that the light and dark laminations preserved in the sediments of the Cariaco basin are the direct result of significant regional changes in climate due to the seasonal shifts in the position of the Intertropical Convergence Zone (ITCZ). Light colored laminae deposit biogenic compounds during the dry upwelling season during the winter and the spring when the ITCZ is the its southernmost position, making trades winds stronger.
Uniquely, individual dark laminae are extremely rich in terrigenous grains and contain a significant amount of titanium. Their interpretation of the titanium content in soil suggests that they can determine the regional hydrologic changes and variations of the mean ITCZ with time in comparison to the Holocene Cariaco record. Similarly, the light laminae have significantly less titanium levels, which suggest a dryer climate at that time period. Haug et al defined very clear parameters towards what the data represented. Dark laminae and higher titanium levels indicated increased water levels as they are deposited during the wet rainy season when the ITCZ is located in the most northerly position, almost directly over the actual basin. On the other hand, light laminae and lower titanium levels suggested lower water levels due to biogenic components that were deposited during the dry upwelling season when the ITCZ is at its southernmost position and there are significantly stronger trade winds along the coast of Venezuela. The connection between rainfall and river sediment input is recorded in the laminated nature of the sediments in the Cariaco Basin. Paired laminations in the sediments are produced by large changes in wind and rainfall due to seasonal changes caused by the position of the ITCZ and its convective activity; therefore, if the ITCZ fails to migrate north then the basin and its surrounding areas will be experience drought due to trade winds.
The well-defined and strict boundaries of the data comparison further strengthen the proxy. Simply put, scientists are able to identify within a bi-monthly scale, the climate, moisture levels and water availability in the center of the Maya civilization during ancient times.
Ultimately, Haug was able to conclude with the help of the data he has gathered that the Maya civilization became too ambitious after a period of productivity and abundant rainfall from AD 550 to AD 750 and that their population expanded way past the land’s carrying capacity; therefore, when a drought occurred, there was not enough water left to sustain the population. As seen in the image below, the evidence supports the theory that megadroughts were one of the causes behind the collapse of the Maya civilization. Additionally, it is arguable that the data provided is significantly substantial and conclusive because other data such as independent paleoclimatic data from similar areas like Lake Valencia and Lake Titicaca, can be criticized for being too vague or showing the natural variability of climate; however, the image clearly shows that the sediments from the Cariaco basin which had the least amount of titanium correlate with other proxies when there was low rainfall, suggesting the presence of light colored laminae instead of dark colored laminae. It is not that the laminae are light but that low Ti means that there is low rainfall, less runoff.
In conclusion, Haug poses an interesting question as to if climate can in fact write history. I firmly believe that climate can because of the abundance of physical evidence that we have found but we see how climate can greatly influence an entire civilization, which creates events that are worthy of being called history.
Britanny Cheng is an incoming junior at the University of Southern California where she is pursuing a degree in Environmental Studies. She attributes her love for the environment to her upbringing in the Philippines where she was exposed daily to the ocean, inspiring her to become a certified advanced water diver, specializing in night dives. In the future, she plans on hopefully research diving for a living whilst increasing awareness for the implementation of marine reserves in the Philippine waters.
May 18, 2012
Underground Evidence for Maya Collapse
The Maya, a Mesoamerican society considered the most advanced Native American civilization of its time, began a complex development around 2000 B.C. Divided into two main periods, Preclassic and Classic, the Mayans underwent a relatively abrupt collapse between 750-900 A.D., known as the Terminal Classic. The cause of rapid decline of such an intricately developed people has since been a topic of interest. One theory in particular is growing increasingly irrefutable: climate change. In 2007, James W. Webster et al., used stalagmite evidence from the Macal Chasm, a cave in Belize, to demonstrate that climate change, specifically drought, may have played an integral role in the Maya demise.
Stalagmite formation in Yok Balum cave, Belize. Photo by Dan Killam.
Stalagmites are formations resultant from the dripping of mineralized solutions into caves through the overlying soil and subsequent deposit of calcium carbonate. Because of the CO2 present in the atmosphere, rainwater is naturally acidic, and thus dissolves calcium carbonate when percolating through calcium carbonate rich soils. Upon entering a cave, water’s concentration of calcium carbonate is so high that some precipitates out when the water drips through the cave, forming stalagmites. Webster et al. used several proxy factors from stalagmites to indicate the climate in Belize during which the stalagmite sample was formed, including reflectance, color, and luminescence of the sample, as well as carbon and oxygen isotopic records.
Luminescence “is produced by organic acids and so is related to productivity in the soil and vegetation cover above the cave [ . . . ] as a proxy for availability of moisture” (Webster et al. 9). The particular sample from the Macal Chasm demonstrates long periods of higher luminescence—moisture—with interjecting periods of lower luminescence—drought. Color has a strong correlation with luminescence. Browner color indicates the accumulation of dust on the stalagmite, implying that there was not enough water in the cave to keep the formations free of dust (9). Dryer climates, as indicated by the color measurements, occurred during the same time periods as lower luminescence, indicating drought. Reflectance also correlates to the two aforementioned values, but is a less dependable indicator because calcium carbonate in general does not reflect much light. Combined, however, the three measurements consistently agree on times of more moisture and times of less moisture, and overall suggest several periods of drought that the Maya faced.
Another important variable measured from the stalagmites are the oxygen and carbon isotopic records, which point to different climate indicators. Oxygen isotopic records signify the amount of rainfall at the time of the stalagmite formation. Oxygen has two common isotopes, oxygen-16 and oxygen-18, the latter being heavier. The stalagmite sample used by Webster et al. was very near the entrance of the cave, meaning a higher likelihood of exposure to outside climate conditions. Because of its lower weight, oxygen-16 is more readily evaporated from the stalagmite than oxygen-18, which leaves behind more oxygen-18 and thus a higher value of the oxygen isotope ratio. More evaporation would occur from the stalagmite in drier climates, so a higher isotopic ratio value suggests less rainfall. The carbon isotopic record, on the other hand, indicates the amount of vegetation present over the cave. Three common isotopes of carbon are carbon-12, carbon-13, and carbon-14, with their weights increasing respectively. Vegetation prefers to use the lightest of the three isotopes because capturing it requires the least amount of energy; therefore, a cave covered with large amounts of vegetation, indicative of a generally wetter climate, would have a lower carbon isotopic ratio. Because both isotope ratios are ultimately suggestive of rainfall levels, they are strongly correlated. The Webster et al. data records lower values (wetter climates) and higher values (drier climates) of each isotope ratio during the same time periods. The matching records greatly increase the dependability of the data.
Data from Webster et al comparing several stalagmite-based datasets during the main periods of Maya civilization.
The Webster et al. data for all five of the proxies measured, shown above, demonstrates remarkable levels of agreement for periods of wet and dry climates. Webster et al. identifies 4 significant periods of drought from the data. The first, occurring around 141 A.D., corresponds to the Preclassic Abandonment, which is archaeologically recorded as a cessation of construction in several major Maya locations (2). The next evident drought comes around 517 A.D., which marks the beginning of a period described as the Maya Hiatus, archaeologically recognized as a period with a decrease in the amount of dedication of monuments. The third drought comes as a series at the peak of the Maya Classic Period, when it is thought that the Maya were at their highest population, and thus, extremely dependent on water for agriculture and consequently vulnerable to drought. The droughts ranged from 780-1139 A.D., with the Maya civilization thought to be completely collapsed around 910 A.D. The fourth and final significant drought identified by Webster et al. around 1472 A.D. comes after the Maya Terminal Classic, but is significant for another reason: it was recorded in Maya Books (14). The confirmation of this portion of the data amplifies the reliability of the rest of the data projected on the Maya Preclassic and Classic periods.
While no theory on the Maya collapse is unquestionably conclusive, strong evidence is building that climate change in the form of drought imposed a significant burden on the civilization, given their degree of dependence on rainfall. Though the collapse of the Mayan society was likely a combination of multiple factors, and perhaps a snowball effect of all the factors combined, it is becoming progressively clearer that climate change as a cause should not be dismissed.
Source:
Webster, James W., George A. Brook, L. Bruce Railsback, Hai Cheng, R. Lawrence Edwards, Clark Alexander, and Philip P. Reeder. “Stalagmite Evidence from Belize Indicating Significant Droughts at the Time of Preclassic Abandonment, the Maya Hiatus, and the Classic Maya Collapse.”Palaeogeography, Palaeoclimatology, Palaeoecology 250.1-4 (2007): 1-17. Print.
Sydney MacEwen, an LA native, is an upcoming Junior pursuing a BS in Environmental Studies and a minor in Geological Hazards. This is her first trip to Belize. She’s particularly interested in climate change and related policy. She hopes to pursue a Master of Arts in Environmental Studies following her undergraduate education.