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.