The Original Proposal
Research Objectives
The main aim of this project is the development of a methodology to analyze geoarchaeological soil samples using an ICP-MS instrument. The principal objective will be to track fluctuations in key elements that are proxies for human-environment interactions. A key marker is mercury, which is rare in the environment but was used widely by the Maya. Cinnabar (HgS) was a key component of red pigments used to decorate buildings, and in ritual contexts. For example, cinnabar is often found in burials as it was used to decorate the bodies of deceased individuals. Thus, even trace amounts could be highly indicative of human impacts, and there is a growing interest among scholars that wish to quantify the legacy of mercury contamination in post-Maya landscapes.
Other elements typically studied by researchers working in the Maya area include magnesium, calcium, iron, aluminum, and manganese. These elements can be used to measure the relative age of soils within a stratigraphic sequence using the ratio: ([Ca]+[Mg])/([Fe]+[Al]+[Mn]). Ca and Mg originate from the underlying local karstic geology, while inputs of Fe, Al, and Mn originate from prolonged exposure to the air notably via Saharan dust storms.
Since we are not sure at what concentrations we will find these analytes within the soil samples, we will first undertake a semi-quantitative analysis of the samples to make sure that total dissolved solid (TDS) counts are kept below the 200 ppm limit of the Agilent 7500ce instrument that we will be using. With the results of the semi-quantitative analysis in mind, I will work with my collaborator, Byron Smith, who collected the core samples that I will be study to select a subset of these samples to subject to a more robust, quantitative, approach with concentrations adjusted to accurately assess concentrations of major and trace analytes. Upon completion of this pilot study, I hope to have a developed methodology that I can use to study soils from excavations across the Maya area. Carbon samples collected from the core will be sent to a third-party commercial laboratory to aid in firmly establishing the chronological sequence of the core and its discrete layers.
Justification
ICP-MS is critical because it is one of the best ways to reconstruct the elemental composition of soil samples, and it is superior to many of the methods that we have at our disposal in the Geography department. The core that we will be studying represents a tremendous opportunity to study the ancient Maya environment. It was collected from a perennially inundated wetland at the base of a limestone ridge (see below). Spanning the top and base of the ridge, above and adjacent to the wetland, is the ancient Maya fortified center of La Cuernavilla. The site may be the most heavily fortified location of ancient America. Archaeological investigations, have revealed probable occupation spanning the Late Preclassic to the Late Classic periods, or the 1st to 8th century AD.
Although the wetland at La Cuernavilla has never been investigated geoarchaeologically, another nearby wetland has. Thus, there is great potential to develop a comparative study and avoid “reinventing the wheel” while hopefully improving existing methods. As stated previously, while mercury is an important marker of human impacts, magnesium, calcium, iron, aluminum, and manganese provide a means of assessing the relative age of soils. Calcium and magnesium come from underlying limestone bedrock but are leached out of soil over time. Meanwhile iron, aluminum, and manganese have external sources (such as Saharan dust storms) and should be higher in soils exposed to the environment (i.e. those that formed an ancient topsoil). Thus, the ratio formed by the sum of calcium and magnesium divided by the sum of iron, aluminum, and manganese provides a useful means of quantifying the degree of natural and anthropogenic inputs within a soil.
Materials and Methods
I will be working in consultation with Byron Smith, when undertaking the necessary sample prep as he has extensive experience with analyzing soils with an ICP-MS. Luckily, he has already sampled the sediment core in Guatemala. Citing concerns with the total extractant of hydrofluoric acid, we will be following a methodology similar to that developed by Battistel and colleagues (2018) on soils from the same region. Starting with running the 92 samples, and later adjusting depending on the results of these initial runs of a smaller subsample.
- Combine 0.5 g of sediment will be combined with 2 mL of Nitric Acid (HNO3) and 6 mL of Hydrochloric Acid (HCl)
- We will heat the sample (either in a microwave or hot bath) at 160C (10 minutes of ramp-up and 5 minutes of hold time) and then at 230C (15 minutes of ramp-up and 45 minutes of hold time).
- After centrifuging (1000 rpm for 5 minutes), we will separate the liquid fraction, and dilute to 40 mL with ultra-pure water.
- We will further dilute, to make the samples 1:100 by volume and run through the instrument in He collision mode.
Expected Outcomes
The analysis of the La Cuernavilla core has considerable intellectual merit, and has the potential to fill many persistent knowledge gaps. The core comes from an ideal setting to understand the ancient environmental impacts of an enigmatic and unique Maya archaeological site. We suspect that the results will show considerable intensification at the site correlating with known conflict periods, and should clarify the important moments in the site’s settlement chronology such as its founding and abandonment. In conjunction with other proxies, we expect that the data will make a substantial contribution to a highly publishable body of work.
Timeframe and Budget
The major time-related impediment remains the unfinished inspection of samples in Guatemala and the need to resolve workspace complications. My goal is to start my sample prep by mid-October, which will give me a month to rectify the difficulties with the lab space and hopefully allow enough time for the samples to arrive. If the Guatemalan authorities do not process the samples in time, I will use my own samples from my master’s research. Because of ample laboratory assistance, I don’t foresee that sample preparation will be particularly time-consuming and I am confident that I can finish within a month. I would like to look at 21 samples for this pilot study, and intend to allow one-hour/sample in the laboratory. Given the $17 rate/sample for the ICP-MS instrument in solution mode, I foresee a total cost of $17 * 92 samples = $1,564. Followed by a further quantitative run of a subsample $17 * 10 samples = $170. Meaning that the final cost will be around $1,734.