Using X-Ray Diffraction to Detect Climate Proxies in Sediments from Basrah Governorate, Southern Iraq. Alex George, Micheal Bizimis, Jennifer R. Pournelle.
Using X-Ray Diffraction to Detect Climate Proxies in Sediments from Basrah Governorate, Southern Iraq Alex George, Micheal Bizimis, Jennifer R. Pournelle Abstract
Conclusions
A geological section sampled by the Sealands Archaeology and Environment Program in southern Iraq, radiocarbon-dated as spanning 6ka calBP – present (4000 BCE – present), reveals largely homogenous, unlaminated fresh-brackish marsh sediments with few visual stratigraphic indicators. However, previous work in the region suggests centennial–millennial scale climatic variation may be detectable from variation in sediment composition. In arid environments, evaporitic minerals, especially calcium carbonates, accumulate in surface soils. Variations in quantity and structure of these minerals can therefore be used as proxies for detecting climatic variation. Eight samples collected from four sites in a marsh sediment section in southern Iraq were analyzed using X-ray diffraction. In this technique, an x-ray beam is fired at the powdered sediment over a range of angles (in this case, 5-60°). The angles at which the rays bend, or diffract, give information about the sample’s mineral composition. Samples with known date were compared to parallel samples with unknown date to further differentiate sedimentary layers. Two samples of unknown date were chosen to be sent for AMS radiocarbon-dating because of significant differences in their composition, particularly percentages of carbonate, quartz, and gypsum. Obtaining these dates will (1) inform the hypothesis that a climatic change occurred circa 2ka BCE and (2) that they represent a chronologically distinctive stratographic unit within the sediment section. Further, these X-ray diffraction results were compared to those found by Al-Sudani and Albadran 2014 at our partner lab in the University of Basrah, in order to check that both labs are using comparable and replicable protocols and equipment calibration and facilitate data-sharing. Significant differences were found between data of the two facilities.
According to Aqrawi and Evans (1994), quartz in the form of detrital grains indicates it was transported there from another place. Quartz has a high resistance to erosion; its presence indicates conditions of fast-flowing water. (Al-Sudani and Albadran 2014) Therefore, differing quartz content between core depths could indicate differing rates of sediment transport and thus, different climates. Looking at the quartz ratios in Figure 6, we see that H1 and H6 are more similar than H5. From this we will infer that between H5 and H6, H6 is more likely to be closer to H1 in date. For that reason, H5 will be sent for radiocarbon dating. Likewise, in Figure 2, quartz content tells us that samples H2 and H8 are likely close in date, so H7 will be sent for radiocarbon dating. Determining the dates of these two samples will help reveal dates of climate changes.
Methods Approximately 10 grams of each of the eight samples were powdered finely with mortar and pestle. The mineral composition of the samples was determined with a Shimadzu LabX XRD-6000 employing CuKα radiation generated at 30kV and 30mA. All clay minerals obtained in the results were combined into one category due to the uncertainty of the technique in identifying specific clays.
Figure 4. Map of Mesopotamia indicating the area of interest. Image sources: Esri, DigitalGlobe, GeoEye, i-cubed, USDA FSA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swistopo, and the GIS User Community.)
Gypsum and calcite ratios also give important information about the climate for a given sample. Relatively high gypsum and/or calcite content indicates a dry period because these minerals form through evaporation (source?) Therefore, it makes sense that sample H5 has low quartz content and high gypsum content, because during a dry period there would be less fastmoving water to deposit quartz and more evaporation to form gypsum. H7 has a higher calcite ratio than H1 and H8, which is consistent with this same pattern. The fact that H7 has less gypsum than H5 could indicate degrees of dryness or could come from our problem of very low sample size. In either case, an approximation of sediment ratio differences is enough to inform the next steps of the project.
Outset: Hareer study area in Basra Governorate, Southern Iraq, showing sites 1-4. Image source: Al-Sudani and Albadran 2014.
Of the samples shown in the graph, only H6 and H8 contained feldspar, or more specifically, albite. According to Aqrawi (1994), feldspar is similar to quartz in that it is deposited in a location through erosion. Therefore, feldspar may be deposited in wetter conditions, but its presence in only two of our Site 1, 3, and 4 samples is a question for further research. Samples from site 2 have not yet been mentioned. These samples were very close in depth and came from two clearly layers (see Figure 5). The visible change in sediment between the two samples suggests a significant change in conditions, but the dates of these samples are both later than Dr. Pournelle’s hypothesized climate change circa 2ka BCE. Figure 1. Loading samples into the x-ray diffractometer. Photo by Carl Frisby.
Figure 5. Lithofacies of Hareer sites 1, 2, 4, and 3. L1, L2, and L3 indicate sediment strata. (L1=grey silt; L2=soft brown mud wih shells; L3=sandy silt). Sediments were visually homogeneous within sampled portions of each facie. After Al-Suldani 2014
Results In the samples overall, calcite was the most common substance, followed by clays, quartz, gypsum, and feldspar (specifically, albite). There were also trace amounts (
