Problems and Solutions: brGDGTs distributions and calibrations for semi-arid environments and application to the wetlands of the Southern Caucasus.

Abstract

<div>The arid and semi-arid mountainous environment of the Southern Caucasus lies between the Black and Caspian Sea with elevations from below sea level to over 5000 m asl and has annual temperatures from -6˚ to 16˚C and precipitation between 200 - 2200 mm a year. Due to the large elevation changes in this mountainous zone, temperature and precipitation shifts occur quickly over short distances. The semi-arid regions here host a number of wetland contexts available for paleo-environmental research. However, recent investigations of branched glycerol dialkyl glycerol tetraethers (brGDGTs) records from a large wetland from the region note the changes in wetland development, from lake to peatland, impact the distribution of brGDGTs and the temperature reconstructions (Robles et al. 2022). To overcome these challenges Robles et al. (2022) applied a combination of global lake and soil calibrations to each section. New research on smaller wetlands, however, have found that these wetland transformations are not always as well defined and include periods of erosional soil inputs, open lake contexts, and eventual transformation to a wetland. In addition, soils from the region record a similar trend found in Gao et al., (2021) who found a temperature relationship with the MBT'6<sub>me</sub> suggesting the MBT'5<sub>me</sub> reconstructions do not fully capture the temperature profile. To address these issues we propose two avenues to reconstruct temperature and present our results. First, we evaluate the feasibility of utilizing a stepwise selection model for local configuration based on mixed samples (lake, wetland, soils) to overcome problems with changes in sediment overtime while capturing the temperature relationships between both the 5-methyl and 6-methyl brGDGT groups. Second, we utilize a probability based machine learning approach to estimate changes in source sediment distribution as the wetland transitions through time. We test both of these applications on downcore wetland sediments to evaluate our results. These results are compared with pollen reconstructions, aquatic pollen, non-pollen polymorphs, and XRF data in order to evaluate their success. We find that both methods can help provide better information for reconstructions.</div><div> </div><p>Robles, Mary, et al. "Impact of climate changes on vegetation and human societies during the Holocene in the South Caucasus (Vanevan, Armenia): A multiproxy approach including pollen, NPPs and brGDGTs." Quaternary Science Reviews 277 (2022): 107297. </p><p>Guo, Jingjing, et al. "Soil pH and aridity influence distributions of branched tetraether lipids in grassland soils along an aridity transect." Organic Geochemistry (2021): 104347. APA  </p><p> </p>