USING DIATOM AND PHYTOLITH ASSEMBLAGES TO INVESTIGATE MOISTURE AVAILABILITY DURING GRASSLAND EVOLUTION IN GRAN BARRANCA, ARGENTINA

Previous work to investigate grassland evolution between 43 and 18 Ma at Gran Barranca, Argentina using phytolith assemblages has shown that vegetation changed over this time period; however, grasses were never abundant. Instead, palms dominate at most levels. Palms today are typically associated with humid climates and are common in tropical wetland habitats. However, isotopic data and a phytolith-based proxy for leaf area index suggest open, dry habitats for Patagonia during this time. Therefore, the relationship between the abundance of certain typically moisture-dependent plant taxa in the phytolith record and moisture availability remains unclear.

Diatoms are also present in some levels of the Gran Barranca section, although it is not clearly a lacustrine or riverine sequence, and may be useful as an independent proxy for moisture availability. Among the diatoms present in the section, those of the genus Orthoseira, known to prefer wetland/bog habitats, may be among the most useful. Other common genera include Staurosira, Navicula, and other fairly ubiquitous benthic taxa as well as the planktonic diatom Aulacoseira in at some levels. Here we compare the abundance of palm phytoliths with the abundance of Orthoseira, as well as the ratio of benthic to planktonic diatoms. High planktonic to benthic ratios and/or high prevalence of Orthoseira co-occurring with transition to palm forest suggests the possibility of humid wet conditions and wetland habitats co-occuring with the spread of palm forest.