Can phytoliths from Sorghum bicolor, a C4 plant, be indicators of past water availability?

Abstract

The reconstruction of past water availability is vital to our understanding of past societies. One of the principal requirements for early state societies is a complex farming system employing irrigation. Early irrigation regimes are likely to have been ephemeral and so would no longer be visible in the archaeological record. C3 plants have been the focus of phytolith research regarding past water availability. Research has proven that phytoliths from C3 plants can indicate past water availability (Madella et al. 2009; Weiskopf et al. 2014; Jenkins et al. 2016). However there has not been a study focusing on phytoliths in C4 plants such as sorghum. This study will determine whether or not the C4 plant sorghum can indicate past water availability. Sorghum grown from three different sites in Jordan, Deir ‘Alla, Ramtha and Salt which has been grown over a period of two years (2009-2010) was taken for phytolith processing. Statistical analyses was undertaken to assess whether variables such as climate, soil and water chemistry affect silica deposition adding further resolution to this discussion. Results show that there were unknown factors that influenced phytolith production between sites and years. However irrigation was found to be the biggest influence in determining the ratio of fixed (short) to sensitive (long) phytolith forms. Samples with a mean fixed to sensitive ratio of above 3.5 are likely to have been unirrigated while samples with a mean short to long form ratio of less than 1.7 are likely to have been irrigated. The most striking observation is that husks have a smaller ratio of fixed to sensitive forms and hence have more sensitive phytolith forms than the leaves and stems. The confidence interval is small for irrigated husks and demonstrates that anything under 0.2 is likely to have been irrigated whereas anything above 0.2 is likely to have been unirrigated. This study demonstrates that phytoliths from sorghum can be used to identify past water availability.