Emma here. I am using stable isotope analysis to investigate past climate change, plant water stress, and food production as part of the TwoRains project. Stable isotope analysis can use chemical signals in charred plants, and human and animal remains to investigate archaeological questions. At the moment, there is not much work I can do on my archaeological questions as my samples are still underground! In the meantime, however, there are lots of experiments I can do that will help us improve the techniques and better interpret the archaeological data.

Which is why Cemre and I are currently growing foxtail millet (Setaria italica) at the Sainsbury Research Laboratory. One of my archaeological questions is to establish whether climate change caused water stress in a wide range of plants, which could have caused crops to fail, in turn contributing to the de-urbanisation of the Indus Civilisation. We know that the climate changed during the Indus period, but we still have many questions. How did it affect our study area? Was severe enough to impact crop yield? Did people water their plants to make up for drier conditions? To answer these questions, Penny’s PhD has examined stable carbon and nitrogen isotope ratios in charred plant remains, work that Cemre and I will be continuing as part of the TwoRains project on new material from our excavations. Before we can do that though, there are several key things that we need to know about each species in order to better interpret our archaeological results.

The problem is that water stress isn’t the only thing that causes stable isotope ratios of plants to change, so if we want to reliably interpret our archaeological data, we need to understand how much variation we would expect due to other factors, basically how much isotopic change do we need to see to interpret it as significant? There are several key things that we just don’t and can’t know about our archaeological samples that will affect the plants’ isotope values:

  • Did the variety of plant change through time, whether through people bringing in new seeds, artificial selection, natural selection or drift?
  • Were the crops grown in the same place(s) throughout the study period?
  • Did the soil degrade or did people manure their fields?

So, what do we do?

Well, we try to quantify how much variation we would expect for these factors for each species by doing experiments involving growing plants under known conditions. In a previous experiment, I have previously shown that: 1. there is a significant amount of variation within foxtail millet plants; and 2. the range of carbon isotope values across different genetic varieties of foxtail millet grown under uniform conditions is about 2‰ (not too bad, similar to the variation within a plant), but in nitrogen is 6‰ (worryingly large, much larger than the variation within a plant and problematic!).

Building on this previous experiment, Cemre and I are doing an experiment to try to quantify the effect of water stress on the plant’s stable isotope values. We are growing four (genetically very similar) replicates of 12 foxtail millet varieties and subjecting them to different watering regimes (see photos below). We are also growing various controls, just to make double check that the differences really are due to the watering regime! So far, the plants have grown and you can see differences between the different watering regimes, but none of them have yet flowered. Hopefully, the isotope values will vary consistently with watering regime. Hopefully, this variation will be larger than the variation within plants and between varieties of plant. And hopefully this will allow us to assess our archaeological data in a way that at least partially accounts for the inherent unknowns.

Wish us luck – now I must go and water the plants!