Friday, August 16, 2013

The Nature of Evidence - Tuesday 13 August 2013

Yesterday afternoon, we tackled our first piece of field work. The reasons for this piece of work take a bit of explanation. As a word of warning, this blog post therefore will have some science detail. 

We're to trying to understand confusing evidence from our longest Rano Kau core. With the help of Maureen Marra, we recovered a few ant fossils in our core. We assumed the presence of the particular ants we found meant that the ant species were transported across the Pacific by Polynesians. Most were at the depth in the core we were convinced represented Polynesian arrival. So we radiocarbon dated the tiny ant fossils. But, the results suggested the ants were roughly 3000 years old, much older than the expected age of 800 years.

One could possibly argue that Polynesians must have transported the ants and arrived 3000 years ago, but this would seem to contradict many decades of careful archaeological work. Before we start to accept a new claim, the process of science requires us to try to disprove the hypothesis representing this claim. One of the most logical explanations for the 3000 year old ants is that the ants were eating 'old' carbon. This would undermine the hypothesis of ants arriving 3000 years ago.
Most of you may need a refresher on how radiocarbon dating works. Radiocarbon is produced in the atmosphere by cosmic rays, establishing a fairly uniform amount of radiocarbon in atmospheric carbon dioxide. Radiocarbon dating works by measuring the radioactive decay that has occurs since the atmospheric carbon dioxide has been taken up and used to build plants.  Subsequently, animals who eat the plants may also be dated.  If an animal, such as one of our ants, eats carbon that was taken from the atmosphere a long time ago, its age will appear much older than expected. This commonly occurs with animals (or humans) eating a seafood diet, since the surface ocean holds dissolved carbon dioxide for hundreds of years.

We descended into Rano Kau crater and laid out some lures to collect ants similar to those we found in our cores. We collected ants at three levels - the crater rim, slope and edge of the lake. We found the most ants at the edge of the lake where it was moist (pictured), and few at both the midslope and crater rim (where Mark is pictured).

Do you think our ants will have radiocarbon ages that make them look thousands of years old? If they do, then the age of the ants can be treated as bad luck and discarded. If that's the case, then the presence of ants in the core can be interpreted as evidence of Polynesian arrival. Or, if the ants we collected look like today's atmosphere, maybe we do have very unexpected evidence of Polynesians reaching Rapa Nui but not settling long enough to leave archaeological evidence?

Do you prefer the idea of the now agreed time of arrival (1200AD), or a so-called 'early arrival' (3000 years ago)? For years, archaeologists and paleobotanists have argued for and against 'early arrival' theories, and the ability to clarify the evidence has been greatly improved by our ability to use accelerator mass spectrometry to count radiocarbon atoms (requiring samples of a milligram or less). This has made it much more feasible to date short lived seeds, twigs, leaves and other materials that are not from big, old trees (like charcoal) or from the sea (like shells). Previously, dating by radioactive decay counting required much larger samples - many grams of material.

Hopefully, the evidence will be clear after we 'date' the ants we have collected.

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