Rats in the radiocarbon (or vice versa)

The story of the New Zealand rat bones is a bit deeper than the press reports (e.g., this AP report). The main idea is that the rat radiocarbon dates support an initial habitation of New Zealand that was relatively late, around 1200 AD. That’s not a big surprise, since no human archaeological site or remains have been found to have earlier dates.

I don’t have any opinion about New Zealand prehistory, really. It seems to me that the rats are a very good source of evidence, because their population growth is potentially much much faster than human population growth. If rats arrive on an island, there’s a good chance of finding them early. I could imagine that humans might escape leaving archaeology for some time. I doubt very much that they could remain invisible for over a thousand years, but that depends on the intensity of archaeological research. But rats are not going to stay invisible. When you have extinct predators who ate rats, and they leave rat bones in their feces that you can sample, and none of those rat bones are more than 800 years old, well that’s a sign.

So what’s the real story here? The Oxford Radiocarbon Accelerator Unit keeps changing sample preparation protocols! These changes have brought in a number of new ways to take contamination and recent carbon out of the sample. I noted the redating of Vindija G1, which was based on a new sample preparation method using filtration to purify collagen from the bone. At the time, this was one among several new methods attempting to improve the accuracy of AMS dates. The cumulative effect of the advent of AMS dating, coupled with these later improvements, has added substantial precision to our knowledge of Europe during the last 40,000 years, as I reviewed here. Tom Higham, who was behind the new dates in the New Zealand paper, also worked out the Vindija G1 redating.

The problem is that every new sampling method raises the prospect that a lot of currently accepted dates are actually wrong. That is what has happened in the case of the New Zealand rats. The rat case demonstrates the depth of the problem: Holdaway (1996) presented seven AMS dates on rat bones whose confidence intervals are significantly older than 1000 AD (calibrated), two that are significantly older than 500 AD. The present study by Wilmhurst et al. must claim that all those rats were contaminated with old carbon.

Since the half-life of carbon-14 is 5730 years, an elevation of more than 500 years in a date represents a very substantial deficit of carbon-14 – on the order of five percent of the maximum amount. Such deficits might be possible, either due to conditions after burial or consumption of marine carbon by the animals during their lives. But in his original study, Holdaway closely considered these effects:

Potential sources of error include the addition of 'old' or reservoir carbon to the bone gelatin before death in the diet, or after deposition via unremoved humics or diagenetic processes in carbonate sedimentary environments, especially for small specimens.
Dietary influences were not apparent. Two individuals of known death date give calibrated ages that include their death dates. In addition, 14C dates on bone gelatin from two herbivorous birds (equilibrium carbon consumers) are not significantly different from those on rat bones from comparable levels. Humic contamination is unlikely, most being removed by gelatinization, but must still be considered fro earlier 'collagen' dates. Environmental carbonates were removed by an acid pre-wash, eliminating carbonate contamination. Measured ages were not related to whole-sample mass.
Longer-term diagenetic changes do not appear to have a significant effect. Samples of moa eggshell (species unknown) and bird bone from close proximity in sediment enclosed by two undisturbed volcanic tephras give indistinguishable ages.... These materials were prepared using different treatments. Finally, a rat dentary excavated from beneith the Taupo Tephra gives an age of 1,775±93 yr BP. In addition to the radiocarbon age being consistent with that of the covering tephra, the bone's position beneath the undisturbed layer provides independent evidence that Pacific rats were established in the North Island before the Taupo eruption (Holdaway 1996:226).

Yes, you read that right. He had a rat under a well-dated volcanic tephra.

The current paper claims that all the oldest dates for rat remains have come from a single lab, all before a single date:

Subsequent dating of Pacific rat bones sampled from both laughing owl (32) and archaeological sites (33-35) failed to duplicate the early series of old rat bone dates (35-38). The most telling criticism of the original dates is that they fall into two distinct groups according to when the bones were processed in the same dating laboratory (22, 36, 37) (see Fig. 1). The early series of rat bone dates processed in 1995 and 1996 are all older than the oldest-dated archaeological evidence (1280 A.D.), but all bones dated after 1996 are younger (36, 37) (Fig. 1). Moreover, some rat bones from archaeological assemblages that were processed in 1995 and 1996 are significantly older than consistent dates on diverse materials from the same stratigraphic contexts (34, 35). Critics argued that this unusual bimodal distribution of ages according to when the bones were processed was due to inadequate pretreatment of small bones (33, 35-37). It has also been argued that some of the old 1995-1996 rat bone dates are older than their "true" age because of dietary uptake of carbon depleted in 14C (e.g., refs. 39-40).

Well, there you have it. The argument has to be that the dates are wrong due to the different sample preparation methods. The “dietary carbon-14” argument can’t be the explanation, because some of the more recently dated samples ought to show the same deficit, and they don’t. I personally don’t see how they deal with the rat under the tephra – they don’t address the question. The only possibility that makes sense with their argument is that the samples were technically processed in a way that led to older dates.

Again, I have no opinion about New Zealand settlement. The recent chronology proposed here sounds reasonable to me, but mainly because people in a massively expanding population shouldn’t remain archaeologically invisible.

I just want to point out how much our knowledge of the archaeological sequence depends on the technical details of dating methods, known only to a small number of researchers. To be sure, technology advances. But we have thrown out an awfully large number of radiocarbon dates in the last few years, due to small but important changes in methods. And the New Zealand case shows that this problem is not confined to the upper limits of AMS dating, where the preserved carbon-14 fraction is at its lowest. In the European case, the biggest problem has been supposed Aurignacian specimens that turned out to be Holocene in age.

This raises the obvious question: how much weight should we give to current date estimates?

References:

Wilmhurst JM, Anderson AJ, Higham TFG, Worthy TH. 2008. Dating the late prehistoric dispersal of Polynesians to New Zealand using the commensal Pacific rat. Proc Nat Acad Sci 105:7676-7680. doi:10.1073/pnas.0801507105

Holdaway RN. 1996. Arrival of rats in New Zealand. Nature 384:225-226. doi:10.1038/384225b0