Irish Oysters

Dear all,

find below a short update on oyster shells which I recently analysed for Chris Hunt (University of Liverpool) and one if his Irish sites. They are O. edulis shells from a small shell midden with samples of mixed preservation. As it is often the case, the hinges were better preserved than other parts, so I went ahead and made sections through left and right valves, which both worked fairly well.

I will not go into each analysed shell but instead show 3 of the specimens that are representative of the various Mg/Ca patterns that I have seen in the overall assemblage.

With further research, I hope to figure out, what elemental distribution is typical or most common in Oyster shells and get a general idea, to then make better interpretations of individual shell-records.

Generally, there are probably two factors that can explain the Mg/Ca ratio in these shells:

• the organic component that is found in higher concentrations within the (often) annual growth lines (the laser does not separate between the organic and the inorganic component of the shell, so there is little we can do to avoid these);

• increases in growth rate (shoutout to Harry Robson (University of York), who saw the same patterns of growth in his Danish oysters). (also check out Durham et al. 2017)

Additionally, there are some spatial distributions of Mg/Ca that are not as common but still turn up in a lot of shells.

In some specimens I found a shift from consistently low Mg/Ca ratios to regular and gradual increases of Mg/Ca after 2 years of growth. I don't know enough about oyster maturation but maybe there's something going on here?

Also, almost every specimen has increased Mg/Ca ratios along the outside of the hinge. At first I tried to explain this with an increased growth rate in this region, as it is usually thicker than other parts of the shell, but it doesn't hold up everywhere. Something else must also be going on here.

Mouchi et al. talk about offsets of temperature curves from six archaeological oysters. They tentatively link the different Mg/Ca ranges to different marine/freshwater conditions, but their values could also be explained by the above factors, which are only visible when mapping the elemental composition or sampling in different parts of the same increment, which rarely happens.

Altogether, I think with LIBS we can provide a quick indicator for season of death, but I would probably want to still base that on a modern reference study, where we actually know when the specimens died, rather than an archaeological assemblage.

Since the Mg/Ca ratios here are not clearly linked to temperature, their interpretation for season of death would be similar to the analysis of growth increments à la Milner and not as straightforward as temperature records. So the training is going to be a wee bit more intensive and the interpretation potentially more subjective as well. Hopefully, once we know more about Irish oysters, we can come back and interpret the archaeological samples in more detail.

The next update will be on some limpets, which is always fun!

Until then,

Niklas