Relative pollen abundance data can be difficult to interpret. If percent Pinus increases, it could be because pine trees became more common, or because other species became rarer. Pollen concentration (or ideally influx rates) can help resolve what is happening.
I’m still exploring the neotoma database and associated R package, and decided to look at how, and how often, pollen concentrations have been calculated (code for all this is on github if you want to see how I did it, and tell me how I can do it better).
There are two basic methods for estimating pollen concentrations. The first is to count all the pollen in a carefully weighed amount of pollen (known in Neotoma as the Jørgensen method). The second is to add a spike of exotic pollen/spores/plastic microspheres and calculate the concentration from the pollen:spike ratio. This spike method is by far the most popular method of calculating concentration in the Neotoma database, with a variety of exotics used.
There is some geographic patterns in the method used. Eucalyptus is, not surprisingly, not used in Australia, and the Jørgensen method is inexplicably popular in eastern Canada.
In my previous post on Neotoma, I noted that the very high concentrations of Eucalyptus in Hockham Mere suggested that the exotic spike had been mislabelled as a tree/shrub. I want to know if this is the only site with this problem.
Several Eucalyptus morphotaxa are found in Neotoma, but all except plain Eucalyptus are only found in Australia.
Assuming that no palynologists working on Australian material would use Eucalyptus as a spike, we only need to consider the 29 non-Australian data-sets. Of these, many have just a few Eucalyptus grains, and these are mostly from recent sediments and so the pollen are perhaps from introduced Eucalyptus trees growing near to the studied site. About ten records have, judging by the abundance of Eucalyptus pollen, mislabelled spikes. In some cases, for example the Lake of the Clouds, most levels have microsphere markers, the others have many Eucalyptus grains.
Lycopodium (clubmoss) is the other taxon commonly used as a spike. Given its worldwide distribution, it is going to be more difficult to identify rogue spikes. Twenty eight Lycopodium taxa/morphotaxa are found in 1421 data sets. The vast majority (95%) of these data sets have a maximum abundance of fewer than 50 spores and so are unlikely to be spikes. Of the remainder, some are almost certainly spikes (e.g. 4296, 4355), others are difficult to tell without going back to be original literature.
In Neotoma, as with any big data set, there are inevitably errors in a small proportion of the data. It is user’s responsibility to know the data well enough that they can recognise unlikely data and to check that their exciting results are not being caused by errors or artefacts.