“Fossil Insect Study Suggests That Los Angeles Climate Has Been Relatively Stable for at Least 50,000 Years”

So sayeth the press release. But what about the paper, and the 182 beetles sampled from La Brea tar pits?

Fossil preservation in the tar pits is exceptional, but the constant stream of gas through the tar deposits mixes the fossils – there is no stratigraphy in the tar. Holden et al have to radiocarbon date each and every beetle they analyse. Naturally, this limits the number of beetles they can analyse, both because of the financial cost and the destruction of the beetles. It also limits the number of species than can be analysed (to seven), as only species with abundant fossils can be vaporised  (we are not told what other species are found – I would have liked this information).

Contrast the situation with the tar pits with a more typical site for beetle analysis, say a section through a peat bog, where large volumes of sediment in stratigraphic order can be collected, with dozens or more beetle fossils from many species in each sample, and only a few dates needed to constrain the chronology. La Brea is a challenging site.

These are dates of the seven beetle species at La Brea.

1-s2-0-s0277379117301671-gr2

Holden et al figure 2. Median calibrated age of each beetle with 2-sigma ranges. Cases where only the error bar is shown are “greater than” ages where the results were very close to background and only a lower limit could be specified. The low quality of the image is because Elsevier are hopeless.

The first thing to note is that no beetles were dated to the last glacial maximum. Holden et al ascribe this to either the lack of insect collections from the pits containing LGM mammal fossils, or a cooler LGM climate making the tar less sticky so fewer insect were caught. If the former explanation is correct, the press release claim that “Los Angeles Climate has been relatively stable for at least 50,000 years” cannot be substantiated as there is no evidence from a critical interval. If the latter explanation is correct, the press release is refuted. A third explanation not considered by Holden et al is that the climate changed such that the seven beetles they use were not present at La Brea. Whatever the reason for the lack of LGM beetles, the headline of the press release is wrong.

There is also a beetle gap in the early Holocene thermal maximum. Again, the lack of evidence precludes a conclusion that the climate was stable.

It is always going to be difficult to reconstruct climate from a just seven species of beetles, selected in part because they were common. So it is not greatly surprising the reconstructed climate, for the intervals where there are data, is similar to modern. Holden et al report “mean summer temperatures within ±5 °C of today’s conditions”. A range wide enough that only the LGM (for which there are no data) could reasonably be expected to exceed.

The method for reconstructing climate is unclear. The paper appears to assume that the assemblage composition is constant through time and hence that the climatic conditions must have remained similar to modern. I would have liked to see a figure showing the assemblage in each time window. Something like this.

Beetles_barplot.png

Number of beetles by time interval. ?Modern beetles have an age of < 200 cal BP, and so might represent accidental modern contamination of the tar pit.

There are distinct shifts in the species composition through time that Holden et al do not explore. I don’t know if these shifts have possible climatic interpretations. Although Holden et al collate modern records for their species, they don’t present the results in an easy-to-interpret way (they present violin plots, each species in a separate file, and scatter plots).

While the species distribution modelling could certainly have been done better, and the method for climate reconstruction made explicit, it is always going to be difficult to work on the stratigraphically-mixed deposits at La Brea, and without a huge amount of money for dating, and a willingness to atomise any beetles for dates, it will be impossible to get the data and reconstruction that could be expected from a more typical site. But that is no excuse for a press release that is unsupported by the paper.

 

 

 

 

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About richard telford

Ecologist with interests in quantitative methods and palaeoenvironments
This entry was posted in climate, Peer reviewed literature and tagged , . Bookmark the permalink.

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