Historical Geology/TEX86

In this article we shall look at the TEX₈₆ temperature proxy, how it works, and how we know that it works.

GDGTs

The TEX₈₆ method is based on glycerol dibiphytanyl glycerol tetraethers (GDGTs). These come in various forms with more or fewer cyclopentane structures (the reader need neither know nor care what these actually are). The GDGTs of interest to us can be denoted as GDGT 1, GDGT 2, GDGT 3 and GDGT 4' (pronounced "four-prime"), where the numbers 1, 2, 3 and 4 correspond to the number of cyclopentanes.

(For the benefit of those readers who wish to research the TEX₈₆ method in the technical literature, I should point out that different papers use different numbering schemes; the one used here seems most suitable, because of the correspondence between the GDGT number and the number of cyclopentanes.)

Crenarchaeota and temperature

In nature these GDGTs are produced by the group of single-celled organisms known as the Crenarchaeota. As with the alkenones discussed in the previous article, the GDGTs resist processes that destroy most organic compounds, and so can be found in marine sediment; and just as with the alkenones, the proportions of the different GDGTs produced by the Crenarchaeota varies with temperature, according to the formula:

T = 56.2 × TEX₈₆ - 10.78

where T is the temperature in °C and TEX₈₆ (an abbreviation of "TetraEther indeX of tetraethers consisting of 86 carbon atoms") is defined as the ratio of the sum of the quantities of GDGTs 2, 3 and 4' to the sum of the quantities of GDGTs 1, 2, 3 and 4'.

It should be noted that this relationship ceases to hold below about 5°C; below this temperature the variation in TEX₈₆ becomes negligible and so measurements of TEX₈₆ can't distinguish between temperatures below that point.

How do we know?

We can measure TEX₈₆ in living organisms and recent sediments, and measure the temperature of the water in which they are found; this is how the formula given above was derived.

However, unlike the U^k'₃₇ method, the relationship is harder to demonstrate experimentally. Experiments do show that TEX₈₆ increases with temperature in the lab; however for some unknown reason lab-grown cultures of Crenarchaeota produce less GDGT 4' than is found in Crenarchaeota in the wild, and so the exact relationship between temperature and TEX₈₆ can't yet be replicated in the laboratory.