Ring Index: A new strategy to evaluate the integrity of TEX86 paleothermometry

Abstract

The TEX86 (tetraether index of 86 carbon atoms) sea surface temperature proxy has been increasingly applied in the reconstruction of Mesozoic and Cenozoic ocean temperatures. However, the archaeal lipids that compose TEX86 indices, glycerol dialkyl glycerol tetraethers (GDGTs), can derive from water column production, terrestrial sources, and/or within sediments. TEX86-sea surface temperature estimates can also be influenced by non-temperature factors, such as growth phase and nutrient levels. Here we show that the weighted average of cyclopentane moieties, known as the Ring Index (RI), can be used to determine if TEX86 temperature estimates are influenced by non-thermal factors and/or deviate from modern analogues. We demonstrate that RI and TEX86 indices from the published global core top data set and mesocosm cultures are significantly correlated, as predicted through the influence of temperature on lipid biochemistry. We further show that when RI and TEX86 indices in modern or ancient records deviate from the modern global TEX86-RI relationship, GDGT distributions are not solely controlled by environmental temperature and/or TEX86-based temperature reconstructions are questionable.