T112 Oceans and Climates through Earth History: Modeling, Proxy, and Proxy Development Studies (Posters)

Monday, 23 September 2024
8:00 AM - 5:30 PM
Hall D

USE OF THE EFFECTIVE TEMPERATURE CONCEPT TO ACCOUNT FOR TEMPERATURE VARIATION IN CLUMPED ISOTOPE MEASUREMENTS OF GAR SCALES

Paper number: T112-25
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

We have recently developed a new calibration for measuring terrestrial surface temperatures using the clumped isotopes Δ47 of the carbonate ion in the bioapatite scales of garfish (Gray and Brandon, in review). Our contribution here is to describe how the effective temperature concept can be used to account for the variable temperature history of our modern samples. Our calibration is based on a sampling of modern gars from a latitudinal transect extending from Mexico and across the US Midwest. Gars scales provide an ideal isotope thermometer for terrestrial settings. Gars live in shallow depths (< 3 m) in rivers and lakes, are non-migratory, and their scales grow continuously over their lifespan (~8-14 years). As ectotherms, their body temperature is equivalent to ambient water temperature.

The effective temperature is defined as the steady temperature that would produce the same Δ47 value as produced by the variable temperature experienced by the sample. The method has a long history: Pallmann et al., 1940; Lee, 1969; McCoy, 1987; Rogers, 2007; 2008. It is designed for any system where growth or production, P, is well-defined by an Arrhenius function, P ∝ exp(-Ea/RT), where Ea is the activation energy, T is temperature (K), and R is Boltzman’s constant. The surface temperature variation is usually represented by a harmonic series with a mean (annual) temperature Tm and sin functions with parameters ΔTd and ΔTs equal to the average range for daily and seasonal temperature variations (Rogers, 2007, 2008). We used the Daymet V4 climate dataset to estimate these parameters for sample locations. The temperature function is then combined with the Arrhenius function to determine an average value for P. The Metabolic Theory of Ecology (Brown et al., 2004) demonstrates that growth rates across a large range of organisms, including fish, have a fixed Ea = 60 kJ/mol. This value allows us to find Te for each of our sample locations. We find that Te is the same as Tm in the tropics, but can be 12 °C greater than Tm in mid-latitude settings. We anticipate that this approach will be useful for calibration and prediction of temperature using other geochemical methods.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-405548

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