Triple oxygen isotope distribution in modern mammal teeth and potential
geologic applications
Abstract
Reconstructing water availability in terrestrial ecosystems is key to
understanding past climate and landscapes, but there are few proxies for
aridity that are available for use at terrestrial sites across the
Cenozoic. The isotopic composition of tooth enamel is widely used as
paleoenvironmental indicator and recent work suggests the potential for
using the triple oxygen isotopic composition of mammalian tooth enamel
(∆’17Oenamel) as an indicator of aridity. However, the extent to which
∆’17Oenamel values vary across environments is unknown and there is no
framework for evaluating past aridity using ∆’17Oenamel data. Here we
present ∆’17Oenamel and δ18Oenamel values from 50 extant mammalian
herbivores that vary in physiology, behavior, diet, and water-use
strategy. Teeth are from sites in Africa, Europe, and North America and
represent a range of environments (humid to arid) and latitudes (34S to
69N), where mean annual δ18O values of meteoric water range from -26.0‰
to 2.2‰ (VSMOW). ∆’17Oenamel values from these sites span 146 per meg
(-283 to -137 per meg, where 1 per meg = 0.001‰). The observed variation
in ∆’17Oenamel values increases with aridity, forming a wedged-shape
pattern in a plot of aridity index vs. ∆’17Oenamel that persists
regardless of region. In contrast, the plot of aridity index vs.
δ18Oenamel for these same samples does not yield a distinct pattern. We
use these new ∆’17Oenamel data from extant teeth to provide guidelines
for using ∆’17Oenamel data from fossil teeth to assess and classify the
aridity of past environments. ∆’17Oenamel values from the fossil record
have the potential to be a widely used proxy for aridity without the
limitations inherent to approaches that use δ18Oenamel values alone. In
addition, the data presented here have implications for how ∆’17Oenamel
values of large mammalian herbivores can be used in evaluations of
diagenesis and past pCO2.