Triple oxygen isotopes and clumped isotopes in modern vertebrate and dinosaur biominerals: Records of paleoecology, paleoaridity, and paleo-carbon-cycling

Abstract:

The parameter Δ¹⁷O describes the departure of δ¹⁷O from an expected equilibrium relationship with δ¹⁸O, which can be caused by factors such as evaporation of parent waters, and photochemical reactions among oxygen-bearing gases in stratosphere. Hence, the Δ¹⁷O of water records information about environmental aridity, and the Δ¹⁷O of atmospheric O₂ is related to atmospheric concentrations of CO₂ and O₂, and gross primary productivity (GPP). Vertebrates incorporate Δ¹⁷O signals of input water (e.g. drinking water and free food water) and atmospheric O₂ into body water through respiration, and biominerals forming in equilibrium with body water can preserve this signal over geological timescales. The preservation of fossil biominerals can be evaluated by clumped isotopes as they record the temperature of mineralization, be it primary mineralization in the living animal (at body temperature), or secondary mineralization during diagenesis. We can distinguish the alteration of samples from the deviation between observed clumped isotope temperatures and plausible body temperatures. Meanwhile, diagenesis tends to moderate Δ¹⁷O of biominerals towards Δ¹⁷O of meteoric waters, such that measured Δ¹⁷O values reflect the minimum anomaly in fossil samples. Thus, preservation of anomalous Δ¹⁷O indicates at least partial preservation of the original signal.

We present Δ¹⁷O data from both modern vertebrate and fossil dinosaur biominerals. We use a ¹⁷O-enabled body water model to explore the influence of aridity and dietary ecology on animal Δ¹⁷O, and to predict the degree of dilution of the atmospheric O₂ Δ¹⁷O signal by other sources of oxygen to the animal. We observe: 1) animals consuming more leaf water than drinking water are “evaporation sensitive” (ES) animals, and have lower Δ¹⁷O relative to “evaporation insensitive” animals in the same climates; 2) ES animals from arid climates have lower Δ¹⁷O values compared to ES animals from humid climates, which forms the basis of a paleoaridity proxy; and 3) anomalously low body water Δ¹⁷O values are observed for some of the dinosaur samples, indicating a significantly different carbon cycle during that time, with pCO₂ up to 3250±1250 ppm during late Jurassic assuming present day GPP and pO₂.