Determinants of blood water δ18O variation in a population of experimental sheep: Implications for paleoclimate reconstruction

Abstract

Mammalian body, blood and hard tissue oxygen isotope compositions (δ18O values) reflect environmental water and food sources, climate, and physiological processes. For this reason, fossil and archaeological hard tissues, which originally formed in equilibrium with body chemistry, are a valuable record of past climate, landscape paleoecology, and animal physiology and behavior. However, the environmental and physiological determinants of blood oxygen isotope composition have not been determined experimentally from large herbivores. This class of fauna is abundant in Cenozoic terrestrial fossil assemblages, and the isotopic composition of large herbivore teeth has been central to a number of climate and ecological reconstructions. Furthermore, existing models predict blood water, or nearly equivalently body water, δ18O values based on environmental water sources. These have been evaluated on gross timescales, but have not been employed to track seasonal variation. Here we report how water, food, and physiology determine blood water δ18O values in experimental sheep (Ovis aries) subjected to controlled water switches. We find that blood water δ18O values rapidly reach steady state with environmental drinking water and reflect transient events including weaning, seasons, and snowstorms. Behavioral and physiological variation within a single genetically homogenous population of herbivores results in significant inter-animal variation in blood water δ18O values at single collection times (1 s.d. = 0.1–1.4 ‰, range = 3.5 ‰) and reveals a range of water flux rates (t1/2 = 2.2–2.9 days) within the population. We find that extant models can predict average observed sheep blood δ18O values with striking fidelity, but predict a pattern of seasonal variation exactly opposite of that observed in our population for which water input variation was controlled and the effect of physiology was more directly observed. We introduce to these models an evaporative loss term that is a function of environmental temperatures. The inclusion of this function produces model predictions that mimic the observed seasonal fluctuations and match observations to within 1.0 ‰. These results increase the applicability of available physiological models for paleoseasonality reconstructions from stable isotope measurements in fossil or archaeological enamel, the composition of which is determined in equilibrium with blood values. However, significant blood δ18O variation in this experimentally controlled population should promote caution when interpreting isotopic variation in the archaeological and paleontological record.

Quantitative reconstruction of seasonality from stable isotopes in teeth

“…Modeling of oxygen throughput in our experimental animal, using available blood water isotopic measurements and δ 18 O values for sources of O to blood water, produces results consistent with observations of drinking water and feed intake through the duration of the experimental switch. Our results are also within the range of body water half-lives (2.2 -9.0 days) previously reported for domesticated sheep using deuterated and tritiated water experiments (MacFarlane et al, 1971;Dawson, 1978;Nicol, 1978;Green et al, 2018). Nevertheless, it is reasonable to assume that parameters such as the magnitude of drinking water intake, evaporative water loss, and oral temperature will vary between breeds, species, and over seasonal and even diurnal cycles (Kohn et al, 1998;Zazzo et al, 2010;Gretebeck et al, 1997).…”

“…( 2)where λb is the decay constant, δb(t) is the blood δ 18 O value at time t, and δb,ss(t) is the theoretical blood δ 18 O steady state at that same time, determined by physiological parameters and feed and air δ 18 O values. Importantly, from equations (1) and (2) we can predict both the δ 18 O value of body water, and its rate of change, from environmental sources and physiological conditions (Green et al, 2018). Blood δ 18 O data from our experimental animal indicate an overall single-pool blood water oxygen half-life of 2.6 days, calculated by fitting λb with data.…”

“…We raised a male high-percentage Dorset sheep at the Concord Field Station in Bedford, Massachusetts, USA, following weaning at two months of age. It was fed from a single batch of dry feed (Central Connecticut Co-op Farmers Association) and provided Bedford drinking water (ranging from -7.4 to −5.9 ‰ throughout the experiment; all  18 O values are reported on V-SMOW scale) originating primarily (85% or more) from the 1.5km 3 Quabbin Reservoir, and to a lesser extent from Bedford ground water (Green et al,, 2018 Aliquots of animal feed were dried at 60 °C and analyzed for oxygen isotope analysis on the TCEA-Conflo IV-Delta V Plus IRMS system at the University of Chicago. Feed δ 18 O was measured to be +25.3 (+/− 0.8 1 s.d.)…”

Modeling enamel matrix secretion in mammalian teeth

“…Nevertheless, complex patterns of enamel thickness on a tooth crown can result from a single developmental process, without the need to evoke specific control or explanations for the thickness of individual features. Combined with in-depth analyses of enamel formation rates [4, 9, 27] and isotope compositions [28, 29] obtainable from sections, our model should help to move studies using enamel thickness towards more mechanistic and predictive science. The model also points to the need to investigate the role of extracellular space and capillaries in regulating diffusion, and to the potential effects of cell-cell tension in the ameloblast layer [30].…”

Isotope Mass Spectrometry Application for the Abiogenic Alcohols Detection in Grape Wines

“…Given that in the molecule in addition to carbon atoms of the alcohol also includes oxygen and hydrogen, it can be assumed that the relationship isotope 18 O/ 16 O and D/H can be significantly different from the relationship of oxygen and hydrogen isotopes alcohols of plant origin. Indeed, the oxygen of the technical water involved in the reaction for obtaining abiogenic alcohol is much easier than the oxygen of the aqueous component of plant raw materials [11][12][13][14][15][16][17][18][19][20][21][22][23][24].…”