Hello Harro,
Heterotrophs produce some metabolic water by "burning" food that may be isotopically very different from local meteoric water. Considering the fact that even some mammals in arid environment rarely or never drink and thus produce most or all of their body water metabolically, it can be expected that body water occasionally has little in common with local meteoric water. There is a nice graph depicting various mammalian sources and sinks of body water in terms of an oxygen isotope mass balance:
Ayliffe L.K. and Chivas A.R., 1990. Oxygen isotope composition of the bone phosphate of Australian kangaroos: Potential as a palaeoenvironmental recorder. Geochimica et Cosmochimica Acta 54, 2603-2609. doi:10.1016/0016-7037(90)90246-H
The same principles hold true for hydrogen.
Consequently diet has a major influence on the isotopic composition of animals' body water, in addition to the influence of drinking water and evapotranspiration (e.g., water vapor loss through lungs). Hydrogen and oxygen in food biomass are incorporated from water by autotrophs using different biosynthetic pathways, with fractionation for oxygen and hydrogen being independent from each other. Different biochemicals show different fractionations. Diets differ greatly among heterotrophs. These may be the chief reasons for your observations. In contrast, the isotopic exchange between body water and exchangeable organic hydrogen and oxygen in food and biomass has little to do with it.
Can you offer some more detailed information about which species' body water isotopic compositions exhibit strong deviation from that of regional meteoric water?
With best wishes,
Arndt

H.A.J. Meijer wrote:
[log in to unmask]" type="cite">Dear colleagues,

My estimate for the isotope values for blood (that is the water component) in humans and animals would be that it has to be pretty close to the local (drinking) water. It is possibly slightly enriched due to evaporation processes in the body as well as in the open water available in nature for animals to drink, but not by more than a few per mil.
In the "bakground" samples we process for doubly labelled water studies, however, relatively high values for 2H occur regularly, sometimes higher than + 50 o/oo. 18O is always in the local natural range that I expect.
Is this an artefact, i.e. contamination by memory effects in the lab, or in the field, or during sample storage (samples are being stored in flame-off capilaries)?. If so, why does it  never happen for 18O?
Or is it real? We know that the hydrogen in body water interacts with H in tissue to some extent, but can tissue be that highly enriched in 2H? And if so, why is it so variable?
Is there any experience with "blood water" isotopes in fields where people do not use enriched water as well, f.i. in forensics?

Thanks on forehand for helping!

best regards,

Harro


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