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Title: Isolating relative humidity: dual isotopes d18O and dD as deuterium deviations from the global meteoric water line

Author
item VOELKER, STEVEN - Southern Oregon University
item BROOKS, J RENEE - Us Environmental Protection Agency (EPA)
item MEINZER, FREDERICK - Forest Service (FS)
item RODEN, JOHN - Southern Oregon University
item PAZDUR, ANNA - Silesian University Of Technology
item PAWELCZYCK, SLAWOMIRA - Silesian University Of Technology
item HARTSOUGH, PETER - University Of California
item Snyder, Keirith
item PLAVOCA, LENKA - Ulm University
item SANTRUCEK, JIRI - University Of South Bohemia

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/16/2013
Publication Date: 7/15/2014
Citation: Voelker, S.L., Brooks, J., Meinzer, F.C., Roden, J., Pazdur, A., Pawelczyck, S., Hartsough, P., Snyder, K.A., Plavoca, L., Santrucek, J. 2014. Isolating relative humidity: dual isotopes d18O and dD as deuterium deviations from the global meteoric water line. Ecological Applications. 24(5):960-975.

Interpretive Summary: Evidence of past climates has been historically reconstructed using information contained in tree ring cellulose. Tree ring cellulose is formed from photosynthate that is formed with water that is evaporated in the leaves. The water evaporated in the leaves will have certain isotopic signature of hydrogen and oxygen, these heavier isotopic forms are commonly referred to as deuterium (dD) and oxygen (d18O). These isotopic signatures of water are a function of the source of precipitation and subsequent evaporation due to relative humidity. Commonly only a single isotope has been used either dD or d18O in models of tree ring cellulose. This study used both isotopes in the cellulose of tree rings to attempt to isolate the effects of relative humidity from the source of precipitation. This dual isotope approach will be useful for understanding variations in relative humidity in past climates.

Technical Abstract: Cellulose d18O and dD can provide insights on climates and hydrological cycling in the distant past and how these factors differ spatially. However, most studies of plant cellulose have used only one isotope, most commonly d18O, resulting in difficulties partitioning variation in d18O of precipitation vs. evaporative conditions that affect leaf water isotopic enrichment. Moreover, observations of pronounced diurnal differences from conventional steady-state model predictions of leaf water isotopic fractionation have cast some doubt on single isotope modeling approaches for separating precipitation and evaporation drivers of cellulose d18O or dD. We explore a dual isotope approach akin to the concept of deuterium-excess (d), to establish deuterium deviations from the global meteoric water line in leaf water ('dl) as driven by relative humidity (RH). To demonstrate this concept, we survey studies of leaf water d18O and dD in hardwood vs. conifer trees. We then apply the concept to cellulose d18O and dD using a mechanistic model of cellulose d18O and dD to reconstruct deuterium deviations from the global meteoric water line ('dc) in Quercus macrocarpa, Q. robur, and Pseudotsuga menziesii. For each species, Ddc showed strong correlations with RH across sites. 'dc agreed well with steady-state predictions for Q. macrocarpa, while for Q. robur, the relationship with RH was steeper than expected. The slope of 'dc vs. RH of P. menziesii was also close to steady-state predictions, but 'dc were more enriched than predicted. This is in agreement with our leaf water survey showing conifer 'dl was more enriched than predicted. Our data reveal that applications of this method should be appropriate for reconstructing RH from cellulose d18O and dD after accounting for differences between hardwoods and conifers. Hence, Adc should be useful for understanding variability in RH associated with past climatic cycles, across regional climates, or across complex terrain where climate modeling is challenging. Furthermore, 'dc and inferred RH values should help in constraining variation in source water d18O.