Dentine oxygen isotopes (δ18O) as a proxy for odontocete distributions and movements

Spatial variation in marine oxygen isotope ratios (δ¹⁸O) resulting from differential evaporation rates and precipitation inputs is potentially useful for characterizing marine mammal distributions and tracking movements across δ¹⁸O gradients. Dentine hydroxyapatite contains carbonate and phosphate that precipitate in oxygen isotopic equilibrium with body water, which in odontocetes closely tracks the isotopic composition of ambient water. To test whether dentine oxygen isotope composition reliably records that of ambient water and can therefore serve as a proxy for odontocete distribution and movement patterns, we measured δ¹⁸O values of dentine structural carbonate (δ¹⁸O_SC) and phosphate (δ¹⁸O_P) of seven odontocete species (n = 55 individuals) from regional marine water bodies spanning a surface water δ¹⁸O range of several per mil. Mean dentine δ¹⁸O_SC (range +21.2 to +25.5‰ VSMOW) and δ¹⁸O_P (+16.7 to +20.3‰) values were strongly correlated with marine surface water δ¹⁸O values, with lower dentine δ¹⁸O_SC and δ¹⁸O_P values in high‐latitude regions (Arctic and Eastern North Pacific) and higher values in the Gulf of California, Gulf of Mexico, and Mediterranean Sea. Correlations between dentine δ¹⁸O_SC and δ¹⁸O_P values with marine surface water δ¹⁸O values indicate that sequential δ¹⁸O measurements along dentine, which grows incrementally and archives intra‐ and interannual isotopic composition over the lifetime of the animal, would be useful for characterizing residency within and movements among water bodies with strong δ¹⁸O gradients, particularly between polar and lower latitudes, or between oceans and marginal basins.     

Variation in the carbon and oxygen isotope composition of plant biomass and its relationship to water‐use efficiency at the leaf‐ and ecosystem‐scales in a northern Great Plains grassland

Measurements of the carbon (δ¹³C_m) and oxygen (δ¹⁸O_m) isotope composition of C₃ plant tissue provide important insights into controls on water‐use efficiency. We investigated the causes of seasonal and inter‐annual variability in water‐use efficiency in a grassland near Lethbridge, Canada using stable isotope (leaf‐scale) and eddy covariance measurements (ecosystem‐scale). The positive relationship between δ¹³C_m and δ¹⁸O_m values for samples collected during 1998–2001 indicated that variation in stomatal conductance and water stress‐induced changes in the degree of stomatal limitation of net photosynthesis were the major controls on variation in δ¹³C_m and biomass production during this time. By comparison, the lack of a significant relationship between δ¹³C_m and δ¹⁸O_m values during 2002, 2003 and 2006 demonstrated that water stress was not a significant limitation on photosynthesis and biomass production in these years. Water‐use efficiency was higher in 2000 than 1999, consistent with expectations because of greater stomatal limitation of photosynthesis and lower leaf c_i/ca during the drier conditions of 2000. Calculated values of leaf‐scale water‐use efficiency were 2–3 times higher than ecosystem‐scale water‐use efficiency, a difference that was likely due to carbon lost in root respiration and water lost during soil evaporation that was not accounted for by the stable isotope measurements.     

Transpiration rate relates to within‐ and across‐species variations in effective path length in a leaf water model of oxygen isotope enrichment

Stable oxygen isotope ratio of leaf water (δ¹⁸O_L) yields valuable information on many aspects of plant–environment interactions. However, current understanding of the mechanistic controls on δ¹⁸O_L does not provide complete characterization of effective path length (L) of the Péclet effect, – a key component of the leaf water model. In this study, we collected diurnal and seasonal series of leaf water enrichment and estimated L in six field‐grown angiosperm and gymnosperm tree species. Our results suggest a pivotal role of leaf transpiration rate (E) in driving both within‐ and across‐species variations in L. Our observation of the common presence of an inverse scaling of L with E in the different species therefore cautions against (1) the conventional treatment of L as a species‐specific constant in leaf water or cellulose isotope (δ¹⁸O_p) modelling; and (2) the use of δ¹⁸O_p as a proxy for g_s or E under low E conditions. Further, we show that incorporation of a multi‐species L‐E scaling into the leaf water model has the potential to both improve the prediction accuracy and simplify parameterization of the model when compared with the conventional approach. This has important implications for future modelling of oxygen isotope ratios.     

δ18O‐derived incubation temperatures of oviraptorosaur eggs

In order to determine the incubation temperature of eggs laid by non‐avian dinosaurs, we analysed the oxygen isotope compositions of both eggshell carbonate (δ¹⁸O_c) and embryo bone phosphate (δ¹⁸O_p) from seven oviraptorosaur eggs with preserved in ovo embryo bones. These eggs come from the Upper Cretaceous Nanxiong Formation of Jiangxi Province, China. Oviraptorosaur theropods were selected because of their known brooding behaviour as evidenced by preserved adult specimens fossilized in brooding posture on their clutch. Incubation temperature of these embryos was estimated based on the following considerations: eggshell δ¹⁸O_c value reflects the oxygen isotope composition of egg water fluid; embryo bones precipitate from the same egg fluid; and oxygen isotope fractionation between phosphate and water is controlled by the egg temperature. A time‐dependent model predicting the δ¹⁸O_p evolution of the embryo skeleton during incubation as a function of egg temperature was built, and measured δ¹⁸O_c and δ¹⁸O_p values used as boundary conditions. According to the model outputs, oviraptorosaurs incubated their eggs within a 35–40°C range, similar to extant birds and compatible with the known active brooding behaviour of these theropod dinosaurs. Provided that both eggshell and embryo bones preserved their original oxygen isotope compositions, this method could be extended to investigate some reproductive traits of other extinct groups of oviparous amniotes.     

Contributions of evaporation, isotopic non-steady state transpiration and atmospheric mixing on the delta18O of water vapour in Pacific Northwest coniferous forests

Changes in the ²H and ¹⁸O of atmospheric water vapour provide information for integrating aspects of gas exchange within forest canopies. In this study, we show that diurnal fluctuations in the oxygen isotope ratio (δ¹⁸O) as high as 4‰ were observed for water vapour (δ¹⁸O_vp) above and within an old‐growth coniferous forest in the Pacific Northwest region of the United States. Values of δ¹⁸O_vp decreased in the morning, reached a minimum at midday, and recovered to early‐morning values in the late afternoon, creating a nearly symmetrical diurnal pattern for two consecutive summer days. A mass balance budget was derived and assessed for the ¹⁸O of canopy water vapour over a 2‐d period by considering the ¹⁸O‐isoflux of canopy transpiration, soil evaporation and the air entering the canopy column. The budget was used to address two questions: (1) do δ¹⁸O values of canopy water vapour reflect the biospheric influence, or are such signals swamped by atmospheric mixing? and (2) what mechanisms drive temporal variations of δ¹⁸O_vp? Model calculations show that the entry of air into the canopy column resulted in an isotopically depleted ¹⁸O‐isoflux in the morning of day 1, causing values of δ¹⁸O_vp to decrease. An isotopically enriched ¹⁸O‐isoflux resulting from transpiration then offset this decreased δ¹⁸O_vp later during the day. Contributions of ¹⁸O‐isoflux from soil evaporation were relatively small on day 1 but were more significant on day 2, despite the small H₂¹⁶O fluxes. From measurements of leaf water volume and sapflux, we determined the turnover time of leaf water in the needles of Douglas‐fir trees as ≈ 11 h at midday. Such an extended turnover time suggests that transpiration may not have occurred at the commonly assumed isotopic steady state. We tested a non‐steady state model for predicting δ¹⁸O of leaf water. Our model calculations show that assuming isotopic steady state increased isoflux of transpiration. The impact of this increase on the modelled δ ¹⁸O_vp was clearly detectable, suggesting the importance of considering isotopic non‐steady state of transpiration in studies of forest ¹⁸O water balance.     

Strong seasonal disequilibrium measured between the oxygen isotope signals of leaf and soil CO2 exchange

The oxygen isotope composition (δ¹⁸O) of atmospheric CO₂ is among a very limited number of tools available to constrain estimates of the biospheric gross CO₂ fluxes, photosynthesis and respiration at large scales. However, the accuracy of the partitioning strongly depends on the extent of isotopic disequilibrium between the signals carried by these two gross fluxes. Chamber‐based field measurements of total CO₂ and CO¹⁸O fluxes from foliage and soil can help evaluate and refine our models of isotopic fractionation by plants and soils and validate the extent and pattern of isotopic disequilibrium within terrestrial ecosystems. Owing to sampling limitations in the past, such measurements have been very rare and covered only a few days. In this study, we coupled automated branch and soil chambers with tuneable diode laser absorption spectroscopy techniques to continuously capture the δ¹⁸O signals of foliage and soil CO₂ exchange in a Pinus pinaster Aït forest in France. Over the growing season, we observed a seasonally persistent isotopic disequilibrium between the δ¹⁸O signatures of net CO₂ fluxes from leaves and soils, except during rain events when the isotopic imbalance became temporarily weaker. Variations in the δ¹⁸O of CO₂ exchanged between leaves, soil and the atmosphere were well explained by theory describing changes in the oxygen isotope composition of ecosystem water pools in response to changes in leaf transpiration and soil evaporation.     

Correlating δ13C and δ18O in cellulose of trees

We measured the carbon and oxygen isotopic composition of stem cellulose of Pinus sylvestris, Picea abies, Fagus sylvatica and Fraxinus excelsior. Several sites along a transect of a small valley in Switzerland were selected which differ in soil moisture conditions. At every site, six trees per species were sampled, and a sample representing a mean value for the period from 1940 to 1990 was analysed. For all species, the mean site δ¹³C and δ¹⁸O of stem cellulose are related to the soil moisture availability, whereby higher isotope ratios are found at drier sites. This result is consistent with isotope fractionation models when assuming enhanced stomatal resistance (thus higher δ¹³C of incorporated carbon) and increased oxygen isotope enrichment in the leaf water (thus higher δ¹⁸O) at the dry sites. δ¹⁸ O-δ¹³C plots reveal a linear relationship between the carbon and oxygen isotopes in cellulose. To interpret this relationship we developed an equation which combines the above-mentioned fractionation models. An important new parameter is the degree to which the leaf water enrichment is reflected in the stem cellulose. In the combined model the slope of the δ¹⁸O-δ¹³C plot is related to the sensitivity of the p_i/p_a of a plant to changing relative humidity.     

Interpreting species‐specific variation in tree‐ring oxygen isotope ratios among three temperate forest trees

Although considerable variation has been documented in tree‐ring cellulose oxygen isotope ratios (δ¹⁸O_cell) among co‐occurring species, the underlying causes are unknown. Here, we used a combination of field measurements and modelling to investigate the mechanisms behind variations in late‐wood δ¹⁸O_cell (δ¹⁸O_lc) among three co‐occurring species (chestnut oak, black oak and pitch pine) in a temperate forest. For two growing seasons, we quantified among‐species variation in δ¹⁸O_lc, as well as several variables that could potentially cause the δ¹⁸O_lc variation. Data analysis based on the δ¹⁸O_cell model rules out leaf water enrichment (Δ¹⁸O_lw) and tree‐ring formation period (Δt), but highlights source water δ¹⁸O (δ¹⁸O_sw) as an important driver for the measured difference in δ¹⁸O_lc between black and chestnut oak. However, the enriched δ¹⁸O_lc in pitch pine relative to the oaks could not be sufficiently explained by consideration of the above three variables only, but rather, we show that differences in the proportion of oxygen exchange during cellulose synthesis (p_ex) is most likely a key mechanism. Our demonstration of the relevance of some species‐specific features (or lack thereof) to δ¹⁸O_cell has important implications for isotope based ecophysiological/paleoclimate studies.     

Evaporation and carbonic anhydrase activity recorded in oxygen isotope signatures of net CO2 fluxes from a Mediterranean soil

The oxygen stable isotope composition (δ¹⁸O) of CO₂ is a valuable tool for studying the gas exchange between terrestrial ecosystems and the atmosphere. In the soil, it records the isotopic signal of water pools subjected to precipitation and evaporation events. The δ¹⁸O of the surface soil net CO₂ flux is dominated by the physical processes of diffusion of CO₂ into and out of the soil and the chemical reactions during CO₂–H₂O equilibration. Catalytic reactions by the enzyme carbonic anhydrase, reducing CO₂ hydration times, have been proposed recently to explain field observations of the δ¹⁸O signatures of net soil CO₂ fluxes. How important these catalytic reactions are for accurately predicting large‐scale biosphere fluxes and partitioning net ecosystem fluxes is currently uncertain because of the lack of field data. In this study, we determined the δ¹⁸O signatures of net soil CO₂ fluxes from soil chamber measurements in a Mediterranean forest. Over the 3 days of measurements, the observed δ¹⁸O signatures of net soil CO₂ fluxes became progressively enriched with a well‐characterized diurnal cycle. Model simulations indicated that the δ¹⁸O signatures recorded the interplay of two effects: (1) progressive enrichment of water in the upper soil by evaporation, and (2) catalytic acceleration of the isotopic exchange between CO₂ and soil water, amplifying the contributions of ‘atmospheric invasion’ to net signatures. We conclude that there is a need for better understanding of the role of enzymatic reactions, and hence soil biology, in determining the contributions of soil fluxes to oxygen isotope signals in atmospheric CO₂.     

Biogeochemical cycling of sulphur in karst and transfer into speleothem archives at Grotta di Ernesto, Italy

Trace amounts of sulphur in speleothems suggest that stalagmites may act as archives of sulphur deposition, thereby recording aspects of atmospheric variability in sulphur content. Accurate interpretation of this novel sulphur archive depends upon understanding how biogeochemical cycling in the soil and epikarst above the cave may modify the precursor atmospheric values of sulphur concentration and isotopic composition prior to incorporation into the speleothem record. Dual isotope analysis of δ³⁴S-SO₄ and δ¹⁸O-SO₄ is used to trace biogeochemical transformations of atmospheric sulphur through the cave system at Grotta di Ernesto in the Italian Alps and builds towards a framework for interpretation of speleothem sulphur archives which depends on overlying ecosystem dynamics and karst hydrological properties. A three component model of atmospheric sulphate signal modification is proposed to be driven by (1). vegetation and soil cycling, (2). the degree of groundwater mixing in the karst aquifer; and (3). redox status. The relative influence of each process is specific to individual drip flow sites and associated stalagmites, rendering each sulphur archive a unique signal of environmental conditions. Under conditions found in the soil and epikarst above Grotta di Ernesto, the dual isotope signatures of sulphate sulphur and oxygen incorporated into speleothem carbonate, closely reflect past conditions of industrial sulphur loading to the atmosphere and the extent of signal modification through biogeochemical cycling and aquifer mixing.     

Responses of the carbon and oxygen isotope compositions of desert plants to spatial variation in soil salinity in Central Asia

We examined the isotopic parameters in two C₃ species (Artemisia diffusa H. Krasch and Tamarix hispida Willd.) and a C₄ species [Haloxylon aphyllum (Minkw.) Iljin.] growing or planted in soils with different levels of salinity in a Central Asian desert. The oxygen isotope ratios of stem water (δ¹⁸O_stem) in T. hispida and H. aphyllum distributed in high-salinity zones were similar to the δ¹⁸O of artesian water (δ¹⁸O_artesian) and different from that in A. diffusa distributed in lower-salinity zones. This indicates that T. hispida and H. aphyllum depend on water with low salinity in the deeper soil layer, whereas A. diffusa depends on water in the shallower soil layer that would be affected by salt accumulation. The carbon isotope composition of leaf organic matter (δ¹³C_om) and oxygen isotope enrichment in leaf organic matter above stem water (Δ¹⁸O_om) were lower in A. diffusa than in the other species. The responses of δ¹³C_om and Δ¹⁸O_om to soil salinity observed for T. hispida suggest that the species decreased its transpiration rate and increased its intrinsic water-use efficiency in response to increasing soil salinity. The δ¹³C_om and Δ¹⁸O_om of H. aphyllum were higher than those of the C₃ species, and were not correlated with soil salinity, suggesting that H. aphyllum reduced its salt uptake by decreasing transpiration—even though it was able to access less saline water in the deeper soil layer. These results indicate that the water-use strategy of desert plants in high-salinity environments can be assessed based on their carbon and oxygen isotope ratios.     

Increased snowfall weakens complementarity of summer water use by different plant functional groups

Winter snowfall is an important water source for plants during summer in semiarid regions. Snow, rain, soil water, and plant water were sampled for hydrogen and oxygen stable isotopes analyses under control and increased snowfall conditions in the temperate steppe of Inner Mongolia, China. Our study showed that the snowfall contribution to plant water uptake continued throughout the growing season and was detectable even in the late growing season. Snowfall versus rainfall accounted for 30% and 70%, respectively, of the water source for plants, on the basis of hydrogen stable isotope signature (δD) analysis, and accounted for 12% and 88%, respectively, on the basis of oxygen stable isotope signature (δ¹⁸O) analysis. Water use partitioning between topsoil and subsoil was found among species with different rooting depths. Increased snowfall weakened complementarity of plant water use during summer. Our study provides insights into the relationships between precipitation regimes and species interactions in semiarid regions.     

High‐resolution isotope analysis of young alewife Alosa pseudoharengus otoliths: assessment of temporal resolution and reconstruction of habitat occupancy and thermal history

Otoliths of age 0 year alewife Alosa pseudoharengus collected in different Lake Michigan habitats were microsampled, and carbon and oxygen isotope ratios (δ¹⁸O_otolith and δ¹³C_otolith) of resulting microsamples were quantified. To assess the temporal resolution of the method, age and otolith growth rates were also estimated by counting otolith daily growth increments. Core and outer intra‐otolith samples averaged 36 and 23 days, respectively. Because of the accretionary nature of otolith growth, a habitat switch by a larva occurring between 0 and 18 days post‐hatch may not be recognized by this approach. Taking this temporal resolution into account, A. pseudoharengus habitat occupancy and thermal history in nearshore Lake Michigan, and a connecting drowned river‐mouth lake were documented. Comparisons between δ¹⁸O_otolith and δ¹³C_otolith profiles, and isotope values of Lake Michigan habitats suggested that movements by individual fish between a nearshore area of Lake Michigan proper and drowned river‐mouth lake habitats were rare. Some individuals evidently moved between habitats, and such movements occurred during different periods of ontogeny. Thermal reconstructions, based on δ¹⁸O_otolith values suggested that during early life (e.g. first month of life) young A.pseudoharengus appeared to inhabit microhabitats with temperatures greater than mean epilimnetic temperatures. This study demonstrates not only the utility of intra‐otolith geochemical analysis to describe the complexity of fish behaviour in fresh water but also identifies limitations of the present approach.     

Oxygen isotope fractionations across individual leaf carbohydrates in grass and tree species

Almost no δ¹⁸O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ¹⁸O relationship between leaf water and cellulose. We measured δ¹⁸O values of bulk leaf water (δ¹⁸O_LW) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ¹⁸O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally ¹⁸O‐enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (ε_bio) of more than 27‰ relative to δ¹⁸O_LW, which might be explained by isotopic leaf water and sucrose synthesis gradients. δ¹⁸O_LW and δ¹⁸O values of carbohydrates and cellulose in grasses were strongly related, indicating that the leaf water signal in carbohydrates was transferred to cellulose (ε_bio = 25.1‰). Interestingly, damping factor p_exp_x, which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ¹⁸O_LW but not if modelled directly from δ¹⁸O of individual carbohydrates. We conclude that δ¹⁸O_LW is not always a good substitute for δ¹⁸O of synthesis water due to isotopic leaf water gradients. Thus, compound‐specific δ¹⁸O analyses of individual carbohydrates are helpful to better constrain (post‐)photosynthetic isotope fractionation processes in plants.     

The role of effective leaf mixing length in the relationship between the δ18O of stem cellulose and source water across a salinity gradient

Previous mangrove tree ring studies attempted, unsuccessfully, to relate the δ¹⁸O of trunk cellulose (δ¹⁸O_CELL) to the δ¹⁸O of source water (δ¹⁸O_SW). Here, we tested whether biochemical fractionation associated with one of the oxygen in the cellulose glucose moiety or variation in leaf water oxygen isotope fractionation (Δ_LW) can interfere with the δ¹⁸O_SW signal as it is recorded in the δ¹⁸O_CELL of mangrove (saltwater) and hammock (freshwater) plants. We selected two transects experiencing a salinity gradient, located in the Florida Keys, USA. The δ¹⁸O_CELL throughout both transects did not show the pattern expected based on that of the δ¹⁸O_SW. We found that in one of the transects, biochemical fractionation interfered with the δ¹⁸O_SW signal, while in the other transect Δ_LW differed between mangrove and hammock plants. Observed differences in Δ_LW between mangroves and hammocks were caused by a longer effective leaf mixing length (L) of the water pathway in mangrove leaves compared to those of hammock leaves. Changes in L could have caused the δ¹⁸O_CELL to record not only variations in the δ¹⁸O_SW but also in Δ_LW making it impossible to isolate the δ¹⁸O_SW signal.     

Oxygen isotope ratios between soil water and stem water of trees in pot experiments

Since the oxygen isotopic ratio of water extracted from stems reflects that of water taken up by roots, the stem water isotope ratio can be used to analyze the source of water for plant growth. However, it is known that the fractionation of isotopes during evaporation from the surface soil increases the isotope ratio in soil water drastically. In this study, it was experimentally confirmed that the stem water of Elaeocarpus sylvestris vs. ellipticus Hara seedlings is not isotopically similar to the water source in the case where evaporation from the soil occurs actively. However, since water in these plant bodies was replaced in about 2 days in the pot experiments, the 2-day-averaged values of the soil water isotope ratio approached the stem water isotope ratio. Thus, time-course samplings of the soil and stems, and measurements of the replacement time of water in the plant body (water volume in plant/transpiration rate) are recommended for correct interpretation of the isotopic signature of soil water and stem water.     

Contrasting water sources and water‐use efficiency in coexisting desert plants in two saline‐sodic soils in northwest China

• Soil degradation resulting from various types of salinity is a major environmental problem, especially in arid and semiarid regions. Exploring the water‐related physiological traits of halophytes is useful for understanding the mechanisms of salt tolerance. This knowledge could be used to rehabilitate degraded arid lands.
• To investigate whether different types of salinity influence the water sources and water‐use efficiency of desert plants (Karelinia caspia, Tamarix hohenackeri, Nitraria sibirica, Phragmites australis, Alhagi sparsifolia, Suaeda microphylla, Kalidium foliatum) in natural environments, we measured leaf gas exchange, leaf carbon and xylem oxygen isotope composition and soil oxygen isotope composition at neutral saline‐sodic site (NSS) and alkaline saline‐sodic site (ASS) in northwest China.
• The studied plants had different xylem water oxygen isotope compositions (δ¹⁸O) and foliar carbon isotope compositions (δ¹³C), indicating that desert plants coexist through differentiation in water use patterns. Compared to that at the NSS site, the stem water in K. caspia, A. sparsifolia and S. microphylla was depleted in ¹⁸O at the ASS site, which indicates that plants can switch to obtain water from deeper soil layers when suffering environmental stress from both salinity and alkalinisation. Alhagi sparsifolia had higher δ¹³C at the ASS site than at the NSS site, while K. caspia and S. microphylla had lower δ¹³C, which may have resulted from interspecific differences in plant alkali and salt tolerance ability.
• Our results suggest that under severe salinity and alkalinity, plants may exploit deeper soil water to avoid ion toxicity resulting from high concentrations of soluble salts in the superficial soil layer. In managed lands, it is vital to select and cultivate different salt‐tolerant or alkali‐tolerant plant species in light of local conditions.

     

18O composition of CO2 and H2O ecosystem pools and fluxes in a tallgrass prairie: Simulations and comparisons to measurements

In this paper we describe measurements and modeling of ¹⁸O in CO₂ and H₂O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the δ¹⁸O value of leaf water, depth‐resolved soil water, atmospheric water vapor, and Keeling plot δ¹⁸O intercepts for net soil‐surface CO₂ and ecosystem CO₂ and H₂O fluxes during three periods of the 2000 growing season. Daytime discrimination against C¹⁸OO, as calculated from measured above‐canopy CO₂ and δ¹⁸O gradients, is also presented. To interpret the isotope measurements, we applied an integrated land‐surface and isotope model (ISOLSM) that simulates ecosystem H₂¹⁸O and C¹⁸OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the δ¹⁸O value of net ecosystem CO₂ and H₂O fluxes. Simulations indicate that incomplete equilibration between CO₂ and H₂O within C₄ plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the δ¹⁸O value of above‐canopy vapor had a small impact on the predicted δ¹⁸O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the δ¹⁸O value of above‐canopy CO₂ substantially affected the predicted ecosystem discrimination. Leaves dominate the ecosystem ¹⁸O‐isoflux in CO₂ during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C¹⁸OO fluxes requires accurate predictions of both soil and leaf ¹⁸O‐isofluxes.     

Reconstruction of source water using the δ18O of tree ring phenylglucosazone: A potential tool in paleoclimate studies

The oxygen isotope ratios of tree ring cellulose have a great potential as proxy for the oxygen isotope ratios of source water, which is related to climate. However, source water isotopic signatures can be masked by plant physiological and biochemical processes during cellulose synthesis. To minimize biochemical effects in the recording of source water, we modified the cellulose molecule to phenylglucosazone, which only has oxygen attached to carbon 3–6 (O_C3–6) of the cellulose glucose moieties, thus eliminating the oxygen attached to carbon 2 of the cellulose glucose moieties (O_C-2). Here we developed a method to use small amounts of inter and intra-annual tree ring cellulose for phenylglucosazone synthesis. Using this new method we tested if the oxygen isotope ratios of source water reconstructed from tree ring phenylglucosazone (δ¹⁸O_sw^PG) and the observed source water (δ¹⁸O_sw^obs) would have a better agreement than those reconstructed from the tree ring cellulose molecule. Annual tree ring samples were obtained from Pinus sylvestris (1997–2003) (Finland) and Picea abies (1971–1992) (Switzerland) and intra-annual tree ring samples were obtained from Pinus radiata (October 2004–March 2006) (New Zealand), each near a meteorological station where precipitation and relative humidity (RH) were measured periodically. The δ¹⁸O of tree ring cellulose and tree ring phenylglucosazone for each of the three species were then used to back calculate the δ¹⁸O of source water according to a previous published empirical equation. As expected, the δ¹⁸O of tree ring phenylglucosazone was superior than cellulose in the reconstruction of source water available to the plant. Deviation between δ¹⁸O_sw^PG and δ¹⁸O_sw^obs was in part correlated with variation in atmospheric relative humidity (RH) which was not observed for the cellulose molecule. We conclude that this new method can be applicable to inter and intra-annual tree ring studies and that the use of the tree ring phenylglucosazone will significantly improve the quality of paleoclimate studies.     

Records of palaeo‐seawater condition from oxygen‐isotope profiles of early Pleistocene fossil molluscs from the Seoguipo Formation (Korea)

Kim, J.K., Khim, B.‐K., Woo, K.S., & Yoon, S.H. 2009: Records of palaeo‐seawater condition from oxygen‐isotope profiles of early Pleistocene fossil molluscs from the Seoguipo Formation (Korea). Lethaia, Vol. 43, pp. 170–181. High‐resolution δ¹⁸O profiles of early Pleistocene fossil molluscs (Mizuhopecten tokyoensis hokurikuensis) from the shallow‐marine sedimentary Seoguipo Formation (Korea) were found to exhibit distinct annual cycles identified by their unique seasonality (δ¹⁸O amplitude). A direct comparison of fossil δ¹⁸O profiles with that of living shells (Amusium japonicum japonicumi) indicated that the palaeoseawater conditions differed from present‐day seawater. Specifically, the positive δ¹⁸O shift in the isotope profile of the fossil specimens relative to that of the living mollusc shell reflected that palaeotemperature was lower than that today. However, a comparison of the coldest palaeotemperatures (determined from the heaviest δ¹⁸O values of fossil shells), with the present‐day winter temperatures indicates that temperature variation alone cannot account for the entire positive δ¹⁸O offset. These findings indicate that variation in the seawater δ¹⁸O_w values plays a dominant role in the biogenic carbonate precipitation of fossils. Thus, the fossil shells obtained from stratigraphic units suggest different palaeoenvironmental conditions, including lower temperatures and ¹⁸O‐enriched glacial seawater, when compared with the present‐day conditions. The Seoguipo Formation records at least five cycles of relative sea‐level fluctuations, with changes in fossil δ¹⁸O amplitudes separated by sequence boundaries likely to reflect variations of unique palaeoseawater condition, although the oxygen‐isotope profile of fossil molluscs appears to provide a snap‐shot of the palaeoclimatic signature. □Early Pleistocene, mollusc fossils, oxygen isotope, palaeoenvironment, seawater temperature.