Identifying drivers of leaf water and cellulose stable isotope enrichment in Eucalyptus in northern Australia

Several previous studies have investigated the use of the stable hydrogen and oxygen isotope composi- tions in plant materials as indicators of palaeoclimate. However, accurate interpretation relies on a detailed understanding of both physiological and environmental drivers of the variations in isotopic enrichments that occur in leaf water and associated organic compounds. To pro- gress this aim we measured δ18O and δ2H values in euca- lypt leaf and stem water and δ18O values in leaf cellulose, along with the isotopic compositions of water vapour, across a north-eastern Australian aridity gradient. Here we compare observed leaf water enrichment, along with pre- viously published enrichment data from a similar north Australian transect, to Craig–Gordon-modelled predic- tions of leaf water isotopic enrichment. Our investigation of model parameters shows that observed 18O enrichment across the aridity gradients is dominated by the relation- ship between atmospheric and internal leaf water vapour pressure while 2H enrichment is driven mainly by variation in the water vapour—source water isotopic disequilibrium. During exceptionally dry and hot conditions (RH 37 °C) we observed strong deviations from Craig– Gordon predicted isotope enrichments caused by partial stomatal closure. The atmospheric–leaf vapour pressure relationship is also a strong predictor of the observed leaf cellulose δ18O values across one aridity gradient. Our find- ing supports a wider applicability of leaf cellulose δ18O composition as a climate proxy for atmospheric humidity conditions during the leaf growing season than previously documented

First continuous shipboard δ18O and δD measurements in sea water by diffusion sampling: cavity ring-down spectrometry

Combined measurements of salinity and the oxygen/hydrogen stable isotope composition of marine waters can characterise processes such as freshwater mixing, evaporation, precipitation and sea-ice formation. However, stable isotope data with high spatial and temporal resolution are necessary for a detailed understanding of mixed water bodies with multiple inputs. So far analysis of δ18O and δD values in water has been a relatively expensive, laboratory-based technique requiring collection of discrete samples. This has greatly limited the scope and scale of field research that can be undertaken using stable isotope analysis. Here, we report the first continuous shipboard measurements of δ18O and δD values in water by diffusion sampling-cavity ring-down spectrometry. Combined with continuous salinity recordings, a data set of nearly 6,000 measurements was made at 30-s intervals during a 3-day voyage through the Great Barrier Reef Lagoon. Our results show that continuous shipboard measurement of δ18O/δD values provides additional discriminatory power for assessing water mass formation processes and histories. Precise identification of river plumes within the Great Barrier Reef Lagoon was only possible because unique δ18O/δD–salinity relationships of individual plumes were measured at high spatial and temporal resolution. The main advantage of this new technique is the ability to collect continuous, real-time isotope data at a small fraction of the cost of traditional isotope analysis of discrete samples. Water δ18O and δD values measured by diffusion sampling-cavity ring-down spectrometry and laboratory-based isotope ratio mass spectrometry have similar accuracy and precision

Mobility and potential bioavailability of traffic-derived trace metals in a ‘wet–dry’ tropical region, Northern Australia

The aqueous mobility and potential bioavailability of metals and metalloids in road runoff in a ‘wet–dry’ tropical location were assessed by analysing metal and\ud metalloid concentrations in particulate, total dissolved and\ud labile dissolved phases in runoff waters. Road-derived Al,\ud Cu, Pb, Sb and Zn concentrations were substantially elevated\ud in runoff when compared to receiving creek waters.\ud Median dissolved concentrations in road runoff exceeded\ud those in creek waters by up to an order of magnitude.\ud Leaching experiments of road sediments confirmed that\ud several metals and metalloids were released in high concentrations from road sediments. Labile Zn and Cu concentrations measured by diffusion gradients in thin films\ud (DGT) showed that almost all dissolved Zn and up to half\ud of dissolved Cu in runoff waters and in road sediment\ud leachate were potentially bioavailable. Comparisons of\ud dissolved metal concentrations in receiving waters affected\ud by road runoff with ecosystem guideline levels, indicated a\ud risk of reaching toxic levels of Cu and Zn in the receiving\ud waters in the absence of adequate treatment or dilution.\ud Low dilution rates of road runoff are likely to occur during\ud late ‘dry’ season/early ‘wet’ season storms which have the\ud potential to produce high metal concentrations derived\ud from long periods of accumulation of road sediment at a\ud time when creek flow rates are at their annual minimum