S.J. Birks

Atmospheric circulation controls on precipitation isotope–climate relations in western Canada

δ18O and δ2H time-series of monthly composite precipitation (July 1975–June 1982) for three stations located in western Canada were examined to characterize the influence of atmospheric circulation on modern isotope–climate relations in the region. Spatially coherent trends in long-term isotope and temperature anomalies were evident among the three stations, with isotope and temperature anomalies showing the strongest correlations when weighted to reflect precipitation amount. Strong correlations were also found between unweighted isotope anomalies and the Pacific–North American (PNA) index, which is a key descriptor of air-mass circulation patterns across North America. Positive δ anomalies and variable temporal δ–temperature relations having relatively shallow slopes occur during periods of intensified meridional circulation (PNA+), especially during winter. Periods of stronger zonal circulation (PNA−), in contrast, are marked by negative δ anomalies and steeper δ–temperature relations, consistent with increased distillation and deepening of the isotope shadowin the lee of the Canadian Cordillera. Significant positive correlations with pressureheight and precipitable-water anomalies located over western Canada provide additional confirmation that seasonal and interannual variability in the strength of the PNA ridge-and-trough pattern profoundly influences the isotopic evolution of moisture reaching the interior. Analogous circulation-dependent shifts in the precipitation isotope–temperature relation in western Canada also occur over much longer timescales, as inferred from isotope palaeorecords in various natural archives.

Maps and animations offer new opportunities for studying the global water cycle

The International Atomic Energy Agency/World Meteorological Organization Global Network for Isotopes in Precipitation (IAEA/WMO GNIP) data base includes more than 100,000 δ18O,δ2H, and 3H measurements performed on monthly precipitation samples collected at 550 stations worldwide. Since 1961, the data base has served as a baseline reference for the distribution of water isotopes in modern precipitation. It is widely used in the fields of isotope hydrology, climatology oceanography and paleoclimatology.

Isotope Hydrology Research in Canada, 2003-2007

This article provides an overview of recent progress in isotope tracer hydrology in Canada during 2003-2008, identifying over 85 published scientific articles. The cornerstone of Canada’s contribution to isotope hydrology has been and continues to be via contributions from independent university-based researchers and students to the peer reviewed literature. Long-standing networks, such as the Canadian Network for Isotopes in Precipitation, and scientific steering groups, such as the Canadian Geophysical Union Committee on Isotopic Tracers, have also been important coordinating bodies for data collection, analysis and dissemination, and have sought to improve awareness of current interests, as well as to promote meetings and community activities. Research linkages to international programs such as the International Atomic Energy Agency (IAEA)/World Meteorological Organization’s Global Network for Isotopes in Precipitation, IAEA Coordinated Research Programs such as Large River Basins and Geostatistical Spatial Analysis, and recent involvement with the International Association of Hydrological Sciences International Commission on Tracers have been some of the more visible contributions to Canada’s international efforts.

Clay aquitards as isotopic archives of Holocene palaeoclimate in the Northern Great Plains: sensitivity analysis

The vertical distributions of 18 O and 2 H in porewater of a glaciolacustrine clay were simulated to explore the potential conservation of signals deriving from changes in the isotopic composition of Holocene precipitation in low-permeability deposits. The simulations used a groundwater velocity of zero, assuming that transport was solely by diffusion. As expected, distributions were highly sensitive to the timing of Holocene events, with recent events having the greatest preservation potential. Simulations show that these signals are confined to the area immediately below the weathered zone, where infiltration of modern water via fractures or fluctuations in the water table could threaten signal preservation in the field. Century-scale events with magnitudes of 6%o or less in δ 18 O have little effect on the isotopic profiles if they occur more than 1000 years ago. Longer duration events occurring more than 1000 years ago are preserved as slight alterations in the shape of the isotopic profiles. Differences in the effective diffusion coefficients for 1 H 2 H 16 O and 1 H 2 18 O result in alteration of the original d-excess (d = δ 2 H-8δ 18 O). Hence this parameter is not conservative in groundwater environments where transport is dominantly by diffusion. On the other hand, simulated vertical profiles of d-excess amplify fluctuations in the isotopic composition of precipitation that are subdued in the individual profiles of δ 18 O and δ 2 H Simulations that include Holocene events with timings, durations and magnitudes estimated for the Hypsithermal, Medieval Warm Period and Little Ice Age result in distributions that differ from the baseline by 0.6‰ in δ 18 O (5‰ in δ 2 H). That these events are preserved as a measurable offset in the isotopic profile suggests that some aquitards can be used to reconstruct or constrain mid to late Holocene palaeoprecipitation. The small magnitude of these positive offsets suggest that exclusion of Holocene events in the input functions used to constrain parameters by fitting measured isotopic profiles with simulated diffusion profiles, probably has a minimal effect on the parameters they are used to estimate, but could result in a slight overestimation of parameters such as velocity or time since deglaciation.