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Clay aquitards as isotopic archives of Holocene palaeoclimate in the Northern Great Plains: sensitivity analysis
Author(s) -
Birks S. J,
Remenda V. H,
Edwards T. W. D
Publication year - 2000
Publication title -
hydrological processes
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.222
H-Index - 161
eISSN - 1099-1085
pISSN - 0885-6087
DOI - 10.1002/1099-1085(20000615)14:8<1523::aid-hyp984>3.0.co;2-m
Subject(s) - holocene , geology , δ18o , groundwater , water table , precipitation , aquifer , stable isotope ratio , paleontology , physics , geotechnical engineering , meteorology , quantum mechanics
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‰ 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. Copyright © 2000 John Wiley & Sons, Ltd.