Preservation of Paleoisotopic Record of Hydrologic History In Aquifer Systems

Climatic change during the Quaternary has been inferred from paleoisotopic compositions of groundwaters in aquifers on various Earth continents. Such isotopic evidence is more likely to be found in confined than in unconfined aquifers, partly because confined aquifers contain older groundwater, but not all confined aquifers seem to preserve a paleoisotopic record.

Paleoisotopic records have been preserved in the transition from updip, generally unconfined parts of aquifers to downdip, more-confined parts; between surficial unconfined aquifers and deeper confined aquifers; or in some combination of the two. At least four different logical approaches have been used to interpret paleoisotopic records: 1) graphs of isotopic composition versus age, 2) graphs of isotopic composition versus down-gradient distance, 3) comparison of dD and d¹⁸O data to the global meteoric water line, and 4) comparison of dD and d¹⁸O between stratified unconfined and confined aquifers. The first two 1D analyses tend to assume no cross-formational flow. The third approach includes comparisons (“deuterium excess”) to the global meteoric water line but not all applications are statistically rigorous. The last approach is appropriate where downward leakage accounts for groundwater in the confined aquifer.

There are five conditions needed to preserve a paleoisotopic record in an aquifer system. 1) Isotopic composition of paleorecharge was distinct from Holocene isotopic composition. 2) Paleoclimatic conditions persisted long enough to emplace groundwater with a unique isotopic composition in both unconfined and deeper confined aquifers. 3) Recharge with a Holocene isotopic composition displaced older groundwater from the more surficial aquifer. 4) Holocene recharge or cross-formational leakage rates have been slow enough that older water at greater depth has not been replaced. 5) Effect of hydrodynamic dispersion is small relative to the change in isotopic composition. How the paleoisotopic record relates to specific climatic variables can be ambiguous for many reasons.