Oxygen isotope ratios in Holocene carbonates across a climatic gradient,eastern Washington State, USA: Evidence for seasonal effects on pedogenic mineral isotopic composition

摘要

Oxygen isotope ratios of pedogenic carbonates were analyzed along a climatic gradient in the Palouse region of eastern Washington State to evaluate changes in their isotopic composition with changing climate. A modern analog approach was used to assess the relationship between the istotope ratios in the pedogenic carbonates with those in soil water (measured in June when we anticipated carbonate formation might occur because of low soil water content), and meteoric water from two National Atmospheric Deposition Program (NADP) sites adjacent to our sequence. Mean annual meteoric water δ~(18)O values (wrt VSMOW) from the two NADP sites varied slightly with elevation, -13.9‰ at the Starkey site (elevation 1253 m) and -13.0‰ at the Palouse site, (elevation 766 m) but showed distinct seasonal trends with weighted means ranging from -11.9‰ during autumn to -15.0‰ during the winter. Carbonate δ~(18)O (wrtVPDB) varied significantly with MAP, ranging from -14.1‰ at the most arid site to -11.4‰ at the wettest site but displayed a trend opposite to that of soil water δ~(18)O (-4 to -12‰ VSMOW for the 70 cm depth). Neither measured soil water δ~(18)O values nor mean meteoric water δ~(18)O values generated realistic temperatures of formation for the observed trend in carbonate δ~(18)O values.When mean annual temperatures across the gradient were used for temperature of formation, calculated soil water δ~(18)O in equilibrium with carbonate δ~(18)O was similar to winter precipitation δ~(18)O at the most arid sites but became increasingly similar to mean annual meteoric δ~(18)O as MAP increased.We suggest that the unexpected trend in carbonate δ~(18)O is largely due to the diminishing influence of winter precipitation/snowmelt on soil moisture as MAP increases across the climatic gradient and that seasonal effects of meteoric water may need to be considered in pedogenic mineral isotopic equilibrium in temperate regions.