Oxygen Isotope Ratios In Holocene Pedogenic Carbonate along a Climatic Sequence, Eastern Washington State, USA: Evidence for Seasonal Variation In Carbonate Precipitation?

A modern analog approach was used to assess the relationships between soil water(sampled at 2 dates in early and late summer), and Holocene pedogenic carbonate δ¹⁸O values across a bioclimatic sequence (200-1000 mm mean annual precipitation) in the Palouse loess region of eastern Washington state USA. Meteoric water samples were also analyzed from two National Atmospheric Deposition Program (NADP) monitoring sites adjacent to our sequence.

Mean annual meteoric precipitation δ¹⁸O values varied slightly with elevation, -13.89 ‰ VSMOW at 1253 m and -12.89 ‰ VSMOW at 766 m, and showed distinct seasonal trends (mean weighted seasonal values ranging from -11.89 ‰ VSMOW during autumn to -15.00 ‰ VSMOW during the winter). Carbonate δ¹⁸O varied significantly with MAP, ranging from -14.13 (‰ VPDB) at the most arid site to -11.38 (‰ VPDB) at the wettest site, but showed an opposite trend to soil water δ¹⁸O values (-4 to -12 ‰ VSMOW for the 70 cm depth). Neither measured (summer) soil water values nor mean meteoric precipitation values generated realistic temperatures of formation for the observed trend in carbonate ¹⁸O values. When mean annual temperatures across the gradient were used for temperature of formation, calculated soil water δ¹⁸O in equilibrium with carbonates was similar to winter precipitation δ¹⁸O at the most arid sites but became increasingly similar to mean annual meteoric precipitation δ¹⁸O as MAP increased. We suggest that our unexpected trend in carbonate δ¹⁸O values is largely due to the diminishing influence of winter precipitation and/or snowmelt on soil moisture as MAP increases across the climatic gradient, and indicates seasonal changes in isotopic composition of meteoric precipitation and soil water may need to be considered in interpretation of oxygen isotope values for paleoclimatic reconstruction of pedogenic minerals in temperate climates.