Changes in Precipitation Sources over Glacial/Interglacial MIS 11 and 12 Examined by Δ17O of SiO2 Obtained from Diatoms along the Valles Caldera Lake Core, NM

Abstract:

Quantitative estimates of the isotopic composition of paleo-lake water have been made using ¹⁸O/¹⁶O (ẟ¹⁸O) in diatom silica (Dodd and Sharp, GCA, 2010). Post-mortem diatom silica equilibrates with ambient lake water within six months, chronicling the bulk oxygen isotope composition of the lake and resulting in silica that is near the quartz-water fractionation line (Dodd et al, GGG, 2012). The δ¹⁸O values of lacustrine diatoms from the Valles Caldera, NM, vary by ~25‰ between glacial and interglacial periods and suggest a collapse of the summer monsoon that currently provides 50% of the modern precipitation in NM. Triple oxygen isotope measurements of diatom silica may serve as a proxy for the isotopic composition of the lake water and as an estimate of paleo-humidity over the precipitation source. The deuterium excess parameter (d= ẟD - 8 ẟ¹⁸O) has been used along ice cores as a source relative humidity index, but is difficult to make in lake sediments. Instead, high precision ¹⁷O-excess (∆¹⁷O) measurements (=ẟ¹⁷O ‑ 0.528 ẟ¹⁸O) may provide paleo-humidity information. Landais et al. (GRL, 2008) found a ∆¹⁷O difference of 0.02‰ in the Vostok ice core between glacial and interglacial times, interpreted as a function of changing relative humidity of the precipitation source. A 0.03‰ change was observed in glacial (∆¹⁷O=-0.22‰) and interglacial (∆¹⁷O=-0.19‰) diatom silica along the Valles Caldera lake core. Further information regarding the δ¹⁸O value of meteoric water can be calculated from paired δ¹⁸O-δ¹⁷O measurements. The combined δ¹⁸O-δ¹⁷O values of interglacial diatoms suggest a δ¹⁸O_{meteoric water} value of -9‰. Modern δ¹⁸O value of monsoonal precipitation in NM is ~-10‰. The δ¹⁸O of glacial diatoms suggest a δ¹⁸O_{meteoric water} = -20‰. Modern δ¹⁸O value of winter precipitation in NM is ~-20‰. These results suggest that the seasonality of precipitation in New Mexico can be inferred based on changes in the relative humidity of the precipitation’s source.