A HYDROGEN ISOTOPE PALEOCLIMATE RECONSTRUCTION OF THE NORTHERN ROCKY MOUNTAINS FROM IRON OXY-HYDROXIDE CHRONOSEQUENCES

Stable isotopes can provide a valuable tool in the development of both regional and small-scale climatic records. Here we assess the use of hydrogen isotopes preserved in iron oxy-hydroxides formed in a naturally acidic iron-rich stream as a means of determining paleoclimate.

Recent advances in analytical techniques for hydrogen isotope analysis using a high-temperature, thermal conversion, elemental analyzer, allows for the rapid and precise measurement of hydrogen isotopes of hydrous minerals. We analyzed the hydrogen isotopes of goethite from two separate 9,000 year old ferricrete chronosequences in southern and central Montana to assess Holocene climate change in the northern Rocky Mountain region. The results of this work show an ~30‰ increase in the dD of goethite cement over the past 9,000 years. This dramatic increase in the dD value suggests an increase in isotopically heavy summer precipitation since the early Holocene, which agrees with palynological studies of the region. This isotopic shift is similar in degree to the ~3‰ increase in the oxygen isotopes of these samples. However, the hydrogen isotope record presents a clearer picture of streamwater isotope changes during this time period than previous oxygen isotope analyses of the same materials, because the hydrogen isotope values are not influenced by the trace quantities of silicate contaminates within the goethite framework, and hydrogen isotope fractionation within these minerals has been shown to be independent of stream temperature. The results of this study demonstrates the utility of hydrogen isotopes in paleoclimate reconstruction and suggests that the combination of oxygen and hydrogen isotope records from iron oxy-hydroxide chronosequences may provide a more robust record of paleoclimatic changes than oxygen isotope analyses alone.