BEHAVIOR OF OSMIUM IN RECENTLY GLACIATED TERRAINS: OS ISOTOPES OF THE CONNECTICUT RIVER WATERSHED

A key component to understanding climate change is to determine past CO₂ levels. Continental weathering consumes CO₂ thus changes in continental weathering rates have important ramifications for atmospheric CO₂ levels and climate change. Osmium (Os) isotopes are potentially useful as a proxy for continental (silicate) weathering. Recently glaciated terrains are regions where rapid weathering is occurring thus play an important role in the consumption of atmospheric CO₂. Rivers draining this actively weathering terrain should give insight into the processes controlling the Os concentration and isotopic composition in river waters. We have begun a sampling campaign along the Connecticut River to investigate the behavior of Os. Both bank sediment and water samples were collected along the Connecticut River from Wells River, VT to Lebanon, NH. The sample at Wells River is well upstream of anthropogenic influences while the sample at Lebanon, NH is downstream of Wilder dam. These samples span across several lithologies of diverse ages and varying degrees of anthropogenic impacts.

Initial results indicate that the isotopic composition and concentration of Os in the Connecticut River water is non-radiogenic and homogenous (¹⁸⁷Os/¹⁸⁸Os = 0.71-0.90, [Os] = 15-27 fg/g). These measured values are surprisingly close to a sample from Wells River measured nearly 10 years ago using a different method (¹⁸⁷Os/¹⁸⁸Os = 0.85), suggesting that the river is homogenous spatially and temporally. In contrast, the sediments show a large variability in both Os isotopic composition (¹⁸⁷Os/¹⁸⁸Os = 0.9-2.2) and concentration (6-29 pg/g). The water and sediments are clearly not in isotopic equilibrium since no measured sediment sample had ¹⁸⁷Os/¹⁸⁸Os values as low as those found in the water samples. Further study will attempt to determine the source(s) of non-radiogenic Os to the river water.