1. Chemical Weathering of Different Watersheds in Western Greenland
Adam Marshall1, Kelly Deuerling2, Cecilia Scribner2, Jonathan Martin2, Mike Davlantes2, Ellen Martin2
Department of Chemistry1, Department of Geological Sciences2, University of Florida, Gainesville, FL
Introduction
Results and Discussion
Conclusions
Geochemical studies of streams can be used to evaluate the intensity of chemical weathering, which has implication for the long term carbon cycle as well as the transfer of dissolved solids and nutrients to the ocean. This study evaluated the geochemistry of a stream system in the Qorlortoq Valley, a deglaciated region of Western Greenland with a negative water balance, and compared the results to data from streams near Sisimuit, Greenland, which is closer to the coast and has a positive water balance (Nielsen, 2010). The hypothesis for the study was that the Qorlortoq streams would record less intense weathering in response to the drier climate. Ultimately, these data will be compared to data from a proglacial river sourced directly from the Greenland Ice Sheet in order to determine whether variations in the geochemistry of material delivered to the ocean by these rivers can be used to understand past variations in the size of the ice sheet.
Increasing conductivity downstream and the offset seen in the regional meteoric water line are both indicative of excess evaporation in the region.
Study of the saturation state of typical minerals based on cation and anion concentrations indicate more weathering is occurring the Qorlortoq region than the Sisimuit region, contrary to predictions based on water balance.
Possible explanations for this discrepancy include 1) differences in the bedrock of the region, and 2) the age and extent of previous weathering of the bedrock given that the Greenland Ice Sheet retreated from the Sisimuit region one to two thousand years before it retreated from the Qorlortoq region (Levy et al., 2012).
Specific Conductivity increases downstream in the Qorlortoq streams and is higher than observed conductivities in Sisimuit streams (outlined in shaded box) The meteoric water line expresses the average relationship between dD2 and DO18 in natural waters. Fractionation of those isotopes occurs through processes of precipitation and evaporation. Values that plot below the line show excess evaporation, while values above the line show excess precipitation. The conductivity trends and regional meteoric water line for Qorlortoq both indicate increasing evaporation downstream.
Future Research
50 km
Sisimiut
Qorlortoq
Sisimiut
Additional studies are ongoing to verify that weathering is more intense in the Qorlortoq region. These include determination of a chemical index of alternation for the bedload sediments and studies of radiogenic isotopes (Sr, Nd and Pb) of the waters that can also record variations in weathering (eg., Harlavan et al., 1998; Blum and Erel, 2003) and which are ultimately preserved in marine sediments and can be analyzed over long time scales to see how weathering varied on Greenland over glacial and interglacial periods in the Pleistocene. Comparisons will also be made to a proglacial stream fed directly from the ice sheet.
Qorlortoq
Qorlortoq
Objectives
These saturation indices illustrate the minerals in the water in the streams at Qorlortoq have much higher saturation states than at Sisimiut. This is a result of increased mineral weathering in Qorlortoq, which increases the saturation state of ions downstream. This combined with increasing conductivity compared to Sisimiut, suggests higher rates of chemical weathering reactions in the Qorlortoq waters.
To use chemical saturation indices to determine the intensity of weathering in the stream and the minerals involved in chemical weathering reactions. To compare the geochemistry of streams in two deglacial areas with varying levels of precipitation and evaporation.
Acknowledgements
Methods
Grant number: NSF ARC (JBM and EEM)
I would like to give a special thanks to Dr. Ellen Martin, Kelly Deuerling, and Cecilia Scribner for their constant support and assistance throughout the entire program. Also, George Kamenov for his help with analyses on the Nu MC-ICP.
Water samples from the Qorlortoq region were collected and analyzed in the field for conductivity, temperature, pH, alkalinity, and dissolved oxygen.. Nineteen water samples were collected and brought back to UF where major ions and stable isotopes were analyzed. These data were processed in a modeling program (PHREEQ), which provided saturation indices for typical minerals that could be reacting in the streams.
An example from one cation and mineral system. Ca is normalized to Cl-, a conservative element, to remove the effects of evaporation and precipitation. Ca/Cl decreases downstream, which is consistent with calcite saturation indices that indicate undersaturation and dissolution of calcite upstream in the Q1 system which ultimately leads to oversaturation and precipitation in the downstream samples.
References
Levy, L. B., Kelly, M. A., Howley, J. A., Virginia, R. A. (2012). Quaternary Science Reviews, 52(0),1 -5. doi:
Nielsen, A. B. (2010) Present conditions in Greenland and the Kangerlussuaq Area. Eurajoki, Finland, Posiva Oy
Harlavan, Erel, and Blum, 1998, GCA, 62,
Blum and Erel, 1997, GCA, 61,