Colloquium Recap: Nathan Anderson

The Bloody Dick gneiss of Montana....

The processes occurring during the Paleoproterozoic (2.5-1.4 Ga) along boundaries of different crustal blocks in North America are not completely understood, a problem that Nathan Anderson is tackling with his master’s thesis.  The occurrence of maybe the most interesting-named gneiss, the Bloody Dick gneiss, and the Maiden Peak gneiss in SW Montana allow for the study of the evolution of the western margin of North America, as they are near the Wyoming craton, the Great Falls tectonic zone, and the Farmington zone. 

Creating the first detailed complete map of the Bloody Dick gneiss, Nathan collected samples to analyze in the lab.  Analyses of zircons in the gneiss, along with field observations showed that the Bloody Dick gneiss consists of multiple gneiss units that were originally igneous rocks emplaced at ~2.47 Ga, and then metamorphosed at ~1.76 Ga. 

Broad scale paleomap of the region. Nathan's map is located at the pink dot... go read his paper or thesis to see the final result.

This would be consistent with previous models suggesting northward-dipping subduction, and the Medicine Hat block being thrust over the Wyoming craton; however, juvenile hafnium values in the zircons suggests the rocks were derived from the mantle shortly before they crystalized.  This would be consistent with a northward dipping subduction model, although if the direction of subduction was instead to the south, that model would fit the new results. 

Nathan uses the results of his thesis to expand previous models, and suggests a new model having a flip in subduction direction in the Paleoproterozoic, similar to modern day New Zealand.

Recap by Ryan Goldsby

Colloquium Recap: Caitlin Rushlow

The rise of greenhouse gas concentrations in the atmosphere and subsequent increases in global temperature is disproportionately affecting the Arctic, as excess heat is driven polewards. As has been observed and predicted globally, this is leading to an amplification of hydrological cycle – manifested as increased river discharges in the Arctic (see Figure 1). These landscapes, which maintain intermittent and perenially frozen substrate known as permafrost, are highly sensitive to these changes. Our very own PhD candidate, Caitlin Rushlow, took to explain that in light of these rapid changes – how does hillslope form influence thermal and hydrological behavior of the Arctic landscape?

Figure 1. Large increases in poleward heat flux are already amplifying runoff response in the Artic. 

To do this Caitlin set up monitoring stations to build time series of soil temperature, water discharge, and water table depth in what are known as water tracks – zero-order basins which intermittently drain water off of Arctic hillslopes. Central to the understanding of how hillslopes produce runoff in response to precipitation is what is known as the “fill-and-spill” hypothesis (see Figure 2). This is the idea that after a certain threshold is exceeded during a precipitation event (or series of them), saturated zones on a hillslope will become connected and produce a punctuated runoff response.

Figure 2. Fill-and-spill hypothesis showing increasing connectivity of saturated zones over a storm event that drives intense runoff response.

What Caitlin found during summer monitoring periods was that water track discharge responded strongly to rainfall events, though not as rapidly as had been suggested in previous studies. The “fill-and-spill” concept seemed to definitely be at play here – strong hydrograph response was only observed after a certain amount of precipitation and water table rise was achieved (see Figure 3). The longer tails of these hydrographs during the spilling phase of these responses also seem to suggest that water tracks are the source of extended streamflow, once thought to be driven by slow hillslope drainage. 

Figure 3. Paired precipitation and runoff time series for a water track and resulting response model.

In order to tackle questions of how water tracks influence the depth and persistence of permafrost on Arctic hillslopes, Caitlin took an approach that used both field data and also utilized a robust hydrologic model to understand the advective versus conductive transfers of heat on hillslopes. What she found through geophysical surveys was that the depth of thaw was greater in water tracks relative to intra-track topography. Based on soil temperature profiles, the timing of permafrost thaw varied greatly between water tracks and adjacent hillslopes. Water tracks appear to both thaw and freeze later in the year than surrounding hillslope domains. This is important to the exchange of materials and biotic cycling that occur within these soil profiles. Finally, Caitlin’s modeling approach showed that advective heat transfer (i.e., the direct movement of heat) could greatly increase summer substrate temperatures (see Figure 4). Water and its transmission of heat in a historically icy landscape could have profound impacts on how this landscape will evolve into the future.

Figure 4.  Advection of heat in water tracks in model runs appears to amplify increases in summer temperatures in water tracks

We appreciate Caitlin’s fantastic science that is highly relevant to understanding the hydrology of sensitive landscapes as well as being a motivated spokesperson for our science community!

-Recap by Jimmy Guilinger