Discussion

The key objective of this method project was to effectively determine the trace element concentrations and regressions (B vs Al, B vs Si) in areas of different luminosity under both reflected light and CL imaging. This objective is important because it sheds light on patterns of boron incorporation into calcite lattices and aluminosilicate minerals under reducing and oxidizing conditions, which in turn informs where is best to sample from to develop the paleosol paleoclimate proxy. This method project successfully distinguished the bright and dull regions from one another on both soil carbonate nodules, with the mean trace element concentrations in the dull zones an order of magnitude higher concentration for almost every element compared to the bright zones. These findings support the idea that soil carbonate nodules in the Dance Bayou grow under changing redox conditions that result in markedly different trace element concentrations in the carbonate (Mintz et al., 2011).

Profiling the bright and dull regions and plotting B vs Al and B vs Si for both nodules confirmed the known, close association between boron and aluminosilicates (Goldberg et al., 1985). Dull regions with higher aluminosilicate concentrations had especially high boron concentrations, and the very high R-squared values for the B vs Al and B vs Si plots demonstrate that the increase in boron concentration is because of aluminosilicate-associated boron. The same plots for the bright regions have high but significantly lower R-squared values, which indicate that although much of the boron present is associate with aluminosilicates, there is a greater amount of the total boron present that is likely incorporated into the calcite lattice. Flipping the axes to make Al vs B plots for the bright regions on both nodules so that Al concentration is a function of B concentration, the y-intercept indicates the approximate B concentration when Al equals zero, or the B concentration in the calcite lattice. For DB-100-2 the B concentration approximated is 1.7ppm, and for DB-110-1 the B concentration approximated is 1.4ppm. Looking to the B element concentrations by region of interest graphs, the approximate percentage of B existing in the calcite lattice for DB-100-2 is 1.7/10.6=16% and for DB-110-1 is 1.4/3.3=42%. This suggests that the bright region of DB-110-1 is the best location to sample calcite for proxy development because that region has the highest total B originating from calcite lattices.

This method project has successfully demonstrated that it is capable of identifying the best areas to sample for B associated with calcite lattices. And those best areas on both nodules are the bright regions formed in reducing conditions, which suggests that with after additional mapping and analysis to confirm the findings of this method project, the brightest parts of soil carbonate nodules in the Dance Bayou can be sampled with the knowledge that a high amount of the total B present in those areas originates from calcite lattices.