The aim of this study is to investigate the spatial relationship between chemical weathering on Basse-Terre and two spatially varying factors; precipitation and bedrock age. Basse-Terre, the large western island of Guadeloupe archipelago, and is part of the Lesser Antilles volcanic arc in the eastern Caribbean. The island is comprised entirely of andesite bedrock that decreases in age to the south. Six swath topographic profiles were computed using the Matlab script DEM2swath (Gallen, 2013). One swath is oriented North-South, along the drainage divide of the island, and displays an increase in elevation along the crest of the volcanic massif toward more recent volcanic centers in the south. Five West-East oriented topographic profiles highlight the asymmetry of Basse-Terre. Plotting the distribution of K/Ar-dated lava flows as a function of distance along the North-South topographic swath profile reveals that bedrock age decreases at a rate of 0.07 ka/km to the south with an R2 value of 0.89.
Upstream basin area was computed for the 27 sample sites investigated in Gaillardet et al. (2011). Precipitation for the island was interpolated from two sources: six weather stations on the island (Metro France), and the Tropical Rainfall Measuring Mission (TRMM) satellite data. Mean basin-wide precipitation was calculated using both measures. Analysis indicates that the precipitation regime and runoff are asymmetrical. The western side of the island receives less precipitation on average than the east. The cross-island asymmetry observed in the five cross-island swath profiles is attributed to spatial variability in precipitation.
Mean precipitation, chemical weathering and mean stream steepness of each of the basins studied here and by Gaillardet et al (2011) were mapped. These variables were also plotted relative to distance along the North-South swath profile. Mean basin-wide annual precipitation for each of the basins increases to the south and at a given latitude is greater in basins draining the eastern side of the island. Mean chemical weathering rate (Gallardet et al., 2011) is higher in the eastern basins, suggesting a positive correlation between chemical weathering rate and precipitation. Mean chemical weathering and stream steepness both increase southward. The asymmetric precipitation patterns result in increases in both runoff and chemical weathering in the east. These results indicate that the relief of Basse-Terre is exerting a first order control on precipitation and runoff, and thus the amount of chemical and physical weathering.
GIS_Map_48_36Jerome Gaillardet et al. 2011. Orography-driven chemical denudation in the Lesser Antilles; evidence for a new feed-back mechanism stabilizing atmospheric CO2. American Journal of Science 311(10):851-89r.
Gallen, Sean, 2014. DEM2Swath script. Matlab Tools and Scripts. https://sites.google.com/site/theseanswebsite/tools-and-scripts