Paper No. 5
Presentation Time: 2:10 PM
TWO SIDES TO EVERY RANGE: OROGRAPHIC INFLUENCES ON CHEMICAL WEATHERING
Many studies have illustrated the importance of mountainous watersheds in the global carbon cycle, but few have discussed the influences of rain shadows on chemical weathering in these watersheds. Here we examine orographic hydrologic forcing as an additional mechanism associated with uplift that controls chemical weathering. Studies of chemical weathering in the Lesser Antilles found that hydrology, not bedrock age, had a larger impact on chemical weathering. We have conducted a study of a large mountainscape in Central America with a rain shadow that causes a four-fold difference in precipitation from one side of the range to the other. Our data from streams draining both sides of the Sierra de las Minas in Guatemala suggest that orographic hydrologic forcing also affects chemical weathering in non-volcanic mountain ranges, warranting further global consideration. To examine this, we measured discharge and collected water samples from streams draining the northerly windward side and the southerly leeward side of the range during the dry season. Water samples were analyzed for major cations, major anions, dissolved silica, and nutrients. These data were then used to calculate silicate chemical weathering rates and associated CO2
consumption from a previously established model.
Stream discharges increased roughly four-fold more per km2 of watershed area in windward streams compared to leeward streams of the Sierra de las Minas. This difference is reflective of the roughly four-fold greater annual rainfall observed on the windward (2000 mm yr-1) compared to the leeward (500 mm yr-1) side of the range. We argue it is this difference in hydrology that influences chemical weathering rates and results in approximately an order of magnitude greater CO2 consumption by silicate weathering for streams draining the windward (110–1100 x 103 mol km‑2 yr‑1) than the leeward (1.1–375 x 103 mol km‑2 yr‑1) side of the range. Bedrock lithology does play an important role in the observed differences in chemical weathering, but hydrology plays a larger role than lithology. Furthermore, we speculate that orographic hydrologic forcing may also enhance physical weathering and particulate carbon burial, and is likely to have the greatest effect in tropical areas where rainfall is generally higher than in other climates.