FLUVIAL KNICKPOINTS AND TRANSIENT EROSION ON BASSE-TERRE, GUADELOUPE, WITH IMPLICATIONS FOR ESTIMATES OF CHEMICAL WEATHERING

The mountainous island of Basse-Terre in the Guadeloupe archipelago of the Lesser Antilles is well suited for investigating the relationships among transient erosion, mean annual precipitation (MAP), bedrock age, and chemical weathering of the critical zone. Bedrock age decreases linearly from north to south parallel to the topographic divide that also defines an orographic precipitation gradient. The MAP varies from 200 to > 6500 mm/a along an east-west oriented gradient meaning that areas of similar bedrock age, but differing MAP, can be compared. Knickpoints are common and define two geographically distinct populations separated by the NW-SE striking Pointe a Sel-Capesterre Fault system. A constructional landscape lies south of the transverse fault system, and many knickpoints there occur at the contacts of mapped Pleistocene to recent lava flows and separate steep upstream reaches from gentler downstream reaches. In contrast, knickpoints north of the transverse fault system separate gentler upstream reaches from steeper downstream reaches and cluster in chi-elevation space (6-8m chi, 350-450m elevation). The region upstream of these knickpoints defines a contiguous ~80 km² region of low slope straddling the topographic divide in the northern portion of Basse Terre that we interpret as a relict landscape. We hypothesize that this low-slope region, accounting for ~10% of the surface area of the island, is mantled by a thick, chemically inert regolith pile where hydrologic flow paths are too shallow to intersect the reactive bedrock. Detailed streamwater sampling along the 700 m length of the Quiock River exhibits lower pH values (<5.6) where the stream flows over the relict topography and higher pH (≥ 6.5) downstream of the knickpoint, indicating that geochemical signatures of chemical weathering are largely absent in streamwater above the knickpoint. Similar trends occur regionally: drainage basins that lie mostly upstream of knickpoints have lower streamwater pH values. Consequently, in drainage basins underlain by significant fractions of relict topography, surface-area-normalized catchment-scale estimates of chemical weathering likely underestimate the magnitudes of chemical weathering in the reactive portions of the critical zone where flow paths intersect fresh bedrock.