Relict landscape resistance to dissection by upstream migrating knickpoints

Abstract

Expanses of subdued topographies are common at high elevation in mountain ranges. They are often interpreted as relict landscapes and are expected to be replaced by steeper topography as erosion proceeds. Preservation of such relict fragments can merely reflect the fact that it takes time to remove any preexisting topography. However, relict fragments could also possess intrinsic characteristics that make them resilient to dissection. We document here the propagation of a wave of dissection across an uplifted relict landscape in Puerto Rico. Using 10Be‐26Al burial dating on cave sediments, we show that uplift started 4 Ma and that river knickpoints have since migrated very slowly across the landscape. Modern detrital 10Be erosion rates are consistent with these long‐term rates of knickpoint retreat. Analysis of knickpoint distribution, combined with visual observations along the streambeds, indicates that incision by abrasion and plucking is so slow that bedrock weathering becomes a competing process of knickpoint retreat. The studied rivers flow over a massive stock of quartz diorite surrounded by an aureole of metavolcanic rocks. Earlier studies have shown that vegetation over the relict topography efficiently limits erosion, allowing for the formation of a thick saprolite underneath. Such slow erosion reduces streambed load fluxes delivered to the knickpoints, as well as bed load grain size. Both processes limit abrasion. Compounding the effect of slow abrasion, wide joint spacing in the bedrock makes plucking infrequent. Thus, the characteristics of the relict upstream landscape have a direct effect on stream incision farther downstream, reducing the celerity at which the relict, subdued landscape is dissected. We conclude that similar top‐down controls on river incision rate may help many relict landscapes to persist amidst highly dissected topographies.

Abrupt Change in Forest Height along a Tropical Elevation Gradient Detected Using Airborne Lidar

“…Knickpoints in the parts of the Luquillo Mountains underlain by quartz diorite are clustered at~600 m elevation across the drainage network. These knickpoints do not coincide with lithological discontinuities, but instead represent the front to an upstream migrating wave of incision that originated at the paleoshoreline [7]. Erosion into the surrounding mountain slopes produces steep hillslopes downstream of the knickpoints, while shallower hillslopes still prevail upstream of the knickpoints.…”

“…The remnants of one such platform surround the Luquillo Mountains at an elevation of about 600 m [6]. The platform encloses the remnant of a paleoisland, "El Yunque Island", which peaks now protrude to 1100 m [6,7]. The uplift of the island and its surrounding platforms sparked vigorous incision of the newly elevated landmass, in the form of upstream migrating waves of river incision.…”

“…have found that a pulse of tectonic uplift which occurred during the rise of the Luquillo Mountains generated waves of erosion that affect soil mineralogy and soil nutrients [6,7]. Using the LiDAR data, we investigate whether these variations in erosion rate affect canopy height along elevation gradients.…”

“…Most of its landmass emerged from the ocean approximately 4.2 My ago, converting an archipelago of low elevation islands and surrounding marine platforms into high mountaintops [7,11]. The remnants of one such platform surround the Luquillo Mountains at an elevation of about 600 m [6].…”

Ideas and perspectives: Strengthening the biogeosciences in environmental research networks

“…Gene surveys of microbial communities in previously underexplored environments in aquifers, soils, and what call the "deep subsurface" continue to indicate the existence of enormous numbers of branches in the tree of life and are prompting fundamental changes in how we understand life's diversity. The new analyses illuminate not only the identity of organisms but their metabolic capacities, with the diversification of bacteria particularly notable (Brown et al, 2015). Nearly all of these ongoing findings are not yet represented in biogeochemical models that simulate microbially mediated processes from local to global scales.…”