The anatomy of weathering profiles on different lithologies in the tropical forest of northeastern Puerto Rico: from bedrock to clouds

Open Access
Author:
Orlando, Joseph James
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 26, 2014
Committee Members:
  • Susan Louise Brantley, Thesis Advisor
Keywords:
  • deep weathering
  • Luquillo Critical Zone Observatory
  • drilling
  • ground penetrating radar
  • metavolcaniclastic
  • hornfels
  • corestone
Abstract:
The Critical Zone is the zone where meteoric fluids change the structure and chemistry of rock formed at depth. Near the surface, these changes result in formation of a mantle of chemically and physically altered material defined here as regolith. Worldwide, the depth of regolith varies from zero to hundreds of meters and is highly affected by lithology and climate. In the Luquillo Mountains in northeastern Puerto Rico, we explored the influence of lithology (quartz diorite versus volcaniclastic sedimentary rocks versus metavolcaniclastic rocks) and climate (influenced by elevation) on weathering in the Rio Icacos watershed. The Icacos watershed is defined by hornfels-facies ridges above a quartz diorite-underlain valley (down to ~610 meters above sea level (masl)). Prominent knickpoints not attributed to lithologic variation with flat reaches directly headward can be identified in all of the watersheds in the Luquillo Experimental Forest that are draining the quartz diorite bedrock. Regolith profiles were studied by drilling deep boreholes (25-40 m) and imaging the subsurface using ground penetrating radar (GPR). In kaolinite-rich quartz diorite derived regolith near the land surface, GPR is a useful tool for imaging subsurface structures (to ~15 m depth) related to weathering. Boreholes in the Upper Icacos revealed a highly heterogeneous subsurface characterized by intervals of decimeter- to meter-sized corestones and centimeter- to meter-thick sections of disaggregated weathered material. Two other boreholes drilled into the quartz diorite revealed that regolith at ridgetops thickened as elevation increased. We also investigated the neighboring Bisley catchment, a sub-watershed of the larger Mameyes watershed where regolith has developed on sedimentary volcaniclastics, with GPR to image the subsurface to depths of about 8 mbls. Along the divide between the Rio Icacos and Mameyes watersheds, where the outcropping rocks are hornfels-grade metavolcaniclastics, the GPR data was less useful, likely due to differences in clay mineralogy or the presence of iron oxides. The depth of regolith on ridgetops was observed to increase with elevation and also to increase from quartz diorite (QD) to hornfels (HF) to volcaniclastics (VC). This trend is similar to the trend of increasing denudation rate of the three lithologies, i.e., HF < QD < VC. At the bottom of each borehole, we always observed oxidized fractures. These were attributed either to the deepest depth of the water table at each location or to deep influxes of oxidized water along fractures. The thick regolith (~36 m) at the highest point of the watershed divide may be caused by the large fluctuations in the water table driving oxidation and fracturing of bedrock at that elevation. Another factor controlling the weathering of rocks in El Yunque National Forest is the base level at which clouds form, as the solute chemistry and the quantity of precipitation are affected by the immersion of the forest and land surface in clouds. The cloud condensation level in the Luquillo Experimental Forest ranges from 600 – 800 masl, i.e., the lowest elevation is coincident with the knickpoint (600 masl) and the flat reach headward on the Rio Icacos. Sulfur concentrations are highest in the soils (0.060 wt % S) developed at the highest elevations in the forest. In the highest elevations, cloud water also contains higher S concentrations (152 eq SO42- /L) than rainwater (43 eq SO42- /L), and is more acidic (pH 4.35 cloud water, pH 4.76 rain water). Sites above the cloud condensation level therefore are frequently exposed to naturally occurring acidic cloud water for long periods of time. A plausible explanation of the architecture of the Critical Zone in the upper Luquillo Mountains of Puerto Rico is that relief is dictated largely by lithology because rock type controls the fracture density. This density in turn mediates the influx of water to rock at depth, influencing the type of weathering. However, fracture density is not just a function of the fracture toughness of each rock type but is also a function of how the minerals in each rock weather. Important positive feedbacks occur when the water table reaches the quartz diorite because oxidative weathering of biotite in that felsic rock drives formation of additional fractures that in turn enhance further weathering. The combination of biotite in quartz diorite exposed to acidic waters in the cloud layer above 600 masl combine to produce some of the fastest weathering rocks in the world and may contribute to the elevation of the flat reach headward of the knickpoints on quartz diorite draining rivers.