Twelve testable hypotheses on the Geobiology of weathering

Brantley S.L., Megonigal J.P., Scatena F.N. et al 2010. Twelve testable hypotheses on the Geobiology of weathering. Geobiology. DOI: 10.1111/j.1472-4669.2010.00264.x

Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth’s surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solar-to-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plant-microbe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experiencing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over the long term.(4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation via biotic processes.(5) Biology shapes the topography of the Critical Zone.(6) The impact of climate forcing on denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates and geomorphological transport laws.(7) Rising global temperatures will increase carbon losses from the Critical Zone.(8) Rising atmospheric PCO2 will increase rates and extents of mineral weathering in soils.(9) Riverine solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through changes in biologically mediated weathering.(10) Land use change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer timescales.(12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur.

Bermeja Complex Serpentinites: Testing Theories of Emplacement

The Bermeja Complex in southwestern Puerto Rico contains large blocks of serpentinite of uncertain origin. Competing theories have emerged to explain the presence of these rocks. One theory is that the serpentinite represents the serpentinized peridotite section of an obducted seafloor ophiolite. An alternative theory states that the rocks are part of a volcanic island arc terrane in which large blocks of serpentinite from a subducted slab were squeezed up through the crust and solidified within the Puerto Rico crust. We have studied the mineralogy, chemical composition, and outcrop-scale geology of our field site in the southernmost belt of the Bermeja Complex, the Sierra Bermeja mountains. Our main goal was to sample the serpentinites and test the concentration of trace elements and rare earth elements to infer the origin of the complex. Our trace element analysis was used to provide constraints on the origin of the serpentinite, being either crust or mantle derived. The combination of mineralogical analysis, bulk chemical analysis, outcrop-scale measurements, and trace element concentration data have provided sufficient evidence to resolve the issue of the Sierra Bermeja serpentinite origin. We have determined that the serpentinites of our field site were emplaced by the obduction of an ophiolite sequence. Our results have implications for the complex tectonic history of the PRVI microplate and the Caribbean region as a whole.


Cardona,W.A., 1984. El Junque mineral prospects, eastern Puerto Rico. Caribbean
Journal of Science 20, 79-87.

known. To date, no extensive reconnaissance or geochemical work has been carried out. In order to promote future exploration, the author searched among spanish historical documents for clues of previous mining operations. Through the research a total of twenty three (23) sites were relocated which reveal a predominantly gold placer mining operation in the region. Furthermore, the data clearly denotes two zones of mineral occurrence: a northern gold-rich deposit zone and a southern copper dominant area. The extensive hydrothermal processes and contact metamorphism associated with the Rio Blanco Stock and its proximity to the southern hydrothermal alteration belt and San Lorenzo Batholith appear to account for this mineral differentiation.

Vertical motions of the Puerto Rico Trench and Puerto Rico and their cause

ten Brink US (2005) Vertical motions in the Puerto Rico trench and
Puerto Rico and their cause. J Geophys Res 100:B06404. doi:

The Puerto Rico trench exhibits great water depth, an extremely low gravity anomaly, and a tilted carbonate platform between (reconstructed) elevations of +1300 m and 4000 m. I argue that these features are manifestations of large vertical movements of a segment of the Puerto Rico trench, its forearc, and the island of Puerto Rico that took place 3.3 m.y. ago over a time period as short as 14–40 kyr. I explain these vertical movements by a sudden increase in the slab’s descent angle that caused the trench to subside and the island to rise. The increased dip could have been caused by shearing or even by a complete tear of the descending North American slab, although the exact nature of this deformation is unknown. The rapid (14–40 kyr) and uniform tilt along a 250 km long section of the trench is compatible with scales of mantle flow and plate bending. The proposed shear zone or tear is inferred from seismic, morphological, and gravity observations to start at the trench at 64.5Wand trend southwestwardly toward eastern Puerto Rico. The tensile stresses necessary to deform or tear the slab could have been generated by increased curvature of the trench following a counterclockwise rotation of the upper plate and by the subduction of a large seamount.

Bivergent thrust wedges surrounding oceanic island arcs: Insight from observations and sandbox models of the northeastern Caribbean plate

ten Brink U, Marshak S, Granja JL (2009) Bivergent thrust wedges
surrounding oceanic island arcs: insights from observations and
sandbox models in the north-eastern Caribbean plate. Geol Soc
Am Bull 121:1522–1536

At several localities around the world, thrust belts have developed on both sides of oceanic island arcs (e.g., Java-Timor, Panama, Vanuatu, and the northeastern Caribbean). In these localities, the overall vergence of the backarc thrust belt is opposite to that of the forearc thrust belt. For example, in the northeastern Caribbean, a north-verging accretionary prism lies to the north of the Eastern Greater Antilles arc (Hispaniola and Puerto Rico), whereas a south-verging thrust belt called the Muertos thrust belt lies to the south. Researchers have attributed such bivergent geometry to several processes, including: reversal of subduction polarity; subduction-driven mantle fl ow; stress transmission across the arc; gravitational spreading of the arc; and magmatic infl ation within the arc. New observations of deformational features in the Muertos thrust belt and of fault geometries produced in sandbox kinematic models, along with examination of published studies of island arcs, lead to the conclusion that the bivergence of thrusting in island arcs can develop without reversal of subduction polarity, without subarc mantle fl ow, and without magmatic infl ation. We suggest that the Eastern Greater Antilles arc and comparable arcs are simply crustalscale bivergent (or “doubly vergent”) thrust wedges formed during unidirectional subduction. Sandbox kinematic modeling suggests, in addition, that a broad retrowedge containing an imbricate fan of thrusts develops only where the arc behaves relatively rigidly. In such cases, the arc acts as a backstop that transmits compressive stress into the backarc region. Further, modeling shows that when arcs behave as rigid blocks, the strike-slip component of oblique convergence is accommodated entirely within the prowedge and the arc—the retrowedge hosts only dip-slip faulting (“frontal thrusting”). The existence of large retrowedges and the distribution of faulting in an island arc may, therefore, be evidence that the arc is relatively rigid. The rigidity of an island arc may arise from its mafi c composition and has implications for seismic-hazard analysis.

Three-dimensional gravity modelling of a Trinidad mud volcano, West Indies

Arafin S., 2005. Three-dimensional gravity modeling of a trinidad mud volcano, West
Indies. Exploration Geophysics 36, 329–333.

The violent eruption of the Piparo mud volcano, Trinidad, in February 1997 demonstrated its destructive capability by completely burying 16 houses and a number of livestock under a 5 m thick mud pile. Unlike magmatic volcanoes, mud volcanoes involve very low energy, making geophysical methods such as seismology unsuitable for monitoring. Three-dimensional gravity modelling over the Tabaquite mud volcano suggests the presence of a large density contrast (–0.70 t.m-3). The density contrast being large and dynamic (i.e., it is absent at recently active mud volcanoes like Piparo) makes the gravity method a potential tool for monitoring mud volcanoes.
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