Bimodal volcanism in northeast Puerto Rico and the Virgin Islands (Greater Antilles Island Arc): Genetic links with Cretaceous subduction of the mid-Atlantic ridge Caribbean spur

Jolly, Wayne T., Edward G. Lidiak, and Alan P. Dickin. 2008. Bimodal volcanism in northeast puerto rico and the virgin islands (greater antilles island arc): Genetic links with cretaceous subduction of the mid-atlantic ridge caribbean spur. Lithos 103 (3-4) (JUL): 393-414.

Bimodal extrusive volcanic rocks in the northeast Greater Antilles Arc consist of two interlayered suites, including (1) a predominantly basaltic suite, dominated by island arc basalts with small proportions of andesite, and (2) a silicic suite, similar in composition to small volume intrusive veins of oceanic plagiogranite commonly recognized in oceanic crustal sequences. The basaltic suite is geochemically characterized by variable enrichment in the more incompatible elements and negative chondritenormalized HFSE anomalies. Trace element melting and mixing models indicate the magnitude of the subducted sediment component in Antilles arc basalts is highly variable and decreases dramatically from east to west along the arc. In the Virgin Islands, the sediment component ranges betweenb0.5 to ∼1% in Albian rocks, and between ∼1 and 2% in succeeding Cenomanian to Campanian strata. In comparison, sediment proportions in central Puerto Rico range between 0.5 to 1.5% in the Albian to 2 toN4% during the Cenomanian-Campanian interval. The silicic suite, consisting predominantly of rhyolites, is characterized by depleted Al2O3 (averageb16%), low Mg-number (molar Mg/Mg+Feb0.5), TiO2 (b1.0%), and Sr/Y (b10), oceanic or arc-like Sr, Nd, and Pb isotope signatures, and by the presence of plagioclase. All of these features are consistent with an anatexic origin in gabbroic sources, of both oceanic and arc-related origin, within the sub-arc basement. The abundance of silicic lavas varies widely along the length of the arc platform. In the Virgin Islands on the east, rhyolites comprise up to 80% of Lower Albian strata (112 to 105 Ma), and about 20% in post-Albian strata (105 to 100 Ma). Farther west, in Puerto Rico, more limited proportions (b20%) of silicic lavas were erupted. The systematic variation of both sediment flux and abundance of crustally derived silicic lavas are consistent with current tectonic models of Caribbean evolution involving approximately perpendicular subduction of the Caribbean spur of the mid-Atlantic Ridge, which was located approximately midway between North and South America until Campanian times. Within this hypothetical setting the centrally positioned Virgin Islands terrain remained approximately fixed above the subducting ridge as the Antilles arc platform swept northeastward into the slot between the Americas. Accordingly, heat flow in the Virgin Islands was elevated throughout the Cretaceous, giving rise to widespread crustal melting, whereas the subducted sediment flux was limited. Conversely, toward the west in central Puerto Rico, which wasconsistently more remote from the subducting ridge, heat flow was relatively low and produced limited crustal melting, while the sediment flux was comparatively elevated.

Geochemical Model of Redox Reactions in a Tropical Rain Forest Stream Riparian Zone: DOC Oxidation, Respiration and Denitrification

Jiménez R.A., Geochemical Model of Redox Reactions in a Tropical Rain Forest Stream Riparian Zone: DOC Oxidation, Respiration and Denitrification. Master's Capstone and Thesis. University of Pennsylvania, 2011.

A geochemical equilibrium model was used to quantify Dissolved Organic Carbon (DOC) electron donors during aerobic respiration and denitrification in a tropical stream riparian zone of the Luquillo Experimental Forest, Puerto Rico. DOC electron donors were measured across three general redox zones (Oxic: slope, Transitional: slope-riparian interface and Anoxic: riparian-floodplain) of the Icacos watershed. Model results suggest that nitrate and oxygen are completely reduced after approximately 10.1 mg/L of DOC have reacted with an initial ground water solution. In order to reach the observed mean oxygen concentration of 3.79 mg/L in the Oxic zone from the modeled equilibrium oxygen concentration of 9.46 mg/L, approximately 5.33 mg/L of DOC need to be oxidized. Additionally, 2.06 mg/L of DOC are oxidized in order to reach the observed mean oxygen concentration of 1.6 mg/L in the Transitional zone. In order to reach the observed mean Anoxic zone oxygen concentration of 1.27 mg/L from the observed mean Transitional zone oxygen concentration, an additional 0.309 mg/L of DOC are oxidized. From modeled equilibrium concentrations of oxygen (9.46 mg/L), approximately 8.8 mg/L of DOC are oxidized by oxygen before nitrate becomes more thermodynamically favorable as the electron acceptor and begins decreasing in concentration. Model simulations suggest that 1.19 mg/L of DOC reduce the observed mean nitrate concentration of 0.47 mg/L found in the Oxic zone to the lowest observed mean nitrate concentration of 0.01mg/L found in the Transitional zone. Differences between the observed DOC concentrations in the field and the modeled DOC concentrations needed to reach zone levels of oxygen and nitrate suggest that field reported values for DOC electron donors could represent residual or unused electron donors. Results also indicate that between 8.68 mg/L and 10.7 mg/L of DOC oxidation, 0.42 mg/L of dissolved N2 are produced, HCO3 increases from 0.33 mg/L to 2.64 mg/L and CO2 concentrations decrease from 13.8 mg/L to 13.7 mg/L before continuing to increase. This pronounced interval of DOC oxidation at which denitrification occurs and beyond which CO2 continues increasing suggests a specific range at which denitrifiers metabolize versus a larger range at which a general heterotrophic population metabolizes.

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.

A Statistical Method for Forecasting Rainfall over Puerto Rico

Carter MM, Elsner JB. 1997. A statistical method for forecasting rainfall over Puerto Rico. Weather Forecasting 12: 515–525.

Using results from a factor analysis regionalization of nontropical storm convective rainfall over the island of Puerto Rico, a statistical methodology is investigated for its potential to forecast rain events over limited areas. Island regionalization is performed on a 15-yr dataset, while the predictive model is derived from 3 yr of surface and rainfall data. The work is an initial attempt at improving objective guidance for operational rainfall forecasting in Puerto Rico. Surface data from two first-order stations are used as input to a partially adaptive classification tree to predict the occurrence of heavy rain. Results from a case study show that the methodology has skill above climatology—the leading contender in such cases. The algorithm also achieves skill over persistence. Comparisons of forecast skill with a linear discriminant analysis suggest that classification trees are an easier and more natural way to handle this kind of forecast problem. Synthesis of results confirms the notion that despite the very local nature of tropical convection, synoptic-scale disturbances are responsible for prepping the environment for rainfall. Generalizations of the findings and a discussion of a more realistic forecast setting in which to apply the technology for improving tropical rainfall forecasts are given.

Hydrologic Modeling of Land Processes in Puerto Rico Using Remotely Sensed Data

Hydrologic Modeling of Land Processes in Puerto Rico Using Remotely Sensed Data
Cruise, J. F.; Miller, R. L.
Journal of the American Water Resources Association, vol. 30, Issue 3, p.419-428

An integrated, multi-disciplinary effort to model land processes affecting Mayaguez Bay in western Puerto Rico is described. A modeling strategy was developed to take advantage of remotely sensed data. The spatial, interannual, and seasonal variability of sediment discharges to the bay were also evaluated. Classified images of remotely sensed data revealed the spatial distribution and quantities of land use classes in the region and aided in the discretization of the watershed into homogeneous regions. These regions were modeled using a geomorphic modeling technique based upon spatially averaged parameters. Simulation results from the modeling effort compared favorably with observations at two locations within the watershed. Results showed that runoff and sediment loads from the area exhibit a marked seasonal trend and that deforested areas located in the foothill regions of the watershed contribute a disproportionate share of the sediment load to the bay. In years when rainfall distributions are uniformly distributed over the area, the sediment yields may be up to 100 percent higher than years when the rainfall is concentrated in the heavily forested mountainous regions.

Development of a Landforms Model for Puerto Rico and its Application for Land Cover Change Analysis

MARTNUZZI, SEBASTIÁN; GOULD, WILLIAM A.; RAMOS GONZÁLEZ, OLGA M.; EDWARDS, BROOK E. 2007. Development of a Landforms Model for Puerto Rico and its Application for Land Cover Change Analysis.. Caribbean Journal of Science, Vol. 43, No. 2, :161-171.

Comprehensive analysis of land morphology is essential to supporting a wide range environmental studies. We developed a landforms model that identifies eleven landform units for Puerto Rico based on parameters of land position and slope. The model is capable of extracting operational information in a simple way and is adaptable to different environments and objectives. The implementation of the landforms model for land cover change analysis represents an advanced step towards understanding the expansion of urban areas and forest cover in Puerto Rico between 1977 and 1994. Expansion of urban areas has typically been associated with low and flat topographies. Forest recovery, on the other hand, has been associated with high elevations and steep slopes. Our study revealed that (1) nearly half of new developments occurred outside the plains, (2) almost all new forests occurred in mountain regions (but not on the steepest slopes), and (3) there are transitional and very dynamic landforms (the side slopes) that experience both important land development and forest recovery. Finally, we present additional examples of the landforms model applications, including vegetation mapping, physiography, and the modeling of vertebrate habitat distributions.


A numerical groundwater model was constructed to simulate groundwater flow in the Yauco Alluvial Valley aquifer. The groundwater flow model was constructed to evaluate future management options, including the potential to increase aquifer firm yield through a conjunctive management of ground and surface water. The aquifer within the Yauco Valley consists of river alluvium deposited over the incised Juana Díaz formation and Ponce limestone. A finite-difference, numerical model was developed to simulate ground water flow in the Yauco Valley. The single-layer model encompasses the entire alluvial deposits of the valley which extends from the Yauco town to the Caribbean Sea. The model was calibrated to October 1960 and 1970 to 1974 water levels. Different management scenarios were modeled to analyze and determine how much water can be extracted from the aquifer and evaluate the conjunctive use potential. Results demonstrate that the aquifer could be subjected to a total extraction in the order of 4.6 to 4.8 mgd (1-1.25 mgd above current extractions) without reducing the water levels to a point that could produce saltwater intrusion. Simulations showed that groundwater extractions could be increased by 5 mgd to 6 mgd during the dry season (March-August) if artificial recharge is provided in the range of 1.3 mgd to 1.95 mgd on a year-around basis. This demonstrates that the potential exists to conjunctively use ground and surface water to increase aquifer yield

A Model for Predicting Daily Peak Visitation and Implications for Recreation Management and Water Quality: Evidence from Two Rivers in Puerto Rico

Santiago LE, Gonzalez-Caban A, Loomis J (2008). “A model for
predicting daily peak visitation and implication for recreation
management and water quality: evidence from two rivers in Puerto
Rico”. Environ. Manage., 41: 904-914.

Visitor use surveys and water quality data indicates that high visitor use levels of two rivers in Puerto Rico does not appear to adversely affect several water quality parameters. Optimum visitor use to maximize visitor defined satisfaction is a more constraining limit on visitor use than water quality. Our multiple regression analysis suggests that visitor use of about 150 visitors per day yields the highest level of visitor reported satisfaction, a level that does not appear to affect turbidity of the river. This high level of visitor use may be related to the gregarious nature of Puerto Ricans and their tolerance for crowding on this densely populated island. The daily peak visitation model indicates that regulating the number of parking spaces may be the most effective way to keep visitor use within the social carrying capacity.

Multi-scale analysis of species introductions: combining landscape and demographic models to improve management decisions about non-native species

Brown, K.A., Spector, S.& Wu, W. (2008)Multi-scale analysis of species introductions:
combining landscape and demographic models to improve management
decisions about non-native species. Journal of Applied Ecology, 45,

1. Non-native, invasive species can affect biological patterns and processes at multiple ecological scales. The multi-scalar effects of invasions can influence community structure, ecosystem processes and function, and the nature and intensity of ecological interactions. Consequently, efforts to assess the spread of invasive species may benefit from a multi-scale analytic approach. 2. We analysed results from landscape- and population-scale models for Syzygium jambos , a nonnative tree in the Luquillo Mountains of Puerto Rico, to demonstrate a multi-scale approach that can be used to inform management decisions about invasive plants. At the landscape-level, we used an Ecological Niche Modelling approach to predict environmentally suitable habitats for the target plant. At the population-level, we constructed matrix projection models to determine the finite rate of population increase ( λ ) for S. jambos . We then extrapolated λ values to the landscape-scale to obtain a distribution map of λ values for the Luquillo forest. 3. The landscape analyses suggested that the most environmentally suitable habitats were those most similar to where S. jambos had already been observed. The population-level analyses showed that four of the seven populations had λ values less than 1, indicating that they were projected to be below replacement. The λ distribution map showed that S. jambos growth was highest in areas where it was most common and lowest in areas where it was most rare. 4. Our analyses further suggested that the importance of different drivers of invasion and the environmental variables that mediate them appear to be strongly scale-dependent. Past disturbances seemed most important for controlling invasions at fine-spatial scales; while abiotic environmental variables modulated coarse-scale invasion dynamics. 5. Synthesis and applications. We have shown that a multi-scale analytic approach can be used to manage invasive species by simultaneously targeting susceptible life stages and rapidly growing populations in a landscape. The utility of this approach stems from an ability to: (i) map the distribution of habitats that can potentially sustain λ values above replacement; (ii) identify populations to manage or monitor during selected stages of an invasion; (iii) forecast the probability for a target species to increase above a critical threshold abundance; and (iv) set priorities for control and monitoring actions.

A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation

Fletcher, R.C., Buss, H.L., Brantley, S.L., 2006.Aspheroidal weathering
model coupling porewater chemistry to soil thicknesses during
steady-state denudation. Earth Planet. Sci. Lett. 244, 444–457.

Spheroidal weathering, a common mechanism that initiates the transformation of bedrock to saprolite, creates concentric fractures demarcating relatively unaltered corestones and progressively more altered rindlets. In the spheroidally weathering Rio Blanco quartz diorite (Puerto Rico), diffusion of oxygen into corestones initiates oxidation of ferrous minerals and precipitation of ferric oxides. A positive ΔV of reaction results in the build-up of elastic strain energy in the rock. Formation of each fracture is postulated to occur when the strain energy in a layer equals the fracture surface energy. The rate of spheroidal weathering is thus a function of the concentration of reactants, the reaction rate, the rate of transport, and the mechanical properties of the rock. Substitution of reasonable values for the parameters involved in the model produces results consistent with the observed thickness of rindlets in the Rio Icacos bedrock (≈2–3cm) and a time interval between fractures (≈200–300 a) based on an assumption of steady-state denudation at the measured rate of 0.01cm/a. Averaged over times longer than this interval, the rate of advance of the bedrock–saprolite interface during spheroidal weathering (the weathering advance rate) is constant with time. Assuming that the oxygen concentration at the bedrock–saprolite interface varies with the thickness of soil/saprolite yields predictive equations for how weathering advance rate and steady-state saprolite/soil thickness depend upon atmospheric oxygen levels and upon denudation rate. The denudation and weathering advance rates at steady state are therefore related through a condition on the concentration of porewater oxygen at the base of the saprolite. In our model for spheroidal weathering of the Rio Blanco quartz diorite, fractures occur every ∼250yr, ferric oxide is fully depleted over a four rindlet set in ∼1000yr, and saprolitization is completed in ∼5000yr in the zone containing ∼20 rindlets. Spheroidal weathering thus allows weathering to keep up with the high rate of denudation by enhancing access of bedrock to reactants by fracturing. Coupling of denudation and weathering advance rates can also occur for the case that weathering occurs without spheroidal fractures, but for the same kinetics and transport parameters, the maximum rate of saprolitization achieved would be far smaller than the rate of denudation for the Rio Blanco system. The spheroidal weathering model provides a quantitative picture of how physical and chemical processes can be coupled explicitly during bedrock alteration to soil to explain weathering advance rates that are constant in time.
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