Tensor time domain electromagnetic resistivity measurements at Ngatamariki geothermal field, New Zealand

Experimental measurements in the Ngatamariki geothermal field, North Island, New Zealand were made to test the applicability of the time domain electromagnetic method for detailed investigation of the resistivity structure within a geothermal field. Low-frequency square wave signals were transmitted through three grounded bipole current sources sited about 8 km from the measurement lines. Despite high levels of electrical noise, transient electric field vectors could be determined reliably for times between 0.02 and 3.3 s after each step in the source current. Instantaneous apparent resistivity tensors were then calculated. Apparent resistivity pseudosections along the two measurement lines show smooth variations of resistivity from site to site. Over most of the field the images consistently show a three-layer resistivity structure with a conductive middle layer (3–10 Ωm) representing the conductive upper part of the thermal reservoir. A deep-seated region of low resistivity in the northwest of the field may indicate a conductive structure at about 1 km associated with a deeper diorite intrusion. Measurements sited closer than about 100 m to drillholes appear to have been disturbed by metallic casing in the holes. A change in resistivity structure in the east of the field may indicate a major geological or hydrothermal boundary.     

A numerical model of trace-element coprecipitation in a physicochemical calcification system: Application to coral biomineralization and trace-element ‘vital effects’

A mixed equilibrium/kinetic steady-state numerical model of coral calcification has been developed to test whether a physicochemical calcification mechanism is able to account for recent geochemical observations, in particular correlated trace-element variations presented in a companion paper [Sinclair, D.J., 2005. Correlated trace-element ‘vital effects’ in tropical corals: a new tool for probing biomineralization chemistry. Geochim. Cosmochim. Acta69 (13), 3265-3284]. The model simulates trace-element partitioning from a CaCO₃ supersaturated extracellular calcifying fluid (ECF) which has been modified by enzymatic input of Ca²⁺ and removal of 2H⁺ by CaATPase. CO₂ input is modelled as a diffusion process, while the ECF is continuously replenished by fresh seawater, which is the sole source of minor and trace-elements (TEs). Trace-element species fully equilibrate in the ECF, and selected trace-element species kinetically compete with Ca²⁺ or at the surface of the growing crystal. Each simulation is run to steady-state, and results are presented for a grid of CaATPase ion pumping rates and seawater replenishment rates. The dominant feature of the model output occurs when CaATPase ion pumping is high while seawater replenishment rates are low. At this point, CO₂ diffusion reaches its maximum, C input becomes limiting, buffering capacity is reduced and the pH of the system rises dramatically; significantly affecting the TE composition of the skeleton. At more modest pumping rates, the model reproduces the relative amplitudes of trace-element variations and slopes of the mutually positive correlations between B, Sr and U observed by Sinclair [Sinclair, D.J., 2005. Correlated trace-element ‘vital effects’ in tropical corals: a new tool for probing biomineralization chemistry. Geochim. Cosmochim. Acta69 (13), 3265-3284], but does not reproduce the negative correlations with Mg. The best fit between model and observation occurs when the coral simultaneously increases ion pumping and seawater replenishment rates: a strategy which allows rapid calcification while avoiding dangerously high pH variations. The model predicts that calcification occurs at only moderate pH elevations (8.3-8.4) with seasonal TE variations being explained by a shift of only 0.3 pH units. The model does not reproduce the full amplitude of diurnal pH variations observed recently. Sensitivity tests show that the model output is relatively insensitive to changes in the composition of the fluid from which the ECF is drawn (such as might occur if photosynthesis or active C transport mechanisms significantly modify the penultimate fluid source). Further research, however, is needed to establish the consequences of active transport of TEs and anions to the calcifying site.     

Clionid sponge surveys on the Florida Reef Tract suggest land-based nutrient inputs

Bioerosion by Cliona delitrix and Cliona lampa was assessed at 43 sites along the Florida Reef Tract, USA, in the summer of 2001. Sponge abundances were estimated using rapid visual assessment. Tissue samples of sponges were taken for analysis of delta15N. Comparison samples were taken from Belize. Annual trends in sponge abundance were estimated from archived videos covering the period from 1996 to 2001. Sites with the greatest boring sponge size and cover were in the Backcountry and Lower Keys, where total nitrogen, ammonium, and delta15N levels were highest. The sites with the largest relative increase of C. delitrix and C. lampa over the 5 year period were in the Upper Keys, where the greatest relative decline in stony coral cover has occurred. Florida sponge delta15N values were 5.2(+/-0.1)%, suggesting the influence of human waste; in comparison, offshore Belize samples were 2.1(+/-0.1)%. These results suggest sewage contamination of the Florida Reef Tract, shifting the carbonate balance from construction to destruction.     

Developing multimetric indices for monitoring ecological restoration progress in salt marshes

Effective tools for monitoring the status of ecological restoration projects are critical for the management of restoration programs. Such tools must integrate disparate data comprised of multiple variables that describe restoration status, including the condition of environmental stressors, landscape connectivity, ecosystem resilience, and ecological structure and function, while communicating these concepts effectively to a wide range of stakeholders. In this paper we describe the process of constructing multimetric indices (MMIs) for monitoring restoration status for restoration projects currently underway on the eastern coast of Saudi Arabia. During this process, an initial suite of measurements is filtered for response and sensitivity to ecosystem stressors, eliminating measurements that provide little information and reducing future monitoring efforts. The retained measurements are rescaled into comparable domain metrics and assembled into MMIs. The MMIs are presented in terms of established restoration theories, including restoration trajectory and restoration endpoint targets.     

Using field analogue soil column experiments to quantify radon-222 gas migration and transport through soils and bedrock of Halifax,Nova Scotia,Canada

Radon gas is a human health hazard; long-term exposure to high radon concentrations through inhalation is the second leading cause of lung cancer. Nova Scotia has been previously identified as a potential high risk region because of the geology. As such, the gas transport through Halifax’s fine grained leucomonzogranite (FGL) unit of the South Mountain Batholith needed to be quantified to further remediation efforts. Using controlled laboratory experiments, four different soil columns were created using the Halifax Regional Municipality’s (HRM) highest producing field tills and bedrock. Permeability, diffusivity, radon-222 gas concentrations, and gas transit time/speed were measured in both dry tills (field moisture) and wet tills (simulated rain event moisture). Columns with HRM till displayed the highest radon concentrations, and were less permeable with additional moisture. Radon diffusivity calculated from CO₂ was 7.52 × 10^?8 m² (dry), and 3.37 × 10^?8 m² (wet); diffusivity calculated from ²²²Rn was 7.30 × 10^?7 m² (dry), and 6.47 × 10^?7 m² (wet). The average FGL transit time in a 60 cm column was 3.57 days (dry), and 3.82 days (wet). Locally this study presents two different methods for diffusivity calculations, for a unit lacking previous diffusivity information. The radon gas concentrations and transport speeds quantified the transport mechanisms within the till. Globally, the correlation between soil moisture, and radon/permeability values was established using these results. The link between diffusivity and permeability was also confirmed using field tills. Implications were made for building foundations, as well as the depth and type of material necessary to reduce radon gas from reaching the surface.     

Health of the coral reefs at the US Navy Base,Guantánamo Bay,Cuba: A preliminary report based on isotopic records from gorgonians

Specimens of the gorgonian Plexaura homomalla were sampled from several areas along the fringing reefs fronting the United States Naval Base at Guantánamo Bay, Cuba. Sample coverage extended from apparently healthy reefs in oceanic waters to declining reefs located in the plume of the drainage from upper parts of Guantánamo Bay. Tentacle tips were excised, and trunk sections were cut and polished. Stable isotope ratios of nitrogen (δ¹⁵N) and carbon indicate a strong correlation of reef health with proximity to the plume of the river.