Oxygen patchiness in a lake

The presence and extent of small scale vertical structure in dissolved oxygen were investigated in a tropical Australian lake. At a 9 m deep site an extremely high oxygen patch appeared near the bottom boundary. Light levels reaching the bottom boundary during the life of the patch exceeded 10 µEinst m^–2 s^–1. The patch remained for 1.5 h before gradually reaching ambient oxygen levels. The patch's disappearance coincided with decreasing light levels during the late afternoon. Differences in the extent of oxygen vertical patchiness between deep (16 m) and shallow (9 m) sites were also observed. At the deep site, the distribution of oxygen steps over the water column coincided largely with the distribution of temperature steps. This indicates the dominance of physical processes at that site. At the shallow site, marked vertical patchiness in oxygen was observed with no similar patchiness in temperature. This indicates the prevalence of biological and chemical processes.     

Epiphytic nitrogen fixation associated with standing dead shoots of smooth cordgrass, Spartina alterniflora

N₂ fixation associated with the epiphytic community on standing dead Spartina alterniflora shoots was examined in both a natural and transplanted salt marsh in North Carolina. Acetylene reduction (AR) assays were conducted over a 24-mo period to estimate N₂ fixation rates on standing dead stems and leaves. In the natural salt marsh, mean AR rates ranged from 0.5 nmol C₂H₄ cm^?2 h^?1 to 14 nmol C₂H₄ cm^?2 h^?1, while in the transplanted marsh mean AR rates ranged from 1 nmol C₂H₄ cm^?2 h^?1 to 33 nmol C₂H₄ cm^?2 h^?1. Diel AR activity of epiphytic communities in both marshes varied seasonally. Midday incubations yielded higher AR rates than nighttime incubations in the spring, while midday incubations in late summer and fall generally yielded AR rates equal to or lower than nighttime incubations. Desiccation during low tides occasionally repressed AR activity, although AR rates quickly rebounded with wetting. AR activity was localized in the epiphytic community, rather than in the underlying Spartina stem material. Based on the measured AR rates and the density of standing dead stems, the annual input of new N to the natural salt marsh via epiphytic N₂ fixation is estimated to be 2.6 g N m^?2 yr^?1. The estimate of annual input of new N to the transplanted marsh is 3.8 g N m^?2 yr^?1. These estimates should be added to previous estimates of N₂ fixation in marsh sediments to estimate the total contribution of new nitrogen to salt marsh nitrogen budgets.     

Global climate policy: will cities lead the way?

While the Conference of the Parties wrangle at an international scale with climate policy, a quiet set of policies and measures is being implemented at a local scale by municipalities across the globe. This study examines the motivation municipalities have for undertaking policies to reduce their greenhouse gas emissions, when the theory of free-riding would predict that local administrations should find it difficult to unilaterally reduce their emissions for the benefit of the global climate. Through interviews with officials and/or staff in 23 municipalities in the United States enacting climate policy, data are gathered that suggest local government abatement policies are primarily a top–down decision based on what officials or staff members believe to be “good business” or rational policy choices. They are primarily driven by the potential for realised or perceived cost savings and co-benefits rather than by public pressure. Economic data from some dozen municipal projects are analyzed, finding, while municipalities lack sophisticated accounting techniques, some justification for the often-disputed claim that at least initial reductions in emissions can be made at cost savings. In the United States, with the lack of national abatement policies, it is municipalities that are leading the way in beginning to implement mitigation strategies, even if only for initial reductions.     

Extensive methane-derived authigenic carbonates in the Irish Sea

Extensive areas of methane-derived authigenic carbonate (MDAC) have been mapped in the Irish Sea. In the Irish Sector, 23 seabed mounds associated with the Codling Fault Zone were identified by multi-beam echo sounder mapping. Inspection by ROV-mounted video showed that these mounds are rocky features rising 5–10 m above the normal seabed; sampling showed that they are comprised of quartz grains bound together by carbonate cement, probably MDAC. Two separate locations have been mapped in the UK Sector. At Texel 11, seabed mounds and a 6–8 m high cliff were mapped geophysically (MBES, SSS and seismic profiler surveys). Video surveys showed that both the mounds and the cliff are rocky reefs colonised by a prolific fauna. Samples proved to be carbonate-cemented sediments, and carbon isotope analysis (δ¹³C −41 to −46% PDB) showed that the cement was MDAC. Similar surveys of the Holden’s Reefs area proved the presence of similar rocky reefs which are also cemented by MDAC. The total area covered by these two MDAC occurrences is estimated to be >500,000 m². These MDAC occurrences are comparable in nature and formation to the ‘bubbling reefs’ of the Kattegat. As the bubbling reefs are “seabed features formed by leaking gas”, one of the marine habitats identified by the European Commission’s Habitats Directive as being sensitive and worthy of protection, it is suggested that the Irish Sea carbonate reefs should also be considered as special habitats.     

Rifting of oceanic crust at Endeavor Deep on the Juan Fernandez microplate

The development of an anomalously deep rift appears to be a common characteristic of the evolution of microplates along the East Pacific Rise, including the Galapagos, Easter, and Juan Fernandez microplates. We investigate crustal rifting at Endeavor Deep on the Juan Fernandez microplate using bathymetry, gravity and side scan sonar data. An initial phase of lithospheric extension accompanied by extensive subsidence results in the formation of a very deep rift valley (up to 4 km of relief, 70 km long and 20 km wide). Morphological observations and gravity data derived from GEOSAT satellite altimetry show the subsequent initiation of crustal accretion and development of a mature spreading center. Recent models of the kinematics of microplate rotation allow the amount of opening across Endeavor Deep over the past 1 m.y. to be quantified. We develop a simple mechanical model of rifting involving block faulting and flexural response to explain the gravity signature over the rift valley. The Bouguer gravity anomaly is asymmetric with respect to the surface topography and requires that a shallow-dipping fault on the western wall of the valley dominate the extension at Endeavor Deep. Consideration of three similar microplate rift valleys leads us to suggest that asymmetric rifting is the characteristic process forming microplate deeps.     

Living on the Edge: Increasing Patch Size Enhances the Resilience and Community Development of a Restored Salt Marsh

Foundation species regulate communities by reducing environmental stress and providing habitat for other species. Successful restoration of biogenic habitats often depends on restoring foundation species at appropriate spatial scales within a suitable range of environmental conditions. An improved understanding of the relationship between restoration scale and environmental conditions has the potential to improve restoration outcomes for many biogenic habitats. Here, we identified and tested whether inundation/exposure stress and spatial scale (patch size) can interactively determine (1) survival and growth of a foundation species, Spartina alterniflora and (2) recruitment of supported fauna. We planted S. alterniflora and artificial mimics in large and small patches at elevations above and below local mean sea level (LMSL) and monitored plant survivorship and production, as well as faunal recruitment. In the first growing season, S. alterniflora plant survivorship and stem densities were greater above LMSL than below LMSL regardless of patch size, while stem height was greatest in small patches below LMSL. By the third growing season, S. alterniflora patch expansion was greater above LMSL than below LMSL, while stem densities were higher in large patches than small patches, regardless of location relative to LMSL. Unlike S. alterniflora, which was more productive above LMSL, sessile marine biota recruitment to mimic plants was higher in patches below LMSL than above LMSL. Our results highlight an ecological tradeoff at ~LMSL between foundation species restoration and faunal recruitment. Increasing patch size as inundation increases may offset this tradeoff and enhance resilience of restored marshes to sea-level rise.     

Steady convective exchange flows down slopes

: Horizontal exchange flows driven by destabilising buoyancy fluxes through the surface waters of lakes and coastal regions of oceans are important in understanding the transport of nutrients, micro-organisms and pollutants from littoral to pelagic zones. Our interest here is in the discharge flow driven by cooling or destabilising forcing at the water surface in a water body with variable depth due to sloping bottom topography. Flow visualisation studies and measurements in a laboratory model enabled us to develop scaling arguments to predict the dependency of discharge upon surface forcing and the angle of bottom slope. The results were used to interpret both the laboratory measurements and field data from a small shallow lake with sloping sides and an essentially flat bottomed interior, as well as published results from the literature. The steady state horizontal exchange can be described by Q = 0.24 B^1/3 (l tan //(1 + tan /))^4/3, where Q is the discharge rate per unit length of shoreline, / is the angle of the bottom slope, B is the surface buoyancy flux and l is the horizontal length of the forcing region over the slope. The flushing timescale of the wedge shaped littoral region was given by Ff ~ l ^2/3 (1 + tan /) ^4/3/ (B tan / ^1/3. While the buoyancy flux in the field is almost never constant in space or time and the slope from the shore is seldom uniform, we found that the exchange rate was relatively insensitive to buoyancy flux changes and only moderately sensitive to slope.     

Basin modeling meets rift analysis – A numerical modeling study from the Jeanne d'Arc basin,offshore Newfoundland,Canada

The Jeanne d'Arc basin, on the Grand Banks offshore Newfoundland, is a confined, failed-rift basin. Its initial development and burial history were controlled by crustal stretching and thinning. This study provides a detailed analysis of the heat flow-, tectonic subsidence-, stretching factor-, maturation- and temperature history, based on 44 1D models- and one full 4D (space and time) model.     

Enhancing the Reliability of 3D Subsurface Models through Differential Weighting and Mathematical Recombination of Variable Quality Data

One of the first stages of the three‐dimensional (3D) subsurface modeling process involves collation and analysis of available borehole and/or outcrop data to identify individual subsurface units, usually distinguished by the grain size of the sediment, and the elevation of their bounding contacts. Input data can come from a variety of sources and may be categorized according to their reliability and/or quality. The output from the 3D model is a prediction of subsurface conditions based on these data and the reliability of the output model is highly dependent on both the quality of input data and the types of interpolation methods used. This article presents a new quality weighting methodology that allows the user to assign a differential weighting factor to data points of variable quality in the modeling process. Input data are categorized into high and low quality datasets which are then recombined using a grid math process in which a differential “weighting” factor is applied. This allows the 3D modeling program to maximize the use and effectiveness of data from all available sources while giving high quality data greater influence on the final model output, and will result in the generation of more accurate and reliable 3D subsurface models.