Short-term effects of a low dissolved oxygen event on estuarine fish assemblages following the passage of hurricane Charley

Hurricane Charley, a category 4 storm, passed directly over the Charlotte Harbor estuary and Peace River watershed on August 13, 2004. Following the storm's passage, dissolved oxygen in the Peace River fell below 1 mg l⁻¹ and hypoxic conditions (<2mgl⁻¹) extended into Charlotte Harbor. A long-term fisheries-independent monitoring program (1989 to present) was already in place in Charlotte Harbor, and sampling was intensified during the month after the storm. Changes in fish assemblages that resulted from the hypoxic event were determined using nonmetric multidimensional scaling and similarity percentages analysis. At the mouth of the Peace River and upper Charlotte Harbor, fish abundance decreased dramatically after the hurricane, and typical estuarine fish assemblages were replaced by those dominated by a few resilient estuarine and freshwater species, including the nonindigenous brown hoplo (Hoplosternum littorale) and sailfin catfish (Pterygoplichthys spp.). Fish assemblages in the estuarine portion of the Maykka River, located only a few kilometers west of the Peace River, were unaffected. The hypoxic event was short lived; dissolved oxygen and estuarine fish assemblages in the Peace River and upper Charlotte Harbor recovered within a month. The results of this study are consistent with other hurricane-related hypoxic events in the literature which have reported acute effects to estuarine systems in the short term, rapid recoveries, and long-term resilience.     

Carbon sequestration via wood harvest and storage: An assessment of its harvest potential

A carbon sequestration strategy has recently been proposed in which a forest is actively managed, and a fraction of the wood is selectively harvested and stored to prevent decomposition. The forest serves as a ‘carbon scrubber’ or ‘carbon remover’ that provides continuous sequestration (negative emissions). Earlier estimates of the theoretical potential of wood harvest and storage (WHS) based on coarse wood production rates were 10?±?5 GtC y^?1. Starting from this physical limit, here we apply a number of practical constraints: (1) land not available due to agriculture; (2) forest set aside as protected areas, assuming 50 % in the tropics and 20 % in temperate and boreal forests; (3) forests difficult to access due to steep terrain; (4) wood use for other purposes such as timber and paper. This ‘top-down’ approach yields a WHS potential 2.8 GtC y^?1. Alternatively, a ‘bottom-up’ approach, assuming more efficient wood use without increasing harvest, finds 0.1–0.5 GtC y^?1 available for carbon sequestration. We suggest a range of 1–3 GtC y^?1 carbon sequestration potential if major effort is made to expand managed forests and/or to increase harvest intensity. The implementation of such a scheme at our estimated lower value of 1 GtC y^?1 would imply a doubling of the current world wood harvest rate. This can be achieved by harvesting wood at a moderate harvesting intensity of 1.2 tC ha^?1 y^?1, over a forest area of 8 Mkm² (800 Mha). To achieve the higher value of 3 GtC y^?1, forests need to be managed this way on half of the world’s forested land, or on a smaller area but with higher harvest intensity. We recommend WHS be considered part of the portfolio of climate mitigation and adaptation options that needs further research.     

Origins of cryptic variation in the Ediacaran–Fortunian rhyolitic ignimbrites of the Saldanha Bay Volcanic Complex,Western Cape,South Africa

The Saldanha eruption centre, on the West Coast of South Africa, consists of 542 Ma, intracaldera, S-type, rhyolite ignimbrites divided into the basal Saldanha Ignimbrite and the partly overlying Jacob’s Bay Ignimbrite. Depleted-mantle Nd model ages suggest magma sources younger than the Early Mesoproterozoic, and located within the Neoproterozoic Malmesbury Group and Swartland complex metasedimentary and metavolcanic rocks that form the regional basement. The Sr isotope systematics suggest that the dominant source rocks were metavolcaniclastic rocks and metagreywackes, and that the magmas formed from separate batches extracted from the same heterogeneous source. No apparent magma mixing trends relate the Saldanha to the Jacob’s Bay Ignimbrites, or either of these to the magmas that formed the Plankiesbaai or Tsaarsbank Ignimbrites in the neighbouring Postberg eruption centre. The magmas were extracted from their source rocks carrying small but significant proportions of peritectic and restitic accessory minerals. Variations in the content of this entrained crystal cargo were responsible for most of the chemical variations in the magmas. Although we cannot construct a cogent crystal fractionation model to relate these groups of magmas, at least some crystal fractionation occurred, as an overlay on the primary signal due to peritectic assemblage entrainment (PAE). Thus, the causes of the cryptic chemical variation among the ignimbrite magmas of the Saldanha centre are variable, but dominated by the compositions of the parent melts and PAE. The preservation of clear, source-inherited chemical signatures, in individual samples, calls into question the common interpretation of silicic calderas as having been formed in large magma reservoirs, with magma compositions shaped by magma mingling, mixing, and fractional crystallization. The Saldanha rocks suggest a more intimate connection between source and erupted magma, and perhaps indicate that silicic magmas are too viscous to be significantly modified by magma-chamber processes.     

Mechanisms and Model Diversity of Trade-Wind Shallow Cumulus Cloud Feedbacks: A Review

Shallow cumulus clouds in the trade-wind regions are at the heart of the long standing uncertainty in climate sensitivity estimates. In current climate models, cloud feedbacks are strongly influenced by cloud-base cloud amount in the trades. Therefore, understanding the key factors controlling cloudiness near cloud-base in shallow convective regimes has emerged as an important topic of investigation. We review physical understanding of these key controlling factors and discuss the value of the different approaches that have been developed so far, based on global and high-resolution model experimentations and process-oriented analyses across a range of models and for observations. The trade-wind cloud feedbacks appear to depend on two important aspects: (1) how cloudiness near cloud-base is controlled by the local interplay between turbulent, convective and radiative processes; (2) how these processes interact with their surrounding environment and are influenced by mesoscale organization. Our synthesis of studies that have explored these aspects suggests that the large diversity of model responses is related to fundamental differences in how the processes controlling trade cumulus operate in models, notably, whether they are parameterized or resolved. In models with parameterized convection, cloudiness near cloud-base is very sensitive to the vigor of convective mixing in response to changes in environmental conditions. This is in contrast with results from high-resolution models, which suggest that cloudiness near cloud-base is nearly invariant with warming and independent of large-scale environmental changes. Uncertainties are difficult to narrow using current observations, as the trade cumulus variability and its relation to large-scale environmental factors strongly depend on the time and/or spatial scales at which the mechanisms are evaluated. New opportunities for testing physical understanding of the factors controlling shallow cumulus cloud responses using observations and high-resolution modeling on large domains are discussed.     

Importance of oceanic resolution and mean state on the extra-tropical response to El Niño in a matrix of coupled models

The extra-tropical response to El Niño in configurations of a coupled model with increased horizontal resolution in the oceanic component is shown to be more realistic than in configurations with a low resolution oceanic component. This general conclusion is independent of the atmospheric resolution. Resolving small-scale processes in the ocean produces a more realistic oceanic mean state, with a reduced cold tongue bias, which in turn allows the atmospheric model component to be forced more realistically. A realistic atmospheric basic state is critical in order to represent Rossby wave propagation in response to El Niño, and hence the extra-tropical response to El Niño. Through the use of high and low resolution configurations of the forced atmospheric-only model component we show that, in isolation, atmospheric resolution does not significantly affect the simulation of the extra-tropical response to El Niño. It is demonstrated, through perturbations to the SST forcing of the atmospheric model component, that biases in the climatological SST field typical of coupled model configurations with low oceanic resolution can account for the erroneous atmospheric basic state seen in these coupled model configurations. These results highlight the importance of resolving small-scale oceanic processes in producing a realistic large-scale mean climate in coupled models, and suggest that it might may be possible to “squeeze out” valuable extra performance from coupled models through increases to oceanic resolution alone.     

The Redan Geophysical Zone: Part of the Willyama Supergroup? Broken Hill,Australia

The Redan Geophysical Zone forms a regional magnetic high in contrast to the regional magnetic low defined by the main part of the Broken Hill Block. The magnetic rocks are interpreted to dip below the remainder of the Broken Hill Block and there has been speculation that they are significantly older than the Early Proterozoic Willyama Supergroup.

Evaluation of lithological mapping and aeromagnetic data permitted interpretation of a stratigraphic sequence within the Redan Geophysical Zone, consisting of three new formations: the Redan Gneiss, Ednas Gneiss and Mulculca Formation, plus the Lady Brassey Formation, part of the Thackaringa Group. The rocks are considered to belong to the lower part of the Willyama Supergroup and are not an older basement.

Although the Redan Geophysical Zone contains some rock types not found elsewhere in the Broken Hill Block, there are some lithological similarities with the lower part of the Willyama Supergroup: an abundance of albite‐rich rocks, the presence of quartz‐magnetite rocks with Cu and trace Co, and abundant amphibolite/ basic granulite in the Lady Brassey Formation.

The boundary between the Redan Geophysical Zone and the remainder of the Broken Hill Block appears to be conformable, with no evidence of major faulting. Similarly no evidence of unconformities or major displacement of stratigraphic boundaries has been found within the Redan Geophysical Zone. Structural history, fold style and orientation, and metamorphic grade within the Redan Geophysical Zone are similar to adjacent areas of the Broken Hill Block.

It is concluded that the Broken Hill Block contains no outcropping equivalent of the first cycle of sedimentary/ igneous rocks recognized in the Early Proterozoic of northern Australia.

Albite‐quartz‐hornblende‐magnetite rocks unique to the Redan Geophysical Zone most likely comprised detritus derived directly from an intermediate volcanic suite. Some were altered considerably, while other rocks retained the dacite/andesite composition, except for the addition of Na, an increase in the oxidation state, and partial leaching of some of the more mobile elements. These modifications could have taken place in shallow alkaline evaporitic lakes.

The Redan Geophysical Zone contains some of the elements of a foreland basin adjacent to a continental volcanic arc: a thick stratigraphic sequence, oxidizing evaporitic conditions, and intermediate volcanic detritus. The change from intermediate‐acid volcanism in the earliest formations, to bimodal acid/basic volcanism in the Thackaringa and Broken Hill Groups could correspond with a change from initial continental arc volcanism into bimodal rift volcanism. The case for the arc volcanism is weakened, however, by the relative scarcity of rocks with andesitic compositions and the lack of basaltic andesite compositions. The alternative is that the intermediate to acid volcanism represents only a variation on the later bimodal rift volcanism.     

A test of independence for stratigraphic sequences with respect to embedded Markov chains

Identification of cyclic sequences gives valuable insight into depositional associations of stratigraphic facies. An embedded Markov chain is a reasonable general model for facies transitions. But a model with independent random occurrences of facies is not an appropriate null hypothesis to be tested to show the presence of cycles because of definitional restriction in transition observations to only those between different facies. This is a common stratigraphic situation and the problem has been raised recently by several authors. We present here a test statistic for null hypothesis derived from the concept of partial independence and inherent to the model of embedded Markov processes.     

Eustatic control of late Quaternary sea-level change in the Arabian/Persian Gulf

Accurate sea-level reconstruction is critical in understanding the drivers of coastal evolution. Inliers of shallow marine limestone and aeolianite are exposed as zeugen (carbonate-capped erosional remnants) on the southern coast of the Arabian/Persian Gulf. These have generally been accepted as evidence of a eustatically driven, last-interglacial relative sea-level highstand preceded by a penultimate glacial-age lowstand. Instead, recent optically stimulated luminescence (OSL) dating suggests a last glacial age for these deposits, requiring > 100 m of uplift since the last glacial maximum in order to keep pace with eustatic sea-level rise and implying the need for a wholesale revision of tectonic, stratigraphic and sea-level histories of the Gulf. These two hypotheses have radically different implications for regional neotectonics and land–sea distribution histories. Here we test these hypotheses using OSL dating of the zeugen formations. These new ages are remarkably consistent with earlier interpretations of the formations being last interglacial or older in age, showing that tectonic movements are negligible and eustatic sea-level variations are responsible for local sea-level changes in the Gulf. The cause of the large age differences between recent studies is unclear, although it appears related to large differences in the measured accumulated dose in different OSL samples.     

Pressure and density gradients in H II Regions

Here we discuss the effects of large ambient pressures and decreasing density gradients on the observed properties of dusty UC hii regions. Dust absorption can effectively reduce the size of the photo ionized region, and density gradients can modify the spectral index of the emission. The effects of the density gradients seem to be present in both galactic and extragalactic HII regions.