A skeletal assemblage classification system for non-tropical carbonate deposits based on New Zealand Cenozoic limestones

Cenozoic limestones in New Zealand are mainly skeletal grainstones and packstones formed under non-tropical climatic conditions in open marine shelf or ramp environments. Following petrographic analysis of the nature and abundance of the skeletal components in nearly 500 samples of these limestones, a complete linkage cluster analysis identified seven major skeletal assemblages that may be regarded as subdivisions of the single foramol skeletal association defined by Lees and Buller (1972) for temperate-region carbonate deposits. The seven assemblages are given contracted names, as follows: (a) BARNAMOL = barnacle/bivalve-dominated; (b) BIMOL = bivalve-dominated; (c) BRYOMOL = bryozoan/bivalve-dominated; (d) ECHINOFOR = echinoderm/benthic foraminiferal-dominated; (e) NANNOFOR = nannofossil/planktonic foraminiferal-dominated; (f) RHODALGAL = calcareous red algal-dominated; and (g) RHODECHFOR = calcareous red algal/echinoderm/benthic foraminiferal-dominated. A composite triangular classification diagram has been devised for naming the skeletal assemblage of an unknown sample on the basis of its three main skeletal components. The diagram successfully characterises more than 85% of the New Zealand Cenozoic limestone samples and also appears to be applicable for the skeletal assemblage designation of many overseas examples of non-tropical carbonate deposits. Limitations relate mainly to locally common skeletal types (e.g. serpulids, brachiopods) that are presently not incorporated into the New Zealand-based scheme. The general ecological preferences of the main skeletal contributors in each of the seven skeletal assemblages form a basis for relating the assemblages to broad shelf habitats. Consequently, as well as the benefits of providing a more consistent skeletal assemblage terminology for comparative studies between different workers, the scheme can assist with the paleoenvironmental interpretation of non-tropical skeletal carbonate facies.     

Numerical models of quiescent normal polarity prominences

We present 2-D numerical models of quiescent solar prominences with normal magnetic polarity. These models represent an extension to the classical Kippenhahn-Schlüter model in that the prominence is treated as having finite width and height and the external coronal field is matched smoothly to the internal prominence field so that there are no current sheets at the prominence sides. Using typical prominence and coronal values we find solutions to the generalised Grad-Shafranov equation which illustrate the necessary magnetic support. We also discuss some extensions to the basic model.     

Magnetic instability of coronal arcades as the origin of two-ribbon flares

The generally accepted scenario for the events leading up to a two-ribbon flare is that a magnetic arcade (supporting a plage filament) responds to the slow photospheric motions of its footpoints by evolving passively through a series of (largely) force-free equilibria. At some critical amount of shear the configuration becomes unstable and erupts outwards. Subsequently, the field closes back down in the manner modelled by Kopp and Pneuman (1976); but the main problem has been to explain the eruptive instability.The present paper analyses the magnetohydrodynamic stability of several possible arcade configurations, including the dominant stabilizing effect of line-tying at the photospheric footpoints. One low-lying force-free structure is found to be stable regardless of the shear; also some of the arcades that lie on the upper branch of the equilibrium curves are shown to be stable. However, another force-free configuration appears more likely to represent the preflare structure. It consists of a large flux tube, anchored at its ends and surrounded by an arcade, so that the field transverse to the arcade axis contains a magnetic island. Such a configuration is found to become unstable when either the length of the structure, the twist of the flux tube, or the height of the island becomes too great; the higher the tube is situated, the smaller is the twist required for instability.     

A conceptual model of depositional,rather than erosional,tidal channel development in the rapidly prograding Skagit River Delta (Washington,USA)

The origin and growth of blind tidal channels is generally considered to be an erosional process. This paper describes a contrasting depositional model for blind tidal channel origin and development in the Skagit River delta, Washington, USA. Chronological sequences of historical maps and photos spanning the last century show that as sediments accumulated at the river mouth, vegetation colonization created marsh islands that splintered the river into distributaries. The marsh islands coalesced when intervening distributary channels gradually narrowed and finally closed at the upstream end to form a blind tidal channel, or at mid‐length to form two blind tidal channels. Channel closure was probably often mediated through gradient reduction associated with marsh progradation and channel lengthening, coupled with large woody debris blockages. Blind tidal channel evolution from distributaries was common in the Skagit marshes from 1889 to the present, and it can account for the origin of very small modern blind tidal channels. The smallest observed distributary‐derived modern blind tidal channels have mean widths of 0·3 m, at the resolution limit of the modern orthophotographs. While channel initiation and persistence are similar processes in erosional systems, they are different processes in this depositional model. Once a channel is obstructed and isolated from distributary flow, only tidal flow remains and channel persistence becomes a function of tidal prism and tidal or wind/wave erosion. In rapidly prograding systems like the Skagit, blind tidal channel networks are probably inherited from the antecedent distributary network. Examination of large‐scale channel network geometry of such systems should therefore consider distributaries and blind tidal channels part of a common channel network and not entirely distinct elements of the system. Finally, managers of tidal habitat restoration projects generally assume an erosional model of tidal channel development. However, under circumstances conducive to progradation, depositional channel development may prevail instead. Copyright © 2006 John Wiley & Sons, Ltd.     

Landscape allometry and prediction in estuarine ecology: Linking landform scaling to ecological patterns and processes

Spatial variation in landforms and associated physical processes can often be described by allometric scaling relationships, similar to those describing organismal allometry. Because plant and animal distribution, abundance, and behavior are generally affected, if not sometimes controlled, by the physical environment, landform scaling likely causes parallel scaling of ecological patterns and process across the landscape, i.e., landscape allometry. Organismal allometry has a long history, well-established tradition, and well-developed body of theory. Landscape allometry is a newly emerging conceptual framework that offers explanation for ecological patterns and utility for practical issues such as allowing landscape-scale replication of experimental and control treatments, providing landscape-scale predictions of ecological pattern and process, providing design guidelines for landscape management, and providing diagnostic methods for assessing historical anthropogenic effects to landscapes. Organismal and landscape allometry could be used in complementary fashion, and perhaps ultimately integrated, to form a useful theoretical framework for ecology.     

Photospheric line-tying conditions for the MHD stability of coronal magnetic fields

Using the localised, ballooning ordering, the effect of a density stratification on the ideal MHD stability of magnetic fields is investigated. It is found that, when the photospheric density is very much greater than the coronal value, the line tying conditons are best simulated by assuming that all coronal disturbances vanish at the photospheric boundary. This is commonly known as the rigid wall conditions.     

The scale size of chondrule formation regions: Constraints imposed by chondrule cooling rates

Abstract— Meteoritic data strongly suggest that most chondrules reached maximum temperatures in a range of 1650–2000 K and cooled at relatively slow rates of 100–1000 K/h, implying a persistence of external energy supply. The presence of fine‐grained rims around chondrules in most unequilibrated chondrites also indicates that a significant quantity of micron‐sized dust was present in chondrule formation regions. Here, we assume that the persistent external energy source needed to explain chondrule cooling rates consists primarily of radiation from surrounding heated chondrules, fine dust, and gas after the formation event. Using an approximate one‐dimensional numerical model for the outward diffusion of thermal radiation from such a system, the scale sizes of formation regions required to yield acceptable cooling rates are determined for a range of possible chondrule, dust, and gas parameters. Results show that the inferred scale sizes depend sensitively on the number densities of micron‐sized dust and on their adopted optical properties. In the absence of dust, scale sizes > 1000 km are required for plausible maximum chondrule number densities and heated gas parameters. In the presence of dust with mass densities comparable to those of the chondrules and with absorptivities and emissivities of ～0.01 calculated for Mie spheres with a pure mineral composition, scale sizes as small as ～100 km are possible. If dust absorptivities and emissivities approach unity (as may occur for particles with more realistic shapes and compositions), then scale sizes as small as ×10 km are possible. Considering all uncertainties in model parameters, it is concluded that small scale sizes (10–100 km) for chondrule formation regions are allowed by the experimentally inferred cooling rates.     

Environmental Models and Public Stakeholders in the Chesapeake Bay Watershed

The Chesapeake Bay is not only North America's largest estuary, but it is also home to one of most comprehensive computational modeling efforts that seeks to study and integrate a very large range of the complex socio-ecological dynamics within its watershed. Known as the Chesapeake Bay Modeling System (CBMS), this suite of models are invaluable to scientists, environmental policymakers, resource managers, and local government and community leaders, all of whom are engaged in efforts to reduce the negative impacts of population growth and development in the watershed on the ecosystems and living resources of the Chesapeake Bay. Until recently, the results of the CBMS were used to guide voluntary efforts to reduce nutrient and sediment runoff into the Chesapeake Bay. However, the results are now being used to guide and evaluate the implementation of Watershed Implementation Plans at the state and county levels, which in turn are based on CBMS estimates of total maximum daily loads (TMDLs) for these sub-watershed regions. The use of the CBMS for these regulatory efforts has also increased the number of public stakeholders who are now able to directly inquire about the how the CBMS works and raise questions about its ability to accurately represent their land use decisions and practices. This "going public" of the CBMS has raised many societal and cultural issues, including the articulation of local expert and scientific knowledge, and modelers, scientists, policymakers, and resource managers are now realizing the need to understand more of the human dimensions arising from the translation and implementation of the CBMS. A multidisciplinary approach is needed to understand these rapidly emerging societal and cultural dimensions of the expanded use of the CBMS. In this paper, we draw upon our collective experience as social and natural scientists, with modeling experience, to describe a range of social, economic, political, and cultural issues that have emerged as a result of the CBMS being used to support mandatory nutrient reduction regulations (TMDL).