Lacustrine sedimentary record of human-induced gully erosion and land-use change at Providence Canyon, southwest Georgia, USA

Providence Canyon, one of a series of large gullies in the upper Coastal Plain of the southeastern United States, formed as a result of deforestation and agricultural development in the early 1800's. Sediment eroded from the canyon aggraded the floodplain downstream, dammed tributary valleys, and formed North and South Glory Hole lakes (NGH, 4.8 ha and SGH, 2.5 ha). Sedimentary sequences in these lakes include a basal unit (I) of layered sand and clayey-sand overlain by 0.05-0.1 m of mud, fine sand and organic matter, with large fragments of wood (Unit IIa). An upper unit (IIb) 0.29-1.61 m thick consists of silt and clay containing discrete layers of sand. We interpret Unit I as floodplain alluvium deposited before the lakes were dammed, Unit IIa as sediment deposited during the early phase of the lakes when detritus from trees killed by flooding was abundant, and Unit IIb as lacustrine mud deposited after lake levels stabilized, with periodic pulses of sand eroded from Providence Canyon introduced to the lake by backflooding events. Basal dates extrapolated from a ²¹⁰Pb chronology for the upper part of SGH core suggests that development of the canyon and formation of the lakes began in the 1840's, and that lake levels stabilized by about 1880. Although consistent with historical accounts of the age of Providence Canyon, these dates must be considered as approximate because of uncertainty in extrapolating dates to the bottom of the core.In contrast, the ²¹⁰Pb chronology for the upper portion of the core (post 1930) is validated independently using historical climatic records, and indicates that lacustrine sedimentation faithfully records recent land-use change visible in historical aerial photography. An 8-fold increase in lacustrine sedimentation occurred after the clearing of forest near SGH in the 1930's and 1940's, and a 1.5-fold increase occurred because of road construction in the l950's. Individual sand layers deposited between 1830 and 1957 correlate with erosion at Providence Canyon during major storm events. Since then, downcutting and headward incision by the stream draining Providence Canyon have reduced backflooding to the Glory Hole lakes. As a result, the thickness of individual sand layers decreased, although increases in mass sedimentation rates for mud correspond more directly with large precipitation events after channel incision cut off the major source of sediment from Providence Canyon. The results of this study illustrate the value of the lacustrine sedimentary record in assessing geomorphic, climatic, and human-induced environmental change in heavily disturbed landscapes.     

Off-shore seismological experiments south of Provence in relation to “Glomar Challenger” deep sea drillings (JOIDES-DSDP,Leg 13)

The evaporitic layer located at 35 km south of Toulon, according to compressional-wave velocity measured in samples from JOIDES-DSDP Leg 13 drillings in the Mediterranean Sea, could be constituted by gypsum or anhydrite, halite having been deposited in the most central parts of the basin. These deductions are in favour of the supposed “bull's eye” structure for evaporitic layers in the Mediterranean Sea.     

Sediment community oxygen consumption in the deep Gulf of Mexico

Sediment community oxygen consumption (SCOC) has been measured from the continental shelf out to the Sigsbee Abyssal Plain in the NE Gulf of Mexico (GoM). SCOC rates on the continental shelf were an order of magnitude higher than those on the adjacent continental slope (450–2750 m depth) and two orders of magnitude higher than those on the abyssal plain at depths of 3.4–3.65 km. Oxygen penetration depth into the sediment was inversely correlated with SCOC measured within incubation chambers, but rates of SCOC calculated from either the gradient of the [O₂] profiles or the total oxygen penetration depth were generally lower than those derived from chamber incubations. SCOC rates seaward of the continental shelf were lower than at equivalent depths on most continental margins where similar studies have been conducted, and this is presumed to be related to the relatively low rates of pelagic production in the GoM. The SCOC, however, was considerably higher than the input of organic detritus from the surface-water plankton estimated from surface-water pigment concentrations, suggesting that a significant fraction of the organic matter nourishing the deep GoM biota is imported laterally down slope from the continental margin.     

Multivariate Analysis of an LA-ICP-MS Trace Element Dataset for Pyrite

Application of multivariate statistics to trace element datasets is reviewed using 164 multi-element LA-ICP-MS spot analyses of pyrite from the Moonlight epithermal gold prospect, Queensland, Australia. Multivariate analysis of variance (MANOVA) is used to demonstrate that classification of pyrite on morphological and other non-numeric factors is geochemically valid. Parallel coordinate plots and correlation cluster analysis using Spearman??s coefficients are used to discover unexpected elemental relationships without making assumptions a priori. Finally, principal component analysis and factor analysis are used to demonstrate the presence of sub-classes of pyrite. Corroborated with geological data, statistical analysis provides evidence for successive generations of hydrothermal fluids, each introducing specific metals, and for partial or complete replacement of different minerals. The data permit reinterpretation of Moonlight as a telescoped system where epithermal-Au (± base metals) is superposed onto early porphyry-Mo mineralization.     

Mangrove (Avicennia marina subsp. australasica) litter production and decomposition in a temperate estuary

Mangrove forests can provide important cross-boundary subsidies of organic matter to adjacent habitats through the production, export, decomposition and assimilation of litter. We quantified two of these components in a temperate mangrove forest in Whangamata Harbour, New Zealand: 1) litter production; and 2) decomposition rates as a function of tidal elevation, sediment type and burial depth. Litter traps sampled monthly for a year measured an annual detrital input of 3.24–5.38 t DW ha^?1, of which 77% occurred in summer. Decomposition rates depended on litter type, with leaves decomposing faster (63 d to decay by 50%) than pneumatophore and wood material (316 and 460 d, respectively). Buried leaf and wood litter decomposed 1.3–1.4 times slower than litter on the sediment surface; however, tidal elevation and sediment type (mud vs. sand) had no effect. The slow decay of litter (an order of magnitude slower than tropical mangrove litter) suggests that its incorporation into temperate marine food webs may be relatively slow.     

Snapper (Chrysophrys auratus): a review of life history and key vulnerabilities in New Zealand

Snapper (Chrysophrys auratus) is an important coastal fish species in New Zealand for a variety of reasons, but the large amount of research conducted on snapper has not been reviewed. Here, we review life history information and potential threats for snapper in New Zealand. We present information on snapper life history, defining stages (eggs and larvae, juvenile and adult), and assess potential threats and knowledge gaps. Overall we identify six key points: 1. post-settlement snapper are highly associated with certain estuarine habitats that are under threat from land-based stressors. This may serve as a bottleneck for snapper populations; 2. the largest knowledge gaps relate to the eggs and larvae. Additional knowledge may help to anticipate the effects of climate change, which will likely have the greatest influence on these early life stages; 3. ocean acidification, from land-based sources and from climate change, may be an important threat to larval snapper; 4. a greater understanding of population connectivity would improve certainty around the sustainability of fishery exploitation; 5. the collateral effects of fishing are likely to be relevant to fishery productivity, ecosystem integrity and enduser value; 6. our understanding of the interrelationships between snapper and other ecosystem components is still deficient.     

Terrace reconstruction and long profile projection: a case study from the Solent river system near Southampton,England

River terrace sequences are important frameworks for archaeological evidence and as such it is important to produce robust correlations between what are often fragmentary remnants of ancient terraces. This paper examines both conceptual and practical issues related to such correlations, using a case study from the eastern part of the former Solent River system near Southampton, England. In this region two recent terrace schemes have been constructed using different data to describe the terrace deposits: one based mainly on terrace surfaces; the other on gravel thicknesses, often not recording the terrace surface itself. The utility of each of these types of data in terrace correlation is discussed in relation to the complexity of the record, the probability of post-depositional alteration of surface sediments and comparison of straight-line projections with modern river long profiles. Correlation using age estimates is also discussed, in relation to optically stimulated luminescence dating of sand lenses within terrace gravels in this region during the PASHCC project. It is concluded that the need for replication at single sites means that this approach has limited use for correlative purposes, although dating of sediments is important for understanding wider landscape evolution and patterns of human occupation.     

Dissipative structures in a nonlinear dynamo

This article considers magnetic field generation by a fluid flow in a system referred to as the Archontis dynamo: a steady nonlinear MHD state is driven by a prescribed body force. The field and flow become almost equal and dissipation is concentrated in cigar-like structures centred on straight-line separatrices. Numerical scaling laws for energy and dissipation are given that extend previous calculations to smaller diffusivities. The symmetries of the dynamo are set out, together with their implications for the structure of field and flow along the separatrices. The scaling of the cigar-like dissipative regions, as the square root of the diffusivities, is explained by approximations near the separatrices. Rigorous results on the existence and smoothness of solutions to the steady, forced MHD equations are given.