Table 1: Soil chemical properties of the analysed depth profiles in the Venice lagoon

Visual traces of iron reduction and oxidation are linked to the redox status of soils and have been used to characterise the quality of agricultural soils.We tested whether this feature could also be used to explain the spatial pattern of the natural vegetation of tidal habitats. If so, an easy assessment of the effect of rising sea level on tidal ecosystems would be possible. Our study was conducted at the salt marshes of the northern lagoon of Venice, which are strongly threatened by erosion and rising sea level and are part of the world heritage 'Venice and its lagoon'. We analysed the abundance of plant species at 255 sampling points along a land-sea gradient. In addition, we surveyed the redox morphology (presence/absence of red iron oxide mottles in the greyish topsoil horizons) of the soils and the presence of disturbances. We used indicator species analysis, correlation trees and multivariate regression trees to analyse relations between soil properties and plant species distribution. Plant species with known sensitivity to anaerobic conditions (e.g. Halimione portulacoides) were identified as indicators for oxic soils (showing iron oxide mottles within a greyish soil matrix). Plant species that tolerate a low redox potential (e.g. Spartina maritima) were identified as indicators for anoxic soils (greyish matrix without oxide mottles). Correlation trees and multivariate regression trees indicate the dominant role of the redox morphology of the soils in plant species distribution. In addition, the distance from the mainland and the presence of disturbances were identified as tree-splitting variables. The small-scale variation of oxygen availability plays a key role for the biodiversity of salt marsh ecosystems. Our results suggest that the redox morphology of salt marsh soils indicates the plant availability of oxygen. Thus, the consideration of this indicator may enable an understanding of the heterogeneity of biological processes in oxygen-limited systems and may be a sensitive and easy-to-use tool to assess human impacts on salt marsh ecosystems.

Mineral and chemical composititon of sediments from ODP Hole 119-740A

ODP Hole 740A is located on the inner part of the East Antarctic continental shelf in Prydz Bay, at the seaward end of a major onshore rift structure known as the Lambert Graben. Drilling at this site led to the recovery of some 65 m of continental sediments (Prydz Bay red beds) that form part of a much thicker (2-3 km) pre-continental breakup sequence, the development of which may be related to the initiation and rifting of the Lambert Graben. Palynological and paleomagnetic studies have not been able to determine the age of the sediments; they may be equivalent to the onshore late Permian Amery Group or younger. The succession consists predominantly of sandstone, siltstone, and claystone arranged in erosively based, pedogenically influenced fining-upward sequences up to 5 m thick. These were deposited by shallow, braided streams draining an extensively vegetated alluvial plain, with sufficient topographic relief to trap fine-grained sediment and inhibit rapid channel shifting. Pedogenic processes were initiated on the alluvial plain, but climatic conditions were generally unsuitable for extensive pedogenic carbonate formation and the development of mature soil profiles. The sediments were probably derived from a rapidly uplifted fault block terrain composed of upper Proterozoic and Archaean gneisses lying to the southeast of the depositional site. Uplift may have taken place along the tectonically active seaward extension of the eastern faulted margin of the Lambert Graben, which passes immediately southeast of Hole 740A. Differences in mineralogical composition between the Amery Group and the Prydz Bay red beds probably reflect differences in rock composition in the source area. The age of the Prydz Bay red beds has still to be resolved.

Chemical composition of zeolites in sandstones at DSDP Sites 445 and 446

Andesitic volcaniclastic sandstones of Middle Eocene age recovered from DSDP Sites 445 and 446, a back-arc basin, contain clinoptilolite, heulandite and analcite as a pore-filling cement. Clinoptilolite and heulandite at Sites 445 and 446 contain different chemical composition from other deep-sea clinoptilolites and heulandites. The dominant cation of both clinoptilolite and heulandite is Na+ ion activity in pore water was greater at the time of their formation. Volcanic glass has acted as a precursor for both clinoptilolite and heulandite at Sites 445 and 446. Biogenic silica is also suggested as a precursor. High Na+ ion activity in pore water also helped to transform clinoptilolite and heulandite to analcite downhole. The necessary Na+ ions for this clinoptilolite, heulandite and analcite at Sites 445 and 446 might have been provided by hydrothermal circulation of seawater through more permeable sandstones deposited during early stages of back-arc basin rifting.