Shell Debris Research Articles

Formation of mud ridge and runnels in the intertidal zone of the Severn Estuary, UK

Flow-parallel linear bedforms, consisting of ridges and runnels have been reported widely in soft and consolidated cohesive sediments within intertidal and subtidal coastal waters. Less frequent are reports of these systems incised into soft bedrock. Despite their common occurrence there are few data concerning the hydrodynamic controls on the initiation and maintenance of ridge–runnel systems. Rather, several hypotheses have been advanced to account for them. In tidal environments a theory based upon the evolution of ‘streamwise vorticity’ transverse to the main flow direction is favoured, and has been supported recently by detailed hydrodynamic data obtained from above a ridge–runnel system on an intertidal zone of the Severn Estuary, UK. In the present paper, a detailed description of the sedimentology of the same ridge–runnel system is provided and interpreted within the framework of the ‘streamwise vorticity’ hypothesis. This objective is accomplished by measurements of the morphology and sampling the bulk characteristics of the sedimentary ridges and runnels but also with reference to ridges and runnels incised into soft bedrock beneath the sediment body. Grain-size composition, sediment compaction, radionuclide distributions and elemental distributions and X-radiography are used to describe the sediment structures within ridges and their inferred histories of development. These descriptions are interpreted with respect to the known hydrodynamic climate and the deposition and erosion behaviour of the sediments in laboratory experiments using an annular flume. Runnels contain liquid mud at low tide but contain very little fine gravel and shell debris. In contrast, fine suspended sediment is present at high concentrations in the flow. Thus fluid evorsion, aided by a dense suspension of fine sediment that impacts the mud bed, must be the primary tool to initiate and maintain the runnels, rather than scour by a coarse bedload. The ridges are composed primarily of fine silt or sand laminae some of which are approximately horizontal but most of which are slightly domed upwards such that they drape the ridge topography. This relationship between lamination and form demonstrates that the ridges are distinct accretion units and not residual masses of un-eroded planar-laminated mud incised by runnels. Laminae thicken, or thin and pinch-out, or are truncated, laterally at the sides of the runnels. Truncation occurs probably by damped spanwise turbulence structures which were recorded at this location. A notable characteristic is that the mud in the upper portion of the ridges is of low density and low compaction whilst the sediment in the lower part is more dense and well-compacted; often the two units are distinct, being separated by major reactivation surfaces; the latter indicative of occasional stripping of the surface sediments from above a residual ‘core’ at the base. The presence of coarse-grained laminae and lenses and minor reactivation surfaces throughout the sediment body indicates that high energy conditions, possibly other than the Neap–Spring cycle, are also important in developing the ridges. In this respect, consideration of the erosion potential of deposited sediment, using field and flume data, shows that the relatively low density, low-compaction mud constituting the crests of the ridges is not readily eroded by the present-day Neap–Spring tidal flow regime. Further, for the same flow regime, when the shear stresses on the bed fall below critical for suspension, potential deposition rates are very low, which supports the interpretation of the coarse-grained layers and reactivation surfaces as indicators of the significant role played by additional high-stress events, such as storm surges, in ridge-erosion and rebuilding.

Traces of Holocene tsunamis across the Sound of Lefkada, NW Greece

This paper gives evidence of multiple tsunami inundation of the Sound of Lefkada (NW Greece) since the mid-Holocene based on the analysis of sediment cores by means of geomorphological, sedimentological, geochemical, micromorphological and micropalaeontological methods. Layers of sand, gravel and shell debris, mostly unsorted, were found intersecting autochthonous lagoonal muds of the sheltered and quiescent inner-sound environment. They are further characterized by erosional unconformities at their base, rip-up clasts from the underlying sediments, fining upward sequences and an upward increase in sorting. The coarse grained high energy deposits include macro- and microfaunal remains typical of open-marine, partly even deep-water conditions which underlines their allochthonous character. Several distinct event layers indicate multiple tsunami passage across the sound. Earth resistivity measurements and vibracore transects revealed that the entire sound has been affected by catastrophic wave events. Thin sections of tsunami-influenced sediments found in lateral parts of the sound show a mixture of marine, lagoonal and terrigenous material. The inner Sound of Lefkada, well protected against storms, thus represents an excellent trap for tsunamigenic deposits. A preliminary local tsunami geochronology is based on 9 radiocarbon dates and diagnostic ceramic fragments. Several early tsunami impacts hit the sound between the 6th and 3rd millennium BC. Younger events seem to be consistent with tsunami landfalls that hit adjacent areas around 1000 cal BC, 395–247 cal BC as well as in Roman and medieval times. Vibracore data document an isthmus-like shallow-water environment which existed in the central sound and which was repeatedly inundated by tsunami wave action. Multiple tsunami passage eroded a natural channel which, we suggest, is the precursor of the famous waterway excavated by the Corinthians in the 7th century BC. Based on historical data, it is concluded that the navigable channel across the sound was repeatedly choked with sediments by tsunamigenic inundation during the following centuries. This study reveals a high tsunami risk for the Sound of Lefkada with at least one strong event in every 500–1000 years.

Post-glacial persistence of turbiditic activity within the Rhône deep-sea turbidite system (Gulf of Lions, Western Mediterranean): Linking the outer shelf and the basin sedimentary records

Emplacement of post-glacial turbidites is commonly controlled by rapid changes in sea level or by seismicity. On the continental rise of the Gulf of Lions (Western Mediterranean), an aseismic area, we identified turbiditic beds deposited during the rising stage and highstand of sea level. Swath bathymetry, sediment cores, in situ Cone Penetrating Tests (CPTU), heavy mineral associations and radiocarbon dating determined the source, composition, distribution and age of the turbiditic beds. Turbidites are composed of homogeneous to positively graded silts to medium sand with quartz (up to 90%), shell debris and shelfal benthic faunas. Their distribution on the sea floor is very patchy and controlled by abundant inherited erosional bedforms. Their source is found in relict regressive sands at the outershelf. Their deposition occurred just after the onset of the post-glacial sea level rise and the concomitant sediment starvation of the Rhône deep sea turbiditic system until recently. Whilst canyons are fed with sand by strong seasonal hydro-sedimentary dynamics on the outershelf, the emplacement of post-glacial turbidites is not controlled by sea level changes but probably by the periodic flushing of the canyons. Our study revealed that this low energy aseismic margin undergoes significant transport of sand, down to the base of slope, during the sea-level rise and the Holocene highstand.

An ENAA and PGAA comparative study of anoxic Black Sea sediments

A comparative study performed by means of both epithermal neutron activation analysis (ENAA) and prompt gamma neutron activation analysis (PGAA) concerning the distribution of 9 elements (Na, Al, Cl, K, Ca, Ti, Mn Fe and Sm) in unconsolidated sediments collected from the Black Sea anoxic region is presented. Principal component analysis (PCA) was used as a supplementary method in interpreting experimental data. The final results have shown that eight of them (Na, Al, Cl, K, Ti, Mn, Fe and Sm) have concentrations which differ by less than 30% from the corresponding concentrations of Upper Continental Crust (UCC), confirming the continental origin of sediments. At the same time, Ca concentration was about six times higher than that of UCC, this significant enrichment being due to the presence of the neighbouring oxygenated zone of considerable deposits of shell debris, characteristic of the Western Black Sea Continental Platform. Excepting Na and Mn, the concentrations of all other elements involved in this intercomparison were coincident within one standard deviation which attests the accuracy of this test. At the same time, PCA revealed the presence of five clusters, which could be put into concordance with the mineralogical composition of sediments, each of them consisting in equal proportions of elements determined by both methods.

Early geological investigations of the Pleistocene Tamala Limestone, Western Australia

Abstract The first geological studies of the Quaternary deposits, which crop out extensively along the coast of Western Australia, were carried out by members of English and French expeditions of discovery, between 1791 and 1836. The exploring parties included scholars with a background in geology, zoology and botany, as well as knowledgeable surgeons and sea captains with a strong interest in the natural sciences. Their collective work established the continuity, over vast distances, of a sequence of sedimentary rocks composed of quartz grains and shell debris, which today form the major part of the Tamala Limestone sequence. Their observations of the internal features of these rocks led some among them to develop views on the nature and origin of the cementing substance that bonds sand grains and shell debris in sedimentary layers and in concretions. There was disagreement among successive parties of visitors on the nature and origin of rhizoliths and other petrified woody matter in calcareous rocks. The finding of well-preserved sea shells in rocks now above sea level provided convincing evidence to investigators that the ocean had, in recent times, retreated from the land. The discovery of species of mollusc, known to be extinct in Europe, raised questions about an assumed worldwide extent of sedimentary sequences.

Decline of springtime abundance of the pileworm Neanthes succinea in relation to hydrographic conditions at the Salton Sea, California

The Salton Sea is the largest lake in California, and its most abundant macroinvertebrate is the pileworm (Neanthes succinea: Polychaeta). This is a major dietary item for three of the four most abundant fish species in the lake, and for at least one waterbird, the eared grebe (Podiceps nigricollis). Pileworm abundance in April 2004 was monitored at depths of 2, 6 and 10 m on seven transects distributed around the perimeter of the lake. Temperature and oxygen showed marked stratification with depth. Densities were compared to those observed in April 1999. Abundances at 2 m stations were similar to those in 1999; however, densities were greatly reduced at 6 and 10 m stations compared to 1999. The development of hypoxic or anoxic bottom conditions apparently occurred earlier in the spring in 2004 than in 1999. This was the most likely cause for the low densities at 6 and 10 m. Worm densities were higher at sites with coarser sediments (sand and/or barnacle shell debris), and lowest at 2 m sites near or downstream of freshwater inflows and at 10 m where near anoxic conditions prevailed. In March 2005, a sampling of 74 stations at depths ranging from 1 m to 10 m all around the perimeter of the lake found only a single pileworm. It was evident that the lake had just turned over and that sulfide levels were very high and had probably killed off most pileworms and many other organisms as well.

Neosabellaria vitiensis, n. sp. (Annelida: Polychaeta: Sabellariidae), from Shallow Water of Suva Harbor, Fiji1

A new species of the genus Neosabellaria Kirtley, 1994, is described from shallow-water locations of Suva Harbor, Fiji. Neosabellaria vitiensis Bailey-Brock, Kirtley, Nishi, & Pohler, n. sp., is a gregarious sabellariid; its tubes are constructed of sand and shell debris and form small “reefs” exposed during low tides. The new species is distinguished by the structure of opercular paleae in the middle row, which are shoe-shaped with circular tips, and paleae in the outer row, which have distal lateral teeth and denticulate median plume. Detailed morphological features of the new species are described and compared with other Pacific sabellariid species, particularly with most closely related N. clandestina (Menon & Sareen, 1966). Neosabellaria vitiensis is endemic to the Fiji Islands.

PURPLE‐DYE PRODUCTION IN LYCIA – RESULTS OF AN ARCHAEOZOOLOGICAL FIELD SURVEY IN ANDRIAKE (SOUTH‐WEST TURKEY)

Summary. In 2003 a substantial deposit of heavily fragmented shells of purple snails was by chance located at the site of Andriake, the ancient port of the Lycian city of Myra near Demre, in southern Turkey. The archaeozoological and archaeological record of a survey project in 2004 proved the identification of a large installation for the production of purple dye. Adjacent to a well preserved Hadrianic granary and partly superimposing the remains of a U‐shaped building, the deposit is wrapped around three sides of a subterranean cistern. Apparently dating to the sixth century AD, the deposit contains approximately 300 m3of shell debris. The unique chance of examining an undisturbed and well preserved manufacturing place for purple dye provides an excellent opportunity to investigate the processual particularities as well as the ecological impact of purple‐dye production in antiquity.

Meiobenthos in Nha trang bay of the south China sea (Vietnam)

The distribution of the taxonomical composition and the density of meiobenthos depending on the sediment type has been studied in bottom sediments of Nha Trang Bay. The maximal population density and the taxonomical diversity were observed in the silted coarse and heterogeneous sand (1031.4μ419.7 ind. 10 cm-2), whereas the minimal level of density and diversity (588.1μ152.5 ind. 10 cm-2) was in the coarse and heterogeneous sand with shell debris and corals. The correlation between the median diameter of sediment particles and population density of meiobenthos has been revealed (r=0.82, p<0.05). In bottom sediments of Nha Trang Bay, twenty six taxonomic groups of meiobenthos were observed. Nematodes dominated in all sediment types. Representatives of four orders, twenty eight families and ninety seven genera of marine nematodes were identified. The vertical distribution of meiobenthos in different sediment types was considered. A sediment column (10 cm height) was sectioned by five 2 cm portions. In the last layer (8-10 cm) the most number of meiobenthic groups was found in sandy sediments. In the lower layers of silt sediments, only nematodes were found.

Sequestration of Carbonate Shell Material in Coastal Dunes on the Gulf of California (Baja California Sur, Mexico)

Atmospheric and oceanographic conditions stimulate high productivity of marine organisms in the Gulf of California that are bulk producers of calcium carbonate. Due to prevailing winter winds, north-facing beaches receive vast amounts of shell debris derived from offshore clam banks above the 50-m isobath. As shells undergo mechanical abrasion, the smaller particles of carbonate material are transferred from beaches by the wind and sequestered in coastal dunes. This study reports on two dune fields from the midriff zone of the Gulf. Sieve analysis is used to describe the grain size and sorting characteristics of seven samples from dunes at Cerro El Gallo near Mulegé and 12 samples from dunes farther south near San Nicolás. Dune sediments also were impregnated with epoxy to simulate rock samples from which thin sections were made to determine composition and relative abundances of constituent grains. The dunes at Cerro El Gallo are carbonate poor (5–15%) compared with those near San Nicolás (26–51%) and possible factors contributing to regional variation are explored. Another part of this study appraises the fecundity necessary to produce any carbonate fraction integrated by coastal dunes from seashells. One of the region's more abundant clams, Megapitaria squalida, was used as a model to estimate the number of individuals of a given size and age class required to generate a cubic meter of pure carbonate sand. Satellite images of the dune fields at El Gallo and San Nicolás were viewed to map surface coverage. With input on the thickness of dune deposits and their composition, it is possible to roughly estimate the number of mature Megapitaria equal to the carbonate fraction of these dunes. The method may be applied to all coastal dunes on the Gulf of California and could be used to assess a part of the region's overall carbon budget heretofore unappreciated.

Will providing a filamentous substratum in the water column and shell litter on the bottom increase settlement and post-larval survival of the scallop Argopecten purpuratus?

The marked variability in the natural recruitment of Argopecten purpuratus, a common characteristic for many marine invertebrates with a pelagic larval stages, with important consequences for community functioning, is a problem for the fishery on this species. We ran experiments in the subtidal zone in Tongoy Bay, Chile, to test whether providing a filamentous settlement substratum in the water column and shell litter on the bottom would increase the settlement and post-larval survival of scallops. We placed collectors made of Netlon® 50 cm above the sand and mud bottoms, and three and a half months later there were significantly more scallop spat on the bottom under the collectors (38.5 ind m − 2 ), than in areas without collectors (0 ind m − 2 ), or in controls where collectors were installed but a bag around the collector prevented the juveniles from falling to the bottom (4.8 ind m − 2 ). Also, the addition of either entire or broken scallop shells to the bottom resulted in increased settlement of juveniles on the bottom (33.7 ind m − 2 with entire shells and 48.1 ind m − 2 with broken shells), compared to plots where no shell debris was added (0 ind m − 2 ). The 2 week survival rate of juveniles (< 3 mm shell height) added to plots covered with entire scallop shells (12.4%) and to plots covered with broken shells (15.1%) was greater than in plots where we did not add shells (3.5%). These results suggest that substrate availability explains spatial variability of recruitment for this species, while temporal variability (between years) is mainly the consequence of larval supply. The manipulation of substrates can locally increase settlement, but will not remove the temporal variability. Whereas our experiments provide useful insights into strategies for managing or establishing local scallop populations, experiments over a longer term and at a large scale are needed to further understand the community functioning in order to develop a strategy for managing this fishery resource.

Linear sand ridges on the outer shelf of the East China Sea

Based on the latest full-coverage high-resolution multi-beam sounding data, the distribution of the linear sand ridges on the outer shelf of the East China Sea (ECS) is studied with quantitative statistical analysis. The study area can be divided into the northeastern part and the southwestern part. Sand ridges in the northeastern area, trending 116°N, show obvious linear character and shrink to the inner shelf. Sand ridges in the southwestern area, trending 120°N–146°N, tend to have net form. Sand ridges gradually become sand sheets in the center part of study area. Sand ridges are distributed landward to the isobath of 60m, distributed seaward to the water depth of 120 m in the northeast and 150 m in the southwest. Immature sand ridges are observed at water depth of 130–180 m in the southwestern depressions. The acoustic reflection properties of the internal high-angle inclined beddings of the sand ridges are analyzed based on the typical seismic profiles close to the research area. Lithological analysis and dating of 4 boreholes and 12 cores indicate that the widely distributed transgressive sand layer with high content of shell debris which was formed in the early-middle Holocene is the main composition of the linear sand ridges on the outer shelf of the ECS. The dominating factor in formation, developing and burying of the sand ridges is the variation of water depth caused by sealevel change and the rate of sediment supply. In 12400 aBP the cotidal lines of the M2 tidal component were closely perpendicular to the strike-directions of the sand ridges in the study area, and the tidal wave system during 12000–8000 aBP might play a key role in the formation of the linear sand ridges which are widely distributed on the outer shelf of the ECS.

THE IMPORTANCE OF SMALL-SCALE HABITAT STRUCTURE FOR MAINTAINING BETA DIVERSITY

Marine soft sediments are often considered to be vast, homogeneous expanses of mud or sand; however, most exhibit small-scale biogenic structures. Generally such structures result from short-term processes, but mollusk shell debris can last for centuries, providing a settlement surface for plants and animals, which in turn stabilize the debris. This study sampled patches of shell debris and the surrounding sediment at increasing distances from the patches to determine the role of the patches in driving beta biodiversity. Sampling was conducted at three similar sites within 1 km in a marine reserve in New Zealand. The shell debris significantly affected beta diversity at all sites, although the magnitude of the effect was dependent on patch size, density, and debris particle size. These results have important implications for estimating and mapping biodiversity. They also have implications for marine conservation. The role of shell debris, suggested by this study, in increasing and maintaining biodiversity, emphasizes a need for reducing disturbance regimes and actively managing seafloor habitats in areas not previously considered for such.

Isotopic microanalysis of seawater strontium in biogenic calcite to assess subsequent rehomogenisation during metamorphism

Precise LAM-MC-ICPMS in situ microanalyses of 87Sr/ 86Sr ratios in biogenic calcite of the bivalve ‘ Atomodesma’ are reported for 14 horizons in low-grade metasediments of late Early Permian to late Late Permian age in South Island, New Zealand. ‘ Atomodesma’ shells (and associated skeletal echinoderm calcite) in zeolite and possibly to lower prehnite-pumpellyite mineral facies material yield isotopic data that are interpreted as primary seawater 87Sr/ 86Sr values. Data for higher metamorphic grade (probable upper prehnite-pumpellyite to lawsonite-albite facies) samples consistently show evidence of Sr isotopic homogenisation between ‘ Atomodesma’ calcite and host matrix and thus seawater 87Sr/ 86Sr values are not retained in shell debris under any circumstances.

Two new species of Agauopsis (Halacaridae, Acari) from Southeastern Brazil

Mites belonging to two new species of Agauopsis (Halacaridae, Acari) were obtained from seaweed, coarse sand and coarse shell debris taken from several sites in Northern Coast of São Paulo State. Agauopsis legionium sp. nov., which belongs to the brevipalpus group and Agauopsis itassussensis sp. nov., closely related to A. okinavensis Bartsch, 1986 are described.

Origin of rippled scour depressions associated with cohesive sediments in a shoreface setting (eastern Bay of Seine, France)

Side-scan sonar investigations in the eastern part of the macrotidal Bay of Seine have revealed the presence of numerous rippled scour depressions (RSDs) at water depths of 5–9 m. The sediments in these depressions consist essentially of coarse-grained shell hash derived from underlying Holocene sediments dated at roughly 6,500 years BP, and arranged in large wave-generated ripples. The shallow marine area where these features occur consists of a wave-generated ravinement surface produced during the marine flooding of the late Holocene transgression. It can be shown that, during the last 20 years at least, erosion of the muddy sand and sandy seabed has exposed underlying relict sediments. These consist of stiff clays, silts and a layer of shell debris which, when exposed, cover the bottom of large scour depressions which appear to be in equilibrium with the local hydrodynamic regime. Morphological and hydrodynamic data suggest that the RSDs are generated by strong cross-shore bottom currents flowing parallel to the features in the direction of the prevailing waves, and probably associated with storm-induced downwelling events.

The stability of tidal flats in Venice Lagoon—the results of in-situ measurements using two benthic, annular flumes

Seabed properties in Venice Lagoon were examined in situ in two multidisciplinary field campaigns. The purpose of this study was to understand the mechanisms controlling the stability of bed types. Two benthic annular flumes (Sea Carousel and Mini Flume) were deployed simultaneously from a floating pontoon at 24 sites during summer (1998), which were considered representative of the range in bed/habitat types. As well, bottom sampling and coring, water-column monitoring and benthic habitat analyses were carried out. All but three sites were on cohesive sediments. Bed types included bare shelly mudflats and regions colonised by the seagrasses Cymodocea nodosa and Zostera noltii, by filamentous cyanobacteria, and by patches of the macrophytes Ulva rigida and Chaetomorpha sp. A subset (13) of these sites was visited during the subsequent winter to evaluate seasonal changes. Six of the sites were intertidal, the remainder were in the sublittoral zone. Water temperature varied between 5 and 30 °C, and salinity varied between 20 and 38 psu. In the absence of waves, turbidity was generally low (<10 mg/l) and was composed of high amounts of organic matter (25–50%). This indicates that the tidal flows were not competent to support estuarine sediment. Higher levels of turbidity were measured during wind events or boat passage as a result of resuspension from the bed. Bed (saturated) density was, on average, 1770 kg/m 3, which was extremely high for estuarine sediments. Sea Carousel and Mini Flume provided comparable results, despite large differences in instrument footprints. Trends from the two instruments were similar and showed that summertime bed strength exceeded the winter by up to five times. Mean summer erosion thresholds for Sea Carousel and Mini Flume were 1.10 and 0.82 Pa, respectively, whereas during winter, they were 0.69 and 0.74 Pa. The northern lagoon had the most resistant tidal flats due to the stabilizing effect of filamentous cyanobacteria (Biostabilization Index: BI=244%), microphytobenthos (BI=153%) and Z. noltii (BI=206%). The stabilizing effects of C. nodosa (BI=74%), U. rigida (BI=115%) and shell debris (BI=115%) were intermediate, while bare sublittoral mud beds were the least resistant (BI=58%). Summer erosion rates (as a function of applied stress) were lower than winter ones, probably due to water temperature changes. The algorithm E= χτ s β , yielded good results and indicated that erosion rates in Venice Lagoon were high, notwithstanding the high erosion thresholds. The mean summertime friction coefficient was φ=62° and was highest in the central lagoon. The wintertime φ=69° showed that there was no seasonal fluctuation in bed stability. Mass settling in Venice Lagoon was a strong function of suspended sediment concentration ( S) and a decay constant ( k) of the exponential function S( t)= S o e − kt ; it was found to be in continuity with examples from other locations worldwide (and therefore normal).

Macrobenthic Distribution Patterns at a Sewage Disposal Site in the Inner Shelf off Mar del Plata (SW Atlantic)*

Abstract The inner shelf (5–15 m depth) off Mar del Plata (Argentina, Southwestern Atlantic) was studied to assess the influence of an intertidal sewage effluent on macrobenthic populations and to provide baseline information for a future monitoring program. Bottom samples were obtained with van Veen grabs at 49 stations. Macrobenthic organisms retained in a 0.5 mm mesh screen were used for biological analysis. Very fine sand-bottoms with a Natica–Owenia association occur in shallower sites, while patchy coarse sand bottoms plus gravel-sized shell debris inhabited by an association of patchy populations of Melita–Prionospio–Kalliapseudes–Caulleriella characterize the rest of the area. Low organic matter content in the area (from 0.78 to 2.05 %), and absence of any known indicator species (such as Prionospio spp.) in relation to distance to effluent, suggest that local benthic populations remain relatively undisturbed in spite of the heavy input of organic matter. The major factor contributing to this situ...

Sedimentology and geochemistry of an urban coastal lake system: Coombabah Lake Nature Reserve, Gold Coast, Queensland

A study was initiated to address environmental concerns associated with changes in land use in the catchment area of Coombabah Lake, a brackish coastal lake system located in southeast Queensland. Sedimentological and geochemical data derived from a series of cores that penetrate the ca. 0–6000 year‐old lacustrine sequence indicate that throughout much of its history, Coombabah Lake has remained a quiet, shallow, water body fed by fine‐grained sediment dropped from suspension. Discrete and laterally continuous, shelly horizons form the basis for the stratigraphy developed for the lake sequence. A lithological transition in the upper 50 cm of the sediment column, from mud to shell‐rich, sandy mud, suggests that the depositional regime changed ∼500 years ago as accommodation space was filled. At this time, the dominant sedimentary processes changed to include reworking, partial bypass of sediment, the intermittent concentration of shell debris by winnowing, and the development of a flood‐tidal delta complex in the lower reaches of the lake. Sharp increases in total organic carbon, total nitrogen, and total phosphorus concentrations below the lithological transition at 50 cm depth suggest that the capacity of the system to preserve organic matter and trap nutrients decreased significantly after the transition in depositional regime. Results indicate that current conditions in Lake Coombabah are governed by natural processes associated with the long‐term evolution of coastal lake systems, and reveal little to suggest that there were any adverse affects resulting from recent human activities in the surrounding catchment.

Spatial distribution of sedimentary P pools in a Mediterranean coastal lagoon ‘Albufera d’es Grau’ (Minorca Island, Spain)

The method for the sequential extraction of P proposed by Jensen et al. [Mcglathery, K.J., Marino, R., Howarth, R.W. (1998) Limnol. Oceanogr. 43, 799–810] was used to study the spatial distribution of sedimentary P in superficial sediments of a mesohaline coastal lagoon located in a watershed formed by carboniferous sandstone in the Western Mediterranean (Minorca Island, Spain). Dissolved inorganic phosphate (IP), dissolved organic phosphate (OP), Fe, Ca, Al, and F were analyzed in the extractions to assess adequacy of this method to clay sediments. The elemental composition of the solid phase (Al, Fe, K, Ti, Si, Mg and Ca) was also analyzed to relate concentrations of P pools to the mineral composition. Samples from marine carbonated sediments, rock and some materials of biological origin (tubes of polychaete Ficopomatus enigmaticus, bivalve shell debris) were analyzed for comparative purposes. The sequential procedure allowed to extract almost all sedimentary P from carbonate sediments and biogenic debris, but only 70% of total phosphorus (TP) from clay sediments and rock. Main IP pools in the lagoon were Fe-bound P (16.6% of TP), CaCO 3-bound P mainly from Ficopomatus tubes (12.0% of TP), and detrital carbonate fluorapatite (7.8% of TP). The most abundant P pool was refractory P (20.3% of TP), which appeared associated to the (Fe,K)Al-silicate fraction and to humic P. This indicated that clay–humic–organic P complexes were the main P reservoir in the lagoon. Spatial distribution of P pools reflected differential sedimentation of allochthonous materials, authigenic precipitation of Fe-oxides, and Fe-bound P as well as the differential distribution of organisms such as Ficopomatus.