Elaine K. Baker

A preliminary study of sedimentation in the tidally dominated Fly River Delta, Gulf of Papua

High resolution seismic profiles, surface sediment samples and cores document sedimentary facies of the tidally dominated Fly River Delta. Tidal currents dominate in the transport of sandy sediments throughout the Fly Estuary. On the delta front (5–17 m water depth) surface waves generated by southeasterly trade winds rework the muds and sands to winnow out the fine grained sediments, from March to November. During the northwest monsoon (December–March), minimal surface wave activity results in the deposition of a mud drape, resulting in seasonal sand-mud interbeds (varves). Prodelta deposits extend below this depth from 17 to 45 m and contain massively bedded muds accreting at a mean rate 4 cm year−1. A facies model for the deltaic sequence is presented and employed to derive a preliminary sediment budget for the delta. The budget demonstrates that of the 85 million tonnes year−1 of sediment discharged by the Fly, about 47 million tonnes year−1 is deposited in the delta area adjacent to the months of the Fly River and less than 2% is deposited in Torres Strait to the south. Suspension load transport removes sediments northeastwards into the Gulf of Papua and westwards along the coast, whereas fluid muds may remove sediments eastwards onto the shelf and, possibly, to the adjacent Coral Sea Basin. The budget also demonstrates that a ∼38 m vertical section, encompassing distal delta, prodelta and delta front facies, is deposited in about 1100 to 1600 years. The application of the facies model in interpreting other tidally dominated deltaic deposits is discussed.

Late Quaternary Deltaic and Carbonate Sedimentation in the Gulf of Papua Foreland Basin: Response to Sea-Level Change

ABSTRACT The rivers that drain the wet, mountainous island of New Guinea discharge about 1.5 billion tonnes/yr of sediments into the adjacent seas, including the foreland basin between New Guinea and Australia. Despite this huge sediment input, there appears to have been only limited deposition in the Gulf of Papua during the (Holocene) postglacial rise in sea level. Seismic and core data indicate that the transgressive systems tract in the Gulf of Papua is thin and patchy. It is confined to regions within and north of an incised, east-west-trending shelf-valley system. Of the possible explanations for the absence of a significant transgressive systems tract, inland storage and along- and off-shelf transport of the sediment are of greatest significance. Reef growth up to the latitude of the east-west-trending incised-valley system in the southern Gulf of Papua is considered to have been facilitated by a northward-flowing coastal boundary current, the Coral Sea Coastal Current. This current now sweeps turbid, brackish waters and terrigenous sediments discharged by the rivers northwards away from the reefs. An observed northward offset in transgressive sediments in relation to the axis of the shelf valleys suggests that such a northward.flowing shelf current operated during the late Pleistocene and early Holocene. The northern limit of the Great Barrier Reef could thus be controlled by the balance between fluvial sediment supply and northward advection of suspended sediment by the Coral Sea Coastal Current. This current may also be imp rtant in maintaining a supply of clear water to the eastern Gulf of Papua, thus enabling photosynthesis and the flourishing of calcareous-algae (Halimeda) bioherms or biostromes at depths of up to 100 m over much of the middle and outer shelf, directly offshore of the modern Fly Delta. These carbonate sediments represent the exposed maximum flooding surface and condensed section. Modern highstand delta deposits have begun to prograde over this layer on the inner shelf.

Copper, lead, and zinc distribution in the sediments of the Fly River Delta and Torres Strait

This study is concerned with the distribution of copper, lead, and zinc in the sediments of the Fly Delta an adjacent continental shelf. In the present study metal concentrations determined for sedimentsfrom both the deltaic and shelf environments are presented.

The nature of sediments forming the Torres Strait turbidity maximum

An area of persistently high relative water turbidity occurs in LANDSAT images from central Torres Strait. Analysis of water, suspended particulate and seabed sediment samples indicates that this turbidity maximum comprises mainly silt‐sized carbonate skeletal fragments derived from the erosion of surficial seabed deposits. Fluvial sediment is characterized by kaolinite, which only occurs in waters adjacent to the Fly Delta and in isolated seabed deposits of the southern Great North East Channel. Since montmorillonite is absent from the suspended sediment, erosion of montmorillonite‐bearing Pleistocene clay, which may crop out on the Torres Strait seabed, is unlikely to be an important source of the observed turbidity. A physical model, based upon frictional decay of tidal energy, is proposed to explain the occurrence of the turbidity maximum in Torres Strait.

Aquatic Ecological and Human Health Risk Assessment of Chemicals in Wet Weather Discharges in the Sydney Region, New South Wales, Australia

Sydney Water has completed a risk assessment to assess the risks to human health and aquatic organisms in creeks, rivers, estuaries and ocean waters affected by wet weather sewage overflows, stormwater and sewage treatment plant discharges. The risk assessment methodology consists of a comparison of measured and predicted concentrations of chemicals with toxicity reference values. Estimates of receiving water chemical concentrations were derived using data from a 10-year period so that the variable rainfall pattern was represented. Computer models were used to simulate and predict wet weather discharges during this ten year period. Risks were validated by bioassays and bioassessments. Risks to aquatic life from wet weather discharges were attributed to 14 chemicals at one or more of the sites and stormwater was the predominate source of the chemicals. There were no risks to people engaged in water based activities. Noncarcinogenic risks from fish ingestion are predicted at three sites. Predicted cancer risks for most individual chemicals were relatively small. Carcinogenic risks were typically associated with organochlorine compounds, polycyclic aromatic hydrocarbons, dichlorobenzene, and arsenic. The predicted cancer risks also appear to be largely the result of stormwater rather than sewage overflow inputs and largely due to historical contamination by organochlorine pesticides. It is expected the concentrations of these chemicals will decrease over time.

Why Map Benthic Habitats

Publisher Summary This chapter provides a synthesis of seabed geomorphology and benthic habitats based on up-to-date information contained in the case studies. It examines the drivers that underpin the need for benthic habitat maps, including threats to benthic habitats. Benthic habitat maps are spatial representations of physically distinct areas of seabed that are associated with particular species, communities, or assemblages that consistently occur together. Habitat maps can illustrate the nature, distribution, and extent of distinct physical environments and importantly, they can predict the distribution of the associated species and communities. The data sets collected for constructing habitat maps provide fundamental information that can be used for a range of management and industry applications, including the management of fisheries, spatial marine environmental management, design of marine reserves, supporting offshore oil and gas infrastructure development, port and shipping channel construction, maintenance dredging, tourism, and seabed aggregate mining. Seafloor habitat mapping provides fundamental baseline information for decision makers working in these sectors. GeoHab is an international association of marine scientists conducting research using a range of mapping technologies in the study of biophysical indicators of benthic habitats and ecosystems as proxies for biological communities and species diversity. Using this approach, combinations of physical attributes of the seabed identify habitats that have been demonstrated to be effective surrogates for the benthic communities. Seafloor geomorphology is one of the most useful physical attributes of the seabed mapped and measured by GeoHab scientists.

Physical properties of sewage particles in seawater

Samples of primary-treated effluent were examined by laser particle sizer, photographic image-analysis and in settling/rinsing experiments in order to determine particle sizes and falling/rising speeds through sea-water. Using a 1.7 m column and 24 h experiments, it was found that approximately 22% of the particles settle out of suspension, 3% rise to the surface and 75% remain in suspension. Between 80 and 100% of the falling particles reach the bottom in less than 6 h. A good correlation (r=0.95) was found between the cumulative mass of particles at the base of a settling column and particle falling velocity. In contrast, a poor correlation (r=0.57) was found between median particle size and falling velocity. The falling particle sizes did not follow a log-normal distribution as might be expected. This is probably explained by the fact that larger particles are composed of flocculated material and vary in density from the smaller particles. Particles with a diameter of < 80 μm make up 90% of the mass of material that sinks. A fairly good correlation was found between percent cumulative mass and rising velocity (r=0.74). However, the relationship between particle size and rising velocity is more complex. There are three main categories of rising particles: 150 μm. The smallest class of particles (< 52 μm) are by far the most abundant, comprising on average 95% of the total number of rising particles. The small particles show a weak logarithmic relationship between size and rising velocity (r=0.53). The mid-size particles (52–150 μm) comprise approximately 4% of the total number of particles and are the most difficult to categorize as there appears to be no relationship between particle size and rising velocity. The large size particles, although the least abundant (∼ 1%), are the most predictable; there is a good correlation between particle size and rising velocity (r=0.77). This is probably because the particles are fairly homogeneous and largely composed of oil and grease.