Western Australian Shelf Research Articles

Contrasting intensity of aragonite dissolution and dolomite cementation in glacial versus interglacial intervals of a subtropical carbonate succession

AbstractAragonite and high‐Mg calcite are abundant in modern, neritic carbonate systems but almost absent in their fossil counterparts. Dissolution of these metastable mineral phases commonly leaves no visible trace in the sedimentary record, compromising the derivation of palaeoenvironmental information from the rock record. The upper 25 m of Integrated Ocean Drilling Program (IODP) Site U1460 on the outer ramp of the western Australian Shelf were investigated to study shallow burial (tens of metres) marine diagenesis in organic‐carbon poor sediments using microscopic, total organic carbon, biomarkers and mineralogical analysis in combination with porewater geochemistry. Aragonite dissolution is negligible at the seafloor but intensifies ca 5 m below, even though bulk porewaters are supersaturated for aragonite. This apparent contradiction likely results from dissolution in undersaturated microenvironments. Aragonite dissolution below 5 to 6 m is on average more intense in interglacial compared to glacial intervals. The presence of disseminated framboidal pyrite and porewater results indicate that minor sulphate reduction is active at IODP Site U1460. Sulphate reduction is probably limited by the low organic matter content (ca 0.2%). It is well‐known from the literature that incipient sulphate reduction can lead to a drop in pH and consequently to carbonate dissolution. It is therefore assumed that the slightly higher concentration of organic matter in the interglacial intervals allowed increased aragonite dissolution during sulphate reduction compared to glacial beds. Low amounts of dolomite cement (<15%) start to form at the same depth (5 to 6 m) as aragonite dissolution intensifies. Dolomite formation and aragonite dissolution also show covariance on a metre‐scale below 5 to 6 m, indicating that a low carbonate saturation state might enhance dolomite formation. This mechanism provides an indirect link between dolomite formation, aragonite dissolution and orbital cycles. The outcome of this study, therefore, contributes to a better understanding of differential diagenesis in marine carbonates.

Influence of the subtropical ridge on directional wave power in the southeast Indian Ocean

AbstractChanges in the intensity and position of subtropical anticyclones are major drivers of wave climate around Australia. The quasi‐stationary ridge of high atmospheric pressure in the mid‐latitudes is known as the Subtropical Ridge (STR). In this study, we decompose the directional wave climate along the Western Australian Shelf (WAS) into three primary modes of variability using a statistical‐synoptic typing approach, and relate the wave power signatures of each to movements in the STR. We find a significant reduction in wave power along the WAS over the past 35 years. This is synonymous with an intensification of the STR resulting in blocking of powerful Southern Ocean Low wave conditions, a small increase in weaker anticyclonic waves, and a reduction in total wave power. A persistent intensification and/or poleward shift in the STR, associated with anthropogenic warming, may result in a continued decrease and anticlockwise rotation in wave power along the WAS. The approach taken in this article relates empirical changes in wave climate to movements in large‐scale climate drivers which can be used to complement the understanding of climate‐model based scenarios of wave climate change.

Diverse depositional and geochemical signatures of the Frasnian-Famennian global event in western Thailand reveal palaeotethyan vs. Western Australian geotectonic affinities

Two studied sections of the Frasnian-Famennian (F-F) boundary beds in western Thailand differ significantly in their depositional and geochemical characteristics. The highly condensed, monotonous Mae Sariang (MS) limestone succession generally corresponds to the event-chemostratigraphic pattern of the F-F biocrisis based primarily on German sections, and brief anoxic episodes are identifiable in trace-metal signatures (but neither organic-rich intercalations nor distinct volcanic signals). In the case of the Thong Pha Phum (TPP) site, two specific features are especially notable: (1) a δ13C positive excursion, comparable only with the peculiar Western Australian biogeochemical signature, and (2) elemental proxies that indicate exclusively oxic and probably largely oligotrophic conditions during both Kellwasser intervals. Therefore, the demonstrated biogeochemical and environmental differentiation confirms the totally different paleogeographic settings of the studied successions, as indicated previously by the Nd isotope composition of Late Devonian seawater. The distinctiveness of the TPP section clearly indicates its affinity with the Western Australian shelf successions, which are characterized by a well-known “atypical” biogeochemical and ecological signature. In terms of provenance, the siliciclastic fraction in the MS succession corresponds to continental margin. The Paleotethyan region was sourced by felsic-volcanic or granite-gneissic massifs. In the case of the TPP section, the continental margin, as well as the partly the continental island arc, are recorded in the detritus, predominantly derived from older, continental sedimentary-metasedimentary terrains. Therefore, the assumed Western Australian geotectonic assignment of TPP corresponds to the variable, partly recycled material supplied abundantly to the “Sibumasu depocenter” from adjoining granite-dominated Archean cratons.

Geomorphic classification of coral reefs in the north western Australian shelf

Coral reefs occur extensively along the northwest Australian continental shelf in the Kimberley Bioregion (KIM), forming major geomorphic features along and just off the coast. These reefs have not been studied in as much detail as the offshore reefs and are poorly known due to the coastal conditions, including extremely high tide regimes, high turbidity and complex coastline morphology. This study aims to establish a regional-scale distribution map of exposed and intertidal reefs of the KIM and to classify the Kimberley reefs into types, adopting widely recognised reef classification and typology schemes. Remote sensing and Geographic Information Systems (GIS) were used in this study to process and produce digital maps as well as to provide some of the first detailed spatial analysis of reef distribution. Outcomes of this study showed that the Kimberley reefs possess strong morphological complexity and clear regional patterns. The study revealed that the number of Kimberley reefs and their area are considerably (60%) greater than previously thought; the total combined reefal area is approximately 1,950 km2. Fringing reefs have been identified as the dominant reef type and are widely distributed throughout the KIM. It was also found that tidal range affected the distribution of reef geomorphologies. The outcomes of this study will contribute to a better understanding of the Kimberley reefs, and provide marine park managers with essential and quality scientific information so that better management decisions can be made in this area.

Seismic and stratigraphic evidence for reef expansion and onset of aridity on the Northwest Shelf of Australia during the Pleistocene

Modern reef (the Great Barrier Reef and Ryukyu Reef) distribution in the Indo-Pacific region is strongly controlled by warm currents (East Australian and Kuroshio Currents) that radiate from the Indo-Pacific Warm Pool. The modern distribution of reefs (south of 15°S) on the Western Australian shelf is related to the presence of the warm Leeuwin Current. However, the age of the reefs south of 15°S, and hence their temporal relationship to the Leeuwin Current, has been largely unknown. Seismic and subsurface stratigraphic data show that reef growth and expansion on the Northwest Shelf of Australia began in the Middle Pleistocene (∼0.5 Ma). The oldest ooids in the region are approximately synchronous with reef growth. We suggest a two stage process for the spread of reefs to higher latitudes on the Western Australian coast; first an increase in Leeuwin Current activity at approximately 1 Ma brought warm waters and a tropical biota to the region; and second, increased aridity after ∼0.6 Ma led to a decline in clastic input and increased alkalinity, triggering ooid formation and reef expansion to higher latitudes associated with the switch to higher amplitude glacio-eustatic cycles at the end of the Middle Pleistocene Transition. The timing and mechanisms for reef expansion south along the Western Australian coast has implications for the origin of the Eastern Australian Middle Pleistocene Great Barrier Reef, the New Caledonia Barrier Reef and Japanese Ryukyu Reef systems.

A simple numerical model for predicting vertical distribution of phytoplankton on the continental shelf

A simple one-dimensional ecosystem model of phytoplankton production and nitrogen recycling is used to predict the steady-state vertical distribution of phytoplankton on the continental shelf (i.e. water depths <100m). For an homogenously mixed water-column, close-to-uniform profiles of phytoplankton biomass are simulated in the model at mixing rates greater than 3×10−3m−2s−1. When mixing is reduced, sub-surface maxima develop either in mid-water or at the seabed depending on the rate of light attenuation. Nitrogen exchange between the water-column and seabed drives a positive feedback in the model between settling velocity and surface phytoplankton productivity. The model is tested against observations made along the Western Australian shelf. Despite the model's simplicity, from 42 profiles in depths between 20 and 100m, we are able to predict vertical distributions of phytoplankton and nitrate to within 1 standard deviation. The model is shown to reproduce observed cross-shelf gradients in depth integrated quantities.

A mixed-layer nutrient climatology of Leeuwin Current and Western Australian shelf waters: Seasonal nutrient dynamics and biomass

We present a seasonal climatology of the nutrient environment for waters off southwestern Australia with the intention of identifying spatial and seasonal characteristics of the nutrient environment and identify situations where the shelf may be susceptible to anthropogenic nutrient stress. The seasonal climatologies were generated from historical hydrographic data contained within the CSIRO Atlas of Regional Seas. The data presented here suggest the surface waters of the southwestern Australian shelf, the Leeuwin Current and offshore are all low in nitrogen (less than 0.5 μM) year round and that primary productivity is nitrogen limited. The shelf waters contain some dissolved phosphate, at relatively low levels (up to 0.25 μM) but diatom production may be limited by low levels of silicic acid (silicate) which are less than 2 μM year round. The Leeuwin Current is largely devoid of phosphate but contains reasonably high levels of silicate (up to 4 μM) and may be a silicate source to surrounding waters. A cross-shelf gradient in chlorophyll a biomass suggests that terrestrial nutrient sources make an important contribution to primary productivity. Offshore, a seasonal (wintertime) increase in chlorophyll a biomass coincides with a deepening of the mixed layer and is presumably supported by the mixing of deep water nutrients or chlorophyll from the deepwater maximum into the euphotic zone associated with this deepening. Further observations, particularly cross-shelf profiles from winter and profiles along the core of the Leeuwin Current, are required to fully separate the influence of the Leeuwin Current from other potential seasonal nutrient sources.

The functional morphology of Bentholyonsia teramachii (Bivalvia: Lyonsiellidae): clues to the origin of predation in the deep water Anomalodesmata

Abstract Bentholyonsia teramachii has hitherto been described from Japan, but a conspecific specimen is herein recorded from Western Australian shelf waters at a depth of ∼100 m. Hitherto placed in the Lyonsiidae, B. teramachii is herein relocated in the Lyonsiellidae, alongside a second genus –Lyonsiella. Features of the anatomy of Bentholyonsiateramachii, including highly sensory siphons, reduced ctenidia unable to collect and transport particulate food, non‐sorting labial palps, and a simplified Type II stomach and muscular gut all suggest that the species is a predator like Lyonsiella formosa. Other features of the anatomy, such as pallial taenioid muscles, radial mantle glands, and the structure of the ctenidia all point to a link between B. teramachii and the Lyonsiellidae and thus with the Verticordiidae and Parilimyidae and, therefore, with some of the oldest anomalodesmatans, the Pholadomyidae. In the possession by B. teramachii of a unique pair of antero‐dorsal suspensory muscles, the link with fossil pholadomyoideans is reinforced. Bentholyonsia thus represents another genus of predatory bivalve, not only expanding our view of the prevalence of this feeding mode in the deep sea by this class but also helping us show how it evolved, in anatomical terms, from a suspension‐ (or deposit‐) feeding ancestor.

A study of tropical cyclone influence on the generation of internal tides

An investigation is made into modifications of the semidiurnal internal tide on the Western Australian Shelf by a passing tropical cyclone. Current mooring observations are presented, taken during the passage of Tropical Cyclone Bobby (1995) over these mooring locations. The observations reveal large vertical excursions at near‐inertial frequency of the isotherm during and following the cyclone's passage. This coincides with diminishing semidiurnal baroclinic tides. To gain dynamical insight into the observations, a fully three‐dimensional, free surface, nonlinear, hydrostatic model is applied to the Western Australian Shelf. The model is initialized with realistic stratification and forced by representative tides and cyclone winds. The wind‐forcing is derived from an idealized analytical cyclone model prescribed from observed cyclone track and central pressure data, and the tidal forcing allows for the generation of internal tides. The cyclone modifies stratification by turbulent mixing, upwelling/downwelling and density advection up or down the shelf slope. This results in significant changes to internal tide characteristic paths and hence the internal tide generation process on the continental slope. Mixing is enhanced over the shelf due to shallow topography with respect to the shelf slope region creating a strong density front at the shelf edge. Observations and modeling show that the water column response to the cyclone passage is most abrupt in shallow water at 125 m, then at 300 m depth. The model reproduces the main features of the observed inertial oscillations concomitant with the dampening of M2 internal tides after the passage of the cyclone.