Search

Coastal upwelling is important for marine ecosystems and the economy, because of its elevated primary and secondary productivity and large potential for fish catch. This study developed a scale-independent and semi-automatic image processing technique to map the upwelling areas along the 4500 km south-eastern coast of Australia from 14-year monthly MODIS Sea Surface Temperature (SST) data. The results show that there is significant spatial variability in the mapped upwelling areas, month to month, season to season and year to year. There is also strong temporal (month to month, seasonal and inter-annual) variability of the upwelling characteristics in area of influence, SST anomaly, chlorophyll-a concentrations and upwelling speed. This study identifies two prominent upwelling systems, the NSW system along the coast of New South Wales and the WVIC/SA system along the coast of western Victoria and adjacent South Australia. The NSW coastal upwelling system occurs more or less continuously from austral spring to autumn. The WVIC/SA coastal upwelling system is a seasonal upwelling system occurring in the austral summer. The NSW coastal upwelling system has a stronger upwelling intensity than the WVIC/SA system, in terms of area of influence, SST anomaly, chlorophyll- a concentrations and upwelling speed. We believe that the NSW coastal upwelling system, especially the northern and central parts, is mainly driven by the East Australian Current (EAC) and its eddies; while, the WVIC/SA coastal upwelling is a typical wind-driven system. In addition, the results indicate that the El Nino Southern Oscillation (ENSO) events are likely to have a low-to-moderate impact on both the NSW and the WVIC/SA coastal upwelling systems. The El Nino (La Nina) events tend to strengthen (weaken) upwelling intensity. • A scale-independent and semi-automatic upwelling mapping technique is developed. • There is strong temporal variability of the upwelling characteristics. • Two persistent upwelling systems off south-eastern Australian coast are identified. • ENSO events are likely to have a low-to-moderate impact on both upwelling systems. [ABSTRACT FROM AUTHOR]

Abstract Northeastern Queensland is a far-field site from ice caps formed during the Last Glacial. However, the region has experienced glacio-hydroisostatically driven coastal elevation change during the Holocene that generated a distinctive relative sea-level record. We tested whether this subtle vertical movement of the coast is recorded in the morphology and stratigraphy of the beach-ridge strandplain at Cowley Beach, which comprises a series of 36 prominent and several subdued or truncated beach ridges. Previous studies have shown that the strandplain records the long-term progradation of the coast, with the ridges comprising coarse-medium sand that was deposited during both fairweather (foreshore and berm facies) and tropical cyclone events (erosional unconformities; storm-surge deposits) over the past approximately 7 ka. We utilised a fine-scale digital elevation model of the strandplain to examine beach ridge morphology and to compare the elevation pattern with published Holocene sea-level data for the region. The overall elevation pattern of the strandplain surface is consistent with the regional sea-level history, that includes a highstand at around 6 ka BP generated by isostatic uplift, and the elevation of the berm facies matches well robust coral sea-level indicator data. We also modelled the strandplain topographic patterns that would result under stable and continually rising Holocene sea-levels. The frequencies of elevation classes across the strandplain are normally distributed when the strandplain is deposited during a period of stable sea level, and negatively skewed with a continually rising sea level, consistent with the expected patterns. The results indicate the sensitivity of the elevation of the top of the berm facies preserved in beach ridges to subtle changes in sea level (tens of cm over a thousand years). Our results further underline the utility of beach-ridge strandplains, a globally distributed coastal landform, for building relatively detailed regional records of sea level during the Holocene at sites currently lacking this information. Highlights • Holocene coastal isostatic uplift is recorded in beach ridges at Cowley Beach. • Beach ridge elevation records a sea-level highstand at 6 ka BP. • Rising and stable sea-level regimes produce distinctive strandplain elevation patterns. • These deposits can provide many more detailed regional sea-level records. [ABSTRACT FROM AUTHOR]

The approach and results of our 2019 paper (Brooke et al., 2019) have been criticised by Dougherty (2022). We examined the sensitivity of a tropical, coarse sand strandplain to changes in relative sea level as expressed through beach-ridge morphology and the elevation of beach berms preserved in the succession of cyclone-generated beach ridges at Cowley Beach, Queensland, Australia. Rather than presenting a new sea-level curve for this region as suggested by Dougherty, our 2019 study of the Cowley Beach strandplain revealed that when compared with the modelled Holocene relative sea-level record and sea-level indicator data for north-eastern Queensland, the strandplain morphology and stratigraphy provide a first-order representation of sea-level position. Importantly, Dougherty (2022) has overlooked the processes involved in the formation of beach ridges at this site and the spatial coverage of our analysis of the strandplain morphology. Both these factors are fundamental to the approach and conclusions of our 2019 paper. We reiterate here that the Cowley Beach strandplain morphology and succession of beach-berm deposits have elevation patterns that match well the broad trend in the regional relative sea-level model for the past 7 ka. We highlight those key beach-ridge formative processes and data limitations neglected in the Dougherty Comment. Finally, we note the challenges and opportunities for further research into the sedimentary records preserved in this distinctive type of tropical strandplain. • First-order changes of sea level can be detected in the Cowley Beach strandplain. • The coarse sandy ridges comprise a berm capped by cyclone-generated surge deposits. • Processes forming these ridges are in marked contrast to temperate strandplains. • There is negligible aeolian deposition contributing to these low-relief ridges. • Uncertainties in ridge elevation, age and GPR data limit sea-level reconstruction. [ABSTRACT FROM AUTHOR]

Seabed sediment predictions at regional and national scales in Australia are mainly based on bathymetry-related variables due to the lack of backscatter-derived data. In this study, we applied random forests (RFs), hybrid methods of RF and geostatistics, and generalized boosted regression modelling (GBM), to seabed sand content point data and acoustic multibeam data and their derived variables, to develop an accurate model to predict seabed sand content at a local scale. We also addressed relevant issues with variable selection. It was found that: (1) backscatter-related variables are more important than bathymetry-related variables for sand predictive modelling; (2) the inclusion of highly correlated predictors can improve predictive accuracy; (3) the rank orders of averaged variable importance (AVI) and accuracy contribution change with input predictors for RF and are not necessarily matched; (4) a knowledge-informed AVI method (KIAVI2) is recommended for RF; (5) the hybrid methods and their averaging can significantly improve predictive accuracy and are recommended; (6) relationships between sand and predictors are non-linear; and (7) variable selection methods for GBM need further study. Accuracy-improved predictions of sand content are generated at high resolution, which provide important baseline information for environmental management and conservation. [ABSTRACT FROM AUTHOR]

Some of the highest density pockmark fields in the world have been observed on the northwest Australian continental shelf (>700/km2) where they occur in muddy, organic-rich sediment around carbonate banks and paleochannels. Here we developed a semi-automated method to map and quantify the form and density of these pockmark fields (~220,000 pockmarks) and characterise their geochemical, sedimentological and biological properties to provide insight into their formative processes. These data indicate that pockmarks formed due to the release of gas derived from the breakdown of near-surface organic material, with gas accumulation aided by the sealing properties of the sediments. Sources of organic matter include adjacent carbonate banks and buried paleochannels. Polychaetes biodiversity appears to be affected negatively by the conditions surrounding dense pockmark fields since higher biodiversity is associated with low density fields. While regional bi-directionality of pockmark scours corresponds to modelled tidal flow, localised scattering around banks suggests turbulence. This multi-scale information therefore suggests that pockmark scours can act as proxy for bottom currents, which could help to inform modelling of benthic biodiversity patterns. [ABSTRACT FROM AUTHOR]

Perth Canyon is Australia's second largest submarine canyon, and its shelf-incising morphology contrasts with the more prolific slope-confined canyons that typify Australia's passive continental margin. The canyon has a sinuous course that extends 120 km from the shelf break (~180 m depth) to its fan at the foot of the continental slope (~4500 m). Though the canyon initiates only 50 km offshore from a major city, its genesis and geomorphic stability have not been well understood. Bathymetry data acquired in 2015 by the Schmidt Ocean Institute enabled the application of a new two-part seafloor classification approach to objectively map the complexity of the system in unprecedented detail. Part 1 used a semi-automated approach to classify the seafloor bathymetry into morphological categories, and Part 2 defined these units as geomorphological features through the interpretation of sub-bottom and seismic images, sediment samples and acoustic backscatter datasets. The resulting geomorphic map reveals an array of aggradational (cyclic steps and sediment waves), incisional (entrenched canyon floor and nick-points) and mass movement (slump and slab failures) features that for the first time provide detailed insights into the canyon's formative processes. Large faults and the Cretaceous palaeobathymetry appear to have strong influence on the canyon's planform, its depth of incision, and the distribution and types of mass failure that characterise its flanks. These data also reveal the Perth Canyon to be a predominantly relict feature; a large Late Cretaceous infilled incised valley (subaerial) beneath the canyon headwall likely initiated the canyon's development and represents its initial and most active phase. Two more infilled incised valleys are stacked above the first, and demonstrate a progressive decrease in scale, and presumably also canyon activity. Each incised valley represents lowstand incisions of the palaeo-Swan River, and their timing is linked to pronounced Late Cretaceous to Cenozoic sea level regression events, palaeoclimatic change, and onshore catchment enlargement. The disconnection of the modern Perth Canyon from the present day Swan River, and the low rates of sediment accumulation on the adjacent shelf and slope, ensure low rates of sediment supply to the canyon, and only infrequent ignition of turbidity currents. Low rates of sediment supply can similarly account for the entrenched morphology of the modern fan and only minimal headwall movement coincident with seismic events in 2018. However, additional core and bathymetry data for the lower canyon reaches are required to conclusively determine the extent of recent canyon activity. • Applies a two-part mapping approach to Australia's second largest canyon. • Integrates new high-resolution bathymetry with subsurface interpretations. • Links canyon evolution to geology, hydrology and eustatic sea level change. [ABSTRACT FROM AUTHOR]