Alan R. Orpin

Sediment yield and impacts from river catchments to the Great Barrier Reef lagoon

Land use intensification is estimated to result in an overall increase in sediment delivery to the Great Barrier Reef lagoon by a factor of approximately four. Modelling suggests that, following land use intensification, croplands cause the greatest increase of sediment yield and sediment concentration, whereas erosion of grazing land is the main contemporary source of sediments, primarily owing to the large spatial extent of this land use. The spatial pattern of sediment yield to the coast after land use intensification is strongly correlated with the pattern under natural conditions, although the greatest increase is estimated to have occurred in the wet-dry catchments. Sediment transport and resuspension processes have led to the development of a strongly sediment-partitioned shelf, with modern mud-rich sediments almost exclusively restricted to the inner and inner-middle shelf, northward-facing embayments and in the lee of headlands. Elevated sediment concentrations increase the potential transport rates of nutrients and other pollutants. Whether increased sediment supply to the coastal zone has impacted on reefs remains a point of contention. More sediment load data need to be collected and analysed in order to make detailed estimates of catchment yields and establish the possible sediment impact on the Great Barrier Reef.

From Oblique Subduction to Intra-Continental Transpression: Structures of the Southern Kermadec-Hikurangi Margin from Multibeam Bathymetry, Side-Scan Sonar and Seismic Reflection

The southern Kermadec-Hikurangi convergent margin, east of New Zealand, accommodates the oblique subduction of the oceanic Hikurangi Plateau at rates of 4–5 cm/yr. Swath bathymetry and sidescan data, together with seismic reflection and geopotential data obtained during the GEODYNZ-SUD cruise, showed major changes in tectonic style along the margin. The changes reflect the size and abundance of seamounts on the subducting plateau, the presence and thickness of trench-fill turbidites, and the change to increasing obliquity and intracontinental transpression towards the south. In this paper, we provide evidence that faulting with a significant strike-slip component is widespread along the entire 1000 km margin. Subduction of the northeastern scrap of the Hikurangi Plateau is marked by an offset in the Kermadec Trench and adjacent margin, and by a major NW-trending tear fault in the scarp. To the south, the southern Kermadec Trench is devoid of turbidite fill and the adjacent margin is characterized by an up to 1200 m high scarp that locally separates apparent clockwise rotated blocks on the upper slope from strike-slip faults and mass wasting on the lower slope. The northern Hikurangi Trough has at least 1 km of trench-fill but its adjacent margin is characterized by tectonic erosion. The toe of the margin is indented by 10–25 km for more than 200 km, and this is inferred to be the result of repeated impacts of the large seamounts that are abundant on the northern Hikurangi Plateau. The two most recent impacts have left major indentations in the margin. The central Hikurangi margin is characterized by development of a wide accretionary wedge on the lower slope, and by transpression of presubduction passive margin sediments on the upper slope. Shortening across the wedge together with a component of strike-slip motion on the upper slope supports an interpretation of some strain partitioning. The southern Hikurangi margin is a narrow, mainly compressive belt along a very oblique, apparently locked subduction zone.

Patterns of mixed siliciclastic–carbonate sedimentation adjacent to a large dry-tropics river on the central Great Barrier Reef shelf, Australia

The Great Barrier Reef represents the largest modern example of a mixed siliciclastic‐carbonate system. The Burdekin River is the largest source of terrigenous sediment to the lagoon and is therefore an ideal location to investigate regional patterns of mixed sedimentation. Sediments become coarser grained and more poorly sorted away from the protection of eastern headlands, with mud accumulation focused in localised ‘hot spots‘ in the eastern portion of embayments protected from southeast trade winds. The middle shelf has a variable facies distribution but is dominated by coarse carbonate sand. North of Bowling Green Bay, modern coarse carbonate sand and relict quartzose sand occur. Shore‐normal compositional changes show Ca‐enrichment and Al‐dilution seawards towards the reef, and shore‐parallel trends show Al‐dilution westwards (across bays) along a Ca‐depleted mixing line. Intermediate siliciclastic‐carbonate sediment compositions occur on the middle shelf due to the abundance of relict terrigenous sand, a pattern that is less developed on the narrow northern Great Barrier Reef shelf. Rates of sediment deposition from seismic evidence and radiochemical tracers suggest that despite the magnitude of riverine input, 80–90% of the Burdekin‐derived sediment is effectively captured in Bowling Green Bay. Over millennial time‐scales, stratigraphic controls suggest that sediment is being preferentially accreted back to the coast.

ENSO/SAM interactions during the middle and late Holocene

A new chronology of large magnitude rainfall events derived from the continuous high-resolution Lake Tutira storm sediment record covers the last 6800 years and provides the first insight into changes in El Niño-Southern Oscillation (ENSO) teleconnections to the higher southern latitudes to be obtained from New Zealand. Synthesis with independent paleoclimate records from the tropical Pacific and Antarctica also reveals a millennial-scale waxing and waning of the teleconnections that were not visible in the narrow historical window previously used to view interactions between ENSO and the Southern Annular Mode (SAM). Consistent with modern ENSO behaviour, we find teleconnections to the Southwest Pacific varied throughout the middle and late Holocene, depending on the strength and phase of ENSO and the phase of the SAM. We suggest that precession-driven changes in the seasonal cycle of solar radiation exert a first-order control on the interaction between the two climate modes. Consequently, their present status may neither be indicative of conditions that prevailed earlier in the Holocene, nor of those that might be associated with future climate changes in the Southern Hemisphere extratropics.

Flushing time of solutes and pollutants in the central Great Barrier Reef lagoon, Australia

The flushing time of the central Great Barrier Reef lagoon was determined by using salinity as a tracer and developing both an exchange model and a diffusion model of the shelf exchange processes. Modelling suggests that the cross-shelf diffusion coefficient is approximately constant for the outer half of the lagoon but decays rapidly closer to the coast. The typical outer-shelf diffusion coefficient is ~1400 m2 s–1, dropping to less than 100 m2 s–1 close to the coast. Flushing times are around 40 days for water close to the coast and 14 days for water in the offshore reef matrix.

Understanding sediment transfer from land to ocean

A new research program focusing on sediment dispersal across the active margin of the New Zealand east coast has provided the foundation for a holistic understanding of the transport and fate of terrestrial materials in the coastal ocean. Field studies began in January 2005 with two acoustic mapping and shallow seabed sampling expeditions to the shelf and slope off the Waipaoa River (Figure l), and in February 2006, the specially designed French research vessel (R/V) Marion Dufresne II collected seven long (up to 25 meters) piston cores from the study area for stratigraphic control. Both the 2005 and 2006 expeditions are part of the U.S. National Science Foundation (NSF) MARGINS Source-to-Sink (S2S) initiative.

Automatic weather stations: Tools for managing and monitoring potential impacts to coral reefs

With recent technological advances and a reduction in the cost of automatic weather stations and data buoys, the potential exists for significant advancement in science and environmental management using near real-time, high-resolution data to predict biological and/or physical events. However, real-world examples of how this potential wealth of data has been used in environmental management are few and far between. We describe in detail two examples where near real-time data are being used for the benefit of science and management. These include a prediction of coral bleaching events using temperature, light and wind as primary predictor variables, and the management of a coastal development where dynamic discharge quality limits are maintained with the aid of wind data as a proxy for turbidity in receiving waters. We argue that the limiting factors for the use of near real-time environmental data in management is frequently not the availability of the data, but the lack of knowledge of the quantitative relationships between biological/physical processes or events and environmental variables. We advocate renewed research into this area and an integrated approach to the use of a wide range of data types to deal with management issues in an innovative, cost-effective manner.

Coherent rainfall response to middle- and late-Holocene climate variability across the mid-latitude South Pacific

Intercorrelated terrestrial and marine records from New Zealand, Chile and Argentina provide the first evidence of a coherent pan-Pacific response to the growing influence that El Niño-Southern Oscillation (ENSO) and multicentury variations in the interaction between ENSO and the Southern Annular Mode (SAM) exerted on rainfall in the Southern Hemisphere mid-latitudes during the middle and late Holocene. Rainfall is also strongly influenced by moisture originating from the prevailing southern westerly winds (SWW), and we find that the variations in rainfall forced by the interaction of ENSO and the SAM are superimposed on an underlying long-term trend induced by a temporal strengthening of the westerly circulation. We conclude that the evolution of rainfall across the South Pacific reflects the influence seasonal insolation exerts on: (1) ENSO/SAM interactions; and (2) the strength of the SWW.

Windows-based software for optimising entropy-based groupings of textural data

EntropyMax is a new 32-bit Windows-based software that groups large matrices of grain-size distribution data into a finite number of groups. The software utilises statistical algorithms that minimise the entropy within a group while maximising the entropy between groups. EntropyMax builds upon an existing DOS-BASIC program through the addition of a more robust computational routine, a graphical user interface, incorporation of an improved and more statistically meaningful method for identifying the optimal number of groupings, and better graphical presentation of results. These refinements add significantly to the functionality and statistical rigour of the program on modern computer platforms and large data sets.

Simulating post-LGM riverine fluxes to the coastal zone: The Waipaoa River System, New Zealand

HydroTrend, a climate-driven hydrologic transport model, is used to simulate the suspended sediment discharge of the Waipaoa River System (WRS) over the last 5.5kyr. We constrain the precipitation input with a paleo-rainfall index derived from the high-resolution Lake Tutira storm sediment record. The simulation is extended to 22ka using a lower resolution version of the model, constrained by terrestrial and marine paleoenvironment indicators and a simulated model of northeast New Zealands climate at the Last Glacial Maximum (LGM). Comparison of the 5.5kyr simulation with the shelf sediment core MD97-2122 suggests that the sediment flux variations observed on the shelf primarily reflect changes in rainfall associated with wetter and drier periods of centuries to millennia duration. Storage of sediment on the Waipaoa River floodplain (Poverty Bay Flats) moderates the signal by reducing the sediment flux reaching the coast. During the LGM conditions were more erosive than the Holocene with tussock and grass dominated vegetation. For erodibility four times the Holocenes and half todays, the LGM Waipaoa River System would have generated approximately half the current sediment yield and about 3 times the amount generated when the catchment was fully forested.