Tim Pietsch

Riparian vegetation and the late Holocene development of an anabranching river: Magela Creek, northern Australia

Many anabranching rivers are characterized by dynamic interactions between fluvial processes and riparian vegetation, but uncertainties surround the processes and time scales of anabranch development. We use geomorphological investigations and optically stimulated luminescence (OSL) dating to determine spatial and temporal trends in the development of anabranching along a 6.5-km-long reach of Magela Creek in the seasonal tropics of northern Australia. Many trees and shrubs that survive the wet-season floods establish on the sandy beds and lower banks, such that anabranches divide and rejoin around numerous ridges and islands that are formed mainly by accretion in the lee of in-channel vegetation and, less commonly, by excision from formerly continuous island or flood plain surfaces. Once ridges and islands form, colonizing vegetation maintains their stability by increasing sediment cohesion and decreasing flow erosivity. Over the Holocene, Magela Creek has vertically aggraded and extended in length by delta progradation into Madjinbardi Billabong, resulting in a time sequence of anabranches and associated ridges and islands from older (upstream) to younger (downstream). OSL ages for islands in the upstream and middle reaches are ca. 1.6 ka and older, and the narrow, deep anabranches (width/depth [w/d] typically ~10–30) have few in-channel obstructions. Farther downstream, island OSL ages are ca. 0.7 ka and younger, anabranches tend to be wider and shallower (w/d >30) with more obstructions, and splays and locally scoured island and floodplain surfaces are more common. Based on these findings, previous flow and sediment-transport measurements, and theoretical analyses, we posit that there is a decline in anabranch efficiency from an upstream equilibrium system in mass-flux balance to a downstream disequilibrium system characterized by bed aggradation and localized island and floodplain erosion. In the downstream reaches, inefficient (high w/d and obstructed) anabranches do not persist because they either aggrade and are abandoned, or they are subdivided into more efficient (lower w/d and less obstructed) anabranches as a result of the interactions between in-channel vegetation growth and ridge and island accretion or local excision. Consequently, a more efficient anabranching system gradually develops with characteristics similar to those in the upstream reaches. This enhances downstream sediment transfer, which enables ongoing delta progradation and provides fresh sediment surfaces for vegetation to colonize and initiate new anabranches. The OSL ages from Magela Creek demonstrate that a recognizable but relatively inefficient anabranching system can develop within a few centuries, while adjustment to a more efficient system occurs over a few millennia.

Rates of Shoreline Progradation during the Last 1700 Years at Beachmere, Southeastern Queensland, Australia, Based on Optically Stimulated Luminescence Dating of Beach Ridges

Abstract The optically stimulated luminescence (OSL) dating method was used to determine the geochronology of seven relict beach ridges that sit immediately behind the modern beach at Beachmere, a low-energy sandy coast within Moreton Bay, Queensland. Between 2600 ± 400 and 1700 ± 130 years ago, the shoreline eroded and foreshore sediment was deposited over the older beach deposit. Subsequently, there was a 1500-year period of shoreline progradation: the shoreline advanced 0.16 m/y between 1700 ± 130 and 1140 ± 80 years ago; and 0.41 m/y between 1140 ± 80 and around 200 years ago. Shortly after 690 ± 60 years ago, a series of well-developed regularly spaced beach ridges gave way to an intertidal flat and then deposition of a set of lower amplitude, closely spaced beach ridges. The younger ridges were deposited between 230 ± 40 and 140 ± 50 years ago, at a rate of around 1.06 m/y. During the last several decades, much of the Beachmere shoreline has eroded into these younger relict ridges. Drivers of these changes in shoreline sedimentary regime are yet to be accurately determined; however, it seems likely they are related to switches that occur in the nearshore sand transport pathway. Our results demonstrate the utility of the OSL method for providing insights into coastal change that occurred in the historical and recent geological period. Better understanding the tempo of shoreline change in the recent past is particularly relevant for assessments of vulnerability to erosion of rapidly developing, low-lying sandy coasts such as northern Moreton Bay.

Delineation of sediment sources to a coastal wetland in the Great Barrier Reef catchment: influence of climate variability and land clearing since European arrival

Environmental context. Undisturbed sediments provide a record to past events in a catchment. In this study we examine changes in sources of sediment and their variation over the past century due to changes in climate and extensive modification of the catchment after European settlement. We also highlight how multiple lines of forensic evidence acquired from the sediments can be used to reconstruct catchment history over a range of timescales. Abstract. Enhanced delivery of sediment and nutrients to the Great Barrier Reef has the potential to profoundly influence ecological processes in this natural icon. Within the Fitzroy River Basin (FRB) of north-eastern Australia, natural impoundments such as Crescent Lagoon provide an invaluable archive of accumulated sediment that can be dated using multiple techniques to reconstruct the history of sediment export. During the last century, net rates of accumulation of sediment remain similar; however, large variations in sediment sources are apparent. A major sedimentary and geochemical discontinuity is present between ~45 to 29 years before present. Within this time interval a redox front is preserved corresponding to a change in organic matter influx; C3 plant detritus derived from the onset of broadscale agriculture within the FRB provided an assimilable carbon source resulting in more reducing conditions within the sediments. Statistical correlations demonstrate a notable correspondence between some sediment fractions supporting the notion of a short-lived disturbance to the sedimentation regime in the 1960–70s.

Development, composition and age of indurated sand layers in the Late Quaternary coastal deposits of northern Moreton Bay, Queensland

Indurated sand in the Late Quaternary coastal plain succession of northern Moreton Bay was examined in sand-mine pits, drillcores and the eroded bank of an estuarine channel. Samples show that the cements usually coat grains and partially infill interstitial pores. Distinctive cement habits reflect different constituents that are dominantly kaolinite and amorphous organic-rich complexes. Trace-metal concentrations in the cements are lower than previously reported for soils and estuarine sediments in the study region. Optically stimulated luminescence and thermoluminescence ages of these deposits indicate that pedogenic induration occurs over long periods, up to approximately 90 000 years, with only incipient induration evident in deposits 16 000 – 2600 years old. However, the rate of induration is far higher in relatively coarse channel fill, in which mineral and amorphous organic-rich cements have precipitated from shallow groundwater that flowed laterally through the deposit. The degree of induration, therefore, is strongly influenced by the original depositional texture and morphology of deposits, with well-indurated gravely channel fill (76 700 ± 6500 y), at least 2 m thick, sitting adjacent to medium-grained sand probably of dune origin, which contains a 30 cm-thick induration horizon (98 000 ± 9900 y).

Digging your own grave: OSL signatures in experimental graves.

Excavation of mock graves in sediments of aeolian and fluvial origin were conducted to test the bleaching efficiency of grave digging in materials that commonly host ancient burials in Australia. Grave-size pits were dug into Pleistocene aeolian sediments at Willandra Lakes and younger fluvial sediments on the Lachlan River, backfilled, and re-excavated. Samples for optical dating were taken from sediment infilling the mock graves and from the adjacent, undisturbed substrate, and analysed using the single aliquot-regenerative dose (SAR) protocol applied to single quartz grains. The resulting equivalent dose (De) distributions revealed that ≤1% of grains had been fully zeroed in both settings, and an additional 1-6% of poorly bleached grains were apparent in the fluvial sediments. Insufficient and heterogeneous bleaching of sediments during excavation and backfilling produced a decrease in the central dose of between 3 and 6 Gy, and an increase in over-dispersion values of between 5 and 10%. These differences were insufficient to clearly distinguish the disturbance event from the effects of bioturbation, biological mixing, or other sources of De variation. The use of the Minimum Age Model substantially over-estimated the burial age (zero years) in both depositional environments, with the degree of over-estimation increasing with the age of the host sediments. These results suggest that optically stimulated luminescence (OSL) techniques will not produce accurate ages for grave infill in a number of forensic and archaeological settings. Further study of the bleaching susceptibility of grains within grave infills, as well as the effectiveness of grave-digging as a bleaching mechanism is required. In other archaeological and geomorphological applications of OSL dating we recommend routine checks on the effective zeroing of sediments in modern equivalent situations.

Insights into the history and timing of post-European land use disturbance on sedimentation rates in catchments draining to the Great Barrier Reef

Sediment runoff has been cited as a major contributor to the declining health of the Great Barrier Reef (GBR), however, climate and land use drivers have not been jointly evaluated. This study used alluvial archives from fluvial benches in two tributaries of the Upper Burdekin catchment together with the best available land use history and climate proxy records to provide insights into the timing of depositional events in this region over the past 500 years. This study suggests that mining and the increased runoff variability in the latter half of the nineteenth century are the likely sources of the original excess sediment that was used to build the bench features in these catchments. Grazing also contributed to increased bench sedimentation prior to 1900, however, the contribution of grazing was likely more significant in the second half of the 20th century, and continues to be a dominant land use contributor today.

An alternative method for interpreting jet erosion test (JET) data: Part 2. Application

This paper reports the results of jet tester experiments on soil samples of uniform properties which allow quantitative application of the new theory proposed in part 1 of these publications. This theory explores the possibly that a more adequate indicator of soil erodibility may be obtained by using the mass (and so volume) of soil eroded by the jet and the depth of scour penetration, rather than by using penetration depth alone, as assumed in the commonly-used data interpretation method. It is shown that scour geometry can be well described using a generalized form of the Gaussian function, defined by its standard deviation and maximum depth. Using a published expression for jet kinetic energy flux, the new theory divides this flux into that used to erode soil, and the remainder which is dissipated in a variety of ways. Jet experiments on a specially-prepared uniform soil sample are reported which provide the key to determining the spatial variability in the profile resistance to erosion offered by field soils. This resistance is expressed in the work required to erode unit mass of soil, denoted as J (in J/kg). The paper also gives results obtained on the profile variation in J for jet tests carried out at riverine sites on the upper Brisbane River, Queensland, Australia. As expected in most natural soil profiles, the results show an increase in J with depth in the profile. The soil resistance (J) is compared to the traditional interpretation of soil erodibility, (kd). The graphical comparison of these two indicators illustrates the inverse type of relationship between them which is expected from their respective definitions, but this relationship is associated with significant scatter. Possible reasons for this scatter are given, together with comments on jet tester experience in a wide variety of soil types. Copyright