Tracing sources of inorganic suspended particulate matter in the Great Barrier Reef lagoon, Australia.

Water clarity on the inshore Great Barrier Reef (GBR) is greatly influenced by terrestrial runoff of suspended particulate matter (SPM). Catchment sediment tracing studies often do not extend into the marine environment, preventing the analysis of preferential marine transport. This study employs novel collection and sediment tracing techniques to examine the transport of the terrigenous 'mineral' component of plume SPM within the GBR lagoon for two flood events. Utilising geochemical, radionuclide and clay mineral analysis, we trace terrigenous mineral sediments > 100 km from the river mouth. We show that the SPM geochemistry is highly influenced by particle-size fractionation, desorption, and dilution within the plume, rendering traditional tracing methods unviable. However, the ratios of rare earth elements (REE) to thorium (Th) provide stable tracers of mineral SPM transported across the catchment to marine continuum and allow the identification of discrete catchment sources for each flood event. Plume sediment radionuclides are also stable and consistent with sub-surface erosion sources.

The effect of the introduction of livestock on the erosion of alpine soils: a comparison of five dating techniques applied to sediments of the Australian alpine Blue Lake.

210Pb and 137Cs dating of bulk sediments obtained from the alpine Blue Lake, located in the Snowy Mountains of southeastern Australia, was applied here to date recent lacustrine sediments. In addition, the presence of Pinus pollen (a taxon introduced in Australia about 150 years ago) down to a sediment depth of 56 cm in the core is used to obtain a chronology for the upper part of the core. Accelerated Mass Spectrometry radiocarbon dates obtained from organic muds from the same core do not agree with the chronology constructed using the three other dating techniques. In addition, optically stimulated luminescence (OSL) dating of single quartz grains, from sediment-core samples collected from the same lake, was applied to date recent lacustrine sediments. The optical age of 185 ± 20 years for a sample at 60-62 cm depth, and 470 ± 50 years at 116-118 cm depth are well over 1000 years younger than the ages inferred from radiocarbon dates. We therefore infer that the 'old' radiocarbon ages result from carbon stored for considerable time within the catchment prior to its transport and deposition on the lake floor. As plant decomposition occurs at much slower rates in high altitude environments, these results bring into question the veracity of previously published radiocarbon dates from Blue Lake and alpine lake sediments in general. The deposition ages inferred from the 210Pb-137Cs and OSL dating, and the first appearance of Pinus pollen, indicate that for the 100-year period after European settlement (from the mid 1800s to early 1900s) the sediment-accumulation rate increased by a factor of about 2, from 0.19 ± 0.01 cm yr-1 to 0.35 ± 0.02 cm yr-1. In the 1900s the accumulation rate increased further to 0.60 cm yr-1. The accumulation rate was particularly rapid in the 20-year period from 1940-1960, reaching a rate 18 times higher than the pre-European rate in the mid-1950s. The increase in sedimentation rate is attributed to changes in land use resulting from European activities in the lake catchment, primarily through sheep and cattle grazing in the Blue Lake catchment.

Vegetation and longitudinal coarse sediment connectivity affect the ability of ecosystem restoration to reduce riverbank erosion and turbidity in drinking water.

It is a substantial challenge to quantify the benefits which ecosystems provide to water supply at scales large enough to support policy making. This study tested the hypothesis that vegetation could reduce riverbank erosion, and therefore contribute to reducing turbidity and the cost of water supply, during a large magnitude flood along a 62 km riparian corridor where land cover differed substantially from natural conditions. Several lines of evidence were used to establish the benefits that vegetation provided to reducing eleven riverbank erosion processes over 1688 observations. The data and analyses confirmed that vegetation significantly reduced the magnitude of the riverbank erosion process which was the largest contributor to total erosion volume. For this process, a 1% increase in canopy cover of trees higher than five metres reduced erosion magnitude by between 2 and 3%. Results also indicate that riverbank erosion was likely to be affected by direct changes to the riparian corridor which influenced longitudinal coarse sediment connectivity. When comparing the impact of these direct changes on a relative basis, sand and gravel extraction was likely to be the dominant contributor to changed erosion rates. The locations where erosion rates had substantially increased were of limited spatial extent and in general substantial change in river form had not occurred. This suggests that the trajectory of river condition and increasing turbidity are potentially reversible if the drivers of river degradation are addressed through an ecosystem restoration policy.

Catchment soils supply ammonium to the coastal zone - Flood impacts on nutrient flux in estuaries.

Erosion of soil from catchments during floods can deliver large quantities of sediment to the coastal zone. The transformations and processes of nutrient release from catchment soils during flooding are not well understood. To test the hypothesis that catchment soils supply nutrients to the coastal zone, we examined nutrient release and transformation following wetting of soils formed from three distinct rock types (basalt, granite and sandstone) with fresh and marine water. The soil samples were collected from eroding areas of a subtropical river catchment. We simulated runoff, transport and deposition by tumbling the fine fraction of the soils in freshwater for three days and settling in seawater for four weeks. We also collected and incubated cores from an adjacent coastal bay and added a layer of catchment soil to simulate deposition of new sediment following flood plume settling. Dissolved nutrients were measured in both simulations. Basalt soils were relatively nutrient rich and released substantial quantities of organic and inorganic dissolved nutrients, particularly phosphate. However when soils were added to estuarine sediment cores and incubated, there was a net influx of phosphate from the overlying water. All soils continually released ammonium in both experiments, indicating that catchment soils may be an important source of ammonium to fuel productivity within the coastal zone. This study provides new insights into increased nitrogen availability in a nitrogen-depauperate coastal zone and identifies catchment geology as an important influence in coastal productivity through delivery of soil nitrogen to downstream estuaries.

Differentiating the sources of fine sediment, organic matter and nitrogen in a subtropical Australian catchment.

Understanding the sources of sediment, organic matter and nitrogen (N) transferred from terrestrial to aquatic environments is important for managing the deleterious off-site impacts of soil erosion. In particular, investigating the sources of organic matter associated with fine sediment may also provide insight into carbon (C) and N budgets. Accordingly, the main sources of fine sediment, organic matter (indicated by total organic carbon), and N are determined for three nested catchments (2.5km2, 75km2, and 3076km2) in subtropical Australia. Source samples included subsoil and surface soil, along with C3 and C4 vegetation. All samples were analysed for stable isotopes (δ13C, δ15N) and elemental composition (TOC, TN). A stable isotope mixing model (SIAR) was used to determine relative source contributions for different spatial scales (nested catchments), climatic conditions and flow stages. Subsoil was the main source of fine sediment for all catchments (82%, SD=1.15) and the main N source at smaller scales (55-76%, SD=4.6-10.5), with an exception for the wet year and at the larger catchment, where surface soil was the dominant N source (55-61%, SD=3.6-9.9), though contributions were dependent on flow (59-680m3/s). C3 litter was the main source of organic C export for the two larger catchments (53%, SD=3.8) even though C4 grasses dominate the vegetation cover in these catchments. The sources of fine sediment, organic matter and N differ in subtropical catchments impacted by erosion, with the majority of C derived from C3 leaf litter and the majority of N derived from either subsoil or surface soil. Understanding these differences will assist management in reducing sediment, organic matter and N transfers in similar subtropical catchments while providing a quantitative foundation for testing C and N budgets.

Progress in Australian dendroclimatology: Identifying growth limiting factors in four climate zones.

Dendroclimatology can be used to better understand past climate in regions such as Australia where instrumental and historical climate records are sparse and rarely extend beyond 100years. Here we review 36 Australian dendroclimatic studies which cover the four major climate zones of Australia; temperate, arid, subtropical and tropical. We show that all of these zones contain tree and shrub species which have the potential to provide high quality records of past climate. Despite this potential only four dendroclimatic reconstructions have been published for Australia, one from each of the climate zones: A 3592year temperature record for the SE-temperate zone, a 350year rainfall record for the Western arid zone, a 140year rainfall record for the northern tropics and a 146year rainfall record for SE-subtropics. We report on the spatial distribution of tree-ring studies, the environmental variables identified as limiting tree growth in each study, and identify the key challenges in using tree-ring records for climate reconstruction in Australia. We show that many Australian species have yet to be tested for dendroclimatological potential, and that the application of newer techniques including isotopic analysis, carbon dating, wood density measurements, and anatomical analysis, combined with traditional ring-width measurements should enable more species in each of the climate zones to be used, and long-term climate records to be developed across the entire continent.

Examining the link between terrestrial and aquatic phosphorus speciation in a subtropical catchment: the role of selective erosion and transport of fine sediments during storm events.

This study examined the link between terrestrial and aquatic phosphorus (P) speciation in the soils and sediments of a subtropical catchment. Specifically, the study aimed to identify the relative importance of P speciation in source soils, erosion and transport processes upstream, and aquatic transformation processes as determinants of P speciation in lake sediments (Lake Wivenhoe). Using a sequential extraction technique, NH(4)Cl extractable P (NH(4)Cl-P; exchangeable P), bicarbonate-dithionite extractable P (BD-P; reductant soluble P), NaOH extractable P (NaOH-rP; Al/Fe oxide P), HCl extractable P (HCl-P; apatite-P), and residual-P (Res-P; organic and residual inorganic P) fractions were compared in different soil/sediment compartments of the upper Brisbane River (UBR) catchment, Queensland, Australia. Multidimensional scaling identified two distinct groups of samples, one consisting of lake sediments and suspended sediments, and another consisting of riverbed sediments and soils. The riverbed sediments and soils had significantly higher HCl-P and lower NaOH-rP and Res-P relative to the lake and suspended sediments (P < 0.05). Analysis of the enrichment factors (EFs) of soils and riverbed sediments showed that fine grained particles (<63 µm) were enriched in all but the HCl-P fraction. This indicated that as finer particles are eroded from the soil surface and transported downstream there is a preferential export of non-apatite P (NaOH-rP, NaOH-nrP, BD-P and Res-P). Therefore, due to the preferential erosion and transport of fine sediments, the lake sediments contained a higher proportion of more labile forms of inorganic-P relative to the broader soil/sediment system. Our results suggest that a greater focus on the effect of selective erosion and transport on sediment P speciation in lakes and reservoirs is needed to better target management strategies aimed at reducing P availability, particularly in P-limited water bodies impacted by soil erosion.

Determining floodplain sedimentation rates using 137Cs in a low fallout environment dominated by channel- and cultivation-derived sediment inputs, central Queensland, Australia.

Fallout (137)Cs has been widely used to determine floodplain sedimentation rates in temperate environments, particularly in the northern hemisphere. Its application in low fallout, tropical environments in the southern hemisphere has been limited. In this study we assess the utility of (137)Cs for determining rates of floodplain sedimentation in a dry-tropical catchment in central Queensland, Australia. Floodplain and reference site cores were analysed in two centimetre increments, depth profiles were produced and total (137)Cs inventories calculated from the detailed profile data. Information on the rates of (137)Cs migration through local soils was obtained from the reference site soil cores. This data was used in an advection-diffusion model to account of (137)Cs mobility in floodplain sediment cores. This allowed sedimentation rates to be determined without the first year of detection for (137)Cs being known and without having to assume that (137)Cs remains immobile following deposition. Caesium-137 depth profiles in this environment are demonstrated to be an effective way of determining floodplain sedimentation rates. The total (137)Cs inventory approach was found to be less successful, with only one of the three sites analysed being in unequivocal agreement with the depth profile results. The input of sediment from catchment sources that have little, or no, (137)Cs attached results in true depositional sites having total inventories that are not significantly different from those of undisturbed reference sites.

New ages for human occupation and climatic change at Lake Mungo, Australia.

Australia's oldest human remains, found at Lake Mungo, include the world's oldest ritual ochre burial (Mungo III) and the first recorded cremation (Mungo I). Until now, the importance of these finds has been constrained by limited chronologies and palaeoenvironmental information. Mungo III, the source of the world's oldest human mitochondrial DNA, has been variously estimated at 30 thousand years (kyr) old, 42-45 kyr old and 62 +/- 6 kyr old, while radiocarbon estimates placed the Mungo I cremation near 20-26 kyr ago. Here we report a new series of 25 optical ages showing that both burials occurred at 40 +/- 2 kyr ago and that humans were present at Lake Mungo by 50-46 kyr ago, synchronously with, or soon after, initial occupation of northern and western Australia. Stratigraphic evidence indicates fluctuations between lake-full and drier conditions from 50 to 40 kyr ago, simultaneously with increased dust deposition, human arrival and continent-wide extinction of the megafauna. This was followed by sustained aridity between 40 and 30 kyr ago. This new chronology corrects previous estimates for human burials at this important site and provides a new picture of Homo sapiens adapting to deteriorating climate in the world's driest inhabited continent.