Browsing by Author "Morgenstern, U"

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    Peru margin palaeoceanography since the Last Glacial Maximum and the long-term implications for El Niño-Southern Oscillation
    (Australian Geosciences Council, 2012-08-05) Skilbeck, G; Gutierrez, D; Rein, B; Sifeddine, A; Salvatteci, R; Fink, D; Druffel, E; Sanchez-Cabeza, JA; Morgenstern, U; Murray, B; Dunbar, R
    Laminated-sediment cores from nine shelf sites along the Peru continental margin between 11◦S and 14.5◦S, reveal a 20,000 yr inter-millennial scale pattern of sediment accumulation consistent with other equatorial and south Pacific-rim indicators of long-term ENSO behaviour. Nearly 200 carbon-14 dates define 6 distinct periods during the Holocene-LGM each characterised by linear sedimentation rates that can be regionally correlated between the cores. The early (∼10–8.5 cal Kyr BP) and late (∼2.5–0cal Kyr BP) Holocene, and early Deglaciation (15.5–20Kyr BP) were periods of relatively rapid accumulation (∼80 and 100+ cm/ka), with thin, rapidly deposited laminae, whereas from ∼4.5 to ∼2.5 cal Kyr BP sediments accumulated at ∼20 cm/ka. The middle Holocene (∼8.5 to 4.5 cal Kyr BP) was characterised by very slow sediment deposition, erosion and slumping. The latter period corresponds with the mid Holocene minima in March-September equatorial insolation. Sedimentation data are supplemented by 2 mm-spaced scanning XRF records that confirm the significance of the millennial-scale associations and support a long-term ENSO-like (alternating flooding and drought from continental south America) interpretation. We suggest that this regional pattern indicates millennial changes in nearshore ocean currents with the middle Holocene representing a period of intensified upwelling.
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    Sources and transit times of water in headwater temperate rainforest streams
    (Copernicus Publications, 2019-04-07) Cartwright, I; Atkinson, AP; Gilfedder, BS; Hofmann, H; Cendón, DI; Morgenstern, U
    Headwater catchments are important sources of water for many river systems. Unlike lower reaches of rivers that are frequently connected to alluvial aquifers, headwater catchments are commonly developed on indurated rocks that lack extensive groundwater systems. The observation, however, that many headwater streams are perennial implies that streamflow is sustained by water contained in fractures, soils, and/or the regolith. Understanding the sources and transit times of water that generates streamflow in headwater streams is important for understanding catchment functioning and predicting the response of catchments to changing climate or land use. This study determines water sources and transit times in first-order streams from a temperate rainforest in the Otway Ranges, southeast Australia. Comparison of the major ion geochemistry of soil water, water flowing through soil pipes (macropores), and groundwater from the riparian zone adjacent to the stream indicates that water from soil pipes is the major contributor to streamflow. By contrast, riparian zone groundwater and water from elsewhere within the soils contributes little to streamflow. The streams are gaining and the lack of riparian zone groundwater inputs may be due to the presence of low hydraulic-conductivity organic-rich streambed sediments or compartmentalisation of shallow groundwater by clays in the weathered rocks. Similarly, pockets of isolated water within the soils that are not connected to the soil pipes also exist. The stream water has tritium (3H) activities of 1.80 to 2.06 TU, with slightly higher activities recorded during the higher winter flows. The water from the soil pipes has 3H activities of 1.80 to 2.25 TU, the riparian zone groundwater has 3H activities of 1.35 to 2.39 TU, and one sample of soil water has a 3H activity of 2.22 TU. The 3H activities of all these catchment waters are significantly lower than those of modern rainfall (2.6 to 3.0 TU), and mean transit times calculated using a range of lumped parameter models are between 3 and 57 years. These mean transit times are consistent with the waters being resident in the catchment for sufficient time for weathering reactions and evapotranspiration to occur. While the discharge from the soil pipes increases following periods of high rainfall, this water is stored for several years within the catchment before discharge (probably within the weathered regolith). Thus, the increase in discharge is not the simple transmission of recent rainfall through the macropores but mobilisation of younger stores of water as the catchment wets up. The long mean transit times of the stream water imply that it is derived from a relatively large store (>108 m3) and is buffered against year-on-year variations in rainfall. However, longer-term variations in rainfall or land use will likely impact streamflow. © Author(s) 2018. CC Attribution 4.0 license.
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    Understanding the sources and transit times of water sustaining streamflow in upland catchments
    (National Centre for Groundwater Research And Training, & Australian Chapter International Association Of Hydrogeologists, 2019-11-24) Cartwright, I; Atkinson, AP; Gilfedder, BS; Hofmann, H; Cendón, DI; Morgenstern, U
    Headwater catchments are important sources of water in many rivers. While headwater catchments are commonly developed on indurated rocks without extensive groundwater systems, the observation that many headwater streams are perennial implies that they are sustained by water in fractures, soils, or the regolith. Understanding the sources and transit times of water in headwater streams is important for understanding catchment functioning and predicting the impacts of changing climate or land use. This study uses major ion geochemistry and tritium (3H) to determine water sources and transit times in first-order streams in the Otway Ranges, southeast Australia. Comparison of the geochemistry of soil water, water from soil pipes (macropores), and riparian groundwater indicates that macropore flow is the major contributor to streamflow. The streams are gaining and the lack of riparian zone groundwater inputs may be due to the presence of low hydraulic-conductivity organic-rich streambed sediments or compartmentalisation of shallow groundwater by clays in the weathered rocks. Similarly, much of the soil water exists in isolated pockets of isolated water that are not connected to the soil pipes. The stream water has tritium (3H) activities of 1.80 to 2.06 TU. These are significantly lower than the 3H activities of modern rainfall (2.6 to 3.0 TU), even during the higher winter flows. The water from the soil pipes has 3H activities of 1.80 to 2.25 TU, the riparian zone groundwater has 3H activities of 1.35 to 2.39 TU, and one sample of soil water has a 3H activity of 2.22 TU. Mean transit times calculated using a range of lumped parameter models are between 3 and 57 years. Relatively long mean transit times are consistent with the major ion geochemistry that implies that waters are resident for sufficient time for weathering reactions and evapotranspiration to have occurred. While the discharge from the soil pipes increases following periods of high rainfall, the long mean transit times implies that this water is stored for several years within the regolith before discharge, with storage volumes estimated as >108 m3. Thus the increase in streamflow is not the simple transmission of recent rainfall through the macropores but mobilisation of existing catchment stores. The streams will be buffered against year-on-year variations in rainfall but are vulnerable to longer-term variations in rainfall or land use. Management of these catchments needs to consider the impacts on the macropores and the delayed responses caused by the large storage volumes. © The Authors
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    Using geochemistry to understand water sources and transit times in headwater streams of a temperate rainforest
    (Elsevier B. V., 2018-12) Cartwright, I; Atkinson, AP; Gilfedder, BS; Hofmann, H; Cendón, DI; Morgenstern, U
    Understanding the sources and transit times of water that generates streamflow in headwater streams is important for understanding catchment functioning. This study determines the water sources and transit times in first-order streams from a temperate rainforest in the Otway Ranges, southeast Australia. Comparison of the major ion geochemistry of soil water, water flowing through soil pipes (macropores), and groundwater from the riparian zone adjacent to the stream suggests that water from soil pipes is the major contributor to streamflow. The tritium (3H) activities of the stream water are between 1.80 and 2.06 TU, the water from the soil pipes has 3H activities between 1.80 and 2.25 TU, the riparian zone groundwater has 3H activities of 1.35–2.39 TU, and one sample of soil water has a 3H activity of 2.22 TU. These 3H activities are significantly lower than those of local modern rainfall (2.6–3.0 TU), and mean transit times calculated using a range of lumped parameter models are between 3 and 57 years. These estimates are consistent with the major ion and stable isotope data, which imply that mean transit times were sufficiently long to allow weathering of minerals and/or organic matter and evapotranspiration to occur. The long mean transit times imply that water flows in this upper catchment are buffered against year-on-year variations in rainfall, but may change due to longer-term variations in rainfall or landuse. © 2018 Elsevier Ltd.