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Browsing by Author "Woodroffe, CD"

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Allochthonous and autochthonous contributions to carbon accumulation and carbon accumulation and carbon store in southeastern Australian coastal wetlands
(Estuarine, Coastal and Shelf Science, 128, 84-92, 2013-08-10) Saintilan, N; Rogers, K; Mazumder, D; Woodroffe, CD
Estimates of carbon store and carbon accumulation rate in mangrove and saltmarsh are beset by issues of scale and provenance. Estimates at a site do not allow scaling to regional estimates if the drivers of variability are not known. Also, carbon accumulation within soils provides a net offset only if carbon is derived in-situ, or would not otherwise be sequestered. We use a network of observation sites extending across 2000 km of southeastern Australian coastline to determine the influence of geomorphic setting and coastal wetland vegetation type on rates of carbon accumulation, carbon store and probable sources. Carbon accumulation above feldspar marker horizons over a 10-year period was driven primarily by tidal range and position in the tidal frame, and was higher for mangrove and saltmarsh dominated by Juncus kraussii than for other saltmarsh communities. The rate of carbon loss with depth varied between geomorphic settings and was the primary determinant of carbon store. A down-core enrichment in delta C-13 was consistent with an increased relative contribution of mangrove root material to soil carbon, as mangrove roots were found to be consistently enriched compared to leaves. We conclude that while surface carbon accumulation is driven primarily by tidal transport of allocthonous sediment, in-situ carbon sequestration is the dominant source of recalcitrant carbon, and that mangrove and saltmarsh carbon accumulation and store is high in temperate settings, particularly in mesotidal and fluvial geomorphic settings. © 2013, Elsevier Ltd.
Assessing soil remobilisation in catchments using a 137Cs-sediment hillslope model
(Taylor & Francis (Routledge), 2008-12) Simms, AD; Woodroffe, CD; Jones, BG; Heijnis, H; Harrison, JJ; Mann, RA
Soil redistribution studies are important, especially in water supply catchments, because the rate at which denudation is occurring has implications for offsite water quality. However, the extent to which soil is redistributed within the landscape can be difficult to determine. This challenge can be overcome using fallout caesium-137 (137Cs). This paper describes the rates of soil loss and remobilisation in two sub-catchments within the Sydney Basin region, namely Kembla and Kentish Creeks, which drain to the Cordeaux reservoir. The total inventories of 137Cs in catchment soils were determined, a 137Cs-regression equation and a theoretical diffusion and migration model were used to established relationships between 137Cs inventories and the rates of soil loss. These relationships revealed relatively low occurrence of soil loss in Kentish Creek, but two slopes in the Kembla Creek sub-catchment had losses that appear to be moderate. However, there was no clear evidence to suggest whether slopes in upper and lower reaches of catchments had specific patterns of soil remobilisation. Qualitative categorisation of the slope elements using a 137Cs-sediment hillslope model can be a useful sentinel for land users and decision makers even if absolute rates of soil loss or gain are not certain. The findings suggest that sediments mobilised in the study sub-catchments are not likely to impact significantly on the water quality in the Cordeaux reservoir. © 2008, Taylor & Francis (Routledge).
Coral microatoll reconstructions of El Niño-southern oscillation: new windows on seasonal and interannual processes
(PAGES Past Global Changes, 2013-08-01) McGregor, HV; Woodroffe, CD; Fischer, MJ; Gagan, MK; Fink, D
Porites corals are the most commonly used genus for reconstructing El Niño- Southern Oscillation (ENSO). This hermatypic coral is found in all tropical reef environments(Veron 2000) with a variety of growth forms. Climate reconstructions of a century or more have been obtained from the most common, dome-shaped Porites growth form, whereby the colonies, beginning from the substrate, grow outward and upward towards the ocean surface(Knutson et al. 1972). Domed structures, however, are not the only Porites growth form. © 2013, Authors.
El Niño-Southern Oscillation (ENSO) from 0-2 ka
(University of Melbourne, 2010-05-31) McGregor, HV; Phipps, SJ; Woodroffe, CD; Gagan, MK; Fink, D
Environmental control of the oxygen isotope composition of Porities coral microatolls
(Elsevier, 2011-07-15) McGregor, HV; Fischer, MJ; Gagan, MK; Fink, D; Woodroffe, CD
Understanding the influence of climatic and non-climatic factors on geochemical signals in corals is critical for assessing coral-derived records of tropical climate variability. Porites microatolls form large, disk-shaped colonies constrained in their upward growth by exposure at or close to mean spring low water level, and occur on Indo-Pacific reefs. Microatolls appear suitable for paleoclimate reconstruction, however the systematics of the microatoll chemistry-climate relationship are yet to be characterized. In this study, the delta(18)O signal in Porites microatolls from well-flushed reef flats on Kiritimati (Christmas) Island, central Pacific was investigated for intra-coral (growth aspect and extension rate effects) and between-coral effects, and to explore the climate signal contained within their skeletons. Samples for delta(18)O analysis were taken from six individual transects from different positions within Porites microatoll XM22. The results show that: (1) the average standard deviation for the mean delta(18)O values of transects that represent the same time periods is 0.03 parts per thousand, and is within measurement error for a single analysis (0.04 parts per thousand); (2) the average standard deviation for time-equivalent, near-monthly samples along the transects within the same microatoll is 0.07 parts per thousand and; (3) comparison of the average delta(18)O values of records for different microatolls from across Kiritimati Island show only a small between-coral differences of 0.04 parts per thousand and 0.11 parts per thousand for different time periods. These differences in mean delta(18)O are within the range for intra-and inter-colony differences in seasonal and interannual delta(18)O reported for dome-shaped Porites. Based on these results, a stacked microatoll delta(18)O record was constructed for the period 1978-2007 for comparison with published coral delta(18)O records for nearby dome-shaped Porites. There is a systematic offset between the two types of records, which is probably due to variations in delta(18)O seawater across Kiritimati Island. Despite the offset, all records show similar amplitudes for the seasonal-cycle of delta(18)O, and there is a strong correlation (r = -0.71) between microatoll delta(18)O and local sea surface temperature (SST). The delta(18)O-SST slope relationship for microatolls is -0.15 parts per thousand/degrees C, very similar to that reported for fast-growing domed corals (-0.18 parts per thousand to -0.22 parts per thousand/degrees C). Statistical analysis of the stacked microatoll delta(18)O record shows that it is correlated with both local and large-scale climate variables (primarily SST) at semiannual, annual and interannual timescales. Our results show that the signal reproducibility and fidelity of skeletal delta(18)O in coral microatolls is comparable to that observed for more conventional coral growth forms. Longer-lived, and fossil, Porites microatolls, where they have grown in suitably flushed environments, are likely to contain delta(18)O signals that can significantly extend instrumental records of tropical climate variability. Crown copyright (C) 2011 Published by Elsevier Ltd.
Incremental accretion of a sandy reef island over the past 3000 years indicated by component-specific radiocarbon dating
(American Geophysical Union, 2007-02-06) Woodroffe, CD; Samosorn, B; Hua, Q; Hart, DE
Low-lying reef islands appear particularly threatened by anticipated sea-level rise, and determining how they formed and whether they are continuing to accumulate sediment is essential for their sustainable management. Depositional chronology of Warraber Island, a small sand cay in Torres Strait, Australia, is re-examined based on AMS radiocarbon dating of specific skeletal components. Whereas radiometric dating of bulk sand samples indicated one or more discrete phases of mid-late Holocene deposition, component-specific AMS radiocarbon dating of sand grains indicates sustained incremental growth over the past 3000 years. Ages on gastropods that lived on the reef flat around the island indicate continuing sediment production and island progradation, in contrast to bulk ages and other components produced at greater distances. Growth of sand cays depends on the rate and pattern of sediment supply, which are functions of the local ecology of the surrounding reef and reef flat, and hydrodynamic constraints. © 2007, American Geophysical Union
Marine reservoir correction for the Cocos (Keeling) Islands, Indian Ocean
(Cambridge University Press, 2004) Hua, Q; Woodroffe, CD; Barbetti, M; Smithers, SG; Zoppi, U; Fink, D
Known-age corals from the Cocos (Keeling) Islands, Indian Ocean, have been analyzed by accelerator mass spectrometry (AMS) for radiocarbon to determine marine reservoir age corrections. The ΔR value for the Cocos (Keeling) Islands is 66 ± 12 yr based on the analyses undertaken for this study. When our AMS and previously published dates for Cocos are averaged, they yield a ΔR of 64 ± 15 yr. This is a significant revision of an earlier estimate of the ΔR value for the Cocos (Keeling) Islands of 186 ± 66 yr (Toggweiler et al. 1991). The (revised) lower ΔR for the Cocos (Keeling) Islands is consistent with GEOSECS 14C data for the Indian Ocean, and previously published bomb 14C data for the Red Sea, Gulf of Aden, and Cocos Islands. The revised ΔR is also close to values for the eastern Indian Ocean and adjacent seas. These suggest surface waters that reach the Cocos Islands might be partly derived from the far western Pacific, via the Indonesian throughflow, and might not be influenced by the southeast flow from the Arabian Sea. Copyright © The Arizona Board of Regents on behalf of the University of Arizona
Microatolls and the record of Holocene sea level that can be derived from them
(Australian Geosciences Council, 2012-08-05) Woodroffe, CD; Smithers, SG; McGregor, HV; Fink, D; Lambeck, K
The Great Barrier Reef expedition in 1973 described living and fossil microatolls on the Great Barrier Reef and recognised their significance as sea-level indicators. John Chappell subsequently surveyed and dated Porites microatolls along the mainland and interpreted them in the context of hydro-isostatic adjustment, inferring gradual falling sea level during the past 6000 years. We re-examine the significance of microatolls, providing a detailed account of more than 100 fossil microatolls from Christmas Island in the central Pacific. We relate their elevations to living modern equivalents and compare the sea-level record derived from them with the modelled Holocene isostatically-corrected sea level for this equatorial location. These long-lived massive corals, up to 9 metres in diameter, preserve a biologically-mediated multi-decadal record of sea-level over the past 5000 years. The upper surface of fossil microatolls on the reef flat lie in a narrow elevation range similar to that observed for their modern, living counterparts, implying negligible change of sea level. Further evidence for the lack of major fluctuations in sea level over the period 5000–1000 years BP is provided by corals from the interior of the island. This second population grew prolifically in a large lagoonal setting and is offset from those in open water by about 50 cm. The continuity of microatoll growth precludes significant oscillations of sea level during this time, and accords with geophysical modelling for this site, implying that the ‘eustatic’ contribution from post-6ka ice melt and the isostatic adjustment of the ocean floor to loading cancel each other at this site.
Mid-Pacific microatolls record sea-level stability over the past 5000yr
(Geological Society of America, 2012-10-01) Woodroffe, CD; McGregor, HV; Lambeck, K; Smithers, SG; Fink, D
There has been geographical variation in sea level since rapid postglacial melting of polar ice ceased similar to 6 k.y. ago, reflecting isostatic adjustments of Earth and ocean surfaces to past (and ongoing) redistribution of ice and water loads. A new data set of over 100 fossil microatolls from Christmas (Kiritimati) Island provides a Holocene sea-level record of unparalleled continuity. Living reef-flat corals grow up to a low-tide level. Adjacent fossil microatolls, long-lived Porites corals up to several meters in diameter, occur at similar elevations (+/-0.1 m), and extensive fossil microatolls in the island interior are at consistent elevations within each population. Collectively, they comprise an almost continuous sequence spanning the past 5 k.y., indicating that, locally, sea level has been within 0.25 m of its present position, and precluding global sea-level oscillations of one or more meters inferred from less stable locations, or using other sea-level indicators. This mid-Pacific atoll is tectonically stable and far from former ice sheets. The precisely surveyed and radiometrically dated microatolls indicate that sea level has not experienced significant oscillations, in accordance with geophysical modeling, which implies that the eustatic contribution from past ice melt and the isostatic adjustment of the ocean floor to loading largely cancel each other at this site. © 2012, Geological Society of America
Millennial to seasonal scale views of El Niño-Southern Oscillation from central Pacific corals
(Australasian Quaternary Association, 2022-12-06) McGregor, HV; Wilcox, P; Fischer, MJ; Phipps, SJ; Gagan, MK; Wittenberg, A; Felis, T; Kölling, M; Wong, HKY; Devriendt, L; Woodroffe, CD; Zhao, JX; Fink, D; Gaudry, JJ; Chivas, AR
El Niño-Southern Oscillation (ENSO) is naturally highly variable on interannual to decadal scales making it difficult to detect a possible response to climate forcing. Despite the high variability, several lines of evidence from tropical corals, mollusc, lake sediments, and foraminifera suggest that 5,000-3,000 years ago ENSO variance was on average reduced by 60-80% compared to the present day. We investigate the seasonal-to-centennial variation in ENSO amplitude and tropical climate during this ENSO ‘quiet period’ 5,000-3,000 years ago using a new Sr/Ca SST record from a 175-year-long 4,300-year-old coral, and new d18O and Sr/Ca results from a similar-aged ~180-year-long Porites sp. coral. Both corals were discovered on Kiritimati (Christmas) Island, an optimal ENSO ‘centre of action’ in the central tropical Pacific. Together, these corals confirm a reduction in ENSO amplitude and that ENSO amplitude is modulated on multi-decadal scales. Composites of month-by-month changes in Sr/Ca-SST show an unprecedented view of ENSO and detail which seasonal-scale features of ENSO are an inherent part of the system, and which are subject to change under altered climate states. We also investigate the millennial timescale changes in ENSO variance using combine coral oxygen isotope (18O) data from central Pacific corals and a suite of forced and unforced simulations conducted using the CSIRO Mk3L and GFDL CM2.1 climate system models. On millennial timescales, the coral data reveal a statistically significant increase in ENSO variance over the past 6,000 years. This trend is not reproduced by the unforced model simulations but can be reproduced once orbital forcing is accounted for. Together these views of past ENSO may contribute to advances in understanding the response of ENSO to future changes in climate forcings.
Millennial to seasonal scale views of El Niño-Southern Oscillation from central Pacific corals
(2022-12-06) McGregor, HV; Wilcox, P; Fischer, MJ; Phipps, SJ; Gagan, MK; Wittenberg, A; Felis, T; Kölling, M; Wong, HKY; Devriendt, L; Woodroffe, CD; Zhao, JX; Fink, D; Gaudry, JJ; Chivas, AR
El Niño-Southern Oscillation (ENSO) is naturally highly variable on interannual to decadal scales making it difficult to detect a possible response to climate forcing. Despite the high variability, several lines of evidence from tropical corals, mollusc, lake sediments, and foraminifera suggest that 5,000-3,000 years ago ENSO variance was on average reduced by 60-80% compared to the present day. We investigate the seasonal-to-centennial variation in ENSO amplitude and tropical climate during this ENSO ‘quiet period’ 5,000-3,000 years ago using a new Sr/Ca SST record from a 175-year-long 4,300-year-old coral, and new d18O and Sr/Ca results from a similar-aged ~180-year-long Porites sp. coral. Both corals were discovered on Kiritimati (Christmas) Island, an optimal ENSO ‘centre of action’ in the central tropical Pacific. Together, these corals confirm a reduction in ENSO amplitude and that ENSO amplitude is modulated on multi-decadal scales. Composites of month-by-month changes in Sr/Ca-SST show an unprecedented view of ENSO and detail which seasonal-scale features of ENSO are an inherent part of the system, and which are subject to change under altered climate states. We also investigate the millennial timescale changes in ENSO variance using combine coral oxygen isotope (18O) data from central Pacific corals and a suite of forced and unforced simulations conducted using the CSIRO Mk3L and GFDL CM2.1 climate system models. On millennial timescales, the coral data reveal a statistically significant increase in ENSO variance over the past 6,000 years. This trend is not reproduced by the unforced model simulations but can be reproduced once orbital forcing is accounted for. Together these views of past ENSO may contribute to advances in understanding the response of ENSO to future changes in climate forcings.
Morphology and formation of relict coral reef on the shelf around Lord Howe Island.
(GeoHab (Marine Geological and Biological Habitat Mapping), 2010-05-06) Woodroffe, CD; Brooke, BP; Linklater, M; Kennedy, DM; Jones, BG; Buchanan, C; Mleczko, R; Hua, Q; Zhao, JX
Coral reefs track sea level and are particularly sensitive to changes in climate. Reefs are threatened by global warming, with those in tropical waters experiencing increased incidences of bleaching. Although it has been suggested that reefs may extend poleward at their latitudinal limit, there has been little evidence to support this contention. In this paper, we report on a much more extensive coral reef that flourished around Lord Howe Island, which presently supports the southernmost coral reef in the Pacific. Multibeam swath mapping and sub-bottom profiling reveal an extensive reef that encircled the island, in the middle of the shelf, rising from water depths of around 50 m to 30 m, with isolated peaks reaching 23.5 m. Coring has indicated that this relict reef is composed of corals that grew between 9200 and 7100 years ago, and that the main phase of reef growth had terminated and the reef backstepped by 7000 years BP. Localised re-establishment of corals over the surface around 2500 and in the past few hundred years provides some indication that corals may be able to recolonise the relict reef and extend further in response to warmer temperatures anticipated later this century and beyond. However, this will depend on the availability of suitable substrate, as well as other environmental factors, as the relict reef is now in greater water depths than when it was most actively growing.
Quantifiying seasonal-scale changes in El Niño southern oscillation for the past millennia
(Australian Meteorological & Oceanographic Society, 2012-01-31) McGregor, HV; Fischer, MJ; Gagan, MK; Woodroffe, CD; Fink, D; Phipps, SJ; Zhao, JX
The El Niño-Southern Oscillation (ENSO) is the greatest source of interannual climate variability, yet model forecasts of the response of this system to global warming are inconsistent. The brevity of the instrumental record and lack of detailed knowledge of ENSO under different background states contribute to the uncertainty. Here we present a sequence of Porites coral microatoll !18O records from Kiritimati (Christmas) Island in the central equatorial Pacific showing ENSO variability during discrete “windows” spaced between 1500 and 6000 years ago (mid- to late Holocene), when background climate conditions were different due to changes in the Earth’s orbit around the sun. Our sequence includes a 175-year monthly-resolved microatoll !18O record showing ENSO variability 4,300 thousand years ago. The record shows a 60% reduction in the ENSO variance, a stronger annual cycle that persisted for the full 175 years of the record, and limited low frequency (multi-decadal) modulation of the ENSO signal. El Niño events were ‘damped’ during their June-December growth phase, but still phaselocked to the seasonal cycle. La Niña events were reduced and together ENSO seasonal phasing was likely similar to that observed during the weak ENSO period of the 1920-1950s.Further, results from corals aged between 1,500 and 2,000 years ago also show reduced ENSO, as well as changes in the contribution of the annual cycle, El Niño and La Niña events to the overall coral !18O signal. Our results show fundamental metrics on the seasonal characteristics of ENSO during the altered background conditions of the mid- to late Holocene. The results suggest that Holocene ENSO responded to changes in orbital forcing and that there was limited, unforced variability. This may have implications for ENSO under future global warming conditions.
Radiocarbon in corals from the Cocos (Keeling) Islands and implications for Indian Ocean circulation
(American Geophysical Union (AGU), 2005-11-02) Hua, Q; Woodroffe, CD; Smithers, SG; Barbetti, M; Fink, D
Annual bands of a Porites coral from the Cocos (Keeling) Islands, eastern Indian Ocean, were analysed by radiocarbon for 1955–1985 AD. A rapid oceanic response of the site to bomb 14C is found, with a maximum Δ14C value of 132‰ in 1975. This value is considerably higher than those for the northwestern Indian Ocean, suggesting that surface waters reaching Cocos are not derived from the Arabian Sea. Instead, Δ14C values for Cocos and those for Watamu (Kenya) agree well over most of the study interval, suggesting that the South Equatorial Current carries 14C-elevated water rather than 14C-depleted water westward across the Indian Ocean. This implies that oceanic upwelling in the northwestern Indian Ocean is spatially confined with little contribution to the upper limb of the global thermohaline circulation. © 2005 by the American Geophysical Union.
Radiocarbon surface ocean reservoir ages over the past 6,000 years from Porites microatolls, Christmas (Kiritimati) Island, central Pacific Ocean
(Australian Geosciences Council, 2012-08-05) Fink, D; Carilli, JE; McGregor, HV; Woodroffe, CD; Zhao, JX; Fallon, SJ
We present radiocarbon reservoir (ΔR) values over the past 5,000 years based on high-precision paired U-series and AMS 14C in modern and fossil Porites coral micro-atolls from Christmas Island (2N, 157W). The data set (n∼25) allows temporal reconstruction of ΔR with ∼250 year spacing. Christmas Island lies within the Equatorial Counter Current and the NINO3.4 region with a climate frequently punctuated by higher precipitation and warmer SSTs during El-Nino. Along its coastal perimeter, and throughout the internal network of tidal flats and lagoons, which in the late Holocene were flourishing reefal environments, fossil and modern microatolls abound. Microatolls are large discoid colonies with horizontal radial growth axes constrained in upwards vertical growth by spring tide low-water level. Consequently, a single microatoll can extend to ∼9 m in diameter representing an exceptional ∼300 years of continuous growth. Sr/Ca and δ18O in modern microatolls faithfully replicate instrumental climate records (SSTs, rainfall). Results from numerous fossil cores (1500–5500 years BP) shows distinct variability in ENSO variance both in frequency and amplitude domains compared to modern microatolls. These fossil populations are used to generate a temporal pattern of ΔR variability. Strict criteria were followed with respect to secondary calcite (via quantitative XRD), unaltered microstructure (via thin sections) and ensuring splits of identical coral chips for dual U-series and AMS 14C. Preliminary results show a ΔR decrease at about 2000 BP. Further analyses of correlations of ΔR deviations from the long term average with changes in regional ocean surface currents and in paleo-ENSO variance are in progress.
Rapid U-series dating of young fossil corals by laser ablation MC-ICPMS
(Elsevier, 2011-04-01) McGregor, HV; Hellstrom, JC; Fink, D; Hua, Q; Woodroffe, CD
We demonstrate the utility of uranium-series age dating using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA MC-ICPMS) to ‘range find’ a large suite of fossil corals in order to select those for subsequent detailed and high resolution paleoclimatological analyses. The high abundance of unaltered and long-lived (100–200 years) microatolls on Kiritimati Island, Kiribati, offers the exciting prospect of constructing a millennial, if not continuous, coral paleoclimate record for the central Pacific spanning the period from the mid-Holocene to present. However, with a multitude of collected corals and Kiritimati sample sites, we required an efficient, cost-effective and reliable analytical method to optimize selection of quality coral belonging to a prescribed age. Two modern and 41 fossil microatoll corals from Kiritimati were analysed by the range-finding LA MC-ICPMS technique with a subset also analysed by high-precision solution MC-ICPMS and AMS radiocarbon to assess the reliability and accuracy of the laser ablation technique. Coral ages ranged from 6400 to 900 yr BP and laser ablation age precisions, constrained by the low 230Th count rate, ranged from 9 to 45% at the 2SE level. These ages generally agreed with ages measured by solution U-series and AMS 14C. However, conflicting results for (234U/238U)initial in samples analysed by LA- and solution-MC-ICPMS indicates that the laser ablation method may not be as suitable as previously thought for detecting subtle geochemical alteration. With 40–50 sample analyses per day, the laser ablation method offers a rapid, efficient and cost-effective means to histogram coral ages with multi-centennial accuracy, even at such low 230Th count rates typical for young Holocene samples. In addition to in situ corals, the LA MC-ICPMS technique could be further applied to corals recovered from drill core. Copyright © 2011 Elsevier Inc.
Spatial variability of coastal wetland carbon
(Coastal & Estuarine Research Federation, 2017-12-09) Owers, CJ; Rogers, K; Mazumder, D; Woodroffe, CD
Blue carbon ecosystems, particularly mangrove and saltmarsh, sequester more atmospheric carbon per unit area than any other natural system in the world. Variation in above and below-ground carbon storage, that relate to the expression of environmental processes across wetland landscapes, are yet to be adequately quantified. We proposed that vegetation structure was a significant control on some of the spatial variation in carbon storage, and that this was a function of the dynamic nature of vegetation change at a site. Initially vegetation structural complexity was delineated using innovative remote sensing techniques. Above-ground biomass of mangrove and saltmarsh were quantified by developing region-specific allometric relationships. Cores were extracted from wetlands on the basis of delineated vegetation complexity to characterise the variation in carbon storage within sediments to 2m depth. We found that above-ground biomass varied with vegetation structural complexity, such that tall mangrove contribute more than 65% of above-ground biomass at some sites. The influence of vegetation structure on below-ground carbon storage was only detected to a depth of 50 cm, however >50% of below-ground carbon exists below this depth. Spatial variation in below-ground carbon storage is primarily due to sedimentary factors associated with estuary evolution and geomorphic setting and the influence these factors have on vegetation distribution over the mid-late Holocene. Current approaches to carbon stock assessment, based on extrapolating mean values of carbon storage to national and global scales, oversimplify carbon stock variation and may significantly under or overestimate carbon storage. Systematic approaches to carbon stock assessments characterising carbon storage variation at various scales will provide required confidence necessary for carbon markets.
Spatial variation in carbon storage: a case study for Currambene Creek, NSW, Australia
(Coastal Education and Research Foundation & Journal of Coastal Research, 2016-03-01) Owers, CJ; Rogers, K; Mazumder, D; Woodroffe, CD
Quantifying carbon storage in coastal wetland environments is important for identifying areas of high carbon sequestration value that could be targeted for conservation. This study combines remote sensing and sediment analysis to identify spatial variation in soil carbon storage for Currambene Creek, New South Wales, Australia to establish whether vegetation structure influences soil carbon storage in the upper 30 cm. Wetland vegetation was delineated to capture structural complexity within vegetation communities using Light detection and ranging (Lidar) point cloud data and aerial imagery with an object-based image analysis approach. Sediment cores were collected and analysed for soil carbon content to quantify below-ground carbon storage across the site. The total soil carbon storage in the upper 30 cm for the wetland (59.6 ha) was estimated to be 3933 ± 444 Mg C. Tall mangrove were found to have the highest total carbon storage (1420 ± 198 Mg C), however are particularly sensitive to changes in sea-level as they are positioned lowest in the intertidal frame. Conservation efforts targeted at protecting areas of high carbon sequestration, such as the tall mangrove, will lead to a greater contribution to carbon mitigation efforts. © Coastal Education and Research Foundation, Inc. 2016
Temperate coastal wetland near-surface carbon storage: spatial patterns and variability
(Elsevier B. V., 2020-04-05) Owers, CJ; Rogers, K; Mazumder, D; Woodroffe, CD
Carbon mitigation services provided by coastal wetlands are not spatially homogeneous, nevertheless are commonly described on the basis of vegetation distribution within the intertidal zone. Distribution of mangrove and saltmarsh varies in response to frequency of tidal inundation, resulting in environmental gradients in edaphic factors that influence vegetation structure, and subsequently affect sedimentary carbon additions by vegetation and carbon losses by decomposition. Current sampling approaches and reporting do not adequately account for variability of carbon storage within a wetland, and assessments need to capture spatial variation associated with carbon storage to improve estimates of potential carbon mitigation services by natural ecosystems. This study quantifies the variation in near-surface carbon storage (i.e. upper 30 cm) across an intertidal gradient using a stratified sampling approach that recognises vegetation structure. Vegetation distribution and structure, as well as sedimentary controls on carbon content, explained variation in carbon storage. Saltmarsh near-surface carbon storage varied considerably between structural form. This was less evident for mangrove structural forms (i.e. tall, shrub, dwarf), which may be due to mangrove roots extending to depths beyond 30 cm. Sedimentary characteristics correlated with carbon content, demonstrating considerable influence on near-surface carbon storage within a wetland. The principal finding of this study was that variation within a wetland corresponds to the variation between sites. Stable carbon isotopes offer a means to identify previous vegetation contributions to sediment, associated with an earlier stage of wetland development, likely reflecting previous environmental conditions. A stratified sampling approach that recognises vegetation structure provides the capacity to account for variability of carbon within a wetland that is inadequately described by current sampling protocols. © 2020 Elsevier B.V.
Use of Pb-210 and Cs-117 to simultaneously constrain ages and sources of post-dam sediments in the Cordeaux reservoir, Sydney, Australia
(Elsevier, 2008-07) Simms, AD; Woodroffe, CD; Jones, BG; Heijnis, H; Mann, RA; Harrison, JJ
Environmental radionuclides can be employed as tracers of sediment movement and delivery to water bodies such as lakes and reservoirs. The chronologies of sediments that have accumulated in the Cordeaux reservoir in Sydney, Australia, were determined by the rate of change of Pb-210(cx) with depth and indicate slow accretion in the reservoir. The ratio of enrichment of radionuclides in sediment cores to Pb-210(cx) and (CS)-C-117 concentrations in a reference soil sample within the Cordeaux catchment indicates that the dominant source of sediment in the Cordeaux reservoir is surface erosion (detachment and removal of sediment at depths less than 30 cm). However, in the Kembla Creek arm of the reservoir a mixture of sources was detected and includes sheet and rill erosion together with sub-soil contributions. Implications for the utility of these radionuclide sedimentation assessments, especially where samples are limited, are that well-constrained chronologies and sources of soil erosion are facilitated. © 2008, Elsevier Ltd.

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