The value of the geological record in determining rates and drivers of coastal lagoon shoreline development

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Title:
The value of the geological record in determining rates and drivers of coastal lagoon shoreline development
Author: Adlam, Kellie
Subjects: Carbon sequestration ; Coastal lagoon evolution ; Ecomorphodynamics ; Holocene stratigraphy ; Shoreline processes ; Wetland ecology
Description: This research investigated the feasibility of using the geological record to determine rates and drivers of morphological change in coastal lagoons. Substrate elevation in these environments is of primary importance for survival of wetland habitats, the effectiveness of drainage and flood mitigation functions delivered by those habitats, and the success of potential carbon sequestration programs. Investigating rates and trajectories of lagoon evolution will become more important given the effects of accelerating sea-level rise and human interventions, direct and indirect, on all coastal depositional environments. Elevation change on coastal lagoon shorelines is the net result of numerous sediment accretion and erosion cycles that are subject to considerable uncertainty. Numerous hydrological, biological, geological and anthropogenic processes interact over a range of timescales, and are subject to complex relationships and non-linear feedbacks. To successfully reproduce and predict long-term shoreline change with numerical models, the net effect of these processes must be captured and attributed to appropriate functions and parameter values. Shoreline processes are typically measured in-situ, and measurements would need to span several decades in order to reach an adequate level of confidence about the representativeness of the results. This is particularly true in regions subject to inter-decadal climate variability, such as the El Niño Southern Oscillation in southeast Australia. Even with a sufficiently long-term empirical dataset, the lasting effect of sediment accumulation for elevation change depends strongly on sub-surface processes (root production, decomposition, compaction and soil water content), which take place over still longer timescales and require sub-surface investigation. Reliance on the depositional history captured in the geological record would improve confidence in longer-term rates of morphological change. It would reduce the time and effort required from years (at least) of field measurements to a few months of laboratory work. The effectiveness of the geological record for model parameterisation and calibration, however, depends on the potential to infer drivers of elevation change as well as rates. For this research, soil samples up to 1.8 m depth were obtained in cross-shore core transects from prograded shorelines in three NSW coastal lagoons: Wooloweyah Lagoon near Yamba; Lake Innes near Port Macquarie; and Neranie Bay within Myall Lake. The three lagoons and the segments of shoreline sampled were selected to be as low-energy as possible by avoiding the effects of fluvial and tidal processes that could render intractable shoreline processes with already complex interactions. Each core sample was split and scanned for high-resolution optical images and down-core profiles of magnetic susceptibility, and geochemistry. These datasets enabled the identification and correlation of depositional units between cores and along cross-shore profiles, and thus high-level analysis of shorelines stratigraphy. From each site or transect at least one representative core was selected for detailed investigation, sub-sampled at 10 mm resolution and analysed for grain size, moisture content, density, organic content, and isotopic activity of 210Pb, 137Cs and 14C which provided the approximate timing of deposition for each sub-sample. Mass accumulation rates (g/cm2/yr) and vertical accretion rates (mm/yr) were calculated for correlation with physical sediment properties. At one site, Neranie Bay, this detailed level of analysis was performed for three cores, covering most of the cross-shore transect. Accretion rates calculated for approximately the last 100 years from 210Pb analysis averaged less than 2 mm/yr, consistent with figures reported for similar environments elsewhere in southeast Australia, and at the lower end of the spectrum for internationally reported rates. Preceding the timing of European settlement, accretion rates at the three sites were considerably lower. Recent rates of sediment mass accumulation mostly ranged from 0.02-0.2 g/cm2/yr, but this figure is rarely reported elsewhere and is therefore difficult to compare. Accretion and mass accumulation rates reduced rapidly down-core in the upper few centimetres of each sample, suggesting a significant role for organic matter decomposition for at least several decades following initial deposition. Changes in moisture content and bulk density were observed over similar depths. This research highlights the importance of analysing soil samples to sufficient depth and ensuring sub-surface processes have ceased to have significant impacts on down-core changes before making interpretations about trends over time. A controlling influence of organic content over vertical accretion (and therefore elevation change) was found for the three sites investigated. This control was independent of the inorganic sediment input, which was often higher (by mass) than the organic input. At Neranie Bay, cross-shore trends in organic content were evident. Organic matter input at the surface of the soil sample was greatest when the sample was taken from a higher elevation with less frequent inundation (i.e. short hydroperiod). The proportion of organic matter retained in the soil profile, however, was lowest where hydroperiod was shortest. On balance, organic matter makes the greatest contribution to elevation change when hydroperiod is longest. It could not be determined whether this was caused by higher rates of sub-surface decomposition with short hydroperiod, or high rates of below-ground productivity with long hydroperiod (or both). Either way the results are counter-intuitive and could not be determined without reliance on the geological record. The cross-shore trend that was established from this research is of vital importance. The relationship between hydroperiod and organically driven elevation change results in self-regulating, negative feedback and therefore greater resilience to increases in hydroperiod when the relationship is as reported here. When the reverse relationship is found, however, resilience to increased hydroperiod, and therefore sea level rise, would be compromised because inundation would continually decrease the ability of organic sedimentation to drive accretion, potentially resulting in habitat loss and exposing the shoreline to the risk of erosion. Previous studies suggest that this cross-shore relationship varies on a site-by-site basis. Determining the direction of the relationship with field measurements would take years and still be subject to much higher uncertainty than the methods employed here. This research has shown that the geological record is not only a feasible source of information about accretion rates and drivers, but also a preferable one. Provided further research can succeed in linking sub-surface retention of organic matter to contemporary primary production at the surface, the geological record will provide a more efficient and effective method of designing and calibrating much-needed predictive models to explore scenarios of shoreline development and wetland survival under changing conditions. Further research should also target a range of geologic and climatic settings to differentiate between drivers that can be generalised across all sites and those that vary on a site-by-site basis.
Publisher: University of Sydney
Creation Date: 2014
Language: English

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The value of the geological record in determining rates and drivers of coastal lagoon shoreline development

Adlam, Kellie

University of Sydney 2014

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