High-resolution reconstruction of recent vegetation dynamics in a Mediterranean microtidal wetland: implications for site sensitivity and palaeoenvironmental research

The analysis of recent sediment sequences from coastal wetlands provides an opportunity to examine the response of these sites to environmental change and events, many of which are independently documented. This also permits an evaluation of rates of response to be made that can help in assessing changes identified in longer-term (Holocene) coastal sediment sequences. A short core from the Mulinello estuary, Augusta Bay, south east Sicily, was dated using 210Pb and 137Cs. Samples were analysed for pollen and spore content, and the results are presented here as both percentage and influx data. Temporal resolution of the pollen data is typically 5-15 years for the first 50 years of the record (circa 1895-1945 AD) and 2-5 years for the last 50 years (1945-1995 AD). Two phases of salt marsh expansion in the Bay occurred, up to the 1940s and from the 1960s to the mid 1980s. In the mid 1940s, the salt marsh underwent a significant decline, marked by a sudden fall in influx and percentage data for Chenopodiaceae. This correlates with an inwashing of catchment-derived pollen, particularly of resistant Lactucae grains, indicating more regular fluvial inundation. Climate records show the occurrence of significantly higher precipitation at this time. Since the construction of a port access road in the 1980s a second decline in the local halophyte community occurred. Pollen influx data enable a precise assessment of how quickly local colonisation of surfaces at the sampling site occurred. During both episodes of salt marsh colonisation, the transition from low-moderate to high Chenopodiaceae influx took less than 6 years. The data show that salt marsh communities can expand and decline very rapidly and that these variations can occur independently of significant changes in relative sea level

Recent estuarine sedimentation rates from shallow inter-tidal environments in western Scotland: implications for future sea level trends and coastal wetland development

During the mid-late Holocene large sections of the Scottish coastline have been characterized by falling relative sea-levels resulting from differential glacio-isostatic uplift of this region of northern Britain. The complex interplay between crustal and sea-level movements continues to influence the morphological development of the Scottish coast. A number of geophysical models predict ongoing uplift of the Scottish landmass. However, a number of recent studies based upon the analysis of satellite altimetry data indicate a late 20th Century acceleration in the rate of eustatic sea-level rise.Detailed geochemistry, radiometric dating, and diatom analysis on selected sediment cores from four mature coastal marsh environments in Argyll, western Scotland, provides an opportunity to investigate the linkages between Twentieth century crustal movements, eustatic sea-level rise and recent rates of sedimentation recorded within marsh sediments across the proposed Scottish glacio-isostatic uplift dome.Solid-phase major and trace element geochemistry has been used to examine the extent to which post-depositional physical disturbance and/or chemical reactions may have influenced the reliability of the radiometric dating methods. Geochemical data indicate that the evolution of these marsh environments has not been significantly influenced by physical disturbance and overall the supply of minerogenic material to the marshes has been quite uniform.Vertical distributions of 210Pbexcess and 137Cs activity have been measured and used to develop models of recent marsh vertical accretion. Dating of the cores reveals subtle variations in the rates of sediment accumulation over the last c. 70 years between sites. For much of the last hundred years or so, sedimentation rates have been in good overall agreement with various estimations for sea-level rise, although at the more easterly sites these estimates are generally exceeded. However, quasi-equilibrium between marsh sedimentation and sea-level rise for much of the Twentieth Century is indicated from the Diatom analysis.Over the most recent period of marsh development (<10 years), a significant increase in the rate of surface sedimentation is recorded at all sites across the study area. Diatom analysis of these surface layers reveals an increase in the relative abundance of marine (polyhalobous) taxa in the near-surface sediments. This signifies a very recent increase in the rate of regional relative sea-level rise indicating that a regional threshold in coastal forcing has now been exceeded.These findings provide clear evidence that recent relative sea-level rise is now outpacing estimated rates of glacio-isostatic adjustment (GIA) across the proposed Scottish uplift dome

The role of chalk in the development of buried (“drift-filled”) hollows

Drift-filled’ hollows are an increasingly recognised hazard in the London Basin. This largely reflects improvements in data quality relating to ground investigations for recent and ongoing development, together with improved access to this data in digital format. Potential problems linked to them include differential subsidence, tunnel face collapse, unexpected water ingress into excavations and possible contamination of groundwater. The hollows occur as large closed depressions in the rockhead – typically London Clay Formation, though sometimes penetrating down to the Chalk. They are infilled by a range of Quaternary (superficial) deposits including river gravel and sand, lacustrine silt and clay, and some organics. Where sample quality has been good, this infill often shows signs of post-depositional, and possibly syn-sedimentary disruption. The hollows’ distribution shows a rough pattern associated with valley floor geomorphology, current or former artesian conditions, thickness of the London Clay Formation, and presence of clay-rich units within the Lambeth Group (Banks et al. 2014). There may also be an association with geological structures. Consideration of the available deeper evidence shows that some, at least, of the hollows are associated with significant disturbance of the underlying ‘solid’ strata, including upwards folding and diapir-like intrusion of disaggregated chalk, together with vertical mixing of reworked Cretaceous, Tertiary and Quaternary materials. The genesis of the hollows is still under debate. This paper will propose a formation hypothesis, with a particular focus on the role of the Chalk. The break up and subsequent mobilisation of chalk will be examined and its possible contribution to the evolution of the hollows, and potential ongoing hazards, will be considered. Ongoing research will use the evidence within this paper to evaluate competing existing and forthcoming hypotheses

Coastal wetlands as recorders of earthquake subsidence in the Aegean: a case study of the 1894 Gulf of Atalanti earthquakes, central Greece

Earthquakes may have a major impact on the morphology and evolution of coasts in tectonically active areas. In the area of the Gulf of Atalanti, central Greece, a damaging earthquake sequence in 1894 is reported to have caused nearshore slumping, tsunami-induced flooding, and decimetre-to metre-scale coastal subsidence. The earthquakes caused major changes to coastal configuration, in particular the separation of the Gaïduronisi peninsula from the mainland.Detailed stratigraphical, microfossil and radiometric (210Pb and 137Cs) analyses of coastal wetlands around the Gulf of Atalanti show that these environments preserve evidence of sudden, lasting subsidence contemporaneous with the 1894 events, with foraminiferal assemblages at a site on the mainland opposite the island indicating an abrupt change from supratidal soil to intertidal marine conditions. The elevation change indicated by this terrestrial to marine transition is 30–80 cm. The coastal stratigraphy in the Atalanti area however lacks laterally extensive sharp soil–mud contacts or tsunami deposits of the type used in identifying past earthquake subsidence events at plate–boundary settings. Despite the major earthquake-induced changes in coastal configuration, the stratigraphical signature of the 1894 earthquakes is relatively subtle, and is consequently difficult to distinguish from other rapid coastal changes (e.g. storm events or barrier breaching) without high-resolution dating control and reference to documentary records. For this reason, coastal wetland stratigraphies are only likely to be useful in identifying pre-historical or poorly documented earthquakes in the Aegean at sites where metre-scale coseismic elevation changes and/or major tsunami inundation have occurred. In addition, the resolution of the biostratigraphic data reported here is insufficient to distinguish between the contrasting fault rupture models for the 1894 events. The coastal elevation changes recorded in these wetlands, however, are best accounted for by a slip of 1 m on a multi-segment fault. <br/

Coastal wetlands as recorders of earthquake subsidence in the Aegean: A case study of the 1894 Gulf of Atalanti earthquakes, central Greece

Earthquakes may have a major impact on the morphology and evolution of coasts in tectonically active areas. In the area of the Gulf of Atalanti, central Greece, a damaging earthquake sequence in 1894 is reported to have caused nearshore slumping, tsunami-induced flooding, and decimetre-to metre-scale coastal subsidence. The earthquakes caused major changes to coastal configuration, in particular the separation of the Gaiduronisi peninsula from the mainland. Detailed stratigraphical, microfossil and radiometric (210Pb and 137Cs) analyses of coastal wetlands around the Gulf of Atalanti show that these environments preserve evidence of sudden, lasting subsidence contemporaneous with the 1894 events, with foraminiferal assemblages at a site on the mainland opposite the island indicating an abrupt change from supratidal soil to intertidal marine conditions. The elevation change indicated by this terrestrial to marine transition is 30-80 cm. The coastal stratigraphy in the Atalanti area however lacks laterally extensive sharp soil-mud contacts or tsunami deposits of the type used in identifying past earthquake subsidence events at plate-boundary settings. Despite the major earthquake-induced changes in coastal configuration, the stratigraphical signature of the 1894 earthquakes is relatively subtle, and is consequently difficult to distinguish from other rapid coastal changes (e.g. storm events or barrier breaching) without high-resolution dating control and reference to documentary records. For this reason, coastal wetland stratigraphies are only likely to be useful in identifying pre-historical or poorly documented earthquakes in the Aegean at sites where metre-scale coseismic elevation changes and/or major tsunami inundation have occurred. In addition, the resolution of the biostratigraphic data reported here iS insufficient to distinguish between the contrasting fault rupture models for the 1894 events. The coastal elevation changes recorded in these wetlands, however, are best accounted for by a slip of 1 m on a multi-segment fault. (C) 2000 Elsevier Science B.V