A Palaeoenvironmental Reconstruction of Galway Bay, Western Ireland, from the Last Glacial Maximum to Present Day

Abstract

Studies of past changes in the environment are essential to understand the speed and scale of modern-day climatic and environmental change. This research investigates the palaeoenvironmental changes throughout the last glacial and current interglacial period in Galway Bay, Western Ireland, using a multiproxy examination of geophysical data and twenty-two sediment cores. The methodologies used in this study include the interpretation of multibeam echo-sounder bathymetry and backscatter, altimetry, hydrological and seismic data, as well as the analysis and interpretation of the sediment cores’ physical properties, X-radiographs, grain size analysis, microfossil content, geochemical signatures (micro-XRF) and radiocarbon dates. These data together allow for an interpretation of the driving forces behind the observed patterns in the distribution of sediments at the seafloor and sub-surface from the last glacial maximum until present day. Such data also provide field data, which can be used to constrain sea level in the bay after 19 ka cal BP. This is particularly valuable as the only available sea-level reconstructions for the area are derived from glacio-isostatic adjustment (GIA) models.
Seismic data cover the mid and outer bay and reveal three distinct seismic units, representing bedrock, glacial till and post-glacial marine sediments. The till unit, overlying the bedrock, shows evidence of a glaciofluvially cut channel along the northern portion of the bay, constraining the relative sea-level depth in the bay during deglaciation to 58m below modern-day levels. This lowstand is in broad agreement with a GIA predicted sea-level position at 15 ka cal BP. The overlying marine sediment unit, in which prograding clinoforms and infilling are visible, represents the sea-level transgression since 15 ka cal BP. Shoreline deposits (rounded pebbles in a silty sand matrix, found in four cores) were deposited in the mid and outer Bay between 12.6 ka cal BP and 8.8 ka cal BP. The identification of such submerged palaeo-shore line sediments in depths of 39-43m reveals a late glacial to early Holocene slowstand. This identified slowstand lasted 2500 years longer than predicted by GIA based predictions. Core data suggests that, due to this delayed subsequent transgression, GIA derived RSL level predictions were not reached until ~4000 years BP. As well as providing important information for RSL reconstructions, the sediments and foraminifera within
allow the identification of palaeoenvironmental conditions over time and space. In general, the pre- and early Holocene period is characterised by high sediment input in warm, productive, estuarine conditions with 2 separate inlets in the bay leading to two distinct environments: (1) a high energy beach environment transitioning to low energy, transitional waters in the north, and (2) a low energy, anoxic subtidal environment in the south. The mid Holocene period is characterised by the opening up of the bay and the presence of the gastropod Turritella sp. across the entire area. This represents a medium energy, warm, productive, environment in transitional waters. The late Holocene is typified by the disappearance of Turritella sp. and a transition to marine conditions. This represents a deepening, productive and high energy environment with warm and wet climatic conditions in the region. Interestingly though, global and regional climatic events such as the Younger Dryas, the 8.2 ka and 4.2 ka event have left no significant imprint on the sedimentary record. Overall, the data acquired from Galway Bay documents constant relative sea-level transgression, with a trend towards a wetter and marine- influenced environment after the initial deglaciation. The more recent Late Holocene sediments and current seabed topography record the influence of tidal currents, storm events and wave action, characteristic of the western Irish coastline.
Results from this research can help validate both climate and sea-level models for the region, which in turn provide predictions for future environmental developments.

Date of Award	Aug 2019
Original language	English
Sponsors	Department of Employment and Learning, Northern Ireland
Supervisor	Sara Benetti (Supervisor) & Ruth Plets (Supervisor)