Browsing by Author "Simms, Alexander R."
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Item A community-based geological reconstruction of Antarctic Ice Sheet deglaciation since the Last Glacial Maximum(Elsevier, 2014) The RAISED Consortium; Bentley, Michael J.; Ó Cofaigh, Colm; Anderson, John B.; Conway, Howard; Davies, Bethan; Graham, Alastair G.C.; Hillenbrand, Claus-Dieter; Hodgson, Dominic A.; Jamieson, Stewart S.R.; Larter, Robert D.; Mackintosh, Andrew; Smith, James A.; Verleyen, Elie; Ackert, Robert P.; Bart, Philip J.; Berg, Sonja; Brunstein, Daniel; Canals, Miquel; Colhoun, Eric A.; Crosta, Xavier; Dickens, William A.; Domack, Eugene; Dowdeswell, Julian A.; Dunbar, Robert; Ehrmann, Werner; Evans, Jeffrey; Favier, Vincent; Fink, David; Fogwill, Christopher J.; Glasser, Neil F.; Gohl, Karsten; Golledge, Nicholas R.; Goodwin, Ian; Gore, Damian B.; Greenwood, Sarah L.; Hall, Brenda L.; Hall, Kevin; Hedding, David W.; Hein, Andrew S.; Hocking, Emma P.; Jakobsson, Martin; Johnson, Joanne S.; Jomelli, Vincent; Jones, R. Selwyn; Klages, Johann P.; Kristoffersen, Yngve; Kuhn, Gerhard; Leventer, Amy; Licht, Kathy; Lilly, Katherine; Lindow, Julia; Livingstone, Stephen J.; Massé, Guillaume; McGlone, Matt S.; McKay, Robert M.; Melles, Martin; Miura, Hideki; Mulvaney, Robert; Nel, Werner; Nitsche, Frank O.; O'Brien, Philip E.; Post, Alexandra L.; Roberts, Stephen J.; Saunders, Krystyna M.; Selkirk, Patricia M.; Simms, Alexander R.; Spiegel, Cornelia; Stolldorf, Travis D.; Sugden, David E.; van der Putten, Nathalie; van Ommen, Tas; Verfaillie, Deborah; Vyverman, Wim; Wagner, Bernd; White, Duanne A.; Witus, Alexandra E.; Zwartz, DanA robust understanding of Antarctic Ice Sheet deglacial history since the Last Glacial Maximum is important in order to constrain ice sheet and glacial-isostatic adjustment models, and to explore the forcing mechanisms responsible for ice sheet retreat. Such understanding can be derived from a broad range of geological and glaciological datasets and recent decades have seen an upsurge in such data gathering around the continent and Sub-Antarctic islands. Here, we report a new synthesis of those datasets, based on an accompanying series of reviews of the geological data, organised by sector. We present a series of timeslice maps for 20 ka, 15 ka, 10 ka and 5 ka, including grounding line position and ice sheet thickness changes, along with a clear assessment of levels of confidence. The reconstruction shows that the Antarctic Ice sheet did not everywhere reach the continental shelf edge at its maximum, that initial retreat was asynchronous, and that the spatial pattern of deglaciation was highly variable, particularly on the inner shelf. The deglacial reconstruction is consistent with a moderate overall excess ice volume and with a relatively small Antarctic contribution to meltwater pulse 1a. We discuss key areas of uncertainty both around the continent and by time interval, and we highlight potential priorities for future work. The synthesis is intended to be a resource for the modelling and glacial geological community.Item Holocene Evolution of the Western Louisiana–Texas Coast, USA: Response to Sea-Level Rise and Climate Change(2022) Anderson, John B.; Wallace, Davin J.; Rodriguez, Antonio B.; Simms, Alexander R.; Milliken, Kristy T.; Geological Society of AmericaAn extensive grid of high-resolution seismic data, hundreds of sediment cores, and a robust radiocarbon-age data set acquired over nearly four decades allows detailed analysis of Holocene coastal evolution of western Louisiana and Texas, USA. Results from this study provide a framework for assessing the response of a myriad of coastal environments to climate change and variable sea-level rise. Climate varies across the region today, spanning four climate zones from humid to semi-arid, and has fluctuated during the Holocene. The most notable changes were alterations between cool/ wet and warm/dry conditions. Sea-level records for the northwestern Gulf of Mexico indicate an average rate of rise during the early Holocene of 4.2 mm/yr, punctuated by rates exceeding 10.0 mm/yr. After ca. 7.0 ka, the rate of rise slowed, and by ca. 4.0 ka, the average rate decreased from 0.6 mm/yr to 0.3 mm/yr. The current rate of sea-level rise in the region is 3.0 mm/yr, marking a return to early Holocene conditions. Despite its incomplete stratigraphic record of coastal evolution during the middle and early Holocene, it is still the most complete record for the Gulf Coast. Bay evolution, as recorded within the offshore Trinity and Sabine incised valleys, was characterized by periods of bayhead delta and tidal delta expansion, followed by episodes of dramatic landward shifts in these environments. The ancestral Brazos, Colorado, and Rio Grande river deltas and coastal barriers also experienced landward stepping during the early Holocene. The widespread nature of these flooding events and their impact on multiple coastal environments suggests that they were caused by episodes of rapid sea-level rise. Similar methods were used to study modern bays, including the acquisition of seismic lines and drill cores along the axes of the bays to examine the magnitudes and timing of transgressive events. Results from Lake Calcasieu, Sabine Lake, Galveston Bay, Matagorda Bay, Copano Bay, Corpus Christi Bay, and Baffin Bay reveal that landward shifts in bayhead deltas, on the order of kilometers per century, occurred between 9.8 ka and 9.5 ka, 8.9–8.5 ka, 8.4–8.0 ka, and 7.9–7.5 ka. These results are consistent with those from offshore studies and indicate that punctuated sea-level rise dominated coastal evolution during the early Holocene. By ca. 7.0 ka, the average rate of sea-level rise in the northern Gulf of Mexico decreased to 1.4 mm/yr, and there was considerable sinuosity of the coastline and variability in the timing of bay and coastal barrier evolution. The diachronous nature of coastal environment migration across the region indicates that sea-level rise played a secondary role to climate-controlled oscillations in river sediment discharge to the coast. At ca. 4.0 ka, the average rate of sea-level rise decreased to 0.5 mm/yr. During this period of slow sea-level rise, coastal bays began to take on their current form, with the exception of changes in the sizes and locations of bayhead deltas caused by changes in sediment supply from rivers. There were also significant changes in the size and configuration of tidal inlets and deltas as a result of barrier growth. The late Holocene was also a time when coastal barriers experienced progradation and transgression on the order of several kilometers. The timing of these changes varied across the region, which is another indication that sea-level rise played a minor role in coastal change during the late Holocene. Instead, barrier evolution during this time was controlled by fluctuations in sand supply to the coast from rivers and offshore sources. Historical records indicate a dramatic reversal in coastal evolution marked by increased landward shoreline migration of chenier plains and coastal barriers across the region. The main cause of this change is accelerated sea-level rise during this century and diminished sediment supply to the coast. Wetlands are also experiencing rapid change due to their inability to keep pace with sea-level rise, especially in areas where subsidence rates are high. Although direct human influence is a factor in these changes, these impacts are more localized. Coastal change is expected to increase over the next several decades as the rate of sea-level rise increases, the climate in Texas becomes more arid, and more severe storms impact the coast.Item Reconstruction of ice-sheet changes in the Antarctic Peninsula since the Last Glacial Maximum(Elsevier, 2014) Ó Cofaigh, Colm; Davies, Bethan J.; Livingstone, Stephen J.; Smith, James A.; Johnson, Joanne S.; Hocking, Emma P.; Hodgson, Dominic A.; Anderson, John B.; Bentley, Michael J.; Canals, Miquel; Domack, Eugene; Dowdeswell, Julian A.; Evans, Jeffrey; Glasser, Neil F.; Hillenbrand, Claus-Dieter; Larter, Robert D.; Roberts, Stephen J.; Simms, Alexander R.This paper compiles and reviews marine and terrestrial data constraining the dimensions and configuration of the Antarctic Peninsula Ice Sheet (APIS) from the Last Glacial Maximum (LGM) through deglaciation to the present day. These data are used to reconstruct grounding-line retreat in 5 ka time-steps from 25 ka BP to present. Glacial landforms and subglacial tills on the eastern and western Antarctic Peninsula (AP) shelf indicate that the APIS was grounded to the outer shelf/shelf edge at the LGM and contained a series of fast-flowing ice streams that drained along cross-shelf bathymetric troughs. The ice sheet was grounded at the shelf edge until ∼20 cal ka BP. Chronological control on retreat is provided by radiocarbon dates on glacimarine sediments from the shelf troughs and on lacustrine and terrestrial organic remains, as well as cosmogenic nuclide dates on erratics and ice moulded bedrock. Retreat in the east was underway by about 18 cal ka BP. The earliest dates on recession in the west are from Bransfield Basin where recession was underway by 17.5 cal ka BP. Ice streams were active during deglaciation at least until the ice sheet had pulled back to the mid-shelf. The timing of initial retreat decreased progressively southwards along the western AP shelf; the large ice stream in Marguerite Trough may have remained grounded at the shelf edge until about 14 cal ka BP, although terrestrial cosmogenic nuclide ages indicate that thinning had commenced by 18 ka BP. Between 15 and 10 cal ka BP the APIS underwent significant recession along the western AP margin, although retreat between individual troughs was asynchronous. Ice in Marguerite Trough may have still been grounded on the mid-shelf at 10 cal ka BP. In the Larsen-A region the transition from grounded to floating ice was established by 10.7–10.6 cal ka BP. The APIS had retreated towards its present configuration in the western AP by the mid-Holocene but on the eastern peninsula may have approached its present configuration several thousand years earlier, by the start of the Holocene. Mid to late-Holocene retreat was diachronous with stillstands, re-advances and changes in ice-shelf configuration being recorded in most places. Subglacial topography exerted a major control on grounding-line retreat with grounding-zone wedges, and thus by inference slow-downs or stillstands in the retreat of the grounding line, occurring in some cases on reverse bed slopes.Item Unprecedented Historical Erosion of US Gulf Coast: A Consequence of Accelerated Sea-Level Rise?(Wiley, 2023) Anderson, John B.; Wallace, Davin J.; Rodriguez, Antonio B.; Simms, Alexander R.Most of the US Gulf Coast is composed of barrier islands, peninsulas, chenier plains, and mainland beaches that are the main line of defense for wetlands, estuaries, and urban and industrial centers from rising sea level and severe storms. These wave-dominated shorelines are currently experiencing widespread erosion. Using newly acquired and existing results from 13 sites spanning south Florida to south Texas, we compare shoreline migration rates during the late Holocene (∼−4000 to 1850 CE) with historical changes since the mid-19th century. The records show an overall trend of seaward growth during the late Holocene followed by landward migration or a decrease in the rate of growth during historical time. Diminishing offshore sand supply, human alteration of rivers and coastal sand transport, and severe storms have contributed to this change in shoreline trajectory, but their influence has been mostly limited in extent. The most likely cause of this reversal from coastal stability and growth to widespread shoreline retreat is the dramatic historical increase in the rate of sea-level rise over the past century.