Browsing by Author "Nittrouer, Jeffrey A"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    The Antarctic Peninsula's Response to Holocene Climate Variability: Controls on Glacial Stability and Implications for Future Change
    (2015-12-02) Minzoni, Becky Lynn; Anderson, John B; Droxler, André; Nittrouer, Jeffrey A; Rudolf, Volker
    The Antarctic Peninsula is one of the most rapidly changing regions in the Cryosphere, with 87% of its glaciers receding and several ice shelves catastrophically collapsing since observations began in the 1960’s. These substantial, well-documented changes in the ice landscape have caused concern for the mass balance of the Antarctic Peninsula Ice Cap. To better understand the significance of these recent changes, I have assimilated a massive database of new and published marine sedimentary records spanning the Holocene Epoch (the last 11.5 kyrs). The database includes 9 coastal embayments with expanded sedimentary packages and well-dated cores. Each site represents an end-member in the wide range of Antarctic Peninsula oceanography, orography, meteorology, and glacial drainage basin characteristics. Multi-proxy analysis, including sedimentology, geochemistry, and micropaleontology, was conducted at each site to reconstruct glacial history at centennial-scale resolution on par with ice-core data. The coastal sites were then compared in the context published ice-core paleoclimate, paleoceanographic, and glaciological records. The first of these sites, Herbert-Croft Fjord, provides an unparalleled opportunity to compare the marine sediment record with a related ice-core in an Antarctic maritime setting. Herbert-Croft Fjord is the southernmost embayment studied on the eastern side of the Antarctic Peninsula and represents an end-member with a cold, dry atmosphere and cold, saline ocean mass. The record from Herbert-Croft Fjord indicates grounded ice receded quickly and early in the Holocene, followed by a floating ice phase that collapsed 10 ± 2.4 calendar kyrs before present (cal kyr BP, where present day is 1950 A.D.) and never re-advanced. The fjord remained open and productive during the prolonged warm intervals of the Mid Holocene, and began to experience greater glacial influence and sea ice cover during Late Holocene cooling, a period termed the Neoglacial. The second site, Ferrero Bay of the Amundsen Sea, is the southernmost end-member on the western side of the Antarctic Peninsula and represents a truly polar coastal setting. Grounded ice receded from the deep basin much earlier than expected, ~10.7 cal kyr BP, due to warm deep water masses that under-melted the extended ice sheet during the Early Holocene. Ferrero Bay was then covered by the extensive Cosgrove Ice Shelf, which remained stable during the Mid Holocene Hypsithermal and did not recede to its current position until ~2.0 cal kyr BP, coincident with Neoglacial cooling. Thus, Ferrero Bay and the Cosgrove Ice Shelf were starkly out of phase with the northern peninsula sites and apparently were not sensitive to Holocene climate variability but rather to impinging warm ocean currents, which are observed in Ferrero Bay today. Comparison of 9 coastal sites, including Herbert-Croft Fjord, Ferrero Bay, and several other embayments of various local settings, shows highly variable glacial response to Early Holocene warming, with difference in response onset of ± 4.2 kyrs and difference in duration of ± 2.5 kyrs. The Mid Holocene was less variable, with the onset of moderately cooler conditions varying by ± 1.7 kyrs and difference in duration of this epidosde of ± 1.9 kyrs. The Late Holocene Neoglacial by contrast was synchronous with difference in onset and duration of ± 0.7 kyrs and ± 0.7 kyrs, respectively. Later historic events of shorter duration, such as the Little Ice Age, were more synchronous with differences in onset and duration of ± 0.3 kyrs and ± 0.2 kyrs, respectively. Regional glacial behavior became more synchronous during the Holocene as the glaciers and their drainage basins became progressively smaller, rendering them more sensitive to climate change and less influenced by various local forcings such as ocean temperature, basin bathymetry, and precipitation differences. The increase in glacier sensitivity helps explain the current widespread glacial recession in response to the rapid regional warming of ~3.5° C over the last century.
  • Thumbnail Image
    Item
    Assessing deltaic landscape management strategies based on studies from the Yellow River delta, China
    (2020-04-21) Moodie, Andrew J; Nittrouer, Jeffrey A
    Deltaic environments possess an abundance of natural resources that promote human welfare, but these delicate environments are threatened by a multitude of natural and anthropogenic factors. This dissertation is motivated by a need to understand the impact of anthropogenic channel management on river-delta resiliency. The unanticipated civil disruption associated with flooding and channel relocation via avulsion is at odds with society's desire for landscape stability, so channel engineering that restricts mobility and floodplain connectivity is common, but this engineering diminishes sediment dispersal necessary to sustain deltas and coastlines. On the Yellow River delta, China, engineered diversions have been implemented to approximate natural delta evolution; the Yellow River fluvial-deltaic system is used throughout this dissertation as a case study, to identify best practices for promoting deltaic sustainability that are exportable to other deltas around the world. In three chapters, this dissertation addresses the following questions: 1) how do sediment transport mechanics change in high concentration flows, and how does this impact diversion effectiveness, 2) what factors dictate the timing and location of natural deltaic avulsions, and 3) what is the channel response to artificial diversions, and what diversion length optimizes societal benefit while minimizing cost. First, a study of density stratification in the Yellow River reveals that fine sediment extracts turbulent energy from the river flow, modulating the vertical distribution of sediment and impacting effectiveness of water and sediment extraction efforts to deliver sediment to the coast. Second, a numerical model is developed to simulate the processes that lead to natural deltaic avulsion, and it is found that deltaic lobe progradation profoundly impacts the location and timing of avulsion. Third, simulating artificial diversions along a deltaic channel course reveals autogenic channel behavior that leads to system nonlinearity and emergence of an optimal diversion length. Overall, this dissertation research indicates that the sustainability of coastal landscapes is achievable, but will require continued study and careful management of delicate coastal landscapes.
  • Thumbnail Image
    Item
    Dynamics of delta building across different scales as informed by the Selenga River Delta, Russia
    (2020-04-24) Dong, Tian Yang; Nittrouer, Jeffrey A
    This dissertation examines dynamics of delta building across different spatial and temporal scales using field measurements and remotely-sensed data collected from the Selenga River delta, Russia. The research topics include evaluating the controls on hydraulic channel geometry for sand-bed rivers, developing a framework to better predict flow partitioning in delta channel networks, as well as deciphering the impacts of tectonic subsidence and variable basin depth on delta lobe building. Field measurements include detailed hydrological and sedimentological data collected within the distributary network of the Selenga Delta over four field campaigns, conducted 2013−2018. Remotely-sensed data include measurements of shoreline position, topset elevation, and basin depth since 1862. This research shows that, for the smallest of channel scales, bankfull width is directly, and depth is inversely, related to the characteristic median bank sediment size. Furthermore, new predicative relations of hydraulic channel geometry for sand-bed rivers that consider bank sediment size are derived, which improve upon previous work. On the channel network scale, channel geometry and planform variables, including cross-sectional area, depth, width, length, and sinuosity are determined to be the best parameters for predicting water and sediment partitioning in a delta system. The best predictive relations for water and sediment partitioning (channel width, length, and sinuosity) are incorporated into an exiting graph theory model, to yield a framework that accurately predicts flux distribution in delta networks. This method is tested by predicting flow partitioning in the Wax Lake delta, the Selenga River delta, and the Lena River delta. Predictions are in good agreement with field measurements. At the delta lobe scale, discrete tectonic subsidence events caused by the Baikal Rift are found to modify avulsion dynamics by varying receiving basin depth. These subsidence events create shallow embayments around the delta, by downdropping a portion of the topset below the average channel depth. As a result, flow is attracted to the partially subsided lobe, as the regional gradient is increased, due to a reduction in delta topset radius. Incisional terraces form as channels re-equalibrate to this new topset gradient. Theory predicts that these terraces reduce the frequency of lobe-scale avulsions by increasing the avulsion-setup threshold. Downstream of the terraces, smaller channel-scale avulsions still occur, and distribute water and sediment to topographic lows between the active channels. Predictive relations and models developed from the studies of this dissertation can be used standalone or incorporated into numerical models to better forecast the response of deltas to perturbations, such as the ongoing climate.
  • Thumbnail Image
    Item
    Evolution of North Malé Atoll Rim during the Last Full Glacial Cycle (Malé Island, Republic of Maldives)
    (2014-07-31) Koksal, Tugba; Droxler, André W; Sawyer, Dale S; Nittrouer, Jeffrey A; Gischler, Eberhard
    My study focuses on the evolution of Malé Island to elucidate the late Pleistocene-Holocene evolution of the discontinuous North Malé Atoll rim. Sample analyses from two boreholes, published information from additional boreholes, and a multi-beam bathymetric map for Malé Island deep surroundings, are available for this study. Facies analyses of the lower sedimentary unit reveal an overall deepening coralgal reef that accumulated probably during the previous interglacial and subsequently was altered by meteoric diagenesis during a 100 ky-long exposure. The upper Holocene unconsolidated coralgal reef, overlying the karstified coralgal MIS 5e lower unit, was initiated at ~8200 kyr BP and vertically grew 25 m high until 6510 kyr BP, protected behind a karstified late Pleistocene reef. A small, though 30-35 m deep, newly formed lagoon started to fill up only at ~5500 kyr BP, when a reef initiated on top of the highest elevated Pleistocene karstified reef and sea level rise stalled.