Browsing by Author "Anderson, John B"
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Item Downstream elimination of gravel in multiple distributary channels on the Selenga River delta topset, Russia: a morphodynamics case study of fan delta at the Baikal Rift basin margin(2015-04-24) Dong, Tian Yang; Nittrouer, Jeffrey A.; Anderson, John B; Dickens, Gerald RThe Selenga River delta, Lake Baikal, Russia, is approximately 600 km² in size and contains multiple distributary channels that receive varying amounts of water and sediment discharge. This delta is positioned along the deep-water (~1600 m) margin of Lake Baikal, a half-graben styled rift basin, qualifying it as a modern analogue of shelf-edge system. This study provides a detailed field survey of channel bed sediment composition, channel geometry, and water discharge. Our data indicate that the delta exhibits downstream sediment fining, ranging from predominantly coarse gravel and sand near the delta apex to silt and sand at the delta-lake interface. An analytical framework is developed utilizing field data to evaluate the downstream elimination of gravel within the multiple distributary channels. Our major findings are: 1.) the Selenga River delta consists of at least eight orders of distributary channels, 2.) with increasing channel order, channel cross-sectional area, width-depth ratio, water discharge, boundary shear stress, and sediment flux all decrease downstream, 3.) the downstream elimination of gravel in distributary channels is caused by reducing boundary shear stress downstream where water discharge is partitioned among bifurcating channels, 4.) Gravel is trapped on the delta topset due to frequent basin subsidence, thus preventing coarse material from being delivered to the axis of the rift basin. The distribution of sediment in deltaic channels and the associated sediment transport processes that construct stratigraphy, combined with the active tectonic setting, allow sedimentary systems like the Selenga delta to be preserved in the long-term geologic record. Therefore, the location of the Selenga River delta along the active Baikal Rift margin renders the opportunity to explore the influence of tectonics and sedimentation on deltaic processes that produce basin stratigraphy.Item Glacial Retreat Patterns and Processes on Antarctic Continental Margins(2018-11-30) Prothro, Lindsay O'Neal; Anderson, John BAntarctic ice margins are complex and dynamic, and Antarctic ice sheets hold a volume of ice equivalent to ~53 meters of sea level rise. The necessity of understanding ice margins and their stability motivates advances in ice-sheet and sea-level modeling, but models must be tested and tuned using observational data. However, contemporary ice margins are difficult to access and offer a limited spatial and temporal glimpse of the processes that occur where the ocean meets the ice. Therefore, geologic investigations of now-deglaciated portions of the Antarctic continental shelf are used to provide a more substantial record of the timing, rates, and patterns of past ice sheet retreat, as well as the mechanisms that have caused instability in the past. This study merges data from sediment cores, bathymetric mapping, and radiocarbon dating to provide records of ice-sheet retreat on Antarctic continental margins. Cores from the Ross Sea have been analyzed to develop a new sediment facies model that is reflective of geomorphic context. This facies model is designed to improve interpretations of events dated with radiocarbon. Analysis of sedimentary facies relative to seafloor geomorphology has also revealed a contrasting retreat style of the East Antarctic Ice Sheet (EAIS) from the West Antarctic Ice Sheet (WAIS) following the Last Glacial Maximum, with the EAIS retreating continuously and the WAIS remaining stable for long periods of time before episodically retreating over large distances. Previously-published and newly-acquired radiocarbon ages have been interpreted within the context of the updated facies model and have been found to reveal a highly complex EAIS retreat pattern that includes a major mid-Holocene ice shelf collapse, reorganization of drainage, and readvance of ice. Furthermore, these new results have mitigated previous discrepancies between marine and terrestrial records of ice sheet retreat. The sediment facies model has also been used to interpret previously-published and newly-acquired radiocarbon ages in Marguerite Bay. The ancestral Marguerite Trough Ice Stream was underlain by a subglacial hydrological system that radiocarbon ages indicate was highly active from ~13-10 cal ka BP, possibly creating instability that prompted a major ice-sheet collapse at ~10 cal ka BP. These new records surpass the resolution of current ice sheet models.Item Last 2000 Year Climate Sediment Record from the Belize Central Shelf Lagoon: A Detailed Archive of Droughts and Floods Linked to the Collapse of the Mayan Civilization and Caribbean Historical Famines(2014-05-30) Agar Cetin, Ayca; Droxler, Andre W; Anderson, John B; Morgan, Julia KIn the past several decades, climate change linked to increasing anthropogenic CO2 emission to the atmosphere, has resulted not only in steady global warming but also in extreme climate events. Heat and cold waves, flash floods and droughts, and catastrophic hurricanes, are some of the extreme climate events the Earth has been experiencing. In the future, those events are expected to become common rather than exceptional. Understanding processes linked to extreme climate is becoming more crucial and analyzing extreme climate paleo-records have become more important. This study is focusing on the last 2000 yr precipitation record archived in the mixed carbonate/siliciclastic sediments accumulated in the Belize Central Shelf Lagoon, partially filling the Rhomboid Reef lagoons and English Caye Channel. The Belize climate is described as subtropical, largely influenced by the seasonal migration of the Intertropical Convergence Zone (ITCZ), triggering alternating winter dry and summer wet seasons. In the late Holocene, the ITZC has been reported to have reached higher latitudes during the Medieval Climate Anomaly (MCA) producing high precipitation on the Yucatan Peninsula, contrasting with periods when the ITCZ remained in low latitudes, generating years of low precipitation and even dramatic droughts, as during the couple of centuries just preceding the MCA, corresponding to the Mayan Terminal Classic (TCC) Collapse and the Little Ice Age (LIA). Two submersible vibrocores, BZE-RH-SVC-58 from Elbow Caye Lagoon, and BZE-ECC-SVC-68 from English Caye Channel, were retrieved, among several additional cores, from the Belize Central Shelf Lagoon. Carbonate content values were determined by carbonate bomb and element (Ti, Si, K, Fe, Al, and Sr) counts via X-Ray Fluorescence (XRF) scans. This study is mainly based upon the detailed analyses of two of these cores with well-constrained timeframe, established by accelerator mass spectrometry (AMS) radiocarbon dating of benthic foraminifera, Quinqueloculina. The mixed sediments in these two cores, based upon the variations in the past 2000 years of elements such as Ti and K counts, have recorded the weathering rate variations of the adjacent Maya Mountain, highly influenced by alternating periods of high precipitation and droughts, linked to large climate fluctuations and extreme events. The 800-900 CE century just preceding the MCA, characterized by unusually low Ti and K counts and interpreted to be triggered by low precipitation and resulting in severe droughts in the Yucatan Peninsula, corresponds well with the Mayan Terminal Classic collapse (TCC). High Ti and K counts, although highly variable, during the MCA (CE 900-1350) are interpreted as an unusually warm period characterized by two 100-to-250 years-long intervals of higher precipitation when the number of tropical storms peaked, separated by a century (CE 1000-1100) of severe droughts and low tropical storm frequency coinciding with the collapse of Chichen Itza (CE 1040-1100). During the LIA (CE 1400-1850), Ti and K counts reach minimum values, with extreme minima during two historical drought times and related Caribbean-wide famines in the year CE 1535 and the last third of the 18th century (CE 1765-1800).Item Late Quaternary Mixed Carbonate-Siliciclastic Sediment Slope Accumulation: Unexpected Responses of Australia and Papua New Guinea Reefs to Glacio-Eustatic Sea Level Fluctuations(2014-06-24) Harper, Brandon Babbel; Droxler, Andre W; Dickens, Gerald R; Anderson, John B; Sawyer, Dale S; Rudolf, Volker H. W.Overtime, the north Queensland (Australia) and Gulf of Papua (Papua New Guinea) margin has been constructed by the biogenic carbonate growth and development of large barrier reefs on the mid-to-outer shelves and offshore platform/atoll tops, in conjunction with latitudinally variable riverine delivery of terrigenous siliciclastic material to the coast and inner shelf. This association forms Earth’s longest tropical mixed carbonate-siliciclastic system. Spatial and temporal variations of neritic carbonate (chiefly aragonite) production and siliciclastic sediment supply are controlled by late Quaternary high amplitude sea level fluctuations and climate changes. Sediment accumulation on the upper slopes adjacent to mixed margins is an important indicator of reef establishment/demise and siliciclastic shelf bypass during a time when glacio-eustatic sea level fluctuations are well understood and sediment dating techniques well developed. Sedimentation variability since the last deglaciation and, to some degree, during the last glacial cycle, is well understood and described by the transgressive and highstand shedding, reciprocal and coeval sedimentation concepts; based on low resolution records. My Ph.D. research results enhance the understanding of reef initiation, accretion, and demise along the Great Barrier Reef and Gulf of Papua margin by adding new cores and elemental (Sr, Ca, Si, K, Al) data from high resolution x-ray fluorescence core scanning. Sea level related reef evolution along this margin is more complex than previously thought. Contrary to established models, the central Great Barrier Reef exited the photic zone during peak interglacial, Marine Isotope Stage (MIS) 5e, resulting in the drowning of the reef and extremely low aragonite mass accumulation on the upper slopes. When sea level fell, reef-tops re-entered the photic zone increased production and export of aragonite to the upper slopes. The precise timing and elevation of sea level during MIS-5a is defined by the presence and absence of aragonite within slope sediments from the flooding and exposure of reef-tops. Additionally, along the Papuan Peninsula, a barrier reef established on a glacial shelf edge delta during re-flooding at 19 ka, followed by the subsequent drowning and back-stepping of the reef, synchronous to Termination I melt water pulses, away from siliciclastics.Item Late Quaternary variability in coastal changes along the Texas Gulf coast(2018-11-26) Odezulu, Christopher Ikechukwu; Anderson, John BThe majority of the coastal barriers in Texas are retreating landwards, only a few are stable or progradational. The barriers are responding to both external (allogenic) forcings such as relative sea-level and sediment supply, and internal (autogenic) basinal mechanisms such as antecedent topography, regional subsidence, wave climate, offshore profile, and anthropogenic effects. The central Texas coast is characterized by stable barriers whereas the upper and south Texas coast is dominated by transgressive barriers. These changes along the Texas coast became prominent in the mid-late Holocene when the rate of sea-level rise decreased. Man-made activities such as the construction of jetties and dams exacerbate the more recent changes. Transects of sediment cores that extend from back-barrier to beyond the toe of the shoreface were investigated to examine the Holocene evolution and the response of Follets Island (an end member transgressive barrier) and Matagorda Peninsula through North Padre Island (an end member of the stable barrier) to allogenic forcings. Radiocarbon ages are used to constrain the timing and rates of shoreface progradation and retrogradation. The Late Quaternary stratigraphic architecture of the Texas coast varies regionally despite a similar sea-level history. Our data shows that sea level and sediment supply played a major role in the evolution of the Texas coast. The central Texas coast-shoreface experienced high-frequency progradation and retrogradation in the mid-late Holocene whereas upper Texas coast experienced continuous flooding and landward shifts in shoreline. Based on sand budget estimates, only barriers on the central Texas coast are currently stable; upper and south Texas coastal barriers have limited sand thickness and are currently retreating landwards. With the impact of anthropogenic effects on sand delivery to the coast, some barriers are on the verge of disappearing within a few centuries given projected rates of sea-level rise.Item Morphodynamic modeling of channel fill and avulsion timescales during early Holocene transgression using Trinity River, TX incised valley stratigraphy(2015-04-24) Moran, Kaitlin Moran Elizabeth; Nittrouer, Jeffrey A.; Anderson, John B; Droxler, Andre W; Lorenzo-Trueba, Jorge; Perillo, Mauricio MThe Trinity River and the sediments that infill its incised valley system are well-constrained in terms of time and space properties of the sediment deposits and resulting stratigraphy. The Trinity River is an excellent natural laboratory to test fluvial morphodynamic concepts that could be used to examine the processes of incised valley infill. We develop a numerical model that links sediment transport processes and the production of stratigraphy to evaluate the effects of Holocene transgression on the development of Trinity stratigraphy. We simulate the mechanics of channel fill and avulsions for the Trinity River, by coupling fluid flow, sediment transport and channel response, constrained by modern and early Holocene conditions. Our results show how non-uniform flow produces loci of sediment deposition, which backstep within the channel as base-level rises, and coincide with the avulsion locations. There is an upstream retreat as the rate of base-level rise increases (transgression). Additionally, we show how including a floodplain parameter within the model framework influences the calculated time for avulsion by changing the amount of sediment deposited within the channel. Our model is applied over century to millennial timescales, and is utilized to evaluate basin scale patterns of known stratigraphic variability. Because the model is well-constrained, our results have application for predicting stratigraphy for other fluvial-deltaic systems undergoing transgression. This is especially important for predicting the valley infill of systems that lack the robust constraint exhibited by the Trinity incised valley system.Item Paleo-Ice Stream Behavior: Retreat Scenarios and Changing Controls in the Ross Sea, Antarctica(2016-03-17) Halberstadt, Anna Ruth Weston; Anderson, John BStudying the history of ice-sheet behavior in Antarctica’s largest drainage basin, the Ross Sea, can improve understanding of patterns, timing, and controls on marine-based ice-sheet dynamics, and provide constraints on numerical ice-sheet models. Newly collected high-resolution multibeam swath bathymetry data, combined with two decades of legacy multibeam and seismic data, are used to map glacial landforms and reconstruct paleodrainage. Last Glacial Maximum grounded ice reached the continental shelf edge in the eastern but not western Ross Sea. Recessional geomorphic features in the western Ross Sea indicate virtually continuous retreat of the ice sheet in contact with the bed. In the eastern Ross Sea, well-preserved linear features and a lack of small-scale recessional landforms record rapid lift-off of grounded ice from the bed. Physiography exerted a first-order control on ice behavior, while seafloor geology played an important subsidiary role. This new analysis of retreat patterns suggests that: (1) a large embayment formed in the eastern Ross Sea; (2) retreat was complex and asynchronous between troughs; and (3) the eastern Ross Sea largely deglaciated prior to the western Ross Sea. Previously published grounding-line retreat scenarios are based on terrestrial observations; however, this work uses Ross Sea-wide geomorphology to constrain marine deglaciation.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, VolkerThe 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.