Browsing by Author "Droxler, Andre W"

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    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 K
    In 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).
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    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.
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    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 M
    The 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.