Browsing by Subject "Holocene"
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Item A 1,500-year record of late Holocene temperature variability and recent warming from Laguna Chingaza, Colombia(2012-08) Bixler, Curtis William; Shanahan, Timothy M.Rapid tropical glacier retreat over the last 50 years has been well documented, and has received significant media attention. Many studies suggest these changes are due to rising global surface air temperatures, however disentangling the effects of temperature and precipitation has hampered scientific consensus. Furthermore, because of the shortness of the instrumental record, it is difficult to assess the larger significance of the climate changes associated with the decline of tropical glaciers. Here, we present a locally calibrated, independent temperature reconstruction for the past 1,500 years from Laguna Chingaza, Colombia based on distributions of branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs) in order to assess the controls on long term temperature variability in the tropical Andes, and their relationship with growth and demise of Andean glaciers. Comparison of reconstructed temperatures with the instrumental record suggests that our proxy record faithfully records decadal to century scale trends in temperature. The largest temperature decline over the last 1,500 years was a decrease of 2.5 ± 0.3 °C during the Little Ice Age (LIA), reaching lowest temperatures during the mid-17th century, and is broadly consistent with terrestrial temperature reconstructions throughout the tropics and the higher latitudes. The structure and timing of temperature changes at Laguna Chingaza are remarkably similar to recent terrestrial temperature reconstructions from elsewhere in the tropics, including sites in the tropical Pacific and equatorial Africa, suggesting that these changes are widespread in the tropics. Together, these records suggest that warming over the last few decades is unprecedented over the last 1,500 years, including the Medieval Climate Anomaly (800-1150 AD). Comparison of these temperature changes with records of Andean glacier limits suggests that temperature is the dominant driver of glacial retreat, particularly over the past few decades. Additionally, paleotemperatures inferred from LIA and recent glacial equilibrium line altitudes (ELAs) underestimate past changes in temperature when compared with brGDGTs reconstructions, suggesting that changes in precipitation complicate the use of glacier ELAs to reconstruct past temperatures. The coupling of temperature and ice extent in South America suggests that with projected future warming, the health of tropical glaciers could be in jeopardy, significantly impacting the communities and ecosystems that depend on them.Item Archaeological assessment reveals Earth’s early transformation through land use(Science, 2019-08-30) Stephens, Lucas; Ellis, ErleEnvironmentally transformative human use of land accelerated with the emergence of agriculture, but the extent, trajectory, and implications of these early changes are not well understood. An empirical global assessment of land use from 10,000 years before the present (yr B.P.) to 1850 CE reveals a planet largely transformed by hunter-gatherers, farmers, and pastoralists by 3000 years ago, considerably earlier than the dates in the land-use reconstructions commonly used by Earth scientists. Synthesis of knowledge contributed by more than 250 archaeologists highlighted gaps in archaeological expertise and data quality, which peaked for 2000 yr B.P. and in traditionally studied and wealthier regions. Archaeological reconstruction of global land-use history illuminates the deep roots of Earth’s transformation and challenges the emerging Anthropocene paradigm that large-scale anthropogenic global environmental change is mostly a recent phenomenon.Item Dune behavior in a multidirectional wind regime : White Sands Dune Field, New Mexico(2014-08) Pederson, Anine Oehlenschlaeger; Kocurek, GaryAs with most dune fields, the White Sands Dune Field in New Mexico forms in a wind regime that is not unimodal. In this study, dune behavior at White Sands was documented from a time series of five lidar-derived digital elevation models (DEM) and compared to a record of wind direction and speed during the same period. For the study period of June 2007 - June 2010, 244 sand-transporting wind events occurred and define a dominant wind mode from the SW and lesser modes from the NNW and SSE. Based upon difference maps and tracing of dune brinklines, overall dune behavior consists of migration to the NE, but with along-crest migration of dune sinuosity to the SE. Permutations of the DEMs allow matching specific dune behavior with wind modes. The SW winds are transverse to dune orientations and cause most forward migration. The NNW winds cause along-crest migration of dune sinuosity and low stoss bedforms, as well as SE migration of NE-trending dune terminations. The SSE winds cause ephemeral dune deformation, especially crestal slipface reversals. Dune deformation occurs because of unequal deposition along the lee face as a function of the incidence angle formed between the wind and the local brinkline orientation. Incidence-angle control on dune deformation and types of lee-face surface processes allows for an idealized model for White Sands dunes. The dunes behave as complex systems in which each wind event deforms the dune shape, this new shape then serves as the configuration for the next wind event.Item Glacial to Holocene climate variability in the southern mid latitudes(2022-12-05) Piatrunia, Natallia; Shanahan, Timothy M.; DiNezio, Pedro N; Partin, Jusdon W; Breecker, Daniel O; Martindale, Rowan CThe Southern Hemisphere westerly winds play an important role in global climate, by influencing the weather in the mid latitudes, transmitting climate changes between high and low latitudes, and have been proposed to influence the carbon storage in the Southern Ocean. Changes in the position and intensity of the westerlies are anticipated to have a large impact in the future and have also been proposed to play significant role in past climate changes. Paleoclimate proxy records of past variation in the westerlies, however, are controversial, and models suggest that they may depend on a complex interplay between high and low latitude drivers as well as factors such as the mean state of the climate system. This dissertation seeks to improve our understanding of the Southern Hemisphere westerly wind and climate variations. To do so, I generate a 45,000-year record of Southern Hemisphere westerly wind and temperature variability on the North Island of New Zealand using compound-specific hydrogen isotope in sedimentary leaf waxes and branched GDGT-derived temperature using lake records. To improve our interpretations of the hydrogen isotope data in proxy records, I first examine climatic controls on the modern isotopic composition of rainfall on the North Island of New Zealand. This work indicates that isotopic ratios of precipitation mainly reflect regional-scale changes in the atmospheric circulation, with higher δ¹⁸Oₚ values reflecting an increased influence of the tropical anticyclonic circulation and lower δ¹⁸Oₚ ratios reflecting a greater contribution from the Southern Hemisphere westerly winds. Based on our improved understanding of isotopic controls, our multi-proxy record shows that Northern Hemisphere climate exerts a dominant control over the westerlies, irrespective of the changes in the southern Hemisphere mid to high latitude climate conditions. Finally, I examine the distribution of isoprenoid GDGT lipids and elemental ratios from the same lake to provide a complimentary set of proxies of lake stratification and mixing.Item Holocene depositional history of Lavaca Bay, central Texas Gulf coast(1975) Byrne, J. R.; Jonas, E. C. (Edward Charles), 1924-Sediments in Lavaca Bay represent a texturally fining upward marine transgressive sequence. The entire sediment sequence is 120 feet (36.4 m) thick at the mouth of Lavaca Bay. Gravel and sand from the Lavaca-Navidad fluvial-deltaic complex comprise the oldest Holocene sediments in the bay. Successively younger sediments are represented by up to 80 feet (24.2 m) of bay-estuarine mud. The bay-estuarine sediments reflect three major mud sources. These are: (1) suspended load transported by the Colorado-Brazos fluvial complex, which was discharged into marine and estuarine bodies over 50 miles (83 km) to the northeast and carried southwest by longshore currents, (2) erosion of Pleistocene deltaic sediments exposed along the bay margins, and (3) suspended load transported by the Lavaca-Navidad fluvial complex, discharging into the head of Lavaca Bay. Comparison of the volumes of mud presently filling this Pleistocene valley, with calculated volumes of mud that have been supplied from these potential sources has resulted in a rough argillaceous sediment budget for this coastal estuary. Sufficient mud had been made available from the Lavaca-Navidad fluvial complex and the bluffs exposed along the bay margins to completely fill Lavaca Bay since Recent stillstand of the Gulf of Mexico. However, the sediment accumulation rate in Lavaca Bay is slow due to the transport of significant quantities of sediment out of Lavaca Bay by waves and surface currents. In addition, lithologic characteristics and facies relationships of the respective muds indicate that discharge of suspended load by the Colorado-Brazos fluvial complex had been an important source as well. Progradation of the Lavaca-Navidad fluvial-deltaic complex since Recent stillstand has resulted in a regressive sediment sequence which overlies transgressive sediments at the head of Lavaca Bay. The clay mineral suite in sediments in Lavaca Bay reflects clay minerals in the source area. No changes in the clay minerals were detected as a result of increasing salinity, sediment depth and time. X-ray diffraction data indicate significant potassium fixation by montmorillonite after treatment with 1N potassium chloride. Sediment facies are the main control on trace element distribution in Lavaca Bay. In general, high trace element concentrations are correlative with clay sediment content and, to a lesser degree, organic carbon content. Data suggest that both "detrital" and "nondetrital" trace elements contribute to trace element accumulations in Lavaca BayItem Letter to H.B. Stenzel from C.O. van Regteren Altena on 1966-05-19(1966-05-19) van Regteren Altena, C.O.Item Modern depositional environments and recent alluvial history of the Lower Colorado River, Gulf Coastal Plain of Texas(1992) Blum, Michael D.; Butzer, Karl W.The Colorado River is the trunk stream of a large polyzonal fluvial system that drains the Edwards Plateau and Gulf Coastal Plain of Texas. This dissertation summarizes previous work on the late Pleistocene and Holocene alluvial history of major valley axes in the upper Colorado drainage, on the Edwards Plateau, then presents a new spatially- and temporally-controlled allostratigraphic framework for late Pleistocene and Holocene alluvial deposits of the lower Colorado River on the Gulf Coastal Plain. The late Pleistocene and Holocene alluvial history of major streams in the upper Colorado drainage on the Edwards Plateau consists of a series of informal allostratigraphic units and bounding disconformities that were defined on the basis of areally persistent soil-geomorphic and stratigraphic relations. Chronological control for this allostratigraphic framework is afforded by numerous radiocarbon ages. Large valleys in the upper Colorado drainage contain late Pleistocene terraces and underlying fills that record an extensive period of deposition centered on the full-glacial time period, ca. 20-14,000 yrs BP. After abandonment of late Pleistocene floodplains, major streams incised bedrock valleys to present depths by ca. 11,000 yrs BP. Since that time there have been two episodes of net deposition, from ca. 11,000-5000 yrs BP, and ca. 4600-1000 yrs BP, whereas the modem incised, and in many cases underfit, channels and associated depositional environments represent the last millennium of fluvial activity. A very similar sequence of allostratigraphic units and bounding disconformities has been identified in the lower Colorado valley, on the Gulf Coastal Plain. The oldest unit of interest has been defined as the Eagle Lake Alloformation, after localities near the town of Eagle Lake that display characteristic soil-geomorphic and stratigraphic relationships with both older and younger deposits. Radiocarbon ages indicate the Eagle Lake unit was deposited during the late Pleistocene from ca. 20-14,000 yrs BP, roughly contemporaneous with full-glacial conditions. The main valley fill of the lower Colorado River has been defined as the Columbus Bend Alloformation, named after localities near the town of Columbus, and subdivided into three members on the basis of soil-geomorphic and stratigraphic relations. Radiocarbon ages indicate that Columbus Bend Member 1 was deposited during the latest Pleistocene through early to middle Holocene, from ca. 13-5000 yrs BP, whereas Member 2 represents the period ca. 5-1000 yrs BP. Columbus Bend Member 3 constitutes the modem depositional system of the lower Colorado River and represents the last millennium. Late Pleistocene and Holocene allostratigraphic units and bounding disconformities in the upper Colorado drainage and through the lower Colorado valley correlate with independently-identified changes in climate, vegetation cover, and the characteristics of upland soil mantles, and are interpreted to reflect a series of morphological and sedimentary responses to changes in the relationship between the discharge regime and the concentration of sediments along valley axes. In the far downstream portion of the lower Colorado valley, allostratigraphic units and bounding disconformities persist but their geometry, or stratigraphic architecture, changes substantially in response to the last glacio-eustatic cycle. In this part of the drainage, genesis of the alluvial sequence is interpreted to reflect adjustments to changes in climate, but sequence architecture reflects the details of the last glacio-eustatic cycle