Land-sea Evidence for Periodical Hot Showers of Ejecta Debris from the 0.8 Ma Impact: Conciliating the Australasian Tektite Contradictions?
Controversial aspects still obscure the 0.8 Ma Australasian impact. The paradox between the 0.8 Ma tektite age and their younger depositional ages, the association of microtektites to unmelted mineral debris and the lack of a large source crater in inland Indochina have yet to be elucidated. Here we present the first conclusions from a microstratigraphic/analytical study of marine and terrestrial records aimed at clarifying the 0.8 Ma impact. Accurately framed sequences within the astronomical time-scale from South China sea and the Indian Ocean date the initial impact-fall at 805 Kyr with an assemblage of melted to unmelted ejecta without microtektites. The repeated occurrence of the impact-assemblage associated to microtektites, sharply interrupting the normal sedimentation before the Matuyama-Brunhes boundary (MBB) and long after above, suggests delayed delivery of the initial ejecta that was launched to space by the hypervelocity impact. The terrestrial sequences from South Europe to Australasia consolidates the periodical fall of the ejecta assemblage from 805 Kyr to later periods, showing an erratic spatial pattern. The association of flow-textured glass, impact breccia, microtektites, tektites, spherules, partly melted and unmelted rockclasts and burning traces suggests disintegration of hot ejecta blocks at the ground surface. In addition to soil, sedimentary, igneous and volcanic clasts, marine mud with a subtropical to subpolar seawater micro-faunal assemblage and a kerogen-rich sulphide mud with fresh water micro organisms suggest that the target source rocks are part of the continental plateau from the Indian/Austral ocean. Discontinuities in the Austral sedimentation at ca. 0.8 Ma BP, filling of the 77°S fore deep grabben by the 0.8 Ma impact breccia and presence of a tasmanite-like kerogen in the ejecta assemblage constrain the impact area between the Kerguelen plateau and the Australian West coast. The metal-rich carbonaceous polymorphs (polymers, vitrous carbon, graphite, diamonds) formed by reduction of the organic terrestrial precursors during the ejecta emplacement. Their thermal instability would explain the explosive pulverisation of the ejecta blocks and local ignition at landing. The metals, carbonaceous polymorphs, c-rich impact breccia and glass offer a unique organo-mineral assemblage for identifying the delayed ejecta fall since the 0.805 Ma parental impact. The ejecta assemblage is found to be associated to the Younger Dryas Boundary layer, the 4 kyr BP dust layer or the 1908 Tunguska peat layer thus explaining the controversial linked air blast. The sequential analysis of the organo-mineral tracers in the host soil matrix allows to link singular environmental changes to carbon-rich aerosol loading of the upper atmosphere for each ejecta fall, i.e. heavy rain, cooling, acidification, dust spikes, wildfires, tsunamis.
Holocene Paleoclimatic Transitions Inferred From Diatom Assemblages in Sediments of Kusawa Lake, Yukon Territory, Canada
The southwest Yukon Territory, Canada, has long been recognized as an important region for recovering sensitive records of Holocene paleoclimatic change, owing to pronounced moisture and elevation gradients, sharp vegetation transitions, and the abundance of glaciated alpine watersheds. More information is required, however, to refine the magnitudes and timing of major Holocene climatic transitions, and associated impacts on ecosystems. Lake sediments are important repositories of paleoclimatic information although watershed- specific processes need to be identified prior to the extraction of the regional climate signal. Smaller lakes and ponds have been the focus of many studies. We used analyses of both physical properties of the sediments and biological proxies in cores from Kusawa Lake (60°19'55°N; 136°4'48°W; 671 m a.s.l) to record the sensitivity of this large (142 km2), deep (max. 135 m) freshwater ecosystem to Holocene climatic and environmental transitions. An depth-age model was constructed for a single core in the deepest part of the lake using five AMS radiocarbon dates and a tephra layer. A high-resolution sequence of paleolimnological change was produced through the analysis of diatoms (microscopic algae) in the core, and measurement of biogenic silica (BSi). Fluctuations in diatom primary production as documented both by BSi and valve concentrations suggest that the Holocene Thermal Maximum (HTM) began ∼9000 cal yrs BP, terminating ∼7000 cal yrs BP. Coincident changes in relative abundance of key planktonic taxa, increases in species diversity, and comparisons of those dates with other records from the region, help to corroborate the existence of these regional climatic changes at that time. Stratigraphy and grain size data show frequent sand beds and laminae alternating with silt-clay laminated sections between 9700 cal yrs BP and ∼6500 cal yrs BP, indicating a period of energetic turbidity currents, alternating with quiet water deposition. The late Holocene shows less frequent sand turbidite inputs but higher rates of primarily silt-clay deposition as distinct couplets reflecting the cooler and wetter Neoglacial climates.
The Sensitivity of Great Lakes hydrology to Sunspots and Orbital Forcing: Explaining Early Holocene Aridity Throughout the Region
The very arid conditions that resulted in the westward expansion of the prairie in the early to mid Holocene were transmitted throughout the Upper Great Lakes by eliminating outflow from Lake Superior (via the St. Marys River). Aridity was more muted in the Lower Great Lakes, but wetlands and small bodies of water in the Lower Great Lakes region, such as the Willoughby Bog and Hamilton Harbour, do show evidence of increased aridity. Peak aridity occurred throughout the Great Lakes basin around 8200 cal. years ago, but the nature of the relationship between climatic and hydrologic changes in this region with the global 8.2k event is unclear. The isolation of the Erie and Ontario basins from the effects of the intense aridity of the western prairie allows us to focus on the sensitivity of Great Lakes hydrology to the more subtle controls exerted by orbital (Milankovitch) parameters as well as sunspot activity.
Early Holocene Paleoclimate and Low Water levels in the Laurentian Great Lakes, an opportunity to Enhance Knowledge of Climate-Lake Level Sensitivity
Review of radiocarbon-dated, upwarped shorelines led to an analytical description of glacial rebound throughout the Great Lakes basin, and the reconstruction and comparison of original elevations of former lakes and outlets through time. The comparison revealed that lake levels fell below basin outlets in the relatively dry, early Holocene climate, contrary to the assumption of previous syntheses that lakes were always overflowing water bodies. For example, water levels in the Huron basin declined into hydrologic closure (>20 m below the basin overflow outlet) between about 7900 and 7500 14C (∼8770 and 8290 cal) years BP. This phase of closed lowstands has been validated by newly-acquired and re-interpreted older evidence of low water levels in sedimentary records. In the Huron and Georgian Bay basins these data, which also apply to the Michigan basin, include seismo- and litho-stratigraphic evidence of a widespread erosion surface attributed to wave erosion during a period of low lake level, microfossil indications of brackish conditions likely related to enhanced evaporation, and in situ tree stumps up to 43 m below present lake level. Seismostratigraphic evidence of mud-buried beaches implying former water levels 40 to 105 m below present in western Lake Superior has been reported. A submerged beach represents the early Holocene lowstand in the Erie basin, and new and published paleo-environmental findings show that shallow-water conditions occurred in Lake Ontario at the same time. Hydrologic closure occurred in the upper Great Lakes basins when the supply of glacial meltwater was diverted permanently from the basins after 8000 14C (8890 cal) BP. The closed lowstands occurred generally when lake water balances became negative as evaporative losses exceeded precipitation in the drier-than-present early Holocene climate. This unique hydrological event provides an opportunity to evaluate the sensitivity of the Great Lakes system under climatic conditions different than present, thereby increasing confidence in its value. The sensitivity is a key parameter for modeling and projecting future lake levels under global warming.
The Terminal Phase of Lake Ojibway in Northern Ontario as Recorded in Lacustrine Sediment Cores
Sediment cores from multiple lakes constrain the stratigraphy of Glacial Lake Ojibway and its subsequent drainage to the Hudson Straits. North of the limit of the Cochrane ice advance there is a consistent and correlatable stratigraphy. Lacustrine sediment cores terminate on the Cochrane till. Above the till is a short series of glaciolacustrine varves (<30) that rapidly thin upsection. These varves cannot be correlated to the Lake Ojibway varve thickness time series. Above these varves is a dark gray, ice distal glaciolacustrine clay. This dark gray clay unit is characterized by numerous ice-rafted silt and clay pellets, but sedimentation rates are lower than within the varves and the flux of ice rated pellets generally decreases upsection. Low sedimentation rates resulted in the complete dissolution of calcite, and incomplete dissolution of dolomite. Ostracodes (Candona subtriangulata) occur within this unit. There is a sharp transition above the dark gray clay to a calcareous light gray clay, which is believed to reflect drainage of Lake Ojibway. Geochemically this light gray clay appears to be homogenized varved clay from lower in the section. This unit grades into post- glacial sediment, which varies depending on the site, but generally these sediments reflect a climate that was drier than modern. Dateable terrestrial macrofossils are absent in the immediate post-glacial sediments. South of the Cochrane ice limit the dark gray, ice distal clay is not conspicuous, but the overlying re-worked varves that occur during drainage are ubiquitous. The implications of this pattern are not yet fully realized.
New Dinoflagellate and Isotopic Data From the Northeast Newfoundland Shelf
The drainage of glacial Lake Agassiz could be responsible for the 8.2 ka cold event recorded in the Greenland Ice Sheet and tentative links with the Preboreal Oscillation have been made. Recent studies show clear evidence for drainage of large volumes of meltwater through Hudson Strait and Coriolis deflection predicts a southward flow over the Labrador shelf. Indeed, detrital carbonate (DC) beds in cores from the Labrador and Newfoundland shelves suggest that plumes of suspended DC sediment were carried through Hudson Strait into the Labrador Current, and as far south as Grand Bank at least. DC beds in cores from Notre Dame Channel have been dated and correspond with known periods of meltwater flow out of Hudson Strait. We re-evaluated the correction for early radiocarbon dates from the Labrador Shelf to range up to -730 years. The correction varies as pre-bomb reservoir ages and/or ice cover durations change. Onshore-offshore correlations of dated palynological records from Newfoundland and the Newfoundland shelf are used to verify our dating scheme and to lend support to the corrections we propose. The timing of meltwater drainage is critical when attempting to link freshwater drainages with cooling events and/or slowing of thermohaline circulation in the North Atlantic. Dinocyst assemblages support our hypothesis that the Labrador Current was significantly enhanced by the ouflow from Lake Agassiz. Detrital carbonate layers show increased proportions of Impagidinium pallidum indicating cooler sea surface conditions and the overall dinocyst assemblages suggest stronger vertical stratification, and hence reduced surface salinity. In the previous DC bed, which is correlated with the Noble Inlet ice advance, the appearance of Spiniferites elongatus suggests more northern influence. Transfer functions based on dinoflagellate cysts also show a reduction in sea surface salinity, before and during the DC bed. Oxygen isotope analyses performed on calcareous dinoflagellate Thoracosphaera heimii from Notre Dame Channel will be presented.
Submerged Archaeological Sites and Water-Level Fluctuations in Rice Lake, Ontario, Canada
Post-glacial and climate-driven water level fluctuations during the Holocene had a dramatic effect on shoreline positions in the Great Lakes basins. In Rice Lake, in southern Ontario, a major lowstand event at ca. 6500 YBP is recorded by pollen and a sediment erosional hiatus in lake cores. The lowstand event corresponds with the Archaic phase of human settlement (9,000-5000 YBP), but sites of these ages are under-represented in the archaeological record. It has been proposed that one reason for the paucity of Archaic sites is that the rise in water-levels following the mid-Holocene lowstand may have inundated many of these sites. In order to gain a better understanding of the water-level fluctuations in Rice Lake and its archaeologic potential, a detailed geophysical survey and sediment coring program was initiated in 2007. Cores extracted from a bay (water depth 2.5 m) in Rice Lake adjacent to a well-documented terrestrial Archaic archaeological site provide evidence that human occupation extended into areas that are now submerged. Small microscopic quartz and chert flakes from stone tool manufacture (microdebitage) were identified in three cores using a SEM and high-resolution grain-size analysis. Paleoenvironmental reconstruction using microfossil analysis (thecamoebians), magnetic susceptibility and radiocarbon dating provide further insights into the extent and timing of the lowstand event that allowed Archaic peoples to exploit the now inundated landscape.
Deciphering dam impacts from the natural background in ultra-high resolution sedimentary records in the NW Gulf of St. Lawrence
Dam construction is an important factor influencing the sedimentary budget in the proximal offshore catchment area. However, not only the timing of dam construction is decisive on the sedimentation, but also the natural hydro-climatic variations. Therefore, there is a major difficulty in deciphering the timing of the dam construction from the natural variations in the sedimentary records of the catchment area. In this study we illustrate a methodology to measure the background natural sedimentary variations and distinguish the timing of the dam construction in the sedimentary regime using a multi-core and multi-proxy approach combined with wavelet analysis. Onboard the R/V Coriolis II, two box cores were collected between 10 and 20 km offshore in the Sept- Īles area (NW Gulf of St. Lawrence): one within (56BC) and one outside (63BC) the catchment area of the dammed Sainte-Marguerite river. The methods comprised 210Pb-derived age models, CAT-scan imagery, grain size, geochemical and isotopic measurements (C, N, 13C). In addition, ultra-high resolution micro-fluorescence X (X-RF) data were acquired on both cores with a 100 μm downcore resolution, corresponding to an annual temporal resolution. In order to study the high frequency sedimentological variations, the X-RF data were submitted to a principal component analysis, over which a wavelet analysis was performed. The results reveal a sharp change in the late 1890's in core 56BC (in the dam catchment area) through a flattening of the d50 grain size parameter. This sharp change is not observed in the other core and is associated with the construction of the SM1 dam, the first dam constructed on the Sainte-Marguerite river in 1906. The only other sedimentological events recorded in the grain size data are dated in 1966 AD and 1972 AD or 1976 AD and are linked to the largest historical hydro-meteorological events. Other high frequency sedimentological variations have been investigated by wavelet analysis and compared with historical climatic time series. NAO-like periods (2 to 6 yrs, 6 to 8 yrs, 8 to 12 yrs, 12 to 16 yrs, 18 to 22 yrs, 30 to 35 yrs, 40 to 40 yrs, 60 to 80 yrs and 95 to 100 yrs) and patterns have been uncovered and suggest a relationship between the Gulf of St. Lawrence sedimentary regime and climatic oscillations. Finally, another striking result is the absence of high frequency (periods <30 years) sedimentological variations after AD 1900 in core 56BC (in the catchment area), highlighting once more the possible impact of the dam construction on the sedimentary regime. These results therefore suggest that the proximal offshore sedimentary records are well-suited for measuring the impacts of dam construction on the environment.