6 research outputs found
Clay Mineral Cycles Identified by Diffuse Spectral Reflectance in Quaternary Sediments From the Northwind Ridge: Implications for Glacial-Interglacial Sedimentation Patterns in the Arctic Ocean
A Quaternary record of fine-grained sediment composition is used to investigate Arctic Ocean climate variability on glacial-interglacial time scales. Diffuse spectral reflectance data from sediment core P1-92AR-P25 from the Northwind Ridge, north of Alaska, demonstrates cyclic variations in mineralogy. Varimax-rotated R-mode factor analysis of down-core data revealed three major mineralogical assemblages, which were then compared with the content of manganese, a proxy for basin ventilation, and thus glacial-interglacial cycles. Results indicate that factor 1, a smectite + chlorite clay assemblage, was delivered to the core site during interglacials, either by fluvial discharge or sea-ice drift from Siberian rivers or inflow from the Bering Sea. Factor 2, an illite + goethite assemblage, is related to glacial periods, and was probably transported from the Laurentide Ice Sheet by icebergs or meltwater. Factor 3, glauconite, might have been sourced from the North Slope region of Alaska during deglacial intervals, or from dolomites associated with Laurentide iceberg-discharge pulses. The observed variations in sediment source and transport mechanisms arise from glacial-interglacial changes in sea level, the size of the terrestrial ice sheets surrounding the Arctic Ocean, the extent of sea-ice cover and altered atmospheric circulation. The reconstructed glacial-interglacial circulation patterns from the Late Quaternary show some similarity with modern circulation changes presumably related to the monthly- to decadally-fluctuating Arctic Oscillation. However, because the Arctic Oscillation operates on much shorter time scales, further research is necessary to better understand the driving mechanism for the changes observed over glacial-interglacial cycles, and the potential role of ocean-atmospheric interaction
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A Multi-Proxy Investigation of the Late Glacial "Mystery Interval" (17.5-14.5 ka)in the Cariaco Basin, Venezuela
The "Mystery Interval" (17.5-14.5 ka) is an unusual time period of abrupt global climate change during the late glacial between Heinrich event 1 and the Bølling-Allerød warm period (~17.5-14.5 ka). This period was characterized by extreme cooling in the North Atlantic region, warming in Antarctica, the rise of atmospheric greenhouse gases, and a variety of hydrologic changes around the globe, all of which may have stemmed from Heinrich event 1 and the possible collapse of the Atlantic's meridional overturning circulation. A distinctive and unique gray clay layer was deposited in Cariaco Basin, Venezuela, within this time period, which has no apparent counterpart in the basin's sediment record for at least the last full glacial-interglacial cycle. One hypothesis for the origin of the gray layer is that the initial pulse of deglacial sea level rise over the shallow Unare Platform, south of the basin, caused remobilization and rapid emplacement of previously deposited shelf sediments. However, analysis of the timing and extent of sea level rise as well as evidence from radiocarbon ages and a comparison of the organic content of gray layer sediments and known turbidites in the basin does not support this hypothesis. The alternative hypothesis, that the gray layer is related to increased fluvial discharge from local rivers as a result of elevated regional rainfall, is supported by a number of lines of evidence. The bulk sediment elemental content measured by scanning X-ray fluorescence (XRF) (this study) and clay mineralogy (Yu, 1996) support input of local river sediments. Coccolith abundances (Mertens et al., 2009), sea surface salinity (SSS) estimates and foraminiferal Ba/Ca analysis (this study) are also consistent with freshening of surface waters caused by elevated river runoff. This implies increased rainfall in the region which is corroborated by elemental and mineralogical ratios that point to increased precipitation and chemical weathering. Average terrigenous grain size and terrigenous fluxes are also in line with modern rainy season data. Despite prior suggestions that the Cariaco Basin region should be dry due to a southward-shifted Inter-tropical Convergence Zone (ITCZ) during cool periods in the North Atlantic, such as the Mystery Interval, data presented in this thesis suggest elevated rainfall and fluvial input related to deposition of the gray layer. Multiple lines of proxy evidence indicate that Cariaco Basin may have been characterized by a drier climate in the first part of the Mystery Interval but then shifted to a wetter climate in the second part, after ~16.5 ka, which might resolve this apparent conflict. The change to wetter conditions is most likely due to a northward shift in the position of the ITCZ, possibly due to warming tropical North Atlantic sea surface temperatures (SSTs) and/or extreme North Atlantic seasonality. Comparison of the Cariaco Basin climate records to climate observations from around the globe reveal a similar shift in climatic conditions around the same time, suggesting that the Mystery Interval may actually have been a two-phase event. Although many of the climatic observations from around the world can be explained by a shift from a southerly position of the ITCZ within the first part of the Mystery Interval to a more northerly position during the later part of the interval, many regions are not directly affected by the ITCZ and other complicating factors may play a role in the rapid climate changes observed globally.</p
Clay mineral cycles identified by diffuse spectral reflectance in Quaternary sediments from the Northwind Ridge: implications for glacial–interglacial sedimentation patterns in the Arctic Ocean
A Quaternary record of fine-grained sediment composition is used to investigate Arctic Ocean climate variability on glacial–interglacial time scales. Diffuse spectral reflectance data from sediment core P1-92AR-P25 from the Northwind Ridge, north of Alaska, demonstrates cyclic variations in mineralogy. Varimaxrotated R-mode factor analysis of down-core data revealed three major mineralogical assemblages, which were then compared with the content of manganese, a proxy for basin ventilation, and thus glacial–interglacial cycles. Results indicate that factor 1, a smectite + chlorite clay assemblage, was delivered to the core site during interglacials, either by fluvial discharge or sea-ice drift from Siberian rivers or inflow from the Bering Sea. Factor 2, an illite + goethite assemblage, is related to glacial periods, and was probably transported from the Laurentide Ice Sheet by icebergs or meltwater. Factor 3, glauconite, might have been sourced from the North Slope region of Alaska during deglacial intervals, or from dolomites associated with Laurentide iceberg-discharge pulses. The observed variations in sediment source and transport mechanisms arise from glacial–interglacial changes in sea level, the size of the terrestrial ice sheets surrounding the Arctic Ocean, the extent of sea-ice cover and altered atmospheric circulation. The reconstructed glacial– interglacial circulation patterns from the Late Quaternary show some similarity with modern circulation changes presumably related to the monthly- to decadally-fluctuating Arctic Oscillation. However, because the Arctic Oscillation operates on much shorter time scales, further research is necessary to better understand the driving mechanism for the changes observed over glacial– interglacial cycles, and the potential role of ocean–atmospheric interaction