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During the late Quaternary, both external and internal forcings have driven major climatic shifts from glacial to interglacial conditions. Nonlinear climatic steps characterized the transitions leading to these extrema, with intermediate excursions particularly well expressed in the dynamics of the Northern Hemisphere cryosphere. Here we document the impact of these dynamics on the north-eastern North Atlantic Ocean, focussing on the 35-10 ka interval. Sea-surface salinities have been reconstructed quantitatively based on two independent methods from core MD95-2002, recovered from the northern Bay of Biscay adjacent to the axis of the Manche paleoriver outlet and thus in connection with proximal European ice sheets and glaciers. Quantitative reconstructions deriving from dinocyst and planktonic foraminiferal analyses have been combined within a robust chronology to assess the amplitude and timing of hydrological changes in this region. Our study evidences strong pulsed freshwater discharges which may have impacted the North Atlantic Meridional Overturning Circulation.

During the late Quaternary, both external and internal forcings have driven major climatic shifts from glacial to interglacial conditions. Nonlinear climatic steps characterized the transitions leading to these extrema, with intermediate excursions particularly well expressed in the dynamics of the Northern Hemisphere cryosphere. Here we document the impact of these dynamics on the north-eastern North Atlantic Ocean, focussing on the 35-10 ka interval. Sea-surface salinities have been reconstructed quantitatively based on two independent methods from core MD95-2002, recovered from the northern Bay of Biscay adjacent to the axis of the Manche paleoriver outlet and thus in connection with proximal European ice sheets and glaciers. Quantitative reconstructions deriving from dinocyst and planktonic foraminiferal analyses have been combined within a robust chronology to assess the amplitude and timing of hydrological changes in this region. Our study evidences strong pulsed freshwater discharges which may have impacted the North Atlantic Meridional Overturning Circulation.

During the late Quaternary, both external and internal forcings have driven major climatic shifts from glacial to interglacial conditions. Nonlinear climatic steps characterized the transitions leading to these extrema, with intermediate excursions particularly well expressed in the dynamics of the Northern Hemisphere cryosphere. Here we document the impact of these dynamics on the north-eastern North Atlantic Ocean, focussing on the 35-10 ka interval. Sea-surface salinities have been reconstructed quantitatively based on two independent methods from core MD95-2002, recovered from the northern Bay of Biscay adjacent to the axis of the Manche paleoriver outlet and thus in connection with proximal European ice sheets and glaciers. Quantitative reconstructions deriving from dinocyst and planktonic foraminiferal analyses have been combined within a robust chronology to assess the amplitude and timing of hydrological changes in this region. Our study evidences strong pulsed freshwater discharges which may have impacted the North Atlantic Meridional Overturning Circulation.

By combining reconstruction of airmass back-trajectories from dust deposition sites in Europe and measurements of the (Nd) isotopic composition of deposited dust particles, potential sources of different Saharan dust events can be identified. The study of “red dust” events collected in France allowed us to identify distinct North African source areas (e.g. Lybia vs. Mauritania). Surprisingly, the airmass trajectory of one dust event (March 6, 1990) was distinct from the others, and revealed a Chinese origin. The Nd isotopic composition of this dust was consistent with the range of isotopic compositions of Chinese loess. Moreover, an atmospheric global model simulation reveals that a dust plume left China before February 25, 1990, flew over North America around the February/March transition and reached the French Alps by March 6, 1990, revealing that intercontinental dust and pollutant transport may occur across the Pacific Ocean and the North Atlantic at the Westerlies latitudes.

A four-year, high-resolution (<2 mo) record of mineralogical and isotopic (Sr and Nd) characteristics of mineral dust deposited at NorthGRIP confirms the seasonal variability in the eastern Asian source regions providing dust to northern Greenland at present. Comparison of the Sr and Nd isotopic compositions of the dust with those of potential source area samples from China and Mongolia support that the Takla Makan desert is the primary source, supplying most if not all of the mineral particles during the dusty spring season. A different source area, however, plays a role during most of year and during the low-dust season ( summer through winter) in particular. Inner Mongolian deserts of northern China, including the Tengger and the Mu Us, are likely candidates but the Mongolian Gobi is ruled out as a significant contributor to Greenland.

By combining reconstruction of airmass back-trajectories from dust deposition sites in Europe and measurements of the (Nd) isotopic composition of deposited dust particles, potential sources of different Saharan dust events can be identified. The study of “red dust” events collected in France allowed us to identify distinct North African source areas (e.g. Lybia vs. Mauritania). Surprisingly, the airmass trajectory of one dust event (March 6, 1990) was distinct from the others, and revealed a Chinese origin. The Nd isotopic composition of this dust was consistent with the range of isotopic compositions of Chinese loess. Moreover, an atmospheric global model simulation reveals that a dust plume left China before February 25, 1990, flew over North America around the February/March transition and reached the French Alps by March 6, 1990, revealing that intercontinental dust and pollutant transport may occur across the Pacific Ocean and the North Atlantic at the Westerlies latitudes.

A four-year, high-resolution (<2 mo) record of mineralogical and isotopic (Sr and Nd) characteristics of mineral dust deposited at NorthGRIP confirms the seasonal variability in the eastern Asian source regions providing dust to northern Greenland at present. Comparison of the Sr and Nd isotopic compositions of the dust with those of potential source area samples from China and Mongolia support that the Takla Makan desert is the primary source, supplying most if not all of the mineral particles during the dusty spring season. A different source area, however, plays a role during most of year and during the low-dust season ( summer through winter) in particular. Inner Mongolian deserts of northern China, including the Tengger and the Mu Us, are likely candidates but the Mongolian Gobi is ruled out as a significant contributor to Greenland.

Udgivelsesdato: 22 March

Udgivelsesdato: 22 March

Twelve glacial dust samples extracted from the Greenland Ice Core Project ice core have been characterized in terms of mineralogy, Sr and Nd isotopic composition, and rare earth element concentrations. The ice samples are taken from the Younger Dryas, the Bølling, the Last Glacial Maximum, and both cold and mild glacial periods back to 44 kyr B.P. A comparison with samples from possible dust source areas determines eastern Asia as being the main source area at all times, hereby confirming and extending earlier results (Biscaye et al., 1997). This finding is partly in agreement with recent atmospheric general circulation model simulations of glacial atmospheric dust transport (Andersen et al., 1998; Mahowald et al., 1999). A low intersample variability in tracer characteristics is discussed in the context of a volcanogenic influence, grain size dependence, and variability in the continental dust source areas. The clay mineralogy of the ice core dust shows a weak climatic dependence, while the isotopic composition of Sr and Nd does not correlate with climate. This is interpreted as a geographical variation of source areas on a regional scale within Asia. Two ice dust samples were size-fractionated at 2 μm, and the grain size dependence on tracer characteristics is in accordance with the literature.

Theories explaining the origin and distribution of Heinrich layers—abrupt, massive discharges of ice-rafted detritus—generally attribute these layers to the Laurentide ice sheet. However, doubt has been cast by the discovery that sediments also originated from European ice sheets. Here we present isotopic (Sr-Nd) fingerprints of Heinrich layers obtained in cores from the European margin. They reveal that for most of the Heinrich layers, the icebergs were calved first from the European ice sheets, predating the Laurentide surges. Accelerator mass spectrometry 14C ages reveal that the oceanic Heinrich events were synchronous with their dust equivalents in the Greenland ice cores, implying a direct ocean-atmosphere link. The typical Laurentide surge seems to have occurred over a short time span, after about 1.5 k.y. of iceberg release from the European ice sheets. This suggests that the Laurentide ice sheet responded to a stimulus created by events in the European sector.

Udgivelsesdato: 22 Dec.

Theories explaining the origin and distribution of Heinrich layers—abrupt, massive discharges of ice-rafted detritus—generally attribute these layers to the Laurentide ice sheet. However, doubt has been cast by the discovery that sediments also originated from European ice sheets. Here we present isotopic (Sr-Nd) fingerprints of Heinrich layers obtained in cores from the European margin. They reveal that for most of the Heinrich layers, the icebergs were calved first from the European ice sheets, predating the Laurentide surges. Accelerator mass spectrometry 14C ages reveal that the oceanic Heinrich events were synchronous with their dust equivalents in the Greenland ice cores, implying a direct ocean-atmosphere link. The typical Laurentide surge seems to have occurred over a short time span, after about 1.5 k.y. of iceberg release from the European ice sheets. This suggests that the Laurentide ice sheet responded to a stimulus created by events in the European sector.

Udgivelsesdato: 22 Dec.

Reactive phosphorus undergoes diagenetic transformation once transferred into marine sediments. The degree of regeneration and redistribution of phosphorus depends on early diagenetic and environmental conditions, which may be linked to larger scale phenomena, such as bottom water circulation, water column ventilation, and organic carbon flux. Phosphorus phases of the <50-μm-sized fraction of deep-sea sediments from core SU 90-09 (North Atlantic, 43°31′N, 30°24′W, 3375 m below sea level) have been analyzed using a sequential extraction technique (SEDEX method) to reconstruct phosphorus geochemistry during Heinrich events 4 and 5. Comparison with Holocene samples from the same site indicates that postdeposition diagenetic transformation has not affected phosphorus distribution in the deep part of the sediments. Total and reactive phosphorus average 0.40 ± 0.04 mg/g and 0.30 ± 0.05 mg/g, respectively, and are comparable to values found in analog deep-sea environments in the North Atlantic. Detrital phosphorus, the phase linked to igneous- and metamorphic-derived material, sharply increases during Heinrich events and covaries with the ice-rafted debris record, whereas authigenic and Fe-bound phosphorus phases, both influenced by redox conditions, decrease or even disappear. These findings suggest that during the deposition of Heinrich layers (HLs), environmental parameters hampered the precipitation of these phases. Large freshwater discharges in relation to iceberg surges may have provoked a temporary stratification of the water column. Accordingly, dysaerobic conditions in the sediments may have fostered the loss of dissolved phosphorus from the sediments to the water column, in a direct and rapid response to the changed conditions. Decreasing trends in organic matter elemental ratios (total organic carbon/organic phosphorus) and Rock-Eval oxygen index values, along with the presence of partly authigenic dolomite and ankerite within HLs, also support this assumption.