Trond Dokken

Rapid changes in the mechanism of ocean convection during the last glacial period

High-amplitude, rapid climate fluctuations are common features of glacial times. The prominent changes in air temperature recorded in the Greenland ice cores are coherent with shifts in the magnitude of the northward heat flux carried by the North Atlantic surface ocean; changes in the oceans thermohaline circulation are a key component in many explanations of this climate flickering. Here we use stable-isotope and other sedimentological data to reveal specific oceanic reorganizations during these rapid climate-change events. Deep water was generated more or less continuously in the Nordic Seas during the latter part of the last glacial period (60 to 10 thousand years ago), but by two different mechanisms. The deep-water formation occurred by convection in the open ocean during warmer periods (interstadials). But during colder phases (stadials), a freshening of the surface ocean reduced or stopped open-ocean convection, and deep-water formation was instead driven by brine-release during sea-ice freezing. These shifting magnitudes and modes nested within the overall continuity of deep-water formation were probably important for the structuring and rapidity of the prevailing climate changes.

FLUCTUATIONS OF THE SVALBARD–BARENTS SEA ICE SHEET DURING THE LAST 150 000 YEARS

Abstract On Spitsbergen, western Svalbard, three major glacial advances have been identified during the Weichselian. All three reached the continental shelf west of the Svalbard archipelago. Radiocarbon, luminescence and amino acid dating of interbedded interstadial and interglacial sediments indicate that these glacial advances have Early (Isotope Stage 5d), Middle (Stage 4), and Late Weichselian ages (Stage 2). An additional, more local, advance has been dated to Isotope Stage 5b. The Late Weichselian ice sheet expanded across the entire Barents Sea. However, in the south-western Barents Sea, the Late Weichselian till is the only till above Eemian sediments, indicating that the Early- and Middle Weichselian ice advances were restricted to the Svalbard archipelago and the northern Barents Sea. A major problem with the onshore sites is the dating of events beyond the range of the radiocarbon method. To overcome this, the onshore record has been correlated with marine cores from the continental slope and the deep-sea west of Svalbard, where a chronology has been established by oxygen isotope stratigraphy. Ice rafted detritus (IRD) was used as the main monitor of glaciation. The IRD record closely mirrors the glaciation history as interpreted from the onshore sections. During the Late Weichselian, the largest IRD peak occurred during deglaciation, a pattern also postulated for the earlier events. Given this, the results from the marine cores indicate that the ages for the first glacial advances during the Weichselian were a few thousand years older than interpreted from the onshore stratigraphy.

RAPID CLIMATIC VARIATIONS DURING MARINE ISOTOPIC STAGE 3 : MAGNETIC ANALYSIS OF SEDIMENTS FROM NORDIC SEAS AND NORTH ATLANTIC

Abstract The bulk magnetic parameters of seven deep-sea cores distributed from the Nordic Seas (67°N) to the North Atlantic as far south as the Bermuda Rise (33°N) exhibit short-term variations which correlate with rapid climatic changes during marine isotopic stage 3 (MIS3). The magnetic mineralogy is uniformly dominated by well sorted low Ti-content magnetites indicating that these variations are due to variations in the relative amount of magnetic minerals. Because the magnetic minerals predominantly originate from one common source area (the Nordic basaltic province), these changes arise from changes in the efficiency of the transport of the magnetic particles by deep currents from the source to the site of deposition. These results therefore show that the fast climatic changes are related to coeval fast changes in the strength of the deep-sea circulation. The latter was active/reduced during the interstadials/stadials and Heinrich events transporting the magnetic particles from the Norwegian Sea into the North Atlantic ocean along a path similar to the present path of the NADW. It is tentatively suggested that the Faeroe-Shetland Channel and the Denmark Strait were the only two active paths for the overflow water during MIS3. The presence of magnetic oscillations in the Bermuda Rise core in phase with those from the North Atlantic indicates that the activity of the southern Newfoundland Basin gyre was linked to that of the NADW during MIS3.

Coherent patterns of ice-rafted debris deposits in the Nordic regions during the last glacial (10–60 ka)

We have synchronized records of ice-rafted rock debris deposits of three sediment cores from the Norwegian Sea and the Irminger Basin during the last glacial period from 10 to 50 ka by combining the use of radiocarbon dates and adjustments of physical properties. Our synchronized records indicate that layers rich in ice-rafted debris were deposited throughout the Nordic regions at times near to synchronous with the major collapses of the Laurentide ice sheet: during the Heinrich events. There are also millennial-scale, coherent and near to synchronous deposits of ice-rafted rock debris into the Norwegian Sea related to repetitive changes of the flux of icebergs from the Fenno-Scandinavian. The correlation with the cold phases of the Dansgaard–Oeschger temperature record points to a close coupling between atmospheric temperature oscillations and variations of iceberg fluxes into the Norwegian Sea during the last glacial. Variations in atmospheric circulation patterns bringing moisture supply to high latitudes and the distribution of this moisture over the different Northern Hemisphere coastal ice sheets and ice shelves could be controlling both the timing of ice sheet advances and the flux of iceberg to the open ocean.

Early Holocene temperature variability in the Nordic Seas: The role of oceanic heat advection versus changes in orbital forcing

Received 7 January 2011; revised 15 July 2011; accepted 21 July 2011; published 22 October 2011. [1] The separate roles of oceanic heat advection and orbital forcing on influencing early Holocene temperature variability in the eastern Nordic Seas is investigated. The effect of changing orbital forcing on the ocean temperatures is tested using the 1DICE model, and the 1DICE results are compared with new and previously published temperature reconstructions from a transect of five cores located underneath the pathway of Atlantic water, from the Faroe‐Shetland Channel in the south to the Barents Sea in the north. The stronger early Holocene summer insolation at high northern latitudes increased the summer mixed layer temperatures, however, ocean temperatures underneath the summer mixed layer did not increase significantly. The absolute maximum in summer mixed layer temperatures occurred between 9 and 6 ka BP, representing the Holocene Thermal Maximum in the eastern Nordic Seas. In contrast, maximum in northward oceanic heat transport through the Norwegian Atlantic Current occurred approximately 10 ka BP. The maximum in oceanic heat transport at 10 ka BP occurred due to a major reorganization of the Atlantic Ocean circulation, entailing strong and deep rejuvenation of the Atlantic Meridional Overturning Circulation, combined with changes in the North Atlantic gyre dynamic causing enhanced transport of heat and salt into the Nordic Seas.

Mg/Ca ratios in the planktonic foraminifer Neogloboquadrina pachyderma (sinistral) in the northern North Atlantic/Nordic Seas

In core top samples in the Nordic Seas, Mg/Ca ratios of N. pachyderma (sin.) are generally consistent with previous high-latitude calibration data but do not reflect the modern calcification temperature gradient from 2°C in the northwest to 8°C in the southeast. This is because Mg/Ca ratios in foraminiferal shells from the central Nordic Seas are ∼0.4 mmol/mol higher than expected from calibrations of Nurnberg (1995) and Elderfield and Ganssen (2000). The enhanced Mg/Ca ratios are observed in an area with low sedimentation rates (<∼5 cm/kyr). Possible factors that may cause this include bioturbation, Holocene variability in old core tops, dissolution, pore water chemistry, occurrence of volcanic ash, and other natural variability. The enhanced foraminiferal Mg/Ca ratios in areas of the Nordic Seas and the northern North Atlantic may also be linked with secondary factors related to the presence of fresher and colder water masses, possibly combined with pore water chemistry in low-sedimentation areas differing from high-sedimentation areas.

Quartz content and the quartz-to-plagioclase ratio determined by X-ray diffraction: a proxy for ice rafting in the northern North Atlantic?

Many paleoceanographic reconstructions of the glacial North Atlantic include estimates of iceberg discharge, which are based on the variable abundance of ice-rafted detritus (IRD) in deep-sea sediments. IRD abundance is most often determined by the mechanical separation and painstaking counting of terrigenous particles larger than a specified threshold grain size, typically 150 mum. Here we present a new proxy for IRD based on X-ray diffraction (XRD) analysis of bulk sediments. This approach complements results obtained from standard techniques while offering several distinct advantages. In addition to the rapid production of objective data, XRD measurements on bulk sediments are sensitive to a broader and more characteristic grain size range than counts of individual coarse lithic fragments. The technique is demonstrated in a study of 12 sediment cores from the North Atlantic. Bulk quartz content and the quartz-to-plagioclase ratio exhibit peak-to-peak correspondence to manual counting results, which verifies the identification of large IRD influxes. The XRD data also reveal variations between the manually identified peaks, suggesting increased sensitivity to low-level, distal, or sea-ice sources of IRD. A saw-tooth pattern emerges in many IRD events, which supports a link between ice rafting and atmospheric temperature changes over Greenland, and providing further evidence of the influence of climate on iceberg discharges

Magnetic anisotropy and environmental changes in two sedimentary cores from the Norwegian Sea and the North Atlantic

Abstract We have studied the magnetic properties and the magnetic anisotropy (AMS and AARM) of cores MD95-2010 from the Voering Plateau and SU90-33 from the south Icelandic basin. In the first core, the study has been focussed on climatic stage 3 during which the record of magnetic susceptibility is correlated with the δ18O signal from the Greenland ice cores. Minima (maxima) in susceptibility, anhysteretic remanent magnetization and isothermal remanent magnetization coincide with cold (warm) periods and reflect a modulation in the amount of deposited magnetite. In the second core six climatic stages have been studied. The AMS and the AARM changes are tightly correlated, showing that both are related to changes in the organization of magnetite grains in the sediment. Both magnetic fabrics are oblate and the variations of the degree of anisotropy are climatically controlled with higher values during short warm events or interglacial periods compared to cold periods. Different possible mechanisms leading to the observed fabrics are discussed. It appears that, although a differential compaction might have been active, the fabrics mainly result from depositional effects, in connection with climatic changes. Although the exact mechanisms of the link between magnetic fabrics and bottom current dynamics are, as yet, open to discussion, the study of magnetic anisotropy, which reflects the sedimentary structure, may ultimately lead to a better understanding of the coupling between atmospheric temperature changes and paleocirculation pattern in the North Atlantic and Nordic Seas.

Palaeoceanography on the European arctic margin during the last deglaciation

Abstract To reveal the palaeoceanographic and palaeoclimatic evolution related to the disintegration of the Barents Sea and Fennoscandian ice sheets, high-resolution sediment cores from the continental margin off western Svalbard, western Barents Sea and northern Norway were investigated. The location of these cores is below the axis of the Norwegian Current and beyond, but close to, glaciated continental areas. Hence they should sensitively reflect the palaeoceanography of the northernmost Norwegian Sea. Between, 14.5–19.5 and 22.5–29 14C ka, a high abundance of planktonic foraminifera and a small content of subpolar species indicate seasonally ice-free and slightly warmed surface water. This phenomenon is related to the advection of surface water of North Atlantic origin. The onset of the deglaciation is characterized by a marked low oxygen event dated to between 15 and 13 ka, interpreted to reflect a surface water freshening produced by meltwater and reduced oceanic mixing. After the onset of deglaciation, the surface ocean warmed in three rapid steps: (1) around 12.5 ka, indicated by increased abundances of planktonic foraminifera; (2) at 10.2 ka and at (3) 10–9.6 ka. The two latter steps are reflected by the increased abundance of subpolar planktonic foraminifera.

Atlantic Ocean circulation changes preceded millennial tropical South America rainfall events during the last glacial

During the last glacial period, Greenlands climate shifted between cold (stadial) and warm (interstadial) phases that were accompanied by ocean circulation changes characterized by reduced Atlantic Meridional Overturning Circulation (AMOC) during stadials. Here we present new data from the western tropical Atlantic demonstrating that AMOC slowdowns preceded some of the large South American rainfall events that took place during stadials. Based on 231Pa/230Th and Ti/Ca measurements in the same sediment core, we determine that the AMOC started to slowdown 1420 ± 250 and 690 ± 180 (1σ) years before the onset of two large precipitation events associated with Heinrich stadials. Our results bring unprecedented evidence that AMOC changes could be at the origin of the large precipitation events observed in tropical South America during Heinrich stadials. In addition, we propose a mechanism explaining the differences in the extent and timing of AMOC slowdowns associated with shorter and longer stadials.