Reidar Løvlie

Holocene glacial and climate history of the Jostedalsbreen region, Western Norway; evidence from lake sediments and terrestrial deposits

The valleys surrounding the Jostedalsbreen ice cap were deglaciated during the latter half of the Preboreal Chronozone. At the end of the Preboreal Chronozone, however, a glacier readvance occurred. Terminal moraines were deposited by outlet valley glaciers from the Jostedalsbreen Plateau up to 1 km beyond Little Ice Age moraines. Inferred from the altitude of lateral moraines formed during this readvance and calculations of the equilibrium-line altitude (ELA) depression based on an accumulation area ratio (AAR) of 0.6, the average depression of the ELA was 325 + 75−115 m below the present. By assuming a similar precipitation pattern as at present, this suggests a mean temperature decline of about 2°C. Palynological investigations from Sygneskardet, Sunndalen, indicate that climate like the present was achieved just after 9000 BP. The Holocene climatic optimum occurred during the Atlantic Chronozone, with elm (Ulmus) stands growing at the present birch (Betula) forest limit in Sunndalen and pine (Pinus) growing at Styggevatnet to an altitude of at least 1160 m. During this period the mean summer temperature is estimated to have been at least 2.7 and 1.8°C warmer than at present, with and without the local climatic effect of Jostedalsbreen, respectively. An inferred rise of the ELA of about 400 m from the present altitude suggests that possibly no glaciers existed on the Jostedalsbreen Plateau during the Holocene climatic optimum. Vegetational changes as deduced from palynological studies, lowered tree limits and increased resedimentation in peat bogs indicate general climatic deterioration since the Late Atlantic Chronozone. A significant Alnus decline 6300 BP and a Betula expansion 5300 BP, as recorded from palynological studies of peat bogs in Sprongdalen, is interpreted to represent the initial Neoglaciation on the Jostedalsbreen Plateau. Lithostratigraphic evidence at a section in the upper part of Glomsdalen may indicate a minor Neoglacial phase close to 6000 BP. The first pronounced Neoglacial event in the Jostedalsbreen region, however, was bracketed between 3700 and 3100 BP. Sporteggbreen, the upper part lying ca. 30 m above the present regional glaciation threshold, was formed around 500 BP at the initiation of the Little Ice Age. Gelifluction at 1000 m altitude began after 3200-2800 BP. During the Medieval Period, glaciers were smaller than now. At the northwestern part of Jostedalsbreen an initial Little Ice Age glacier expansion is dated to have occurred after 890 ± 60 BP (A.D. 1030–1220). From the early 14th century, and especially from the mid-17th century, a severe climatic deterioration during the Little Ice Age is historically documented. Around Jostedalsbreen this advance culminated during the mid-18th century. Documents suggest that the western outlet glaciers from Jostedalsbreen reached their maximum Little Ice Age position some years before the longer eastern outlet valley glaciers. A typical depression of the ELA during the Little Ice Age of 100–150 m indicates a mean temperature decline from the present of ca. 0.5–1°C.

Manganese and color cycles in Arctic Ocean sediments constrain Pleistocene chronology

Sequential variations in manganese (Mn) content and color of deepsea sediments retrieved from the Lomonosov Ridge (87°N) in the central Arctic Ocean apparently mimic low-latitude δ 18 O glacial-interglacial cyclicity, thereby providing stratigraphic information that together with biostratigraphic data permit the construction of a detailed chronological model. Correlation of this Mn and color chronology to established apparent Brunhes-age estimates of geomagnetic excursions reveals a remarkable fit between these two independently derived time scales. The Mn and color cycles probably provide paleoenvironmental information about material fluxes in the Arctic Ocean over the past 1 m.y. We suggest that the primary source for the observed manganese variations in our sediment core is northern Siberia, which has extensive peat bogs and boreal forests. These Siberian source areas could operate in an off and on mode tuned to Pleistocene glacial and interglacial periods. Contrasts in ventilation of Arctic Ocean waters during interglacial-glacial cycles probably could also enhance the observed Mn and color variability.

Age model and core‐seismic integration for the Cenozoic Arctic Coring Expedition sediments from the Lomonosov Ridge

Cenozoic biostratigraphic, cosmogenic isotope, magnetostratigraphic, and cyclostratigraphic data derived from Integrated Ocean Drilling Program Expedition 302, the Arctic Coring Expedition (ACEX), are merged into a coherent age model. This age model has low resolution because of poor core recovery, limited availability of biostratigraphic information, and the complex nature of the magnetostratigraphic record. One 2.2 Ma long hiatus occurs in the late Miocene; another spans 26 Ma (18.2–44.4 Ma). The average sedimentation rate in the recovered Cenozoic sediments is about 15 m/Ma. Core-seismic correlation links the ACEX sediments to the reflection seismic stratigraphy of line AWI-91090, on which the ACEX sites were drilled. This seismostratigraphy can be correlated over wide geographic areas in the central Arctic Ocean, implying that the ACEX age model can be extended well beyond the drill sites.

Pleistocene stratigraphy and paleoenvironmental variation from Lomonosov Ridge sediments, central Arctic Ocean

Abstract High resolution seismoacoustic chirp sonar data and piston cores were collected from the Lomonosov Ridge in the central Arctic Ocean (85°–90°N; 130°–155°E). The chirp sonar data indicate substantial erosion on the ridge crest above 1000 mbsl while data from deeper sites show apparently undisturbed sedimentation. Piston cores from both the eroded ridge crest and the slopes have been analyzed for a variety of properties, permitting inter-core correlation and description of paleoenvironmental change over time. Based on the evidence of extensive sediment erosion at depths above 1000 mbsl, we infer that the top of the Lomonosov Ridge has been eroded by grounded ice during a prominent glacial event that took place during MIS 6 according to a newly published age model. This event is coeval with a dramatic shift from low amplitude glacial–interglacial variability to high amplitude variability recorded in the sedimentary record. The new age model used in our study is based on nannofossil biostratigraphy and correlation between sedimentary cycles and a low-latitude oxygen isotope record and confirmed by paleomagnetic polarity studies where negative paleomagnetic inclinations are assigned to excursions. Due to the controversy between this age model and age models that assign the negative paleomagnetic inclinations to polarity reversals, we provide a correlation to Lomonosov Ridge core PS2185-6 [Spielhagen et al., Geology, 25 (1997) 783]. According to the latter age models, the Lomonosov Ridge was eroded by ice grounding much earlier, at MIS 16.

Post-depositional remanent magnetization in a re-deposited deep-sea sediment

Abstract Re-deposition of some deep-sea clay material in the earths magnetic field is described. At the time of an induced reversal of the ambient field declination, calcium carbonate layers were introduced for identification purposes. NRM and bulk susceptibility measurements along the still moist sediment cores showed the presence of a time lag of around 10 days between a change in the ambient field direction and a corresponding change in the stable magnetization. AF-demagnetization of discs sampled from a dried core suggested that this time lag was caused by a post-depositional remanent magnetization mechanism. Magnetomineralogical investigations show the presence of the thermally unstable magnetic mineral phase, probably titano-maghemite. A positive correlation between NRM-intensities and water temperature during deposition is discussed. It is suggested that consolidation-rate is a major factor influencing the NRM intensity in deep-sea sediments.

Magnetostratigraphy in three Arctic Ocean sediment cores; arguments for geomagnetic excursions within oxygen-isotope stage 2–3

Two zones with shallow to steep negative palaeomagnetic directions have been detected within similar lithological units in three high latitude gravity cores from the Arctic Ocean. Coinciding azimuthal distributions of declinations relative to arbitrary oriented sub-sampling surfaces are attributed to the systematic acquisition of magnetic components by frictional deformation of the unconsolidated sediment during the sub-sampling procedure. Alternating field demagnetization to 50 mT was only partly successful in obtaining stable single component directions. Magnetic polarity zones are deduced by extrapolation of directional trends of inclinations during progressive af-demagnetization. High delta δ18O values in sinistrally coiled N. pachyderma range between 4.30–4.68‰ PDB (mean values), reflecting sediment accumulation during continental glaciation. Amino acid ratios of N. pachyderma suggest ages less than approximately 60 000 years ago. When seen in conjunction, these observations suggest that the sediments were deposited within oxygen isotope stages 2–3. The reversed palaeomagnetic polarity zones are concluded to represent two short duration excursions of the geomagnetic field which occurred less than 60 000 years ago. Likely known candidates are the Lake Mungo (28 000–30 000 years ago) and Laschamp/Olby (35 000–40 000 years ago) excursions all confined within oxygen isotope stage 3 (24 000–59 000 years ago).

Magnetic properties and mineralogy of four deep-sea cores

Ferromagnetic minerals have been extracted from 17 samples from four deep-sea cores. Measurements of bulk susceptibility and saturation isothermal remanent magnetization were used as semiquantitative indicators of the efficiency of the described technique. This was found to be around 60%. X-ray diffraction and fluorescence analysis and determination ofJs-T curves revealed that the dominant ferromagnetic mineral extracted is probably a high Curie temperature form of titanomagnetite. Scanning electron micrographs of the separations showed detrital grains, more than 40% of which were finer than one micron in size. Redeposition experiments and magnetic susceptibility anisotropy measurements were used to investigate the origin of the NRM, which in these cores was probably post-depositional or depositional rather than chemical.

The intensity pattern of post-depositional remanence acquired in some marine sediments deposited during a reversal of the external magnetic field

Abstract Re-deposition of four different marine sediments during an instantaneous reversal of the external magnetic field revealed that the reversals of the remanent magnetization in the partially dried cores were encountered at stratigraphic levels deposited 2–6 days prior to the actual reversal of the external field. The lag is attributed to post-depositional alignment of the magnetic grains. The remanent intensity patterns show pronounced minima associated with the appearance of intermediate directions, probably caused by the presence of deviating and partly opposing magnetic components within each specimen. A post-DRM model is suggested involving the presence of a zone of gradual consolidation in which the magnetic grains become permanently aligned. One implication of the model is that the intensity decrease associated with polarity transitions observed in deep-sea sediments do not necessarily reflect the behaviour of the geomagnetic field but is a property of the magnetization processes operating in deep-sea sediments.

Holocene glacier activity at the southwestern part of Hardangerjøkulen, central-southern Norway : evidence from lacustrine sediments

Lacustrine sediments from Lake Isdalsvatn downstream from the southwestern margin of the plateau glacier Hardangerjøkulen, central-southern Norway, show that the lake received glacial meltwater until 7890 ± 95 BP. A thin, bluish-grey minerogenic silt layer probably reflects a short glacial (re)advance between 7005 ± 90 and 6835 ± 150BP. After the latter date, inflow of glacial meltwater ceased, indicating that small or, more probably, no glaciers existed on the southwestern Hard angerjøkulen plateau. A drop in organic content and a slight increase in detrital content inferred from magnetic susceptibility at 120cm depth in the core, may indicate some glacier activity in the catchment from about 3400 BP. Textural changes at 50cm depth indicate increased Neoglacial activity in the upper catchment area from 2265 ± 80 BP. Differences in glacier chronologies between the southern and northern sectors of Hardangerjøkulen may be explained partly by different glacier responses to the precipitation and wind distribution at opposite sides of the ice cap.

Paleoclimate deduced from a multidisciplinary study of a half-million-year-old stalagmite from rana, northern Norway

A 7-cm-thick flowstone sequence has been dated by extended Uranium series techniques to less than 1.25 myr, with a probable growth interval of ≥350,000–≤730,000 yr B.P. The time span is in accordance with paleomagnetic results revealing normal polarity for the whole sequence, i.e., a depositional age of <730,000 yr B.P. Oxygen isotope variations suggest deposition during three warm periods, interrupted by two isotopically cold hiatuses characterized by bulk resolution and detrital laminae. Calculations suggest that climatic transitions may have involved a shift of 1.1–2.4°C in mean annual temperatures. Contemporary flooding of the cave due to adjacent glacier expansion provides evidence for the widely held view that speleothem deposition is halted by glacier proximity. The flowstone matrix contained appreciable amounts of pollen (pine, birch) as well as larger amounts of charcoal dust. The pollen assemblage suggests a climate comparable with the present, implying that Norway has experienced at least three glacial/interglacial transitions during the growth interval of the speleothem. The warmer periods were characterized by a forest environment, with ample evidence of fire.