Frank Niessen

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth’s orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the ‘warmer-than-present’ early-Pliocene epoch (∼5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, ∼40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ∼3 °C warmer than today and atmospheric CO2 concentration was as high as ∼400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO2.

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3–4 °C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1–23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

Interhemispheric synchrony of Late-glacial climatic instability as recorded in proglacial Lake Mascardi, Argentina

Several high-resolution continental records have been reported recently in sites in South America, but the extent to which climatic variations were synchronous between the northern and southern hemispheres during the Late-glacial–Holocene transition, and the causes of the climatic changes, remain open questions. Previous investigations indicated that, east of the Andes, the middle and high latitudes of South America warmed uniformly and rapidly from 13 000 14C yr BP, with no indication of subsequent climate fluctuations, equivalent, for example, to the Younger Dryas cooling. Here we present a multiproxy continuous record, radiocarbon dated by accelerated mass spectroscopy, from proglacial Lake Mascardi in Argentina. The results show that unstable climatic conditions, comparable to those described from records obtained in the Northern Hemisphere, dominated the Late-glacial–Holocene transition in Argentina at this latitude. Furthermore, a significant advance of the Tronador ice-cap, which feeds Lake Mascardi, occurred during the Younger Dryas Chronozone. This instability suggests a step-wise climatic history reflecting a global, rather than regional, forcing mechanism. The Lake Mascardi record, therefore, provides strong support for the hypothesis that ocean–atmosphere interaction, rather than global ocean circulation alone, governed interhemispheric climate teleconnections during the last deglaciation.

Late Quaternary sedimentation in Kejser Franz Joseph Fjord and the continental margin of East Greenland

Abstract The marine sedimentary record in Kejser Franz Joseph Fjord and on the East Greenland continental margin contains a history of Late Quaternary glaciation and sedimentation. Evidence suggests that a middle-shelf moraine represents the maximum shelfward extent of the Greenland Ice Sheet during the last glacial maximum. On the upper slope, coarse-grained sediments are derived from the release of significant quantities of iceberg-rafted debris (IRD) and subsequent remobilization by subaqueous mass-flows. The middle-lower slope is characterized by hemipelagic sedimentation with lower quantities of IRD (dropstone mud and sandy mud), punctuated episodically by deposition of diamicton and graded sand/gravel facies by subaqueous debris flows and turbidity currents derived from the mass failure of upper slope sediments. The downslope decrease of IRD reflects either the action of the East Greenland Current (EGC) confining icebergs to the upper slope, or to the more ice-proximal setting of the upper slope relative to the LGM ice margin. Sediment gravity flows on the slope are likely to have fed into the East Greenland channel system, contributing to its formation in conjunction with the cascade of dense brines down the slope following sea-ice formation across the shelf. Deglaciation commenced after 15 300 14C years , as indicated by meltwater-derived light oxygen isotope ratios. An abrupt decrease in both IRD deposition and delivery of coarse-grained debris to the slope at this time supports ice recession, with icebergs confined to the shelf by the EGC. Glacier ice had abandoned the middle shelf before 13 000 14C years with ice loss through iceberg calving and deposition of diamicton. Continued retreat of glacier-ice from the inner shelf and through the fjord is marked by a transition from subglacial till/bedrock in acoustic records, to ice-proximal meltwater-derived laminated mud to ice-distal bioturbated mud. Ice abandoned the inner shelf before 9100 14C years and probably stabilized in Fosters Bugt at 10 000 14C years . Distinct oxygen isotope minima on the inner shelf indicate meltwater production during ice retreat. The outer fjord was free of ice before 7440 14C years . Glacier retreat through the mid-outer fjord was punctuated by topographically-controlled stillstands where ice-proximal sediment was fed into fjord basins. The dominance of fine-grained, commonly laminated facies during deglaciation supports ablation-controlled, ice-mass loss. Glacimarine sedimentation within the Holocene middle-outer fjord system is dominated by sediment gravity flow and suspension settling from meltwater plumes. Suspension sediments comprise mainly mud facies indicating significant meltwater-deposition that overwhelms debris release from icebergs in this East Greenland fjord system. The relatively widespread occurrence of fine-grained lithofacies in East Greenland fjords suggests that meltwater sedimentation can be significant in polar glacimarine environments. The ice-distal continental margin is characterized by meltwater sedimentation in the inner shelf deep, iceberg scouring over shallow shelf regions, winnowing and erosion by the East Greenland Current on the middle-outer shelf, and hemipelagic sedimentation on the continental slope.

GLACIAL VARVE THICKNESS AND 127 YEARS OF INSTRUMENTAL CLIMATE DATA: A COMPARISON

Annually laminated sediments (glacial varves) from Lake Silvaplauna, a High Alpine proglacial lake in the Central Swiss Alps, were compared with glacier monitoring data and instrumental climate data from 1864 to 1990. Long-term and short-term responses to climatic change as well as anthropogenic influence can be traced separately in the varve succession. Economic development in the lake catchment has resulted in higher autochthonous production in recent years. Autochthonous components contribute around 10% to the total amount of sediment accumulated annually since 1960 but their contribution is negligible before this date. Decadal-scale varve thickness trends correlate with glacier size-variations. A stepwise, running multiple regression analysis demonstrates that interannual changes in varve thickness are strongly correlated with changes in mean summer temperatures, but cannot be sufficiently explained without considering summer precipitation and the number of days with snow per year. The wide range of observed correlation coefficients reveals the sensitivity of the archive to temporal variability of the climatic forcing factors and makes the development of transfer functions ambiguous.

The Late Weichselian glaciation of the Franz Victoria Trough, northern Barents Sea: ice sheet extent and timing

High resolution seismic profiles (PARASOUND, 4 kHz) and three sediment cores from the Franz Victoria Trough and the adjacent continental slope were studied in order to constrain the timing and extent of the northern Svalbard/Barents Sea ice sheet during the Late Weichselian glaciation. Stacked debris flow lobes and layers of glacimarine diamicton on the lower continental slope indicate that large quantities of glacially derived sediments were deposited by the northern Svalbard/Barents Sea ice sheet directly onto the upper continental slope at approximately 23 14 C ka. A grounding-line advance to the shelf break is supported by the identification of diamicton, interpreted as till, in the seismic profile near the shelf break. After several ice sheet instabilities marked by significant input of ice rafted detritus to the continental margin, the disintegration of the northern Svalbard/Barents Sea ice sheet (Termination Ia) is indicated by a distinct pulse of ice rafted detritus at 15.4 14 C ka and the transition to an isotopically defined meltwater signal. The drastic change in sedimentary pattern on the upper continental slope, dated to about 13.4 14 C ka, is interpreted as grounding-line retreat from the shelf edge. A further stepwise retreat of the northern Svalbard/Barents Sea ice sheet is indicated by pulses of ice rafted detritus which appear to be contemporaneous with the onset of distinct ice rafting events in adjacent areas and pulses of glacimarine sedimentation in the southwestern Barents Sea. q 2000 Elsevier Science B.V. All rights reserved.

Palaeolimnological studies of the eutrophication of volcanic Lake Albano (Central Italy)

We use palaeolimnological techniques to reconstruct the eutrophication history of a volcanic lake (Lake Albano, central Italy) over the past three centuries. The presence of annual varves down to the bottom of the core (c. 1700 A.D.) indicated the lack of bioturbation and likely long-term meromixis. Sedimentation rates were estimated by varve counts (calcite/diatom couplets), indicating a mean rate of 0.15 cm yr−1. The reconstruction of eutrophication was traced using past populations of algal and photosynthetic bacteria (through their fossil pigment), and geochemistry, as well as fossil remains of chironomids. Phaeophorbidea and the red carotenoid astaxanthin were used to detect past zooplankton development.The first sign of trophic change related to human activities is datedc. 1870 A.D. From that period onward a sharp increase of authigenic CaCO3, nitrogen, N:P ratio, and dinoxanthin, a characteristic carotenoid of Chrysophyceae and Dinophyceae, is observed.Chironomid analyses showed the near absence of a deep water fauna throughout the core length. The populations of chironomid larvae are restricted to oxygenated littoral zones. In fact, the few fossil remains found are primarily of littoral origin, representing shallow water midges that were transported to profundal waters. The reduction of total chironomid in the uppermost layers of the core is to be related to human land uses.

Evidence for ice-free summers in the late Miocene central Arctic Ocean

Although the permanently to seasonally ice-covered Arctic Ocean is a unique and sensitive component in the Earths climate system, the knowledge of its long-term climate history remains very limited due to the restricted number of pre-Quaternary sedimentary records. During Polarstern Expedition PS87/2014, we discovered multiple submarine landslides along Lomonosov Ridge. Removal of younger sediments from steep headwalls has led to exhumation of Miocene sediments close to the seafloor. Here we document the presence of IP25 as a proxy for spring sea-ice cover and alkenone-based summer sea-surface temperatures >4 °C that support a seasonal sea-ice cover with an ice-free summer season being predominant during the late Miocene in the central Arctic Ocean. A comparison of our proxy data with Miocene climate simulations seems to favour either relatively high late Miocene atmospheric CO2 concentrations and/or a weak sensitivity of the model to simulate the magnitude of high-latitude warming in a warmer than modern climate.

The eastern extent of the Barents-Kara ice sheet during the Last Glacial Maximum based on seismic-reflection data from the eastern Kara Sea

We present sub-bottom profiling (sparker and Parasound) results from the eastern Kara Sea, on the Eurasian Arctic margin, which enable the identification of the Last Glacial Maximum (LGM) ice extent. The analysed profiles show that glacigenic diamicton is ubiquitous at the seafloor, east of about 95°E and 78°N. The eastern margin of this diamicton is expressed in a conspicuous morainic ridge at the entrance to the Vilkitsky Strait, and to the south the diamicton projection aligns with the LGM limit mapped at the north-western Taymyr. The bottom of the Voronin Trough further north is also covered with diamicton and has numerous erosional bedforms, indicating a streamlined flow of grounded ice along the trough. Accurate dating of the diamicton is not attainable, but the correlation of pre-diamict sediments to well-dated sections in the Laptev Sea, and available 14C ages from sediments on top of the diamicton, indicate its LGM age. These results support the palaeogeographic reconstruction that assumes the extension of the LGM Barents–Kara ice sheet as far east as Taymyr. This configuration implies that LGM ice blocked the drainage of the Ob and Yenisey rivers on the Kara shelf. This inference is consistent with the presence of large (>100 km wide) lenses of basin infill adjacent to the southern margin of the diamicton. However, the limited distribution of the eastern Kara ice lobe, not extending on Severnaya Zemlya, suggests that the ice was fairly thin and short-lived: insufficient for the accumulation of the gigantic proglacial lakes that occurred during earlier glaciations.

The Late Quaternary evolution of the western Laptev Sea continental margin, Arctic Siberia—implications from sub-bottom profiling

Abstract High-resolution seismic profiles (PARASOUND, 4 kHz) of the western Laptev Sea continental margin and the adjacent Vilkitsky Strait were studied in order to gain new evidence about the Weichselian glaciations in Central Siberia and to test reconstructions of maximum ice sheet extents. Four regionally correlatable seismic units, named I (youngest) to IV, were identified in the upper Quaternary sedimentary succession: (I) a thin drape; (II) prograding wedge-shaped deposits along the shelf edge; (III) layered sediments of draping and infilling character with increasing thickness towards the western Laptev Sea shelf edge and the Vilkitsky Strait; (IV) stacked debris-flow deposits. The thin drape of unit I is radiocarbon-dated to Holocene and mainly deposited during the transgression of the Laptev Sea. The wedge-shaped deposits of unit II are interpreted as river deltas, referring to point sources along the shelf edge during the Late Weichselian sea-level lowstand. This indicates that the river input across the Laptev Sea shelf was continuous during marine isotope stage (MIS) 2. The layered sediments of unit III suggest hemipelagic conditions indicative of a sea-level highstand. The pronounced thickening of unit III towards the shelf edge of the western Laptev Sea reflects the lowering of the global sea level during MIS 3. This is associated with increased riverine input due to the northward shift of the Siberian coastline. The stacked debris-flow deposits of unit IV extend continuously from the shelf edge in the Vilkitsky Strait to the continental rise of the western Laptev Sea continental margin. They indicate that large quantities of sediments were deposited directly on the upper continental slope during advances of the Kara Sea ice sheet to the shelf break. These ice-proximal conditions are presumably linked to the Middle Weichselian glaciation (MIS 4). Our evidence confirm earlier reconstructions, suggesting that in central Siberia, the Middle Weichselian glaciation (MIS 4) was of larger extent than the Late Weichselian glaciation (MIS 2).