Bjørg Risebrobakken

Late Holocene surface ocean conditions of the Norwegian Sea (Vøring Plateau)

[1] Late Holocene sea surface ocean conditions of the eastern Norwegian Sea (Voring Plateau) are inferred from planktic stable isotopes and planktic foraminiferal assemblage changes in cores JM97-948/2A and MD952011 (66.97� N, 7.64� E). Strong covariance between the planktic stable oxygen isotopic record and abundance changes of N. pachyderma (sin) show that major changes in surface ocean conditions are reflected both in the geochemical composition of the foraminiferal tests as well as in the composition of the foraminiferal fauna. Surface ocean conditions warmer than present were common during the past 3000 years. During the so-called Medieval Warm Period, surface conditions were highly variable with marked changes in sea surface temperature. The warmest sea surface temperatures during this period occurred between 800 and 550 years BP (0 BP = AD 2000). Climatic deterioration, recorded as decreases in sea surface temperature, occurred at about 2750, 1550, 400, and 100 years BP. The cooling events at about 2750 and 1550 years BP appear to correlate with increases in ice-rafted debris in the North Atlantic. Based on the results from JM97-948/2A and MD952011, the onset of the Little Ice Age cooling trend seems to have occurred around 700–600 years BP. Faunal changes indicate two cooling events during the Little Ice Age (at 400 and 100 years BP) that correspond to decreases in Fennoscandian summer temperatures and increases in ice-rafted debris in the eastern North Atlantic. INDEX TERMS: 3030 Marine Geology and Geophysics: Micropaleontology; 4267 Oceanography: General: Paleoceanography; 4870 Oceanography: Biological and Chemical: Stable isotopes; KEYWORDS: paleoceanography, stable isotope, Holocene, micropaleontology Citation: Andersson, C., B. Risebrobakken, E. Jansen, and S. O. Dahl, Late Holocene surface ocean conditions of the Norwegian Sea (Voring Plateau), Paleoceanography, 18(2), 1044, doi:10.1029/2001PA000654, 2003.

Climate and oceanographic variability in the SW Barents Sea during the Holocene

The Holocene section of the marine sediment core PSh-5159N, located in the SW Barents Sea, has been studied at high resolution with a multiproxy approach. A well-stratified water column occurred at the site 11—9.8 ka BP. The stratification was probably a result of a winter sea ice cover and/or fresh, warm surface waters during summer. Stratification and resultant reduction in air—sea interaction allowed for warmer bottom water temperatures. The general situation 11—9.8 ka BP could have been associated with an anomalous high-pressure system over the Nordic Seas and the Arctic Ocean. During the 11—10.5 ka BP interval the polar front was located close to the Barents Sea margin. The polar front moved towards the site from 10.5 ka BP, and from 9.8 to 7.5 ka BP it was probably located close to the site. At 7.5 ka BP the polar front retreated eastwards as the present-day oceanographic pattern established. The mid Holocene was in general characterized by rather stable conditions. In contrast, highly variable conditions are recorded throughout the late Holocene. Episodic expansions of the coastal water influenced zone are typical for the last 2.5 ka BP. Predominantly cold conditions and reduced southwesterly wind strength are suggested during these episodes. The Holocene temperature variability seems in general to be of larger amplitude than instrumentally recorded temperature changes in the SW Barents Sea.

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.

Reconstruction of the postglacial environments in the southwestern Barents Sea based on foraminiferal assemblages

Environmental changes in the surface and bottom water layers of the Ingøydjupet Basin and the history of the Atlantic Water inflow to the southwestern Barents Sea during the last 16 ka are reconstructed based on planktic and benthic foraminiferal assemblages. The multiproxy study of sediment cores PSh-5159R and PSh-5159N, including AMS 14C dating, provides a time resolution of about 200 years for the deglaciation, 100 years for the Holocene, and 25–50 years for the last 400 years. Stable polar conditions with the sea ice at the surface were typical for the Early Deglaciation period. Unstable bottom settings and the onset of ice rafting marked the Oldest Dryas. The cold Atlantic Water inflow increased notably during the Bölling-Alleröd interstadial nearby the site location and then decreased during the Younger Dryas. The initial Holocene was characterized by abrupt warming in bottom and surface water layers, especially ∼9.7–7.6 ka BP. Stable conditions prevailed during the Middle Holocene. Remarkable changes in the sea-surface temperature and bottom environments occurred during the last 2.5 cal. ka BP.

Holocene Climate Variability in the Northern North Atlantic Region: A Review of Terrestrial and Marine Evidence

The Holocene epoch, which followed the last major pulse of glaciation (the Younger Dryas) at the end of the last glaciation, encompasses a period before there was any substantial anthropogenic forcing of climate. A synthesis of climatic development during the Holocene (ca. 11,500 cal. yr BP to the present) is based on pollen-based quantitative temperature reconstructions, tree-line variations, chironomids, tree-ring records, speleothem data, glacier variations, and marine records (stable isotopes, species abundance, lithological changes) from the Nordic Seas. The Holocene has been regarded as a period of relatively stable climate, but recent evidence suggests there have been several significant millennial-scale climate fluctuations (larger than the post mid-19th century warming trend) throughout the Holocene. A general climate warming in the first part of the Holocene was punctuated by a few, abrupt climate reversals, centred at 10,000, 9,700, and 8,200 cal. yr BP. The data suggest there was a period of relatively warm conditions in the first half of the Holocene, in many areas warmer than in the 20th century, after which temperatures generally declined. The temperature decline was punctuated by centennial-scale warmer and colder periods with the most recent cold episode (∼AD 1550-1925), including the Little Ice Age, being one of the coldest of the entire Holocene. The kind of data presented here can be used for detecting mechanisms and forcing factors behind the reconstructed climate variations and to study leads and lags in the Earths climate system.

Low-frequency Pliocene climate variability in the eastern Nordic Seas

The Pliocene (5.3–2.6 Ma) is often described as a relatively stable climatic period, with warm temperatures characterizing high latitudes. New suborbital resolved stable isotope records from Ocean Drilling Program Hole 642B in the eastern Nordic Seas document that the Pliocene was not a stable period characterized by one climate. Rather, seven distinct climate phases, each lasting between 150,000 and 400,000 years, are identified and characterized in the time interval 5.1–3.1 Ma. Four of the transitions between the defined climate phases occurred close to an eccentricity minimum and a minimum in amplitude of change for Northern Hemisphere summer insolation, while two occurred around an eccentricity maximum and a maximum in amplitude in insolation change. Hence, a low-frequency response of the Nordic Seas to insolation forcing is indicated. In addition, paleogeographic and related paleoceanographic changes, expansion of the Arctic sea ice cover, and onset of Northern Hemisphere glaciation were important factors behind the evolving Pliocene low-frequency variability in the eastern Nordic Seas. It is likely that the identified climate phases and transitions are important beyond the Nordic Seas, due to their association with changes to both insolation and paleogeography. However, a strong and variable degree of diagenetic calcite overgrowth is documented for the planktic foraminifera, especially influencing the planktic δ18O results; the absolute values and amplitude of change cannot be taken at face value.

Extent and Variability of the Meridional Atlantic Circulation in the Eastern Nordic Seas During Marine Isotope Stage 5 and its Influence on the Inception of the Last Glacial

Variable climatic and oceanographic conditions characterized the last interglacial at high northern latitudes, probably related to changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC). The magnitudes of these changes are comparable to the Holocene variability, and were thus significantly subdued compared to glacial climate changes. A thermal optimum occurred during the early part of the interglacial, followed by a period of reduced Atlantic inflow to the northernmost Nordic Seas. Subsequently, a new period with increased strength of the AMOC occurred. Significant amounts of Ice-Rafted Debris (IRD) were deposited in the northernmost Nordic Seas before any major change of the global ice volume. This implies an early onset of local ice sheet growth, probably the result of enhanced inflow of Atlantic water to the northernmost Nordic Seas contemporary with a Northern Hemisphere summer insolation minimum. Contrasting sea-land conditions provided large moisture fluxes towards land, giving rise to rapid, early glacial growth. Throughout the glacial part of Marine Isotope Stage (MIS) 5, millennial-scale cold events occurred along the axis of the warm water transport, from the subtropics all the way to the northernmost Nordic Seas. Correlation of IRD events from sites in the Fram Strait, on the Voring Plateau, and in the North Atlantic provides evidence that the major Northern Hemisphere ice sheets at times responded coherently to the same forcing. The widespread distribution of these events highlights the importance of the oceanic influence on the regional climate system.

Palaeoclimate constraints on the impact of 2°C anthropogenic warming and beyond

Over the past 3.5 million years, there have been several intervals when climate conditions were warmer than during the pre-industrial Holocene. Although past intervals of warming were forced differently than future anthropogenic change, such periods can provide insights into potential future climate impacts and ecosystem feedbacks, especially over centennial-to-millennial timescales that are often not covered by climate model simulations. Our observation-based synthesis of the understanding of past intervals with temperatures within the range of projected future warming suggests that there is a low risk of runaway greenhouse gas feedbacks for global warming of no more than 2 °C. However, substantial regional environmental impacts can occur. A global average warming of 1–2 °C with strong polar amplification has, in the past, been accompanied by significant shifts in climate zones and the spatial distribution of land and ocean ecosystems. Sustained warming at this level has also led to substantial reductions of the Greenland and Antarctic ice sheets, with sea-level increases of at least several metres on millennial timescales. Comparison of palaeo observations with climate model results suggests that, due to the lack of certain feedback processes, model-based climate projections may underestimate long-term warming in response to future radiative forcing by as much as a factor of two, and thus may also underestimate centennial-to-millennial-scale sea-level rise.A review of Earth system changes associated with past warmer climates provides constraints on the environmental changes that could occur under warming of 2 °C or more over pre-industrial temperatures.

Mid‐Piacenzian Variability of Nordic Seas Surface Circulation Linked to Terrestrial Climatic Change in Norway

During the mid-Piacenzian, Nordic Seas sea surface temperatures (SSTs) were higher than today. While SSTs provide crucial climatic information, on their own they do not allow a reconstruction of potential underlying changes in water masses and currents. A new dinoflagellate cyst record for Ocean Drilling Program (ODP) Site 642 is presented to evaluate changes in northward heat transport via the Norwegian Atlantic Current (NwAC) between 3.320 and 3.137 Ma. The record is compared with vegetation and SST reconstructions from Site 642 and SSTs from ODP Site 907, Iceland Sea, to identify links between SSTs, ocean currents and vegetation changes. The dinocyst record shows strong Atlantic water influence via the NwAC corresponds to higher-than-present SSTs and cool temperate vegetation during Marine Isotope Stage (MIS) transition M2–M1 and KM5. Reduced Atlantic water inflow relative to the warm stages coincides with near-modern SSTs and boreal vegetation during MIS M2, KM6 and KM4–KM2. During most of the studied interval, a strong SST gradient between sites 642 and 907 indicates the presence of a proto-Arctic Front (AF). An absent gradient during the first half of MIS KM6, due to reduced Atlantic water influence at Site 642 and warm, presumably Atlantic water reaching Site 907, is indicative of a weakened NwAC and EGC. We conclude that repeated changes in Atlantic water influence directly affect terrestrial climate and that an active NwAC is needed for an AF to develop. Obliquity forcing may have played a role, but the correlation is not consistent.

Icebergs in the Nordic Seas throughout the Late Pliocene

Abstract The Arctic cryosphere is changing and making a significant contribution to sea level rise. The Late Pliocene had similar CO2 levels to the present and a warming comparable to model predictions for the end of this century. However, the state of the Arctic cryosphere during the Pliocene remains poorly constrained. For the first time we combine outputs from a climate model with a thermodynamic iceberg model to simulate likely source regions for ice‐rafted debris (IRD) found in the Nordic Seas from Marine Isotope Stage M2 to the mid‐Piacenzian Warm Period and what this implies about the nature of the Arctic cryosphere at this time. We compare the fraction of melt given by the model scenarios with IRD data from four Ocean Drilling Program sites in the Nordic Seas. Sites 911A, 909C, and 907A show a persistent occurrence of IRD that model results suggest is consistent with permanent ice on Svalbard. Our results indicate that icebergs sourced from the east coast of Greenland do not reach the Nordic Seas sites during the warm Late Pliocene but instead travel south into the North Atlantic. In conclusion, we suggest a continuous occurrence of marine‐terminating glaciers on Svalbard and on East Greenland (due to the elevation of the East Greenland Mountains during the Late Pliocene). The study has highlighted the usefulness of coupled climate model‐iceberg trajectory modeling for understanding ice sheet behavior when proximal geological records for Pliocene ice presence or absence are absent or are inconclusive.