Katrine Husum

Enhanced Modern Heat Transfer to the Arctic by Warm Atlantic Water

Water flow from the Atlantic Ocean into the Arctic through the Fram Strait is warmer than at any time in the past 2000 years. The Arctic is responding more rapidly to global warming than most other areas on our planet. Northward-flowing Atlantic Water is the major means of heat advection toward the Arctic and strongly affects the sea ice distribution. Records of its natural variability are critical for the understanding of feedback mechanisms and the future of the Arctic climate system, but continuous historical records reach back only ~150 years. Here, we present a multidecadal-scale record of ocean temperature variations during the past 2000 years, derived from marine sediments off Western Svalbard (79°N). We find that early–21st-century temperatures of Atlantic Water entering the Arctic Ocean are unprecedented over the past 2000 years and are presumably linked to the Arctic amplification of global warming.

A continuous marine record 8000-1600 cal. yr BP from the Malangenfjord, north Norway: foraminiferal and isotopic evidence

Holocene oceanography and climate were studied in the continuous marine sediment sequence in a piston core from the high-latitude Malangenfjord, north Norway, using benthic foraminifera and stable isotopes. Radiocarbon dates indicate that the investigated sequence covers the period from c. 8000 to 1600 cal. BP and has a sedimentation rate of 0.3-12.7 m/1000 years. Bottom-water temperatures reconstructed from benthic foraminiferal stable isotopes in the Malangenfjord show a cooling of c. 3°C ranging from 10°C and reaching c. 7°C at 1600 cal. yr BP which is similar to present bottom-water temperature at the core location. A planktic foraminiferal °18O record was established from c. 6400 to 1600 cal. yr BP generally following the same pattem as the benthic record. This long-term cooling correlates to the decreasing insolation at 70°N following the orbital forcing. The long-term cooling also corresponds well with atmospheric proxy records implying a close ocean-atmosphere coupling. It is also indicated both by marine and atmospheric temperature and precipitation proxy records that the climate was more oceanic during the early Holocene. Several cold events are interpreted as decreased influence of Atlantic Water in the fjord.

MODERN FORAMINIFERAL DISTRIBUTION IN THE SUBARCTIC MALANGEN FJORD AND ADJOINING SHELF, NORTHERN NORWAY

Living (stained) and dead (unstained) foraminifera have been investigated along a NW–SE transect from the inner fjord head to the adjoining shelf of the subarctic Malangen fjord, northern Norway. The outer sill of the fjord is deep and allows good exchange with water masses from the Norwegian Coastal and Norwegian Atlantic currents. The aim of this study is to elucidate the recent foraminiferal fauna and its relation to sediment properties and water mass characteristics. Three benthic foraminiferal assemblages have been distinguished on the basis of the dominating living and dead benthic species. The Bulimina marginata–Cassidulina reniforme assemblage is found in the inner fjord basin, where it is correlated with relatively cold (6–6.9°C) and slightly less saline water masses (34–34.75‰) compared to the Norwegian Atlantic Water masses. The Brizalina skagerrakensis assemblage is found in the outer fjord basin, an area influenced by relatively stable temperate (6–7.9°C) and saline (34–35.1‰) basin water caused by inflows of Norwegian Atlantic Water. The Cassidulina obtusa assemblage is found outside the outer sill on the shelf and reflects a higher temperature (5.7–9.8°C) on the shelf compared to the outer fjord. Two planktic foraminiferal assemblages have also been distinguished. The Globigerinata uvula assemblage is found in the outer fjord basin and on the inner shelf. The Neogloboquadrina pachyderma (dextral) assemblage is found on the outer shelf, where the influence of Norwegian Atlantic Current is the largest.

Reconstructions of surface ocean conditions from the northeast Atlantic and Nordic seas during the last millennium

We undertake the first comprehensive effort to integrate North Atlantic marine climate records for the last millennium, highlighting some key components common within this system at a range of temporal and spatial scales. In such an approach, careful consideration needs to be given to the complexities inherent to the marine system. Composites therefore need to be hydrographically constrained and sensitive to both surface water mass variability and three-dimensional ocean dynamics. This study focuses on the northeast (NE) North Atlantic Ocean, particularly sites influenced by the North Atlantic Current. A composite plus regression approach is used to create an inter-regional NE North Atlantic reconstruction of sea surface temperature (SST) for the last 1000 years. We highlight the loss of spatial information associated with large-scale composite reconstructions of the marine environment. Regional reconstructions of SSTs off the Norwegian and Icelandic margins are presented, along with a larger-scale reconstruction spanning the NE North Atlantic. The latter indicates that the ‘Medieval Climate Anomaly’ warming was most pronounced before ad 1200, with a long-term cooling trend apparent after ad 1250. This trend persisted until the early 20th century, while in recent decades temperatures have been similar to those inferred for the ‘Medieval Climate Anomaly’. The reconstructions are consistent with other independent records of sea-surface and surface air temperatures from the region, indicating that they are adequately capturing the climate dynamics of the last millennium. Consequently, this method could potentially be used to develop large-scale reconstructions of SSTs for other hydrographically constrained regions.

Early Holocene cooling events in Malangenfjord and the adjoining shelf, north‐east Norwegian Sea

A high resolution study of early Holocene climate and palaeoceanography has been performed on two combined sediment cores from Malangenfjord, northern Norway. The fjord provides a regional oceanographic climatic signal reflecting changes in the North Atlantic heat flux at this latitude because of its deep sill and the relatively narrow adjoining continental shelf. Fauna and stable oxygen and carbon isotopes indicate cool, meltwater-depleted water masses in the fjord from 12000 to 11400 cal. yr BP followed by a warming between 11400-10300 cal. yr BP. The climatic variability can be explained partly by freshwater forcing hampering the North Atlantic heat conveyor, and partly by changing solar irradiance. A major cooling event at 11500-11400 cal. yr BP, followed by a rapid warming, is correlated to the Preboreal Oscillation, a widespread signal in the North Atlantic region which is probably linked to the increased meltwater flux to the northern North Atlantic at this time. Brief and small-scale cooling events between 10 300 and 10100 cal. yr BP, correlated to the onset of increased 10Be flux in the Greenland ice cores, suggest a response to solar forcing.

A global multiproxy database for temperature reconstructions of the Common Era

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850–2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.

Asynchronous response of marine-terminating outlet glaciers during deglaciation of the Fennoscandian Ice Sheet

This is the accepted manuscript version. Published version is available at http://dx.doi.org/10.1130/G35299.1

Sub sea surface temperatures in the Polar North Atlantic during the Holocene: Planktic foraminiferal Mg/Ca temperature reconstructions

Holocene sea surface temperatures in the eastern Fram Strait are reconstructed based on Mg/Ca ratios measured on the planktic foraminifer Neogloboquadrina pachyderma (sin). The reconstructed sub sea surface temperatures (sSSTMg/Ca) fluctuate markedly during the earliest Holocene at ~11.7 and 10.5 kyr BP. This is probably in response to the varying presence of sea-ice and deglacial meltwater. Between ~10.5–7.9 kyr BP, the sSSTMg/Ca values are relatively high (~4°C) and more stable reflecting high insolation and intensified poleward advection of Atlantic Water. After 7.9 kyr BP, the sSSTMg/Ca values decline to an average of ~3°C throughout the mid-Holocene. These changes can be attributed to a combined effect of reduced poleward oceanic heat advection and a decline in insolation as well as a gradually increased influence of eastward migrating Arctic Water. The sSSTMg/Ca values increase and vary between 2.1°C and 5.8°C from ~2.7 kyr BP to the present. This warming is in contrast to declining late-Holocene insolation and may instead be explained by factors including increased advection of oceanic heat to the Arctic region possibly insulated beneath a widening freshwater layer in the northern North Atlantic in conjunction with a shift in calcification season and/or depth habitat of N. pachyderma (sin).

A 2000 year record of Atlantic Water temperature variability from the Malangen Fjord, northeastern North Atlantic

A high-resolution sedimentary record from the subarctic Malangen fjord in northern Norway, northeastern North Atlantic has been investigated in order to reconstruct variations in influx of Atlantic Water for the last 2000 years. The fjord provides a regional oceanographic climatic signal reflecting changes in the North Atlantic heat flux at this latitude because of its deep sill and the relatively narrow adjoining continental shelf. The reconstructions are based on oxygen and carbon isotopic studies of benthic foraminifera from a high accumulation basin in the Malangen fjord, providing subdecadal time resolution. A comparison between instrumental measurements of bottom water temperatures at the core location and the reconstructed temperatures from benthic foraminiferal δ18O for the same time period demonstrates that the stable isotope values reflect the bottom water temperatures very well. The reconstructed temperature record shows an overall decline in temperature of c. 1°C from c. 40 bc to ad 1350. This cooling trend is assumed to be driven by an orbital forced reduction in insolation. Superimposed on the general cooling trend are several periods of warmer or colder temperatures. The long-term fluctuations in the Malangen fjord are concurrent with fluctuations of Atlantic Water in the northern North Atlantic. Although they are not directly comparable, comparisons of atmospheric temperatures and marine records, indicate a close coupling between the climate systems. After ad 1800 the record shows an unprecedented warming within the last 2000 years.

Exceptional ocean surface conditions on the SE Greenland shelf during the Medieval Climate Anomaly

Diatom inferred 2900 year long records of August sea surface temperature (aSST) and April sea ice concentration (aSIC) are generated from a marine sediment core from the SE Greenland shelf with a special focus on the interval ca. 870–1910 Common Era (C.E.) reconstructed in subdecadal temporal resolution. The Medieval Climate Anomaly (MCA) between 1000 and 1200 C.E. represents the warmest ocean surface conditions of the SE Greenland shelf over the late Holocene (880 B.C.E.(before the Common Era) to 1910 C.E.). It was characterized by abrupt, decadal to multidecadal changes, such as an abrupt warming of ~2.4°C in 55 years around 1000 C.E. Temperature changes of these magnitudes are rare on the North Atlantic proxy data. Compared to regional air temperature reconstructions, our results indicate a lag of about 50 years in ocean surface warming either due to increased freshwater discharge from the Greenland ice sheet or intensified sea ice export from the Arctic as a response to atmospheric warming at the beginning of the MCA. A cool phase, from 1200–1890 C.E., associated with the Little Ice Age, ends with the rapid warming of aSST and diminished aSIC in the early twentieth century. The results show that the periods of warm aSST and aSIC minima are coupled with solar minima suggesting that solar forcing possibly amplified by atmospheric forcing have been behind the variability of surface conditions on the SE Greenland over the last millennium. The results indicate that the SE Greenland shelf is a climatologically sensitive area where extremely rapid changes are possible and highlights the importance of the area under the present warming conditions.