Camilla S. Andresen

High-resolution analyses of an early Holocene climate event may imply decreased solar forcing as an important climate trigger

Early Holocene lacustrine, tree-ring, ice-core, and marine records reveal that the Northern Hemisphere underwent a short cooling event at 10 300 calendar yr B.P. (9100 14 C yr B.P.). The records were compared on a common high-resolution time scale and show that the event lasted less than 200 yr, with a cooling peak of 50 yr, and the event coincides with a distinct Holocene thermohaline disturbance recognized in the North Atlantic Ocean. In spite of wellknown freshwater forcings at the time of the event, the negligible difference between the modeled D 14 C record, based on the GISP2 (Greenland Ice Sheet Project 2) 10 Be data, and the measured values, does not allow for detectable D 14 C changes related to global ocean ventilation. We can, however, show that the onset of the cooling coincides with the onset of one of the largest Holocene 10 Be flux peaks. This finding may imply that the climate system is more

Spatial and temporal distribution of mass loss from the Greenland Ice Sheet since AD 1900

The response of the Greenland Ice Sheet (GIS) to changes in temperature during the twentieth century remains contentious, largely owing to difficulties in estimating the spatial and temporal distribution of ice mass changes before 1992, when Greenland-wide observations first became available. The only previous estimates of change during the twentieth century are based on empirical modelling and energy balance modelling. Consequently, no observation-based estimates of the contribution from the GIS to the global-mean sea level budget before 1990 are included in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Here we calculate spatial ice mass loss around the entire GIS from 1900 to the present using aerial imagery from the 1980s. This allows accurate high-resolution mapping of geomorphic features related to the maximum extent of the GIS during the Little Ice Age at the end of the nineteenth century. We estimate the total ice mass loss and its spatial distribution for three periods: 1900–1983 (75.1 ± 29.4 gigatonnes per year), 1983–2003 (73.8 ± 40.5 gigatonnes per year), and 2003–2010 (186.4 ± 18.9 gigatonnes per year). Furthermore, using two surface mass balance models we partition the mass balance into a term for surface mass balance (that is, total precipitation minus total sublimation minus runoff) and a dynamic term. We find that many areas currently undergoing change are identical to those that experienced considerable thinning throughout the twentieth century. We also reveal that the surface mass balance term shows a considerable decrease since 2003, whereas the dynamic term is constant over the past 110 years. Overall, our observation-based findings show that during the twentieth century the GIS contributed at least 25.0 ± 9.4 millimetres of global-mean sea level rise. Our result will help to close the twentieth-century sea level budget, which remains crucial for evaluating the reliability of models used to predict global sea level rise.

Anomalously mild Younger Dryas summer conditions in southern Greenland

The first late-glacial lake sediments found in Greenland were analyzed with respect to a variety of environmental variables. The analyzed sequence covers the time span between 14400 and 10500 calendar yr B.P., and the data imply that the conditions in southernmost Greenland during the Younger Dryas stadial, 12800-11550 calendar yr B.P., were characterized by an arid climate with cold winters and mild summers, preceded by humid conditions with cooler summers. Climate models imply that such an anomaly may be explained by local climatic phenomenon caused by high insolation and Fohn effects. It shows that regional and local variations of Younger Dryas summer conditions in the North Atlantic region may have been larger than previously found from proxy data and modeling experiments.

Interaction between subsurface ocean waters and calving of the Jakobshavn Isbræ during the late Holocene

A marine sediment core from Vaigat in Disko Bugt, West Greenland, has been analysed in terms of lithology, dinoflagellate cysts and foraminifera in order to evaluate the influence of oceanographic variability on West Greenland glacier stability. The data show that during the past 5200 years the Atlantic foraminiferal abundance in the subsurface waters of the West Greenland Current (WGC) episodically increased, indicating periods of increases in the inflow of subsurface warm Atlantic water at 2000—1500 cal. yr BP and 1300 cal. yr BP as well as periods of less pronounced increased bottom-water temperatures around 4700—4000 cal. yr BP, 3100—2800, 2600, 1000—800, 500—400, and at 200 cal. yr. The sedimentological and dinoflagellate cyst data indicate that these episodes with enhanced advection of Irminger Sea-derived waters are accompanied by increased iceberg rafting, which we link to increased iceberg calving in relation to destabilization of the Jakobshavn Isbrae. The long-term trend in the data documents the end of a late-Holocene Thermal Maximum between 5200 and 4300 cal. yr BP and a final onset of the Neoglaciation at 3500 cal. yr BP. Increased responses of the iceberg rafting after 3500 cal. yr BP, reflects a westward/seaward advance of the glacier margin in relation to onset of Neoglaciation and a development of the glacier into a floating tongue after 2000 cal. yr BP. A comparison of our record with a record from the eastern North Atlantic indicates that a NAO-like anomaly pattern between subsurface waters in West Greenland and atmospheric temperature in the Eastern North Atlantic may have been operating during most of the late Holocene. However, during the past 1000 years the NAO signal may have weakened as some other mode of climate variability overprints the anti-phase climate signal in this region.

What do ?14C changes across the Gerzensee oscillation/GI-1b event imply for deglacial oscillations?

High-resolution studies were performed on late-glacial sediments from a small lake in western Denmark with respect to lithology, geochemistry, stable isotopes, pollen stratigraphy and radiocarbon dating on terrestrial macrofossils. One purpose was to detect the so-called Gerzensee oscillation, or the GI-1b event, in the later part of the Allerod warm period, and to describe the environmental impact of this short cooling. The other aim was to test the hypothesis that considerable Δ14C changes occur over this time, which can be related to ocean ventilation/thermohaline circulation changes. We find that the GI-1b event had a major impact on both terrestrial and limnic ecosystems: large vegetation changes, increased soil erosion and lowered aquatic production. By correlations to events in the GRIP ice-core and 14C patterns in the Cariaco basin we also transformed our 14C dated record into calendar years to calculate Δ14C values. The 14C dates show that the GI-1b event both preceded, and was part of, the 11 400–11 300 14C yr BP radiocarbon plateau, and was followed by the 11 000–10 900 14C yr BP plateau; thus the later part of the event coincides with a distinct age decline. This delayed age drop (Δ14C rise) in relation to the hypothetical triggering mechanism behind the event, decreased ocean ventilation, could be explained by redeposited macrofossils at the onset of GI-1b. This phenomenon, also seen at the onset of Younger Dryas, may also reflect increased soil erosion and redeposition at the start of cold periods. The independent Cariaco Basin record, however, implies that the very end of the cool GI-1b event is related to a distinct rise in Δ14C. Likewise, the 10Be record from GISP2 shows a distinct rise in the middle of the event, precluding decreased solar forcing as the trigger of the climate event, but making it likely that high cosmic ray flux (low solar activity) may be the cause of the rising atmospheric 14C content. We thus conclude that the Δ14C changes over the Gerzensee oscillation (GI-1b), being one of several coolings during the Last Termination, does not seem to be related to ocean ventilation changes. The reason behind this lack of coincidence between rising Δ14C and a fairly distinct Northern Hemisphere cooling may be due to the fact that the oceanic changes during some of these coolings are too subtle to give an atmospheric 14C imprint, or that an anti-phase relationship between the two hemispheres blurs the Δ14C signal, or, finally, that a partly unknown mechanism may lie behind such coolings. Copyright

Sholocene climate variability in the Denmark Strait region - A land-sea correlation of new and existing climate proxy records

ABSTRACT. Two well dated Holocene sediment records bordering the Denmark Strait region have been used to reconstruct past climate variability. The content of biogenic silica, classic and organic material and moss in a lacustrine record from Lake N14 has been used to infer past variability in precipitation and temperature in southern Greenland. Sedimentologic and petrologic composition of sand in a shelf sediment record from the Djúpáll trough is used to infer past variability in the northwestern storm activity on northwestern Iceland, which probably also affected the inflow of polar waters from the East Greenland Current. Our evaluation of these records with a number of previous studies from the region documents Holocene climatic optimum conditions peaking between 8000 and 6500 calendar years before present (cal yr BP). Mid‐Holocene climate deterioration set in around 5000 cal yr BP followed by a further marked setback around 3500 cal yr BP. A stacking of climate variability on a centennial timescale from previous studies in the area shows a fairly good correspondence to the timing of marked cold and warm events as evidenced from the Lake N14 and the Djúpáll trough records. Cooler periods are explained as the response to marked incursions of ice‐laden polar water from the Arctic Ocean to the Denmark Strait region. Cool northerly and northwesterly winds along the East Greenland coast in relation to frequent strong atmospheric low pressure in the Barents Sea, coupled with strong high pressure over Greenland, would have favoured southward export of polar waters. A comparison with the proxy records of nuclide production (14C and 10Be) suggests that solar activity may have had some influence on the atmospheric pressure distribution in the Denmark Strait region.

Rapid Holocene climate changes in the North Atlantic: evidence from lake sediments from the Faroe Islands

Holocene records from two lakes on the Faroe Islands were investigated to determine regional climatic variability: the fairly wind-exposed Lake Starvatn on Streymoy and the more sheltered Lake Lykkjuvotn on Sandoy. Sediment cores were analysed for content of biogenic silica, organic carbon and clastic material, and magnetic susceptibility. In addition, a new qualitative proxy for past lake ice cover and wind activity was developed using the flux of clastic grains that are larger than 255 mu m. Both long-term and short-term climatic developments were similar between the two lakes, suggesting a response to a regional climate signal. The long-term climate development is characterized by early Holocene rapid warming followed by Holocene climatic optimum conditions ending around 8300 cal. yr BP. A more open landscape as evidenced from increased sand grain influx in the period 8300-7200 cal. yr BP could reflect the aftermath of the 8200 cal. yr BP event, although the event itself is not recognized in either of the two lake records. From around 7200 cal. yr BP the mid-Holocene climate deterioration is observed and from 4200 cal. yr BP the climate deteriorated further with increased amplitude of centennial cooling episodes. (Less)

Variability and seasonality of North Atlantic climate during the early Holocene: evidence from Faroe Island lake sediments

Based on their position in relation to major ocean currents, palaeoclimatic archives in the Faroe Islands are expected to be sensitive recorders of variations in North Atlantic circulation. The multiproxy analysis (magnetic susceptibility, total carbon, nitrogen and sulphur, biogenic silica, grains >255 µm) of a lacustrine record with both winter and summer climate indicators illustrates the variability of Faroese climate during the early Holocene (c. 11 300—10 240 cal. yr BP) and allows comparison with other records in the region. Our high-resolution record suggests a period of predominantly winter cooling and increased storminess centred at c. 11 150 cal. yr BP followed by a period of stability between c. 11 000 and 10 680 cal. yr BP, which correlates with a time when the North Atlantic was particularly free of meltwater incursions. After c. 10 680 cal. yr BP, both winter and summer indicators show an increase in variability broadly synchronous with increased variability at other North Atlantic sites. Within this time period, a predominantly winter cooling centred at c. 10 600 cal. yr BP, a winter/summer cooling centred at c. 10 450 cal. yr BP and a winter/summer cooling centred at c. 10 300 cal. yr BP are recorded. These distinctive coolings correspond to periods of ice core δ18O inferred lowered atmospheric temperatures over Greenland and the reoccurrence of meltwater outbursts at c. 10 600, 10 500 and 10 350 cal. yr BP.

Fluctuations of sediment accumulation rates in front of an Arctic delta in Greenland

An automated layer counting technique is developed to estimate the chronology of a marine sediment core and this technique is validated with Pb210 chronology. The marine sediment core was sampled in front of the delta of Mittivakkat Glacier meltwater river in the Sermilik Fjord, SE Greenland, and is a proxy of the sediment delivery from a glacial drainage basin to a fjord. The estimated time series was based on automatic lamination detection (varves) on a line scan of the core using gray scale intensities, and covered the last two centuries. The estimated time series of sediment accumulation rates was coupled to modelled runoff from the Mittivakkat Glacier and compared with local climatic parameters (air temperature and precipitation) and with the Atlantic Multidecadal Oscillation (AMO) index. Maxima in the sediment accumulation rate at the bottom of the side-fjord, about 1 km from the delta, mostly depended on glacier ablation and consequently on changes in river runoff, which were initiated by the air temperature. This was especially the case during transition from colder periods towards warmer, where short-lived maxima in sediment accumulation rates were followed by lower rates, even though the temperature remained high. This suggested a quite rapid glacial response to changes in climatic forcing, and/or a hysteresis effect, where sediment stored in the glacier/valley system was evacuated soon after a temperature dependent increase in discharge. The air temperature was in turn controlled by the AMO index.

Modern solar maximum forced late twentieth century Greenland cooling

The abrupt Northern Hemispheric warming at the end of the twentieth century has been attributed to an enhanced greenhouse effect. Yet Greenland and surrounding subpolar North Atlantic remained anomalously cold in 1970s to early 1990s. Here we reconstructed robust Greenland temperature records (North Greenland Ice Core Project and Greenland Ice Sheet Project 2) over the past 2100 years using argon and nitrogen isotopes in air trapped within ice cores and show that this cold anomaly was part of a recursive pattern of antiphase Greenland temperature responses to solar variability with a possible multidecadal lag. We hypothesize that high solar activity during the modern solar maximum (approximately 1950s–1980s) resulted in a cooling over Greenland and surrounding subpolar North Atlantic through the slowdown of Atlantic Meridional Overturning Circulation with atmospheric feedback processes.