According to the paper:
4.4 Reconstruction of sea-surface conditions
Summer SST [sea surface temperature] reconstructions (Fig. 6c) are characterised by a decreasing trend between ∼ AD 1855–1960 and reconstructed SST between ∼ AD 1885–1935 are warmer by up to 3◦C with respect to the average modern temperature at the coring site which is ∼ 4.1◦C. During ∼ AD 1935–1975 reconstructed SSTs are ∼ 1◦C below the modern value. Within the next 10 yr, the temperature increases up to 5.4◦C (∼ 1987) and gradually decreases towards the modern value of 4.1◦C.
The reconstructed SIC [sea ice coverage] trend mirrors that of reconstructed SSTs (Fig. 6b). The root mean squared error (RMSE) calculated on SIC values, which is the difference between reconstructed and observed values, is 1.43 months yr−1, and reﬂects the accuracy of the approach. For the period ∼ AD 1887–1945, reconstructed SIC values are an average 8.3 months yr−1 which is 1.1 months yr−1 lower than the modern values. In contrast, the period AD 1945–1975 is marked by reconstructed SIC values closer to the modern conditions, with an average value of 8.8 months yr−1
A decrease in SIC characterises the period AD 1975–1995, with an average value of 7.6 months yr−1, which is 1.8 months yr−1 below the modern value. Sea ice cover duration then gradually increases toward the modern value. All above reconstructed values are within or very close to the conﬁdence limits of the method.
|Sea ice coverage in months per year is shown in the 3rd graph from the left. Modern average sea ice coverage is shown at the top and is amongst the highest values of the entire 150 year record.|
Quantitative reconstruction of sea-surface conditions over the last 150 yr in the Beaufort Sea based on dinoflagellate cyst assemblages: the role of large-scale atmospheric circulation patterns
1Institut des sciences de la mer (ISMER), Université du Québec à Rimouski, 310 allée des Ursulines, Rimouski, QC, Canada
2LOCEAN Institut Pierre-Simon Laplace, UMR7159, 4 Place Jussieu, BP100 75252 Paris Cedex, France
3Université de Bordeaux, EPOC, UMR5805, 33400 Talence, France
4Takuvik Joint Laboratory, Université Laval, Pavillon Alexandre-Vachon, 1045, avenue de la Médecine, local 2064, Quebec city, Canada
Abstract. Dinoflagellate cyst (dinocyst) assemblages have been widely used over the Arctic Ocean to reconstruct sea-surface parameters on a quantitative basis. Such reconstructions provide insights into the role of anthropogenic vs natural forcings in the actual climatic trend. Here, we present the palynological analysis of a dated 36 cm-long core collected from the Mackenzie Trough in the Canadian Beaufort Sea. Dinocyst assemblages were used to quantitatively reconstruct the evolution of sea-surface conditions (temperature, salinity, sea ice) and freshwater palynomorphs fluxes were used as local paleo-river discharge indicators over the last ~ 150 yr. Dinocyst assemblages are dominated by autotrophic taxa (68 to 96%). Cyst of Pentapharsodinium dalei is the dominant species throughout most of the core, except at the top where the assemblages are dominated by Operculodinium centrocarpum. Quantitative reconstructions of sea-surface parameters display a series of relatively warm, lower sea ice and saline episodes in surface waters, alternately with relatively cool and low salinity episodes. Variations of dinocyst fluxes and reconstructed sea-surface conditions may be closely linked to large scale atmospheric circulation patterns such as the Pacific Decadal Oscillation (PDO) and to a lesser degree, the Arctic Oscillation (AO). Positive phases of the PDO correspond to increases of dinocyst fluxes, warmer and saltier surface waters, which we associate with upwelling events of warm and relatively saline water from Pacific origin. Freshwater palynomorph fluxes increased in three phases from AD 1857 until reaching maximum values in AD 1991, suggesting that the Mackenzie River discharge followed the same trend when its discharge peaked between AD 1989 and AD 1992. The PDO mode seems to dominate the climatic variations at multi-annual to decadal timescales in the western Canadian Arctic and Beaufort Sea areas.