Arto Miettinen

Multicentennial Variability of the Sea Surface Temperature Gradient across the Subpolar North Atlantic over the Last 2.8 kyr

A 2800-year-long August Sea Surface Temperature (aSST) record based on fossil diatom assemblages is generated from a marine sediment core from the northern subpolar North Atlantic. The record is compared with the aSST record from the Norwegian Sea to explore the variability of the aSST gradient between these areas during the late Holocene. The aSST records demonstrate the opposite climate tendencies towards a persistent warming in the core site in the subpolar North Atlantic and cooling in the Norwegian Sea. At the multicentennial scale of aSST variability of 600–900 years, the records are nearly in anti-phase with warmer (colder) periods in the subpolar North Atlantic corresponding to the colder (warmer) periods in the Norwegian Sea. At the shorter time scale of 200–450 years, the records display a phase-locked behaviour with a tendency for the positive aSST anomalies in the Norwegian Sea to lead by ~30 years the negative aSST anomalies in the subpolar North Atlantic. This apparent aSST seesaw might have an effect on two major anomalies of the European climate of the past Millennium: Medieval Warm Period (MWP) and the Little Ice Age (LIA). During the MWP warming of the sea surface in the Norwegian Sea occurred in parallel with cooling in the northern subpolar North Atlantic whereas the opposite pattern emerged during the LIA. The results suggest that the observed aSST seesaw between the subpolar North Atlantic and the Norwegian Sea could be a surface expression of the variability of the eastern and western branches of the Atlantic Meridional Circulation (AMOC) with a possible amplification through atmospheric feedback.

Palaeoenvironment of the Karelian Isthmus, the easternmost part of the Gulf of Finland, during the Litorina Sea stage of the Baltic Sea history

The palaeoenvironment of the Karelian Isthmus area during the Litorina Sea stage of the Baltic Sea history, between 8.0 and 4.5 kyr BP (8.8-5.2 cal. kyr BP), was reconstructed by studying four sites located on the Karelian Isthmus in Russia. Methods included diatom and pollen analyses, sediment lithostratigraphical interpretation and 14C dating. The brackish-water (Litorina) transgression began c. 7.7 kyr BP (8.45 cal. kyr BP) in the Karelian Isthmus area. The transgression maximum occurred between 6.7 and 5.7 kyr BP (7.6-6.5 cal. kyr BP), depending on the glacio-isostatic land uplift rate. Regarding the vegetation, the maximum occurrence of temperate deciduous trees took place at the same time. The transgression was interrupted by a short-lived sea-level standstill during the middle phase of the main transgression, c. 6.3 kyr BP (7.2 cal. kyr BP), on the eastern part of the isthmus. The highest Litorina shoreline is located between 8 and 13 m above present sea-level and the amplitude of the Litorina transgression has varied between 5 and 7 m. The 8.2-kyr cold event is not evident, but the sea-level standstill around 6.3 kyr BP (7.2 cal. kyr BP) could reflect a cool episode at that time in the Karelian Isthmus area.

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.

The Origin and Evolution of Lake Vähä-Pitkusta, SW Finland – A Multi-Proxy Study of a Meromictic Lake

Lake Vähä-Pitkusta is a 35-m deep meromictic kettle-hole lake in SW Finland. The topmost 15cm of the sediment stratigraphy consists of black gyttja, representing the last ca. 600years. A sediment core from the deepest point was examined with multi-proxy methods to determine the origin and evolution of the basin, the onset of the meromictic conditions and the primary factors that initiated the meromictic processes. The results indicate that Lake Vähä-Pitkusta became isolated from the Baltic basin during the Yoldia Sea stage. After initial unstable sedimentary conditions the development continued steadily until ca. 2500–2300 cal.BP when there is indication of lake level rise and/or increased surface runoff, probably resulting from a cooler and moister climate. The diatom and cladoceran results indicate only a slight rise in the trophic state following the increased erosion. Pollen evidence suggests weak anthropogenic activity in the catchment during the last 1000 years, and this had only little effect on the trophic state, which remained rather low. Thus, it is most unlikely that the trophic state was the primary reason for the origin of meromixis, although it may have contributed to it. We tentatively suggest that the primary reason was the peculiar morphometric dimensions of Lake Vähä-Pitkusta (small, deep, sheltered from winds) which made it very sensitive and that even a very slight increase in soluble electrolytes may push it over the edge into meromixation. It is possible that this did not occur during the cold/moist period around ca. 2300 cal.BP but during the Medieval Climate Anomaly. Studies on sediment geochemistry are needed to verify this assumption.

Quantitative reconstruction of Holocene sea ice and sea surface temperature off West Greenland from the first regional diatom data set

Holocene oceanographic conditions in Disko Bay, West Greenland, were reconstructed from high-resolution diatom records derived from two marine sediment cores. A modern data set composed of 35 dated surface sediment samples collected along the West Greenland coast accompanied by remote sensing data was used to develop a diatom transfer function to reconstruct April sea ice concentration (SIC) supported by July sea surface temperature (SST) in the area. Our quantitative reconstruction shows that oceanographic changes recorded throughout the last ~11,000 years reflect seasonal interplay between spring (April SIC) and summer (July SST) conditions. Our records show clear correlation with climate patterns identified from ice core data from GISP2 and Agassiz-Renland for the early to middle Holocene. The early Holocene deglaciation of western Greenland Ice Sheet was characterized in Disko Bay by initial strong centennial-scale fluctuations in April SIC with amplitude of over 40%, followed by high April SIC and July SST. These conditions correspond to a general warming of the climate in the Northern Hemisphere. A decrease in April SIC and July SST was recorded during the Holocene Thermal Optimum reflecting more stable spring-summer conditions in Disko Bay. During the late Holocene, high April SIC characterized the Medieval Climate Anomaly, while high July SST prevailed during the Little Ice Age, supporting previously identified antiphase relationship between surface waters in West Greenland and climate in NW Europe. This antiphase pattern might reflect seasonal variations in regional oceanographic conditions and large-scale fluctuations within the North Atlantic Oscillation and Atlantic Meridional Overturning Circulation.

Younger Dryas ice margin retreat triggered by ocean surface warming in central-eastern Baffin Bay

The transition from the last ice age to the present-day interglacial was interrupted by the Younger Dryas (YD) cold period. While many studies exist on this climate event, only few include high-resolution marine records that span the YD. In order to better understand the interactions between ocean, atmosphere and ice sheet stability during the YD, more high-resolution proxy records from the Arctic, located proximal to ice sheet outlet glaciers, are required. Here we present the first diatom-based high-resolution quantitative reconstruction of sea surface conditions from central-eastern Baffin Bay, covering the period 14.0–10.2 kyr BP. Our record reveals warmer sea surface conditions and strong interactions between the ocean and the West Greenland ice margin during the YD. These warmer conditions were caused by increased Atlantic-sourced water inflow combined with amplified seasonality. Our results emphasize the importance of the ocean for ice sheet stability under the current changing climate.High-resolution proxy records are essential for understanding the interactions between ocean, ice sheet and atmosphere. Here the authors present a high-resolution record of Younger Dryas sea surface conditions in Baffin Bay and show that ocean surface warming triggered the retreat of Jakobshavn Isbræ ice stream.

The Holocene marine diatom flora of Eastern Newfoundland bays

In total, 39 diatom species belonging to 22 genera were identified and photographed from Holocene marine sediment cores of Eastern Newfoundland bays. The cores were retrieved from Placentia Bay in the southeast and Bonavista Bay in the northeast of Newfoundland. The study area lies at the meeting point of the cold Labrador Current and the warmer Gulf Stream and is sensitive to changes in ocean circulation. It is thus an ideal location for paleoceanographic reconstructions based on the analysis of (sub)fossil diatoms, for which a good taxonomic framework is essential. The studied sediment cores encompass different age ranges and together represent the entire Holocene epoch. The most abundant species belonged to the genera Thalassiosira, represented by 10 different species, Fragilariopsis (four species), Detonula, Thalassionema and Odontella (one species each). This study provides an annotated list of the most commonly identified diatom species and references to further literature. All taxa are documented by high quality photomicrographs.