Karl Ljung

A Holocene peat record in the central South Atlantic: an archive of precipitation changes

Peat deposits from the littoral part of the wetland 2nd Pond on Nightingale Island in the central South Atlantic have been analysed to investigate the Holocene climate development on the island and to test a hypothesis about regionally persistent humidity variations. A variety of proxies were analysed – total carbon and nitrogen, biogenic silica, diatoms, amount of organic matter, macrofossils and magnetic susceptibility – and together with the lithology they are interpreted as a record reflecting changes in humidity/precipitation. Early Holocene (10,000–8500 cal. BP) was possibly significantly drier than today, probably caused by a southerly displacement of the Southern Hemisphere Westerlies (SHW) during the Antarctic climate optimum. From 8500 cal. BP and onwards, the climate became generally more humid and surface run-off increased due to higher precipitation, possibly as an effect of increased influence from the SHW as it moved north. During this generally humid period, our data disclose a distinct pattern of recurrent centennial- to millennial-long events of increased precipitation and the results corroborate the only other study in the region with a similar humidity record. The events might represent large-scale climate oscillations in the Southern Hemisphere, such as latitudinal shifts of the SHW, but may also be related to changes in sea surface temperatures.

Possible Late Pleistocene volcanic activity on Nightingale Island, South Atlantic Ocean, based on geoelectrical resistivity measurements, sediment corings and (14)C dating

Tristan da Cunha is a volcanic island group situated in the central South Atlantic. The oldest of these islands, Nightingale Island, has an age of about 18 Ma. In the interior of the island, there are several wetlands situated in topographic depressions. The ages of these basins have been unknown, and their genesis has been debated. Aiming towards the reconstruction of the geomorphological history of these basins, we conducted geoelectrical resistivity measurements to map the subsurface topography, extracted peat and sediment cores and dated the onset of sedimentation applying the radiocarbon method. The irregular shapes of the basins and the lack of clear erosional features indicate that they are not eruption craters and were not formed by erosion. Instead, we regard them as morphological depressions formed between ridges of trachytic lava flows and domes at a late stage of the formation of the volcanic edifice. The onset of sedimentation within these basins appears to have occurred between 24 and 37 ka with the highest situated wetland yielding the highest ages. These ages are very young compared to the timing of the main phase of the formation of the island, implying volcanic activity on the island during the Late Pleistocene.

Decalcification and survival of benthic foraminifera under the combined impacts of varying pH and salinity

Coastal areas display natural large environmental variability such as frequent changes in salinity, pH, and carbonate chemistry. Anthropogenic impacts - especially ocean acidification - increase this variability, which may affect the living conditions of coastal species, particularly, calcifiers. We performed culture experiments on living benthic foraminifera to study the combined effects of lowered pH and salinity on the calcification abilities and survival of the coastal, calcitic species Ammonia sp. and Elphidium crispum. We found that in open ocean conditions (salinity ∼35) and lower pH than usual values for these species, the specimens displayed resistance to shell (test) dissolution for a longer time than in brackish conditions (salinity ∼5 to 20). However, the response was species specific as Ammonia sp. specimens survived longer than E. crispum specimens when placed in the same conditions of salinity and pH. Living, decalcified juveniles of Ammonia sp. were observed and we show that desalination is one cause for the decalcification. Finally, we highlight the ability of foraminifera to survive under Ωcalc < 1, and that high salinity and [Ca2+] as building blocks are crucial for the foraminiferal calcification process.

A lacustrine record of the Pleistocene-Holocene boundary in southernmost Greenland

Abstract A sediment core from a lake in the Kap Farvel area on South Greenland was analysed with respect to pollen and microfossil content, loss-on-ignition, magnetic susceptibility and radiocarbon age. The aim was to reconstruct the limnic and terrestrial changes taking place at the Pleistocene-Holocene boundary. Organic sedimentation started between 12 200 and 12800 cal yr B.P. Break-up of sea-ice around Kap Farvel, as indicated by the appearance of marine dinoflagellates (Hystrix), took place some hundred years after the transition into Preboreal. The pollen analysis indicates that the area was barren during the Younger Dryas and that the Holocene vegetation establishment was slightly delayed.