Age determination of sediment cores from the Skagerrak

Paleoceanographic and paleoenvironmental interpretations based on foraminifera, sedimentary data, radiocarbon dates, and stable isotope measurements were derived from two sections in the Skagerrak: a 115-m-thick Holocene marine section drilled onshore at Skagen near the northernmost tip of Jutland, Denmark, and a 9-m piston core from the Skagerrak, north of Skagen. The foraminiferal data show that arctic-subarctic environments in the deep Skagerrak-Kattegat area were succeeded by boreal conditions at 9.6 ka. This was a result of northward migration of the Atlantic polar front and inflow of warm Atlantic water into the area through the Norwegian Channel. A gradual warming of the water masses after 9.6 ka is indicated by the data. Rare foraminifera and high sedimentation rates are found between approximately 8.6 ka and 7.6 ka at both core locations. The modern foraminiferal assemblages of the area were fully established at 7.6 ka indicating that the modern circulation pattern in the Skagerrak-Kattegat after the opening of the English Channel and the Danish Straits was not established before this date. At 5.5 ka a sudden change to coarser sediments (higher-energy environments) and the appearance of the foraminifer Eoeponidella laesoeensis is recorded in the Skagen core. This indicates a rapid change in the hydrography reflecting altered meteorological and hydrographic conditions in the Skagerrak-Kattegat, including a strengthening of the Jutland Current and increased inflow of North Sea water into the Kattegat. The event is interpreted as a response to cooling at the end of the Holocene climatic optimum in late Atlantic time and possibly reflects a rapid cooling event of North Atlantic surface water masses.

Stable carbon and oxygen isotope record od IODP Hole 306-U1313

The Mid-Pleistocene transition (MPT) was the time when quasi-periodic (? 100 kyr), high-amplitude glacial variability developed in the absence of any significant change in the character of orbital forcing, leading to the establishment of the characteristic pattern of late Pleistocene climate variability. It has long been known that the interval around 900 ka stands out as a critical point of the MPT, when major glaciations started occurring most notably in the northern hemisphere. Here we examine the record of climatic conditions during this significant interval, using high-resolution stable isotope records from benthic and planktonic foraminifera from a sediment core in the North Atlantic (Integrated Ocean Drilling Program Expedition 306, Site U1313). We have considered the time interval from late in Marine Isotope Stage (MIS) 23 to MIS 20 (910 to 790 ka). Our data indicate that interglacial MIS 21 was a climatically unstable period and was broken into four interstadial periods, which have been identified and correlated across the North Atlantic region. These extra peaks tend to contradict previous studies that interpreted the MIS 21 variability as consisting essentially of a linear response to cyclical changes in orbital parameters. Cooling events in the surface record during MIS 21 were associated with low benthic carbon isotope excursions, suggesting a coupling between surface temperature changes and the strength of the Atlantic meridional overturning circulation. Time series analysis performed on the whole interval indicates that benthic and planktonic oxygen isotopes have significant concentrations of spectral power centered on periods of 10.7 kyr and 6 kyr, which is in agreement with the second and forth harmonic of precession. The excellent correspondence between the foraminifera d18O records and insolation variations at the Equator in March and September suggests that a mechanism related to low-latitude precession variations, advected to the high latitudes by tropical convective processes, might have generated such a response. This scenario accounts for the presence of oscillations at frequencies equal to precession harmonics at Site U1313, as well as the occurrence of higher amplitude oscillations between the MIS22/21 transition and most of MIS 21, times of enhanced insolation variability.

Vegetation cover and radiocarbon dates of palsa and peat plateaus in the Hudson Bay Lowlands

The Holocene development of a treed palsa bog and a peat plateau bog, located near the railroad to Churchill in the Hudson Bay Lowlands of northeastern Manitoba, was traced using peat macrofossil and radiocarbon analyses. Both sites first developed as wet rich fens through paludification of forested uplands around 6800 cal. yr BP. Results show a 20th-century age for the palsa formation and repeated periods of permafrost aggradation and collapse at the peat plateau site during the late Holocene. This timing of permafrost dynamics corroborates well with that inferred from previous studies on other permafrost peatlands in the same region. The developmental history of the palsa and peat plateau bogs is similar to that of adjacent permafrost-free fens, except for the specific frost heave and collapse features associated with permafrost dynamics. Permafrost aggradation and degradation is ascribed to regional climatic, local autogenic and other factors. Particularly the very recent palsa development can be assessed in terms of climatic changes as inferred from meteorological data and surface hydrological changes related to construction of the railroad. The results indicate that cold years with limited snowfall as well as altered drainage patterns associated with infrastructure development may have contributed to the recent palsa formation.

Stable carbon and oxygen isotope record of IODP Site 306-U1313, MIS 23-19

Since the seminal work by Hays et al. (1976), a plethora of studies has demonstrated a correlation between orbital variations and climatic change. However, information on how changes in orbital boundary conditions affected the frequency and amplitude of millennial-scale climate variability is still fragmentary. The Marine Isotope Stage (MIS) 19, an interglacial centred at around 785 ka, provides an opportunity to pursue this question and test the hypothesis that the long-term processes set up the boundary conditions within which the short-term processes operate. Similarly to the current interglacial, MIS 19 is characterised by a minimum of the 400-kyr eccentricity cycle, subdued amplitude of precessional changes, and small amplitude variations in insolation. Here we examine the record of climatic conditions during MIS 19 using high-resolution stable isotope records from benthic and planktonic foraminifera from a sedimentary sequence in the North Atlantic (Integrated Ocean Drilling Program Expedition 306, Site U1313) in order to assess the stability and duration of this interglacial, and evaluate the climate system's response in the millennial band to known orbitally induced insolation changes. Benthic and planktonic foraminiferal d18O values indicate relatively stable conditions during the peak warmth of MIS 19, but sea-surface and deep-water reconstructions start diverging during the transition towards the glacial MIS 18, when large, cold excursions disrupt the surface waters whereas low amplitude millennial scale fluctuations persist in the deep waters as recorded by the oxygen isotope signal. The glacial inception occurred at ~779 ka, in agreement with an increased abundance of tetra-unsaturated alkenones, reflecting the influence of icebergs and associated meltwater pulses and high-latitude waters at the study site. After having combined the new results with previous data from the same site, and using a variety of time series analysis techniques, we evaluate the evolution of millennial climate variability in response to changing orbital boundary conditions during the Early-Middle Pleistocene. Suborbital variability in both surface- and deep-water records is mainly concentrated at a period of ~11 kyr and, additionally, at ~5.8 and ~3.9 kyr in the deep ocean; these periods are equal to harmonics of precession band oscillations. The fact that the response at the 11 kyr period increased over the same interval during which the amplitude of the response to the precessional cycle increased supports the notion that most of the variance in the 11 kyr band in the sedimentary record is nonlinearly transferred from precession band oscillations. Considering that these periodicities are important features in the equatorial and intertropical insolation, these observations are in line with the view that the low-latitude regions play an important role in the response of the climate system to the astronomical forcing. We conclude that the effect of the orbitally induced insolation is of fundamental importance in regulating the timing and amplitude of millennial scale climate variability.