Boreas最新文献

This study uses a multiproxy approach including the first use of ¹⁸⁷Os/¹⁸⁸Os, %C_37:4 biomarkers, carbonate content, sedimentological grain size, geochemical X-ray fluorescence and microfossil benthic foraminifera species combined with radiocarbon dating, measured on six cores from across the Skagerrak, in order to study the Lateglacial to Middle Holocene history of the area. A new chronostratigraphic framework is developed based on the appearance of specific benthic foraminifera species along with changes in carbonate/X-ray fluorescence and grain size data. This allowed the correlation of cores based on a series of radiocarbon dated tie points. Analysing the cores together reveals several events recorded in the Skagerrak including: (i) an increased freshwater input (bracketed between 13.3 and 11.3 cal. ka BP) signified by radiogenic ¹⁸⁷Os/¹⁸⁸Os values, high %C_37:4 values and an increase in sand content; (ii) the Glomma drainage event, signified by a sudden appearance of Valvulineria as well as higher %C_37:4; and (iii) the opening of the Danish Straits and English Channel leading to the development of modern-day conditions and circulation patterns in the Skagerrak, signified by the appearance of Hyalinea balthica and a fall in %C_37:4.

Investigating past interglacial climatic and environmental changes can enhance our understanding of the natural rates and ranges of climate variability under interglacial boundary conditions. However, comparing past interglacial palaeoclimate records from different regions and archives is often complicated by differing and uncertain chronologies. For instance, the duration of the Last Interglacial in Europe is still controversial as southern European palaeoclimate records suggest a duration of ~16 500–18 000 years, while a length of only ~11 000 years in northern-central Europe was previously inferred from the analysis of partly annually laminated (varved) palaeolake sediments recovered at Bispingen, northern Germany. To resolve this discrepancy, we here present sediment microfacies, geochemistry and pollen data from a new sediment core from the Bispingen palaeolake sediment succession, covering the entire Last Interglacial (Eemian) and the earliest part of the Last Glacial (Weichselian). In particular, we provide evidence that the duration of the Last Interglacial at Bispingen must have been hitherto underestimated due to the investigation of an incomplete sediment core. Using microscopic varve counting and sedimentation rate estimates for non-varved sections on the new sediment core, we show that the Eemian in northern-central Europe probably lasted at least ~15 000 years, about 4000 years longer than previously thought. This new duration estimate is in much better agreement with results from southern European palaeoclimate records, clarifying the enigma of a steep trans-European vegetation gradient for several millennia at the end of the Last Interglacial.

Northern fens, that host unique biota and form a remarkable carbon stock, are sensitive to changes in the moisture balance and, therefore, may be strongly affected by climatic fluctuations. However, long-term monitoring and palaeoecological studies of fens are relatively rare and, as a result, their responses to past and current climatic fluctuations are poorly known. In this study, we examined the recent vegetation change as well as changes in testate amoeba communities in the mire margin of a subarctic fen in Finnish Lapland with four peat profiles. Testate amoebae were used as indicators of past fluctuations in water table depth. The vegetation showed a drastic shift from sedge-dominated fen to Sphagnum-dominated communities during the late 20th and the early 21st centuries. This shift was accompanied by a turnover in the testate amoeba community. Testate amoeba-based water table reconstructions indicated recent drying. This may be due to the lowering of the water table either from accelerated Sphagnum increment or enhanced evaporation. The observed hummock establishment concurs with the documented hemisphere-wide expansion trend of hummock communities in fens. This change may strengthen the carbon sink and storage capacity of these peatlands, which could be viewed as a welcome negative feedback process to the ongoing climate warming. However, the change also poses a threat to biodiversity since fens are not only species-rich habitats but are also endangered ecosystems.

The dating of moraine deposits can present challenges to standard geochronological methods; terrestrial cosmogenic nuclide (TCN), sediment luminescence and radiocarbon dating may suffer from problems of incomplete resetting (by inheritance, intermittent cover/exposure, transport under unfavourable conditions) and/or a lack of suitable (organic) material. Rock surface luminescence burial dating (RSLBD) offers an alternative approach with considerable potential in dating moraines. In RSLBD, large cobbles/boulders are targeted, rather than smaller grains usually used in luminescence dating. The age limit of RSLBD is much higher than that of radiocarbon dating, and rocks are much more readily available than organic material. In contrast to TCN dating, the effect of exposure prior to deposition can be measured. In this study, we sampled a broad selection of primarily granitic boulders of various sizes and shapes (e.g. different degrees of roundness and sphericity) from the Vimmerby Moraine, a prominent and accessible feature in southern Sweden dated using TCN to 14.4±0.9 ka. Our study was designed to test whether morphological characteristics can be used to discriminate in favour of the most light-exposed boulders and minimize measurements of non-exposed boulders. As expected, not all RSLBD ages can be attributed to the same depositional event, but the majority of the resulting ages provide a mean age of 13.0±1.5 ka, consistent with the cosmogenic nuclide dating of the Vimmerby Moraine. Despite the apparently successful TCN study, the luminescence–depth profiles measured in the buried surfaces of the sampled clasts indicate that >50% of these moraine boulders were exposed to light (and cosmic radiation) before final deposition, implying some (presumably small) TCN inheritance; seven of the 16 boulders identified as light exposed were sufficiently bleached to be useful for RSLBD. These results and their implications in regard to transport and deposition of the sampled cobbles are critically discussed and evaluated.

The regional loess stratigraphy in the Vojvodina region, in the southeastern Carpathian Basin, has often been successfully correlated to the global palaeoclimate. This is a quasi-continuous sedimentary record that provides detailed environmental reconstruction during the last four glacial/interglacial cycles. In this study, we present a standardized loess stratigraphy and illustrate how it correlates with the marine oxygen isotope and Chinese loess stratigraphical records. We argue that the loess stratigraphy in Vojvodina region is an important link in the integration of European terrestrial stratigraphical schemes and the deep-sea stratigraphical model. We highlight how the loess record can better illustrate terrestrial environmental change through multiple glacial cycles than other records, such as glacial records. The investigated loess record enables direct links to be made between the loess sediments and their glacial sources. This reveals evidence of glaciations during every glacial cycle of the Saalian Stage complex, equivalent to Marine Isotope Stage (MIS) 10, 8 and 6. Therefore, Serbian loess has the potential to provide a direct link between terrestrial glaciations and wider records of global climate change, which is an enigma for many other continental records. These loess records display a strong relationship with the intensity of European glaciations during different glacial cycles. Loess sedimentation rates are highest in the most intensive European glaciation of the Saalian complex (MIS 6) and much lower during the weaker ‘missing’ glaciations equivalent to MIS 8 and 10. A key observation from the Vojvodina loess is the gradual increase in interglacial aridity through the late Middle Pleistocene. The explanation for the progressively increasing aridity in the investigated region at this time is still unclear. However, this trend is consistent with the idea of the Saalian complex as representing a 400 ka mega glacial cycle modulated by shorter classic 100 ka glacial cycles.

Subsurface deposits in recently glaciated marine areas are highly heterogeneous, representing a range of glacial and postglacial environments and processes including glacitectonism, overconsolidation, fluvial and lacustrine deposition, and transgression. These heterogeneities are often linked to variations in engineering properties, with important implications for the design and installation of offshore wind infrastructure. In this study we present an integrated geological characterization of the Sørlige Nordsjø II offshore wind site, located in waters of 50–70 m depth along the southern border of the Norwegian North Sea, focusing on the evolution of the area's depositional setting during the late Quaternary period and its implications for offshore wind development. We integrate interpretations from a marine geological data set acquired in 2022 with legacy 3D seismic data and a review of the current understanding of the southern North Sea's complex glacial history. A preliminary ground model and accompanying risk map for the site is presented with five main geological units: (i) homogeneous and layered marine sands covering most of the site, with patchy distribution and coarser-grained deposits in the east; (ii) buried, layered channel deposits containing organic material and possible associated shallow gas; (iii) buried, stiff glacilacustrine clay deposits; (iv) a buried, layered, glacitectonized unit incised by tunnel valleys, with a sandy marine infill; and (v) mounded tills and glacitectonized deposits containing boulders, exposed to shallowly buried in the east. Salt diapirism and gas migration were also found to be important potential geohazards at the site. Three-dimensional seismic attribute maps were found to be a powerful aid to understanding the distribution and genesis of seismic facies identified on 2D high-resolution sub-bottom profiles. This type of data integration is an under-utilized methodology for generating detailed preliminary ground models, which can inform more cost-effective site survey and early foundation concept planning at geologically complex offshore wind sites.

A marked sedimentological change in subsurface sediments from the entire Baltic Proper, the Baltic Sea, has been previously noted. Our detailed work on a variety of multi-cores from basin-wide transects indicates that this sedimentological change was caused by a large shift in environmental conditions during the 1950s. Until the 1950s, the water column was rather weakly stratified and winter-time convection – although weakened during the post Little Ice Age warming – was still able to ventilate the bottom waters of the Baltic Proper. Therefore, complete sediment sequences only accumulated in calm waters deeper than 150–160 m. High-resolution benthic foraminiferal records of subsurface sediments obtained along the saline water inflow pathway in combination with historical data indicate that the depositional environment changed drastically owing to the giant saline water inflow in AD 1951. The accompanied sharpening of the halo(pycno)cline triggered a collapse in the ventilation of the basin, resulting in oxygen-deficient bottom waters. This deficiency, in turn, caused the onset of phosphate release from the sediments, which accelerated primary production. The ventilation collapse also enabled the onset of deposition of organic carbon-rich sediments also in shallower water areas as calm conditions prevailed up to the modern winter mixing depth (60–70 m). A slight return to Little Ice Age-type conditions was observed during the late 1980s when temperatures decreased and stratification weakened. These conditions gave rise to a reduction in hypoxic areas and to a bottom-water ventilation, most pronounced in the north of the so-called Baltic Sea Klint, a hydrographic and topographic barrier. However, the general environmental conditions essentially have not changed since the 1950s. Remarkably, external (temperature and stratification) in combination with internal factors (e.g. ventilation collapse and phosphate release) were able to change the redox conditions of the Baltic Proper from oxic to hypoxic within less than 10 years.

The principal aim of this paper is to present a critical overview of the stratigraphical subdivision and correlation of the Late Mid-Pleistocene (LMP) sediments in Lithuania. In recent decades, a number of studies of LMP sediments have been carried out, allowing clarification of the stratigraphical position of individual sedimentary sections. Particular efforts have been made to determine the age of sediments using a variety of absolute age determination methods. However, there are still several outstanding questions. It is still unclear how many warm periods occurred during the Saalian in Lithuania and of what magnitude. The least problematic issue is identification of the Butėnai (Holsteinian) Interglacial as it correlates well biostratigraphically with the contemporaneous sections of adjacent areas. The most controversial unit is the Snaigupėlė Interglacial, the age of which continues to be a subject of debate. Recently, more evidence has emerged that the sediments considered to be of the Snaigupėlė Interglacial are contemporaneous with the Merkinė (Eemian) Interglacial. The chronological placement of these sediments closely hinges on the unresolved stratigraphical classification of the Medininkai deposits (MIS 6 or MIS 8?) and the Žemaitija tills (MIS 8 or MIS 10?). The debate is centred on whether these two tills were formed during two entirely different glaciations or represent different phases of a single major glaciation event. Traditionally, these two tills were attributed to distinct glaciation events separated by the Snaigupėlė Interglacial. However, no sedimentary sections have been discovered that contain both these tills and the interglacial sediments that separate them. Given the similar lithological, geochemical and petrographic composition of the Medininkai and Žemaitija tills some research has supported their possible stadial rank. The clarification of these stratigraphical issues in the future may be facilitated by the application of new proxies and re-examination of the existing LMP key sections.

Surface exposure–age dating was applied to rock surfaces associated with ice-marginal moraines at elevations of ~1520–1780 m a.s.l. on the slopes of Galdhøpiggen and Glittertinden, the two highest mountains in Scandinavia located in the Jotunheimen mountains of central southern Norway. This is important for understanding the pattern and timing of wastage of the Scandinavian Ice Sheet at the Younger Dryas–Holocene transition. Cosmogenic exposure dating (here ¹⁰Be dating) of boulders from the moraine ridges yielded overall mean ages (corrected for glacio-isostatic uplift, surface erosion and snow shielding) of ~11.6 ka from Galdhøpiggen and ~11.2 ka from Glittertinden. Similar ¹⁰Be ages were also obtained from additionally collected proximal and distal erratic boulders and bedrock samples. These enabled age calibration of Schmidt-hammer R-values and independent Schmidt-hammer exposure-age dating (SHD) of the moraine ridges, which yielded comparable mean SHD ages of ~10.8 and ~10.6 ka from the Galdhøpiggen and Glittertinden sites, respectively. Taking account of the age resolution and other limitations of both dating techniques, the results suggest that the two sets of moraines have approximately the same age but that neither technique can distinguish unambiguously between moraine formation in the late Younger Dryas or Early Holocene. Together with features of moraine-ridge morphology and estimates of equilibrium-line altitude depression of ~360–575 m (corrected for land uplift), the results imply moraine formation during short-lived re-advances of active glaciers, at least the lower reaches of which were warm-based. It is concluded that the local glaciers remained active and advanced during deglaciation either very late in the Younger Dryas or very early in the Holocene, possibly in response to the Preboreal Oscillation at ~11.4 ka. The study supports the concept of a thin Younger Dryas ice sheet and places time constraints on the timing of final deglaciation in southern Norway.

Denmark has been subject to complex interactions of isostatic uplift and eustatic sea level changes since the last deglaciation. Prominent coastal beach ridges as well as lagoonal and lake deposits from this period have been investigated at a number of sites in the region to constrain the relative sea level (RSL) changes. However, despite the common occurrence of former coastal lagoons and lakes in proximity to raised beach ridges, they have rarely been studied in combination. In this study, we use a multiproxy approach including geospatial data, lake sediment coring, ground penetrating radar and optically stimulated luminescence dating to investigate the Holocene coastal evolution and RSL history at Rugård in Mols Bjerge National Park, on the east coast of the Jutland Peninsula. Our results show that the coastal area at Rugård was transgressed between c. 7.6 and 7.0 cal. ka BP and that RSL was ~4.5 m higher than present between c. 6.6 and 5.9 ka ago, when the highest section of the beach ridge plain was deposited. The elevation and timing of this relative highstand are in good agreement with previous estimates of the Littorina transgression and contribute to our combined knowledge about RSL history and coastal evolution in the southern Kattegat. Subsequently, isostatic adjustment has caused uplift and erosion of the beach ridge plain, but renewed progradation and deposition of a lower beach plain have taken place since c. 1740 CE. Our results demonstrate the value of using a multiproxy approach to study RSL changes and coastal evolution.