| 1. |
- Dalton, April S., et al.
(författare)
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The marine δ18O record overestimates continental ice volume during Marine Isotope Stage 3
- 2022
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Ingår i: Global and Planetary Change. - Amsterdam : Elsevier. - 0921-8181 .- 1872-6364. ; 212, s. 103814-103814
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Tidskriftsartikel (refereegranskat)abstract
- There is disagreement in the Quaternary research community in how much of the marine δ18O signal is driven by change in ice volume. Here, we examine this topic by bringing together empirical and modelling work for Marine Isotope Stage 3 (MIS 3; 57 ka to 29 ka), a time when the marine δ18O record indicates moderate continental glaciation and a global mean sea level between −60 m and −90 m. We compile and interpret geological data dating to MIS 3 to constrain the extent of major Northern Hemisphere ice sheets (Eurasian, Laurentide, Cordilleran). Many key data, especially published in the past ~15 years, argue for an ice-free core of the formerly glaciated regions that is inconsistent with inferences from the marine δ18O record. We compile results from prior studies of glacial isostatic adjustment to show the volume of ice inferred from the marine δ18O record is unable to fit within the plausible footprint of Northern Hemisphere ice sheets during MIS 3. Instead, a global mean sea level between −30 m and − 50 m is inferred from geological constraints and glacial isostatic modelling. Furthermore, limited North American ice volumes during MIS 3 are consistent with most sea-level bounds through that interval. We can find no concrete evidence of large-scale glaciation during MIS 3 that could account for the missing ~30 m of sea-level equivalent during that time, which suggests that changes in the marine δ18O record are driven by other variables, including water temperature. This work urges caution regarding the reliance of the marine δ18O record as a de facto indicator of continental ice when few geological constraints are available, which underpins many Quaternary studies.
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| 2. |
- Hall, Adrian M., 1955-, et al.
(författare)
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Late Cenozoic deep weathering patterns on the Fennoscandian shield in northern Finland : A window on ice sheet bed conditions at the onset of Northern Hemisphere glaciation
- 2015
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Ingår i: Geomorphology. - : Elsevier BV. - 0169-555X .- 1872-695X. ; 246, s. 472-488
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Tidskriftsartikel (refereegranskat)abstract
- The nature of the regolith that existed on the shields of the Northern Hemisphere at the onset of ice sheet glaciation is poorly constrained. In this paper, we provide the first detailed account of an exceptionally preserved, deeply weathered late Neogene landscape in the ice sheet divide zone in northern Finland. We mine data sets of drilling and pitting records gathered by the Geological Survey of Finland to reconstruct regional preglacial deep weathering patterns within a GIS framework. Using a large geochemical data set, we give standardised descriptions of saprolite geochemistry using a variant of the Weathering Index of Parker (WIP) as a proxy to assess the intensity of weathering. We also focus on mineral prospects and mines with dense pit and borehole data coverage in order to identify links between geology, topography, and weathering.Geology is closely linked to topography on the preglacial shield landscape of northern Finland and both factors influence weathering patterns. Upstanding, resistant granulite, granite, gabbro, metabasalt, and quartzite rocks were associated with fresh rock outcrops, including tors, or with thin (< 5 m) grusses. Plains developed across less resistant biotite gneisses, greenstones, and belts of alternating rock types were mainly weathered to thick (10–20 m) grusses with WIPfines values above 3000 and 4000. Beneath valley floors developed along mineralised shear and fracture zones, weathering penetrated locally to depths of > 50 m and included intensely weathered kaolinitic clays with WIPfines values below 1000.Late Neogene weathering profiles were varied in character. Tripartite clay–gruss–saprock profiles occur only in limited areas. Bipartite gruss–saprock profiles were widespread, with saprock thicknesses of > 10 m. Weathering profiles included two discontinuities in texture, materials and resistance to erosion, between saprolite and saprock and between saprock and rock. Limited core recovery when drilling below the soil base in mixed rocks of the Tana Belt indicates that weathering locally penetrated deep below upper fresh rock layers. Such deep-seated weathered bands in rock represent a third set of discontinuities. Incipient weathering and supergene mineralisation also extended to depths of > 100 m in mineralised fracture zones. The thin weathering crusts found extensively beneath till may represent types of early or middle Pleistocene palaeosols.We confirm that glacial erosion has been very limited (< 20 m) in northern Finland and has been widely restricted to the partial stripping of saprolith. The Fennoscandian Ice Sheet in this ice-divide zone remained cold-based and unerosive throughout the Pleistocene. The large-scale shield geomorphology developed before glaciation and is a product of differential weathering and erosion acting on diverse rock types and structures through the Neogene. The first ice sheets did not advance across planar, uniformly soft, deeply kaolinised beds as proposed in recent models of the Laurentide ice sheet. Instead, in northern Finland, the shield topography comprised broad plains and valleys with isolated hills and hill masses, with a relative relief of several hundred metres. Weathered rock was restricted in its distribution and thickness and provided diverse bed materials for ice sheets, including rock, broken saprock, permeable gruss, and linear zones of impermeable clay, with multiple discontinuities. Glacial erosion and local glacial transport led to widespread incorporation of this saprolith material into tills.
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| 3. |
- Parducci, Laura, 1965-, et al.
(författare)
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Proxy comparison in ancient peat sediments : pollen, macrofossil and plant DNA
- 2015
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Ingår i: Philosophical Transactions of the Royal Society of London. Biological Sciences. - : The Royal Society. - 0962-8436 .- 1471-2970. ; 370:1660, s. 20130382-
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Tidskriftsartikel (refereegranskat)abstract
- We compared DNA, pollen and macrofossil data obtained from Weichselian interstadial (age more than 40 kyr) and Holocene (maximum age 8400 cal yr BP) peat sediments from northern Europe and used them to reconstruct contemporary floristic compositions at two sites. The majority of the samples provided plant DNA sequences of good quality with success amplification rates depending on age. DNA and sequencing analysis provided five plant taxa from the older site and nine taxa from the younger site, corresponding to 7% and 15% of the total number of taxa identified by the three proxies together. At both sites, pollen analysis detected the largest (54) and DNA the lowest (10) number of taxa, but five of the DNA taxa were not detected by pollen and macrofossils. The finding of a larger overlap between DNA and pollen than between DNA and macrofossils proxies seems to go against our previous suggestion based on lacustrine sediments that DNA originates principally from plant tissues and less from pollen. At both sites, we also detected Quercus spp. DNA, but few pollen grains were found in the record, and these are normally interpreted as long-distance dispersal. We confirm that in palaeoecological investigations, sedimentary DNA analysis is less comprehensive than classical morphological analysis, but is a complementary and important tool to obtain a more complete picture of past flora.
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