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| 3. |
- Abbott, P. M., et al.
(författare)
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Re-evaluation and extension of the Marine Isotope Stage 5 tephrostratigraphy of the Faroe Islands region : The cryptotephra record
- 2014
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Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - : Elsevier BV. - 0031-0182 .- 1872-616X. ; 409, s. 153-168
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Tidskriftsartikel (refereegranskat)abstract
- Previous studies of marine sequences from the Faroe Islands region have identified a series of coarse-grained tephra horizons deposited during Marine Isotope Stage (MIS) 5. Here we reassess the MIS 5 tephrostratigraphy of the Faroe Islands region and focus on the cryptotephra deposits preserved within the fine-grained fraction of marine core LINK 16. We also extend the record to encompass the late MIS 6 and early MIS 4 periods. A density separation technique, commonly used for tephra investigations in lacustrine settings but rarely applied to marine sediments, is utilised to explore the fine-grained material and EPMA and LA-ICP-MS are employed to determine the major and trace element composition of individual tephra shards. In total, 3 basaltic and 3 rhyolitic Icelandic cryptotephra deposits with homogeneous geochemical compositions are identified - all of which have the potential to act as isochronous tie-lines. Geochemical results highlight that the Grimsvotn volcanic system of Iceland is the predominant source of the basaltic horizons and the Oraefajokull or Torfajokull systems are the likely sources of the rhyolitic deposits. Three of the horizons have been previously recognised in Faroe Islands region marine sequences, with two of these deposits traceable into a Norwegian Sea sequence. An early MIS 4 rhyolitic horizon is the most widespread deposit as it can be traced into the Norwegian Sea and to the south into a record from the Rockall Trough. Basaltic and rhyolitic horizons deposited during late MIS 6 have not been recognised in other sequences and represent new additions to the regional tephrostratigraphy.
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| 5. |
- Buylaert, J. -P, et al.
(författare)
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Luminescence dating of the PASADO core 5022-1D from Laguna Potrok Aike (Argentina) using IRSL signals from feldspar
- 2013
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 71, s. 70-80
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Tidskriftsartikel (refereegranskat)abstract
- We have measured and tested a luminescence chronology for the PASADO core 5022-1D from the maar lake of Laguna Potrok Aike. Because of unsuitable quartz OSL characteristics, sand-sized K-feldspar extracts were chosen as a dosimeter and the dose was measured using a post-IR IRSL (pIRIR(290)) measurement protocol. Using this approach we were able to access a stable signal and thus avoid the ubiquitous problem of feldspar signal instability. Extensive laboratory tests show that the chosen pIRIR(290) protocol is applicable to these samples. We also developed a new criterion based on known relative bleaching rates of the conventional IRSL signal (IR50) and the pIRIR(290) signal and the relationship between resulting equivalent doses; this is used to identify and reject poorly bleached samples. Eighteen samples out of 47 were rejected based on this criterion, without reference to absolute doses or stratigraphy; the resulting age-depth profile is self-consistent, increases smoothly with depth and is in agreement with independent age control based on volcanic ash layers (Reclus, Mt Burney and Hudson tephras) at the top and middle of the core. Our new luminescence chronology suggests that the 5022-1D core reaches back to similar to 65 ka at similar to 96 m below lake floor.
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- Davies, Siwan M., et al.
(författare)
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Tracing volcanic events in the NGRIP ice-core and synchronising North Atlantic marine records during the last glacial period
- 2010
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 294, s. 69-79
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Tidskriftsartikel (refereegranskat)abstract
- Abstract Five basaltic tephra horizons have been identified in the NGRIP ice-core during the last glacial period. Geochemical analyses indicate that these tephras are of Icelandic origin with one horizon identified as the Faroe Marine Ash Zone III (FMAZ III) — an important marker horizon in several marine records within the Faroe Islands margin. The FMAZ III is a few millimetres thick and is constrained to 38,122 ± 723 yr b2k (before year AD 2000) according to the Greenland Ice Core Chronology (GICC05). This event falls directly at the thermal peak of Greenland Interstadial 8 (GI-8) and a marine-ice correlation, based on the position of this tephra, suggests a close coupling of the marine and atmospheric systems at this time. The remaining tephras originate from the Eastern Volcanic Zone with the Katla volcanic system pinpointed as the dominant source. These are thought to represent previously unknown volcanic events which add considerably to our knowledge of Icelandic volcanism during this period. Independent ages are assigned to these events and together with major element characterisation, form the backbone of the tephrochronology framework presented here. Despite the identification of these tephras, this study also demonstrates that the sulphate record may not always act as a suitable proxy for the identification of tephra horizons in ice-core records.
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- Gebhardt, Andrea Catalina, et al.
(författare)
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Seismic evidence of up to 200 m lake-level change in Southern Patagonia since Marine Isotope Stage 4
- 2012
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Ingår i: Sedimentology. - : Wiley. - 0037-0746 .- 1365-3091. ; 59:3, s. 1087-1100
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Tidskriftsartikel (refereegranskat)abstract
- Maar lake Laguna Potrok Aike is located north of the Strait of Magellan (south-eastern Patagonia). Seismic reflection profiles revealed a highly dynamic palaeoclimate history. Dunes were identified in the eastern part of the lake at approximately 30 to 80 m below the lake floor, overlying older lacustrine strata, and suggest that the region experienced dry conditions probably combined with strong westerly winds. It is quite likely that this can be linked to a major dust event recorded in the Antarctic ice cores during Marine Isotope Stage 4. The dunes are overlain by a series of palaeo-shorelines indicating a stepwise water-level evolution of a new lake established after this dry period, and thus a change towards wetter conditions. After the initial, rapid and stepwise lake-level rise, the basin became deeper and wider, and sediments deposited on the lake shoulder at approximately 33 m below present-day lake level point towards a long period of lake-level highstand between roughly 53.5 ka cal. bp and 30 ka cal. bp with a maximum lake level some 200 m higher than the desiccation horizon. This highstand was then followed by a regressional phase of uncertain age, although it must have happened some time between approximately 30 ka cal. bp and 6750 yrs cal. bp. Dryer conditions during the Mid-Holocene are evidenced by a dropping lake level, resulting in a basin-wide erosional unconformity on the lake shoulder. A second stepwise transgression between ca 5.8 to 5.4 ka cal. bp and ca 4.7 to 4 ka cal. bp with palaeo-shorelines deposited on the lake shoulder unconformity again indicates a change towards wetter conditions.
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- Johansson, Hans, 1956-
(författare)
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Towards a Holocene tephrochronology for the Azores
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Azores is situated in the North Atlantic Ocean and is one of the most active volcanicregions in the Northern Hemisphere. The volcanic history of the islands is fairly wellknown and several explosive trachytic eruptions have been reported but the geochemicalcompositions of the glass component of the tephra as well as the dispersal oftephras to distal areas are less well known. The purpose of this study is twofold: (1) topresent major element geochemistry of the glass component from several historic aswell as prehistoric tephras, including the trachybasaltic Capelinhos AD 1957‐58eruption on the island of Faial, and the trachytic explosive eruptions of Sete Cidades(c. AD 1440), Fogo A (c. 5000 BP), Fogo AD 1563 and Furnas AD 1630 on the island ofSão Miguel; (2) to present a refined tephrostratigraphy for the island of Pico. Analyses ofmajor element geochemistry suggest that tephras from the three active stratovolcanoeson São Miguel can be separated in biplots showing e.g. FeOtot vs.TiO2 and FeOtot vs. CaO.The tephrostratigraphy of Caveiro bog on the island of Pico is based on a radiocarbondated core with eight tephra layers extending back to c. 7000 BP. All tephras are oftrachybasaltic/basaltic trachyandesitic composition except the oldest layer, which is ofbasanitic composition. An attempt was made to correlate the tephra record of Caveirobog with the previously investigated Lake Caveiro. A tephra‐based correlation betweenthe Caveiro bog and Lake Caveiro is not straightforward and only three tephras inCaveiro bog can possibly be correlated with tephras found in the sediments of LakeCaveiro.Proximal glass data from the Furnas volcano on São Miguel suggest that distal cryptotephrasfound in Ireland may have an origin in the Azores and not on Jan Mayen aspreviously has been suggested. The similarity of the proximal tephras on São Miguel anddistal tephras in Ireland is demonstrated by high similarity coefficients (>0.95) andbiplots showing major element composition also support a correlation between Azoreaneruptions and distal tephras in Ireland. Thus, trachytic tephras erupted from explosiveeruptions on the island of São Miguel may have a potential to contribute to theconstruction of a European‐wide tephrochronology framework.Trachytic tephras erupted from explosive eruptions on the Azores may be more widelydispersed than previously thought and may provide useful isochrones for correlation ofpaleoclimate archives in the north‐central Atlantic, North Africa and the Iberian regions.The trachybasaltic/basaltic trachyandesitic tephras erupted from cinder cones on PicoIsland are probably only useful for a local tephrochronology in the Azores region, andnot for a wider Atlantic or European framework.
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| 10. |
- Kliem, P., et al.
(författare)
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Magnitude, geomorphologic response and climate links of lake level oscillations at Laguna Potrok Aike, Patagonian steppe (Argentina)
- 2013
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 71, s. 131-146
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Tidskriftsartikel (refereegranskat)abstract
- Laguna Potrok Aike is a large maar lake located in the semiarid steppe of southern Patagonia known for its Lateglacial and Holocene lake level fluctuations. Based on sedimentary, seismic and geomorphological evidences, the lake level curve is updated and extended into the Last Glacial period and the geomorphological development of the lake basin and its catchment area is interpreted. Abrasion and lake level oscillations since at least similar to 50 ka caused concentric erosion of the surrounding soft rocks of the Miocene Santa Cruz Formation and expanded the basin diameter by approximately 1 km. A high lake level and overflow conditions of the lake were dated by luminescence methods and tephra correlation to the early Lateglacial as well as to similar to 45 ka. The lowest lake level of record occurred during the mid-Holocene. A further lake level drop was probably prevented by groundwater supply. This low lake level eroded a distinct terrace into lacustrine sediments. Collapse of these terraces probably caused mass movement deposits in the profundal zone of the lake. After the mid-Holocene lake level low stand a general and successive transgression occurred until the Little Ice Age maximum; i.e. ca 40 m above the local groundwater table. Frequent lake level oscillations caused deflation of emerged terraces only along the eastern shoreline due to prevailing westerly winds. Preservation of eolian deposits might be linked to relatively moist climate conditions during the past 2.5 ka. Precisely dated lake level reconstructions in the rain-shadow of the Andes document high Last Glacial and low Holocene lake levels that could suggest increased precipitation during the Last Glacial period. As permafrost in semiarid Patagonia is documented and dated to the Last Glacial period we argue that the frozen ground might have increased surficial runoff from the catchment and thus influenced the water balance of the lake. This is important for investigating the glacial to Holocene latitudinal shift and/or strengthening of the Southern Hemispheric Westerlies by using lake level reconstructions as a means to assess the regional water balance. Our interpretation explains the contradiction with investigations based on pollen data indicating drier climatic conditions for the Last Glacial period.
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| 11. |
- Langdon, P. G., et al.
(författare)
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A chironomid-based reconstruction of summer temperatures in NW Iceland since AD 1650
- 2011
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Ingår i: Quaternary Research. - : Cambridge University Press (CUP). - 0033-5894 .- 1096-0287. ; 75:3, s. 451-460
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Tidskriftsartikel (refereegranskat)abstract
- Few studies currently exist that aim to validate a proxy chironomid-temperature reconstruction with instrumental temperature measurements. We used a reconstruction from a chironomid percentage abundance data set to produce quantitative summer temperature estimates since AD 1650 for NW Iceland through a transfer function approach, and validated the record against instrumental temperature measurements from Stykkisholmur in western Iceland. The core was dated through Pb-210, Cs-137 and tephra analyses (Hekla 1693) which produced a well-constrained dating model across the whole study period. Little catchment disturbance, as shown through geochemical (Itrax) and loss-on-ignition data, throughout the period further reinforce the premise that the chironomids were responding to temperature and not other catchment or within-lake variables. Particularly cold phases were identified between AD 1683-1710. AD 1765-1780 and AD 1890-1917, with relative drops in summer temperatures in the order of 1.5-2 degrees C. The timing of these cold phases agree well with other evidence of cooler temperatures, notably increased extent of Little Ice Age (LIA) glaciers. Our evidence suggests that the magnitude of summer temperature cooling (1.5-2 degrees C) was enough to force LIA Icelandic glaciers into their maximum Holocene extent, which is in accordance with previous modelling experiments for an Icelandic ice cap (Langjokull). (C) 2010 University of Washington.
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| 12. |
- Wastegård, S, et al.
(författare)
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Different phases of the Yoldia Sea in the north-western Baltic Proper
- 1995
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Ingår i: Quaternary International. - 1040-6182 .- 1873-4553. ; 27, s. 121-129
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Tidskriftsartikel (refereegranskat)abstract
- Four sediment cores from the north-western Baltic Proper, covering the time from the Weichselian deglaciation to the present have been studied. Main interest is focused on the Yoldia stage, and especially the duration of the marine phase of this stage. The lithological compositions of the sediments and results of different analyses (magnetic, carbonate, ostracods and foraminifera) have resulted in a subdivision into five stratigraphical units (A-E, from older to younger). Unit A was deposited in a glaciolacustrine environment, probably during the first non-saline phase of the Yoldia stage. The first ingression of saline water is recorded in unit B. Brackish water ostracods and foraminifera occur exclusively in this unit, which represents the only saline phase of the Yoldia stage. This phase lasted for less than 120 clay varve years. Unit C was deposited in fresh water, probably during the last phase of the Yoldia stage and the Ancylus stage. The uppermost units, D and E are of Holocene age and represent different phases of the Litorina and Post-Litorina stages.
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