Naki Akçar

Last Name

Akçar

First Name

Naki

ORCID

0000-0002-5604-3179

Organisational unit

01659 - Lehre Erd- und Planetenwissenschaften

Show all metadata

Search Results

Publications 1 - 10 of 33

Last Glacial Maximum glacier fluctuations on the northern Alpine foreland: Geomorphological and chronological reconstructions from the Rhine and Reuss glacier systems
Kamleitner, Sarah; Ivy-Ochs, Susan; Manatschal, Lucia; et al. (2023)
Geomorphology
Fresh glacial landforms of the Alpine forelands evidence the presence and extent of large piedmont glaciers during the Last Glacial Maximum (LGM) and yield valuable insights into LGM glacier dynamics. This study assesses widespread ice marginal landforms preserved within the limit of the former LGM Rhine glacier and the eastern lobes of the LGM Reuss glacier system by means of geomorphological mapping. Timing of formation of the studied ice margins in Rhine and Reuss systems are chronologically framed by new 10Be and 36Cl surface exposure ages, as well as new radiocarbon dates. This includes redating of radiocarbon samples first determined in the 1980s. Results of this and an earlier study focussing on outwash deposits downstream of the outer LGM ice margin, suggest that the Rhine glacier advanced to and reached its LGM maximum between ca. 26–22 ka, thereby forming a broad >100 km wide foreland piedmont lobe and constructing prominent and largely continuous chains of frontal moraines (outer Schaffhausen moraines). The eastern lobes of the Reuss glacier likely advanced to their LGM maximum position by 25/24 ± 2 ka as indicated by published luminescence dates. Stabilization of the corresponding Untertannwald ice margin and the slightly internal yet more prominent Mellingen moraines (Reuss glacier system) occurred no later than 22 ± 1 ka and 21 ± 1 ka, respectively. Glacier oscillations following the LGM maximum position are evidenced in both Rhine and Reuss systems but show varying degrees of preservation. Largely contemporaneous, late LGM readvances of Rhine (Stein am Rhein stadial) and Reuss (Bremgarten stadial) glaciers occurred after 20.6 ± 1.7 ka and 20.8 ± 1.3 ka, respectively. Absence of upstream moraines suggests rapid ice decay without marked stabilization, thereafter. Despite clear differences in the size and nature of their foreland lobes, with Rhein glacier as a broad piedmont lobe and narrow valley glacier like lobes characterising the eastern Reuss system, a remarkable similarity in timing is shown between the two.
Rapid post-glacial bedrock weathering in coastal Norway
Andersen, Jane Lund; Margreth, Annina; Fredin, Ola; et al. (2022)
Geomorphology
Quantifying bedrock weathering rates under diverse climate conditions is essential to understanding timescales of landscape evolution. Yet, weathering rates are often difficult to constrain, and associating a weathered landform to a specific formative environment can be complicated by overprinting of successive processes and temporally varying climate. In this study, we investigate three sites between 59 degrees N and 69 degrees N along the Norwegian coast that display grussic saprolite, tafoni, and linear weathering grooves on diverse lithologies. These weathering phenomena have been invoked as examples of geomorphic archives predating Quaternary glaciations and consequently as indicators of minimal glacial erosion. Here we apply cosmogenic nuclide chronometry to assess the recent erosional history. Our results demonstrate that all three sites experienced sufficient erosion to remove most cosmogenic nuclides formed prior to the Last Glacial Maximum. This finding is inconsistent with preservation of surficial (<1-2 m) weathered landforms under non-erosive ice during the last glacial period, while simultaneously demonstrating that post-glacial weathering and erosion rates can be locally rapid (4-10 cm kyr(-1)) in cold temperate to subarctic coastal locations. (C) 2021 The Authors. Published by Elsevier B.V.
Quantifying sediment supply at the end of the last glaciation: Dynamic reconstruction of an alpine debris-flow fan
Savi, Sara; Norton, Kevin P.; Picotti, Vincenzo; et al. (2014)
GSA Bulletin
In this paper we quantify the sediment dynamics in the formerly glaciated Zielbach catchment in the Italian Alps from the end of the Last Glacial Maximum (LGM) until today. As a basis for our quantification, we use the stratigraphic record offered by a 3.5 km2 large fan that we explore with a seismic survey, stratigraphic analyses of drillhole material, and 14C ages measured on organic matter encountered in these drillings. In addition, we calculate past denudation rate variability in the fan deposits using concentrations of cosmogenic 10Be. We merge this information into a scenario of how the sediment flux has changed through time and how this variability can be related to climatic variations, framed within well-known paraglacial models. The results document a highly complex natural system. From the LGM to the very early Holocene, ice-melted discharge and climate variability promoted a high sediment flux (sedimentation rate up to 40 mm/yr). This flux then dramatically decreased toward interglacial values (0.8 mm/yr at 5–4 calibrated kyr B.P.). However, in contrast to the trend of classic paraglacial models, the flux recorded at Zielbach shows secondary peaks at 6.5 ka and 2.5 ka, with values of 13 mm/yr and 1.5 mm/yr, respectively. Paleo-denudation rates also decrease from ∼33 mm/yr at the beginning of the Holocene to 0.42 mm/yr at 5 ka, with peaks of ∼6 mm/yr and 1.1 mm/yr at 6.5 ka and 2.5 ka. High-amplitude climate change is the most likely cause of the secondary peaks, but anthropogenic activities may have contributed as well. The good correlation between paleo-sedimentation and paleo-denudation rates suggests that the majority of the deglaciated material destocked from the Zielbach catchment is stored in the alluvial fan.
Seismic Activity of the Manisa Fault Zone in Western Turkey Constrained by Cosmogenic 36Cl Dating
Mozafari, Nasim; Özkaymak, Çağlar; Tikhomirov, Dmitry; et al. (2021)
Geosciences
This study reports on the cosmogenic 36Cl dating of two normal fault scarps in western Turkey, that of the Manastır and Mugırtepe faults, beyond existing historical records. These faults are elements of the western Manisa Fault Zone (MFZ) in the seismically active Gediz Graben. Our modeling revealed that the Manastır fault underwent at least two surface ruptures at 3.5 ± 0.9 ka and 2.0 ± 0.5 ka, with vertical displacements of 3.3 ± 0.5 m and 3.6 ± 0.5 m, respectively. An event at 6.5 ± 1.6 ka with a vertical displacement of 2.7 ± 0.4 m was reconstructed on the Mugırtepe fault. We attribute these earthquakes to the recurring MFZ ruptures, when also the investigated faults slipped. We calculated average slip rates of 1.9 and 0.3 mm yr−1 for the Manastır and Mugırtepe faults, respectively.
Late pleistocene glacial history of Mount Karadağ, SW Türkiye
Bayrakdar, Cihan; Çılğın, Zeynel; Sarış, Faize; et al. (2024)
Geomorphology
Glacial landforms in the southwestern part of the Anatolian Peninsula are found at lower elevations than the rest, suggesting that glaciers were present during the Late Pleistocene. Karadağ, located west of the Western Taurus Mountains, provides evidence of the climatic conditions that facilitated this extensive glaciation. It is characterized by numerous peaks exceeding 2300 m above sea level (asl), with the highest peak reaching 2418 m, making it the region's largest glaciation area. This high mountain mass is composed mainly of limestone and dolomite. This study focuses on the glacial landforms in Karadağ to reconstruct the Late Pleistocene glacial chronology and palaeoclimate of the Western Taurus Mountains. We employed detailed UAV photogrammetry, extensive fieldwork and mapping, surface exposure dating with cosmogenic ³⁶Cl, meteorological measurements, palaeoglacier reconstruction, and palaeo-equilibrium line altitude (pELA) calculations. Karadağ, a topographic barrier to humid air masses from the Mediterranean Sea, ranks as the second wettest area in the Western Taurus Mountains. At a meteorological station we installed in Karadağ, an annual precipitation of 1700 mm or more was recorded, highlighting its significant precipitation. In Karadağ, we have identified two glacial valleys with a maximum length of 4.5 km and six cirques. The glaciers reached their maximum extent around 22.4 ± 2.8 ka during the Last Glacial Maximum (LGM), facilitated by the lowering of the pELA to 2090 m. During the LGM, the glaciers covered an area of approximately 3.5 km² and reached a maximum thickness of about 140 m. Sample TRKR 10 at 2015 m probably belongs to the LGM period. Although a single sample does not conclude definitive conclusions, the presence of the moraine at its highest position indicates the maximum thickness of the glacier. Following the LGM, two more significant glacier advances occurred during the Lateglacial (15.5 ± 2.7 ka) and the Younger Dryas (12.4 ± 1.1 ka). During the late Pleistocene glaciations, and especially during the LGM, Karadağ probably received more precipitation than today, leading to the formation of glaciers at relatively lower elevations than on the Anatolian Peninsula.
The Anatolian Mountains: glacial landforms during deglaciation (18.9–14.6 ka)
Akçar, Naki (2023)
European Glacial Landscapes
High Anatolian terrain in the Eastern Mediterranean region hosted glaciated mountains during the Late Pleistocene. Evidence for these glaciations exist in the Eastern Black Sea Mountains, Taurus Mountains, Eastern Anatolian Mountains, at Uludağ in the NW peninsula and on isolated extinct volcanic cones in the interior parts such as Mount Erciyes. In many of these mountains, glacial deposits and landforms show a single glacier advance during the deglaciation. The associated ELA depression was less than about 800 m. The palaeoglaciers were a few km long and less extensive than the LGM, that is, up to 70%–80% of that of LGM. Thin cirque glaciers, however, responded more sensitively to the climate evolution during the deglaciation. For example, cirque glaciers in Northwestern Anatolia fluctuated at least four times between 16.2±0.3 and 15.1±0.5 ka.
The Quaternary Period in Switzerland
Schlüchter, Christian; Akçar, Naki; Ivy-Ochs, Susan (2021)
World Geomorphological Landscapes ~ Landscapes and Landforms of Switzerland
The evolution of the Swiss landscape during the Quaternary Period over the past 2.6 million years is controlled by multiple glaciations and deglaciations with at least 15 drastic environmental changes between glacier advances (with yearly average temperatures of –16° compared with today) and warm phases (with yearly average temperatures of +2° compared with today). During the Most Extensive Glaciation (MEG) several hundred thousand years ago, Switzerland was almost completely ice-covered with the exception of the area around Basel and the most external parts of the Jura Mountains. During the warmest interglacial periods the glaciers were, most likely, completely gone. The feeding mechanism for the inner-alpine ice accumulation was a southerly (foehn) circulation. Vegetation cover during interglacials was comparable to today except for the Last Interglacial when Fagus (beech) was missing and during at least one older interglacial when a Fagus/Petrocary-forest was growing in the Central Plateau. The age for the deep valley erosion in the northern Alpine Foreland is several millions of years younger than in southern Alpine Insubria.
The first major incision of the Swiss Deckenschotter landscape
Akçar, Naki; Ivy-Ochs, Susan; Alfimov, Vasily; et al. (2014)
Swiss Journal of Geosciences
The Swiss Deckenschotter (“cover gravels”) is the oldest Quaternary units in the northern Swiss Alpine Foreland. They are a succession of glaciofluvial gravel layers intercalated with glacial and/or overbank deposits. This lithostratigraphic sequence is called Deckenschotter because it “covers” Molasse or Mesozoic bedrock and forms mesa-type hill-tops. Deckenschotter occurs both within and beyond the extent of the Last Glacial Maximum glaciers. The Swiss Deckenschotter consist of two sub-units: Höhere (Higher) and Tiefere (Lower) Deckenschotter. Although the Höhere Deckenschotter sub-unit (HDS) is topographically higher than the Tiefere Deckenschotter, it is older. The only available age for the Swiss Deckenschotter is 2.5–1.8 Ma based on mammal remains found in HDS at the Irchel site. In this study, we present an exposure age for the topographically lowest HDS, calculated from a cosmogenic 10Be depth-profile. Our results show that the first phase of the Deckenschotter glaciations in the Swiss Alps terminated at least 1,020+80−120 ka ago, which is indicated by a significant fluvial incision. This line of evidence seems to be close to synchronous with the beginning of the Mid-Pleistocene Revolution, when the frequency of the glacial-interglacial cyclicity changed from 41 to 100 ka and the amplitude from low to high, between marine isotope stages 23 and 22.
A Special Issue of Geosciences: Cutting Edge Earth Sciences—Three Decades of Cosmogenic Nuclides
Akçar, Naki; Ivy-Ochs, Susan; Schlunegger, Fritz (2022)
Geosciences
LGM Glaciations in the Northeastern Anatolian Mountains: New Insights
Reber, Regina; Akçar, Naki; Tikhomirov, Dmitry; et al. (2022)
Geosciences
Barhal Valley belongs to the Çoruh Valley System in the Kaçkar Mountains of northeastern Anatolia. This 13 km long valley is located to the south of the main weather divide and to the east of Mt. Kaçkar, with the highest peak of the mountain range being 3932 m. Today, source of an average yearly precipitation of 2000 mm of moisture is the Black Sea, situated approximately 40 km to the north of the study site. Glaciers of the Last Glacial Maximum (LGM) descended directly from Mt. Kaçkar and reached an altitude of ca. 1850 m a.s.l. (above sea level). In this study, we are exploring whether the position of Barhal Valley to the south of the main weather divide and its east–west orientation have an influence on the existence and expansion of paleoglaciers. Here, we present 32 new cosmogenic 36Cl dates on erratic boulders from the Çoruh Valley System. We reconstructed three geomorphologically well-contained glacier advances in the Barhal Valley, namely at 34.0 ± 2.3 ka, 22.2 ± 2.6 ka, and 18.3 ± 1.7 ka within the time window of the global LGM. Field evidence shows that the glacier of the 18.3 ± 1.7 ka advance disappeared rapidly and that by the latest time, at 15.6 ± 1.8 ka, the upper cirques were ice-free. No evidence for Lateglacial glacier fluctuations was found, and the Neoglacial activity is restricted to the cirques with rock glaciers. A range of 2700 to 3000 m for the Equilibrium Line Altitude (ELA) at the LGM was reported based on modeling of the glacial morphology. We determined that the most likely position of the LGM ELA in the Çoruh Valley System was at 2900 m a.s.l. We suggest an alternative moisture source to the direct transport from the Black Sea for the ice accumulation in the Eastern Black See Mountains. The shift of the Polar Front and of the Siberian High Pressure System to the south during the LGM resulted in the domination of easterly airflow to the Caucasus and Kaçkar Mountains with moisture from expanded lakes in central–western Siberia and from the enlarged Aral-and Caspian Seas.

Publications 1 - 10 of 33

Naki Akçar

Last Name

First Name

ORCID

Organisational unit

Filters

Search Results