Naki Akçar

The Rhone Glacier was smaller than today for most of the Holocene

We present the development and application of the novel in situ cosmogenic 14 C/ 10 Be chronometer to recently exposed proglacial bedrock of the Rhone Glacier, Switzerland. Results show that during the Holocene, the glacier was smaller than today for 6500 ± 2000 yr and larger than today for 4500 ± 2000 yr. This pattern is consistent with limited data from other techniques for glaciers in the Alps and Scandinavia, but in contrast to glaciers from the Southern Alps of New Zealand, emphasizing the high sensitivity of large mountain glaciers to small climate changes. The 14 C/ 10 Be chronometer also shows that abrasion rates beneath the Rhone Glacier increased with ice speed.

Holocene glacier culminations in the Western Alps and their hemispheric relevance

The natural variability of Holocene climate defi nes the baseline to assess ongoing climate change. Greenland ice-core records indicate warming superimposed by abrupt climate oscillations in the early Holocene, followed by a general cooling trend throughout the middle and late Holocene that culminated during the Little Ice Age (LIA). Tropical precipitation changes correlate with these patterns throughout the Holocene. Here we use mountain glaciers in the European Alps to reconstruct the regional Holocene climate evolution and to test for a link between mid-latitude, North Atlantic, and tropical climate. Our precise 10 Be chronology from Tsidjiore Nouve Glacier, western Swiss Alps, indicates a glacier culmination during the earliest Holocene ~11.4 k.y. ago, likely related to the Preboreal Oscillation. Based on our data, no Holocene glacier advance of similar amplitude occurred until ~3.8 k.y. ago, when the glacier reached LIA limits. The 10 Be ages between 500 and 170 yr correspond to the LIA, while the youngest 10 Be ages overlap with the historically recorded post-LIA glacier positions. Integrating our data with existing records, we propose a hemispheric climate link between the Alps, North Atlantic temperature, and tropical precipitation patterns for the Holocene, supporting the concept of a pervasive climate driver. These fi ndings from northern mid-latitudes are consistent with the hypothesis formulated for the tropics that the Earth’s thermal equator, responding to North Atlantic temperature changes, might have migrated southward throughout the Holocene, reaching the southern turning point toward the end of the LIA.

High‐resolution sequence stratigraphic correlation in the Upper Jurassic (Kimmeridgian)–Upper Cretaceous (Cenomanian) peritidal carbonate deposits (Western Taurides, Turkey)

Upper Jurassic (Kimmeridgian)±Upper Cretaceous (Cenomanian) inner platform carbonates in the Western Taurides are composed of metre-scale upward-shallowing cyclic deposits (parasequences) and important karstic surfaces capping some of the cycles. Peritidal cycles (shallow subtidal facies capped by tidal-¯at laminites or fenestrate limestones) are regressive- and transgressive-prone (upward-deepening followed by upward-shallowing facies trends). Subtidal cycles are of two types and indicate incomplete shallowing. Submerged subtidal cycles are composed of deeper subtidal facies overlain by shallow subtidal facies. Exposed subtidal cycles consist of deeper subtidal facies overlain by shallow subtidal facies that are capped by features indicative of prolonged subaerial exposure. Subtidal facies occur characteristically in the Jurassic, while peritidal cycles are typical for the Lower Cretaceous of the region. Within the foraminiferal and dasyclad algal biostratigraphic framework, four karst breccia levels are recognized as the boundaries of major second-order cycles, introduced for the ®rst time in this study. These levels correspond to the Kimmeridgian±Portlandian boundary, mid-Early Valanginian, mid-Early Aptian and mid-Cenomanian and represent important sea level falls which affected the distribution of foraminiferal fauna and dasyclad ¯ora of the Taurus carbonate platform. Within the Kimmeridgian±Cenomanian interval 26 third-order sequences (types 1 and 2) are recognized. These sequences are the records of eustatic sea level ¯uctuations rather than the records of local tectonic events because the boundaries of the sequences representing 1±4 Ma intervals are correlative with global sea level falls. Third-order sequences and metre-scale cyclic deposits are the major units used for long-distance, high-resolution sequence stratigraphic correlation in the Western Taurides. Metre-scale cyclic deposits (parasequences) in the Cretaceous show genetical stacking patterns within third-order sequences and correspond to fourth-order sequences representing 100±200 ka. These cycles are possibly the E2 signal (126 ka) of the orbital eccentricity cycles of the Milankovitch band. The slight deviation of values, calculated for parasequences, from the mean value of eccentricity cycles can be explained by the currently imprecise geochronology established in the Cretaceous and missed sea level oscillations when the platform lay above fluctuating sea level.

36Cl exposure dating of paleoearthquakes in the Eastern Mediterranean: First results from the western Anatolian Extensional Province, Manisa fault zone, Turkey

Based on historical and instrumental data, societies in the Eastern Mediterranean and Middle East have survived at least 150 large earthquakes (generally M > 6) during the past 2500 yr. Beyond this time span, an earthquake chronology is mostly unknown, which hampers the production of reliable long-term earthquake models. Since the only remaining evidence of seismic activity is a bedrock scarp, cosmogenic 36 Cl is the only suitable nuclide to be applied in the determination of the seismic history and slip rate of an active limestone fault plane. In this study,

Synchronous Last Glacial Maximum across the Anatolian peninsula

Abstract Uludağ is a prominent mountain in northwestern Turkey where glacial deposits have been documented in the Kovuk Valley and the glacial history has been reconstructed based on 31 cosmogenic 10Be exposure ages from glacially transported boulders and bedrock. The results suggest that the Kovuk Glacier began advancing before 26.5±1.6 ka. It reached its maximum extent at 20.3±1.3 ka, followed by a re-advance at 19.3±1.2 ka, both during the global Last Glacial Maximum (LGM) within Marine Isotope Stage 2. The timing of the LGM glaciations in the Kovuk Valley is consistent with the investigated LGM glaciations in other mountains of Anatolia, the Mediterranean and the Alps. Based on the geomorphological ice margin reconstruction and using the accumulation/ablation area ratio (AAR) approach, the equilibrium line altitude (ELA) of the Kovuk LGM glacier was c. 2000 m above sea level for an estimated AAR of 0.67. This indicates a c. 1000 m lowering of the ELA for the LGM compared with the modern ELA estimate. These lines of evidence are consistent with the LGM glaciers that have been documented elsewhere in the Anatolian Mountains. Supplementary material: An input file for exposure age calculation in the CRONUS-Earth online calculator and an KML file for sample locations are available at http://www.geolsoc.org.uk/SUP18878

Quantifying sediment supply at the end of the last glaciation: Dynamic reconstruction of an alpine debris-flow fan

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.

The Frozen Bosphorus and its Paleoclimatic implications based on a summary of the historical data

Historically, the first evidence of a frozen Bosphorus was noted during the time of Herodotus. Analysis of the historical data about freezing events in the Bosphorus (at Istanbul) reveals the existence of four main cold periods since 1 AD. The first occurred around the 1st cen tury. Although the temperature was close to, or perhaps a little lower than, that of the present, three succes- sive freezing events are indicated between 7-17 AD. The second cold period was in the 4th century, when another freezing event was reported in 401 AD. After a slight temperature increase up to the beginning of the 8th century, the third cold period extended from the mid-8th to the 13th century, during which the Bosphorus and even parts of the Black Sea were repeatedly frozen, and floating ice masses entered the Sea of Marmara. Winters were markedly mild for 400 years starting from the mid-13th century. The fourth cold period began early in the mid-17th century and has lasted to the present day; it has been characterized by severe winters, however, the intensity of the winter cold has gradually diminished during this interval. Our aim is to evaluate existing historical data on these modern cold periods, to analyze instrumental meteoro- logical data, and to provide suitable data for future correlations with the amplitude and frequency of paleoglacier advances both in the Alps and in Anatolia. These four periods are more or less coeval with the phases of glacial advance in the Northern Hemisphere. As the accuracy of the historical data increases with time, evidence is more detailed for the fourth period. During this interlude, which coincides with the Little Ice Age, freezing events were not all coeval with reliable central European evidence. This can be explained by the low index of the North Atlantic Oscillation that resulted in higher precipitation ratios.

Isochron-burial dating of glaciofluvial deposits: First results from the Swiss Alps

In this study, we use isochron-burial dating to date the Swiss Deckenschotter, the oldest Quaternary deposits of the northern Alpine Foreland. Concentrations of cosmogenic 10Be and 26Al in individual clasts from a single stratigraphic horizon can be used to calculate an isochron-burial age based on an assumed initial ratio and the measured 26Al/10Be ratio. We suggest that, due to deep and repeated glacial erosion, the initial isochron ratio of glacial landscapes at the time of burial varies between 6.75 and 8.4. Analysis of 22 clasts of different lithology, shape, and size from one 0.5 m thick gravel bed at Siglistorf (Canton Aargau) indicates low nuclide concentrations: <20000 10Be atoms/g and <150000 26Al atoms/g. Using an 26Al/10Be ratio of 7.6 (arithmetical mean of 6.75 and 8.4), we calculate a mean isochron-burial age of 1.5 ± 0.2 Ma. This age points to an average bedrock incision rate between 0.13 and 0.17 mm/a. Age data from the Irchel, Stadlerberg, and Siglistorf sites show that the Higher Swiss Deckenschotter was deposited between 2.5 – 1.3 Ma. Our results indicate that isochron-burial dating can be successfully applied to glaciofluvial sediments despite very low cosmogenic nuclide concentrations.

Isochron-burial dating of glaciofluvial deposits: First results from the Swiss Alps: ISOCHRON-BURIAL DATING OF GLACIOFLUVIAL DEPOSITS

In this study, we use isochron-burial dating to date the Swiss Deckenschotter, the oldest Quaternary deposits of the northern Alpine Foreland. Concentrations of cosmogenic 10Be and 26Al in individual clasts from a single stratigraphic horizon can be used to calculate an isochron-burial age based on an assumed initial ratio and the measured 26Al/10Be ratio. We suggest that, due to deep and repeated glacial erosion, the initial isochron ratio of glacial landscapes at the time of burial varies between 6.75 and 8.4. Analysis of 22 clasts of different lithology, shape, and size from one 0.5 m thick gravel bed at Siglistorf (Canton Aargau) indicates low nuclide concentrations: <20000 10Be atoms/g and <150000 26Al atoms/g. Using an 26Al/10Be ratio of 7.6 (arithmetical mean of 6.75 and 8.4), we calculate a mean isochron-burial age of 1.5 ± 0.2 Ma. This age points to an average bedrock incision rate between 0.13 and 0.17 mm/a. Age data from the Irchel, Stadlerberg, and Siglistorf sites show that the Higher Swiss Deckenschotter was deposited between 2.5 – 1.3 Ma. Our results indicate that isochron-burial dating can be successfully applied to glaciofluvial sediments despite very low cosmogenic nuclide concentrations.

10Be-inferred paleo-denudation rates imply that the mid-Miocene western central Andes eroded as slowly as today

Terrestrial cosmogenic nuclide concentrations of detrital minerals yield catchment-wide rates at which hillslopes erode. These estimates are commonly used to infer millennial scale denudation patterns and to identify the main controls on mass-balance and landscape evolution at orogenic scale. The same approach can be applied to minerals preserved in stratigraphic records of rivers, although extracting reliable paleo-denudation rates from Ma-old archives can be limited by the target nuclide’s half-life and by exposure to cosmic radiations after deposition. Slowly eroding landscapes, however, are characterized by the highest cosmogenic radionuclide concentrations; a condition that potentially allows pushing the method’s limits further back in time, provided that independent constraints on the geological evolution are available. Here, we report 13–10 million-year-old paleo-denudation rates from northernmost Chile, the oldest 10Be-inferred rates ever reported. We find that at 13–10 Ma the western Andean Altiplano has been eroding at 1–10 m/Ma, consistent with modern paces in the same setting, and it experienced a period with rates above 10 m/Ma at ~11 Ma. We suggest that the background tectono-geomorphic state of the western margin of the Altiplano has remained stable since the mid-Miocene, whereas intensified runoff since ~11 Ma might explain the transient increase in denudation.