Capabilities of the Lamont–Doherty Earth Observatory in situ¹⁴C extraction laboratory updated

We report on the status and capabilities of the Lamont–Doherty Earth Observatory in situ¹⁴C extraction laboratory. In late 2006 we began, in collaboration with the AMS group at the University of Arizona, construction of a new laboratory to extract in situ cosmogenic ¹⁴C from terrestrial silicates. Long-term measurements of the process blank over the last two years give an arithmetic mean and standard deviation of 125 ± 43 × 10³ atoms ¹⁴C (n = 9) and show significant improvement in the number of atoms, as well as stability compared to initial measurements of the process blank. We report long-term measurements of the intercomparison material CRONUS-A, which has been developed as part of the CRONUS-Earth effort to characterize inter- and intra-laboratory variability. We interpret the standard deviation (5%) of six replicate measurements of CRONUS-A as the reproducibility of in situ¹⁴C extractions in our laboratory

Using in situ Chlorine-36 cosmonuclide to recover past earthquake histories on limestone normal fault scarps: a reappraisal of methodology and interpretations

International audienceCosmic-ray exposure dating of preserved, seismically exhumed limestone normal fault scarps has been used to identify the last few major earthquakes on seismogenic faults and recover their ages and displacements through the modelling of the content of in situ [36Cl] cosmonuclide of the scarp rocks. However, previous studies neglected some parameters that contribute to 36Cl accumulation and the uncertainties on the inferred earthquake parameters were not discussed. To better constrain earthquake parameters and to explore the limits of this palaeoseismological method, we developed a Matlab® modelling code (provided in Supplementary information) that includes all the factors that may affect [36Cl] observed in seismically exhumed limestone fault scarp rocks. Through a series of synthetic profiles, we examine the effects of each factor on the resulting [36Cl], and quantify the uncertainties related to the variability of those factors. Those most affecting the concentrations are rock composition, site location, shielding resulting from the geometry of the fault scarp and associated colluvium, and scarp denudation. In addition, 36Cl production mechanisms and rates are still being refined, but the importance of these epistemic uncertainties is difficult to assess. We then examine how pre-exposure and exposure histories of fault-zone materials are expressed in [36Cl] profiles. We show that the 36Cl approach allows unambiguous discrimination of sporadic slip versus continuous creep on these faults. It allows identification of the large slip events that have contributed to the scarp exhumation, and provides their displacement with an uncertainty of +/- ~25 cm and their age with an uncertainty of +/-0.5-1.0 kyr. By contrast, the modelling cannot discriminate whether a slip event is a single event or is composed of multiple events made of temporally clustered smaller size events. As a result, the number of earthquakes identified is always a minimum, while the estimated displacements are maximum bounds and the ages the approximate times when a large earthquake or a cluster of smaller earthquakes have occurred. We applied our approach to a data set available on the Magnola normal fault, Central Italy, including new samples from the buried part of the scarp. Reprocessing of the data helps to refine the seismic history of the fault and quantify the uncertainties in the number of earthquakes, their ages and displacements. We find that the Magnola fault has ruptured during at least five large earthquakes or earthquake clusters in the last 7 ka, and may presently be in a phase of intense activity

Holocene deglaciation of the northern Fildes Peninsula, King George Island, Antarctica

The timing and magnitude of Holocene glacial oscillations in most currently ice-free areas of Antarctica remain unknown. This work focuses on the recent deglaciation in the northern sector of the Fildes Peninsula, King George Island, northern Antarctic Peninsula. The ice cap covering ca. 90% of the island has receded since the Last Glacial Maximum and exposed ca. 29 km2 of ice-free land. We reconstruct its glacial history based on a dataset of 12 36Cl exposure ages obtained through cosmic-ray exposure (CRE) dating of moraine boulders, polished surfaces and erratic boulders surrounding the peninsula's northern plateau. Results reveal that the deglaciation of the northern Fildes Peninsula took place during the Holocene Thermal Maximum at 7–6 ka, when warm conditions promoted a massive glacial retreat. The present arrangement of ice-free areas was in place by 6 ka. Small cirque moraines suggest the subsequent occurrence of favourable climate conditions for glacial expansion fed by intense snow deflation at 4.6 and 1 ka at the foot of the northern plateau. The deglaciation pattern of the Fildes Peninsula resulted from the combined shrinkage of different ice masses, rather than of the long-term retreat of the King George Ice Cap. No evidence of glacier expansion during more recent cold periods (i.e. the Little Ice Age) was found. These results fit well with regional deglacial histories inferred from lacustrine sediments and raised beaches and complement the existing chronological framework to help better understand the peninsula's Holocene geoecological dynamics.info:eu-repo/semantics/acceptedVersio

Holocene glacier culminations in the Western Alps and their hemispheric relevance

The natural variability of Holocene climate defines 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 10Be 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 10Be ages between 500 and 170 yr correspond to the LIA, while the youngest 10Be 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 findings 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

Glacial evolution in King George and Livingston Islands (Antarctica) since the Last Glacial Maximum based on cosmogenic nuclide dating and glacier surface reconstruction-CRONOANTAR project

EGU General Assembly 201

Reversible glacial-periglacial transition in response to climate changes and paraglacial dynamics: a case study from Héðinsdalsjökull (northern Iceland)

The objective of this work is to chronologically establish the origin of the different glacial and rock glacier complex landforms deposited by Héðinsdalsjökull glacier (65°39′ N, 18°55′ W), in the Héðinsdalur valley (Skagafjörður fjord, Tröllaskagi peninsula, central northern Iceland). Multiple methods were applied: geomorphological analysis and mapping, glacier reconstruction and equilibrium-line altitude calculation, Cosmic-Ray Exposure dating (in situ cosmogenic 36Cl), and lichenometric dating. The results reveal that a debris-free glacier receded around 6.6 ± 0.6 ka, during the Holocene Thermal Maximum. The retreat of the glacier exposed its headwall and accelerated paraglacial dynamics. As a result, the glacier terminus evolved into a debris-covered glacier and a rock glacier at a slightly higher elevation. The front of this rock glacier stabilized shortly after it formed, although nuclide inheritance is possible, but its sector close the valley head stabilized between 1.5 and 0.6 ka. The lowest part of the debris-covered glacier (between 600 and 820 m altitude) collapsed at ca. 2.4 ka. Since then, periods of glacial advance and retreat have alternated, particularly during the Little Ice Age. The maximum advance during this phase occurred in the 15th to 17th centuries with subsequent re-advances, namely at the beginning of the 19th and 20th centuries. After a significant retreat during the first decades of the 20th century, the glacier advanced in the 1960s to 1990s, and then retreated again, in accordance with the local climatic evolution. The internal ice of both the debris-covered and the rock glacier have survived until the present day, although enhanced subsidence provides evidence of their gradual degradation. A new rock glacier developed from an ice-cored moraine from around 1940–1950 CE. Thus, the Holocene coupling between paraglacial and climatic shifts has resulted in a complex evolution of Héðinsdalsjökull, which is conflicting with previously proposed models: a glacier, which had first evolved into a debris-covered and rock glacier, could later be transformed into a debris-free glacier, with a higher sensitivity to climatic variability.info:eu-repo/semantics/publishedVersio

Teleconnections between boreal cooling and tropical glaciers in the Cordillera Blanca (Peru)

30 samples have been collected for the cosmogenic dating of expanding and shrinking glacial phases southwesternward of Nevado Hualcán (9º, 12'S; 77º, 31'W, 6122 m), in the Cordillera Blanca (Peru).The set includes 28 moraine boulder surface samples and 2 polished bedrock surface samples. The resistivity of 3 samples of moraine boulder surfaces, measured in the AMS facility ASTER, did not allow us to estimate the accumulation of 10Be. Therefore, it was not possible to estimate the exposure age of these samples. However, it was possible to obtain 27 10Be glacial ages: 25 boulder ages from 5 moraine sets (M5-M1) and 2 polished bedrock ages (between M2-M1). 5 boulder ages seem too high, probably because of cosmogenic inheritance. Nevertheless, despite the uncertainties, the remaining 22 ages seem consistent with their geomorphological context. Nowadays, the oldest moraines (M5) are higher than younger moraines, because they define a glacial paleo-valley cut by the current valley in which the other groups of moraines (M4-M1) are becoming younger and younger and in higher altitudes. M5 ages (~132-61 ka) suggest the existence of at least two glacial phases long before the Last Glacial Maximum (LGM). The oldest phase could be traced back to the beginning of the Last Glacial Cycle (LGC). The M4-M1 moraines chronologies seem to correlate with paleolake transgressions in the Bolivian Altiplano. They can be interpreted as a reflection of wetter paleoclimatic phases than the current phase, and are linked to cooling periods in the Northern Hemisphere (Placzek et al., 2013). This link suggests the existence of a boreal/tropical teleconnection through a large shift towards the south of the Inter-Tropical Convergence Zone (Kelly et al., 2012) and/or of the Bolivian High (Martin et al., 2018). The M4 moraines define the maximum glacier advance on the south-west area of Nevado Hualcán. Their ages (~32-24 ka) are consistent with the early LGM, shown by various proof around the world (Clark et al., 2009). The M3 and M2 moraines show smaller and thinner readvances than previous moraines. Their ages (~20-12 ka and ~13-11 ka) are contemporaneous to Tauca (~18-14 ka) and Coipasa transgressions (~13-11 ka; Blard et al., 2011). The deglaciation after M3 could be the results of a weakening of tropical circulation related to boreal heating Bølling-Allerød. In that period (ages ~3-1 ka), the glaciers retreated to above their current limit. The M1 moraines pinpoint the last glacier advances, whose exposure ages without cosmogenic inheritance (~1.0-0.3 ka) are consistent with lichenometry-based dating in nearby valleys (Jomelli et al., 2008) and the ITCZ southward shift during the Little Ice Age (Sachs et al., 2009)

El registro cosmogénico glacial del cambio climático en los Andes peruanos: resultados del proyecto FONDECYT 144- 2015

Este trabajo presenta 71 edades de exposición a la radiación cósmica (berilio-10) de fases de avance y retroceso de glaciares andinos. Proporcionan cronologías glaciares para 3 áreas de estudió alineadas en un transecto norte-sur de la cordillera occidental de los Andes Centrales: al suroeste de los nevados Hualcán (Cordillera Blanca; 6122 m; 9ºS; región Ancash) y Pariacaca (5758 m; 12ºS; región Lima) y al sur del Cerro Ticllo Ticllo (5400 m; región Arequipa). Las cronologías fueron obtenidas por investigadores del proyecto FONDECYT 144-2015. En dichas áreas de estudio se recogieron muestras de 1-2 kg de roca, en superficies de bloques morrénicos (datan avances glaciares) y lechos rocosos con pulimento glaciar (datan fases de deglaciación)

Cronologías glaciales de los Andes Centrales occidentales de Perú estimadas por el Proyecto FONDECYT 144-2015

Este trabajo presenta edades de exposición a la radiación cósmica (berilio-10) de fases de avance y retroceso de glaciares en montañas de altitud decreciente, alineadas en un transecto norte-sur de la cordillera occidental de los Andes Centrales. Las muestras se recogieron en valles comprendidos por grandes morrenas de avances glaciares, al suroeste del Nevado Hualcán (región Ancash), al suroeste del Nevado Pariacaca (región Lima) y al sur del Nevado Chila (región Arequipa). Los resultados reflejan máximos avances glaciares (M5) hace 120-130 ka; 60-70 ka y 40-50 ka, anteriores al Last Glacial Maximum global (LGM; ~30-19 ka). También hay edades LGM (M4; 32-24 ka), 2 fases posteriores de reavances menores (M3-M2) y una última pulsación M1 de la Pequeña Edad del Hielo (PEH). Los avances glaciares parecen bastante bien correlacionados con transgresiones de los paleolagos en Bolivia y eventos fríos en el hemisferio norte. Ese hecho sugiere la existencia de una teleconexión enfriamiento boreal-humedad tropical a través de un desplazamiento hacia el Sur de la Zona de Convergencia Inter-Tropical (ZCIT)