Geology of the Nevado Coropuna volcanic complex

The Nevado Coropuna volcanic complex (NCVC), located in the northernmost CAVZ (15°3’ S, 72º39’ W), includes several edifices, aligned WNW-ESE above Neogene ignimbrites. Adjacent composite volcanoes include the Late Pliocene, eroded Sunjillpa to the WNW and the glacially eroded, 0.6-0.25 Ma-old Cunciacha to the ESE. Located on the west flank of the Western Cordillera, the asymmetric volcanic complex shows stubby lava flows overlying the NE, 4500 m-high plateau, contrasting with long, inverted lava flows and debris-avalanche deposits filling deep valleys draining the steep SW flanks. The central, highest NCVC is a cluster of five, aligned lava domes reaching 6160 and 6330 masl. The dome cluster and its voluminous lava flows overlie an old stratovolcano with inverted lava flows dated at 1.02 Ma. The 0.4 Ma-old base of dome cluster is overlain by lower lava flows c. 270 - 254 ka, middle lava flows c. 118 - 108 ka, and the uppermost lava domes 70 – 60 ka. A high-spatial resolution DEM shows six vents on the domes and one collapse scar open to the south. NCVC has grown atop a caldera as shown by AMS data collected on Early Quaternary ignimbrites dipping away west, south and east of NCVC, and by abnormal contacts with both adjacent volcanoes to the WNW and the ESE. All lavas show two major compositional fields of high-K andesites and dacites (SiO2 57-67%wt). Harker diagrams and trace elements suggest AFC magmatic processes. Although CNVC tephra and PDC deposits represent a small volume, we observed Late Glacial Plinian fallout are scattered and Holocene ashfall layers are associated with three lava flows, the youngest being 1700 to 2400 yr old. The Nevado Coropuna ice cap c. 44.1 km2 is arguably the largest in the world tropical belt

Reconstructing the History of Reedy Glacier, Antarctica

This thesis describes the distribution and sedimentologic characteristics of glacial deposits at Reedy Glacier (86 s) in the southern Transantarctic Mountains. Reedy Glacier is an outlet glacier of the East Antarctic Ice Sheet which flows into the West Antarctic Ice Sheet ~100 km behind the Ross Sea grounding line. This position means the flow and thickness of Reedy Glacier are controlled, in part, by the damming effect of the West Antarctic Ice Sheet. This thesis is an integral part of a larger study aimed at assessing the current stability of the West Antarctic Ice Sheet through recent changes in the surface of Reedy Glacier. Glacial geologic mapping and 10Be surface-exposure dating provide a record of glaciation at Reedy Glacier spanning the last ~290 Ma. The earliest glacial landforms were deposited when the Antarctic climate was still temperate. Since the inception of cold polar conditions, Reedy Glacier has expanded on at least six occasions. Each expansion has been less extensive than the previous, suggesting changes in the size of the West Antarctic Ice Sheet and perhaps downcutting of the bed of Reedy Glacier. The most recent of these episodes occurred during the Late Pleistocene and is contemporary with the Last Glacial Maximum throughout the Transantarctic Mountains. At this time, the ice sheet was as much as 500 m thicker than it is today. Subsequent deglaciation of Reedy Glacier has been controlled by recession of the Ross Sea grounding line and has lagged the onset of global deglaciation by several millennia

Comment on ‘Was Scotland deglaciated during the Younger Dryas?’ by Small and Fabel (2016)

The course of climatic events in Scotland and the broader North Atlantic region during the glacial termination has important implications for our understanding of the causes and mechanisms of abrupt climate change but remains in debate. One example is the timing of the late-glacial 'Loch Lomond Readvance' (LLR), during which an ice cap and numerous cirque glaciers were nourished in the Scottish Highlands. Exactly when the LLR occurred and culminated has been disputed for several decades and has been addressed via several different types of chronologic evidence (e.g., Lowe and Walker, 1976; Golledge et al., 2007; MacLeod et al., 2011; Bromley et al., 2014). Recently, Small and Fabel (2016) presented a suite of six 10Be surface-exposure ages from moraine ridges on Rannoch Moor, central Scottish Highlands, that questioned whether two different dating techniques - 10Be and 14C - yield the same result for the timing of final deglaciation of the Scottish ice cap. In that study, Small and Fabel (2016) concluded that the 10Be data show deglaciation occurred at the close of the Younger Dryas (YD) stadial (∼11.6 kyr), as much as a millennium later than the scenario presented by Bromley et al. (2014) based on minimum-limiting 14C data. While the issue of which, if either, is a more reliable age for deglaciation cannot be resolved fully in a short note, we comment on several points raised by Small and Fabel (2016) and suggest a means to resolve this question.peer-reviewe

In situ10Be production-rate calibration from a 14C-dated late-glacial moraine belt in Rannoch Moor, central Scottish Highlands

An objective of terrestrial in situ cosmogenic nuclide research is to obtain precise and accurate production-rate estimates on the basis of geological calibration sites from a diverse range of latitudes and altitudes. However, a challenge has been to establish production rates on the basis of landforms for which independent ages have been determined directly using absolute isotopic dating techniques. Here we present a 10Be production-rate calibration from a recessional moraine belt located in Rannoch Moor, central Scottish Highlands (56.63°N, 4.77°W; ∼310 330 m a.s.l.). This moraine belt was deposited at the margin of the disintegrating late-glacial West Highland ice field (WHIF) during the final stages of deglaciation. Minimum-limiting 14C dates on macrofossils of the earliest terrestrial vegetation to arrive on the landscape place the timing of moraine abandonment, and hence exposure of morainal boulder surfaces to the cosmic-ray flux, to no later than 12,480 ± 100 calendar years before C.E. 1950 (cal yrs BP). Maximum-limiting 14C dates on marine shells incorporated into basal tills deposited during expansion of the WHIF to its full late-glacial extent place the onset of deglaciation, and thus deglaciation of Rannoch Moor, to no earlier than 12,700 ± 100 cal yrs BP. After removal of a single high-concentration outlier, surface 10Be concentrations of 11 boulders rooted in two sub-parallel moraine ridges exhibit a high degree of internal consistency and affords an arithmetic mean of 6.93 ± 0.24 [x104] atoms g−1 (1σ). This data set yields a site-specific 10Be production rate of 5.50 ± 0.18 at g−1 yr−1, based on the midpoint age 12,590 ± 140 cal yrs BP of the bracketing 14C chronology. Transforming this result to sea-level/high-latitude (SLHL) neutron-spallation 10Be production-rate values using Version 3 of the University of Washington (UW) Online Production-Rate Calculator yields upper and lower bounds, and a mid-point rate. Maximum-limiting SLHL 10Be production rates, based on minimum-limiting 14C age control, are 3.95 ± 0.11 (2.7%) at g−1 yr−1 for the commonly used Lm and St scaling protocols. The corresponding (non-dimensional) correction factor for a reference production rate determined by the LSDn scaling model is 0.79 ± 0.02 (2.7%). Minimum-limiting SLHL reference 10Be production rates, based on maximum-limiting 14C age control, are 3.88 ± 0.11 (2.7%) at g−1 yr−1 (St) and 3.89 ± 0.11 (2.7%) at g−1 yr−1 (Lm). The corresponding correction factor for LSDn scaling is 0.77 ± 0.02 (2.7%). SLHL reference production-rate values based on a midpoint age of 12,590 ± 140 yrs are 3.91 ± 0.11 (2.8%) at g−1 yr−1 (St) and 3.92 ± 0.11 (2.8%) at g−1 yr−1 (Lm). The corresponding correction factor for LSDn scaling is 0.78 ± 0.02. The production-rate calibration data set presented here for Scotland yields SLHL values that agree with those determined from calibration data sets based on directly dated landforms from northeastern North America, the Arctic, the Swiss Alps, the Southern Hemisphere middle latitudes, and from the high tropical Andes. We suggest that this production-rate calibration data set from the central Scottish Highlands, used together with the UW online calculators, will produce accurate 10Be surface-exposure ages in the British Isles.We acknowledge support from the Dan and Betty Churchill Exploration Fund and the Lamont-Doherty Earth Observatory (LDEO) Climate Center. A.E.P and G.R.M.B. each acknowledge support from the LDEO Postdoctoral Fellowship. A.E.P. acknowledges the Lenfest Foundation, the Comer Family Foundation, and the Quesada Family Fund. J.M.S. acknowledges support from the Lamont Climate Center. We thank R. Schwartz and J. Frisch for assistance in the laboratory, as well as D. Duerden, E. Watson, and H. Senn for their help during field work at Rannoch Moor. This is LDEO contribution number 8267.2020-11-2

Interstadial rise and Younger Dryas demise of Scotland\u27s last ice fields

Establishing the atmospheric expression of abrupt climate change during the last glacial termination is key to understanding driving mechanisms. In this paper, we present a new 14C chronology of glacier behavior during late-glacial time from the Scottish Highlands, located close to the overturning region of the North Atlantic Ocean. Our results indicate that the last pulse of glaciation culminated between ~12.8 and ~12.6 ka, during the earliest part of the Younger Dryas stadial and as much as a millennium earlier than several recent estimates. Comparison of our results with existing minimum-limiting 14C data also suggests that the subsequent deglaciation of Scotland was rapid and occurred during full stadial conditions in the North Atlantic. We attribute this pattern of ice recession to enhanced summertime melting, despite severely cool winters, and propose that relatively warm summers are a fundamental characteristic of North Atlantic stadials.his work was supported by NSF grant EAR‐9118375 and National Geographic/WAITT Foundation grant 450‐16. A.E. Putnam acknowledges support from the Comer Family Foundation, the Lenfest Foundation, a Lamont‐Doherty Earth Observatory postdoctoral fellowship, and NSF grant EAR‐1554990. The data reported and discussed in this paper are listed in the references, tables, and supporting information

Late glacial fluctuations of the Laurentide Ice Sheet in the White Mountains of Maine and New Hampshire, U.S.A.

Prominent moraines deposited by the Laurentide Ice Sheet in northern New England document readvances, or stillstands, of the ice margin during overall deglaciation. However, until now, the paucity of direct chronologies over much of the region has precluded meaningful assessment of the mechanisms that drove these events, or of the complex relationships between ice-sheet dynamics and climate. As a step towards addressing this problem, we present a cosmogenic 10Be surface-exposure chronology from the Androscoggin moraine complex, located in the White Mountains of western Maine and northern New Hampshire, as well as four recalculated ages from the nearby Littleton Bethlehem moraine. Seven internally consistent 10Be ages from the Androscoggin terminal moraines indicate that advance culminated ~ 13.2 ± 0.8 ka, in close agreement with the mean age of the neighboring Littleton Bethlehem complex. Together, these two datasets indicate stabilization or advance of the ice-sheet margin in northern New England, at ~ 14 13 ka, during the Allerød/Greenland Interstadial I