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Abstract. In situ cosmogenic 14C (in situ 14C) in quartz provides a recently developed tool to date exposure of bedrock surfaces of up to ∼ 25 000 years. From outcrops located in east-central Sweden, we tested the accuracy of in situ 14C dating against (i) a relative sea level (RSL) curve constructed from radiocarbon dating of organic material in isolation basins and (ii) the timing of local deglaciation constructed from a clay varve chronology complemented with traditional radiocarbon dating. Five samples of granitoid bedrock were taken along an elevation transect extending southwestwards from the coast of the Baltic Sea near Forsmark. Because these samples derive from bedrock outcrops positioned below the highest postglacial shoreline, they target the timing of progressive landscape emergence above sea level. In contrast, in situ 14C concentrations in an additional five samples taken from granitoid outcrops above the highest postglacial shoreline, located 100 km west of Forsmark, should reflect local deglaciation ages. The 10 in situ 14C measurements provide robust age constraints that, within uncertainties, compare favourably with the RSL curve and the local deglaciation chronology. These data demonstrate the utility of in situ 14C to accurately date ice sheet deglaciation, and durations of postglacial exposure, in regions where cosmogenic 10Be and 26Al routinely return complex exposure results. 

Abstract. Over the last 30 years, in situ cosmogenic nuclides (CNs) have revolutionizedsurficial processes and Quaternary geologic studies. Commonly measured CNsextracted from common mineral quartz have long half-lives (e.g.,10Be, 26Al) and have been applied over timescales from a fewhundred years to millions of years. However, their long half-lives alsorender them largely insensitive to complex histories of burial and exposure ofless than ca. 100 kyr. On the other hand, in situ cosmogenic 14C (in situ 14C) isalso produced in quartz, yet its 5.7 kyr half-life renders it very sensitiveto complex exposure histories during the last ∼25 ka, aparticularly unique and powerful tool when analyzed in concert withlong-lived nuclides. In situ 14C measurements are currently limited torelatively coarse-grained (typically sand-sized or larger, crushed or sieved tosand) quartz-bearing rock types, but while such rocks are common, they arenot ubiquitous. The ability to extract and interpret in situ 14C fromquartz-poor and fine-grained rocks would thus open its unique applicationsto a broader array of landscape elements and environments. As a first step toward this goal, a robust means of interpreting in situ 14Cconcentrations derived from rocks and minerals spanning wider compositionaland textural ranges will be crucial. We have thus developed aMATLAB®-based software framework to quantifyspallogenic production of in situ 14C from a broad range of silicate rock andmineral compositions, including rocks too fine grained to achieve purequartz separates. As expected from prior work, production from oxygendominates the overall in situ 14C signal, accounting for >90 %of production for common silicate minerals and six different rock types atsea level and high latitudes (SLHL). This work confirms that Si, Al, and Mgare important targets but also predicts greater production from Na thanfrom those elements. The compositionally dependent production rates for rockand mineral compositions investigated here are typically lower than that ofquartz, although that predicted for albite is comparable to quartz,reflecting the significance of production from Na. Predicted productionrates drop as compositions become more mafic (particularly Fe-rich). This framework should thus be a useful tool in efforts to broaden the utility ofin situ 14C to quartz-poor and fine-grained rock types, but futureimprovements in measured and modeled excitation functions would bebeneficial. 

Abstract. Extraction procedures for in situ cosmogenic 14C (in situ 14C) fromquartz require quantitative isotopic yields while maintaining scrupulousisolation from atmospheric and organic 14C. These time- and labor-intensiveprocedures are ripe for automation; unfortunately, our original automatedin situ 14C extraction and purification systems, reconfigured and retrofittedfrom our original systems at the University of Arizona, proved less reliablethan hoped. We therefore installed a fully automated stainless-steel system(except for specific borosilicate glass or fused-silica components)incorporating more reliable valves and improved actuator designs, along witha more robust liquid nitrogen distribution system. As with earlier versions,the new system uses a degassed lithium metaborate (LiBO2) flux to dissolvethe quartz sample in an ultra-high-purity oxygen atmosphere, after alower-temperature combustion step to remove atmospheric and organic 14C. We compared single-use high-purity Al2O3 against reusable90 %Pt / 10 %Rh (Pt/Rh) sample combustion boats. The Pt/Rh boats heat moreevenly than the Al2O3, reducing procedural blank levels andvariability for a given LiBO2 flux. This lower blank variability alsoallowed us to trace progressively increasing blanks to specific batches offluxes from our original manufacturer. Switching to a new manufacturerreturned our blanks to consistently low levels on the order of (3.4 ± 0.9) × 104 14C atoms. We also analyzed the CRONUS-A intercomparison material to investigatesensitivity of extracted 14C concentrations to the temperature andduration of the combustion and extraction steps. Results indicate that 1 hcombustion steps at either 500 or 600 ∘C yield results consistentwith the consensus value of Jull et al. (2015), while 2 hat 600 ∘C results in loss of ca. 9 % of the high-temperature14C inventory. Results for 3 h extractions at temperatures rangingfrom 1050 to 1120 ∘C and 4.5 h at 1000 ∘Cyielded similar results that agreed with the nominal value andpublished results from most laboratories. On the other hand, an extractionfor 3 h at 1000 ∘C was judged to be incomplete due to asignificantly lower measured concentration. Based on these results, ourpreferred technique is now combustion for 1 h at 500 ∘C followedby a 3 h extraction at 1050 ∘C. Initial analyses of the CoQtz-Nintercomparison material at our lab yielded concentrations ca. 60 % lowerthan those of CRONUS-A, but more analyses of this material from this andother labs are clearly needed to establish a consensus value. 

Abstract The impact of late Cenozoic climate on the East Antarctic Ice Sheet is uncertain. Poorly constrained patterns of relative ice thinning and thickening impair the reconstruction of past ice-sheet dynamics and global sea-level budgets. Here we quantify long-term ice cover of mountains protruding the ice-sheet surface in western Dronning Maud Land, using cosmogenic Chlorine-36, Aluminium-26, Beryllium-10, and Neon-21 from bedrock in an inverse modeling approach. We find that near-coastal sites experienced ice burial up to 75–97% of time since 1 Ma, while interior sites only experienced brief periods of ice burial, generally <20% of time since 1 Ma. Based on these results, we suggest that the escarpment in Dronning Maud Land acts as a hinge-zone, where ice-dynamic changes driven by grounding-line migration are attenuated inland from the coastal portions of the East Antarctic Ice Sheet, and where precipitation-controlled ice-thickness variations on the polar plateau taper off towards the coast.