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Abstract. Investigating North Pacific climate variability during warmintervals prior to the Common Era can improve our understanding of thebehavior of ocean–atmosphere teleconnections between low latitudes and theArctic under future warming scenarios. However, most of the existing icecore records from the Alaskan and Yukon region only allow access to climateinformation covering the last few centuries. Here we present asurface-to-bedrock age scale for a 210 m long ice core recovered in 2013from the summit plateau of Begguya (Mt. Hunter; Denali National Park,Central Alaska). Combining dating by annual layer counting with absolutedates from micro-radiocarbon dating, a continuous chronology for the entireice core archive was established using an ice flow model. Calibrated14C ages from the deepest section (209.1 m, 7.7 to 9.0 ka cal BP)indicate that basal ice on Begguya is at least of early Holocene origin. Aseries of samples from a shallower depth interval (199.8 to 206.6 m) weredated with near-uniform 14C ages (3 to 5 ka cal BP). Our resultssuggest this may be related to an increase in annual net snow accumulationrates over this period following the Northern Hemisphere Holocene ClimateOptimum (around 8 to 5 kyr BP). With absolute dates constraining thetimescale for the last >8 kyr BP, this paleo-archive will allowfuture investigations of Holocene climate and the regional evolution ofspatial and temporal changes in atmospheric circulation and hydroclimate inthe North Pacific.

US maize and soy production have increased rapidly since the mid-20th century. While global warming has raised temperatures in most regions over this time period, trends in extreme heat have been smaller over US croplands, reducing crop-damaging high temperatures and benefiting maize and soy yields. Here we show that agricultural intensification has created a crop-climate feedback in which increased crop production cools local climate, further raising crop yields. We find that maize and soy production trends have driven cooling effects approximately as large as greenhouse gas induced warming trends in extreme heat over the central US and substantially reduced them over the southern US, benefiting crops in all regions. This reduced warming has boosted maize and soy yields by 3.3 (2.7–3.9; 13.7%–20.0%) and 0.6 (0.4–0.7; 7.5%–13.7%) bu/ac/decade, respectively, between 1981 and 2019. Our results suggest that if maize and soy production growth were to stagnate, the ability of the crop-climate feedback to mask warming would fade, exposing US crops to more harmful heat extremes.

Hydrogen (δD) and oxygen (δ¹⁸O) isotopic ratios are strongly correlated in precipitation over time and space, defining the meteoric water line, and the slope of this δD‐δ¹⁸O relationship reflects covariations of deuterium excess (d‐excess) with δD or δ¹⁸O. This δD‐δ¹⁸O line provides a tool for inferring hydrologic processes from the evaporation source to condensation site. Here, we present δD‐δ¹⁸O relationships on seasonal and annual timescales for daily precipitation, snow pits, and a 15‐m ice core (Owen) at Summit, Greenland. Seasonally, precipitation δD‐δ¹⁸O slopes are less than 8 (summer = 7.70; winter = 7.77), while the annual slope is greater than 8 (8.27). We suggest that intra‐season slopes result primarily from Rayleigh distillation, which, under prevailing conditions, produces slopes less than 8. The summer line has a greater intercept (higher d‐excess) than the winter line. This separation causes annual slopes to be greater than seasonal ones. We attribute high summer d‐excess primarily to contributions of vapor sublimated from the Greenland Ice Sheet and other terrestrial sources. High sublimated moisture proportions result in a large separation between seasonal δD‐δ¹⁸O lines, and thus high annual slopes. Inter‐seasonal weighting of precipitation amount also influences annual slopes because slopes are weighed by the number of storms each season. Using snow pit measurements, we demonstrate that precipitation isotopic signals translate to the snowpack. We generate indices to determine Sublimation Proportion Index and Precipitation Weighting Index, and find that annual Owen core δD‐δ¹⁸O line slopes are significantly related to these indices, demonstrating that these factors are recorded in ice cores.

The relationship between firn microstructure and water movement is complex: firn microstructure controls the routing of meltwater through the firn while continuously being altered by liquid water flow processes. Importantly, microstructural transitions within the firn column can stall vertical meltwater percolation, which creates heterogeneities in liquid water content resulting in different rates of firn metamorphism. Physics‐based firn models aim to describe these processes to accurately predict ice layer or firn aquifer formation, but require detailed observations of firn structure to validate and inform percolation schemes. Here, we present grain size measurements and ice layer stratigraphy from seven firn cores collected in western Greenland's percolation zone during the 2016 Greenland Traverse for Accumulation and Climate Studies (GreenTrACS). Grain size transitions within the cores are negatively correlated with all temperature proxies for meltwater supply. Additionally, the number of grain size transitions are strongly anticorrelated with the number of ice layers within each core, despite these transitions, particularly fine‐over‐coarse transitions, promoting meltwater ponding and potential ice layer formation. To investigate if these negative correlations can be understood with firn model physics, we simulate water movement along stratigraphic transitions using the SNOWPACK model. We find that grain size transitions diminish from rapid grain growth in wet firn where ice layers can form, suggesting these microstructural transitions are unlikely to survive repeated meltwater infiltration. Incorporating these microstructure—meltwater feedbacks in firn models could improve their ability to model processes such as ice slab formation or firn aquifer recharge that require accurate predictions of meltwater infiltration depth.

A large volcanic sulfate increase observed in ice core records around 1450 C.E. has been attributed in previous studies to a volcanic eruption from the submarine Kuwae caldera in Vanuatu. Both EPMA–WDS (electron microprobe analysis using a wavelength dispersive spectrometer) and SEM–EDS (scanning electron microscopy analysis using an energy dispersive spectrometer) analyses of five microscopic volcanic ash (cryptotephra) particles extracted from the ice interval associated with a rise in sulfate ca. 1458 C.E. in the South Pole ice core (SPICEcore) indicate that the tephra deposits are chemically distinct from those erupted from the Kuwae caldera. Recognizing that the sulfate peak is not associated with the Kuwae volcano, and likely not a large stratospheric tropical eruption, requires revision of the stratospheric sulfate injection mass that is used for parameterization of paleoclimate models. Future work is needed to confirm that a volcanic eruption from Mt. Reclus is one of the possible sources of the 1458 C.E. sulfate anomaly in Antarctic ice cores.