Reply on RC2

Julien Westhoff

doi:10.5194/cp-2021-89-ac2

Abstract

We present a record of melt events obtained from the EastGRIP ice core, in central north eastern Greenland, covering the largest part of the Holocene. The data were acquired visually using an optical dark-field line scanner. We detect and describe bubble free layers and -lenses throughout the ice above the bubble-clathrate transition, located at 1100 m in the EastGRIP ice core, corresponding to an age of 9720 years b2k. We distinguish between melt layers (bubble free layers continuous over the width of the core), melt lenses (discontinuous), crusts (thin and sharp bubble free layers) and attribute three levels of confidence to each of these, depending on how clearly they are identified. Our record of melt events shows a large, distinct peak around 1014 years b2k (986 CE) and a broad peak around 7000 years b2k corresponding to the Holocene Climatic Optimum. We analyze melt layer thicknesses and correct for ice thinning, we account for missing layers due to core breaks, and ignore layers thinner than 1.5 mm. We define the brittle zone in the EastGRIP ice core from 650 m to 950 m depth, where we count on average more than three core breaks per meter. In total we can identify approximately 831 mm of melt (corrected for thinning) over the past 10,000 years. We compare our melt layer record to the GISP2 and Renland melt layer records. Our climatic interpretation matches well with the Little Ice Age, the Medieval and Roman Warm Periods, the Holocene Climatic Optimum, and the 8.2 kyr event. We also compare the most recent 2500 years to a tree ring composite and find an overlap between melt events and tree ring anomalies indicating warm summers. We open the discussion for sloping bubble free layers (tilt angle off horizontal > 10°) being the effect of rheology and not climate. We also discuss our melt layers in connection to a coffee experiment (coffee as a colored substitute for melt infiltration into the snow pack) and the real time observations of the 2012 CE rain event at NEEM. We find that the melt event from 986 CE is most likely a large rain event, similar to 2012 CE, and that these two events are unprecedented throughout the Holocene. Furthermore, we suggest that the warm summer of 986 CE, with the exceptional melt event, was the trigger for the first Viking voyages to sail from Iceland to Greenland.

Highlights

1.1 Greenland melt layer recordsMelt layers provide a robust record of warm summer days on the Greenland ice sheet, meaning that the surface temperature 25 exceeded the melting point and/or that insolation was very high (Langway and Shoji, 1990)
We present a record of melt events obtained from the EastGRIP ice core, in central north eastern Greenland, covering the largest part of the Holocene
We find that the melt event from 986 CE is most likely a large rain event, similar to 2012 CE, and that these two events are unprecedented throughout the Holocene

Summary

Introduction

1.1 Greenland melt layer recordsMelt layers provide a robust record of warm summer days on the Greenland ice sheet, meaning that the surface temperature 25 exceeded the melting point and/or that insolation was very high (Langway and Shoji, 1990). The first full Holocene melt layer record from a Greenlandic ice core was presented by Alley and Anandakrishnan (1995) on the Greenland Ice Sheet Project 2 (GISP2) ice core, who applied visual inspection during ice core processing. Visual inspection was used by Herron et al (1981), some years earlier, on the DYE3 ice core from southern Greenland, to create a 2200 year melt record. Studies of melt 35 features in the ablation zone of the Greenland ice sheet have been conducted using multiple shallow ice cores (e.g. Graeter et al, 2018), or snow pits (e.g. Humphrey et al, 2012). Combined micro computer tomography (CT, Schaller et al, 2016) and visual analysis using line scan images (see methods section) for melt layer detection was applied on the Renland Ice Cap (RECAP) ice core, coastal eastern Greenland, by Taranczewski et al (2019), combining one deep and two shallow cores. Melt layer records have been established for many glaciated sites around the world, e.g. in Canada (Koerner and Fisher, 1990; Fisher 40 et al, 1995, 2012), Alaska (Winski et al, 2018), and Arctic Russia (Fritzsche et al, 2005)

Methods

Results

Conclusion