Origin of englacial stratigraphy at three deep ice core sites of the Greenland Ice Sheet by synthetic radar modelling

During the past 20 years, multi-channel radar emerged as a key tool for deciphering an ice sheet's internal architecture. To assign ages to radar reflections and connect them over large areas in the ice sheet, the layer genesis has to be understood on a microphysical scale. Synthetic radar trace modelling based on the dielectric profile of ice cores allows for the assignation of observed physical properties’ variations on the decimetre scale to radar reflectors extending from the coring site to a regional or even whole-ice-sheet scale. In this paper we rely on the available dielectric profiling data of the northern Greenland deep ice cores: NGRIP, NEEM and EGRIP. The three records are well suited for assigning an age model to the stratigraphic radar-mapped layers, and linking up the reflector properties to observations in the cores. Our modelling results show that the internal reflections are mainly due to conductivity changes. Furthermore, we deduce fabric characteristics at the EGRIP drill site from two-way-travel-time differences of along and across-flow polarized radarwave reflections of selected horizons (below 980 m). These indicate in deeper parts of the ice column an across-flow concentrated c-axis fabric

Volcanism and the Greenland ice cores: A new tephrochronological framework for the last glacial-interglacial transition (LGIT) based on cryptotephra deposits in three ice cores

Chemical profiles from Greenland ice cores show that the frequency of volcanism was higher during the last glacial-interglacial transition (LGIT) and early Holocene, (17–9 ka b2k) than in any other period during the last 110 kyr. This increased frequency has partly been linked to climate-driven melting of the Icelandic ice sheet during the last deglaciation, with regional isostatic changes thought to alter mantle viscosity and lead to more eruptions. Our study is the first to construct a comprehensive tephrochronological framework from Greenland ice cores over the LGIT to aid in the reconstruction of volcanic activity over this period. The framework is based on extensive high-resolution sampling of three Greenland ice cores between 17.4 and 11.6 ka b2k and comprises a total of 64 cryptotephra deposits from the NGRIP, GRIP and NEEM ice cores. We show that many of these tephras are preserved within the core without an associated chemical signature in the ice, which implies that reconstructions of volcanism based solely on glacio-chemical indicators might underestimate the number of events. Single glass shards from each deposit were geochemically characterised to trace the volcanic source and many of these deposits could be correlated between cores. We show that the 64 deposits represent tephra deposits from 42 separate volcanic events, and of these, 39 are from Iceland, two from the north Pacific region (Japan and USA) and one has an unknown source. Six deposits can be correlated to terrestrial and/or marine tephra deposits in the Northern Hemisphere and the remaining 36 are unreported in other archives. We did not locate tephra from the compositionally distinctive Laacher See eruption (∼13 ka b2k) in our records. Combining our new discoveries with the previously published tephra framework, raises the number of individual tephra horizons found in Greenland ice over this interval to 50. This significantly improves the regional tephrochronological framework, our knowledge of the eruptive history of Iceland during the LGIT and provides new tephra constraints over key LGIT climate events. Consequentially, this framework can guide sampling strategies of future tephra studies in the terrestrial and marine realms aiming to link these records to the Greenland ice cores to assess regional climate synchroneity

A first chronology for the East Greenland Ice-core Project (EGRIP) over the Holocene and last glacial termination

This paper provides the first chronology for the deep ice core from the East Greenland Ice-core Project (EGRIP) over the Holocene and the late last glacial period. We rely mainly on volcanic events and common peak patterns recorded by dielectric profiling (DEP) and electrical conductivity measurement (ECM) for the synchronization between the EGRIP, North Greenland Eemian Ice Drilling (NEEM) and North Greenland Ice Core Project (NGRIP) ice cores in Greenland. We transfer the annual-layer-counted Greenland Ice Core Chronology 2005 (GICC05) from the NGRIP core to the EGRIP ice core by means of 381 match points, typically spaced less than 50 years apart. The NEEM ice core has previously been dated in a similar way and is only included to support the match-point identification. We name our EGRIP timescale GICC05-EGRIP-1. Over the uppermost 1383.84 m, we establish a depth–age relationship dating back to 14 967 years b2k (years before the year 2000 CE). Tephra horizons provide an independent validation of our match points. In addition, we compare the ratio of the annual layer thickness between ice cores in between the match points to assess our results in view of the different ice-flow patterns and accumulation regimes of the different periods and geographical regions. For the next years, this initial timescale will be the basis for climatic reconstructions from EGRIP high-resolution proxy data sets, e.g. stable water isotopes, chemical impurity or dust records

Permittivity measured with the dielectric profiling (DEP) technique on the Talos Dome ice core, Antarctica

Dielectric Profiling (DEP) of the Talos Dome Ice Core (TALDICE) were recorded in the field during the 2005-2007 field seasons with the DEP device described by Wilhelms et al. (1998). The permittivity and conductivity of DEP data are calculated by their respective densities and conductivities (Wilhelms, 2005). The resolution of DEP data is 5 mm depth intervals. The DEP was not processed at a consistent temperature due to the varying temperature in the field seasons. For more information on the calibration procedure see Mojtabavi et al, 2020

A first chronology for the East Greenland Ice core Project (EastGRIP)

Polar ice cores are unique archives recording immediate past atmospheric conditions. They provide, most prominently, reconstructions of past temperature evolution, volcanic eruptions, greenhouse gas concentrations and many other parameters that determine the environmental conditions. The dating of the ice core provides the respective chronology to the recorded past climate conditions. Here, we establish a first chronology for the East Greenland Ice-core Project (EGRIP) core based on matching of electrical conductivity peaks of volcanic origin as recorded by dielectric profiling (DEP) in the EGRIP, NGRIP and NEEM ice cores from Greenland. Our initial dating relies on the transfer of the Greenland Ice Core Chronology 2005 (GICC05) from the NGRIP and NEEM cores to the uppermost 352 m of the EGRIP core from the 2017 field season

DiElectric Profiling with rapid access drilling: in-situ DEP in the borehole

Dielectric profiling (DEP) is a fast, non-destructive method to precisely scan the dielectric properties of an ice core in as high depth resolution as a few millimetres. Initially being proposed to acquire conductivity profiles, the method has been extended to also interpret relative permittivity and ultimately determine the firn’s density and its pure ice phase’s conductivity by inversion of the measured properties with the specifically developed mixing model DECOMP. The conductivity profile itself exhibits time markers from volcanic eruptions and facilitates, by integrating the density along depth, the calculation of average accumulation rates in between these time markers. The modelling of synthetic radar traces is another application of dielectric profiling data, that establishes a high precision link between the icecore and geophysical ground penetrating radar surveys in the vicinity of the drilling site. In the light of these applications the aim to log the dielectric properties directly in the borehole has been out there for a few decades. Mainly motivated in acquiring records without missing data due to core breaks and to even better extent the depth scale beyond sections of possibly missing core sections. Along with the development of rapid access drilling, where no ice core but only cuttings are taken, there is an even more urgent desire to acquire the high quality electrical data directly from in-situ measurements in the borehole. The traditional DEP method, as described in the literature, uses commercial auto-balancing-bridge devices to measure the electrical circuit properties of the DEP capacitor with the icecore as a dielectric filling. The commercial devices typically have 19” rack size dimensions and extensive circuit design would have been required to adapt the electronics to fit into a typically 100 mm diameter tube of a borehole logger. We developed a lock-in amplifier based electronics with a small circuit layout and successfully tested it in an icecore DEP device . We will inter-compare the records with the results of an auto-balancing-bridge based measurement and estimate the expected measurement performance of our new lock-in amplifier circuit. The electrostatic theory to determine the borehole-wall ice properties has been described before and allows to predict the precision of the ice parameters as measured in a borehole log. We will also lay out the mechanical and electrical design to build and operate a borehole DEP in a dry and a liquid filled hole