Local-scale deposition of surface snow on the Greenland ice sheet

Abstract. Ice cores from polar ice sheets and glaciers are an important climate archive. Snow layers, consecutively deposited and buried, contain climatic information from the time of their formation. However, particularly low-accumulation areas are characterised by temporally intermittent precipitation, which can be further redistributed after initial deposition, depending on the local surface features at different spatial scales. Therefore, the accumulation conditions at an ice core site influence the quantity and quality of the recorded climate signal in proxy records. This study aims to characterise the local accumulation patterns and the evolution of the snow height to describe the contribution of the snow (re-)deposition to the overall noise level in climate records from ice cores. To this end, we applied a structure-from-motion photogrammetry approach to generate near-daily elevation models of the surface snow for a 195 m2 area in the vicinity of the deep drilling site of the East Greenland Ice-core Project in northeast Greenland. Based on the snow height information we derive snow height changes on a day-to-day basis throughout our observation period from May to August 2018 and find an average snow height increase of ∼ 11 cm. The spatial and temporal data set also allows an investigation of snow deposition versus depositional modifications. We observe irregular snow deposition and erosion causing uneven snow accumulation patterns, a removal of more than 60 % of the deposited snow, and a negative relationship between the initial snow height and the amount of accumulated snow. Furthermore, the surface roughness decreased by approximately a factor of 2 throughout the spring and summer season at our study site. Finally, our study shows that structure from motion is a relatively simple method to demonstrate the potential influences of depositional processes on proxy signals in snow and ice.

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Local scale depositional processes of surface snow on the Greenland ice sheet

10.5194/tc-2021-36 ◽

2021 ◽

Author(s):

Alexandra M. Zuhr ◽

Thomas Münch ◽

Hans Christian Steen-Larsen ◽

Maria Hörhold ◽

Thomas Laepple

Keyword(s):

Spatial Scales ◽

Ice Core ◽

Negative Relationship ◽

Greenland Ice Sheet ◽

Ice Cores ◽

Snow Accumulation ◽

Data Set ◽

Climate Signal ◽

Depositional Processes ◽

Snow Deposition

Abstract. Ice cores from polar ice sheets and glaciers are an important climate archive. Snow layers, consecutively deposited and buried, contain climatic information of the time of their formation. However, particularly low-accumulation areas are characterised by temporally intermittent precipitation, which can be further re-distributed after initial deposition. Therefore, the local conditions of accumulation at an ice core site influence the quantity and quality of the recorded climate signal in proxy records. Local surface features at different spatial scales further affect the signal imprint. This study therefore aims to characterise the local accumulation patterns and the evolution of the snow height to describe the contribution of snow (re-)deposition to noise in climate records from ice cores. By using a photogrammetry Structure-from-Motion approach, we generated near-daily elevation models of the snow surface for a 195 m2 area in the vicinity of the deep drilling site of the East Greenland Ice Core Project in northeast Greenland. Based on the snow height information we derived snow height changes on a day-to-day basis throughout our observation period from May to August 2018. Specifically, the average snow height increased by ~11 cm. The spatial and temporal data set allowed an investigation of snow deposition versus depositional modifications. We observed irregular snow deposition, erosion, and the re-distribution of snow, which caused uneven snow accumulation patterns, a removal of more than 60 % of the deposited snow, and a negative relationship between the initial snow height and the amount of accumulated snow. Furthermore, the surface roughness decreased from 4 to 2 cm throughout the spring and summer season at our study site. Finally, our study further shows that our method has several advantages over previous approaches, making it possible to demonstrate the importance of accumulation intermittency, and the potential influences of depositional processes on proxy signals in snow and ice.

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The Effect of Surface Sublimation on the Snow Isotope Signal

10.5194/egusphere-egu21-9701 ◽

2021 ◽

Author(s):

Sonja Wahl ◽

Alexandra Zuhr ◽

Maria Hörhold ◽

Anne-Katrine Faber ◽

Hans Christian Steen-Larsen

Keyword(s):

Isotopic Composition ◽

Climate Modeling ◽

Ice Core ◽

Greenland Ice Sheet ◽

Layer By Layer ◽

Climate Signal ◽

North East ◽

Depositional Processes ◽

Surface Snow ◽

Water Isotope

Post-depositional processes affect the stable water isotope signal of surface snow between precipitation events.&#160;Combined vapor-snow exchange processes and isotope diffusion influence the top layer of snow as well as buried layers below.&#160;This implies, that ice core isotope climate proxy records can not be interpreted as a precipitation weighted temperature signal alone.Here we present to what extend surface sublimation can explain in-situ observed changes of the stable water isotope signal in the snow. We use direct observations of the isotopic composition of the sublimation flux together with surface snow samples taken in the North-East of the Greenland Ice Sheet accumulation zone throughout the summer months of 2019 to demonstrate sublimation impacts. We show that, contrary to the understanding of effectless layer-by-layer removal of snow, sublimation involves fractionation and therefore influences the isotopic composition of the snow.&#160;Complementary measurements of humidity as well as isotope fluxes constrain the local vapor snow exchange and allow for the quantification of post-depositional influences while the snow is exposed to the atmosphere. This improved process understanding of the formation of the climate signal found in snow is important for merging climate modeling and ice core proxies.&#160;

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On the occurrence of annual layers in Dome Fuji ice core early Holocene ice

Climate of the Past ◽

10.5194/cp-11-1127-2015 ◽

2015 ◽

Vol 11 (9) ◽

pp. 1127-1137 ◽

Cited By ~ 5

Author(s):

A. Svensson ◽

S. Fujita ◽

M. Bigler ◽

M. Braun ◽

R. Dallmayr ◽

...

Keyword(s):

Ice Core ◽

Flow Analysis ◽

Ice Cores ◽

Snow Accumulation ◽

Early Holocene ◽

High Temporal Resolution ◽

High Accumulation ◽

Data Set ◽

Antarctic Plateau ◽

Annual Layer

Abstract. Whereas ice cores from high-accumulation sites in coastal Antarctica clearly demonstrate annual layering, it is debated whether a seasonal signal is also preserved in ice cores from lower-accumulation sites further inland and particularly on the East Antarctic Plateau. In this study, we examine 5 m of early Holocene ice from the Dome Fuji (DF) ice core at a high temporal resolution by continuous flow analysis. The ice was continuously analysed for concentrations of dust, sodium, ammonium, liquid conductivity, and water isotopic composition. Furthermore, a dielectric profiling was performed on the solid ice. In most of the analysed ice, the multi-parameter impurity data set appears to resolve the seasonal variability although the identification of annual layers is not always unambiguous. The study thus provides information on the snow accumulation process in central East Antarctica. A layer counting based on the same principles as those previously applied to the NGRIP (North Greenland Ice core Project) and the Antarctic EPICA (European Project for Ice Coring in Antarctica) Dronning Maud Land (EDML) ice cores leads to a mean annual layer thickness for the DF ice of 3.0 ± 0.3 cm that compares well to existing estimates. The measured DF section is linked to the EDML ice core through a characteristic pattern of three significant acidity peaks that are present in both cores. The corresponding section of the EDML ice core has recently been dated by annual layer counting and the number of years identified independently in the two cores agree within error estimates. We therefore conclude that, to first order, the annual signal is preserved in this section of the DF core. This case study demonstrates the feasibility of determining annually deposited strata on the central East Antarctic Plateau. It also opens the possibility of resolving annual layers in the Eemian section of Antarctic ice cores where the accumulation is estimated to have been greater than in the Holocene.

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Exploring the role of snow metamorphism on the isotopic composition of the surface snow at EastGRIP

10.5194/tc-2021-344 ◽

2021 ◽

Author(s):

Romilly Harris Stuart ◽

Anne-Katrine Faber ◽

Sonja Wahl ◽

Maria Hörhold ◽

Sepp Kipfstuhl ◽

...

Keyword(s):

Isotopic Composition ◽

Pore Space ◽

Greenland Ice Sheet ◽

Ice Cores ◽

Water Isotopes ◽

Climate Signal ◽

Depositional Processes ◽

Decay Model ◽

Surface Snow ◽

Snow Metamorphism

Abstract. Stable water isotopes from polar ice cores are invaluable high-resolution climate proxy records. Recent studies have aimed to improve knowledge of how the climate signal is stored in the water isotope record by addressing the influence of post-depositional processes on the surface snow isotopic composition. In this study, the relationship between changes in surface snow microstructure after precipitation/deposition events and water isotopes is explored using measurements of snow specific surface area (SSA). Continuous daily SSA measurements from the East Greenland Ice Core Project site (EastGRIP) situated in the accumulation zone of the Greenland Ice Sheet during the summer seasons of 2017, 2018 and 2019 are used to develop an empirical decay model to describe events of rapid decrease in SSA, driven predominantly by vapour diffusion in the pore space and atmospheric vapour exchange. The SSA decay model is described by the exponential equation SSA(t) = (SSA0 −26.8) e−0.54t + 26.8. The model performance is optimal for daily mean values of surface temperature in the range 0 °C to −25 °C and wind speed < 6 m s−1. The findings from the SSA analysis are used to explore the influence of surface snow metamorphism on altering the isotopic composition of surface snow. It is found that rapid SSA decay events correspond to decreases in d-excess over a 2-day period in 72 % of the samples. Detailed studies using Empirical Orthogonal Function (EOF) analysis revealed a coherence between the dominant mode of variance of SSA and d-excess during periods of low spatial variability of surface snow over the sampling transect, suggesting that processes driving change in SSA also influence d-excess. Our findings highlight the need for future studies to decouple the processes driving surface snow metamorphism in order to quantify the fractionation effect of individual processes on the snow isotopic composition.

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The role of sublimation as a driver of climate signals in the water isotope content of surface snow: Laboratory and field experimental results

10.5194/tc-2021-87 ◽

2021 ◽

Author(s):

Abigail G. Hughes ◽

Sonja Wahl ◽

Tyler R. Jones ◽

Alexandra Zuhr ◽

Maria Hörhold ◽

...

Keyword(s):

Isotopic Composition ◽

Laboratory Experiments ◽

Ice Core ◽

Water Isotopes ◽

Driving Mechanism ◽

Climate Signal ◽

Depositional Processes ◽

Surface Snow ◽

Isotope Content ◽

Water Isotope

Abstract. Ice core water isotope records from Greenland and Antarctica are a valuable proxy for paleoclimate reconstruction, yet the processes inﬂuencing the climate signal stored in the isotopic composition of the snow are being revisited. Apart from precipitation input, post-depositional processes such as wind-driven redistribution and vapor-snow exchange processes at and below the surface are hypothesized to contribute to the isotope climate signal. Recent ﬁeld studies have shown that surface snow isotopes vary between precipitation events and co-vary with vapor isotopes, which demonstrates that vapor- snow exchange is an important driving mechanism. Here we investigate how vapor-snow exchange and sublimation processes inﬂuence the isotopic composition of the snowpack. Controlled laboratory experiments under dry air ﬂow show an increase of snow isotopic composition of up to 8 ‰ δ18O in the uppermost layer, with an attenuated signal down to 3 cm snow depth over the course of 4–6 days. This enrichment is accompanied by a decrease in the second-order parameter d-excess, indicating kinetic fractionation processes. Using a simple mass-balance and diffusion box model in conjunction with our observed laboratory vapor isotope signals, we are able to reproduce the observed changes in the snow. This conﬁrms that sublimation alone can lead to a strong enrichment of stable water isotopes in surface snow and subsequent enrichment in the layers below. To compare laboratory experiments with realistic polar conditions, we completed four 2–3 day ﬁeld experiments at the East Greenland Ice Core Project site (Northeast Greenland) in summer 2019. High-resolution temporal sampling of both natural and isolated snow was conducted under clear-sky conditions, and demonstrated that the snow isotopic composition changes on hourly timescales. A change of snow isotope content associated with sublimation is currently not implemented in isotope-enabled climate models and is not taken into account when interpreting ice core isotopic records. However, our results demonstrate that post-depositional processes such as sublimation play a role in creating the climate signal recorded in the water isotopes in surface snow. This suggests that the ice core water isotope signal may effectively integrate across multiple parameters, and the ice core climate record should be interpreted as such.

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Post-depositional processes visible in the integration of EGRIP high-resolution water isotope record and visual stratigraphy

10.5194/egusphere-egu21-14131 ◽

2021 ◽

Author(s):

Valerie Morris ◽

Julien Westhoff ◽

Bruce Vaughn ◽

Ilka Weikusat ◽

Tyler Jones ◽

...

Keyword(s):

High Resolution ◽

Ice Core ◽

Ice Cores ◽

Strong Relationship ◽

Dark Field ◽

Scanning System ◽

Climate Signal ◽

Depositional Processes ◽

Isotope Record ◽

Water Isotope

With recent advances in analytical techniques, water stable isotope ratios can be measured in astounding detail in ice core records (~mm scale or equivalent to subannual resolution). While this has enabled the study of past climates across a vast range of timescales, the full set of processes driving the highest frequency variability in these water isotope records remains poorly understood. In the EastGRIP ice core, we observe a strong relationship between high-frequency water isotope anomalies (sharp transitions on the scale of cms) and variability in the visual stratigraphy of the ice. The water isotope timeseries reveals these anomalies that would otherwise be missed using traditional lower resolution discrete sampling methods (5-50 cm scale).&#160; A comparison with the dark-field imaging of stratigraphic layers (high-resolution line-scanning system; 50&#181;m/pix) from the EGRIP ice core indicates a correlation between bubble-free ice layers and the sharp transitions observed in the isotope record.&#160; Prior to this comparison, such anomalies in high-resolution isotope records were often dismissed as analytical artifacts. The striking correspondence to the bubble-free ice layers, which is a parameter measured independently from the isotopes, suggests the isotope variability is real. We are investigating a range of depositional and post-depositional processes that may may be able to explain the origin of this variability and its relationship to the physical properties of the ice. This study has implications for frequency analysis of the isotope data, and the related analysis of isotope diffusion and its effects on the recorded climate signal. Understanding these anomalies opens new doors to the interpretation of climate signals in ice cores.

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A 485 year record of atmospheric chloride, nitrate and sulfate: results of chemical analysis of ice cores from Dyer Plateau, Antarctic Peninsula

Annals of Glaciology ◽

10.3189/s0260305500015792 ◽

1995 ◽

Vol 21 ◽

pp. 182-188 ◽

Cited By ~ 5

Author(s):

Jihong Cole Dai ◽

Lonnie G. Thompson ◽

Ellen Mosley-Thompson

Keyword(s):

Sea Water ◽

Accumulation Rate ◽

Linear Regression Analysis ◽

Ice Core ◽

Ice Cores ◽

Snow Accumulation ◽

Sea Salt ◽

Volcanic Emissions ◽

Data Set ◽

Snow Chemistry

Detailed ionic analyses of Dyer Plateau snow show that major soluble impurities in snow consist of sodium (Na+), chloride (Cl−), nitrate (NO3−), sulfate (SO42−), and acidity (H+). The ratios of Na+ to Cl− concentrations are close to that of sea water, indicating little or no fractionation of sea-salt aerosols. The analyses of core sections from three sites along a 10 km transect show that local spatial variation of snow chemistry in this area is minimal and that temporal (decadal, inter-annual and sub-annual) variations in snow chemistry are very well preserved.Anion analyses of the upper 181 m section of two 235 m ice cores yield a data set of 485 years (1505-1989) of annual snow accumulation and fluxes of Cl−, NO3−, and non-sea-salt (nss) SO42−. No significant long-term trends are observed in any of the anion fluxes. This is consistent with other Antarctic ice-core records showing no significant anthropogenic atmospheric pollution in the high southern latitudes. Linear regression analysis shows that Cl− flux is independent of snow-accumulation rate. Significant positive correlations are found between accumulation rate and both NO3− flux and background nss-SO42− flux. These results suggest that dry deposition is primarily responsible for air-to-ground Cl− flux while wet deposition dominates the NO3− and nss-SO42− flux (≥90% and ≥75%, respectively). The nss-S042− fluxes provide a chronology of explosive volcanic emissions reaching the Antarctic region for the past 485 years.

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Quantifying the role of post-depositional processes on the isotopic composition of surface snow – new findings from the SNOWISO project

10.5194/egusphere-egu2020-11774 ◽

2020 ◽

Author(s):

Hans Christian Steen-Larsen ◽

Maria Hörhold ◽

Sonja Wahl ◽

Abigail Hughes ◽

Anne-Katrine Faber ◽

...

Keyword(s):

Isotopic Composition ◽

Time Scales ◽

Ice Core ◽

Atmospheric Temperature ◽

Snow Surface ◽

Climate Signal ◽

Depositional Processes ◽

Surface Snow ◽

Temperature And Humidity

The goal of the SNOWISO project is to quantify the role of the post-depositional processes, which are influencing the isotopic composition of the surface snow and hence the ice core water isotope climate signal. Here we are reporting on findings from field campaigns carried out at EastGRIP over the four summers 2016-2019. We have collected a suite of observations containing the isotopic composition of the surface snow and the snowpack, together with direct observations of atmospheric water vapor isotopes and fluxes between the snow surface and the atmosphere. To support the analysis of the isotopic data we also collected meteorological observations comprising of atmospheric temperature and humidity gradients alongside with sub-surface and snow surface temperature along with atmospheric temperature and humidity gradients. With this dataset we are able to document significant changes in the snow isotopic composition, which are driven by post-depositional processes. The changes in the snow surface isotopic composition is observed to occur on time scales ranging from diurnal to several days. The changes in the snow surface isotopic composition is observed to occur on time scales ranging from diurnal to several days. We can show that the changes in the snow surface is consistent with the flux of the isotopologues between the snow surface and the atmosphere. This gives us confidence that we will be able to develop parameterizations of post-depositional effects, and model their influence on the ice core isotopic climate signal.<div>&#160;</div><div>&#160;</div>

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A 485 year record of atmospheric chloride, nitrate and sulfate: results of chemical analysis of ice cores from Dyer Plateau, Antarctic Peninsula

Annals of Glaciology ◽

10.1017/s0260305500015792 ◽

1995 ◽

Vol 21 ◽

pp. 182-188 ◽

Cited By ~ 22

Author(s):

Jihong Cole Dai ◽

Lonnie G. Thompson ◽

Ellen Mosley-Thompson

Keyword(s):

Sea Water ◽

Accumulation Rate ◽

Linear Regression Analysis ◽

Ice Core ◽

Ice Cores ◽

Snow Accumulation ◽

Sea Salt ◽

Volcanic Emissions ◽

Data Set ◽

Snow Chemistry

Detailed ionic analyses of Dyer Plateau snow show that major soluble impurities in snow consist of sodium (Na+), chloride (Cl−), nitrate (NO3 −), sulfate (SO4 2−), and acidity (H+). The ratios of Na+ to Cl− concentrations are close to that of sea water, indicating little or no fractionation of sea-salt aerosols. The analyses of core sections from three sites along a 10 km transect show that local spatial variation of snow chemistry in this area is minimal and that temporal (decadal, inter-annual and sub-annual) variations in snow chemistry are very well preserved. Anion analyses of the upper 181 m section of two 235 m ice cores yield a data set of 485 years (1505-1989) of annual snow accumulation and fluxes of Cl−, NO3 −, and non-sea-salt (nss) SO4 2−. No significant long-term trends are observed in any of the anion fluxes. This is consistent with other Antarctic ice-core records showing no significant anthropogenic atmospheric pollution in the high southern latitudes. Linear regression analysis shows that Cl− flux is independent of snow-accumulation rate. Significant positive correlations are found between accumulation rate and both NO3 − flux and background nss-SO4 2− flux. These results suggest that dry deposition is primarily responsible for air-to-ground Cl− flux while wet deposition dominates the NO3 − and nss-SO4 2− flux (≥90% and ≥75%, respectively). The nss-S04 2− fluxes provide a chronology of explosive volcanic emissions reaching the Antarctic region for the past 485 years.

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Reconstructing Antarctic snow accumulation using nitrogen isotopes of nitrate

10.5194/egusphere-egu21-672 ◽

2021 ◽

Author(s):

Pete D. Akers ◽

Joël Savarino ◽

Nicolas Caillon ◽

Mark Curran ◽

Tas Van Ommen

Keyword(s):

Nitrogen Isotopes ◽

Full Range ◽

Ice Core ◽

Ice Cores ◽

Snow Accumulation ◽

East Antarctica ◽

Accumulation Rates ◽

Sea Levels ◽

Antarctic Snow ◽

Parallel Reconstruction

Precise Antarctic snow accumulation estimates are needed to understand past and future changes in global sea levels, but standard reconstructions using water isotopes suffer from competing isotopic effects external to accumulation. We present here an alternative accumulation proxy based on the post-depositional photolytic fractionation of nitrogen isotopes (d15N) in nitrate. On the high plateau of East Antarctica, sunlight penetrating the uppermost snow layers converts snow-borne nitrate into nitrogen oxide gas that can be lost to the atmosphere. This nitrate loss favors 14NO3- over 15NO3-, and thus the d15N of nitrate remaining in the snow will steadily increase until the nitrate is eventually buried beneath the reach of light. Because the duration of time until burial is dependent upon the rate of net snow accumulation, sites with lower accumulation rates have a longer burial wait and thus higher d15N values. A linear relationship (r2 = 0.86) between d15N and net accumulation-1 is calculated from over 120 samples representing 105 sites spanning East Antarctica. These sites largely encompass the full range of snow accumulation rates observed in East Antarctica, from 25 kg m-2 yr-1 at deep interior sites to >400 kg m-2 yr-1 at near coastal sites. We apply this relationship as a transfer function to an Aurora Basin ice core to produce a 700-year record of accumulation changes. Our nitrate-based estimate compares very well with a parallel reconstruction for Aurora Basin that uses volcanic horizons and ice-penetrating radar. Continued improvements to our database may enable precise independent estimates of millennial-scale accumulation changes using deep ice cores such as EPICA Dome C and Beyond EPICA-Oldest Ice.

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