Signals from the Surface Snow: Post-Depositional Processes Controlling the Ice Core Isotopic Fingerprint

Informazioni relative al progetto

SNOWISO

ID dell’accordo di sovvenzione: 759526

DOI

10.3030/759526

Progetto chiuso

Data della firma CE 7 Settembre 2017

Data di avvio 1 Gennaio 2018

Data di completamento 30 Giugno 2024

Finanziato da

EXCELLENT SCIENCE - European Research Council (ERC)

Costo totale

€ 1 497 260,00

Contributo UE

€ 1 497 260,00

1 497 260,00

Coordinato da

UNIVERSITETET I BERGEN
Norway

Periodic Reporting for period 4 - SNOWISO (Signals from the Surface Snow: Post-Depositional Processes Controlling the Ice Core IsotopicFingerprint)

Periodo di rendicontazione: 2022-07-01 al 2024-06-30

For the past 50 years, our use of ice core records as climate archives has relied on the fundamental assumption that the isotopic composition of precipitation deposited on the ice sheet surface determines the ice core water isotopic composition. Since the isotopic composition in precipitation is assumed to be governed by the state of the climate this has made ice core isotope records one of the most important proxies for reconstructing the past climate. Society is faced with the consequences of climate change. Knowledge about how the past climate has varied before anthropogenic emissions of greenhouse gasses will inform us about the internal dynamics, which govern the climate. Understanding these dynamics better will allow for climate predictions to improve.

New simultaneous measurements of snow and water vapor isotopes have shown that the surface snow exchanges with the atmospheric water vapor isotope signal, altering the deposited precipitation isotope signal. This severely questions the standard paradigm for interpreting the ice core proxy record and gives rise to the hypothesis that the isotope record from an ice core is determined by a combination of the atmospheric water vapor isotope signal and the precipitation isotope signal.

The SNOWISO project has verify this new hypothesis by combining laboratory and field experiments with in-situ observations of snow and water vapor isotopes in Greenland and Antarctica. This has enabled us to quantify and parameterize the snow-air isotope exchange and post-depositional processes. SNOWISO has implemented these results into a snowpack module coupled with a regional and global general circulation model and benchmarked against in-situ observations. Using the coupled snow-atmosphere isotope model SNOWISO has establish the magnitude of the isotopic shift due to post-depositional processes under different environmental conditions.

By establishing how the water isotope signal is recorded in the snow, the SNOWISO project has built the foundation for future integration of isotope-enabled General Circulation Models with ice core records; this opens a new frontier in climate reconstruction.

The backbone of the SNOWISO project is the in situ observations of the snow and atmospheric conditions on top of the Greenland and Antarctic Ice Sheets carried out during field campaigns. The field work has spanned four individual campaigns with three campaigns in Greenland at the EastGRIP station and one campaign in Antarctica at the Kohnen station. The data gathered have consisted of both atmospheric water vapor isotope and snowpack water isotope measurements in addition to measurements of water fluxes between the snow surface and the atmosphere.

A main goal for the sampling of water isotope data on top of the ice sheets has been focused on closing the water isotope budget i.e. making sure that we are able to link changes in the snow pack isotopes with the flux of water isotopes between the snow surface and atmosphere. To achieve this goal, SNOWISO has been developing novel methodology for directly measuring the water isotope flux by combining three-dimensional wind measurements with laser spectroscopy isotope measurements on the water vapor above the snow. The suite of measurements has then been used as datasets to test our understanding of the processes responsible for the post-depositional processes.

A secondary objective of the SNOWISO project has been to develop a snowpack water isotope model that incorporates the relevant processes driving changes in snow isotopic composition, allowing for the simulation of snow isotopic composition. This model development has been linked with the field observation focused on linking the snow surface water isotope signal with the climate signal that is being archived in the snow pack and subsequently is making up the ice core water isotope climate record. To do this, SNOWISO has collected more than ten thousand samples of the snow surface and snowpack in order to have a sufficient dataset to separate the climate process signal from the deposition noise originating when the snow is deposited and redistributed on the surface.

The SNOWISO snowpack model has been coupled with inputs from both global isotope-enabled General Circulation Models and with snow-atmosphere exchange processes from regional climate models. Using this model we have been able to document the influence both spatially and temporally of post-depositional processes on the recorded water isotope climate signal in the ice from both Greenland and Antarctica. The result shows that the alteration of the snow isotopic composition compared to the initial precipitation isotope signal is significant on both annual and seasonal time scales. Our results exemplify the importance of taking into consideration post-depositional processes, when interpreting ice core climate records from the water isotopic composition.

The SNOWISO project has gone beyond the state of the art by challenging the fundamental assumption by which ice core isotope records have been interpreted based on for the last 50 years. Through a combination of direct field observations, the collection of an extensive sample set, and targeted modeling, the SNOWISO project has established that post-depositional processes play a significant role in creating the climate signal recorded by the ice core water isotope record. To achieve this result, the SNOWISO project has developed and executed cutting-edge measurement campaigns on top of the Greenland and Antarctic Ice Sheet, to obtain directly quantifiable evidence of post-depositional processes and their role in driving the climate signal recorded in the ice core.

Specifically, the SNOWISO project has documented: 1) the existence of isotopic fractionation during snow sublimation based on direct observations, 2) that the isotopic signal of the original precipitated snow alters on daily time scale, 3) that using our understanding of the key driving processes we can explain up to 50% of the day to day variability, 4) through direct observations that post-depositional processes significantly alters the mean annual and seasonal isotope value, 5) that post-depositional processes are not only influencing the mean isotopic composition but also the inter-annual variability, and 6) that the influence of post-depositional processes on the isotope signal is varying spatially and temporally.

Field Work

Periodic Reporting for period 4 - SNOWISO (Signals from the Surface Snow: Post-Depositional Processes Controlling the Ice Core IsotopicFingerprint)

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