Ice core research in Antarctica sheds new light on role of sea ice in
carbon balance

Date:
June 22, 2020
Source:
University of Bonn
Summary:
New research findings underline the crucial role that sea ice
throughout the Southern Ocean played for atmospheric CO2 in times
of rapid climate change in the past. An international team has
shown that the seasonal growth and destruction of sea ice in a
warming world increases the biological productivity of the seas
around Antarctica by extracting carbon from the atmosphere and
storing it in the deep ocean.



FULL STORY ==========================================================================
New research findings underline the crucial role that sea ice throughout
the Southern Ocean played for atmospheric CO2 in times of rapid
climate change in the past. An international team of scientists with the participation of the University of Bonn has shown that the seasonal growth
and destruction of sea ice in a warming world increases the biological productivity of the seas around Antarctica by extracting carbon from
the atmosphere and storing it in the deep ocean. This process helps to
explain a long-standing question about an apparent 1,900-year pause in
CO2 growth during a period known as the Antarctic cold reversal. The
research results have now been published in "Nature Geoscience."

========================================================================== Surrounding the remote continent of Antarctica, the Southern Ocean is
one of the most important yet poorly understood components of the global
carbon cycle.

Having captured half of all human-related carbon that has entered the
ocean to date, the Southern Ocean is crucial to regulating human-induced
CO2. Therefore, understanding the processes that determine its
effectiveness as a carbon sink through time are essential to reducing uncertainty in climate projections.

After the Last Ice Age, around 18,000 years ago, the world transitioned naturally into the warm interglacial world we live in today. During this period, CO2 rose rapidly from around 190 ppm to 280 ppm over around 7,000 years. This rise was not steady, and was interrupted by rapid rises and intermittent plateaus, reflecting different processes within the global
carbon cycle.

Antarctic Cold Reversal One period stands out: a 1,900-year plateau of near-constant CO2 levels at 240 ppm starting some 14,600 years ago called
the Antarctic Cold Reversal. The cause of this plateau remains unknown,
but understanding the processes may be critical for improving projections surrounding climate-carbon feedbacks.

"We found that in sediment cores located in the sea-ice zone of the
Southern Ocean biological productivity increased during this critical
period, whereas it decreased farther north, outside of the sea-ice
zone," says Michael Weber, co- author of the study from the Institute
for Geosciences at the University of Bonn. "It was now important to find
out how climate records on the Antarctic continent depict this critical
time period." To resolve this question researchers from Keele University, U.K., and the University of New South Wales (UNSW) in Sydney, Australia, travelled to the Patriot Hills Blue Ice Area to obtain new records of
marine biomarkers captured in ice cores. Chris Fogwill, lead author of
the study from Keele University, says "the cause of this long plateau
in global atmospheric CO2 levels may be fundamental to understanding
the potential of the Southern Ocean to moderate atmospheric CO2. Whilst
recent reductions in emissions due to the Covid-19 pandemic have shown
that we can reduce CO2, we need to understand the ways in which CO2
levels have been stabilised by natural processes, as they may be key to
the responsible development of geoengineering approaches and remain
fundamental to achieving our commitment to the Paris Agreement."
Horizontal ice core analysis Blue ice areas are created by fierce,
high-density katabatic winds that erode the top layer of snow effectively
and expose the ice below. As a result, ice flows up to the surface,
providing access to ancient ice below. While most Antarctic researchers
drill down into the ice to extract samples with a conventional ice core,
this team used a different method: horizontal ice core analysis. Chris
Turney (UNSW, Sydney) says "Instead of drilling kilometres into the ice,
we can simply walk across a blue ice area to travel back through time.

This provides the opportunity to sample large volumes of ice necessary
for studying new organic biomarkers and DNA that were blown from
the Southern Ocean onto Antarctica and preserved in the blue ice."
The results demonstrated a marked increase in the number and diversity
of marine organisms across the 1,900 year period of the CO2 plateau, an observation never seen before. The team also conducted climate modelling revealing that this period coincided with the greatest seasonal changes in
sea ice extent from summer to winter. Together with the marine cores,
these findings provide the first evidence of increased biological
productivity record and suggest that processes in the Antarctic Zone of Southern Ocean may have caused the CO2 plateau.

The team will use this work to underpin the development of climate models
that seek to improve our understanding of future climate change. The
inclusion of sea ice processes that control climate-carbon feedbacks in
a new generation of models will be crucial for reducing uncertainties surrounding climate projections and help society adapt to future warming.


========================================================================== Story Source: Materials provided by University_of_Bonn. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. C. J. Fogwill, C. S. M. Turney, L. Menviel, A. Baker, M. E. Weber,
B.

Ellis, Z. A. Thomas, N. R. Golledge, D. Etheridge, M. Rubino, D. P.

Thornton, T. D. van Ommen, A. D. Moy, M. A. J. Curran, S. Davies,
M. I.

Bird, N. C. Munksgaard, C. M. Rootes, H. Millman, J. Vohra,
A. Rivera, A.

Mackintosh, J. Pike, I. R. Hall, E. A. Bagshaw, E. Rainsley,
C. Bronk- Ramsey, M. Montenari, A. G. Cage, M. R. P. Harris,
R. Jones, A. Power, J.

Love, J. Young, L. S. Weyrich, A. Cooper. Southern Ocean carbon sink
enhanced by sea-ice feedbacks at the Antarctic Cold Reversal. Nature
Geoscience, 2020; DOI: 10.1038/s41561-020-0587-0 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200622132937.htm

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