Reduced ocean circulation during the ice age caused anoxic conditions
and increased carbon storage in the deep sea

Date:
December 8, 2021
Source:
MARUM - Center for Marine Environmental Sciences, University
of Bremen
Summary:
The movement of water masses in the ocean, its circulation, is
an essential component of the global climate system. Researchers
have now been able to show that circulation in the deep ocean was
significantly slowed down during the last glacial period. Analyses
of sediment samples show that the decomposition of organic carbon
in the water masses of the deep sea consumed the oxygen available
there.



FULL STORY ==========================================================================
As a natural sink for carbon, the ocean is a central element of the
Earth's climate system. The amount of carbon removed from the system
in the long run depends on how much particles containing carbon are
stored in the seabed. Here, the availability of dissolved oxygen is of
central importance, as it is consumed during the microbial decomposition
of previously formed biomass. The distribution of oxygen in the water
column is primarily determined by the vertical circulation. To answer
the question of whether the corresponding conditions in the deep ocean
were subject to changes in the recent history of the Earth, the authors
of the new study examined sediment samples. Chemical elements that can
be used as indicators for oxygen-free conditions and are preserved in
the sediment over thousands to millions of years were analyzed.


========================================================================== Sediment cores from biologically highly productive area analyzed The
sediment cores available to the team came from the Cape Basin off the
west coast of southern Africa, from water depths between 1,000 and
2,500 meters. Due to the ocean currents, the area is one of the most biologically productive ones: Cold, nutrient-rich water from the depth increases the productivity of phytoplankton. Sinking particles of dead
organic material is processed by microorganisms in the water column,
as well as on the seabed. This process mostly consumes oxygen. If large
amounts of organic material sink, this can require more oxygen than
is supplied by the ocean currents. The water column becomes "anoxic,"
which means oxygen-free.

Oxygen deficiency also detected in the deep-sea during ice age Using geochemical signatures in the sediments, the researchers were able to
prove that much less oxygen must have been available in the deep ocean
during the last glacial period compared to warmer phases. Until now,
glacial periodswere known to have a stronger temperature gradient between
the poles and the equator that was directly related to an increase in
wind circulation, thus a stronger upwelling of nutrient-rich water and, in turn, more intensive biological production. It was also known that due to
the formation of polar ice caps and the resulting lower sea level in cold periods, the near-shore upwelling shifted towards the continental slope
and thus the deeper parts of the ocean. "What is new about the current
study is that the depletion of oxygen is not limited to water depths
of a few hundred to a thousand meters but has now also been detected at
the bottom of the ocean," says co-author Dr. Matthias Zabel from MARUM.

More organic carbon stored at depth This can essentially be attributed
to two causes: Intensive decomposition processes of the biomass that was increasingly produced during glacial periods consumed a lot of oxygen. The increased content of organic carbon in the sediments studied can be
seen as a clear indication that the availability of oxygen must have
been severely restricted at the same time. "Today, oxygen-free zones are
found on the shallow shelf up to a water depth of a few hundred meters,
that is at the transition from the continental shelf to the open ocean.

During the Ice Age, on the other hand, the water of the open ocean was
anoxic at greater depths," emphasizes Dr Florian Scholz. The GEOMAR biogeochemist is co-author of the study and head of the Emmy Noether
research group ICONOX - - Iron cycling in continental margin sediments
and the nutrient and oxygen balance of the ocean.

Implications for the global carbon cycle "From the sediment samples,
we understand that during glacial periods, organic material was degraded
less effectively in the deep ocean and consequently more organic carbon
was buried in the seabed sink," says Dr Scholz. "By analyzing these
processes from Earth's history in more detail, we can better assess
whether slower circulation could also lead to increased storage of human- released carbon in deep-sea sediments in the future," adds Dr Zabel, summarizing the significance of the new study for research. "Against the background of the anthropogenic CO2 increase current climate change,
it is crucial to determine and evaluate processes and mechanisms that
impact the oceanic bottom water oxygen content," the paper states.

========================================================================== Story Source: Materials provided by MARUM_-_Center_for_Marine_Environmental_Sciences,
University_of_Bremen. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Natascha Riedinger, Florian Scholz, Michelle L. Abshire, Matthias
Zabel.

Persistent deep water anoxia in the eastern South Atlantic during
the last ice age. Proceedings of the National Academy of Sciences,
2021; 118 (49): e2107034118 DOI: 10.1073/pnas.2107034118 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211208123420.htm

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