Why did ocean productivity decline abruptly 4.6 million years ago?


Date:
January 18, 2022
Source:
Uppsala University
Summary:
By drilling deep down into sediments on the ocean floor researchers
can travel back in time. A research team now presents new clues as
to when and why a period often referred to as the 'biogenic bloom'
came to an abrupt end. Changes in the shape of the Earth's orbit
around the Sun may have played a part in the dramatic change.



FULL STORY ==========================================================================
By drilling deep down into sediments on the ocean floor researchers
can travel back in time. A research team led from Uppsala University
now presents new clues as to when and why a period often referred to
as the 'biogenic bloom' came to an abrupt end. Changes in the shape of
the Earth's orbit around the Sun may have played a part in the dramatic
change.


========================================================================== Healthy ocean systems contain healthy primary producers, such as the
single- celled algae diatoms and coccolithophores, which sustain all
other life in the oceans through the marine food webs. Primary producers
also release oxygen and regulate the climate by taking up CO2 and binding carbon into solid components that are buried in deep-sea sediments, which
is an effective long-term solution for carbon removal from the atmosphere.

Most of these algae use sunlight, CO2 and inorganic nutrients to build up
their body mass. However, these nutrients are quickly exhausted in the
sun-lit surface waters, if not replenished by ocean mixing or renewed
by river supply.

Throughout Earth's history, paleoceanographers reconstruct changes in
primary productivity by looking at algal remains buried in ocean-floor sediments.

Although only a small fraction of surface water production is recorded in marine sediments, on geological time scales, changes in the accumulation
of biogenic sediments (including the calcareous scales of coccolithophores
and siliceous shells of diatoms) are linked to past changes in ocean productivity.

It is important to understand what factors influence ocean productivity
on a global scale, but also how fast or slow this complex system may
respond to environmental changes.

For many decades, geoscientists have known about an extended period of
time when ocean productivity was much higher than today. This occurred
during the late Miocene to early Pliocene (from 9 to 3.5 million years
ago) and the period is often called the 'biogenic bloom'. However,
to date, scientists still do not fully understand the reasons why
productivity was so much higher in the past, or why this period came to
an end.

A group of scientists collaborating with Boris-Theofanis Karatsolis, a PhD student at Uppsala University, combined multiple deep-sea sediment drill
cores from all major oceans to investigate what caused the end of the
high ocean productivity. Using scientific ocean drilling, it is possible
to operate in waters as deep as 4 kilometres and still drill 1 kilometre
down into the sediments, recovering millions of years of ocean history.

The sediments studied by Karatsolis and colleagues were recovered
from 200-350 metres below the seafloor on the northwestern Australian
shelf. The researchers measured the accumulation rates of biogenic
particles and combined their data with those previously collected using
similar methods at 16 additional locations. The accuracy of the ages of
each dataset was first critically assessed, to ensure reliable comparisons across the different regions.

Their results show that productivity declined abruptly 4.6 million years
ago in the tropics. A possible explanation for this rapid decrease may
involve reduced East Asian monsoon intensity and decreased riverine
nutrient supply, coinciding with changes in the shape of the Earth's
orbit around the Sun.

The study adds new pieces to the larger puzzle but the mechanisms behind
this event will need further study.

"Understanding the natural pace of past events provides a good comparative measuring stick for the changes we observe in our environment today,"
says Boris-Theofanis Karatsolis.

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


========================================================================== Journal Reference:
1. B. - Th. Karatsolis, B. C. Lougheed, D. De Vleeschouwer,
J. Henderiks.

Abrupt conclusion of the late Miocene-early Pliocene biogenic
bloom at 4.6-4.4 Ma. Nature Communications, 2022; 13 (1)
DOI: 10.1038/ s41467-021-27784-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220118125150.htm

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