Records from Lake Magadi, Kenya, suggest environmental variability
driven by changes in Earth's orbit

Date:
September 21, 2021
Source:
Geological Society of America
Summary:
Rift Valley lakes within eastern Africa range from freshwater to
highly alkaline systems and are homes to diverse ecosystems. These
Rift Valley lakes are also sedimentary repositories, yielding
a high-resolution environmental record that can be targeted to
better understand the environmental and climatic context of human
evolution over the past few million years in eastern Africa.



FULL STORY ==========================================================================
Rift Valley lakes within eastern Africa range from freshwater to highly alkaline systems and are homes to diverse ecosystems. These Rift Valley
lakes are also sedimentary repositories, yielding a high-resolution environmental record that can be targeted to better understand the environmental and climatic context of human evolution over the past few
million years in eastern Africa.


==========================================================================
A new study published yesterday in Geology examines the geochemical record
of drill core sediments collected from Lake Magadi -- a saline, alkaline
lake in the southern Kenya Rift -- that provides a nearly one-million-year paleoenvironmental record from an unusual Rift Valley lake system.

Lead author Dan Deocampo of Georgia State University and a group of international co-authors drilled Lake Magadi as part of the Hominin
Sites and Paleolakes Drilling Project, which collected deep sediment
cores from lake basins in the East African Rift.

"We're trying to understand how the Earth's surface environment
has changed over the last several million years and how that has
impacted early hominin habitats," said Deocampo. "We are using many
different proxies of the ancient environments to understand how the
environment has changed, how habitats have changed, and therefore how
the hazards and resources for early hominins changed through time."
The geochemical analysis of the Lake Magadi samples showed some of the
highest concentrations of elements like molybdenum, arsenic, and vanadium
ever reported in lake sediments. Hyperaccumulation of these elements has
not previously been observed in other East African lakes and generally
requires euxinic conditions.

Euxinic conditions occur when the lake water is both anoxic and sulfidic, typically triggered during negative water balance episodes like droughts.

"The amount of molybdenum accumulated in a sulfide-rich sediment in
the lake is not going to tell us habitat structure, where the hominins
were living, but fluctuations between those euxinic conditions and
fresher water conditions, that can tell us something about the pace of environmental change," said Deocampo.

Deocampo and co-authors found that euxinia became common after about
700,000 years ago and subsequently tended to occur during intervals when Earth's orbit was more elliptical, which occurs over a 100,000-year
cycle. As Earth's orbit becomes more elliptical, Earth can become
farther away from the sun, which causes greater variations in seasonal
climate. The episodes of euxinia provide an important indicator of
intense droughts in the region during periods of extensive glaciations.

These high-amplitude environmental fluctuations driving shifts between
euxinic and well-mixed lake conditions would have profoundly affected
moisture availability and vegetation over evolutionary timescales.

The environmental variability suggested by the geochemical record of
Lake Magadi is associated in time with mammal species turnover and the
first appearance of Middle Stone Age technology in the southern Kenya
Rift between 500,000 and 320,000 years ago.

"Now that is kind of a touching point with the paleoanthropologists
who are thinking about changes in the amplitude of environmental change
and how that relates to gene pool modifications and changes in habitat structure, first appearances, and last appearances," said Deocampo.

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


========================================================================== Journal Reference:
1. D.M. Deocampo, R.B. Owen, T.K. Lowenstein, R.W. Renaut,
N.M. Rabideaux,
A. Billingsley, A. Cohen, A.L. Deino, M.J. Sier, S. Luo,
C.-C. Shen, D.

Gebregiorgis, C. Campisano, A. Mbuthia. Orbital control of
Pleistocene euxinia in Lake Magadi, Kenya. Geology, 2021; DOI:
10.1130/G49140.1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210921134350.htm

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