Records from Lake Magadi, Kenya, suggest environmental variability
driven by changes in Earth’s orbit
Boulder, Colo., USA: 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 (HSPDP; https://hspdp.asu.edu), 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.
FEATURED ARTICLE
Orbital control of Pleistocene euxinia in Lake Magadi, Kenya
Authors:
Daniel Deocampo (CONTACT: deocampo@gsu.edu), Richard Owen, Tim Lowenstein,
Robin Renaut, Nate Rabideaux, Anne Billingsley, Andrew Cohen, Alan Deino,
Mark Sier, Shangde Luo, Chuan-Chou Shen, Daniel Gebregioris, Christopher
Campisano, and Anthony Mbuthia
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G49140.1/607797/Orbital-control-of-Pleistocene-euxinia-in-Lake
GEOLOGY articles are online at
https://geology.geoscienceworld.org/content/early/recent
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