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Batuan kuno merekam bukti pertama untuk fotosintesis yang membuat oksigen
Date:
October 6, 2015
Source:
University of Wisconsin-Madison
Summary:
Sebuah studi baru menunjukkan bahwa bantalan batuan-besi yang terbentuk pada dasar laut 3,2 miliar tahun yang lalu membawa bukti jelas tentang oksigen . Satu-satunya sumber yang logis untuk oksigen yang merupakan contoh paling awal dikenalnya fotosintesis oleh organisme hidup , kata geoscientists .
.............. " Batu dari 3,4 miliar tahun yang lalu menunjukkan bahwa laut yang terdapat pada dasarnya tidak ada oksigen bebas , " kata Clark Johnson , profesor geosains di UW- Madison dan anggota dari NASA Astrobiology Institute . " Kerja terbaru menunjukkan sedikit kenaikan oksigen pada 3 miliar tahun . Batuan yang kami pelajari berusia 3230000000 tahun , dan cukup baik diawetkan , dan kami percaya mereka menunjukkan tanda-tanda yang pasti untuk oksigen di lautan jauh lebih awal dari penemuan sebelumnya . "....more
Ancient rocks
record first evidence for photosynthesis that made oxygen
Date:
October 6, 2015
Source:
University of Wisconsin-Madison
Summary:
A new study shows that iron-bearing rocks that formed at the ocean floor
3.2 billion years ago carry unmistakable evidence of oxygen. The only logical
source for that oxygen is the earliest known example of photosynthesis by
living organisms, say geoscientists.
...................
A new study shows that iron-bearing rocks that formed at the ocean floor 3.2
billion years ago carry unmistakable evidence of oxygen. The only logical
source for that oxygen is the earliest known example of photosynthesis by
living organisms, say University of Wisconsin-Madison geoscientists.
"Rock from 3.4 billion years ago showed that the ocean contained
basically no free oxygen," says Clark Johnson, professor of geoscience at
UW-Madison and a member of the NASA Astrobiology Institute. "Recent work
has shown a small rise in oxygen at 3 billion years. The rocks we studied are 3.23
billion years old, and quite well preserved, and we believe they show definite
signs for oxygen in the oceans much earlier than previous discoveries."
The most reasonable candidate for liberating the oxygen found in the iron
oxide is cyanobacteria, primitive photosynthetic organisms that lived in the
ancient ocean. The earliest evidence for life now dates back 3.5 billion years,
so oxygenic photosynthesis could have evolved relatively soon after life
itself.
Until recently, the conventional wisdom in geology held that oxygen was
rare until the "great oxygenation event," 2.4 to 2.2 billion years
ago.
The rocks under study, called jasper, made of iron oxide and quartz, show
regular striations caused by composition changes in the sediment that formed
them. To detect oxygen, the UW-Madison scientists measured iron isotopes with a
sophisticated mass spectrometer, hoping to determine how much oxygen was needed
to form the iron oxides.
"Iron oxides contained in the fine-grained, deep sediment that formed
below the level of wave disturbance formed in the water with very little
oxygen," says first author Aaron Satkoski, an assistant scientist in the
Geoscience Department. But the grainier rock that formed from shallow,
wave-stirred sediment looks rusty, and contains iron oxide that required much
more oxygen to form.
The visual evidence was supported by measurements of iron isotopes,
Satkoski said.
The study was funded by NASA and published in Earth and Planetary
Science Letters.
The samples, provided by University of Johannesburg collaborator Nicolas
Beukes, were native to a geologically stable region in eastern South Africa.
Because the samples came from a single drill core, the scientists cannot
prove that photosynthesis was widespread at the time, but once it evolved, it
probably spread. "There was evolutionary pressure to develop oxygenic
photosynthesis," says Johnson. "Once you make cellular machinery that
is complicated enough to do that, your energy supply is inexhaustible. You only
need sun, water and carbon dioxide to live."
Other organisms developed forms of photosynthesis that did not liberate
oxygen, but they relied on minerals dissolved in hot groundwater -- a far less
abundant source than ocean water, Johnson adds. And although oxygen was
definitely present in the shallow ocean 3.2 billion years ago, the
concentration was only estimated at about 0.1 percent of that found in today's
oceans.
Confirmation of the iron results came from studies of uranium and its decay
products in the samples, says co-author Brian Beard, a senior scientist at
UW-Madison. "Uranium is only soluble in the oxidized form, so the uranium
in the sediment had to contain oxygen when the rock solidified."
Measurements of lead formed from the radioactive decay of uranium showed
that the uranium entered the rock sample 3.2 billion years ago. "This was
an independent check that the uranium wasn't added recently. It's as old as the
rock; it's original material," Beard says.
"We are trying to define the age when oxygenic photosynthesis by bacteria
started happening," he says. "Cyanobacteria could live in shallow
water, doing photosynthesis, generating oxygen, but oxygen was not necessarily
in the atmosphere or the deep ocean."
However, photosynthesis was a nifty trick, and sooner or later it started
to spread, Johnson says. "Once life gets oxygenic photosynthesis, the sky
is the limit. There is no reason to expect that it would not go
everywhere."
Story Source:
The above post is reprinted from materials provided byUniversity
of Wisconsin-Madison. The original item was written by David
Tenenbaum. Note: Materials may be edited for content and length.
Journal Reference:
1.
Aaron M. Satkoski, Nicolas J. Beukes, Weiqiang Li, Brian L. Beard, Clark M.
Johnson. A redox-stratified ocean 3.2 billion years ago. Earth
and Planetary Science Letters, 2015; 430: 43 DOI: 10.1016/j.epsl.2015.08.007