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Research: Recent Highlights from UW Astrobiology Program Researchers
Date: September 2007
Kenneth
H. Williford*, Peter
D. Ward*, Geoffrey H. Garrison, Roger
Buick*
* UW Astrobiology Program member
Summary:
Changes in the isotopic composition of organic carbon in marine sedimentary rocks deposited 200 million years ago during the Triassic-Jurassic mass extinction suggest that this biological catastrophe was accompanied by major perturbations in the global carbon cycle. Long term isotopic change in these rocks from British Columbia shows evidence for rising atmospheric carbon dioxide due to the eruption of a massive volcanic province associated with the breakup of Pangea. Shorter term isotopic changes were likely caused by a collapse in photosynthetic productivity followed by a decline in calcification due to acidification of the oceans and a shift in the types of organisms contributing organic matter to marine sediments.
Abstract:
New lithologic and organic carbon-isotope data
are presented for the Triassic–Jurassic boundary section at
Kennecott Point, Queen Charlotte Islands, British Columbia, Canada.
The previously reported Late Norian to earliest Hettangian record
is extended by over 130 m, and three new isotopic features are revealed.
The record now shows a negative offset in baseline carbon isotope
values from approximately - 29‰ in the Late Norian to - 31‰
in the Hettangian. This offset is accompanied by the previously
reported 2‰ negative excursion at the Triassic–Jurassic
boundary and a 5‰ positive excursion in the early Hettangian.
There is a significant long-term negative isotopic trend in the
Hettangian interval of the section, which may be due to CAMP volcanism.
The positive excursion is attributed to a decline in bio-calcification
as well as changes in microbial ecology, both related to the mass
extinction at the Triassic–Jurassic boundary.
Details of the Research:
During a mass extinction 200 million years ago
at the boundary between the Triassic and Jurassic periods, Earth's
biodiversity was reduced by nearly three quarters. The dinosaurs
were just beginning to emerge in earnest at this time, and the extinctions
among other land animals likely played an important role in this
process. Many land plants also went extinct at this time. Triassic-Jurassic
extinctions were most pronounced in the oceans, however. Ammonites,
a cephalopod relative of the modern chambered Nautilus,
suffered near complete extinction, with only a single genus surviving
into the Jurassic. The ammonites would go on to acheive fantastic
diversity and size in the Jurassic and Cretaceous periods, and then
disappear completely along with the dinosaurs during the Cretaceous-Tertiary
mass extinction, 65 million years ago. Conodonts, tiny eel-like
creatures that had swum the world's oceans for hundreds of millions
of years, were completely eliminated at the Triassic-Jurassic boundary.
There were also major extinctions among radiolaria, single-celled
zooplankton with beautiful glassy skeletons.
Fossil ammonite from the Queen Charlotte
Islands
At the same time as the Triassic-Jurassic mass
extinctions, the supercontinent of Pangea was beginning to break
up. During this breakup, an enormous volcanic province erupted in
what is now New Jersey, Brazil, and Morocco. These areas were relatively close
together 200 million years ago, but have since drifted apart as
the Atlantic Ocean was formed. The volcanoes released
huge quantities of CO2 into the atmosphere, causing extreme
global warming. This warming is thought to be the primary cause of
the Triassic-Jurassic mass extinction.
Like other mass extinctions, the Triassic-Jurassic
boundary was accompanied by a major perturbation in the global carbon
cycle. We find evidence for this in the form of changes in the ratio
of the stable isotopes of carbon (13C to 12C)
preserved in sedimentary organic matter. Because it moves and reacts
more readily, the lighter isotope of carbon (12C) is
concentrated in organisms during the complex biochemical reactions
of photosynthesis. This process of isotopic discrimination is called
"fractionation." During times of increasing photosynthetic
productivity, more 12C is removed from the ocean-atmosphere
system and buried in marine sediments, and the carbon pool becomes
isotopically heavier, or enriched in 13C. During a collapse
in photosynthetic productivity, the opposite is true, and the ocean-atmosphere
system becomes isotopically lighter. This may have been the case
during mass extinctions, when photosynthesizing organisms in the
surface oceans widely perished and were decomposed by bacteria,
returning more isotopically light biological carbon to the ocean-atmosphere
system.
As organisms photosynthesize in the surface ocean,
the carbon with which they build their bodies develops an isotopic
composition controlled by the isotopic compositions of the
ocean and atmosphere and the photosynthetic frationation effect
imparted by the many processes involved in converting inorganic
carbon from outside the organism into a biological molecule inside
the organism. The organism dies and is buried in the sediments,
which eventually become rock. Over millions of years the sedimentary
rock layers are uplifted by tectonics and exposed by erosion so
that we are able to visit and sample them. We gather the samples
in the field and return them to the laboratory in Seattle where
we grind them and remove inorganic carbon by treating them with
acid, leaving behind the organic carbon (a mixture of the dead bodies
of many marine and terrestrial organisms that fell into the sediments
on the ocean floor at the time of deposition). We then take a tiny bit
of the treated rock powder and put it in a mass spectrometer which
measures its isotopic composition. Finally, we plot the isotopic
composition of each sample next to a diagram of the sedimentary
rock sequence that has been reconstructed to show the oldest rocks
on the bottom and the youngest on top. When we see major changes, or "excursions,"
in isotopic composition, we interpret these as perturbations in
the carbon cycle or a change in the composition of organic matter
that reached the ocean floor.
Isotope sampling in the Queen Charlotte
Islands
An intriguing pattern of isotopic change has emerged
over several seasons in the field and laboratory working on an organic
rich sequence of Triassic-Jurassic boundary rocks from the Queen
Charlotte Islands, British Columbia, Cananda. The rocks show a "baseline"
isotopic composition varying by less than one part per thousand
(permil, ‰) and persisting for millions of years on either
side of the boundary, interrupted by a negative excursion of 2‰
immediately coincident with a mass extinction among radiolaria and
a longer term positive excursion of 5‰ encompassing the earliest
Jurassic, pre-recovery interval which is largely devoid of fossils.
As the isotopic composition returns to baseline values, a diverse
and abundant fossil ammonite fauna reappears. There is also a long
term negative isotopic trend in the baseline composition, spanning
several million years, which we attribute to the volcanism associated
with the opening of the Atlantic Ocean that occurred as the supercontinent
Pangaea began to separate. The Central Atlantic Magmatic Province
(CAMP) released an estimated 9000 billion tonnes (Gt) of carbon
as CO2 into the atmosphere over the course of less than
a million years. Photosynthetic fractionation (preferential uptake
of the lighter isotope) increases with increasing ambient CO2
concentration, and the organic matter produced under such conditions
becomes progressively lighter. We attribute the large, positive
excursion to a combination of possible factors, including a change
in microbial ecology (e.g. vast algal blooms in the wake of the
extinctions could have lowered local CO2 concentrations
leading to decreased fractionation) and a decline in biocalcification
due to global warming and ocean acidification, leading to a decrease
in the export of isotopically-heavy carbon from the oceans as calcium
carbonate.
click
for full size version of this figure
The next step in this research is to extract the
organic matter and separate it into its individual constituent molecules.
These "molecular fossils" or biomarkers, are indicators
of their parent organisms, often things like bacteria and algae
that would not otherwise become fossils. An ongoing survey of molecular
fossils from these rocks has revealed a decrease in bacterial activity
and an increase in the contribution of land plants to marine sediments
just above the Triassic-Jurassic boundary. This could have been
caused by a drop in sea level (effectively bringing the sedimentary
sequence closer to the coastline and the source of land plants)
or a catastrophic die-off of land plants associated with the mass
extinction. With further work on the molecular fossils from this
boundary sequence and others, we plan to continue our investigation
of the relationship between microbial and metazoan habitability
and the dynamics of biogeochemical cycling during mass extinctions.
The study of mass extinctions is important to Astrobiology
because these events seem to represent one of the most fundamental
patterns in the evolution of life on Earth. Since the appearance
of animals nearly 600 million years ago, biodiversity through time
has neither remained constant nor steadily increased. Instead, episodic
intervals of extinction and recovery have wiped the ecological slate
clean and made way for basic reorganizations, resulting for instance,
in the ascent of dinosaurs after the Triassic-Jurassic boundary
and the rise of mammals after the Cretaceous-Tertiary boundary.
Mass extinctions become obvious in the geologic record only after
the evolution of the mineral skeleton 540 million years ago at the
dawn of the Phanerozoic Eon (the "time of visible life")
made possible an abundant and diverse sequence of macroscopic fossils.
Soft bodied organisms such as bacteria and archaea, only very rarely
fossilize. For this reason, we do not know whether or not there
were mass extinctions in Precambrian times.
The Phanerozoic Eon represents only slightly more
than 10% of Earth history, and readily fossilizing organisms such
as animals and plants represent only a tiny fraction of biological
diversity on Earth. What is the effect of mass extinctions on microbial
communities? It may be that, due to the greatly enhanced environmental
tolerance and metabolic diversity among the bacteria and archaea
relative to animals and plants, microbial mass extinction is extraordinarily
difficult to acheive.
Perhaps more importantly, what is the effect of
microbial communities on mass extinctions? Recent research on the
Permian-Triassic mass extinction has revealed that an expansion
among sulfate reducing bacteria in marine sediments led to massive
releases of hydrogen sulfide gas, possibly killing animals and plants
on land directly through sulfide toxicity and indirectly through
ozone destruction. New and developing techniques in organic geochemistry
will allow us to address these questions like never before. The
ensuing discoveries will be an important part of our quest to understand
basic planetary habitability and the distribution of life in the
cosmos.
Reference:
Kenneth H. Williford,
Peter D. Ward, Geoffrey H. Garrison and Roger Buick
Palaeogeography, Palaeoclimatology, Palaeoecology
Volume 244, Issues 1-4, 9 February 2007, Pages 290-296
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