Severe Selenium Depletion Detected During Ordovician, Devonian and Triassic Mass Extinctions

Severe Selenium depletion in the Phanerozoic oceans as a factor in three global mass extinction events

Authors:

Long et al

Abstract:

Selenium (Se) is one of the key trace elements required by all animal and most plant life, and Se deficiencies in the food chain cause pathologies or death. Here we show from new geochemical analyses of trace elements in Phanerozoic marine pyrite that sustained periods of severe Se depletion in the past oceans correlate closely with three major mass extinction events, at the end of the Ordovician, Devonian and Triassic periods. These represent periods of Se depletion > 1.5-2 orders of magnitude lower than current ocean abundances, being within the range to cause severe pathological damage in extant Se-reliant organisms. Se depletion may have been one of several factors in these complex extinction scenarios. Recovery from the depletion/extinction events is likely part of a natural marine cycle, although rapid rises in global oxygen from sudden major increases in marine productivity and plant biomass after each extinction event may also have played a crucial role.

Double Impact Crater Found in Sweden From Floian/Dapingian Ordovician, Possibly Linked to Proposed Ordovician Meteor Event

Double rainbows have nothing on Earth’s newest dynamic duo: the double crater.

Researchers from the University of Gothenburg have uncovered two impact craters in Jämtland, Sweden, which they believe to have occurred simultaneously around 460 million years ago. Double meteor impacts are rare events, and the discovery in Sweden is the first proven instance of its kind.

“Information from drilling operations demonstrates that identical sequences are present in the two craters, and the sediment above the impact sequences is of the same age. In other words, these are simultaneous impacts,” said Erik Sturkell, professor of geophysics at the University of Gothenburg and a member of the team that found the double crater, in a university statement.

Although these two meteorites struck at the same time, that does not mean they are physically alike, however. One crater measures a massive 4.7 miles in diameter, while the other, which was located nearly ten miles away, was a much a smaller 2,300 feet across.

As for how it happened, it all started in the stars.

“Around 470 million years ago, two large asteroids collided in the asteroid belt between Mars and Jupiter, and many fragments were thrown off in new orbits. Many of these crashed on Earth, such as these two in Jämtland,” said Sturkell.

link.

There are several impacts in North America and another in Estonia that roughly line up.  This hypothesized event is termed the "Ordovician Meteor Event."  They are all supposedly within a million years of each other and I wonder though whether or not they line up given the paleogeography. 

Evidence of Heavy Metal Poisoning From Ordovician Mass Extinction in Marine Plankton

Several Palaeozoic mass extinction events during the Ordovician and Silurian periods (ca. 485 to 420 to million years ago) shaped the evolution of life on our planet. Although some of these short-lived, periodic events were responsible for eradication of up to 85% of marine species, the exact kill-mechanism responsible for these crises remains poorly understood.

An international team led by Thijs Vandenbroucke (researcher at the French CNRS and invited professor at UGent) and Poul Emsbo (US Geological Survey) initiated a study to investigate a little known association between 'teratological' or 'malformed' fossil plankton assemblages coincident with the initial stages of these extinction events.

In a paper just published in Nature Communications, they present evidence that malformed fossil remains of marine plankton from the late Silurian (415 million years ago) contain highly elevated concentrations of heavy metals, such as iron, lead, and arsenic. These are well-known toxins that cause morphologic abnormalities in modern aquatic organisms; which led the authors to conclude that metal poisoning caused the malformation observed in these ancient organisms and may have contributed to their extinction and that of many other species.

link.

Early Terrestrial Plants Could NOT Have Caused the Ordovician Glaciations

Constraining the role of early land plants in Palaeozoic weathering and global cooling

Authors:

Quirk et al

Abstract:

How the colonization of terrestrial environments by early land plants over 400 Ma influenced rock weathering, the biogeochemical cycling of carbon and phosphorus, and climate in the Palaeozoic is uncertain. Here we show experimentally that mineral weathering by liverworts—an extant lineage of early land plants—partnering arbuscular mycorrhizal (AM) fungi, like those in 410 Ma-old early land plant fossils, amplified calcium weathering from basalt grains threefold to sevenfold, relative to plant-free controls. Phosphate weathering by mycorrhizal liverworts was amplified 9–13-fold over plant-free controls, compared with fivefold to sevenfold amplification by liverworts lacking fungal symbionts. Etching and trenching of phyllosilicate minerals increased with AM fungal network size and atmospheric CO2 concentration. Integration of grain-scale weathering rates over the depths of liverwort rhizoids and mycelia (0.1 m), or tree roots and mycelia (0.75 m), indicate early land plants with shallow anchorage systems were probably at least 10-fold less effective at enhancing the total weathering flux than later-evolving trees. This work challenges the suggestion that early land plants significantly enhanced total weathering and land-to-ocean fluxes of calcium and phosphorus, which have been proposed as a trigger for transient dramatic atmospheric CO2 sequestration and glaciations in the Ordovician.

Marine Productivity and Redox Conditions During Ordovician Hirnantian Glaciation

Changes in marine productivity and redox conditions during the Late Ordovician Hirnantian glaciation

Authors:

Zhou et al

Abstract:

Changes in marine productivity and redox conditions during the end-Ordovician (Hirnantian) glaciation and Ordovician–Silurian transition were investigated through Mo-isotope and major- and trace-element analyses of the Wangjiawan (Hubei Province) and Nanbazi (Guizhou Province) sections from the Yangtze Platform of South China. Katian shales of the Wufeng Formation, which yield the graptolites Dicellograptus complanatus, Dicellograptus complexus, and Paraorthograptus pacificus, were deposited under euxinic conditions at both localities, as shown by high MoEF, UEF, and δ98Mo values. A major sea-level regression during the Hirnantian glaciation resulted in shallowing and a shift toward better-oxygenated conditions within the Yangtze Sea, as well as deposition of thin-bedded siliceous sediments, calcirudite debris flows, and limestone turbidites of the Kuanyinchiao Formation, the base of which correlates with the first phase of the end-Ordovician mass extinction. The termination of the Hirnantian glaciation at the top of the Kuanyinchiao Formation was associated with a major sea-level transgression, a rapid expansion of euxinia in the Yangtze Sea (as documented by a return of high MoEF, UEF, and δ98Mo values), and the second phase of the mass extinction, during which the cool-adapted Hirnantian Fauna went extinct.

The long-term cooling trend of the Middle and Late Ordovician, which culminated in the Hirnantian glaciation, was driven by enhanced burial of organic carbon, as documented by δ13Ccarb, δ13Corg, and δ34Spy records. Increased organic carbon burial was linked to high rates of marine productivity, as shown by high TOC and biogenic Ba concentrations especially at the deeper, less-restricted Wangjiawan locale, producing the HICE (Hirnantian Isotopic Curve Excursion) δ13Ccarb excursion. The locus of organic carbon burial during the Hirnantian crisis shifted to deeper-water environments that were located outside the study region. The relatively rapid onset and termination of the Hirnantian glaciation were probably due to crossing of tipping points in the Late Ordovician climatic–oceanic system.

No Sign of Progressive Katian Ordovician Cooling

Oxygen isotopes from Conodont Apatite of the midcontinent, us: implications for late ordovician climate evolution

Authors:

Quinton et al

Abstract:

The major glaciation at the end of the Ordovician is associated with the 2nd largest mass extinction event of the Phanerozoic. Growth of Late Ordovician ice sheets requires a dramatic cooling from the ‘greenhouse’ conditions that prevailed for most of the Ordovician, but when and how fast this cooling occurred is controversial. The controversy is due in large part to a lack of good geochemical constraints on the temperature history of the Katian (453–445.2 Ma). To address this uncertainty, we measured phosphate δ18O values from 3 conodont species collected from sections in the midcontinent region of the United States that span an ~ 5.7 m.y. long interval covering most of the Katian. Results reveal a statistically significant offset in δ18O values between some taxa and show up to 2‰ differences among samples. However, there are no apparent long-term trends within or between sections; rather, values fluctuate around a δ 18O mean of ~ 19‰ VSMOW. Our study provides the longest, relatively high resolution, species specific conodont record generated for this interval, and we found no evidence supporting progressive cooling during the Katian.

Ordovician Extinction was Caused by the Kitchen Sink?

End Ordovician extinctions: A coincidence of causes

Authors:

Harper et al

Abstract:

The end Ordovician (Hirnantian) extinction was the first of the five big Phanerozoic extinction events, and the first that involved metazoan-based communities. It comprised two discrete pulses, both linked in different ways to an intense but short-lived glaciation at the South Pole. The first, occurring at, or just below, the Normalograptus extraordinarius graptolite Biozone, mainly affected nektonic and planktonic species together with those living on the shallow shelf and in deeper water whereas the second, within the N. persculptus graptolite Biozone, was less focused, eradicating faunas across a range of water depths. In all about 85% of marine species were removed. Proposed kill mechanisms for the first phase have included glacially-induced cooling, falling sea level and chemical recycling in the oceans, but a general consensus is lacking. The second phase is more clearly linked to near-global anoxia associated with a marked transgression during the Late Hirnantian. Most recently, however, new drivers for the extinctions have been proposed, including widespread euxinia together with habitat destruction caused by plate tectonic movements, suggesting that the end Ordovician mass extinctions were a product of the coincidence of a number of contributing factors. Moreover, when the deteriorating climate intensified, causing widespread glaciation, a tipping point was reached resulting in catastrophe.

Was There a Third Pulse to the Ordovician Extinction?

The early Rhuddanian survival interval in the Lower Silurian of the Oslo Region: A third pulse of the end-Ordovician extinction

Author:

B. Gudveig Baarli

Abstract:

Ordovician/Silurian boundary layers with Rhuddanian strata are exposed as a long, continuously fossiliferous sequence in the Solvik Formation in the Asker area, central Oslo Region, Norway. Brachiopods belonging to Benthic Assemblage 5 are preserved in the lower parts of the formation. This level is investigated for the presence of a survival interval after the last end-Ordovician extinction event. The criteria for a survival interval include taxa that are dwarfed, long ranging, eurytopic, often opportunistic, as well as assemblages that show low density and diversity.

Four species, Isorthis prima, Leangella scissa, Dicoelosia osloensis and Eoplectodonta duplicata, were collected and measured. The three former were dwarfed as compared to a younger Aeronian fauna belonging to the same Benthic Assemblage. Detailed investigations showed a statistically significant two-step reduction in size for all but D. osloensis. The first interval, immediately above the last end-Ordovician extinction event, display dwarfed brachiopods, but diversity is high and the number of long-ranging and eurytopic species is low due to the presence of globally and locally “relict” Ordovician species. The second interval, the upper parts of lower Rhuddanian, shows all the characteristics of a survival interval in which the sizes are statistically significantly smaller than both those in the interval below and those in the Aeronian interval above. No lithological change within lower Rhuddanian strata could be linked to the pronounced reduction in size. The second step in size reduction may be related to global occurrence of anoxia in the deep oceans with a pulse of anoxic water pushed onto the shelf at that time. It is coeval with a short negative ∂13C excursion found in some locales, signifying a brief period of global warming. The results suggest a protracted extinction event through parts of the early Rhuddanian and a third and final extinction event followed by a clear survival interval.

Ecological Implications of Mass Extinction are Separate From Taxonomic Implications

Contrasting the ecological and taxonomic consequences of extinction

Authors:

Christie et al

Abstract:

Extinction in the fossil record is most often measured by the percentage of taxa (species, genera, families, etc.) that go extinct in a certain time interval. This is a measure of taxonomic loss, but previous work has indicated that taxonomic loss may be decoupled from the ecological effects of an extinction. To understand the role extinction plays in ecological change, extinction should also be measured in terms of loss of functional diversity. This study tests whether ecological changes increase correspondingly with taxonomic changes during the Late Ordovician M4/M5 extinction, the Ordovician/Silurian mass extinction, and the Late Devonian mass extinction. All three extinctions are evaluated with regional data sets from the eastern United States. Ecological effects are measured by classifying organisms into ecological lifestyles, which are groups based on ecological function rather than evolutionary history. The taxonomic and ecological effects of each extinction are evaluated with additive diversity partitioning, detrended correspondence analysis, and relative abundance distributions. Although the largest taxonomic changes occur in the Ordovician/Silurian extinction, the largest ecological changes occur in the Late Devonian extinction. These results suggest that the ecological consequences of extinction need to be considered in addition to the taxonomic effects of extinction.

Ordovician Glaciations Were Complex, Implicated in Mass Extinction

Nd isotope records of late Ordovician sea-level change—implications for glaciation frequency and global stratigraphic correlation

Authors:

1. C. Holmden (a)

2. C.E. Mitchell (b)

3. D.F. LaPorte (a)

4. W.P. Patterson (a)

5. M.J. Melchin (c)

6. S.C. Finney (d)

Affiliations:

a. Saskatchewan Isotope Laboratory, Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, S7N 5E2 966-5712

b. Department of Geology, University at Buffalo, 876 Natural Science Complex, Buffalo, NY 14260

c. Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS, , B2G 2W5Canada

d. Department of Geological Sciences, California State University at Long Beach2, 1250 Bellflower Boulevard, Long Beach, CA 90840

Abstract:

Detailed records of ice-sheet advances and retreats are reconstructed for the Hirnantian and Katian ages of the Late Ordovician using Nd isotopes (εNd) as a sea-level proxy in three study sections from the western margin of Laurentia: two shallow water platform sections located south and north of the paleo-equator, and one deep water section located in a continental slope-rise setting. When sea-level was high and paleo-shorelines had migrated eastward, the εNd value of seawater in the vicinity of each of the study sections shifted toward the εNd value of the eastern Panthalassa Ocean (~ –4.0). By contrast, when sea-level was low and paleo-shorelines had migrated westward, the εNd value of seawater shifted toward the εNd value of the continental weathering flux from Laurentia (–8.5 ± 0.2, 2σmean). These stratigraphic patterns of changing εNd values are interpreted to reflect the eustatic sea-level fluctuations that previous studies have documented in response to Gondwanan ice-sheet advances and retreats, thus linking the εNd sea-level proxy to Late Ordovician global-scale climate changes. The εNd profiles for the two platform sections yielded similar proxy sea-level curves with five cycles of oscillation recorded during the latest Katian and Hirnantian. Three additional cycles of late Katian sea-level change are recognized in the εNd profile of the deep water continental slope-rise section.

The combination of εNd, δ13C and graptolite biostratigraphic data facilitates a precise interregional correlation of the Hirnantian Age and the paleoclimate changes that took place during this interval. The new correlations support previous findings that the Hirnantian ice age comprised two major glacial periods separated by a minor interglacial during the early part of the M. persculptus Biozone. The younger glacial (confined to mid M. persculptus Biozone time) led to more extensive sea surface cooling than did the earlier one, and resulted in extensive eustatic sea-level drawdown and C-cycle changes. It records the strata most often recognized as HICE (the Hirnantian Carbon Isotope Excursion) in sedimentary successions worldwide, such as Anticosti Island, Scotland, Estonia, Siberia, and South China. The results of this study support and strengthen the view that glaciation predated the Hirnantian Age in the Late Ordovician, and that the record of small positive δ13C excursions in Katian successions from Baltica and eastern North America are themselves proxy indicators of glaciation frequency and eustatic sea level changes.

A New Study of the Ordovician Mass Extinction

The second-largest mass extinction in Earth's history coincided with a short but intense ice age during which enormous glaciers grew and sea levels dropped. Although it has long been agreed that the so-called Late Ordovician mass extinction—which occurred about 450 million years ago—was related to climate change, exactly how the climate change produced the extinction has not been known. Now, a team led by scientists at the California Institute of Technology (Caltech) has created a framework for weighing the factors that might have led to mass extinction and has used that framework to determine that the majority of extinctions were caused by habitat loss due to falling sea levels and cooling of the tropical oceans.

The work—performed by scientists at Caltech and the University of Wisconsin, Madison—is described in a paper currently online in the early edition of the Proceedings of the National Academy of Sciences.

The researchers combined information from two separate databases to overlay fossil occurrences on the sedimentary rock record of North America around the time of the extinction, an event that wiped out about 75 percent of marine species alive then. At that time, North America was an island continent geologists call Laurentia, located in the tropics.

Comparing the groups of species, or genera, that went extinct during the event with those that survived, the researchers were able to figure out the relative importance of several variables in dictating whether a genus went extinct during a 50-million-year interval around the mass extinction.

"What we did was essentially the same thing you'd do if confronted with a disease epidemic," says Seth Finnegan, postdoctoral scholar at Caltech and lead author of the study. "You ask who is affected and who is unaffected, and that can tell you a lot about what's causing the epidemic."

As it turns out, the strongest predictive factors of extinction on Laurentia were both the percentage of a genus's habitat that was lost when the sea level dropped and a genus's ability to tolerate broader ranges of temperatures. Groups that lost large portions of their habitat as ice sheets grew and sea levels fell, and those that had always been confined to warm tropical waters, were most likely to go extinct as a result of the rapid climate change.

"This is the first really attractive demonstration of how you can use multivariate approaches to try to understand extinctions, which reflect amazingly complex suites of processes," says Woodward Fischer, an assistant professor of geobiology at Caltech and principal investigator on the study. "As earth scientists, we love to debate different environmental and ecological factors in extinctions, but the truth is that all of these factors interact with one another in complicated ways, and you need a way of teasing these interactions apart. I'm sure this framework will be profitably applied to extinction events in other geologic intervals."

The analysis enabled the researchers to largely rule out a hypothesis, known as the record-bias hypothesis, which says that the extinction might be explained by a significant gap in the fossil record, also related to glaciation. After all, if sea levels fell and continents were no longer flooded, sedimentary rocks with fossils would not accumulate. Therefore, the last record of any species that went extinct during the gap would show up immediately before the gap, creating the appearance of a mass extinction.

Finnegan reasoned that this record-bias hypothesis would predict that the duration of a gap in the record should correlate with higher numbers of extinctions—if a gap persisted longer, more groups should have gone extinct during that time, so it should appear that more species went extinct all at once than for shorter gaps. But in the case of the Late Ordovician, the researchers found that the duration of the gap did not matter, indicating that a mass extinction very likely did occur.

"We have found that the Late Ordovician mass extinction most likely represents a real pulse of extinction—that many living things genuinely went extinct then," says Finnegan. "It's not that the record went bad and we just don't recover them after that."

Ordovician Extinction Cause: War between Appalachians and Volcanoes

Researchers here have discovered the pivotal role that volcanoes played in a deadly ice age 450 million years ago.

Perhaps ironically, these volcanoes first caused global warming -- by releasing massive amounts of carbon dioxide into the atmosphere.

When they stopped erupting, Earth's climate was thrown off balance, and the ice age began.

The discovery underscores the importance of carbon in Earth's climate today, said Matthew Saltzman, associate professor of earth sciences at Ohio State University.

The results will appear in the journal Geology, in a paper now available online.

Previously, Saltzman and his team linked this same ice age to the rise of the Appalachian Mountains. As the exposed rock weathered, chemical reactions pulled carbon from Earth's atmosphere, causing a global cooling which ultimately killed two-thirds of all species on the planet.

Now the researchers have discovered the other half of the story: giant volcanoes that formed during the closing of the proto-Atlantic Ocean -- known as the Iapetus Ocean -- set the stage for the rise of the Appalachians and the ice age that followed.

"Our model shows that these Atlantic volcanoes were spewing carbon into the atmosphere at the same time the Appalachians were removing it," Saltzman explained. "For nearly 10 million years, the climate was at a stalemate. Then the eruptions abruptly stopped, and atmospheric carbon levels fell well below what they were in the time before volcanism. That kicked off the ice age," he said.

This is the first evidence that a decrease in carbon from volcanic degassing -- combined with continued weathering of the Appalachians -- caused the long-enigmatic glaciation and extinction in the Ordovician period.

Here is the picture the researchers have assembled: 460 million years ago, during the Ordovician, volcanoes along the margin of what is now the Atlantic Ocean spewed massive amounts carbon dioxide into the atmosphere, turning the world into a hothouse. Lava from those volcanoes eventually collided with North America to form the Appalachian Mountains.

Acid rain -- rich in carbon dioxide -- pelted the newly exposed Appalachian rock and wore it away. Chemical reactions trapped the carbon in the resulting sediment, which formed reefs in the vast seas that covered North America.

For about 10 million years, the volcanoes continued to add carbon to the atmosphere as the Appalachians removed it, so the hothouse conditions remained stable. Life flourished in the warm oceans, including abundant species of trilobites and brachiopods.

Then, 450 million years ago, the eruptions stopped. But the Appalachians continued weathering, and atmospheric carbon levels plummeted. The Earth swung from a hothouse to an icehouse.

By 445 million years ago, glaciers had covered the south pole on top of the supercontinent of Gondwana (which would eventually break apart to form the continents of the southern hemisphere). Two-thirds of all species had perished.

When they started this research, Saltzman and his team knew that Earth's climate must have changed drastically at the end of the Ordovician. But they didn't know for certain that volcanoes were the driving force, explained Seth Young, who did this research for his doctoral degree at Ohio State. He is now a postdoctoral researcher at Indiana University.

"This was not necessarily what we expected when we started investigating, but as we combined our data sources, the story began to fall into place," Young said.

Using a computer model, they drew together measurements of isotopes of chemical elements -- including strontium from rocks in Nevada and neodymium from rocks in Virginia and Pennsylvania -- with measurements of volcanic ash beds in the same locations. Then they factored in temperature models developed by other researchers.

The ash deposits demonstrated when the volcanoes stopped erupting; the strontium levels indicated that large amounts of volcanic rock were being eroded and the sediment was flooding Earth's oceans during this time; and the neodymium levels pinpointed the Appalachians as the source of the sediment.

The new findings mesh well with what scientists know about these ancient proto-Atlantic volcanoes, which are thought to have produced the largest eruptions in Earth's history. They issued enough lava to form the Appalachians, enough ash to cover the far ends of the earth, and enough carbon to heat the globe. Atmospheric carbon levels grew 20 times higher than they are today.

This study shows that when those volcanoes stopped erupting, carbon levels dropped, and the climate swung dramatically back to cold. The timing coincides with today's best estimates of temperature fluctuations in the Ordovician.

"The ash beds start building up at the same time the Appalachian weathering begins, but then the record of volcanism ends, and the temperature drops," Saltzman said. "Knowing these details can help us understand how carbon in the atmosphere is changing Earth's climate today."

Next, the researchers will examine the role of the ancient volcanic ash more closely. While the ash was in the atmosphere -- before it settled around the globe -- it might have blotted out the sun, and cooled the earth somewhat. Saltzman and his team want to make some estimate of this short-term cooling effect to refine their computer model.

Meanwhile, Young is just starting to re-analyze the same rock samples, this time looking for a different isotope -- sulfur. This, he hopes, will offer clues to how much oxygen was in the oceans, and how that oxygen may have affected life in the Ordovician.

Death by carbon cycle. Nice. No time to comment beyond saying that this would mesh well with what we know of the Ordovician Extinction and its probably causes. Also, it strongly argues against using broad sweeping, horribly time stepped models to draw detailed conclusions from. Especially ones that you do not at least attempt to correlate them to the very thing they are supposed to be modeling. ahem.

Symptom or Complication: Algae Mats at Mass Extinctions

ROLE OF TOXIN-PRODUCING ALGAE IN PHANEROZOIC MASS EXTINCTIONS: EVIDENCE FROM MODERN ENVIRONMENTS AND THE GEOLOGIC RECORD

CASTLE, James W.,

Dept. of Environmental Engineering and Earth Sciences, Clemson University, 340 Brackett Hall, Clemson, SC 29634-0919, jcastle@clemson.edu

RODGERS, John H. Jr,

Dept. of Forestry and Natural Resources, Clemson University, 261 Lehotsky Hall, Clemson, SC 29634-0317

Abstract:

Observations from modern environments and evidence from the geologic record support the hypothesis that toxin-producing algae were present in the geologic past and played an important role in Phanerozoic mass extinctions. Mass mortalities of invertebrates, fish, birds, and mammals caused by algal-produced toxins are occurring in modern environments. Several types of human illness, some resulting in death, are attributed to toxins produced by algae. In addition to direct effects of these toxins, the large mass of organic material produced by algal blooms can result in dissolved oxygen depletion during decay and indirectly cause death of some biota. Toxin-producing algae occupy a wide range of modern marine, brackish, and freshwater environments. Their growth in aquatic environments is favored by warm water temperatures, increased inorganic carbon concentrations (e.g. CO2), and abundant bioavailable nutrient supplies. Modern, toxin-producing algal blooms are occurring at increasing frequency, which may be related to global warming. Cyanobacteria (blue-green algae) are responsible for most of the disease and death caused by algal toxicity today. The rock record demonstrates a pronounced increase in abundance and environmental range of algae, including stromatolitic cyanobacterial mats, coincident with major Phanerozoic mass extinctions. During these past events of algal expansion, declines in populations of metazoan taxa may have been caused by lethal effects of algal blooms, including algal-produced toxins, at a scale sufficient to generate a fossil record of mass extinction. Past environmental changes such as climatic warming, sea-level change, and increased nutrient supply may have promoted algal blooms over vast expanses of marine to freshwater environments. Environmental stressors including UV irradiation, drought, physical injury, and changes in water chemistry can induce algae in modern environments to produce increased quantity and potency of toxins. Catastrophic events such as volcanism and impacts may have been a source of environmental stress that caused or contributed to increased production or potency of algal-produced toxins in the geologic past.

Symptom or complication?

Is this a symptom of the mass death: the lack of grazers allows the return of bacterial mats? Or is it one of the complicating causes of mass death during extinctions? Or both?

Ordovician Extinction Caused by Gamma Ray Burst?

A new study has suggested that a brilliant burst of gamma rays may have caused a mass extinction event on Earth 440 million years ago, and a similar celestial catastrophe could happen again in the future.

Most gamma-ray bursts are thought to be streams of high-energy radiation produced when the core of a very massive star collapses. According to a report in National Geographic News, the new computer model shows that a gamma-ray burst aimed at Earth could deplete the ozone layer, cause acid rain, and initiate a round of global cooling from as far as 6,500 light-years away.

Such a disaster may have been responsible for the mass die-off of 70 percent of the marine creatures that thrived during the Ordovician period (488 to 443 million years ago), suggests study leader Brian Thomas, an astrophysicist at Washburn University in Kansas.

The simulation also shows that a significant gamma-ray burst is likely to go off within range of Earth every billion years or so, although the stream of radiation would have to be lined up just right to affect the planet.

Currently WR104, a massive star 8,000 light-years away in the constellation Sagittarius, is in position to be a potential threat, according to Thomas.

Study author Thomas’ former graduate advisor, Adrian Melott, first proposed in 2004 that a gamma-ray burst near Earth wiped out Ordovician life.

Since then, both researchers have been tackling pieces of the puzzle.

According to their newest models, gamma radiation from a nearby burst would quickly deplete much of Earth’s protective ozone layer, allowing increased ultraviolet radiation (UV) from the sun to reach the surface.

In the longer term, chemical reactions in the atmosphere would produce dark, nitrogen-based gases that would block the sun’s heat and trigger global cooling, even as the gamma rays continued to deplete ozone and let in UV rays, the authors suggested.

Some of the pollution would fall as damaging acid rain, which can severely disrupt ecosystems.

From what I can see here, this looks like its just a sim. Do they have any field work to support their assertion? Anyone have this paper? Or is it even a paper at all?

CCSM Paleoclimate Ordovician

This is presented by Christine Shields of NCAR. She is speaking on her simulations of he Late Ordovician. This is also done with Dr Scotese of the paleomap project and Dr Kiehl of NCAR. She is exploring why the Ordovician Extinction is interesting. This is the only extinction to take place during glaciation. This causes a very mixed ocean and this apparently brought up toxins from the sea bed as well as provide lots nutrients. Algae go nuts making more toxins. Causes extinction.

Ice extent is close to the subtropical during the time period and reaches up to 30 deg s. There are no continents in the northern hemisphere. They literature seems to indicate a high CO2 content, 15x pre industrial. This is a big dilemma. How do you get glaciation with uber carbon dioxide? They ran with T31 resolution for 100 years. They chose a cold orbit with low eccentricity. They found that solar forcing seems to be 15.5 watts/m^2 total. Model seems to be a proof of concept: first time for a fully coupled model on the Ordovician.

Poles are warmer than now in the simulation. This is a warm world. They actually get a very strong circumpolar current: something like a roaring 40s there. The ocean mixing is extremely strong. Northern hemisphere overturns down to the very bottom of the ocean. Polar regions seem to show that it takes 10 years for overturn (ideal age). Modern Atlantic is 31 years +/-.

They're changing the model. Adding ocean topography and adjusting the CO2 estimates: they need to get the ice to grow, don't get it yet. They think they will get it. Shields is very optimistic. because of the weathering outbalancing the dominating the solar forcing. They need to change the ground cover model.

I'll save commentary until later.

*sighs* YAGUMET: Sea Rise & Fall

If you are curious about Earth's periodic mass extinction events such as the sudden demise of the dinosaurs 65 million years ago, you might consider crashing asteroids and sky-darkening super volcanoes as culprits.

But a new study, published online June 15, 2008 in the journal Nature, suggests that it is the ocean, and in particular the epic ebbs and flows of sea level and sediment over the course of geologic time, that is the primary cause of the world's periodic mass extinctions during the past 500[sc1] million years.

"The expansions and contractions of those environments have pretty profound effects on life on Earth," says Shanan Peters, a University of Wisconsin-Madison assistant professor of geology and geophysics and the author of the new Nature report.

In short, according to Peters, changes in ocean environments related to sea level exert a driving influence on rates of extinction, which animals and plants survive or vanish, and generally determine the composition of life in the oceans.

Despite what Peters' statements, this is not a new theory. This is actually a rather old one. One that had some nontrivial work done on it by Dr Anthony Hallam et al. He referred to this as the 'regression-transgression' scenario. He talks about it in Catastrophes and Lesser Calamities published in 2004. By this point he'd been advocating the correlation between the sea level changes and mass extinctions for more than 20 years. The problem is that the sea level changes are a correlation and sometimes even a symptom of something else happening.

Frex, the sea level fall in the Ordovician that appears to have driven the mass extinction, yes, but it was a symptom caused by glaciation. Furthermore, how does sea level change effect the planktonic fossils? Specifically those that are photosynthetic that got mopped at the end of the KT/K-Pg? Or the benthic fossils during the PT Extinction? I said it before and I will say it again: the cause of a mass extinction needs to fit the evidence or symptoms of the extinction, not just be chronologically convenient.