December 5. Paleocene-Eocene Thermal Maximum

To start with today, an update on Archaeopteryx, the first bird. Or maybe not. New research is showing that the transition from dinosaurs to birds was complex, and some lines of thought suggest that Archaeopteryx was a feathered, gliding dinosaur rather than a true bird. This year’s Society of Vertebrate Paleontologists’ meeting in Berlin included new studies of Archaeopteryx. Here's a link to a report from the journal Nature on current thinking about the possible first bird.

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Our Cenozoic topic today is the climate at the end of Paleocene and beginning of Eocene time, about 55 or 56 million years ago. Just about at the boundary between the two epochs there was a short, intense, 170,000-year period of warming called the Paleocene-Eocene Thermal Maximum, or PETM.

Temperatures rose quickly, over about 20,000 years, by about 6°C, and carbon isotope ratios suggest that the oceans underwent acidification because of increased CO2 that was dissolved in ocean water. Sea levels rose because of thermal expansion of the water. Some extinctions are associated with the Thermal Maximum, especially among deep-water marine microorganisms, but some shallow-water calcareous microorganisms actually became more abundant. There’s no evidence for significant extinction on land, and in fact shortly after it began, there is a dramatic radiation of some mammals such as camels, horses, pigs, primates, and other groups. 

We don’t really know what caused the changes in CO2 and temperature during this Thermal Maximum. There are some volcanic eruptions that are at essentially the same time, including some in Canada and Greenland that might have accounted for the spike in carbon dioxide, but the volcanism is more suited to explaining the more gradual warming that was occurring through the Paleocene. Spikes are always challenging to explain.

Speculations about impacts have found no evidence. One good possibility is that warming oceans reached a threshold temperature at which methane hydrates – essentially, methane gas trapped in ice within sea-floor sediment – melted and released their methane, a more potent greenhouse gas than carbon dioxide. Such a release could have occurred over a short period of time – hundreds to thousands of years – making it a reasonable candidate for the spike in temperatures. But changes in carbon isotope ratios suggest that the effects of the event worked their way from shallow waters into deep waters over a span of around 10,000 years, the opposite of what would be expected with a deep-water methane release. Exactly what was going on in the oceans is hard to quantify – things that might support the methane idea include changes in ocean circulation that might have mixed warm tropical waters more thoroughly in the world ocean. The Isthmus of Panama did not yet exist, so there would have been more connection between the Atlantic and Pacific.

The Paleocene-Eocene Thermal Maximum is being studied quite intensely because it may serve as a model for the modern increases in temperature and carbon dioxide that are occurring as a result of human activities. I have a link below to an article in Paleontology Online by Phil Jardine, which is a nice overview of the facts of the Thermal Maximum at the end of the Paleocene.

—Richard I. Gibson

Paleocene-Eocene Thermal Maximum
Carbon releases and PETM  

November 4. Paleogeography and climate

Because it’s so long – 80 million years – it’s definitely not cool to imply anything constant about the distribution of lands and seas nor about the climate of the Cretaceous, but as usual I’m happy to make some generalizations. As we said at the end of the Jurassic, the globe was beginning to become recognizable in terms of the modern continents. We’ll talk about some specifics, like the opening of the South Atlantic Ocean, over the course of the month, but at least by the middle of the Cretaceous, about 100 million years ago, the basic shapes of most of the continents were close to their modern forms.

Cretaceous globe (105 million years ago) by Ron Blakey, used under CC-BY-SA & GDFL. 

India and Australia were still way south of the Equator, having just separated from Antarctica, and the Tethys Ocean was still between them and the main mass of Asia. Antarctica itself was near or at the South Pole as it is today, and the northern margins of North America and Eurasia were in the north polar region as they are today.


But the climate was much less extreme than it is today, with only a few notable exceptions, throughout most of the 80 million years the Cretaceous lasted. There were no widespread continental glaciers during the Cretaceous, although there were probably mountain glaciers in high latitudes, but even that was probably limited to the very early Cretaceous in what is now Australia. Most of the rest of the period was warm, and the temperature change from poles to equator was not extreme. One probable consequence of the weak temperature gradient would be less extreme winds and perhaps fewer intense storms like modern hurricanes. That, in turn could lead to oceans with poorer circulation – though the general distribution of land masses, at least until the late Cretaceous, might have a greater control on that. This is a possible explanation for the extensive black shales, related to anoxic events in the oceans, which are observed in the Cretaceous rock record.

The equable climate at the poles is reflected in the presence of dinosaurs there, as well as temperate plants. Southernmost Australia today was within the Antarctic Circle in early Cretaceous time, and mud flats that formed there in the rift between Australia and Antarctica have yielded diverse dinosaur bones, and Cretaceous dinosaurs are also known from Antarctica itself.

Temperate forests were present within a few degrees of the poles. Remember that whatever the temperature, the poles still had long seasons of darkness just as they do today. Cretaceous greenhouse conditions allowed plants to grow in polar regions, but it might have been the light and dark cycles that provided evolutionary pressure for plants to evolve. The early Cretaceous forests were much like those of the Jurassic, with cycads, ferns, and conifers. But within 15 or 20 million years, dramatic evolution of the angiosperms, flowering plants, had begun. We’ll talk more about that in a few days.

By late Cretaceous time, average temperatures had risen even more, to perhaps 4 to 6 degrees Centigrade warmer than today. This time is called the Cretaceous Thermal Maximum, and it occurred during the Turonian stage. Exactly what caused it is debated, but two favored factors are both related to rifting. As more and more rifts formed, and oceanic crust was generated, the volcanism associated with rifting could have put more and more CO2 into the atmosphere, enhancing an already strong greenhouse effect. It’s also possible that changing patterns of land and sea allowed for more efficient movement of warm surface water around the globe. There is considerable variation in the patterns of temperature even during the Turonian, so this event is definitely not fully understood.

In North America, the Cretaceous was dominated by the Western Interior Seaway, a long shallow sea that extended from Alaska to Alberta to Colorado to West Texas and into Mexico. We’ll talk more about that when we talk about the mountain-building events that were defining the western margin of North America, later this month.

—Richard I. Gibson

LINKS:
Rifting and Cretaceous Thermal Maximum 

Cretaceous Thermal Maximum

Dinosaur Cove, Australia 

Cretaceous globe (105 million years ago) by Ron Blakey, used under CC-BY-SA & GDFL. 

October 22. Climate and extinctions

You really can’t fully characterize the climate of an entire 50-million-year period like the Jurassic with a few sentences, but we can make some generalities. With the break-up of Pangaea dominating the planet’s tectonics during Jurassic time, there was more volcanism associated with the rifting, and because the supercontinent was fragmenting, there were much longer coastlines. As sea levels rose because new mid-ocean ridges displaced large volumes of water, there were more shallow seas spreading over low-lying areas of the continents. This created many ecological niches for marine and near-shore life.  And with smaller continents, more regions were relatively close to the sea and its moisture, so the desert conditions of the Triassic were much less widespread during the Jurassic.  

That’s the general picture, but it’s punctuated by variations from that general trend at least a few times during the Jurassic. 

There’s no evidence of glaciation anywhere on earth during the Jurassic. It was a warm time, if not so exceedingly hot as the Triassic appears to have been. The atmosphere contained both more oxygen and more carbon dioxide than today, leading more or less to greenhouse conditions – Jurassic plants flourished, and eventually became coal and oil. Many plants were distributed worldwide, suggesting that the climate was probably more uniform than today, both in terms of geographic changes from the poles to the equator and perhaps in terms of seasonal changes as well. With an average temperature as much as 3°C above the present, winters would have been mild and summers hot. There may have been winter snow in the polar regions, but probably nothing like today. Temperate conditions and vegetation extended much further north and south than they do today.  

There were a few relatively minor extinction events during the Jurassic. One, during the Toarcian age toward the end of the Early Jurassic epoch, impacted diversity of brachiopods, crinoids, bivalves, ammonites, and other groups. We talked about this extinction on October 5 in connection with the eruption of the Karoo Volcanics about 183 million years ago.

The Tithonian is the last stage of the Jurassic, with the end of the period at about 145 million years ago. It is marked by an extinction event that appears to have killed off at least 7 of the 11 ammonite families living at the time. About a quarter of the mollusk families in Europe died in this event. There was a regression of the sea at the same time, at least in much of Europe, which could be an important factor in the extinctions. This wasn’t a world-wide sea-level change, and in South America the sea actually transgressed over the land in places. There is actually no evidence for a mass extinction among bivalves in South America at this time, quite a contrast to the situation in Europe. So I think at best, we have to see the end of the Jurassic as a time when regional extinctions happened, but there was nothing huge, nothing of global extent.

—Richard I. Gibson

LINKS:
Smithsonian
Declining Oxygen Levels and Jurassic Extinction
Toarcian biological crises 
Minor extinctions of the Jurassic
Jurassic climate

September 24. The Carnian Pluvial Event

You have probably gotten the impression that the Triassic was a pretty awful time, hot and dry on land and with tropical seas that might have been too hot for life. But I hope I have also made it clear that things were not uniform, and not necessarily so bad, all over the entire globe, and also not through time during the Triassic. During the late Triassic, during the Carnian stage, at about 230 million years ago, there was a big-time disturbance in the carbon cycle that among other things suggests that the climate was dramatically more humid and rainy, at least for a while. It’s called the Carnian Pluvial Event, and like most geological “events,” we’re probably talking about a million years or more. 

The evidence comes mostly from changes in carbon isotope ratios. Different carbon isotopes are taken up at different rates by organisms including plants and calcite-secreting animals such as clams and corals. These rates of uptake are proportional to temperature and can be used to infer temperatures in past geologic times. 

Geological evidence for this humid interval includes the presence of paleosols, ancient soil horizons that would require increased weathering and water for them to form. Fossil spores suggest an increase in more humid-adapted plants, and there is also a world-wide pulse in sediment such as sand and silt, implying that there was greater runoff and erosion.

Map from The Accreted Wrangellia Oceanic Plateau in Alaska, Yukon, and British Columbia

There are two possible causes suggested for this event. First, the carbon cycle could have been disrupted by a massive eruption of volcanic rocks, which would put lots of CO2 into the atmosphere. There is indeed a possible smoking gun for this scenario. A complex block of material called the Wrangellia Terrane, which is now in Alaska, Yukon, and British Columbia, in the Triassic was hanging around offshore western North America. It was a mess, a combination of island arcs, oceanic crust, and other stuff, but it also contains an extensive pile of basaltic lava flows. The flows were flood basalts, similar to but less extensive than the Siberian Flood Basalts that were extruded near the end of the Permian. The Wrangellia Flood Basalts are dated to 230 to 225 million years ago, virtually coincident with the start of the Carnian Pluvial Event. The CO2 released by the volcanism could have brought on a period of global warming, more favorable to evaporation and an active hydrologic cycle, which is a fancy way of saying it would have rained more. Enough CO2 could have impacted the ocean’s waters to change the depth where calcium carbonate can precipitate out – and that could have resulted in the changes we see at this time in the way carbonate platforms, reefs and such, changed to low-relief ramps with less calcium carbonate available.

The changes in the ocean led to extinctions in some groups, including crinoids, bryozoa, and ammonites, but after the Carnian event, some groups including corals diversified dramatically. And on land, it is impossible to ignore the fact that the dinosaurs seem to have begun at just about this time as well. And they also diversified quite rapidly.

In addition to the Wrangellia basalt flows, the Cimmerian continental blocks were beginning to collide with Asia. You recall Cimmeria, a long narrow disjointed strip that rifted off the northeast edge of Pangaea in late Permian time. Since then, it had been chugging across the ocean, closing the Paleo-Tethys ahead of it and opening the Neo-Tethys behind it. Well, it was finally colliding with something big – Eurasia – by late Triassic time. It’s kind of like the collision of India with Asia, in miniature, but high mountains were uplifted. The mountains could have served as a barrier to moisture-laden winds, resulting in a prominent monsoon with lots of rain, which in turn would have produced active erosion and the thick piles of erosional debris, sand and silt and mud, that we see in the rock record.

There’s actually a lot of work going on about the Carnian Pluvial Event and its possible causes and consequences. I doubt quite a bit if this is fully understood yet.

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Philip B. King was born September 24, 1903, in Chester, Indiana. His career with the U.S. Geological Survey was marked by extensive work in West Texas, especially the Marathon region, where he worked out much of the stratigraphy and tectonics. He became a synthesizer of geologic data, and compiled the Tectonic Map of the United States, Tectonic Map of North America, and the 1974 Geologic Map of the United States. These maps were and still are of great importance to our broad understanding of how continents are constructed and evolve.

—Richard I. Gibson

Links:
Carnian event and Wrangellia basalts 

Wrangellia oceanic plateau 

Map from The Accreted Wrangellia Oceanic Plateau in Alaska, Yukon, and British Columbia [Excellent resource]

September 6. Triassic Climate

You can imagine that whatever the causes of the great extinction at the end of the Permian, the effects continued at least a while into the Triassic. It looks like it took maybe 4 million years or so for the recovery to be clearly underway. Extinctions are double-edged swords – while they decimate many species and eliminate countless individuals, they also clear the slate, opening up ecological niches, so that when conditions allow, the survivors that can change and adapt to the new circumstances have great opportunities to expand into vacated ecological realms. Extinctions favor the opportunistic, to some degree at least. 

So what were the conditions during the Triassic that life had to adapt to? To a large extent, Pangaea was still assembled into one big continent. We heard about small blocks rifting off to form the Cimmerian continent, and there is evidence that Pangaea had started to rift apart between today’s Greenland and Scandinavia – the first hints of the North Atlantic Ocean. But on the whole, it was still one big continent, and its tropical interior, distant from the sea was in many places pretty hot and arid. The red beds that characterize much of the Triassic are clear evidence for this.

On average, carbon dioxide levels in the atmosphere were high. Depending on which geochemical model you use, the level could have been as much as 1500 parts per million, 4 times today’s value. And oxygen levels had plummeted during the Permian, to possibly as low as 15% of the total versus 21% today. That continued during the Triassic, increasing slowly through the period. The overall average temperature was higher than today, probably at least 3ºC higher, and perhaps more. All of the geochemical models and actual measurements indicate hot, dry, carbon-dioxide-rich settings during the Triassic, and that meshes well with the kinds of rocks – commonly, red beds and evaporites – and terrestrial life, adapted to such conditions, that we actually observe. 

Arid and dry does not necessarily mean conditions like the middle of a modern desert. There had to be some water, both for life and to give the alternating wet-dry conditions that produce red beds and evaporites. And even modern deserts, with some relatively small exceptions, support life. The Triassic seems to have been an intensely seasonal time – in part because of the world ocean, Panthalassa, which would have generated monsoons bringing seasonal rains to at least the coastal parts of Pangaea. Large woody trees, hit hard by the Permian extinction, appear to have recovered to a large extent within about 6 million years, about 245 million years ago during the early Triassic.

You can and should imagine that we would not expect the 50-million-year time span of the Triassic climate to be uniform and boring. There was always variety, depending on where you were on the globe, and there was plenty of variation in time as well. There’s even evidence that at times during the Triassic the climate became much wetter. We’ll talk about one such period in the late Triassic later this month.

—Richard I. Gibson

April 26. Silurian deserts?

We know from all that salt in Michigan that the climate during part of the Silurian was hot enough in places to evaporate small seas. Were there deserts?  We don’t really know. There’s a lot of work on Silurian sea-level changes and geochemistry that attempts to unravel the nature of planet earth during the Silurian – and there is still quite a bit of controversy, as far as I can tell.

Scientists studying oxygen and carbon isotopes find that variations in them during the Silurian coincide with some relatively small extinction events. If nothing else, it’s beginning to give a picture of the Silurian as a time with repeated climate changes, in contrast to what I was taught 45 years ago that the Silurian was stable, uniform, and hot. Changes in temperature affect the uptake of different isotopes of carbon and oxygen in rocks and organisms such as the shells of animals – that’s how we use isotope ratios to infer changes in climate. But what is clearly not clear is what was causing those climate changes.

Most of what I have read suggests that indeed, there must have been periods of arid conditions and high temperatures, which fits with those vast salt deposits. So it’s reasonable to suppose that there were deserts somewhere during at least parts of the Silurian Period. But so far as I can tell, there’s little other direct evidence for such deserts – no vast deposits of wind-blown sand, for example.

As usual, research continues.

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Today, April 26, is the birth date of Leopold von Buch, in 1774, at the castle of Stolpe, Brandenburg, Germany. You probably haven’t heard of von Buch, but he was a prominent and important geologist in the early 19th Century. He studied the volcano Vesuvius and recognized the volcanic origin of basalt – at the time, many scientists thought basalt, and virtually everything else, crystallized from water. And von Buch defined the subdivisions of the Jurassic Period of the Mesozoic Era, which we’ll get to in October.

It’s also the birthday of a geophysicist you probably have heard of – Charles Richter, born this day in 1900, in Ohio. He invented the Richter scale, the first real quantitative way of estimating the magnitude of earthquakes. Seismologists use the moment magnitude scale today, but the Richter scale was the principal way of evaluating earthquakes from 1935 into the 1980s.

—Richard I. Gibson

References
Paper by Munnecke et al. 2010
Silurian climate