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Wednesday, September 24, 2014

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
Carnian event and Wrangellia basalts 

Wrangellia oceanic plateau 

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

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