Bad things come in threes, it is said, and nothing shows that more than The Carnian Age; the first stage of three in the mass extinction era of the Late Triassic Epoch 228 to 217 million years ago.

It had three features; dramatic climate change with much higher humidity, the first dinosaurs appeared, and gigantic volcanic eruptions called the Wrangellia large igneous province spewed out greenhouse gases.

For dinosaurs, climate change was a good thing because plant life grew, thanks to the humid conditions and higher carbon dioxide. That event, now called the Carnian Pluvial Episode, may have been caused by the Wrangellia volcanic eruptions. The confluence of events seems to have spurred the early diversification of dinosaurs.

Muddy footprints along the shoreline of a lake during a rainstorm some 234 million years ago in northwestern Argentina. Image: Jorge Gonzalez/NHMU

There are challenges in studying the ancient past today. Precise dates for many Carnian sediments makes comparisons difficult and few detailed paleoclimatic data exist outside of Europe, making it unclear whether the Carnian Pluvial Episode was truly a global climate event or more European Exceptionalism when it comes to science. Equally challenging is conclusively linking it to dinosaur diversification but a new paper suggests that the Carnian Pluvial Episode affected the southern hemisphere, specifically South America, which strengthens the case that it was a global climate event.

The team studied Carnian rocks of the Los Rastros Formation, which are preserved in the Ischigualasto-Villa Unión Basin in northwest Argentina. For the first time, the team precisely dated volcanic ash preserved in lake sediments and pieced together the paleoclimate at this time. They focused on those because they had the perfect combination of a good fossil record, dateable ash layers, and rich climate data preserved in lake sediments.

In order to date the ash layer, the scientists isolated small needle-like crystals of zircon, minerals that act like time-capsules. When zircon crystallizes during an eruption, it traps the element uranium in its crystal structure, but never incorporates lead. Any lead preserved in the crystals today is a result of the radioactive decay of uranium. Because scientists know this decay rate, they can measure the ratio of uranium and lead in each zircon crystal and calculate how far back in time the crystals formed. In the present study, this measurement was done on a precise mass spectrometer at the Berkeley Geochronology Center.

The scientists then obtained paleoclimatic data by looking at detailed features in the sediments-- the types of clay preserved, and carbon and oxygen isotopes in freshwater limestone layers. With these measurements, the researchers estimated temperature, humidity and aridity and observed a distinct interval of particularly warm and humid conditions. Based on the absolute date from the same strata, they concluded that it matched in time with the CPE in the northern hemisphere. By using a variety of different analyses, the resulting paleoclimate inferences were more robust than previous assertions made from a single line of evidence.

This warm/humid interval also fortuitously includes layers that preserve fossil footprints of early dinosaurs or their closest relatives.

"Our study suggests that the appearance of dinosaurs in South America could be linked to the CPE, but the data available worldwide remains inconclusive. To make a more robust global link between the CPE and dinosaur diversification will require many more detailed studies of paleoclimate with precise ages like we were able to do for the Los Rastros Formation in Argentina," says Adriana Mancuso, a National Scientific and Technical Research Council (CONICET) researcher at the Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales in Mendoza, Argentina.