Harrow and Hillingdon Geological Society
Global Warming Events
Global warming events in earth history
There have been a number of global warming events over the last 200M years, as evidenced from oxygen isotope records. This presentation covered 3 of those events, detailing their context in the geological records, their characteristics and their consequences.
Climate warming has a number of different causes:
Analysis of the earth’s climate from oxygen isotope records shows that it was much hotter 120M years ago than today and also in the mid-Palaeogene. Peak warming was in the mid-Cretaceous when temperatures were 10-20oC warmer than today.
Earth’s climate for the past 150-200M years (after Jenkyns, 2010)
Palaeocene – Eocene Thermal Maximum (PETM)
The PETM occurred at about 56 million years (Ma), when continents were in much their modern positions except that India and Australia were further south and there was a trans-continental seaway from Central Asia through to the high Arctic and the Tethys Ocean joining it to the Atlantic and Indian Oceans, via southern Europe, the Mediterranean and the Middle East.
Eocene palaeogeography (after Scotese, 2010)
The PETM was characterised by:
The PETM was caused by a massive release of CO2 to the atmosphere, probably derived from:
It has been estimated that between 3,000 and 6,800 Gtonnes of carbon were released during the PETM. In comparison, the deep ocean today holds 90,000Gtonnes of dissolved carbon.
One of the major consequences of global warming events is de-oxygenation of the deep oceans. Analysis of molybdenum isotopes indicates that 1% of the sea floor was completely anoxic (twice the modern extent) and that there were extensive areas of sub-oxic conditions in the South Atlantic and Indian Oceans.
On land, there was a significant weathering response. Since it was hotter and wetter, there was more weathering and this is clearly indicated by osmium isotope analysis, which is used as a proxy for weathering.
While we know quite a lot about the PETM, there are still a number of unknowns:
Ocean Anoxic Event 2 (OAE-2)
This anoxic event occurred in the late Cretaceous at 94Ma, when the Atlantic was only partly open, Australia was still joined to Antarctica, India was in mid-latitudes, there was a Western Interior Seaway in North America and a much bigger peri-Tethys Ocean.
Late Cretaceous palaeogeography (after Scotese, 2010)
It was characterised by;
It was associated with activity from the Caribbean, Ontong-Java and Arctic large igneous provinces. It was first recognised at Furlo in Italy, where 1.5m of black shale (the Volcanic Bonarelli Black Shale) overlies a carbonate sequence. It has also been found by the Ocean Drilling Programme off Newfoundland.
During OAE-2, the temperature gradient in the tropics was from 35 to 30oC from 5oN to 30oN. This compares with the modern maximum of about 27oC. De-oxygenation has been calculated using various methods;
Osmium isotopes were higher before the event, indicative of weathering and lower in the event, indicating volcanism. The OAE-2 is very intimately associated with the emplacement of volcanism in large igneous provinces.
Still unknown are:
The early Toarcian Oceanic Anoxic Event (T-OAE)
This occurred in the early Jurassic at 195Ma and all the sea floor at the time has now gone. All the continents were joined together and there was only one seaway between North America and Eurasia.
Early Jurassic palaeogreography (after Scotese, 2010)
It is characterised by:
Much of the data concerning this event is derived from the Jurassic mudrocks of the Yorkshire coast. It was driven by release of carbon and marine carbonates and terrestrial wood isotopes followed each other. There was a strong weathering response, with a 400-800% increase in the average rate of weathering.
Release of carbon is believed to have been from:
Best estimates are that between 6,000 and 27,000Gtonnes of carbon were released during this event.
The speaker summarized his presentation in the following terms: