HHGS Global Warming Events

Global warming events in earth history

Alex Dickson

Introduction

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:

Sunspot cycles –years to decades;
Greenhouse gases, especially CO₂ from volcanic activity, methane hydrates and weathering of old organic carbon –years to hundreds of thousands of years;
Periodic oscillations of the orbit of earth through space (Milankovitch forcing) – tens of thousands to millions of years; and
Tectonic changes – thousands to millions of years.

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 10-20^oC 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:

5-8^oC global warming;
ocean acidification (deep ocean);
stronger hydrological cycle (more precipitation);
benthic extinctions;
enhanced adaptation and evolutionary turnover in surface ocean organisms;
enhanced productivity near continental margins; enhanced oligotrophy in central gyre regions;
rapid evolution, migration and adaptation in terrestrial fauna and flora; and
greater weathering due to the hydrological cycle.

Research has indicated extreme tropical warming during the PETM, with tropical ocean temperatures of 36-43^oC, the highest temperatures known from the Cenozoic and possibly too hot for organisms to survive.

The PETM was caused by a massive release of CO₂ to the atmosphere, probably derived from:

methane hydrate destabilisation (~50-60‰);
combustion of organic carbon by volcanic intrusions (~20-35‰);
direct volcanic CO₂ input (~5‰);
methanogenesis from peatlands; and
erosion and oxidation of terrestrial organic carbon.

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:

How much carbon was added to the atmosphere?
Where did the excess carbon come from?
What caused carbon release (oceanic circulation, tectonic threshold, gradual volcanic warming)?
How extreme did the weather become?
How was the excess carbon sequestered after 200,000 years (weathering or organic carbon drawdown)?

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;

large enrichments in organic carbon in marine sediments, which are now black shale;
one of the major Phanerozoic extinction events, with widespread loss of foraminifera and the death of shallow marine carbonate ecosystems;
2-3^oC of oceanic warming in the tropics; and
a drop in atmospheric CO₂ concentrations;

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 30^oC from 5^oN to 30^oN. This compares with the modern maximum of about 27^oC. De-oxygenation has been calculated using various methods;

5-12% of the sea floor (from molybdenum isotopes, 20-40 times the modern extent);
<5% of the sea floor (sulphur isotopes, 17 times modern);
1% of the sea floor (uranium isotopes, 3 times modern); and
GENIE modelling shows >50% of the sea floor (170 times modern).

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:

What caused tropical ocean warming?
How much carbon was buried to cause the drop in atmospheric CO₂ concentrations during the event?
What triggered OAE-2? Was more than one large igneous province involved?
How extensive was seawater anoxia during OAE-2? What were the processes that caused anoxia?
How did paleogeography pre-condition the Cretaceous world to extensive anoxia and warming?

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:

a large (6 per mil) negative carbon isotope excursion in marine carbonates and organic matter;
large enrichments in organic carbon in many marine sediments that now appear as black shales;
one of the major Phanerozoic extinction events, with widespread loss of marine and terrestrial organisms, including calcareous nannoplankton;
7-13^oC surface ocean warming in the northern mid-latitudes;
initial drop in atmospheric CO _concentrations, followed by a huge increase; and
it is associated with activity from the Caribbean, Ontong-Java, and Arctic large igneous provinces.

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:

Methane hydrate destabilisation (-50 to -60‰);
Combustion of organic carbon by volcanic intrusions (-20 to -35‰); and
Direct volcanic CO₂ input (-5‰).

Best estimates are that between 6,000 and 27,000Gtonnes of carbon were released during this event.

Summary

The speaker summarized his presentation in the following terms:

Short-term global warming episodes have punctuated the geological record.

The prominent episodes lasted for a few hundreds of thousands of years.
Global warming occurred quickly, over <50,000 years, and amounted to a few C o of globally averaged temperature increase.
Warming drove many climate-system feedbacks (weathering, ocean circulation, atmospheric circulation).
Climate-system feedbacks caused ecological perturbations (extinctions, turnover, morphological adaptation).
Ultimate causes are debated, but volcanism remains a major candidate.
The fastest rate of past warming was an order of magnitude slower than today.