The Environment of the Cretaceous

The world during the Cretaceous looked very different. Its continental arrangement was different, as was the 'flora and fauna' that survived in a very different climate. The Cretaceous Period as a whole, and the significance of some specific environmental aspects of the time, [are now discussed].

The following discussion covers:

• The Appearance of the World
• The High Sea Level
• Climate and the High Temperature

The Appearance of the World

From space, the Cretaceous Earth looked markedly different. Although the continental arrangement was beginning to resemble that of today, an equatorial-seaway existed around the globe, dividing the land mass into Northern, and Southern, continents. Within this arrangement of continental separation by an oft-narrow seaway, Africa was still welded to South America, Antarctica and Australia were still attached to Africa, and North America and Eurasia was one.
(For more detail, please go to the Palaeogeography page)

Both the Atlantic and Indian Oceans were young and small, and the Pacific (the previous Panthallassic "super-ocean", now renamed) correspondingly wider than today.
Furthermore; there was little emergent land, little (or no) ice at the poles, and equatorial land was arid - unlike today's equatorial rainforests.
The biota was a mixture of the exotic and familiar - luxuriant green forests of now-extinct trees flourished within the Arctic Circle and dinosaurs roamed. However; flowering plants (angiosperms) were making their mark, and many marine animals, such as the crustaceans and crocodiles of the time, closely resemble those that thrive today.

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The High Sea Level

Perhaps the most significant factor of the Cretaceous world was the very high sea levels. The global sea level was at its highest ever during the Cretaceous (though was very high in the preceding Jurassic, too), peaking during the Late Cretaceous around [86]Ma ago. Various estimates have suggested height increases (above today's level) of many 100's metres, and although it is now believed that many of these estimates are excessive, it is certain that the eustatic (global) sea level was well over 200m higher during the Cretaceous than it is today.

Considering that the highest point of Portsdown Hill is 131m, and [another example! ] it is clear that vast tracts of what is emergent land today would have been flooded during that time.
(Remember, however, that at that time, Portsdown was (almost certainly) not a hill, but a flat sea-floor. Its present-day topography was the result of later tectonic movements - the Alpine orogeny, that included the collision of Africa into Europe - that folded the Cretaceous sea floor).

During the peak of the high sea levels, only isolated areas of the Highlands of Scotland, and possibly Wales, in the United Kingdom, were land. The rest of our island was under the warm, shallow, tropical sea that flooded much of the Eurasian continent.

There are three major factors which lead to this high sea level. In reverse order of importance,

Firstly, the higher temperatures that we know [existed] at the time resulted in the 'thermal expansion' of the ocean waters. Today, the ocean bottom waters are close to freezing temperature, at about 4°C. Then, sea-floor temperatures were nearer [15°C?]. Thus, as with most matter, the warmer temperature led to a greater volume for a given mass of water.

Secondly, again because of the higher global temperature, there was either very little, or no, polar land ice at the time. (Sea-ice has no effect on sea levels, as it displaces no more water as ice, than it would occupy as water. However; land-ice, as on Antarctica today, removes a huge volume of water from the oceans, thus lowering the global sea level.)
What is now polar land ice was then water; so the volume of water in the oceans was much higher then.
However; current estimates suggest that if all land ice were to melt (following our attempts to create another Greenhouse world), the sea levels would rise by 'only' about [80m]; so clearly other factors that affected the sea level were also at play in the Cretaceous.

The higher temperatures thus led to both more water in the oceans, and a given mass of water occupying a greater volume; but even these could not account for the very high levels of water, for which a third explanation is required.
[Do i really want, or need, this short para?].

The third factor is the most important, and relates to tectonic processes, rather than atmospheric conditions.

During the Cretaceous, the great landmass of Pangea was breaking up, and the new oceans all had volcanically-active ocean ridges, [pumping | extruding |ex-thingying]]. lava onto the sea floors, where they build up submarine mountain ranges.
[insert an image of a present-day ocean ridge].
These new oceans were all shallow (as they resulted from land splitting apart), and the mountainous ocean ridges displaced water from them.
Furthermore; because the rifting of the land was initiated by super-plumes from deep within the Earth, the whole sea-floor domed upwards as hot mantle material impinged upon the lithosphere; contributing to the decreased shallowing of the basin.

The Atlantic sea-floor has got, down its middle, the 'zipper' of the mid-Atlantic Rift, but other than this mountain chain, it is relatively flat. Conversely, in the Pacific - the Western Pacific particularly - there are 1000's of volcanos, some of which are in chains, others isolated. The Mid-Pacific Mountains (in its West!) [consist of | .......]. a collection of volcanos, themselves sitting on a plateau that is 1-2 km higher than the surrounding ocean floor. These mountains, which formed between 125 - 75Ma ago, show the degree of volcanic activity at the time.
Also, both the mountains, and the existence of the high plateau on which they sit, indicate how so much water was displaced over the low-relief continental land of the Cretaceous, leading to its characteristic high sea levels.

Thus, the capacity of the shallow ocean basins, with their high ocean ridges, was much less than that of the deep oceans today - or of the single ocean (the Panthallassic Ocean) that covered much of the Earth's surface before the break-up of Pangea.

• Tectonic activity and the geography of the Earth at the time reducing the capacity of the numerous but shallow ocean basins,
• The lack of polar ice because of a warmer climate, and
• thermal expansion of water in that warmer climate,

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Climate and the High Temperature

The higher temperature of the Cretaceous has already been referred to. Estimates suggest that at the beginning of the Cretaceous, the Global Mean Surface Temperature (GMST) was around 20°C (about 5° hotter than today's value of 15°C), and was about the same at the period's end - but peaked to a high of 25°C in the Upper Cretaceous.
These high temperatures were due to the much higher level of carbon dioxide in the atmosphere at the time - which has been suggested as being 4 times as much as is in our air today.
The Cretaceous was thus an intense "Greenhouse world", and we have a long way to go before reaching those conditions.

The causes of the different climate and higher temperatures are, like the high sea levels, varied.
The continental arrangement, with its equatorial seaway, was partially responsible, in that it limited the oceanic circulation. The transport of heat, via the circulation of warm equatorial waters to polar regions is a major factor in today's climate.
(This is the reason why the areas of Western Europe will become colder before they get hotter, as global warming proceeds, The Gulf Stream and warm equatorial waters that give us our relatively warm climate for our northerly latitude, will cease to flow. Thus, when we no longer gain the advantage of those warming waters from the South, our climate will get colder.
There is even a theory that this effect could tip the Earth back into an Ice Age, rather than warming up; but there is no consensus for this yet).

However; the whilst the different conditions of circulation affect the different climates around the world, they do not explain the overall average higher temperature.
This was due, again, to the break-up of Pangea.
The volcanically-active mid-ocean ridges not only spilled lava onto the ocean floors, but also [exhaled??...] vast quantities of volcanic gases, including sulphur dioxide and carbon dioxide. These gases - as we now well know - trap more of the solar energy that reaches the Earth in its atmosphere, thus causing an increase the global mean surface temperature.

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For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk