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Modelling ancient Earth climates
Alan Haywood, Leeds University
Introduction
At present we are living in an interglacial stage of the late Cenozoic ice age that began around 30 million years ago and has not yet ended. We know that global warming is happening now, even if we are not sure of the cause. Governments and scientists worry about its possible effects: heat waves, sea-level rise, coastal flooding, glaciers melting, diseases spreading, but what do we know of ancient global warming before the last ice age? And why should we want to know? In fact the problem is simple: we are right in the middle of it, so we can’t see the wood for the trees. We therefore need to consider palaeoclimates to try to find the best analogues in order to a) describe what might happen; b) to validate the models over a wide range of climatic situations; and c) to better understand all mechanisms which might influence climates warmer than today.
The Pliocene
The Pliocene era is the period in the geologic time scale that extends from 5.332 million to 1.806 million years before present. During the Pliocene era, three million years ago, the world was warmer than it is today. The degree of warming wasn’t uniform, and appears to have been greatest at the mid to high latitudes (30° to 90° north and south of the equator). If you’d taken a summer swim off the southern coast of Great Britain at this time, the waters would have felt almost Mediterranean.
We can tell the world was warmer from a range of fossil information. For example, tiny foraminifera (organisms a bit like an amoeba with a chalk shell) can be used to map out the surface temperature of the sea waters in which they lived. This is because different species, or assemblages of species, thrive at different temperatures. By comparing fossil foraminifera in seafloor samples from the Pliocene with foraminifera alive today, we can reconstruct how warm the surfaces of the oceans were. The technique suggests that the North Atlantic may have been up to 5°C warmer than it is now.
Why were mid and high latitudes so warm? There are two main ideas. One suggests that ocean circulation was stronger at this time, and that currents flowing from the tropics towards the poles carried more warm surface waters with them. The other explanation suggests that there was more carbon dioxide in the atmosphere, warming the world through a greenhouse effect.
The pattern of sea surface temperature change three million years ago gives a clue to the correct explanation. If Pliocene warmth was due only to the ocean currents, we would expect to see the tropics cooler and the higher latitudes warmer than they are today. This is indeed the pattern fossil foraminifera show But sophisticated numerical models of climate, known as general circulation models, and which are run on powerful super computers, suggest that both the tropics and the higher latitudes warmed. This result is now supported by new evidence which also suggests that sea surface temperatures were higher everywhere, including the tropics.
Tiny haptophyte algae (the best known being coccoliths that are a main constituent of chalk) that live near the sea surface produce carbon-based compounds which decay very slowly. These chemicals are incorporated into the seafloor mud. If you analyse a core taken from the seafloor, you can estimate sea surface temperatures when the algae were alive. These temperature estimates for the Pliocene sometimes disagree with those from foraminifera. They suggest that the sea surface was warmer in both the tropics and higher latitudes three million years ago—the same pattern of warming predicted by climate models. The pattern is also consistent with the greenhouse explanation, rather than the ocean currents idea, because carbon dioxide will cause warming at all latitudes.
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Antarctica glaciation and global change
Around 34 million years ago, at the boundary between the EOCENE and OLIGOCENE epochs, the earth experienced a fundamental switch in its climate system from a “GREENHOUSE” state, where there were no significant polar ice sheets, to the current “ICEHOUSE” climate with a large, if variable, Antarctic ice cap.
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So now there appears to be evidence that high levels of the greenhouse gas carbon dioxide were at least partly responsible for Earth's warmer climate three million years ago. Exploring the Pliocene world with advanced computer models, and investigating climate signals derived from fossils, is a good way to understand what Earth's climate may be like in the future, as a result of man-made greenhouse gas emissions.
Causes or coincidence
El Niño: Recent models suggest that El Niño–like conditions (an abnormal warming of surface ocean waters) may have contributed to Pliocene warming and that the termination of this state may have influenced northern hemisphere glaciation. However El Niño Southern Oscillation events are clearly expressed by the model. Sensitivity tests indicate that a prescribed permanent El Niño-like condition increases global mean annual surface temperatures by a maximum of 0.6°C. Tropical warming is in part, or wholly, compensated for by high-latitude cooling. Therefore, if the Pliocene were characterised by a permanent El Niño-like state, it is questionable that it provided a significant contribution to global warmth at that time and, therefore, it us uncertain that the termination of this state contributed significantly to the onset of northern hemisphere glaciation.
Panama Seaway: A variety of palaeontological data points to the fact that the final closure of the Panama Seaway and the onset of northern hemisphere glaciation happened at approximately the same time, during the Pliocene. It has been widely suggested that there is a causal link between the two events based on the fact that the closure of the seaway effectively cut off exchange of Atlantic Ocean water into the Pacific and vice-versa. It also strengthened the western boundary current in the northern Atlantic Ocean, the Gulf Stream (thus more northward heat transport) and initiated the development of the thermohaline 'oceanic conveyor belt' as we know it today — all processes necessary to 'feed' the growing ice sheets. However model results indicate that the closure of the Panama Seaway did not directly cause Northern Hemisphere Glaciation during the Pliocene.
Conclusion
The advantage of studying geological examples of global warming is that we can see the whole story played out in one take. We can work out how long it took and what its effects were — how much the sea level rose, how the atmosphere changed and how the plants and animals adapted (or died out). We can see the whole process on a 'panoramic' scale. Quantitative information derived from geological studies directly complements computer modelling of climate change. Worst of all, there is geological evidence that modern computer climate models seriously understate the magnitude of future climate change.
Furthermore, climate variations provided living conditions on Earth that ranged from 'snowball' to 'hot-house' climates. Such variations are also known to have triggered biological evolution as noted by a) the warming in the Cambrian (600 Ma) and b) the world-wide spreading of Homosapiens early in the Holocene (13 Ka).
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Submarine channels: giants of the deeps
Jeff Peakall, Leeds University
Introduction
Submarine channels (‘deep-sea rivers’) form troughs and valleys on the seafloor and behave in the most unusual way; improving our understanding of these systems is critical for improved oil and gas extraction. Because of their inaccessible location on the deep ocean floor, little is known about their dynamics and geomorphology, and existing conceptual models tend to consider submarine meanders as straightforward analogues of their fluvial counterparts. The channels on the seafloor, which are typically bounded by levees, can be kilometres wide and thousands of kilometres long. Also they provide significant conduits for sediment transfer from the continental shelf to the deep oceans, and are scoured by dense underwater flows (‘turbidity currents’) formed from muddy river floodwaters or underwater avalanches. These flows have formed meandering systems of a scale similar to or surpassing the greatest fluvial meanders on earth, and supply submarine fans, the largest sedimentary fans on Earth.
Over the last 75,000 years sediment from the Amazon River has created this 800 km long channel beneath the Atlantic. The image shows a 200 km long 4 km wide section of the channel with banks 100’s m high.
Credit: Amos and Peakall (2006) Savoye, IFREMER.
Sedimentary rocks formed from ancient seafloor deposits contain preserved seafloor channels, and these sandy deposits form some of the world’s major oil and gas reservoirs. The local Pendle Grit is a good example of an ancient sea-floor turbidite sand body. Recent research by our team at the University of Leeds has shown that flow and sedimentation in seafloor channels is dramatically and unexpectedly different, with the flow spiralling in the opposite direction to the flow of water in rivers, where the sediment eventually settles. However, perhaps the most spectacular difference is that the wiggleyness (‘sinuosity’) of submarine channels, seems to vary globally with latitude, with ones close to the equator spectacularly sinuous, and those towards the poles very straight. In contrast, wiggly rivers can be found anywhere on Earth, controlled in the main by slope. This observation of global variations overturns decades of thinking that sea-floor channels like their rivery cousins were primarily controlled by slope angles. Possible controls include climate, sediment supply, and Coriolis (due to the planet’s spin). If Coriolis is the control then Earth may have always shown this distribution, whilst if due to climate/sediment type then non-glacial periods may show a different pattern.
In the case of the Pendle Grits, these were formed in a glacial period, and at a time when Northern England was in the equatorial region, so in either case we can predict that channels were quite wiggly (unless slopes were so steep in the small Craven basin, that only straight channels could form). The existing model of Sims (1988) predicts almost straight, braided channels; this recent work of our team predicts wiggly, single thread channels. Fieldwork is just starting to see if these predictions are correct!
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The end-Guadalupian Mass Extinction
Paul Wignall, Leeds University
Introduction
All mass extinctions of the past 300 million years, and many of those before this time, coincide with large igneous province (LIPS) eruptions, but demonstrating a causal link has proved difficult. However, the connection is one of great relevance in earth sciences and the wider community because it allows the cause and consequence of high-amplitude climatic changes to be assessed. Many extinction scenarios involve a cascade of volcanically-driven environmental changes that include global warming from CO2 release and volcanic winters from pyroclastic eruptions. Such conflicting causes form the core of extinction debates that began in the early 1880’s. They have proved difficult to resolve. Many workers have implicated high-volume flood basalt eruptions of the Siberian Traps as a cause of the end-Permian mass extinction, whereas others “blame” the extensive Tuffaceous Series that underlies the basalts. It is difficult to test this link because the timing relationship relies on radiometric dating and its associated errors. Thus, the end-Permian mass extinction is best dated in its Chinese type section where data indicate an age of 252.6 MA. In contrast, dates for the Siberian Traps flood basalt eruptions indicate an age of 249.4 MA. Even allowing for problems when comparing two different radiometric systems, the resolution is still not sufficient to resolve which style of volcanism shows the closest temporal link with extinction. The ideal test occurs when the extinction and volcanism record can be examined in the same sections.
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The Emeishan Flood Basalts are the dark coloured rocks to the left of the roadside cutting. The basalt unconformably overlies the late Middle Permian Maokou limestone to the right. Photo taken at Ebian, Sichuan.
© Professor Paul Wignall (2007).
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Fortunately, such a test is available in the form of the end-Guadalupian (Guadalupian-Lopingian) extinction / Emeishan volcanism link. Hitherto, there has been very little work on this Permian extinction event because it lies “in the shadow” of the great end-Permian mass extinction peak, and for a long time the Guadalupian extinction losses were incorporated into one protracted, late Permian phase of extinctions. Only in 1994 did two teams independently identify a separate mass extinction late in the Middle Permian followed by a phase of radiation and recovery prior to the end-Permian even.
The Guadalupian-Lopingian mass extinction (Mid-Late Permian boundary) has emerged as being a separate event constituting an estimated 70% minimum loss amongst marine species. This crisis is best known from shallow marine, equatorial carbonate settings and was particularly severe for brachiopods, corals, echinoderms (blastoids, echinoids, crinoids), reef-forming sponges and foraminifera (especially fusulinids). Intriguingly, at around the same time as the identification of this extinction event, a flood basalt province was discovered in SW China. This is the Emeishan Province and it was initially considered a contributory factor in the end-Permian crisis. However, improvements in dating indicated that there is a better temporal link with the Guadalupian event.
Although geologists are still in the early stages of establishing detailed extinction models for the crisis, initial results suggest that eustatic regression definitely played a significant part in the extinction of faunas. The evidence is just as compelling for both basaltic and acidic volcanism to have played its part in this extinction event. There remains a degree of uncertainty on the impact (if any) of global cooling and as for oceanic anoxia, the evidence suggests not.
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