C. Mesozoic Earth, The Dinosaur's Cradle Flashcards
1
Q
Describe the geography at the start of the Mesozoic.
A
At the start of the Mesozoic, the continents were amalgamated into one landmass, the supercontinent Pangea. This was surrounded by the equivalent of today’s Pacific Ocean, called the Panthalassic Ocean and an embayment in the east called the Tethys Ocean. This gives Pangea an appearance of the letter “C”. The fragmentation of this landmass would dominate the ocean-climate system throughout the Mesozoic.
2
Q
How does Pangea begin to break apart? Describe the geography by Late Triassic.
A
Pangea starts to “unzip” westwards throughout the Triassic, creating Gondwana to the south and Laurasia (comprising North America and Eurasia) to the north. The Tethys Ocean moved into this rift and started to divide the supercontinent in half. By the Late Triassic, North America and Africa had started to show definite separation.
3
Q
Describe the geography during Late Triassic- Early Jurassic
A
During the Late Triassic - Early Jurassic the newly formed North Atlantic Ocean was located in low tropical latitudes.
During the Late Triassic and Jurassic, Antarctica and Australia (which remained sutured together) began separating from South America and Africa. At the same time, India began to rift from Gondwana and drift northwards.
The separation of the continents continued during the Jurassic, allowing water from Tethys to start to flow more continuously into the young Atlantic Ocean and ocean water from the Pacific to flow into the embryonic Gulf of Mexico.
4
Q
What analogy is similar to what the primitive Altantic Ocean may have resembled in Late Triassic?
A
By looking at the East African Rift System today. Here, hot mantle rocks are causing the continental crust to thin and rift apart. The Red Sea went through a similar history where thinning progressed to such an extent that the continental rocks split apart, Africa to the southwest, the Middle East to the northwest. Oceanic crust is now forming and spreading on the floor of the Red Sea.
5
Q
When did the rifting of North America from South America begin?
A
It began during the latest Triassic and Early Jurassic.
6
Q
Describe the Hispanic Corridor. Who proposed this name? What is there to be certain vs uncertain about?
A
This name was proposed by Dr. Paul Smith (UBC Professor Emeritus) in 1983.
It is the seaway between North and South America.
There is no doubt that this seaway existed for many millions of years before finally closing again when the Isthmus of Panama formed between North and South America about 3 million years ago.
What isn’t quite clear though is the timing of the opening of the Hispanic Corridor. Was it open in the very latest Triassic, earliest Jurassic or not until about 6 million years later, in the late Early Jurassic?Many different scientists have used fossil evidence to try to shed light this question over the last few decades. Specifically bivalves and ammonites are useful in this regard, as they are mobile in at least part of their life cycle, and so, could be expected to move through a marine waterway if it was a good corridor for dispersal.
7
Q
Describe sunrisites.
A
They are spirals, it is a depressed ammonite.
Found in Hispanic Corridor.
8
Q
Most common fossil types used to help determine when the Hispanic Corridor formed?
A
Bivalves and ammonites
9
Q
What does the presence of Sunrisites in the old rock of the western side of the Tethys Ocean suggest?
A
May have been a route through, between the Eatern Pacific (the common location of sunrisites) to the Tethys ocean.
10
Q
Why does the distribution of the fossil genus discussed in the recording suggest the Hispanic Corridor may have been open as early as the earliest Jurassic?
A
Found on the western side of Tethys ocean. During Jurassic the separation of continents had been continuing, hence why it could be earliest Jurassic.
11
Q
Describe the geography during Jurassic times.
What was Jurassic and the Tethys ocean relation?
A
During Jurassic times, South America and Africa start the process of rifting that eventually forms the South Atlantic.
The Jurassic marks the beginning of the end for the Tethys Ocean with Laurasia rotating counterclockwise and Africa drifting northwards to close the ocean.
12
Q
Describe the geography by the end of the Cretaceous.
A
By the end of the Cretaceous, Antarctica and Australia had separated and India was moving towards the equator. Greenland was now a separate landmass and Africa and South America were fully separated by the South Atlantic. Pangea had truly been split apart.
13
Q
What are the remains of the Tethys Ocean?
A
The Tethys Ocean would continue to close as Africa and Europe continued to move together. All that is left of Tethys today are fragments trapped between the closing continents, namely the Mediterranean and Black Seas.
14
Q
What is the west coast of North America comprised of?
A
The west coast of North America is made up of fragments of continents and various chains of volcanic islands that have been moving across (what is today) the Pacific, due to plate tectonics and have now all collided and “stuck” onto the edge of Western North America.
i.e. it is comprised of exotic terranes
15
Q
Define exotic terrane
A
An exotic terrane is a fragment of crustal material formed on or broken off from one tectonic plate and accreted to the crust of another plate. This crustal fragment has its own geological history which is different from that of surrounding areas.
16
Q
Describe how terrane accretion happens with an example.
A
The Insular Plate subducting beneath the North American Plate near the western margin of what will be British Columbia. The Bridge River Ocean is contained between North America and the Insular Islands.
30 million years later, the Insular Plate is entirely subducted and the Bridge River Ocean is completely closed. All that remains of this ocean is the Bridge River terrane, which is now accreted on to the edge of North America.
17
Q
Define the term faunal association.
A
In paleontology, the term faunal association refers to the whole group of living creatures coming from the same environment, the same bed, and the same geological outcrop.
One of the methods commonly used by paleontologists involves examining the various faunal associations in sedimentary rocks in some of the terranes.
18
Q
Describe the research of Dr. Paul Smith.
A
He focused on the study of Early and Middle Jurassic ammonite faunas.
Their study:
If one looked at the ammonite faunas in Jurassic rocks that have been attached to North America since they formed, it is possible to recognize a faunal association that is characteristic of more northerly latitudes (such as Western Canada).
These marine animals (now extinct) thrived in relatively cold waters and are referred to as boreal (or northern) faunas. Their distribution is shown in brown on the maps below. Faunal assemblages that formed in warmer waters much farther south (e.g., in Texas) are distinctly different, and are referred to as Tethyan assemblages. In between the boreal and tethyan provinces one finds examples of both.
Smith and colleagues noted that several of the accreted terranes in the Canadian Cordillera contained Jurassic ammonite faunas that were typical of faunas formed far to the south of their present locations. By using the approximate boundaries between where the boreal, tethyan, and mixed faunas occur in non-transported parts of western North America, these scientists were able to postulate how far south the different terranes had originated.
19
Q
Define orogeny and its cause.
A
Mountain building
Is usually the result of the movement of lithospheric plates.
20
Q
What is an orogenic belt?
A
In geology, an orogenic belt (i.e. a mountain belt) could also refer to the roots of an ancient mountain belt that has been eroded down.
21
Q
Describe the mechanisms for the growth of orogenic belts.
A
There are 4 main mechanisms for the growth of orogenic belts:
- Volcanic activity (usually in volcanic arcs along convergent margins)
- Regions undergoing crustal extension where the continental crust is stretched, leaving a series of uplifted and downdropped blocks. This produces narrow mountain belts.
- Regions undergoing crustal shortening due to compression
- Collision between two continental plates drives up mountains where the two plates meet (this is occurring today in the Himalayas)
22
Q
How are stratovolcanoes formed? Why do these magmatic arcs not produce a continuous mountain belt? describe the conditions of these magmatic arcs.
A
Formation of Stratovolcanoes. Water released from the subducting oceanic crust rises up and hydrates the overlying mantle; water released deeper down causes partial melting of rock to magma that erupts in the arc volcanoes.
At a convergent margin where oceanic crust is being subducted, the oceanic plate that is sinking down into the upper mantle begins to heat up. Water is driven off the down-going plate, and this water passes up into the overlying wedge of mantle material (see figure below), where it causes that mantle material to start to melt. This melt, or magma, is hot and has relatively low density, so it rises buoyantly into the overlying crust.
the volcanoes are typically quite widely spaced (10s to 100s km apart), so this process does not produce a continuous mountain belt, but rather a chain of isolated volcanoes
23
Q
What are plutons / intrusions?
A
Large bodies of crystallized (cooled) magma that exist within the crust; majority of magma ends up cooling like this, not all magma generated at a convergent margin erupt to form volcanoes.
24
Q
What is a batholith?
A
A group of adjoining plutons of similar composite extent
25
What is the Principle of Isostasy?
This emplacement of plutons, the batholith, inflates the crust with relatively low density rock masses, which both thickens the crust and reduces its overall density.
According to the Principle of Isostasy, this portion of the Earth's crust must float higher on the underlying mantle, creating a continuous mountain belt.
26
What are reverse faults?
What is a thrust fault?
Reverse fault: Faults formed during crustal shortening
These are faults where the hanging wall block has moved UP with respect to the footwall block. If sufficient displacement occurs on any reverse fault, it will lead to the uplift of the hanging wall block sufficient to cause a mountain belt to form.
Thrust fault: A special case of reverse faulting is when the angle of inclination of a reverse fault is less than 30°
27
What are the ways that crustal compression creates mountans?
Reverse faults and thrust faults.
Both reverse faulting (including thrust faulting) and by folding, where rocks are forced to fold or bend
28
How were the Coast Ranges mountains in BC formed?
The Coast Ranges were built by subduction of oceanic crust and formation of a magmatic arc, a major batholithic belt, and associated crustal thickening and uplift.
29
How were the Canadian Rocky Mountains built?
The Canadian Rocky Mountains were built by crustal compression and associated folding and thrust faulting.
30
Describe the foreland thrust and fold belt of BC
31
What is a foreland basin? How is it formed?
A foreland basin is a depression that develops adjacent and parallel to a mountain belt as the lithosphere is bent downwards. It forms when large amounts of rock are pushed over the edge of the continent, resulting in the thickening of the continental crust along the edge. The large rock mass loads the edge of the continent and causes it to be pushed down (or subside). This produces a large basin inland from the fold and thrust belt, which as called a foreland basin.
32
What is the name of the ancestral North American continent?
Which of the four main mechanisms of mountain belt formation caused the Appalachian mountains around 200 million years ago?
The addition of which fluid into the mantle during plate subduction causes melting and, eventually, volcano formation?
The San Andreas fault formed between North America and which oceanic plate?
1. Laurentia
2. The Appalachian Mountains formed from faulting and folding due to compression at formation of Pangea
3. water
4. Pacific plate
33
Describe North America in the Cambrian about 500 million years ago.
What would change by the Carboniferous period?
During the Cambrian, about 500 million years ago, the west coast of North America (North America at that time is called Laurentia) was not tectonically active – there was no active subduction. It was what we call a “passive” margin. In addition, North America was located around the equator and the whole continent was oriented in a different manner with (what is today) the west coast arranged East-West rather than North-South. British Columbia did not exist as a land mass, the shoreline would have lain somewhere quite a distance away. Around the modern border of BC and Alberta, the Burgess Shale animals (that we met in an earlier lecture) lived at the top of a submarine cliff.
Carboniferous to Permian: This “passive” tectonic set-up would change by the Carboniferous period when volcanic islands were advancing towards Laurentia, and North America was now in a more familiar “North-South” orientation. This tectonic activity was also initiating uplift (early stages of mountain building) in the interior of Laurentia.
34
What is a passive margin?
A margin is considered ‘passive’ when the margin between continental and oceanic crust is not an active margin.
35
Describe Western Canada from Early to Middle Triassic. (~240 Ma)
In Early to Middle Triassic time, oceanic crust was being subducted under the western margin of N. America, producing a volcanic arc (or possibly a complex series of parallel arcs). This arc magmatism appears to have mostly been manifested by the construction of chains of volcanic islands along the margin of the continent.
36
Describe Western Canada in Early Jurassic (~180 Ma)
This same general scenario continued in Early Jurassic time. However, at this time a large terrane (which is basically a large block of crust) called Wrangellia (W on the diagram on the left) was beginning to move towards the edge of the continent. Most of this terrane was submerged at this time (as shown by the lighter coloured ocean) except for isolated volcanic islands. The western edge of North America was covered with shallow seas, with isolated chains of volcanic islands offshore.
37
Describe Western Canada in Middle Jurassic (~160 Ma)
By Middle Jurassic time, there was a series of volcanic islands fringing the western edge of N. America, and Wrangellia was continuing to move toward the continent. In fact, the southern end of Wrangellia began to collide with the edge of North America, at approximately the latitude of California at this time. There are two important aspects to note from the figure below. First, scientists have a pretty good idea of what the relative plate motion vector was between North America and the oceanic plate(s) to the west.
The shallow sea now extends far into the interior of North America during this time.
38
Describe Western Canada in Late Jurassic (~145 Ma)
By Late Jurassic time, Wrangellia was continuing to collide with the margin of North America. This process was causing a mountain belt to grow along the collision zone. At this time the oceanic plate to the west (the Farallon Plate), which was underlying part of the Pacific Ocean, was converging obliquely to the south (light yellow-coloured arrow), so Wrangellia began to slide southwards along the coast. The inland sea has shrunk considerably.
39
Describe Western Canada in Early Cretaceous (~125 Ma)
The Farallon Plate is now converging almost orthogonally (perpendicular; light yellow-coloured arrow) to the coast, so southwards motion of Wrangellia stops and it begins to be pushed (accreted) against the margin. This collision of Wrangellia with the edge of North America causes the Canadian Rocky Mountains to start to grow along the western edge of the continent. These mountain belts form from crustal compression.
40
What is the Cretaceous Seaway (or 'Western Inland Seaway' or 'Western Interior Seaway')?
A shallow sea (see the last page for information if you want to review how this happened), the so-called Cretaceous Seaway, forms to the east of this growing mountain chain extending from the Arctic Ocean south to Southern Wyoming, and another in Northern Mexico.
41
Describe Western Canada in Middle Cretaceous (~105 Ma)
By Middle Cretaceous time, the Farallon Plate was converging slightly obliquely to the north, so Wrangellia and more inboard terranes begin to be pushed slightly to the north. The whole margin was under compression, so mountains were being built by arc magmatism along the western edge of the continent. Farther to the east, the Canadian Rocky Mountains were being formed by crustal thickening (formation of a fold and thrust belt) .
In Middle Cretaceous time, the Rocky Mountains were still growing because the crust was still being compressed and thickened in that area, and the area to the east was subsiding, forming an even larger foreland basin.
42
Describe Western Canada in Late Cretaceous (~85 Ma)
By Late Cretaceous time, the Farallon Plate had split into a southern part (still called the Farallon Plate) and a northern part (now called the Kula Plate). The split allowed the diverging motions of the oceanic plates with the Kula Plate moving almost parallel to the coast (light yellow-coloured arrow) and the Farallon Plate moving obliquely towards the continent. Terranes within the Cordillera began to be pushed strongly to the north, thus the margin continued to experience both compression and shortening.
At this time the Cretaceous Seaway has extended the entire length of Western North America and was separated from the Pacific Ocean basin by a continuous mountain chain. The dinosaurs were happy!
43
Describe Western Canada in Cretaceous/Paleogene boundary (~65 Ma)
By the end of the Cretaceous, the Kula Plate was still moving to the north, dragging all the terranes along with it. There was no convergence between the Kula Plate and North America now, so no subduction was occurring and therefore no arc magmatism was being produced. By the end of the Cretaceous, convergence of oceanic plates from the west had mostly stopped, so the crustal compression that had continued to build the Rocky Mountains up to this point also stopped.
With the cessation of crustal shortening and thickening, subsidence of the foreland basin region stopped. Also, as inevitably happens, the Rocky Mountains began to erode down, and most of this material ended up being carried by rivers eastwards into the foreland basin. As a result, the Cretaceous Seaway, was ultimately filled up and ceased to exist, very much reducing the suitable habitat for terrestrial dinosaurs in western Canada.
44
Describe one Cretaceous and one modern factor that contribute to Alberta's excellent dinosaur fossil collection.
What time period are the rocks in Alberta's badlands from?
Using your knowledge from this video and the material above, would you expect to find terrestrial, marine, or a mixture of fossils in Alberta's badlands?
1. Perfect because sedimentary rock like sandstone, mudstone, ironstone; the environment was perfect for piling up sediments.
2. Late Cretaceous (high dino diversity)
3. mixture
45
What did Dr Peter Mustard from SFU discover?
One discovery was made in northwestern BC several years ago. Dr. Peter Mustard from Simon Fraser University discovered both fossil turtles and some impressive dinosaur tracks in a locality north of Terrace, BC.
Dino tracks are from raptors.
46
Where are most of the dino fossils found in Western Canada found? What are the exceptions?
Most of the dinosaur fossils that have been discovered in Western Canada are from within or immediately around the sedimentary rocks of the Cretaceous Seaway, east of the Rocky Mountains. However, there have been several recent discoveries of dinosaur fossils from west of the Rocky Mountains too and it isn't at all clear how they got there.
47
What does the Bowser Basin contain?
Jurassic and Cretaceous sedimentary rocks more than 5000m thick, with an area of 65k km^2
48
What was the discovery in Yukon and Alaska?
Another recent discovery was made near the Village of Ross River in Southeastern Yukon. It consisted of a large number of dinosaur tracks (a so-called dinosaur trackway). Note from the paleogeographic reconstructions below that both this and the previous locality would have been well to the west of the Rocky Mountains.
Dinosaur tracks and some bones have also been discovered farther north in the Yukon (Bonnet Plume Basin) and also in Central Alaska. The bottom line is that dinosaurs clearly existed much farther west and north than previously thought.
49
How did dinosaurs get into the Northern Cordillera?
There is some speculation that dinosaurs may have crossed into the Northern Cordillera via land bridges that connected North America and Siberia in Cretaceous time.
50
How do modern-day researchers determine temperatures in the Cretaceous Arctic?
What is histology and why is it important for the study of dinosaurs in the Arctic?
What evidence suggests Arctic dinosaurs did not migrate far (i.e., they were year-round inhabitants)?
What adaptations do researchers think Troodon had to survive in the Artic?
1. from paleosols or fossil soils; it was warmer, mean temp of 6.3 deg C; use carbon and oxygen isotopes to know what temps they formed; also, fossil plants provide climate clues in their shape and size of leaves
2. histology is looking at microscopic of bone tissue. tell us how animal grew and the conditions.
3. mass bonebeds of juvenile edmontosaurus in Alaska, would have been too small to migrate very far.
4. Large forward facing eyes to take in lots of light, binocular vision, brain size within extant birds, covered in feathers that insulate against cold, larger bodies than southern troodon, with larger teeth, also lack of predators
51
How can analyzing bone internal structure tell us how animal grew?
When food is more abundant, bone tissue deposited faster; when scarce, bone laid down less rapidly
52
phenotypic plasticity
when organism's physiology, behaviour, morphology change with environment; larger size when prey more abundant or larger
53
why was nanuqsaurus smaller?
If it were as big as TREX, would require more energy to survive; i.e. there is a Goldilocks size for life in Arctic
54
Describe what typically for a megathrust earthquake to occur
For Southwestern BC, the megathrust type of earthquake is thought to represent the greatest hazard. What typically happens prior to a megathrust earthquake is that frictional forces along the upper surface of the subducting plate and the leading edge of the overriding plate (North America in this case) cause the thrust fault zone to become locked, such that no displacement can occur.
As the two plates continue to move towards each other, the upper plate will be shortened in an east-west direction, and will begin to buckle, causing the leading edge of the upper plate to experience uplift. Eventually, however, the accumulated stress and strain will overcome the frictional forces that are keeping the zone locked, and the accumulated strain will be released instantaneously as a giant earthquake. The leading edge of the upper plate that was being uplifted will suddenly downdrop, and move rapidly to the west. The abrupt uplift generates the tsunami, and the collapse of the bulge causes the subsidence recorded in buried coastal marshes.
55
What are the dangers of megathrust earthquakes?
A megathrust earthquake could result in powerful shaking that could last several minutes. This could result in the loss of structural integrity and possible collapse of many buildings, bridges and other manmade structures. There could also be extensive faulting of land masses. Liquefaction in many areas is also likely.
Tsunamis.
56
What is liquefaction and what is evidence for it?
Liquefaction occurs when flat lying, loose to moderately saturated, layered sediments (such as sands, gravels, muds, etc.) are strongly shaken, essentially transforming the solid layers into a slurry with little or no strength. When a sand layer liquefies, the more solid layers above it may break-up and rotate (tip), or simply collapse as in a landslide (see figure below). Good evidence of liquefaction are the so-called sand volcanoes, which form when liquefied sand is ejected up along a fracture in the overlying layers and erupts onto the surface.
57
How are tsunami generated?
Tsunami are generated when an earthquake occurs beneath the water, usually along a coastline, which then causes a major displacement of the seafloor (either because of reverse fault motion, as shown in the figure below, or from triggering a large underwater landslide). The displacement of the seafloor causes a large volume of water to be displaced as well, resulting in a series of large waves to move through the water away from the area of seafloor displacement.
Out in the open ocean these waves are typically not very high (amplitude of only up to a few meters at most) but have extremely long wavelengths, so they represent the involvement of enormous volumes of water. When one of these low amplitude, long wavelength waves moves from the deep ocean into shallow coastal waters, the amplitude becomes greatly amplified, up to several tens of meters high.
58
What evidence of the megathrust earthquakes occurring along our coastline in the past?
Oral tales past down through generations of First Nations People, evidence of tsunami sands and drowned ancient forests on our coastlines, evidence from drilling of cores down through the subduction zone, etc. As the last major quake is believed to have happened in 1700AD, we are 'due' for another one anytime.
59
Until recently, which 2 sources was it believed water on Earth came from?
1. Water would have been present in the material that formed the Earth. Degassing via volcanic activity would also release some of this water to the Earth's surface and atmosphere.
2. Water was also brought to our planet by extraterrestrial objects as they collided with our planet. There is some debate as to the specific source, but both comets and carbon‐rich chondrites contain lots of water. Water was also likely brought to the Earth upon impact with the object that formed the Moon. Water is common in the solar system and it is likely that large quantities of water were available to the early Earth.
60
According to recent sources, where does our water come from?
More recent work is suggesting that the majority (50 - 75%) of Earth's water may have came from particles emitted by the sun. The idea is that solar wind (charged hydrogen particles streaming out from the sun) could have combined with dust grains to create water. This water then subsequently travelled to Earth sometime after its creation 4.6 billion years ago!
61
What is the solar wind?
What did the solar wind potentially combine with to create water?
By mass, how much of the water on Earth needs to have come from the solar wind?
What type of water on Earth is most critical for life?
What did the scientist, Dr. Hope Ishii, analyze to discover her data?
What are the hydrogen ions doing when they run in to the rock?
How is the water held within the asteroid? Is it as water or within the chemistry of the rock itself?
Why are asteroids that fall to Earth quite regularly not as useful to study water as the piece of asteroid collected during the Japanese mission?
How much water is there per cubic meter of asteroid?
How is it possible to say that the water probably came from the surface of asteroids and not some other source?
"solar wind" — charged hydrogen particles streaming out from the sun — may have combined with dust grains to create water, which would have then travelled to Earth after the planet's formation 4.6 billion years ago.
Solar wind striking rock, solar wind
Isotopic light water
Results suggest that somewhere between 50 and 75 per cent by mass of the water [on Earth] needs to come from solar wind water, in order to match or reproduce what we see in the Earth's oceans," said Hope Ishii.
2 ways a rock can hold water: 1) chemically bound to atoms within the rock (OH), 2) as a molecule (H2O)
Meteroites have friction going thru atmosphere so solar wind water area was heated.
Asteroid rock could hold up to 20L of water per cubic meter.
The water is only present on surface mineral grains instead of instead, means a surface process; also generated results in lab.
62
How did the oceans get salty?
Our planet's oceans was originally composed of freshwater. "Salt" or the dissolved solids in seawater was added by the erosion of rocks on land and from volcanic activity on the ocean floor. The total salt content in the current oceans is estimated to be around 50 quadrillion tons (50 million billion tonnes), making our oceans truly "salty".
63
What property causes rain to dissolve continental rocks?
Describe the rolls of rivers and oceans in the movement and accumulation of salt.
Rainwater is slightly acidic
Rivers deliver minerals to ocean
64
What information can we get from sedimentary rocks?
The oldest water-deposited sedimentary rocks ever discovered are about 3.8-3.9 billion years old. This indicates that standing water must have existed on the planet by at least that time. In fact, isotopic evidence suggests that there was standing water around from around 4.4 billion years ago! Interestingly, these rocks contain chemical signatures that could also indicate that life was already present. This suggests that the biosphere formed pretty soon after Earth was cool enough for bodies of standing water to form.
65
Evidence from sedimentary rocks account for the water, what about the ocean basins?
Before the advent of plate tectonics, it was thought that the age of the oceans would be the same all over the planet. We now know that ocean crust is being continuously recycled with new crust being produced at spreading centres and old crust returning to the mantle at subduction zones. Although still debated by some, the oldest ocean crust comes from the Herodotus Basin in the eastern Mediterranean Sea and is about 340 million years old. The oldest undisputed ocean crust is about 200 million years old.
66
What is sonar and how did it affect our understanding of seafloor depth?
What did Mary Tharp map by hand?
What feature did Mary Tharp identify on the seafloor and what did it reminder her of on land?
What was the other prominent theory used to explain the spreading of continents?
What supporting evidence coincided with Mary Tharp's discovery?
Sonar used to measure depth of ocean; before, it was sounding (drop weight and mark rope when it reached bottom) which was inaccurate and also only gave 1 point of data. Sonar is sound waves, measure the amount of times the sonar bounced off, to measure the depth.
Mary Tharp made hand-drawn diagrams that plotted the ship's paths and their depth measurements. Then stitched them together, created topographical maps of the sea floor.
She found a cleft in the center of the North Atlantic (now called the Mid Atlantic Ridge, kms wide and '00s of meters deep; and it reminded her of a rift valley; a feature that first forms on land when two tectonic plates pull away from each other.
At the time, there was the idea of the "expanding Earth" where continents were pulling apart because the actual volume of the Earth was increasing, like a balloon.
Another assistant in the lab was making a map of earthquake epicenters, and the earthquakes were forming in the same spot as the rift valley. Since earthquakes are formed by the movement of tectonic plates, this made it clear that the rift valley was between two tectonic plates and they were pulling apart.
67
What is a transgression in terms of the sea? Regression?
A sea level rise is called a transgression and a sea level fall a regression.
68
Describe how changes in sea levels impact us.
Over geological time, the average sea level has changed many times over several hundred metres.
Changes in sea level can have significant effects on global climate if areas of continents are alternately flooded or exposed. Many continental margins are very flat (for example Bangladesh and the Bay of Bengal) and vertical elevation changes of just 1 part vertical for every 1000 horizontal can result in a transgression of the ocean onto land of 1 km.
69
What can cause local effects of sea level changes?
Give examples of it occurring in real life.
Local effects can be caused by tectonic processes such as mountain building, which, by increasing continental elevation, would give the appearance of sea level regression. In addition, loading of the crust by ice or large packages of sediment can cause local down-warping of the crust and transgression of the ocean into those areas.
Turakirae Head, New Zealand. These raised beaches in New Zealand, bare areas running parallel to the coast, are beach ridges that have been raised above sea level by large earthquakes. The youngest raised beach (closest to the present-day shoreline) was uplifted 6.4 metres by the 1855 Wairarapa earthquake. Older ridges have been dated and are associated with earthquakes occurring in 2900 BC, 1100 BC and 1460 AD.
70
What are eustatic (global) sea level changes?
Eustatic variations in sea level are broadly the result of changes in the volume of the oceans or the volume of the ocean basins (or a combination of the two).
71
What evidence do we have of eustatic variations in sea level?
Evidence of high global sea level comes from the presence of marine sediments that were deposited at the same time (coeval) in the interior of continents on different continental land masses.
72
Describe how ocean ridges are the centres for the generations of new ocean crust. What is the average ridge crest depth?
Ocean ridges are the centres for the generation of new ocean crust. As they are heated by hot mantle rocks from below, the rocks expand and rise. As the seafloor spreads to either side of the ridge, it cools and subsides forming a typical ridge crest profile. Today, the average ridge crest depth is about 2,500 m but by the time the crust is around 60 million years old it has subsided to about 5,500 m.
73
Explain how the speed of seafloor spreading will result in different sea level profiles.
Periods of rapid seafloor spreading produce high elevation (broad or fat) profiles. These ridges occupy volume in the ocean basin and displace water onto continents. Slower seafloor spreading rates produce lower elevation (narrow or thin) profiles and displace less water. As a result, sea level will be higher during times of accelerated sea floor spreading.
So: fast seafloor spread -> higher sea level;
slow seafloor spread -> lower sea level
74
Describe the collision of continental plates and why they form mountains when they collide.
How do the high plateaus and mountains of continent-continent collision areas form?
How does crustal thickening affect the ocean basin and continental area?
When was the most recent collision event?
Continental crust has a much lower density than the mantle. As a result, when two continental plates collide to form a mountain range, no continental plate material is subducted. Instead the continental rocks thicken from about 30 km to around 60 km, building plateaus high above sea level. An associated deep mountain "root" that presses down into the mantle is also formed.
The high plateaus and mountains of continent-continent collision areas are created by faults that shear off slivers of continental crust and stack them on top of one another in process called crustal thickening. At deeper levels (below 15 km) thickening occurs as the hot plastic rocks are squeezed together. Crustal thickening results in a net loss of continental area and an equivalent increase in the area of the ocean basins. This increase in the accommodation space in the ocean basins results in a eustatic reduction in sea level, a regression.
The most recent collision event occurred about 55 million years ago when India collided with the Eurasian Plate to form the Himalayas and the Tibetan Plateau. This increase in the area of the available ocean basins has caused global sea level to fall about 40 m.
75
Explain how volcanic plateaus in the ocean are formed. Are they formed by the volcanism at ocean ridges?
Some parts of the ocean floor are covered with large upstanding volcanic areas that are *not* directly related to the volcanism occurring at ocean ridges. It is thought that these features were produced by mantle plumes, upwelling of hot rock within the Earth's mantle that can cause melting of the underside of the crust and generation of magmas.
Like ridge crests, when these volcanic features are young, they form topographically high areas that can displace large volumes of sea water. As they become less active and cool, they subside.
76
What is likely one of the main causal factors in producing warm climates during the Middle Cretaceous? How?
It has been proposed that mantle plumes may be one of the main causal factors in producing the warm climates during the Middle Cretaceous. Plume activity would have accelerated sea floor spreading and also generated volcanism in continental areas in addition to oceanic volcanic plateaus. All this additional volcanic activity would have released large quantities of greenhouse CO2 into the atmosphere.
77
How does ice affect sea level?
The formation of large quantities of ice is an easily visible way in which water can be locked away from the world's oceans. The ice sheets on Antarctica holds about 66 m of global sea level, Greenland is holding a further 6 m. During the last Ice Age, sea level was around 130 m lower than present-day levels. (So, there was more ice back then)
78
Describe the thermal expansion of seawater. How has thermal contraction of seawater affected global sea level since the Middle to Late Cretaceous?
Depending on its temperature, water can occupy different volumes as described by the thermal expansion coefficient of water. This coefficient describes the fractional change in water volume per degree of change in temperature, which averages out about 1 part in 7000 for each 1°C of temperature change. Since the highest temperatures achieved in the Middle to Late Cretaceous, the thermal contraction of seawater has reduced global sea level by about 7m.
79
How does describing ancient changes in eustatic ocean depth important for geologists? What did Peter Vail in particular do?
Sea level change has important implications for how sedimentary rocks are deposited. Of interest are the patterns of sedimentary rocks (stratigraphy) that can help geologists focus on geological formations that might contain oil. It is not surprising that the most comprehensive study of past sea level was carried out by Peter Vail, an employee of Exxon. It is for Vail that sea level change curves are named, Vail Curves.
80
Describe the ocean condition by the end of the Permian and the formation of the supercontinent Pangea.
By the end of the Permian (coincident with the end of the Paleozoic) and the formation of the supercontinent Pangea, sea levels were very low. This is partly due to the slow rate of sea floor spreading and associated reduction in the volume occupied by mid ocean ridge crests. In addition, several mountain chains had been formed by the amalgamation of Pangea and, as was described previously in the formation of the Himalayas, this led to an increase in the area of the ocean basins. As the oceans withdrew from the continental margins, many shallow marine ecosystems were lost, which in part may be responsible for the mass extinction that occurred at the end of the Permian.
81
Describe the ocean condition throughout the Triassic through the Jurassic.
Sea level would remain fairly constant throughout the Triassic and vary somewhat through the Jurassic.
82
Describe the ocean condition throughout the Cretaceous.
Despite a few variations, there was a steady rise in sea level through the Cretaceous with ocean depth reaching a maximum by the Middle to Late Cretaceous. At this time coastlines and the interiors of continents were flooded by an ocean some 200m higher than today. Put another way, the land was flooded some 40% more than present.
Ocean waters covered much of North Africa and Europe. Seaways penetrated into the interior of North America. Much of Western Europe and probably parts of Africa were reduced to a series of tropical islands. These warm shallow oceans are often referred to as epicontinental or epeiric seas.
83
Describe the factors that caused the sea level changes during the Cretaceous.
The rise in sea level is due to a number of factors:
At this time were are starting to see an accelerated fragmentation of Pangea. This increase in sea floor spreading would generate larger and longer ocean ridges that would displace more water onto the continents.
This period also saw the formation of a number of oceanic volcanic plateaus that would have a similar water displacement effect. One of the largest is the Ontong Java Plateau, which was at peak activity some 119 - 125 million years ago (Middle Cretaceous times), and is roughly the same size as Alaska.
The oceans were warm and as such occupied more volume (remember the thermal expansion coefficient).
There were no ice sheets and limited or perhaps no areas of standing ice on the planet.
84
What was the overall trend in Cretaceus oceanic evolution? What are epeiric seas? What kind of sediments were deposited in these oceans?
The overall trend in Cretaceous oceanic evolution was towards deeper inter-continental oceans and transgression across shallow continental areas, creating large shallow intra-continental oceans, often called epeiric seas.
All types of sediments were deposited in these oceans but many were characterized by sediments composed of calcium carbonate (CaCO3), often referred to as carbonates.
85
What was the dominant reef producing organism during the Cretaceous? What allowed them to be so common?
An important source of carbonate sediments today are coral reefs. Although the modern group of corals (scleractinian corals) had evolved by the Cretaceous, the dominant reef producing organism was a type of clam called a rudist bivalve. Rudists may have dominated the reef environment as they were more able to adapt to the warm and probably more saline water conditions of this time. Rudists became extinct at the end of the Cretaceous and today scleractinian corals are the primary reef builders.
86
What are rudists?
Rudists are a group of bizarrely shaped marine heterodont bivalves that arose during the Jurassic, and became so diverse during the Cretaceous that they were major reef-building organisms in the Tethys Ocean. They were among the many animal groups that perished during the Cretaceous–Paleogene extinction event.
87
The most important carbonate sediment-producing organisms during the Cretaceous?
The most important carbonate sediment-producing organisms during the Cretaceous were a lot smaller than corals or clams. Coccolithophores are single-celled phytoplankton that surround themselves with plates of calcium carbonate called coccoliths.
88
What did carbonates compose of during the Cretaceous?
Carbonates composed of coccoliths were extremely common during the Cretaceous. When transformed into rock they are called by a name we are all very familiar with, chalk. These chalk epeiric seas are very characteristic of the Cretaceous. Although similar calcareous sediments are being produced in the center of ocean basins today, it does not compare with the vast quantities of carbonate sediment produced during the Cretaceous.
89
What do the white cliffs of Dover exemplify?
The famous White Cliffs of Dover provide a good example of sedimentary rocks made of CaCO3, called limestone. When looking at limestone under a microscope, we find that it consists mainly of fossil shells of marine plankton, which were deposited on the seafloor during the Cretaceous (144-65 Ma). These fossils are very similar to those found in today's ocean.
90
Why were carbonate rocks so common in the Cretaceous?
Today with lower sea levels and more land exposed, erosion is producing a lot of clastic sediment that is transported down rivers and eventually deposited into the oceans on the continental margins. This high sediment supply is also aided by the large number of mountains present on today's continents that supply vast quantities of sediment.
In contrast, the Cretaceous continents were over 40% more flooded than today. Any sediment that got produced by continental erosion would be deposited along the continental margins in shallow seas. In addition, there were fewer tall mountainous areas during the Cretaceous and consequently less sediment input overall. Given the extent of the oceans, there was insufficient sediment being produced to cover them all. This left a lot of open ocean with no input of sediment. These warm and shallow ocean waters, unclouded with continental sediment are perfect for the deposition of calcium carbonate sediments.
The generation of such large amounts of carbonate sediment probably helped regulate the Cretaceous climate with coccolithophores extracting CO2 from the ocean to help build their CaCO3 shells. This biochemical process that would allow the ocean to soak up more CO2 from the atmosphere.
91
Where is the modern analogy to the Cretaceous world today?
The closest modern analogy to the Cretaceous world today is Australia. Australia has a low mean elevation and enjoys a relatively warm climate. Its margins are rich in carbonate sediment.
92
How do ice cores tell us about the climate in the past? How far back can it measure?
In areas where snow cover is not removed during the summer, it is slowly buried over time and recrystallized into ice. As this occurs, bubbles of air are trapped within the ice, preserving a record of past atmospheric composition. In this manner, ice cores have preserved climatic records going back hundreds of thousands of years, perfect for unraveling the recent glacial history of our planet. Sadly, the record does not go back far enough in time for us to read the climate of Earth over 65 million years ago.
93
Describe denrochronology and what it tells us.
Dendrochronology, more popularly known as tree ring dating, can be used as a proxy for climate change over time. Tree rings are the result of different rates of growth during the seasons, rapid growth during the summer and slower growth during the winter. Since one ring represents 1 growth year, counting the rings of a tree gives its age. In addition to age, the thickness of the tree ring also gives an impression of the climate. Thick rings are produced during warm times and thin during cold.
94
Dendrochronology vs ice core information-wise?
Unfortunately, tree rings present the same problem as with ice cores in that continuous tree ring dates only go back thousands of years. Unlike ice though, Mesozoic trees have been preserved as fossils, which give a brief glimpse of the climate at the time when that particular tree was growing. This however, is far removed from providing a continuous record of climatic variation through the Mesozoic.
95
How does coral give us information?
Like trees, corals lay down annual layers of calcium carbonate (CaCO3) that can be used as proxies for seasonal changes over time. Coral data provide a more continual climatic history than tree rings but only to about the same level as that of ice core data. Just like petrified trees, fossilized Mesozoic corals can provide some climatic insights into the past but only where those corals are found preserved in the fossil record.
96
Other fossil remains, plants in particular, can be useful climatic indicators. In large accumulations, plant material can be compressed by geological processes to form coal. Where does coal form commonly?
A common location for the formation of coal is hot, humid environments. It is important to note however that coal can also form in cooler environments. Thus scientists must consider the types of fossil plants and/or animals that might be contained within the coal before making a climatic determination.
97
How are leaves used to determine climactic conditions?
Leaf shape has also been used to determine climatic conditions. Today, leaves with a smooth outline dominate in warm tropical areas whereas increasing proportions of jagged-edged leaves occur at higher and cooler latitudes.
Another climatic indicator from leaves is stomatal density. Stomata are the tiny pores through which plants exchange CO2 and H2O with the atmosphere. Plants balance CO2 uptake against H2O loss, responding to variations in CO2 levels by adjusting the number of stomata on their leaves.
In periods of high atmospheric CO2, plants have a lower stomatal density and to retain more water but maintain the same rate of CO2 uptake. This inverse relationship between the number of stomata and atmospheric CO2 allows the estimation of ancient atmospheric CO2 concentration from the density of stomata on fossil leaves.
98
How do sediments record the climate?
Se dimentary rocks are effectively Earth’s tape recorder. Sediments will often reflect the climatic conditions under which they were deposited. For example, red sandstones that are mostly composed of well rounded quartz grains are often indicative of hot, arid desert deposits. The additional presence of evaporites, produced as ephemeral lakes evaporate, may also confirm hot and arid conditions.
99
What can sand dunes tell us about the land?
Large structures, such as entire regions sand dunes, can be preserved in the geological record. In addition to providing clues to climatic conditions, depending on the orientation of these features, they can also indicate the direction of prevailing winds. Sand gets blown along the back of a sand dune and then avalanches down the steep slip face. This creates a number of layers that dip in the direction that the wind was blowing called cross bedding.
100
What is calcrete (aka caliche)?
Another indicator of hot and arid climate is calcrete, sometimes called caliche. Calcretes form when minerals are leached out of the soil by intense evaporation. Calcretes are commonly composed of calcium carbonate (CaCO3). Initially, CaCO3 is precipitated on small grains to form lumps of carbonate. With continued precipitation, a layer or bed of calcrete is formed.
101
What are evaporites?
Evaporites are salt deposits. They are evidence of the evaporation of inland seas and the precipitation of various minerals as the water concentrates and dwindles.
102
What are limestones?
Limestones that are composed of the skeletons of tropical animal species are a fairly good indicator of shallow, clear, and fairly warm conditions. A good analogue would be the limestones that are forming from the coral skeletons of Australia’s Great Barrier Reef.
They are carbonates which give indications of climatic conditions.
103
What is glendonite? What do they indicate.
It is important to note that carbonates can form across a range of different climatic conditions. For example, the mineral ikaite (CaCO3·6H2O) only forms over a very specific temperature range of 0-7°C. At warmer temperatures (including when sediments are transformed into rock) the ikaite looses the 6H2O part from its crystal structure and becomes calcite (CaCO3).
Calcites that retain the original distinctive crystal form and structure of the original ikaite are called glendonite and are often used as an indicator of cold climatic conditions.
104
What are bauxites?
Climatic conditions play a significant role in the manner in which rocks erode and form soils. Evidence of ancient erosion surfaces or ancient soils (called paleosols) are important climatic indicators. Bauxite (the most important ore of aluminum) and laterites often form in hot, humid, tropical conditions from the erosion of igneous rocks such as granite or metamorphic rocks like gneiss
105
What is tillite?
Cold climatic conditions also leave behind clues in sediments. For example, the sedimentary rock tillite (lithified glacial till) is indicative of past glacial conditions. Glacial till is a poorly sorted sediment composed of fragments of rocks eroded by a glacier.
106
What is a ventifact?
A ventifact is a rock that has been etched and polished by wind that contains sand particles.
107
What are glacial striations?
Other types of evidence of erosion of glacial ice can also be preserved in the geological record. Rocks embedded in glaciers scratch parallel lines on the surfaces that the glacier is moving over. This not only gives an indication of cold climates but also the general direction of the movement of that ice.
108
What are dropstones and how are they formed?
As glaciers move out over water they create rafts that contain rocks frozen into the ice. As these ice rafts melt, the rocks held in them fall to the bottom of the lake or ocean that they were moving over. When these rocks impact the soft sediment they create characteristic dropstone structures that disrupt any layers or laminations that were in the sediment.
109
What do unconformities indicate?
Even though sediments can be excellent climatic indicators, we have to be aware that periods of erosion and tectonic activity have made the sedimentary record incomplete. Sedimentary sequences are often broken by large gaps or unconformities, representing breaks in the sedimentary record or periods of active erosion and removal of sediment.
110
Sedimentary record in ocean?
The sedimentary record is more continuous in the oceans. However, due to the destruction of oceanic crust at subduction zones, the oldest confirmed open ocean crust, and therefore oceanic sediment, is no older than about 200 million years old (placing it in the Jurassic). This gives us a well documented sedimentary record through a little over half of the Mesozoic, but still leaves parts of the Jurassic and the entire Triassic with only the patchy continental record. (Note that there are relatively small disputed areas of oceanic crust that are potentially thought to be older. For example, there is ocean crust that is potentially 340Ma in the Herodotus Basin, found under the Eastern part of the Mediterranean sea.
111
Describe the Phanerozoic Eon's climate:
How did climate change through the Mesozoic?
Hot houses and cold houses (see images for more)
Climate over the last 2 billion years can (broadly) be described as being either in a warm Hot House or cold Ice House (also sometimes called a Cold House) state. During the Phanerozoic (everything younger than about 542 million years) there have been around four Ice House periods and four Hot House periods. In general, there were more Hot House than Ice House periods during the Mesozoic.
112
Does being in Ice House mean always glacial conditions?
No.
Being in an Ice House does not necessarily mean being only under glacial conditions but that glacial periods commonly occur during these times.
113
Describe climate in Early to Middle Triassic?
The Early to Middle Triassic was mostly warm and dry, with Pangea straddling the equator. The continents that made up Pangea had fairly high elevations and sea level was fairly stable. As a result, there were very few shallow inland continental seas. The interior of the supercontinent was very dry and covered by hot, arid deserts. This means that red desert sandstones were very common rocks throughout the Triassic.
In addition to the red desert sandstones, other telling features in rocks from the Early Triassic include ventifacts and calcretes. These features also point towards hot, arid conditions.
Recall that oceans have a great capacity for regulating the temperature of continental regions. They help warm continental areas during the winter and help cool and moisten continental areas during the summer. As the oceanic effect was minimized during the Early Triassic, Pangean summers were very hot and dry, with warm conditions extending into polar regions, though regions near the coast most likely experienced seasonal monsoons.
During the winter, the interior of Pangea may have become quite cold. As there were no polar ice caps, the temperature gradient in the north-south direction was probably more gradual than today. Overall, it appears that the climate included both arid dune environments and some river and lake habitats with gymnosperm forests.
114
Describe climate of Late Triassic
By the Late Triassic there were three broadly defined climatic belts: (1) an equatorial year-round dry belt; (2) a zone north and south of this dry belt with strong seasonal rainfall; and (3) more humid higher latitude belts.
Evidence for the humid high latitudinal areas comes from coal-rich sequences, as well as the fossils of large amphibians. Fossils of species of ferns that today prefer wet, shady areas (as are found under forest canopies) are also found in these areas.
Super monsoons: During boreal (Northern Hemisphere) summer, the northern landmass of Pangea became warmer than the surrounding seas and oceans creating a low atmospheric pressure cell. Land on southern Pangea became colder (winter in the Southern Hemisphere), creating a high atmospheric pressure cell. As a result, moisture evaporating from the warm equatorial Tethys Sea, was carried to the northern part of the continent where it produced rain (summer monsoons). Since winds were blowing away from the cold southern Pangea, preventing warm oceanic air from reaching these parts, the southern part of continent would have stayed dry.
This situation would have reversed 6 months later, resulting in alternating, hemisphere-wide wet and dry seasons.
115
Describe the climate of Early Jurassic
The hot arid/seasonal conditions continued into the Early Jurassic. However, it was probably getting a little more moist. This was because of the continued opening of the Tethys Ocean, bringing oceanic conditions closer to the interior of Pangea. Evidence for this comes from Late Triassic/Early Jurassic rocks in China that indicate conditions were lush and verdant, due to the presence of moisture-bearing winds.
In summary, the period from the Triassic through the Early Jurassic was a time of heat and aridity but with clearly defined seasons and probably no ice at the poles. Another feature of Pangea’s climate at this time was the Super Monsoon.
Also, there was CAMP, increased fragmentation of Pangea.
Probably more significant to changes in climate associated with this volcanism was the production of large amounts of carbon dioxide gas, possibly up to 10 times current atmospheric CO2 levels. As we have already discussed, CO2 is a very effective greenhouse gas and such an increase in the levels of this gas would have led to global warming effects.
116
What could have been the cause of the high temperatures in the Late Triassic to Early Jurassic?
This is still an area of debate but it is likely that an increase in the rate of fragmentation of Pangea may be a causal factor.
117
What is the Central Atlantic Magmatic Province (CAMP) in Early Jurassic and how did it form?
The central parts of Pangea were experiencing increased volcanism and would later start to separate to form the Atlantic Ocean. This volcanism produced very large amounts of basaltic lava, at least 2 × 106 (2 million) cubic kilometers, which covered large areas of Pangea. Lava produced in this magnitude, eventually covering large stretches of land or ocean floor, are called flood basalts.
The flood basalts in this area are referred to as the Central Atlantic Magmatic Province (CAMP). CAMP was probably one of the largest igneous events in Earth's history. It stretched from the east to west coasts of what are now northwestern Africa, eastern America (north and south), and parts of Europe.
Probably more significant to changes in climate associated with this volcanism was the production of large amounts of carbon dioxide gas, possibly up to 10 times current atmospheric CO2 levels. As we have already discussed, CO2 is a very effective greenhouse gas and such an increase in the levels of this gas would have led to global warming effects.
CAMP may be related to a mass extinction event that occurred at the end of the Triassic.
118
Describe the climate in Middle to Late Jurassic
During the Late Jurassic, global climate began to change due to the breakup of Pangea. The interior of Pangea became less dry, and seasonal snow and ice frosted the polar regions. Even so, the climate compared to today was still more equable and less variable. Tropical-subtropical conditions extended far into the present temperate regions and temperate conditions occurred into the present polar regions.
119
Describe the climate of Late Jurassic to Early Cretaceous, and what kinds of evidence we have for its climate
Temperatures dropped through the Late Jurassic and into the Early Cretaceous, to produce what was, by Mesozoic standards, an Ice House world. It is important to note however that this Ice House was still very warm and humid by today's standards. The poles may have had small permanent ice caps, and snow probably fell on the cool temperate forests that surrounded them.
This was the coldest time of the entire Mesozoic. The climate would fluctuate from Ice House to Hot House through the Early Cretaceous before starting a general warming trend later on in the Cretaceous (during the Aptian age division of the Cretaceous).
Evidence for these cooler conditions comes from glendonites and from glacial dropstones found in various areas that were located in high latitudes during the Early Cretaceous. Oxygen isotope data from Early Cretaceous sediments in Australia (Eromanga Basin) suggests that temperatures may have been as low as -6°C, although the climate was probably seasonally rather than uniformly cold.
Further evidence of cooler conditions comes from plants. Growth rings from fossil conifer wood are conspicuously narrow and contain a record of very seasonal growth. An increase in the number of jagged-edged leaves has also been noted for broad leaf plants during this part of the Cretaceous.
120
Describe the climate in Middle to Late Cretaceous
By the Middle Cretaceous, high sea levels and high CO2 concentrations (perhaps 18× higher than today) produced warm global climates. The concept of a very warm Middle to Late Cretaceous world is well supported by many lines of evidence including the very low stomatal density found on fossil leaves of that time. This period of the Cretaceous is often referred to as a Hot House World.
It is likely that the planet had no permanent ice, had plants adapted to the cool temperate climate, and dinosaurs living very close to the poles. The planet could be divided into three major biogeographic regions, the northern boreal (temperate), tethyan (tropical), and southern boreal provinces.
It is not known if tropical conditions were warmer than today; however, it is widely accepted that tropical conditions did extend in a wide band north and south of the equator. The opening of the Atlantic Ocean and continued expansion of the Tethys Ocean helped regulate temperatures in the now fragmenting Pangea. As a result, the extreme temperature differences experienced early in the Mesozoic where oceanic water was unable to penetrate to the interior of the landmass, were not a factor at this time in the Cretaceous.
121
Describe the climate in Late Cretaceous
The last 30 million years of the Cretaceous saw a mild deterioration of the warm equable conditions of the Middle Cretaceous. Sea levels fell, a likely reason why the planet was experiencing more pronounced seasonality at this time. At the very end of the Cretaceous there may have been another period of increased global warming, this time related to elevated atmospheric CO2 from the volcanic eruption of the Deccan Traps Flood Basalts in India.
The large igneous provinces include continental flood basalt provinces, volcanic passive margins, oceanic plateaus, aseismic submarine ridges, ocean basin flood basalts, and seamount groups.
