Abstract were being deposited at this time.

Abstract

Throughout
Geological history global environments have undergone a lot of change due to
many reasons, including the movement of continents. While it is widely known
what the environment in Britain was like during certain times in history, for
example during the ice age, it is a lot harder to deduce what the environment
was like millions of years ago. The Jurassic period is mainly famous for being
the time when dinosaurs walked the Earth, and that during this time the Earth
was experiencing a much warmer climate, with the majority of life environments being
those of a tropical sea environment. However, did the environment in Britain
follow this general global trend or is there evidence to be found that could
suggest something different altogether.

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Introduction

Due to
continental drift and the splitting apart of major continents, Britain has
experienced a lot of movement and therefore has also experienced a lot of
change in terms of what the environment was like during this time, as well as
experiencing a large shift in the species that were living there. This in turn
would influence the rock types that were being deposited at this time. Broken
up continents are highly likely to be able to create niches and new favourable
conditions that are likely to encourage biodiversity. (James Valentine et al,
1970) During the Jurassic, a lot of significant global change occurred
including Pangaea splitting up and the eventual formation of the Atlantic Ocean.
(figure 1) The extensive plate tectonics that happened throughout all this
movement led to a large amount of volcanic activity as well as mountain
building events. (Carol Marie Tang, 2017) This report will focus on what the
environment was like in Britain during the Jurassic period and will look into
evidence that supports this including rock types and fossil specimens. This
will also focus on discussing possible reasons as to why the environment may
have been like this.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Jurassic
Britain Palaeoenvironment

The
Jurassic Period occurred between 201 and 145 million years ago and is likely most
famous in Britain due to the Jurassic coast which is a well-known area of
southern Britain found to contain fossils of Jurassic age marine life. At the
start of the Jurassic there was a significant rise in sea level which caused a
transformation from the desert that had previously existed in the Triassic into
a tropical sea environment. (JurassicCoast.org, n.d.) At this time in history,
Britain was 30-40o north of the equator, and experienced annual
temperatures of 12-29oC (BGS, n.d.). New oceans had opened and there
was formation of mountains on the sea floor which caused the water to be
displaced and to rise onto the continent helping trees to grow in the new
subtropical atmosphere that was created. In Southern England there was a
general trend of deepening marine conditions. (BGS, n.d.) Ginkgoes were
particularly common in the northern latitudes. The newly formed shallow seas at
this time were full of highly diverse life and on land dinosaurs became more
abundant. The late Jurassic also came with the presence of the earliest known
bird- the Archaeopteryx (National Geographic, n.d.) Rock strata that was laid
down during this time have been known to contain many natural resources including
petroleum, gold and coal which is known to form in a warm environment. During
the early stages of the Jurassic, the animals and plants in both the land and
sea environments were having to recover from one of the largest mass
extinctions that Earth has experienced throughout its history. In higher
latitudes, frost sensitive plants have indicated that there was a less significant
difference between the temperature at the poles and the temperature at the
equator than there is now. The coolest temperatures that were experienced
during this time occurred in the middle Jurassic whereas the warmest
temperatures occurred in the late Jurassic. (Tang, 2017) Some environments of
deposition have been found to include, shallow marine, tropical, lagoonal as
well as more arid.

 

Fossil
and Rock Evidence

There is a
lot of important rock and fossil evidence from the Jurassic that can be found
in Britain that can be used to help pinpoint what the environment would have
been like during this time. Rocks from the Jurassic period are widely
distributed and include rocks of all types. Due to subduction, rocks that were
found to be from the middle Jurassic oceanic crust tend to be the oldest
sediments that are found to remain in the deep sea. Igneous rocks of Jurassic age
can be found around spreading centres, in the areas where the Atlantic Ocean
was opening for example. Jurassic sedimentary rocks can be found everywhere,
but in Europe black shales are found to be particularly common due to restricted
circulation of water in shallow marine basins, which eventually would lead to
the bottom waters to become deficient in oxygen, enough to create the anoxic
environment needed for the deposition of black shales. As the sea spreading
continued and the Tethys ocean widened, is caused the facilitation of the
formation of deeper water sediments, including Jurassic carbonates, to become
very common in England. Terrestrial deposits were found to consist mainly of
sandstones, red beds and mudstones that would have been deposited under
fluvial, lacustrine or Aeolian environments, also there were found to be
accumulations of coal beds and other continental sediments. (Tang, 2017) During
the early Jurassic the seas were full of life, which lead to lots of deposits
of Jurassic age fossils, most of the fossil remains would be found to occur in
mudstones or limestones which suggests a quiet water environment. However, some
fossil remains have been found to include coarse ribbed bivalves which were
more adapted to turbulent conditions. Some beds of the Mendips have been found
to contain fossilised ripple marks which would suggest that the environment of
deposition is that of a shallow marine environment. (BGS, n.d.) Fossils that
have been found in the Great Estuarine Group, Scotland have been found to be
consistent with the deposition patterns of a lagoonal environment. Results from
measurements taken of Sr-87/Sr-86, Sr/Ca, delta (18)O and delta (13)O suggest
this due to being consistent with a lagoonal hydrology controlled by seasonal
changes in processes including evaporation rather than by direct inputs of
water i.e. seawater. (Holmden, C, 2003) Britain contains multiple GSSP’s that
show the first appearance of a stage of the Jurassic by correlating species of
a very important Jurassic fossil- the ammonite. For example the base of the Sinemurian
(figure 2) can be found at East Quantoxhead and is associated with the
appearance of the ammonite Bifericas
donovani and Apoderoceras sp.
(Geological society, n.d.)

 

 

 

 

 

 

 

 

One place
in particular that holds particular importance is the Jurassic coast, which
holds a variety of fossils including fish, insects, mammals, echinoderms,
molluscs and brachiopods etc. The Jurassic coast also happens to be one of the
most important sources for reptile fossils in the world, including
ichthyosaurus- shown in figure 3, plesiosaurus and a unique dinosaur-
Scelidosaurus.

 

 

 

 

 

 

 

 

 

 

The most
common fossils to be found on the Jurassic coast happen to be trace fossils
which can be used to help reconstruct habitats on the sea bed, they also help
to show how the marine ecosystems were able to recover after the Triassic/
Jurassic mass extinction. In certain rock layers it has been found to be very
common to find dinosaur foot prints- shown in figure 4. (JurassicCoast.org,
n.d.)

 

 

 

 

 

 

 

One site
in particular that is very fossiliferous is the Kimmeridge clay formation,
which is a section of rock deposited in the upper Jurassic. (Gallois, 2000)
onshore formations of this rock type tend to outcrop as mudstones, thin
siltstone and cementstone beds, whereas offshore is tends to mainly appear as
black, organic rich mudstone which is the source of the North Sea oil. (British
Geological Survey, n.d.) The Kimmeridge clay formation has been found to be the
source of many remarkable fossils including bones of dinousaurs, icthyosaurus
and others as well as many invertebrates. Among the fossils found is a very
well preserved specimen of an ichthyosaur paddle (figure 5).

 

 

 

 

 

 

 

 

 

 

 

As
well as this there are many deposits of crushed ammonite shells, with
associated oysters. At the time this was deposited, the sediment would have
been in sea water that was oxygenated. The ammonite is pelagic so would have
been swimming however the oysters would have been living on the sea floor so
have been used to indicate that the sea floor Palaeoenvironment was habitable.
(Figure 6). As well as the crushed ammonites, there were multiple other species
found of ammonites as well at this site. (Figure 7) (Ian West, 2017)

 

 

 

 

 

 

 

The
rise and fall of sea level during this time (figure 8) can be correlated by
looking into the deposition of marine shales in non-marine sediments, also by
looking at the concentration of fossils as well as glauconite and phosphorite. (Hallam,
2000)

Discussion

The
Jurassic climate has been suggested to be a lot calmer than that of the present
climate, with tropical/ subtropical conditions present in what are now
temperate belts and temperate belts in the current day polar regions. In the
Jurassic, there is no evidence found of there ever being polar ice caps at this
time, although the melting of ice caps could be one of the causes of the sea
level rise. (Hallam, 2000) The ocean level at this time was a lot higher with
the majority of Britain being covered. There could be problems with making
precise statements about what the environment was like as we do not have the
necessary knowledge needed to do this. For example, Jurassic fossils have been
extinct for too long for them to have close enough modern relatives to use to
be able to compare how they live now to how they would have lived before.
(Hallam, 1982) During the Jurassic, there was a lot of plate movement happening
due to the splitting of Pangea and the subsequent formation of Laurasia and
Gondwana. This movement caused the plates to subduct causing volcanism and the melting
of rocks which led to CO2 being released into the atmosphere. The CO2
would have trapped any escaping heat energy causing the Earth to warm up
and therefore could be able to prove that the Jurassic period would have been
very warm and humid. (van der Meer, 2014) Warm temperatures could be found to
also be related to the Tethys ocean, which would have, due to ocean currents,
distributed warm, tropical waters around the world. (Tang, 2017) The best
climatic indicators that can be analysed from deposits of sedimentary rocks
would have been evaporates and coal. (Frakes, 1979) Large salt deposits, in
particular halite, anhydrite and gypsum are found to represent areas of high
aridity while the coal indicates areas of high precipitation. (Tang, 2017) Some
of the fossil evidence could be used as a climatic indicator including
hermatypic corals and ferns. For the reef building corals to be able to form
and be visible in the fossil record there would have to have been a minimum
water temperature of around 20oC, and the types of ferns that were
found in terrestrial deposits have living relatives that are known to not be
able to tolerate living in a cold environment with frost. (Hallam, 1975)
Analyses of oxygen isotopes collected from the shells of marine fossils suggest
that the global temperatures were generally warm, with surface waters being
around 20oC and deeper waters lower at around 17oC.
(Tang, 2017) While
this would have been a general trend, a recent report has actually suggested
that there would have been quite a lot of variation in sea temperatures during
at least the early Jurassic. New carbon and oxygen isotope records collected
from compositions of carbonate and belemnites- which were taken from the
Sinermurian-Aalenian stage from an are in the northern Tethys Ocean.
Temperatures that have been inferred from looking at the ?18O suggest a much lower water temperature in the
Late Sinemurian that what would be expected from a tropical environment (10-13oC),
this then had a lot of variation, rising by 4oC in the early Pliensbachian and
then again cooling in the late Pliensbachian (Arabas et al, 2017). This could
suggest that the environment was a lot more complex that just being a constant
tropical sea environment throughout the entirety of the Jurassic. These varying
cold and warm periods have been suggested because of paleo biogeographic and
isotopic evidence. To support the colder periods of time it has been shown that
glendonite has been found which is a pseudomorph of calcite that it only found
in old water. On the other hand, the Early Jurassic- Toarcian- Oceanic anoxic
event is a very warm period in comparison. (Korte et al, 2015)

 

Conclusion

While it is widely
known that the environment during the Jurassic was one that was warm and humid,
and also that of a tropical sea environment, and there is a lot of evidence
around to be able to support this fact including the rocks that were deposited
for example carbonates found that are typical of a shallow marine deposit. Also,
the fossils found during this time are those you would expect with the marine
life as well as the species that are found on land. Climate changes throughout
the past indicate events happening in the world and the changes that happened
during this period mainly coincide with the drift of the continents which
subsequently caused a chain of events behind including volcanic activity which
caused the drastic change in environment from desert to tropical/subtropical. (Tang,
2017) While it is the case that from evidence we can assume that the climate
was like this, there is also evidence coming out that can look deeper into any
minor changes that may have occurred to take it away from being tropical, for
example the evidence found from the isotopes that suggest there were also
colder periods during the Jurassic. (Korte et al,
2015)

 

References

 

Arabas, A, Schlogl, J, Meister, C, November 2017, Early Jurassic carbon and oxygen isotope records
and seawater temperature variations: Insights from marine carbonate and
belemnite rostra (Pieniny Klippen Belt, Carpathians), Paleogeography,
Paleoclimatology, Paleoecology, Volume 485, Pages 119-135 Accessed December
2017

Available at: http://www.sciencedirect.com/science/article/pii/S003101821630921X