AbstractThroughoutGeological history, global environments have undergone a lot of change due tomany reasons, including the movement of continents.
While it is widely knownwhat the environment in Britain was like during certain times in history, forexample during the ice age, it is a lot harder to deduce what the environmentwas like millions of years ago. The Jurassic period is mainly famous for beingthe time when dinosaurs walked the Earth, and that during this time the Earthwas experiencing a much warmer climate, with the majority of life environments beingthose of a tropical sea environment. However, did the environment in Britainfollow this general global trend or is there evidence to be found that couldsuggest something different altogether.
IntroductionDue tocontinental drift and the splitting apart of major continents, Britain hasexperienced a lot of movement and therefore has also experienced a lot ofchange in terms of what the environment was like during this time, as well asexperiencing 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. Brokenup continents are highly likely to be able to create niches and new favourableconditions that are likely to encourage biodiversity. (James Valentine et al,1970) During the Jurassic, a lot of significant global change occurredincluding the formation of the Atlantic Ocean and the separation of Pangea.(figure 1) The extensive plate tectonics that happened throughout all thismovement led to variety of events including high levels of volcanism as well asthe formation of mountains. (Carol Marie Tang, 2017) This report will focus onwhat the environment was like in Britain during the Jurassic period and willlook into evidence that supports this including rock types and fossilspecimens. This will also focus on discussing possible reasons as to why theenvironment may have been like this. JurassicBritain PalaeoenvironmentTheJurassic Period occurred between 201 and 145 million years ago and is likely mostfamous in Britain due to the Jurassic coast which is a well-known area ofsouthern Britain found to contain fossils of Jurassic age marine life.
At thestart of the Jurassic, there was a significant rise in sea level which caused atransformation from the desert that had previously existed in the Triassic intoa tropical sea environment. (JurassicCoast.org, n.d.) At this time in history,Britain was 30-40o north of the equator, and experienced annualtemperatures of 12-29oC (BGS, n.
d.). New oceans had opened and therewas the formation of mountains on the sea floor which caused the water to bedisplaced and to rise onto the continent helping trees to grow in the newsubtropical atmosphere that was created. In Southern England, there was ageneral trend of deepening marine conditions. (BGS, n.d.) Ginkgoes wereparticularly common in the northern latitudes. The newly formed shallow seas atthis time were full of highly diverse life and on land, dinosaurs became moreabundant.
The late Jurassic also came with the presence of the earliest knownbird- the Archaeopteryx (National Geographic, n.d.) Rock strata that were laiddown during this time have been known to contain many natural resources includingpetroleum, gold and coal which is known to form in a warm environment. Therewas a large mass extinction at the Triassic-jurassic boundary that species werehaving to recover from in the beginning parts of the Jurassic. In higherlatitudes, frost sensitive plants have indicated that there was a less significantdifference between the temperature at the poles and the temperature at theequator than there is now.
The coolest temperatures that were experiencedduring this time occurred in the middle Jurassic whereas the warmesttemperatures occurred in the late Jurassic. (Tang, 2017) Some environments ofdeposition have been found to include, shallow marine, tropical, lagoonal aswell as more arid. Fossiland Rock EvidenceThere is alot of important rock and fossil evidence from the Jurassic that can be foundin Britain that can be used to help pinpoint what the environment would havebeen like during this time. Rocks from the Jurassic period are widelydistributed and include rocks of all types. Due to subduction, rocks that werefound to be from the middle Jurassic oceanic crust tend to be the oldestsediments that are found to remain in the deep sea.
Igneous rocks of Jurassic agecan be found around spreading centres, in the areas where the Atlantic Oceanwas opening for example. Jurassic sedimentary rocks can be found everywhere,but in Europe black shales are found to be particularly common due to the restrictedcirculation of water in shallow marine basins, which eventually would lead tothe bottom waters to become deficient in oxygen, enough to create the anoxicenvironment needed for the deposition of black shales. As the sea spreadingcontinued and the Tethys ocean widened, is caused the facilitation of theformation of deeper water sediments, including Jurassic carbonates, to becomevery common in England. Terrestrial deposits were found to consist mainly ofsandstones, red beds and mudstones that would have been deposited underfluvial, lacustrine or Aeolian environments, also there were found to beaccumulations of coal beds and other continental sediments. (Tang, 2017) Duringthe early Jurassic, the seas were full of life, which lead to lots of depositsof Jurassic age fossils, most of the fossil remains would be found to occur inmudstones or limestones which suggests a quiet water environment. However, somefossil remains have been found to include coarse ribbed bivalves which weremore adapted to turbulent conditions. Some beds of the Mendips have been foundto contain fossilised ripple marks which would suggest that the environment ofdeposition is that of a shallow marine environment. (BGS, n.
d.) Fossils thathave been found in the Great Estuarine Group, Scotland have been found to beconsistent with the deposition patterns of a lagoonal environment. Results frommeasurements taken of Sr-87/Sr-86, Sr/Ca, delta (18)O and delta (13)O suggestthis due to being consistent with a lagoonal hydrology controlled by seasonalchanges in processes including evaporation rather than by direct inputs ofwater i.e.
seawater. (Holmden, C, 2003) Britain contains multiple GSSP’s thatshow the first appearance of a stage of the Jurassic by correlating species ofa 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 theappearance of the ammonite Bifericasdonovani and Apoderoceras sp. (GeologicalSociety, n.d.) One placein particular that holds particular importance is the Jurassic coast, whichholds a variety of fossils including fish, insects, mammals, echinoderms,molluscs and brachiopods etc.
The Jurassic coast also happens to be one of themost important sources for reptile fossils in the world, includingichthyosaurus- shown in figure 3, plesiosaurus and a unique dinosaur-Scelidosaurus. The mostcommon fossils to be found on the Jurassic coast happen to be trace fossilswhich can be used to help reconstruct habitats on the seabed, they also help toshow how the marine ecosystems were able to recover after the Triassic/Jurassic mass extinction. In certain rock layers, it has been found to be verycommon to find dinosaur footprints- shown in figure 4. (JurassicCoast.org,n.d.
) One site, inparticular, that is very fossiliferous is the Kimmeridge clay formation, whichis a section of rock deposited in the upper Jurassic. (Gallois, 2000) onshoreformations of this rock type tend to outcrop as mudstones, thin siltstone andcement stone beds, whereas offshore is tends to mainly appear as black,organic-rich mudstone which is the source of the North Sea oil. (BritishGeological Survey, n.d.) The Kimmeridge clay formation has been found to be thesource of many remarkable fossils including bones of dinosaurs, ichthyosaurusand others as well as many invertebrates. Among the fossils found is a verywell preserved specimen of an ichthyosaur paddle (figure 5). Aswell as this there are many deposits of crushed ammonite shells, withassociated oysters.
At the time this was deposited, the sediment would havebeen in seawater that was oxygenated. The ammonite is pelagic so would havebeen swimming however, the oysters would have been living on the sea floor sohave been used to indicate that the sea floor Palaeoenvironment was habitable.(Figure 6). As well as the crushed ammonites, there were multiple other speciesfound of ammonites as well at this site. (Figure 7) (Ian West, 2017) Therise and fall of sea level during this time (figure 8) can be correlated bylooking into the deposition of marine shales in non-marine sediments, also bylooking at the concentration of fossils as well as glauconite and phosphorite.
(Hallam,2000)DiscussionTheJurassic climate has been suggested to be a lot calmer than that of the presentclimate, with tropical/ subtropical conditions present in what are nowtemperate belts and temperate belts in the current day polar regions. In theJurassic, there is no evidence found of there ever being polar ice caps at thistime, although the melting of ice caps could be one of the causes of the sealevel rise. (Hallam, 2000) The ocean level at this time was a lot higher withthe majority of Britain being covered. There could be problems with makingprecise statements about what the environment was like as we do not have thenecessary knowledge needed to do this. For example, Jurassic fossils have beenextinct for too long for them to have close enough modern relatives to use tobe 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 happeningdue to the splitting of Pangea and the subsequent formation of Laurasia andGondwana. This movement caused the plates to subduct causing volcanism and the meltingof rocks which led to CO2 being released into the atmosphere. The CO2would have trapped any escaping heat energy causing the Earth to warm upand therefore could be able to prove that the Jurassic period would have beenvery warm and humid. (van der Meer, 2014) Warm temperatures could be found toalso be related to the Tethys ocean, which would have, due to ocean currents,distributed warm, tropical waters around the world.
(Tang, 2017) The bestclimatic indicators that can be analysed from deposits of sedimentary rockswould have been evaporates and coal. (Frakes, 1979) Large salt deposits, inparticular halite, anhydrite and gypsum are found to represent areas of higharidity while the coal indicates areas of high precipitation. (Tang, 2017) Someof the fossil evidence could be used as a climatic indicator includinghermatypic corals and ferns. For the reef-building corals to be able to formand be visible in the fossil record there would have to have been a minimumwater temperature of around 20oC, and the types of ferns that werefound in terrestrial deposits have living relatives that are known to not beable to tolerate living in a cold environment with frost. (Hallam, 1975)Analyses of oxygen isotopes collected from the shells of marine fossils suggestthat the global temperatures were generally warm, with surface waters beingaround 20oC and deeper waters lower at around 17oC.(Tang, 2017) Whilethis would have been a general trend, a recent report has actually suggestedthat there would have been quite a lot of variation in sea temperatures duringat least the early Jurassic.
New carbon and oxygen isotope records collectedfrom compositions of carbonate and belemnites- which were taken from theSinermurian-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 theLate 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 andthen again cooling in the late Pliensbachian (Arabas et al, 2017). This couldsuggest that the environment was a lot more complex that just being a constanttropical sea environment throughout the entirety of the Jurassic. These varyingcold and warm periods have been suggested because of paleo biogeographic andisotopic evidence. To support the colder periods of time it has been shown thatglendonite has been found which is a pseudomorph of calcite that it only foundin old water. On the other hand, the Early Jurassic- Toarcian- Oceanic anoxicevent is a very warm period in comparison.
(Korte et al, 2015) ConclusionWhile it is widelyknown 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 evidencearound to be able to support this fact including the rocks that were depositedfor 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 marinelife as well as the species that are found on land. Climate changes throughoutthe past indicate events happening in the world and the changes that happenedduring this period mainly coincide with the drift of the continents whichsubsequently caused a chain of events behind including volcanic activity whichcaused 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 climatewas like this, there is also evidence coming out that can look deeper into anyminor changes that may have occurred to take it away from being tropical, forexample the evidence found from the isotopes that suggest there were alsocolder periods during the Jurassic. (Korte et al,2015)