Increased Seawater Anoxia During The T Oae Biology Essay


O levels in seawater become lower when atmospheric CO levels increase due to the carbon cycle. The carbon cycle involves the global movement of carbon through the atmosphere and through the sea and it is vital that the carbon cycle remains in relative balance. The reason why is that if it became unbalanced, a runaway effect could be produced causing an extinction event. A resident time is the period of time that carbon remains in a certain area. This is calculated by dividing the reservoir size at a steady state by the rate of respiration and decomposition (inflow rate) and the rate of photosynthesis (outflow rate) both of which have been given a in the Kump et al as 60gton(c)/yr. This effectively means a balanced system with carbon & oxygen cycling through at the same rate. The CO/O balance is also influenced by the seasons. During the northern hemisphere's summer photosynthesis is increased so there for O rises, this then decreases in the winter with CO increasing. This is mirrored in the Southern hemisphere just with the summer being at the opposite time.

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Dependant on a gases solubility which is dependent on both the salinity and temperature, these gases are in constant equilibrium with the atmosphere dependant on their relative concentrations. If salinity increases, the percentage of gas dissolved decreases due to the immobilisation of water molecules by the salt ion. If the temperature of the water was to increase the gas molecules would evaporate from the water so reducing the amount of gas dissolved.

Furthermore, the biological pump - photosynthesis is undertaken at the surface creating oxygen organic matter settles down into the deep ocean where it is consumed by consumers giving off co2 and upwelling of nutrients occurred.

(Kump et al, 2011)

(b) What is the geological evidence for increased seawater anoxia during the T-OAE? Why is the phenomenon considered to have been a global one? (Your answer should be up to 270 words in length.)

The Evidence for seawater anoxia during the T-OAE can be seen in widespread accumulation of organic -rich marine deposits, these co-insided with short intervals of severe paleoenviromental change. During these periods abrupt global warming would be 5-10 °c higher than that of continental weathering, mass extinction and high level perturbation of the global carbon cycle. Although not entirely confirmed (Pearce, et al.,2008) identified that how alterations in molybdemum seawater isotope ratios during the Toarican- early Jurassic indicated the onset and progression of oxygen deficient conditions. The data showed that the anoxia expanded and contracted periodically in accordance with precision-driven changes in 13corg. (Pearce, et al., 2008)

Futhermore the reason the T-OAE was considered a global one was as a result of rocks deposited on the continental shelf and epicontental seas rather than those from the deep ocean floor. This was due to most of the ocean floor during the Toarcian being subducted with the exception of Japan.

Further evidence that this was a global event was that these rocks were deposited in both Northern and Southern hemispheres. There location provides a compelling case for enhanced accumulation worldwide of marine organic matter during a relatively brief interval of time indicating a mass extinction event, T-OAE. Furthermore Perturbation to the global carbon cycle during the toarcian is also marked by a substantial and abrupt CIE. (Cohen, et al., 2007)

(c) Describe, using diagrams where appropriate, how the increase in anoxia during the T-OAE has been quantified using geochemical proxy data. Briefly state two reasons why the estimate is an approximate one. (Your answer should be up to 270 words in length.)

The increase in anoxia during the T-OAE is quantified through the alterations in molybdemum seawater isotope ratios. This isotope is specific and the changes found indicate the onset and progression of oxygen deficient conditions.

By looking at the graph we can see how the T-OAE developed.....

Environmental change began due to the movement of the movement of the karoo-ferar large igeous province.

(Pearce, et al., 2008) graph

Interval 1- decrease in δ13Corg & environmental change show onset anoxia due to the abrupt shift in δ13Corgas a result of large scale methane hydrate dissociation. During this period marine anoxia contracted and expanded. T-OAE ended between intervals 2 and 3.

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Interval 3 - high anoxic conditions in the Cleveland basin, levels of mo where raised, marine anoxia contracted but global levels increased.

Interval 4 - mo & mo water values decrease, re/mo ratios increase, indicating anoxia is less promident in the basin.

Although this isotope is known to be present in low oxygen conditions there is no data on the extent of sediment accumulation that occurred during this period and instead it uses todays oceans as a reference point.

They further go on to express that it is not possible to quantify the extent of marine anoxia or to define the relationship between the onset and duration of reducing conditions and the major perturbation to the global carbon cycle due to this the reason the way that the T-OAE came about is unclear.

(Pearce, et al., 2008)

(d) Some marine organisms are particularly susceptible to seawater deoxygenation; identify the general type of organism and explain why they are particularly susceptible to deoxygenation. Other marine organisms thrive during periods of decreased oxygen supply. Explain briefly what they are and why their survival is favoured by such conditions. (You can obtain the information you need to answer this question from the set book (Kump et al.), and in particular from Chapter 8. Your answer should be up to 270 words in length.)

Marine organism that are susceptible to seawater deoxygenation include Zooplankton, these include organisms such as foraminifera and radiolarians. These zooplankton live at the bottom of the ocean out of the range of sunlight so therefore cannot photosynthesis and are classed as marine consumers. In place of photosynthesis these zooplankton use the conversion of chemical energy for survival. They utilise dead organic matter that has settled on the bottom of the ocean through chemical decomposition. They produce fecal pellets that settle through the water column along with other particles. These zooplankton are particularly susceptible to deoxygenation as consumers as they require oxygen to initiate chemical reactions in order for them to metabolise chemical energy.

Organisms that would thrive well during periods of deoxygenation include phytoplankton. Some species of phytoplankton include diatoms and coccolithophoirds and inhabit close to the surface where sufficient light can penetrate. They metabolise CO2 and produce 02 through photosynthesis. As they consume CO2 instead of 02 they are less effected by deoxygenation and furthermore are able to use their ability to photosynthesise to produce the oxygen that they require.

(Kump et al ,2011)

Question 2

(a) Figure 1 is a systems feedback diagram that illustrates very simply how the phosphorous content of the oceans exerts what many consider to be the dominant control on biological productivity in the oceans.

Figure1 Systemfeedbackdiagramforthemarinephosphoruscycle.(Kumpetal.,2010,p.171)

Describe in your own words the processes that are thought to control the dissolved phosphorous content of the oceans, and explain how these processes are linked to the level of biological productivity in the oceans. (Your answer should be up to 300 words in length.)

Phosphorous is released from igneous, metamorphic and sedimentary rocks through the process of chemical weathering. The phosphorous then leaches from the soil into ground water, rivers and out to the ocean in the form of phosphate ions. Phosphorous is taken up by the aquatic plants and animals and processed from in organic phosphorous into organic. Species utilise the phosphorous by using it as a component involved in energy storage.

Phosphorous can become in excess when this occurs such things as algae blooms can form damaging the rest of the species in the ocean. This is because the bacteria feed of the algae, growing and spreading eventually depleting the oxygen in the sea.

With the delivery of nutrients into the oceans biological productivity increases, this mirrors the high productivity in regions of upwelling as deep water is also nutrient rich.

In other areas the surface ocean is nutrient poor due to the aggressive uptake by plankton or through attachment to settling oxidised iron nutrient. These nutrients are transferred to the deep ocean through biological and iron pumps. A small percentage of phosphorous settles on the ocean floor via the biological pump and is buried with sediments

the rest is released to the deep ocean when the organic matter decomposes.

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(Kump et al, 2011)

(b) From your studies of the material presented in Block 2 you will now know that past hyperthermal events were associated with increased levels of continental weathering, and thus with an elevated fluvial supply of nutrients to the oceans. Using this information, and with reference to Chapter 8 of Kump et al. (2010), describe in your own words how the sequence of events and processes that are associated with hyperthermal events are thought to have increased the levels of marine anoxia. (Your answer should be up to 300 words in length.)

During a hyperthermal event environmental changes may occur. Organic matter from the surface dies and decays. This could be as a result of increasing temperatures on the surface or another factor. This organic matter is leached from the soil and carried into rivers in the form of phosphate ions eventually passing into the oceans. This matter starts to settle at the top of the water column where it is aggressively taken up by phytoplankton and converted from inorganic phosphorous into organic. The phytoplankton then use this phosphorous in their energy store.

The phytoplankton starts to multiply and spread rapidly forming large extensive algae blooms which bacteria start feeding on depleting the oxygen in the water further. A percentage of the decomposing matter is not taken up by the phytoplankton, this reaches the bottom of the ocean causing a further reduction in oxygen and significant accumulations of peat. This dead matter increases CO as oxygen is used up as the dead matter decays, therefore increasing sea water anoxia.

Furthermore during a hyperthermal event the surface temperatures tend to be greater than today, meaning that dissolved oxygen levels where lower making it easier to achieve an anoxic event. The hotter the surface and the water the less oxygen content in the ocean.

Another effect a hyperthermal event has is an increase in continental weathering. This weathering creates and elevated supply of nutrients to the oceans. Furthermore with the increase in CO this increases pH so affecting corals through bleaching, killing of the protozoa on the coral. This is likely to turn the ocean acidic. Moreover, with the increase in temperatures on the surface, rain increases. Co2 rapidly dissolves in the rain reaching the ocean where it undergos rapid transformation to other forms of inorganic carbon.

(kump et al,2011)

(c) Worldwide, there has been an enormous increase in the use of fertilisers over the past 60 years, and consequently a much higher riverine flux of nutrients to many coastal and estuarine localities. From your reading of the core references for Block 2 and the other material that you have studied, explain how our understanding of the processes associated with the development of past OAEs might cast light on the likely effects of greatly increased fertiliser usage. (Your answer should be up to 300 words in length.)

During past oceanic anoxic events an increase in surface temperature would increase the amount of detritus and so therefore the amount of nutrients leached into the ocean. This as previously stated causes an increase in phosphorous which is taken up by phytoplankton. The phytoplankton rapidly increase depleting the oxygen in the water. A percentage of dead matter sinks to the bottom; this dead matter gives of CO thus lowering the O levels further.

This process that happened in the oceanic anoxia event could also occur if excessive fertilisers where leached into the rivers and oceans. In addition fertilisers, rich in both nitrogen and phosphorous, also giving off methane could cause an increase in temperatures and if leached into the soil could cause eutrophication. Furthermore nutrients in the fertiliser would result in excessive algae blooms; it could affect the food web as oxygen becomes reduced. The phytoplankton releases toxins which would cause further plant and animal death thus increasing the amount of detritus and so there for increasing the amount of CO released further depleting the O.

(kump et al,2011)

Question 3

(a) Based upon these two articles and your reading of the core references for Block 2, describe what is likely to happen in the rivers, oceans and the atmosphere when a large amount of olivine dust is made available to weathering in the manner described by Köhler et al. (2010). Use simple chemical equations, as appropriate, to illustrate your explanations.

(b) Explain whether you consider the described geoengineering process to be practicable in terms of its financial cost, technical feasibility, energy demand, social impact and political achievability.

(c) Consider whether the impact of the proposal made by Köhler et al. (2010) (in terms of its estimated maximum theoretical capability of reducing atmospheric CO2) is likely to be matched by the actual impact on atmospheric CO2 levels. In your essay, explain why you think the theoretical and actual impacts might (or might not) vary, and try to estimate by how much they may differ.

(d) Considering all aspects associated with this proposed geoengineering procedure, rank the procedure on a scale of 1 to 10 (1 = a very bad idea, 10 = a very good idea), and explain the reasoning behind your answer.

Question 4

The final question of this eTMA is formative; in other words it does not carry any marks contributing towards your final results. However, it is an important opportunity for you to continue the preparation of your end-of module assessment (EMA).

Choose the subject for your EMA from the list of topics you were given in Question 4 of eTMA 01. You can opt for the same topic as you wrote about in eTMA 01, or can choose a different one from the list of six that you saw there. (Note: you will still be able to change the subject of your EMA to another of the six topics after completing eTMA 02 if you wish, but if you do continue to work on the same topic you will be able to collect valuable resources now, ready for the EMA.)

Write an outline plan that details the aspects of the subject that you will be covering in your 3000-word critical review for the EMA, including the main questions and hypotheses you intend to investigate and the sources (core papers, set book, databases, OU Library, internet search) you used to find information about the subject. Provide a list of articles (around 8-10) that you consider will be most relevant to your critical review.


Paper 1:

Doney, S.C. and Schimel, D.S., 2007. Carbon and Climate System Coupling on Timescales from the Precambrian to the Anthropocene. Annual Review of Environment and Resources, 32, pp.31-66

Paper 2:

Zachos, J.C. et al., 2008. An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics. Nature, 451, pp.279-283

Paper 3:

Cohen, A.S. et al., 2007. The Late Palaeocene-Early Eocene and Toarcian (Early Jurassic) carbon isotope excursions: a comparison of their time scales, associated environmental changes, causes and consequences. Journal of the Geological Society, 164, pp. 1093-1108

Paper 4:

Zachos, J.C. et al., 2005. Rapid Acidification of the Ocean During the Paleocene-Eocene Thermal Maximum. Science, 308, pp. 1611-1615

Paper 5:

Schmitz, B. and Pujalte, V., 2007. Abrupt increase in seasonal extreme precipitation at the Paleocene-Eocene boundary. Geology, 35(3), pp.215-218 

Paper 6:

Pearce, C.R. et al., 2008. Molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the Early Jurassic. Geology, 36(3), pp.231-234

Paper 7:

Whitney, F.A. et al., 2007. Persistently declining oxygen levels in the interior waters of the eastern subarctic Pacific. Progress in Oceanography, 75, pp.179-199

Paper 8:

Doney, S.C., 2006. The Dangers of Ocean Acidification. Scientific American, 2006(3), pp.58-65

Kump, L R, Kasting, J F & Crane, G The Earth System (3rd edn) Pearson Education 2011