Deforestation may also have a duel effect, in that vegetated areas remove
carbon dioxide from the atmosphere by photosynthesis, however, if the
deforestation opens up the land for chemical weathering, then further carbon
dioxide is removed from the atmosphere. So although it is clear that human
activities add carbon dioxide and other greenhouse gasses into the
atmosphere, it is unclear as to how much of a contributory factor this is to
global warming, and how much of the warming trend is purely a natural cycle of
the Earth’s changing temperature.
Question 3
Three processes which could effect global sea level changes are the formation
of submarine plateaux, the creation of shallow oceans and ocean floor
spreading, and global warming.
Submarine plateaux or flood basalt provinces which have been encompassed in
the ocean basins, are formed by what is termed a super plume, at subduction
zones the subducted materials interrupt the normal convection of magma,
resulting in the ejection of massive amounts of basalt lava. An example of this
is the Ontong-Java Plateau which was formed approximately 112 million years #
ago. The volume of lava erupted on a coastal area was sufficient enough to
displace sea water in ocean basins, effectively reducing the amount of oceanic
crust that the oceans could reside over. This displacement caused the sea level
to rise by approximately ten metres. Submarine plateaux are categorised by
unusually thick areas of oceanic crust, which is an indication that at these
areas the ocean is shallower and therefore reduced in its capacity for sea
water. Submarine plateaux and flood basalt provinces can be erupted over
periods of ten to a hundred years.
The movement of land masses through continental drift, especially following
the break up of Pangea and later Gondwanaland, are also thought to be
responsible for increasing sea level. This in combination with sea floor
spreading at convergent plate boundaries reduces the capacity of the oceans
to seawater. Greater coastal areas lead to greater areas of continental shelf
which are essentially shallow areas of the ocean surrounding land masses. Less
water is able to be held in a series of shallow seas in comparison to a single of a
few deep oceans. The break up of continents in the future due to unpredictable
changes in plate tectonic activities could cause this to happen again. The time
scale of an event like this is hundreds of millions of years, the continents are
still moving from the break up of Pangea 175 million years ago. An increase in
the production of ocean floor and of oceanic volcanic activity which can also be
attributed to plate tectonic activities would also reduce the volume of the
oceans by increasing the volume of rock at divergent plate boundaries and at
volcanic island chains the reducing the volume and depth of the ocean.
The most commonly known and most familiar factor attributing to possible sea
level rising is that associated with global warming. This has two contributing
factors to the increase in the sea level, firstly the melting of water held in ice
caps and glaciers, secondly the expansion of the warm thermohaline surface
layer of the ocean. Ice caps and glaciers retreat and grow over periods of
10,000 to 100,000 years in a cyclic pattern. Ice caps are formed by
precipitation of snow and hail, which is then compacted, water which is frozen
in this way is not returned to the ocean basins, as water which is precipitated
as rain. It is stored over land, or as frozen sheets of ice. The melting of the
ice caps and glaciers associated with global warming would therefore increase
the volume of water in the oceans causing sea level to rise. Warm water also
expands in volume and decreases in density according to the physical
properties of the chemical, it therefore takes up more space, and has a higher
sea level when the water is warmer. The melting of the ice caps would also
reduce the amount of incident radiation reflected back into the stratosphere
and so would increase the temperature further and increase the effect of
global warming and then so increase the sea level further due to expansion. A
temperature rise of the ocean waters of ten degrees would cause a sea level
rise of approximately ten metres globally.
Question 4
If an eruption occurred in Iceland that lasted for a period of 50 years and produced 300km3 of lava the gases and ash erupted would mainly be associated with the Northern Hemisphere. This is due to the effects of the polar jet stream in higher latitudes where Iceland is situated. The gases and ashes would primarily be carried around the globe in the polar jet stream having a substantial climatic and environmental effect on the areas of the Earth which were affected by that atmospheric wind pattern, there is little likelihood that the atmospheric particles would effect areas of the globe south of the intertropical convergence zone, and those below the effect of the polar jet stream would experience only a reduced level of atmospheric anomaly.
The lava would be basaltic in composition, and the type of lava eruption as described is a basaltic flood of average proportion. As the eruption is not especially explosive in this type of eruption the gases and ash would not enter the stratosphere, instead they would circulate within the weather systems of the troposphere and would have a greater climatic effect than a more explosive eruption, even if that eruption produced more aerosols.
Assuming that the Laki Fissure in Iceland is of a comparable composition to the lave erupted from this hypothetical eruption and other basaltic eruptions such as the Deccan Traps are also similar due to the nature of these eruptions, it is possible to use data from these eruptions to extrapolate estimates of the amount of carbon dioxide and sulphur dioxide (and therefore sulphuric acid) emitted from these lava flows both annually and in total.
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From the Deccan Traps there is 0.002 kg of CO2 per kg of lava erupted
- From the Laki Fissure there is 0.0013 kg of S per kg of lava erupted.
The volume of the lava stream is 300km3 and has a density of 2650 kg m-3, this means that the total mass of lava erupted can be calculated to
3x1011 m3 x 2650 = 7.95x1014 kg of lava
From the mass of the lava flow it is possible to calculate the total amount of CO2 and of Sulphur aerosols emitted from the eruptions annually
7.95x1014 x 0.002 = 3.18x1010 kg CO2 yr-1 (i)
50
7.95x1014 x 0.0013 = 2.067x1010kg S yr-1 (ii)
50
Because of the different masses of carbon dioxide and of sulphur due to their molecular and atomic masses the actual mass of their emissions does not give an entirely accurate measure of the difference in the amounts of the products emitted. By transferring the masses of the gases emitted into their amount in Moles it is easier to see how much more carbon dioxide is emitted than sulphur, it also makes it possible to use simple calculations using molecular masses to work out the mass of sulphur dioxide and of sulphuric acid aerosols produced by the emitted sulphur atoms.
3.18x1010 kg = 7.23x1011 Mol of CO2
0.044
2.067x1010 kg = 6.64x1011 Mol of SO2
0.032
There are almost 10% more carbon dioxide molecules emitted each year compared to sulphur dioxide, however, the sulphur emissions and their resulting aerosols are likely to have a greater climatic and environmental effect compared to the carbon dioxide because of the reaction between sulphur dioxide and water in order to form sulphuric acid aerosols (SO2 + 2H2O → H2SO4) The resulting hydrosulphuric acid aerosols are involved in the nucleation of rain, and would result in the formation of acid rain over a period of at least fifty years. Making the broad assumption that all sulphur dioxide molecules emitted from the eruption are converted into hydrosulphuric acid then the mass of this would be
6.64x1011 M x [(1x2) + (32) + (16x4)] = 6.5072x1013g yr-1 or
6.5072x1010 kg H2SO4 yr-1
This over the total length of time of the eruption is a massive 3.254x1012 kg H2SO4
The annual forcing of the Iceland eruption would be –9.3Wm2 and in total over the fifty years the accumulative effect would be a forcing factor of 465.3Wm2 .
The amount of sulphur dioxide in the atmosphere is unlikely all to form hydrogen sulphate aerosols due to the amount of water vapour that is required for the chemical reaction to take place. It is likely that the acidic rain that does fall would have a similar effect on the affected environment as the Laki Fissure eruptions. The climatic effects of the Lake Fissure would be very similar to those in this eruption, although the Laki Fissure was relatively short term and small in size compared to this hypothetical eruption over fifty years.
The effects on climate as described in the aftermath of the Laki Fissure eruption were based upon an eruption of approximately 1.5x1011 kg of sulphide aerosols in a period of a year, less is produced annually in this eruption, however it is sustained over a period of fifty years which makes the circumstances and the environmental effect much more severe, as the atmospheric and climatic effects will be sustained long enough to have a cascading effect on the northern hemisphere, which in turn would then begin to effect the southern hemisphere. The ashes in the atmosphere would provide a dull haze preventing the full penetration of incident sunlight, the aerosols themselves would also act to reflect some solar radiation, lowering temperatures. The estimate of the annual forcing factor based on the values obtained in the text are about 1 degree Celsius over the Northern Hemisphere, however this is difficult to predict due to the individual nature of different eruptions. It is likely both that areas of the Northern hemisphere would experience periods colder than average and warmer than average due to the greenhouse effect of the carbon dioxide and sulphur dioxide remaining in the atmosphere.
When considering the carbon dioxide increase of 3.18x1010kg of CO2 yr. –1 as previously calculated this is a small addition compared to the atmospheric reservoir, and is only actually an increase of 0.004%. Unlike the sulphate aerosols, carbon dioxide is removed from the atmosphere both by the surface ocean and then the deep ocean by means of the seeking of equilibrium by the ocean and atmosphere reservoirs. The chemical weathering of the silicon rich basalt rocks erupted by the Iceland lava would also remove carbon dioxide from the atmosphere. It is possible that the cooling of the atmosphere and therefore the ocean surface areas would also facilitate an increased absorption rate into the seawater as cooler liquids can support more dissolved gases.
The main contributing factor to the effect of this eruption is the amount of sulphate aerosols outgassed, but also the addition of the extended timescale. Many of the Earth’s processes require a period of years or decades in order to be initiated and altered, this is why some huge eruptions by scale to this hypothetical one have had little overall or extended effect on the environment and climate. The Laki Fissure eruption produced more aerosols and the effect of this was felt over a period of three years or so in the changes of climates in the Northern Hemisphere, these changes in the Northern Hemisphere were not sufficient in timescale to effect the southern hemisphere or global climate. Over a period of fifty years of continua increases in the amount of carbon dioxide and sulphides in the atmosphere, the climatic changes would be catalyse other reactions. If global cooling occurred enough for glaciers to form and the polar caps to increase in size, then this would also increase the albedo of the Earth and reduce the amount of radiation remaining within the temperature and the ambient temperature would drop even further. The growth of ice caps in the Northern Hemisphere would reduce sea level eustatically, and this in turn would effect the amount of carbon dioxide that the oceans could remove from the atmosphere, which would suggest a period of warming due to the greenhouse effect, so it is impossible to say which of the possible climatic changes would take effect over the period of fifty years. As a drop in temperature could cause an eustatic fall in sea level due to removal of water from the oceans into polar ice caps and glaciers, it is also possible that the mass of lava erupted onto Iceland would cause an isostatic sea level rise for Iceland. It is clear that basaltic floods such as this have had been linked to periods of mass extinction. Although the Iceland eruption is not as large as the Deccan Traps or the Ontong-Java basalt flood plains, it is possible that this eruption would have a long term and continuous effect on the Northern Hemisphere causing disruption, starvation and death, and a lesser effect on the Southern Hemisphere.
Question 5
To work out the 87Sr/86Sr ratios of seawater the formula
Rsw = (F1 x R1) + (F2 x R2)
(F1 + F2)
Where F1 and R1 are the river flux and the corresponding ratio, and F2 and R2 are the hydrothermal flux and its corresponding strontium ratio.
The current seawater strontium ratio for sea water can be worked out using the following information:-
Rivers:- flux is 290x107 kg yr-1 and the strontium ratio is 0.7119
Hydrothermal:- flux is 145x107 kg yr-1 and the strontium ratio is 0.7034
By inserting these figures into the equation above the calculation gives the strontium ratio for current seawater as:-
Rsw = [(290x107) x 0.7119] + [(145x107) x 0.7034]
[(290x107) + (145x107)]
Rsw = 0.7091
In pre-Himalayan times, the rivers Bramaputra and Ganges were not present in the river flood, these rivers have a combined flux of 13x107 and a strontium ratio of 0.7326
Rsw = [{(290x107) x 0.7119}-{(13x107) x 0.7326}] + [(145x107) x 0.7034]
(290x107) – (13x107) + (145x107)
Rsw = 0.70834 (before the uplift of the Himalayans)
- The pre-Himalayan seawater has a strontium ratio less than that of post Himalayan seawater. Calculations show that prior to the formation of the Himalayan mountains the strontium ratio was 0.70834 compared to 0.7091 currently. This is due to the increase in the amount of water and the physical erosion of the newly formed mountain ranges increasing the relative ratios. Prior to the uplift of the mountain ranges the continental rocks would not have been as readily available for such erosion. Considering the method in which the Himalayans were formed, when the Indian Subcontinent effectively collided with Asia, the rocks which were folded and uplifted into the mountain range were not newly erupted from magmatic sources, they were existing rocks. This means that the rubidium in these rocks would have time to decay prior to the building of the mountain ranges and the Tibetan plateau, increasing the amount of strontium 87, compared to new igneous rocks which have a higher concentration of rubidium which is yet to undergo radioactive decay.
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The difference between strontium ratios before and after the building of the Himalayan mountains is an increase of 0.0008. If the pre-Himalayan hydrothermal flux was increased prior to the formation of the mountains then the overall ratio of the seawater would be lower. This is because the hydrothermal flux comes from a magmatic source with more rubidium which has not yet decayed than the rivers which carry the sediments of existing rock which have undergone decay. If the overall seawater strontium ratio is lower, then the increase in the flux from the rivers would have a greater effect as it is higher than both the hydrothermal and the average river strontium ratios. The Bramaputra and Ganges rivers at the ratio of 0.7326 would have a greater impact upon the overall seawater ratios following the formation of the Himalayan mountains. This can be seen mathematically substituting the hydrothermal flux of 145x107 to an increased figure of 160x107 and using the same equations as in question (a) for calculating both the pre and post Himalayan seawater ratios.
Pre Himalayan seawater ratio becomes :- 0.6627
Post Himalayan seawater ratio becomes:- 0.6830
This increase in hydrothermal flux of approximately 10% has given to a difference in the seawater ratios of 0.0203, which is an approximate rise in strontium ratio of 3%, whereas with the lowered hydrothermal flux the rise in strontium ratio before and after the Himalayan build was only 0.1%.
- A period of arid climate would reduce the overall flux of the rivers, but would not affect the hydrothermal flux into the oceans. A lower flux from the rivers would mean that there is less erosion on the rocky areas of the land, the volume of water flowing would be decreased. This decrease in the river flow would also mean that waterways moved more slowly, and the sediments that were eroded would not be carried as far down stream, and more would be deposited prior to entrance into the oceans. Assuming that the arid period was only temporary and normal weather conditions returned at some stage either during or after the mountain building, then this would cause a greater difference in the seawater ratios. However, there would still be a greater difference, because, reducing the amount of water and therefore the flux of the river systems, it is effectively increasing the percentage of the total flux which comes from hydrothermal sources. The formation of the mountains themselves would actually affect the climate of that area and due to the uprising of wind currents over the peaks, it is likely that there would be an increased rainfall across the southward side of the Himalayans into the Ganges and Bramaputra rivers.
- The 87/86 strontium ratio is heavier in post Himalayan sea water due to the increase in continental rock available for erosion, and the mountains being built from old continental rocks. A greater amount of sediment would be carried into the oceans and this sediment would have a higher 87/86 ratio because the rubidium in the Himalayan rocks has had time to undergo radioactive decay.
- From calculations the 87/86 strontium ratio for pre Himalayan sea water due to the increase in the Amazon flux is 0.7082, this is a difference of only 0.0006 compared to the figure of 0.7088, and only a difference of 0.00014 compared to the previously calculated figure of pre-Himalayan sea water as shown in question (a) at 0.70834.
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The change in the flux of the Amazon River does not have a major effect on the overall change in the river. The increase in water flow is only 5.5x107kg in a total incoming flux of 290x107. This is an increase of only 1.8% of the total flux. The flux of the Amazon river has a ratio of 0.7109 compared to the total flux of 0.7119, so the actual impact of this change is small. The Amazon River picks up sediments from the Andes Mountain range, where the bared rock is of a younger age than the bared rock in the Himalayans, this means that the strontium ratio in the Amazon flux is reduced compared to that of the Ganges and Bramaputra, so the impact of the increase is small.
