Oxygen isotopes provide fundamental evidence about the earths previous climates. They can be used to reconstruct past sea levels, ice quantity, and marine temperatures. Oxygen isotopes can vary substantially according to location due to a number of factors. This variation in the oxygen isotopes can be seen in the Bay of Bengal and the Arabian Sea which both illustrate two contrasting examples. The northern area of the Bay of Bengal has high influxes of freshwater and the result is a depleted 18O ratio and the southern region of the Arabian Sea is affected by poor circulation of the water body which results in more depleted 18O ratio. Interglacial events have also affected these isotopic ratios since long term variation of 16O and 18O are controlled by variations in ice sheet quantity.
My area of study will be the region of the Indian Ocean, in particular comparing and contrasting the oxygen compositions of two sub locations; the Arabian Sea and the Bay of Bengal.
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The Arabian Sea is surrounded by India on the east, Pakistan and Iran on the north, the Arabian Peninsula on the west and on the south the north eastern part of Somalia. The surface area of the Arabian Sea is approximately 3,862,000Â km2. Indus River is the largest river flowing into the Arabian Sea. There are also two important branches visible on the map, the Gulf of Oman and the Gulf of Aden which connect the Persian Gulf and Red sea to the Arabian Sea. The Bay of Bengal spreads over an area of 2,172,000Â kmÂ² and is the largest bay in the world with a significant number of large rivers flowing into the bay. The bay is bordered by Bangladesh, India and Sri Lanka to the west and Burma and the Andaman and Nicobar Islands to the east.
Figure 2: Eurasian, Arabian, African & Indian plates
The Arabian Sea and Bay of Bengal both share similar geological settings. The two locations both lie on the Indian plate. However the Arabian Sea does tip out towards the west onto the Arabian plate.
A significant quantity of stable isotopes of oxygen and hydrogen in the earth's atmosphere and hydrosphere is necessary for isotopic fractionation from physical and chemical processes. The evaporation and condensation of water triggers the fractionation process, which creates lighter and heavier isotopes. Fractionation tends to turn lighter isotopes into vapour and the heavier isotopes into liquid. Temperature is the key driver for fractionation, converting liquid to vapour and vapour to liquid.
Foraminifera also hold key information as they contain calcite shells which precipitate and fractionate oxygen isotopes in sea water. The 18O values in the calcite of the foraminifera can give us composition and temperature of the location at the time the shell was precipitated.
TemperatureFigure 3: Annual sea surface temperature and salinity in the Indian Ocean
Looking at figure 3 temperature graph we can see very little variation in sea surface temperature between the Arabian Sea and Bay of Bengal. The temperature ranges around 25.67oC and 29.78oC. The low end of the range falls around the Arabian Sea region and the higher end of the temperature range seems to fall around the Bay of Bengal region.
Figure 3 salinity graph is a total contrast to the temperature graph with a distinct difference between the two regions. The Bay of Bengal shows the lowest salinity value of 27.33â€° and the Arabian Sea shows the highest salinity value of 39.65â€°.
Figure 4: Water salinity cycle
This figure shows that some locations increase salinity and others decrease salinity. Sea surface salinity is controlled by geographic differences. In general locations dominated by precipitation have low sea surface salinity and locations dominated by evaporation have high sea surface salinity. Ocean currents can also alter sea surface salinity.
Figure 4 portrays what is happening in the two locations. A large amount of evaporation is taking place in the Arabian Sea and the Bay of Bengal is being inputted by a large quantity of freshwater. However on a million year scale, the data from both locations would show a reasonably constant level of salinity simply because salt has an approximate residence time of 130Ma in seawater.
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Discussion: Bay of Bengal
Figure 5: Salinity Vs Latitude; Longitude 85.5E, 86.5E and 87.5E
Figure 5 shows a dictinct dramataic difference in the level of salinity between the north and the south region of the Bay of Bengal. The south has a high level of salinity and the north has a significantly low salinity level. This is a result of large amounts of fresh water inputed into the Ganges which then is mixed resulting in reduced salinity in the north. Sea water mixing with water from the indian ocean which is of normal salinity is the reason for the high salinity level in the south.
Figure 6: Temperature Vs Latitude; Longitude 82.5E, 83.5E and 84.5E
Figure 6 illustrates an overall linear trend line, increasing in temperature as the latitude increases towards the north. Therefore the left side of the graph must be where the Bay of Bengal meets the Indian Ocean as the temperature appears to be lower in consideration to the right side of the graph.
Figure 7: Î´ 18O Core-Top & Î´ 18O LGM Vs Latitude
Figure 7 clearly shows that Î´ 18O core-top values in the north of the Bay of Bengal are more depleted in Î´ 18O than the waters in the south of the Bay. The Last Glacial Maximum (LGM) took place approximately 20,000 years ago. According to figure 7 plot the LGM was much depleted in the surface sea water in comparison with Î´ 18O values of today. This can be deduced from the close relationship in trend between the LGM and the more negative core-top values of Î´ 18O.
Discussion: Arabian Sea
Figure 8: Salinity Vs Latitude
Looking at figure 8 we can see that the Arabian Sea has the lowest salinity levels in the south and gradually increases towards the north. This may be a result of the high levels of evaporation in this region.
Figure 9: Sea Surface Temperature (Present) Vs Latitude
Figure 9 indicates overall temperature decreases towards the north of the Arabian Sea, which is in contrast to the temperatures in the Bay of Bengal. From latitude 20 there seems to be an irregular rise in sea surface temperature. The highest Temperature on the graph is in the south.
Figure 10: Î´ 18O Core-Top Vs latitude
Figure 10 emphasises the earlier differences we distinguished between Î´ 18O values in the Arabian Sea and the Î´ 18O values in the Bay of Bengal. The planktonic foraminifera in the Arabian Sea show further depletion of Î´ 18O as you move south and less depletion of Î´ 18O towards the northern region of the Arabian Sea.
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The Î´ 18O data taken from the planktonic foraminifera in the Bay of Bengal suggest that Î´ 18O is more depleted in the northern regions than the southern regions. The salinity data clearly show that salinity levels are higher in the southern regions than the northern which is mainly a result of a significant level of freshwater being inputted by the Ganges and other major rivers. Isotopic fractionation causes Î´ 18O to become more depleted in freshwater as evaporation takes place, therefore to suggest that the low level of salinity in the Bay of Bengal is due to the freshwater input is perfectly reasonable.
What was very noticeable to me was that generally the Bay of Bengal and the Arabian Sea seem to be doing the opposite to each other. The present day Î´ 18O values in the Arabian Sea show that the surface sea water is more depleted in the south than the north and salinity is higher in the north than the south which may have been caused by a higher evaporation rate in this region. I can also distinguish that Î´ 18O concentration were higher in the north than the south. This may be due to the north having shallower seas therefore being unable to circulate water as well as the open deeper seas.
Looking at the LGM and the Î´ 18O of planktonic foraminifera in the Bay of Bengal, it is clear that Î´18O of planktonic foraminifera is more depleted than the LGM which may be due to the vast amounts of ice during the LGM. Ice is highly depleted in Î´18O so during periods of interglacial periods this ice must have released Î´ 18O into the sea, causing the sea to have a higher concentration of Î´ 18O.
Sea water compositions can be effected by varying oxygen isotopes. This variation in the oxygen isotopes is dependent on a number of factors, one key factor being temperature. The influx of high levels of fresh water has caused the Bay of Bengal to become less depleted in Î´ 18O and to have a reduced salinity level. This was the case generally in the northern regions in the Bay of Bengal. The poor sea water circulation has caused the Arabian Sea to have higher concentrations of Î´ 18O and lower levels of Î´ 16O; whereas the high level of evaporation has caused a depleted Î´ 16O and the ice acts as a reservoir storing this Î´ 16O. The Î´ 18O isotopic values of the Planktonic foraminifera show evidence of interglacial periods which have effected isotopic composition of the seawater. There is clear evidence that Î´ 18O levels were higher during this period than today, as a result evaporation has taken place which has produced freshwater with depleted Î´ 18O ratios.