Oxygen Isotope Perturbation in Bay of Bengal - Essay Example

Oxygen Isotope Perturbation in Bay of Bengal www.academia-research.com Sumanta Sanyal d: 11/03/2006 Report: Oxygen Isotope Perturbation and Its Effects on Temperature and Salinity in the Bay of Bengal Table of Contents 1. Introduction 3 2. The Isotope Ratio 3-4 3. The 'Foram' Fractionation Factor 4-5 4. Methodology 5-7 5. Results 7 6. Discussion 8-11 6.1 Temperature 6.2 Salinity 6.3 Temperature 6.4 (Foram) 6.5 (Foram) and Temperature 6.6 (Foram) and Salinity 7. Conclusion 11 8. Reference 12 9. Bibliography 13 10. Appendix (Tables and Graphs) 14-20 1. Introduction Oxygen has three isotopes - , and . 16, 17 and 18 signify the number of neutrons in the nuclei of the oxygen atoms which all have the same proton number, as they are all isotopes of oxygen. All three isotopes exist in the water of the seas and oceans combined with hydrogen. is the heaviest while is the lightest. Thus, the two isotopes evaporate variantly and their relative proportions, the isotope ratio expressed as : , in water vapour emanating from a particular stretch of seawater depends upon the temperature of the seawater and the existing isotope ratio in it. Oxygen isotope ratio is a climatic proxy, an environmental variable, and it assists climatologists, including paleoclimatologists, to determine certain important climatic parameters, both existing and past, from it. (UNST 232a Mentor Section Assignment 5) Another phenomenon the isotopes are involved in is that oxygen accumulates in the calcium carbonate (CaC) of the shells of marine animals called foraminifera -"Forams". The isotope ratio in the shells depends upon on the temperature and existing isotope ratio of the water in which the animals live or lived. Over periods of time these animals die and their shells descend to the bottom of the sea and form layers that present researchers with important data from which past temperature and isotope ratios can be determined. (UNST 232a Mentor Section Assignment 5) At the very onset the report states that it will account for only the evaporation side of the isotope ratio in seawater as the precipitation side is not relevant to the topic at hand. 2. The Isotope Ratio The isotope ratio in any singular stretch of water changes over time. Water molecules made up of the lighter isotope evaporate easily leaving water molecules with the heavier one back in the seawater. When the evaporated water precipitates back into the sea there is little change in the ratio but when the water vapour precipitates on land the lighter isotope is transported to land from where it may come back to the sea via rivers and other waterways or it may remain trapped for thousands of years in the form of ice if it is precipitated as snow on places like the polar icecaps and high mountain tops. (UNST 232a Mentor Section Assignment 5) During normal times the isotope ratio in sea and ocean water is fairly constant but during glacial periods when the icecaps advance beyond normal limits the isotope ratio shifts in favour of for obvious reasons. This is in a comparative sense to normal times. (Oceanography 540, 2002) The isotope ratio is expressed as delta (lower case) with the isotope contents expressed in parts per thousand (per mille) as the following equation demonstrates. = - / x 1000 Higher negativity in indicates greater depletion. (UNST 232a Mentor Section Assignment 5) The standard in this case is of 'Standard Mean Ocean Water' or 'SMOW'. (Oceanography 540, 2002) 3. The 'Foram' Fractionation Factor The foraminifera species being investigated in this report is the planktonic Globigerinoides ruber. As mentioned earlier varying ratios of isotopic ratios are found in planktonic and benthic foram depending upon the temperature and existing isotopic ratio of the water in which the animals have lived. Harold Urey and Toshiko Mayeda of the University of Chicago in 1947 (UNST 232a Mentor Section Assignment 5) demonstrated that the isotopic fractionation during CaC crystallization was temperature dependent. This has a simple explanation. Atoms with differing masses have differing chemical potential with the lighter atoms going into reaction more easily than the heavier ones. In this instance the exchange of oxygen is between the water molecules (O) and the carbonate molecules (CaC). Hydrogen has a higher affinity to oxygen than calcium and as a consequence, at lower energies (temperature), tends to retain the lighter atoms () in favour of the heavier ones (). Thus, at lower temperatures at carbonate formation in foram there is considerable accumulation of . The isotope ratio of the carbonates expressed in parts per thousand is as that of water: = - / x 1000 The standard for carbonates is either SMOW or fossil belemnite from the Pee Dee formation or PDB. (Oceanography 540, 2002) 4. Methodology The report relies upon three primary sets of data as hereunder. Table 1 - Temperature Data for Surface Waters of the Bay of Bengal (Monthly Means August 1994). Table 2 - Salinity Data for Surface Waters of the Bay of Bengal (Monthly Means February 1994). Table 3 - Oxygen Isotope Data for Planktonic Foram Globigerinoides ruber in Deep Sea Cores from the Bay of Bengal (Sorted according to Latitude S-N). These values have been sorted according to latitude. There are two sets of values of - the core-top values from the top of the sample and the LGM values from the lower parts of the samples. Additionally, Table 4 has been constructed with latitudinal values ranged at 2.5 and the corresponding core-top and LGM values together with the monthly mean temperature (88.5E) and salinity (88.5E) values have been averaged to fit within the relevant latitudinal range. It is to be noted that for the purpose of the analysis the difference in the month for sampling of the monthly mean temperature taken for August 1994 and the monthly mean salinity taken for February 1994 is not taken to be of any significance. It is also to be noted that for monthly means of temperature and salinity the values taken at 88.5E longitude have been accepted for the purpose of analysis. These represent mid-sea values where there is minimal presence of freshwater, a principal cause for anomalies in data presentation for the Bay. The principal mode of analysis is plotting graphs utilising the various data sets available and interpreting the results thereof with the assistance of literature available on the subject. 5. Results The results are available in the six graphs appended to the report and are utilised in the discussion. 6. Discussion The Bay of Bengal is a body of water that lies mostly in the Northern Hemisphere but starts closely from the Equator from the larger expanse of the Indian Ocean and nestles closely in a wide curve in the landmass of the Indian subcontinent on the eastern side. On the extreme northern part of the Bay a number of large rivers bringing water from the Himalayas meets it supplying it with a substantial amount of freshwater. It is also to be noted that the Bay area is fairly ringed by rivers discharging their waters into it. (Gupta, S.K., Deshpande, R.D, 2005) This is a salient feature of the bay and it should be strongly noted as much of the interpretation of the results relies on it. The discussion begins with the tables. 6.1 Temperature: Table 1 contains the monthly mean temperature data. It can be observed from Graph 6 that the mean temperature of the surface water gradually increases from lower latitudes, peaks at mid-sea latitudes and falls again at the higher latitudes. The explanation for this may be that the lower latitudes are very near the Indian Ocean, a large body of water, and are cooler than the mid-values for the Bay, that is a confined body of water. Thus, mean values at the middle of the Bay tend to be higher than those near the Indian Ocean or the coast. For the coastal waters, influx of cooler river water may have a cooling effect. What is also observable from Table 1 is that the mean values peak at the mid-longitudinal values. That is, values that are for surface waters of the mid-sea regions have the higher values compared to adjacent longitudinal values for the same latitude that are for waters nearer the shoreline. Mean values at the extreme longitudinal ends are for coastal areas and here the surface waters mostly tend to cool down a bit because of some influx of cooler freshwater from rivers. No graph has been constructed as the feature is distinctly observable from the table. 6.2 Salinity: It can be observed from Table 2 of the Appendix that the salinity of the waters of the Bay has a unique variability pattern. All latitudes lying close to the coastal areas have low salinity as rivers end there and discharge freshwater. Places with co-ordinates at the most northerly positions tend to have the lowest mean salinity values as it is here that the large Himalayan rivers like the Ganges ends. (Gupta, S.K., Deshpande, R.D, 2005) Example: 21.5N, 91.5E with monthly mean salinity at 29.14. This is the lowest salinity recorded. Places nearer to the equator tend to be far away from land and thus acquire a high level of salinity. Example: 5.5N, 81.5E with monthly mean salinity - 34.27. This is the highest salinity recorded. Places spread between these two points vary in salinity as per their nearness to coasts with large runoffs from rivers with the larger rivers producing less saline waters. Graph 1 amply demonstrates this. Means from the lower latitudes have high values as evaporation is high here at mid-sea regions and there is very little influx of freshwater. 6.3 Temperature and Salinity: Graph 2 gives a demonstration of the unique relationship between the mean temperature and mean salinity values against gradual increase in latitudinal values. The explanation is simple. The lower salinity mean values correspond to low temperature mean values that are for higher latitude regions near the coast. As the salinity mean values rise with higher temperature means in the mid-bay area the line curves up. The line then peaks off at mid-bay values and the higher salinity mean values for the lower latitudes correspond with the lower temperature mean values there (the Indian Ocean effect as discussed earlier) . Thus the singular nature of the curve is more due to the low mean temperature values at both ends of the curve with high mean temperatures at the middle section. 6.4 (Foram): It should be noted from the very start that the Bay of Bengal has an almost difference in favour of the lighter oxygen isotope compared to the Arabian Sea which has a lesser influx of river water compared to the Bay. (Gupta, S.K., Deshpande, R.D, 2005) The explanation is simple: freshwater tends to have a higher level of in consequence of rainwater which itself has a higher level of as per discussions earlier in the essay. Table 3 that contains data of obtained from samples taken at places with different co-ordinates demonstrates this position. Graph 3 shows that the negativity of the core-top values increase with increase in latitude. This is because values at lower latitudes are of mid-sea regions where, with the high evaporation rates, there is also no influence of freshwater which influences values most with larger river discharges signifying higher negative values. The combination of low salinity and freshwater influx at the higher latitudes produces the highly negative values there. From Table 3 it is found that the core-top values are always more negative than the LGM ones. This is because the LGM values are from an older period nearer to glacial periods when there was much depletion of the lighter isotope leading to higher percentage in the shell. The lighter isotope during glacial periods is retained in the larger ice masses on the planet and the oceans have high content and low negativity. (Oceanography 540, 2002) More information on which period the samples are from and the geographic and climatic data corresponding for that period for the Bay may reveal more accurate details of the values for both core-tops and LGMs. 6.5 (Foram) and Temperature: Graph 4 shows a somewhat clear relationship between and mean temperature. With rise in temperature values the negativity increases. This is because the evaporation rate increases and the lighter isotope participates more in the change of state than the heavier one. Another factor that must be noted is that graph 4 is obtained from averaged values in Table 4 where temperature is seen to increase with increase in latitude. This apparent phenomenon has been discussed earlier with the influence of the Indian Ocean taken into account. Otherwise, temperature usually decreases with increase in latitude. 6.6 (Foram) and Salinity: Graph 5 demonstrates the inverse relationship between mean salinity values and the negativity of the values. Negativity increases as mean salinity decreases. This is also simple to explain. Higher salinity values signify higher evaporation rates and higher retention of compared to .(Variations in Salinity and Oxygen Isotopes) Thus, as salinity decrease with latitude the values also show higher negativity signifying more presence of . In both cases of relationship between mean temperature and salinity with (Foram) it must be noted that the higher influx of freshwater nearer the coasts is a very significant compounding factor. 7. Conclusion The report amply demonstrates that the(Foram) fractionation factor derived from samples of deep-sea cores can reveal a wealth of information, both past and present, of the body of water from under which the samples are taken. The Bay of Bengal participates actively in the Indian Southwest Monsoon, the most powerful monsoonal phenomenon in the world. Thus, the findings of the report has some special significance in the sense that it can be accurately ascertained that most of the water vapour is generated at the mid-Bay regions up to the borders of the Indian Ocean. This is the region where the(Foram) fractionation factor has least negativity signifying highest evaporation rates. More study of inland hydrological resources and calcareous remains for oxygen isotope ratio composition can reveal more useful facts about the existing hydrological cycle that drives the monsoonal engine in the region. Reference Gupta, S.K. and R.D. Deshpande, The need and potential applications of a network for monitoring of isotopes in the waters of India, 2005. Extracted on 8th March, 2006, from: http://www.ias.ac.in/currsci/jan102005/107.pdf#search='S.%20K.%20Gupta%20and%20R.%20D.%20Deshpande%20monitoring%20of%20isotopes' Oxygen and Carbon Isotopes as Paleotracers, Oceanography 540, 2002. Extracted on 8th March, 2006, from: http://www2.ocean.washington.edu/oc540/lec02-19/ UNST 232a Mentor Section Assignment 5, paleoclimate proxy data and insolation. Undated. Extracted on 8th March, 2006, from: http://web.pdx.edu/karps/Karps/Paleoclimatology.htm Variations in salinity and Oxygen Isotopes, Exploring Earth, McDougal Littell, Undated. Extracted on 8th March, 2006, from: http://www.classzone.com/books/earth_science/terc/content/investigations/es2307/es2307page03.cfmchapter_no=investigation Bibliography Sinha, Ashish, et al, Variability of Southwest Indian summer monsoon precipitation during the Bolling Allerod, Geological Society of America, Vol. 33, No. 10, P. 813-816, 2005. extracted on 8th March, 2006, from: http://geology.geoscienceworld.org/cgi/content/abstract/33/10/813 Appendix Table 1: Temperature Data for Surface Waters of the Bay of Bengal (Monthly Means August 1994) coordinates 81.5E 82.5E 83.5E 84.5E 85.5E 86.5E 87.5E 88.5E 89.5E 90.5E 91.5E 92.5E 93.5E 94.5E 95.5E 96.5E 5.5N 27.7 27.85 27.93 27.99 28.05 28.1 28.16 28.21 28.26 28.31 28.37 28.45 28.55 28.7 29.1 6.5N 27.87 27.97 28.03 28.09 28.14 28.19 28.23 28.27 28.29 28.32 28.37 28.45 28.56 28.7 28.88 7.5N 27.92 28.05 28.12 28.17 28.2 28.24 28.27 28.29 28.3 28.3 28.31 28.34 28.41 28.51 28.65 8.5N 28.01 28.1 28.18 28.23 28.26 28.29 28.31 28.33 28.34 28.33 28.29 28.26 28.25 28.27 28.34 28.42 9.5N 28.1 28.24 28.31 28.34 28.35 28.37 28.39 28.41 28.41 28.37 28.31 28.24 28.18 28.16 28.19 28.24 10.5N 28.3 28.37 28.4 28.41 28.42 28.44 28.46 28.48 28.47 28.43 28.35 28.24 28.14 28.09 28.09 28.13 11.5N 28.49 28.47 28.46 28.45 28.46 28.47 28.5 28.52 28.52 28.48 28.38 28.26 28.13 28.04 28.02 28.07 12.5N 28.6 28.54 28.5 28.47 28.47 28.48 28.51 28.54 28.54 28.5 28.41 28.26 28.11 27.99 27.94 27.96 13.5N 28.67 28.58 28.52 28.48 28.47 28.48 28.5 28.53 28.54 28.5 28.41 28.26 28.09 27.93 27.83 27.82 14.5N 28.68 28.63 28.56 28.51 28.49 28.48 28.5 28.52 28.53 28.49 28.41 28.26 28.07 27.85 27.69 27.59 15.5N 28.8 28.71 28.64 28.58 28.54 28.52 28.52 28.53 28.53 28.5 28.42 28.28 28.07 27.76 27.51 27.4 16.5N 28.81 28.73 28.69 28.64 28.6 28.58 28.57 28.56 28.52 28.45 28.33 28.19 17.5N 28.81 28.86 28.77 28.7 28.65 28.61 28.59 28.55 28.48 28.37 28.18 18.5N 29.17 28.92 28.78 28.68 28.62 28.58 28.53 28.48 28.47 28.55 19.5N 28.96 28.83 28.61 28.54 28.49 28.43 28.37 28.43 28.36 20.5N 28.39 28.37 28.33 28.25 28.12 21.5N 28.2 28.15 28.08 27.99 Table 2: Salinity Data for Surface Waters of the Bay of Bengal (Monthly Means February 1994) coordinates 81.5E 82.5E 83.5E 84.5E 85.5E 86.5E 87.5E 88.5E 89.5E 90.5E 91.5E 92.5E 93.5E 94.5E 95.5E 96.5E 5.5N 34.27 34.38 34.4 34.38 34.31 34.21 34.12 34.07 34.04 34 33.92 33.77 33.56 33.29 32.44 6.5N 34.25 34.25 34.23 34.17 34.09 33.99 33.92 33.87 33.81 33.7 33.54 33.33 33.08 32.76 32.5 7.5N 34.03 34.07 34.08 34.04 33.97 33.89 33.8 33.73 33.63 33.5 33.34 33.15 32.96 32.75 32.56 8.5N 33.74 33.82 33.88 33.91 33.91 33.87 33.79 33.7 33.6 33.48 33.33 33.17 33.01 32.87 32.74 32.64 9.5N 33.55 33.61 33.68 33.73 33.76 33.74 33.69 33.59 33.48 33.33 33.18 33.03 32.9 32.8 32.73 32.67 10.5N 33.35 33.41 33.46 33.53 33.58 33.6 33.57 33.48 33.35 33.19 33.03 32.89 32.78 32.73 32.71 32.74 11.5N 33.21 33.2 33.24 33.3 33.36 33.41 33.41 33.34 33.2 33.04 32.88 32.75 32.66 32.62 32.64 32.75 12.5N 32.93 32.93 32.96 33.02 33.08 33.17 33.2 33.17 33.06 32.9 32.74 32.61 32.5 32.46 32.48 32.59 13.5N 32.53 32.57 32.63 32.7 32.8 32.93 32.98 32.97 32.9 32.77 32.62 32.47 32.34 32.25 32.23 32.3 14.5N 32.01 32.14 32.26 32.39 32.55 32.69 32.72 32.73 32.66 32.58 32.47 32.33 32.17 31.98 31.89 31.94 15.5N 31.33 31.7 31.9 32.09 32.3 32.4 32.41 32.4 32.35 32.31 32.26 32.17 32.03 31.63 31.41 31.4 16.5N 31.44 31.51 31.72 31.96 32.05 32.04 31.98 31.94 31.95 31.96 31.97 32.1 17.5N 30.89 31.18 31.49 31.61 31.6 31.52 31.52 31.55 31.58 31.64 31.77 18.5N 30.46 30.91 31.1 31.19 31.15 31.14 31.15 31.19 31.2 31.36 19.5N 30.53 30.6 30.95 30.89 30.8 30.77 30.82 30.65 30.71 20.5N 31.18 30.69 30.41 30.32 30.7 21.5N 30.3 29.82 29.43 29.14 Table 3: Oxygen Isotope Data for Planktonic Foram Globigerinoides ruber in Deep Sea Cores from the Bay of Bengal (Sorted according to Latitude S-N) Core Latitude N Longitude E d18O core-top d18O LGM RC14 35 0050'S 8957' -2.25 -0.51 RC14 36 0028'S 9000 -2.25 -0.93 RC14 37 0128' 9010' -2.86 Dodo 200 0255' 9103' -2.15 Dodo 197 0258' 8851' -1.73 Dodo 201 0259' 9141' -2.26 V29 30 0305' 7615' -2.34 -0.81 Dodo 204 0309' 9406' -2.57 V29 29 0507' 7735' -2.36 -0.68 RC14 39 0551' 9031' -2.64 -0.94 MD77164 0605' 9337' -2.89 MD77191 0730' 7643' -2.61 -0.84 RC12 239 0908' 9002' -2.65 -0.93 RC 12347 0920' 9320' -2.78 MD77169 1013' 9503' -3.14 -1.02 MD77171 1146' 9409' -3.24 -0.99 V29 15 1157' 8844' -2.68 -0.74 MD77185 1224' 9203' -3.07 RC12 340 1242' 9001' -2.84 -1.04 RC12 344 1246' 9604' -3.28 -0.88 RC12 341 1303' 8935' -2.71 -0.75 MD77176 1431' 9308' -3.18 -1.17 RC12 343 1510' 9034' -2.84 -1.04 MD77177 1625' 9324' -3.41 -1.02 MD77178 1712' 9305' -3.7 -1.12 MD77181 1724' 9029' -3.34 -0.99 MD77179 1822' 9101' -3.5 -1.02 MD77180 1828' 8951' -3.41 -0.9 Table 4: Averaged d18O Core-top, LGM values with Averaged Monthly Means of Temperatures and Salinity at 88.5E Longitude within Latitude Range. Latitude Range (N) Averaged d18O Core-top Averaged d18O LGM Averaged Monthly Mean Temperature (88.5E) Averaged Monthly Mean Salinity (88.5E) 5.1-7.5 2.63 0.82 28.24 33.93 7.6-10 2.72 0.93 28.37 33.65 10.1-12.5 3.04 0.93 28.51 33.33 12.6-15 2.95 0.96 28.53 32.85 15.1-17.5 3.32 1.04 28.57 31.97 17.6-20 3.46 0.96 28.58 31.02 Graph 1: The Monthly Mean Salinity of the surface waters of the Bay of Bengal (At 88.5E) for February 1994 plotted against Latitude Graph 2: Monthly Mean Temperature (August 1994) plotted against Monthly Mean Salinity (February 1994) for 88.5E. Graph 3: The Oxygen Isotope Ratio plotted against Latitude (Sorted) Blue Line: Core Top Values Pink Line: Core Bottom Values Graph 4: The Averaged d18O Core-top values plotted against Averaged Monthly Mean Temperature at 88.5E for August 1994. Graph 5: The Averaged d18O Core-top values plotted against Averaged Monthly Mean Salinity at 88.5E for February 1994. Graph 6: The Monthly Mean Temperature (88.5E) plotted against Latitude Read More