UGC-NET (NTA) Environmental Sciences, December-2019

Among the most interesting techniques for analyzing the earth's climatic history on a scale of hundred's to thousands of years are the study of ocean floor sediments and oxygen isotope analysis. Both methods are used to reconstruct past temperatures and each in part is related to the other.

The seafloor sediments contain the remains of organisms that once lived near the surface. When such near surface organisms die, their shell slowly settle to the floor of the ocean, where they become part of the sedimentary record. One reason why seafloor sediments are useful recorders of worldwide climatic change is that the numbers and types of organisms living near the sea surface change as the climate changes.

We would expect that in any area of the ocean/ atmosphere interface the average annual temperature of the surface water of the ocean would approximate that of the contiguous atmosphere. The temperature equilibrium established between surface sea water and the air above it should mean that changes in climate should be reflected in changes in organisms living near the surface of the deep sea.

The second technique, oxygen isotope analysis, is based on precise measurement of the ratio between two isotopes of oxygen: ¹⁶0, which is the most common, and the heavier ¹⁸0. Because the lighter isotope. ¹⁶0, evaporates more readily from the oceans, precipitation (and hence the glacial ice that it may form) is enriched in ¹⁶O. Of course, this leaves a greater concentration of the heavier isotope, ¹⁸0, in the ocean water.

Thus during periods when glaciers are extensive the concentration of ¹⁸0 in seawater increases; conversely, during warmer interglacial periods when the amount of glacial ice drops dramatically, the amount of ¹⁸0 relative to ¹⁶O in ocean water also drops. Ascertain microorganisms secrete their shells of calcium carbonate (CaCO₂), the prevailing ¹⁸0/ ¹⁶0 ratio is reflected in the composition of these hard parts. Consequently, periods of glacial activity can be determined from variations in the oxygen isotope ratio found in shells of certain microorganisms buried in deep-sea sediments. A second use of the ¹⁸0/¹⁶0 ratio technique is its application to the study of cores taken from ice sheets, such as the one covers Greenland.

Here another cause for variation in the oxygen isotope ratio is used-namely, the ratio influenced by temperature. More ¹⁸0 is evaporated from the oceans when temperatures are high and less is evaporated when temperatures are low. Thus the heavy, isotope is more abundant in the precipitation of warm eras and less abundant during colder periods.

Which of the following is expected during the warmer periods?
A. Higher ¹⁸O/¹⁶O inocean waters as compared to that in cold periods
B. Higher ¹⁸O/¹⁶O in the calcium carbonate shells of organisms as compared to that
in cold periods
C. Higher ¹⁸O/¹⁶O in the glacial ice as compared to that in cold periods
D. Lower ¹⁸O/¹⁶O in the glacial ice as compared to that in cold periods

(a) It is greater than one in ocean waters.
(b) It is less than one in glacial ice.
Which of the above statements are correct?

Assertion (A): Seafloor sediments are useful recorders of worldwide climate change.
Reason (R) : Organisms living near the surface of deep sea are sensitive to variations in water temperature.
In the light of above two statements, choose the correct options.

(a) Water molecules with 160 evaporate more readily from the oceans.
(b) Water molecules with 180 are found in greater concentration in glacial ice than in oceans.
Which of the above statements are correct?

(a) The average annual temperature of the ocean surface water is close to the temperatures of contiguous atmosphere
(b) Changes in climate can be estimated from ¹⁸O/¹⁶O ratio in the calcium carbonate shells of organisms
(c) ¹⁸O/¹⁶O ratio in warmer periods is less in oceans as compared to that in cold periods.
Choose the correct option from those given below:

Ozone and ultraviolet radiation are inseparable. It takes UV photons to form ozone from oxygen and UV photons to release the chlorine from CFCs to destroy ozone. Ozone depletion causes ground level UV radiation-an environmental hazard.

The wavelength of the electromagnetic radiation that is visible to the human eye and photosynthetically active for plants lies between 400-700 nm. Wavelengths below 400 nm in the ultraviolet are invisible to the unaided human eye. The UV region is conventionally subdivided into UV-A (320-400 nm), UV-B (280-320 nm) and UV-C (200-280 nm).

Natural UV is supplied exclusively by the sun. which radiates energy as a black body with an effective surface temperature of 6000 K. Outside the earth's atmosphere, the total solar radiation flux density of 1367 Wm⁻ ² is the sum total of visible and IR (1254 Wm⁻ ²), UV-A (85.7 Wm⁻ ²), UV-B (21.1 Wm⁻ ²) and UV-C (6.4 Wm⁻ ²). Close to the top of the earth's atmosphere, the spectrum is essentially extra-terrestrial.

By an altitude of 40 km. Os absorption has already made a significant difference, reducing the flux most around 250 nm because that corresponds to the centre of the absorption band. At 20 km. above the surface, photon flux is almost same as the surface value, except for the residual peak at 200 nm.

At this level, UV-C is effectively eliminated. Although UV-B is attenuated, the UV-C source flux increases towards the blue and Ozone absorption band weakens towards 300 nm. Both these influences combine to give a significant UV-B flux which is very sensitive to small shift in ozone aborption.

Which one of the following statement is NOT correct?