1.If measurements of the O18/O16 ratio in a marine fossil shell show a decrease
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1.If measurements of the O18/O16 ratio in a marine fossil shell show a decrease in value (O18/O16 decreases), would this be characteristic of an interglacial warm period or a cooling period in Earth history? Explain your answer in detail. Why is the oxygen isotope ratio in calcium carbonate organisms significant in the fossil record? 1.If measurements of the O18/O16 ratio in a marine fossil shell show a decrease in value (O18/O16 decreases), would this be characteristic of an interglacial warm period or a cooling period in Earth history? Explain your answer in detail. Why is the oxygen isotope ratio in calcium carbonate organisms significant in the fossil record?Explanation / Answer
Sea water is abundant in 18O whereas water from the terrestrial parts is enriched in 16O. Glacial periods represent more 16O in glaciers and more 18O in terrestrial land. We know that since 16O is characteristically abundant in terrestrial water, subsequently, 16O is characteristic of glacial ice too. During glacial periods, there is formation of glaciars which would entrap 16O within the glaciers when they freeze. Thus, sea water is enriched in 18O. Subsequently, the oxygen isotope ratio increases. When the glaciers melt in the interglacial period, 16O is released and as the glaciers flow to the sea, they take with them the 16O to the sea. Thus, the sea is supplied with 16O. So, the ocean becomes less abundant in 18O. Thus, oxygen isotope ratio decreases.
The oxygen isotope ratio in calcium carbonate organisms is significant in the fossil record because it helps in the palaeoclimatic studies and thus, we can know about the climate where these organisms lived in the past. The change in the ratio 0f 18O/16O i.e. the ratio of heavy oxygen to light oxygen is a constraint to the changing climatic patterns over time. It is also an important constraint to the abundance of the calcium carbonate organisms in the previous times. Colder climates indicate the abundance of 18O in the calcium carbonate organisms.
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