Choose the CORRECT factor(s) from the following, which affect(s) the \(^{18}O/^{16}O\) isotopic ratio in foraminifera growing in an ocean.
Show Hint
- Temperature
- Oxygen isotopic composition of water
- Growth of continental ice sheets
- Continental detritus input
The Correct Option is A, B, C
Solution and Explanation
Foraminifera build their calcite shells in equilibrium, or close to it, with the seawater they live in. The ratio of \(^{18}O\) to \(^{16}O\) recorded in that calcite depends on both the temperature of the water and the isotopic makeup of the water itself.
Step 2: Check option (A), temperature.
Calcite precipitation involves temperature dependent fractionation. Cooler water favors more \(^{18}O\) in the shell, warmer water favors less. This is the basis of the oxygen isotope paleothermometer, so temperature is a real control. Option A is correct.
Step 3: Check option (B), isotopic composition of water.
If the seawater itself is enriched or depleted in \(^{18}O\), the shell built from that water inherits the same signal on top of the temperature effect. So the water composition is a direct control. Option B is correct.
Step 4: Check option (C), growth of continental ice sheets.
Ice sheets form from evaporated ocean water, and evaporation preferentially removes the lighter \(^{16}O\) into the vapor that becomes snow and ice. This leaves the remaining ocean water enriched in \(^{18}O\) during glacial periods. Forams growing in that ocean water record this ice volume signal, so ice sheet growth is a genuine control. Option C is correct.
Step 5: Check option (D), continental detritus input.
Detrital sediment washed in from land changes the clastic content of a sample, but it does not change the oxygen isotope ratio locked inside the foram calcite itself, since that ratio was already fixed when the shell grew in the water column. Option D is not a control on the isotopic ratio.
Step 6: Final conclusion.
Temperature, water isotopic composition, and ice sheet growth all affect the \(^{18}O/^{16}O\) ratio in foraminifera, while detrital input does not.
\[ \boxed{\text{(A), (B) and (C) are correct}} \]
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