Question

1. In deep sea paleoceanographic records, oxygen isotope ratios are commonly studied from both planktonic and benthic foraminifera. What does each record indicate? Are the resulting curves the same or different? Explain.

Answer 1

Different paleoceanographic records can be recovered from the foraminiferal shells like paleo-temperature (by studying the oxygen isotopes), paleo-nutrient condition (by studying the Carbon isotopes) etc. Why we can study these things from a foraminiferal shell? This is because foraminifera secrets their shells in an isotopic equilibrium condition, so they preserve that particular isotopic composition of sea/ocean water in their shells. So by studying their shells, we can infer the paleo-temperature and paleo-climate like things.

Both the planktic and benthic foraminifera shows a similar trend of record.

We use oxygen isotopic study for knowing the paleo-temperature condition. We take O16 and O18 isotopes for this study. O16 is the lighter isotope and O18 is the heavier isotope. The record is represented as delO18 which is the ration of O18/O16. During evaporation O16 being the lighter isotope goes to the atmosphere first, hence the sea/ocean water gets enriched with O18 at that time. We know ice generally take freshwater for its formation, and freshwater is rich in O16 than O18. So during the glacial period more of the O16 from seawater gets incorporated into the formation of ice, hence the seawater becomes enriched with O18, hence the delO18 ratio also increases. During interglacials, O16 from ice comes and mixes with the seawater, hence the delO18 value decreases.

Similarly, we use Carbon isotope to study the paleo-nutrient condition. Here we use C12 and C13 as two isotopes and delC13 as the ratio which is C13/C12. The less the delC13 value the more the biological input as more biological plants are made up of C12, whereas the more the delC13 value the more the tehrmogenic input. Also less delC13 value suggests upwelling or high nutrient rich condition at the upper ocean.  

Like this we use foraminiferas for the study of paleoclimate.