The advent of deep-sea drilling in the 1950's prompted the use of planktonic foraminifera (unicellular protozans) as palaeoceanographic indicators. They provide a natural archive of past environmental changes due to their global distribution, their prolific productivity and sensitivity to environmental variations.
The most obvious change in relative abundance in core BAR9403 (off the coast of Sumatra) is shown by sub-polar to transitional 'upwelling' species Ga. bulloides with abundances of 26% at approximately 14,000yrs BP and 22.7% during the Holocene. This is compared to the periods from Marine Isotope Stage (MIS) 3 to the Last Glacial Maximum (LGM) where the relative abundance of Ga. bulloides is generally <10%. Gr. menardii, a tropical 'upwelling' species, also increases its relative abundance during MIS 2 from <8% during MIS 3 to a peak abundance of 16% at ~17,000yrs BP.