Is the distribution of planktonic foraminifera guided by sea-water density?

In 1977, Bé and Hudson wrote a significant article on the distribution of planktonic foraminifera in the Indian Ocean. They provided modern-day data on live collections for the entire ocean and established the ranges of salinities and temperatures for each of the 32 planktonic foraminifera based on 164 plankton tows. These authors also compared the live distribution of these foraminifera with the tests of the same species found on the sea floor in some 190 core tops, assuming they were of ‘modern’ age. More recent studies contributed to a discussion concerning the importance of seawater density on planktonic foraminifera distributions in the water column. These are discussed in detail. The Indian Ocean data are used to determine the range of water densities these foraminifera live in and an attempt is made here to determine the sea water densities the same foraminifera must have lived in during the Last Glacial Maximum (LGM) when surface seawater was at least 1 practical salinity unit higher as a result of a global sea level drop of some 125 m (compared with today), and oceanic temperatures had decreased significantly. Examination of planktonic foraminifera assemblages recovered from ten core tops and samples deposited during the LGM in the same cores from the Australian region in the Eastern Indian Ocean are used to evaluate if the distribution of those foraminifera had indeed been controlled by sea water density. In order to determine whether the Indian Ocean foraminifera at the LGM cores could live under high salinities, comparison is made with the distribution of live planktonic foraminifera in the Red Sea and the Gulf of Aden, as in these two ‘restricted’ areas salinity is extremely high due to intense evaporation. The contrasting foraminifera assemblages between modern and glacial conditions in all the Indian Ocean cores, with assumed density changes, clearly indicate that seawater density does not control the distribution of planktonic foraminifera. Instead, oceanic currents and oceanographic conditions such as nutrients and ephemeral upwelling as well as temperature changes, clearly affect the distribution of foraminifera when near-surface conditions change between interglacial and glacial conditions, especially concerning salinity.