Protozoans hold clue to cooling in the sea

By analysing the magnesium-calcium ratio in some marine protozoans, a team of researchers has revealed that around 3.5 to 2.95 million years ago, there was pronounced sub-surface cooling in the eastern tropical Indian Ocean¹. The finding could significantly impact our present knowledge of the regional climate.

The cooling theory partially contradicts earlier assumptions² that there was a change in surface throughflow in line with the continuous plate tectonic reorganisation in that area between four and three million years ago.

Marine geologists have been keenly studying the opening and closing of ocean gateways that impact regional and global climates. In the geological past, this phenomenon has driven long-term global changes. For instance, opening of the Drake Passage led to the expansion of ice sheets in the Antarctic continent.

The importance of the Central American and Indonesian Seaways has been extensively debated with respect to global climatic reorganisations during the past 5 million years. These reorganisations, geologists feel, may have led to the initiation of major Northern Hemisphere Glaciation (NHG) in the middle Pliocene (4-3 million years ago). The mid-Pliocene climate transition marks the gradual change from a climate with no ice cap in the Northern Hemisphere to a climate state with extended continental ice sheets at high northern latitudes.

Earlier work had suggested a switch in the source of Indonesian subsurface throughflow (ITF) water from warm southern to cold northern Pacific during four to three million years ago in response to the constriction of the Indonesian Seaway. The cold water may have cooled the Central Indian Ocean surface water thus driving aridity in the African region and intensification of the NHG.

The cooling theory was established by the analysis of Mg/Ca and isotopic ratios in planktic foraminifera Globigerinoides ruber, G. sacculifer, Globorotalia crassaformis and Globoquadrina venezuelana obtained from the tropical eastern Indian Ocean Deep Sea Drilling Program (DSDP) Site 214. In contrast to the conditions at the surface of the ocean, the team observed a distinct cooling of the subsurface (thermocline) with little or no effect in the surface waters.

"The decoupled evolution of subsurface and surface water masses causes an increasing thermal and isotopic gradient between shallow and deep dwelling species from about 3.5 million years onwards, pointing to a shoaling and cooling of the thermocline," says one of the researchers Anil Kumar Gupta. It is speculated that the major temperature drop in the tropical eastern Indian Ocean subsurface waters might have affected the surface temperature in the western Indian Ocean via equatorial and/or coastal upwelling off Somalia.

A sea surface temperature drop in the western Indian Ocean triggered by cooled tropical eastern Indian Ocean subsurface waters might have caused a reduction in evaporation and hence, less precipitation over eastern Africa and in consequence, a change in hominid evolution, he adds.

The authors of this work are from: Leibniz Institute of Marine Sciences, University of Kiel,Wischhofstrasse, Kiel, Germany; Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur, India; AlfredWegener Institute for Polar and Marine Research, Bremerhaven, Germany and Zentrum für Marine Umweltwissenschaften, Universität Bremen, Germany.