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Oxygen isotope analysis was originally conceived as a proxy for past temperatures.
But later scientists pointed out that the ratio was more heavily influenced by how much ice there was at the time the foraminifera were alive than by the temperature per se - more of the lighter isotope tends to get locked up in ice as it formed, so the water left in the oceans has more of the heavier one, and foraminifera shells preserve this difference.
They then washed away the mud from each sample, picked out the particular kind of fossil foraminifera they were interested in, cleaned them and fed them into a mass spectrometer, which measured the relative quantities of magnesium and calcium.
The colder the water, the less magnesium the foraminifera absorb and build into their shells.
So by analysing the ratio between the two elements at depth intervals, each equivalent to around a few hundred years, throughout the length of the core, they could get an accurate record of the deep ocean temperature at this location over the past 1.5 million years.
Evidence for deposition of these layers by turbidity currents is as follows: (1) the layers occur in submarine canyons, in deltalike features at the terminal ends of canyons, in basins and depressions, never on isolated rises; (2) they are interbedded with late Pleistocene sediments of abyssal facies; (3) they are well-sorted and commonly graded; and (4) they commonly contain organic remains of shallow-water origin.
These are distorted by measuring both the Earth's temperature and the amount of its water locked up in glaciers and icecaps.
For the first time, the new technique allows scientists to disentangle the two effects, giving a much more detailed view of how the climate has fluctuated between warm and cold periods.
H Elderfield, P Ferretti, M Greaves, S Crowhurst, IN Mc Cave, D Hodell, AM Piotrowski.
Studies of lithology, particle-size distributions, and micropaleontology and chemical analyses of 221 Atlantic and Caribbean deep-sea cores lead to new conceptions of processes of sedimentation, rates of sediment accumulation, Pleistocene chronology, and pre-Pleistocene history of the Atlantic Basin.