A VITAL clue to the riddle of when life began may have surfaced in Greenland. A geologist believes he has found brine from the Earth鈥檚 primordial ocean there, trapped in lavas that erupted on the seafloor 3.8 billion years ago.
Jacques Touret of the Free University in Amsterdam discovered the fluid in rocks from the Isua formation of western Greenland. This is the oldest known oceanic crust, formed right after an intense bombardment by giant meteorites. Life is thought to have emerged shortly after this period 鈥� what may be the earliest fossils are 3.5 billion years old 鈥� but it is not known just how quickly it appeared.
If life is found in the 3.8-billion-year-old rocks at Isua, it must have appeared almost as soon as was possible on our planet. That would make it much more likely for life to arise on other planets too, even if they are only habitable for a very short time. 鈥淚n that case, I would say the chance of finding remnants of life on Mars is 100 per cent,鈥� says Touret.
Advertisement
When Touret analysed rock samples from Isua, he found tiny cavities just a few micrometres across that contained methane gas, as well as a thick brine containing up to 15 per cent salt. He says there must have been hydrothermal vents near the undersea volcano, and that when lava erupted on the ocean floor, methane from the vents was trapped in the lava along with seawater as it cooled (Precambrian Research, vol 126, p 219). The presence of hydrothermal vents is a promising sign that organisms could have been present, says Touret. 鈥淭his environment is very favourable for life.鈥�
These results are questioned by David Vanko of Towson University in Maryland. Vanko is studying fluid inclusions from 3.2 and 3.5-billion-year-old rocks in Australia and South Africa. These rocks are well preserved for their age, but the Isua rocks were recrystallised and deformed by intense heat and pressure after being deposited. This means, says Vanko, that the fluids could have been altered since they were trapped.
Touret鈥檚 droplets come from rock containing distinct structures called pillow lavas, formed when lava meets seawater. These have not been deformed, so the rock may not have experienced such severe heat and pressure, he says.
Touret is confident his results suggest the primordial sea was saltier than today鈥檚. But Vanko argues that the metamorphosis could have concentrated the brine.
Another specialist on fluid inclusions, John Mavrogenes from the Australian National University in Canberra, says the droplets may not have been trapped as the lava formed, but instead separated out.
Touret plans to try to prove the inclusions do date from when the rock first formed, and find direct evidence of life. Fossils would be conclusive, but it is unlikely any microbes could have survived the heat of the lava intact. Another option is to check levels of chlorine and bromine in the water. Inorganic processes don鈥檛 affect the ratio of chlorine to bromine in seawater, whereas living cells tend to accumulate bromine.