CLIMATES THROUGH TIME : THE PRECAMBRIAN

Isotopic data from sediments are problematic for reconstructing Precambrian climate because of:

Heating of rocks through time

Changes in the composition of seawater

Changes in the temperature of seawater

The Archean

We know that there was liquid water on the Archean Earth because

* there are water-lain sediments

* there are water-precipitated chemical sediments.

Therefore the temperature at the surface was between 0 and 100 deg.C.

More than that about Archean climate we really don't know. The Archean is not a good analogue for our more modern planet however.

The faint-sun paradox

The Archean sun was about 25% less bright than present day. But the climate regime was not extreme: there was liquid water. In fact it may have been very hot: Some tests using O2 isotopes suggest very hot (60 to 100 °C) oceans in the Proterozoic. We don't yet know whether those are good results or whether they reflect resetting.

The Proterozoic

The Proterozoic climate is much better known than that of the Archean, but much less well-known than the Phanerozoic history.

From the sedimentary rock record we know that there were (at least) two glacial periods in the Proterozoic.

Huronian glaciation

The earliest known unequivocal glacial deposits date to about 2.3 Ga. The type example is the Gowganda Fmn. in Canada, which includes lithified tillite, glacial striae and dropstones. The controls on paloelatitude for this time period are not great, so we are not sure if we are looking at a non-execptional polar glaciation or a true ice-house.

After the Huronian, it appears that the earth was ice-free (or there may have been no polar continents). We know that there were warm conditions because limestone deposits are known from all continents.

Late Proterozoic (Varangian) glaciation

he Late Proterozoic is remarkable because on every continent we see glacial sediments just below rocks containing the earliest fossil record.

Between 750-550 Ma, there is evidence for ice cover found on all continents, extending to paleolatitudes below 30° (coral reefs are found today at these latitudes), and possibly even to the equator. There were several (at least 4) glacial intervals, separated by very warm interglacials.

The probable average Earth surface temperature in the Vendian was probably about 5 °C, although if the tropical oceans were frozen it may have been lower. The mean surface temperature in the interglacials may have been more than 20°C (20° is the inferred temp. for the Miocene, when the current icecaps are throught to have first appeared).

During the late Proterozoic the sun's luminosity, which has been increasing linearly through time, was about 6-7% less than now. Modeling the planetary climate then is difficult. If cloud cover was the same, then the warmer temperatures could only be explained by an enhanced greenhouse effect. If cloud cover were less, more solar radiation would have been incident on the surface and so there would have been more direct warming.

In the early Proterozoic the atmosphere may still have contained abundant ammonia (NH3) and methane (CH4). Both are strong greenhouse gasses, but both are reduced species. As the atmosphere became more oxygenating due to the activities of plants, these gasses would have declined.

Also, as organisms proliferated, the CO-2 concentration of the atmosphere may have been lowered, further reducing the greenhouse effect.

The low-latitude glacial deposits are interbedded with warm-water limestones, indicating that climatic variation must have been dramatic. Possible forcing mechanisms include terrestrial volcanism: this was a tectonically active period in Earth history. The combination of organic extinctions and explosive volcanism may have forced the CO2 concentrations in the atmosphere back up to very high levels. The ice melted completely etc. etc.

Recent work (Hoffman et al. 1998, summarised in NYT article in reading packet) suggests that the Late Proterozoic glaciations may have acted as a spur for metazoan evolution. The massive icehouses and succeeding greenhouses would have caused mass extinctions in the oceans. But a few organisms would have been able to survive. Recent research suggests that environmental stress can trigger genetic changes in organisms, and rapid, repeated mutations may have resulted in the first metazoan organisms.

Once multi-cellular life began to proliferate, CO2 burial may have been retarded, aiding the buildup of greenhouse gasses in the atmosphere.