To promote a wider interest in the science of geology through organised lectures, field excursions and social activities.
To provide a link between the amateur, the student, the teacher and the professional geologist.
To foster interest in geological sites within the area with a view to their study and wise conservation.
To establish and maintain good relations with organisations that have common interests.

 

 

 
 
 

 

 

 

LIFE ON A YOUNG PLANET

 

Andrew H Kroll

 

ISBN: 0691009783

Princeton University Press
 

Book Review by Joyce Ali
 
A new book this year with the latest information is not often found on bookshelves, so this was very much a "must have" item. The author is now a professor at Harvard and has spent more than twenty years working on the earliest rocks to be found. The map shows the location of up to 10 sites which are described in detail. Several of these are now on continental coasts which have escaped tectonic activity so far, though when the next period of collisions arrive they could be lost, as many such sites must have been before. The youngest sequences are found in Siberia where the Kotuikan River is cutting down through over 300' of early Cambrian rocks or up to 20 MY of limestones and lime mud, once a warm shallow sea. Few fossils here with some trails to be found but at the top nearly 100 different types of shells exist.

One picture taken along the river showing an angular unconformity at the Precambrian boundary really left an impression of the vast periods of evolutionary time about to be revealed. A chapter sums up the latest research on attempts to compile a "tree of life" for the early bacteria, extremophiles, and other organisms, many of which are still being discovered. Chemical reactions are the fuel for some of these organisms as well as the more familiar photosynthetic variety. Their preservation in the rocks they created in life as they and the environment evolved is examined in brilliant fashion in the ensuing pages.

 

In north eastern Spitzbergen the stromatolites, chert bands, mats of cyanobacteria, intertidal and lagoon sediments with patch reefs are stacked up in vast thicknesses producing sequences never seen by most geologists. It was quite a relief to hear mention of "tepee" structures found in some of the beds and be able to recognise a feature! As the microoganisms weave their mats and trap the fine mud and sand the columns slowly build and bacterial action on the buried cells changes the chemistry and carbonate crystals grow. It takes a very long time to build a 7000' succession such as is found in this Akademikerbreen Group of rocks. The chert nodules here are common, form in the sediments and contain microfossils. Cyanobacterial mats can still be found in modern day equivalent environments.

 

Its a long jump from 800MY rocks in Spitzbergen to North West Australia both in time and space for the 3.5BY year old rocks of the Warrawoona Group. The sediments here are associated with volcanic rocks which have been used to confirm the age. A technique using lasers to examine growth layers on zircons has provided dates. These minerals can survive many rock cycles without showing signs of wear and tear so these measurements are needed. Here the cherts were formed from heated fluids and silica has replaced the gypsum crystals that formed in evaporites in the lagoons. Here the debate is whether the structures that resemble stromatolites are biological or formed physically. The description as "Microstructures" is a half way solution and much work goes on here and on similar rocks in the ancient Barberton Ranges of South Africa. The chemistry and mineral deposits of Oceanic ridge vents are also providing information for the debate.

 

At 1.9BY the Gunflint cherts formed in iron rich waters before oxygenation and were the last to precipitate directly onto the sea floor. The structures resemble stromatolites and contain microfossils in the laminations but whether they were actively involved in construction or just passively preserved has to be decided. Iron loving bacteria are found with fossil cyanobacteria inside black chert nodules around Hudson Bay.

 

A chapter on the evolution of oxygen in the atmosphere and oceans reveals that for a long period sulphate reducing bacteria were active in the deep sea while surface waters slowly became oxygenated.

 

In the rest of the book, the "Great Wall" of the Kotuikan river cliffs 1.5BY old stromatolites were built by cyanobacteria and now they are distinguished by different rates of sedimentation around the mats and reefs. Some were preserved rapidly before they collapsed, others were squashed.

 

Exchanging frozen Siberia for Namibia takes in the Nama reefs which were built up to 200' high and are now being exhumed in the deserts. Cap Carbonates, strange associations of fine laminations disrupted by crystal layers are also here. Their relationship to widespread Glacial deposits both here and in other places round the globe is discussed in detail.

 

South West China has an amazing deposit of phosphate beds mined for fertilizer which has multicellular algae remains, fossil eggs and embryos. At 590-600MY years these beds have only been researched for about 10-15 years so a lot more work needed here.

 

Not to be overlooked are up to date ideas on the Mars rock structures and their associated puzzles.

Eight colour plates are included and show a comprehensive range of early microfossils and where relevant their modern counterparts. Other pictures and data are displayed, the references and index are exhaustive.

 

This has been only the tip of the "Iceberg of information" in this book and I found every one of the 250 pages fascinating. If you are wondering what to do with your Xmas book tokens look no further because this is one volume I won't be selling at the A.G.M.!