by Gabrielle Walker Crown Publishers. N.Y., 2003 CDN$37.95, hardcover. ISBN 0-609-60973-4

1 This book recounts the development of the Snowball Earth theory from its beginnings up to press time and even beyond (several articles in print or progress are cited!). It is pitched at the layperson's level and close to half of the text is devoted to the lives and motivations of the scientists involved. Most readers of this review will know the main Snowball protagonist - the GSC's former Precambrian virtuoso, Paul Hoffman. Many will have heard him in his present role of Harvard professor as he has expounded on the theory at national meetings and in university settings across the continent and around the world. The progress of the theory, its adherents and challengers, flashbacks to other scientists with theories ahead of their times (e.g. Wegener's continental drift) are cleverly interspersed and told in an exciting, suspenseful way by a skilled, informed and experienced narrator. Author Gabrielle Walker, a Cambridge Ph.D. and well-known science editor, has visited key Snowball field sites around the world and has had lengthy interviews with the main players over the past several years. Her book will appeal to scientists and non-scientists alike.

2 Evidence for a world-wide Late Proterozoic glacial event (or events) originated with the late Brian Harland of Cambridge University. His fieldwork, beginning over 60 years ago in the Svalbard Archipelago, Norway, and Greenland, recognized obvious glacial marine deposits bounded above and below by oolitic limestones that suggested tropical deposition. When he presented a paper in 1963 on a "Great Infra-Cambrian Glaciation" he was shot down by critics who interpreted his glacial dropstones as products of marine mudslides. Paleomagnetism was not advanced enough to prove that his "ice rocks" had originated near the equator so he reached a dead end.

3 Next on the scene was brilliant ("nutty" he says) Joe Kirschvink from Cal Tech. A paper contending that Precambrian glacial deposits in South Australia showed the flat magnetic fields indicative of equatorial origin intrigued him. Tests for magnetic overprinting had not been applied, so Joe sent a graduate student to study and sample the site. The flat magnetic fields proved to be original and, hence, equatorial. All he wrote was a two page article in an obscure monograph. In it, he indicated that volcanism would continue throughout global glaciation and so CO₂ would build up to enormous levels as there was no mechanism to remove it from the atmosphere. The Greenhouse affect would kick in, the atmosphere would become like a furnace, the global ice would melt and the scorching climate would set the stage for the emergence of a whole new kind of life. He christened the event "Snowball Earth". Before dropping its advocacy and moving to other "nutty projects", Joe sat with Paul Hoffman at a dinner and told him about the Snowball theory.

4 Enter the book's hero, our Paul. Leaving the Geological Survey of Canada after an altercation with its director and spending a couple of years at University of Victoria, he accepted a professorship at Harvard. Then, where to find a field project? Namibia, in southwest Africa, with vast, unstudied, desert exposures of 600– 700 Ma rocks was a natural. His initial purpose in 1993 was to measure timing of continental shifts, but he became more and more intrigued with exposures of glacial marine deposits bounded below and above with tropical carbonates. A literature search found similar phenomena reported on all continents and unearthed Brian Harland's and Joe Kirschchvink's ponderings on global glaciation. The carbonates above and below fascinated Paul with their anomalous C13:C12 ratios, and those above with their rose-coloured crystal fans and mysterious vertical tubes. All was explained when he teamed up with an insightful young Harvard colleague, Dan Schrag, a carbonate and paleoclimate specialist. This produced the theory many readers of this review have heard from Paul's own lips or read in Science or Scientific American a few years ago. In short: 1) the C13:C12 ratios showed greatly diminished life immediately before and after global glaciation; 2) volcanic CO₂ wrapped Earth in a blanket of warmth causing rapid melting; 3) scorched air sucked up moisture and hyper-hurricanes spread sheets of acid rain on glacial rock dust and swept it into the oceans; 4) saturated oceans deposited cap carbonates all around our planet. Their crystal fans resemble those found near acid hot springs, the vertical tubes were produced by gas bubbles. A good story but science has doubters!

5 "The Present is the Key to the Past" is an anchor of our science and a catastrophic Snowball Earth was not welcomed by many practitioners. Two chapters are devoted solely to the challenges and the challengers. There were many, e.g. sand wedges in the "ice rocks" of Australia suggest freezing and thawing and hence seasonal variations. Another was the question of how life could survive global glaciation. Paul and his colleague, Dan, either found answers to the queries or modified their theory to accommodate them. Sometimes an attempt to disprove the theory backfired - a student working on the Precambrian "ice rocks" in Australia found evidence of seven polar reversals, providing welcome proof of the long duration of the glacial event. She was a student of Columbia University's Nick Christie-Blick, who seems obsessed with melting the Snowball theory. He even presented a seminar at M.I.T. entitled "The Snowball Earth: A Neoproterozoic Snow Job"! Paul now has evidence that at least two, and possibly as many as five successive global glaciations occurred between 750 and 590 Ma. This is now widely (but not universally) accepted. So, what effect did it have on development of multicellular life?

6 A chapter entitled "Creation" takes the reader on field trips to Ediacara, Australia, the White Sea, and finally to Mistaken Point, Newfoundland to learn details of the multicellular life forms that appeared before the "Cambrian Explosion". The Newfoundland fossils are well-dated by their overlying volcanic ash at 575 Ma, just 15 Ma after the end of global glaciation. Coincidence, or did climatic extremes trigger the beginning of complexity in life forms? Biologists, just as geologists, are conditioned to thinking of evolution progressing slowly, and catastrophic causes arouse suspicion. There have been many challenges; for example, reports of complex life in pre-Snowball times, and "DNA Clock" experiments that initially suggested a more ancient origin of complexity. However, recent DNA experiments carried out by a doubter at Dartmouth University seem to tie the beginning of complex life to the Snowball era.

7 A final chapter reviews recent speculations on the causes of global glaciation. The current favourite involves congregation of the continents around the equator causing sunlight to be reflected and resulting in multiple global glaciations until dispersal approximately 590 Ma ago.

8 Most readers of this review know from first hand or geo-gossip something of Paul Hoffman. They will learn much more about this famed Boston Marathon runner, jazz enthusiast, hot-tempered genius and his continual quest for recognition and fame. In contrast, his closest colleague, Dan Schrag, is pleasant, easy going and adept at smoothing out "snowbrawls". The person who got it all going, Brian Harland, is "a slight man with pinched features and a nervous disposition" who worries over details and plans to excess. He has survived many dangerous mishaps in his Arctic explorations. Good- humoured Joe Kirschvink picks topics that others shun, continually makes discoveries that bring him into the spotlight and then moves on to something else. His students love him! Then there is Nick Christie-Blick, soft-spoken Brit, who devotes himself to "picking away at the threads of theories until he finds a detail that unravels the whole thing". Many more participants are described. All are sufficiently different to shatter any stereotypes as Gabrielle Walker tells the story of the most hotly contested theory in Earth science today and the scientists, pro and con.

9 Her final lines: "Theories like the Snowball often languish for decades without being properly probed. They need champions to drag them into the scientific limelight and expose them to scrutiny. They need people like Paul."