above those reached 2. 1 biUion years ago, 

 fossil soils do not provide a firm guide to 

 atmospheric composition, and thus we are 

 unable to measure oxygen levels directly. 

 Fortunately, however, we can look for evi- 

 dence of environmental processes capable 

 of affecting the amount of oxygen in the 

 atmosphere. Principal among these is the 

 burial of organic matter in the sea floor by 

 sediments. When organisms die. they de- 

 compose. Normally, the amount of oxygen 

 produced in photosynthesis is balanced by 

 the amount of oxygen consumed by respir- 

 ing organisms, including decompsers. 

 Burial, however, shields organic remains 

 from respiring organisms, thus disrupting 

 the balance of oxygen production and con- 

 sumption and — if enough organic matter 

 gets buried — tipping the balance toward 

 production. Second is the conversion of 

 sulfate ions (abundant in seawater) to oxy- 

 gen-free forms of sulfur that accumulate in 

 sediments as pyrite. also known as fool's 

 gold. Every time a sulfate ion combines 

 with a metal in seawater to produce a sul- 

 fide ion. two molecules of oxygen are 

 freed. The geological history of these 

 processes is written in the language of iso- 

 topes, the variants in atomic composition 

 exhibited by individual elements. 



A decade ago, my colleagues and I 

 chanced on a telling fragment of this 

 record near the other, better-known North 

 Pole. Limestone from the Arctic island of 

 Spitsbergen contained distinctive isotopic 

 compositions of carbon and strontium that 

 hinted at large-scale global change be- 

 tween 750 and 550 million years ago. 

 Since then, we have sampled sediments 

 from around the world, and they all tell a 

 consistent story. Just prior to the radiation 

 of large animals, tremendous amounts of 

 organic matter (the remains of dead organ- 

 isms) were buried beneath shallow seas. 

 The high rates of organic carbon burial are 

 related to rapid sediment accumulation, as 

 shifting tectonic plates built both major 

 mountain chains and new ocean basins. 

 Recently, Gerry Ross, of the Canadian Ge- 

 ological Survey, has .shown that the bal- 

 ance of the sulfur cycle shifted at the same 

 time, depositing unusually large amounts 

 of pyrite in deep-sea sediments. 



As a result of these events, the environ- 

 ment must have expanded once more, 

 adding a richly oxygenated surface 

 layer — the atmosphere — in which the 

 manifold physiological needs of large ani- 

 mals could be met. The Phanerozoic 

 eon — the age of visible animal life that 

 continues to the present — was ushered in. 

 The first animals to appear in the fossil 



American Museum of Natural History 



IMAGES OF INDONESIA 



September 17 - October 1, 1994 



Indonesia comprises over 13,000 

 islands spread out like an emerald 

 crescent between the Malay 

 Peninsula and Australia. Created by 

 powerful geological processes, it is 

 a land of distinct cultures, some vir- 

 tually untouched by outside influ- 

 ence, and remote islands with some 

 of the world's most unusual 

 species. 



Following in the footsteps of such 

 luminaries as renowned naturalist 

 Alfred Russel Wallace and 

 American Museum anthropologist 

 Margaret Mead, the American 

 Museum is offering an exciting 

 opportunity this September to 

 explore these enchanting islands 

 aboard the first-class, 110-passen- 

 ger Caledonian Star. 



BALI 



SULAWESI 



SALAYAR 



KABAENA 



KAKABIA 



ALOR 

 LOMBLEM 



SAVU 

 KOMODO 



BRUNEI 



MALAYSIA 



MALAYSIA 







\ 



, Singapore 







SINGAPORE 







\ 



BORNEO 



Palopo _ 





IRIAN 





*-— sAL^zr 



INDONESIA 



JAYA 





JAVA 









■ KOMODO, ,,^„ 

 ' ' ,BALI SUMBAWA ALOR 









SUMBA , 









SAVU 







American 

 Museum of 

 Natural 

 History 

 Discovery Cruises 



Central Park West at 79th Street 

 New York, NY 10024-5192 

 Toll-free (800) 462-8687 or 

 (212) 769-5700 in New York 



19 



