Geology Reference
In-Depth Information
why sandstones were overlooked. Most fossils are preserved in fine-grained rocks like
chert or shale, or in limestone reefs—hence the focus on black chert, black shale, and stro-
matolites.Sand,bycontrast,isrelativelycoarse,withmineralgrainsmuchlargerthanmost
microbes. What's more, sand tends to concentrate at the beach, in the turbulent tidal zone,
where most signs of life are quickly erased—eroded, washed away, and dispersed. But
Noffke has spent two decades studying modern tidal flats and their rich ecosystems and
found that tough, fibrous microbial mats impose distinctive structures on shallow, sandy
shorelines. They imprint a crinkly texture to the sand surface, not unlike a wrinkled table-
cloth; they bind and trap sediment grains in a thick, resilient mass of algal strands; they
alter the pattern of ripple marks in the sand; and they fragment in storms, tearing into dis-
tinctive geometric chunks and rolling up like little Persian carpets.
Most sandstone outcroppings appear smooth or gently rippled, devoid of anything obvi-
ously biological. But once Noffke learned to spot the distinctive wrinkled and cracked sur-
faces characteristic of fossilized microbial mats in ancient rocks, she spied subtle features
almosteverywhereshelooked.In1998sheidentified thediagnosticcrinklytexturesonthe
surfaces of 480-million-year-old rocks of the Montagne Noire in the French Alps. In 2000,
after moving to Harvard University for postdoctoral work, she pushed the record even fur-
ther back, identifying similar patterns in 550-million-year-old rocks of Namibia. The fact
that microbial mats existed a half-billion years ago was not particularly newsworthy; all
paleontologists would have agreed that microbial mats must have been around to decor-
ate coastal regions much earlier than that. But no one before Noffke had taken the time to
scrutinize modern mat systems, then recognize the similar traces preserved as unambigu-
ous fossils in ancient rocks.
In2001Noffkemadethefirstofaseriesofgroundbreakingmicrobialmatdiscoveriesin
formations more than three billion years old from South Africa and Australia—a time long
before the presumed Great Oxidation Event. Such features are difficult to spot in the over-
headglareofthemiddaySun,butlateintheafternoonattheendoflongandoftenfruitless
days of searching, as the sunlight rakes across barren rock, the telltale wrinkled sandstone
surfaces stand in stark relief. “The structures seemed to pop out everywhere,” she recalls
of one thrilling discovery, consummated at the final hour of the final day of one arduous
African field excursion.
Nora first came to me in 2000 at the suggestion of her Harvard mentor, paleontologist
Andy Knoll. Andy and I had been friends since graduate student days in the 1970s; for a
time, our careers had taken us in different scientific directions, but our mutual interest in
astrobiology had renewed our conversations. Knoll realized that Noffke's case for ancient
mats wasbasedalmost entirely onsurface features that, while suggestive, attimes required
a speculative imagination. The average paleontologist, lacking Noffke's extensive experi-
encewithmodernmats,couldeasilyoverlookordismissoddripplemarksorwrinkledrock
