Abstract
Students of geology learn early on about the macro- and microscopic features of sands and sandstones, imparted by physics at work on land and in the sea. The core problem investigated here arises because many sands and sandstones display features that physics alone cannot explain. These are microbially induced sedimentary structures, or MISS, bedding features whose explanation requires consideration of biological as well as physical processes. Beginning in the 1970s, Wolfgang Krumbein and Gisela Gerdes of Oldenburg University began to explore microbial features in the sand flats of northern Germany. Nora Noffke joined this effort as a student and, in time, made its study her own.
Microbially induced sedimentary structures provide an instructive case study of the logic that provides much of geology's explanatory power. Earth scientists want to understand the past in terms of interacting states and processes, but the geological record provides only pattern, and lots of it. If geologists are to interpret the patterns in ancient rocks in terms of underlying processes and conditions, they must investigate modern environments where comparable patterns form today. Only by careful observation, experiment, and modeling can we build an understanding of how physical, chemical, and biological processes give rise to pattern in sedimentary rocks. This understanding firmly in hand, we can interpret with confidence preserved patterns in ancient rocks and even, potentially, on planets beyond our own.
With this in mind, Noffke begins her discussion of MISS in coastal environments where sands accumulate today, especially northern Germany and the tropical coast of Tunisia. Repeatedly throughout the year, storms deposit sand and whisk it away. These physical processes of deposition and erosion leave unexpected patterns because the mechanical properties of sand beds are influenced by microbial communities on, and within, surface sediments. As Noffke explains lucidly, MISS record the interactions of microbial and physical processes, imparting a distinct set of textural signatures to accumulating beds. The resulting features provide a fingerprint of microbial communities, potentially as informative as stromatolites, microfossils, and organic biomarkers, but dramatically less well studied. Having established the relationship between pattern and process in modern environments, Noffke takes readers backwards in time, from the Pleistocene, where assumptions about life and environments are fairly secure, to the Archean, where much less can be assumed with confidence. Throughout Earth's historical record, the signature of MISS is clear.
Geobiology is no stranger to astrobiologists. Indeed, astrobiology might fairly be defined as the application of geobiology to worlds beyond our own. If, for example, the Kepler mission or its technological descendants ever detect life on an extrasolar planet, the evidence will reflect the influence of biological processes on atmospheric composition. Most geobiological research focuses on chemistry as the bridge between Earth and life, an endeavor built on field and laboratory research on the metabolic processes that cycle carbon, sulfur, nitrogen, and iron through ecosystems, and biogeochemical efforts to reconstruct Earth's deep microbial past. Given this central tendency, it is enlightening to read a geobiological treatise that focuses instead on physics. Noffke's discussions are expertly drawn and abundantly illustrated, offering lessons for professional geobiologists as well as the wider scientific community, not least those who comb through images of sedimentary rocks on Mars.
Noffke's book provides a state-of-the-art perspective on an important line of geobiological research, but what might we expect in the future? Having demonstrated that MISS can be found in Archean sandstones, has she exhausted the scientific potential of these features? Clearly not, although her book offers only a brief glimpse of future prospects. One key question concerns the physiological and phylogenetic interpretation of MISS in ancient rocks. Documenting MISS in 2.9-billion-year-old rocks, Noffke interprets them as evidence of cyanobacteria. If correct, this is an important inference, as other lines of geobiological inquiry have left most workers uncertain about evidence for oxygenic photosynthesis much before oxygen began to pervade the atmosphere and surface ocean 2.4 billion years ago. Noffke builds her case on the observed relationship between pattern and process in modern sediments: in present-day sands cyanobacteria play a key role in MISS generation, so why not interpret ancient MISS in terms of the same organisms? This question, of course, goes to the heart of actualistic programs for geobiological interpretation. In today's oxygen-suffused oceans, cyanobacteria dominate microbial mats in coastal marine environments, but what did mat communities look like before the rise of oxygen? Interpretations of Archean stromatolites have foundered on this issue; and, absent actualistic observations, experiments seem to provide the way forward. Flume experiments incorporating photosynthetic bacteria grown under oxygen-free conditions might tell us whether the fit between cyanobacteria and MISS is unique—or whether MISS might be expected for diverse microbial communities.
Indeed, there is potentially rich ground for integrated studies of stromatolites and MISS. Stromatolites have been studied for decades, and surely our hard-won lessons on stromatolitic pattern and process have implications for microbially induced sedimentary structures? Conversely, observations of MISS in formation might illuminate details of stromatolite morphogenesis. The deep integration here is between physics and chemistry, the former governing lamina formation but the latter playing a key role in the accretion of stromatolitic laminae through time. To this end, siliciclastic stromatolites, rare structures generally treated as biosedimentary exotica, might provide an informative intermediate.
One application of MISS that has received much attention by other researchers but gets only limited discussion in this book is the relationship between microbially influenced sediments and animals, both in the preservation of Ediacaran macrofossils (work of Jim Gehling and others) and on the effect of changes in benthic substrates on Cambrian evolution (for example, thoughtful papers by Dolf Seilacher, David Bottjer, and others). As Noffke herself has shown, MISS are not ubiquitous in ancient sandstones, so knowing the controls on their distribution in space as well as time might help to refine our understanding of marine ecosystems at the dawn of animal evolution.
Finally, there is the question of astrobiology. Noffke illustrates a Mars Exploration Rover and rightly holds that MISS could provide evidence for extraterrestrial life that is not easily mimicked by physical processes. Certainly, of the many gigabits of data sent back to Earth by Spirit and Opportunity, the most promising subset to search for signs of life is the high-resolution Pancam images of exposed sedimentary rocks. To date, nothing convincing has shown up in Mars Exploration Rover images. This could, of course, indicate that there is no biological signature to be found in these rocks, but there is a persistent problem. Sandblasting inexorably erodes exposed sedimentary rocks, so exposed bedding surfaces soon become modified beyond recognition. For the moment, then, it is the details of rocks observed in cross-sectional exposure and microscopic images of sandstone grains that might best repay geomicrobiological scrutiny.
Clearly, studies of microbially induced sedimentary structures remain in their infancy, with maturity only dimly glimpsed in the distance. As geobiologists and astrobiologists think about how to apply and sharpen the tools afforded by MISS, we can be thankful that Nora Noffke has provided us with a detailed but accessible road map for continuing research.