WAS SOME PROTEROZOIC AND CAMBRIAN SEAWATER ANOMALOUSLY VISCOUS?

A number of distinctive sedimentary features in Proterozoic and Cambrian strata remain enigmatic; among these are giant ooids, edgewise flat-pebble conglomerate, a variety of synsedimentary fissures and cracks, and oscillation ripples with unusually sharp crests. We tested the hypothesis that anomalously viscous seawater may have played a key role in the formation of these features. In aquaria experiments, we manipulated the viscosity of water using the soluble polymer Polyvinyl Pyrrolidone. Results of these experiments allow us to quantify the effects of water viscosity on two phenomena: (1) settling times of graded glass spheres, which were used to constrain parameters to model the growth of giant ooids, and (2) the formation of oscillation ripples in fine-grained quartz sand under viscous conditions, in order to test the theory that conspicuously sharp wave-ripple crests seen in Proterozoic quartzite and other rock may be attributable to depositional conditions, rather than metamorphism. When results of settling experiments are input into an ooid growth model, ooid size increases as a result of decreased grain-grain abrasion, increased rest time (in part through suppression of turbulence) and increased precipitation during extended fall time through the water column. Results of wave-ripple experiments were more straightforward: viscous water (~ 4 cp) resulted in perceptibly and statistically pointier ripple crests (as measured as radius of curvature). Seawater viscosity seems an obvious, if elusive, sedimentary variable. During Snowball Earth episodes, oceans were likely very salty and cold. Likewise, abundant cryptmicrobial features in Proterozoic and Late Cambrian rock show that benthic microbial films were commonplace, and it follows that a nepheloid viscous microbial-gel may have also existed, which would have locally influenced processes at the sediment-water interface. Increased seawater viscosity would have had profound consequences for early planktonic life (including larval forms of early metazoa). Viscous seawater would have also affected the settling of fine particulate matter, and thus would have significant implications for carbon and nutrient cycling in Proterozoic and Cambrian oceans.