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Project Progress

The oldest known fossil communities of large, complex, multicellular life forms are found on the coastal exposures of eastern Newfoundland. Although frond-like, these fossils did not photosynthesize, as they were deposited in the deep sea at depths where light cannot penetrate. In addition, the complex “fractal” surfaces of these forms has led to the interpretation that they take up their nutrition osmotically—they are absorbers of dissolved nutrients. This leads to the question as to why these large forms are competitive with bacterial films which do the same job of absorbing. In short, the fossils raise the question: Why be big? To address this question, we reconstructed flow in the fossil community, which required addressing the question of how the community itself affected flow. Assessing community density allowed us to determine that a particular type of flow, referred to as canopy flow, occurred in the rangeomorph community. Canopy flow generates a very well-mixed fluid in the community despite very low flow velocities. It also generates gradually increasing flow with height above the sea floor. We were then able to further model the uptake response of organism surfaces under these conditions, and found that nutrient uptake rates are strongly controlled by fluid velocity. Consequently, there was a strong advantage to increased exposure to flow through growth to higher height above the bottom. These heights could only be achieved by multicellular eukaryotic architecture—potentially explaining the advantage of eukaryotic size in the late Precambrian, just prior to the dawn of animal life.

Our paper has been accepted to Current Biology. The journal has commissioned a preview of the article, and we are working with them to prepare cover artwork and a video abstract. Thus, we anticipate this to be a high-impact publication.

The work provides an exciting interdisciplinary integration that has its basis in Neoproterozoic geochemical and oceanographic conditions, which were yielding an increasingly oxygenated deep sea at this time. This research also incorporates sedimentology and detailed reconstruction of the fossil community, which provide the parameters constraining the flow models. However, some of the most interesting aspects of our results relate to the seemingly counterintuitive aspects of canopy flow, which is quite distinct from turbulent boundary layer flow more typically applied to the marine benthos. Canopy flows such as those reconstructed for the rangeomorph community are formed by different flow velocities within and above a community “canopy”, which can be composed of any set of elements of sufficient density sticking into the flow.

Canopy flows are characterized by linear arrays (streets) of large vortices, produced by what is known as the Kelvin–Helmholtz instability. These are produced where the faster flow above the canopy effectively trips over the slower flow below. These flows are not typically thought about, as our intuition is largely trained by boundary layer processes associated with turbulent boundaries, formed by small chaotic vortices as opposed large regular ones. Yet Kelvin Helmholtz vortices are all around us. They are responsible for the waving of grain fields in the wind, and the repeated vertical swirls of clouds. The Japanese have a word the movement of grain fields in canopy flow, Honami, but there appears to be no comparable word in Western languages. Nevertheless such waves inspired great art, such as wheat fields depicted by van Gogh. They also generate distinct physical properties. The large vortices produced generate large vertical mixing, more so than turbulent flow. Despite very low flow velocities, very well mixed bottom waters are produced in our models. This mixing refutes alternative hypotheses of rangeomorph height advantage that require changing concentrations of nutrients with height in the community.

Our modeling of surface uptake processes demonstrated that flow is critical for nutrition. In the absence of flow, a stagnant “blanket” around the organism limits uptake. Osmotic exchange to the organism is then effectively controlled by the thickness of this layer of motionless water. This blanket is narrowed and eliminated by flow itself. In low-flow regimes (such as the environment at Mistaken Point), any incremental increase in flow improves exchange and organismal uptake, and provides an impetus for rangeomorphs to grow upwards to access higher flow velocity with height above the bottom.

We anticipate that similar modeling will be important for understanding a range of processes in other fossil and living communities, including soft coral communities in the deep sea today, and processes locally controlling coral bleaching. These models could also pertain to geochemical phenomena such as crystal growth. Overall, we feel that this work has broad applications for the study of organic and inorganic processes of complex surfaces in flow.