Abstract
Stromatolites can grow under the influence of microbial processes, but it is often unclear whether and how the macroscopic morphology of these rocks records biological processes. Conical stromatolites, which formed in the absence of sedimentation, provide a comparatively simple record of the interplay between microbial growth and lithification. Here, we show that the dynamics shaping conical stromatolites result from diffusive gradients within the overlying microbial mat. These gradients cause minerals to precipitate faster in regions of high curvature, resulting in measurable properties of the shapes of stromatolite laminas. This model allows us to estimate the thickness of ancient stromatolite-forming mats to be approximately , consistent with modern systems. Proceeding from the assumption that the ubiquitous process of diffusion is recorded in the translating form of a stromatolite, we derive the shape of a diffusion-driven stromatolite. The conical morphology—a distinctive feature of stromatolites growing in the absence of sedimentation—arises from these dynamics. This form is quantitatively consistent with the shape of conical stromatolites that grew for more than yrs of Earth history.
- Received 9 August 2012
DOI:https://doi.org/10.1103/PhysRevX.3.041012
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Published by the American Physical Society
Synopsis
Eons of Diffusive Growth
Published 13 November 2013
Models of fossils called stromatolites reveal details about the ancient microbes that produced them.
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Popular Summary
Stromatolites are laminated, lithified rocks that are commonly regarded as the fossilized remains of ancient microbial mats. A particular class of stromatolites with conical shapes can be as old as and remain in hot springs today. How did such a conical form come about? Did this persistence of form over billions of years imply some fundamental generality in the physical, chemical, and biological processes underlying the formation? In this paper, we show that diffusion of calcium ions and molecules of inorganic carbon species is one of the keys to addressing these questions.
Understanding the growth dynamics of rocks that stopped growing billions of years ago seems a daunting task. Fortunately, the color and texture of the precipitated minerals in stromatolites, in the shape of periodic mineral bands, provide a very good record of their growth and indications of the physical and chemical conditions for the growth as well as biological makeup of the microbial mats. This information not only gives us clues about how to make our theoretical hypotheses about the ancient growth process but also allows us to verify our predictions against it.
Our theory views the ancient growth process of a conical stromatolite as follows: Each growing stromatolite had a thin microbial mat as its surface layer. Mineral precipitation and their diffusion through the mat were the two primary processes. However, as mineral precipitation was limited by diffusion through the microbial mat, the rate of precipitation was faster in regions of high curvature. Mathematical formulation of this conceptual picture in terms of a differential equation about the form of the stromatolite leads to the prediction of conical shapes that compare quantitatively well with those observed in fossil records. Equally compelling is our estimation, based on the model and its comparison with the fossil records, of the 1-mm thickness of ancient microbial mats. This number is quite consistent with the thickness of modern-day microbial mats.
Our work highlights the microscopic diffusion process as a dynamical basis for the ubiquity of conical stromatolites throughout Earth’s history.