University of Manchester

CT imaging of 2.5 billion year old cuspate stromatolites (Gamohaan Formation, South Africa)

Russell Garwood
Project Completed

Scientific Case

During the late Neoarchean era of Earth’s history (ca. 2.5 billion years ago) the planet’s ocean-atmosphere system was anoxic and the most complex life forms on Earth were communities of microbial mats which are preserved in the geological record as finely laminated rocks called ‘stromatolites’. These microbial mat communities are thought to have contained photosynthesising cyanobacteria as it is at this time that the first evidence for trace amounts of oxygen appears in the rock record. However, no direct geochemical evidence for photosynthesis has ever been observed within the stromatolites themselves.

We have examined the geochemistry of ‘cuspate’ stromatolites from the Neoarchean Gamohaan Formation in South Africa and found elemental distributions that seem suggestive that these microbial mats were photosynthesising and hence producing free oxygen. In addition to this intriguing geochemical data, the Gamohaan ‘cuspate’ stromatolites have a very unusual ‘tent-like’ morphology, consisting of a central stalk from which radial drapes of microbial descend. Both the stalk and drape material – the biogenic structures - show strong enrichments in iron and manganese relative to the host dolomite matrix, meaning that a strong density contrast should exist between the biogenic structures and the surrounding rock. Therefore we propose to use CT imaging to produce the first three dimensional image of the structure of these unusual cuspate stromatolite forms, which were among Earth’s first photosynthetic communities.

Experiment Design

Samples are dolomite (CaMg(CO3)2) rocks with minor calcite cements (CaCO3), which have been sampled from drill core slabs. The dimensions of the region of interest are: height = 25 mm, width = 20 mm, depth = 12 mm. Hence resolution (as 1/3400th of the width) will be ~0.006 mm or ~6 µm. The features of interest are mm-scale iron rich drapes within the rock, thus the resolution is sufficient to pick out these mm-scale features.

The samples are inert rock samples and present no safety risk.
Scanners and Rigs
Nikon XTH225
Not Required
Based on the requirement of spatial resolution and sample size, we propose using the Nikon XTH225 cabinet system, which will provide ample resolution to resolve the features of interest. We propose concatenating two scans for regions of interest to acquire maximum resolution, starting with half a day of machine time for an exploratory scan of a single region of interest. This will demonstrate whether there is sufficient attenuation contrast. If this is a success we will request another half day on the same scanner, to complete a second region of interest scan (we have two stromatolites in this single rock same). No rigs or other equipment will be required. Scanning will be conducted by Russell Garwood.

Sample & Safety

Sample Description: samples are dolomite (CaMg(CO3)2) rocks with minor calcite cements (CaCO3), i.e. inert rock samples. There are no hazards or risks associated with these inorganic rock samples.
Low Hazard

Scan Records

Project Report