A Tale of Two Stromatolites

Stromatolites, classically interpreted to represent a macroscopic manifestation of microbial processes, are quintessential geo/astrobiologic structures—large enough to be imaged remotely and robust enough to survive post-depositional alteration. On the one hand, there is no doubt that some (perhaps most) stromatolites on Earth were formed with biologic influence. On the other, recent work has demonstrated that stromatolite-like structures have formed without biologic input. When faced with determining the presence of life elsewhere in our solar system, “extraordinary claims require extraordinary evidence”—do stromatolites pass this test?

Here, we compare/contrast two different stromatolites as a cautionary tale for exobiologic studies. The stromatolites are basically identical at the hand sample scale (e.g., the scale most easily imaged by a rover). Both are small (~15 cm tall) branching columnar structures that display finely laminated “light-dark” lamination on the order of ~100 microns thick. The laminae tend to thicken over the tops of the columns and thin towards the sides.

Microscopic examination reveals the laminae in one stromatolite, collected from a subsidiary pool to Obsidian Hot Spring in Yellowstone National Park, are composed entirely of silicified microbial filaments—a classic biogenic structure, with an active cyanobacterial mat on the surface. The laminae in the other stromatolite, collected from a hydrothermal vein in the Tempaiute Range, Nevada, are composed of interlocking bladed calcite—a classic abiotic precipitated fabric. Furthermore, the Tempaiute stromatolites grew surface normal to a high-angle hydrothermal vein deep in the subsurface, in the absence of light.

While the geologic context provides some hint to their potential biogenicity (hot spring/active mat versus subsurface vein), the context may not always be apparent (e.g., a Mars boulder field). At the hand sample scale, many stromatolites may fail the “extraordinary evidence” test—if possible, such structures should be examined in-situ microscopically to help guide our decision.