Taking the Pulse of Archean Rocks - Iron and Carbon in the Deep Terrestrial Biosphere

Objectives:(1) Describe microbial life in anoxic brines in deep fractured rock, and (2) understand how microbial communities repsond to and potentially alter geochemical gradients in mats.

Site Characteristics: Archean rocks of the Canadian Shield overlain by 2.7 Ga (billion year old) Banded Iron Formations

The site is in Soudan, MN at ~0.5 miles depth in 2.7 billion year old rock. James Essig (Minnesota Department of Natural Resources, Park Manager) was our tour guide. Professor Calvin Alexander (Geology, UMN) arranged for a group of cavers to join in the fun.

Calvin Alexander, Ryan Lesniewski, and I explored the mine with Jim Essig and a troop of cavers.

Mural - High Energy Physics Lab, University of Minnesota

On the 27th level in the West Tunnel of the mine, we sample groundwater, minerals, and microbial biofilms along a flow path fed by at least three springs. The springs were formed by exploratory bore holes drilled prior to the mine closure in 1962. The springs are composed of anoxic calcium chloride brines loaded with dissolved iron.

floating biofilms - west tunnel.

The aqueous geochemistry and mineralogy changes along the flow path. The oxidation-reduction (redox) potential slowly increases, the pH decreases, and iron-bearing minerals vary from ferrihydrite to goethite and lepidocrocite. The major rock type, spring water composition, and cultured microbes are very different between the east (chlorite/sericite schists) and west (hematite/quartz Banded Iron Formation) tunnels.

Iron is beautiful - east tunnel.

This is a microbial ecosystem driven by iron and characterized by the absence of sunlight. We do not know whether the old rocks are connected to the world above by water moving through fractures - this will affect microbial access to nutrients. We hypothesize that the microbial communities along the redox gradient transition from iron-based autotrophy to frothy floating biofilms full of heterotrophic microbial activity. For example, Ryan and Jeff isolated more than 10 different kinds of Marinobacters from the west tunnel.

Brandy Toner.