Iron and Carbon in the Deep Terrestrial Biosphere

The Soudan Underground Mine State Park is a 1/2 mile deep window into the microbial world of ancient rocks. With springs composed of anoxic brines carrying high concentrations of dissolved iron and methane, the site represents an accessible analog environment to deep-sea hydrothermal vents and Mars. Our collaborative team (Toner, Gralnick, Alexander) is asking questions like this:

How old are the anoxic brines and how do they form?

Why are the microbial communities of the east (chlorite/sericite schists) and west (hematite/quartz Banded Iron Formation) tunnels different?

What is the microbial biogeography along the iron and oxygen gradient?

September 14, 2008

We traveled to Soudan, MN to sample of iron-rich microbial mats at 0.5 miles depth in 2.7 billion year old rock (Peterson et al. 2001). James Essig (MN DNR) 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, UMN

On the 27th level in the West Tunnel of the mine, we sampled fluids, 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 rapidly 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.

We will travel to Soudan, MN again this fall - watch for updates!