Science Daily:

New research from the University of Washington suggests that early microbes might have been widespread on land, producing oxygen and weathering pyrite, an iron sulfide mineral, which released sulfur and molybdenum into the oceans.
"This shows that life didn't just exist in a few little places on land. It was important on a global scale because it was enhancing the flow of sulfate from land into the ocean," said Eva Stüeken, a UW doctoral student in Earth and space sciences.

The significance of this finding?

"It supports the theory that oxygen was being produced for several hundred million years before the Great Oxidation Event. It just took time for it to reach higher concentrations in the atmosphere," Stüeken said.
I think it has long been hypothesized that oxygen was being produced before the GOE (this is the moment when free oxygen began accumulating in the atmosphere—the British biochemist Nick Lane has an excellent chapter about this event in his book Life Ascending). So what the UW scientists are providing in this paper is more evidence for this hypothesis.