Evidence for a naturally occurring post-glacial acid sulfate weathering event in northwestern Indiana, USA

- A natural acid sulfate weathering (ASW) event occurred south of Lake Michigan.
- ASW accounts for some “unproductive black soils” encountered by early settlers.
- ASW explains why highly leached soils in ultic subgroups occur in the area.
- ASW may explain why arsenic is anomalously high in the streambed of the Kankakee River.
- Areas of unproductive black soils still occur in the area today due to ASW.

We review the evidence for a naturally occurring acid sulfate weathering event along the southern shore of Lake Michigan that was initiated ~ 15,000 years BP (before present) at the close of the Wisconsin Glaciation. The evidence includes: (1) “unproductive black soils” that were encountered when settlers drained the wetlands in the area for agriculture, (2) areas of soils that have anomalously low base saturation and are classified into ultic subgroups in a region where typic subgroups are the norm, (3) unexpectedly high arsenic concentrations in streambed sediments of the Kankakee River, (4) pyrite, jarosite, and gypsum in cobbles from outwash deposits in the areas, and (5) identification of a modern day “unproductive black soil” with a surface soil pH of 2.1. We propose that the acid sulfate weathering event was initiated when the Lake Michigan Lobe of the Wisconsin Glacier eroded pyrite-rich bedrock and deposited it in an outwash fan in front of the Valparaiso Moraine, where sorting by water depleted calcium carbonate-rich fine material relative to pyrite-rich coarse material. Acid sulfate weathering on the higher landscape positions led to well-drained soils depleted of calcium and magnesium and resulted in anomalously low base saturation and soils now classified into ultic subgroups. Calcium, iron and sulfate moved to adjacent wetlands in the lower lying landscape positions, where reduction of iron and sulfate resulted in the precipitation of secondary pyrite in the accumulating organic material. Drainage of these wetlands for agriculture beginning in the late 1800s exposed the secondary pyrite to oxygen and initiated a second cycle of acid sulfate weathering that led to some of the reports of “unproductive black soils.” These soils developed extremely low pH upon drainage. Some of these highly acid conditions exist today. Arsenic in the original pyrite is moving through the landscape and is manifested in the unexpectedly high arsenic contents in streambed sediments of the Kankakee River that drains the area. This work shows that a whole landscape approach is necessary to understand how seemingly unrelated observations are all manifestations of a single natural phenomenon.