University:
Arizona State University
Course:
ABS 370 | Ecology
Academic year:

2024
Views:

190

Pages:

1
Author:

Thuyoul US

BAND IRON FORMATION Proterozoic mineral deposits are layered or stratified, in contrast to Archean vein deposits. Iron, the fourth most abundant element in the Earth's crust, is leached from the continents and dissolved in seawater under conditions of reduction (deoxidation). When iron reacts with oxygen in the oceans, it settles (becomes insoluble) in large deposits at shallow continental margins. Alternating bands of iron-rich and iron-poor sediments give ores a banded appearance, thus encouraging the formation of named banded iron or BIF. These deposits, mined extensively around the world, provide more than 90 percent of the iron reserves that can be mined. As a result, biological activity is responsible for iron deposits because photosynthetic organisms produce oxygen. When plants began to produce oxygen (Figure 37), it combined with iron, kept oxygen levels in the ocean within limits tolerated by early prokaryotes. Throughout the Archean, the amount of oxygen may remain less than 1 percent due to this regulatory mechanism. Then, between 2.5 and 2 billion years ago, photosynthesis produced enough oxygen to react with iron on a large scale. The BIF deposits, consisting of alternating layers of iron and silica, formed about 2 billion years ago at the height of the early Proterozoic ice age. For unknown reasons, the main episodes of iron deposition coincided with periods of glaciation. Average ocean temperatures may be warmer than today. When warm ocean currents rich in iron and silica flow towards glacial polar regions, the suddenly cooled water can no longer hold minerals in solution. As they settle out of seawater, they form alternating layers on the seafloor due to differences in deposition rates between silica and iron, which are the heavier of the two minerals. Once most of the dissolved iron was locked up in the sediment, oxygen levels began to rise steadily, spawning the evolution of more advanced species. Biochemical activity in the ocean is also responsible for stratified sulfide deposits (Fig. 38). Sulfur-metabolizing bacteria living near underwater hydrothermal vents oxidize hydrogen sulfide to elemental sulfur and various sulfates. Copper, lead, and zinc, which were much more abundant in the Proterozoic than in the Archean, also reflect underwater volcanic origins.