Timefulness by Marcia Bjornerud

Timefulness by Marcia Bjornerud

Author:Marcia Bjornerud
Language: eng
Format: epub
ISBN: 9780691184531
Publisher: Princeton University Press


ONE BILLION YEARS OF LASSITUDE

After the upheavals of the GOE, Earth’s atmosphere seems to have settled into a long period of geochemical stability. Although the main period of iron formation deposition ended around 1.8 billion years ago, oxygen levels seem to have remained about constant, and far below the current value, for another billion years after that.15 Such sustained equilibrium—akin to a national economy that experiences no inflation, recessions, or market turmoil for decades—points to a remarkably fine-tuned balance between the oxygen supplied by hardy one-celled photosynthesizers and oxygen consumed by covetous metals, sulfurous volcanic gases, and decaying organic matter. This steady state may have been enforced by a regime of austerity—in particular, severe limitation on the availability of phosphorus, an essential nutrient for all life.

While shallow ocean waters had become oxygenated, there is evidence that deeper reaches remained in the transitional state of the Early Proterozoic. In these stratified conditions, precious phosphorus would have been continuously removed from deeper waters, stolen away on the surfaces of iron minerals, like currency smuggled out of a poor country in the linings of pilferers’ coats. This in turn created chronic shortages of phosphorus in the shallow ocean. Biological productivity was thus kept in check, which limited organic carbon burial and in turn prevented atmospheric oxygen levels from rising.16

This lean eon encouraged organisms to pursue low-phosphorus lifestyles and new recycling strategies. In other ways, however, evolution seemed to be biding its time. The biosphere was diverse but still entirely unicellular; planktonic species—including some eukaryotic giants called acritarchs, up to 0.8 cm (0.3 in.) in diameter—proliferated in the oceans, and stromatolites quietly blanketed coastlines around the world. This peaceful stretch of the Proterozoic Eon has come to be known informally among geologists as the “Boring Billion.” But this Homer Simpson–inspired designation is unfair, and misleading—akin to history books that focus only on war and skip over the much longer intervening periods of peace when “nothing happened.”

First, maintaining such long-term equipoise is something that we humans in the Holocene might look to as a template for amending our own biogeochemical habits, since our looming environmental crises are the result of unchecked consumption of scarce resources and an extreme imbalance between the production and removal of an atmospheric gas. The Proterozoic Earth somehow “understood” the fundamental principles of sustainability; geochemical trading flourished, but all commodities flowed in closed loops—the waste products of one group of microbial manufacturers were the raw materials of another.

Second, the Boring Billion was the period when the durable cores of the modern continents were assembled, as the new plate tectonic system swept together pieces of Archean crust and then constructed additions in the form of volcanic arcs. The basement rocks beneath my feet here in Wisconsin—and buried under younger sediments across the Midwest and Great Plains—are almost entirely Proterozoic, formed by mountain-building events during the Boring Billion, when vast areas of continental crust were annexed to the old Canadian Shield (see figure 11). Boring, perhaps, but it was a productive time of infrastructure development—another practice we modern Earth-dwellers could profitably adopt.



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