Two papers in Geology this month cast serious doubt on assumptions used to date rocks.

Hundreds, Not Hundreds of Thousands

A “giant ore deposit” in Hungary thought to require hundreds of thousands of years of slow, gradual deposition to form has been re-examined.  Conclusion: it formed in just hundreds of years due to the action of microbes.  The abstract posted today in the journal Geology says it all:

The Úrkút (Hungary) manganese (Mn) ore, hosted by Jurassic black shale, was studied using high-resolution mineralogical, microtextural, and chemical methods. Two independent superimposed biostructures were identified consisting of rhythmic laminations that provide important proxies for paleoenvironments and duration of ore formation. Millimeter-scale laminae reflect a depositional series of Fe-rich biomats, mineralized microbially produced sedimentary structures. These biomats formed at the sediment-water interface under dysoxic and neutral pH conditions by enzymatic Fe2+ oxidizing processes that may have developed on a daily to weekly growth cycle. The early diagenetic sedimentary ore is composed of Ca rhodochrosite, celadonite, and smectite, and also shows a 100-μm-scale element oscillation that produces Mn(Ca)-rich and Si(Fe clay)-rich microlaminae. This microlamination may reflect a 10 h to daily rhythmicity produced by the growth of microbial communities. If true, then the giant Úrkút ore deposit may have formed over hundreds of years, rather than hundreds of thousands of years as previously thought.

Source: Polgári et al., Microbial action formed Jurassic Mn-carbonate ore deposit in only a few hundred years (Úrkút, Hungary), Geology, 10.1130/G33304.1 v. 40 no. 10 p. 903–906 .

Cosmogenic Clock Reset

A dating method that relies on constant bombardment by cosmic rays has new troubles.  Geologists had thought that cosmogenic radiation damage in the rocks accumulated at a steady rate.  They overlooked the scrambling of data due to debris flows, reports a Swiss team.  In Geology, the abstract of their paper, “Debris-flow–dependent variation of cosmogenically derived catchment-wide denudation rates” explains the problem:

Catchment-wide denudation rates (CWDRs) obtained from cosmogenic nuclides are an efficient way to determine geomorphic processes quantitatively in alpine mountain ranges over Holocene time scales….

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