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. These rate estimations assume steady geomorphic processes. Here we use a time series (3 yr) in the Aare catchment (central Swiss Alps) to test the impact of spatially heterogeneous stochastic sediment supply on CWDRs. Our results show that low-frequency, high-magnitude debris-flow events significantly perturb cosmogenic nuclide (10Be, 14C) concentrations and thus CWDRs. The 10Be concentrations decrease by a factor of two following debris-flow events, resulting in a doubling of inferred CWDRs. The variability indicates a clear time and source dependency on sediment supply, with restricted area-weighted mixing of sediment. Accordingly, in transient environments, it is critical to have an understanding of the history of geomorphic processes to derive meaningful CWDRs. We hypothesize that the size of debris flows, their connectivity with the trunk stream, and the ability of the system to sufficiently mix sediment from low– and high-order catchments control the magnitude of CWDR perturbations. We also determined in situ 14C in a few samples. In conjunction with 10Be, these data suggest partial storage for colluvium of a few thousand years within the catchment prior to debris-flow initiation.
Source: Kober et al., Debris-flow–dependent variation of cosmogenically derived catchment-wide denudation rates, Geology, doi: 10.1130/G33406.1 v. 40 no. 10 p. 935–938.
Most of us learn the lesson, “never assume,” the hard way. Both dating methods assumed simple, uniform, slow-and-gradual processes produced these deposits. In one case, errors were found at least 3 orders of magnitude, leading to the conclusion that a giant ore deposit took only a few hundred years to form, not hundreds of thousands of years. In the second case, assumption of “steady geomorphic processes” was exaggerated by a factor of two or more.
None of this is to allege that these geologists have changed their minds about the standard evolutionary billions-of-years timeline. The first authors, for instance, believe the quick deposition of the ore occurred in the Jurassic, over 100 million years ago. The second authors are only cautioning about calculating dates with steady-state assumptions. The lesson here concerns philosophy of science: measurements by fallible humans who weren’t there and don’t know all the factors can produce erroneous conclusions that become ensconced in textbooks as The Truth About the World. No; the second paper states: “it is critical to have an understanding of the history of geomorphic processes to derive meaningful” rates. But there’s the rub; is this possible? What human being is capable of gaining “an understanding of the history of geomorphic processes” for any location, when he or she was not there to watch? There could always be some other geomorphic (landscape-forming) process that was not considered, such as the debris flows discussed here. What else will be suggested in future studies? Can anybody except an Eyewitness ever claim that, yes, “now we know” ALL the processes that contributed to a given landscape? The question answers itself.