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Monday, October 31, 2011

Gasping for oxygen, and an argument: AiG's 'News to Note' and the Permo-Triassic Extinction

As an aside to my current series of posts, I thought it might be worth commenting on a recent news update at Answers in Genesis (News to Note, Oct. 22). Therein, Dr. Elizabeth Mitchell tried unsuccessfully to subvert the work of Dr. Ariel Anbar (Arizona State University) and his research group to corroborate her own view that a recent, catastrophic flood deposited nearly all Phanerozoic strata. According to Mitchell, this includes Permian and Triassic rocks, which are differentiated by an abrupt disappearance of most fossil groups. The Permo-Triassic extinction event is the largest known from geologic history, and was responsible for the loss of some 70% of terrestrial species and more than 90% of marine species (including the iconic trilobite).

Ocean anoxia has long been cited as a major cause of the mass extinction, since there is widespread evidence for the deposition of black shales, a shutdown in marine productivity, and enhanced burial of organic carbon—all of which precede or coincide with the extinction horizon. Black shales are extremely rich in organic material, which can only be preserved in low-oxygen conditions. They also provide evidence of slow burial, since rapid sedimentation tends to 'dilute' the concentration of organic remains from microscopic marine organisms (e.g., algae, phytoplankton, radiolarians). Carbon isotopic signatures and the abundance of trace metals have been used to monitor the recovery of marine microorganisms after the extinction event. Geologists have been eager, however, to refine this hypothesis quantitatively. What caused the ocean anoxia? How long did it last? How much of the ocean became anoxic?

A recent study by Brennecka et al. (2011) employed a rather novel technique in the study of ocean anoxic events. In short, two major isotopes of uranium (238 and 235) are naturally separated (fractionated) to small extent when positively charged uranium (6+) is reduced (i.e. gains electrons) in the absence of oxygen. The latter form (tetravalent uranium, with a charge of 4+) is highly insoluble in water, and so is drawn down into the sediments. Since the heavier isotope (238U) is preferred in the tetravalent form, sediments deposited under anoxic conditions will yield a slightly higher isotopic signature (δ238U). Conversely, the uranium isotopic signature (δ238U) of the ocean should decrease when anoxic conditions are sufficiently prevalent to perturb the uranium cycle.

Uranium is present in low concentrations in the ocean, so the time needed for uranium isotopic signatures to respond to anoxic events is relatively short (compared to isotopes of carbon, for example, which take upward of ~100,000 years to equilibrate). This makes it an ideal proxy for quantifying past changes in ocean chemistry. Only recently have uranium isotopes been employed in the study of paleoceanography, however, since the technology required to precisely measure δ238U was not previously available (Montoya-Pino et al., 2010). Brennecka et al. (2011) analyzed δ238U in a section of carbonate rock (limestone) from southern China that contained the Permo-Triassic boundary. They concluded that anoxic conditions increased by ~6 times normal for the Permian ocean, and further that these conditions prevailed for only ~50,000 years following the mass extinction. Previous hypotheses assumed a more pervasive and long-lasting event.

"Abruptness of ancient oceanic alterations fit the Flood"?

Dr. Mitchell's response to the article was unfortunately ignorant of the geology/chemistry behind the study. When summarizing key assumptions, for example, she states:

"[Brennecka et al., 2011] assumed that there was an “isotopically constant U input from rivers..." as well as “a constant isotope fractionation between seawater” and the various places where precipitated uranium gets deposited. In other words, their interpretation that sudden global depletion of oceanic oxygen caused mass extinction assumes that nothing happened to suddenly change the amount of water flowing into the sea or to stir up the oceans more than usual."

There is a logical disconnect in Dr. Mitchell's reasoning. Changes in the flux of water (and hence dissolved uranium) into the oceans would not affect the uranium isotopic signature of the oceans to any significant degree. The average δ238U value of crustal materials (basalt/granite) is -0.3‰, while the value of dissolved U in modern seawater is -0.4‰ (Montoya-Pino et al., 2010). The difference is almost too small to be measured. One can visualize the effect by analogy of mixing paints at the hardware store. Adding one bucket of yellow paint (U in rivers) to another bucket of yellow paint (U in the oceans) yields one large bucket of yellow paint. Dr. Mitchell has essentially noted that the authors 'assumed that mixing yellow paint with yellow paint would not make the paint green'.

The assumption that uranium input from rivers was "isotopically constant" has nothing to do, therefore, with how much water enters the ocean. Dr. Mitchell does not explain, moreover, why "stir[ring] up the oceans more than usual" would falsify the assumption that isotopic fractionation remained constant between oxic and anoxic environments. Isotopic fractionation depends rather on the geochemistry of the water: temperature, pH, and concentration of dissolved ions. Temperature has little effect on the fractionation of uranium, because the mass difference between 238U and 235U is negligible. Since uranium is deposited in ionic complexes with carbonate anions (CO32-), the latter two factors would only affect isotopic fractionation at very low carbonate concentrations (unrealistic for marine settings). The isotopic exchange of uranium between uranyl species (i.e. the oxidation and reduction of UO22+ in oxic/anoxic environments) is thus favored as the primary cause of isotopic fractionation.

Dr. Mitchell must explain, therefore, how δ238U could suddenly shift negative by 0.28‰ in a carbonate sequence that was supposedly deposited catastrophically during the Flood. Instead, she overlooks the significance of the data and appeals to her starting position (petitio principii):

"The biblical record however tells of a sudden global change in the oceans—the Flood. The global Flood not only sends all the stated assumptions by which the investigators have interpreted their data out the window but actually explains their findings."

We have already seen how this scenario does nothing to falsify the assumptions of Brennecka et al. (2011), so one is left to wonder how Dr. Mitchell got from point A to point B. What changes specifically does she refer to? We are completely justified in asking for a viable, scientific explanation for the observed trends in uranium isotopes according to her views on geologic history. But Dr. Mitchell's silence is telling; she does not understand the data and so she cannot explain them scientifically. She continues:

"The Permian layers are at the top of the Paleozoic rock sequence, a sequence dominated by marine invertebrate fossils. In the upper layers of these Paleozoic rocks, amphibians and land animals do make their appearance."

Paleozoic rocks are no more "dominated by marine invertebrate fossils" than the overlying Mesozoic sequence. It just so happens that terrestrial depositional environments are better preserved in the Mesozoic (a phenomenon predicted by plate tectonic theory). Numerous species of land animals (and plants) are known from the Paleozoic, but they are less familiar to most people outside the field of paleontology. Nonetheless, should we be surprised that terrestrial organisms become more diverse and abundant in time? Is there an argument hidden in these blank, misleading statements? She writes:

"The Paleozoic...layers...are dominated by marine creatures because those would have been the first buried by oceanic upheavals as the earth’s crust cracked as described in Genesis 7:11. The distribution of fossils in the higher layers would have depended in part upon animals’ abilities to flee the rising waters."

Technically, land animals and plants should have been the first to be buried, since they would be the first to be overwhelmed with sediment-saturated waves from the ocean masses. Apparently Dr. Mitchell is unfamiliar with the effect of tsunamis on land inhabitants—how exactly does one's "ability to flee the rising waters" of such catastrophes factor in? At best, we might expect to find fossils organized hydrodynamically—intermingled marine and terrestrial forms—but we find just the opposite. Fossils are not sorted by the forces of flowing water, but divided neatly by ecological habitat. Dr. Mitchell's uninformed musings aside, I can't help but to notice that she has completely evaded any meaningful discussion of uranium isotopes and the Permo-Triassic boundary. Perhaps there is hope in her final paragraph:

"If these abrupt changes in Permian uranium are a snapshot of abrupt global changes at the time those Permian rock layers were laid down, then those changes are a snapshot of the turbulent conditions of a part of the Flood year, perhaps even related volcanic outpourings of lavas and chemical-laden hot waters at the time. The Bible explains these sudden catastrophic changes to the earth’s surface, the resulting massive death toll, and apparently some significant geochemical changes as well."

It pains me to think how many readers will take these words for granted—uncritically and without reservation. Dr. Mitchell's description has no scientific basis and appeals rather to the ignorance of her audience (i.e. she must assume that her audience does not grasp the science behind the study—a very disingenuous move). She implies here that "volcanic outpourings" may have something to do with the results of the study. What is the connection? Please tell us! In fact, there is no connection—these phenomena would have no effect on uranium isotopes in Permo-Triassic carbonate sequences. Dr. Mitchell thus appeals to ignorant conjecture over against the evidence she cites. Yet because of her credentials (which have nothing to do with geology), many a reader will conflate the two. Worst of all, her association of the biblical text with a false interpretation of geological data is counterproductive to the gospel message. It is hardly surprising that so many have deemed the church "antagonistic to science".

The big picture: why study ocean anoxic events in the first place?

To conclude, I want to consider briefly how Dr. Mitchell begins her review:

"While the cause of this extinction event has eluded secular geologists, hypotheses have generally held that millions of years of oceanic oxygen depletion preceded the deaths."

Evidence for severe climatic change at the Permo-Triassic boundary is overwhelming. A more difficult task is assigning the proper causal relationship between these changes and the extinctions that accompanied them. Ocean anoxia is but one among several factors that may have been partially responsible for the sharp reduction in marine taxa at the end of the Permian. The new finding that anoxia may have been more abrupt than previously hypothesized is very helpful, but does not preclude other factors from playing a part. All in all, it is fairly misleading to say that the cause(s) of the extinction has/have "eluded secular geologists".

One of the key words in the study by Brennecka et al. (2011) is 'sensitivity'—that is, sensitivity of the oceans and its life to environmental changes. The relevance of this study to our own time cannot be overstated, and I think our time would be better spent in awe of the fragility of life, and our ultimate responsibility over it. One of the best ways to appreciate our current position (or predicament) in the cosmos, I think, is to study the geologic past in light of the commission found in Genesis 1:26.

References Cited:

Brennecka, G. A., Herrmann, A.D., Algeo, T. J., Anbar, A. D., 2011, Rapid expansion of oceanic anoxia immediately before the end-Permian mass extinction: Proceedings of the National Academy of Sciences.

Montoya-Pino, C., Weyer, S., Anbar, A.D., Pross, J., Oschmann, W., van de Schootbrugge, B., Arz, H.W., 2010, Global enhancement of ocean anoxia during Oceanic Anoxic Event 2: A quantitative approach using U isotopes: Geology, v. 38, p. 315–318.

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