The SPICE Event: global disruption in the Late Cambrian carbon cycle

In the last post, we explored how geologists use chemostratigraphy to correlate sedimentary layers, especially limestone, from one part of the world to another. Now, I would like to provide you with a specific example of how this is done. All carbonate rocks (CaCO₃ and MgCO₃) contain carbon, which itself exists in various forms, called isotopes. The ratio of ‘heavy’ carbon (¹³C) to ‘light’ carbon (¹²C) within limestone rock does not change spontaneously over time, because these two isotopes are stable and will not decay to other elements. Unlike oxygen, carbon-isotope ratios are also resistant to diagenetic alteration, such as recrystallization that may occur after burial. Therefore, by analyzing successive layers of limestone (Fig. 1) for their carbon-isotope ratio (represented by the term δ¹³C), we can reconstruct the carbon-isotope chemistry of the oceans over time. This isotopic chemistry is sensitive to geological factors that affect how much carbon enters or leaves the oceans, such as climate, tectonics, sea-level change, and ecological changes. Perturbations to the ancient carbon cycle are thus recorded in chemostratigraphic profiles as isotopic excursions.

Figure 1: How does chemostratigraphy work? Samples of limestone are collected at regular intervals, such as from this Cambrian-age section in western Utah. The limestone is then analyzed by mass spectrometry to determine the stable-isotope ratio (δ13C), and these data are plotted on a time series to reveal geochemical trends.

Story of the SPICE event

The Late Cambrian SPICE (Steptoean Positive Carbon Isotope Excursion) event is one such excursion, first documented in limestone rocks from the western United States. In a span of ~3 million years, the δ¹³C value shifted from around 0‰ to more than 5‰, before returning to equilibrium. Conventional explanations for this isotopic excursion cite the role of organic carbon burial (e.g. Saltzman et al., 1998), especially in the deep oceans. You may recall from the last post that the formation of organic matter via photosynthesis preferentially removes ‘light’ carbon from the oceans, leaving them enriched in ‘heavy’ carbon. If this organic matter remains in the ocean, where in the presence of oxygen it simply decays back into carbon dioxide and methane, then the δ¹³C value of the oceans is unaffected. Using my M&M’s analogy, this would be like taking a handful of candy from the bowl and then just throwing it back (bad manners, but who will notice?). On the other hand, a small amount of organic carbon (~1% of the total produced each year) is removed permanently as it becomes buried in ocean sediments. If the rate of burial increases for an extended period of time, therefore, the relative abundance of ‘heavy’ carbon will likewise increase little by little.

Several factors are known from the Steptoean stage of the Late Cambrian, which may have increased the rate of carbon burial in oceanic sediments. First of all, sea level was falling, reaching its lowest point at the peak of the excursion. When sea level is low, a greater area of the continents is exposed to erosion, which increases sediment supply to the ocean. We also know from strontium-isotope values (a similar tool in chemostratigraphy) that several mountain ranges, which had been growing for tens of millions of years, were being eroded at a greater rate during the Late Cambrian than the preceding ~50 million years.

In addition to increasing sediment supply to the ocean, which increases burial rate, these factors would have also increased the nutrient supply to the surface oceans. The productivity of the oceans (i.e. how fast organisms, especially algae, reproduce) is limited by several nutrients, such as phosphorous, nitrogen, and iron. Normally, the addition of these nutrients to the ocean would be no different than throwing a little fertilizer into a lake: the algae bloom in response, but the extra food supply is quickly used up, after which things return to normal. In a basin so massive as the global ocean, however, there is a potential for positive (reinforcing) feedbacks to cause everything to spiral out of control. The longer the added food supply is sustained, the more organic matter accumulates in the ocean. When that organic matter decays, it uses up dissolved oxygen in the water column. The less oxygen available in the water column, the more likely organic matter is to reach the ocean bottom, where it is buried in the sediments. In addition, anoxic basins release abundant phosphorous back into the oceans, providing even more food to the surface ocean. The more phosphorus is released, the more critters grow, the more critters decay, the greater the extent of anoxia, and the more phosphorus is released—so the cycle continues (Saltzman et al., 2005).

Several researchers took this model, derived primarily from carbon-isotope data, to predict similar trends in sulfur-isotopes during the Late Cambrian. When they analyzed sulfate minerals in limestone, along with pyrite in shales, Gill et al. (2011) found definitive evidence that the SPICE event had caused the oceans to lose much of their oxygen and build up toxic sulfur gas (similar to the modern Black Sea). These data help to explain why so many species of trilobite and other marine organisms disappear abruptly at the end of the Steptoean stage (i.e. the peak of the SPICE event). The story of the SPICE event is consistent, therefore, between disciplines. Paleontologists long hypothesized that an abrupt shift in ocean chemistry occurred during the Steptoean stage, leading to widespread extinction, and the chemostratigraphers provided the definitive evidence to corroborate that hypothesis.

So how did the SPICE event end? Well, there is a flip side to removing lots of organic carbon and pyrite from the oceans: it actually adds oxygen to the atmosphere (see Saltzman et al. 2011). Ultimately, this boost in atmospheric oxygen provides the shut-off switch, or negative feedback, required to reverse the growing anoxia of the oceans. Coincidentally, this “pulse of oxygen” was followed shortly by a diversification of marine animals at the beginning of the Ordovician period. Are the two related? We may never know, but the scenario is more than plausible.

The SPICE event and Flood geology: never the twain shall meet

Isotopic excursions in the sedimentary record provide definitive evidence against the young-Earth interpretation of the geologic column. There is simply no way to explain large shifts in carbon-isotope ratios, either in fossils or in lime mud. But the SPICE event also provides an excellent example of how event stratigraphy can be used to confirm biostratigraphic correlations. Biostratigraphy is the process by which layers of rock are correlated from one point to the next, based on the types of fossils they contain. For the Paleozoic (542–251 million years ago), trilobites and brachiopods are among the most popular fossils used, because so many species exist that are easily distinguishable from one another, and individual species are confined to relatively short intervals of geologic time. Any single stage of the Paleozoic, for example, may be defined by several dozen species of trilobites and brachiopods, which do not appear elsewhere in the geologic column.

But what if biostratigraphers have falsely concluded that the order of trilobite species is determined by evolutionary patterns over long periods of time? What if these various species of shelled organisms actually appear in a certain order due to something like hydrodynamic or ecological sorting during Noah’s Flood? Well, chemostratigraphy provides the ultimate test. Imagine you have two outcrops of limestone rock—one on each side of the world—which have been attributed to a single stage during the Paleozoic based on certain species of trilobites and brachiopods they contain. So far, we are concluding that these layers are the same age, based on the reasonable assumption that these fossil species evolved and then went extinct at approximately the same time across the globe. Now, imagine that you construct a stable-isotope profile from one of the outcrops (as in Fig. 1) and identify a large isotopic excursion. If these layers are the same age, and if the excursion reflects an ancient perturbation to global ocean chemistry, then the same geochemical trends should appear in the outcrop on the other side of the world.

Using the example of the SPICE event, let’s take a look at how these profiles match up, starting with the western United States (Fig. 2 from Saltzman et al., 1998):

And how about from the Appalachian Mountains, around Tennessee (Glumac and Walker, 1998)?

The following compilation by Cowan et al. (2005) from several papers illustrates the excursion in five more locations from around the world:

And this figure from Kouchinsky et al., (2008) reveals that even the Siberian platform recorded the global perturbation:

Finally, the SPICE event shows up not only in the chemistry of limestone, but in the very organic matter being buried simultaneously. This fact provides corroboration of the interpretation given above, because we should expect the carbon-isotope composition of photosynthetic organisms to shift in parallel to the limestone, if indeed both were formed from the same ocean. Figure 4 from Ahlberg et al. (2009) documents the isotopic excursion in organic-rich shales from Sweden:

These figures are but a sampling of the literature available on the Late Cambrian SPICE event, which has also been documented in Texas, Missouri, Argentina, and France. The well known global excursion has aided substantially in stratigraphic correlations and provides a fascinating example of how paleoceanographers can investigate Earth’s dynamic past. In the context of Flood geology, however, the ubiquitous shift in δ¹³C that accompanies unique fossil patterns makes absolutely no sense. We may conclude, therefore, that evidence for the SPICE event—one of dozens of isotopic excursions correlated around the globe—is definitive proof that the geologic column was not formed catastrophically in a worldwide Flood. Creation scientists can speculate all they want about how thick sedimentary sequences were deposited in rapid succession, or how index fossils were buried in the same order on every continent. But these fanciful tales are merely distractions, if one cannot explain why chemostratigraphy continues to work so well.

 

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