It all began with breakfast in the park.Years ago, when I was in the middle of writing my master’s thesis, I took my then future wife on a trip to meet my family and visit the place where I grew up. We were invited to a picnic with some very close friends, whom I had not seen in far too long. The meal was pleasant, but I found myself having to avoid a potentially awkward confrontation upon being asked to explain my research.”Well…”, I formulated by sentence carefully, “I am basically analyzing the chemistry of limestone rocks to interpret how the atmosphere and ocean changed while the rock layers were being laid down.”I wasn’t lying, but I did want to avoid stating explicitly that the rock formation was ~490 million years old or that these changes occurred over the course of 3 million years. You see, this is the family that first introduced me to young-Earth creationism and Flood geology—especially the works of Henry Morris, John Whitcomb, Duane Gish, and others. What they didn’t realize was that I no longer held this view, so I was hoping not to interrupt the festivities with critical examination of my new ‘heresy’. Still, the question came:”Have you considered what this has to do with the Flood?”
I gave my quiet response “No, not really” with hesitation, because this time, it was something of a lie. Before graduate school, I had never heard the term chemostratigraphy, but by this point I knew well that this obscure subdiscipline in geology completely undermined every young-Earth interpretation of the geologic column. Regardless, the field has gone largely unmentioned by Flood geologists, who continue to provide fanciful explanations for how miles of sediment were laid down by Noah’s flood and sorted into neatly organized fossil zones. Since that day in the park, I’ve learned much about chemostratigraphy, particularly the use of stable-isotope geochemistry to correlate sedimentary rocks and study the ancient ocean and atmosphere. It truly is the silent objector to Flood geology, and that’s what I hope to change.
A crash course in chemostratigraphy
Let’s begin with the second half of this term. Stratigraphy is a field in geology devoted to correlating sedimentary rock layers from one point to the next, even across the globe. For a simple example, imagine two mountains separated by a large valley. Each mountain contains four sedimentary layers with unique characteristics. The fact that the same layers appear in the same order allows us to correlate the layers across the valley, despite that the physical connection has been eroded away. This is the basic method by which geologists may construct a geologic column for any given region, and a quick Google image search for “stratigraphic cross section” will yield countless examples.
|Simple cross section with correlation lines, from Wikipedia commons.|
|In the absence of mountainous outcrops, geologists can use boreholes to access the subsurface. Various instruments take measurements down each well to describe the sedimentary layers, and the resulting “well logs” are used to correlate rocks across the subsurface, as in this example from Michigan.|
In addition to matching layers, we might also be interested in their relative and absolute ages. The principle of superposition (i.e. younger layers overly older layers, in their original position) has been utilized since the time when geologists began to divide Earth history into various eras, periods, and epochs. Stratigraphers are the ones who argue, for example, about where to place the boundary between the Jurassic and the Cretaceous periods, into how many stages each period should be divided, and the precise age of each boundary. To accomplish this, stratigraphy borrows from geochronology, which may be used to date markers such as volcanic rocks within the sedimentary sequence.
Determining the relative age of sedimentary strata over a wide region is no simple task, hence oil companies still pay top salaries to professional stratigraphers. The sedimentary characteristics of a given layer (such as its composition) will change, for example, as you trace it over a wide geographic area, or the layer could disappear altogether. Therefore, geologists use additional age markers to define geological periods and sedimentary sequences. Perhaps the best known markers are fossils, and so biostratigraphers are those who use fossil assemblages to identify coeval layers across a great distance.
Geologists typically study the chemical makeup of rocks to obtain details about their origin, but certain chemical ratios are ideal markers to aid in stratigraphic correlation. If the chemical composition of a rock is related to the chemistry of the ocean, for example, perturbations to the ocean system will be recorded in rock layers over time. Sedimentary strata thus work like tape recorders listening to a thunderstorm, during which each lightning strike causes a unique imprint to be made on the tape. Likewise, perturbations to ocean chemistry show up as excursions in the chemical record, so they can used to identify events in geological history and determine which rock layers were laid down simultaneously. This relationship provides the fundamental principle behind chemostratigraphy, which has long been used to correlate ancient strata, particularly those devoid of characteristic fossils.
Carbonate chemostratigraphy: for the love of limestone
Consider now the specific example of carbonates, which make up some 22% of all sedimentary rocks and whose primary mineral constituent is either calcite (CaCO3) or dolomite ((Mg,Ca)CO3). Most carbonates, such as limestone, were deposited in shallow oceans with limited river input—a process that occurs today in places like the Bahamas and the Florida Bay. Limestone is comprised of calcite either in the form of shells from marine critters or minerals precipitated directly from the water column. In either case, the calcium, carbon, and oxygen that combined to make limestone originated from dissolved salts in the ocean. The chemistry of limestone is thus linked organically to the chemistry of the ocean, making it the perfect tool for the chemostratigrapher. But what kind of distinct chemical changes might show up in layers of limestone?
All three elements found in calcite are themselves comprised of multiple stable isotopes. For any given element, various isotopes contain the same number of protons and electrons, but a unique number of neutrons. This causes each isotope to have a different mass, which affects its chemical behavior ever so slightly (the impact is greater for lighter elements like carbon). While geochronologists are interested particularly in radioactive isotopes, because they decay to daughter elements at a known rate, chemostratigraphers mainly study stable isotopes, which never change. Using a mass spectrometer, we can analyze specifically the ratio between stable isotopes in rock samples, such as the relative abundance of “heavy” carbon (13C) versus “light” carbon (12C). In the modern ocean, the average ratio is about 1:99, making heavy carbon by far the rarer form. This ratio is quantified and described by the term δ13C (deviation from a standard value in parts per thousand), which is higher when a sample contains more heavy carbon than the standard and lower when it contains less.
Perhaps the best way to illustrate isotopes of carbon in the ocean is with a bowl of red and green M&M’s, where each color corresponds to a different stable isotope of carbon. For the sake of discussion, this bowl contains precisely 50% green M&M’s (light carbon) and 50% red M&M’s (heavy carbon), for a ratio of 1:1. Now, imagine you leave the room and return later to find that the ratio has shifted to 0.9:1.1, meaning the bowl has been enriched in red M&M’s. There are two possibilities that could explain the shift: either someone added a sample containing more than 50% red M&M’s, or someone removed a sample containing less than 50% red M&M’s. Perhaps you have a child, therefore, who prefers one color to the other, so every handful he takes is biased to that color. This process will leave the bowl preferentially enriched in the other color. If every handful contained precisely half green and half red M&M’s, then the ratio of green to red in the bowl would never change. Likewise, any process that removes carbon from the ocean will change the δ13C value of oceanic carbon, so long as the isotopic ratio of the sample differs from that in the bulk ocean.
There are many processes that add carbon to or remove it from the ocean, but the most relevant to this discussion are the burial of organic carbon in marine sediments and the formation of limestone. Organic carbon is any form derived from living tissues, whether algae, bacteria, or whale remains. By far, photosynthetic organisms in the surface layer of the ocean contribute the vast majority of dissolved organic carbon to the oceans. This fact is important, because the process of photosynthesis prefers light carbon to heavy carbon, so organic carbon is heavily depleted in “heavy carbon” relative to the ocean. Whenever organisms like algae remove CO2 from the ocean and convert it to organic matter, they remove CO2 containing the light isotope of carbon at a slightly greater rate. The preference can be explained by the greater mass of 13C, since the CO2 must be absorbed through a cellular wall/fluid—a process made easier for the lighter molecule.
Since organic carbon contains relatively less of the heavy isotope, removing it in large quantities causes the rest of the ocean to become enriched in heavy carbon (just like your child’s preference for green M&M’s, leaving the bowl a little more red). Normally, this process is offset by the formation of limestone, for which the isotopic preference is the opposite, or the weathering of organic carbon back into the ocean. Perturbations to the normal ocean cycle, however, may cause the carbon-isotope ratio of the ocean as a whole to shift in one direction or another over a long period of time. For example, if photosynthetic productivity in the surface layer was increased for a sustained period (due to enhanced nutrient supply, shifts in ocean currents, etc.) or the rate of sedimentary deposition was increased for a sustained period (due to enhanced delivery of sediments from rivers, climate change, etc.), then the δ13C of the ocean should become more positive (i.e. enriched in the heavy isotope of carbon). This phenomenon is well illustrated by records of the past million years or so, since the δ13C value of the ocean fluctuates predictably (albeit somewhat chaotically) alongside glacial-interglacial cycles.
Chemostratigraphy: ardent objector to Flood geology
Now that you have a grasp on the principles behind chemostratigraphy, consider the implications for the Flood geologist. Since at least the 1930’s, modern ‘creation scientists’ have been working to reinterpret the geologic column in terms of a catastrophic, worldwide flood. These explanations typically involve giant waves of water depositing sediments over wide regions, only to rinse and repeat. Somehow, sediment types were neatly divided to form distinct layers of gravel, sand, silt, clay, and lime, replete with fine sedimentary structures, including mud cracks, eolian dunes, and beachside cross-bedding. Most impossibly, this chaotic process is said to have produced a precise ordering of fossils, which various young-Earth creationists have tried to rationalize through tales of hydrodynamic sorting, ecological niching, stepwise inundation of the continents, and intelligence gaps. Yes, even thousands of species of flowering plants universally managed to outsmart their gymnosperm counterparts and escape the initial waves of the flood (that is, until the end of the Jurassic, around day 160 of the flood).
Despite the persistent arm-waving of Flood geologists on this point, they have not grown tired in the face of numerous counter examples, where fossil zones are simply too precise to be explained by global flood waters. There is no hydrodynamic or ecological factor that could sort foraminifera, pollen, and trilobites into neat zones that occur in the same order from one side of the continent to another. But even if we do grant the impossible scenario, chemostratigraphy provides a final test and falsification.
In the conventional geological interpretation, we can use index fossils and radiometric dating alongside stratigraphy to determine that, for example, a rock layer in Nevada is the same age as a rock layer in southern China (perhaps they both contain a unique assemblage of Cambrian-aged trilobites). According to the flood geologist, these layers were deposited in a single year, less than 5,000 years ago, but were not necessarily deposited simultaneously. If they both contain the same types of fossils in the same order, it can only be due to the living arrangement of organisms prior to the Flood. Regardless of how the order arose, one thing is certain: if these marine organisms were all buried in a global flood, then all of them made their shells from the same ocean and the same reservoir of carbon with approximately the same isotopic ratio. So when fossilized shells of trilobites, brachiopods, molluscs, etc. are analyzed across the Phanerozoic (542 Ma – Present) for carbon isotopes, are they isotopically homogenous (as predicted by Flood geology) or do patterns emerge?
|Phanerozoic evolution of δ13C in seawater, from Veizer et al., 1999.|
As we can see from the figure above, the carbon-isotope ratio in carbonate fossils—and therefore the ocean itself—varied substantially over the past 500 million years. Though excluded from this plot, Precambrian variations are even greater in magnitude, though less frequent. Now, if this range in δ13C values on the order of 6–10 parts per thousand does not seem impressive, consider that to increase the oceanic δ13C value by only 5‰ requires a sustained doubling in the rate of organic carbon burial for about 1 million years. Because the carbon reservoir in the ocean is so large (today, about 39,000 billion tons of carbon), the color of this bowl of M&M’s does not change appreciably on a whim—certainly not in the space of a 370 days. Therefore, Flood geologists are left with the impossible task of explaining two features of this plot:
- Variations in the carbon-isotope ratios of fossils are far too great to be explained by shifting ocean chemistry within a single year, meaning these organisms could not have lived in the same ocean at the same time.
- The pattern of carbon-isotope variations from Cambrian to Quaternary is the same across the entire globe. Whether you’re sampling rocks from Texas or Tanzania, layers of limestone determined to be the same age according to their fossil content also exhibit the same pattern of δ13C values over time. These values are invariably high for Permian-aged carbonates and invariably low for Ordovician-aged carbonates.
|Figure 2 from Saltzman et al. (2005). Composite of carbon-isotope records from carbonate rocks of the Great Basin, USA.|
When we examine carbon-isotope records on a finer scale, the advantage of chemostratigraphy in correlating rock layers becomes more apparent. In this figure from Saltzman et al. (2005), numerous isotopic excursions can be distinguished at various stages in the Paleozoic. These are periods when a perturbation to the ocean system caused the average δ13C value of oceanic carbon to shift for a sustained period of time (perhaps 1–3 million years) before returning to an equilibrium value. Whatever the mechanism, these paleoceanographic events show up in the chemical record like thunderous lightning on an audio recording. Any tape players that happen to be running in the vicinity of the storm will record the same events in the same order. As we examine sedimentary records across the globe, therefore, we can synchronize the various tape players with great precision and even determine which players stopped recording for a brief interval (i.e. a depositional hiatus or unconformity). Therefore, we can be confident that biostratigraphic zones indeed represent unique, coeval periods of Earth history, when particular assemblages of organisms were living together in ancient oceans. Otherwise, we would not find a common signal of ocean chemistry, but a jumbled mess of relatively homogenous isotopic values throughout the geologic column. The concept of Flood geology, therefore, is entirely inconsistent with this peculiar subdiscipline called chemostratigraphy.
In the next post, I will briefly explore the means by which a single isotopic excursion—the Steptoean Positive Carbon Isotope Excursion, or SPICE event—has been traced around the globe, being predicted by the handful of trilobite species that defined the Steptoean stage of the Late Cambrian. In establishing that the oceanic carbon reservoir shifted by 5‰ during a single stage of the Cambrian period, we can be all the more confident that thick formations of limestone were deposited over millions of years—not dozens of days.