October 5, 2009
An important finding was reported last week in the same issue of Science as the new studies of Ardipithecus, and unfortunately, overshadowed by the news of the 4-million-year-old hominid. This finding may turn out to be even more important because it relates not to the evolution of a single species, but to the recovery of life in general on Earth following one of the greatest catastrophes ever.
I’m referring to a paper by Julio Sepúlveda and others called “Rapid Resurgence of Marine Productivity After the Cretaceous-Paleogene Mass Extinction.”
Sepúlveda and colleagues examined marine sediments in Denmark that date to the period following the K-T mass extinction event. That event consisted of an impact on the Earth of a large asteroid 65 million years ago and the subsequent extinction of many species including all the dinosaurs. It is thought that there was a huge drop in the biological activity in the oceans after the event because the sun was largely blocked out, reducing photosynthesis in ocean-living algae. Without sun, the algae would have died off, and without algae, which are at the base of the oceanic food chain, other life forms in the ocean would die off or become very rare. The more widely accepted reconstructions of what happened indicate that this oceanic die-off did indeed happen, and that it took up to three million years for the ecosystems of the open ocean to recover from this impact. (Near-shore ecosystems have been thought to recover much more quickly.) The relatively lifeless post-impact open ocean is sometimes referred to as the “Stangelove ocean” in reference to the character in the apocalyptic movie “Dr. Strangelove.”
That previous research, however, was based on the examination of fossils of marine organisms including algae that leave an easily fossilized “skeleton” of silica, which indeed are sparse for a very long time after the impact. However, it is possible that certain types of organisms that do not leave behind fossils, such as cynobacteria, were abundant and would remain undetected in the fossil record.
The paper by Sepúlveda and colleagues used a different kind of evidence to look for open ocean biological activity and found it, in abundance, possibly within a century after the impact. If this proves to be true, then the darkening of the sky following the impact must have been fairly short term, and the observed long-term disruption of the ocean’s ecosystems must have a different explanation.
“Primary productivity came back quickly, at least in the environment we were studying,” according to Roger Summons, one of the paper’s authors. “The atmosphere must have cleared up rapidly. People will have to rethink the recovery of the ecosystems. It can’t be just [because of] the lack of food supply.”
The method this research team used was to look for isotopically distinct materials in the ocean sediments they examined, as well as molecules that could only have been formed by living things.
The sediments they looked in consist of a 37-centimeter-thick layer of clay in Denmark. Within this clay, which was deposited in relatively shallow near-shore environments, are hydrocarbon molecules produced by living organisms that are reasonably well preserved from 65 million years ago. These molecules indicate the existence of extensive open oceanic photosynthesis that would not have been possible under the “Strangelove ocean” model.
The way the analysis works can be understood this way: The ocean has a lot of dissolved carbon in it. This carbon exists in the form of more than one isotope. An isotope is a version of an element that is only a tiny bit different in its nuclear composition, and most elements lighter than Uranium have multiple non-radioactive isotopes. If there was no life in the ocean, the carbon would reach a certain equilibrium with respect to the proportion of each isotope, so sediments that included carbon would have a predictable ratio of these isotopes. (Note: This has nothing to do with radiocarbon dating. See this blog post for more on the potential confusion about that issue.)
Living forms use carbon, but when carbon is taken from the surrounding environment certain isotopes are incorporated into biological tissue more readily than others. Which isotopes are used and in what way by biological systems, and the exact reason for this, is complex and far beyond the scope of a mere blog post! Suffice it to say that when a geochemist looks at a sample of carbon, using very sensitive instruments, she can tell if this carbon has come from a non-biological system vs. a biological system. Beyond this, it is even possible to tell what kind of biological system is represented.
Sepúlveda’s team was able to tell that the carbon in these post-impact sediments could only have been assembled into these hydrocarbons (and other compounds) in a functioning open ocean ecosystem with plenty of algae photosynthesizing away at a pretty good clip. Since these sediments were deposited right after the impact, the “Strangelove” ocean theory, with a vast lifeless sea, is highly unlikely.
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