Crucial Geological Proxy May Be Misinterpreted
Many inferences about past climate and evolution depend on a measurement with dubious interpretations.
Two scientists just published a bombshell paper in PNAS that could undermine many inferences about past climate and evolution. Geyman and Maloof title their paper, “A diurnal carbon engine explains 13C-enriched carbonates without increasing the global production of oxygen.” Specialists in paleogeochemistry and paleoclimate should pay attention. Evolutionists should worry.
Since nobody can observe the past in a time machine, scientists use proxies to make inferences about past conditions. A proxy is a measurement that theoretically traces what happened in the unobservable past. Examples are tree rings, stalactites, and isotopic ratios in sediments and ice cores. One proxy is particularly important, because it is widely trusted in everything from evolutionary theory to climate history. Its scientific shorthand is δ13C (delta-carbon-13), a measure of isotopic carbon ratios as they change over time (the Greek letter delta is shorthand for “change” in scientific lingo). Carbon, which is element 12 in the periodic table with 6 protons and 6 neutrons, has two famous isotopes. Carbon-14, with two extra neutrons, is unstable and radioactive, and used for ‘radiocarbon’ dating. Carbon-13 has one extra neutron. It is a stable isotope, but much less abundant than carbon-12. Because of its slightly heavier atomic weight, some processes can increase its abundance relative to carbon-12 in certain conditions. This is called fractionation: carbon-13 exhibits a higher fraction of total carbon than normal. One of these processes is photosynthesis.
When geologists or climatologists see an “excursion” in δ13C in the sediment record, they want to know how it got there. Theories rush into the vacuum of the unknown. The most popular theory is that photosynthetic organisms concentrate carbon-13, such that inorganic carbon will have less of it than organic carbon. Why is this important?
The δ13C of shallow carbonates is a crucially important tool for global chemostratigraphic correlation, especially before the appearance of index animal fossils.
Based on measurements of sediments taken now, scientists routinely envision the chemistry of the oceans 500 million Darwin years ago and older. For example, they believe that rising oxygen in the oceans (inferred from δ13C measurements) triggered the Cambrian explosion. The extra oxygen “permitted” complex life to evolve after billions of years of low-oxygen water held them back (so goes the just-so story). Climatologists, too, use carbon-13 ratios to infer past climate conditions, teaching the peasants about geologic eras when temperatures were hot or cold, and when animals survived droughts and ice ages. A lot of interpretive weight is put on those carbon-13 excursions!
Geyman and Maloof have a simpler explanation. It involves days, not millions of years. They identified a “diurnal cycle” that can enrich or deplete carbon-13 in shallow water sediments – the very locations where geologists typically measure it, or infer that the rock strata were formed in such sediments. (Deep marine sediments cannot be used because of plate tectonics and subduction.)
We present stable carbon isotope (δ13C) data from modern carbonate sediment that require a decoupling of the carbon cycles in the global ocean versus shallow carbonate shelves. This realization is important because, for the first 97% of Earth history, many inferences about global paleoclimate and seawater chemistry rely on interpretations of shallow carbonates. We use modern observations and a simple model to show how ordinary diurnal carbon cycling in shallow waters is sufficient to produce anomalously positive δ13C on shelves today, and in the geological record. Our results alleviate the need to interpret positive δ13C excursions in the geological record as global reorganizations of the carbon cycle and instead link δ13C to local and/or global paleoenvironmental and paleoecological controls.
If Geyman and Maloof are right, a good deal of interpretation about past climate and evolution could be vulnerable to falling like a house of cards.
Evaluating the significance of this paper is going to take deeper analysis than is possible here. We want to bring it to the attention of non-Darwinian geologists in order to stimulate discussion and raise questions. Here are some possible questions:
- What are the extremes of interpretation involved? (i.e., no significance vs highly significant).
- What affect could this have on dating the geologic column?
- How does the new theory affect interpretation and dating of ice cores?
- What does it do to the interpretation of early earth conditions before fossils?
- How does it affect the theory of evolution?
Laymen need to be cautious not to run off announcing a ‘scientific revolution’ regarding the age of the earth, climate, or the fossil record. When investigating a proxy like this, there are all kinds of complicating factors: e.g., the effects of temperature on deposition, the abundance of photosynthetic life, variations in erosion, saturation changes of carbon in sea water, how that affects minerals, and much more.
Creationists need to acknowledge that we weren’t there during the “early earth” (however old one believes it is), and so we have similar issues understanding what δ13C means. Knowledgeable geochemists should first study the paper and determine its potential significance.
For now, the take-home message should be caution about proxy measurements and interpretations of the unobservable past. What scientists confidently assert about earth history may rely on a “broken reed of a staff which will pierce the hand of any man who leans on it” (II Kings 18:21).