The Mesoproterozoic
"... never in the course of Earth’s history did so little happen to so much for so long."
Buick et al. (1995), quoted in Huntley et al. (2006).
The Mesoproterozoic Era of the Proterozoic Eon: 1600 to 1000 million years ago
The Mesoproterozoic was the first period of Earth's history with a respectable geological record. Continents existed in the Paleoproterozoic, but we know little about them. The continental masses of the Mesoproterozoic are more or less the same ones that are with us today. This was the Era of the formation of the first identifiable supercontinent, Rodinia; the first large mountain building episode about which we have detailed knowledge, the Grenville Orogeny (although the Paleoproterozoic Wopmay Orogeny might also qualify); and the high point of the stromatolites, huge mushroom or tree-like colonies of bacteria. It was also the Era when cells discovered sex and, possibly, the joys of communal living as Metazoan organisms. Finally, it was an Era of apparently critical, but still poorly understood, changes in the chemistry of the sea, the sediments of the earth, and the composition of the air. Most significantly, oxygen levels had risen to perhaps 1% of today's levels at the beginning of the Mesoproterozoic and continued rising throughout the Era.
The basic outline of Mesoproterozoic history looks like this:
The subdivisions of the Mesoproterozoic are, obviously, arbitrary divisions based on time. They are not geo- or biostratigraphic units. The base of the Mesoproterozoic is defined chronometrically, in terms of years, rather than by the appearance or disappearance of some organism. This gives us an illusory sense of certainty. Radiometric dating is a good tool, and gets better each decade. However, it creates certain problems. As a practical matter, radiometric dates have an error margin of 1-2%. That sounds good, but it means that two sites, both measured to be at the exact base of the Ectasian, might differ in age by over 50 My. Since the Ectasian is only 200 My long, that's a serious matter. And this accounts only for random error. Systematic errors can be caused by extraterrestrial events, by geochemical or biochemical sorting of isotopes, and human error. Thus far, biostratigraphy has usually proved considerably more exact. In addition, a thoughtful choice of biological marker can be used as a signal to expect a whole host of ecological changes. The difference between a Changhsingian and an Induan deposit isn't just a matter of a few years. The world changed hugely at the end of the Permian.
By contrast, the transition from Calymmian to Ectasian has no meaning beyond calendar time. The usual reason given for the use of a chronometric system is that we don't have enough biological activity or geochemical change to find useful markers. That is a position which is now a little uncertain and is going to become increasingly tenuous over the next few years. For example, we have a number of good potential markers in the rise and decline of "Christmas tree" stromatolites, in the coming and going of banded iron formations, the appearance of stable carbon isotope (13C) excursions, and so on. These have real meaning for the geologist and paleontologist.
For that matter, we are not completely without biological markers. There has been considerable progress in studying and identifying fossil bacteria and Eukarya. The cyanobacterium Archaeoellipsoides is one relatively common form, apparently known from several species. It is probably related to the extant Anabaena and indicates the presence of significant free oxygen. Oxygen levels also had significant effects on ocean chemistry: increasing continental weathering rates and providing sulfates and nitrates as nutrients. It would be remarkable if this didn't result, in turn in new populations of both bacterial and eukaryotic organisms. Since the presence of these cells would be tied directly to important geochemical events, they would make ideal organisms for biostratigraphy.
Clearly, we can't set our clocks by hypothetical organisms depending on, as yet, poorly understood chemistry. We don't know nearly enough about these matters yet. Still, it may not be long before we should think about telling Proterozoic time a different way.
ATW041001. Text public domain. No rights reserved.
Resources
Chemistry of the Mesoproterozoic Ocean and Links to Biospheric ... brief & technical, but useful summary of some important geochemistry.
The Grenville. An incredible collection of information and links on the Grenville Orogeny.
Kah - Introduction: the mainpage of Prof Kah's superb website.
last modified ATW041001, edited RFVS111203
checked ATW050821
