Let’s get one thing out of the way right now: The question of whether or not a new geologic epoch has “really started” is precisely as stupid as the question of whether or not Pluto is “really” a planet. The definitions of geologic eons, eras, and epochs are not objective truths about the history of the planet that are simply waiting for us in the rock. The geologic time scale is made up by geologists, for geologists as a matter of convenience. So Greg Laden is making a category error when he dismisses both the Anthropocene and the Holocene as scientifically invalid – the worst thing you can really say is that they’re inelegant.
Okay, the Holocene is inelegant, and the Anthropocene is unbearably narcissistic. What does that mean to our future cockroach geologist overlords? And am I really going to argue that the geologic time scale is racist?
When constructing a geologic time scale, we try to align its divisions with what we think are important changes to the character of rocks and fossils, and we want to be consistent about mapping big changes to big divisions and little changes to little divisions. We also want to set up a time scale that is useful for scientific communication among geologists, so we try to reflect the divisions and terms in actual use.
This is where we can get into trouble. For example, in North America, sediments from 360-320 million years old are mostly limestones which outcrop in the Mississippi River Valley. Sediments from 320-300 million years old, by contrast, form many of the coal mines in the Eastern U.S. To reflect what seemed to be a natural division, North American geologists divided these rocks into the Mississippian and Pennsylvanian periods. Meanwhile, Western Europe from 360-300 million years ago was one long unbroken organic-rich sediment deposition machine, so European geologists simply called that period of time the Carboniferous. Eventually, the geographic peculiarities of North America were enshrined in the time scale as the Mississippian and Pennsylvanian sub-periods of the Carboniferous. If there are natural divisions in the Carboniferous stratigraphy of Asia, Africa, Australia, or South America, we don’t see them in the time scale because
racism colonialism geologists from those continents did not have much influence when the time scale was first put together.
Regardless of its place within our preferred metanarrative of time, the Holocene exists because it has a practical purpose. Of course, many of the Holocene’s charms are restricted to temperate latitudes: if you live in an area where the most recent Ice Age cycle crapped all over your landscape, then of course you will want to draw a line at the place where “glaciers ate my homework” is no longer a plausible excuse.
I have to admit, I’m partial to the Anthropocene. If we adopt it, though, we leave the Holocene looking not just inelegant, but so enfeebled as to be utterly useless. At 11,000 years long, the Holocene would be ten times as short as the next-longest Cenozoic epoch, the Pleistocene – which, at 1.8 million years, is piddling short in its own right. Plus, the Holocene looks much like any other Pleistocene interglacial period. Wooly mammoths, saber-tooth cats, and giant sloths may have gone extinct near the base of the Holocene, but even though they were awesome, I don’t think they were widespread enough to make good index fossils. There is absolutely no way such a piece of crap epoch as the Holocene will survive the scrutiny of our future cockroach geologist overlords.
So assuming that cockroaches prefer to see Earth history as a series of distinct periods rather than as a continuum, will they lump the Holocene in with the Pleistocene, or with the Anthropocene? My bet is on the Pleistocene – the changes to atmospheric chemistry, climate, and sedimentation pointed out by Zalasiewicz et al., and the spread of uniquely anthropogenic fossil markers (beer bottle caps), are all global phenomena whose geologic record will be readily accessible, making the Holocene-Anthropocene boundary a very good candidate for a golden spike. If it turns out that cockroach geologist children are reeeeeeally enamored of the Pleistocene megafauna, though, all bets are off.