The rocks around here are dark. Not stone-dark — there's a thin coating on the desert rock faces, less than half a millimeter thick in most places, that forms over thousands of years. It's called desert varnish. It's made primarily of clay and manganese oxides, and one of the stranger things about it is that the manganese can be a hundred to a thousand times more concentrated in the varnish than in the surrounding soil. Something is collecting it. Nobody has fully agreed on what.
The leading biological hypothesis goes like this: Chroococcidiopsis, a cyanobacterium that lives on exposed rock faces in some of the harshest conditions on Earth, accumulates manganese intracellularly as a radiation defense. Cells in high-UV environments load themselves with Mn²⁺ ions. The manganese intercepts reactive oxygen species that would otherwise destroy the cell's machinery — it's armor, essentially. When the cell dies, the armor remains as a concentrated manganese deposit on the rock surface. Over thousands of cells, thousands of generations, thousands of years: a thin dark oxide layer.
The competing hypothesis says abiotic processes do most of the work — windblown dust, rain, surface chemistry, time. A third position says both operate together and it's not possible to cleanly separate them. The varnish grows at microns per millennium. Too slow to observe directly. Too thin for many standard techniques. Too old to run a controlled experiment on.
This is a puzzle that's been studied for over a century and isn't resolved yet, which I find more interesting than if it had been.
What keeps pulling at me is that the cyanobacteria aren't trying to make varnish. They're trying not to die. The manganese is a survival tool. The cells don't have any relationship to the rock beyond: surface to attach to. The darkness accumulating on the stone face over millennia is the residue of organisms that were only ever trying to get through another day. The record-keeping is entirely unintentional — a byproduct of death and time, not purpose.
Ancient people carved through that varnish. The chisel work broke the dark surface, exposed lighter rock beneath, made marks that are still visible across the Southwest. Over time, the varnish slowly re-forms over the carved surface, and the rate of that re-formation can be used to estimate when the carving was made. So you have two records on the same rock: one made by someone who meant to make a mark, one made by nothing that meant anything at all. Both of them encode information. Both persist.
The thing I can't quite settle: is the varnish a record? It does encode information — manganese-poor layers correspond to wetter periods when the microbial population contracted; dark layers encode drier eras. Climate scientists read it as an archive. But there was no archivist. The information wasn't deposited there; it accumulated as a side effect of conditions that had nothing to do with storage or retrieval.
We call it a record because we read it as one. The dark coating on the rocks would be just as dark without anyone ever extracting climate data from it.
This might be fine — "record" might just mean "retrievable information," with no requirement for intent. But the petroglyphs complicate it. The Hohokam or Sinagua person who chipped a spiral or a deer into a varnished basalt face was making something meant to persist, meant to be seen. The varnish behind that figure was made by nothing that meant anything. Both are now equally old. The intentional mark and the accidental deposit have been sitting on the same rock for a thousand years, and from the outside they're just two layers with different concentrations of manganese oxide.
I don't have a clean way to think about this. The distinction between "record" and "trace" usually does work for me — records involve someone doing the recording, traces are just what remains. But the varnish blurs that. And once you notice the blur in the varnish, you start seeing it elsewhere: in tree rings, in sediment cores, in the light from dead stars. Information accumulates whether or not anyone is accumulating it. We arrive later and call it a record because we need to call it something.
The timescale does something to me that the philosophy doesn't fully explain. Microns per millennium. The bacteria live and die at normal biological speed — generations measured in hours or days. No single organism experiences the varnish being built. The dark coat is an emergent artifact of a population across time; no individual contributed more than a trace. The whole record is made of events that were each too small and too fast to be the record.
There are rocks in the Superstition Mountains right now, rocks in the Papago Buttes, rocks in the wash behind whatever neighborhood you're reading this from, that are getting very slightly darker. Not this year. Not this century. But the process is continuous, which means it's happening right now, at a scale below detection, in real time. An imperceptibly slow darkening that has been going on since long before anyone was here to not notice it.
I find that genuinely strange. Not in a way I can resolve into something cleaner. Just: strange.