Every sixty million years, the biodiversity of our planet's oceans mysteriously crashes. This strange boom and bust cycle goes back 500 million years, and we now might know why: rising continents make the oceans too shallow for species to survive.
The key to this mystery, according to new research led by University of Kansas physicist Adrian Melott, is the isotope strontium-87. This is one of four stable isotopes of the element strontium, although its less common (7.0%) than strontium-86 (9.86%) and much, much less abundant than strontium-88 (82.58%). The researchers, which also included paleontologist Richard Bambach and earth scientists Kenni D. Petersen and John M. McArthur, found that the concentration of Sr-87 relative to Sr-86 in marine fossils seems to increase every 60 million years, in lockstep with the periodic wave of extinction.
According to Melott, one way to produce Sr-87 is through the radioactive decay of the element rubidium. We know that rubidium is common in the igneous rocks of the continental crust, so the spikes in Sr-87 imply that something was happening to those igneous rocks every 60 million years. If these rubidium-rich rocks underwent an unusual erosion event, lots of Sr-87 would be released and dumped into the ocean, and that's what the researchers are seeing in the marine fossils.
The question then is what could be causing such massive erosion. Melott suspects the continents were actually undergoing a process known as tectonic uplift, which describes a number of different possible geological processes in which the elevation of a region increases. Melott explains how this process might play out:
"Continental uplift increases erosion in several ways. First, it pushes the continental basement rocks containing rubidium up to where they are exposed to erosive forces. Uplift also creates highlands and mountains where glaciers and freeze-thaw cycles erode rock. The steep slopes cause faster water flow in streams and sheet-wash from rains, which strips off the soil and exposes bedrock. Uplift also elevates the deeper-seated igneous rocks where the Sr-87 is sequestered, permitting it to be exposed, eroded, and put into the ocean. What we're seeing could be evidence of a 'pulse of the earth' phenomenon. There are some theoretical works which suggest that convection of mantle plumes, rather like a lava lamp, should be coordinated in periodic waves."
The notion of a "pulse of the earth" caused by this convection process deep within the Earth is intriguing, but how exactly is it causing all these extinctions? Well, as the continentals periodically increase in elevation, the ocean depths along the continental shelf would in turn become much shallower. Most marine species are found along the continental shelfs, and the sudden loss of available space could account for these periodic extinction events. It's far from the only explanation, and indeed the researchers say these results only definitely apply to North America. But it's still a fascinating possibility - particularly when it all flows from the changing fortunes of a single isotope.