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The Gal谩pagos Islands are home to a tremendous diversity of plants and animals found nowhere else in the world. But why this is, and when it all began, remains something of an open question. Now scientists may have at least one more piece of the puzzle. According to , the geologic formation of one particular part of the archipelago--the part responsible for the huge biodiversity--formed, approximately 1.6 million years ago.
The lead author of the study is Fellow Kris Karnauskas, who you might say has a thing for these islands. He鈥檚 studied them extensively, authoring six peer-reviewed scientific papers with 鈥淕al谩pagos鈥� in the title. But one question in particular kept nagging at him: When did the Gal谩pagos become the Gal谩pagos?
鈥淚 asked around and couldn鈥檛 get a straightforward answer,鈥� says Karnauskas, who鈥檚 also an Assistant Professor in the Department of Atmospheric and Oceanic Sciences at the University of Colorado Boulder. 鈥淢y geology friends said anywhere between half a million to twenty million years ago, depending on what feature we鈥檙e talking about.鈥�
The age of one particular island, or even the whole chain, wasn鈥檛 quite what Karnauskas was looking for. 鈥淚 wasn鈥檛 really interested in when the very first island breached the surface, but when this ecosystem developed,鈥� he says. He wanted to put a finger on the geologic event or moment that turned the Gal谩pagos from just another set of ordinary oceanic islands into one of the most biologically diverse spots in the world. 鈥淭hat鈥檚 not the customary way to ask questions in geology, nor does it lend itself to the usual toolbox.鈥�
Reading the current
To start with the basics, the Gal谩pagos sit on the Nazca tectonic plate, off the coast of South America. The plate is slowly moving from west to east (about 4 cm each year), and happens to be traveling over a hotspot, a point at which magma from the Earth鈥檚 core makes it all the way through the crust, forming volcanic islands. The oldest of the Gal谩pagos islands, now eroded and no longer above water, is millions of years old; the youngest island, farther west, currently sits on top of the hotspot.
Karnauskas and his colleagues hypothesized that the critical event that caused a biological explosion in the Gal谩pagos came about when the Equatorial Undercurrent (EUC) began colliding with the archipelago. The EUC is a current that, because of the laws of physics鈥攖he shape of the Earth and the way it spins鈥攊s virtually stuck to the equator. But what happens when something gets in the way?
鈥淭hat鈥檚 what occurred with the Gal谩pagos,鈥� says Karnauskas. At some point, a large enough island (or possibly a cluster of them) rose high enough from the seafloor to block the current. Today, it鈥檚 the island of Isabela that serves that role. 鈥淚t鈥檚 a pure accident of geography that Isla Isabela is so large and stands right on the equator, right where the EUC is trying to pass through. This is enough to drive cold, nutrient-rich water up to the surface where it can fuel marine productivity. We can easily see it today from space; the water is very cold and productive just west of the Gal谩pagos along the shores of Isabela. It鈥檚 no surprise that you鈥檒l find all the penguins jumping in the water there.鈥�
A helping hand
Finding out exactly when the Gal谩pagos blocked the EUC required help from some the paleoceanography community. Karnauskas and his colleagues used previously collected data from sediment cores鈥攄eep samples of the sea floor鈥攖hat had been pulled up from sample sites near the Gal谩pagos Islands and South America. The data files, which are hosted by NOAA Boulder's National Centers for Environmental Information, provided information on changes in sea surface temperatures over millions of years.
Lo听and behold, approximately 1.6 million years ago, they saw shifts in the chemical composition of the fossil bugs in the sediment suggesting a significant change in those temperatures. Cold water that had once been upwelling off the coast of South America was suddenly upwelling along the western shores of the Gal谩pagos instead. That sounded familiar to Karnauskas and coauthors; they knew from their own model experiments conducted over the past decade that this was the fingerprint of the Gal谩pagos blocking the EUC. Coauthor Eric Mittelstaedt, Assistant Professor of Geological Sciences at the University of Idaho, then developed a new computer model of the archipelago鈥檚 geologic evolution; by combining that model with Karnauskas鈥� ocean circulation model, the team was able to independently corroborate the timing.
At that moment in time (geologically speaking, of course), the Gal谩pagos ecosystem was forever changed. Since the EUC could no longer keep going straight toward the mainland, some of it rushed upward, carrying with it those cold, nutrient-rich waters to the surface, and creating conditions in which the fish, plants and penguins that now call the island chain home could thrive.
鈥淭ypically, we use known geologic constraints to help explain past changes in the environment such as ocean circulation,鈥� says Karnauskas. 鈥淚n this case, we flipped the problem on it鈥檚 head, combined models that aren鈥檛 normally combined, and discovered a new constraint for piecing together the bigger picture of the evolution of, and on, the islands over time. It contributes a unique data point not only for geology but also for ecology and biogeography鈥攚here and when life is distributed.鈥�
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Gal谩pagos giant tortoise at the Charles Darwin Research Station. Photo by Kristopher B. Karnauskas
