Posted on Categories Discover Magazine
This article was originally published in Eos.
New Zealand straddles the boundary of two tectonic plates and as a result is in a constant state of upheaval. As mountains rise and fall, rivers are split, diverted, and joined. In some cases, they have even reversed flow.
A geologist, a biologist, and an ecologist recently put their expertise together to simultaneously trace the movement of fish and gold through the country’s rivers. The results point to hidden riches, and the team’s approach has informed research around the world.
Caught amid New Zealand’s geological turbulence are tiny native freshwater fish—species in the genus Galaxias. Over millions of years, galaxiid populations have been split by river capture—the diversion, by geological change, of rivers into different catchments.
In the southern province of Otago, rivers also carry gold eroded from the basement schist across the landscape. Nineteenth century miners flocked to Otago to dredge, pan, and sluice its lucrative waters.
Gold is also found in Otago’s neighboring province, Southland. Because Southland has far less gold-bearing basement rock, geologists have deduced that a large river system once carried most of this gold from Otago to Southland.
Today, though, the two provinces are separated by a mountain range, described by University of Otago geologist Dave Craw as a “Hadrian’s Wall,” which prevents water from flowing between them.
In the late 1990s, Craw met up for a drink with his University of Otago colleague biologist Jon Waters. Waters was struggling to explain another unlikely connection between Southland and Otago—a galaxiid species he’d found living on both sides of the drainage divide.
Craw had an explanation for him: The river the fish lived in, the Nevis, had, at some time in the distant past, been reversed by the rising mountains, cutting the fish population in two. The pair immediately realized they could use the rate of genetic divergence between the two fish populations to pinpoint the date of that reversal. “We essentially wrote a paper straightaway,” Craw said.
They went on to apply the same techniques elsewhere in Otago. “We started looking for more clues, and we’ve continually found more,” Waters said. “There are places where the fish record is strong and the geology isn’t, and places where the geological record is strong and the fish are less informative. But by putting the two together, we can put the pieces of the jigsaw puzzle together.”
From numerous studies, the team calibrated a “geogenomic clock” that they can now apply to river capture queries around New Zealand. It’s also helped them predict where old, gold-bearing rivers may be buried.
Fish genes, Craw said, are often far more useful than rocks for narrowing down the exact timing of geological events—something that is crucial when looking for mineral deposits. “You need to know what rivers went where, and when,” he said.
A freshwater galaxiid of a newly discovered, undescribed species swims in the Pomahaka River in southern New Zealand. Credit: Daniel Jack
Craw and Waters, along with ecologist Ciaran Campbell at the Otago Regional Council, recently turned their attention to Southland. In their latest study, published in the New Zealand Journal of Geology and Geophysics, the group used fish dating as one tool to predict where a number of old, gold-bearing rivers flowed. In doing so, they also identified a probable new species of galaxiid.
Pedro Val, a geomorphologist with the City University of New York who was not involved in the study, compares fish genes with sediments to date river changes in the Amazon Basin. “It makes total sense,” he said. “Fish are passively inhabiting the river. If the rivers are changing, fish are going along with it, and the same with sediment. It’s kind of the perfect scenario where you get two independent things showing the same process.”
The collaborative approach Craw and Waters developed is now used in other countries. James Albert, an ecologist at the University of Louisiana at Lafayette, studies how freshwater fish in South America have evolved in response to the uplift of the Andes.
The New Zealand team, Albert said, has “probably the most well developed system” of matching fish genetics with geology in the world.
Albert, who was not involved in the New Zealand research, explained that the close agreement between galaxiids and gravel in the New Zealand work confirmed the accuracy of the team’s methods.
“One of the assumptions,” he said, “is that the only way the fishes can move among river basins is when there’s river capture. But it’s also possible, in principle, that birds can move fish around.”
That, he said, “could screw up this kind of analysis. But the patterns [they] have found here are very reasonable interpretations of the underlying geology.”
The study results suggested that an ancient riverbed is lying under farmland southwest of the modern-day Mataura River. The buried river may well be loaded with gold, Craw said. Extraction, however, is not likely anytime soon.
“There’s probably 50 meters (164 feet) or more of gravel sitting on top,” he said. “That’s a lot of diesel [fuel] to remove that.”
If the economics of mining were to change, however, such buried rivers may one day prove lucrative. Craw pointed out that a nearby active gold mine needed 40 meters (130 feet) of gravel removed to become operational. “It can be done,” he said. “So maybe in 20 years’ time, they’ll take the top off it.”