In 1981 geologists proposed that the chemical weathering of rocks like granite can draw the greenhouse gas carbon dioxide (CO2) out of the atmosphere and, in the process, cool Earth. As cooling progressed, chemical weathering reaction rates would decrease, more CO2 would remain in the air, and warming of the planet would begin again. Although this sort of natural “thermostat” could help explain such puzzles as why CO2 from volcanic eruptions does not accumulate in the atmosphere unceasingly, physical evidence for a mechanism that moderates the planet’s temperature has been lacking.
But new research presented on 14 August at the Goldschmidt2017 conference in Paris showcased telltale chemical data from ancient rocks. The data indicate that falling temperatures during a glaciation about 445 million years ago triggered a process leading to a period of renewed warming, much like a chilly afternoon might trigger your thermostat to turn on the furnace for the night.
Combining these new findings with prior modeling studies, “for the first time we have actual empirical data for the thermostat, and therefore of the mechanism that keeps the Earth habitable,” isotope geochemist Philip Pogge von Strandmann of the University College London in the United Kingdom, who led the new research, told Eos.
He added, however, that finding a thermostat does not offer hope for humanity’s current climate change predicament. The weathering thermostat helps warm and cool the planet on the scale of hundreds of thousands of years—too slow to help moderate the modern meteoric rise in atmospheric CO2 caused by human activity. “While this is the process that will allow the climate to recover from man-made warming, it’s too slow to help us right now,” Pogge von Strandmann said.
Setting the Thermostat
Silicate rocks like granite react with CO2 dissolved in rainwater, explained Lee Kump, a geochemist at Pennsylvania State University in University Park who was not involved in the new research. “Carbon dioxide in rainwater is an acid, and that acid attacks rocks and is neutralized. When it attacks silicate rocks like granites and basalts,” a reaction incorporates the CO2 into limestone, which removes the molecule from the atmosphere, he said.
The reaction also releases isotopes of the chemical element lithium from the rocks, Pogge von Strandmann explained. Although it’s difficult to tease out weathering-driven deterioration of rock from other factors in the stratigraphic record, lithium isotopes offer a relatively straightforward, parallel record of weathering taking place.
Investigating a period of ice buildup known as the Hirnantian glaciation, which happened about 445 million years ago at the end of the Ordovician period and likely triggered life’s first big mass extinction, Pogge von Strandmann and his team tested three stratigraphic sections for their lithium isotope content. Their analysis of two sections on Anticosti Island in Canada and a third near Moffat in Scotland revealed that less lithium weathered out of silicate rocks around the time of the glaciation.
“We found that lithium isotope ratios show a…peak around this time, and we can model that as being due to a decrease in silicate weathering during this cooling time period,” Pogge von Strandmann said.
At Sixes and Sevens
The team found in strata from the cooling period less lithium-6 than usual compared to lithium-7, both of which are stable isotopes. Although, generally, “there’s 12 times more lithium-7 than lithium-6 on Earth,” Pogge von Strandmann said, during the cooling period the ratio increased to about 12.2 to 12.4.
That difference resulted from a rise in the formation of clays, such as illite, on land, which typically happens when less weathering is occurring, he explained. The greater abundance of clays reduced how much lithium-6 was transported to the ocean because the clays take up and retain some of the lithium-6 from nearby rock. Consequently, streams transported less lithium-6 compared to lithium-7 to the oceans where the deposition that created the strata took place.
“Temperatures initially dropped because of decreasing CO2 degassing from volcanoes. This caused weathering to decrease by about 4 times globally, allowing CO2 and temperatures to recover, terminating the glaciation,” Pogge von Strandmann said. This weathering “minimum,” as he described it, occurs right at the end of the glaciation, which is unsurprising as long as temperature controls weathering rates.
Before the glaciation, silicate weathering sequestered about 150 megatons of CO2 per year. “At the peak of the Hirnantian, we model a rate of 35 megatons per year—so around a 4 to 5 times decrease,” Pogge von Strandmann said. Over long time spans, this change was enough to help restore warming of the planet, he added.
“For me what is compelling about this study is that it presents not just one but three records for the same time period from different parts of the world, and they all appear to show the same response,” said Rachael James, a geochemist at the University of Southampton in the United Kingdom who was not involved in the work. “This hasn’t been done before, for any time period, and it really gives me hope that lithium isotopes can be used to trace silicate weathering rates far back in time and consequently assess the role of weathering in setting Earth’s thermostat.”
Thermostats regulate temperature in both directions, not only turning up the heat when temperatures drop, as this new evidence suggests happened in the Hirnantian, but also triggering cooling when temperatures rise.
In previous work published in 2013 in Nature Geoscience, Pogge von Strandmann and a different team of colleagues used geochemical simulations to determine that an increase in weathering during an era of warming 93.5 million years ago known as ocean anoxic event 2 may have led to sequestering of carbon and cooling of the planet. “That gave the first half of the story,” he said.
“By itself, [that’s] not full evidence of the thermostat,” Pogge von Strandmann acknowledged. However, “now we have evidence that weathering decreases during cooling,” showing both sides of thermostat operation.
Tricky to Measure
Despite this new evidence, forces other than temperature might also control the thermostat, according to Kump. For instance, another study suggested that the number of silicate rocks exposed at a particular time on the planet’s surface may exert the most influence on the rate of weathering. During the Ordovician, mountain uplift brought silicates rocks to the surface in droves. “The uplift of these mountain belts—some expressions of which are in the East Coast of the United States—led to enhanced weathering because it’s like a conveyor belt bringing these rocks to the surface,” Kump said. “That exposes them to this naturally acidic rain, and that stimulates weathering.” Then, as atmospheric CO2 concentrations decreased, ice sheets grew and covered the rocks, “and that reduces silicate weathering,” said Kump.
Regardless of whether mountain building or temperature change is the main thermostat driver, Pogge von Strandmann explained, the main take-home from his team’s work—also reported in a recent Geochemical Perspectives Letters paper—is that they have solid evidence that a thermostat exists at all. “But it doesn’t mean that [temperature change] always is dominant,” Pogge von Strandmann said.
“We never actually had any real information that thermostats even exist in the past. What we’ve done is show that at least the thermostat does exist, and in this particular area it stabilized the climate rapidly” on a geological timescale. The team’s findings, he explained, take scientists a step further toward understanding a stabilizing force that may help Earth escape from extreme climatic conditions.
—Lucas Joel (email: [email protected]), Freelance Writer