Rock Weathering and Climate: What a New Study Found

The takeaway is uncomfortable: erosion is not a one-way carbon sink. Under the right conditions, weathering rock can push warming further rather than reining it in.

Reading the climate in 183-million-year-old mud

To settle the question, the team did not model the future. They reached into the deep past, to a volcanically triggered episode of warming in the early Jurassic known as the Toarcian Ocean Anoxic Event.

The Toarcian is one of roughly a dozen ancient climate upheavals set off by enormous volcanic phenomena called large igneous provinces. Several of these are tied to mass extinctions, including the Great Dying at the end of the Permian, caused by the Siberian Traps. The Toarcian event itself was triggered by massive eruptions across South Africa and Antarctica, which were joined together at the time.

The result was brutal. Global temperatures climbed 6 to 7 degrees Celsius. The makeup of plant and dinosaur species on land was reshuffled, and corals and other marine species suffered a mass extinction.

As co-author and Oxford professor Bob Hilton put it, this event had been well studied before. Researchers already understood its drivers, how it caused mass extinctions, and that it was driven by a large igneous province release. That existing understanding made it a clean test case for the new question about carbon weathering.

How rhenium isotopes became a carbon tracer

The clever part is the measurement. Organic carbon in rock ranges from visible bits of fossil leaves and wood down to the molecular remains of plankton, algae, and microbes. During events like the Toarcian, so much organic matter was buried at sea that the resulting shales turned black with carbon.

To measure how much of that organic carbon was later weathered on land, the team used isotopes of the element rhenium. Rhenium works as a tracer because it binds chemically with organic matter in seabed sediments.

The mechanism is elegant. When organic carbon weathers on eroding land, the carbon escapes as CO2 gas. But the rhenium that was attached to it does not go into the air. Instead it washes through rivers into the ocean and gets built into new seabed sediments. There it records what happened. The intensity of organic carbon oxidation shifts the ratio of rhenium 187 to rhenium 185, so that ratio in the sediment becomes a measure of how much organic carbon was weathering at the time.

A 1,300-meter core and a measurement at the edge of possible

The raw material was a 1,300-meter rock core spanning the late Triassic to the early Jurassic, drilled in the 1960s in Wales and now stored by the British Geological Survey. The team chipped shale samples from different points along the core, each one representing a different moment during the warming event.

Those samples were digested in a succession of acids to break down minerals and organic matter, then measured using Inductively Coupled Plasma Mass Spectrometry. The sensitivity required is extreme: rhenium concentrations in the rock run as low as one billionth of a gram per gram of rock. Hilton noted that recent improvements in mass spectrometer sensitivity have been a bit of a game changer in making studies like this possible at all.

The rhenium values shifted as the Toarcian event unfolded, indicating that organic carbon weathering intensified as the climate heated up. In other words, the warming itself appears to have fed more carbon-releasing erosion, which amplified the warming further.

What this means and what happens next

Outside experts framed the result as a methodological opening as much as a finding. Katherine Grant of Lawrence Livermore National Laboratory, who was not involved, called it one of the first papers to use rhenium isotopes in a past geologic context, opening the system up to study further back in time and across other anomalous events. Jeremy Caves Rugenstein of Colorado State University, also not involved, said Hilton's group has revolutionized our understanding of how geologic organic carbon interacts with climate.

Over the coming months and beyond, expect the rhenium-isotope approach to be pointed at other ancient warming events to see whether the amplifying effect is a one-off or a pattern. The headline implication is that the same erosion-driven carbon release may apply to modern climate change. The honest caveat from the researchers is that the extent to which the past is prologue remains uncertain. The geological record suggests warming can feed on itself through weathering, but how strongly that translates to today's fast, human-driven warming is the open question the next round of work will chase.

Source: Ars Technica