In Brief

Key to the past

About 56 million years ago, Earth experienced an extraordinary episode of global environmental change

Words by Jerry Dickens
16 May 2022

Could mercury anomalies in sediments deposited on the Norwegian margin during the PETM signal unprecedented delivery of fluvial components?

About 56 million years ago, Earth experienced an extraordinary episode of global environmental change. Over a geological instant, surface temperatures soared by 6 °C, ecosystems, both terrestrial and marine, evolved dramatically, and tremendous quantities of carbon dioxide entered the ocean and atmosphere. The event, now called the Paleocene-Eocene Thermal Maximum (PETM), likely represents our best past analogue for understanding the effects of current climate change associated with anthropogenic fossil fuel emissions. As highlighted by three recent studies, the PETM also forces our broad geoscience community to contemplate a commonly taught axiom: the present is the key to the past.

The first study by Sev Kender (at the University of Exeter, UK and the British Geological Survey) and colleagues reconstructs mercury (Hg) records from sediment cores that span the PETM and come from the North Sea. The records reveal that Hg accumulation increased significantly but sporadically before and during the PETM. Assuming most sedimentary Hg derives from volcanoes, the authors conclude that pulsed volcanism, particularly in the North Atlantic Igneous Province, drove the PETM, including a substantial portion of the carbon emissions. This general idea was pushed to the forefront nearly two decades ago with the discovery of numerous buried craters on the Norwegian continental margin, which apparently contain sediment deposited during the PETM. In the recent paper, though, sedimentary Hg contents are relatively low at the onset of the PETM, precisely when stable carbon isotope records suggest massive carbon emissions were occurring. The authors therefore suggest volcanism triggered additional carbon release from other sources.

A recurring and fascinating find is that, along continental margins, the PETM manifests as an expanded horizon of terrestrially derived material

A recurring and fascinating find is that, along continental margins, the PETM manifests as an expanded horizon of terrestrially derived material. A second study by Simin Jin at the China University of Geosciences and colleagues spectacularly shows this aspect for cores from the North Sea. Indeed, the onset and main phase of the PETM, which lasted less than 200,000 years, corresponds to 140 metres of stratigraphic thickness in one of the cores. Moreover, this record displays cycles in sedimentary components with an apparent beat to Earth’s precession. Consistent with results from general circulation models for Earth’s future, the authors argue that greatly enhanced rainfall seasonality characterised the PETM, and this led to immense sedimentary discharge from rivers to continental margins.

The third paper, which comes from Maodian Liu at Peking University, China, never mentions the PETM. Indeed, it entirely omits reference to the geological past. Instead, it is a remarkable and comprehensive examination of Hg riverine discharge to modern continental margins. The team shows convincingly, and in contrast to dogma, that most Hg on modern continental margins derives from rivers rather than from atmospheric sources, such as emitted by volcanoes. Crucially, they also suggest that rivers disproportionately add Hg during events of extreme discharge.

Individually, the three papers represent interesting works, each of which moves a disparate field forward. Read collectively, they exemplify how we might misread the sedimentary record during times of extraordinary change. For example, it seems that sedimentary filling of craters along the Norwegian margin and Hg anomalies in the North Sea could potentially represent an underappreciated and perhaps unprecedented delivery of fluvially derived components to continental margins, rather than near instantaneous and voluminous volcanism. For the PETM, a better axiom might be: the future is the key to the past.