IMPULSE expedition sails

IMPULSE is an international collaboration to "Take the Pulse of the Icelandic Mantle Plume". Led by Dr Steve Jones, it involves scientists at Cambridge and Southampton Universities, plus collaborators in USA and Iceland.  Doctoral researcher Hazel Knight also sails.   

The 3-year project kicks off with a 38-day expedition on board the US research vessel Marcus G Langseth. She is an ex-industry seismic acquisition ship now owned and run by Lamont Doherty Earth Observatory – the only one of her type available for academic research. The expedition marks the culmination of a 2-year preparation period of working with NERC National Marine Facilities to barter with US authorities to obtain access to the Langseth's deep Earth imaging facility.

The mantle is the largest component of the Earth, comprising 84% of our planet's volume. Although the mantle is solid, over geological time it churns vigorously like a fluid in a process known as mantle convection, driven by heating from radioactive decay in Earth's interior and cooling from above. Mantle convection deforms the Earth's entire surface into an interlocking pattern of swells and depressions known as "dynamic topography", with diameters of several thousand kilometres and heights of several kilometres. Dynamic topography influences oceanic current patterns, land surface erosion and accumulation of the eroded sediment, and these effects are known to control the distribution of valuable natural mineral resources. Volcanic activity also usually occurs in association with the hot, rising elements of the convective circulation, known as mantle plumes. The most vigorous mantle plumes give rise to Large Igneous Provinces (LIPs) – episodic huge outpourings of lava accompanied by voluminous release of greenhouse gases to the atmosphere. LIPs coincide in time with some of the most remarkable perturbations to global climate, ecosystems and the carbon cycle in Earth's history, including mass extinctions, ocean anoxic events, and the largest natural global warming event of Cenozoic time.

Whilst it is widely accepted that mantle convection has influenced Earth's surface and climate processes over geological time periods (tens of millions of years or more), these time frames are too slow to explain the rapid onset and short duration of the environmental changes that usually coincide with LIPs. But growing evidence now suggests that patterns of mantle convection, dynamic topography and igneous outpouring can evolve in less than a million years. Key to this theory is a process known as "Thermal Plume Pulsing", in which hotter and cooler blobs of mantle are carried along with the convective circulation within a mantle plume. The hottest pulses within the biggest mantle plumes, such as the Icelandic Mantle Plume, can rise at speeds in excess of 200 mm/yr, which is faster than the motion of tectonic plates, and can cause changes in local sea-level of over 1 mm/yr, similar to modern mean global sea-level change. At such speeds, past pulsing of the Icelandic Mantle Plume could have activated greenhouse gas generation from the North Atlantic LIP rapidly enough to explain the Paleocene-Eocene Thermal Maximum extreme global climate change event, the best natural analogue to anthropogenic climate change.

However, the Plume Pulsing hypothesis is not universally accepted for Iceland or Earth's other major mantle plumes as key data is lacking. High-quality measurements of seafloor features near Iceland known as the "V-Shaped Ridges" (VSRs) that comprise the world's best record of the suggested hot pulses will address this gap. Working with the lead advocates of the alternative models for VSRs, we have devised an experiment to determine the origin of the VSRs by measuring both the thickness and the chemical composition of the crust that builds the VSRs. A high-quality geochemical survey of the basaltic seafloor was made recently by dredging and by scientific drilling under the International Ocean Discovery Program. Now, IMPULSE will measure the variation in thickness and seismic velocity (hence bulk composition) of the entire crust beneath several VSRs for the first time.

Our pilot work indicates that IMPULSE will provide firm evidence for fluctuations in mantle temperature on a million-year timeframe to give the first definitive proof of the Mantle Plume Pulsing hypothesis. Furthermore, by formally correcting for the complicating effect of mid-ocean ridge tectonic processes on VSR crustal thickness for the first time, our new VSR record will determine the shortest time period for fluctuations in mantle temperature.

These results will be crucial to test hypotheses for how mantle convection has influenced Earth's past climate. They will feed directly into other Birmingham-led endeavours, especially the C-FORCE Large Grant, which is investigating the Carbon-Cycle Feedbacks from Response to Carbon Emissions that resulted from the Icelandic Mantle Plume's biggest pulse.