Variable magma and carbon fluxes at mid-ocean ridges by David Rees Jones

21/11/2019

Eyjafjallajokull volcano in Iceland

Variable magma and carbon fluxes at mid-ocean ridges and on Iceland between glacial and interglacial cycles by David Rees Jones

Abstract:

David Rees Jones and his team looked at the influence of glacial and inter-glacial cycles on the melt production and composition at mid-ocean ridges. As the sea-level varies with the cycles the water column above the mid-ocean ridge varies thus leading is small changes in confining pressure. As the magma is formed by decompression melting the depth and percentage of melt is affected by variations of pressure thus responding to the glacial cycles. This studies tried to quantify this influence and it managed to achieve it for long periods but still overestimated it for shorter ones.

Main:

Mid-ocean ridge volcanis has been often looked at in the light of a driver of plate tectonics as well as a response to plate divergeance. Many previous studies have been realized in order to understand the processe involved with the formation of the magma at mid-ocean ridges and their compositions. Glacial and inter-glacial cycles are thought along wit hother factors to be a consequence of the variation in volcanic activity. The study conducted by David Rees Jones and his team composed of Nestor Cerpa, Rich Katz and John Rudge looks at the influence of the glacial and inter-glacial cycles on the formation and composition of the melt, which brings in a new feedback perspective.



David and his team already hade previous work experience in geodynamics. They looked at fluid mechanic related fields of work like the Earth's outer core magnetic processes or the understanding of glacial cycles. This gave them the background necessary to try and answer this question: "Does the glacial and inter-glacial cycles have an influence on magma production and composition at mid ocean ridges?".

First lets look at the basics of mid-oceanic ridge magma formation. The main process to create melt at mid ocean ridges is decompression melting. Decompression melting is the process where the asthenospher is hot enough to melt but the pressure keep it in solid state at greater depth. As it rizes towards the surface by convection the rock column above become smaller up to the point where the confining pressure drops enough to allow melt to form. Trough this process the fertile asthenosphere (lhertzolite) melts between 5% and 20% melt leaving depleted mantle rocks behind (hartzburgite). Because it is only partial melting there is a partition of elements between the solid (rock) and liquid phase (melt), which depends with the percentage of melt.

Then the climate cycles pattern needs to be understood. During the last 2 My there is a good record of sea surface temperatures illustrating glacial and inter-glacial cycles (from E.Lisiecki et al 2005 and Peter U. Clark et al 2006*). The cycles alternate with a rapid heating of the climate into an inter-glacial period with a slower cooling back into a glacial period. These cycles are on average 41 ky long before the Mid-Pleistocene where they switch to 100 ky cycles. These are thought to be linked with Milankivic cycles where ~40 ky corresponds to obliquity, and the ~100 ky cycles could correspond to the eccentricity having to most influence. But what could be the link with the mid-ocean ridge volcanism?

Variations of delta O18 from benthic foraminifera as a proxy for sea surface temperature during the last 3 Myr (from Peter U. Clark et al 2006). Up to ~1.5 Myr the cycles last ~41 ky but after the Mid-Pleistocene transition the cycles last ~100 ky. This is most likely caused by the relative inflence of Milankovic cycles.

Between ice ages the mean sea-level can have around 100m variations! This changes the water culumn that lies above the mid-ocean ridge which in turn changes the pressure gradient where decompression melting occur. During glacial periods the sea-level is lower, thus the pressure is also smaller which means that the decompression melting happens deeper, thus marginally increasing the melt %. The reverse would happen during an inter-glacial period. This would have an impact on the magma formation at the mid-ocean ridge thus creating a feedback on climate.
The melting by decompression only (without lowering the melting temperature with the help of water) is referred to as "dry melting". During dry melting the carbon gets concentrated in melt as it is incompatible. This results in volcanic gases that emane from the magma to be highly concentrated in CO2. This plays a direct role in climate regulation as it is a well known greenhouse gas.
By looking in the variations of CO2 composition of the melts, David and his team found that there was a 10 to 15 years lag between the sea-level change and the variation in the decompression melting depth response.
They made a model to quantify the melt production variation caused by sea-level change. This model seemed to be accurate for long periods but for shorter ones had the tendency to overstimate the melt production, up to twice the actual amount. This model thus sets a maximum to the melt production variation.

In Iceland, the same processes can be applied but the parameters are different as the water column change has less inluence now as the ridge is above sea-level but ice sheets now are an additional mass to take into account. An additional comprexity is the imput of the hot spot below Iceland. It is still difficult to quantify the influence of the glacial and inter-glacial cycles
on the melt production of iceland.

References and further readings:

*A Pliocene-Pleistocene stack of 57 globally distributed benthic D18O records by Lorraine E. Lisiecki and Maureen E. Raymo
The Middle Pleistocene transition: characteristics, mechanisms, and implications for long-term changes in atmospheric pCO2 by Peter U.Clark et al.