Enriched Carbon Source Detected in the Deep Mantle

Australia: The Land Where Time Began

Enriched Carbon Source Detected in the Deep Mantle

The carbon levels in the deep mantle that have been estimated vary by more than an order of magnitude. A carbon-rich mantle plume source region beneath Hawai’i was revealed by coupled volcanic CO₂ emission data and the rates of supply of magma to have contained 40 % more carbon than previously estimated.

The deep carbon cycle transports carbon from the mantle to the surface of the Earth over geologic time scales by way of volcanism, following which it is carried back into the interior by subduction. In contrast to the near surface carbon cycle between the atmosphere, oceans and continents, the deep carbon cycle has remained poorly constrained having received very little attention from researchers. Carbon concentrations in the deepest portions of the mantle, those regions that feed mantle plumes and ocean island basalts, that are the most poorly studied, are the most poorly constrained, because of the near-impossibility of retrieving samples that are near pristine. Plumes originating from the deep mantle that have been less degassed, feed hotspot volcanoes, such as Hawai’i, and they represent the best opportunity to investigate deep carbon. Understanding of deep mantle structures, primary sources of volcanism, and the deep carbon cycle of the Earth are limited by the ambiguity in the deep carbon budget. Anderson & Poland (Anderson & Poland, 2017) present in Nature Geoscience, a physical model that incorporates existing carbon data from Hawai’i, and suggests that the deep carbon content of the Earth may be significantly higher than had previously been believed.

In the deep mantle carbon is stored as mineral carbonate, diamond, metal carbide or dissolved in metals (Dasgupta & Hirschmann, 2010). There are many ways in which the amount of deep mantle carbon can be estimated, the most direct way being by geochemical observations of surface volcanic manifestations. To determine the carbon content of the source region deep beneath an active volcano, such as Hawai’i is, however, difficult, as samples lose much of their carbon by degassing during ascent and eruption (Hilton, McMurtry & Kreulen, 1997). Valuable information can be recovered by the use of indirect geochemical measurements, by determining the ratio of carbon to other elements that are similarly incompatible, such as He (Marty & Tolstikhin, 1998; Shaw et al., 2003), niobium and chlorine (Saal et al., 2002). Volcanic CO₂ measurements can in this way be inverted to estimate carbon concentrations, taking into account the effects of partial melting, evolution of the magma, and degassing. Geophysical measurements such as electrical conductivity (Gaillard et al., 2008), can also be used, but these are limited to the shallowest regions of the upper mantle.

A different approach was taken by Anderson & Poland (Anderson & Poland, 2017); they combined geochemical surface observations that had been published of (CO₂ and SO₂) and satellite data that was available (InSAR) and physical model for magma flow and gas flow through the Kilauea Volcano. Their approach doesn’t rely on rare, degassed rock and mineral samples, and doesn’t require degassing models to reconstruct carbon contents of the mantle, which differs from previous modelling studies (Macpherson & Mattey, 1994). They used a Bayesian statistical model instead and this allowed for uncertainties in the parameters of the model to infer higher undegassed sources of carbon concentrations in the magma and mantle. They estimate, based on this statistical model, that the CO₂ content is about 1%wt, roughly 40 % higher than previously thought, in supplying magma that is derived from the mantle to the active volcanoes of Hawai’i. It is suggested by Anderson & Poland that this magma is supplied for a source region in the mantle that has a carbon concentration of 263^+81/_-62 ppm, which is more than an order of magnitude higher than estimates for mid-ocean ridges (Marty, 2012) of 20 ± 8 ppm.

A critical contribution to the field of mantle geochemistry is made by this study for 2 reasons.

1. It effectively syntheses the many multidisciplinary carbon studies that have been undertaken in the Hawai’ian region.

2. It sheds light on a critical question that is poorly understood about source of carbon in the deep plume-derived mantle.

The field has struggled to reach consensus whether the mantle that is plume-related is carbon-rich or carbon-poor relative to basalt from the mid ocean ridge that is depleted (MORB) mantle, with estimates that vary by 2 orders of magnitude (Barry et al., 2014). In the future it will be interesting to see the performance of this model on data from other volcanic systems that are plume-influenced, in particular those being investigated with the most up to date geochemical techniques. Additional research into the deep carbon cycle will be helped by this work and further understanding of the redistribution and budgets of all elements in the mysterious deep interior of the Earth.

On human timescales, emissions of anthropogenic carbon emissions are significantly (about 135 times) larger than modern volcanic carbon fluxes (Gerlach, 2011). However, volcanic emissions on million-year timescales, volcanic emissions are probably the dominant control on climate (Sobolev, 2011). Little is known about the long-term variability of deep volcanic carbon outputs, though the different inputs, such as volcanic versus respiratory carbon, and feedbacks to the global carbon cycle play a fundamental role in the regulation of the climate system of the Earth.

It is implied by the findings of Anderson & Poland that there are higher rates of CO₂ emissions from volcanoes that are related to Large Igneous Provinces (LIPs) and plumes. The deep plume source is, however, likely to be heterogeneous and that only a snapshot of modern deep CO₂ is represented by the estimated mantle plume. Carbon emissions from volcanic eruptions may have been dramatically higher in the past, with substantial and rapidly pulsating (10^4—10⁵) volcanogenic degassing of carbon during emplacements (Schaller, Wright & Kent; Percival, 2017) of Large Igneous Provinces, and with profound impacts on the climate of the Earth. There is still much that is not known about the evolution of the carbon cycle over time; by accurately estimating fluxes of modern deep carbon, it will allow better understanding volcanic contributions in the past of the Earth, which will have important implications for historical reconstructions.

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