We have a new open-access study about geothermal heat flow beneath the Greenland and Antarctic ice sheets in the Journal of Geophysical Research: Earth Surface.
Presently, there’s a lot of uncertainty about the magnitude and pattern of geothermal heat flow beneath both ice sheets. That’s because it has only been sampled at a handful of widely spaced deep ice cores (Figure 1). While the average value of geothermal heat flow is relatively small, getting it right is essential for ice-flow models. If you run a computer simulation of an ice sheet with a severe over- or under-estimation of the geothermal heat flow, you can easily end up generating an ice sheet that is either too warm or too cold. Ice flow is very sensitive to temperature – particularly near the bed – so geothermal heat flow is a critical variable for simulating the form and flow of Earth’s ice sheets.
Our study took a fresh look at changes in geothermal heat flow across space, but not those due to the subtle variations in Earth’s crust and mantle properties over tens of km. Instead, we examined the effect of the ice-sheet bed’s topographic relief on geothermal heat flow to generate the first comprehensive snapshot of changes in geothermal heat flow at scales of hundreds of meters due to that relief. It’s been known for over a century that geothermal heat flow is greater in valleys and smaller on ridges. Basically, if the heat escaping Earth’s interior is looking for the quickest way to radiate into the atmosphere, a deeply incised valley provides the fastest exit. This effect is readily observable from the fact that geotherms – surfaces of constant temperature – are packed more closely together beneath valleys, indicating a stronger temperature gradient there (and hence heat flow) in comparison to ridges.
We created a simple statistical model to estimate this topographic influence on geothermal heat flow. This model essentially uses a digital elevation model of the bedrock topography to assess local topographic relief and then converts this local relief into a fractional correction for geothermal heat flow. It produces a positive correction – an increase – for valleys, and vice versa for ridges. Our approach is admittedly simple and empirical – a literal “first-order” approximation – but it seems to reliably reproduce the topographic variability in geothermal heat flow in all the settings for which we could find previous studies. So, we applied this statistical model to digital elevation models for Greenland and Antarctica. This revealed much more detail in a geothermal heat flow map than we are used to seeing.
Across both Greenland and Antarctica, we see patterns of increased geothermal heat flow within deeply incised glacier valleys and decreased geothermal heat flow along ridges and mountains. In many regions, most notably the Antarctic Peninsula (Figure 2) and Central East Greenland (Figure 3), we find that local topography routinely modifies regional geothermal heat flow by more than ~50%.
In Greenland, we estimate that there are ~100 outlet glaciers that are both sufficiently narrow and deeply incised to more than double local geothermal heat flow relative to that of the regional average value. The model also suggests that – especially deep within the interior of the Greenland Ice Sheet – local geothermal heat flow may be sufficiently suppressed along prominent subglacial ridges to cause subglacial water to refreeze. (At least, in ice-sheet areas where the ice-bed interface is near the freezing point.)
The topographic correction for geothermal heat flow that we model is only as a good as the topographic relief that we derive from subglacial digital elevation models. Generally, in areas where subglacial topography is best resolved, the effect of the topography on the local geothermal heat flow is greater than uncertainty in the underlying regional geothermal heat flow model (Figure 4). There are still large swaths of the ice sheets where subglacial topography remains poorly resolved. In these areas, our model is not tremendously useful; the existing uncertainties between regional geothermal heat flow models are still larger than any local topographic correction that we can estimate.
The topographic corrections for geothermal heat flow in Greenland and Antarctica that we have calculated are now available as dimensionless fields in NetCDF format, with grids that are the same resolution as BedMachine, for each region via the PROMICE data portal (www.promice.dk). This means that they can be anonymously downloaded and applied to any regional geothermal heat flow model of the user’s choice. We hope that these topographic corrections for geothermal heat flow will be adopted into ice-flow models to improve both present-day ice-sheet simulations, as well as our understanding of the role of geothermal heat flow in the feedback between ice flow and topography on geologic timescales.
It has certainly been a long and winding road to this publication – the original draft of this article was first submitted in June 2019 – and we are grateful for Noah Finnegan (University of California Santa Cruz) and Olga Sergienko (Princeton University) for serving as editors to four very helpful peer-reviewers. Interdisciplinary projects can clearly provide a bumpier ride than staying in your own lane, but – in this instance – the journey seems to have taken us to a very different view of geothermal heat flow in Greenland and Antarctica.
Development of this data product was funded by the award “HOTROD: Prototype for Rapid Sampling of Ice-Sheet Basal Temperatures” provided by the Experiment Programme of the Villum Foundation. Improved understanding of the spatial variability in subglacial geothermal heat flow helps us optimize drill site selection and analyze ice temperature measurements. This data product was also supported by the Danish Ministry for Climate, Energy and Supplies through the Programme for Monitoring of the Greenland Ice Sheet (PROMICE).
Colgan, W., J. MacGregor, K. Mankoff, R. Haagenson, H. Rajaram, Y. Martos, M. Morlighem, M. Fahnestock and K. Kjeldsen. 2021. Topographic Correction of Geothermal Heat Flux in Greenland and Antarctica. Journal of Geophysical Research. 125: e2020JF005598. doi:10.1029/2020JF005598.
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