The Earth's Energy

by Rob Harris from Oregon State University.

I am interested in understanding the energy budget associated with geologic processes.  I think this understanding leads to better insights into how the Earth works.  For example, plate tectonics – the creation, motion, and destruction of plates – reflects Earth cooling.  About 70% of the Earth’s heat loss is through the ocean floor and is reflected by the cooling and subsidence of oceanic plates as they move away from spreading centers.  The upper layer of these plates, the oceanic crust, is cooled efficiently by hydrothermal circulation.  It turns out that the entire volume of the global ocean circulates through the oceanic crust every few hundred thousand years.  This hydrothermal circulation is important because it leads to significant exchanges in energy, mass, and solutes between the ocean and crust.  These exchanges modify the chemistry of the ocean, the chemical and physical properties oceanic crust, and supports a globally significant biosphere.

 

Rob with his heat probe. Note the thin thermistor string supported by the thick lance.

I am excited about participating on the OSCAR project because it touches on many aspects of these processes, the cooling and evolution of oceanic plates, hydrothermal circulation, and the impact of heat exchange between the ocean and crust.  Specifically, I am using heat flow measurements to better understand how and where fluids are moving in the oceanic crust and how this fluid flow changes as the plate ages.  One mystery is how and why the fluid flow wanes as the plate ages.  Clearly part of the reason is that the plate cools so there is less energy to drive the system, but other processes are involved as well.  This data will also be used to better understand the nature of energy transfer between the crust and oceans.  One idea is that warms fluids emanating from the crust may stimulate the flow of bottom water.

 

The heat probe being deployed, showing the weight stand at the top, and the long lance and thermistor string pointing downwards

The picture shows the heat flow probe I use.  The weight stand contains the data logger, power, and acoustic telemetry so we can monitor its performance from the ship.  These heat flow measurements are made by plunging the probe, under the force of gravity, into sediments.  The lance supports the thermistor string keeping it straight.  We house the thermistor string in a small tube so that the thermal response time is relatively fast, decreasing the time each measurement is made.  This style of probe with a sensitive thermistor string supported by a larger mass is called a violin-bow probe. 

Heat flow is the product of the thermal gradient and thermal conductivity.  Once the probe is in the sediment, we measure the thermal gradient by measuring the temperature at each of the 11 thermistors in the thermistor string and knowing the distance between them.  A heater wire also extends along the thermistor string that generates a short heat pulse.  The way the heat decays lets us determine the thermal conductivity.  These two measurements yield the heat flow.  Low heat flow measurements can indicate areas where cold bottom water enters the oceanic crust.  High heat flow measurements can indicate area where the now warm water exits the oceanic crust.