Friday, January 2, 2015

The Science Plan

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In 2013 we drilled through the Whillans Ice Stream (WIS) into Subglacial Lake Whillans and both instrumented the hole and took sediment and water samples.  Last year we drilled two holes and instrumented them but didn't take samples from the subglacial environment.  This year we will be drilling to the base of the ice stream again to take samples, but at the grounding zone.  The grounding zone is the transition between ice which is thick enough to sit on the ground and the ice shelf, which floats above the sea floor.  The stability of an ice stream can be characterized by the tendency for the grounding zone to retreat inland.  Grounding zone retreat is typically accompanied by loss in ice mass and a contribution to sea level rise.  It is unclear how long the grounding zone of the WIS was at its present position, but there has been little mass loss from the WIS in recent decades.  By studying the ice-ocean interface, we can start to address the questions:
     How stable is the WIS?
     What kind of substrate is the ice grounded?
     How does the sediment coming from upstream influence this substrate?
     At what rate is warmer seawater melting the ice at the grounding zone?
     In what manner is subglacial water discharged into the ocean?

The WISSARD team found life in the subglacial water and sediments of Subglacial Lake Whillans, and we want to know what kinds of life inhabit the grounding zone.  Subglacial water and melting ice provide fresh water and ocean currents bring in salty water.  How saline is this environment?  The weathering of rocks as the ice flows over them provides a source of nutrients to microbes that Martyn Tranter will study.  We plan to take samples of the sediment from the seafloor, seawater in the grounding zone, and basal ice.  Ice at the base of the ice sheet tends to have layers of dirty ice that it picked up along the way.  In some places at the base of the ice it is melting and in other places it is freezing.  Often when the subglacial water freezes onto the ice, it does so with pieces of the substrate, rock fragments of whatever size is available.  These basal ice samples thus tell us something about what substrate lies upstream.  This dirty ice eventually flows over sea water and starts to melt at the base, raining out these rocks.  This ice-rafted debris, as the science community calls it, has been identified in sediment cores further out on the ocean floor to reconstruct how the extent of ice has changed through time.  

We know comparatively little about the geology of Antarctica because it is buried beneath the ice sheet.  West Antarctica is home to the largest rift zone in the world and we know there are still active volcanoes in the region.  Geothermal heat flux can provide us important information about the geologic history of the region and give us a better idea of the rate of melting at the base of the ice sheet.  There are few measurements of geothermal heat flux beneath the ice sheet, because it requires drilling to the rocks below the ice.  However, the measurement that we took two years ago at Subglacial Lake Whillans and the measurement from the West Antarctic Ice Sheet Divide, located further east, are much higher than magnetic studies and seismology had predicted.  The UCSC team is excited to get another measurement of geothermal heat flux.  If it is also high, that lends more credence to the hypothesis that the whole region has high heat flow.

The UCSC team also aims to monitor the ice stream throughout the year.  We have been doing this through passive seismology and GPS at stations scattered over the Whillans Ice Plain.  We have been returning to the last three holes to take measurements Some of these sensors we will be using this year are the same as those in other holes; temperature, pressure, tilt, seismometers; and the turbidity sensor is a new addition.  The more sensors we have, the more cables we need to simultaneously lower down the hole.  The more independent cables we have, the more winches we need to lower them and more hands to operate the winches.  Because of this challenge, we spent some time in Santa Cruz taping two of the cables together so they could go off of the spool at once.  As we lower all the cables down the hole, we'll also be taping them together so they will all hang together and we'll know exactly what the spacing is between them.
This turbidity sensor looks at the amount of light that is reflected to determine how murky the water is.  We want to know how the amount of sediment being discharged from the subglacial environment changes through time.

We received news today that the visibility is only 30 feet at the grounding zone due to blowing snow. To minimize drifting we store our cargo in lines parallel to the wind direction.   If the bad weather continues, I may not get into the field with the rest of the team tomorrow.  The drillers plan to carry on, working around the clock in two 12 hour shifts. We hope to have the borehole open in time to fit 8 days of sampling and deploying our instruments.  All of this planning for only 8 days!  We hope everything goes smoothly, but more likely than not there will be some kinks.

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