Identifying Freshwater Sources and Dissolved Iron Input along the Western Antarctic Peninsula

We are now 4 days into our 5 days of science time aboard the ARSV Laurence M Gould.  The overarching goal of this project is to quantify the rate and chemical signature of the freshwater discharging into the coastal area near Anvers Island, on the western side of the Antarctic Peninsula.  As part of this project and while we are aboard the Gould, we hope to evaluate the rate at which water is mixing from the shoreline to the open ocean (what we call the “continental shelf”).  This lateral mixing from the coastal zone to waters offshore will transport land-derived chemical components such as iron (Fe).  Iron is referred to as a “micronutrient”, which means it is needed by marine phytoplankton (small plants) in very small amounts.  As it turns out, iron is thought to be “bio-limiting” in the Southern Ocean, i.e., there is often not enough iron in the ocean for phytoplankton to grow.

 

To quantify the mixing of iron across the shelf, and ultimately the actual source of  the water, such as ice sheet melting and groundwater, we are using different chemical elements (actually, naturally occurring radioactive and stable isotopes) as tracers of water movement.  The variation in concentration and isotopic signature of these tracers in samples collected across the shelf will provide information on the age and the source (fresh vs. ocean) of the water.

 

We have set up several transects that run perpendicular from the shoreline across the continental shelf (see the picture of one of our charts that we are currently using…the colors represent differences in depth, known as bathymetry; where red is shallow, blue is deep, and gray is land)  Note in this photo the sites labeled with marker, site names, and the orientation).  To prepare for the cruise, we had selected ~20 stations that we planned to sample.  However, this will typically change slightly while at see…sometimes additional sites will be added, but other times weather or mechanical failures might shorten the amount of time and prevent sampling all the stations.  Fortunately for us, we have a great crew and perfect weather, so we have been actively adding sites to our sampling grid…providing additional insight into the “cross-shelf mixing”.

 

At each station, we collect large volumes of water for analysis of these water tracers.  The water is filtered and analyzed in the laboratories on the ship or stored for analysis back at our home institutions.  We are currently analyzing samples on the ship for the element radium (Ra), specifically the short-lived Ra-224 (see second picture of the analysis).  We are also measuring the radioactive gas radon (Rn), the isotope Rn-222 (see the third picture).  The source of these tracers will either be from the sediments on the ocean floor or from groundwater onshore.  We will also collect sediments from each station to determine if these are a significant source of the tracers. Other samples to be analyzed back home, including those for nutrients (nitrogen, phosphorus), helium (He), and isotopes of oxygen (O-18) and hydrogen (H-2), will help decipher the source of the water.

 

Once we get to Palmer Station, our work will focus on many of these same tracers, but our major scientific goal will be slightly different.  Again, our offshore work is primarily focused on evaluating mixing across the shelf.  Our research at Palmer Station will focus on determining the amount of freshwater entering the coastal ocean and its source, groundwater or direct run off from ice sheet melting.  See more about Palmer Station soon…

DRCorbett