Award: OCE-0927754

One of the main goals in oceanography is to understand why the ocean has its particular chemical makeup and how various elements enter and leave the ocean. In this project, we study how a particular group of elements is transported within and removed from the ocean. As it turns out, most of the oceans have the same amont of uranium regardless of depth or geographic location. Because of the constant uranium concentration, a number of elements are produced from uranium in the ocean at constant and known rates. These include varieties of the elements thorium (thorium-230) and protactinium (protactinium-231). In this study, we analyzed the concentrations of these varieties of these elements in seawater collected from across the north Atlantic Ocean from Europe and north Africa to the East Coast of the U.S. Because they are produced at a known rate, our measurements give us information about how they are transported from one portion of the ocean to another and how they are removed from the ocean. In addition, we measured concentrations of another variety of the element thorium (thorium-232). This variety of thorium is contained within dust blown onto the ocean from the continents (for example from the Sahara desert) and within sediments such as small clay particles transported to the ocean in rivers. Measurements of this variety of thorium gave us information about the rate at which thorium has been added to the ocean from dust and sediments. In order to carry out the project, we needed to refine our skills in measuring these elements because their concentrations in sea water are vanishingly small. For example, imagine you collected a bucket of sea water and wanted to concentrate the protactinium in that bucket to high levels. Suppose you took one millionth of that bucket of water, which contained every atom of protactinium in that bucket. You would not be anywhere close to having a concentrated solution of protactinium. You would need to take one millionth of that one millionth (containing all the original protactinium), then one millionth of that one millionth (containing all the protactinium) before you would have a concentrate of the protactinium contained in the original bucket. In the course of this project, we refined our measurement skills to do just that. We were then able to carry out the project successfully. Once we refined our measurement skills and measured hundreds of sea water samples, we were rewarded by a remarkable picture of the concentrations of the varieties of these elements across the Atlantic. Illustrated in the accompanying diagram is one of these pictures (for thorium-232). Just from a visual perusal of the diagram, one can see the main outcomes/conclusions of our work. High concentrations near the surface, particularly off of Africa are indicative of input from dust blown over the ocean by winds. High concentrations along the seafloor are from sediments on the ocean bottom brought up into the deep ocean by currents along the seafloor. Low concentrations above the Mid-Atlantic Ridge (the high point in the seafloor in the middle of the picture) are caused by small particles that incorporate thorium within them, then settle to the sea floor. These particles form when water from submarine hot springs (along the Mid-Atlantic Ridge) mix with seawater. Low concentrations in deep waters along the African coast are caused by sinking particles that carry thorium on their surfaces, effectively removing it from the ocean and burying it in the accumulating sediments of the sea floor. This picture and similar ones for other elements tell us a lot about the cycles of pertinent elements in the ocean, just at first glance. However, these measurements will merit careful attention and further analysis in coming years. They will further our understanding of how the ocean works. Last Modified: 06/25/2014 Submitted by: R. Lawrence Edwards