Thorium-230 is produced throughout the oceans by the radiocactive decay of a naturally occuring isotope of uranium and chemically adheres to marine particles that sink to the seafloor. According to theory, 230Th should increase steadily as you go deeper, as it does at 47?S (left). But at 22?N (right), 230Th concentrations level off in the abyss. The North Atlantic Ocean?s deep southward circulation (blue arrow) is probably influencing what would otherwise be a steady increase of 230Th concentration to the bottom. (Illustration by E. Paul Oberlander, Woods Hole Oceanographic Institution)[back]
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