Dierdre Toole covers the global phosphorus cycle in this MIT session from 12.742 Marine Chemistry, Fall 2006. She frames phosphorus as chemically simpler to track than carbon or nitrogen because it lacks a significant gaseous phase, so the cycle can be traced almost entirely through solid and dissolved forms. The lecture follows phosphorus from weathering rock through soils and rivers into the ocean, where it settles into sediments before eventually being uplifted back into crust over geologic time. Toole discusses the biological uptake of phosphorus by marine organisms and how burial and recycling in sediments regulate its ocean concentration. The session is part of a graduate marine chemistry course and assumes some familiarity with oceanography and chemistry, presented as a straightforward chalkboard-style lecture rather than a polished production.