Karen Casciotti's MIT course 12.742, Marine Chemistry, turns to the seafloor in this session on early diagenesis, the chemical transformations sediments undergo just after burial. She argues that the surface sediment layer, though only 10 to 20 centimeters thick, functions as a boundary as important to ocean chemistry as the air-sea interface or an estuary, mediating exchange between the water column and the deeper sediment record. The lecture runs 78 minutes and builds on earlier sessions in the course, working through the processes that control what gets buried, what gets recycled back into the water, and how that balance shapes the chemical signals oceanographers read out of marine sediment cores. Recorded at MIT in fall 2006 as part of OpenCourseWare, the session assumes some prior grounding in chemical oceanography but stays focused on explaining why this thin layer matters so much to the larger ocean chemistry picture.