ORGANIC matter preserved in marine sediments provides a molecular record of marine biological processes(1), accounts for approximately 20% of all carbon burial(2) and plays a key role in balancing the long-term flux of oxygen to the atmosphere(3). Only recently has it been appreciated that more than 90% of the organic matter preserved in most marine sediments is intimately associated with mineral surfaces(4). Little is known, however, of the effect that sorption to mineral surfaces might have in controlling either the lability or-quantity of-organic matter in the marine sedimentary record. The preserved organic material could be either intrinsically stable, or stabilized through interactions with mineral matrices. We show here that sorption of organic matter to mineral surfaces in marine sediments stabilizes the component molecules, slowing remineralization rates by up to five orders of magnitude. Sorptive protection can therefore account for the enigmatic preservation of intrinsically labile molecules such as amino acids and simple sugars in marine deposits(5,6) and links the preservation of organic carbon in marine sediments to the deposition of mineral surfaces.