Phosphorus (P) plays an important role in fueling life, including microbial life in the deep subseafloor environment, which is estimated to contain up to 1% of Earth’s total biomass. These microorganisms play a significant role in controlling the chemical composition of the deep ocean and atmosphere on geological timescales by selectively degrading organic matter through metabolic respiration. Consequently, understanding P geochemistry in subseafloor sediments is important, as P bioavailability can impact microbial activity. This study focuses on characterizing and quantifying the main reservoirs of solid-phase P in open-ocean sediments. The sediment samples used in this study were collected during Integrated Ocean Drilling Program (IODP) Expedition 336 to North Pond, a sediment pond located on the western flank of the Mid-Atlantic Ridge. We characterized solid-phase P reservoirs in sediments from four holes (U1384A, U1382B, U1383D, and U1383E) using the sedimentary extraction (SEDEX) sequential extraction scheme. This method quantitatively separates five distinct sedimentary P reservoirs: (1) loosely sorbed P, (2) ferric iron-bound P, (3) authigenic carbonate fluorapatite + biogenic apatite + CaCO3-associated P, (4) detrital apatite, and (5) refractory organic P. The separation of these P-bearing phases is based on the reactivity of each targeted phase to a particular extractant solution.