Project: Phosphorus cycling in ridge-flank sediments

Phosphorus (P) plays a key 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, since P bioavailability can impact microbial activity. This project focuses on characterizing and quantifying the main reservoirs of solid-phase P in open ocean sediments, and uses stable oxygen isotopes in phosphate as a proxy for studying P cycling in deep sea sediments. The sediment samples used in this study were collected during the International Ocean Discovery Program (IODP) Expedition 336 to North Pond, a sediment-filled depression in the oceanic crust located on the western flank of the Mid-Atlantic Ridge, and during IODP Expedition 327 to the Juan de Fuca Ridge flank.  We characterized solid-phase P reservoirs in sediments using the 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. We then conducted stable phosphatic oxygen isotope ratio analyses on a subset of those samples from borehole U1382B at North Pond (X336) to further constrain the biotic and abiotic factors influencing P cycling at this site.

NSF C-DEBI Award #156246 to Dr. Adina Paytan

NSF C-DEBI Award #157598 to Dr. Delphine Defforey