169-4 Was hydrothermal circulation a source or a sink of phosphorus to the Archean biosphere?

Session: Phantastic Phosphorus and Its Historical Biogeochemistry

Presenting Author:

Benjamin Johnson

Authors:

Johnson, Benjamin William¹, Som, Sanjoy², Anderson, Rika³, Stüeken, Eva⁴

(1) Earth, Atmosphere, Climate, Iowa State University, Ames, IA, USA, (2) Blue Marble Space, c/o NASA Ames Research Center, Moffett Field, CA, , (3) Carleton College, Northfield, MN, , (4) School of Earth & Environmental Sciences, University of St. Andrews, St. Andrews, United Kingdom,

Abstract:

Phosphorus is a critical nutrient, but is often limiting since it can only be delivered from geologic reservoirs. While continental weathering is important in modern sites, this source was likely less large in the Archean due to limited crustal emergence. Recent studies have suggested that hydrothermal circulation of seawater through oceanic crust may have released substantial P into the ocean during the Archean. Modern hydrothermal activity, though, is a sink for P. Further, at high temperatures during alteration apatite can form, potentially trapping P. To assess this possibility, we used the 3.24 Ga Panorama District volcanogenic massive sulfide (VMS) to constrain the behavior of P in a well preserved hydrothermal setting. Panorama preserves 5 distinct hydrothermal cells, with units that capture the full depth of these cells, spanning the paleoseafloor down several km to a granitic intrusion which provided heat to drive circulation. We find that P and K are anticorrelated, suggesting potassic alteration at relatively low temperatures. In contrast, P correlates with C, with P:Ca ratios matching the stoichiometry of fluorapatite in high-temperature zones. Yet, reactive transport modeling indicates that phosphite may be liberated during reaction with andesitic rocks, which are also found at Panorama. Our work indicates that there may be competing effects on P during circulation, and that P released may have been phosphite, rather than phosphate. 

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