Enhanced phosphorus recycling during past oceanic anoxia amplified by low rates of apatite authigenesis

Abstract

Enhanced recycling of phosphorus as ocean deoxygenation expanded under past greenhouse climates contributed to widespread organic carbon burial and drawdown of atmospheric CO 2 . Redox-dependent phosphorus recycling was more efficient in such ancient anoxic marine environments, compared to modern anoxic settings, for reasons that remain unclear. Here, we show that low rates of apatite authigenesis in organic-rich sediments can explain the amplified phosphorus recycling in ancient settings as reflected in highly elevated ratios of organic carbon to total phosphorus. We argue that the low rates may be partly the result of the reduced saturation state of sediment porewaters with respect to apatite linked to ocean warming and acidification and/or a decreased availability of calcium carbonate, which acts as a template for apatite formation. Future changes in temperature and ocean biogeochemistry, induced by elevated atmospheric CO 2 , may similarly increase phosphorus availability and accelerate ocean deoxygenation and organic carbon burial.

Biological diversification linked to environmental stabilization following the Sturtian Snowball glaciation

“…Furthermore, microbial sulfate reduction under euxinic water column conditions, or within sulfidic sediment pore waters, promotes the preferential release of P from organic matter and limits the formation of unsulfidized Fe minerals to which P can readily re-adsorb ( 36 , 51 , 52 ). Sulfidic pore water conditions, elevated temperature, and reduced pH may also limit P uptake through inhibition of apatite authigenesis ( 53 ). In this way, sulfidic conditions, ocean warming, and acidification all have the potential to supplement nutrient input for primary production in surface waters by driving extensive bioavailable P recycling from sediments ( 48 , 54 ).…”

“…In a low-sulfate, post-Sturtian global ocean, the gradual decrease in weathering-derived nutrient and sulfate input would have reduced the shallow marine area conducive to the development of euxinic water column and sulfidic porewater conditions, leading to enhanced sedimentary P retention in shelf environments. Global cooling and decreasing atmospheric pCO 2 may also have led to enhanced apatite authigenesis ( 53 ). This was accompanied by a corresponding reduction in the global areal extent of shallow seafloor conducive to bioavailable P recycling, which persisted in lower productivity deep basin environments ( Fig.…”

Oxygen rise in the tropical upper ocean during the Paleocene-Eocene Thermal Maximum

“…However, rapid warming intervals are typically associated with reduced rates of ventilation ( 42 ), which would tend to work against the observed PETM oxygenation, exacerbating the need for lower tropical productivity. With regard to nutrient concentration, there have been suggestions of a global increase in nutrient reservoir during the PETM ( 43 ); this alone would increase nutrient supply to tropical surface waters, the opposite of what is needed to explain the Site 865 data and the ODZ contraction.…”

A Double‐Edged Sword: The Role of Sulfate in Anoxic Marine Phosphorus Cycling Through Earth History

“…Indeed, low‐sulfate lakes retain P in anoxic sediments (Caraco et al., 1989). The reasons for this may include: diminished Fe‐sulfide formation, enabling precipitation of authigenic Fe(II)‐phosphates (Xiong et al., 2019), greater apatite authigenesis in non‐euxinic conditions (Papadomanolaki et al., 2022), and limited organic remineralization due to muted sulfate reduction. Whatever the mechanism(s), sulfate affects whether P is released or retained by anoxic sediments.…”