The size distribution of framboidal pyrite in modern sediments : an indicator of redox conditions

Relationship between inferred redox potential of the depositional environment and geochemistry of the Upper Pennsylvanian (Missourian) Stark Shale Member of the Dennis Limestone, Wabaunsee County, Kansas, U.S.A.

The phosphorus cycle, phosphogenesis and marine phosphate-rich deposits

The biogeochemical cycling of phosphorus in marine systems

Phosphorus (P) is an essential element to all life, being a structural and functional component of all organisms.1-4 P provides the phosphate-ester backbone of DNA and RNA, and it is crucial in the transmission of chemical energy through the ATP molecule (Figure 1). P is also a structural constituent in many cell components such as phosphoproteins, and phospholipids in cell membranes, teeth, and bones (Figure 1). In some organisms it can also be present as intracellular polyphosphate storage granules (Figure 1). Phosphorus availability can impact primary production rates in the ocean as well as species distribution and ecosystem structure.5-8 In some marine and estuarine environments, P availability is considered the proximal macronutrient that limits primary production.8,9 Specifically, in recent years it has been recognized that phosphorus limitation in the ocean may be more prevalent than previously thought. For example, it is generally accepted that orthophosphate (PO4) is the limiting nutrient in the eastern Mediterranean Sea.10-12 In addition, phosphate is suspected to play an important role in limiting production in the Sargasso Sea13 and may also be limiting in bodies of water receiving large freshwater inputs or fertilizer runoff from agriculture, as in the Chesapeake Bay.14,15 Furthermore, research in the Pacific Ocean gyres indicates that biological P uptake rates far surpass the combined input from atmospheric and deep water sources, suggesting that P is efficiently recycled within oligotrophic euphotic zones.16 It has also been suggested that transitions over the last two decades from nitrogen (N) limitation to P limitation have taken place in the North Pacific subtropical gyre and that this may be responsible for the observed succession of prokaryotic picophytoplankton such as Prochlorococcus and Synechococcus in oligotrophic waters.6 In other marine environments, P may only be limiting a subset of organisms within the ecosystem.17-19 Phosphorus, in the form of orthophosphate, plays a key role in photosynthesis (i.e., primary productivity). The chemical equation representing average ocean photosynthesis can be written as

The oceanic phosphorus cycle.

Early diagenesis of organic matter in Peru continental margin sediments: Phosphorite precipitation

The present day formation of apatite in Mexican continental margin sediments

Modern sea-floor phosphorite nodules sampled from off the coast of Peru are shown to grow at rates of millimeters per thousand years in spite of the fact that the measured accumulation rates of the underlying sediment are two to four orders of magnitude faster. Phosphate nodules grow downward into soft sediment, and the rates of phosphorus accumulation into nodules are approximately equal to the upward diffusive flux of dissolved phosphate inferred from pore water profiles. These results demonstrate that phosphorus in nodules originates from regeneration in sediments rather than as a result of direct precipitation from bottom waters.

https://science.sciencemag.org/content/sci/215/4540/1616.full.pdf

Growth rates of phosphate nodules from the continental margin off peru.

Phosphorites on the current sea floor are known to occur in both upwelling and non-upwelling environments. Uranium-series and rare earth element (REE) studies display certain characteristics which may be a consequence of the type of phosphorite formation prevalent in a coastal upwelling environment. Quaternary phosphate nodules exposed at the sediment-seawater interface in the upwelling zone off Peru/Chile are shown to: (a) display unidirectional growth downward into the underlying sediments at rates which are slow compared to rates of associated sediment accumulation; (b) contain small but measurable amounts of excess 231Pa relative to 230Th; and (c) have very low concentrations of REE with a shale-like pattern rather than one resembling seawater. Results from phosphorites sampled from low-productivity areas (e.g., Pacific seamounts, the Agulhas Bank) indicate extreme differences in these characteristics when compared to those from Peru/Chile. Radiochemical and geochemical approaches thus offer promise for distinguishing “upwelling” from “non-upwelling” suites of phosphorites.

Kevin K. Roe

Papers

Early diagenesis of organic matter in Peru continental margin sediments: Phosphorite precipitation

The present day formation of apatite in Mexican continental margin sediments

Growth rates of phosphate nodules from the continental margin off peru.

Upwelling and Phosphorite Formation in the Ocean

Uranium geochemistry and dating of Pacific island apatite