P in marine sediments Overview: P Cycling in the Ocean Inputs and Outputs and Residence Time Sediment Chemistry of P Selective leach procedures & implications A quick look at P preservation in sediments

Phosphorus in marine sediments P : an abundant element in the crust: ~ 0.1% Like Nitrogen, Phosphorus is an essential nutrient There is evidence that P is a limiting nutrient --- in some locations in the modern ocean It is argued that P is the limiting nutrient on long time scales --- because of the large pool of N 2 in the atmosphere --- if that’s true, then P may influence atmospheric O 2 and CO 2 on long time scales

P cycle - overview Ruttenberg, 2003

What is the residence time of P in the ocean? Oceanic P reservoir: 3000 x mol Inputs of reactive P x mol P / y (mostly from rivers; most of that is particulate P) Burial of reactive P in sediments: x mol P /y ==> Res time = Amount in reservoir / flux = 10, ,000 y based on input rate = 12, ,000 y based on burial rate

Inputs - from continents

How much of this particlulate input is reactive?

Removal of P from the oceans

Removal with organic matter Most C org burial Ratio =4x10 -3 Calculate removal with o.m.: (P/C org ) x C org burial rate From Froelich 1982

Removal by hydrothermal processes

… into metalliferous sediments

Pore water chemistry of reactive P Ruttenberg, 2003

The principal flux of reactive P to the sediment-water interface arrives with organic matter ==> C:P released to pore Waters by organic matter Oxidation ~ 106:1 ==> in the deep sea, P is efficiently recycled: no formation of authigenic P minerals… Central equatorial Pacific MANOP Site C

Continental margin sediments: * large organic matter flux * electron acceptors other than O 2 Let’s consider a sediment dominated by sulfate reduction: Defining P as the production rate of a solute, What would we predict pore water profiles of these 3 solutes to look like? Solution:

Solve the equation for each solute: Assume porosity = 0.8 and D sed = D sw x (porosity) 2 … then D HCO3- = 323 cm 2 /y, D NH4+ = 543 cm 2 /y, D HPO42- = 208 cm 2 /y Assume P 0 = 100, p 1 = 0.2

Plotting the concentration of one solute vs. another… Interpreting the slopes: At any depth, Therefore, the slopes imply

What is observed in high-Corg flux sediments? P cycling apparently decoupled from C org remineralization Red lines = Predicted from o.m. stoichiometry

Why are PO4 concentrations so much higher than predicted by organic matter stoichiometry? Benthic flux chambers: one with O2 kept high, one not NH4 flux ~ same in both Fluxes of PO4 and Fe both increase Dramatically when O2 drops. PO4 adsorbs readily to Fe oxides: PO4 is cycled with Fe in high-Corg flux sediments

The formation of marine apatites In marine sediments: “carbonate fluorapatites X = Fluorine CO 3 2- substitutes for PO 4 3- For example, an equation for the formation of francolite

A picture of the mechanism of apatite formation in marine sediments Need: 1) large flux of organic matter (P source) 2) mechanism for concentrating P To exceed solubility of apatite Froelich et al Mar. Geol. 80,

Example -- upwelling region in northeast Pacific Jahnke et al. (1983) GCA 47, Is the solubility product of apatite (with a realistic composition) exceeded? 2.Pore water evidence for the formation of CFA

The solubility of CFA Solubility increases as CO 3 2- increases

Pore water evidence for CFA formation Coincident removal of F and PO 4

Another example from the northeast Pacific Schuffert et al.(1994) GCA 58, ** more pore water evidence ** confirmation by XRD

Pore water data Note: ** subsurface PO4 maxima: enhancement of [PO4] by cycling with Fe ** apparent coincident removal of P and F

Combine pore water data with solid phase measurements Grey bands: phosphorite layers (> 5% P) determined by XRD Contemporary phosphorite formation Earlier phosphorite formation

Removal of P from the oceans

Partitioning solid phase P…

Even though you can’t measure apatite directly in sediments of non-upwelling regions,there’s probably important formation of smaller quantities in other margin sediments Results of selective leaches showing coincident decrease in (A) organic P and increase in authigenic CFA an (B) Decrease in Fe-bound P and increase in auth. CFA AB

Recycling of PO4 to the water column