169-2 Redox Reorganization of Sedimentary Phosphorus Cycling from the Archean to the Mesoproterozoic

Session: Phantastic Phosphorus and Its Historical Biogeochemistry

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Abstract:

Few elements connect the biochemistry of life to planetary surface evolution as directly as phosphorus. Over geologic timescales, phosphorus is regarded as an ultimate limiting nutrient, linking organic carbon fixation and burial to ocean-atmosphere redox coevolution. Because geological processes govern its supply, the sedimentary record archives past phosphorus cycling. Yet phosphorus is effectively monoisotopic, so its sources, sinks, and transformations must be reconstructed from elemental concentrations, mineral hosts, and proxy relationships.

Precambrian phosphorus records appear contradictory. Low phosphorus concentrations in shales and low phosphorus-to-iron ratios in iron formations suggest persistent phosphate scarcity. In contrast, elevated carbonate-associated phosphate indicates appreciable incorporation during carbonate precipitation, while the near-synchronous global appearance of phosphorites after the Great Oxidation Event documents phosphorus concentration and burial in a new sedimentary host. This paradox partly reflects comparisons among archives that record different processes. Resolving it requires distinguishing changes in sedimentary phosphorus inventory from shifts in its delivery, recycling, retention, remobilization, burial, and preservation across depositional environments.

We compiled approximately 16,000 sedimentary geochemical analyses spanning 4.0–1.0 Ga from the Sedimentary Geochemistry and Paleoenvironments Project and published datasets. Bulk phosphorus in shales, fine-grained siliciclastics, carbonates, and chemical sediments varies by orders of magnitude and shows a gradual, nonmonotonic increase through time. Era-level distributions are statistically distinct, but the uneven pattern indicates dynamic evolution rather than a single GOE-associated rise. Phosphorus-to-iron ratios show a stepwise shift, suggesting that phosphorus relationships with iron-bearing phases changed more strongly than bulk concentrations alone. Phosphorus-to-sulfur ratios follow a contrasting trajectory, rising nearly 2000% from the Archean into the GOE, declining sharply afterward, and rebounding in the Mesoproterozoic.

These patterns suggest that oxygenation reorganized phosphorus cycling through competing redox-sensitive pathways rather than initiating a simple positive feedback in bioavailability. Across the GOE, oxidative weathering likely increased phosphate and sulfate delivery, while Fe(III) oxyhydroxides scavenged phosphate and promoted burial. Concurrent sulfate reduction generated sulfide, drove reactive iron sulfidization and pyrite formation, weakened iron-associated phosphorus retention, and enhanced phosphate remobilization. Paleoproterozoic phosphogenesis may therefore have developed where increased nutrient delivery coincided with sulfur cycling, iron sulfidization, Ca-rich pore waters, and limited clastic dilution. However, the rarity of Mesoproterozoic phosphorites despite higher bulk phosphorus and renewed phosphorus-to-sulfur enrichment indicates that phosphorus abundance alone was insufficient to drive widespread phosphogenesis.

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