BIOLOGIC IRON LIMITATION IN THE LATE PERMIAN AND EARLY TRIASSIC OCEAN

The sparsely productive Late Permian and Early Triassic (LP/ET) oceans were probably strongly iron limited because of significant reductions in iron input to oceans and intense sulfide burial that removed iron from oceanic waters. Support for this suggestion, includes: 1) inferences from chemical weathering and modeled atmospheric behavior which suggest that iron input to oceans was minimal, 2) sulfur isotope systematics and analogy to the Mesoproterozoic “Canfield” ocean which favored intense burial of iron as sulfides, and 3) model results for Late Permian anoxia which predict abundant availability of phosphorus in anoxic waters that are not characterized by high productivity.

Diminished chemical weathering of silicate rock in a world characterized by expanded desert belts and a lack of continental collision orogenies would have reduced the availability of terrestrial iron in the LP/ET. Despite widespread desert settings in the Late Permian and Early Triassic, delivery of fine particles of iron oxide to oceans via windblown dust was probably greatly reduced. Model results (Kidder and Worsley, 2004) suggest a 50% reduction in planetary wind shear. Remaining detrital iron oxide input as particulate matter by rivers settled quickly from the water column and was buried in coastal sediments. Poleward retreat of Late Permian terrestrial vegetation further curtailed chemical weathering, and the plant extinctions at the boundary would have shut it down even further.

Iron limitation in the LP/ET was probably even more intense than that proposed by Canfield (1998) for the Mesoproterozoic ocean. Atmospheric oxygen levels were probably fairly low in the LP/ET, but they were much higher than Mesoproterozoic levels. The resulting higher levels of oceanic sulfate in LP/ET surface waters coupled with lower hydrothermal supplies than in the Mesoproterozoic fueled euxinic conditions in deeper ocean waters, totally removing the hydrothermal iron supply.

The predicted abundance of available phosphorus in LP/ET oceans that resulted from modeling of anoxia in those waters (Hotinski et al., 2001) is consistent with the iron-limitation hypothesis for the interval. If phosphorus were limiting, an abundance of it in ocean water would be unlikely.