PHOSPHORUS, NITROGEN AND THE REDOX EVOLUTION OF THE PALEOZOIC OCEANS

Oceanic d¹³C_carb fluctuations during the Paleozoic Era were transitional between the highly unsettled Neoproterozoic and the increasingly stable Mesozoic and Cenozoic periods. The record shows several major excursions to values as high as +7‰, which are on par with the Neoproterozoic peaks but generally shorter in duration and lacking in associated large negative shifts. A new high-resolution record from the exceptionally well-exposed and well-dated Paleozoic (Middle Cambrian to Early Permian) succession in the Great Basin is presented here. Eight d¹³C_carb excursions are identified, which are all known outside the region and considered to represent important paleoceanographic events. These excursions fall into three separate time intervals characterized by d¹³C volatility in the Late Cambrian (Steptoean), Late Ordovician-Silurian, and Late Devonian-Early Mississippian, all of which are separated by relatively stable periods with values between —2 and +2‰.

The excursions are most simply interpreted to reflect increases in the fraction of marine carbon buried as organic matter, commonly attributed to enhanced primary production. The control of primary production in the world oceans by either nitrogen or phosphorus is a longstanding debate in paleoceanography. P is usually considered the ultimate limiting nutrient on geologic timescales due to its much longer residence time. However, rates of denitrification would have been much higher than today in the Paleozoic due to widespread oceanic anoxia, which could lower N/P below Redfield ratios for geologically significant time periods. Nitrogen fixers would raise N/P ratios but may have been inhibited by a lack of essential trace elements (i.e., Fe or Mo) that resulted from scavenging in euxinic (anoxic-sulfidic) waters or lower eolian delivery during climatic optima. A nitrogen limited ocean, in comparison to P-limitation, would be characterized by a general stability in d¹³C because of the negative feedback on oxygen levels (denitrification). Intervals of large excursions are thus interpreted to reflect a switch from N to P limitation driven by redox state transitions in the Paleozoic oceans.