HOW COMMON IS A NEGATIVE NITROGEN ISOTOPE EXCURSION DURING THE ONSET OF MAJOR BIOGEOCHEMICAL PERTURBATIONS?

Major biogeochemical events in Earth history leave clear signals in the chemostratigraphic record. Most of these major events were first identified by their carbon isotope signal that recorded the perturbation to the global carbon cycle that is typically associated with these events. These perturbations are most often marked by a pronounced positive excursion in the carbonate and organic carbon isotope record that are traditionally interpreted to represent increased burial of organic carbon that preferentially removed ¹²C from seawater and transferred it to the sedimentary record. Enhanced burial of organic carbon in reducing environments is favored during episodes of expanding anoxia/euxinia that are best known during the Cretaceous and referred to as Oceanic Anoxic Events (OAEs). Whereas the initial trigger of some events has begun to be identified, many of the most significant biogeochemical events of the Paleozoic remain enigmatic.

Here, we compare carbon and nitrogen isotope data from multiple biogeochemical events throughout the Paleozoic and Mesozoic including the Cambrian SPICE Event, Silurian Ireviken Event, Devonian-Carboniferous Hangenberg Event, and Cretaceous OAE2 that demonstrate a remarkably consistent pattern of perturbation. In each case a significant negative excursion in nitrogen isotopes occurs during the overall event initiation that indicates a fundamental reorganization of local nutrient cycling at the onset of these events. In total, this comparison demonstrates that changes in nutrient cycling and primary productivity were likely a significant component of event initiation and may play a common role in many, if not most, major biogeochemical events in Earth history.