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Project Progress

A fundamental control on marine productivity is the supply and recycling of bio-available nitrogen (N) in the photic zone. Thus, understanding N-cycling throughout Earth’s history has critical implications for the evolution of complex marine ecosystems on geologic timescales. Records of the marine N cycle are preserved in the isotopic composition (δ¹⁵N) of sedimentary organic material. In the modern ocean, the average seawater nitrate isotope value of ~ +5‰ is driven by water column denitrification, which preferentially removes ¹⁴N, enriching ¹⁵N in the remaining nitrate pool. However, during broad stretches of Paleozoic, there were several extended intervals with δ¹⁵N values < 0‰, and the Proterozoic remains almost entirely unexplored. Modern N-cycling processes, mainly denitrification, can explain fluctuating values above 0‰. In contrast, the exceptionally low values for δ¹⁵N during the early-mid Paleozoic (≤ -3‰) cannot be explained through the modern framework of competing effects of denitrification and N-fixation. These patterns lead to the question: What process causes sediments to have δ¹⁵N values < 0‰? We are exploring this question under the hypothesis that the δ¹⁵N record may be a robust indicator of the history of Earth’s oxygenation.

Sedimentary porphyrins, the tetrapyrrole skeletons of ancient chlorophyll molecules, preserve primary N isotopic signals. We have developed a rapid approach to measure δ¹⁵N values of nitrogen in porphyrins (Higgins et al., 2009). These measurements allow us to evaluate two fundamental biogeochemical controls on the ancient marine nitrogen cycle: patterns in the absolute value of δ¹⁵N supplied to the photic zone, and the type of organism responsible for most of this production (Eukaryotes or Cyanobacteria). The latter is evaluated using the parameter εpor, which is the approximate difference between δ¹⁵N of total organic nitrogen in the source rock (measured as whole rock or kerogen) and δ¹⁵N of the porphyrin: εpor ≈ δ¹⁵Ntotal – δ¹⁵Npor.

This value is taxonomically distinct and likely is related to intracellular fractionations associated with biosynthesis. Eukaryotes have εpor = 5 ± 2‰, and marine Cyanobacteria have εpor = 0 ± 2‰ (Higgins et al., 2011). Using this approach, we were able to determine three key features of episodes of negative δ¹⁵N values: (1) ¹⁵N excursions can be out of phase with ¹³C excursions; (2) values of ¹⁵N were < 0‰ cannot be explained by a change in the type of primary producer; and (3) export production to sediments appears to be dominantly eukaryotic throughout the Phanerozoic. These results lead to some obvious questions: When did the δ¹⁵N signature of primary production become > 0‰? Our recent modeling work suggests that negative or near-zero values of δ¹⁵N indicate an ocean with significant (possibly > 50%) anoxic deep basins dominated by ammonia cycling (Naafs et al., in prep). Ventilation of the deep oceans with oxic waters is an important turning-point in Earth’s redox history.

Additional data on this problem is being collected by Greg Henkes, a post-doctoral investigator in the Pearson lab. In collaboration with UC-Riverside team members Noah Planavsky (Yale University), Gordon Love (UC-Riverside), and a group from the Georgia Institute of Technology, including Ellery Ingall, we have obtained a number of Proterozoic and Paleozoic shales which we already know contain sufficient porphyrin for isotopic measurements (Table 1). We anticipate completing the analyses in 2016.