Evidence from chlorin nitrogen isotopes for alternating nutrient regimes in the Eastern Mediterranean Sea
Date of Original Version
Nitrogen isotopes of chlorins, degradation products of chlorophyll, reflect the isotopic composition of nutrient N utilized by marine phytoplankton communities. Here we show that in sediments of the eastern Mediterranean Pleistocene and Holocene, values of δ15N for chlorins and total nitrogen vary in concert, with a consistent offset of ∼ 5‰ reflecting the fractionation imparted during chlorophyll biosynthesis. Samples from the Integrated Ocean Drilling Program Sites 964 and 969 were analyzed at a sampling resolution of ∼ 4-10 cm, clustered around sapropel events 2, 3, 4 and 5 (∼ 100-170 ka). In low organic content sediments, chlorin values of ∼ 0‰ coincident with total nitrogen values of ∼+ 5‰ indicate that the latter reflects the original biomass and is not a consequence of diagenetic isotope enrichment. In sapropel horizons, the chlorin and total nitrogen values are 5‰ more negative (∼-5‰ and ∼ 0‰, respectively), resembling previously-reported, modern-day water-column particulates (∼ 0‰). We suggest that nutrient conditions in the Eastern Mediterranean correspond to three scenarios and that the similarity between sapropel and modern-day bulk δ15N is coincidental. Organic-poor marl sediments formed under oligotrophic conditions where surface productivity resulted from upwelling of Atlantic-sourced nitrate. Sapropels were characterized by enhanced diazotrophy that was likely fueled by increased riverine P fluxes to surface waters. Present-day conditions are dominated by anthropogenic N sources. These scenarios agree with a model of sapropel formation in which stratification caused by increased fresh-water inputs led to N fixation due to P:N nutrient imbalance. Enhanced production combined with stratification promoted and maintained anoxic deep waters, consequently increasing organic matter preservation. Such a model may be relevant to interpreting other episodes of intense organic matter deposition in past oceans. © 2010 Elsevier B.V. All rights reserved.
Publication Title, e.g., Journal
Earth and Planetary Science Letters
Higgins, Meytal B., Rebecca S. Robinson, Susan J. Carter, and Ann Pearson. "Evidence from chlorin nitrogen isotopes for alternating nutrient regimes in the Eastern Mediterranean Sea." Earth and Planetary Science Letters 290, 1-2 (2010). doi: 10.1016/j.epsl.2009.12.009.