Black Sea nitrogen cycling and the preservation of phytoplankton δ¹⁵N signals during the Holocene

The stable isotopic compositions of bulk, clay‐bound, organic, and compound‐specific nitrogen were determined for mid to late Holocene Black Sea sediments from a set of box and gravity cores. The data demonstrate that cyanobacterial N 2 fixation provided ∼55% of phytoplankton‐derived N preserved in the top 1–2 cm of the sediments. Prior to widespread agricultural and industrial development in the catchment, N 2 fixation was more prominent, providing 70–80% of phytoplankton N. Organic and clay‐bound nitrogen fractions record different down‐core δ 15 N trends that reflect phytoplankton and detrital sources, respectively, and in samples with low organic matter content, the clay‐bound fraction comprises up to 38% of bulk nitrogen. Compared with bulk samples, pyropheophytin a (Pphe a ), which is a chlorophyll a (Chl a ) degradation product, provides a more accurate record of changing phytoplankton δ 15 N values during the Holocene. An examination of the δ 15 N Pphe a values in light of published and new estimates of the isotopic difference between biomass and Chl a suggests that most of the preserved Pphe a was derived from eukaryotic algae, not cyanobacteria. We infer from these data that cyanobacterial biomass is rapidly recycled in the photic zone, with 15 N‐depleted NH 4 + released during heterotrophy and assimilated by other phytoplankton. A conceptual model for N 2 fixation in the Black Sea is presented, drawing upon water column nutrient and hydrographic data as well as regional climate variability to explain the proposed temporal variability in N 2 fixation.