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

Abstract

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 N2fixation 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, N2fixation was more prominent, providing 70–80% of phytoplankton N. Organic and clay‐bound nitrogen fractions record different down‐coreδ15N 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, pyropheophytina (Pphe a), which is a chlorophyll a (Chl a) degradation product, provides a more accurate record of changing phytoplankton δ15N values during the Holocene. An examination of the δ15NPphe 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 15N‐depleted NH4+ released during heterotrophy and assimilated by other phytoplankton. A conceptual model for N2 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 N2 fixation.