Abstract. The eastern Mediterranean Sea sedimentary record is characterised by
intervals of organic-rich sapropel sediments, indicating periods of severe
anoxia triggered by astronomical forcing. It has been hypothesised that
nitrogen fixation was crucial in injecting the Mediterranean Sea with
bioavailable nitrogen (N) during sapropel events. However, the evolution of
the N biogeochemical cycle of sapropels is poorly understood. For example,
the role of the complementary removal reactions like anaerobic ammonium
oxidation (anammox) has not been investigated because the traditional lipid
biomarkers for anammox, ladderane fatty acids, are not stable over long
periods in the sedimentary record. Using an alternative lipid biomarker for
anammox, bacteriohopanetetrol stereoisomer (BHT isomer), we present here for
the first time N removal throughout the progression, e.g. formation,
propagation, and termination, of basin-wide anoxic events. BHT isomer and
ladderanes were analysed in sapropel records taken from three eastern
Mediterranean sediment cores, spanning S1 to Pliocene sapropels. Ladderanes
were rapidly degraded in sediments, as recently as the S5 sapropel. BHT
isomer, however, was present in all sapropel sediments, as far back as the
Pliocene, and clearly showed the response of anammox bacteria to marine
water column redox shifts in high-resolution records. Two different N
removal scenarios were observed in Mediterranean sapropels. During S5,
anammox experienced Black Sea-type water column conditions, with the peak of
BHT isomer coinciding with the core of the sapropel. Under the alternative
scenario observed in the Pliocene sapropel, the anammox biomarker peaked at
onset and termination of said sapropel, which may indicate sulfide
inhibition of anammox during the core of sapropel deposition. This study
shows the use of BHT isomer as a biomarker for anammox in the marine
sediment record and highlights its potential in reconstructing anammox
during past anoxic events that are too old for ladderanes to be applied,
e.g. the history of oxygen minimum zone expansion and oceanic anoxic events.
Biomarker evidence for the occurrence of anaerobic ammonium oxidation in the eastern Mediterranean Sea during Quaternary and Pliocene sapropel formation
