Ladderane records over the last century in the East China sea: Proxies for anammox and eutrophication changes

Abstract

Anaerobic ammonium oxidation (anammox), an important process for converting fixed nitrogen to N₂, plays an important role in the present-day marine nitrogen cycle. However, little is known about anammox activities in the past, especially in regions that were strongly affected by human activities, evidenced by eutrophication and hypoxia, which promote anammox bacteria growth. In this study, ladderanes have been measured in a sediment core and suspended particulate matter (SPM) in the East China Sea (ECS), to reconstruct the anammox record and to evaluate its responses to eutrophication and hypoxia. The detection and distribution of different ladderane lipids in SPM provide additional evidence that ladderanes were mostly produced in the water column and could reflect anammox activities. Summed ladderane content from the core varied between 11 and 300 ng/g dry weight (dw) sediment, with C20-[5]-ladderane fatty acid methyl esters (FAME) as the predominant compound (5-150 ng/g dw), followed by C20-[3]-ladderane FAME (1-110 ng/g dw), C18-[3]-ladderane FAME (1-32 ng/g dw) and C18 -[5]-ladderane FAME (3-11 ng/g dw). The detection of ladderanes over the last century indicate the existence of anammox in the past. The rapidly increasing trend of ladderanes since the 1960s correlates with an increase in phytoplankton biomarkers (Σ(B + D + A), brassicasterol (B), dinosterol (D) and C₃₇ alkenones (A)), indicating that eutrophication exacerbated anammox growth. The co-variation between our ladderane record and published records of low-oxygen tolerant foraminiferal microfossils and hypoxia events over the past 60 years suggested that sediment ladderanes are a useful indicator for past changes of oxygen depletion or hypoxia in the ECS.

Keywords: Anammox; Hypoxia; Ladderanes; Nitrogen cycle; Phytoplankton.