Sediment microfabric records mass sedimentation of colonial cyanobacteria and extensive syndepositional metazoan reworking in Pliocene sapropels

Abstract

The sapropel record of the eastern Mediterranean provides unique insight into the primary climatic, oceanographic, and biological drivers of organic carbon enrichment in marine sediments. The dominant source of organic matter, timing of oxygen depletion at the sea floor, and extent of metazoan reworking of these deposits remain unclear. These questions are addressed by combining microbeam imaging with bulk and molecular geochemical characterization of several Pliocene sapropels, revealing four microfacies which record distinct palaeoceanographic conditions, phytoplankton assemblages, and degrees of postdepositional reworking. The most organic‐rich, carbonate‐lean sapropel intervals consist of alternating 10–60‐μm‐thick organic and detrital mineral laminae. Petrographical features consistent with a pelagic origin, δ15N <−1.8 and the presence of 2α‐methylhopanes indicate that organic laminae formed by mass sedimentation of N‐fixing colonial cyanobacteria (Trichodesmium). The association of framboidal pyrite with organic laminae suggests that mass sedimentation led to the abrupt development of sea floor anoxia. Unlike similar Quaternary sapropels, no petrographic or biomarker evidence is found for a mat‐forming diatom contribution to these organic‐rich, laminated intervals. However, episodic mass sedimentation was not found to be a universal ingredient of sapropel formation. Carbonate‐lean, weakly laminated intervals contain >50 μm organomineral aggregates, interpreted as marine snow, whereas carbonate microfossil‐rich intervals record periods of nitrogen fixation and moderately increased primary production by a diverse assemblage of calcareous, organic‐walled, and siliceous plankton. The results presented here further show that burrowing by microscopic meiofauna impacted most sapropels, extending into seemingly laminated intervals below obvious disruption from burrowing macrofauna, indicating that metazoans influence organic carbon burial in oxygen‐depleted settings even where physical displacement of sediment is not visible.

Linking diatom-diazotroph symbioses to nitrogen cycle perturbations and deep-water anoxia: Insights from Mediterranean sapropel events

“…By contrast, a sapropel-like scenario would amplify the more strongly reducing pathway of nitrite reduction via anammox, leading to an abundance of BHT-II and suppression of 2Me-hopanoid deposition. This is supported by the fact that 2Me-hopanoids were not detected in the sapropel samples studied here and have been reported only in very low concentrations for other sapropel events (Löhr et al, 2018). Importantly, regardless of whether anammox or heterotrophic denitrification consumes fixed nitrogen, diazotrophy-including algae-diazotroph symbioses-would still proliferate under either scenario.…”

Vitamin B 12 -dependent biosynthesis ties amplified 2-methylhopanoid production during oceanic anoxic events to nitrification

“…S9). Finally, Neogene sapropel events in the subtropical Mediterranean Sea that featured conditions superficially similar to OAEs (low N/P, water column stratification, anoxia, enhanced nitrogen fixation) together with an expansion of heterocystous, nitrogen-fixing cyanobacteria (Nostocales) (72) resulted in the deposition of only trace amounts of methylhopanoids (73) relative to large amounts of nonmethylated hopanoids (74,75).…”

“…Two distinct modes of nitrogen cycling during ocean anoxia. Despite their relative abundance in OAE sediments, 2-methylhopanoids are rare in modern anoxic environments and recent anoxic events such as Pliocene-Pleistocene Mediterranean sapropels (73), which like OAEs, are associated with marine nitrogen cycle perturbations (80,81). This contrasting behavior raises questions about oceanographic influences on 2-methylhopanoid production.…”

The response of water column and sedimentary environments to the advent of the Messinian salinity crisis: insights from an onshore deep-water section (Govone, NW Italy)

“…Studies of modern (Wakeham et al 1993;Sinninghe Damsté et al 2003) and ancient sediments (e.g. Freeman et al 1990;Kenig et al 1995;Behrooz et al 2018;Löhr et al 2018) have shown that preservation of lycopane is highest when bottom waters are anoxic.…”