Carbon isotope excursions during the late Miocene recorded by lipids of marine Thaumarchaeota, Piedmont Basin, Mediterranean Sea

Mathia Sabino,Francesco Dela Pierre,Marcello Natalicchio,Daniel Birgel,Jörn Peckmann

doi:10.1130/g49360.1

Mathia Sabino, Francesco Dela Pierre + Show 3 more

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https://doi.org/10.1130/g49360.1

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Journal: Geology	Publication Date: Jan 1, 2022
Citations: 3	License type: cc-by

Affiliation: Universität Hamburg, University of Turin

Abstract

Abstract Group I mesophilic Thaumarchaeota fix dissolved inorganic carbon (DIC), accompanied by a biosynthetic fractionation factor of ~20‰. Accordingly, the δ13C signature of their diagnostic biomarker crenarchaeol was suggested as a potential δ13CDIC proxy in marine basins if input from nonmarine Thaumarchaeota is negligible. Semi-enclosed basins are sensitive to carbon-cycle perturbations, because they tend to develop thermohaline stratification. Water column stratification typified the semi-enclosed basins of the Mediterranean Sea during the late Miocene (Messinian) salinity crisis (5.97–5.33 Ma). To assess how the advent of the crisis affected the carbon cycle, we studied sediments of the Piedmont Basin (northwestern Italy), the northernmost Mediterranean subbasin. A potential bias of our δ13CDIC reconstructions from the input of soil Thaumarchaeota is discarded, since high and increasing branched and isoprenoid tetraether (BIT) index values do not correspond to low and decreasing δ13C values for thaumarchaeal lipids, which would be expected in case of high input from soil Thaumarchaeota. Before the onset of the crisis, the permanently stratified distal part of the basin hosted a water mass below the chemocline with a δ13CDIC value of approximately −3.5‰, while the well-mixed proximal part had a δ13CDIC value of approximately −0.8‰. The advent of the crisis was marked by 13C enrichment of the DIC pool, with positive δ13CDIC excursions up to +5‰ in the upper water column. Export of 12C to the seafloor after phytoplankton blooms and limited replenishment of remineralized carbon due to the stabilization of thermohaline stratification primarily caused such 13C enrichment of the DIC pool.

Highlights

Thaumarchaeota are among the dominant archaea in marine environments, where their occurrence can be traced by crenarchaeol, the thaumarchaeal-specific isoprenoid glycerol dialkyl glycerol tetraether
−23‰ and −18‰ (δ13CBp-cren; Schouten et al, 2013). These values originate from autotrophic fixation of dissolved inorganic carbon (DIC) as bicarbonate, with a biosynthetic fractionation factor (ε) of ∼20‰ (K önneke et al, 2012)
A high branched and isoprenoid tetraether (BIT) may imply high input of crenarchaeol sourced from group I soil

Summary

Introduction

Marine group I (MGI) mesophilicThaumarchaeota are among the dominant archaea in marine environments, where their occurrence can be traced by crenarchaeol, the thaumarchaeal-specific isoprenoid glycerol dialkyl glycerol tetraether (iGDGT; e.g., Besseling et al, 2020). The δ13C values of the cyclohexane ring–containing tricyclic biphytane (Bp-cren) derived from ether cleavage of crenarchaeol typically fall between. −23‰ and −18‰ (δ13CBp-cren; Schouten et al, 2013) These values originate from autotrophic fixation of dissolved inorganic carbon (DIC) as bicarbonate, with a biosynthetic fractionation factor (ε) of ∼20‰ (K önneke et al., 2012). Δ C 13 Bp-cren values could, be a powerful (paleo-)δ13CDIC proxy, as demonstrated by successful applications in tracing past carbon-cycle perturbations (Kuypers et al, 2001; Schoon et al, 2013; Elling et al., 2019). Carbon-cycle perturbations are common for modern semi-enclosed basins, which tend to develop thermohaline stratification, altering the δ13CDIC of the water column (e.g., Fry et al, 1991). Thermohaline stratification may have caused the alteration of δ13CDIC pools in ancient semienclosed basins (e.g., Schoon et al, 2013)

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