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

Group I mesophilic Thaumarchaeota fix dissolved inorganic carbon (DIC), accompanied by a biosynthetic fractionation factor of similar to 2 parts per thousand. Accordingly, the delta C-13 signature of their diagnostic biomarker crenarchaeol was suggested as a potential delta C-13(DIC) 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 delta C-13(DIC) 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 delta C-13 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 delta C-13(DIC) value of approximately -3.5%0, while the well-mixed proximal part had a delta C-13(DIC) value of approximately -0.8 parts per thousand. The advent of the crisis was marked by C-13 enrichment of the DIC pool, with positive delta C-13(DIC) excursions up to +5 parts per thousand in the upper water column. Export of C-12 to the seafloor after phytoplankton blooms and limited replenishment of remineralized carbon due to the stabilization of thermohaline stratification primarily caused such C-13 enrichment of the DIC pool.

Automated summary

This study investigates the impact of the crisis on the carbon cycle in the Mediterranean Sea. The results show that the crisis led to C-13 enrichment of the DIC pool, primarily due to the export of C-12 to the seafloor after phytoplankton blooms, and limited replenishment of remineralized carbon due to the stabilization of thermohaline stratification.