10:15 AM - 10:30 AM
[MIS10-06] Porphyrins reveal modes of nitrogen cycle controlled by chemocline depth under density-stratified condition during the Messinian Salinity Crisis
Keywords:the Messinian Salinity Crisis, nitrogen cycle, porphyrins, nitrogen isotopic composition
The chemical species and the carbon and nitrogen isotopic compositions of porphyrins, degradation products of chlorophylls, exhibit a common pattern in the two shale beds of gypsum–shale alternation, indicating the predominance of particular phototrophic community. These porphyrins show δ15N values (−7.6‰ to −4.7‰) indicating that N2-fixation was the dominant process supplying nitrogen for phototrophs under nitrate-depleted surface environment, formed primarily due to denitrification in the chemocline. On the other hand, the δ13C values of porphyrins derived from chlorophyll c show approximately 5‰ difference between the two shales, probably reflecting the chemocline depth controlled by the amount of freshwater discharge.
By contrast, the porphyrins purified from a shale bed of halite–shale alternation exhibit substantially higher δ15N value (17.2‰). We interpret this to reflect the supply of 15N-enriched ammonium from the subsurface hypersaline water to the photic zone, due to the shallower chemocline depth. We suggest that the chemocline depth, fluctuating in response to changes in regional evaporation–precipitation balance, have the potential to shift the mode of nitrogen cycle during the MSC (nitrification–denitrification–N2-fixation coupling vs. phototrophic assimilation of ammonium).