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On October 22, 2024, 11:47:13 AM UTC, Gravatar ckan_admin:

  • Changed title to Assessing marine nitrogen cycling dynamics under altering redox conditions using depositions of sapropels S1 and the ambiguous S2 in the Eastern Mediterranean Sea (previously Nitrogen cycling dynamics under altering redox conditions during sapropels S1 and the ambiguous S2 in the Eastern Mediterranean Sea)


  • Updated description of Assessing marine nitrogen cycling dynamics under altering redox conditions using depositions of sapropels S1 and the ambiguous S2 in the Eastern Mediterranean Sea from

    In the eastern Mediterranean Sea (EMS), the organic-poor sedimentary record is periodically interspersed with organic-rich layers, known as sapropels. Sapropels are characteristic of basin-wide anoxic events, triggered by precession-forced insolation maxima. In the late Quaternary, relatively subdued insolation maxima are not always expressed as distinct sapropels. Late Quaternary EMS sedimentary records hereby offer the opportunity to investigate tipping points of anoxia and associated nitrogen (N) cycling dynamics. To this extent, we have investigated a ~68 kyr sediment record from the EMS (64PE406-E1), which contains the well-established Holocene sapropel S1 (deposited in two parts: S1a [~10.5─8.5 kyr BP] and S1b [~7.8─6.1 kyr BP]) and the sediments timed to the ambiguous S2 sapropel of the late Pleistocene (~53 kyr BP). We focus on lipid biomarkers of microorganisms to reconstruct changes in key components of the N cycle: (1) anaerobic ammonium oxidation (anammox) using ladderanes and a stereoisomer of bacteriohopanetetrol (BHT-x), (2) dinitrogen gas (N2) fixation using heterocyte glycolipids (HGs), and (3) nitrification by Thaumarchaeota using crenarchaeol. Elevated crenarchaeol at the start of S1a indicates enhanced nitrification by Thaumarchaeota, likely promoted by an influx of nutrients via enhanced river discharge. Anammox occurred throughout S1, but was most intense ~1.5 kyr after the onset of anoxia (at ~10.5 cal. kyr BP; as indicated by redox-sensitive trace elements and benthic foraminifer assemblages). High organic carbon levels in S1a likely resulted in anammox being initially outcompeted by heterotrophic denitrification. N2-fixation by heterocytous cyanobacteria appeared predominantly at the S1a termination and in the S1 interruption. Ladderanes suggest additional episodes of bioavailable N removal between ~69 to 39 cal. kyr BP. These episodes correspond to brief periods of anoxia at the sediment-water interface, with maximum deoxygenation occurring in sediments timed to S2 (53─53 cal. kyr BP; as indicated by Mn/Al and benthic foraminifera). During these episodes, BHT-x is absent, but, ladderanes co-occur with a later eluting BHT stereoisomer (BHT-34R). BHT-34R compound-specific δ13C values indicate an anammox source, potentially synthesized by relatively poorly studied marine sedimentary anammox bacteria. Taken all together, our results highlight various modes of operation of the N cycle during the different deoxygenation events. During S1a, a combination of anammox, denitrification and an excess supply of P may have been a reinforcing feedback for anoxia, by favoring the appearance of diatom-diazotroph symbiotic consortia. While during S1b and S2, a temporal coupling between anammox and N2-fixation was not observed, either because loss of bioavailable N was not sufficiently extensive or diazotrophs were nutrient limited. During these periods, anammox may have acted as a negative feedback on anoxia by quenching primary production.
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    The eastern Mediterranean Sea (EMS) sedimentary record is periodically interspersed with organic- rich ‘sapropel’ layers. Sapropels are characteristic of basin-wide anoxic events, triggered by precession-forced insolation maxima. Relatively subdued insolation maxima, however, are not always expressed as distinct sapropel events. The EMS sedimentary record hereby allows anoxia and nitrogen (N) cycling tipping points to be investigated, which may act as analogues for modern deoxygenation. To this end, we investigated a ~68 kyr EMS sedimentary record, containing the well-established sapropel S1 (deposited in two phases: S1a [~10.5─8.5 ka BP] and S1b [~7.8─6.1 ka BP]) and sediments timed to the ambiguous S2 sapropel (~53 ka BP). We focus on lipid biomarkers of microorganisms to reconstruct key components of the N cycle: (1) anaerobic ammonium oxidation (anammox) using ladderanes and a stereoisomer of bacteriohopanetetrol (BHT-x), (2) dinitrogen gas (N2) fixation using heterocyte glycolipids (HGs), and (3) nitrification by Thaumarchaeota using crenarchaeol. During S1, export-productivity (indicated by the barium to aluminum ratio) and anoxia (indicated by redox-sensitive trace elements and benthic foraminifer assemblages) are enhanced. Thaumarchaeota are most abundant in S1a, while anammox is enhanced throughout S1. N2-fixation (indicated by bulk sedimentary δ15N) occurs throughout S1, while the highest heterocyte cyanobacteria abundance is at the S1a termination and S1 interval. Ladderane presence suggests additional episodes of bioavailable N removal between ~69 to 39 cal ka BP. These episodes correspond to brief periods of water column deoxygenation, with anoxia occurring at the sediment-water interface in sediments timed to S2 (53─51 cal ka BP). During these episodes, ladderanes co-occur with the BHT-34R stereoisomer, but not BHT-x. Compound-specific δ13CBHT-34R indicates an anammox source. Our results highlight various modes of operation of the N cycle during the different deoxygenation events. During S1a, a combination of N-loss and P-supply may have reinforced anoxia, by favoring diatom-diazotroph symbiotic consortia. Conversely, a coupling between N2-fixation and anammox was not observed during S1b and the S2-timed interval, either because loss of bioavailable N was insufficient or diazotrophs were nutrient limited. During these periods, anammox may have provided negative feedback on anoxia by quenching primary production.


  • Changed the version of Assessing marine nitrogen cycling dynamics under altering redox conditions using depositions of sapropels S1 and the ambiguous S2 in the Eastern Mediterranean Sea to 2 (previously 1)