Gas chromatography-mass spectrometry selected ion monitoring and gas chromatography-tandem mass spectrometry selected reaction monitoring analyses of mono-, di- and tri-unsaturated C25 highly branched isoprenoid alkene biomarkers in sea ice and sediment samples: A comparative study.

RATIONALE: The efficiency of selected ion monitoring (SIM) and selected reaction monitoring (SRM) analyses for the quantification of three mono-, di- and tri-unsaturated highly branched isoprenoid (HBI) alkenes (IP25 , IPSO25 and HBI III, respectively), often used as proxies for the occurrence of Arctic and Antarctic sea ice or the adjacent open waters, was compared. METHODS: Gas chromatography (GC)-mass spectrometry (MS)/SIM and GC/MS/MS/SRM analyses were carried out on dilute solutions made from purified standards of these three HBIs, and then on hydrocarbon fractions of several sediment and sea ice sample extracts. More efficient and specific SRM transitions were selected after collision-induced dissociation of each precursor ion at different collision energies. RESULTS: SRM analysis avoided any overestimation of IP25 resulting from the contribution of the coeluting 13 C mass isotopomer of IPSO25 (M+ ˙ + 2) to the SIM target ion. In contrast, SRM analysis is less reliable for IPSO25 quantification in cases where several regio-isomers are present, likely due to intense double bond migrations following electron impact. In the case of HBI III, SRM analysis constitutes a potentially suitable alternative to SIM analysis, especially in terms of improving limit of detection. CONCLUSIONS: Despite the intense migrations of HBI double bonds under electron ionization, the selected SRM transitions should be more suitable than SIM target ions for IP25 and HBI III quantification in complex hydrocarbon fractions of natural samples. However, the advantage is less evident for IPSO25 due to the presence of numerous regio-isomers.

Sea ice-ocean coupling during Heinrich Stadials in the Atlantic-Arctic gateway.

The variability of Arctic sea-ice during abrupt stadial-interstadial shifts in the last glacial period remain poorly understood. Here, we investigated the millennial-scale relationship, with a focus on Heinrich Stadials (HS), between sea-ice cover and bottom water temperature (BWT) during Marine Isotope Stages (MIS) 3 and 2 (64-13 ka) in the Fram Strait using new molecular sea ice biomarker data and published benthic foraminiferal BWT records. Widespread spring sea-ice cover (SpSIC) dominated the studied interval, especially in mid-late MIS 3 (45-29 ka). Yet, warm interstadials were characterized by relatively more open-ocean conditions compared to cold stadials. At the transition between a HS and the subsequent interstadial, sea ice was tightly linked to BWT with rapid reductions in SpSIC coinciding with lower BWT at the end of HS. The relative timing of the events, especially during HS 1, points to ocean warming as the key controlling factor for sea ice reduction at millennial timescales.

Antiphased dust deposition and productivity in the Antarctic Zone over 1.5 million years.

The Southern Ocean paleoceanography provides key insights into how iron fertilization and oceanic productivity developed through Pleistocene ice-ages and their role in influencing the carbon cycle. We report a high-resolution record of dust deposition and ocean productivity for the Antarctic Zone, close to the main dust source, Patagonia. Our deep-ocean records cover the last 1.5 Ma, thus doubling that from Antarctic ice-cores. We find a 5 to 15-fold increase in dust deposition during glacials and a 2 to 5-fold increase in biogenic silica deposition, reflecting higher ocean productivity during interglacials. This antiphasing persisted throughout the last 25 glacial cycles. Dust deposition became more pronounced across the Mid-Pleistocene Transition (MPT) in the Southern Hemisphere, with an abrupt shift suggesting more severe glaciations since ~0.9 Ma. Productivity was intermediate pre-MPT, lowest during the MPT and highest since 0.4 Ma. Generally, glacials experienced extended sea-ice cover, reduced bottom-water export and Weddell Gyre dynamics, which helped lower atmospheric CO2 levels.

Type II photosensitized oxidation in senescent microalgal cells at different latitudes: Does low under-ice irradiance in polar regions enhance efficiency?

Comparison of Type II photosensitized oxidation of lipids (the photodynamic effect) and photodegradation of chlorophyll (sensitizer photobleaching) in samples of particulate matter collected previously from locations representing a diverse range of latitudes reveals an enhancement of the photooxidation of lipids at the expense of chlorophyll photodegradation in the polar regions. The efficiency of the photodynamic effect appears to be particularly high in sinking particles collected under sea ice and is attributed to the rapid settling of highly aggregated sympagic algae to depths of low light transmission favouring the photodynamic effect at the expense of photobleaching of the sensitizer. Paradoxically, the low efficiency of Type II photosensitized oxidation of lipids observed in temperate and equatorial regions is associated with high solar irradiances in these regions. Type II photosensitized oxidation of lipids in senescent phytoplankton seems thus to be strongly dependent of the intensity of solar irradiance.

Electron ionization mass spectrometric fragmentation and detection of autoxidation products of 2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5-ene in Arctic sediments.

RATIONALE: Some highly branched isoprenoid (HBI) alkenes are commonly used as proxies for palaeoceanographic reconstructions. However, there is a need to identify compounds that are sufficiently stable and abundant to be used as tracers of HBI oxidation in sediments. 2,6,10,14-Tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5(Z/E)-en-4-ols resulting from 2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5-ene appear to be useful for this purpose. METHODS: Comparison of electron ionization (EI) mass spectra and retention times with those of standards allowed formal identification of autoxidation products of 2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5-ene. EI-MS fragmentations of TMS ethers of the main oxidation products (2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5(Z/E)-en-4-ols) were deduced by gas chromatography/electron ionization mass spectrometry (GC/EI-MS), low-energy collision-induced dissociation tandem mass spectrometry (CID-MS/MS) and accurate mass measurements. These compounds were then quantified in Arctic sediment samples in MS/MS multiple reaction monitoring (MRM) mode using transitions based on the main fragmentation pathways elucidated. RESULTS: 2,6,10,14-Tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5(Z/E)-en-4-ols were identified after autoxidation of the HBI alkene 2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5-ene. Low-energy CID-MS/MS analyses and accurate mass measurements allowed the EI-MS fragmentation pathways of their trimethylsilyl (TMS) derivatives to be elucidated. Some specific fragment ions and chromatographic retention times were also useful for further characterization. As an application of some of the described fragmentations, TMS derivatives of these metabolites were characterized and quantified in MRM mode in Arctic sediments. CONCLUSIONS: Due to their production in high proportions during autoxidation of their parent HBI diene, their apparent stability in sediments, and their specific EIMS fragmentations, (Z and E)-2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5-en-4-ol TMS derivatives appeared to be useful tracers of HBI autoxidation in sediments.