Distributions of highly branched isoprenoid alkenes and other algal lipids in surface waters from East Antarctica: Further insights for biomarker-based paleo sea-ice reconstruction

The occurrence and variable abundance of certain di- and tri-unsaturated C₂₅ highly branched isoprenoid (HBI) biomarkers in Antarctic marine sediments has previously been proposed as a possible proxy measure of paleo sea-ice extent in the Southern Ocean. In the current study, we obtained 47 near-surface (ca. 0-10 m) water samples taken from locations in East Antarctica with different sea ice settings and analysed them for their HBI, sterol and fatty acid content. Sampling locations ranged from the permanently open-ocean zone (POOZ), with no seasonal sea-ice cover, the near-shore summer sea ice zone (SIZ), where sea ice remains long into the summer melt season, and the marginal ice zone (MIZ), located between the POOZ and the SIZ, and with a highly variable latitudinal sea-ice edge throughout the season. A di-unsaturated C₂₅ HBI (diene II) was only identified in surface waters from the MIZ and the SIZ, consistent with a sea-ice diatom origin for this biomarker. In contrast, a tri-unsaturated C₂₅ HBI (triene III) was detected in all samples from the POOZ, the MIZ and the SIZ, and with a stable isotopic composition (δ¹³C = -35 ± 1.5‰) consistent with a phytoplankton source. The highest concentrations of diene II and triene III were in samples from the SIZ and the MIZ, respectively, thus providing further insights into the sea-ice conditions likely favourable for their production and how their relative abundances (the II/III ratio) in underlying sediments might be better interpreted for paleo sea-ice reconstruction. In this respect, relatively high II/III might be a good indicator of extended (into summer) seasonal sea-ice cover, while lower II/III may provide a better indicator of the MIZ. However, the observation of highly variable II/III within the polynya setting of the SIZ may also have significant impacts on sedimentary values. Distributions of diatom sterols and fatty acids were also variable between the three sampling zones, but these were not as distinctive as those observed for the HBIs.10 page(s

Quantitative estimates of sinking sea ice particulate organic carbon based on the biomarker IP<inf>25</inf>

Sea ice-derived particulate organic carbon (iPOC) represents an important contribution of carbon to Arctic ecosystems, yet our ability to obtain realistic quantitative estimates of iPOC outside the sea ice matrix is currently somewhat limited. To address this challenge, we applied a novel approach to quantifying iPOC within the water column under melting sea ice by first measuring the proportion of the sea ice diatom biomarker IP25 within iPOC in bottom ice samples obtained from Resolute Passage during spring 2012. We then compared this value with corresponding values obtained from a time series of water samples. Together, these reflected a period of ice melt and rapid release of iPOC, indicated by changing ice temperature and thickness, in addition to changes in the stable carbon isotope composition and concentration of iPOC, IP25 and chlorophyll a within bottom ice. Estimates of iPOC in seawater were highest (0.15 to 0.22 mg l-1) in the upper 2 m, coincident with the reduction of iPOC in sea ice near the beginning of sampling, with iPOC accounting for an estimated 84 to 125% of total POC (tPOC). Collectively, this biomarker approach yielded realistic estimates of %iPOC, both numerically and in the context of melting sea ice following a spring bloom in the Canadian Arctic. We describe some assumptions of this approach and consider the impacts of possible caveats on quantitative estimates of iPOC derived using this methodology

EIMS Fragmentation and detection of autoxidation products of 2,6,10,14‐tetramethyl‐7‐(3‐methylpent‐4‐enyl)‐pentadec‐5‐ene in Arctic sediments

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Biotic and abiotic degradation of the sea ice diatom biomarker IP 25 and selected algal sterols in near-surface Arctic sediments

International audienceThe organic geochemical IP 25 (Ice Proxy with 25 carbon atoms) has been used as a proxy for Arctic sea ice in recent years. To date, however, the role of degradation of IP 25 in Arctic marine sediments and the impact that this may have on palaeo sea ice reconstruction based on this biomarker have not been investigated in any detail. Here, we show that IP 25 may be susceptible to autoxidation in near-surface oxic sediments. To arrive at these conclusions, we first subjected a purified sample of IP 25 to autoxidation in the laboratory and characterised the oxidation products using high resolution gas chromatography-mass spectrometric methods. Most of these IP 25 oxidation products were also detected in near-surface sediments collected from Barrow Strait in the Canadian Arctic, although their proposed secondary oxidation and the relatively lower abundances of IP 25 in other sediments probably explain why we were not able to detect them in material from other parts of the region. A rapid decrease in IP 25 concentration in some near-surface Arctic marine sediments, including examples presented here, may potentially be attributed to at least partial degradation, especially for sediment cores containing relatively thick oxic layers representing decades or centuries of deposition. An increase in the ratio of two common phytoplanktonic ster-ols-epi-brassicasterol and 24-methylenecholesterol-provides further evidence for such autoxidation reactions given the known enhanced reactivity of the latter to such processes reported previously. In addition, we provide some evidence that biodegradation processes also act on IP 25 in Arctic sediments. The oxidation products identified in the present study will need to be quantified more precisely in down-core records in the future before the effects of degradation processes on IP 25-based palaeo sea ice reconstruction can be fully understood. In the meantime, a brief overview of some previous investigations of IP 25 in relatively shallow Arctic marine sediments suggests that overlying climate conditions were likely dominant over degradation processes, as evidenced from often increasing IP 25 concentration downcore, together with positive relationships to known sea ice conditions

EIMS Fragmentation and MRM quantification of bacterial metabolites of the sea ice biomarker proxy IP25in Arctic sediments.

RATIONALE: 3,9,13-trimethyl-6-(1,5-dimethylhexyl)-tetradecan-1,2-diol and 2,8,12-trimethyl-5-(1,5-dimethylhexyl)-tridecanoic acid appear to be produced during the bacterial metabolism of IP25, a highly branched isoprenoid lipid often employed for past Arctic sea ice reconstruction. Characterization and quantification of these metabolites in sediments are essential to determine if bacterial degradation may exert a significant influence on IP25-based palaeo sea ice reconstructions. METHODS: EIMS fragmentation pathways of 3,9,13-trimethyl-6-(1,5-dimethylhexyl)-tetradecan-1,2-diol and 2,8,12-trimethyl-5-(1,5-dimethylhexyl)-tridecanoic acid TMS derivatives were investigated. These pathways were deduced by: (i) low energy CID-GC/MS/MS, (ii) accurate mass measurement and (iii) deuterium labelling. RESULTS: CID-MS/MS analyses, accurate mass measurement and deuterium labelling experiments enabled us to elucidate the EIMS fragmentations of 3,9,13-trimethyl-6-(1,5-dimethylhexyl)-tetradecan-1,2-diol and 2,8,12-trimethyl-5-(1,5-dimethylhexyl)-tridecanoic acid TMS derivatives. Some specific fragment ions useful in addition to chromatographic retention times for further characterization could be identified. As an application of some of the described fragmentations, the TMS derivatives of these metabolites were characterized and quantified in MRM mode in different Arctic sediments. CONCLUSIONS: EIMS fragmentations of 3,9,13-trimethyl-6-(1,5-dimethylhexyl)-tetradecan-1,2-diol and 2,8,12-trimethyl-5-(1,5-dimethylhexyl)-tridecanoic acid TMS derivatives exhibit specific fragment ions, which appear to be very useful for the quantification of these bacterial metabolites of the palaeo tracer IP25in sediments

A novel biomarker-based proxy for the spring phytoplankton bloom in Arctic and sub-arctic settings – HBI T25

Source at https://doi.org/10.1016/j.epsl.2019.06.038. The spring phytoplankton bloom is a characteristic feature of mid-high latitudes in modern times, but can be challenging to identify in palaeo records. In the current study, we investigated the absolute and relative distributions of two diatom-derived tri-unsaturated highly branched isoprenoid (HBI) lipids, at least one of which has previously been suggested to be a possible proxy for the productive region of the marginal ice zone (MIZ) in the Polar Regions. Based on a comparison of their distributions in surface sediments from the Barents Sea and neighbouring regions with a range of oceanographic parameters, we identify, via principal component analysis, a strong association between the relative proportion of the two HBIs and satellite-derived spring chlorophyll a (chl a) concentration. Further, based on agglomerative hierarchical clustering, we identify two clusters of HBI biomarker ratios and spring chl a together with a potential threshold biomarker ratio (termed HBI TR25) for the spring phytoplankton bloom. A modified version of HBI TR25 (i.e. HBI T25) provides a potentially more straightforward binary measure of the spring phytoplankton bloom. Analysis of HBI TR25 and HBI T25 values in a series of short (spanning recent centuries) and long (Holocene) sediment cores from the region provides an initial evaluation of the applicability of this novel proxy in the palaeo record. Outcomes are mainly consistent with the findings from the surface sediments and with other proxy-based reconstructions, including estimates of past sea ice cover, which is well-known to influence primary production in the region. Indeed, we suggest that the new HBI T25 phytoplankton bloom proxy may also represent an important new tool for characterising the MIZ in palaeo records, especially when used alongside well-established sea ice proxies, such as IP25 and PIP25. Despite the largely empirical nature of the study, we also provide a possible explanation for the observed biomarker ratio-chl a relationship. Thus, a previous laboratory investigation showed that the distributions of the same two HBIs analysed herein in their likely source (viz. Rhizosolenia setigera) was strongly influenced by culture temperature and growth rate. Confirmation of the generality of our findings and of the causal relationship between HBI T25 and the spring phytoplankton bloom will, however, require further laboratory- and field-based studies in the futur