Identification and characterisation of a novel mono-unsaturated highly branched isoprenoid (HBI) alkene in ancient Arctic sediments
Introduction
In recent years, analysis of the biomarker IP25 (II; Fig. 1) in Arctic marine sediments has emerged as a valuable proxy measure of the past occurrence of seasonal Arctic sea ice (Belt et al., 2007, Belt and Müller, 2013). A key attribute of IP25 is its apparent source selectivity (certain Arctic sea ice diatoms) and, indeed, individual diatom sources of it have been recently identified (Brown et al., 2014b). In addition, IP25 appears relatively stable in Arctic sediments, and has been reported in marine core material representing timeframes ranging from recent decades and the Holocene (see Belt and Müller, 2013 for a review) to the Pliocene/Pleistocene boundary (Stein and Fahl, 2013) and even older timescales (Knies et al., 2014).
The basis for all such paleo sea ice studies is the identification and quantification of IP25 in Arctic marine sediments following extraction and analytical measurement using gas chromatography–mass spectrometry (GC–MS). However, amongst the outcomes from a recent inter-laboratory investigation of the measurement of IP25 in marine sediments (Belt et al., 2014), it was demonstrated that aspects relating to instrumental calibration need to be considered carefully if quantitative IP25 data are to be reported and interpreted with confidence. In addition, some laboratories reported anomalies with respect to the identification of IP25, suggesting that this aspect also needs due care, especially as its presence alone is used routinely to provide evidence for the past occurrence of seasonal Arctic sea ice (Belt and Müller, 2013).
We describe here the structural assignment of a previously uncharacterised C25 highly branched isoprenoid (HBI) monoene that co-occurs with IP25 in ancient Arctic sediments from the eastern flank of the Yermak Plateau, NW Spitsbergen. The site was chosen as a suitable location for studying past changes in sea ice since it represents the present summer ice margin in the Atlantic–Arctic gateway (Fram Strait). As part of that study (Knies et al., 2014), a hydrocarbon additional to IP25, also believed to be a mono-unsaturated C25 HBI alkene, was noted, but not reported. Here, we have identified it following synthesis from a closely related C25 HBI diene. The extremely similar GC and MS characteristics to those of IP25 highlight the requirement for extreme care when utilising the latter for paleo sea ice reconstruction.
Section snippets
Sediment
Sediment material was obtained from the ODP Leg 151 borehole (Hole 910C) recovered from the eastern flank of the Yermak Plateau, NW Spitsbergen (80°15.896N, 6°35.430E, 556 m water depth). The age of the material covered the interval ca. 5.8–2.6 Ma (Knies et al., 2014).
Extraction and purification
Analysis of HBIs in Hole 910C sediments was carried out as described by Belt et al. (2012) with minor modification. Thus, internal standard (9-octylheptadec-8-ene; 9-OHD; 0.1 μg) was added to dried sediment material (ca. 2–3 g) which
Identification of novel HBI monoene in Arctic sediments
As part of an investigation into the sea ice history of Fram Strait (Knies et al., 2014), we analysed the hydrocarbon fractions of sediment extracts from Hole 910C and found that, in addition to the presence of the sea ice biomarker IP25, as expected, there was a further hydrocarbon with similar GC and MS (m/z 350; SIM) properties to those of IP25, suggesting the occurrence of a further C25 HBI monoene (Fig. 2). However, although the retention index (RIHP5ms 2084) was slightly different from
Discussion
This study describes the occurrence of a novel C25 HBI monoene in ancient sediments from Fram Strait, together with its structural determination following synthesis from a close analogue; however, a number of other outcomes can be recognised. First, in contrast to IP25, which has been identified in a large number of marine sediments from around the Arctic Ocean (Belt and Müller, 2013), this is the first time that IV has been reported in Arctic sediments, and adds to the growing number of HBIs
Conclusions
We report the presence and structure of a previously uncharacterised HBI monoene (IV; Fig. 1) in ancient marine sediments from the Fram Strait in the Eurasian Arctic. In most 910C extracts, IV co-occurred with the well established sea ice proxy IP25, but we suggest that it has a different source, probably certain marine diatoms. The extremely similar chromatographic and mass spectral properties of the new HBI monoene to those of IP25 mean that due care must be taken to ensure that the two
Acknowledgments
We acknowledge the University of Plymouth for financial support and thank T. Brown and S. Rowland for providing samples of HBI III and the C19 ketone (Fig. 4), respectively. We also thank S.W. Rampen and an anonymous reviewer for helpful suggestions, including the use of the RuO4 oxidation reaction to confirm the double bond position in IV.
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