The Arctic sea ice proxy, IP25, in particulate organic matter (POM) from surface waters and bottom sea-ice of the Central Arctic Ocean during the MOSAiC expedition in 2019/2020

Due to the low concentrations of POM in the water column, the volumes that needed to be filtered were often >10 L and filtration of replicate samples was not feasible. The HBI GC-MS profiles were compared to those from purified standards obtained from previous laboratory diatom cultures (e.g. IPSO25, HBI Trienes Z and E) and Arctic sediment extracts (IP25), and peak areas were normalised to those of a laboratory synthesised internal standard (e.g. 9-octyl-8-heptadecene) for quantification purposes. The four key HBIs can be separated via their number of double-bonds using the following mass spectral ions: m/z 350.3 for the monoene, IP25; m/z 348.3 for the diene, IPSO25; and m/z 346.3 for the two trienes, HBI III - Z and HBI III - E. However, in this set of samples, we did not find any HBI trienes. Moreover, the HBI diene IPSO25, although often present, was not quantifiable due to a combination of low abundances and/or chromatographic overlap with other HBIs. Further, previous studies have shown that IPSO25 is susceptible to rearrangement in some environmental samples (e.g. Belt and Cabedo-Sanz 2015), thus making it difficult to estimate the absolute amount of IPSO25. This appears to be the case in the samples analysed, therefore we note here that IPSO25 was produced in the CAO, but was non-quantifiable. In contrast, the monoene IP25 is the least reactive of the four considered HBIs and could be readily quantified.