DOI: 10.5285/8bd08e69-7839-4096-85fd-9adfcaa85e5a
title: 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

description: During the MOSAiC expedition in the Central Arctic Ocean (CAO, 2019-2020), POM was sampled weekly to fortnightly from surface waters and the Chlorophyll a maximum layer (Chl a max) via CTD casts and from bottom sea ice of the floe via ice coring (first- and second-year ice, two layers nearest to the water-ice interface). The POM was filtered onboard (GF/F filters) and deep frozen for the subsequent analysis of a suite of lipid biomarkers, including IP25 and other highly-branched isoprenoids (HBI), fatty acids (FA) and sterols. These biomarkers can provide valuable information about the nutritional value, the taxonomic composition (e.g. diatoms vs flagellates), and the origin of the POM that represents the basis of the Central Arctic food web. This dataset comprises the results from the HBI analysis only, while the FA dataset is already published and the sterol data will be submitted shortly. The separation of the various lipid biomarkers was carried out at the University of Plymouth. After addition of internal standards for each of the 3 components, the filters were saponified with KOH.
Thereafter, non-saponifiable lipids (HBI and sterols) were extracted with hexane and purified by open column chromatography (SiO2). Fatty acids were obtained by adding concentrated HCl to the saponified solution and re-extracted with hexane. The analysis of IP25 was carried out using an Agilent 7890A gas chromatograph (GC), coupled to an Agilent 5975 mass selective detector (mass spectrometry, MS), fitted with an Agilent HP-5ms column with auto-splitless injection and helium carrier gas. Identification of IP25 and other HBIs was achieved by comparison of their individual GC retention indices and mass spectra with those obtained from purified standards. IP25 was quantified by, first, integrating individual ion responses in selected-ion monitoring mode (m/z 350.3), second, normalising these to the corresponding peak area of the internal standard and, third, applying an instrumental response factor obtained from a purified standard. These IP25 quantities per filter can be normalised to the volume of filtered seawater or melted ice core water. 

Contributions by KS were funded by the UK's Natural Environment Research Council MOSAiC Thematic project SYM-PEL: ''Quantifying the contribution of sympagic versus pelagic diatoms to Arctic food webs and biogeochemical fluxes: application of source-specific highly branched isoprenoid biomarkers''/ (NE/S002502/1)
description: The MOSAiC cruise: The MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate; tag MOSAiC20192020 and the Project ID: AWI_PS122_00) expedition represents the first year-round interdisciplinary study of the atmosphere, the sea ice, the ocean, the ecosystem, and biogeochemical processes during the transpolar drift across the CAO with a unique opportunity for intensive field sampling (Shupe et al. 2022; Rabe et al. 2022; Nicolaus et al. 2022). The observational year was divided into 5 legs: Leg 1 started on October 4, 2019 with the setup of the first Central Observatory (CO1) and installations on the research icebreaker RV Polarstern north of the Laptev Sea (Krumpen et al. 2020). The winter Leg 2 and spring Leg 3 continued the work on CO1, before RV Polarstern had to leave the floe, for logistical reasons, on May 16, 2020. The vessel returned to the original ice floe on June 19, 2020, but at a different location some hundred meters away. Leg 4 continued the drift with the new CO2 over the summer until the disintegration of the floe in the Fram Strait on July 31, 2020. During Leg 5, RV Polarstern travelled back into the ice and started the setup of CO3 on August 21, 2020, near the North Pole. The third drift ended on September 20, 2020, when the vessel started the return voyage.

The water column sampling: Particulate organic matter samples for lipid biomarker analysis were collected from 12 L Niskin bottles attached to the shipboard 24-bottle CTD-rosette (PS-CTD). During legs 1-3, additional water column sampling was conducted via a 5 L Niskin bottle-CTD-rosette from Ocean City (OC), an in-ice hole 300 meters from RV Polarstern (OC-CTD). In the period between mid-March and mid-May, all the water column samples were collected at OC due to the loss of the ice hole alongside the ship (see Rabe et al. 2022). Pelagic particulate organic matter (POM) for FA analysis was collected at 2 m and/or the Chl a max (based on CTD fluorescence sensor profiles). Volumes of 7 to 10 L of seawater were filtered via a vacuum pump (20 kPA) onto pre-combusted (3h, 550°C) 47 mm Whatman GF/F filters and stored in aluminium foil at -80°C until further processing. 
Additional under-ice sampling took place during MOSAiC Leg 5 from August 26 to September 15, 2020. An in-situ automatic pump phytoplankton sampler (PPS; serial number 12697-01, McLane; USA) was deployed at a depth of 11 m to collect suspended particles. In 2-3 day intervals, 4-6 L seawater were filtered through combusted GF/F filters under different pre-set sample parameters of the PPS.
Sea ice sampling: During Leg 1, areas of first year ice (FYI) and second year ice (SYI) were identified that were safely accessible, relatively homogeneous, and large enough to accommodate repeat visits, potentially for the entire drift. The sites were located away from RV Polarstern to minimise the human impacts (e.g. artificial lights, traffic, fumes, noise). Cores for biological properties were collected using a 9-cm diameter KOVACS Mark II ice corer. Cores were usually sectioned and parsed into sterile Whirlpak bags directly inside the ice coring tent under low and/or red-light conditions to minimize artefacts. Small-scale horizontal variability was reduced by pooling 3-4 ice cores, creating a more homogeneous master sample from which related properties were derived. FA profiles of sea ice POM were derived from the two bottom 5 cm intervals (0-5 cm, 5-10 cm) of the pooled ice cores (ECO Pool 1), alongside with samples for pigment analysis (Chl a, HPLC), particulate organic matter (POC/PON) and flow cytometry. All pooled samples were melted after the addition of filtered surface seawater (typically 50 ml per 1 cm of core section) to reduce the impact of osmotic stress and cell loss (Garrison and Buck 1986, Campbell et al. 2019). Ice core sections in bags were melted in the dark at room temperature (18-22°C) and checked every 4-6 hours. Upon completed melt, which took from 12 to 40 hours, bags were transferred into dark, temperature-controlled lab containers, and parsed for sub-sampling of biological properties under red light to minimize artificial light stimulation of biological activities. The subsamples were filtered via a vacuum pump (20 kPA) onto pre-combusted 47 mm Whatman GF/F filters and stored in aluminium foil at -80°C until further processing.
description: In situ-sample collection: 12 L Niskin bottles attached to the shipboard 24-bottle CTD-rosette; automatic pump phytoplankton sampler (PPS; serial number 12697-01, McLane; USA); 9-cm diameter KOVACS Mark II ice corer. 

IP25 analysis: The analysis of partially purified non-polar lipids containing IP25 and other HBIs by gas chromatography-mass spectrometry (GC-MS) was carried out using an Agilent 7890A gas chromatograph (GC), coupled to an Agilent 5975 mass selective detector (mass spectrometry, MS), fitted with an Agilent HP-5ms column with auto-splitless injection and helium carrier gas. 
Chromatogram data evaluation: Agilent chromatography software system (MSD ChemStation Data Analysis, United States)
description: Due to the low concentrations of POM in the water column, the volumes that needed to be filtered were often &gt;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.