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We describe the results of an inter-laboratory investigation into theidentification and quantification of the Arctic sea ice biomarker proxyIP25 in marine sediments. Seven laboratories took part in the study, whichconsisted of the analysis of IP25 in a series of sediment samples fromdifferent regions of the Arctic, sub-Arctic and Antarctic, additionalsediment extracts and purified standards. The results obtained allowed 4 keyoutcomes to be determined. First, IP25 was identified by alllaboratories in sediments from the Canadian Arctic with inter-laboratoryvariation in IP25 concentration being substantially larger than withinindividual laboratories. This greater variation between laboratories wasattributed to the difficulty in accurately determining instrumental responsefactors for IP25, even though laboratories were supplied withappropriate standards. Second, the identification of IP25 by 3laboratories in sediment from SW Iceland that was believed to represent ablank, was interpreted as representing a better limit of detection orquantification for such laboratories, contamination or mis-identification.These alternatives could not be distinguished conclusively with the dataavailable, although it is noted that the precision of these data wassignificantly poorer compared with the other IP25 concentrationmeasurements. Third, 3 laboratories reported the occurrence of IP25 ina sediment sample from the Antarctic Peninsula even though this biomarker isbelieved to be absent from the Southern Ocean. This anomaly is attributed toa combined chromatographic and mass spectrometric interference that resultsfrom the presence of a di-unsaturated highly branched isoprenoid (HBI)pseudo-homologue of IP25 that occurs in Antarctic sediments. Finally,data are presented that suggest that extraction of IP25 is consistentbetween Accelerated Solvent Extraction (ASE) and sonication methods and thatIP25 concentrations based on 7-hexylnonadecane as an internal standardare comparable using these methods. Recoveries of some more unsaturated HBIsand the internal standard 9-octylheptadecene, however, were lower with theASE procedure, possibly due to partial degradation of these more reactivechemicals as a result of higher temperatures employed with this method. Forfuture measurements, we recommend the use of reference sediment materialwith known concentration(s) of IP25 for determining and routinelymonitoring instrumental response factors. Given the significance placed onthe presence (or otherwise) of IP25 in marine sediments, some furtherrecommendations pertaining to quality control are made that should alsoenable the two main anomalies identified here to be addressed

climate of the past

An inter-laboratory investigation of the Arctic sea ice biomarker proxy IP&amp;lt;sub&amp;gt;25&amp;lt;/sub&amp;gt; in marine sediments: key outcomes and recommendations