Abstract
Abstract. We describe the results of an inter-laboratory investigation into the identification and quantification of the Arctic sea ice biomarker proxy IP25 in marine sediments. Seven laboratories took part in the study, which consisted of the analysis of IP25 in a series of sediment samples from different regions of the Arctic, sub-Arctic and Antarctic, additional sediment extracts and purified standards. The results obtained allowed 4 key outcomes to be determined. First, IP25 was identified by all laboratories in sediments from the Canadian Arctic with inter-laboratory variation in IP25 concentration being substantially larger than within individual laboratories. This greater variation between laboratories was attributed to the difficulty in accurately determining instrumental response factors for IP25, even though laboratories were supplied with appropriate standards. Second, the identification of IP25 by 3 laboratories in sediment from SW Iceland that was believed to represent a blank, was interpreted as representing a better limit of detection or quantification for such laboratories, contamination or mis-identification. These alternatives could not be distinguished conclusively with the data available, although it is noted that the precision of these data was significantly poorer compared with the other IP25 concentration measurements. Third, 3 laboratories reported the occurrence of IP25 in a sediment sample from the Antarctic Peninsula even though this biomarker is believed to be absent from the Southern Ocean. This anomaly is attributed to a combined chromatographic and mass spectrometric interference that results from 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 consistent between Accelerated Solvent Extraction (ASE) and sonication methods and that IP25 concentrations based on 7-hexylnonadecane as an internal standard are comparable using these methods. Recoveries of some more unsaturated HBIs and the internal standard 9-octylheptadecene, however, were lower with the ASE procedure, possibly due to partial degradation of these more reactive chemicals as a result of higher temperatures employed with this method. For future measurements, we recommend the use of reference sediment material with known concentration(s) of IP25 for determining and routinely monitoring instrumental response factors. Given the significance placed on the presence (or otherwise) of IP25 in marine sediments, some further recommendations pertaining to quality control are made that should also enable the two main anomalies identified here to be addressed.
Highlights
The reconstruction of past sea ice conditions in the Arctic and Antarctic is key for understanding past environmental changes on Earth and for informing climate prediction models
The identification of IP25 in all S1-S3 sediments is encouraging from a basic analytical point-of-view and the generally good agreement (< 10 % %RSD) for triplicates within laboratories provides a useful outcome when it comes to how relative changes of IP25 are interpreted. %RSDs for individual laboratories were slightly lower overall when IP25 concentrations were determined using 9-OHD compared to 7-HND, but this trend was not systematic for each laboratory so we find no compelling reason to recommend the use of either internal standard over the other (note: the exception to this concerns the use of 9-OHD using the Accelerated Solvent Extraction (ASE) extraction method)
A growing number of laboratories have carried out the analysis of the Arctic sea ice biomarker IP25 in marine sediments and we anticipate that this will increase in the future
Summary
The reconstruction of past sea ice conditions in the Arctic and Antarctic is key for understanding past environmental changes on Earth and for informing climate prediction models. A number of proxy-based approaches to sea ice reconstruction have been developed and employed to provide new insights into sea ice conditions (and changes to these) for both the Arctic and the Antarctic One of the most recent sea ice proxy developments has been the analysis of a biomarker lipid, termed IP25 (Fig. 1), that is biosynthesised by Arctic sea ice diatoms during the spring bloom and, upon ice melt, is deposited into underlying sediments (Belt et al, 2007). IP25 has not been observed in sea ice or sediments from the Antarctic or from open water phytoplankton from both polar regions, so its occurrence in Arctic sediments appears to provide a selective signal of seasonal Arctic sea ice. A further feature of IP25 is its distinctive isotopic (13C) signature, which is characteristic of a sea ice origin (Belt et al, 2008). This isotopic signature is retained for sedimentary IP25 (Belt et al, 2008), which provides further evidence for an exclusive sea ice source
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