The C25 HBI Monoene AKA "IP25" biomarker in marine sediment cores - a valuable contribution?

I know at the end of the last post I promised a move away from palaeoclimate studies to ecosystem change, but there's a couple of subjects I've recently come across that I thought would be interesting to consider before moving on (sorry!). The first one is the use of IP₂₅ from Marine sediment cores to reconstruct a history of ice extent in the Arctic region.

Skeletal formula of the IP₂₅ biomarker

The potential of 'IP₂₅' in palaeo- ice extent studies was first outlined by Belt et al (2007) who found a specific Highly Branched Isoprenoid (HBI) monoene organic structure with 25 carbons (shown above) present in sea ice and in sediment under sea ice, but absent from other ice-free regions. They hypothesised that this organic chemical was biosynthesized by specific diatom species living on and under sea ice, and was therefore present in the ice itself and in the sediment underneath it. The specific ecological processes were still unknown, however, including the reasons why the same diatom species would incorporate this chemical when in sea-ice regions but not in ice-free areas. Overall, it was proposed that IP₂₅ could be a direct indicator of sea-ice extent in marine cores.

Masse et al (2008) tested this proposal using a 14C and tephra (volcanic ash layers) dated core taken from the Northern Icelandic Shelf. Results from IP₂₅ analysis were compared both with historical sea-ice records and two diatom inferred Sea Surface Temperature (SST) reconstructions. Here are their results:

The red line is one of the SST reconstructions and the bars along the base of the diagram shows recorded 'harsh' weather as blue and 'mild' as red. IP₂₅ results are shown in black with centennial means in blue. As is clear, the IP₂₅ record broadly agrees with both of these records as well as the broader phenomena of the Medieval Warm Period (MWP) and Little Ice Age (LIA). Also, the results challenge the assumption that the 'no record' periods shown as grey bar were favourable and therefore there was little to report. For example, during the period dated 1400-1550AD where few records are available, the IP₂₅ results indicate relatively high abundance of sea ice in the region.

In another assessment, Axford et al (2011) found strong negative correlations between two IP25 inferred sea ice abundance records and 17 other temperature records for the Icelandic region. This was taken to be in support of the IP₂₅ proxy by the authors.

A similar method has also be used by a number of other authors since, all finding positive results. However, the best approach would be to proceed with caution until the ecological processes involved are more fully understood, and like always, care must be taken not to accept the full temporal resolution as given. For example, Masse et al have included a mean centennial IP₂₅ abundance, and in both papers have been careful not to comment on sub-centennial trends in the record.

But what can IP₂₅ contribute to palaeoclimate studies? Firstly, if the assumptions made so far are correct, this biomarker would represent a direct proxy for sea ice abundance in the Arctic region. Other proxies, such as diatom assemblages (counting species), give indirect proxies for ice extent via, for example, SST reconstructions. Also, other botanical and zoological indicators require highly skilled experts in order to identify species correctly and these methods are very time-consuming! Therefore, biomarker methods are less prone to mis-identification and could be used in regions where dissolution hinders the use of botanical proxies. In time, however, it is more than likely that problems will be found with this method (as they have been with all other techniques) so the advice is likely 'proceed with caution!'

Despite this, the process seems promising and confidence is sure to rise as more studies are published using the technique. Due to it's recent development, most of the published studies using the method are by the original 'Belt et al' authors and therefore may not be completely objective! Repeat studies by other groups would therefore be desirable.