Sea-ice is an important component of the Earth’s climate system, for example, it greatly increases the proportion of sunlight reflected at high latitudes – the albedo of ice is ~0.6 whereas it is only 0.1 for open water. Because of this importance, palaeoclimatologists want to be able to reconstruct its past extent.
This week, many of these palaeoecologists met in Bremerhaven for the third and final Pages Sea-Ice Proxy workshop, hosted by Rainer Gersonde at the AWI. There were about 30 presentations, some of which were about modelling sea-ice which I won’t discuss here. A diverse range of techniques for reconstructing sea-ice were presented, summarised here as coralline algae, biomarkers, ice-core proxies and biotic assemblages. I apologise if I have omitted or misrepresented anything.
If you have been to a rocky shore, you may have noticed a red crust on some of the rocks. That was probably coralline red algae. In most places, coralline red algae (which are not related to corals) form only a thin calcareous crust. In cold water at high latitudes where grazing rates are low, they can form a thick crust with a layer of reproductive cells marking each year’s growth and permitting the age to be calculated in the same way as a tree can be dated by counting the rings. Unfortunately the variability in layer widths is too small for crusts of different ages to be cross-dated. Jochen Halfar, who has found algal crusts up to 646-years old, presented his work using the width of the growth bands and the calcium magnesium ratio of the algal crusts to reconstruct sea ice extent (growth is slow in years when ice blocks the sun) and temperature.
I thought this was a really nice proxy, unfortunately it is limited to the last ~1000 years because of the availability of material, and there can be hiatuses in the crust caused by severe grazing or iceberg damage. The best material comes from 20-25 m water depth, where the growth rate is high enough, but grazing not too severe. This limits the proxy to coastal areas, so it may be better at recording fast ice attached to the land rather than drift ice which occurs over most of the ocean.
A biomarker called IP25 was the most popular proxy, with at least seven presentations discussing it, beginning with an overview of recent developments by Simon Belt. IP25 stands for Ice Proxy with 25 carbon atoms and is apparently produced only by algae that live in sea-ice. The diatoms that produce IP25 have now been identified and constitute only a small proportion of the diatom community in sea ice. The presence of IP25 seems to be a reliable indicator of the presence of sea ice. Its absence is trickier to interpret: there might be no sea ice; or the sea ice might be so thick that the base is dark and the diatoms cannot grow. Fortunately other biomarkers derived from phytoplankton can be used to distinguish these situations: if these markers are present, but IP25 is not it indicates open water conditions. A schematic figure by Juliane Müller showing this relationship between sea-ice IP25 and other biomarkers was by far the most popular figure of the workshop, included in every IP25 presentation. Kerstin Fahl discussed alternative sources of the phytoplankton biomarkers, including algae in the sea-ice and freshwater algal sources. Despite these complications, the method appears to work.
One potential problem with IP25, as with all biomarkers, is the potential for degradation. Xiaotong Xiao showed some records from the central Arctic, where sedimentations rates are low, that all showed an increase in IP25 towards the present. Simon Belt queried whether this was a real sea-ice signal or the result of progressive degradation.
IP25-producing diatoms only live in the Arctic. Fortunately Lukas Smik showed that there are other biomarkers in the Southern Ocean that can be used to indicate sea ice there.
Ice cores are excellent archives of a wide variety of proxies, including several with a link to sea-ice. The challenge is to understand the mechanisms behind this link so that the proxy can be interpreted correctly. Eric Wolff discussed the sodium record in ice cores. On inter-annual time scales, the record is to be dominated by meteorology – how many salt-bearing storms pass over the ice core site. On longer time scales, sea-ice extent seems to be more important. There are three major potential sources of Na: sea-spray from the open ocean; frost flowers that form on sea ice that freezes in calm conditions; and from salty snow blown off sea-ice after it has wicked up some sea water. Understanding the proxy requires quantifying relative importance of these sources. Marcus Frey presented some work on the latter source – measuring the amount of snow whipped up in a winter storm over sea-ice does not sound like my idea of fun.
Paul Vallelonga showed some new work on halogen deposition in ice. It looks like a complex process, with summer peaks in bromine but winter peaks in iodine concentration despite a summer maxima in emissions. Morgane Philippe had to cancel her talk, so I didn’t hear the latest news about the potential of sulphate and methanesulphonic acid (MSA; produced by phytoplankton) for reconstructing sea-ice, but I did hear that MSA is not stable over long time periods, limiting the duration of records that can be generated. A new ice core from Renland, a small ice cap near the eastern coast of Greenland, to be cored next year has great promise for reconstructing sea-ice as it is much closer to the sea-ice than the central Greenland cores.
A range of taxonomic groups were shown as indicators of sea-ice. Ostracods, crustaceans with calcareous shells like a bivalve, were mentioned briefly by Marit-Solveig Seidenkrantz. Some species are restricted to perennial sea-ice. Reconstructions based on their presence were not not rated as very confident in her compilation of Arctic sea-ice records from the Holocene. She also mentioned a strain of the planktonic foraminifera Neogloboquadrina pachyderma (sin) that has been identified from DNA evidence as being associate with sea-ice. Unfortunately it cannot be identified on morphological grounds, and any DNA found in the sediment will suffer from the usual problems of variable preservation of biomarkers.
Diatoms got more discussion, with Verena Benz, Oliver Esper and Rainer Gersonde presenting work on Southern Ocean diatoms, and Beth Caissie and Jian Ren presenting work from the North Pacific. Some of the diatoms grow in sea-ice, so there are grounds to expect a mechanistic link to sea-ice. Unfortunately the same taxa can also be found remote from sea-ice, complicating interpretation as Jian Ren showed. The team working on the Southern Ocean have amassed an impressive collection of diatom stratigraphies (Oliver Esper made Beth Caissie rather envious when he said he could count a slide in half an hour rather than half a day, such is the material in the Southern Ocean siliceous oozes). I would have liked to see some diatom stratigraphies and reconstruction diagnostics rather than just the reconstructions. The three research groups that work on diatoms in the Southern Ocean are collaborating to generate a combined transfer function and a synthesis of the reconstructions.
For all their importance as a proxy of sea-ice, dinocysts hardly got a mention. Jian Ren showed that the classic dinocyst sea-ice indicators were abundant south of the ice margin. Anne de Vernal was present but did not present. My argument that the uncertainty on dinocyst-based reconstructions is much larger than suggested by naïve cross-validation was not challenged.
Jian Ren’s talk was one of the few to show several sea-ice proxies from the same core. Of course they did not agree. The challenge is to understand what is going on in these multiproxy studies rather than generating case-by-case explanations of why the proxies don’t agree. These explanations could never be applied to a core with only a single proxy!
I think that covers the main news from the meeting.