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Analysis of the organic geochemical biomarker IP 25 in marine sediments is an established method for carrying out palaeo sea ice reconstructions for the Arctic. Such reconstructions cover timescales from decades back to the early Pleistocene, and are critical for understanding past climate conditions on Earth and for informing climate prediction models. Key attributes of IP 25 include its strict association with Arctic sea ice together with its ubiquity and stability in underlying marine sediments; however, the sources of IP 25 have remained undetermined. Here we report the identification of IP 25 in three (or four) relatively minor (o5%) sea ice diatoms isolated from mixed assemblages collected from the Canadian Arctic. In contrast, IP 25 was absent in the dominant taxa. Chemical and taxonomical investigations suggest that the IP 25 -containing taxa represent the majority of producers and are distributed pan-Arctic, thus establishing the widespread applicability of the IP 25 proxy for palaeo Arctic sea ice reconstruction.

This study provides the ®rst detailed estimate of riverine organic carbon¯uxes in British rivers, as well as highlighting major gaps in organic carbon data in national archives. Existing data on organic carbon and suspended solids concentrations collected between 1989 and 1993, during routine monitoring by the River Puri®cation Boards (RPBs) in Scotland and the National River Authorities (NRAs) in England and Wales, were used with annual mean¯ows to estimate¯uxes of dissolved and particulate organic carbon (DOC and POC) in British rivers. Riverine DOC exports during 1993 varied from 7 . 7±103 . 5 kg ha À1 year À1 , with a median¯ux of 31 . 9 kg ha À1 year À1 in the 85 rivers for which data were available. There was a trend for DOC¯uxes to increase from the south and east to the north and west. A predictive model based on mean soil carbon storage in 17 catchments, together with regional precipitation totals, explained 94% of the variation in the riverine DOC exports in 1993. This model was used to predict riverine DOC¯uxes in regions where no organic carbon data were available. Calculated and predicted¯uxes were combined to produce an estimate for exports of DOC to tidal waters in British rivers during 1993 of 0Á68+0Á07 Mt. Of this total, rivers in Scotland accounted for 53%, England 38% and Wales 9%. Scottish blanket peats would appear to be the largest single source of DOC exports in British rivers. An additional 0 . 20 Mt of organic carbon were estimated to have been exported in particulate form in 1993, approximately two±thirds of which was contributed by English rivers. It is suggested that riverine losses of organic carbon have the potential to aect the long-term dynamics of terrestrial organic carbon pools in Britain and that rivers may regulate increases in soil carbon pools brought about by climate change.

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The presence of a di-unsaturated highly branched isoprenoid (HBI) lipid biomarker (diene II) in Southern Ocean sediments has previously been proposed as a proxy measure of palaeo Antarctic sea ice. Here we show that a source of diene II is the sympagic diatom Berkeleya adeliensis Medlin. Furthermore, the propensity for B. adeliensis to flourish in platelet ice is reflected by an offshore downward gradient in diene II concentration in >100 surface sediments from Antarctic coastal and near-coastal environments. Since platelet ice formation is strongly associated with super-cooled freshwater inflow, we further hypothesize that sedimentary diene II provides a potentially sensitive proxy indicator of landfast sea ice influenced by meltwater discharge from nearby glaciers and ice shelves, and re-examination of some previous diene II downcore records supports this hypothesis. The term IPSO25—Ice Proxy for the Southern Ocean with 25 carbon atoms—is proposed as a proxy name for diene II.

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