Signature of organic matter exported from naturally Fe-fertilised oceanic waters

Biomarker distributions and organic carbon and nitrogen isotopic signatures of organic matter (OM) produced in surface waters around the Crozet Plateau (Southern Ocean) are significantly different between a Fe-fertilised region (north) and a high nutrient low chlorophyll region (HNLC, south). If these OM signatures are exported to and preserved in surface sediments, they could potentially be used as palaeo-proxies for identifying Fe-fertilisation events in the past. Here, we assess the alteration of the OM signature through the water column and at the sediment–water interface by comparing organic signatures in surface waters, sediment traps and surface sediments. Our results suggest that there is significant degradation of OM during transport to the sea floor, which causes reduced fluxes of biomarkers to sediments north and east of the Crozet Plateau. Sterols, alkenones and C27 and C29 12-hydroxymethyl alkanoates, and C28 1,14-diols appear to be less labile than total organic carbon (TOC), except to the north, where alkenones and sterols are more rapidly degraded than TOC. Sedimentary bulk and compound specific δ13C values also reflect surface water productivity patterns, with elevated values occurring in sediments underlying the Fe-fertilised waters. In contrast, δ15N values appear to be strongly biased by degradation and grazing during export and burial. Thus, only some of the differences observed in surface waters between the Fe- fertilised and HNLC areas are exported to deep waters and preserved in the sedimentary record, suggesting that caution is required in the application of these proxies to studies of ocean palaeoproductivity.