Bacteriohopanepolyols as tracers for continental and marine organic matter supply and phases of enhanced nitrogen cycling on the late Quaternary Congo deep sea fan

Abstract

Few studies have successfully identified bacteriohopanepolyols (BHPs) in continuous sedimentary records older than 100 ka. Here we expand significantly on previous work and report high resolution records of a variety of fully functionalised BHPs from ODP Site 1075 in the Congo deep sea fan. BHPs are present throughout the core in sediments up to ca. 1.2 Ma, including composite and unsaturated homologues. Although adenosylhopane, a proposed tracer for soil organic matter input, seems to undergo significant diagenesis with increasing depth below 65 m below the sea floor (mbsf), most BHPs appear to be relatively resilient to degradation, highlighting their potential as sedimentary tracers for past bacterial population distributions. An unusual unsaturated BHT–cyclitol ether is present in high concentration between 500 and 600 ka and is interpreted as a possible biomarker for nitrogen-fixing Trichodesmium cyanobacteria. This previously unrecognised ecosystem change may have been partially driven by a decrease in nutrient supply to surface waters, resulting in more intense pelagic N fixation.