Tracing the Holocene environmental evolution on the island of South Georgia by lipid biomarkers and compound-specific radiocarbon analysis

Abstract:

We present lipid biomarker data of a sedimentary record that spans the entire Holocene and was recovered from a marine inlet located at the northern coast of South Georgia (Cumberland West Bay) to adress two major objectives, namely: a) the identification of temporal variations in reservoir effects and b) the reconstruction of regional environmental changes during the last deglaciation. For objective a) we compare compound-specific radiocarbon ages of terrigeneous lipid biomarkers (n-alcohols) with ¹⁴C ages of bulk sedimentary organic matter to detect potential reservoir effects and changes over time. The study site experienced different environmental conditions following the retreat of the local glaciers in the early Holocene. After an initial freshwater stage when the inlet was separated from the sea, it passed into brackish and finally fully marine conditions. These hydrological changes probably resulted in variable reservoir ages. For objective b) we investigate biomarkers, comprising n-alkanes (C₁₅-C₃₅) and n-alcohols (C₁₄-C₂₈), including high molecular weight compounds typically derived from land plants and low molecular weight compounds like C₁₇ n-alkanes and C₁₆ n-alcohols, mostly originating from aquatic organisms. We noticed an abrupt increase in TOC values and sums of biomarkers after the glacier retreat, culminating in highest concentrations at the transition from freshwater to brackish conditions. Subsequently, the concentrations decline and vary at low levels until the inlet becomes fully marine. The marine stage is characterized by relatively constant TOC values but variable biomarker concentrations. Lowest biomarker levels occur around 1870±134 cal yr BP and between 6204±115 and 9666±161 cal yr BP and correspond to phases of glacier advance as suggested by studies of lake and peat deposits from South Georgia.