Controls on the age of vascular plant biomarkers in Black Sea sediments

Abstract

Transfer of organic carbon (OC) from the terrestrial to the oceanic carbon pool is largely driven by riverine and aeolian transport. Before transport, however, terrigenous organic matter can be retained in intermediate terrestrial reservoirs such as soils. Using compound-specific radiocarbon analysis of terrigenous biomarkers their average terrestrial residence time can be evaluated. Here we show compound-specific radiocarbon ( 14C) ages of terrigenous biomarkers and bulk 14C ages accompanied by geochemical proxy data from core top samples collected along transects in front of several river mouths in the Black Sea. 14C ages of long chain n-alkanes, long chain n-fatty acids and total organic carbon (TOC) are highest in front of the river mouths, correlating well with BIT (branched and isoprenoid tetraether) indices, which indicates contribution of pre-aged, soil-derived terrigenous organic matter. The radiocarbon ages decrease further offshore towards locations where organic matter is dominated by marine production and aeolian input potentially contributes terrigenous organic matter. Average terrestrial residence times of vascular plant biomarkers deduced from n-C 29+31 alkanes and n-C 28+30 fatty acids ages from stations directly in front of the river mouths range from 900 ± 70 years to 4400 ± 170 years. These average residence times correlate with size and topography in climatically similar catchments, whereas the climatic regime appears to control continental carbon turnover times in morphologically similar drainage areas of the Black Sea catchment. Along-transect data imply petrogenic contribution of n-C 29+31 alkanes and input via different terrigenous biomarker transport modes, i.e., riverine and aeolian, resulting in aged biomarkers at offshore core locations. Because n-C 29+31 alkanes show contributions from petrogenic sources, n-C 28+30 fatty acids likely provide better estimates of average terrestrial residence times of vascular plant biomarkers. Moreover, sedimentary n-C 28 and n-C 30 fatty acids appear clearly much less influenced by autochthonous sources than n-C 24 and n-C 26 fatty acids as indicated by increasing radiocarbon ages with increasing chain-length and are, thus, more representative as vascular plant biomarkers.