Characterization of Siberian Arctic coastal sediments: Implications for terrestrial organic carbon export

Surface sediments were collected during the 2000 TransArctic Expedition along the Siberian Arctic coastline, including the Ob, Yenisey, Khatanga, Lena, and Indigirka estuaries. Sediments were characterized for elemental composition (total organic carbon, TOC, black carbon, BC, and total N, as well as major and trace elements), isotopic signature ((delta)13C, (delta)15N, (delta)14C, -Nd, 87Sr/86Sr), and organic molecular composition to better understand river export variations over the large spatial scale of the Siberian Arctic. On average, 79 +- 9% of the total C in sediments was organic while 21 +- 9% was inorganic. BC made up 9 +- 4% of the TOC pool, with a general increasing trend from west to east along the Siberian coast. The combined Nd- and Sr-isotopes (Nd and 87Sr/86Sr) were used to define two distinct sediment sources between east and west Siberian regions with the Khatanga River as a boundary. Data from pyrolysis-GC/MS of the sedimentary organic carbon (SOC) indicated an increase in the freshness of the organic matter from west to east on the Siberian Arctic coast, with increasing relative abundance of furfurals (polysaccharides) with respect to nitriles. Values for the (delta)13C of SOC ranged from -27.1‰ (mostly terrigenous) to -23.8‰, while (delta)15N increased from east to west (3.1 to 5.2‰) with a significant correlation with C/N ratio. Values for the (delta)14C of SOC ranged from -805 to -279‰, with a consistent trend increasing from the east (Indigirka River) to the west (Ob River). These (delta)14C values corresponded to a 14C age of 2570 +- 30 yBP in the Ob estuary and 13,050 +50 yBP in the Indigirka estuary. Most importantly, (delta)14C values were significantly correlated with the ratio of BC/TOC (R^2 = 0.91, n = 6), consistent with the distribution pattern of increasing permafrost zone from the west to the east along the Siberian coast. Together, our results suggest that older OC was derived from the release of recalcitrant BC during permafrost thawing and riverbank and coastal erosion, likely enhanced by ongoing environmental changes in the northern ecosystem.