Distribution and Origin of Organic Matter Preserved in Modern Surface Sediments throughout Coastal SE Alaska

In the summer of 2004, modern surface sediments (0-2cm) were obtained by multicoring 27 sites beneath inland waters of SE Alaska (55° to 61°N). To evaluate the distribution pattern and source contribution of organic matter throughout the region, these samples were analyzed for total organic carbon (TOC) and biogenic silica (bioSi) content as well as elemental (C/N), stable isotopic (δ13C) and plantwax lipid composition of TOC. TOC ranged from 0.3% to 8.0% (by weight) with sediments from the region south of 58°N off the coast of the Queen Charlotte and Baranof Islands much more TOC-enriched than those deposited in the region to the north. bioSi content, a gauge of diatom input, ranged from 0.1% to 33.1% (by weight) and displayed high correlation with TOC (r = +0.98). δ13C values ranged from 24.5o/oo to 19.8o/oo with most 13C-depleted values typically found in sediments deposited to the north of 58°N from Malispina Shelf to Prince William Sound. C/N (molar ratio) varied from 7.7 to 12.7 with values typically increasing in sediments to the north. The observed distributional patterns for bioSi, δ13C and C/N fit the interpretation that organic matter deposited in sediments north and south of 58°N is relatively more terrestrial-enriched and marine-enriched, respectively. Plantwax lipid signatures produced a contrasting picture for the organic matter source contribution to this region. Long chain, odd predominant n-alkanes (C23-C35) and even predominant n-acids (C20- C28) characteristic of the epicuticular waxes produced by higher plants were most concentrated (per g TOC) in sediments south of 58°N. Although highest concentrations of long chain n-alkanes were actually measured in sediments to the north, the series of compounds displayed very low odd-to-even chain length predominance, a common fingerprint of fossil organic matter. The plantwax n-alkane signature preserved in sediments south of 58°N was accompanied by a pentacyclic triterpene-β(H),21β(H)-hop-22(29)-ene (diploptene). Although the origin of diploptene is yet to be established in the case of SE Alaska, organic matter eroded from soils is the recognized source of this compound in an analogous compositional pattern for hydrocarbons in coastal regions farther south in the Pacific Northwest. Bulk elemental and terrestrial biomarker data clearly show a conflicting perspective on the terrestrial and marine organic matter distribution in sediments from coastal SE Alaska. Resolution of this environmentally intriguing, organic geochemical ambiguity clearly warrants further investigation.