ALKANE, TERPENE, AND POLYCYCLIC AROMATIC HYDROCARBON GEOCHEMISTRY OF THE MACKENZIE RIVER AND MACKENZIE SHELF - RIVERINE CONTRIBUTIONS TO BEAUFORT SEA COASTAL SEDIMENT

To study the largest source of river sediment to the Arctic Ocean, we have collected suspended particulates from the Mackenzie River in all seasons and sediments from the Mackenzie shelf between the river mouth and the shelf edge. These samples have been analyzed for alkanes, tri terpenes, and polycyclic aromatic hydrocarbons (PAHs). We found that n aturally occurring hydrocarbons predominate in the river and on the sh elf These hydrocarbons include biogenic alkanes and triterpenes with a higher plant/peat origin, diagenetic PAHs from peat and plant detritu s, petrogenic alkanes, triterpenes, and PAHs from oil seeps and/or bit umens, and combustion PAHs that are likely relict in peat deposits. Be cause these components vary independently, the season is found to stro ngly influence the concentration and composition of hydrocarbons in th e Mackenzie River. While essentially the same pattern of alkanes, diag enetic hopanes, and alkyl PAHs is observed in all river and most shelf sediment samples, alkane and triterpene concentration variations are strongly linked to the relative amount of higher plant/peat material. Polycyclic aromatic hydrocarbon molecular-mass profiles also appear to be tied primarily to varying proportions of peat, with an additional petrogenic component which is most likely associated with lithic mater ial mobilized by the Mackenzie River at freshet. Consistent with the g eneral lack of alkyl PAHs in peat, the higher PAHs found in the river are probably derived from forest and tundra fires. A few anthropogenic /pyrogenic compounds are manifest only at the shelf edge, probably due to a weakening of the river influence. We take this observation of py rogenic PAHs and the pronounced source differences between two sedimen t samples collected at the shelf edge as evidence of a transition from dominance by the Mackenzie River to the geochemistry prevalent in Arc tic regions far removed from major rivers.