Terrestrial and marine biomarkers in a seasonally ice-covered Arctic estuary — integration of multivariate and biomarker approaches

Abstract

Hydrocarbons, including alkanes, alkenes, hopane triterpenes and polycyclic aromatic hydrocarbons (PAHs), plus sterols, n-alcohols and a number of higher plant triterpenoids have been determined for suspended particulate, sediment trap and sediment samples taken from the Mackenzie River and the adjacent Mackenzie shelf in the Beaufort Sea. These biomarkers are valuable tracers of terrigenous and petrogenic inputs from the river to the shelf water column and are also useful for assessing marine production in the Mackenzie River estuary. We use Principal Component Analysis (PCA) to provide a robust classification of biomarkers according to their primary source (e.g. terrigenous, marine) and to identify which biomarkers covary. The Mackenzie River is the dominant source for n-alkanes, n-alcohols, sterols and triterpenoids from higher plants, diagenetic hopanes, petrogenic isoprenoids and parent (unsubstituted) PAHs to the Mackenzie shelf. The riverine hydrocarbon signature of these markers is modified by preferential settling out of lithic material relative to less dense higher plant debris. Seasonal marine production of a suite of alkenes, sterols and alcohols from phytoplankton and Zooplankton is evident in water column and sediment trap samples, but these labile compounds tend not to be preserved in surficial sediments. Although few individual sterols provide unambiguous markers of terrestrial or marine organic matter, PCA successfully classifies sterols as either principally marine or principally terrestrial. n-Alcohols are often overlooked as biomarkers, but we find that they too reflect the relative contributions of Zooplankton and terrigenous inputs in the Mackenzie estuary. To quantify dispersal of riverine material on the adjacent shelf in the context of dilution by marine production, we develop a multivariate Partial Least Squares (PLS) path model. We find that the PLS results strengthen geochemical interpretations based on individual biomarkers by providing a quantitative representation of the differences among samples. PLS successfully models the increases in terrigenous particulate on the shelf with river flow and the effect of autochthonous production.