Sources of organic matter in a coastal marine environment: Evidence from n-alkanes and their δ 13C distributions in the Hauraki Gulf, New Zealand

Abstract

The distribution and δ 13C composition of sedimentary n-alkanes (C 14–C 34) were investigated to determine the origins of organic matter in surface sediments in a subtropical, non-riverine dominated coastal shelf. Core-top sediments were collected in the Hauraki Gulf, New Zealand, from three nearshore sites and four offshore sites that represent a range of depths and distances to land. Total n-alkane concentrations varied 2-fold ranging from 326 to 819 μg g − 1 and n-alkane δ 13C ranged from − 21.2‰ to − 38.9‰. Source apportionment was calculated using marine and terrestrial end-member values for n-alkane chain lengths and isotopic composition to derive an inventory of marine versus terrestrial organic matter. Using this multi proxy approach, marine inputs were shown to be relatively constant across the Hauraki Gulf. In contrast, concentrations of terrestrial n-alkanes varied 4-fold across the study sites. Terrestrial contributions were 56–65% of total n-alkanes, dominating at all but those sites most remote from land where terrestrial n-alkanes levels still comprised ~ 40–45% of the characterized organic matter. Bulk analyses (e.g. δ 13C of total organic carbon and C/N ratios) poorly resolved terrestrial inputs in this complex coastal environment. The success of the multi-proxy approach in identifying terrestrial input is attributed to the ability to resolve inputs from coastal sources of organic matter such as mangroves and sea grasses that can mask terrestrial sources. Organic carbon accumulation rates were relatively constant on the shelf (4.0 to 4.5 mg cm − 2 year − 1), highest at a deeper site on the outer shelf (10.7 mg cm − 2 year − 1), and lowest on the upper slope (3.2 mg cm − 2 year − 1). Alkane accumulation rates (19 to 140 μg cm − 2 year − 1) were more variable, with burial rates of terrestrial alkanes on the shelf averaging 1.5 times that of marine accumulation rates. The isotopic and biomarker data indicate that even in what is considered a marine-dominated coastal environment, an appreciable amount of terrestrial organic matter is readily transported from its locus of input across the shelf to be buried in outer-shelf depocenters.