Sources and diagenetic status of organic matter in the Hauraki Gulf, New Zealand: Evidence from the carbon isotopic composition of d- and l-amino acids

The origin of sedimentary organic matter and the influence of microbial reworking in the Hauraki Gulf, New Zealand, were determined using the stable carbon isotopic compositions of sedimentary d- and l-amino acids. Core top sediments from three offshore and five inner Gulf sites were collected using a multi-corer. After correction for microbial alteration of original source materials, source apportionment of l-amino acids indicated that the organic matter at the outer Gulf sites was predominantly derived from a marine source, whereas the inner Gulf sites reflected a terrestrial source. The isotopic compositions of d-amino acids were consistent with preferential microbial utilization of marine organic matter in the outer Gulf and preferential utilization of terrestrial material in the inner Gulf. Microbial reworking was most prevalent at the outer Gulf sites where up to 58% of the total hydrolyzable amino acids (THAA) were derived from microbial synthesis. Although the inner Gulf sites contained relatively less reworked material, the microbial contribution to the THAA pool ranged from 40 to 50%. This study showed that the sediment carbon dynamics in the Hauraki Gulf are complex as microbial reworking influences the isotopic compositions of the sedimentary organic matter throughout the Hauraki Gulf.