Characterization of a flood-associated deposit on the Waipaoa River shelf using radioisotopes and terrigenous organic matter abundance and composition

Abstract An ephemeral oceanic-flood deposit adjacent to a well-studied small mountainous river (SMR), the Waipaoa River in northeastern New Zealand, was characterized using multiple proxies, including radioisotopes ( 234 Th, 7 Be, and 210 Pb), bulk organic carbon abundance and isotopic signature (%OC, δ 13 C), as well as a biomarker of terrigenous organic matter (lignin). Field sampling was conducted within two weeks after a 1-in-8 year flood that occurred between January 30 and February 6, 2010. Geochemical analyses indicated that initial deposition of fresh riverine material extended alongshore to the north and south from the river mouth. A comparison of prior- and post-flood 7 Be inventories revealed that flood sediments were widely dispersed between 20 and 70 m water depth, accounting for 50–80% of the estimated flood load. Surface (0–2 cm) isotopic carbon values increased with distance from Poverty Bay, positively correlating with total 210 Pb activities, potentially reflecting increasing marine influence with water depth. Abundances of sedimentary organic carbon (OC) were 0.18–0.76% dry weight, and the total nitrogen varied from 0.02 to 0.13%. Stable isotope signatures of carbon (δ 13 C OC ), nitrogen (δ 15 N), and lignin abundances (λ 6 ) throughout the study area ranged from −23.6 to −27.7‰, 1.9 to 5.3‰, and 0.93 to 9.0 mg 100 mg OC −1 , respectively. The spatial distribution pattern of terrigenous organic matter (OM) abundance and interclass ratios (indicative of freshness of organic matter) varied along and across-shelf. Lignin abundances were high and interclass ratios were low in the southern depocenter and inner shelf areas, suggesting that this zone had recently received vascular-plant enriched OM, minimally altered by shelf-bed mixing processes. In contrast, sediments in the northern depocenter and outer shelf also contained elevated amounts of terrigenous sedimentary OM, but this material was generally lower in lignin abundance and had higher interclass ratios (greater degradation). Collectively, these results suggest that the flood-derived sediment and fresh terrigenous OM were mostly constrained between 20 and 70 m water depth, with enhanced deposition overlapping the tectonic-controlled depocenters located to the northeast and southeast of Poverty Bay.