Foraminiferal insights into the complexities of the turbidity currents triggered by the 2016 Kaikoura Earthquake, New Zealand

Abstract

The 2016 Mw 7.8 Kaikōura Earthquake triggered simultaneous turbidity currents down ten submarine canyons along a 200 km stretch of the continental slope, east of New Zealand. They discharged into the abyssal Hikurangi Channel which flows >1500 km northwards along the trench floor as part of the Hikurangi Subduction Margin. One hundred and thirty-six foraminiferal samples from the 2016 turbidites and pre-turbidite sediment from 25 canyon/channel cores, collected during surveys 3 days, 2.5 and 8 months after the event, were used to provide insights into the complexities of these submarine gravity flows.Transport in the turbidity currents produced no significant additional breakage of foraminiferal tests, even when transported 650 km down channel. We conclude that highly variable planktic foraminiferal fragmentation index (FI) values indicate that test breakage in turbidite samples is primarily inherited from dissolution on the seafloor at their source area, which is often anomalously high because of lowered sediment pH as a result of the breakdown of the high organic carbon flux. All turbidite and pre-turbidite faunas have strong signatures of downslope displacement (presence of shallow-water benthics, size-sorted test distributions, low percentage of planktics). This is not surprising as all cores were taken in the canyon or channel floor pathways to collect turbidite sequences, with turbidity flows estimated to have a recurrence time of ∼150 yrs. Pre-turbidite faunas can be distinguished from the 2016 turbidites' faunas by having greater percentage of planktics and lower ratios of shallow to deep benthic tests (S/D Index). 210Pb profiles show that the laminated pre-turbidite sediment accumulated slowly and the foraminiferal content indicates it is composed of reworked earlier turbidite sediment with the addition of contemporaneous deep-water benthic tests and the rain of planktic tests mixed in.There are no consistent trends in any foraminiferal faunal parameters with distance displaced in the turbidity current (10–650 km). Rather, intra-turbidite (10–75 cm thick) faunal variability in a single core is often considerable and is attributed to erosion and entrainment of deep-seafloor tests by the passage of turbulent turbidity current heads, variable source canyon depths, different source canyon provenances, and test-size sorting during transport and deposition resulting in vertical and longitudinal fractionation of the smallest tests and finest sediment.This study suggests that a single foraminiferal fauna is often, but not always, insufficient to characterise a turbidite or for use in correlation with turbidites in other cores or exposures.