3D characterisation and quantification of an offshore freshened groundwater system in the Canterbury Bight

Mark Person,Zahra Faghih,Amir Haroon,Carl W. Gable,Tanita Averes,Ashwani Kumar Tiwari,Susanne Woelz,Aaron Micallef,Shuangmin Duan,Marion Jegen,Denis Cohen,Katrin Schwalenberg,Bradley A. Weymer,Joshu J. Mountjoy

doi:10.1038/s41467-020-14770-7

Abstract

Although offshore freshened groundwater (OFG) systems have been documented in numerous continental margins worldwide, their geometry, controls and emplacement dynamics remain poorly constrained. Here we integrate controlled-source electromagnetic, seismic reflection and borehole data with hydrological modelling to quantitatively characterise a previously unknown OFG system near Canterbury, New Zealand. The OFG system consists of one main, and two smaller, low salinity groundwater bodies. The main body extends up to 60 km from the coast and a seawater depth of 110 m. We attribute along-shelf variability in salinity to permeability heterogeneity due to permeable conduits and normal faults, and to recharge from rivers during sea level lowstands. A meteoric origin of the OFG and active groundwater migration from onshore are inferred. However, modelling results suggest that the majority of the OFG was emplaced via topographically-driven flow during sea level lowstands in the last 300 ka. Global volumetric estimates of OFG will be significantly revised if active margins, with steep coastal topographies like the Canterbury margin, are considered.

Highlights

Offshore freshened groundwater (OFG) systems have been documented in numerous continental margins worldwide, their geometry, controls and emplacement dynamics remain poorly constrained
We identify a number of resistive features in the controlled-source electromagnetic (CSEM) inversion models, which we define as bodies of resistivity of >2 Ωm43 that extend across a minimum horizontal distance of 5 km (Fig. 4)
The main offshore freshened groundwater (OFG) body extends up to a distance of 60 km perpendicularly from the coast, has a maximum thickness of at least 250 m, and its top reaches a maximum depth of 50 m bsf

Summary

Introduction

Offshore freshened groundwater (OFG) systems have been documented in numerous continental margins worldwide, their geometry, controls and emplacement dynamics remain poorly constrained. There are at least five mechanisms known to be responsible for the emplacement of OFG These include active groundwater migration across topographic gradients via present day, permeable connections between offshore and onshore aquifers[6], recharge during Pleistocene sealevel lowstands[7,8], sub-glacial and pro-glacial injection[9], entrapment of connate water in subsiding basins[10], and gas hydrate dissociation[11]. Global volumetric estimates of OFG were derived in passive margins and are on the order of 105 km[3] This is two orders of magnitude greater than the volume of groundwater that has been extracted globally from continental aquifers since 1900 Industrial sectors involving seafloor engineering, carbon dioxide sequestration, and ore deposit and petroleum exploration have a direct interest in the evolution of OFG systems because these can place better constraints on past fluid migration histories[16]. OFG can provide potential archives of former environmental conditions[18] and contribute to advance our understanding of human settlement and migration in the past[19]

Objectives

Methods

Results

Conclusion