Inorganic geochemistry of saline substrates associated with coastal turf, Otago Peninsula, New Zealand

ABSTRACT

Halophytic plant communities have developed on exposed coastal landforms to form low-stature turf in several locations around New Zealand. We quantify the inorganic geochemistry, mineralogy and simulated ephemeral surface water composition of saline soil-free bare ground at a well-preserved site at Maori Head on Otago Peninsula, where halophytes have colonised from adjacent coastal turf. The community at this site is dominated by Leptinella dioica, Samolus repens, Disphyma australe and Selliera radicans. Maori Head substrates are dominated by clay-altered basalt colluvium and show a general trend from high electrical conductivity (EC > 10 mS/cm) and weakly acid pH (< 6) on bare ground to low EC (< 1 mS cm⁻¹) and higher pH (> 7) in turf and adjacent pasture soils. High salinities on dry substrates and in simulated ephemeral surface waters result from surface evaporation of airborne sea spray and contain halite (NaCl) with some gypsum (CaSO₄.2H₂O). Chemical interactions with the substrate are minor. The high surface salinities were maintained despite a major winter rain event with strong onshore winds. However, surface salinity is frequently leached by rain and shallow groundwater, whereas inland saline sites (pH 8–11) have substantial salt accumulations driven by lower rainfall, high evaporation and chemical interaction with Otago Schist-derived substrates.

KEYWORDS:

• Salt
• halite
• halophyte
• electrical conductivity
• pH
• basalt
• schist
• evaporation

Acknowledgements

Logistical support and data archiving by the Queen Elizabeth II National Trust were instrumental in developing this research project. Access to the Maori Head site was generously provided by the landowners. Discussions with Clement Lagrue (Department of Conservation) and Scott Jarvie (Otago Regional Council) assisted in developing ideas about links between inland and coastal saline sites. Constructive comments from two journal referees and the handling editor substantially improved the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability

All data relevant to this study are included within this paper and within cited references.

Additional information

Funding

Funding for this research was provided by the University of Otago.