Geochemical evolution of high-pH sodic salt pans in Central Otago, New Zealand

ABSTRACT

Soil-free sodic salt pans with local pH >10 have formed on loess and schist-derived clay surfaces in the semi-arid climate of Central Otago. This study identifies evaporative mineral distributions and associated variations in surface geochemistry, and these results are relevant to survival of rare halophytic plants on some of the pans. Evaporative salts on pan surfaces are dominated by NaCl from marine aerosols, and Na-sulphates and Na-carbonates. Some of the Na in evaporites was derived from alteration of schist-derived albite and Na-bearing smectite formed from albite. The high pH pan surfaces result from a combination of long-term (decades to millennia) albite alteration and short-term (minutes to months) Na-carbonate dissolution and recrystallisation. This muscovite is variably cemented by evaporative salts, and some crusts have abundant salt-dissolution voids. High pH has facilitated dissolution of aluminium and silica from albite and smectite, with evaporative precipitation of Al-oxyhydroxide contributing to crust cementation. Different dissolution rates of evaporative minerals during rain events and damp seasons results in differential mobility of salt components downslope. The salt pans are physically and chemically dynamic features that have formed on time scales of decades to centuries.

KEYWORDS:

• Sodic
• evaporite
• halite
• loess
• clay
• schist
• rain shadow
• halophyte

Acknowledgements

We thank Euan Hind and Scott Jarvie of ORC for their interest and support of our research project. We appreciate the enthusiasm of Ellery Mayhence, Clement Lagrue, Sasha Roselli and Ross Curtis (Department of Conservation) who provided useful discussions and accompanied us on site visits to scientific reserves. Nick Loughnan and Faith Gray kindly allowed access to their land for part of this study. Central Otago District Council staff provided guidance for safe working on roadside outcrops. SEM observations were made at Otago Micro and Nanoscale Imaging (OMNI), University of Otago. Stephen Read ably assisted with image construction. Thoughtful and constructive reviews by two journal referees improved the presentation of the ms.

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

This research was funded by University of Otago and Otago Regional Council.