Hydrogeological controls on formation of Patearoa saline site in Central Otago, New Zealand and definition of geoecological salt lines

ABSTRACT

The Patearoa saline site in Maniototo basin has developed on variably clay-altered Otago Schist. Decomposition and sedimentary redistribution of schist outcrop components has led to formation of bare soil-free substrates (upper metre scale) with contrasting permeability to rain and shallow groundwater percolation. Relatively impermeable clay-rich substrates, with a surface crust (cm scale) of detrital clay and muscovite, are saline with electrical conductivity (EC) of 1–35 mS/cm and locally hyperalkaline (pH > 10), with evaporative mineral accumulations. These saline substrates are downslope of more permeable substrates consisting of coarse schist debris and fractured outcrops, which have lower salinity (EC < 1 mS/cm) and lower pH (<7). A salt line can be mapped at metre scale between these substrate types on hillsides. The geochemical contrasts across the salt line have strong effects on plant communities that are colonising bare ground. Initial colonisation of bare saline substrates by some plants changes the geochemical signatures of the surficial few centimetres to form proto-soil with lower EC and pH. Proto-soil development facilitates further colonisation by pasture grasses, but the underlying substrates remain saline and highly alkaline. Salt lines similar to the one at Patearoa are mappable elsewhere in the Otago area and constitute fundamental geoecological boundaries.

KEYWORDS:

• Geoecology
• evaporation
• Schist
• greywacke
• groundwater
• halophyte
• pH
• salinity

Introduction

Bare soil-free ground is widespread in semi-arid Central Otago, especially in areas of natural or anthropogenic erosion (Mather 1982; Hewitt 1996; Walker 2000; McGlone 2001; Hewitt et al. 2021; McConnell et al. 2021; Craw et al. 2022a). This bare ground becomes progressively colonised by plants. Plant establishment facilitates incipient soil development (called proto-soil herein), followed by development of more substantial, albeit still primitive, soil profiles (Rogers et al. 2002; Rufaut et al. 2018; Hewitt et al. 2021; Craw et al. 2022a). Some of bare ground has relatively impermeable substrates and accumulates evaporative minerals to become variably saline, and these salt pans have been a topic of recent geological, geochemical, and mineralogical research (e.g. Druzbicka et al. 2015; Law et al. 2016; Rufaut et al. 2018; Craw et al. 2022a). These salt pans are also of interest to the biological community as they locally host rare endemic salt-tolerant plants (halophytes) and some sites have been legally protected for this reason (Allen et al. 1997; Allen and McIntosh 1997; Allen 2000; Rogers et al. 2000).

Most of this previous research on dryland bare ground, especially the saline sites, has focused on surficial features and processes, in the upper few centimetres (Hewitt 1996; Allen et al. 1997; Druzbicka et al. 2015; Law et al. 2016; Craw et al. 2022a, 2022b; Craw et al. 2023a, 2023b). In this study, we extend this work to encompass a three-dimensional approach, albeit still involving <20 depth and mostly <1 m depth. In addition, while we concentrate on saline substrates at surface and at shallow depth, we also integrate these saline substrates with the surrounding non-saline landscape in a three-dimensional manner. We do this in detail for the Patearoa saline site in the Maniototo basin (Figure 1A), which we have previously documented in a more general survey of saline bare substrates of the area (Craw et al. 2022b). The Patearoa site proved to be ideal for this three-dimensional extension of our research, and allowed us to define what we call a geoecological ‘salt line’, controlled by the underlying three-dimensional geology, that separates entirely different substrates and entirely different plant communities. We then go on to develop this salt line concept for some other areas with variably saline bare ground and limited soil development in Central Otago. Finally, we show that the inland geoecological salt line concept can be extended to a regional scale, arising from mineralogical differences in underlying basement rocks.

Figure 1. Location and general setting of the Patearoa salt site. A, Oblique view from southeast of topography of a transect across the southern South Island, New Zealand, showing solar irradiation variations (from Solargis; http://globalsolaratlas.info) that affect evaporation and contribute to formation of the inland rain-shadow east of the Southern Alps. B, Hillshade digital elevation model of the Maniototo basin, with Patearoa salt site on the SE margin. C, Annual rainfall variations at Ranfurly, near Patearoa (from NIWA; cliflo.co.nz). Regression indicates negligible change on decadal time scale. D, Oblique drone view (2022) of some Patearoa open salt pans (white) and weed-colonised pans (=ex-pans, purple) including a well-defined wetland (dark green). E, Vertical aerial photograph in 1975 (from Retrolens.co.nz), showing salt pan areas (white) including the wetland which is viewed in (D) along the yellow arrow. Additional historical photography is presented in Supplementary Figure S1.

General setting

Like other reserves set up to protect inland saline habitats (mostly near Alexandra; Figure 1A), the Patearoa site is located in the prominent rain-shadow of the Southern Alps mountains, where abundant sunshine, and dry winds lead to potential evaporation that exceeds rainfall (Figure 1A,C; NIWA 2022). Consequently, impermeable bare substrates can develop surficial salt encrustations that favour halophyte communities (Allen and McIntosh 1997; Rogers et al. 2000), especially where substrate electrical conductivity (EC) > 1 mS cm⁻¹ helps to exclude other species (Allen et al. 1997; Rufaut et al. 2018). The Patearoa site is a Queen Elizabeth II National Trust (QEII Trust) covenant on private land, and was set up primarily because of its saline habitats on soil-free substrates (salt pans; Figure 1D,E) and associated rare and endangered native plants (Supplementary Table 1; Allen and McIntosh 1997; Rogers et al. 2000; Craw et al. 2022b). The site has been fenced, and minor grazing by sheep occurs periodically.

The Patearoa site is on the margin of the Rock and Pillar Range, at the eastern edge of the Taieri River flood plain (Figure 1B; Craw et al. 2022b). The site is underlain by Otago Schist basement which has been variably oxidised and clay-altered by groundwater (Figures 1 and 2A,B; Chamberlain et al. 1999). Contrasting degrees of schist alteration govern the topography, groundwater flow, and most surface substrates at the site (Figure 2A–D). Strongly altered schist is soft, easily eroded, and contributes to clay-rich impermeable surficial sediments, whereas weakly altered schist is more erosion-resistant, permeable to groundwater, and forms coarse-grained permeable surficial sediments (Figure 2D). The topography of the Patearoa site was formed where a now-abandoned channel of the Taieri River eroded into the soft strongly clay-altered schist valley margin, to create a lower flat (Figures 1 and 2C,D), while adjacent harder weakly clay-altered schist resisted that erosion to preserve a flat upper terrace ∼20 m above the lower flat (Figure 2D). A curved terrace scarp separates these two levels (Figure 2A<C,D). The upper terrace was planed flat by an older Taieri River course that left minor remnants of schist-derived river gravels. Contrasts in substrate permeability have resulted in formation of a boundary within the terrace scarp, herein called the ‘salt line’ between saline substrates below it and non-saline substrates above it (Figure 2C,D).

Figure 2. Hydrogeological setting for the Patearoa salt site. A, Topographic setting of the site at the edge of the active Taieri River flood plain, where the river has cut into schist basement, forming a scarp between lower and upper parts of the saline site. B, Sketch cross section through the schist basement showing generalised groundwater alteration profile. C, Oblique drone view of Patearoa salt pans and adjacent terrace scarp, from the lower part of which groundwater seeps enhance grass growth, and contribute to evaporative salts below the salt line. A salt line occurs below this seepage zone where less permeable substrates occur, as seen on salt pans. D, Sketch cross section through the terrace scarp, showing the contrasts in schist basement clay alteration and associated clay-rich sediments, and their effects on the hydrogeology and formation of the salt line. EC values in mS cm⁻¹.

Despite the reserved status of the site, areas of saline bare ground on the lower flat have been progressively colonised by exotic plant species, forming areas with surficial proto-soil adjacent to remaining salt pans, and these areas are called ‘ex-pans’ herein (Figure 2C). The lower flat also has some ephemeral wetland depressions abandoned by the meandering Taieri River course, and these were dry salt pans in 1975 (Figure 1E; Figure 2A). There has been a general change over time to a wetter surficial environment, and this change appears to have occurred principally in the 1980s, based on historical aerial photography (Figure 1D,E; Supplementary Figure 1). The local rainfall record is highly variable, but there has been no consistent increase in annual rainfall since the 1970s (Figure 1C). Instead, this change to wetter lower slopes coincides with development of agricultural irrigation in the area following opening of a major irrigation scheme in 1984. Water stored in Lake Logan on the Rock & Pillar Range passes the saline site in a water race ∼600 m upslope (Figure 1B; Figure 2A). This irrigation water now apparently contributes to the hydrogeological setting of the Patearoa saline site (Figure 2B; Supplementary Figure 1).

Methods

Photographic documentation of the Patearoa site was obtained with drone flights in November 2022. Central portions of >600 vertical photographs were compiled digitally into a georeferenced distortion-free mosaic of the site with pixel resolution of ∼50 cm. A summary mosaic image is presented in Figure 3A. Additional drone photography was conducted to obtain higher resolution views of parts of the site, especially the salt pans, and some oblique photography was obtained to document the various substrates in the context of the relief of the site. The focus of this drone imaging was to define the spatial distribution of salt pans, ex-pans, other bare substrate and proto-soil areas, and their spatial relationships to one another. Grassy turf and soil substrates were also mapped for comparison to the other substrate types. The drone photography was further used as a basis for field checking at the site, using holes dug with a spade, in order to construct a generalised surficial geological map of the principal substrates.

Figure 3. Geoecological maps of the Patearoa saline site, showing distribution of vegetation and substrates. A, Photograph mosaic derived from a drone survey of the site, showing the principal substrates, salt line, and some figure locations. B, Generalised map of distribution of vegetation and substrates, based on mosaic in a, field observations, and EC and pH data.

Spade holes were dug as deep as was practical (<50 cm), generally through soft or firm substrate as far as a hard layer. Some schist debris and schist-derived gravel substrates were too hard to dig deeper than ∼5 cm. Excavations were made away from native plants, and surficial material was kept intact and replaced after investigations were complete to minimise site disturbance. Surficial turf was turned over beside the excavated sites, then re-turned and replaced.

Substrate chemical characterisation was done in the field by measuring electrical conductivity (EC) as a proxy for salinity, and pH. These key parameters were originally chosen by the pioneering botanists and soil scientists to characterise saline sites in inland Otago (e.g. Allen et al. 1997; Allen and McIntosh 1997), and our subsequent work follows this approach (Druzbicka et al. 2015; Law et al. 2016; Rufaut et al. 2018). These measurements were made with a portable Oakton PC450 metre with separate pH and EC probes, calibrated in laboratory conditions (20°C) with standard solutions. Measurements were obtained at 12-16°C in spring; and 15-25°C in summer on slurries made with 20 ml of solids and 30 ml of added distilled water. This solid/liquid mix agitated into a uniform paste slurry pre-measurement has been shown in our previous work (e.g. Craw et al. 2022a, 2022b, 2023a, 2023b) to be the most practical and representative of surface conditions at time of sampling.

To extend our concept of geoecological salt lines developed at Patearoa, three additional inland saline sites, Chapman Road, Springvale, and Mahaka Katia (Figure 1A; Craw et al. 2023b), were revisited to obtain extra information to allow comparisons with Patearoa. This extra information involved land-based and drone-based photography, three-dimensional characterisation of substrates via spade holes, and associated EC and pH measurements of substrates on either side of mapped salt lines. In particular, a specific geoecological map was constructed from drone photographs for part of the Springvale site that crosses a salt line, augmented with shallow holes dug to characterise substrates and obtain subsurface EC and pH measurements.

Results

Definition of the Patearoa salt line and substrate separation

The Patearoa salt line is a distinct mappable hydrogeological feature that separates the site into two geologically different parts that have contrasting substrates and, consequently, contrasting vegetation (Figure 2C,D; Figure 3A,B; Supplementary Table 1). The salt line has developed within the terrace scarp and is highlighted by a belt of tall green pasture grass (Figure 2C; Figure 3A). These patches of green grass under summer conditions reflect minor shallow groundwater seepage towards the surface, in contrast to the dry surrounding areas (Figure 2D), although no water was seen at surface or in holes dug to ∼50 cm during this study. The groundwater seepage at the salt line is controlled by the underlying contrast in substrate permeability, with clay-rich sediments on clay-altered schist below the line, and more permeable weakly altered schist and schist debris above the line (Figure 2D; described in more detail below). These features can be traced across the whole site, around the terrace scarp at approximately the same level (Figure 3A,B). The salt line rises almost to the top of the upper terrace in a narrow zone towards the south-eastern end of the covenant, where clay-altered schist and associated clay-rich sediments are locally preserved (Figure 3A,B).

The salt line can be detected chemically in substrates as a sharp change in EC and pH (Figure 2D). Below the salt line, EC is >1 mS cm⁻¹ and pH is 7–10 on bare substrate surfaces and beneath (>5 cm) incipiently vegetated substrates (Figure 2D). In contrast, EC above the salt line is <1 mS cm⁻¹ and pH is typically <6.5 (Figure 2D). A generalised EC and pH transect up the terrace scarp at the northeast end of the site (location in Figure 3A) demonstrates these chemical contrasts to within ±10 cm of the salt line, highlighting a clear separation of substrate types (Figure 4A,B). There is a clear contrast in shallow substrates below and above the salt line at this point, from clay-rich sediment below the line to silty and even locally sandy sediment above the line (Figure 4A,B). Similar EC and pH contrasts (Figure 4C) have been mapped in a transect up-slope of the main wetland (Figure 2C; Figure 3A). Contrasts in EC and pH are less well defined, but still recognisable, in a transect up-slope at the southern end of the site, especially in subsurface samples (∼10 cm depth; Figure 4D) rather than proto-soil surface samples (Figure 4E).

Figure 4. Chemical and physical definition of the salt line (EC ∼1 mS cm⁻¹) near the base of the terrace scarp on a largely soil-free schist hillside at Patearoa saline site (Figure 3A). Typical regional soil pH range (∼5 to 7) is shown for comparison. A, EC and pH transects (logarithmic scale) up-hill across the salt line, measured at ∼10 cm depth. B, Closely-spaced samples across salt line in a. C, Transect up-hill from the main wetland across the salt line towards the schist terrace (Figure 3A), for near-surface samples (∼5 cm). D, Transect up-hill across the salt line towards the schist terrace gravels (Figure 3A), for surface samples. E, Same transect as in d, with samples from ∼10 cm depth, showing generally higher EC and pH below the salt line.

Patearoa salt pan characterisation

Salt pans are soil-free areas below the salt line, with exposed substrates consisting of impermeable clay-rich sediments derived from, and overlying, clay-altered schist (Figure 2D; Figure 5A). The extant pans are areas of active erosion during rain and wind events, and this erosion periodically redistributes the fine sediments in a dynamic surficial environment (Figure 5A). The pans are gently sloping, with visible downslope drainage features (Figure 5A). The sedimentary surface of the active pans is dominated by detrital mica flakes (50 µm scale) and clay minerals, mostly kaolinite (<10 µm scale), with the clay forming a binding matrix that limits permeability (Figure 5B). Rain water with marine aerosols and shallow groundwater that percolates slowly through the sediments contribute dissolved ions to the surface, where evaporation causes precipitation of salt minerals (Figure 2D; Figure 5A). The most common salt minerals are halite (NaCl), sylvite (KCl) and Na-carbonates, with minor Na-sulphates (Craw et al. 2022b).

Figure 5. Variably vegetated clay-rich saline substrates from below the salt line compared to fully vegetated grassy turf and/with soil above the salt line. EC in mS cm⁻¹. A, Oblique drone view of salt pans (white; clay-rich sediment, erosion = blue arrows) with endemic halophytes on margins, grading into surrounding ex-pans and pasture grasses in proto-soil. B, Scanning electron microscope image of clay-rich sediment from surface of a salt pan. Coarser particles are mica flakes, with background clay. C, Section through an ex-pan with exotic vegetation and proto-soil formation on clay-rich sediment. D, Section through an old salt pan with current wetland grasses (as in Figure 1D; Figure 7E), showing thin cap of proto-soil on clay-rich sediment that was historically saline (Figure 1E). E, Soft silt and clay sediment with a developed soil layer beneath a patch of well-established pasture grasses in a gully above the salt line.

The surface substrates of salt pans at Patearoa show a wide range of EC and pH values, similar to other salt pans in inland Otago (Figure 6A). There is a general trend in Patearoa data from near-neutral pH and EC near to 1 mS cm⁻¹ towards high pH (∼10) and high EC (>10 mS cm⁻¹) in the most salt-encrusted substrates (Figure 5A; Figure 6A). Elevated EC and pH persist below the surface in firm clay-rich sediment and very hard clay-rich basal material occurs at ∼20 to 40 cm depth (Figure 7A,B). This hard basal material is either well-consolidated fine grained clay-rich sediment or clay-altered schist basement with dispersed coarser (mm-cm) quartz-rich debris.

Figure 6. Plots of pH versus EC for various substrates at Patearoa site, with comparisons to typical regional data. Note logarithmic scales in A,B,E. A, Salt pan surface data at Patearoa compared to similar salt pans farther inland (mostly near Alexandra; Figure 1A; Craw et al. 2023a). B, Comparison of Patearoa salt pan surfaces with adjacent ex-pans with proto-soil at surface and at ∼10 cm depth. Coastal bare ground trend is from Rufaut and Craw (2023). C, Ex-pan data as in b, with expanded linear scales. D, Comparison of pasture grass substrates from above and below the salt line. Higher EC and pH data from below the salt line are from >5 cm depth. Wetland substrate data are similar to grass substrates below salt line. E, Comparison of Patearoa salt pan data with schist debris and schist gravel substrates from the top of the Patearoa terrace. Data also shown for surface water pools on the lower flat after a major rain event. F, Patearoa terrace data (as in E) on expanded scale, comparing schist debris, schist gravel, and silt-rich grass soil.

Figure 7. Variations of EC and pH with depth in some Patearoa substrates. A,B, Depth profiles through an active salt pan and immediately adjacent ex-pan (10 cm above pan surface) with incipient exotic vegetation cover. C,D, Depth profiles through ex-pan areas with protosoil (Figure 5C) and grass cover, respectively. E, Depth profile through a wetland floor (Figure 5D). F, Depth profile through substrate beneath well-established grass soil in a gully above the salt line (Figure 5E).

Patearoa ex-pan evolution

Ex-pans also occur below the salt line and represent extensive areas that were once saline bare ground (Figure 5A; Supplementary Figures S1, S2). The ex-pan areas are visible at larger scale as the numerous dark areas below the salt line in Figure 3A (also Supplementary Figure S3). The ex-pans exhibit similar underlying sediment profiles to active pans (Figure 5C; Figure 7A–C). Ex-pans have laterally widely ranging surface EC and pH that are distinctively lower (pH ∼8; EC <1) than adjacent pans (Figure 5A,C; Figure 6B,C). Mats of the salt-tolerant exotic weed species Plantago coronopus dominate ex-pans, with roots that extend up to 10 cm below surface into clay-rich substrates with EC of 1–2 mS cm⁻¹ and pH of 8–10 (Figure 5C; Figure 6B,C; Figure 7A–C). Initially, proto-soil is merely surficial accumulation of dark organic matter from decomposing P. coronopus leaves (Figure 5A; Supplementary Figure S2). The abundance of P. coronopus and an associated veneer of decaying foliage results in profound transformation of the substrate surfaces with formation of the associated proto-soil layer that has less extreme chemistry than the subsurface and comparative active pans (Figure 5A,C; Figure 6B,C; Figure 7A–C). Part of this surface transformation is also physical, as the roots of invading plants prevent the periodic erosion that maintains the dynamic surfaces of the active salt pans (Figure 5A).

Further lowering of EC and pH on ex-pan surfaces occurs where taller grasses become established, initially in conjunction with P. coronopus (Figure 5A). Hence, ex-pans dominated by P. coronopus grade laterally to full grass cover with associated soil development (Figure 3A,B). The EC and pH of the surficial zone in grassed ex-pans are typical of grassland soils, but underlying clay-rich substrates remain variably saline with elevated pH (Figure 4A–E; Figure 6D; Figure 7D).

Patearoa wetlands and surface water

The lower flat has several shallow depressions that are remnants of channels left by the Taieri River when that course was abandoned (Figure 2A) and these are now ephemeral wetlands (Figure 3A,B) that are saturated, locally with surface water pools after major rain events. Historically, some of these wetlands developed prominent salt pans on bare substrates on their floors (e.g. Figure 1E), but the wetlands are now fully vegetated with exotic grasses and merge with adjacent grassed ex-pans (e.g. Figure 1D; Figure 3A,B). Surficial EC and pH are generally low compared to the adjacent pans and ex-pans (Figure 6D). Surficial organic-rich proto-soils in most of the wetland areas have lower EC (<0.5 mS cm⁻¹) and pH <8 (Figure 5D; Figure 6D; Figure 7E; Figure 8A). Ephemeral (over hours-days) pools of water that accumulated in surface depressions on the lower flat after a major rain event in April 2023 (>20 mm in 24 h at Ranfurly) had a wide range of EC and pH values that reflected the variations across the whole site (Figure 6E). The highest pH and EC values in these waters occurred either on salt pan surfaces or in ephemeral wetlands immediately (<10 m) downslope.

Figure 8. Surface and shallow groundwater hydrogeology of the two principal wetlands at the Patearoa saline site. A, Drone-derived contour map of part of the lower flat, showing surface drainage directions into the two main wetlands. Contours (orange) have 0.5 m spacing above an arbitrary datum (∼350 m above sea level). Contour positions are affected by tall grasses, indicated with irregular lines, whereas pans and ex-pans have smoother contours. B, Vertical drone map of the base of the terrace scarp at north end of the site (as in A), showing variations of substrates and vegetation adjacent to the saline wetland and across the salt line above. Shallow groundwater seeps facilitate luxuriant grass growth immediately above the salt line, and establishment of mats of native halophyte Selliera radicans (pale green) below the salt line. C, Photograph of the salt line at area in B, immediately (∼1 m) above the S. radicans patch. Transect uphill from the saline wetland in Figure 4A crosses the salt line at this point, indicated on ground with a blue marker to right of spade.

The only wetland area that retains saline surficial substrates is at the northern edge of the site (Figure 8A,B). The floor of this wetland is normally dry, and has surface EC of 1–2 mS cm⁻¹ and pH ∼9 (Figure 4A; Figure 8A). Some water pools accumulate along the roadside at the downstream end of this wetland and outside the site, and this water has lower EC and pH than the wetland (Figure 6D). The saline wetland occurs at the lowest point of the lower flat, and receives any surface runoff and shallow groundwaters that have flowed past large areas of pans and ex-pans on the lower flat (Figure 8A), although this area has a low gradient. In addition, this wetland is at the foot of a steep slope on its eastern side, and receives any surface runoff and seeps of shallow groundwater from sloping saline pans immediately below the salt line (Figure 8A–C). The only patch of the native halophyte Selliera radicans at the Patearoa site (Figure 3A,B) forms a mat and proto-soil layer on saline substrates immediately above the saline wetland (Figure 8A,B). This halophyte occurs commonly in Otago coastal areas that have higher rainfall than Patearoa (Partridge and Wilson 1987; Rogers and Wiser 2010; Rufaut and Craw 2023), but is rare or absent from dry inland Otago sites. The occurrence at Patearoa may be indicating the location of additional, and saline, shallow groundwater seepage immediately above the saline wetland (Figure 8A–C).

All wetland substrates below their surficial proto-soils consist of the same clay-rich substrates as the adjacent pans and ex-pans (Figure 5D). These substrates show progressively higher EC and pH with depth, and can exceed 1 mS cm⁻¹ (e.g. Figure 7E). The depth profile in Figure 7E was dug in the base of the most prominent non-saline wetland (Figure 1D; Figure 8A) and the excavation reached down to damp, but not wet, substrate near to the water table.

Patearoa schist outcrops and schist debris

Weakly clay-altered schist outcrops are crumbly and form hard but disintegrating surfaces on the terrace scarp and locally on the upper terrace top (Figure 9A,B). Outcrops are surrounded by debris derived by decomposition of these outcrops, and this debris forms highly permeable substrates that consist of cm-scale angular fragments of schist in a matrix of finer grained debris (Figure 8B). The schist is pervasively foliated and segregated on the mm to cm scale into quartz-albite and micaceous (muscovite + chlorite) laminae (Figure 9B). Most of the coarser clasts in the debris (cm scale) are dominated by quartz and albite, with the micaceous debris decomposing into smaller flakes (typically <0.5 mm). Clay alteration products, from albite, muscovite and chlorite, are even finer and locally form a loose cementing matrix to the debris. Rain and wind erosional action removes surficial finer grained debris, leaving an armoured surface of the coarser fragments (Figure 9B). These deposits form irregular areas of bare substrates and associated gravelly substrates with thin (cm scale) proto-soil (Figure 9A–D). The schist debris substrates are typically 5–20 cm thick, although downslope creep causes locally thicker surficial horizons. Similarly, rain and wind redeposition of finer sediments downslope forms thicker accumulations of sandy silt, especially in gullies in the terrace scarp, and on the lower slopes of the terrace scarp near to the salt line (Figure 3A,B; Figure 4B; Figure 9A).

Figure 9. Photographs of the principal substrates with little or no soil above the salt line at the Patearoa site. A, Oblique drone view of terrace scarp on a schist spur, with outcrops of weakly clay-altered schist and debris derived from decomposition of those outcrops. Intensity of clay alteration increases on both sides of the spur, controlling the topography. The salt line occurs at the base of this hill, with grass immediately above that line indicating localised shallow groundwater seepage. B, Schist outcrop with native broom (Carmichaelia sp) growing in fractures (bottom centre), and bare soil-free schist debris (top). Darker parts of outcrop are micaceous foliation surfaces; pale brown portions are quartz-albite veins. C, Schist debris with active erosion of proto-soil, and a specimen of endemic Lepidium solandri. D, Schist-derived river gravel forming top of terrace, with gravel proto-soil (cm scale) predominantly hosting invasive exotic plants and scattered native tussock.

Bare schist debris substrates and these substrates with thin proto-soil host some scattered individuals and clusters of native broom and tussocks (Figure 9A–C; Supplementary Table S1). These are locally accompanied by the rare endemic herb Lepidium solandri that also grows in bare schist debris substrates with thin proto-soil (Figure 9C). However, most of the schist debris substrates with proto-soil are sparsely vegetated with combinations of invasive exotic species (Supplementary Table S1). Thicker sandy silt substrates have developed downslope on the terrace scarp (Figure 4B; Figure 5E; Figure 9A) and these now host exotic pasture grasses, locally made more luxuriant by shallow groundwater seeps (Figure 1D; Figure 8C).

All the above-described substrates consisting of schist debris above the salt line have low EC and pH typical of regional upland soils developed on Otago Schist (Figure 5E; Figure 6E,F; Figure 7F; Hewitt 1982; Weightman et al. 2020; Rufaut et al. 2022). The Patearoa EC above the salt line is typically ∼0.1 mS cm⁻¹, and pH ranges from ∼7 down to as low as 4.9 (Figure 6E,F). Most of these schist debris substrates are too hard, with coarse debris, to dig deeper than 5–10 cm, and these near-surface proto-soils have the same general EC ∼0.1 mS cm⁻¹, and pH ∼ 5.5 (Figure 6F). Sandy silt accumulations with grass and turf formation are sufficiently soft to permit excavation, and these show similar EC at surface and at depth in the same range as the surficial proto-soils (Figure 4D,E; Figure 6F; Figure 7F). These grassy silt substrates immediately above the salt line show up the very strong distinction in EC and pH across that salt line (Figure 4A–E).

Salt lines in other areas of Central Otago

The Patearoa salt line that separates contrasting bare substrates described above is the best-defined such feature in the Maniototo basin. Other saline sites in the areas are on the Taieri River flood plain where saline substrates are susceptible to short-term surface and groundwater fluctuations (Craw et al. 2022b) and any ‘salt line’ is temporary. To extend the salt line concept to a wider area, we have gathered extra information on inland saline sites and integrated that with previous data to delineate geologically controlled salt lines that are geologically different from the Patearoa one, but result in similar geoecological contrasts.

Saline sites on clay-altered schist in the Alexandra area (Figure 1A) have more pronounced underlying geological controls on their surficial expressions (Craw et al. 2022a), and therefore salt lines can be defined more readily. For example, the Chapman Road saline site is partially controlled by a fault that has juxtaposed fractured and permeable schist against intensely clay-altered schist (Figure 10A,B). The surface fault trace is a geoecological salt line that separates substrates with low EC and pH, hosting exotic vegetation, from saline clay substrates that host halophytes (Figure 10A,B; Supplementary Figure S4). Like the Patearoa site, below the salt line at the Chapman Road site the saline substrates persist below surface even beneath exotic weed incursions (Supplementary Figure S4).

Figure 10. Generalised geoecological salt lines at Central Otago saline sites with their geological controls. A, Chapman Road site (Figure 1A), with a salt line formed by a late Cenozoic fault that has uplifted permeable schist (background) against clay-altered schist (white) that has become saline and hosts halophytes (as at Patearoa). Schist clay surface exposure is a result of historic gold mining (Craw et al. 2022a, 2022b, 2023b). B, Sketch cross section along X-Y in a. Colour gradient purple-orange-grey represents increasing schist oxidation and clay alteration. C, Overview of the Mahaka Katia Reserve (Figure 1A) on Pleistocene glacial outwash terraces made of schist-derived gravel (Craw et al. 2023a), with a salt line at the foot of the upper terrace scarp. D, Close view of an active salt pan on loess on the middle terrace at Mahaka Katia. E, Close view of bare gravel with proto-soil above salt line on upper terrace at Mahaka Katia.

The Mahaka Katia reserve northwest of Alexandra (Figure 1A; Craw et al. 2023b) is a natural river terrace saline site that is geometrically similar to Patearoa, but is underlain by glacial outwash gravels (Figure 10C–E). At Mahaka Katia, a terrace scarp has an approximate equivalent of a ‘salt line’ at the base, with rare endemic dryland species on bare non-saline substrates at the top of the terrace, and halophyte-bearing salt pans on a flat below the ‘salt line’ (Figure 10C–E). However, the salt pans are developed only locally on wind-blown and water-washed silt where an impermeable soil-free crust has formed. Immediately adjacent similar silt without a crust remains non-saline at surface and at depth (Supplementary Figure S4). Hence, while there is similar geoecological separation of dryland plant communities to that at Patearoa, large areas below the ‘salt line’ at Mahaka Katia are non-saline both at the surface and below the surface. Some now-vegetated ex-pans are also evident at this site.

Historic placer gold mining at Springvale (Figure 1A) has partially removed Pleistocene gravel cover from underlying Miocene mudstone, and impermeable mudstone surfaces have become saline (Figure 11A,B; Supplementary Figure S4; Law et al. 2016). Halophytes have colonised saline surfaces of pans eroded from mudstone outcrops, and some mudstone outcrops where biofilm and lichen have stabilised surfaces to form proto-soil (Figure 11A,B). A geoecological salt line can be mapped at the boundary between gravel remnants with exotic vegetation (above) and mudstone with halophytes (below; Figure 11A,B). Halophyte-hosting proto-soil on mudstone outcrops has relatively low surficial EC and pH, but values are substantially higher at 5–10 cm depth (Figure 11A,B). Erosional transport of debris from the gravel cover on to downslope mudstone also forms veneers with low EC and pH and exotic vegetation (Figure 11A,B), but the mudstone beneath these veneers remains generally saline with pH near 8.

Figure 11. Variations of substrates that form a geoecological salt line at part of the Springvale saline site (Figure 1A), Central Otago, with details of habitats of rare endemic halophyte, Lepdium kirkii. A, Geoecological map of substrate distribution, with EC and pH measured on the surface veneer (proto-soil) and at 5–10 cm depth. B, Oblique drone view of the upper portion of the map (dashed box at right in a), showing the erosional relief in mudstone outcrops. Ridges and gullies were left by historic mining activity, and the gullies have become enhanced by subsequent erosion.

Discussion

A regional salt line

In a more regional context than salt lines outlined above, there is a large-scale dryland ‘salt line’ between inland Otago where saline substrates are widespread, and inland Canterbury (e.g. Mackenzie basin; Figure 1A) where saline substrates are absent. Both these inland areas have similar climates in the rain shadow of the Southern Alps (Figure 1A). The key difference is defined by the underlying geology: Otago Schist basement and schist-derived sediments in inland Otago, and greywacke basement and greywacke-derived sediments in Mackenzie basin. The latter areas host numerous dryland bare substrates or substrates with proto-soil, including abundant loess and rare areas of clay-altered greywacke (Figure 12A,B; Supplementary Figure S5). However, these substrates do not become saline because they are permeable to rain percolation and do not develop an impermeable surface crusts. EC is typically <0.01 mS cm⁻¹ and pH is commonly <6 on these substrates (Figure 12A,B; Supplementary Figure S5). The permeability of these substrates is maintained by abundant sand and silt particles that have been liberated from the basement greywacke. In addition, there is insufficient clay to bind these particles together (Figure 12A,B; Supplementary Figure S5). Even the most intensely clay-altered greywacke basement is pervaded by sand-sized quartz-rich particles, and EC and pH remains low on outcrops and sediments derived from them (Supplementary Figure S5). Further, micas in greywacke-derived substrates are typically much smaller (micron scale; Turnbull et al. 2001) than those in schist-derived substrates (tens of microns; e.g. Figure 5B). The fine greywacke mica flakes are not individually liberated during erosion, preventing development of a micaceous desiccation crust of the type found on schist substrates (Figure 5B).

Figure 12. Regional salt line features in the Mackenzie basin area (Figure 1A). A, Pleistocene greywacke-derived glacial outwash sediment in Mackenzie basin, with wind-blown desiccated crusted surface. B, Similar material to E, without a crust in wind-eroded outcrop. C, View looking northwest of a road-cut in Otematata area at southern end of MacKenzie basin (Figure 1A). A salt line is defined by a fault between greywacke (right) and saline substrates on clay-altered low-grade schist (TZ 2; left). D, Bare saline substrate (as in C) with desiccation cracks and abundant gypsum-rich evaporative precipitates (white) immediately below surface (here exposed by shallow excavation). E, Natural erosion scar near Otematata with saline bare ground on clay-altered low grade schist, showing localised gypsum-rich evaporative precipitates (white; mostly under overhangs). F, Close view of a sample of gypsum-rich evaporative precipitates, from site in E.

The regional salt line occurs where greywacke-derived substrates abut schist-derived substrates, and this is best exposed in the Otematata area at the southern end of the Mackenzie basin (Figure 1A). Greywacke basement is faulted against low-grade Otago Schist of Textural Zone (TZ) 2 of Turnbull et al. (2001) and Forsyth (2001) in this area (Figure 12C). Clay alteration of this low grade schist yields surficial substrates that are sufficiently impermeable to develop saline crusts (Figure 12C–F). Bare ground in natural erosional scars locally hosts some salt-tolerant plants including the halophytic herb Atriplex buchananii, which also occurs in inland Otago sites including Patearoa (Supplementary Figure S2; Peat and Patrick 2001). Apparently, even the incipient metamorphic recrystallisation and foliation development in TZ 2 schists (Turnbull et al. 2001) has been sufficient to yield micas coarse enough to contribute to formation of desiccation crusts. Evaporative salt encrustations are dominated by gypsum (Figure 12D–F) but halite, other sulphates, and carbonates have been inferred to form as well on these substrates (Craw et al. 2022c).

The contrasts between schist-derived and greywacke-derived substrate formation, as outlined above, are summarised in Figure 13A–D to illustrate the key differences across the regional salt line in Figure 1A, as well as the physical processes that contribute to salt lines across inland Otago. Most of the saline sites in inland Otago, including the Patearoa site, are on TZ 4 schist (Figure 9B; Figure 13A), which is highly segregated and has coarse metamorphic micas (typically >100 µm; Turnbull et al. 2001). This schist decomposes during clay alteration and erosion to yield coarse quartz-rich debris and fine clay-rich micaceous sediment (Figure 13A–C). At the scale of an individual site such as Patearoa, this physical separation of schist components leads to development of the geological structure that underpins a salt line (Figure 2D; Figure 13A–C). These physical separation processes do not occur during greywacke alteration and erosion because the greywacke rock texture is dominated by original clastic quartz and feldspar particles, with a matrix of micron-scale micaceous material (Figure 13D). Substrates derived from this greywacke remain dominated by the residual sand component (Figure 12A,B; Figure 13D) which has relatively high permeability. Accumulation of fine clay-rich sediment with a micaceous clay crust from schist facilitates evaporation-dominated evolution of the saline substrates (Figure 13E), but this does not occur for greywacke-derived substrates (Figure 12A,B; Figure 13D).

Figure 13. Summary flow diagram showing contrasting physical controls on salt line development (in A–D) and chemical processes below the salt line in E. A, Decomposing schist outcrop yields quartz-rich coarse debris and clay + mica sediments, which are separated during sedimentary mobilisation. B, Coarse quartz debris forms a permeable substrate. C, SEM view of mica accumulations, cemented with clay, that forms impermeable saline surface crust of a salt pan. D, SEM image of a sample of typical greywacke (Torlesse terrane; after Large et al. 2012) showing sedimentary texture that yields nonsaline sandy permeable substrates on decomposition. E, Sketch of a typical salt pan, such as at Patearoa saline site, showing lateral variations in surface chemistry and mineralogy that result from vegetation incursion and rainwater runoff. Underlying substrate remains chemically broadly consistent. NaCl ratios are molar.

No outcrops of non-marine Cenozoic mudstone, similar to the mudstone at Springvale (Figure 11A,B) are known in the Mackenzie basin, so comparison of these substrates across this basement-defined salt line is not possible. Further, the lack of gold-mining exposure of unweathered impermeable surfaces (as occur in Otago; Craw et al. 2023a) in the Mackenzie basin inhibits comparisons of such features across the regional salt line. Limited natural exposures of Cenozoic calcareous marine mudstones near to, and to the southeast of, Otematata (Figure 1A; Forsyth 2001) are highly fractured and friable, and therefore poor sites for accumulation of evaporative salts.

Mineralogical variations on salt pans

Salt encrustations on inland Otago pans, including at Patearoa, are dominated by halite (Craw et al. 2022a, 2022b). Halite encrustations typically have pH of ∼7 or lower, and are responsible for the highest EC values, locally >50 mS cm⁻¹ (Babel and Schreiber 2014; Craw et al. 2022c, 2023b). The Patearoa salt pans, like most other inland Otago pans, have highly variable pH and EC on the metre scale. Patearoa pans show a broadly consistent trend from low EC and pH to high EC and pH (Figure 6A,B). This trend is opposite to that observed on halite-dominated bare coastal Otago substrates, where highest EC values coincide with lowest pH (Figure 6B; Rufaut and Craw 2023). Hence, the observed variations in pH and EC on Patearoa pans are not just a result of variable halite distribution, although halite redistribution almost certainly does occur during rain events (Figure 6E; Figure 13E; Craw et al. 2023a, 2023b). Instead, the high pH of evaporative encrustations at Patearoa, along with other high-pH sites in inland Otago, is a result of the presence of Na-carbonate minerals (Craw et al. 2022b; 2023a, 2023b). The Na-carbonate minerals contribute relatively little to the EC, but have a strong effect on pH (Figure 14A). The relative proportions of halite and Na-carbonates in these encrustations are not known in detail, but halite is observed in SEM samples more commonly than Na-carbonate (Craw et al. 2022b). However, irrespective of relative abundances, it is the Na-carbonate that controls the higher pH values that locally verge on hyperalkaline, and halite controls the higher salinity values (Figure 14A).

Figure 14. Geoecological features of substrate mineralogy and chemistry below the salt line. A, Depiction of variations in surface and subsurface pH and EC as a result of different proportions of the three principal pH-governing minerals. Generalised ranges for clay-altered schist substrates (white; e.g. Patearoa) are compared to the two other principal saline substrates. B, Phosphorus and sodium concentrations in background Otago Schist basement rocks typical of the Patearoa area. Apparent high Na in weakly altered rocks reflects sampling bias for more solid quartz-albite-rich specimens (e.g. Figure 8B). Clay alteration leads to lowering of apatite contents, lowering P, and alteration of albite to kaolinite with mobilisation of Na and raising pH. C, Geochemical model (from Geochemists Workbench®) showing the effects of high pH on substrate chemstry for low dissolved Ca and Mg levels in Patearoa substrate leach water. D, SEM image of Patearoa evaporative Mg-phosphate (Craw et al. 2022b). E, Endemic halophyte P. raroflorens at Patearoa site with red-purple colouration that may reflect nutrient stress such as P deficiency and/or Al toxicity.

Despite the wide variability of EC and pH on the Patearoa pan surfaces, the underlying substrates are much less variable, allowing for the surficial changes associated with proto-soil development (Figures 5 and 7). Similar consistency of EC and pH at depth is observed in substrates below salt lines elsewhere in inland Otago (Supplementary Figure S4). High pH in deeper substrates results from alteration of schist albite by shallow groundwater moving through the schist basement and schist-derived sediments (Figure 2D; Craw et al. 2022c; 2023b). Albite alteration can lead to pH > 10 in the substrates through which the shallow groundwater is moving (Mamonov et al. 2020; Craw et al. 2023b), such as the clay-altered schist and its eroded debris on the lower flat at Patearoa (Figure 2D). This albite alteration leaches Na from the schist (Figure 14B) to give Na/Cl molar ratios >1, and contributes to the high sodicity of the sites, with common Na-sulphates as well (Figure 13E; Figure 14A; Craw et al. 2023a, 2023b). The ambient high pH in underlying substrates controls formation of Na-carbonate minerals in the surface encrustations (Figure 13E).

The sodic and locally hyperalkaline pans are distinctly different from most dryland schist-hosted substrates where Ca and Mg rich solutions prevail (Weightman et al. 2020; Rufaut et al. 2022; Craw et al. 2022b,c). In those settings, Ca-carbonate and Mg-carbonate minerals predominate in the evaporative encrustations (Weightman et al. 2020; Craw et al. 2022b). Carbonate mineral equilibria control pH near to 8, but have little effect on EC, which is mainly affected by halite dissolution (Figure 14A; Weightman et al. 2020; Craw et al. 2022b, 2023a, 2023b). This carbonate-dominated chemistry affects areas above the salt lines, at the Patearoa site and other such sites with salt lines, wherever relatively unaltered carbonate-bearing schist and schist-derived substrates occur (Figure 9A–D; Figure 10A–E). This also includes the Miocene mudstone and overlying gravels at Springvale (Figure 11A,B), where calcite is abundant and maintains pH ∼8 in most substrates (Supplementary Figure S4; Law et al. 2016).

Geoecological effects of high pH

Clay alteration of schist results in kaolinite-rich residues that are depleted in Na and other components (Figure 14B; Chamberlain et al. 1999). From a geoecological perspective, leaching of the essential nutrient phosphorus from the substrates is likely to limit plant colonisation and is probably partially responsible for persistence of bare ground on clay-altered schist substrates such as at Patearoa, Chapman Road, and Otematata area, along with the elevated EC (Figure 3A,B; Figure 4A; Figure 10A; Figure 12C–E; Mains et al. 2006; Craw and Rufaut 2017). High pH may exacerbate this issue by causing precipitation of phosphate minerals in the substrates, thereby sequestering the little remaining P and making it unavailable for plants (Figure 14C). For example, apatite precipitation is predicted to occur at progressively lower levels of dissolved P as pH increases (Figure 14C). The resultant theoretical apatite stability field overlaps experimentally-determined Patearoa saline substrate water composition (Figure 14C; Craw et al. 2022b). However, this model is for ideal apatite, and while more complex natural phosphate minerals may deviate from this, the general trend for decreasing solubility with increasing pH probably still applies.. Surficial evaporation makes such precipitation even more likely to occur (Figure 14C). Likewise, Mg-phosphate precipitates have similar geochemical stability to that of apatite (Figure 14C) and Mg-phosphate crystals have been identified in Patearoa salt encrustations (Figure 14D; Craw et al. 2022b). Hence P-deficiency through precipitation and/or non-dissolution of phosphate minerals may be an important factor in maintaining alkaline bare saline pans.

In addition, substrate toxicity to plants may result from high pH at some saline sites such as Patearoa. High pH facilitates dissolution of aluminium (as Al[OH]₄^- ions) from kaolinite clay (Figure 14C) and also from albite (Mamonov et al. 2020). Al-hydroxide precipitates have been observed in high-pH (>10) evaporative precipitates at Mahaka Katia, attesting to this type of Al mobilisation in saline substrates (Craw et al. 2023b). High dissolved Al can lead to Al-toxicity, resulting in damaging of plant rootlets and therefore limitation of nutrient uptake (Brautigan et al. 2012; Lauchli and Grattan 2012). It is notable that the native halophyte Puccinellia raroflorens can show red-purple colouration on some alkaline salt pans (Figure 14D). This colouration is characteristic of moisture and nutrient deficiency, as coloured anthocyanin is produced in the plant as a result of the stress (Osborne et al. 2002; Ticconi and Abel 2004). Such stress may result from e.g. low P uptake as outlined above. The salt-tolerant exotic weed Plantago coronopus, which dominates ex-pans at Patearoa, can also show signs of this reddening effect (Supplementary Figure S2) and may be similarly stressed in high-pH substrates. Nevertheless, P. coronopus survives sufficiently robustly to contribute organic debris that changes the EC and pH of proto-soil on ex-pans, facilitating incursion of other more chemically intolerant plants (Figure 5A,C; Figure 7C,D; Supplementary Figure S2).

Conclusions

This study has shown that there is a clear distinction at Patearoa, based on the underlying geological materials and hydrogeological processes, between non-saline and saline substrates with bare ground and proto-soil, and differing plant communities reflect this contrast. The saline substrates on the lower levels of the site are separated from non-saline upper levels by a well-defined geoecological salt line that is mappable in the field at less than the metre scale across the whole site (Figure 3A,B). Saline bare substrates form salt pans that have high EC (>1 mS cm⁻¹) and high pH (up to ∼10), whereas bare substrates above the salt line have low EC (typically <0.2 mS cm⁻¹) and lower pH (<6.5).

The salt-tolerant exotic species Plantago coronopus stabilises surfaces below the salt line and its leaf litter causes lowering of the EC and pH in the uppermost centimetre. This facilitates establishment of other, salt-intolerant, exotic species. Likewise, long-term (decades) environmental changes, possibly induced by agricultural irrigation, have resulted in infilling of wetlands with exotic vegetation and low EC and pH proto-soil. Nevertheless, substrates below the evolving surficial proto-soils remain saline and alkaline, and could be re-exposed by removal of the proto-soils. A pilot study to investigate the potential for soil removal and enhancement of halophyte habitat has been conducted at Patearoa with some early success (Supplementary Figures S3, S6).

The concept of a geoecological salt line, so well displayed at Patearoa, can be extended to other saline sites in inland Otago although the exact geological controls are different at each site. Salt lines arise because of contrasting substrate permeabilities (Figure 13A–C). This can be controlled by juxtaposing rocks with different degrees of clay alteration, as at Chapman Road (Figure 10A,B) or different sedimentary layers such as loess on gravel at Mahaka Katia (Figure 10C–D) or gravel on mudstone at Springvale (Figure 11A,B). All salt lines can be partially obscured by development of veneers of other substrates, but the fundamental contrasts persist in the underlying geology and can be exposed by natural or anthropogenic erosion.

A more general regional scale salt line appears to exist between inland Otago and inland Canterbury because of contrasting basement rocks in these areas with similar semi-arid climate. The salt line coincides with the boundary between greywacke and low grade schist, and no saline sites occur on greywacke-derived substrates because of their relatively high permeability. The geoecological significance of this regional salt line has not been investigated but for an example in the genus Lepidium, the rare endemic dryland species L. solandri that occurs above, but not below, the salt line on schist at Patearoa (Figure 9C) also occurs on greywacke-derived substrates in the Mackenzie basin. In contrast, the rare endemic species L. kirkii only occurs on saline sites in Central Otago (Figure 11A,B).

Acknowledgements

This project was initiated by personnel from Queen Elizabeth II National Trust, and we are grateful for the on-going enthusiasm and input from Aalbert Rebergen, and Rob and Kate Wardle. Discussions with Scott Jarvie (Otago Regional Council) were helpful in developing this research project. We appreciate the ready access to the site granted by landowner Charlie Hore. Gemma Kerr (University of Otago) ably helped with laboratory work and electron microscopy at the Otago Micro and Nanoscale Imaging (OMNI) facility, University of Otago.

Disclosure statement

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

Data availability statement

All data directly relevant to this study are included within this paper or within cited references. Supplementary figures mentioned in the text, and a table of plant species above and below the Patearoa salt line is available at: https://doi.org/10.6084/m9.figshare.23846472.v1.

Additional information

Funding

This research was funded by Queen Elizabeth II National Trust and University of Otago.