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Chemically distinctive features such as high salinity, low nutrient contents, and extremes of pH can limit the development of a vegetation cover on soil-free bare ground. This study synthesises our previous work on the geoecology of numerous chemically distinctive bare sites in the South Island and adds a temporal component within the last century. The rates of development of surficial cover on decadal time scales have been traced from available historical aerial photography of varying but adequate quality. Rates are highly variable but range between zero and 2 metres per year laterally, with some sites showing localised expansion of bare ground at times. Cover evolution involves the development of a cm-scale proto-soil layer that is at least partially biologically mediated. Geochemical thresholds in surficial substrates control the biological components of encroaching cover. Dynamic surface environments, especially steep and actively eroding slopes, help to maintain bare ground and resist cover development. Some chemically distinctive bare ground, and evolving adjacent proto-soil areas, host specialist tolerant native plant communities and shrinkage of these areas threaten this biodiversity. In contrast, some bare ground sites have elevated arsenic and/or highly acidic pH, and the development of proto-soil and vegetation cover is environmentally positive from a human perspective. [ABSTRACT FROM AUTHOR]

Long-distance dispersal (LDD) is thought to be a key driver of biogeographic processes, yet few direct natural observations have been made of this process. Several studies have characterised diverse benthic epibiotic communities associated with buoyant macroalgae and have proposed that kelp rafting may be an important LDD mechanism for such coastal species. We test for LDD by combining biological, genetic, and geological data from rafted bull-kelp (Durvillaea antarctica) specimens collected in southern New Zealand following a March 2018 storm. Genetic and ecological data strongly indicate that three of 29 detached kelp specimens sequenced (and their associated live epifaunal taxa) had rafted from the sub-Antarctic to mainland New Zealand, traversing both oceanographic and phylogeographic barriers, over the course of an approximately 4-week journey. Numerous additional epifaunal taxa were detected from rafts that had geologically-distant mainland origins. The successful trans-oceanic rafting documented for sub-Antarctic brooding sea-star, chiton and sea-slug taxa presents a mechanism for their broad but phylogeographically disjunct Southern Hemisphere distributions. Moreover, the detection of several such LDD events over the last decade suggests that such journeys are very common over evolutionary timeframes. Although geological and genetic data were informative over different scales, we detected no conflict between the inferences from these distinct data sets, a finding that reinforces the value of integrative approaches to marine biogeography.

The Rakaia River in the South Island of New Zealand is a unique catchment in which to trace active detrital gold transport, deformation and concentration from near-coeval orogenic sources. The catchment hosts widespread small late Cenozoic hydrothermal alteration zones, some of which are gold-bearing, in actively rising and eroding mountain headwaters. Detrital gold liberated from the sources is distinctively porous with abundant micron-scale silicate inclusions, especially albite. Fine gold (∼200–100 µm) has been transported >100 km downstream, but coarse gold remains highly diluted in proximal fluvial sediments. Fine gold is also diluted by coarse fluvial sediments that form the upper kilometre of the Canterbury sedimentary basin southeast of the mountain front. Minor re-concentration of fine gold occurs in a bedrock gorge at the mountain front, and on steep ocean beaches beyond the river mouth. Despite the long-distance transport, the fine gold particles have been only superficially modified by surface smearing and minor flattening. The gold may have travelled in suspension in the river, buoyed by electrostatically adhering fine micas, in contrast to detrital garnets that were rounded in bed-load. In contrast, detrital gold on the western side of the mountains has been flattened to thin flakes during bed-load transport. [ABSTRACT FROM AUTHOR]

Aeolian concentration of iron-titanium oxides (black sand) is a common phenomenon on coasts around the world but associated aeolian detrital gold is relatively rare. In contrast, Holocene and active dunes at Waipapa beach on the coast of the southern South Island of New Zealand host aeolian black sand layers with locally abundant finely particulate (<200 µm) gold and platinum, some of which have been mined historically. Aeolian concentration of black sand, gold and platinum is driven by strong prevailing westerly winds (gusts >100 km/h) on annual or decadal time scales. Holocene fluvial recycling of aeolian sands has been important in upgrading the black sand, gold, and platinum contents. Surf concentration of black sand on the beach is minor and merely feeds the aeolian system, and this differs from other beach gold deposits around the world, which are surf-dominated. Most of the Waipapa beach gold (>90%) has flaky morphology with abundant superficial transport-related deformation reflective of fluvial transport, probably down the nearby Mataura River (∼200 km transport) between Miocene and Holocene. Minor distinctive wind-sculpted toroidal gold, and the platinum, arrived from the west (∼300 km transport) via the Waiau River and Foveaux Strait. [ABSTRACT FROM AUTHOR]

The Glenorchy area of northwest Otago consists of a low-relief Holocene fluvial delta complex below glaciated mountains of Otago Schist basement, and has locally elevated arsenic (As) from two geogenic pathways: Mesozoic hydrothermal processes and Holocene shallow groundwater transport. Soils on the delta complex are primitive and contain abundant sand and silt derived from unoxidised schist basement, with background As contents generally <30 mg/kg. Some soils in wetland areas on the delta have naturally elevated (geogenic) arsenic contents that are locally >300 mg/kg on the cm2 scale. Arsenic enrichment has occurred in the upper 30 cm where dissolved As and Fe from low-redox chemical conditions in shallow groundwater has become oxidised, thereby lowering their solubility. Both As and Fe are being mobilised from background sulfide minerals in schist-derived debris, and groundwater As concentrations up to 0.8 mg/L have been recorded. Historic mining of scheelite for tungsten on the surrounding mountains has exposed small volumes (typically tens to hundreds of m3) of arsenopyrite-bearing mine wastes with spot As contents from <500 to >10 000 mg/kg. Water discharging from mine adits have pH∼8 and dissolved As up to 0.3 mg/L, but these waters are rapidly diluted downstream to <0.02 mg/L. [ABSTRACT FROM AUTHOR]

Reprocessing of mine wastes is a growing procedure around the world, in order to extract more resources from already-mined rocks. Mineralogical and geometallurgical investigations are useful for planned reprocessing, and in this study at Round Hill placer gold mine we characterise fine (<100 µm) and super-fine (micron-scale) particulate gold that has passed through the mine separation stages without being extracted. The gold was discharged with water and suspended sediments to settling ponds, separate from the more voluminous gravel and sand tailings. Authigenic clay and pyrite are abundant in the placer deposit and the settled suspension, and oxidised pyrite yielded gypsum and ferric oxyhydroxide. These minerals coat surfaces of gold particles and are locally intergrown with micron-scale authigenic gold. Most of the gold has toroidal shapes characteristic of beach derivation, and many of these are hollow or clay-filled. Minor super-fine cinnabar and mercury-bearing gold occur with clay. Annealing of gold has liberated silver, and micron-scale authigenic native silver occurs in the settled sediment. Reconcentration of fine and super-fine gold used abundant clean water and slow reprocessing and may be currently uneconomic at this site. The observations on suspended gold loss are relevant to gold recovery at placer mines around the world. [ABSTRACT FROM AUTHOR]

Understanding the landscape factors governing population connectivity in riverine ecosystems represents an ongoing challenge for freshwater biologists. We used DNA sequence analysis to test the hypothesis that major geomorphological features underpin freshwater‐limited fish diversity in a tectonically dynamic region of New Zealand. Phylogeographic analysis of 101 Galaxias depressiceps cytochrome b sequences, incorporating 55 localities from southern New Zealand, revealed 26 haplotypes, with only one shared among rivers. We detect strong hierarchical genetic differentiation both among and within river systems. Genetic structuring is particularly pronounced across the Taieri River system (63 individuals from 35 sites, 18 haplotypes), with 92% of variation partitioned among locations. Distinctive within‐river genetic clusters are invariably associated with major subcatchment units, typically isolated by substantial gorges. The anomalous distribution of a single lineage across a major drainage divide is consistent with local, tectonically driven headwater capture. We conclude that major landscape features such as gorges can strongly partition riverine fish diversity and constrain freshwater biodiversity. [ABSTRACT FROM AUTHOR]

We integrate multidisciplinary observations to provide a regional-scale synthesis of river drainage reconfiguration in Southland (New Zealand) between the Miocene and the Holocene. Distributions of sedimentary clasts, including alluvial gold, garnets and chromite, are combined with freshwater fish genetics, surface geomorphology, and basement geology to constrain river drainage directions and interconnections through time. River evolution in much of the South Island has been dominated by localised tectonic uplift linked to distant Alpine Fault tectonism. In the southeast, the rise of ranges between Central Otago and Southland severed initial long-distance southward drainage and gold transport and isolated fish species. Plio-Pleistocene range uplift caused the recycling of older alluvial gold deposits into the new Southland river systems. The most significant Southland drainage reorientations have occurred through the Pleistocene, in parallel with similar processes in Central Otago. In particular, the Pomahaka River has evolved geomorphologically from a Southland-linked catchment to become a major tributary of the Clutha catchment, with associated biological evolution of distinctive endemic freshwater fish. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Freshwater-limited fish populations are often tightly constrained by river drainage boundaries. As a case in point, the distribution of lineages within New Zealand's diverse Galaxias vulgaris complex is broadly structured by geographic barriers, reflecting tectonic processes. However, several drainages of the Central Otago region have been locally modified by past gold-mining activities, with artificial water races connecting formerly isolated headwaters of distinct river drainage systems. Here we synthesise published genetic data to highlight the role of anthropogenic catchment modification in redistributing fish diversity. These data show that several local phylogeographic anomalies for stream-resident Galaxias fishes are closely linked to anthropogenic connections across major drainage divides. While these anthropogenic translocations may parallel natural geologically-driven mixing events that occur on deeper time scales, they nevertheless have potential conservation implications given the increasing fragmentation of native fish populations. Our review also highlights the role of sub-catchment units in shaping the hierarchical structure of intraspecific biodiversity. [ABSTRACT FROM AUTHOR]