Dave Craw

Climatic and tectonic controls on chemical weathering in the New Zealand Southern Alps

Abstract Climatic and tectonic controls on the relative abundance of solutes in streams draining the New Zealand Southern Alps were investigated by analyzing the elemental and Sr isotope geochemistry of stream waters, bedload sediment, and hydrothermal calcite veins. The average relative molar abundance of major cations and Si in all stream waters follows the order Ca2+ (50%) > Si (22%) > Na+ (17%) > Mg2+ (6%) > K+ (5%). For major anions, the relative molar abundance is HCO3− (89%) > SO42− (7%) > Cl− (4%). Weathering reactions involving plagioclase and volumetrically small amounts of hydrothermal calcite define the ionic chemistry of stream waters, but nearly all streams have a carbonate-dominated Ca2+ and HCO3− mass-balance. Stream water Ca/Sr and 87Sr/86Sr ratios vary from 0.173 to 0.439 μmol/nmol and from 0.7078 to 0.7114, respectively. Consistent with the ionic budget, these ratios lie solely within the range of values measured for bedload carbonate (Ca/Sr = 0.178 to 0.886 μmol/nmol; 87Sr/86Sr = 0.7081 to 0.7118) and hydrothermal calcite veins (Ca/Sr = 0.491 to 3.33 μmol/nmol; 87Sr/86Sr = 0.7076 to 0.7097). Streams draining regions in the Southern Alps with high rates of physical erosion induced by rapid tectonic uplift and an extremely wet climate contain ∼10% more Ca2+ and ∼30% more Sr2+ from carbonate weathering compared to streams draining regions in drier, more stable landscapes. Similarly, streams draining glaciated watersheds contain ∼25% more Sr2+ from carbonate weathering compared to streams draining non-glaciated watersheds. The highest abundance of carbonate-derived solutes in the most physically active regions of the Southern Alps is attributed to the tectonic exhumation and mechanical denudation of metamorphic bedrock, which contains trace amounts of calcite estimated to weather ∼350 times faster than plagioclase in this environment. In contrast, regions in the Southern Alps experiencing lower rates of uplift and erosion have a greater abundance of silicate- versus carbonate-derived cations. These findings highlight a strong coupling between physical controls on landscape development and sources of solutes to stream waters. Using the Southern Alps as a model for assessing the role of active tectonics in geochemical cycles, this study suggests that rapid mountain uplift results in an enhanced influence of carbonate weathering on the dissolved ion composition delivered to seawater.

Influence of exhumation on the structural evolution of transpressional plate boundaries: An example from the Southern Alps, New Zealand

Concentration of erosional activity along transpressional plate boundaries can significantly alter the pattern of mechanical behavior through the influence of exhumation on crustal strength. Three-dimensional numerical modeling of an obliquely convergent orogen shows that a single oblique plate-bounding structure is stable if asymmetric erosion patterns, such as those observed in orographic mountain belts, pertain, and if Earth9s crust has a strong-on-weak rheology. In early stages of oblique convergence of an initially laterally homogeneous material, lateral (boundary-parallel) strain is accommodated along a near vertical structure and convergent (boundary-normal) strain is concentrated on structures dipping at moderate angles into the orogen. Exhumation of deep crustal material along the convergent structure results in thermal weakening along this dipping structure. When the upper crust beneath the orogen is significantly weakened by exhumation, lateral strain abandons the vertical structure and shifts to the dipping structure, combining with the convergent strain to form a single oblique fault that accommodates the plate motion in the upper crust, as is the case along the Alpine fault, New Zealand. The process of thermal thinning is controlled by advection and occurs on time frames of ∼1–2 m.y. The two components of strain remain separate in the lower crust. During active convergence, exhumation of lower crustal material occurs only along those structures accommodating convergent strain. Consequently, material exhumed from lower regions of ductile deformation, as is the case along the Alpine fault, contains lineations that indicate a greater component of convergence than predicted from the total plate motion. Postorogenic exhumation of the roots of an oblique plate boundary will expose two parallel shear zones, one dominantly convergent and one dominantly strike slip. Widely reported orogen-parallel transport in the late stages of ancient oblique convergence may represent not a change in plate vector, but the exhumation of the lateral transport zone.

Infrared spectroscopic characterisation of arsenate (V) ion adsorption from mine waters, Macraes mine, New Zealand

Abstract Processing waters contain up to 10 mg l−1 dissolved As at the Macraes mine, New Zealand, and this is all removed by adsorption as the water percolates through a large earth dam. Laboratory experiments were set up to identify which mineral is the most effective substrate for this adsorption of As. The experiments were conducted using infrared (IR) spectroscopy of thin mineral films adhering to a ZnSe prism. Silicates, including kaolinite, adsorbed only small amounts of As which was readily washed off. Hydrated Fe oxides (HFO) were extremely effective at adsorbing As, particularly the natural amorphous HFO currently being deposited from dam discharge waters at the Macraes mine. An adsorption isotherm determined for this natural material has the adsorption constant, Kads=(1.9±0.4)×104 M−1, and the substrate becomes saturated with adsorbed As when solution concentrations exceed about 50 mg l−1. Saturation is not being reached at the Macraes mine. Arsenic adsorbed on to natural HFO has a distinctive IR spectrum with the absorption peak varying from 800 cm−1 (alkaline solutions) to 820 cm−1 (neutral to acid solutions). Much of this adsorbed As is strongly bound and difficult to wash off. Arsenate ions adsorb in a bidentate structure which may be a precursor for scorodite crystallisation.

An empirical test of freshwater vicariance via river capture.

River capture is a geomorphological process through which stream sections are displaced from one catchment to another, and it may represent a dominant facilitator of interdrainage transfer and cladogenesis in freshwater‐limited taxa. However, few studies have been conducted in a manner to explicitly test the biological significance of river capture. Here we present a multispecies phylogeographical analysis to test whether the nonmigratory fish fauna of the Von River (South Island, New Zealand) is the product of a well‐documented, Late Quaternary capture of a section of the Oreti River (Southland drainage). Specifically, we predict that nonmigratory fishes of the Von River will exhibit closer genetic affinities with those of Southland, rather than those of the Clutha system, into which the Von River presently drains. Mitochondrial DNA phylogeography (control region and cytochrome b sequence data) and analysis of nuclear orthologues of mtDNA sequences indicate that ‘flathead’Galaxias of the Von River (n = 31, three sites) have greatest genetic affinities with those of Southland (Galaxias ‘southern’, n = 216, 38 sites), rather than with those of the Clutha River (Galaxias sp. ‘D’, n = 73, 32 sites). Likewise, Von River ‘roundhead’Galaxias (n = 52, four sites) have greatest genetic affinities with those of Southland drainages (Galaxias gollumoides, n = 223, 58 sites), rather than with those of the Clutha River (Galaxias pullus, Galaxias anomalus, Galaxias gollumoides of the Nevis tributary; n = 68, 32 sites). These findings are consistent with our predictions that genetic affinities of the nonmigratory fish fauna in the Von River would reflect past, rather than present, drainage connections. Consequently, river capture is responsible for the nonmigratory fish fauna of the Von River. In a broader context, river capture has frequently influenced the distribution of genetic lineages among catchments in New Zealand freshwater‐limited fish, and its biogeographical significance may have been underestimated in other regions.

DOES FISH ECOLOGY PREDICT DISPERSAL ACROSS A RIVER DRAINAGE DIVIDE

Abstract Obligate freshwater taxa are frequently distributed among catchments isolated by marine and terrestrial barriers. Such distributions can arise through vicariant changes in drainage geometry, or dispersal via intermittent freshwater connections. We employed two adjacent rivers in southern New Zealand to test for interdrainage dispersal while controlling for historical drainage geometry, and analyzed four ecologically distinct freshwater-limited fish taxa to assess any relationship with habitat preference. Individuals from the Mararoa and Oreti catchments (n >100 per species) were sequenced for a minimum of 1297 bp of mitochondrial DNA (cytochrome b and control region). Phylogeographic relationships were consistent with ecological expectations of interdrainage dispersal capability, with the two obligate riverine taxa each exhibiting reciprocal monophyly between catchments, whereas the two facultative swamp dwellers revealed paraphyletic relationships, one of which shared a haplotype between catchments. Statistical phylogeography, accommodating taxon-specific mutation rates and the known age of the last major riverine connection between these catchments, rejected complete isolation of populations for one of the swamp dwellers. Therefore, dispersal across a young (145–240 kyr) drainage divide is inferred for one species, and can be predicted to some extent by species ecology. Moreover, our study highlights the importance of historical drainage geometry when assessing the causes of contemporary genetic structuring in freshwater taxa.

Geochemical controls on the environmental mobility of Sb and As at mesothermal antimony and gold deposits

Abstract Antimony and arsenic are commonly mobilised into the environment from mesothermal mineral deposits. Both these metalloids are potentially toxic in the environment when dissolved in water at low levels (<0·01 mg/l). Mobility of antimony, in comparison to that of arsenic, is documented at mine sites in four different mesothermal systems in low grade Palaeozoic-Mesozoic metamorphic terranes of New Zealand (Globe Hill, Reefton; Macraes, Otago; and Endeavour Inlet, Marlborough) and Australia (Hillgrove, New South Wales). Dissolved antimony can reach ~50 mg/kg in mine waters where evaporative concentration occurs in oxidised near-neutral pH mine waters in contact with stibnite. Such waters are chemically saturated with respect to antimony oxides, and antimony oxide precipitation occurs locally. Most mine waters have lower dissolved Sb concentrations, especially where high rainfall causes dilution. However, high rainfall areas have widespread diffuse mobilisation of both Sb and As to give elevated background levels (0⋅1–0⋅01 mg/kg) in downstream waters. Dissolved Sb is decreased by adsorption of Sb to hydrated iron oxide (HFO) precipitates in streams. Bulk distribution coefficient, Kd, for this Sb adsorption ranges up to at least 105, and is similar to that of As adsorption in the same settings. Attenuation of dissolved Sb by HFO results in an order of magnitude decrease in Sb concentrations on a scale of metres. In the absence of HFO, dissolved Sb can be transported in streams for many kilometres. Elevated dissolved Sb concentrations can arise distant (tens of kilometres) from a mine site because of dissolution of antimony-bearing minerals physically transported downstream. Hence, without HFO attenuation, environmentally toxic levels of Sb can be readily leached from mine sites and distributed widely.

Arsenic, copper and zinc occurrence at the Wangaloa coal mine, southeast Otago, New Zealand

Abstract Waste piles, created from open cast coal mining activities at the abandoned Wangaloa mine in SE Otago, have exposed pyrite (FeS2) to atmospheric conditions. This has led to the acidification of the surface tailings and nearby drainage waters (acid mine drainage, AMD). Mobilisation of trace metals arsenic (As), copper (Cu), and zinc (Zn) has occurred, partly as a result of the low pH levels (ca. pH 2–4), leading to elevated concentrations of these metals in receiving waters. Authigenic pyrite deposited in a marginal marine coal-forming environment is enriched in As with levels reaching up to 100 ppm. Copper and Zn in solid solution are not elevated above background levels in either coal measures or associated pyrite. Water discharges, sediments, waste rock and background samples were sampled and analysed during the driest (summer) and wettest (winter) seasons of 1998 and 1999. During the winter season, water discharging from the waste piles contained up to 0.7 ppm (mg/kg) As, as measured in 1998. During the 1999 wettest season, no such levels of As were observed, with the highest level attaining 0.07 ppm As. Copper and Zn were locally elevated in waters, with Zn concentrations reaching 1 ppm. During the summer season of 1999, only one sampling site recorded elevated metal concentrations. Adverse effects from the remnant waste piles appear to be highly localised due to downstream natural remediation processes occurring in a wetland area. The absence of strongly elevated metal concentrations during the drier season is a result of strongly depressed water levels within the waste piles. Flushing of acid and metals occurs when the water levels increase with the onset of the winter season. During the summer season, pyrite within the waste piles has been readily decomposing from the increased availability and transport of atmospheric oxygen.

Diagenetic pyrite as a source for metals in orogenic gold deposits, Otago Schist, New Zealand

Abstract Framboidal diagenetic pyrite occurs in low-grade turbidites on the northeast margin of the Otago Schist. Laser ablation inductively coupled plasma mass spectrometry (ICP-MS) analyses of these framboids show that the main metals present, in addition to Fe, are As, Pb, Zn, Te, Ag and Mo; Au is present at between 0.5 and 2 ppm. Minor Au and As anomalies also occur sporadically in the turbidite matrix. Porphyroblastic pyrite at the nearby Macraes mine contains micron-scale native gold and solid solution gold up to 30 ppm. These pyrite porphyroblasts are also enriched in As, W, Ni, Co and Bi. Lead, Zn, Te and Mo contents are distinctly lower than in the low-grade framboids. These results show that low-grade turbidites were a fertile source for Au and As for orogenic gold deposits such as Macraes. The pyrite to pyrrhotite transition, which occurred mainly under greenschist facies conditions, was probably the most effective As and Au mobilisation reaction during Mesozoic metamorphism.

Genetic ages for Quaternary topographic evolution: A new dating tool

All eukaryote populations accumulate mutations in their mitochondrial DNA (mtDNA) over time, so reproductively isolated populations become characterized by distinct mtDNA lineages. In addition, the degree of genetic differentiation among distinct populations can be used to estimate time elapsed since their isolation. We have identified an informative system for calibrating the mtDNA “clock” by genetically comparing freshwater galaxiid fish populations isolated in different river drainages. Calibration using a range of Quaternary geological events in southern New Zealand shows that the mtDNA divergence rate in galaxiid fishes is between 1% and 2%/100 k.y. up to 250 k.y., with the rate decreasing with increasing age. The estimated divergence rate slows to around 4%/m.y. for the middle Quaternary, although calibration is poor. A calibration curve has been fitted to all data: divergence (%) = −2.2 e −9 t + 2.5 t + 2.2, where t is isolation age (in m.y.). This molecular clock has potential as a dating tool for glacially related and active tectonic events that have caused river drainage changes in the late Quaternary in the Southern Hemisphere, where galaxiids are widespread. An application of this dating tool to an example in northern South Island uses three different species of freshwater-limited fish, and all three data sets imply formation of a drainage divide at 320 ± 110 ka, at about the time of a major glacial advance though the divide (oxygen isotope stage 8).

Water–rock interaction and acid neutralization in a large schist debris dam, Otago, New Zealand

Abstract Routine water chemistry monitoring of a large mine tailings impoundment dam at the Macraes mine provided a site for a 5-year study of progressive water–rock interaction with Otago Schist greenschist facies schist debris. Water of known composition was added at the top, and discharging waters had different compositions due to different pathways. Water which had most interaction with the rock showed progressively increasing sulphate (to 1500 ppm), bicarbonate (to 500 ppm), calcium (to 250 ppm) and magnesium (to 130 ppm). Sodium and potassium were fully adsorbed initially then transmission of alkalis increased, but no decomposition of albite or muscovite was detectable. Oxidation of pyrite produced the dissolved sulphate and hydrogen ions. The hydrogen ions were consumed immediately by decomposition of calcite and chlorite intergrown with pyrite. The rates of combined calcite and chlorite decomposition are governed by the rate of pyrite oxidation, and match that rate, so that pH does not change perceptibly. The acid neutralizing capacity (ANC) of these rocks depends on the combined calcite and chlorite content, and other minerals present have no significant effect on neutralization on the 5-year time scale. Decomposition rate of chlorite in these rocks is relatively high compared to experimental observations and other mine sites because the Macraes mine chlorite is intimately intergrown with acid-producing pyrite at the millimetre scale.