Books by Stephanie de Villiers
H2O-CO2 energy equations for South Africa
Papers by Stephanie de Villiers
South African Journal of Science, 2016
The polar regions are more critically affected by climate change than any other region on our pla... more The polar regions are more critically affected by climate change than any other region on our planet. On the Antarctic continent and in its surrounding oceans, the effects of climate change are likely to be dramatic, 3 and include largescale catastrophic ice melt, loss of habitat and biodiversity, and global sea level rise. The 'Southern Ocean' refers to the region where Atlantic, Indian and Pacific Ocean waters come together to encircle Antarctica. These waters connect the different ocean basins by linking the shallow and deep limbs of the global ocean current system ('overturning circulation') and play a critical role in storing and distributing heat and carbon dioxide (CO 2 ). The Southern Ocean thus regulates not only the climate of the Antarctic, but of the entire earth system. By extension, the capacity of the global ocean to ameliorate earth's changing climate is strongly controlled by the Southern Ocean.
Water SA, 2019
The upper catchment area of the Berg River in the Western Cape, South Africa, supplies most of Ca... more The upper catchment area of the Berg River in the Western Cape, South Africa, supplies most of Cape Town and its suburbs with freshwater, in addition to providing water for irrigation purposes along the middle and lower reaches of the river. This study investigates the nutrient status of the Berg River and long-term trends therein. It is shown that inorganic nitrogen and phosphorus levels increase downstream by a factor of more than 10, in response to anthropogenic inputs. Similarly, nutrient levels fluctuate seasonally by more than an order of magnitude, in response to input from diffuse and point sources of pollution. These changes of more than 1 000% far exceed the 15% maximum change stipulated by the South African water quality guidelines for aquatic ecosystems. Total phosphorus levels indicate that hypertrophic conditions prevail at least episodically at all of the Berg River monitoring stations and most of the time at some of them. Additionally, river water phosphate levels show a dramatic increase over the past 20 years. There is also strong evidence that the trophic status of the Berg River is very sensitive to reduced river runoff. The implication is that the construction of the new Berg River Dam in the upper catchment area of the Berg River will exacerbate the existing situation, threatening ecosystem services, human health and lucrative agricultural activities.
Water SA, 2009
Water quality monitoring in the Olifants River catchment, Mpumalanga, is evaluated using river wa... more Water quality monitoring in the Olifants River catchment, Mpumalanga, is evaluated using river water dissolved sulphate levels, one of the best indicators of pollution related to acid mine drainage. Assessment of long-term water quality records shows that monitoring has not been carried out systematically. In that it fails one of the most fundamental criteria of good environmental monitoring practices. At some monitoring stations sampling frequency has been scaled down from approximately weekly to monthly intervals over time, despite evidence for increasing and problematic levels of pollution. At the Loskop Dam dissolved sulphate levels have increased more than 7-fold since the 1970s evidently due to increasing levels of pollution within the Little Olifants River catchment. At 4 of the 7 long-term monitoring stations river water sulphate levels exceed the 100 mg/â„“ threshold value for aquatic ecosystem health most of the time for the duration of the record, and all of the time since about 2001. At these stations river water sulphate levels also exceed the 200 mg/â„“ threshold for human consumption 27 to 45% of the time, for the duration of the long-term record. These observations necessitate more frequent and improved monitoring, not evidently reduced efforts. A major concern is the location of a recently re-opened copper mine outside Phalaborwa, just upstream from the confluence of the Ga-Selati River and the Olifants River. Levels of copper sulphate, highly toxic to aquatic species, should be urgently investigated as a probable cause of recent fish and crocodile deaths in the Kruger National Park. In river systems subject to intensive mining activity, such as the Olifants River, toxic constituents such as copper, arsenic, chrome-VI, etc., currently not routinely measured by the Department of Water Affairs (DWA) need to be included in monitoring efforts as a matter of urgency. This will require drastic improvements in current water quality monitoring efforts, including the acquisition of modern analytical instrumentation.
South African Journal of Geology, 2007
This study of the geochemistry of rivers draining the Archaean granite-gneissic terrains of Swazi... more This study of the geochemistry of rivers draining the Archaean granite-gneissic terrains of Swaziland reports the first anomalous Sr-87 Sr/ 86 Sr data for a river system outside the Himalayas. Both major cation and calcium mass balance approaches suggest that the source of the radiogenic high strontium load of Swaziland rivers is predominantly, or exclusively, silicate weathering. However, a compilation of bedrock compositional data for river catchments in Swaziland demonstrates that the uncertainties associated with these mass balance approaches are large and presently underestimated in global chemical weathering studies. Consideration of the strontium mass balance, based on silicate Na/Sr end member values, demonstrates the potential magnitude of the error involved. Importantly, a compilation of both the Na/Sr and Sr content of silicate rocks in Swaziland demonstrates that the general relationship between (Na/Sr) sil and Sr content is poor and that high silicate versus carbonate X/Sr end member values cannot be assumed to imply relatively lower Sr weathering yields from silicate rocks.
The strontium isotope systematics of the Orange River, Southern Africa
South African Journal of Geology, 2000
The 87Sr/86Sr ratio of Orange River water increases from 0.7081 at its headwaters in the basalt o... more The 87Sr/86Sr ratio of Orange River water increases from 0.7081 at its headwaters in the basalt of the Drakensberg mountains, to a value of 0.7126 before its confluence with the Vaal River tributary. The concomitant increase in Sr concentration is from 0.08 to 1.28 μM. The Vaal River, which drains Precambrian and Permian/Triassic rocks, has higher Sr concentrations, from 0.95 to 6.69 μM, and more radiogenic 87Sr/86Sr values, from 0.713 to 0.731. The geochemistry of the dissolved load of the Orange and Vaal rivers corresponds with that of the dominant catchment lithologies: Jurassic basalt, Permian/Triassic Karoo sediments and Proterozoic/Archaean para- and ortho-gneisses/granitoids. Chemical weathering rates in the Orange River system are at least a factor of 3 lower than the global average value of 36 ton/km2/yr. The highest chemical and physical weathering rates occur in the Upper Orange River, associated with high runoff and relief. In the low-relief Vaal River, chemical weathering predominates over physical weathering.
Mineralogical Magazine, 1998
Diffuse, relatively low-temperature hydrothermal fluxes on ridge flanks play a potentially signif... more Diffuse, relatively low-temperature hydrothermal fluxes on ridge flanks play a potentially significant role in the mass balance of a number of chemical components in seawater. For example, assuming a steady state ocean, oceanic mass balance models based on 3He and on-axis mid-ocean ridge heat fluxes indicate an input deficiency of 16 • 1012 mol/yr for Ca (on the same order of magnitude as the riverine flux), a sink for only 10-40% of estimated riverine Mg input, and flux of Sr that is 5 to 10 times too low to maintain a Sr isotope mass balance in the ocean . The identification of these mass balance discrepancies provides indirect support for suggestions that the balance may be provided via the low-temperature, diffuse, off-axis hydrothermal flux. We present Sr isotope and elemental composition of pore fluids collected from an advective hydrothermal system on the flank (1-4 Ma crust) of the Juan de Fuca ridge. The data show a strong signature of basaltic Sr input into and Mg removal from seawater associated with these low-temperature systems. Additionally, depth-concentration profiles of the 'conservative' elements Ca and Mg through the water column above the East Pacific Rise show mid-depth anomalies in (salinity normalized) concentrations. This may also be interpreted as qualitative support for a significant off-axis sink for Mg and input for Ca. Sr isotope and major element profiles of sediment pore waters were measured for six cores taken during the Flank Flux cruise in 1990 . The cores were taken from two areas approximately 40 and 110 km east of the ridge axis, with estimated basement temperatures of 40~ and 70-90~ respectively. Mg and Ca ocean water
Geochimica et Cosmochimica Acta, 1994
This paper provides an evaluation of two of the most likely pitfalls of Sr/Ca thermometry, i.e., ... more This paper provides an evaluation of two of the most likely pitfalls of Sr/Ca thermometry, i.e., the effect of biogenic cycling of Sr vs. Ca in the surface ocean and the effect of variable extension rate on Sr incorporation in coralline aragonite. We also report cahbration of the Sr/Ca-temperature relationship for three coral species, Porites lobata, P~~Ilo~ora eydok, and Pavona claws, collected from the Hawaiian and Galapagos islands. Analyses of seawater samples show significant spatial and depth variability in the Sr:Ca ratio. The uncertainty introduced by this effect is estimated to be <0.2'C for corals located in tropical oligotrophic waters, and potentially larger for corals located in upwelling areas. Sr/Ca analyses along two different growth axes of a Galapagos Puvonu cluvus, with annual extension rates of-6 and 12 mm/y, respectively, indicate an offset of l-2'C, with higher Sr/Ca values associated with slower extension rates. The offset observed between the two growth axes may be the result of variations in extension and/or calcification rate.
Excess dissolved Ca in the deep ocean: a hydrothermal hypothesis
Earth and Planetary Science Letters, 1998
Variations in seawater alkalinity and dissolved calcium provide information essential to establis... more Variations in seawater alkalinity and dissolved calcium provide information essential to establishing the integrated deep ocean calcium carbonate dissolution flux. High-precision Ca measurements reported here confirm earlier suggestions that the deep ocean contains higher levels of dissolved Ca than expected from calcium carbonate dissolution only. I propose that the mid-depth Ca excess is a manifestation of the circulation of seawater
South African Journal of Marine Science, 1998
Water SA, 2007
The chemistry of the major elements of KwaZulu-Natal river water draining the eastern Drakensberg... more The chemistry of the major elements of KwaZulu-Natal river water draining the eastern Drakensberg Escarpment was monitored in October 2003 and compared to data obtained previously along the western Drakensberg Escarpment, i.e. the Caledon and Upper Orange Rivers. The data obtained in these two surveys reveal no significant differences in the Mg, Ca, Na, K and HCO 3 content of rivers draining similar lithologies, despite slightly different climatic regimes and different suspended loads. The implication is that lithology is the dominant control on the major element chemistry of river water draining the Drakensberg. However, in the northwestern part of KwaZulu-Natal, drought-stricken at the time of sampling, evaporation-induced concentration results not only in evaporite formation, but dramatic changes in river and stream water chemistry. Elevated levels of minor constituents such as NO 3 also indicate that in cultivated areas anthropogenic activities have an impact on water quality and composition.
This study of the geochemistry of rivers draining the Archaean granite-gneissic terrains of Swazi... more This study of the geochemistry of rivers draining the Archaean granite-gneissic terrains of Swaziland reports the first anomalous Sr-87 Sr/ 86 Sr data for a river system outside the Himalayas. Both major cation and calcium mass balance approaches suggest that the source of the radiogenic high strontium load of Swaziland rivers is predominantly, or exclusively, silicate weathering. However, a compilation of bedrock compositional data for river catchments in Swaziland demonstrates that the uncertainties associated with these mass balance approaches are large and presently underestimated in global chemical weathering studies. Consideration of the strontium mass balance, based on silicate Na/Sr end member values, demonstrates the potential magnitude of the error involved. Importantly, a compilation of both the Na/Sr and Sr content of silicate rocks in Swaziland demonstrates that the general relationship between (Na/Sr) sil and Sr content is poor and that high silicate versus carbonate X/Sr end member values cannot be assumed to imply relatively lower Sr weathering yields from silicate rocks.
Hydrography and biogeochemistry during S.A. Agulhas II cruise Agu011, in the SW Indian Ocean, April 2014
Collapse of the African Weathering Surface Across the Congo Basin Triggered Changes in Cenozoic Ocean Chemistry
An annual and a seasonal biogeochemical climatology had been constructed for the Southern Benguel... more An annual and a seasonal biogeochemical climatology had been constructed for the Southern Benguela Upwelling System, from in situ data collected along a 12 station monitoring line, sampled at monthly intervals from 2001 to 2012. The monitoring line reaches a maximum offshore distance of almost 190 km, with monitoring station depths ranging from 27 to 1 465 m. In addition to temperature, salinity and oxygen CTD profile data, archived monitoring data for the macro-nutrients (phosphate, nitrate + nitrite, silicate) and chlorophyll-a was evaluated. The climatologies exhibit clear spatial and seasonal variability patterns for all parameters, that yield important insight into the SBUS upwelling cycle. These data sets comprise valuable additions to our knowledge base, and will aid both future modelling efforts and studies of biogeochemical processes in upwelling systems. Data for the constructed climatologies has been made available via the PANGAEA Data Archiving
Biological Controls on Coral Sr/Ca and delta18O Reconstructions of Sea Surface Temperatures
Science, 1995
The correct use of Sr isotopes in river-groundwater mixing models : a Breede River case study : rapid communication
Water S a, 2005
The nutrient status of South African rivers: concentrations, trends and fluxes from the 1970's to 2005
South African Journal of Science, 2007
H2O-CO2-energy equations for South Africa
A 425 kyr record of foraminiferal shell weight variability in the western equatorial Pacific
Paleoceanography, 2003
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Books by Stephanie de Villiers
Papers by Stephanie de Villiers