Stephanie de Villiers - Academia.edu (original) (raw)

Uploads

Books by Stephanie de Villiers

Papers by Stephanie de Villiers

South African Journal of Science, 2016

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.

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.

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.

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

South African Journal of Science, 2007

South African Journal of Science, 2016

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.

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.

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.

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

South African Journal of Science, 2007