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Abrupt cessation of North Pacific upwelling with Northern Hemisphere Glaciation recorded by silicon isotopes
A530
Goldschmidt Conference Abstracts 2006
Silicon isotope variations in river waters: A weathering proxy? B.C. REYNOLDS1, R.B. GEORG1, K.W. BURTON2, R.H. JAMES2, A.N. HALLIDAY3 1
IGMR, ETH Zu¨rich, Switzerland ([email protected]; [email protected]) 2 Department of Earth Sciences, The Open University, UK ([email protected]; [email protected]) 3 Department of Earth Sciences, University of Oxford, UK ([email protected]) It has been shown that rivers contain Si with heavier dissolved isotope compositions relative to the continental crust, but reasons for this enrichment remain unclear. We compare Si isotope variations with physico-chemical parameters, such as river discharge, the water chemistry and the chemical weathering flux in order to constrain the processes controlling the Si geochemical budgets. The relative Si isotope compositions were measured using the high-resolution NuPlasma 1700 MC-ICP-MS. An initial study to investigate the role of catchment lithology and relief, focussing on rivers in the Swiss Alps found that the main mechanism controlling the Si isotope composition of a mountainous river was a two component mixing between the seepage of soil/ground waters, with heavier Si produced by clay formation and superficial runoff with lighter Si during high discharge events. The mechanisms controlling the Si isotope composition in the lowland rivers display increasing complexity as the proportion of carbonate rocks in the catchment lithology and the proportion of forested and cultivated landscapes increase. In these instances it is impossible to resolve the extent of the isotopic fractionation and contributed flux of Si contributed by biological processes as opposed to abiotic weathering. To investigate if the conclusions drawn form our Swiss study are widely applicable we have investigated d30Si variations in rivers from Iceland and the high Himalayan. Our results indicate that the d30Si of the Iclandic rivers (average d30Si: +0.63 ± 0.38&) seems to be mainly controlled by weathering of silicates and the formation of secondary clay minerals as the dissolved d30Si depends on the Al content and scales with Al/X ratios. A similar relation has been found in one Himalayan catchment, whilst the general situation is more complex. There is also a surprising positive correlation between the d30Si composition of the Himalayan rivers (average d30Si: +0.92 ± 0.28&) and the altitude, where high d30Si values have been measured for samples from the highest altitudes draining the Tibetan sedimentary series, and are correlated with d7Li variations. doi:10.1016/j.gca.2006.06.976
Abrupt cessation of North Pacific upwelling with Northern Hemisphere Glaciation recorded by silicon isotopes B.C. REYNOLDS1, S.L. JACCARD2, A.N. HALIDAY3 1
IGMR, ETH Zu¨rich, Switzerland ([email protected]) Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada ([email protected]) 3 Department of Earth Sciences, University of Oxford, UK ([email protected]) 2
The abrupt cessation of high opaline accumulation marks the initiation of Northern Hemisphere Glaciation in the Subarctic North Pacific at 2.7 Ma, and the beginning of ice-rafted debris deposition. The decreased opal flux has been linked to a decline in the exposure of nutrient-rich deep water to the surface with an increased stratification of the water column within the subarctic gyre. The restricted nutrient supply is considered to lead to more complete utilization of nutrients with a resulting increase in the d15N and d30Si values in the marine sediment record. Both Si and N isotopes vary by about half the mass fractionation factor estimated for nutrient uptake across this transition. Whilst nitrogen isotopes from the bulk sediment document a heavy isotope enrichment, there is an inverse correlation with silicon isotopes. This inverse correlation cannot be reconciled with the inferred nutrient utilization signal for the stable isotope variations. The silicon stable isotope composition of the total diatom export, and thus the underlying sediment, generally does not record a ‘utilization’ signal in high productivity areas. This is because nutrient supply to surface waters and export of organic material are generally near steady-state conditions, with near complete transfer of the upwelling silicic acid (and isotopic composition) to the opal export from the surface water. Hence, sedimentary silicon isotope variations appear to be predominantly forced by external changes in the supply (and isotopic composition) of nutrients to upwelling waters. The decrease in diatomite d30Si values at 2.7 Ma are thus interpreted to reflect a reduced supply of nutrient-rich waters with high d30Si values into the surface waters of the North Pacific, rather than a decrease of Si utilization. The increase in d15N values from low d15N prior to 2.7 Ma may reflect an elimination of diazotrophic N-fixation. The stratification and cooling of the surface waters may have destroyed diatom-diazotroph symbioses, enhancing the effects of water stratification on diatom productivity. Recorded variations on both glacial-interglacial and longer time-scales can thus be attributed to oceanographic re-organisation. The stable isotope variations of this core illustrate the complex inter-dependencies between productivity, ocean circulation and nutrient dynamics and their effects on proxies for nutrient utilization. doi:10.1016/j.gca.2006.06.977
Goldschmidt Conference Abstracts 2006
Silicon isotope variations in river waters: A weathering proxy? B.C. REYNOLDS1, R.B. GEORG1, K.W. BURTON2, R.H. JAMES2, A.N. HALLIDAY3 1
IGMR, ETH Zu¨rich, Switzerland ([email protected]; [email protected]) 2 Department of Earth Sciences, The Open University, UK ([email protected]; [email protected]) 3 Department of Earth Sciences, University of Oxford, UK ([email protected]) It has been shown that rivers contain Si with heavier dissolved isotope compositions relative to the continental crust, but reasons for this enrichment remain unclear. We compare Si isotope variations with physico-chemical parameters, such as river discharge, the water chemistry and the chemical weathering flux in order to constrain the processes controlling the Si geochemical budgets. The relative Si isotope compositions were measured using the high-resolution NuPlasma 1700 MC-ICP-MS. An initial study to investigate the role of catchment lithology and relief, focussing on rivers in the Swiss Alps found that the main mechanism controlling the Si isotope composition of a mountainous river was a two component mixing between the seepage of soil/ground waters, with heavier Si produced by clay formation and superficial runoff with lighter Si during high discharge events. The mechanisms controlling the Si isotope composition in the lowland rivers display increasing complexity as the proportion of carbonate rocks in the catchment lithology and the proportion of forested and cultivated landscapes increase. In these instances it is impossible to resolve the extent of the isotopic fractionation and contributed flux of Si contributed by biological processes as opposed to abiotic weathering. To investigate if the conclusions drawn form our Swiss study are widely applicable we have investigated d30Si variations in rivers from Iceland and the high Himalayan. Our results indicate that the d30Si of the Iclandic rivers (average d30Si: +0.63 ± 0.38&) seems to be mainly controlled by weathering of silicates and the formation of secondary clay minerals as the dissolved d30Si depends on the Al content and scales with Al/X ratios. A similar relation has been found in one Himalayan catchment, whilst the general situation is more complex. There is also a surprising positive correlation between the d30Si composition of the Himalayan rivers (average d30Si: +0.92 ± 0.28&) and the altitude, where high d30Si values have been measured for samples from the highest altitudes draining the Tibetan sedimentary series, and are correlated with d7Li variations. doi:10.1016/j.gca.2006.06.976
Abrupt cessation of North Pacific upwelling with Northern Hemisphere Glaciation recorded by silicon isotopes B.C. REYNOLDS1, S.L. JACCARD2, A.N. HALIDAY3 1
IGMR, ETH Zu¨rich, Switzerland ([email protected]) Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada ([email protected]) 3 Department of Earth Sciences, University of Oxford, UK ([email protected]) 2
The abrupt cessation of high opaline accumulation marks the initiation of Northern Hemisphere Glaciation in the Subarctic North Pacific at 2.7 Ma, and the beginning of ice-rafted debris deposition. The decreased opal flux has been linked to a decline in the exposure of nutrient-rich deep water to the surface with an increased stratification of the water column within the subarctic gyre. The restricted nutrient supply is considered to lead to more complete utilization of nutrients with a resulting increase in the d15N and d30Si values in the marine sediment record. Both Si and N isotopes vary by about half the mass fractionation factor estimated for nutrient uptake across this transition. Whilst nitrogen isotopes from the bulk sediment document a heavy isotope enrichment, there is an inverse correlation with silicon isotopes. This inverse correlation cannot be reconciled with the inferred nutrient utilization signal for the stable isotope variations. The silicon stable isotope composition of the total diatom export, and thus the underlying sediment, generally does not record a ‘utilization’ signal in high productivity areas. This is because nutrient supply to surface waters and export of organic material are generally near steady-state conditions, with near complete transfer of the upwelling silicic acid (and isotopic composition) to the opal export from the surface water. Hence, sedimentary silicon isotope variations appear to be predominantly forced by external changes in the supply (and isotopic composition) of nutrients to upwelling waters. The decrease in diatomite d30Si values at 2.7 Ma are thus interpreted to reflect a reduced supply of nutrient-rich waters with high d30Si values into the surface waters of the North Pacific, rather than a decrease of Si utilization. The increase in d15N values from low d15N prior to 2.7 Ma may reflect an elimination of diazotrophic N-fixation. The stratification and cooling of the surface waters may have destroyed diatom-diazotroph symbioses, enhancing the effects of water stratification on diatom productivity. Recorded variations on both glacial-interglacial and longer time-scales can thus be attributed to oceanographic re-organisation. The stable isotope variations of this core illustrate the complex inter-dependencies between productivity, ocean circulation and nutrient dynamics and their effects on proxies for nutrient utilization. doi:10.1016/j.gca.2006.06.977