A high resolution record of seawater osmium isotopic composition (35 samples over the past 191,00... more A high resolution record of seawater osmium isotopic composition (35 samples over the past 191,000 yr) has been obtained from sediments from Ocean Drilling Program (ODP) Leg 165 Site 1002 in the Cariaco Basin, Venezuela. Strong correlations are observed between 187Os/188Os and both the local and global oxygen isotopic records, with less radiogenic values observed during glacial intervals. In conjunction with previous analyses of sediments from the East Pacific Rise, the Santa Barbara Basin, and the Japan Sea, the data demonstrate conclusively that there have been ocean-wide changes in seawater 187Os/188Os over the Pleistocene. Global seawater excursions to relatively unradiogenic 187Os/188Os, indicating decreased continental inputs, occur during cold intervals, times of increased aridity and increased ice cover of old (radiogenic) continental masses. The data thus indicate a direct climatic control on chemical weathering rates and/or composition of weathering products on a glacial-i...
Radiocarbon measurements suggest that 14C-free carbon enters from beneath Mono Lake at a rate of ... more Radiocarbon measurements suggest that 14C-free carbon enters from beneath Mono Lake at a rate of about 1 mol/m2/yr. An input of this magnitude should be manifested in the inorganic carbon budget of the lake and with this in mind we have devised a model to reconstruct the evolution of the partial pressure of CO2 (pCO2) over the past 150 years. This encompasses a period (1945 to present) during which major diversions of source waters via the Los Angeles aqueduct have been in effect, significantly increasing the salinity of the lake and hence its pCO2. The model has been constrained by experimental characterization of the carbonate chemistry of the lake water, by the temperature dependence of pCO2 for the lake water, and by pCO2 measurements made on the lake water in 1966, 1969, 1981, and 1989. Our calculations suggest that prior to 1945 the pCO2 of Mono Lake water was about 1.3 times the atmospheric value. To produce this excess, an input of CO2 of about 3.3 mol/m2/yr is required. Volcanic activity beneath the lake is a probable source of this input.
Estimates of osmium residence time in the oceans that are based on oceanic mass balance calculati... more Estimates of osmium residence time in the oceans that are based on oceanic mass balance calculations (35–50 kyr) appear irreconcilable with those inferred from the recent evolution of the osmium isotope composition of seawater (3–4 kyr). It is argued that the osmium budget of the oceans is currently close to steady state and thus that the estimates made by the two methods should agree. As the inventory of osmium in the oceans is relatively well constrained, these disparate residence time estimates imply wildly different osmium input fluxes to the oceans. An osmium residence time of 8–10 kyr is proposed by evaluating the uncertainties and limitations of both methods, and it is argued that osmium inputs to the ocean are currently underestimated by a factor of ∼3. This reflects in part the underestimation of the river input of osmium to the oceans owing to a bias within the existing data set and in part the probable existence of sources of osmium to the oceans that have not yet been identified. The very short residence time of 3–4 kyr inferred from the postglacial change in seawater composition (assuming a single step change in input flux) is rejected as it implies unreasonably high osmium input fluxes to the oceans. It is concluded that a postglacial spike in osmium flux, associated with a meltwater event, must have driven part of the change in seawater composition. However, it is also shown that such a spike cannot be the dominant cause of the most recent shift in seawater 187Os/188Os.
The first osmium isotope data are presented for marine sediments spanning the past 200,000 years ... more The first osmium isotope data are presented for marine sediments spanning the past 200,000 years (two full glacial cycles) and establish that there have been ocean-wide variations in seawater osmium isotope composition over this time. While the osmium isotope composition of sea water has been relatively invariant over much of this interval (187 Os= 186 Os of 8.5 to 8.6), excursions to lower isotope ratios (187 Os= 186 Os ³ 8.1) occur late in the last two glacial periods at 20 ka and 160 ka. These are times of high global aridity, low global temperature and maximum ice extent. While various interpretations are possible, a likely explanation is that the low seawater ratios reflect reduced chemical weathering of the continents during times of extreme cold and aridity. There is no evidence that there is a net enhancement of chemical weathering during periods of continental glaciation. The response of ocean composition to changes in input flux constrain the oceanic residence time of osmium to be less than 12,000 years.
A high resolution record of seawater osmium isotopic composition (35 samples over the past 191,00... more A high resolution record of seawater osmium isotopic composition (35 samples over the past 191,000 yr) has been obtained from sediments from Ocean Drilling Program (ODP) Leg 165 Site 1002 in the Cariaco Basin, Venezuela. Strong correlations are observed between 187 Os/ 188 Os and both the local and global oxygen isotopic records, with less radiogenic values observed during glacial intervals. In conjunction with previous analyses of sediments from the East Pacific Rise, the Santa Barbara Basin, and the Japan Sea, the data demonstrate conclusively that there have been ocean-wide changes in seawater 187 Os/ 188 Os over the Pleistocene. Global seawater excursions to relatively unradiogenic 187 Os/ 188 Os, indicating decreased continental inputs, occur during cold intervals, times of increased aridity and increased ice cover of old (radiogenic) continental masses. The data thus indicate a direct climatic control on chemical weathering rates and/or composition of weathering products on a glacial-interglacial timescale.
A model of the sediment mixed layer is used to quantify the glacial-Holocene changes in calcite p... more A model of the sediment mixed layer is used to quantify the glacial-Holocene changes in calcite preservation (productivity minus dissolution) at water depths that span the transition between lysocline and calcite compensation depth in the equatorial Pacific. The mean preservation fluxes required by the model to account for the observed changes in sediment composition indicate, in most cases, that calcite dissolution exceeds supply and that there has been net removal of calcite from surface sediments over the Holocene. The sediment mixed layer is therefore not at steady state with respect to CaCO3 fluxes. The model is also used to show that an observed increase in the radiocarbon age of the core top sediments with increasing extent of dissolution cannot be explained by chemical erosion of underlying glacial material. Dissolution taking place at or just below the sediment-water interface is the probable cause of this phenomenon.
The dissolution kinetics of three plagioclase feldspars (An,i, Andh, and AnT6) were studied in fl... more The dissolution kinetics of three plagioclase feldspars (An,i, Andh, and AnT6) were studied in flow-through reactors over the pH range 3-7. In accordance with the surface complexation model, dissolution rate was described by the equation
A high resolution record of seawater osmium isotopic composition (35 samples over the past 191,00... more A high resolution record of seawater osmium isotopic composition (35 samples over the past 191,000 yr) has been obtained from sediments from Ocean Drilling Program (ODP) Leg 165 Site 1002 in the Cariaco Basin, Venezuela. Strong correlations are observed between 187Os/188Os and both the local and global oxygen isotopic records, with less radiogenic values observed during glacial intervals. In conjunction with previous analyses of sediments from the East Pacific Rise, the Santa Barbara Basin, and the Japan Sea, the data demonstrate conclusively that there have been ocean-wide changes in seawater 187Os/188Os over the Pleistocene. Global seawater excursions to relatively unradiogenic 187Os/188Os, indicating decreased continental inputs, occur during cold intervals, times of increased aridity and increased ice cover of old (radiogenic) continental masses. The data thus indicate a direct climatic control on chemical weathering rates and/or composition of weathering products on a glacial-i...
Radiocarbon measurements suggest that 14C-free carbon enters from beneath Mono Lake at a rate of ... more Radiocarbon measurements suggest that 14C-free carbon enters from beneath Mono Lake at a rate of about 1 mol/m2/yr. An input of this magnitude should be manifested in the inorganic carbon budget of the lake and with this in mind we have devised a model to reconstruct the evolution of the partial pressure of CO2 (pCO2) over the past 150 years. This encompasses a period (1945 to present) during which major diversions of source waters via the Los Angeles aqueduct have been in effect, significantly increasing the salinity of the lake and hence its pCO2. The model has been constrained by experimental characterization of the carbonate chemistry of the lake water, by the temperature dependence of pCO2 for the lake water, and by pCO2 measurements made on the lake water in 1966, 1969, 1981, and 1989. Our calculations suggest that prior to 1945 the pCO2 of Mono Lake water was about 1.3 times the atmospheric value. To produce this excess, an input of CO2 of about 3.3 mol/m2/yr is required. Volcanic activity beneath the lake is a probable source of this input.
Estimates of osmium residence time in the oceans that are based on oceanic mass balance calculati... more Estimates of osmium residence time in the oceans that are based on oceanic mass balance calculations (35–50 kyr) appear irreconcilable with those inferred from the recent evolution of the osmium isotope composition of seawater (3–4 kyr). It is argued that the osmium budget of the oceans is currently close to steady state and thus that the estimates made by the two methods should agree. As the inventory of osmium in the oceans is relatively well constrained, these disparate residence time estimates imply wildly different osmium input fluxes to the oceans. An osmium residence time of 8–10 kyr is proposed by evaluating the uncertainties and limitations of both methods, and it is argued that osmium inputs to the ocean are currently underestimated by a factor of ∼3. This reflects in part the underestimation of the river input of osmium to the oceans owing to a bias within the existing data set and in part the probable existence of sources of osmium to the oceans that have not yet been identified. The very short residence time of 3–4 kyr inferred from the postglacial change in seawater composition (assuming a single step change in input flux) is rejected as it implies unreasonably high osmium input fluxes to the oceans. It is concluded that a postglacial spike in osmium flux, associated with a meltwater event, must have driven part of the change in seawater composition. However, it is also shown that such a spike cannot be the dominant cause of the most recent shift in seawater 187Os/188Os.
The first osmium isotope data are presented for marine sediments spanning the past 200,000 years ... more The first osmium isotope data are presented for marine sediments spanning the past 200,000 years (two full glacial cycles) and establish that there have been ocean-wide variations in seawater osmium isotope composition over this time. While the osmium isotope composition of sea water has been relatively invariant over much of this interval (187 Os= 186 Os of 8.5 to 8.6), excursions to lower isotope ratios (187 Os= 186 Os ³ 8.1) occur late in the last two glacial periods at 20 ka and 160 ka. These are times of high global aridity, low global temperature and maximum ice extent. While various interpretations are possible, a likely explanation is that the low seawater ratios reflect reduced chemical weathering of the continents during times of extreme cold and aridity. There is no evidence that there is a net enhancement of chemical weathering during periods of continental glaciation. The response of ocean composition to changes in input flux constrain the oceanic residence time of osmium to be less than 12,000 years.
A high resolution record of seawater osmium isotopic composition (35 samples over the past 191,00... more A high resolution record of seawater osmium isotopic composition (35 samples over the past 191,000 yr) has been obtained from sediments from Ocean Drilling Program (ODP) Leg 165 Site 1002 in the Cariaco Basin, Venezuela. Strong correlations are observed between 187 Os/ 188 Os and both the local and global oxygen isotopic records, with less radiogenic values observed during glacial intervals. In conjunction with previous analyses of sediments from the East Pacific Rise, the Santa Barbara Basin, and the Japan Sea, the data demonstrate conclusively that there have been ocean-wide changes in seawater 187 Os/ 188 Os over the Pleistocene. Global seawater excursions to relatively unradiogenic 187 Os/ 188 Os, indicating decreased continental inputs, occur during cold intervals, times of increased aridity and increased ice cover of old (radiogenic) continental masses. The data thus indicate a direct climatic control on chemical weathering rates and/or composition of weathering products on a glacial-interglacial timescale.
A model of the sediment mixed layer is used to quantify the glacial-Holocene changes in calcite p... more A model of the sediment mixed layer is used to quantify the glacial-Holocene changes in calcite preservation (productivity minus dissolution) at water depths that span the transition between lysocline and calcite compensation depth in the equatorial Pacific. The mean preservation fluxes required by the model to account for the observed changes in sediment composition indicate, in most cases, that calcite dissolution exceeds supply and that there has been net removal of calcite from surface sediments over the Holocene. The sediment mixed layer is therefore not at steady state with respect to CaCO3 fluxes. The model is also used to show that an observed increase in the radiocarbon age of the core top sediments with increasing extent of dissolution cannot be explained by chemical erosion of underlying glacial material. Dissolution taking place at or just below the sediment-water interface is the probable cause of this phenomenon.
The dissolution kinetics of three plagioclase feldspars (An,i, Andh, and AnT6) were studied in fl... more The dissolution kinetics of three plagioclase feldspars (An,i, Andh, and AnT6) were studied in flow-through reactors over the pH range 3-7. In accordance with the surface complexation model, dissolution rate was described by the equation
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