- Email: [email protected]
Isotopic tracers of chemical weathering and consequences for marine geochemical budgets
Applied Geochemistry 26 (2011) S311–S313
Contents lists available at ScienceDirect
Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem
Isotopic tracers of chemical weathering and consequences for marine geochemical budgets D. Vance ⇑ Bristol Isotope Group, Department of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol BS8 1RJ, UK
a r t i c l e
i n f o
Article history: Available online 24 March 2011
a b s t r a c t The mechanical breakdown of rock by physical weathering exerts a significant control on chemical weathering rates because it produces surface area. During periods of icehouse conditions on Earth, the grinding of rock by glacial processes should lead to faster chemical weathering of the continents, perhaps particularly during periods of pronounced climatic variability, like the Quaternary. Evidence is reviewed here for both high and cyclical chemical weathering rates during the Quaternary, and the implications for both marine geochemical budgets and climate-chemical weathering feedbacks are discussed. Ó 2011 Published by Elsevier Ltd.
1. Introduction Chemical weathering of the continental crust is a key Earth System process, being one of the prime mechanisms for the transfer of material between the solid Earth and its more dynamic fluid envelope. For example, drawdown from the atmosphere via chemical weathering of silicate rocks is the principal long-term control (P105 a) on the atmospheric concentration of CO2 (e.g. Walker et al., 1981). Moreover, the resultant flux of dissolved material, fed via rivers, is the main input for many biogeochemically important elements to the oceans. Chemical weathering rates are positively correlated with Earth’s surface temperature and with runoff. The paradigmatic view of the long-term C cycle is thus that chemical weathering and atmospheric CO2 together thermostatically regulate the Earth’s surface temperature via a negative feedback (e.g. Walker et al., 1981). In this view, the climatic impact of any perturbation in the volcanic source of CO2 to the surface Earth is negated by the response of the weathering sink from the atmosphere to changed greenhouse gas loading. Though the relative temporal stability of Earth’s climate on the longest timescales clearly points to the importance of such a feedback, the detailed history of climate and atmospheric chemistry also requires some variation in its efficiency. The rate at which rock is ground up by physical weathering (thus producing surface area) is an additional variable that might be key in explaining this detail. In a popular set of ideas from the 1990s (Raymo and Ruddiman, 1992), this additional control was invoked to explain one of the most prominent features of atmospheric chemistry in recent Earth history, the marked decline in CO2 concentrations during the Cenozoic (e.g. Pagani et al., 2005). These ideas linked
⇑ Tel.: +44 117 954 5418; fax: +44 117 625 3385. E-mail address: [email protected] 0883-2927/$ - see front matter Ó 2011 Published by Elsevier Ltd. doi:10.1016/j.apgeochem.2011.03.090
Cenozoic mountain-building, and the associated high rates of physical erosion, to fast chemical weathering that could potentially weaken the dominant negative feedback. Here, the focus is on another aspect of the link between physical and chemical weathering rates: the potential role of repeated continental glaciation in providing fine-grained substrate. The central hypothesis is that cyclical climate change in the icehouse world of the late Cenozoic, and perhaps during earlier periods of icehouse conditions, leads to faster chemical weathering rates as a result of repeated episodes of fresh sediment production during periods of continental glaciation. In the extreme case, such a linkage could temporarily switch the sign of the chemical weathering-CO2-climate feedback.
2. Temporal dynamism of physical and chemical weathering rates during late Cenozoic climate change It is well-established that glaciation of the continents leads to substantially increased rates of physical erosion at high latitudes (e.g. Bell and Laine, 1985). Related phenomena have also been documented in mid- to low-latitude mountain belts. In the European Alps, records of sedimentation rates in peri-Alpine basins document dramatic changes during the last two glacial cycles (e.g. Hinderer, 2001), with particularly prominent pulses of sediment production at deglaciations as mountain glaciers retreat. At lower latitudes, including the Himalaya and perhaps the Andes, a similarly dramatic deglacial increase in sediment production may be related to both glacial advance and retreat as well as temporallylinked variation in the strength of monsoonal rainfall (e.g. Goodbred and Kuehl, 2000). Laboratory experiments, as well as studies of soil chronosequences (e.g. Taylor and Blum, 1995; White and Brantley, 2003; Porder et al., 2007), demonstrate that chemical weathering rates
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D. Vance / Applied Geochemistry 26 (2011) S311–S313
show a power-law dependence on substrate age, with rates declining by 2–3 orders of magnitude, from high values on first exposure of the substrate, to much lower values over the subsequent 104–105 a. Taken together, all these findings point to the possibility of both generally high and temporally dynamic chemical weathering rates during the Quaternary, because of the repeated physical renewal of landscapes and the re-supply of fresh fine-grained sediment during successive glacial periods. 3. Evidence for temporal dynamism in chemical weathering rates during the late Cenozoic Three approaches are discussed here that provide tests of the importance of the ideas put forward above. 3.1. The isotope geochemistry of modern rivers Early interglacials should see very rapid weathering rates when fresh glacially-ground substrate is first exposed, suggesting that the dissolved load of modern rivers, just 10–15 ka out of the last glacial period, may be perturbed from the long-term average. The dissolved major and trace element chemistry of major rivers is controlled by a complex interplay of processes that are not often easy to tease apart. However, two types of isotopic study have revealed features that are of significance here. Rate information is accessible through U–Th isotopic studies of the dissolved and suspended load of major rivers. A number of such studies have been conducted in the last decade (many summarised in Dosseto et al. (2008)), and have revealed departures from steady state weathering that necessitate a step increase in chemical weathering rate in the past 5–20 ka, a timescale that encompasses the last deglaciation. Importantly, this finding holds for low latitude river systems that drain young mountain belts (like the Andes) that were glaciated during the last glacial cycle. It is also clear that some lowland tropical rivers are close to steady state at the present day, including parts of the Amazon catchment that do not drain the Andean highlands. With reference to other isotope studies, the assessment of the extent of recent disequilibrium is often rendered difficult by uncertainties over the variability of the rock source material. This uncertainty is perhaps least serious for some of the new metal isotope systems where values for rocks are homogeneous and predictable. Thus, a recent study of Mo isotopic compositions of major rivers (Archer and Vance, 2008), and the finding that the dissolved load is universally heavier than rocks, clearly points to isotopic fractionation at some point during weathering and transport. The exact origin of these fractionations, however, and the degree to which they are supportive of significant isotopic disequilibrium during chemical weathering as opposed to transport, is still the subject of ongoing studies of Mo and other isotopes in soils.
composition of the oceans over long timescales. All of these constraints together suggest that the marine isotopic composition of Sr should be evolving to radiogenic values at a rate almost an order of magnitude faster than is observed in real records. A possible solution to this problem that has recently been suggested (Vance et al., 2009) is that the dissolved chemistry of modern rivers does not accurately reflect the long-term flux (i.e. the multi-million year timescale to which Sr in the ocean responds). If correct, this finding is wholly consistent with dynamic Quaternary chemical weathering rates and the suggestion that modern (early interglacial) rivers are delivering more dissolved material to the oceans than over the long-term. The impact on marine Sr isotopes is potentially re-enforced by a further isotopic phenomenon. The same studies of soil chronosequences that established the power-law dependence of chemical weathering rate on substrate age have also shown that early chemical weathering of silicate soils preferentially leaches radiogenic Sr held in the interlayers of biotite (Blum and Erel, 1997), and that isotopic compositions evolve back to the bulk soil value over the same timescales as overall weathering rates decrease. Thus, modern rivers, are not only delivering more Sr to the oceans than the long-term average, but are perhaps also delivering Sr that is more radiogenic. 4. Cyclical changes in the marine isotope geochemistry of short residence time elements The picture presented above also predicts glacial-interglacial cyclicity in marine Sr isotopes, but this is so massively dampened by the large marine reservoir of Sr that it is not resolvable with current analytical techniques. However, this is not the case for the isotopic systems of other elements that have much smaller oceanic inventories and shorter residence times. One such system is Pb. For Pb isotopes, like Sr, soil chronosequence studies have revealed time-dependent incongruent release of isotopes (Harlavan et al., 1998). In this case, early rapid weathering leaches radiogenic Pb from damaged sites in the lattices of U–Th-rich accessory phases, while subsequently the Pb isotopic composition of the weathering products evolve back to the bulk soil. Unlike for Sr, however, the cyclical changes in the marine isotopic composition of Pb that should result are easily discernible if the right location is chosen. Several recent studies (e.g. Foster and Vance, 2006) have documented quite substantial fluctuations in the Pb isotope geochemistry of the deep North Atlantic that are consistent with pulses of radiogenic Pb during the early part of interglacial periods, followed by a slow return to non-radiogenic values as ice builds up again on the continents. Modelling of these isotope variations, and the accompanying changes in absolute weathering rate, has suggested consistency with the idea of dynamic Quaternary chemical weathering rates driven by changes in physical weathering rates. 5. Conclusions
3.2. The strontium isotope budget of the modern and recent oceans Though one of the best-studied isotope systems in the marine realm (see Vance et al. (2009) for a summary), and one that has previously been used extensively in attempts to understand the large scale pattern of chemical weathering rates on the continents, detailed work on the Sr isotope budget of the modern ocean reveals a significant imbalance. The riverine flux of Sr to the oceans and its isotopic composition is known better than any other element. It has also been possible to place increasingly tight constraints on other fluxes of Sr to the oceans, such as the non-radiogenic exchange fluxes at mid-ocean ridges that partially balance the radiogenic flux from the continents. Finally, very good records are available of the secular evolution of the Sr isotopic
This paper has summarised recent findings that all point to the possibility that chemical weathering rates in the late Cenozoic are significantly impacted by fluctuations in the rate of physical rock breakdown. The emphasis above has been on the geochemical tests of this idea that derive from the predicted cyclicity in Quaternary chemical weathering rates. But a corollary of these ideas is that overall rates across several glacial cycles are higher than in the absence of glaciation. In this respect, and though likely to be controlled by numerous other processes, the widespread evidence for departures from steady-state in secular records of the chemistry of the late Cenozoic oceans is significant. Indeed, the start of the Cenozoic rise in marine Sr isotopes occurs at a time of incipient glaciation in another part of the Earth, Antarctica (Zachos et al., 1999).
D. Vance / Applied Geochemistry 26 (2011) S311–S313
It seems feasible, therefore, that the long-term decline in atmospheric CO2 through the Cenozoic might be a consequence of changes in the nature of the chemical weathering-CO2-climate feedback that controls the Earth’s climate through much of its history. References Archer, C., Vance, D., 2008. The global riverine Mo isotope flux and quantitative estimates of anoxia in the ancient global ocean. Nat. Geosci. 1, 597–600. Bell, M., Laine, E.P., 1985. Erosion of the Laurentide region of North America by glacial and glaciofluvial processes. Quatern. Res. 23, 154–174. Blum, J.D., Erel, Y., 1997. Rb–Sr isotope systematics of a granitic soil chronosequence: the importance of biotite weathering. Geochim. Cosmochim. Acta 61, 3193–3204. Dosseto, A., Bourdon, B., Turner, S.P., 2008. Uranium-series isotopes in river materials: insights into the timescales of erosion and sediment transport. Earth Planet. Sci. Lett. 265, 1–17. Foster, G.L., Vance, D., 2006. Negligible glacial-interglacial variation in continental chemical weathering rates. Nature 444, 918–921. Goodbred, S.L., Kuehl, S.A., 2000. Enormous Ganges–Brahmaputra sediment discharge during strengthened early Holocene monsoon. Geology 28, 1083– 1086. Harlavan, Y., Erel, Y., Blum, Y.D., 1998. Systematic changes in lead isotopic composition with soil age in glacial granitic terrains. Geochim. Cosmochim. Acta 62, 33–46.
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Hinderer, M., 2001. Late Quaternary denudation of the Alps, valley and lake fillings and modern river loads. Geodinam. Acta 14, 231–263. Pagani, P., Zachos, J.C., Freeman, K.H., Tipple, B., Bohaty, S., 2005. Marked decline in atmospheric carbon dioxide concentrations during the Paleogene. Science 309, 600–603. Porder, S., Hilley, G.E., Chadwick, O.A., 2007. Chemical weathering, mass loss, and dust inputs across a climate by time matrix in the Hawaiian Islands. Earth Planet. Sci. Lett. 258, 414–427. Raymo, M.E., Ruddiman, W.F., 1992. Tectonic forcing of Cenozoic climate. Nature 259, 117–122. Taylor, A., Blum, J.D., 1995. Relation between soil age and silicate weathering rates determined from the chemical evolution of a glacial chronosequence. Geology 23, 979–982. Vance, D., Teagle, D.A.H., Foster, G.L., 2009. Variable Quaternary chemical weathering rates and imbalances in marine geochemical budgets. Nature 458, 493–496. Walker, J.C.G., Hays, P.B., Kasting, J.F., 1981. A negative feedback mechanism for the long-term stabilization of Earth’s surface temperature. J. Geophys. Res. 86, 9776–9782. White, A.F., Brantley, S.L., 2003. The effect of time on the weathering of silicate minerals: why do weathering rates differ in the laboratory and the field? Chem. Geol. 202, 479–506. Zachos, J.C., Opdyke, B.N., Quinn, T.M., Jones, C.E., Halliday, A.N., 1999. Early Cenozoic glaciation, Antarctic weathering, and seawater 87Sr/86Sr: is there a link? Chem. Geol. 161, 165–180.
Contents lists available at ScienceDirect
Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem
Isotopic tracers of chemical weathering and consequences for marine geochemical budgets D. Vance ⇑ Bristol Isotope Group, Department of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol BS8 1RJ, UK
a r t i c l e
i n f o
Article history: Available online 24 March 2011
a b s t r a c t The mechanical breakdown of rock by physical weathering exerts a significant control on chemical weathering rates because it produces surface area. During periods of icehouse conditions on Earth, the grinding of rock by glacial processes should lead to faster chemical weathering of the continents, perhaps particularly during periods of pronounced climatic variability, like the Quaternary. Evidence is reviewed here for both high and cyclical chemical weathering rates during the Quaternary, and the implications for both marine geochemical budgets and climate-chemical weathering feedbacks are discussed. Ó 2011 Published by Elsevier Ltd.
1. Introduction Chemical weathering of the continental crust is a key Earth System process, being one of the prime mechanisms for the transfer of material between the solid Earth and its more dynamic fluid envelope. For example, drawdown from the atmosphere via chemical weathering of silicate rocks is the principal long-term control (P105 a) on the atmospheric concentration of CO2 (e.g. Walker et al., 1981). Moreover, the resultant flux of dissolved material, fed via rivers, is the main input for many biogeochemically important elements to the oceans. Chemical weathering rates are positively correlated with Earth’s surface temperature and with runoff. The paradigmatic view of the long-term C cycle is thus that chemical weathering and atmospheric CO2 together thermostatically regulate the Earth’s surface temperature via a negative feedback (e.g. Walker et al., 1981). In this view, the climatic impact of any perturbation in the volcanic source of CO2 to the surface Earth is negated by the response of the weathering sink from the atmosphere to changed greenhouse gas loading. Though the relative temporal stability of Earth’s climate on the longest timescales clearly points to the importance of such a feedback, the detailed history of climate and atmospheric chemistry also requires some variation in its efficiency. The rate at which rock is ground up by physical weathering (thus producing surface area) is an additional variable that might be key in explaining this detail. In a popular set of ideas from the 1990s (Raymo and Ruddiman, 1992), this additional control was invoked to explain one of the most prominent features of atmospheric chemistry in recent Earth history, the marked decline in CO2 concentrations during the Cenozoic (e.g. Pagani et al., 2005). These ideas linked
⇑ Tel.: +44 117 954 5418; fax: +44 117 625 3385. E-mail address: [email protected] 0883-2927/$ - see front matter Ó 2011 Published by Elsevier Ltd. doi:10.1016/j.apgeochem.2011.03.090
Cenozoic mountain-building, and the associated high rates of physical erosion, to fast chemical weathering that could potentially weaken the dominant negative feedback. Here, the focus is on another aspect of the link between physical and chemical weathering rates: the potential role of repeated continental glaciation in providing fine-grained substrate. The central hypothesis is that cyclical climate change in the icehouse world of the late Cenozoic, and perhaps during earlier periods of icehouse conditions, leads to faster chemical weathering rates as a result of repeated episodes of fresh sediment production during periods of continental glaciation. In the extreme case, such a linkage could temporarily switch the sign of the chemical weathering-CO2-climate feedback.
2. Temporal dynamism of physical and chemical weathering rates during late Cenozoic climate change It is well-established that glaciation of the continents leads to substantially increased rates of physical erosion at high latitudes (e.g. Bell and Laine, 1985). Related phenomena have also been documented in mid- to low-latitude mountain belts. In the European Alps, records of sedimentation rates in peri-Alpine basins document dramatic changes during the last two glacial cycles (e.g. Hinderer, 2001), with particularly prominent pulses of sediment production at deglaciations as mountain glaciers retreat. At lower latitudes, including the Himalaya and perhaps the Andes, a similarly dramatic deglacial increase in sediment production may be related to both glacial advance and retreat as well as temporallylinked variation in the strength of monsoonal rainfall (e.g. Goodbred and Kuehl, 2000). Laboratory experiments, as well as studies of soil chronosequences (e.g. Taylor and Blum, 1995; White and Brantley, 2003; Porder et al., 2007), demonstrate that chemical weathering rates
S312
D. Vance / Applied Geochemistry 26 (2011) S311–S313
show a power-law dependence on substrate age, with rates declining by 2–3 orders of magnitude, from high values on first exposure of the substrate, to much lower values over the subsequent 104–105 a. Taken together, all these findings point to the possibility of both generally high and temporally dynamic chemical weathering rates during the Quaternary, because of the repeated physical renewal of landscapes and the re-supply of fresh fine-grained sediment during successive glacial periods. 3. Evidence for temporal dynamism in chemical weathering rates during the late Cenozoic Three approaches are discussed here that provide tests of the importance of the ideas put forward above. 3.1. The isotope geochemistry of modern rivers Early interglacials should see very rapid weathering rates when fresh glacially-ground substrate is first exposed, suggesting that the dissolved load of modern rivers, just 10–15 ka out of the last glacial period, may be perturbed from the long-term average. The dissolved major and trace element chemistry of major rivers is controlled by a complex interplay of processes that are not often easy to tease apart. However, two types of isotopic study have revealed features that are of significance here. Rate information is accessible through U–Th isotopic studies of the dissolved and suspended load of major rivers. A number of such studies have been conducted in the last decade (many summarised in Dosseto et al. (2008)), and have revealed departures from steady state weathering that necessitate a step increase in chemical weathering rate in the past 5–20 ka, a timescale that encompasses the last deglaciation. Importantly, this finding holds for low latitude river systems that drain young mountain belts (like the Andes) that were glaciated during the last glacial cycle. It is also clear that some lowland tropical rivers are close to steady state at the present day, including parts of the Amazon catchment that do not drain the Andean highlands. With reference to other isotope studies, the assessment of the extent of recent disequilibrium is often rendered difficult by uncertainties over the variability of the rock source material. This uncertainty is perhaps least serious for some of the new metal isotope systems where values for rocks are homogeneous and predictable. Thus, a recent study of Mo isotopic compositions of major rivers (Archer and Vance, 2008), and the finding that the dissolved load is universally heavier than rocks, clearly points to isotopic fractionation at some point during weathering and transport. The exact origin of these fractionations, however, and the degree to which they are supportive of significant isotopic disequilibrium during chemical weathering as opposed to transport, is still the subject of ongoing studies of Mo and other isotopes in soils.
composition of the oceans over long timescales. All of these constraints together suggest that the marine isotopic composition of Sr should be evolving to radiogenic values at a rate almost an order of magnitude faster than is observed in real records. A possible solution to this problem that has recently been suggested (Vance et al., 2009) is that the dissolved chemistry of modern rivers does not accurately reflect the long-term flux (i.e. the multi-million year timescale to which Sr in the ocean responds). If correct, this finding is wholly consistent with dynamic Quaternary chemical weathering rates and the suggestion that modern (early interglacial) rivers are delivering more dissolved material to the oceans than over the long-term. The impact on marine Sr isotopes is potentially re-enforced by a further isotopic phenomenon. The same studies of soil chronosequences that established the power-law dependence of chemical weathering rate on substrate age have also shown that early chemical weathering of silicate soils preferentially leaches radiogenic Sr held in the interlayers of biotite (Blum and Erel, 1997), and that isotopic compositions evolve back to the bulk soil value over the same timescales as overall weathering rates decrease. Thus, modern rivers, are not only delivering more Sr to the oceans than the long-term average, but are perhaps also delivering Sr that is more radiogenic. 4. Cyclical changes in the marine isotope geochemistry of short residence time elements The picture presented above also predicts glacial-interglacial cyclicity in marine Sr isotopes, but this is so massively dampened by the large marine reservoir of Sr that it is not resolvable with current analytical techniques. However, this is not the case for the isotopic systems of other elements that have much smaller oceanic inventories and shorter residence times. One such system is Pb. For Pb isotopes, like Sr, soil chronosequence studies have revealed time-dependent incongruent release of isotopes (Harlavan et al., 1998). In this case, early rapid weathering leaches radiogenic Pb from damaged sites in the lattices of U–Th-rich accessory phases, while subsequently the Pb isotopic composition of the weathering products evolve back to the bulk soil. Unlike for Sr, however, the cyclical changes in the marine isotopic composition of Pb that should result are easily discernible if the right location is chosen. Several recent studies (e.g. Foster and Vance, 2006) have documented quite substantial fluctuations in the Pb isotope geochemistry of the deep North Atlantic that are consistent with pulses of radiogenic Pb during the early part of interglacial periods, followed by a slow return to non-radiogenic values as ice builds up again on the continents. Modelling of these isotope variations, and the accompanying changes in absolute weathering rate, has suggested consistency with the idea of dynamic Quaternary chemical weathering rates driven by changes in physical weathering rates. 5. Conclusions
3.2. The strontium isotope budget of the modern and recent oceans Though one of the best-studied isotope systems in the marine realm (see Vance et al. (2009) for a summary), and one that has previously been used extensively in attempts to understand the large scale pattern of chemical weathering rates on the continents, detailed work on the Sr isotope budget of the modern ocean reveals a significant imbalance. The riverine flux of Sr to the oceans and its isotopic composition is known better than any other element. It has also been possible to place increasingly tight constraints on other fluxes of Sr to the oceans, such as the non-radiogenic exchange fluxes at mid-ocean ridges that partially balance the radiogenic flux from the continents. Finally, very good records are available of the secular evolution of the Sr isotopic
This paper has summarised recent findings that all point to the possibility that chemical weathering rates in the late Cenozoic are significantly impacted by fluctuations in the rate of physical rock breakdown. The emphasis above has been on the geochemical tests of this idea that derive from the predicted cyclicity in Quaternary chemical weathering rates. But a corollary of these ideas is that overall rates across several glacial cycles are higher than in the absence of glaciation. In this respect, and though likely to be controlled by numerous other processes, the widespread evidence for departures from steady-state in secular records of the chemistry of the late Cenozoic oceans is significant. Indeed, the start of the Cenozoic rise in marine Sr isotopes occurs at a time of incipient glaciation in another part of the Earth, Antarctica (Zachos et al., 1999).
D. Vance / Applied Geochemistry 26 (2011) S311–S313
It seems feasible, therefore, that the long-term decline in atmospheric CO2 through the Cenozoic might be a consequence of changes in the nature of the chemical weathering-CO2-climate feedback that controls the Earth’s climate through much of its history. References Archer, C., Vance, D., 2008. The global riverine Mo isotope flux and quantitative estimates of anoxia in the ancient global ocean. Nat. Geosci. 1, 597–600. Bell, M., Laine, E.P., 1985. Erosion of the Laurentide region of North America by glacial and glaciofluvial processes. Quatern. Res. 23, 154–174. Blum, J.D., Erel, Y., 1997. Rb–Sr isotope systematics of a granitic soil chronosequence: the importance of biotite weathering. Geochim. Cosmochim. Acta 61, 3193–3204. Dosseto, A., Bourdon, B., Turner, S.P., 2008. Uranium-series isotopes in river materials: insights into the timescales of erosion and sediment transport. Earth Planet. Sci. Lett. 265, 1–17. Foster, G.L., Vance, D., 2006. Negligible glacial-interglacial variation in continental chemical weathering rates. Nature 444, 918–921. Goodbred, S.L., Kuehl, S.A., 2000. Enormous Ganges–Brahmaputra sediment discharge during strengthened early Holocene monsoon. Geology 28, 1083– 1086. Harlavan, Y., Erel, Y., Blum, Y.D., 1998. Systematic changes in lead isotopic composition with soil age in glacial granitic terrains. Geochim. Cosmochim. Acta 62, 33–46.
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Hinderer, M., 2001. Late Quaternary denudation of the Alps, valley and lake fillings and modern river loads. Geodinam. Acta 14, 231–263. Pagani, P., Zachos, J.C., Freeman, K.H., Tipple, B., Bohaty, S., 2005. Marked decline in atmospheric carbon dioxide concentrations during the Paleogene. Science 309, 600–603. Porder, S., Hilley, G.E., Chadwick, O.A., 2007. Chemical weathering, mass loss, and dust inputs across a climate by time matrix in the Hawaiian Islands. Earth Planet. Sci. Lett. 258, 414–427. Raymo, M.E., Ruddiman, W.F., 1992. Tectonic forcing of Cenozoic climate. Nature 259, 117–122. Taylor, A., Blum, J.D., 1995. Relation between soil age and silicate weathering rates determined from the chemical evolution of a glacial chronosequence. Geology 23, 979–982. Vance, D., Teagle, D.A.H., Foster, G.L., 2009. Variable Quaternary chemical weathering rates and imbalances in marine geochemical budgets. Nature 458, 493–496. Walker, J.C.G., Hays, P.B., Kasting, J.F., 1981. A negative feedback mechanism for the long-term stabilization of Earth’s surface temperature. J. Geophys. Res. 86, 9776–9782. White, A.F., Brantley, S.L., 2003. The effect of time on the weathering of silicate minerals: why do weathering rates differ in the laboratory and the field? Chem. Geol. 202, 479–506. Zachos, J.C., Opdyke, B.N., Quinn, T.M., Jones, C.E., Halliday, A.N., 1999. Early Cenozoic glaciation, Antarctic weathering, and seawater 87Sr/86Sr: is there a link? Chem. Geol. 161, 165–180.