Marine geochemical budgets, chemical weathering, and the carbon cycle

Marine geochemical budgets, chemical weathering, and the carbon cycle in the ice-house world of the Quaternary Derek Vance Bristol Isotope Group (BIG), Department of Earth Sciences, University of Bristol and: Marcus Gutjahr, Andrew Keech, Florian Kurzweil, Jörg Rickli Gavin Foster and Damon Teagle (Southampton), Alan Matthews (Jerusalem), Joel Blum (Michigan)

Outline: 1. Big picture – a hypothesis: Chemical weathering-CO2 feedback in an icehouse world • A consequence and test of the hypothesis: • Sr isotopes in the oceans – long-standing budget problems 3. Other consequences and tests: (a) River chemistry (b) Marine isotope records of short(er) residence time elements (e.g. Pb, Os) 4. Back to Sr isotopes: Can island arcs and ocean islands explain the budget problems instead?

The natural “long”-term carbon cycle Chem Chemical weathering of silicate rocks Ocean-Atmosphere-Biosphere Total carbon = 41,000 Gt Cycled between reservoirs very quickly (geologically-speaking) Atmosphere (597) 120 70 90.2 119.6 70.6 101 Terrestrial biota, soils (2300) Surface ocean (900) 11 Deep ocean (37,100) Volcanism Corg burial Metamorphism Earth’s crust Organic sediments and fossil fuels: 10,000,000 Sedimentary carbonates: 50,000,000 Subduction Earth’s mantle

CO2 feedbacks in a cooling climate Feedbacks  surface temperature varies within narrow bounds Ruddiman (2001)

Controls on chemical weathering rates West et al. (2005)

Atmospheric CO2 over the past ~50 million years Pagani et al. (2009)

Northern Hemisphere Ice Caps at the Last Glacial Maximum e.g. sediment supply to sedimentary basins around Laurentice ice-sheet estimated as an order of magnitude higher than in the absence of the ice-sheet (Bell and Laine 1985)

Consequences for chemical weathering fluxes Vance et al. (2009) Nature

Vance et al. (2009) Nature Modeled high-latitude chemical weathering fluxes High deglacial physical weathering rates in mountain belts G-B and Alpine data from Goodbred and Kuehl (2000); Hinderer (2001)

Modern rivers and budget problems in the oceans - 87Sr 87Sr/86Srseawater Time (Myr) compiled from many literature sources If the modern global riverine flux of Sr, and its isotope ratio, is representative of the long-term flux then: the high-T hydrothermal water flux has to be of the order of 1014 kg yr-1

Estimates of unradiogenic Sr exchange by hydrothermal alteration at ridges and on ridge flanks Davis et al. (2003)

balancing riverine budgets Sr8,9 S in oceanic crust6 Tl isotopes in oceanic crust5 Li isotopes in oceanic crust4 Sr isotopes in oceanic crust1,3 Thermal constraints1,2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Independent estimates of the hydrothermal flux from ridges: High-T hydrothermal water flux (1014 kg yr-1) Sources: 1Davis et al. (2003) 2Mottl (2003) 3Teagle et al (2003) 4Chan et al. (2002) 5Nielsen et al. (2006) 6Teagle et al. (1998) 7 Alt and Teagle (1999) 8Palmer and Edmond (1989) 9this study 10Holland (2005) 11Tipper et al. (2006)

87Sr/86Srseawater Time (Myr) Modern rivers and budget problems in the oceans - 87Sr Best estimates of all fluxes imply evolution of Sr isotopes 8 times faster than observed

Vance et al. (2009) Nature Modeled high-latitude chemical weathering fluxes High deglacial physical weathering rates in mountain belts G-B and Alpine data from Goodbred and Kuehl (2000); Hinderer (2001)

Potential for isotopic disequilibrium in modern soils Sr release from Wyoming soils (Blum and Erel 1997) These Precambrian soils: release of Sr today with 87Sr/86Sr 0.9% higher than whole rock Younger terrains (young mountain belts): release of Sr with 87Sr/86Sr 0.05-0.2‰ higher than whole rock

Consequences for oceanic Sr isotope budget Data (filled squares) from Henderson et al. (1994) Vance et al. (2009) Nature Variable shelf flux (Stoll and Schrag 1998): Simulated as 10 times increase in the diagenetic flux for 70% (“glacial”) of a G-IG cycle 87Sr/86Sr = 0.7084

Isotopic disequilibrium in modern rivers Mackenzie river basin Vigier et al. (2001) - U-Th isotopes Chemical weathering 2-11 times faster now than glacial “Age” of chemical weathering = 9-28 kyr

Isotopic disequilibrium in modern rivers Dosseto et al. (2006) - U-Th isotopes - Amazon drainage basin Chemical weathering has been out of steady-state for 4-20 kyr in Andean rivers But, importantly, lowland rivers in steady-state

Mo isotopes: 92Mo 14.84% 94Mo 9.25% 95Mo 15.92% 96Mo 16.68% 97Mo 9.55% 98Mo 24.13% 100Mo 9.63% Variation quoted as: 98/95Mo = [(98Mo/95Mosample/ 98Mo/95Mostd ) - 1]x1000

Isotopic disequilibrium in modern rivers – Mo isotope data Ottawa (St. Lawrence) Volga Chang Jiang Kalix Global riverine flux d98/95Mo ~ 0.7 ‰ Brahmaputra d98/95Mo (‰) Amazon Clear Creek (CO, USA) 1/Mo(nM-1) Archer and Vance (2008) Nature Geoscience

What controls Mo isotopes in rivers? SE England estuary d98/95Mo (‰) 1/Mo(nM-1) Archer and Vance (2008) Nature Geoscience

Consequences for oceanic Sr isotope budget Data (filled squares) from Henderson et al. (1994) Vance et al. (2009) Nature Variable shelf flux (Stoll and Schrag 1998): Simulated as 10 times increase in the diagenetic flux for 70% (“glacial”) of a G-IG cycle 87Sr/86Sr = 0.7084

Huge inertia in oceanic Sr budgets Residence time - and response time - several million years

Pb in the oceans (from Frank 2000) Essentially all from the continents Pb tres = 30 years; N. Atlantic water residence time - 100s years

BM1969 Alvin 539 Less Radiogenic 18.8 18.9 19 206Pb/204Pb Reynolds et al. (1999) 0.70885 19.1 19.2 0.70895 More Radiogenic 19.3 87Sr/86Sr 0.70905 Hodell et al. (1989, 1990) 2 0.70915 3 d18O 4 5 0 1 2 3 4 5 6 Zachos et al. (2001) Time (Ma) Low resolution Pb isotopic data from Fe-Mn crusts

0.82 1.00 ~WR 0.81 0.95 WR 0.8 207Pb/206Pb soil 0.90 207Pb/206Pb soil 0.79 0.85 0.78 0.80 0.77 Wind River, ~2.8 Ga granodiorite From: Harlavan et al. (1999) Sierra Nevada, Cretaceous granodiorite From: Harlavan et al. (1999) 0.75 0.76 0.75 0.70 0 100 200 300 400 0 200 400 600 800 1000 Soil Age (ka) Soil Age (ka) Potential for isotopic disequilibrium in modern soils Weak acid leaching of a granitoid (Erel et al. 2004) Pb isotopes Uranogenic (radiogenic) Pb has: High 206Pb/204Pb Low 207Pb/206Pb These two ratios very tightly correlated

BM1969.05 (1800m) ALVIN539 (2665m) TR079 D-14 (2000m)

Zachos et al. (2001) BM1969.05 ALV 539A ALV 539B Laser ablation Pb isotopes in the deep North Atlantic - Foster and Vance (2006) Nature glacial conditions unradiogenic Pb

Modelled Pb isotopes in the deep North Atlantic (Foster and Vance, 2006 Nature)

Pb isotope records in the deep North Atlantic (from Bristol) Blake Ridge: Gutjahr et al. (2009) EPSL Laurentian Fan: Kurzweil et al. (2010) EPSL

Os isotope records from the Quaternary oceans 187Re 187Os Behaves (in a basic sense) like radiogenic Sr isotopes: continents enriched in parent 187Re Burton et al. (2010) Higher continental input From known inputs and inter-basin similarity of Os isotopes Os residence time in oceans = 35-40 kyr

Do measurements of modern rivers tell us the modern chemical weathering flux to the oceans? Based on 24 rivers. 96 rivers in Vance et al. (2009) Average riverine 87Sr/86Sr ~ 0.7114 The simple mass balance approach is wrong. The oceans are demonstrably not in steady-state for Sr isotopes. Allegre et al. (2010)

Do measurements of modern rivers tell us the modern chemical weathering flux to the oceans? The mass balance proposed as a solution requires a 40% contribution at ocean islands from sub-surface “hydrothermal groundwater” with Sr characteristics like the bottom figure Is this likely to be close to reality? Why consider “groundwater” sources to the oceans at ocean islands and ignore them everywhere else? These are important elsewhere too, and elsewhere they are radiogenic – like rivers. Allegre et al. (2010)

An argument based on Nd isotopes eNd: deviations of 143Nd/144Nd due to decay of 147Sm to 143Nd Old continental crust has negative eNd Continental crust recently extracted from mantle has positive eNd Like Pb, for Nd the only important source is the continents Their 24 rivers have eNd = -12 Average oceanic eNd = -7 Fix by adding additional mantle-like Nd with eNd =+6 But rivers aren’t -12 in eNd- just as rivers are not 0.7136 for Sr isotopes. The (very small) database is biased for both. Completely ignores a now well-established additional source of Nd to the oceans: isotopic exchange between sediments and water at ocean margins (many papers by Catherine Jeandel and others)

Yet another isotope system – stable Sr isotopes 84Sr ~0.56% 86Sr ~9.86% 87Sr ~7.00% 88Sr ~82.58% Only 87Sr is radiogenic – contributed to by radioactive decay of 87Rb Like Mo, there are mass-dependent variations in all isotope ratios In doing a conventional radiogenic 87Sr/86Sr analysis we eliminate these by normalising all analyses to a single 88Sr/86Sr ratio But, as with Mo, there is extra information in the mass-dependent variations

New data for stable Sr isotopes in the oceans (Krabbenhoft et al. 2010)

Take home messages The modern global riverine flux is not a good measure of the long-term chemical weathering flux from the continents. Chemical weathering is playing catch-up with the massive production of surface area during the last glacial Non-steady-state weathering processes can solve the marine Sr isotope budget Quantitative tests on the extent of departure from steady-state weathering from the marine budgets of other elements with isotope systems Does the negative atmospheric CO2-chemical weathering feedback weaken (perhaps even change sign) in an icehouse world?

Marine geochemical budgets, chemical weathering, and the carbon cycle