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Long-term stability of global erosion rates and


Vol 465 | 13 May 2010 | doi:10.1038/nature09044

LETTERS
Long-term stability of global erosion rates and weathering during late-Cenozoic cooling
Jane K. Willenbring1 & Friedhelm von Blanckenburg1
Over geologic timescales, CO2 is emitted from the Earth’s interior and is removed from the atmosphere by silicate rock weathering and organic carbon burial. This balance is thought to have stabilized greenhouse conditions within a range that ensured habitable conditions1. Changes in this balance have been attributed to changes in topographic relief, where varying rates of continental rock weathering and erosion1,2 are superimposed on fluctuations in organic carbon burial3. Geological strata provide an indirect yet imperfectly preserved record of this change through changing rates of sedimentation1,2,4. Widespread observations of a recent (0–5-Myr) fourfold increase in global sedimentation rates require a global mechanism to explain them4–6. Accelerated uplift and global cooling have been given as possible causes2,4,6,7, but because of the links between rates of erosion and the correlated rate of weathering8,9, an increase in the drawdown of CO2 that is predicted to follow may be the cause of global climate change instead2. However, globally, rates of uplift cannot increase everywhere in the way that apparent sedimentation rates do4,10. Moreover, proxy records of past atmospheric CO2 provide no evidence for this large reduction in recent CO2 concentrations11,12. Here we question whether this increase in global weathering and erosion actually occurred and whether the apparent increase in the sedimentation rate is due to observational biases in the sedimentary record13. As evidence, we recast the ocean dissolved 10Be/9Be isotope system as a weathering proxy spanning the past 12 Myr (ref. 14). This proxy indicates stable weathering fluxes during the lateCenozoic era. The sum of these observations shows neither clear evidence for increased erosion nor clear evidence for a pulse in weathered material to the ocean. We conclude that processes different from an increase in denudation caused Cenozoic global cooling, and that global cooling had no profound effect on spatially and temporally averaged weathering rates. Studies of both the suspended and the dissolved loads of the world’s largest rivers and of hill-slope denudation have shown a strong link between physical erosion and chemical weathering8,9. Even though the exact mechanisms linking physical erosion rates with chemical weathering fluxes are still unknown, there is general agreement that high rates of physical erosion supply fresh mineral surfaces to the weathering environment9. Steep mountain slopes such as those in areas of active uplift often have the highest rates of total denudation. Thus, tectonically active areas should be coupled with large silicate weathering fluxes2,4, and the atmospheric CO2 withdrawn in this way should then be disposed of in the oceans’ carbonate sediments1. Similarly, in basins surrounding active mountain belts, a substantial fraction of atmospheric CO2 is sequestered through the terrestrial biosphere in the form of buried particulate organic carbon3. The consequences of the suggested fourfold global increase in Pliocene–Quaternary sedimentation rates5,6 and, by inference, erosion rates should be associated with a similar increase in silicate weathering, carbonate sedimentation, organic carbon burial and, consequently, increased CO2 drawdown (Fig. 1).
1

Atmospheric CO2 concentrations derived from ocean palaeo-pH and stomatal indices do not testify to a significant decrease over this period. Concentrations were around 300 parts per million by volume (p.p.m.v.) during the Pliocene and Miocene epochs11. The significant drop from ,1,000 p.p.m.v. occurred long before the apparent increase in erosion. In particular, during the Pliocene and the Quaternary period, the fourfold increase in erosion was accompanied by only a minor drop in atmospheric CO2 (refs 11, 12; Fig. 1). One proposed source of abated CO2 drawdown during the past 12 Myr is a feedback caused by land plants that accelerated chemical weathering, attenuated long-term CO2 concentration changes and prevented a transition into ice-house conditions during this apparent increase in erosion15. Another hypothesis is that chemical weathering and physical erosion are not linked in as straightforward a way as previously
1,000 900 800 25 Atmospheric CO2 (p.p.m.) 700 600 500 15 400 300 200 5 100 0

30 Terrigenous sediment mass (1018kg)

20

10

0

0

5

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15 Age (Myr)

20

25

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Figure 1 | Terrigenous sediment input into the oceans through the lateCenozoic era and atmospheric CO2. Yellow bars show global terrigenous sediment accumulation in oceans5. Values are separated into bins with an interval of 5 Myr and appear to increase abruptly during the past 5 Myr. The CO2 data compilation is derived from a number of independent proxies and shows steady atmospheric concentrations from the mid-Miocene epoch to today (pre-industrial values) despite the observed increase in terrigenous sediment accumulation. Records include the stable boron isotopes in planktonic foraminifera (purple band), the stomatal distribution in the leaves of C3 plants (green band), the stable carbon isotopes in alkenones (blue band) and air trapped in ice cores from Antarctica (thin black line). Each colour band spans the associated data points and their uncertainties. See Supplementary Information for additional details, uncertainties and references.

Deutsches GeoForschungsZentrum GFZ, Section 3.4: Earth Surface Geochemistry, Telegrafenberg, D-14473 Potsdam, Germany.

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LETTERS

NATURE | Vol 465 | 13 May 2010

thought. For example, in quickly eroding Himalayan-style environments, frequent, high-magnitude mass-wasting events decrease the soil cover and potentially cause lowered rates of silicate weathering16. Yet this plug of unweathered detritus and terrestrial carbon could still contribute to global CO2 drawdown, depending on the contribution
a 12
10 Mass/time (1016kg Myr–1) 8 6 4 2 0 0 20 40 Age (Myr) 60 80 10
Global river loads

Oceans

100

Mass/time = 15 × age–0.5 R2 = 0.86

1

1

10

100

b 12
Volume/time (104km3 Myr–1)

Alps

100 10 1

Volume/time = 7.9 × age–0.5 R2 = 0.6 Cosmogenic nuclide data

9

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0.1

0.1

1

10

100

3

0 0 10 20 Age (Myr) 30 40

c
Volume/time (104km3 Myr–1)

4

Asia

10

Volume/time = 2.2 × age–0.5 R2 = 0.95

3 1 2

0.1 0.1

1

10

100

of silicate mineral dissolution to CO2 drawdown that may take place in the basins within and surrounding mountain belts. Variations in the silicate weathering flux have so far been difficult to quantify using sedimentary and isotope proxies2. The additional CO2 flux from the burial of recent organic matter in the depositional basins surrounding active mountain belts, however, has been quantified using the stable isotopes of carbon measured in marine carbonates deposited over time3. Even though these reconstructions do not detect the absolute rates of organic carbon erosion and burial, an increase in the supply and burial of non-fossil carbon from young biomass that might have accompanied the apparent late-Cenozoic fourfold increase in erosion in basins experiencing high rates of terrigenous erosion and sedimentation17 would show up as a similar increase in the size of the reservoir. However, at least for the past few million years, such an increase in the burial of young biomass is not apparent3. The impetus for the suggested global link between erosion and Quaternary global climate change described above stems from widespread observations that the mass of sediment and rock accumulated in oceans in the past 5 Myr is substantially larger than that accumulated in any previous interval4–7 (Fig. 1). However, we suggest here that this synchronous increase may be an artefact introduced by observation and measurement biases. First, sedimentation rates today are usually measured where deposition is ongoing and rates are currently high. Second, the timescale over which rates are measured has a profound effect on measured rates10,13. Observations of real changes in accumulation rate through geologic time, such as the postulated fourfold rate increase over the past 5 Myr, require that, once deposited, sediment is completely preserved through geological time. Even after correcting for compaction of the lower sections of sedimentary cores, this is rarely the case because of the resulting decreased probability of preservation with age10,13,18. Hence, apparent deposition rates depend on the timescale of the measurement. Third, owing to the vagaries of chronological control through geological time, the timescale over which accumulation is measured is positively related to the age of the strata18. As such, rates are rarely measured over a constant time interval throughout a stratigraphic section and tend to decrease with geological age13. Fourth, given sufficient time, even depositional basins may be inverted and the reverse process (erosion of sediment) observed, leading to progressively decreasing strata volumes. Figure 2 investigates this relationship for large temporal and spatial scales such as ocean deposits5 (Fig. 2a), sedimentary deposits surrounding the Alps19 (Fig. 2b), India20 (Fig. 2c) and global recentto-Cambrian rock volumes21 (Fig. 2d). The decrease in deposition rate with geological time is apparent and follows the power law rate~constant|timec{1 ?1? which has a slope of c 2 1 in logarithmic coordinates13. In all these cases (Fig. 2), the power coefficient is 20.5 (c 5 0.5); together they

1

0 0 10 20 30 Age (Myr) 40 50

d 80
70 Denudation rate (m Myr–1) 60 50 40 30 20 10 0 0

World

1,000 100 10 1 0.1

Denudation rate = 76 × age–0.5 R2 = 0.82

10

1,000

100

200 300 Age (Myr)

400

500

Figure 2 | Sediment accumulation rates and erosion rates as functions of geological time. Four representative environments in which erosion and sedimentation rates change with time. Values are plotted at age midpoints. Insets, same data plotted on a log–log scale. a, Global values for ocean basin sediment accumulation5. The ellipse in the inset shows the modern riverine flux range22, which matches that expected for the flux plotted on a ,105–106yr timescale. b, Volumetric erosion rates for the past 10 Myr from the European (Eastern and Western) Alps19. Rates were estimated from measurements of sediment accumulation in basins around the Alps and were corrected for compaction. In the inset, average cosmogenic isotope measurements23 provide an estimate for recent millennial-scale denudation rates. c, Mass accumulation rates from 18 mostly offshore sedimentary basins in Asia after the initial India–Asia collision20. d, Global Pliocene-toCambrian land and ocean accumulations (by volume)21 and Quaternary ocean accumulation5. All rock volumes were translated to denudation rate by normalizing for the area of continents that were exposed above sea level at that time22. All four environments share a common exponent of 20.5 (equation (1)), which is consistent with processes of stochastic deposition and erosion within the sedimentary deposits10,13.

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NATURE | Vol 465 | 13 May 2010

LETTERS

show similar scaling that spans four orders of magnitude and that is consistent with random net deposition, hiatus and erosion of sediment such that the net accumulation rate is the sum of all surface change divided by its age13. An exponential decrease in rates with time would explain much—but not all—of the observed recent (0–5-Myr) increase in terrigenous ocean sediment5–7. An exponential decrease, however, only takes into account fractional destruction of exposed rock area with time18; stochastic sediment transport and decreased preservation with age is described best by a power law (equation (1))13. In the logarithmic presentation (Fig. 2), the interception with the y axis provides an erosion rate on a timescale that is so close to modern times that recently measured rates potentially offer an independent benchmark with which this interpolated rate can be compared. Recent advances in our ability to determine modern and lateHolocene rates of erosion provide a wealth of such erosion rates. Indeed, these curves appear to intersect modern erosion rates of 40–70 m Myr21 obtained from river load data22 (Fig. 2a). In the Alps, where a large number of rates have been measured from basin accumulation19, cosmogenic isotopes and lake infills23, remarkable agreement exists between time-averaged and more recent rates (Fig. 2b), given this power-law relationship. The implication of this agreement is that even if these mountains are eroding at a rapid modern pace, they may have been doing so for the past 10–20 Myr. A common assumption remains, namely that Quaternary glaciation increased the global delivery of sediment4,7. However, even in settings where glaciers can efficiently erode and transport material, a growing body of work demonstrates that only parts of the margins of such continents or orogens made of weak, friable bedrock formerly covered with ice were eroded significantly during glaciations24,25. The interiors of Northern Hemispheric ice sheets, analogous to the modern East Antarctic ice sheet, were often frozen to the underlying substrate and acted as a protective cover during the majority of the glacial intervals even near the southern margin of the Laurentide ice sheet24,25. In those cases in which ice did produce thick piles of glacial debris, these glacial deposits were often recycled from previous glaciations26 and contributed little to the net delivery of sediment to the oceans. The hypothesis of a lack of a recent increase in the global erosion rate and the inferred suggestion of steady silicate weathering rates requires an independent test not compromised by timescale issues. Geochemical proxies for global weathering rates potentially provide such a test2. However, radiogenic isotopes (Sr, Nd, Hf, Os and Pb) as measured in ocean sediment time series are proxies for many processes related to denudation style and source-rock isotope composition but are not necessarily good indicators of the weathering flux magnitude2,26. We suggest here that ocean records of the ratio of the sea water’s dissolved stable isotope 9Be, derived from continental denudation, to the sea water’s dissolved constant-flux meteoric isotope 10Be (ref. 27) show no increase in weathering flux over the past 12 Myr. This decay-corrected 10Be/9Be ratio remained constant over the past 12 Myr when measured in chemical ocean deposits such as Fe–Mn crusts precipitated from sea water or the authigenic phase of deep-sea sediments14 (Fig. 3). Both faithfully record the sea water’s dissolved 10Be/9Be ratio at the time of precipitation. Most of the flux of cosmogenic 10Be to the ocean has a direct atmospheric origin. Although variations in geomagnetic field strength and changes in solar modulation produce fluctuations in its flux, these are averaged out over the sampling intervals in the data sets14,28 such that the flux over the oceans is roughly constant at 1 3 106 atoms cm22 yr21. Fluvial erosion also adds 10Be to the oceans, but this flux is also roughly constant at steady state between production and removal by either erosion or radioactive decay, regardless of the denudation rate27. Hence, continental erosion is unlikely to introduce long-term variations in the ocean’s 10Be budget. In contrast, 9Be in the ocean has a terrestrial origin, with most derived from fluvial inputs to the oceans27–29 and an insignificant portion from dust (see Supplementary Information for the full

a

2

1.5 1 Initial 10Be/9Be (10–7) 0.5 0 0 2 4
Modern Pacific Ocean range

b
2 1.5 1 0.5 0 0 2 4

6 Age (Myr)

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Modern Atlantic Ocean range

6 Age (Myr)

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Figure 3 | Palaeo-ocean dissolved 10Be/9Be ratios as weathering proxies. Data from Pacific Ocean marine cores (RC12-65, violet circles) and hydrogenous ferromanganese crusts (F7-86-HW/CD29-2, blue diamonds; F10-89-CP/D11-1, white squares; VA13-2/KD237, green triangles; Nova IX/ D137-01, grey squares; F10-89-CP/D27-2-1, brown/green diamonds) (a) and from the Atlantic Ocean hydrogenous ferromanganese crusts (ALV-539, yellow triangles; BM-1969.05, orange squares) and the Arctic Ocean marine core (ACEX, pink circles) (b). See Supplementary Information for data and references. Most individual measurements have a maximum 2-s analytical error of ,10%, with lower uncertainty for recent measurements. Several outlier measurements within the data set have greater uncertainties. Because 10 Be decays with a half-life of 1.39 Myr, the original 10Be/9Be ratio at the time of deposition was calculated by assuming constant crust growth rates or sediment accumulation rates and correcting for decayed 10Be for each sample interval14. An apparent circularity in this approach can be discounted because 10Be-derived Pacific Fe–Mn crust growth rates agree with crust growth rates from Os isotope stratigraphy; decay-corrected Pacific Fe–Mn crust 10Be/9Be ratios agree with the corrected deep-sea core RC12-65 10 Be/9Be ratios, where magnetostratigraphy yields an independent age estimate; and ratios agree with each other within an ocean basin and also with young Fe–Mn surfaces within these basins28. These high-fidelity records of dissolved 9Be and meteoric cosmogenic 10Be in the open oceans imply ratios that fluctuate about a mean of 1 3 1027 for Pacific sites and 0.5 3 1027 for Atlantic and Arctic sites28 (shown as horizontal bands shaded blue in a and pink in b). A fourfold increase in the recent flux of 9Be-bearing terrigenous material (Fig. 1) would cause a decreasing trend in the ratio towards recent time, which is not observed in the three ocean basins sampled through time here. See Supplementary Information for additional details.

quantification of this mass balance). Initially, 9Be accumulates in soils from partial dissolution of silicate minerals that host ,2 p.p.m. 9Be and either binds to particles or remains in the dissolved form. The total amount of 9Be that is either adsorbed to suspended particles in rivers or is transported in the dissolved form is directly dependent on the weathering extent of the source rock and river chemistry27,29. Once particles enter the surface ocean, a fraction of the 9Be they carry is available for partial redissolution. If particles and the 9Be they adsorbed are buried in marine sediment, some of the 9Be is recycled back into deep water during early diagenesis. In each ocean basin, continental 9Be is transferred from its fluvial point source by circulating ocean surface gyres that also carry 10Be. These gyres rapidly homogenize the 10Be and 9Be to a characteristic isotope ratio on an ocean basin scale30. Importantly, scavenging of the element by particles affects both isotopes equally30, such that variations in productivity or removal efficiency do not change the 10 Be/9Be ratio, which represents only the differences in the delivery of terrigenous 9Be to the deep ocean. Consequently, the 10Be/9Be ratio is lower in both the Atlantic and Arctic ocean basins than in the Pacific Ocean, owing to the larger ratio of coast length and 9Be input to ocean area in the Atlantic, but both ratios were relatively
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NATURE | Vol 465 | 13 May 2010

stable throughout the past 12 Myr (Fig. 3a, b). This stability demonstrates that the delivery of 9Be derived from silicate weathering remained constant over this period when averaged over the millionyear sampling interval of the Fe–Mn crusts. We have provided evidence that on a global scale the chemical weathering flux was essentially constant over the past 10 Myr, and have found a mechanism that supports our questioning of observations of large increases in global physical erosion over the same interval. Therefore, we suggest that neither global erosion nor chemical weathering have been increased in any straightforward manner by climate change over the long (million-year) averaging timescale of our analysis, and that any simultaneous pulses in mountain building did not change the erosion or weathering flux globally. Hence, pulses in mountain uplift over this period might have been neither a direct cause nor an inevitable consequence of climate change4.
Received 7 August 2009; accepted 22 March 2010.
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9. 10.

Supplementary Information is linked to the online version of the paper at www.nature.com/nature. Acknowledgements J.K.W. gratefully acknowledges an Alexander von Humboldt Postdoctoral Fellowship. Author Contributions J.K.W. and F.v.B. contributed equally to every aspect of the study. Author Information Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. Readers are welcome to comment on the online version of this article at www.nature.com/nature. J.K.W. is at the Department of Earth & Environmental Sciences, University of Pennsylvania, from July 2010. Correspondence and requests for materials should be addressed to F.v.B. (fvb@gfz-potsdam.de) or J.K.W. (jane.willenbring@sas.upenn.edu).

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