doi: 10.1111/ter.12185 Debate Article The null hypothesis: globally steady rates of erosion, weathering fluxes and shelf sediment accumulation during Late Cenozoic mountain uplift and glaciation Jane K. Willenbring and Douglas J. Jerolmack Department of Earth and Environmental Science, University of Pennsylvania, 240 South 33rd Street, Philadelphia, PA 19104-6316, USA ABSTRACT At the largest time and space scales, the pace of erosion and over the last 10 Ma and support the null hypothesis – that glo- chemical weathering is determined by tectonic uplift rates. bal rates of landscape change have remained constant over this Deviations from equilibrium arise from the transient response time period, despite global climate change and mountain build- of landscape denudation to climatic and tectonic perturbations. ing events. This work undermines the hypothesis that increased We posit that the constraint of mass balance, however, makes weathering due to mountain building or climate change was it unlikely that such disequilibrium persists at the global scale the primary agent for a decrease in global temperatures. over millions of years, as has been proposed for late Cenozoic erosion. We synthesize weathering fluxes, global sedimentation Terra Nova, 28: 11–18, 2016 rates, sediment yields and tectonic motions to show a remarkable constancy in the pace of Earth-surface evolution FORUM: The Editors of Terra Nova invite readers to contribute to a discussion of this topic on our forum at https://terranova- debates.wordpress.com/ (Berner, 1991; Berner and Caldeira, rates (Herman et al., 2013). Introduction 1997; Maher and Chamberlain, 2014) Researchers have attempted to cau- The evolution of the Earth’s surface and physical erosion (which causes sally link these trends to global cool- occurs through the erosion of rock unloading that can induce a tectonic ing with two related hypotheses. In and redistribution of mass. This is response; Montgomery, 1994; Whip- the first, rapid erosion associated ultimately driven by energy from two ple, 2009). In order to unravel these with Himalayan uplift acted to drive sources: tectonics, which contributes feedbacks and linkages, many Cenozoic cooling by reducing CO2 to erosion indirectly through uplift; researchers have turned to global levels through increased silicate and climate, which influences erosion datasets of the relevant parameters. weathering (Raymo and Ruddiman, and weathering via temperature and Of particular interest has been the 1992). The second reverses the arrow precipitation. Global changes in tec- late Cenozoic period, when a global of causality: in this case it is argued tonic motions or climate might be cooling trend, of unknown origin, climatic cooling and glacial-intergla- expected to produce global changes began at 50 Ma and culminated in cial swings acted to increase denuda- in the rate of landscape evolution; continental glaciations during the last tion rates, perhaps even inducing however, feedbacks among climate, 2–3 Ma in Eurasia and North and enhanced mountain uplift (Molnar tectonics and geomorphology may South America (Ruddiman, 2010). and England, 1990; Zhang et al., enhance or obscure the relation Global rates of tectonic motion have 2001; Molnar, 2004). between ‘signal’ and ‘response’. For been either constant over this inter- The climatic cooling trend of the example, although the engine for val, as determined from ages of crust late Cenozoic is not in dispute. We plate tectonics is mantle convection, (Fig. 1; Rowley, 2002, 2013), or challenge the premise that global researchers have proposed that rates decreasing, as determined from slab rates of landscape denudation have of tectonic uplift may depend and flux models that incorporate the changed, however, on both concep- feedback on global climate through geometry of the subduction zone tual and empirical grounds. There silicate weathering (which draws (Van Der Meer et al., 2014). Obser- are numerous possibilities for feed- down CO2 and leads to cooling) vations also point to an apparent backs and linkages among climate, increase (c.f. Sadler, 1981) in land- tectonics, erosion and weathering. scape denudation over the same time However, mass balance places a con- Correspondence: Dr. Jane Kathryn Wil- period, as indicated by global sedi- straint on the magnitude and persis- lenbring, Department of Earth and Envi- ment accumulation curves from the tence of denudation rates. Enhanced ronmental Science, University of world’s oceans and sedimentary weathering and erosion reduces Pennsylvania, 240 South 33rd Street, basins (Hay et al., 1988; Molnar, slopes and grows valleys, which act Philadelphia, PA 19104-6316, USA. Tel.: 2004) and, more recently, a global to decrease denudation back towards +001 215 746-8197; e-mail: erosion@sas. compilation of mountain erosion being in balance with tectonic uplift upenn.edu © 2015 John Wiley & Sons Ltd 11 Steady rates of erosion for the last 10 Ma • J. K. Willenbring and D. J. Jerolmack Terra Nova, Vol 28, No. 1, 11–18 ............................................................................................................................................................. Crust production these results for understanding feed- production (km mean rate of crust mean rate backs between climate and geomor- ) phology are discussed. –1 27 3.2 26 3.0 Evidence from geochemical 2 25 2.8 year proxies –1 rate (mm year rate mean half-spreading mean half-spreading 0 ) Recent proxies suggest stabilization 0246810 of atmospheric CO2 at near-modern Age (My) levels since the late Oligocene, despite continued global cooling Fig. 1 Rates of tectonic forcing over the last 10 Ma are steady. Figure adapted (Beerling and Royer, 2011; Fig. 2A). from Rowley (2002, 2013) and data digitized from Rowley (2002; Fig. 4). This stationary greenhouse gas com- position implies nearly constant chemical weathering over the last (Whipple et al., 1999). As with many ple question of how climate impacts 10–20 Ma, unless there were signifi- problems, it is an issue of scale. landscape probably has as many cant changes in the organic carbon Large regions of the globe may sus- answers as there are landscapes. cycle (Pagani et al., 2009) that some- tain denudation rates that are far The simplest, that is, null, hypoth- how compensated for changes in from being in equilibrium with tec- esis is that global rates of erosion, weathering. Several recent hypotheses tonics, but for only limited periods weathering and sediment accumula- attempting to relate global cooling to of time. Alternatively, at-a-point tion have been constant over the the Northern Hemisphere glaciation rates of denudation may be much time interval in question. This con- have therefore abandoned increased larger than tectonic rates over long stancy is expected from the mass weathering fluxes as a cause (Knies periods of time, but are compensated conservation constraint, given that et al., 2014; Woodard et al., 2014). by lower rates elsewhere. To make global rates of tectonic motion have Other proxies brought to bear on this the case for sustained and enhanced not varied over the late Cenozoic issue have ambiguities that limit our denudation rates globally over sev- (Rowley, 2002, 2013). Ultimately, the interpretive ability. In the past, the eral millions of years, one needs to long time-scales needed to affect glo- rising late Cenozoic 87Sr/86Sr record demonstrate that: (1) erosion and bal heat production, and the correla- was thought to be an indicator of weathering rates are somehow decou- tion of mantle viscosity to mantle increased continental weathering pled from tectonics at the largest temperature, preclude any global (Raymo et al., 1988; Capo and scales; and (2) there is a convincing changes in tectonic rates. We do not DePaolo, 1990). However, this record and causal relation between climate have sufficient knowledge of the has been shown to be sensitive to the and denudation rates at the global mechanisms of weathering, erosion, isotopic ratio of the sedimentary scale. Regarding (1), while numerous geodynamics and climate to address source area (Richter et al., 1992) and studies have reported disequilibrium competing hypotheses from a theo- to the proportion of physical to between erosion and tectonics over retical/modelling perspective. Rather, chemical weathering (Derry and shorter timescales, models and data we must rely on empirical findings, France-Lanord, 1996). Similarly, Nd show a convergence of the two over which fall into three main categories. and Pb isotopes indicate only a timescales greater than 106 years The first approach is finding a geo- change in weathering style or source, (Koppes and Montgomery, 2009; chemical proxy that indirectly not a total weathering flux (Willen- Whipple, 2009), although transience records processes such as the flux of bring and von Blanckenburg, 2010a). may also persist for millions of years erosion or weathering products to Two proxies that are thought to (Willenbring et al., 2013). Further, the oceans and integrates over large describe the total weathering flux are for studies that infer denudation areas (continents, ocean basins or the 10Be/9Be ratio and the d7Li from basin-integrating (i.e. spatially global). The second approach is mea- record. 10Be/9Be systematics are rela- averaging) sediment yields, global suring erosion or weathering rates tively simple, thus may finally con- rates do not show any significant (or total denudation rate) in as many strain the flux of weathering trend over timescales longer than