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Erosion-driven drawdown of atmospheric carbon dioxide: The organic pathway
Applied Geochemistry 26 (2011) S285–S287
Contents lists available at ScienceDirect
Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem
Erosion-driven drawdown of atmospheric carbon dioxide: The organic pathway Niels Hovius a,⇑, Albert Galy a, Robert G. Hilton b, Robert Sparkes a, Joanne Smith a, Kao Shuh-Ji c, Chen Hongey d, Lin In-Tian d, A. Joshua West e a
Department of Earth Sciences, University of Cambridge, United Kingdom Department of Geography, University of Durham, United Kingdom c Research Centre for Environmental Changes, Academia Sinica, Taipei, Taiwan d Department of Geosciences, National Taiwan University, Taipei, Taiwan e Department of Earth Sciences, University of Southern California, Los Angeles, USA b
a r t i c l e
i n f o
Article history: Available online 13 April 2011
a b s t r a c t Rapidly eroding, coastal mountain belts, where steep rivers and submarine channels connect upland sources to nearby marine sinks are hotspots of organic carbon transfer from life biomass, soil and exhumed bedrock into geological storage. Using observations from the Southern Alps of New Zealand, and Taiwan, we have mapped this organic pathway to geological carbon sequestration, and can evaluate the magnitude and efficiency of transfers between sources and sinks. We demonstrate that POC is harvested by landsliding, but importantly also by common and widespread surface runoff on steep hillslopes. Although terrestrially sourced POC is found in many sedimentary environments associated with mountain belts and frontier basins, it appears to be most abundantly trapped and preserved in marine turbidites. The loss of all forms of POC in onward transport through short, steep routing systems to this repository is limited. This is in marked contrast to larger routing systems, in which only the most resilient forms of POC survive into long-term deposition. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The burial of particulate organic C (POC) in marine sediments represents the second largest geological sink of atmospheric CO2 after carbonate deposition induced by excess alkalinity due to continental silicate weathering (Berner, 1990). Approximately half of this organic C is derived from the terrestrial environment (Schlünz and Schneider, 2000; Burdige, 2005), where high yielding areas coincide with active mountain belts at low and intermediate latitudes (Bowman et al., 2009). It is becoming increasingly clear that in these orogenic settings significantly more C may be implied in the erosion and burial of non-fossil POC than in concomitant silicate weathering (Galy et al., 2007). In this presentation mechanisms and rates of harvesting, routing and burial of POC from active mountain belts will be examined, addressing five questions.
2. How much non-fossil POC is mobilized by mass wasting in mountain belts? Where rock uplift and fluvial incision outpace weathering, bedrock landsliding is the principal erosion mechanism. Landslides ⇑ Corresponding author. Address: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, United Kingdom, Tel.: +44 1223 333453; fax: +44 1223 333450. E-mail address: [email protected] (N. Hovius). 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.03.082
harvest organic C from above ground biomass and soil while acting to refresh the landscape surface and forest ecosystems. Using a time series of landslide maps covering 4 decades of mass wasting in the western Southern Alps, New Zealand, it is estimated that landslides mobilised 7.6 ± 2.9 tC km 2 a 1 of modern organic C. Approximately 30% of this material was delivered to river channels, where it was available for onward transport to sites of longterm deposition. Rapid turnover of the landscape, with rates of 3.3% of the surface area per century also aids high rates of net ecosystem productivity in the montane forest of the western Southern Alps of 94 ± 11 tC km 2 a 1.
3. Do other erosion processes play a role? POC from soil and standing biomass makes up 2/3 of the total POC in the suspended load of rivers draining the western Southern Alps. The erosional flux of non-fossil POC from these catchments represents a transfer of about 39 tC km 2 a 1 averaged over the west flank of the mountain belt ((Hilton et al., 2008a). Although these estimates may be high due to undersampling of high river discharges, river organic loads are likely to be significantly higher than the decadal average landslide yields. Other erosion processes may play an important role in the mobilization of organic matter on hillslopes. A high resolution time series of river suspended load samples from the Liwu River, Taiwan has revealed that during heavy rainfall
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the percentage non-fossil organic C in the river suspended load jumps from less than 10% to more than 40% (Hilton et al., 2008b). Thus, rainfall is an essential driver of mobilization of organic matter, through surface runoff rather than mass wasting. This finding is confirmed by small catchment studies outside the inter-tropical convergence zone, where runoff-driven erosion of litter and especially soil dominates the POC flux even where landsliding is intense. 4. How much fossil and non-fossil POC is delivered at the mountain front, and when? In Taiwan C yields estimated from range front sampling match high values obtained in New Zealand. River POC in Taiwan comprises, on average, 70% rock-derived fossil POC and 30% recently photosynthesized POC from the terrestrial biosphere. Most nonfossil organic matter is transferred in rivers during large floods, with event outputs > 1x104 tC from individual mountain catchments occurring most years. Crucially, these floods are intensely turbid with fluid densities greater than seawater. Long-term C output is dominated by the largest floods, such as those caused by typhoon Morakot in SW Taiwan in 2009. In addition to a large efflux of fine grained organic matter, this typhoon produced vast volumes of driftwood in Pacific Asia. Coarse woody debris output from Taiwan totaled 3.8–8.4 Mt, carrying 1.8–4.0 Mt of organic C (West et al., 2011). In addition to the local effects on the marine and coastal environment from such a highly concentrated flux of C and nutrients, storm-driven mobilization of woody debris may represent a significant, if infrequent, transfer of terrestrial biomass to the oceans. Mountain building exposes fossil organic C in exhumed sedimentary rocks. Oxidation of this material releases CO2 from longterm geological storage to the atmosphere. Fossil organic C is mobilized along with non-fossil C on hillslopes by mass wasting and transferred to the particulate load of rivers. In large fluvial systems it is thought to be oxidized in transit, but in short, steep rivers draining mountain islands, fossil organic C may escape oxidation and re-enter geological storage due to rapid fluvial transfer to the ocean. Annual fossil organic C yields in Taiwan vary from 12 ± 1 to 246 ± 22 tC km 2 a 1. Efficient transfer of this material ensures that 1.3 ± 0.1 MtC a 1 of fossil organic C exhumed in Taiwan is delivered to the ocean, with < 15% loss due to weathering in transit (Hilton et al., 2011). This suggests that erosion of coastal mountain ranges can force efficient transfer and long-term reaccumulation of fossil organic C in marine sediments.
during the Last Glacial Maximum as well as the Holocene, showing that long-term preservation of organic C after deposition represents a progressive transfer of atmospheric CO2 into geological storage. In addition, short transfers from source to sink in coastal mountain belts like Taiwan allow preservation and reburial of fossil C in all states of graphitization, limiting release of CO2 from geological storage during orogenic cycling of sediments. This contrasts with long sediment routing systems where only graphitized C survives prolonged transport (Galy et al., 2007).
6. Where is terrestrial POC located in exhumed foreland basins? Exhumed foreland basins allow a more complete investigation of the distribution and abundance of POC in geological storage. In the Late Cretaceous and Tertiary southern Pyrenees foreland basin, POC is found in all depositional environments from river braidplains to deepsea fan. Everywhere, POC is enriched in first cycle material, reflecting input of non-fossil C during basin filling. However, POC concentrations are commonly low, 0.25%, except for turbidites, with POC concentrations around 1%. This confirms the special importance of turbidites in trapping and long-term storage of fossil and non-fossil POC eroded from active mountain belts. Mid Tertiary Turbidites of the southern foreland basin of the European Alps, now exposed in the Apennines, have 0.1% first cycle POC in sandy units and 0.5% in muds. Individual turbidites in this sequence have volumes of 1–100 km3, and may contain up to 101 Mt first cycle C. These volumes are unlikely to be sourced direct from Alpine catchment floods. Instead, accumulation of clastic sediment and POC in deltas and shallow marine deposits, and subsequent remobilization in large submarine transport events is a likely mechanism for the formation of these large turbidites, 1000 of which are exposed in the Apennines (Talling et al., 2007). These observations illustrate an organic pathway for erosiondriven drawdown of atmospheric CO2 in which landsliding and soil erosion by surface runoff combine to mobilize fossil and non-fossil POC from upland catchments, storm floods transfer this POC together with large volumes of clastic sediment to coastlines where onward transport occurs in turbidity currents, possibly amplified by remobilized deltaic and shallow marine deposits, and effective long-term storage of POC is optimized by high deposition rates of deep sea turbidites. This mechanism is likely to be most effective in coastal mountain ranges, where trapping and recycling of POC may have a notable impact on the global C cycle.
References 5. Where does POC eroded from coastal mountain ranges end up? With limited potential for long-term onshore storage, most POC eroded from coastal mountain ranges is transferred to the oceans, but observations of offshore transfer events are rare. Typhoon Morakot generated hyperpycnal river discharge giving rise to turbidity currents in the Kaoping canyon off SW Taiwan. During the event, both the upper canyon and the surrounding shelf were inundated with terrestrial material, whilst turbidity currents in the canyon destabilized shelf walls, causing admixture of terrestrial and marine C and bulking of canyon turbidites. Many other storm events may follow a similar pattern. Their large load of fossil and non-fossil organic C, paired with extreme turbidity facilitates bypass of the continental shelf and preservation of C in the deep ocean. Radioactive and stable isotopic compositions of organic C in marine sediments off NE Taiwan confirm that fluvial POC is transferred into fine grained sea floor sediments without significant loss, and mixed with recent marine organic C. This mechanism has operated
Berner, R.A., 1990. Atmospheric CO2 levels over Phanerozoic time. Science 249, 1382–1386. Burdige, D.J., 2005. Burial of terrestrial organic matter in marine sediments: A reassessment. Glob. Biogeochem. Cycles 19 (GB4011), 1–7. Bowman, D.M.J.S., Balch, J.K., Artaxo, P., Bond, W.J., Carlson, J.M., Cochrane, M.A., D’Antonio, C.M., DeFries, R.S., Doyle, J.C., Harrison, S.P., Johnston, F.H., Keeley, J.E., Krawchuk, M.A., Kull, C.A., Marston, J.B., Moritz, M.A., Prentice, I.C., Roos, C.I., Scott, A.C., Swetman, T.W., van der Werf, G.R., Pyne, S.J., 2009. Fire in the Earth system. Science 324, 481–484. Galy, V., France-Lanord, C., Beyssac, O., Faure, P., Kudrass, H., Palhol, F., 2007. Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system. Nature 450, 407–410. Hilton, R.G., Galy, A., Hovius, N., 2008a. Riverine particulate organic carbon from a pristine, active mountain belt: the importance of vegetation and landslides. Glob. Biogeochem.Cycles. doi:10.1029/2006GB002905. Hilton, R.G., Galy, A., Hovius, N., Chen, M.C., Horng, M.J., Chen, H., 2008b. Tropical cyclones drive erosion of terrestrial organic carbon from mountain slopes to the deep ocean. Nature Geosci.. doi:10.1038/nge. Hilton, R.G., Galy, A., Hovius, N., Horng, M.J., Chen, H., 2011. Efficient transport of fossil organic carbon to the ocean by steep mountain rivers: an organic carbon sequestration mechanism. Geology 39, 71–74. Schlünz, B., Schneider, R.R., 2000. Transport of terrestrial organic carbon to the oceans by rivers: Re-estimating flux and burial rates. Internat. J. Earth Sci. 88, 599–606.
N. Hovius et al. / Applied Geochemistry 26 (2011) S285–S287 Talling, P.J., Amy, L.A., Wynn, R.B., Blackbourn, G., Gibson, O., 2007. Evolution of Turbidity Currents Deduced from Extensive Thin Turbidites: Marnoso Arenacea Formation (Miocene), Italian Apennines. J. Sed. Res. 77, 172–199.
S287
West, A.J., Lin, C.W., Lin, T.C., Hilton, R.G., Tanaka, M., Chang, C.T., Lin, K.C., Galy, A., Sparkes, R., Hovius, N., 2011. Mobilization and transport of coarse woody debris by large storms. Limnol. Oceanog. 56, 77–85.
Contents lists available at ScienceDirect
Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem
Erosion-driven drawdown of atmospheric carbon dioxide: The organic pathway Niels Hovius a,⇑, Albert Galy a, Robert G. Hilton b, Robert Sparkes a, Joanne Smith a, Kao Shuh-Ji c, Chen Hongey d, Lin In-Tian d, A. Joshua West e a
Department of Earth Sciences, University of Cambridge, United Kingdom Department of Geography, University of Durham, United Kingdom c Research Centre for Environmental Changes, Academia Sinica, Taipei, Taiwan d Department of Geosciences, National Taiwan University, Taipei, Taiwan e Department of Earth Sciences, University of Southern California, Los Angeles, USA b
a r t i c l e
i n f o
Article history: Available online 13 April 2011
a b s t r a c t Rapidly eroding, coastal mountain belts, where steep rivers and submarine channels connect upland sources to nearby marine sinks are hotspots of organic carbon transfer from life biomass, soil and exhumed bedrock into geological storage. Using observations from the Southern Alps of New Zealand, and Taiwan, we have mapped this organic pathway to geological carbon sequestration, and can evaluate the magnitude and efficiency of transfers between sources and sinks. We demonstrate that POC is harvested by landsliding, but importantly also by common and widespread surface runoff on steep hillslopes. Although terrestrially sourced POC is found in many sedimentary environments associated with mountain belts and frontier basins, it appears to be most abundantly trapped and preserved in marine turbidites. The loss of all forms of POC in onward transport through short, steep routing systems to this repository is limited. This is in marked contrast to larger routing systems, in which only the most resilient forms of POC survive into long-term deposition. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The burial of particulate organic C (POC) in marine sediments represents the second largest geological sink of atmospheric CO2 after carbonate deposition induced by excess alkalinity due to continental silicate weathering (Berner, 1990). Approximately half of this organic C is derived from the terrestrial environment (Schlünz and Schneider, 2000; Burdige, 2005), where high yielding areas coincide with active mountain belts at low and intermediate latitudes (Bowman et al., 2009). It is becoming increasingly clear that in these orogenic settings significantly more C may be implied in the erosion and burial of non-fossil POC than in concomitant silicate weathering (Galy et al., 2007). In this presentation mechanisms and rates of harvesting, routing and burial of POC from active mountain belts will be examined, addressing five questions.
2. How much non-fossil POC is mobilized by mass wasting in mountain belts? Where rock uplift and fluvial incision outpace weathering, bedrock landsliding is the principal erosion mechanism. Landslides ⇑ Corresponding author. Address: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, United Kingdom, Tel.: +44 1223 333453; fax: +44 1223 333450. E-mail address: [email protected] (N. Hovius). 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.03.082
harvest organic C from above ground biomass and soil while acting to refresh the landscape surface and forest ecosystems. Using a time series of landslide maps covering 4 decades of mass wasting in the western Southern Alps, New Zealand, it is estimated that landslides mobilised 7.6 ± 2.9 tC km 2 a 1 of modern organic C. Approximately 30% of this material was delivered to river channels, where it was available for onward transport to sites of longterm deposition. Rapid turnover of the landscape, with rates of 3.3% of the surface area per century also aids high rates of net ecosystem productivity in the montane forest of the western Southern Alps of 94 ± 11 tC km 2 a 1.
3. Do other erosion processes play a role? POC from soil and standing biomass makes up 2/3 of the total POC in the suspended load of rivers draining the western Southern Alps. The erosional flux of non-fossil POC from these catchments represents a transfer of about 39 tC km 2 a 1 averaged over the west flank of the mountain belt ((Hilton et al., 2008a). Although these estimates may be high due to undersampling of high river discharges, river organic loads are likely to be significantly higher than the decadal average landslide yields. Other erosion processes may play an important role in the mobilization of organic matter on hillslopes. A high resolution time series of river suspended load samples from the Liwu River, Taiwan has revealed that during heavy rainfall
S286
N. Hovius et al. / Applied Geochemistry 26 (2011) S285–S287
the percentage non-fossil organic C in the river suspended load jumps from less than 10% to more than 40% (Hilton et al., 2008b). Thus, rainfall is an essential driver of mobilization of organic matter, through surface runoff rather than mass wasting. This finding is confirmed by small catchment studies outside the inter-tropical convergence zone, where runoff-driven erosion of litter and especially soil dominates the POC flux even where landsliding is intense. 4. How much fossil and non-fossil POC is delivered at the mountain front, and when? In Taiwan C yields estimated from range front sampling match high values obtained in New Zealand. River POC in Taiwan comprises, on average, 70% rock-derived fossil POC and 30% recently photosynthesized POC from the terrestrial biosphere. Most nonfossil organic matter is transferred in rivers during large floods, with event outputs > 1x104 tC from individual mountain catchments occurring most years. Crucially, these floods are intensely turbid with fluid densities greater than seawater. Long-term C output is dominated by the largest floods, such as those caused by typhoon Morakot in SW Taiwan in 2009. In addition to a large efflux of fine grained organic matter, this typhoon produced vast volumes of driftwood in Pacific Asia. Coarse woody debris output from Taiwan totaled 3.8–8.4 Mt, carrying 1.8–4.0 Mt of organic C (West et al., 2011). In addition to the local effects on the marine and coastal environment from such a highly concentrated flux of C and nutrients, storm-driven mobilization of woody debris may represent a significant, if infrequent, transfer of terrestrial biomass to the oceans. Mountain building exposes fossil organic C in exhumed sedimentary rocks. Oxidation of this material releases CO2 from longterm geological storage to the atmosphere. Fossil organic C is mobilized along with non-fossil C on hillslopes by mass wasting and transferred to the particulate load of rivers. In large fluvial systems it is thought to be oxidized in transit, but in short, steep rivers draining mountain islands, fossil organic C may escape oxidation and re-enter geological storage due to rapid fluvial transfer to the ocean. Annual fossil organic C yields in Taiwan vary from 12 ± 1 to 246 ± 22 tC km 2 a 1. Efficient transfer of this material ensures that 1.3 ± 0.1 MtC a 1 of fossil organic C exhumed in Taiwan is delivered to the ocean, with < 15% loss due to weathering in transit (Hilton et al., 2011). This suggests that erosion of coastal mountain ranges can force efficient transfer and long-term reaccumulation of fossil organic C in marine sediments.
during the Last Glacial Maximum as well as the Holocene, showing that long-term preservation of organic C after deposition represents a progressive transfer of atmospheric CO2 into geological storage. In addition, short transfers from source to sink in coastal mountain belts like Taiwan allow preservation and reburial of fossil C in all states of graphitization, limiting release of CO2 from geological storage during orogenic cycling of sediments. This contrasts with long sediment routing systems where only graphitized C survives prolonged transport (Galy et al., 2007).
6. Where is terrestrial POC located in exhumed foreland basins? Exhumed foreland basins allow a more complete investigation of the distribution and abundance of POC in geological storage. In the Late Cretaceous and Tertiary southern Pyrenees foreland basin, POC is found in all depositional environments from river braidplains to deepsea fan. Everywhere, POC is enriched in first cycle material, reflecting input of non-fossil C during basin filling. However, POC concentrations are commonly low, 0.25%, except for turbidites, with POC concentrations around 1%. This confirms the special importance of turbidites in trapping and long-term storage of fossil and non-fossil POC eroded from active mountain belts. Mid Tertiary Turbidites of the southern foreland basin of the European Alps, now exposed in the Apennines, have 0.1% first cycle POC in sandy units and 0.5% in muds. Individual turbidites in this sequence have volumes of 1–100 km3, and may contain up to 101 Mt first cycle C. These volumes are unlikely to be sourced direct from Alpine catchment floods. Instead, accumulation of clastic sediment and POC in deltas and shallow marine deposits, and subsequent remobilization in large submarine transport events is a likely mechanism for the formation of these large turbidites, 1000 of which are exposed in the Apennines (Talling et al., 2007). These observations illustrate an organic pathway for erosiondriven drawdown of atmospheric CO2 in which landsliding and soil erosion by surface runoff combine to mobilize fossil and non-fossil POC from upland catchments, storm floods transfer this POC together with large volumes of clastic sediment to coastlines where onward transport occurs in turbidity currents, possibly amplified by remobilized deltaic and shallow marine deposits, and effective long-term storage of POC is optimized by high deposition rates of deep sea turbidites. This mechanism is likely to be most effective in coastal mountain ranges, where trapping and recycling of POC may have a notable impact on the global C cycle.
References 5. Where does POC eroded from coastal mountain ranges end up? With limited potential for long-term onshore storage, most POC eroded from coastal mountain ranges is transferred to the oceans, but observations of offshore transfer events are rare. Typhoon Morakot generated hyperpycnal river discharge giving rise to turbidity currents in the Kaoping canyon off SW Taiwan. During the event, both the upper canyon and the surrounding shelf were inundated with terrestrial material, whilst turbidity currents in the canyon destabilized shelf walls, causing admixture of terrestrial and marine C and bulking of canyon turbidites. Many other storm events may follow a similar pattern. Their large load of fossil and non-fossil organic C, paired with extreme turbidity facilitates bypass of the continental shelf and preservation of C in the deep ocean. Radioactive and stable isotopic compositions of organic C in marine sediments off NE Taiwan confirm that fluvial POC is transferred into fine grained sea floor sediments without significant loss, and mixed with recent marine organic C. This mechanism has operated
Berner, R.A., 1990. Atmospheric CO2 levels over Phanerozoic time. Science 249, 1382–1386. Burdige, D.J., 2005. Burial of terrestrial organic matter in marine sediments: A reassessment. Glob. Biogeochem. Cycles 19 (GB4011), 1–7. Bowman, D.M.J.S., Balch, J.K., Artaxo, P., Bond, W.J., Carlson, J.M., Cochrane, M.A., D’Antonio, C.M., DeFries, R.S., Doyle, J.C., Harrison, S.P., Johnston, F.H., Keeley, J.E., Krawchuk, M.A., Kull, C.A., Marston, J.B., Moritz, M.A., Prentice, I.C., Roos, C.I., Scott, A.C., Swetman, T.W., van der Werf, G.R., Pyne, S.J., 2009. Fire in the Earth system. Science 324, 481–484. Galy, V., France-Lanord, C., Beyssac, O., Faure, P., Kudrass, H., Palhol, F., 2007. Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system. Nature 450, 407–410. Hilton, R.G., Galy, A., Hovius, N., 2008a. Riverine particulate organic carbon from a pristine, active mountain belt: the importance of vegetation and landslides. Glob. Biogeochem.Cycles. doi:10.1029/2006GB002905. Hilton, R.G., Galy, A., Hovius, N., Chen, M.C., Horng, M.J., Chen, H., 2008b. Tropical cyclones drive erosion of terrestrial organic carbon from mountain slopes to the deep ocean. Nature Geosci.. doi:10.1038/nge. Hilton, R.G., Galy, A., Hovius, N., Horng, M.J., Chen, H., 2011. Efficient transport of fossil organic carbon to the ocean by steep mountain rivers: an organic carbon sequestration mechanism. Geology 39, 71–74. Schlünz, B., Schneider, R.R., 2000. Transport of terrestrial organic carbon to the oceans by rivers: Re-estimating flux and burial rates. Internat. J. Earth Sci. 88, 599–606.
N. Hovius et al. / Applied Geochemistry 26 (2011) S285–S287 Talling, P.J., Amy, L.A., Wynn, R.B., Blackbourn, G., Gibson, O., 2007. Evolution of Turbidity Currents Deduced from Extensive Thin Turbidites: Marnoso Arenacea Formation (Miocene), Italian Apennines. J. Sed. Res. 77, 172–199.
S287
West, A.J., Lin, C.W., Lin, T.C., Hilton, R.G., Tanaka, M., Chang, C.T., Lin, K.C., Galy, A., Sparkes, R., Hovius, N., 2011. Mobilization and transport of coarse woody debris by large storms. Limnol. Oceanog. 56, 77–85.