Robert G. Hilton

Efficient transport of fossil organic carbon to the ocean by steep mountain rivers: An orogenic carbon sequestration mechanism

Mountain building exposes fossil organic carbon (OC fossil ) in exhumed sedimentary rocks. Oxidation of this material releases carbon dioxide from long-term geological storage to the atmosphere. OC fossil is mobilized on hillslopes by mass wasting and transferred to the particu- late load of rivers. In large fl uvial systems, it is thought to be oxidised in transit, but in short, steep rivers that drain mountain islands, OC fossil may escape oxidation and re-enter geological storage due to rapid fl uvial transfer to the ocean. In these settings, the rates of OC fossil transfer and their controls remain poorly constrained. Here we quantify the erosion of OC fossil from the Taiwan mountain belt, combining discharge statistics with measurements of particulate organic carbon load and source in 11 rivers. Annual OC fossil yields in Taiwan vary from 12 ± 1 to 246 ± 22 tC km −2 yr −1 , controlled by the high physical erosion rates that accompany rapid crustal shortening and frequent typhoon impacts. Effi cient transfer of this material ensures that 1.3 ± 0.1 ◊ 10 6 tC yr −1 of OC fossil exhumed in Taiwan is delivered to the ocean, with <15% loss due to weathering in transit. Our fi ndings suggest that erosion of coastal mountain ranges can force effi cient transfer and long-term re-accumulation of OC fossil in marine sediments, further enhancing the role of mountain building in the long-term storage of carbon in the lithosphere.

Seismic mountain building: Landslides associated with the 2008 Wenchuan earthquake in the context of a generalized model for earthquake volume balance

Here we assess earthquake volume balance and the growth of mountains in the context of a new landslide inventory for the Mw7.9 Wenchuan earthquake in central China. Coseismic landslides were mapped from high-resolution remote imagery using an automated algorithm and manual delineation, which allows us to distinguish clustered landslides that can bias landslide volume calculations. Employing a power-law landslide area-volume relation, we find that the volume of landslide-associated mass wasting (~2.8+0.9/-0.7 km3) is lower than previously estimated (~5.7-15.2 km3) and comparable to the volume of rock uplift (~2.6±1.2 km3) during the Wenchuan earthquake. If fluvial evacuation removes landslide debris within the earthquake cycle, then the volume addition from coseismic uplift will be effectively offset by landslide erosion. If all earthquakes in the region followed this volume budget pattern, the efficient counteraction of coseismic rock uplift raises a fundamental question about how earthquakes build mountainous topography. To provide a framework for addressing this question, we explore a group of scaling relations to assess earthquake volume balance. We predict coseismic uplift volumes for thrust-fault earthquakes based on geophysical models for coseismic surface deformation and relations between fault rupture parameters and moment magnitude, Mw. By coupling this scaling relation with landslide volume-Mw scaling, we obtain an earthquake volume balance relation in terms of moment magnitude Mw, which is consistent with the revised Wenchuan landslide volumes and observations from the 1999 Chi-Chi earthquake in Taiwan. Incorporating the Gutenburg-Richter frequency-Mw relation, we use this volume balance to derive an analytical expression for crustal thickening from coseismic deformation based on an index of seismic intensity over a defined area. This model yields reasonable rates of crustal thickening from coseismic deformation (e.g.~0.1-0.5 km Ma-1 in tectonically active convergent settings), and implies that moderate magnitude earthquakes (Mw≈6-8) are likely responsible for most of the coseismic contribution to rock uplift, because of their smaller landslide-associated volume reduction. Our first-order model does not consider a range of factors (e.g., lithology, climate conditions, epicentral depth and tectonic setting), nor does it account for viscoelastic or isostatic responses to erosion, and there remain important uncertainties on the scaling relationships used to quantify coseismic deformation. Nevertheless, our study provides a conceptual framework and invites more rigorous modeling of seismic mountain building.

Erosion of organic carbon in the Arctic as a geological carbon dioxide sink

Soils of the northern high latitudes store carbon over millennial timescales (thousands of years) and contain approximately double the carbon stock of the atmosphere. Warming and associated permafrost thaw can expose soil organic carbon and result in mineralization and carbon dioxide (CO2) release. However, some of this soil organic carbon may be eroded and transferred to rivers. If it escapes degradation during river transport and is buried in marine sediments, then it can contribute to a longer-term (more than ten thousand years), geological CO2 sink. Despite this recognition, the erosional flux and fate of particulate organic carbon (POC) in large rivers at high latitudes remains poorly constrained. Here, we quantify the source of POC in the Mackenzie River, the main sediment supplier to the Arctic Ocean, and assess its flux and fate. We combine measurements of radiocarbon, stable carbon isotopes and element ratios to correct for rock-derived POC. Our samples reveal that the eroded biospheric POC has resided in the basin for millennia, with a mean radiocarbon age of 5,800 ± 800 years, much older than the POC in large tropical rivers. From the measured biospheric POC content and variability in annual sediment yield, we calculate a biospheric POC flux of teragrams of carbon per year from the Mackenzie River, which is three times the CO2 drawdown by silicate weathering in this basin. Offshore, we find evidence for efficient terrestrial organic carbon burial over the Holocene period, suggesting that erosion of organic carbon-rich, high-latitude soils may result in an important geological CO2 sink.

Controls on fluvial evacuation of sediment from earthquake-triggered landslides

Large earthquakes in active mountain belts can trigger landslides, which mobilize large volumes of clastic sediment. Delivery of this material to river channels may result in aggradation and flooding, while sediment residing on hillslopes may increase the likelihood of subsequent landslides and debris flows. Despite recognition of these processes, the controls on the residence time of coseismic landslide sediment in river catchments remain poorly understood. Here we assess the residence time of fine-grained ( 5 mm day–1). Together with previous observations from the C.E. 1999 Chi-Chi earthquake in Taiwan, our results demonstrate the importance of landslide density and runoff intensity in setting the duration of earthquake-triggered landslide impacts on river systems.

Connectivity of earthquake‐triggered landslides with the fluvial network: Implications for landslide sediment transport after the 2008 Wenchuan earthquake

Evaluating the influence of earthquakes on erosion, landscape evolution, and sediment-related hazards requires understanding fluvial transport of material liberated in earthquake-triggered landslides. The location of landslides relative to river channels is expected to play an important role in postearthquake sediment dynamics. In this study, we assess the position of landslides triggered by the Mw 7.9 Wenchuan earthquake, aiming to understand the relationship between landslides and the fluvial network of the steep Longmen Shan mountain range. Combining a landslide inventory map and geomorphic analysis, we quantify landslide-channel connectivity in terms of the number of landslides, landslide area, and landslide volume estimated from scaling relationships. We observe a strong spatial variability in landslide-channel connectivity, with volumetric connectivity (ξ) ranging from ~20% to ~90% for different catchments. This variability is linked to topographic effects that set local channel densities, seismic effects (including seismogenic faulting) that regulate landslide size, and substrate effects that may influence both channelization and landslide size. Altogether, we estimate that the volume of landslides connected to channels comprises 43 + 9/−7% of the total coseismic landslide volume. Following the Wenchuan earthquake, fine-grained ( 90% of the total landslide volume) may be more significantly affected by landslide locations.

Seismically enhanced solute fluxes in the Yangtze River headwaters following the A.D. 2008 Wenchuan earthquake

Large earthquakes alter physical and chemical processes at Earths surface, triggering landslides, fracturing rock, changing large-scale permeability, and influencing hydrologic pathways. The resulting effects on global chemical cycles are not fully known. Here we show changes in the dissolved chemistry of the Min Jiang, a river in the Yangtze River (China) headwaters, following the A.D. 2008 M w 7.9 Wenchuan earthquake. Total solute fluxes transported by the Min Jiang increased after the earthquake, accompanied by an ∼4× increase in Na*/Ca ratios (where Na* is Na+ corrected for atmospheric and evaporite contributions) and a 0.000644 ± 0.000146 increase in 87Sr/86Sr isotopic ratios. These changes are consistent with enhanced contribution from silicate sources. We infer that the CO2 consumption rate via silicate-derived alkalinity increased 4.3 ± 0.4 times. If similar changes are associated with other large earthquakes, enhanced solute export could directly link tectonic activity with weathering and alkalinity fluxes that supply nutrients to ecosystems, influence seawater chemistry evolution, and steer Earths long-term carbon cycle and climate.

Decadal carbon discharge by a mountain stream is dominated by coarse organic matter

Rapid erosion in mountain forests results in high rates of biospheric particulate organic carbon (POC) export by rivers, which can contribute to atmospheric carbon dioxide drawdown. However, coarse POC (CPOC) carried by particles >∼1 mm is rarely quantified. In a forested pre-Alpine catchment, we measured CPOC transport rates and found that they increase more rapidly with water discharge than fine POC (<1 mm) and dissolved organic carbon (DOC). As a result, decadal estimates of CPOC yield of 12.3 ± 1.9 t C km–2 yr–1 are higher than for fine POC and DOC, even when excluding 4 extreme flood events. When including these floods, CPOC dominates organic carbon discharge (∼80%). Most CPOC (69%) was water logged and denser than water, suggesting that CPOC has the potential to contribute to long-term sedimentary burial. Global fluxes remain poorly constrained, but if the transport behavior of CPOC shown here is common to other mountain streams and rivers, then neglecting CPOC discharge could lead to a large underestimation of the global transfer of biospheric POC from land to ocean.

Earthquake-triggered increase in biospheric carbon export from a mountain belt

On geological time scales, the erosion of carbon from the terrestrial biosphere and its burial in sediments can counter CO2 emissions from the solid Earth. Earthquakes may increase the erosion of this biospheric carbon and supply it to mountain rivers by triggering landslides, which rapidly strip hillslopes of vegetation and soil. Over the long term, elevated river sediment loads may promote more efficient carbon burial. However, riverine export of earthquake-mobilized carbon has remained poorly constrained. Here we quantify biospheric carbon discharge by the Zagunao River following a large earthquake with a unique set of samples collected before and after the A.D. 2008 M w 7.9 Wenchuan (China) earthquake. Radioactive and stable carbon isotopes are used to isolate the biospheric carbon, accounting for rock-derived organic carbon inputs. River discharge of biospheric carbon doubled in the downstream reaches, characterized by moderate landslide impact, following the earthquake. The rapid export of carbon from earthquake-triggered landslides appears to outpace its degradation on hillslopes while sediment loads are elevated. This means that enhanced river discharge of biospheric carbon following large earthquakes can link active tectonics to CO2 drawdown.

Mountain glaciation drives rapid oxidation of rock-bound organic carbon

Mountain glaciation greatly enhances oxidative weathering rates of organic carbon in rocks and associated carbon dioxide release. Over millions of years, the oxidation of organic carbon contained within sedimentary rocks is one of the main sources of carbon dioxide to the atmosphere, yet the controls on this emission remain poorly constrained. We use rhenium to track the oxidation of rock-bound organic carbon in the mountain watersheds of New Zealand, where high rates of physical erosion expose rocks to chemical weathering. Oxidative weathering fluxes are two to three times higher in watersheds dominated by valley glaciers and exposed to frost shattering processes, compared to those with less glacial cover; a feature that we also observe in mountain watersheds globally. Consequently, we show that mountain glaciation can result in an atmospheric carbon dioxide source during weathering and erosion, as fresh minerals are exposed for weathering in an environment with high oxygen availability. This provides a counter mechanism against global cooling over geological time scales.

Erosion of organic carbon from the Andes and its effects on ecosystem carbon dioxide balance

Productive forests of the Andes are subject to high erosion rates that supply to the Amazon River sediment and carbon from both recently photosynthesized biomass and geological sources. Despite this recognition, the source and discharge of particulate organic carbon (POC) in Andean rivers remains poorly constrained. We collected suspended sediments from the Kosnipata River, Peru, over one year at two river gauging stations. Carbon isotopes (14C, 13C, 12C) and nitrogen to organic carbon ratios of the suspended sediments suggest a mixture of POC from sedimentary rocks (POCpetro) and from the terrestrial biosphere (POCbiosphere). The majority of the POCbiosphere has a composition similar to surface soil horizons and we estimate is mostly younger than 850 14C years. The suspended sediment yield in 2010 was 3500 ± 210 t km-2 yr-1, >10 times the yield from the Amazon Basin. The POCbiosphere yield was 12.6 ± 0.4 tC km-2 yr-1 and the POCpetro yield was 16.1 ± 1.4 tC km-2 yr-1, mostly discharged in the wet season (December to March) during flood events. The river POCbiosphere discharge is large enough to play a role in determining whether Andean forests are a source or sink of carbon dioxide. The estimated erosional discharge of POCpetro from the Andes is much larger (~1 Mt C yr-1) than the POCpetro discharge by the Madeira River downstream in the Amazon Basin, suggesting oxidation of POCpetro counters CO2 drawdown by silicate weathering. The flux and fate of Andean POCbiosphere and POCpetro needs to be better constrained to fully understand the carbon budget of the Amazon River Basin.