大地震对龙门山构造带剥蚀-风化-碳循环的深远影响

PROLONGED IMPACTS OF LARGE EARTHQUAKES ON EROSION, WEATHERING AND THE CARBON CYCLE ACROSS THE LONGMEN SHAN OROGEN

构造运动对地表剥蚀、硅酸盐风化、长尺度碳循环以及气候系统的影响一直存在争论和不确定性。其主要原因在于，已有关于“构造-风化-碳循环-气候变化”耦合的证据主要基于河水化学计算的和海洋沉积物记录的硅酸盐风化以及有机碳输移通量，一直缺乏对构造事件前后剥蚀、风化通量的直接观测和对比研究，更不清楚构造运动对流域侵蚀和风化过程影响的幅度、范围及持续时间。龙门山位于构造活动活跃的青藏高原东缘，是学术界长期关注的热点区域，拥有大量的基础资料与背景数据。发生于龙门山地区的2008年汶川地震和2013年芦山地震引发了近8万个山体滑坡；这些滑坡体快速向河流系统输送大量碎屑物质，极大地影响了地表过程，为评价构造事件对流域剥蚀和风化的影响提供了非常珍贵的机会。文章通过定量评估地震滑坡对流域侵蚀、河流颗粒有机碳（POC）输移和河水溶质的影响及持续时间，结合滑坡体积与地震震级和复发频次关系，提出了地震滑坡对以龙门山为代表的活跃造山带长时间尺度剥蚀速率控制的新机制。结果表明，在2008年汶川地震之后的5年里，龙门山地区主要河流的悬浮物通量较地震前增加了3～7倍，指示了地震滑坡对流域侵蚀的直接贡献；清空单次地震产生的龙门山地区细颗粒滑坡物质需要几十年至数百年、粗颗粒则可能需要数千年，而滑坡密度和高强度径流是控制滑坡物质搬运时间的主要因素。通过模拟建立了龙门山地区地震震级一滑坡体积与复发频次的关系，计算得到该地区理论上的长时间尺度平均地震滑坡剥蚀速率，约为0．5～1．0mm／a。这一理论值与宇宙成因核素和低温热年代学获得的长尺度剥蚀速率大致相当，并且汶川滑坡的空间分布与测得的长尺度剥蚀速率的空间分布相似，这表明在构造时间尺度上，龙门山构造带的剥蚀通量很可能主要是由地震引发的滑坡碎屑物质持续提供的，为解释构造活跃区的高剥蚀通量提供了一个全新的机制。与地震滑坡剥蚀同步的是河流现代POC和河水溶质的成倍增加。汶川地震造成岷江上游杂谷脑河现代POC供给增加了2—4倍，且只有少部分会被氧化降解；同时，震后岷江河水碱度和硅酸盐风化通量则增加了4倍左右，由此造成的CO：消耗通量和87Sr／86Sr比值分别增加了4．3±0．4倍和0．000644±0．000146。本研究将构造活动与长时间尺度山脉剥蚀、流域风化和碳循环直接地联系到一起，为“构造-风化-碳循环-气候变化”耦合提供了直接有力的观测证据和基于滑坡的全新机制解释。如果其他的地震也有类似汶川地震引发的有机碳埋藏和硅酸盐碳消耗成倍增加的双重碳汇效应，那么地震及其引发的滑坡可能在调节长时间尺度大气CO2和全球气候中的确起着重要的作用。

The linkage between tectonic activities and erosion and silicate weathering remains a matter of debate. Hypotheses for a tectonic trigger are based mainly on the temporal coincidence of the uplift and erosion of the world＇s great mountain ranges（ mainly the Himalayas）and the seawater 87Sr/S6Sr rise from 40 Ma, resulting from high radiogenic Sr fluxes by enhanced silicate weathering along the river waters draining these uplifted mountains. However, evidences so-far for supporting or doubting the hypothesis are indirect, either from the oceanic records preserved in marine sediment, or from the modern-day studies of physical erosion, chemical weathering, organic carbon transfer, and their relationships in tectonically active regions; the latter has been mainly focused upon the dissolved loads of the rivers draining the Himalayas-Tibetan Plateau, because these tectonically active areas have the highest rates of total denudation coupling with large silicate weathering fluxes and large sedimentary organic carbon burial. However, direct empirical evidence still lacks following high magnitude tectonic events （e. g., earthquakes）. Such direct studies are rare on what magnitude large tectonic events impact erosion and weathering processes and its mechanism, because those events occur rarely and often lack sufficient background pre-event data. The 2008 Mw7.9 Wenchuan and 2013 Mw6.6 Lushan earthquakes occurred along the Longmen Shan orogen at the eastern margin of the Tibetan Plateau, triggered massive landslides, and caused significant changes in surface processes and the carbon cycle. With sufficient studies in this region from before the earthquakes, the two recent earthquakes provide a good opportunity to explore how high magnitude tectonic events affect erosion, weathering and the carbon cycle over various timescales. In this study, we assess erosion, transfer of biospheric particulate organic carbon （ POCbiosphere ） and riverine solute in the context of the widespread landslides triggered by the Wenchuan earthquake during five years following the main shock. We propose that long-term erosion rates along a high-relief active plateau margin may be regulated by the resulting earthquake-triggered landslides, by combining with the close relation of earthquake-triggered landslide volume with earthquake magnitude and recurrence interval, as we observe for the Longmen Shan case. The results showed that suspended sediment discharge within five years following the earthquake was elevated 3-7 times compared to prior to the Wenchuan earthquake, indicating a significant contribution of earthquake- triggered landslides to catchment erosion, with an emphasis of the importance of landslide density and runoff intensity in setting the duration of earthquake-triggered landslide impacts on river systems. Following the post- earthquake erosion rate （ ca. 0.5 - 1.0 mm/a） calculated by sediment flux and estimated using the Longmen Shun- specific landslide volume model, we find that the residence times of sediment associated with landslides in the Longmen Shan catchments range from decades （for fine sediment transported as suspended load）to thousands of years（for coarser materials transported as bedload）, and the time scale is comparable to the estimated recurrence interval for Wenchuan-alike events. Accompanying enhanced sediment flux, the riverine fluxes of POC biosphere and solute multiplied following the earthquake. The riverine POC biosphere flux in the lower reaches of the Zagunao River （one of the major tributaries of the Minjiang River） enhanced 2 - 4 times following the large earthquake and the rapid export of POC biosphere from earthquake-triggered landslides likely outpaced its degradation on hillslopes. Meanwhile, the riverine fluxes of alkalinity and silicate weathering solute redoubled after the earthquake, meaning 4. 3±0. 4 times increase in CO2 consumption rate via silicate-derived alkalinity and a 0. 000644±0. 000146 increase in 87Sr/86Sr isotopic ratios. To extrapolate the short-term observations to longer geological timescales, we combined landslide and seismic catalog data, modelled the long-term seismically induced landsliding rate （i. e. ,＂ seismic erosion rate＂）, and compared with the regional long-term erosion rates as measured from cosmogenic nuclides and low temperature thermochronology. We propose that long-term erosion across the Longmen Shan is likely sustained by earthquake- triggered landslides, and highlight the role of earthquakes and seismogenic faults in the erosional budget in steep mountains along high-relief active plateau margins. Overall, our study provides a direct link between seismotectonic activities, erosion, weathering and the carbon cycle. If such changes happen in other tectonically active mountain ranges, earthquakes and associated landslides likely play an important but underestimated role in regulating Earth＇s long-term carbon cycle and the evolution of the global climate system.