Anthropocene Landscape Change and the Legacy of Nineteenth- and Twentieth-Century Mining in the Fourmile Catchment, Colorado Front Range

Abstract

Human impacts on earth surface processes and materials are fundamental to understanding the proposed Anthropocene epoch. This study examines the magnitude, distribution, and long-term context of nineteenth- and twentieth-century mining in the Fourmile Creek catchment, Colorado, coupling airborne LiDAR topographic analysis with historical documents and field studies of river banks exposed by 2013 flooding. Mining impacts represent the dominant Anthropocene landscape change for this basin. Mining activity, particularly placer operations, controls floodplain stratigraphy and waste rock piles related to mining cover >5% of hillslopes in the catchment. Total rates of surface disturbance on slopes from mining activities (prospecting, mining, and road building) exceed pre-nineteenth-century rates by at least fifty times. Recent flooding and the overprint of human impacts obscure the record of Holocene floodplain evolution. Stratigraphic relations indicate that the Fourmile valley floor was as much as two meters higher in the past 2,000 years and that placer reworking, lateral erosion, or minor downcutting dominated from the late Holocene to present. Concentrations of As and Au in the fine fraction of hillslope soil, mining-related deposits, and fluvial deposits serve as a geochemical marker of mining activity in the catchment; reducing As and Au values in floodplain sediment will take hundreds of years to millennia. Overall, the Fourmile Creek catchment provides a valuable example of Anthropocene landscape change for mountainous regions of the Western United States, where hillslope and floodplain markers of human activity vary, high rates of geomorphic processes affect mixing and preservation of marker deposits, and long-term impact varies by landscape location.

人类对于地表过程和物质的冲击, 对于理解提出的人类世时期而言相当关键。本研究检视十九与二十世纪在科罗拉多州的四里溪流域採矿的程度、分布与长期脉络, 并将空中 LiDAR 製图分析与历史文件以及 2013 年洪泛所暴露的河岸之田野调查进行配对。採矿的冲击, 呈现出人类世在此一流域中的支配性地景变迁。採矿活动, 特别是砂矿床的作业, 控制了洪泛平原的地层, 而与採矿相关的废弃沙石堆, 则覆盖了流域中百分之五以上的山坡地。源自採矿活动 (探测、採矿以及建筑) 的山坡表面扰动之总比率, 至少超过十九世纪之前五倍。晚近的洪泛与人类冲击的套印, 使得全新世的洪泛平原演化纪录黯然失色。地层关系显示, 四里溪谷底在过去两千年中竟增高达两公尺, 而砂矿床的重整、侧蚀或微小下切侵蚀, 自全新世晚期主宰至今。砷与金集中于山坡土壤、採矿相关沉积与河流沉积的细粒部分, 作为该流域採矿活动的地理化学标记; 降低洪泛平原沉积物中的砷与金价, 将需耗费数百年至上千年。总体而言, 四里溪流域提供了美国西部山区的人类世地景变迁之宝贵案例, 其中坡地与洪泛平原的人类活动印记不尽相同, 高比率的地形形成过程影响了标记物沉淀的混合与保存, 而长期的影响则随着不同的地景区位而有所不同。

Los impactos humanos sobre los procesos y materiales superficiales de la tierra son fundamentales para entender la época antropocénica propuesta. Este estudio examina la magnitud, distribución y contexto a largo plazo de la minería de los siglos XIX y XX en la cuenca del Fourmile Creek, Colorado, acoplando el análisis topográfico aéreo LiDAR con documentos históricos y estudios de campo de las bancas del río que la inundación de 2013 dejó expuestas. Los impactos de la minería representan el paisaje de cambio dominante en esta cuenca durante el Antropoceno. La actividad minera, en particular las operaciones tipo minería de placer, controla la estratigrafía de la planicie inundable y los amontonamientos roca residual que cubren >5% de las laderas de la cuenca. Las tasas totales de perturbación superficial en las laderas debidas a la actividad minera (prospección, minería y construcción de vías) exceden las tasas anteriores al siglo XIX en por lo menos cincuenta veces. Las recientes inundaciones y la sobrecarga de impactos humanos oscurecen el registro de la evolución de la planicie de inundación en el Holoceno. Las relaciones estratigráficas indican que el fondo del valle del Fourmile era por lo menos dos metros más alto en los pasados 2.000 años y que el trabajo de minería de placer, la erosión lateral, o la menor erosión de profundidad, dominaron desde el final del Holoceno hasta el presente. Las concentraciones de As y Au en la fracción fina del suelo de las laderas, los depósitos relacionados con minería y los depósitos fluviales sirven como un marcador geoquímico de la actividad minera en la cuenca; reducir los valores de As y Au en los sedimentos de la planicie de inundación puede tomar desde cientos de años hasta milenios. En general, la cuenca del riachuelo Fourmile provee un valioso ejemplo del cambio del paisaje en el Antropoceno para las regiones montañosas del oeste de los Estados Unidos, donde los marcadores de actividad humana en ladera y planicie de inundación varían, las altas tasas de los procesos geomórficos afectan la mezcla y preservación de los depósitos de marcación y el impacto a largo plazo varía según la localización del paisaje.

Key Words:

• Anthropocene
• human impacts
• landscape change
• LiDAR
• mining

关键词：:

• 人类世
• 人类冲击
• 地景变迁
• LiDAR
• 採矿。

Palabras clave:

• Antropoceno
• impactos humanos
• cambio del paisaje
• LiDAR
• minería

Additional information

Funding

NSF

Notes on contributors

David P. Dethier

DAVID P. DETHIER is Professor in the Department of Geosciences, Williams College, Williamstown, MA 01267. E-mail: [email protected]. His research interests are chemical weathering processes, human influences on erosion rates, and periglacial processes.

William B. Ouimet

WILLIAM B. OUIMET is Assistant Professor in the Department of Geography and Center for Integrative Geosciences, University of Connecticut, Storrs, CT 06269. E-mail: [email protected]. His research interests include earth surface processes, human–environment interactions, landscape evolution, and tectonic geomorphology.

Sheila F. Murphy

SHEILA F. MURPHY is a Research Hydrologist at the U.S. Geological Survey, Boulder, CO 80303. E-mail: [email protected]. Her research focuses on hydrology and water quality of watersheds and how they are affected by both natural factors and disturbance.

Maneh Kotikian

MANEH KOTIKIAN is currently a graduate student in the Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071. E-mail: [email protected]. Her primary research interest is hydrogeophysics. She was an undergraduate in the Department of Geology at Mount Holyoke College at the time this research was conducted.

Will Wicherski

WILL WICHERSKI is currently a graduate student in the Department of Geography, University of Colorado Boulder, Boulder, CO 80309. E-mail: [email protected]. His main research interest is fluvial geomorphology. He was an undergraduate in the Department of Geosciences at Williams College at the time this research was conducted.

Rachel M. Samuels

RACHEL M. SAMUELS is currently a graduate student in Civil Engineering, Georgia Institute of Technology, Atlanta, GA 30332. E-mail: [email protected]. Her primary research interest is infrastructural and societal resilience to human-induced disasters. She was an undergraduate in the Department of Geology at Washington and Lee University at the time this research was conducted.