Climatic and tectonic influences on ocean island erosion inferred from a global dataset

Abstract:

Volcanic ocean islands are well suited for studying climatic and tectonic influences on erosion because they often have homogeneous bedrock, dramatic rainfall gradients, and remnant surfaces that constrain their age, initial topography, and vertical motions relative to sea level. Because subsiding islands are inherently transient landscapes, island erosion should respond sensitively to climatic and tectonic forcing. However, island erosion also depends on intrinsic factors, such as the slope and geometry of the volcanic shield, and varies through time as island topography evolves. Understanding the temporal evolution of island landscapes is therefore required to isolate climatic and tectonic controls. We examine the relationship between erosion, vertical motion, and precipitation on 59 volcanic ocean islands worldwide, 47°S to 40°N, spanning 35 Myr in age. Erosion rates estimated from the volume of river canyons and the radiometric age of bedrock, normalized by initial island volume, decrease with island age. This timescale dependence is probably not due to episodic erosion since our measurements integrate over entire island landscapes rather than individual channel reaches that may be periodically alluviated. Instead, we posit that island erosion rates vary progressively through time in response to island-wide changes in slope. Rivers initially dissect gentle volcanic shields to produce steep topography. Once local relief begins to decline, slopes become gentler and erosion rates decrease. Erosion may be relatively insensitive to ongoing subsidence. Fractional island erosion shows no clear relationship with net subsidence recorded by the slope break formed at each island’s shoreline during shield building. Erosion rates and fluvial erosional efficiency K increase with mean annual precipitation. We use a landscape evolution model to test the relationships between climate, tectonics, and erosion suggested by our global island dataset.