Biota, fractures, and thresholds: Emergent self-organization in landscape evolution?

本提案是我们正在进行的EarthShape第一阶段工作的延续，在该阶段中，我们研究了河流对切口和沉积物运输的生物控制。我们的研究重点是流量变异性和侵蚀阈值对河流切口的综合影响。在第1阶段，我们利用地球形状气候和植被梯度来破译流量变异和侵蚀效率的生物特征。然而，第1阶段的结果也表明，河流沉积物大小（侵蚀阈值）和风化层厚度在沿地形梯度和跨地形梯度上存在巨大的空间变异性。在各个EarthShape研究地点，观测到的风化层厚度的空间梯度似乎遵循裂缝密度的梯度。因此，我们假设生物群通过其对化学风化的影响，也可能影响沉积物的粒度，但这种影响可能受到断裂密度的限制。此外，地球形状遗址的剥蚀率较低（~10 m/Myr）。这意味着我们研究的景观是在长达数百万年的时间内形成的，当时的条件可能与现在不同。为了揭示生物对化学风化、沉积物大小和侵蚀阈值的影响，并避免虚假的相关性，我们承认裂缝密度和景观演化的时间依赖性的潜在重要影响。在此，我们建议(1)量化裂缝间距、风化层厚度和沉积物大小之间的关系；(2)确定次流域尺度上山坡剥蚀率的时空变异；(3)将这些观测结果结合景观演化建模，量化生物群对河流切口的影响。为此，我们将广泛利用第一阶段从所有项目中收集的数据和观测结果，并将这些数据与新的现场测量结果结合起来，采用新的建模方法，明确考虑水文和地形对化学风化和沉积物大小的影响。我们的结果将为EarthShape站点的可比性提供关键约束，我们的建模方法将提供一个新的接口，有助于将各种科学方法整合到EarthShape项目中。

This proposal is a continuation of our ongoing work in EarthShape phase 1, in which we investigate the biotic controls on incision and sediment transport by rivers. Our research focuses on the combined effect of discharge variability and erosion thresholds on river incision. During phase 1, we exploit the EarthShape climate and vegetation gradient to decipher the biotic signature in discharge variability and erosional efficiency. However, results from phase 1 also indicate enormous spatial variability in river sediment size (erosion thresholds) and regolith thickness both along and across the EarthShape gradient. Within individual EarthShape study sites, observed spatial gradients in regolith thickness appear to follow gradients in fracture density. We thus hypothesize that biota, through its influence on chemical weathering, may also affect sediment grain sizes, but that this influence may be limited by fracture density. Furthermore, denudation rates in the EarthShape sites are low (~10 m/Myr). This means that the landscapes we study have been formed over time periods of up to several million years, during conditions that were likely different from the present. To unravel the biotic influence on chemical weathering, sediment size, and hence erosion thresholds, and to avoid spurious correlations, we acknowledge the potentially important influence of fracture density and the time-dependent nature of landscape evolution. Here, we propose to (1) quantify the relationship between fracture spacing, regolith thickness, and sediment size, (2) determine the sub-basin scale spatial and temporal variability in hillslope denudation rates, and (3) combine these observations in landscape evolution modeling to quantify the influence of biota on river incision. To do so, we will make extensive use of data and observations collected from all projects during phase 1 and combine these with new field measurements in a new modeling approach in which we explicitly account for hydrologic and topographic effects on chemical weathering and sediment size. Our results will provide key constraints on the comparability of the EarthShape sites, and our modeling approach will provide a new interface that helps integrating the diverse scientific approaches in the EarthShape program.