Comment on gchron-2021-31

Claire E Lukens

doi:10.5194/gchron-2021-31-rc2

Abstract

We consider measurements of both in situ produced cosmogenic nuclides and dissolved load flux to characterize the processes and pace of landscape change in central Cuba. The tropical landscape of Cuba is losing mass in multiple ways, making it difficult to quantify total denudation rates and thus to assess the impact of agricultural practices on rates of contemporary landscape change. Long-term sediment generation rates inferred from 26Al and 10Be concentrations in quartz extracted from central Cuban river sand range from 3.7–182 tons km−2 yr−1 (mean = 62, median = 57). Rock dissolution rates (24–154 tons km−2 yr−1; mean = 84, median = 78) inferred from stream solute loads exceed measured cosmogenic nuclide-derived sediment generation rates in 15 of 22 basins, indicating significant landscape-scale mass loss not reflected in the cosmogenic nuclide measurements. 26Al / 10Be ratios lower than that of surface production are consistent with the presence of a deep, mixed, regolith layer in the five basins that have the greatest disagreement between rock dissolution rates (high) and sediment generation rates inferred from cosmogenic nuclide concentrations (low). Our data show that accounting for the contribution of mineral dissolution at depth in calculations of total denudation is particularly important in the humid tropics, where dissolved load fluxes are high, and where mineral dissolution can occur many meters below the surface, beyond the penetration depth of most cosmic rays and thus the production of most cosmogenic nuclides. Relying on cosmogenic nuclide data or stream solute fluxes alone would both lead to underestimates of total landscape denudation in the central Cuba, emphasizing the importance of combining these approaches to fully capture mass loss in tropical landscapes.

Highlights

This study presents measurements of cosmogenic nuclides in river sand and solute fluxes in central Cuba that highlight the importance of both physical and chemical weathering in humid, tropical landscapes
Relying on cosmogenic nuclide data or stream solute fluxes alone would both lead to underestimates of total landscape denudation in the central Cuba, emphasizing the importance of combining these approaches to fully capture mass loss in tropical landscapes
This comparison shows that mass loss is occurring largely by solution in central Cuba, which is consistent with observations from other tropical landscapes (White et al, 1998; Von Blackenburg et al, 2004; Salgado et al, 2006; Regard et al, 2016)

Summary

Introduction

This study presents measurements of cosmogenic nuclides in river sand and solute fluxes in central Cuba that highlight the importance of both physical and chemical weathering in humid, tropical landscapes. 40 (Regard et al, 2016), referred to as chemical weathering, under the assumption that both occur primarily within the uppermost meter or two of Earth’s surface, the penetration depth of the cosmic ray neutrons responsible for producing most cosmogenic nuclides via spallation reactions (Bierman and Steig, 1996). Such rates are often assumed to represent total landscape denudation, but failure to account for rock dissolution at depth and the export of mass as dissolved load below the spallation-dominated production zone (below ~2 m) can result in a 45 low bias for cosmogenic nuclide-derived erosion rate estimates (Small et al, 1999; Riebe et al, 2001a; Dixon et al, 2009a).

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