¡Cuba! River Water Chemistry Reveals Rapid Chemical Weathering, the Echo of Uplift, and the Promise of More Sustainable Agriculture

Abstract

For the first time in more than half a century, a joint Cuban/American science team has worked together to quantify the impacts of chemical weathering and sustainable agriculture on river water quality in Cuba-the largest and most populous Caribbean island. Such data are critical as the world strives to meet sustainable development goals and for understanding rates of landscape change in the tropics, an understudied region. To characterize the landscape, we collected and analyzed water samples from 25 rivers in central Cuba where upstream land use varies from forested to agricultural. Cuban river waters bear the fingerprint of the diverse rock types underlying the island, and many carry exceptionally high dissolved loads. Chemical denudation rates are mostly among the top 25% globally and are similar to those measured in other Caribbean islands. High rates of solute export and the distinct composition of the waters in specific basins suggest flow paths that bring river source waters into contact with fresh, weatherable rock-unusual in a warm, wet, tropical climate where weathering should extend deep below the surface. Tectonically driven uplift likely maintains the supply of weatherable material, leading to channel incision and, thus, to the exposure of bedrock in many river channels. Despite centuries of agriculture, the impact on these rivers' biogeochemistry is limited. Although river water in many central Cuban rivers has high levels of E. coli bacteria, likely sourced from livestock, concentrations of dissolved nitrogen are far lower than other areas where intensive agriculture is practiced, such as the Mississippi River Basin. This suggests the benefits of Cuba's shift to conservation agriculture after 1990 and provides a model for more sustainable agriculture worldwide.

Cosmogenic nuclide and solute flux data from central Cuba emphasize the importance of both physical and chemical denudation in highly weathered landscapes

“…Prior knowledge of mass loss at the basin scale is limited to measurements of suspended sediment discharge for short periods between 1964 and 1983 for 32 Cuban rivers (Pérez Zorrilla and Ya Karasik, 1989), and measurements of dissolved loads in five limestone basins with karst (Pulina and Fagundo, 1992). In central Cuba, underlying basin rock type is the primary control on surface water geochemistry (Betancourt et al, 2012), a finding supported by geochemical analyses of river waters from the same basins sampled in this study (Bierman et al, 2020). Dissolved load fluxes carried by Cuban rivers (Bierman et al, 2020), and rock dissolution rates inferred from these fluxes, are consistent with rates reported for other Caribbean islands [Dominica, Guadeloupe, and Martinique from Rad et al (2013) and Puerto Rico from White and Blum (1995)], and high compared to global data compiled by Larsen et al (2014).…”

“…In central Cuba, underlying basin rock type is the primary control on surface water geochemistry (Betancourt et al, 2012), a finding supported by geochemical analyses of river waters from the same basins sampled in this study (Bierman et al, 2020). Dissolved load fluxes carried by Cuban rivers (Bierman et al, 2020), and rock dissolution rates inferred from these fluxes, are consistent with rates reported for other Caribbean islands [Dominica, Guadeloupe, and Martinique from Rad et al (2013) and Puerto Rico from White and Blum (1995)], and high compared to global data compiled by Larsen et al (2014).…”

“…2; Supplement T1-2). Channel morphologies varied, but most streams were incised, and many had exposed bedrock (see Bierman et al (2020) for photos/descriptions of select field sites). Three sample sites are near previously-gauged hydrologic stations with discharge and suspended sediment records spanning 9-15 years (Pérez Zorrilla and Ya Karasik, 1989).…”

“…1). Bierman et al (2020) attribute high rock dissolution rates and the relationship between stream water chemistry and bedrock type in Cuba to extensive rock-groundwater interaction along subsurface flow paths, controlled by ongoing bedrock uplift and associated rock fracturing. The strong negative correlation between chemical denudation rates and average basin slope also supports the importance of deep weathering in landscape denudation, since such weathering occurs when rock dissolution progresses faster than weathering products are removed from the landscape, a process favored in low relief settings (Ollier, 1988).…”

“…The five basins underlain by marine rocks also demonstrate the greatest disagreement between high rock dissolution rates and low sediment generation rates (9-32X). Water geochemistry data from four of these basins (CU-120, 121 122, 132) suggest the presence of evaporites due to high concentrations of Cl, SO4, Br, and Na (Bierman et al, 2020), and the fifth basin (CU-106) is primarily underlain by the same marine unit as those the basins, presumptive evidence of the presence of evaporite deposits somewhere in the basin.…”