Reconstructing the impact of environmental changes on the river sediment dynamics of the Amazon River and Rio de la Plata drainage basins with radiogenic isotopes

Abstract

Billions of tons of sediment are transported annually through river systems worldwide. The river sediment dynamics are tightly coupled to the global climate and changes in the precipitation intensity or region generate variations in the transported sediment amount or the source area. Humans have also done severe modification to the river sediment dynamics. Actively by rerouting waterways and damming rivers and passively by enhancing surface erosion through soil destabilization (i.e. deforestation, agriculture). River sediment has an important role in the riverine and coastal ecosystem and changes in physical and chemical properties can have long-lasting consequences. With the prognosis of fast-pacing and drastic climate changes in the near future and the anthropogenic impact proceeding to increase, a study of how past processes have change the river sediment dynamics and a monitoring of the modern situation, is a good approach for predictions about future developments. This PhD thesis aims to reveal the modern and past sediment supply situation for the Amazon River and Rio de la Plata drainage basins, the two biggest river basins of South America that considerably influence the global sediment supply into the world oceans. To achieve this we analyzed radiogenic isotopes (Sr, Nd and Pb) and element concentrations of river sediments and marine sediment cores that archive the last 30,000 to 40,000 years of river sediment supply to the Atlantic Ocean. Radiogenic isotopes are not significantly affected by weathering, transport or depositional processes, making them useful tracers to study sediment provenances. In agreement with studies of other chemical parameters of the modern Amazon River sediment, two main sediment source areas (the Andes and the cratonic Shield) were identified. Hereby, the Andes were the main sediment supplier in the last 40 kyr, however, a prominent offset between the past and modern radiogenic isotope signatures was determined. A climate induced shift, due to a wetter western or drier eastern basin sector, during the late Holocene is proposed as the cause. Furthermore, a change from a Madeira River to a Solimoes River dominated sediment signal is observed in the last decades, implying a possible impact of modern river dams on the sediment dynamics in the Amazon River basin. Additionally, the results also show a shift in the precipitation locus during the second part of the Heinrich Stadial 1 from the Andes to the cratonic Shield areas. The radiogenic isotope signatures of the modern Rio de la Plata drainage basin sediment establish three sediment source areas, i) the upper Parana River, ii) the Uruguay River and iii) the Andean draining rivers Salado, Pilcomayo and Bermejo. The down-core results reveal several pronounced sediment source changes in the last 30 kyr that are attributed to millennial scale climate events. A strong South American Summer Monsoon during Heinrich Stadial 1 and the Younger Dryas intensified the sediment supply of the Andean draining rivers. During times of strong SE Trade winds (i.e. the Bolling-Allerod interstadial) the southeastern sector of the basin (i.e. Uruguay River drainage basin) dominated the sediment supply. In addition, the results confirm an external (southern Argentina) sediment deposition in front of the Rio de la Plata estuary due to a maximum sea level stand during the late Holocene. The third manuscript deals with the requirements of the methodology to extract a detrital signature from a marine sedimentation setting. Moreover, it emphasizes that the radiogenic isotope signatures of the carbonate and Fe-Mn oxyhydroxide fraction should be interpreted with caution. Based on the high strontium (Sr) concentration in marine carbonates, on the one hand a decarbonization to obtain a detrital isotopic signature is extremely important, on the other hand the Sr isotopic signature of the carbonate leach fraction is resilient to contamination and does represent a paleo seawater signal. Furthermore, the results suggest that the Fe-Mn oxyhydroxide fraction of marine sediments in river influenced settings does not represent a paleo seawater signal, but rather either a river signature through leaching off pre-formed river-borne Fe-Mn oxyhydroxides (Amazon River basin) or a contamination through the leaching of detrital material (Rio de la Plata drainage basin). Overall, this thesis illustrates the past sediment supply evolution and modern sediment dynamics of the Amazon River and Rio de la Plata drainage basins. Additionally, the results extend the current database of radiogenic (Sr, Nd and Pb) isotope signatures on river sediment. Moreover, the study emphasizes the importance of the methodological approach.