Links between international market trends, agricultural intensification and extreme sedimentation rates in a transboundary South American coastal lagoon

Abstract

In response to global food demand, crop cultivation in the South American Pampa Biome rapidly expanded, intensifying erosion linked to land-use change and mechanisation. In Uruguay and SE Brazil, the most significant expansion occurred after 2000. This study examines the economic, environmental, and productive changes in an agricultural basin over the past two decades, focusing on the expansion of the primary sector of the economy, particularly soybean cultivation. It also investigates the cause of not only maximum but also abnormally extreme sedimentation rates (SR) observed between 2005 and 2009 CE.

Findings indicate that SR trends reflect land-use changes in the basin. A notable shift occurred in the 1980s after the lagoon was converted to a freshwater system, leading to a twofold increase in SR. The peak in SR observed in core MIR2 (i.e., 400 % increase) was recorded around 2005–2009 CE, alongside a 2000 % increase in terrestrial palynomorphs (pollen, spores, and other organic-walled microfossils from terrestrial sources) in core MIR4. For this period, changes in sediment grain size distribution appear to resemble drastic shifts in land cover. This coincided with the approval of glyphosate-resistant seeds (2005/2006 harvest) and a rise in soybean prices post-2006. The extreme sedimentation rate likely resulted from soybean expansion and intensified unsustainable practices like ploughing, which increased the export of fine terrestrial material. Our findings highlight how both land-use changes and climate variability (particularly extreme precipitation events) contributed to the observed SR anomalies, thus suggesting a reinforcing interaction between agricultural expansion and hydrological extremes.