Last millennium intensification of decadal and interannual river discharge cycles into the Southwestern Atlantic Ocean increases shelf productivity

Highlights

• The Río de la Plata mud depocenter holds an excellent high resolution paleoceanographic record.

• Continental input was related to climatic modes of oscillation and intensified over the last 1000 yr.

• River discharge frequency/intensity was modulated by the ENSO and exhibited decadal cycles from 1500 to 1800 CE.

• After onset of the Current Warm Period, cyclicity changed to half-decade cycles.

• Since 1970, river discharge attained maximum magnitude/frequency as a sign of combined natural and anthropogenic forcing.

Abstract

Large rivers represent transitional environments between the coast and the open ocean which discharge is influenced by both climate and anthropogenic impacts. In general, historical information on river discharges does not extend beyond 100-yr data series. This is the case of the Río de la Plata (RdlP) watershed, which represents a very important geographic region sensitive to El Niño Southern Oscillation (ENSO). Thus, this study analyzes a continuous millennial, high-resolution record of river discharge oscillation cycles into the ocean and associated productivity changes. We used two 10-m-long sediment cores retrieved from the high-resolution RdlP mud depocenter from the inner continental shelf off Uruguay, namely GeoB 13813-4 and GeoB 13817-2. The first sediment core yielded a mean linear sedimentation rate of 9.7 mm yr⁻¹, while the second exhibited a lower resolution, with a mean rate of 7.1 mm yr⁻¹. Differences in the sedimentation rate are attributed to the distance to the continental source. We performed 2-mm-step-size XRF scanning and used the Ti/Al, Fe/K, Fe/Ca and Si/Al element ratios as proxies for terrigenous supply into the Southwestern Atlantic Ocean (SWAO), and Fe/Mn as a proxy of the redox conditions. At the same time, Ba/Al ratios recorded productivity changes. We detected significant cycles of 50, 20, 10, 2–7 yr, and less than 1 yr for most of the element ratios of both cores. We inferred that cyclicity was related to the Climatic Modes of Oscillation (CMO): Atlantic Multidecadal Oscillation (AMO), Pacific Decadal Oscillation (PDO), Southern Annular Mode (SAM) and ENSO. The CMO play a crucial role in modulating the geochemical characteristic of the terrigenous fine sediments, building up the RdlP mud depocenter. The process of millennial intensification of river discharge, and the associated increase in productivity, were both modulated by the ENSO mode of variability, particularly after the onset of the Little Ice Age (LIA) especially after 1500 CE. During the LIA, ENSO displayed mostly decadal cycles, whilst after the end of the LIA and onset of the Current Warm Period (CWP), the cyclicity intensified to half-decade cycles. After 1970s, river discharge attained maximum magnitude and frequency as a sign of combined natural and anthropogenic forcing, but we also inferred a concomitant increase in productivity. Herein, we introduce evidence that global warming is causing an intensification of the inter-annual hydro-climatic variability within SESA and land-use practices (mainly intensification of soya crops and deforestation) are producing significant soil erosion after 1970. This intensification represents an early warning signal to anticipate a further increase in continental input and productivity within SWAO, which holds both regional and global implicances.

Introduction

The study of natural systems aiming to assess global climatic changes requires data acquisition and analysis at different scales of variability, which encompass daily, annual, decadal, centennial, and millennial processes. Environmental databases dealing with direct observations are mostly available from daily to decadal variability and effectively resolve scientific questions of monitoring relevance. From multi-decadal and centennial to millennial processes, one of the most important sources of evidence is the sedimentary record (Smol, 2008). The statistical properties of such records arise from the resolution and continuity of the information. Since marine mud depocenters hold outstanding high-resolution continuous records of millennial information (Hanebuth et al., 2015, Hanebuth et al., 2019), inter-annual, decadal and centennial oscillation cycles can be appropriately investigated (Perez et al., 2016, Perez et al., 2018: Mourelle et al., 2018). Given the availability of non-invasive millimetric scanning step-size techniques (μXRF; Croudace et al., 2019), such mud depocenter environments represent an excellent study case to demonstrate how paleoceanographic data can replace long-term oceanographic and hydrologic direct observations (Croudace et al., 2019).

River discharge into the oceans is a fundamental process for maintaining the estuarine hydrological dynamics and associated geochemical and sedimentary cycles, carbon uptake and primary production (Milliman and Meade, 1983; Bianchi, 2007; Hoffmann et al., 2010). According to Clark et al. (2015) South America contributes with about 30% of the world's river flow into the ocean. This is related to the fact that the continent contain two major drainage basins, i.e., the Amazon and Río de la Plata (RdlP), which are respectively ranked as 1st and 9th in terms of global water discharge, and 1st and 5th in terms of drainage basin size (Milliman and Meade, 1983; Milliman et al., 2008). Coastal oceanographic systems exhibit transitional characteristics such as salinity and turbidity gradients developing from the river discharge in the form of plumes (Burrage et al., 2008; Tudurí et al., 2018). Such a riverine discharge is affected by human activities and natural forcing and carries vast amounts of fine sediments eroded from coastal watersheds (Syvitski et al., 2005; Walling, 2006; Depetris and Pasquini, 2007a, 2007b; Milliman et al., 2008; Viers et al., 2009; Hoffmann et al., 2010; Sun et al., 2016). In this sense the RdlP, is the 14th largest sediment discharge source into the ocean worldwide (Milliman and Meade, 1983). The RdlP is formed by the confluence of the Paraná and Uruguay River, where the main sediment source comes from the Paraná River (Depetris and Pasquini, 2007a, 2007b). The retention/dispersal mechanisms modulating the sediment budget indicate that about one-third of the terrigenous material is deposited within the mud depocenter and the remaining fraction is exported to the ocean (Hanebuth et al., 2015). The RdlP paleo-valley mud depocenter formation started at about 1500 yr BP and was classified as a mid-shelf depocenter growing at a 600–850 cm/kyr as a consequence of increased precipitation and weakening of currents (Lantzsch et al., 2014; Hanebuth et al., 2015, Hanebuth et al., 2019). Therefore, the climatic forcing is identified as a critical control for both mud depocenter formation and growth, and exerts differential conditions for shelf sedimentation (Perez et al., 2018; Mourelle et al., 2018). However, there is still a general lack of studies dealing with the influence of the Climatic Modes of Oscillation (CMO) on the long-term river discharge changes and associated productivity (Milliman and Farnsworth, 2011).

In the case of the Southern hemisphere, especially Southeastern South America (SESA), there is still only weak evidence of hydrological responses to CMO at inter-annual, decadal and multi-decadal time scales (Milliman and Fransworth, 2011; Newkom and Gergis, 2012). The most important CMO modulating SESA are the inter-annual El Niño Southern Oscillation (ENSO) and Southern Annular Mode (SAM), and the decadal and multi-decadal Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) (Depetris and Pasquini 2007 b; Garreaud et al., 2009; Chiessi et al., 2009; Milliman and Fransworth, 2011). ENSO and PDO are indices of the sea surface temperature (SST) changes from the Pacific Ocean, and both play an important role in the variability of the South Atlantic Convergence Zone (SACZ) (Robertson and Mechoso, 2000; Carvalho and Jones, 2009). SACZ is a convective band extending from the Amazon to the Southwest Atlantic Ocean (SWAO), closely related to the South American Monsoon System (SAMS) (Garcia and Kayano, 2010), which increases precipitation mainly over the Paraná River during intense activity, i.e., the austral summer season (Fig. 1 A) (Carvalho et al., 2004). Furthermore, AMO is an index of the SST from the North Atlantic Ocean, which also modulates SACZ and SAMS activities at multi-decadal time scales (Chiessi et al., 2009; Novello et al., 2012). SAM is associated with a latitudinal displacement of the Southern Hemisphere westerly wind belt system (SWW), related to a southward/northward contraction in their positive/negative phases which determined higher cyclonic activity in the high/mid-latitudes (Fig. 1 A and B) (Nunes et al., 2009).

The combined effect of all the different CMO modulates the precipitation regime in SESA, which exerts a direct impact on the magnitude of river flow, especially in the upper reaches of the RdlP (García-Rodríguez et al., 2014). Although there are instrumental data available on mean annual river discharge (Schuerch et al., 2016), the first observations come from the beginning of the 21st Century. Therefore, in this paper we introduce millennial proxy data on inter-annual river discharge oscillation cycles of SESA into the SWAO, with the aim of assessing changes in continental terrigenous exports and associated productivity changes related to natural variability and anthropogenic influence. We have re-visited Perez et al. (2018), who had previously performed time-series analyses at 10 mm step-size scanning resolution. Since we realized that core GeoB 13813-4 represents an excellent source of continuous ultra-high resolution of paleoceanographic information, we re-scanned this core, as well as core GeoB 13817-2 (Fig. 1, Fig. 2) at 2 mm step-size and focus our findings on inter-annual to annual resolution for the last millennium.