Sedimentation rate and climate variability
The observed changes in the sedimentation rate are clearly related to the centennial climatic variability (Figs. 2, 3 and 4). That is, the lowest sedimentation rates of 1 mm yr− 1 recorded between 1360 and 1880 CE are most likely a consequence of the impact of the dry conditions inferred for the LIA on the hydro-climatology of SE-Uruguay (Fig. 7, del Puerto et al. 2011, 2013, Lüning et al. 2018). LIA was a globally studied climatic period, characterized by the lowest temperatures recorded for the last millennium (Mann et al. 2009, Lüning et al. 2018) with a strong impact on rainfall and wind patterns in a wide region of SESA, generating significant hydrological changes (Piovano et al. 2009; Lopez-Blanco et al. 2020). For SE- Uruguay, a colder and less humid phase than the present climate was inferred, which exhibited two peaks of minimum temperature and humidity at ∼ 1600 and 1900 CE (del Puerto et al. 2011, 2013). According to Piovano et al. (2009), LIA was associated with lower rainfall over the southernmost area of influence of SALLJ, which was responsible for the humidity advection in Uruguay mainly during the austral summer (Nunes et al. 2009). In this sense, a weakening of SALLJ during LIA was inferred, which is also a common feature occurring nowadays during negative anomalies of the sea surface temperature (SST) in the South Western Atlantic Ocean (Doyle and Barros 2002). As a consequence, during LIA we observed the lowest sedimentation rate due to a low rainfall pattern and low input of clastic material from the basin. After that, the intermediate sedimentation rate of 2 mm yr− 1 recorded between 1880 and 1980 CE is a consequence of the end of LIA and the onset of CWP, where a significant and sustained increase in temperature and humidity occurred in the region compared to the precedent period (Figs. 3 and 7; del Puerto et al. 2011, 2013).
The noticeable increase in sedimentation rate observed after the 1980s (i.e., 4.5 mm yr− 1) is explained by the local but also regional increasing trend of total annual rainfall, mainly associated with an increase in austral spring (SON) and summer (DJF) rainfall (Figs. 2 and 4). During this time, a clear shift was also observed in the accumulated rainfall of 1200 mm yr− 1 after the 1980s, while the previous average had been lower, i.e., 1000 mm yr− 1. Such results were previously recorded by García-Rodríguez et al. (2002) for the meteorological Rocha station rainfall time series, which is located less than 20 km away from Laguna de las Nutrias. This observed hydroclimatic shift was widely recorded worldwide with contrasting hydrological changes depending on the region (i.e., Huang et al. 2005; Vance et al. 2015). The “1970´s climatic shift” is related to a change in the SST of the tropical Pacific and Atlantic oceans, with a predominance of positive ENSO (El Niño events) and PDO phases and negative AMO phases (Huang et al. 2005; Vance et al. 2015; Perez et al. 2021a, b). In particular, it affected significantly the precipitation regime of different regions of South America in different ways (e.g., Jacques-Coper and Garreaud 2014; Córdoba et al. 2014; Perez et al. 2021 a, b). For instance, the RdlP drainage basin displayed more than a 10% increase in rainfall, which led to ∼ a 30% increase in the river flow, thus modulating the highest runoff values recorded in the RdlP mud depocenter during the last millennium (ADT 2016; Perez et al. 2021 a, b). This increase in rainfall is consistent with the concomitant increase in SALLJ frequency and intensity (ADT 2016), which explains the high rainfall levels over SE-Uruguay after 1980 CE (García-Rodríguez et al. 2002; Nunes et al. 2009) and a wide region of the SESA (Piovano et al. 2009; Córdoba et al. 2014). To this end, we further observed statistical differences for the standardized rainfall anomalies for pre and post-1970, with negative and positive anomalies recorded, respectively (Fig. 4b).
Sedimentary proxies and historical hydroclimatic variables
Rainfall time series was positively correlated with both positive PDO and ENSO phases (Fig. 2). In this sense, it is well documented that inter-annual ENSO oscillations modulate both SALLJ frequency and intensity, which exerts an impact on Uruguayan rainfall, i.e., El Niño events increase rainfall in SE Uruguay (Nunes et al. 2009; Barreiro 2010). Within the sedimentary record, we detected some significant correlations between proxies and the standardized rainfall anomalies time series for the last century (Fig. 4c). We observed that increases in rainfall, in part related to El Niño events, led to an increase in lake level, which probably generated less oxygenated bottom conditions in summer (i.e., DJF), possibly as a consequence of the stratification of the water column due to rainfall and temperatures increase, as revealed by, Ti/Ca, and Fe/Mn sedimentary proxies, respectively (García- Rodríguez et al. 2021; Perez et al. 2021 b). In addition, we detected a positive correlation between Ti/K and annual rainfall, especially for the austral winter (JJA), which is related to the fact that rainfall throughout the year ranges within the same order of magnitude, but moisture sources vary significantly from the SALLJ region during the austral spring and summer seasons with a NW direction, to the Atlantic Ocean region during the austral winter with a SE direction (Marengo et al. 2009; Nunes et al. 2009). During austral winter there is also high a frequency of cold fronts related to southerly winds. Such winds bring into the sedimentary record of Laguna de las Nutrias not only rainfall from the south but higher grain size from the adjacent dunes, which are localized southward the lagoon (Fig. 1c). Furthermore, rainfall was negatively correlated with the organic matter content proxy (i.e., S/Ti). Thus, dryer periods appear to be associated with more oxidative conditions and mixed water column and, therefore, higher light penetration, which led to increasing microalgae autochthonous productivity and higher associated trophic state (Haberzettl et al. 2007; Moreno et al. 2007; Unkel et al. 2010). This study also shows that an increase in rainfall, watershed runoff and lake level would generate a decrease in the trophic state, interpreted as a dilution process of nutrients due to an increase in the water column and turbidity, as already reported by Stutz et al. (2012) in the Pampa plain.
Relationship between ENSO and sedimentary proxies during the last 700 yr
We identified two groups of XRF sedimentary proxies (Figs. 5 and 6): Group 1 related to higher watershed runoff, lake level, and lower water column oxygenation (i.e., higher Ti/Al, Ti/Ca, and Fe/Mn, respectively), all of them showing a positive significant correlation to each other over the last 700 year. On the other hand, Group 2 consisted of those variables related to higher trophic state, organic matter content, grain size, and influence of marine spray (i.e., higher Si/Ti, S/Ti, Ti/K, and Br/Ti, respectively), which displayed not only a positive significant correlation to each other but also were negatively correlated with Group 1. The highest/lowest record of Group 2/1 was recorded approximately between 1360–1800 CE/1800 CE- present. After 1360 CE, and climatically related to LIA (1400–1800 CE, Mann et al. 2009, Lüning et al. 2018). During that period, there was a prevalence of colder and less humid conditions in SE Uruguay as first documented by del Puerto et al. (2011) and (2013). This condition led to the lowest lake levels, associated with a lower watershed runoff and influence of major storms of marine influence, together with the highest inferred trophic state (Figs. 5 and 7; Haberzettl et al. 2007; Moreno et al. 2007; Unkel et al. 2010; Perez et al. 2021b). Oliveira et al. (2014) introduced evidence of a notorious increase in storms on the SE coast of Brazil, associated with a more intense passage of cold fronts during LIA, which was translated into an increase in marine spray influence, grain size, and oxidation which was reflected in the sedimentary record of Laguna de las Nutrias. Nowadays, this condition of higher cyclonic activity occurs during austral winter and spring (Grimm 2009). In addition, Laprida et al. (2009) recorded similar patterns of decrease in the water column level and increase in grain size for shallow Pampean lakes during LIA, also consistent with the results of other Pampean lakes (Fig. 7, Córdoba et al. 2014; Guerra et al. 2019; Lopez-Blanco et al. 2020). Limnologically, Laguna de las Nutrias would be interpreted as a clear water phase lake during LIA (Stutz et al. 2012; Mourelle et al. 2020), dominated by macrophytes with a high OM content as recorded by Azcune (2019).
After 1800 CE, with the end of the LIA arid period in SE Uruguay, we recorded a steady increasing trend in the precipitation, watershed runoff, lake level and redox condition, and a decreasing trend in the trophic state. Such conditions could be related to a limnological change towards turbid conditions, dominated by phytoplankton and primary productivity limited by light (Stutz et al. 2012; Mourelle et al. 2020). Such conditions of higher precipitations after the middle of the 19th century were recorded in the Pampa plain lakes as well (e.g., Córdoba et al. 2014). For instance, the Mar Chiquita Lake exhibited elevated lake levels, while the RdlP mud depocenter displayed significant continental input onto the shelf (Piovano et al. 2009; Perez et al. 2021a). These conditions were associated with the onset of CWP, during which we inferred higher temperatures and precipitations for the SESA region (Fig. 7, Piovano et al. 2009; Córdoba et al. 2014; Guerra et al. 2019; Lopez-Blanco et al. 2020; Perez et al. 2021a, b). SESA precipitation trend during CWP is partly attributable to SALLJ intensification arising from natural variability and anthropogenic forcing (Varuolo-Clarke et al. 2022). Furthermore, the positive trend in South American precipitations was even more notorious after the 1970s (Jacques-Coper and Garreaud 2014), when the strongest El Niño events occurred for the last millennium (i.e., 1982/1983 y 1997/1998). This climatic shift in both the intensity and dynamics of ENSO has a clear sedimentary expression in several regional records (Córdoba et al. 2014; Perez et al. 2021 a, b). Accordingly, we introduce evidence here of the highest watershed runoff, lake level change, and lower trophic state, which may be a consequence of nutrient dilution and increased turbidity in Laguna de las Nutrias after 1980, as a consequence of the increasing trend in local summer and spring precipitations (Fig. 2., García-Rodríguez et al. 2002; Perez et al. 2021a).
In the present study, significant centennial, multidecadal, decadal, and interannual cycles were observed (Fig. 5), which were also inferred from the paleoenvironmental reconstruction of the continental export into the RdlP and were related to the Parana River hydrological changes related to the climate variability modes (i.e., ENSO, PDO, SAM, and AMO; Perez et al. 2021a, b). Centennial cycles are most probably related to the solar forcing, i.e., the 200 year de Vries/Suess cycle, which was previously described as the centennial forcing of SMS activity (Novello et al. 2016), as well as the position of the southern westerly wind belt- SWW (i.e., it migrates south during increased solar activity) (Varma et al. 2011). Likewise, multidecadal variability of 110 − 60 year is attributable to AMO (Deser et al. 2010), which has already been identified as the climate forcing of the multi-decadal rainfall variability over SESA, as it modulates SALLJ intensity (Seager et al. 2010). On the other hand, decadal and interannual variability of around 10-yr cycles and 6 to-2 year cycles are attributable to PDO and ENSO, respectively (Deser et al. 2010). PDO and ENSO are related to changes in the Pacific SST ( Deser et al. 2010), and PDO is associated with ENSO as both modes of variability seem to produce similar climatic effects, e.g., enhance SESA precipitation during El Niño (warm-phase ENSO events, Barreiro 2010), although their intrinsic mechanisms are not yet fully understood (Garreaud et al. 2009). ENSO variability and impact on the sedimentary record of Laguna de las Nutrias was further evidenced by correlating the proxy data with the ENSO reconstruction of Mann et al. (2009) for the last centuries (Fig. 6). Here, we registered a positive significant Spearman correlation between ENSO reconstruction and the proxies of watershed runoff, lake level change and redox conditions, and negative with proxies of trophic state. Thus, ENSO events appear to play a fundamental role in the interannual variability of SALLJ, and therefore they both modulate rainfall over Uruguay (Grimm 2009; Nunes et al. 2009; Barreiro 2010), which explains the limnological changes of Laguna de las Nutrias over the last 700 years.
Chronostratigraphic correspondence between Laguna de las Nutrias and regional centennial hydroclimatic records of SESA
Laguna de las Nutrias exhibited a good correspondence with the centennial SESA hydroclimatic variability (Fig. 7) observed for the Argentinean Pampa plains, El Niño 3 region and Rio de la Plata. The trends observed for these regions during the LIA are most probably attributed to a strengthening of the South Atlantic Convergence Zone (SACZ) and wakening of SALLJ (both components of SMS) observed during such a climatic period (Piovano et al. 2009; Perez et al. 2021b). There is a climatic dipole, as observed in the Soth American maps of Fig. 7, between the SACZ region and SE Uruguay and SE Brazil i.e., when SACZ is strengthened, there are lower rainfalls in SE Uruguay due to the weakening of SALLJ (Marengo et al. 2009). The aforementioned dipole explains the lowest Laguna de las Nutrias and other Pampa plain lake levels during LIA, but high continental input into the inner shelf adjacent to the RdlP. With the beginning of the CWP, all regional hydroclimatic reconstructions displayed increasing values of lake level and runoff, even higher after the 1970´s due to this climatic shift, which led to a multidecadal hyper-humid phase in SESA (Córdoba et al. 2014). Such a pattern is consistent with the intensification of El Niño events and the concomitant strengthening of SALLJ, which are the modulators of regional rainfall patterns, mainly during austral spring and summer (Marengo et al. 2009; Barreiro 2010). Future scenarios in accordance with anthropogenic warming due to greenhouse gases suggest a higher occurrence and intensity of SALLJ with increased rainfall, which in turn will promote even higher lake levels for the regions influenced by SMS.
Final considerations
The studied sedimentary record documented three clear climatic periods during the last 700 year, namely LIA, CWP and “1970´s climatic shift”. The LIA was recognized as a dry climatic period which included the lowest observed sedimentation values, watershed runoff, and lake level, explained by low rainfall due to the weakening of SALLJ. During the CWP, a sustained increase in humidity was established in the region and the lake experienced increased sedimentation rate values, watershed runoff, and water levels. The maximum expression was observed after ¨the 1970´s climatic shift” where the highest rainfall values were documented, and the lake exhibited the highest sedimentation rate values. The increased rainfall and watershed runoff after 1880 CE, and even more noticeable since 1970 CE, was consistent with the intensification of El Niño events as recorded by Mann et al. (2009). Our results are in close agreement with other regional studies where similar hydrological changes were inferred for the last centuries, pointing toward the significance of SMS controlling the hydrological balance in SESA.