Environmental and climate history of Laguna Polo
According to CONNISS analyses, four periods were established in POL-18 core (Figure 4 and 5):
Period 1 (1300-1420 AD): A mesotrophic lake during warm and dry conditions
Analyses of biological and non-biological proxies in the deepest sediments (from 54 to 49 cm, 1300 to 1400 AD) shows low values of terrigenous input K/coh mean (x̄) = 0.27, Ti/coh (x̄)=0.96) (Davies et al. 2015). δ15N (x̄=1.55‰) also show low values and N (x̄= 80%), TOC (x̄=9.57%), δ13C (x̄=-23.93‰) and C/N (x̄=14.50) are moderately high. δ15N is used as a proxy of the nitrification process, past oxygen concentrations and respiration process in the environment (Heyng et al. 2012) whereas N and δ13C are indicators of productivity conditions. C/N is an index of origin of the organic matter. Therefore, our results show for this period high productivity and local organic matter production proxies such as K/coh (Figure 7). Bioproxies agree with these findings. The dominance of testate amoebae taxa associated with TOC (Centropyxids and L. vas) and low abundance of D. glans “glans”(Caffau et al. 2022) (Figure 4) together with chironomid assemblages are dominated by Tanytarsini morpho B and Parapsectrocladius, which are taxa related to temperatures close to 10 °C (Massaferro et al. 2014, 2018; Williams et al. 2019), could be interpreted as conditions with high biological activity suggesting high levels of organic matter and mesotrophic conditions related to higher temperatures (Figure 5).
Based on these results, we infer warm conditions during period 1. Considering the age model, this period corresponds to the termination of the Medieval Climate Anomaly (MCA). While the MCA seems to have had uneven effects across the extra-Andean region (Ozán et al. 2022), results from Laguna Polo indicate that the MCA was warm and possibly dry according to the K/coh record (Figure 3), as described in other regions of Patagonia (Moy et al. 2008; Moreno et al. 2009; Kastner et al. 2010; Zolitschka et al. 2018; Daga et al. 2020).
Period 2 (1420-1720 AD): An oligotrophic lake during a cold and wet period
The first part of this zone (1420-1511 AD, 48-43) begins with an increase in K/coh (x̄= 0.65) suggesting input of terrigenous sediments (Davies et al. 2015). This interpretation is supported by C/N values ranging from 12-14, which indicates autochthonous production probably composed by a mixture of algae and littoral macrophytes (Meyers 2003). However, TOC values (x̄=6.83%) and δ13C (x̄=-25.98‰) are indicating a decrease in organic matter which reflects a decrease in lake productivity. This finding is agreed with testate amoebae DCA axis 2 scores with a negative trend since 46 cm. (Figure 4) and the increase in abundance of D. glans “glans” and a decrease of Centropyxids taxa (Figure 4). This DCA shift is interpreted as a decrease in productivity suggesting more oligotrophic conditions. Also, isotope records δ15N and δ13C, both are indicatives of past oxygen concentrations and respiration process (Heyng et al. 2012). During this period the increase of δ15N and the decrease of δ13C are indicating an acceleration of anoxic processes such as methanogenesis and denitrification (Fig, 8). Tanytarsini morpho B and Parapsectrocladius decrease whereas Cricotopus and Dicrotendipes appear during this zone, therefore the negative chironomid DCA axis 1 is interpreted as a cold period, probably with temperatures close to 3-5° (Massaferro and Larocque-Tobler 2013; Williams et al. 2020).
Between 42-33 cm (1525-1670 AD), a peak in δ15N suggests a period of very low oxygen conditions nearly reaching anoxia (Figure 7). Associated with this, TOC values decrease (3.49%), affecting the composition of the testate amoebae communities with the decrease of L. vas and low Shannon index (<1.5), reflecting stressful conditions for this protists (Patterson and Kumar 2002). During the peak of δ15N, Dicrotendipes increase, Parapsectrocladius decrease and Cricotopus disappears (Figure 5). The causes are still unclear, however, it could be related to anoxic conditions, as some species of Dicrotendipes are tolerant to low oxygen availability (Verbruggen et al. 2011).
Biological assemblages and geochemical records altogether indicate that this period was cold and humid. Indeed, the period is coincident to the Little Ice Age (LIA) described for the Southern Hemisphere during ~1550-1800 AD (Haberzettl et al. 2005; Meyer and Wagner 2008; Moy et al. 2008; Fey et al. 2009; Waldmann et al. 2010; Daga et al. 2020).
In Laguna Polo, the peak of δ15N at 40 cm is interpreted as denitrification linked to long cold winters during the LIA. In recent times, Laguna Polo has exhibited ice-covering during winter. Prolonged ice cover on the lake and snow cover in the catchment area have notable implications for biological communities (Fey et al. 2009). For instance, the prolonged reduction of annual fluvial activities, potentially decreases the clastic input to the lake which in turn, coincides with the K/coh record shown in Figure 3. Furthermore, these ice cover conditions lead to prolonged periods of lake water stratification under ice creating extended anoxic conditions at the water/sediment interface, thereby potentially reducing organic matter mineralization as reflected by the bioproxy assemblages.
The final phase of this period shows a chironomid turnover at 35 cm DCA axis 1, Figure 5) that is interpreted as an increase in temperature specially by the increase of taxa like Parapsectrocladius (Figure 5). Also, a decrease of δ15N (increase of nitrification processes) and the increase of δ13C (increase of photosynthesis), both are indicating more biological activity related to higher oxygen availability (Figure 7). Future studies are needed to confirm these findings.
Period 3 (1720-1960): Volcanic activity from Lautaro Volcan
This period (34-15 cm) is characterized by the presence of two conspicuous tephra from Lautaro Volcano, confirmed by the Ca/coh record with peaks of >0.9% during these layers. Each tephra has a different thickness showing different effects on Laguna Polo (Figure 3). Due to the massive volume of volcanic material, other climatic or environmental signals remain clouded by the ash deposition, for instance Ti/coh is affected by ash deposition therefore difficult to use as a proxy for terrigenous input. (Figure 3).
According to DCA axis 1, the record of testate amoebae revealed the strong effects of volcanic events on freshwater communities (Figure 4). Recently, Charqueño-Celis et al. (2022) evidenced the response of testate amoebae to the Lautaro eruptions in Laguna Verde, Argentina showing similar results. For both records, the diversity of amoeba shows a significant decrease but a recover soon after the tephra deposition. Contrarily, chironomids have different responses after the ash events. In the first tephra, between 32-25 cm (1886, AD) chironomid head capsules are absent, whereas in the second tephra between 19-17 cm (1933 AD), chironomids do not show any change in their abundances. Previous studies from northern Patagonia document different responses of chironomids to volcanic events (Massaferro et al. 2005, 2018; Williams et al. 2016; Serra et al. 2021). Massaferro et al (2005) and Williams et al (2016) show that diversity and abundance of chironomid assemblages respond differently to different volcanic events, in Lake Morenito. Massaferro et al. (2018) suggested that susceptibility of chironomid assemblages to disturbance and ecosystem resilience depends on several local factors such as lake watershed, morphology, hydrology, and vegetation cover. Serra et al (2021) mentioned that chironomid community response depends indirectly on factors that determine the way the volcanic ash is deposited, like wind direction and velocity, rainfall after ash deposition or the presence of a macrophytes belt surrounding the lake.
Also, the effect of ash deposition could alter the transparency of the water column reducing the incoming light and thus the phototrophic biomass distribution along the water column provoking a decrease in light (Modenutti et al. 2013) This, in turn, will affects the dynamic of macrophytes in the deep lake and fauna associated as is the case for chironomids and certain taxa of testate amoeba.
Period 4 (1960- to present): Recent climatic warming
This final period begins with the eruption of the Lautaro volcano in 1959. The resulting 6-cm-tephra layer has the strongest impacts on the Laguna Polo. Testate amoebae are absent between 12-10 cm and no chironomid head capsules were recovered within 13-10 cm (Figure 5). After the tephra layer, amoeba DCA axis 2 scores suggest less productive conditions compared to period 1, which is reflected by an increase of Centropyxids as well as L. vas until top of the core (Figure 4). However, in the first 4 cm there is a decrease of D. glans “glans” and an increase of C. aculeata “aculeata” suggesting an increase of productivity to present times. This is reflected in δ15N values (2.57‰-3.07‰) showing changes in the nitrification process whereas in the case of δ13C values (> 26‰), the lowest values recorded on the first four centimeters of the core suggest an increase on respiration processes (Figure 7).
Chironomid records agree with these results. Parapasectrocladius increases and Chironomus and Riethia appear, both related with high organic matter (Mauad et al. 2021). The absence of littoral Cricotopus during this period is attributed to a decrease in precipitation and probably, as an increase in temperature as reflected by DCA axis 1 (Figure 5). The increasing temperatures are consistent with other regional studies. For example, Minowa et al. (2017) determined a warming trend of 0.059°C a−1 for air temperature from 1999 and 2013 at the glacier Perito Moreno located 139 km to the south of Laguna Polo. This trend is also reflected by air temperature and relative humidity values collected at the meteorological station “Los Huemules” for the years 2006-2018 (Figure 1). These findings agree with the increasing Global surface temperature recorded for the first two decades of the 21st century (IPCC 2023).
Atmospheric circulation, climate and environmental forcings during the last millennium in southern Patagonia
The SAM can be described as an atmospheric variability characterized by a predominantly east-west symmetric movement of air masses, displaying opposite pressure anomalies between the mid-latitudes (45-60°S) and Antarctica. This phenomenon influences strength and position of the SWW. A positive phase of SAM is associated with a more southerly movement of the SWW, causing drier and warmer than average conditions in Patagonia, as described for the MCA (Ozán et al. 2022). Indeed, our results of Period 1 from Laguna Polo support dry and warm conditions which could be related to a positive phase of SAM. This phase was described for other lakes in southern Patagonia, including Lago Futalaufquen (Daga et al. 2020), Laguna Guanaco (Moy et al. 2008; Moreno et al. 2009),Lago del Desierto (Kastner et al. 2010) and Laguna Azul (Zolitschka et al. 2018). The negative phase of the SAM is associated with a northward displacement of the SWW, and it is climatically reflected in Patagonia as a cold and humid period (Waldmann et al. 2010; Daga et al. 2020). These conditions have been described for the LIA in several lakes of south Patagonia, such as Laguna Azul (Zolitschka et al. 2018), Las Vizcachas (Fey et al. 2009; Quintana et al. 2018), Cháltel (Ohlendorf et al. 2014), but also for Guanaco (Moreno et al., 2010), del Desierto (Kastner et al., 2010), Argentino (Caffau et al. 2022), Puyehue (Boës and Fagel 2008) and Cardiel (Ariztegui et al., 2010), as well as for Valle de Andorra bog (Chambers et al. 2014). For Laguna Polo, the cold and wet period 2 is observed between ~1400-1700 AD and it is aligned to the negative phase of SAM. However, these findings need to be further tested as the presence and timing of the LIA in the southern part of South America is not entirely clear. The LIA is recorded in the Northern Hemisphere between the 13th and 19th century whereas for southeastern Patagonia the LIA is shifted to the period between 1550 and 1800 AD (Meyer and Wagner 2008) and some records reported this cold period between 1700 and 1900 AD (Haberzettl et al. 2005; Mayr et al. 2005; Daga et al. 2020).
The volcanic eruptions of the Lautaro volcano during the 19th and 20th century masked the climatic signal. However, during period 4 in Laguna Polo, which represents the most recent times, it could be possible to infer a warming spell similar to the one recorded in period 1 of this record, which could also be linked to a positive SAM phase. Indeed, Fogt and Marshall (2020) found that the SAM index displays a marked positive trend during austral summers since 1979, associated with stratospheric ozone loss. These recent and positive SAM index values are unprecedented during the last millennium and ranged outside of natural climate variability. Indeed, in recent decades, SWW have been shifting southward and have been more intense (Koffman et al. 2014) Although this interpretation is still speculative, it is possible that such variation may be reflected in the lake's dynamics and its communities in a period not too distant from the present.