West Antarctica plays a pivotal role in sea level rise in projected scenarios with global socioenvironmental and economic consequences. Therefore, describing its recent and past climate history and changes is essential for identifying the main climate controls and past responses. To better understand the climatic sensitivity of this region, we analyzed sedimentary records from newly surveyed proglacial lacustrine profiles dated from the middle Holocene to the present on the North Antarctic Peninsula. We combined atmosphere-glacier-land databases that evidenced a close coupling among these compartments during intense melting events. Our analyses furthermore suggest that a short Neoglacial phase from ∼2,000 kyr BP until the onset of the 20th century was possibly influenced by a combined long-term decline in total solar irradiance, Andean volcanism and ENSO.
Research Article
A synchronous atmosphere-cryosphere-land response to climate on the northern Antarctic Peninsula since the mid-Holocene
https://doi.org/10.21203/rs.3.rs-3601682/v2
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The future changes in the West Antarctic Ice Sheet (WAIS) play a pivotal role in global sea level rise (Watchman et al., 2011, Lüning et al., 2019), and since the end of the 20th century, the northern Antarctic Peninsula (NAP) and WAIS continental inland ice as a whole have been considered one of the most rapidly warming regions of the planet (Bromwich et al., 2012). As a consequence, the WAIS witnessed unprecedented surface snow melting events (Abram et al., 2013) and increasing ice sheet destabilization (Feldmann and Levermann, 2015). However, in recent decades, ground-based meteorological surveys at the NAP have documented variable regional temperature trends, including a steep warming of 0.32 ± 0.20°C between 1979 and 1997 and a relatively cooling period of − 0.47 ± 0.25°C between 1999 and 2014 (Turner et al., 2016), followed by relatively uniform temperatures until 2019. In February 2020, Esperanza Base at NAP recorded a maximum air temperature of + 18.3°C (SNM-Servico Meteorológico Nacional of Argentine - https://www.smn.gob.ar/), officially the warmest temperature ever instrumentally recorded in Antarctica as announced by the United Nations World Meteorological Organization (UN WMO). These short-term successive temperature increases and reversals illustrate how unpredictable the climate at the NAP has become in recent decades, a region that maintains rich polar biological diversity and glaciers under great dynamic stress.
Although much effort has been devoted to the modeling of NAP temperature changes, the relative weighing of key drivers acting on annual ice loss and rapid temperature changes has remained poorly constrained to date (Abram et al., 2013). Actually, several players (or combined effects of them) have been proposed to explain the climate variability at NAP (and WAIS), which includes the atmosphere-ocean coupling linked to the Tropical Pacific (Ding and Steig, 2013), ENSO (El Niño Southern Ocean) (Rahaman et al., 2019), anthropogenic amplification of the circumpolar westerlies via greenhouse gases (GHG) and ozone depletion (Turner, 2006), the Southern Hemisphere Annular Mode (SAM) (Russell and McGregor, 2010), phase coupled ENSO-SAM (Pike et al., 2013), action of atmospheric rivers (Wille et al., 2022) and intrusions of Warm Circumpolar Deep Water (Assmann et al., 2019). From the glaciological viewpoint, reconstructing the past climate variability at the NAP would be a key step toward a better understanding of the processes involved. Nevertheless, revealing its climate on the Holocene timescales at the NAP is challenging since most of the ice sheets in the region have been subjected to (1) very high precipitation rates that prevent accumulation of long ice records with centennial and millennial resolution (Potocki et al., 2016) and (2) melting temperatures during austral summers that modify the glacio-chemical ice core record due to vertical ionic elusion along the ice core profiles (Simões et al., 2004), thus obscuring the history of chemical species deposition. One exception is the James Ross Island (JRI) ice core record, which covers the glaciological history of the entire Holocene (Mulvaney et al., 2012). Studies of marine sediment cores retrieved from the Southern Ocean surrounding the NAP also allow reconstructing climate change at longer timescales, but they are constrained by relatively low temporal resolution and by the strong oceanographic influence (Shevenell et al., 2011). In contrast, analysis of near-shore glaciomarine sediment deposits may reveal indistinguishable interplay among glaciogenic, oceanographic, climatic and physiographic sedimentary controls (Hass et al., 2010). A recognized complexity for the description of the climate of the NAP is, in part, due to its location in a climatic transition zone influenced by both the temperate North and a polar-influenced South (Braun and Gossmann, 2002).
In the geographical context of the NAP, existing lakes in proglacial environments are promising archives for reconstructing regional past climate and environmental changes (Bentley et al., 2009) and, consequently, shed light on both past and modern climate variability. In this study, we revisited Long Lake at the Fildes Peninsula/King Georg Island (KGI)/South Shetlands Islands for sediment coring and presented additional new data from a set of shallow sediment cores retrieved at Admiralty Bay/KGI. We integrated sedimentary records to obtain a composite signal to be compared with (1) in situ glacier mass balance data (MB) from 40 years of continuous monitoring at Bellingshausen Dome/KGI, one of the longest MB records in Antarctica, (2) ice core records from the Antarctic Peninsula, especially from JRI (Mulvaney et al., 2012) in the northern part, and the Gomez ice core in the southern part (Thomas et al., 2009) (Fig. 1). In this work, we present a climate change investigation from a joint project between the International Atomic Energy Agency (IAEA)/Food and Agriculture Organization (FAO) and the Brazilian Antarctic Program to survey proglacial processes at KGI at different time scales: one part derived from an isolated lake covering the mid-to-late Holocene (Long Lake) and a second part integrating nine cores retrieved from glacier-dammed lakes covering the modern epoch/Industrial Age (the last 150 years). In concert with ice core databases, our sedimentary record composite allows investigation of the impact of natural climate forcing controlling the regional climate, such as the total solar irradiance, insolation, ENSO (El Niño Southern Oscillation), insolation and volcanism as well as the CO2 changes from the mid-Holocene to the modern period. Using this approach, we analyzed synchronicities among atmospheric air changes, lacustrine sedimentary records and glacial responses to improve our knowledge of the regional climate variability in the NAP.
Figure 1
Synchronous climate response for atmosphere-cryosphere-terrestrial environment
The first weather stations on the South Shetlands were set up by the end of the 1940s, which allowed seven decades of continuous data (see Supplementary material A2). This time interval is compatible with dating of modern sediment layers using the excess 210Pb technique and the constant rate of supply (CRS) chronological model (see methods) when applied to sediments at undisturbed depositional systems as lacustrine sites (Appleby and Oldfield, 1978). Applying this method, a mean feature of the modern sedimentation pattern obtained from 9 lakes at proglacial sites in KGI (see Supplementary material A3) depicted a slow and steady increase in sedimentation since the end of the 19th century accompanied by a steep acceleration at the end of the 1980s (Fig. 2). This increase reached a maximum in the 1990s and during the first decade of the 21st century, followed by a fast decrease. In the Maritime Antarctic region, austral summer air temperatures control melting and consequently the glaciofluvial erosions of fine sediments transported to proglacial lakes. This mean feature of the sedimentation rates mirrors both variabilities of the measured regional air temperature and the measured equilibrium line altitude (ELA) - the altitude where accumulation equals ablation over a 1-year period - at Bellingshausen Dome (Fig. 2), indicating a high sensitivity of the whole proglacial/glacial system to the local increased air temperatures. The most pronounced melting events (indicated here by the peaks in sedimentation rate) coincided with increases in sedimentary Ti/Ca and Ti/Al ratios identified in the Lake Uruguay sediment core retrieved in the vicinity of the Bellingshausen dome (García-Rodríguez et al., 2021). Our continuous 40-year monitoring of the Bellingshausen Dome ELA at KGI represents one of the longest in situ ELA monitoring campaigns for all South Shetland Islands (since 1970).
Figure 2
The NAP is subject to different climatic influences depending on latitude and geographical position, i.e., western or eastern sectors. The western NAP is warmer and more humid and is characterized by a lesser sea ice seasonal extent, whereas the eastern sector is colder and more susceptible to Antarctic inland air flow with lower air temperatures and humidity (Turner et al., 2016). Despite these differences, a comparison of the 20th century KGI glacier melting calculated from the sedimentary record at KGI (see Supplementary material A3), JRI (melting events) (Abram et al., 2013) and Gomez ice core oxygen isotopes (d18O – a proxy for air temperature) (Thomas et al., 2009) shows good agreement (Fig. 3). The sedimentary response to air temperature increase and the glaciochemical records were synchronous at these sites, especially during the warm period in the mid-1990s. These results indicate that the proglacial sedimentation rates can be considered a reliable proxy of glacier dynamics and isotopic glaciochemistry associated with warming events at the NAP. This finding reinforces previous evidence showing how modern climate changes may affect the terrestrial ecosystem, as observed in the Dry Valleys/Ross Sea Sector, which includes changes in the primary productivity of lakes and the numbers of soil invertebrates (Doran et al., 2002).
Figure 3
Mid-to-Late Holocene climatic oscillations
Additional knowledge on the KGI mid-to-late Holocene climatic history and sedimentation was inferred from a 1 m-long core retrieved at the center of Long Lake (Fig. 1), an isolated lake that is not fed by glacial melt. Its chronological model based on 14C analysis and previous works (Watcham et al., 2011, and Yoon et al., 2006) points to a time span of top sediments for the period 2,049–5,822 year BP (Supplementary material A4). These data reveal a lack (or negligible amounts) of recently deposited material in the center of the lake during the last ∼2,000 years according to the core-top sediment age. We interpret the lack of sedimentation during the last 2,000 year as a persistent cold phase, probably marked by cold summer temperatures preventing significant melting of the winter snow and associated erosion. This assumption is corroborated by other regional proxy records that suggest a “neo-glacial” phase during this period at NAP, rather than local. For example, δ2H values documented for the JRI Ice core (Mulvaney et al., 2012), a proxy for air temperature, imply cooling between 200 year BP and ≈ 2,000 year BP, which marks the coldest period on the NAP during the entire Holocene. Additionally, radiocarbon data of sediments throughout the Fildes Peninsula corroborate the cooling at approximately 2,000 to 3,000 year BP when glacier fronts advanced over presently ice-free areas on the Fildes Peninsula (Barsch and Mäusbacher, 1986).
The long lake sediment core exhibited a well-laminated sedimentary feature (from the X-ray image) with roughly even-spaced light and dark density band formation. This structure reveals sequential deposition of organic clay-rich material (dark-colored) interbedded with silty to sandy material (light-colored). The low-energy hypoxic to anoxic bottom water conditions and the low inner circulation strength, typical of Sub-Antarctic lakes, may promote the laminated sedimentary profile and its preservation. The formation of these laminated structures commonly found in temperate, subpolar and proglacial environments represents successive cold-to-warm climatic phases (Zolitschka et al., 2015). Relationships between laminated sediment layers (varved type sediments) and climate variability are well described in the literature through several examples, mainly in the Northern Hemisphere during the last 2,000 year (e.g., Haltia-Hovi et al., 2007). The light-colored deposits correspond to the melting (warmer) periods, and the dark-colored laminae correspond to colder periods, as supported by the covariance detected in the upper part of the record for the C(%) and N(%) data (Supplementary material 5). TEX86 recorded colder temperatures in this phase, probably due to cold waters apportioning from runoff during melting (Supplementary material 6). The laminated pattern is observed throughout the core and is more evident within the upper 50 cm depth.
The physical and biogeochemical properties of the sediment core indicate both marine and terrestrial/lacustrine sedimentation histories (Fig. 4). The detection of the diatom Achnanthidium australexiguum occurs almost exclusively in the top 50 cm of the core. This species inhabits freshwater environments of islands located at the NAP (Taylor et al., 2014), suggesting a terrestrial phase of the lake. This is corroborated by the abrupt change in total C and N in the sediment core that was enhanced substantially when the terrestrial phase (approximately 3,950 year cal BP) was established. Long Lake is located approximately 300 m from the shoreline at an elevation of only 14 m a.s.l. and the isolation of the lake from the ocean is due to postglacial rebound in response to glacial melt on KGI since 15,500 ± 2,500 year, as inferred from cosmogenic 36Cl on exposed rocky surfaces (Seong et al., 2008). Since no important traces of bird nests are present in the lake catchment, one source of dissolved and particulate forms of C and N in the lake primarily comes from autochthonous algal production, especially for dissolved organic nitrogen (DON) (Downes et al., 1986), as observed for other Antarctic lakes (Carrizo et al., 2019, García-Rodríguez et al., 2021). In addition, there is input from benthic biological production and the supply of fragments of moss and lichen species from the few melt stream waters (Casanova-Katny et al., 2016). Input attributed to wind-blown organic particulates may be an additional source since nutrient-rich Antarctic Ocean coastal waters promote high microalgal productivity around West Antarctica during the whole austral summer season (Deppeler and Davidson, 2017).
Radiocarbon dating for the 1 m section of the sediment core indicates a near linear sedimentation rate (Supplementary material - A4). This allowed us to establish an age-depth linear relationship and investigate the characteristics of the well-laminated sedimentary features. For this purpose, we employed methods to search cyclicity patterns throughout the laminations, strongly imprinted in our record, such as the Morlet Wavelet Analysis (see methods) and the Signal Decomposition technique (see Methods). Applying the Morlet wavelet analysis to the top 50 cm, we detected statistically significant cycles at α = 0.05 from 55.6 year to 147 year (Fig. 5). This cyclicity band is detected not only in the laminated structure but also in the geochemical data of C and N in Fig. 4 (Supplementary material A6). Among the possible natural forcing mechanisms associated with this frequency band is the well-known Gleissberg solar activity cycle (Ruzmaikin and Feynman, 2015). Gleissberg cyclicity is characterized by an average cycle of 88 year and two harmonics (50–80 year and 90–140 year). Imprints of the Gleissberg cyclicities in lacustrine environments have already been documented elsewhere, for example, in the paleo-Lake Pannon in the Alpine region (Kern et al., 2013), at similar temporal resolution as our record and at Lake Holzmaar located in the West Eifel volcanic field in Europe (Vos et al., 1997). Additionally, solar activity cycles are well recognized in modern varve-thickness sequences in Alaska (Hughes et al., 2018). The markedly periodic pattern observed in the density band formation, C and N is an unambiguous indication that during the mid-to-late Holocene, the terrestrial environment experienced cyclic climatic oscillations, changing from warm to cold phases until a gap was established at approximately 2,000 year BP. The Gleissberg double cycle structure is considered a persistent feature in the terrestrial environment throughout the Holocene, as observed by cosmogenic radionuclides, especially 14C (Peristykh and Damon, 2003; Ruzmaikin and Feynman (2015), with higher amplitudes during the mid-Holocene, lower amplitudes during the interval from 3,500 year to ∼500 year BP and increasing its power again towards the present (Ma, 2009). Most Sun-Earth climate connection models attribute the twisted bundles of magnetic energy that boost ultraviolet irradiance UVR by 4–8% during the solar maximum as a driver of the chemistry and air circulation of the middle atmosphere (Andresson et al., 2014; Peck et al., 2015). During the 11-yr solar cycle, changes in UVR reach the atmosphere, primarily controlling ionization, dissociation and excitation processes involving O and N molecules and consequently playing a pivotal role in the atmosphere's vertical thermal structure (Floyd et al., 2002) through exothermic photochemical reactions. These processes may further couple to atmospheric dynamics and propagate to low levels, changing polar winds and atmospheric wave propagation (Andersson et al., 2014). Moreover, since ozone acts on the absorption of UV radiation, it heats the stratosphere near the equator, which may affect global wind patterns (Zastrow, 2015). Lessons from the four decades of ozone depletion above Antarctica revealed a close relationship of this process with the strengthening of Antarctic westerly winds (Turner et al., 2009) with consequences for the Southern Ocean CO2 sink (Le Quéré et al., 2007), the loss of ice shelves fringing at WAIS (Holland et al., 2019), and Antarctic Peninsula summer temperatures (Marshall et al., 2006), among other far-reaching impacts in Antarctica and surrounding continents.
In view of a possible relationship between the sedimentary sequence and solar forcing, we compared the grayscale pattern (for the upper 50 cm) with the reconstructed total solar irradiance (TSI). The 14C-based sediment age model corresponds to the interval 2,049 − 5,822 year cal BP. We used the TSI reconstruction data proposed by Vieira et al. (2011), which is a physically consistent compilation calibrated from combined satellite data, observations of the sun since the 18th century and the use of natural proxies covering the pretelescopic era. Throughout the Holocene, the reconstruction was based on the evolution of the solar magnetic flux that is described by cosmogenic isotope records (mostly 14C and 10Be) in natural archives, including sediment cores (Mann et al., 2012), ice cores (Steinhilber et al., 2010) and tree rings (Suess, 1980).
Figure 5
To investigate the synchronicity of both signals in time (at 10-year resolution), we applied a signal/frequency decomposition model (see Method section) to each time series and superimposed them in seven decomposition levels (Fig. 6). Each decomposition level was then analyzed by iterative regression analysis (ARIST) (Wolberg, 1967; Rigozo and Nordemann, 1998; and Rigozo et al., 2005) (see Methods) to determine their particular cyclicities. From the decomposition of each time series, a crosswavelet analysis revealed well marked signal covariabilities at the L2, L3 and L4 levels, characteristics of Gleissberg and Suess solar cycles. The centennial-scale Suess cycle (or de Vries cycle), quasi-200 year, is believed to be one of the most intense solar cycles imprinted in terrestrial processes (Raspopov et al., 2008). A high signature of this cycle was recognized throughout the Holocene globally (Raspopov et al., 2008) but markedly from δ18O in speleothems of China (Dongge Cave) and Turkey (Sofular Cave). In both cases, δ18O records mainly reflected precipitation intensities controlled by moisture transport during the monsoon system (Knudsen et al., 2011). The very close covariability identified at levels L2, L3 and L4 from the cross-wavelet analysis as well as in the longer cycles in Fig. 5 (L5-L8) underscore the need to better understand the role of solar-terrestrial processes influencing the polar climate on the decadal-to-centennial time scale in the Maritime Antarctic region during the late Holocene, as evidenced at other global sites, as documented above.
Figure 6
Another point to consider is the proximity of the NAP to the southern Patagonian region. Both sites are influenced by the westerly winds, the SAM and the ENSO, which control the transport of moisture and heat from mid-latitudes to high-latitudes (Marshall, 2003). A recent large survey of annually laminated lacustrine cores covering the past 5,000 years from Lago Argentino (latitude 50°S), a Patagonian proglacial lake, revealed varved structures in the sedimentation process with statistically significant cyclicities at 200 ± 20, 150 ± 16, and 85 ± 9 years (Van Wyk de Vries et al., 2023). These ciclicity patterns are very similar to ours detected at KGI. According to the authors, the variabilities reveal centennial changes in the Southern Annular Mode. However, SAM index reconstructions are constrained to the last 2,000 years (King et al., 2023) and do not allow the description of broad variability during the Holocene. With respect to ENSO, the few existing holocenic reconstructions so far do not have sufficient time resolution to provide a detailed decomposition signal analysis at the decadal-to-centenal scale (e.g., Zhu et al., 2017).
Previous studies have initially described the climatic phases of the NAP region in the Holocene. Most of them provided multiproxy analysis (e.g., Bentley et al., 2009) that evidenced warmer conditions in the early and mid-Holocene. However, clarifications on the relative contributions of the main climate drivers associated with these characteristics remain highly uncertain. Herein, we combined glacial records and new lacustrine sedimentation histories for the NAP to improve this gap. Accompanying reconstructed climate forcings were added to the analysis: TSI, ENSO, CO2 concentrations, summer insolation and volcanic episodes identified at Siple Dome A (SDM-A ice core record) (Kurbatov et al., 2006) located in West Antarctica (Fig. 7). The combined database identified 3 distinct climatic phases at NAP since the mid-Holocene: (1) a relatively stable climatic regime between ∼5,822 year and 2,049 kyr BP, characterized by air temperature anomalies not higher than 0.5°C as recorded at JRI. The KGI proglacial Long Lake recorded a climatic regime of a relatively constant sedimentation rate and regular cold and warm phases, as depicted by the well-preserved varved structures shown in Fig. 4; (2) an important drop in the air temperature from 2 kyr BP to nearly the onset of the 20th century, characterizing a short Neoglacial period; and (3) a steady increase in air temperature during the 20th century with measurable impacts on ice sheet mass loss (e.g., Gudmundsson et al., 2019). We identified a concomitant increase in volcanic events (Fig. 7d) during the Late Holocene Neoglacial period, detected at SDM-A, mostly of moderate VEI (Volcanic Explosive Index) and derived from two main sources: the Southern Andes volcanic sector and from the Antarctic continent. Moderate but recurrent volcanism, to a lesser extent, may also act as climate forcing since their aerosol loadings may reach the lower stratosphere, as experimentally demonstrated in previous works (Berthet et al., 2017). The proximity of these sites to NAP volcanic eruptions may make this mechanism more favorable since polar stratospheric temperatures are more favorable for chlorine activation and enhanced ozone loss (Hanson et al., 1994). The SDM-A location is of relatively lower elevation, 974 m, compared to other sites on the Antarctic Plateau but very representative of the atmospheric transport and circulation pattern enclosing the continental regions closer to the Southern Ocean and the WAIS. The constant action of cyclonic systems around latitude 60 °S favors northerly air mass incursions to West Antarctica and the advection of air parcels and terrigenous aerosols from meridional South America (Dixon et al., 2011). Concomitant with the increased volcanism, the last 2,000 year period is also characterized by decreasing solar activity (Fig. 7c). In Fig. 7, we show that the synchronism in solar irradiance and volcanism are consistent with the timing when air temperature decreases, and sedimentation ceases at the isolated Long Lake. Such a combination of volcanism and solar irradiance reveals a “Little Ice Age (LIA)-like event” similar to processes that occurred in the Northern Hemisphere when the Wolf, Spörer, Maunder and Dalton solar minima were nearly coincident with volcanic eruption sequences (Mann et al, 2009), although other theories have been recently posed (Miles et al., 2020). In addition to these two climatic forcing mechanisms, an intensified Niño 3 + 4 index accompanied these trends (Fig. 7e). The impact of El Niño events over West Antarctica has been widely reported (e.g., Paolo et al., 2018). However, such an impact is not spatially uniform. For the Amundsen and Bellingshausen Sea sectors that border the Antarctic Peninsula, intense El Niño events may lead to ice mass gain near shelves and marginal coastal zones (Paolo et al., 2018), whereas apparent stability is observed in Eastern Antarctica (Bodart and Bingham, 2019). We generated correlation maps of Niño 3 + 4 and air temperature (Fig. 7c) as well as Niño 3 + 4 and SST (sea surface temperature) (Fig. 7d) using the NCEP-NCAR reanalysis II database. These results suggest that more ENSO events can strengthen the cooling at the NAP. Therefore, combined forcings from solar irradiance, volcanism and ENSO may have acted together at NAP during the Neoglacial period and probably have surpassed the influence/forcing of the slight increase in CO2 (on the order of 10 ppm, Fig. 7g) and the increase in austral summer insolation (maximum of 20 W m− 2, Fig. 7f). The geographical domain related to the Late Holocene cooling is not restricted to the NAP but was recognized in several other regions to the south and east, as shown in Fig. 7a,b. The climate mechanism behind the recovery to a warming phase at the NAP by the late 20th century to the present is an issue not yet fully explained. Important controls postulated for this process are (a) the increased influence of the westerlies and wind stress in the Southern Ocean associated with the positive phase of the SAM and enhancements derived from Antarctic ozone depletion (Thompson et al., 2005; Yang et al., 2007) and (b) a higher frequency of northerly warm air mass advections, as revealed by the increasing amounts of mineral dust particles reaching the NAP (McConnell et al., 2007) and WAIS (Dixon et al., 2011), both processes resulting from the southward shift of the westerlies. Looking at the long-term variability of the climate forcings depicted in Fig. 7, the impressive increases in sedimentation rates can only be explained by the CO2 changes. This finding reinforces previous conclusions attributed to the modern positive trend of the SAM that falls outside the 2σ range of the prior 2000 years, inferred from multiproxy analysis, whose behavior is attributed to anthropogenic climate change (King et al., 2023). The rapid air temperature increases since the late 20th century documented at the NAP are also observed in southern Patagonia, reconstructed from intra-annual bands in the wood of Nothofagus pumilio (Masiokas and Villalba, 2004).
Figure 7
Comparing the climate pattern at the NAP during the last 2,000 year with other records from sub-Antarctic and temperate sites in South Georgia at 54°S (Foster et al., 2016) and the South and North Patagonian Icefield at 48°S and 47°S (Glasser et al., 2004), we identify similar variabilities. In South Georgia, GDGT-derived temperatures for the last 2,000 year exhibited a prevalent cold phase without an obvious Medieval Warm Period, which was represented only as a short-term event in comparison to its Northern Hemisphere signal recorded in the GISP2 ice core (Grootes and Stuiver, 1997). In Patagonia, the last 3,000 year (except for the modern time) of the prolonged cold phase has previously been recognized from glacier advances at 2,700-2,000 year BP; 2,300 year BP; 1,600-1,400 year BP and during the Little Ice Age (Glasser et al., 2004).
In summary, combining the climatic histories from ice cores and lacustrine records preserved in proglacial lakes may considerably improve our understanding of the past climate in the NAP Maritime region and vicinity. Our results showed that climate changes at the NAP since the mid-Holocene were probably controlled by multiple forcing mechanisms, while the short Neoglacial period encompassing the last 2,000 year BP to the mid-20th century was markedly represented on both sides of the Antarctic Peninsula. Other sites in Antarctica have also exhibited the same behavior as implied by the d2H values of the Remote Plateau (84°S; 43°E − 3330 m; Mosley-Thompson, 1995) and Dominion Range (85°15S; 166°10E – 2,700 m; Mayewski et al., 1995). A potential cause of this event is a combination of a sharp decrease in TSI in concert with relatively more frequent regional (Southern Andean) volcanism and a more active ENSO, following a hypothesis largely postulated for an analogous scenario during the Little Ice Age in the Northern Hemisphere. In contrast, recent air temperature oscillations observed at the NAP are still an issue of debate. In light of our results, we suggest a possible anthropogenic influence on the regional climate that would be consistent with assumptions to explain unprecedented modern trends in SAM.
Sediment coring
Long lake sediment coring was performed during the austral summer of 2015 in the context of the UN/FAO/IAEA project “Assessing the Impact of Climate Change and its Effects on Soil and Water Resources in Polar and Mountainous Regions, INT/5/153”, with the support of the Russian Antarctic Program and the Brazilian Antarctic Program. Sediment cores were retrieved using a hammer-gravity version of a UWITEC corer equipped with a 1.2 m plastic tube installed on a fixed tripod over a floating platform. Sediment cores were refrigerated and transported to Brazil where nondestructive analyses were first conducted (lithological description, X-ray and magnetic susceptibility) following subsampling at 1 cm resolution under clean room conditions. Shallow lake sampling and characteristics are presented in Supplementary material A1.
Sediment core chronology
Sediment core chronologies were established using the constant rate of supply (CRS) model at 1 cm core depth resolution and, when detected, corrected by the 137Cs peak attributed to the most active years of atomic bomb testing from the standpoint of the total explosive yields (1961/62, UNSCEAR, 2000). For the Antarctic margin, the nuclear peak corresponds to the year 1965 (Laurence et al., 2012). Sedimentation data from all lakes were normalized and averaged to obtain a local sedimentation pattern. Two chronological methods were applied to the present work. For Long Lake, we achieved ages of sedimentary layers based on measurements of 14C and comparison to chronologies from two other studies from the same site. For the shallow lakes, we employed the 210Pbexcess (half-life of 22.2 years) method using high-resolution gamma spectroscopy. Considering the subsampling of the sediment cores in slices of 1 cm, the 210Pb method provides annual to semi-decadal time resolution. In the model, total 210Pb was measured as two components: unsupported 210Pb activity derived from atmospheric deposition and supported 210Pb activity that is delivered from mineral grains containing 226Ra supplied to the lake via runoff during melt events. The unsupported component is the activity that is effectively used in the sedimentary dating model (Appleby and Oldfield 1978). The basic concept of the model is provided by the relationship 210PbUnsupported = 210PbTotal - 210PbSupported. Successive layers of newly deposited sediments are enriched with 210PbUnsupported, which exponentially decays in the lake sediment profile. Therefore, the accumulation rate of the sediments being deposited can be estimated by a simple decay equation as proposed by Ivanovich and Murray et al. (1992): ActivityPb-210(t) = ActivityPb-210(t = 0)*e-(λ Pb-210)t, where λPb-210 is the radioactive decay constant of 210Pb equal to 0.03114 year-1. Sediment layer ages are obtained by Eq. 1 as follows:
where ΣAm represents the integrated 210Pb activity from the surface to depth (m) and ΣA∞ is the total integrated concentration of unsupported 210Pb. In natural environments, factors influencing undisturbed downcore exponential decay profiles include bioturbation, diffusion, or slumping (Farmer 1978). The fact that shallow Subantarctic proglacial lakes freeze almost entirely during the winter season also causes disturbance of the bottom sediment and needs to be considered as a parameter in sediment mixing.
All activities of 210Pb and 226Ra used for the CRS model were determined by high-resolution gamma spectrometry. The detector used was a hyper pure germanium (GeHP) of coaxial geometry with a diameter of 56 mm and a length of 38.5 mm. The gamma system (detector and electronics) was installed in the Laboratory of Radioecology and Global Change (LARAMG / Rio de Janeiro State University). The relative efficiency is 20%, with a resolution of 1.8 keV at 1.33 MeV and 0.850 keV at 122 keV. A very low background lead shield of 12 cm with an internal copper lining on all walls was used. Detector efficiency curves were constructed from a NIST cocktail of radionuclides composed of 133Ba, 57Co, 139Ce, 85Sr, 137Cs, 54Mn, 88Y and 65Zn in a solution of 0.5 M HCl. In addition to the natural radionuclides, 137Cs was determined in all samples.
Elemental carbon and nitrogen and isotope measurements
Samples were dried at 40°C, and inorganic carbon was removed prior to analysis of %C and 13C by acid fumigation following the method of Harris et al. (2001). Moistened subsamples were exposed to the exhalation of HCl in a desiccator overnight; afterwards, the samples were dried at 40°C before measurement. The %N and 15N analyses were performed with unacidified samples. The stable N and C isotope ratios and the C and N concentrations were measured using an elemental analyser (Vario Isotope Select, Elementar, Langenselbold, Germany) coupled to an isotope ratio mass spectrometer (Isoprime 100, Elementar, Langenselbold, Germany) located at the Soil and Water Management and Crop Nutrition Laboratory of the Joint FAO/IAEA Centre of Nuclear Techniques in Food and Agriculture. The N isotope ratios were calibrated against the reference materials IAEA-N-1 and IAEA-N-2. Calibration of C isotope ratios was performed using two laboratory standards, a sugar beet standard with δ13C -26.07‰ and a sugar cane standard with δ13C -10.95‰. Both laboratory standards were calibrated against IAEA-CH-6 and IAEA-CH-7. The C and N concentrations were calibrated against the reference material NIST 1547. The analytical accuracies of the measured standards and reference materials were δ15N -0.05‰ and δ13C -0.03‰, and the analytical precisions were +/- 0.19‰ for 15N and +/- 0.12‰ for 13C isotopes. Stable isotope data were reported as delta values (‰) relative to air for 15N and VDPB for δ13C.
XRF scanning
XRF Core Scanner data were collected every 2 mm downcore using generator settings of 20, 30 and 50 kV, a sampling time of 10 seconds directly on the split core surface of the archive half with XRF Core Scanner III (AVAATECH Serial No. 12) at MARUM, University of Bremen. The split core surface was covered with a 4-micron thin SPEXCerti Prep Ultralene1 foil to avoid contamination of the XRF measurement unit and desiccation of the sediment. The reported data were acquired by an SGX Sensortech Silicon Drift Detector (ModelSiriusSD® D65133Be-INF with 133 eV X-ray resolution), a Topaz-X High-Resolution Digital MCA, and an Oxford Instruments 100 W Neptune X-ray tube with rhodium (Rh) target material. Raw data spectra were processed by the analysis of X-ray spectra by the iterative least square software (WIN AXIL) package from Canberra Eurisys. We selected Zr, Rb, Al, Si, K, Ca, Ti, Al, Fe and Mn as elements, from which we calculated several ratios as proxies for paleoenvironmental change. High-resolution XRF scanning techniques are useful for inferring interannual paleolimnological changes in Subantarctic proglacial lakes during the processes of centennial glacier retreat (García-Rodríguez et al., 2021). XRF is especially appropriate for inferring events and increased catchment erosion inputs (i.e., Ti/Ca and Ti/Al, Haberzettl et al., 2009; Pérez et al., 2016; Pérez et al., 2018).
Spectral analysis
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The FFT (fast Fourier transform) analysis operates by decomposing an N point time domain signal into an N time domain signal to an N frequency domain corresponding to these N time domain signals. The N spectra are synthesized into a single frequency spectrum. Herein, the FFT was calculated using PAST software.
-
Complex Morlet wavelet analysis is recognized as an adequate technique to detect changes in time patterns and cyclicities even in nonstationary signals. The Morlet wavelet is a plane wave modulated by a Gaussian function (Torrence and Compto, 1998; Percival and Walden, 2000), which is defined as:
$$\psi (0{)}_{\eta }={\pi }^{-\frac{1}{4}}{e}^{{\text{iw}}_{o}\eta }{e}^{-\frac{{\eta }^{2}}{2}}$$
Wavelet analysis can be used for analyzing localized variations in power within a time series at many different frequencies. For the wavelet spectrum, we estimated the significance level for each scale using only values in the domain of the 95% cone of influence (COI). The COI is the region of the wavelet spectrum in which edge effects become important and is defined here as the e-folding time for the autocorrelation of wavelet power at each scale. This e-folding time was chosen so that the wavelet power for a discontinuity at the edge drops by a factor “e− 2” and ensures that the edge effects are negligible beyond this point.
(c) We also applied the iterative regression analysis (ARIST) method to the TSI and grayscale decomposed series. The method of the minimum squares iterative regression process is applied for every valid frequency region to a single sine function at a time, with peak frequency, phase and amplitude used as starting values for the first iteration. The iteration process leading to the best fit values for these parameters is then repeated to reach the convergence or no convergence criteria. The iterative regression offers the possibility to weight data points according to their experimental uncertainties and to give every searched parameter its own uncertainty interval. The ARIST method uses simple sine functions with three unknown parameters. The starting point of the method is the definition of the so-called conditional function (F) given by:
F = Y- a0Nsin(a1Nt + a2N)
where Y is the observed signal, filtered or unfiltered, a0N, a1N and a2N are the amplitude, frequency and phase, respectively, and t is the time. The F parameter represents the difference between the measured value Y and the fit curve for the corresponding abscissa, and N is the Nth frequency searched. This method also calculates the standard deviation of each estimated parameter. Further details about the ARIST method can be found in Wolberg (1967), Rigozo and Nordemann (1998), Rigozo et al. (2005) and Echer et al. (2012).
Signal decomposition method
The techniques for spectral analysis used in this study were the wavelet transform (Torrence and Compo, 1998), the classical FFT method and the iterative regression analysis—ARIST (Rigozo et al., 2005). The wavelet analysis was performed using the orthonormal discrete Meyer wavelet transform (Kumar and Foufoula-Georgiou, 1995; Percival and Walden, 2000). The Meyer wavelet is an orthonormal transform, which is composed of a mother wavelet and a scaling function, necessary for performing multiresolution analysis (MRA). It is suitable for decomposing a signal into orthonormal frequency levels. Permitting the operation of bandpass filtering in different frequencies, each one is limited by powers of 2 (i.e., 2n, where n is related to the time series steps). In the wavelet decomposition analysis, the signal (S) is decomposed into approximations (A) and in detail (L). The details contain the high-frequency part of the signal, they are inserted in a period range limited by 2n and 2(n+1), and the approximations contain most of the characteristic frequencies of the signal. In the first step of the decomposition, the signal has a low bandpass filtered component, the A1 level, and a high bandpass filtered component, the D1 level, S = A1 + L1. In the next step, A1 is then split into frequency levels, resulting in an A2 + L2 signal; one has the L2 level as the next bandpass high-frequency level, A1 = A2 + L2, thus S = A2 + L2+L1, and so on (Kumar and Foufuola-Georgiou, 1995). The process is equivalent to a filtering procedure. In the first step, the signal has a low bandpass filtered component, the A1 level, and a high bandpass filtered component, the L1 level. When A1 is then split in frequency levels, resulting in an A2 + L2 signal, one has the L2 level as the next bandpass high-frequency level. The most suitable decomposition of a given signal is selected on an entropy-based criterion, and the process is repeated until this criterion is reached. In our analysis, wavelet decomposition was performed until the L7 level with the A7 approach (Kumar and Foufoula-Georgiou, 1995; Scafetta and West, 2005; Echer et al., 2008, 2012).
Acknowledgements
This work was supported by the IAEA Technical Cooperation Program through the INT/5/153 project “Assessing the Impact of Climate Change and its Effects on Soil and Water Resources in Polar and Mountainous Regions”. We also thank the Bellingshausen Russian Station and the Antarctic Program/SECIRM/PROANTAR-CNPq/MMA/MCTIC for logistic support at King George Island/Antarctica and INCT-Criosfera/MCTIC/CNPq. We also express our thanks to Mr. Vasiliy Boldin for building the floating platform used for drilling the Antarctic lakes. The contribution of Russian researcher S. Verkulich to the article preparation was carried out with the financial support of the Russian Science Foundation, project No. 22-27-00437 and Roberta Priori for figure editing.
Author contributions
Heitor Evangelista. General concept of the manuscript/Paleoclimate description and text writing
Sergey Verkulich. General concept of the manuscript/Paleolimnological description and text writing
Mariza P. Echer. Use of frequency decomposition methods, spectral analysis and text writing
Bulat Mavlyudov. Long term in situ measurements of Equilibrium Line Altitude at King George Island and text writing
Stephanie Kusch. Lipid analysis, 14C model and text revision
Anna B. J. Oaquim. Text and graphic formatting
Marcus Vinicius Licinio. 210Pb dating and melting model
Gerd Dercon. Project PI in the IAEA/FAO/; C and N isotope analysis
Zinaida Pushina. Diatom analysis
Felipe García-Rodríguez. Elemental composition analysis in sediment cores by XRF and text writing
Arthur A. Neto. Magnetic susceptibility analysis and text comments
Sérgio J. Gonçalves Jr. Sediment coring in Antarctica
Rodrigo C. Abuchacra. Magnetic susceptibility analysis and text comments
Maria Heiling. C and N isotope analysis
Johanna Slaets. General text revision and text comments
Christian Resch. C and N isotope analysis
Alejandra Castillo. Additional gamma spectrometry for 210Pb
Roman Gruber. C and N isotope analysis
Marília H Shimizu. Use of climate computational model for the study region
Competing Interests
The authors declare no competing interest in this work.
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- SMIAEALongLake12nov2023.docx
A synchronous atmosphere-cryosphere-land response to climate on the northern Antarctic Peninsulasince the mid-Holocene (Supplementary material)