INTRODUCTION
Patagonia has been the focus of paleoclimate research since Caldenius (Reference Caldenius1932) started to reconstruct past glacier advances in the 1930s. Since then, the number of climate archives and proxies applied to unravel the regional climate history increased distinctly (cf. review of Kilian and Lamy, Reference Kilian and Lamy2012). During the last decades, studies concentrated on the reconstruction of latitudinal shifts of the Southern Hemispheric westerlies (SHWs) at the Pleistocene-to-Holocene transition (Mayr et al., Reference Mayr, Lücke, Wagner, Wissel, Ohlendorf, Haberzettl and Oehlerich2013), as well as on the role of Patagonia as a dust source for the Southern Hemisphere (Petit et al., Reference Petit, Jouzel, Raynaud, Barkov, Barnola, Basile and Bender1999).
With the discovery of significant variations of dust in Antarctic ice cores during the last 800 ka, Patagonia was suggested as the main dust source (Petit et al., Reference Petit, Jouzel, Raynaud, Barkov, Barnola, Basile and Bender1999; EPICA Community Members, 2004, 2006). This hypothesis is supported by data from geochemical fingerprinting (Grousset et al., Reference Grousset, Biscaye, Revel, Petit, Pye, Joussaume and Jouzel1992; Basile et al., Reference Basile, Grousset, Revel, Petit, Biscaye and Barkov1997; Gaiero, Reference Gaiero2007; Sugden et al., Reference Sugden, McCulloch, Bory and Hein2009; Delmonte et al., Reference Delmonte, Andersson, Schöberg, Hansson, Petit, Delmas, Gaiero, Maggi and Frezzotti2010).
Increased Patagonian dust production during the glacial period is related to several factors—for example, increased glacial and fluvial erosion in mountain regions (Petit et al., Reference Petit, Jouzel, Raynaud, Barkov, Barnola, Basile and Bender1999), exposure of the South American shelf as a potential source area for continental dust (Kaiser and Lamy, Reference Kaiser and Lamy2010), increased proglacial meltwater dynamic (Sugden et al., Reference Sugden, McCulloch, Bory and Hein2009), and intensified foehn winds increasing aridity (Kaiser and Lamy, Reference Kaiser and Lamy2010) and reducing atmospheric dust outwash (Lambert et al., Reference Lambert, Delmonte, Petit, Bigler, Kaufmann, Hutterli, Stocker, Ruth, Steffensen and Maggi2008).
After mobilization at the surface, cyclones elevate aerosols to the upper troposphere where they move to the east and to the southeast as far as 80°S (Iriondo, Reference Iriondo2000). Here, dust-transporting air masses sink to the ground under the influence of the Antarctic anticyclone. Trajectory studies demonstrate that dust is transported eastward by moving low-pressure systems. Today, this takes about 7 days from Patagonia to East Antarctica (Li et al., Reference Li, Ginoux and Ramaswamy2010) on pathways crossing the Scotia Sea (Fig. 1; Reijmer et al., Reference Reijmer, van den Broeke and Scheele2002). Consequently, marine dust records from the Scotia Sea (Fig. 1), located midway between the source region (Patagonia) and East Antarctica, correlate with Antarctic dust variations (Hofmann, Reference Hofmann1999; Weber et al., Reference Weber, Kuhn, Sprenk, Rolf, Ohlwein and Ricken2012).
Map with sites mentioned in Figure 3 along the dust trajectory from the South Pacific coast (ODP site 1233, MD07-3128) and Patagonia (5022-2CP) via the Scotia Sea (MD07-3134) to Antarctica (East Antarctic Dronning Maud Land [EDML]). The red arrow indicates the pathway of dust based on a 5-day backward air-parcel trajectory for EDML after Reijmer et al. (Reference Reijmer, van den Broeke and Scheele2002). Additionally, the extent of the Patagonian shelf during the sea-level lowstand of the last glacial maximum (LGM; Iriondo, Reference Iriondo2000) and the limit of the Patagonian Ice Field during the LGM (Hein et al., Reference Hein, Hulton, Dunai, Sugden, Kaplan and Xu2010) are indicated. NPI, modern Northern Patagonian Ice Field; SPI, modern Southern Patagonian Ice Field. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Suitable proxies to compare records are the non-sea salt Ca flux established as an indicator for atmospheric dust variations during glacial times for Antarctic ice cores (Lambert et al., Reference Lambert, Bigler, Steffensen, Hutterli and Fischer2011) and the magnetic susceptibility (MS) obtained for the marine record from the Scotia Sea (Hofmann, Reference Hofmann1999; Weber et al., Reference Weber, Kuhn, Sprenk, Rolf, Ohlwein and Ricken2012). These two proxies have also been suggested to document dust activity for sediments of Laguna Potrok Aike (LPA) at 52°S, located at the southern limit of the Patagonian dust source region (Haberzettl et al., Reference Haberzettl, Anselmetti, Bowen, Fey, Mayr, Zolitschka and Ariztegui2009; Hahn et al., Reference Hahn, Kliem, Ohlendorf, Zolitschka and Rosén2014; Lisé-Pronovost et al., Reference Lisé-Pronovost, St-Onge, Gogorza, Haberzettl, Jouve, Francus, Ohlendorff, Gebhardt and Zolitschka2015).
The main argument for using Ca and MS as LPA dust proxies is the long-term correlation with regional dust proxies of the Scotia Sea and Antarctica (Haberzettl et al., Reference Haberzettl, Anselmetti, Bowen, Fey, Mayr, Zolitschka and Ariztegui2009; Hahn et al., Reference Hahn, Kliem, Ohlendorf, Zolitschka and Rosén2014; Lisé-Pronovost et al., Reference Lisé-Pronovost, St-Onge, Gogorza, Haberzettl, Jouve, Francus, Ohlendorff, Gebhardt and Zolitschka2015). However, the interpretation of MS is complicated because of several confounding factors (Lisé-Pronovost et al., Reference Lisé-Pronovost, St-Onge, Gogorza, Haberzettl, Jouve, Francus, Ohlendorff, Gebhardt and Zolitschka2015). Ca reflects the dust activity of LPA shore sediments (Hahn et al., Reference Hahn, Kliem, Ohlendorf, Zolitschka and Rosén2014). During the Holocene and the late glacial period, autochthonous carbonates were precipitated in the lake (Hahn et al., Reference Hahn, Kliem, Ohlendorf, Zolitschka and Rosén2014). For these time intervals, Ca cannot be used as an indicator for clastic input. However, the glacial sediments deposited between 51 and 15 cal ka BP are carbonate-free (Hahn et al., Reference Hahn, Kliem, Ohlendorf, Zolitschka and Rosen2013). Therefore, we focus our study on this time window. Regarding individual limitations, this study analyzed both parameters together. The Ca record is based on X-ray fluorescence detection and was published by Hahn et al. (Reference Hahn, Kliem, Ohlendorf, Zolitschka and Rosén2014). The new MS record was obtained by point sensor detection on split cores. For further characterization of the parameters Ca and MS, the new dry density (DD) record is presented.
This study focuses on the frequency analysis of Ca and MS records. The analyzed time window from 51 to 15 ka is well known for rapid warming events: Dansgaard-Oeschger (DO) cycles (Dansgaard et al., Reference Dansgaard, Johnsen, Clausen, Dahl-Jensen, Gundestrup, Hammer and Hvidberg1993) from the North Greenland δ18O record (North Greenland Ice Core Project) and the Southern Hemisphere (Alley et al., Reference Alley, Anandakrishnan and Jung2001). The location of LPA offers the opportunity to identify short-term climate cycles within the dust source area. Furthermore, Quaternary climate variations, probably expressed as displacements of the SHWs, can be tracked at Southern Hemispheric midlatitudes (Anderson et al., Reference Anderson, Ali, Bradtmiller, Nielsen, Fleisher, Anderson and Burckle2009; Toggweiler, Reference Toggweiler2009).
SITE DESCRIPTION
LPA is located on the leeside of the Andes in southeastern Patagonia (85 km southwest of the city of Rio Gallegos and ~80 km north of the Strait of Magellan), Argentina. South of 40°S, low-level westerly winds prevail over the continent throughout the year (Garreaud, 2009). The upper-level jet stream moves between 45°S and 55°S during austral summer and between 20°S and 40°S in winter. Regionally, the amount of precipitation and the strength of westerly winds correlate at the Pacific coast and the western slope of the Andes. This correlation decreases dramatically toward the east and reaches negative values across the Patagonian steppe (Garreaud et al., Reference Garreaud, Lopez, Minvielle and Rojas2013). Consequently, the west–east precipitation gradient is less prominent in years with weak westerly winds.
Within the ~200 km² semiarid catchment area of LPA (Zolitschka et al., Reference Zolitschka, Schäbitz, Lücke, Corbella, Ercolano, Fey and Haberzettl2006), annual precipitation is <300 mm (Mayr et al., Reference Mayr, Wille, Haberzettl, Fey, Janssen, Lücke and Ohlendorf2007b) and reaches 150 mm at a local meteorological station near LPA (Zolitschka et al., Reference Zolitschka, Schäbitz, Lücke, Corbella, Ercolano, Fey and Haberzettl2006). A recent time series of precipitation measurements (1999–2005) at LPA shows that easterly wind directions (from the South Atlantic) are often combined with precipitation, whereas west winds carry considerable less moisture per rainfall event (Mayr et al., Reference Mayr, Wille, Haberzettl, Fey, Janssen, Lücke and Ohlendorf2007b). Easterly wind directions can be caused by atmospheric blocking of the SHWs, which are more likely during winter seasons (Schneider et al., Reference Schneider, Glaser, Kilian, Santana, Butorovic and Casassa2003; Garreaud et al., Reference Garreaud, Lopez, Minvielle and Rojas2013). Furthermore, blocking of SHWs enables cold Antarctic air masses to migrate into Patagonia from the South. The mean annual air temperature at Rio Gallegos (6 meters above sea level [m asl], 85 km northeast of the study site) is 7.4±0.7°C, with a July (winter) minimum of 1.0±1.5°C and a January (summer) maximum of 13.0±1.2°C (Zolitschka et al., Reference Zolitschka, Schäbitz, Lücke, Corbella, Ercolano, Fey and Haberzettl2006). Mean annual wind speeds of 7.4 m/s occur at Rio Gallegos with a dominantly western direction (Weischet, Reference Weischet1996; Baruth et al., Reference Baruth, Endlicher and Hoppe1998).
The sediment basin containing LPA is of volcanic origin and resulted from a phreatomagmatic eruption dated to 0.77±0.24 Ma by 40Ar/39Ar (Zolitschka et al., Reference Zolitschka, Schäbitz, Lücke, Corbella, Ercolano, Fey and Haberzettl2006). The area is characterized by extensive backarc volcanism and belongs to the Pali Aike Volcanic Field (Mazzarini and D’Orazio, Reference Mazzarini and D’Orazio2003; Ross et al., Reference Ross, Delpit, Haller, Nemeth and Corbella2011). Scoria cones, plateau lavas, and maar volcanoes are common. They are covered by glaciofluvial deposits and basal till of Pliocene to middle Pleistocene glaciations. However, according to geomorphological and stratigraphic evidence, glaciers did not reach the catchment area of LPA at least for the last five glacial periods (Caldenius, Reference Caldenius1932; Mercer, Reference Mercer1976; Rabassa and Clapperton, Reference Rabassa and Clapperton1990; Meglioli, Reference Meglioli1992; Coronato et al., Reference Coronato, Ercolano, Corbella and Tiberi2013). Widespread sand sheets are the youngest deposits and demonstrate Late Holocene eolian dynamics (Favier-Dubois, Reference Favier-Dubois2007; Kliem et al., Reference Kliem, Buylaert, Hahn, Mayr, Murray, Ohlendorf, Veres, Wastegård and Zolitschka2013a).
In a limnological context, LPA is a polymictic and subsaline maar lake (Zolitschka et al., Reference Zolitschka, Schäbitz, Lücke, Corbella, Ercolano, Fey and Haberzettl2006), and clastic sedimentation dominated throughout the past 51 ka (Hahn et al., Reference Hahn, Kliem, Ohlendorf, Zolitschka and Rosén2014). In 2003, the lake level was at an elevation of 116 m asl and had a maximum diameter of 3.5 km and a water depth of 100 m (Zolitschka et al., Reference Zolitschka, Schäbitz, Lücke, Corbella, Ercolano, Fey and Haberzettl2006). Numerous subaerial and subaqueous lake-level terraces demonstrate rapid and sizable past hydrologic variations and a paleo-outflow channel related to an exceptional high lake level (Haberzettl et al., Reference Haberzettl, Fey, Lücke, Maidana, Mayr, Ohlendorf, Schäbitz, Schleser, Wille and Zolitschka2005, Reference Haberzettl, Corbella, Fey, Janssen, Lücke, Mayr and Ohlendorf2007; Anselmetti et al., Reference Anselmetti, Ariztegui, De Batist, Gebhardt, Haberzettl, Niessen, Ohlendorf and Zolitschka2009; Kliem et al., Reference Kliem, Buylaert, Hahn, Mayr, Murray, Ohlendorf, Veres, Wastegård and Zolitschka2013a). Presently, the lake has neither a permanent tributary nor a surficial outflow. Episodic or ephemeral surface runoff enters the lake through gullies mainly after spring snowmelt that deeply eroded into the subaerial terraces (Haberzettl et al., Reference Haberzettl, Fey, Lücke, Maidana, Mayr, Ohlendorf, Schäbitz, Schleser, Wille and Zolitschka2005; Mayr et al., Reference Mayr, Lücke, Stichler, Trimborn, Ercolano, Oliva and Ohlendorf2007a).
METHODS
Lithology and chronology
The sediment record of LPA (5022-2CP) with a length of 106.08 m composite depth was the key site of the International Continental Scientific Drilling Program (ICDP) lake deep drilling expedition 5022 (Potrok Aike Maar Lake Sediment Archive Drilling Project [PASADO]) performed from August to November 2008 (Zolitschka et al., Reference Zolitschka, Anselmetti, Aristegui, Corbella, Francus, Ohlendorf and Schäbitz2009). Site 5022-2CP consists of three different sediment types: pelagic sediments, mass-movement deposits (MMDs), and tephra layers (Kliem et al., Reference Kliem, Enters, Hahn, Ohlendorf, Lisé-Pronovost, St-Onge, Wastegård and Zolitschka2013b).
Three main lithostratigraphic units (A, B, and C) were distinguished by Kliem et al. (Reference Kliem, Enters, Hahn, Ohlendorf, Lisé-Pronovost, St-Onge, Wastegård and Zolitschka2013b). Sediments of unit A are characterized by laminated silts with inorganic carbonate contents of up to 2% that were deposited from 8.3 cal ka BP to the present day. The highest concentration of plant macroremains occurs in lithostratigraphic unit B, which is characterized by an abrupt onset at 17.2 cal ka BP; this unit represents the transition from the last glacial period to the early Holocene. Unit C comprises the glacial time slice of the record from 51.2 to 17.2 cal ka BP. This unit is composed of pelagic sediments with a down-core increase of MMDs.
The sediments were radiocarbon (14C) dated, and ages were calibrated applying the CalPal_2007_HULU data set (Weninger and Jöris, Reference Weninger and Jöris2008). For age modeling, MMDs and tephra layers were excluded from the profile (Fig. 2). The age model of the event-corrected record is based on a mixed-effect regression procedure (Heegaard et al., Reference Heegaard, Birks and Telford2005; Kliem et al., Reference Kliem, Enters, Hahn, Ohlendorf, Lisé-Pronovost, St-Onge, Wastegård and Zolitschka2013b). The age-depth relationship for the investigated time interval (51.2 to 15 cal ka BP) is based on 22 accelerator mass spectrometry 14C dates. However, preliminary correlation of optically stimulated luminescence dates from drill site 1 (Gebhardt et al., Reference Gebhardt, Ohlendorf and Buylaert2012; Buylaert et al., Reference Buylaert, Murray, Gebhardt, Sohbati, Ohlendorf, Thiel and Zolitschka2013) and results from diatom studies (Recasens et al., Reference Recasens, Ariztegui, Maidana and Zolitschka2015) support that the record may extend into an older Antarctic warm period (i.e., Antarctic Isotope Maximum [AIM] 14; Fig. 3).
(color online) Radiocarbon-based age model for the event-corrected (after removal of event deposits with a thickness >2 cm) sediment record of Potrok Aike Maar Lake Sediment Archive Drilling Project site 5022-2CP (modified after Kliem et al., Reference Kliem, Enters, Hahn, Ohlendorf, Lisé-Pronovost, St-Onge, Wastegård and Zolitschka2013b).
Northwest–southeast transect from the South Pacific entrance of the Magellan Strait to East Antarctica (for location of sites, see Fig. 1). (a) Ice-rafted debris (IRD) at the Pacific entrance of the Magellan Strait (Caniupán et al., Reference Caniupán, Lamy, Lange, Kaiser, Arz, Kilian and Urrea2011). Records from Laguna Potrok Aike (5022-2CP) are displayed from b to f with individual data points and the 200-yr running mean as a bold line. (b) Sedimentation rate (Kliem et al., Reference Kliem, Enters, Hahn, Ohlendorf, Lisé-Pronovost, St-Onge, Wastegård and Zolitschka2013b). (c) Biogenic silica (BSi). (d) Dry density. (e) Magnetic susceptibility (MS). (f) X-ray fluorescence (XRF) calcium (Ca) (c and f: Hahn et al., Reference Hahn, Kliem, Ohlendorf, Zolitschka and Rosén2014). (g–i) Scotia Sea sediment core MD07-3134 (g and h: Weber et al., Reference Weber, Kuhn, Sprenk, Rolf, Ohlwein and Ricken2012): MS (g), XRF-Ca (h), and BSi flux (i) (Sprenk et al., Reference Sprenk, Weber, Kuhn, Rosén, Frank, Molina-Kescher, Liebetrau and Röhling2013). (j) East Antarctic Dronning Maud Land (EDML) non–sea salt (nss) Ca flux (Fischer et al., Reference Fischer, Fundel, Ruth, Twarloh, Wegner, Udisti and Becagli2007). (k) EDML δ18O including orange-numbered Antarctic Isotope Maxima (EPICA Community Members, 2006, 2010). Intervals with increased Ca and MS for 5022-2CP are shaded. An ideal 1.5 ka cycle is represented by the sinusoidal curve between d and e for comparison. MIS, Marine Oxygen Isotope Stage. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
MS and DD
MS was measured nondestructively at 0.5-cm increments on split core halves using a Bartington point sensor (MS2F) mounted on an automated scanner as described by Funk et al. (Reference Funk, von Dobeneck and Reitz2004). In contrast, the data set published by Lisé-Pronovost et al. (Reference Lisé-Pronovost, St-Onge, Gogorza, Haberzettl, Jouve, Francus, Ohlendorff, Gebhardt and Zolitschka2015) was measured on U-channels. DD was calculated from volumetric subsamples that were taken in consecutive 2 cm increments for the entire sediment sequence. Each subsample was weighed, freeze-dried, and weighed again to obtain DD values.
Spectral analysis
Cycles in sediment records can be studied by cyclostratigraphic methods (Prokopenko et al., Reference Prokopenko, Williams, Karabanov and Khursevich2001; Weedon, Reference Weedon2003; Lenz et al., Reference Lenz, Wilde and Riegel2011). The cyclic nature of a sedimentary sequence is investigated by spectral analysis. The spectral analysis is calculated for a certain interval that is defined by the window size. After the interval was analyzed, the window is shifted downward continuously at a specific step size. The calculation is repeated, and the results are displayed at the center of each window, resulting in a three-dimensional (3D) spectral plot (Molinie and Ogg, Reference Molinie and Ogg1990; Wonik, Reference Wonik2001; Weedon, Reference Weedon2003; Baumgarten and Wonik, Reference Baumgarten and Wonik2015).
For MS and Ca of the LPA sediment record, a spectral analysis is calculated for an interval of specific duration (window size=14 ka), and the window is moved downward continuously with a step size of 1 ka. The analysis is repeated at consecutive intervals, and spectra are displayed at the central depth of each window. Results are presented in a 3D spectral plot. The optimal window size was determined by empirical testing.
Generally, a small window size maximizes the length of the resulting plot. However, the contained signal needs to be covered and cannot be determined if a window size is chosen that is too small (e.g., only half of the signal length). Based on the age-depth relationship (Kliem et al., Reference Kliem, Enters, Hahn, Ohlendorf, Lisé-Pronovost, St-Onge, Wastegård and Zolitschka2013b), the data were analyzed from 51.2 to 15 cal ka BP. For this study, 200 yr running means of the new MS and published Ca (Hahn et al., Reference Hahn, Kliem, Ohlendorf, Zolitschka and Rosén2014) data were analyzed.
As spectral analysis by sliding window method can be applied on evenly spaced data only, the unevenly spaced data sets were resampled by linear interpolation prior to further analysis. Spectral analysis for identification of characteristic periodicities (Jenkins and Watts, Reference Jenkins and Watts1969; Priestley, Reference Priestley1981) was performed as fast Fourier transform using MATLAB (MathWorks®).
RESULTS
To detect sedimentary cycles, continuous sedimentation is required. Therefore, all presented data are based on the event-corrected composite profile of LPA (Kliem et al., Reference Kliem, Enters, Hahn, Ohlendorf, Lisé-Pronovost, St-Onge, Wastegård and Zolitschka2013b). Because of the location of the drill site within the flat profundal zone of the lake (accumulation area of MMDs), a neglectable erosion of pelagic sediments by mass-movement events is assumed.
DD, Ca and MS
The DD record of LPA sediments reflects a minor compaction trend and ranges from 0.6 g/cm3 at ~16 cal ka BP to 1.5 g/cm3 at ~48 cal ka BP (Fig. 3d). This trend is interrupted by decreases in DD of ~0.5 g/cm3 for four periods: (1) 51.0–49.0 cal ka BP, (2) 46.9–45.1 cal ka BP, (3) 40.5–37.1 cal ka BP, and (4) 17.5–15 cal ka BP. The same intervals are characterized by increased biogenic silica (BSi) values (Fig. 3c) published by Hahn et al. (Reference Hahn, Kliem, Ohlendorf, Zolitschka and Rosen2013). Both parameters exhibit an anticorrelation (r=−0.62).
The records of DD and MS correlate weakly (r=0.51 for a 200 yr running mean). Moreover, DD shows no correlation with Ca (r=0.33). A good correlation was only identified between Ca and MS (r=0.78).
Spectral analysis
The 3D spectral plots of Ca and MS range from 44 to 22 cal ka BP because half of the window length of 14 ka cannot be displayed. For both records, different high amplitudes have been detected, whereas the frequency of 1.5 ka occurs in both data sets.
The 3D spectral plot of the MS data (Fig. 4a) documents the 1.5 ka cycle starting at ca. 36 cal ka BP with an increased amplitude toward the top. An additional cycle of 6 ka occurs in the youngest section, establishes with increasing energy, and persists toward the top.
The sliding window plot of the Ca data (Fig. 4b) shows that the 1.5 ka cycle establishes around 36 cal ka BP with an increasing energy level toward the top of the analyzed time interval. The amplitude of 3.0 ka has high energy prior to 36 cal ka BP, weakens afterward, and reoccurs around 25 ka.
DISCUSSION
Ca and MS
Ca and MS records of LPA reflect chemical/magnetic changes of the clastic input during the glacial period (Hahn et al., Reference Hahn, Kliem, Ohlendorf, Zolitschka and Rosén2014; Lisé-Pronovost et al., Reference Lisé-Pronovost, St-Onge, Gogorza, Haberzettl, Jouve, Francus, Ohlendorff, Gebhardt and Zolitschka2015). Ca is mainly derived from eroded local basaltic rocks and accumulates as shore sediment (Hahn et al., Reference Hahn, Kliem, Ohlendorf, Zolitschka and Rosén2014). During low lake levels, strong winds triggered the transport of basaltic sand from the lakeshore to the lake center. The identification of the source of clastic input with high values of MS is complicated because of several confounding factors (Lisé-Pronovost et al., Reference Lisé-Pronovost, St-Onge, Gogorza, Haberzettl, Jouve, Francus, Ohlendorff, Gebhardt and Zolitschka2015). However, because of a good visual coincidence (Fig. 3e and f) and a good correlation between Ca and MS (r=0.78), MS very likely also reflects dust input blown out by strong winds from basaltic shore sediments.
Strong winds (deduced from high levels of Ca and MS) likely result from intensified katabatic winds because of glacier advances of the Southern Patagonian Ice Field (SPI; Fig. 1) covering large areas of the eastern Andean foreland. Data from a record of ice-rafted debris in the Pacific support a coincidence of SPI dynamics (Fig. 3a) with Ca and MS records (Fig. 3e and f). The glacier dynamics of the SPI likely reflect climate variations because of shifts of the atmospheric circulation (Caniupán et al., Reference Caniupán, Lamy, Lange, Kaiser, Arz, Kilian and Urrea2011). Glacier advances were associated with the influence of the polar easterlies, whereas glacier retreats reflect intensified SHWs.
Very strong SHWs during AIM 12 and AIM 8 were also inferred from high levels of the lacustrine paleoproductivity indicator BSi (Fig. 3c; Hahn et al., Reference Hahn, Kliem, Ohlendorf, Zolitschka and Rosen2013) and coincide with very low Ca and MS levels (i.e., with a glacier retreat). The DD of the sediment likely decreased because of the high content of BSi (r=−0.62). Thus, low Ca and MS levels because of dilution effects by high BSi content or other nonclastic compounds cannot be excluded during these periods.
Ca and MS variations of the clastic component in the LPA sediment record are indirectly linked to shifts of the polar easterlies and the SHWs. Increased Ca and MS levels reflect intensified easterlies, whereas decreased Ca and MS reflect intensified SHWs.
A robust 1.5 ka periodicity dominates MIS 2
The parameters MS and Ca document a 1.5 ka cycle from 36 to 22 cal ka BP (Fig. 4). However, because of the 14 ka spectral time window the center of the spectral analysis is limited to 22 cal ka BP, even if including data up to 15 cal ka BP. Indeed, the distinct 1.5 ka signal is visually traceable between 31 and 17 cal ka BP (Fig. 3e and f).
Persistent 1–2 ka climate cyclicities during MIS 3 and MIS 2 show a mean periodicity of 1476 yr±585 yr (1σ) between 65 and 15 ka at site DSDP 609. They were inferred from hematite-stained grains of ice-rafted debris (IRD-HSG) records recovered from the North Atlantic (50°N) and were suggested to pace DO events known from Greenland ice cores (Bond et al., Reference Bond, Showers, Elliot, Evans, Lotti, Hajdas, Bonani and Johnson1999). A chronology update and statistical analysis on the classic DSDP 609 data sets revised earlier results and showed that the original ~1.5 ka periodicity is a mixture of two cyclicities (~1000 yr and ~2000 yr; Obrochta et al., Reference Obrochta, Miyahara, Yokoyama and Crowley2012). Moreover, and not corresponding to LPA, a ~2 ka cyclicity prevails between 30 and 22 ka.
The difference between LPA and the reinterpreted DSDP 609 record is of major scientific interest regarding ongoing discussions about the origin of this 1–2 ka periodicity. A combination of different solar frequencies (Braun et al., Reference Braun, Christl, Rahmstorf, Ganopolski, Mangini, Kubatzki, Roth and Kromer2005; Clemens, Reference Clemens2005) might be an explanation for the origin of a 1.5 ka periodicity. However, Holocene atmospheric 14C production rates and ice core 10Be fluxes suggest solar variations with periodicities of ~1000 and ~2000 yr (Obrochta et al., Reference Obrochta, Miyahara, Yokoyama and Crowley2012). It is possible that variations of the geomagnetic field modified this pattern at least for the Holocene (St-Onge et al., Reference St-Onge, Stoner and Hillaire-Marcel2003; Snowball and Muscheler, Reference Snowball and Muscheler2007).
The LPA record reveals a robust ~1.5 ka climate periodicity prevailing during MIS 2. If the reinterpretation of DSDP 609 reports the same climate signal, differences might result from climate cycles not recovered at the North Atlantic IRD-HSG sites or from chronological uncertainties (Fig. 2). For comparison, expected 1.5 ka paced DO events often fail during MIS 3 (Schulz, Reference Schulz2002). Alley et al. (Reference Alley, Anandakrishnan and Jung2001) explained this property of Arctic and Antarctic ice-core records with stochastic resonance—that is, the climate switches between warm and cold if a combination of a weak periodicity plus noise reach a certain threshold (the noise characteristic is of importance and summarizes all relevant factors).
Implications for atmospheric circulations
The 1.5 ka periodicity dominates MIS 2 and occurs with diminished power during the late MIS 3 at LPA. In contrast, distinct climate variations paced by an ~1.5 ka periodicity were only dominant during MIS 3 in the polar regions (Grootes and Stuiver, Reference Grootes and Stuiver1997) and not during the constantly cold temperatures of MIS 2 (Fig. 5a and e). Why did the 1.5 ka paced climate variations not occur during MIS 2 in the Antarctic?
Recently, the displacement of the SHWs was linked with Quaternary climate variations. As a poleward shift of the SHWs increases upwelling of the circumpolar deep waters, the released CO2-rich deep waters influence the greenhouse effect (Anderson et al., Reference Anderson, Ali, Bradtmiller, Nielsen, Fleisher, Anderson and Burckle2009; Toggweiler, Reference Toggweiler2009). Increased upwelling identified in the Southern Ocean coincides with the distinct warm events of AIM 8 and AIM 12 (Anderson et al., Reference Anderson, Ali, Bradtmiller, Nielsen, Fleisher, Anderson and Burckle2009). Frequent upwelling during MIS 3 has also been inferred from Scotia Sea BSi fluxes (Fig. 3i; Sprenk et al., Reference Sprenk, Weber, Kuhn, Rosén, Frank, Molina-Kescher, Liebetrau and Röhling2013). In contrast, upwelling in the Scotia Sea was distinctly reduced during MIS 2. With the SHWs in a much more northerly position during MIS 2, it is possible that the (minor) latitudinal shifts paced by a 1.5 ka periodicity of this wind system did not reach the required latitude to release CO2-rich deep waters into the atmosphere. Therefore, an increased greenhouse effect did not occur, and thus the prominent climate signals disappeared.
In contrast, the provenance changes of the detrital input at LPA reflect a climate signal with a 1.5 ka periodicity during MIS 2. Glacier advances of the SPI suggest a northward displacement of the SHWs that probably increased the influence of polar easterlies delivering cold and/or wet air masses to southern Patagonia. The influence of polar easterlies decreased during minor southward displacements with a 1.5 ka periodicity, resulting in temporary temperature increases and/or precipitation decreases. Periodic 1.5 ka provenance changes did not occur under an overall southward displacement of the SHWs during MIS 3.
However, the episodic 3 ka (Ca) and the persistent 6 ka (MS) periodicities (multiples of the 1.5 ka periodicity; Fig. 4) suggest that provenance changes respond to a 1.5 ka pacing even during MIS 3. However, inconsistencies between Ca and MS spectral plots, fewer recurrences, and increased dating errors at the limit of the radiocarbon dating method (Fig. 2) complicate the interpretation of these frequencies.
CONCLUSIONS
The Ca and MS records reflect strong wind-triggered dust input of basaltic shore sediments to the lake. Strong winds likely result from intensified katabatic winds caused by an expansion of the SPI into the eastern Andean foreland. The SPI glaciers advanced because of an increased influence of the easterlies (=Ca and MS increase in LPA sediments), whereas glacier retreats suggest intensified SHWs (=Ca and MS decrease in LPA sediments).
Based on spectral analysis of Ca and MS records, a strong 1.5 ka periodicity during MIS 2 (31–17 cal ka BP) has been identified for the first time in the Southern Hemisphere. Our data support a link between the latitudinal position of the SHWs band, greenhouse gas emission from the Antarctic bottom waters, and thus global climate variations. Because of a major northward displacement of the SHWs during MIS 2, the zone of CO2-rich deep water was not sufficiently affected by the SHWs. Therefore, an increased greenhouse effect caused by intensified upwelling did not occur at this time, which resulted in the constantly cold climate of MIS 2.
During MIS 3, however, an overall southward displacement of the SHWs increased the probability of upwelling driven by minor latitudinal shifts of the SHWs. A 1.5 ka periodicity paces these small-scale shifts and thus the prominent climate signal. This southward displacement caused a general change of the geodynamic framework in the catchment, and the dominance of SHWs probably also reduced the influence of polar easterlies. This finally stopped provenance variations with 1.5 ka periodicity at LPA.
(color online) Three-dimensional spectral plots of magnetic susceptibility (a) and Ca (b) data for the last glacial period (51.2 to 15 cal ka BP). Dominant cycles are labeled with their frequency.
North–south transect from Greenland via the Atlantic Ocean to East Antarctica. (a) Northern Hemisphere (North Greenland Ice Core Project) δ18O (per mil Standard Mean Ocean Water [SMOW]) including brown-numbered Dansgaard-Oeschger cycles (North Greenland Ice Core Project members, 2004). (b) Hematite-stained grains (HSG) of ice-rafted debris records recovered from the North Atlantic at 50°N (Bond et al., Reference Bond, Showers, Elliot, Evans, Lotti, Hajdas, Bonani and Johnson1999) with updated chronology (Obrochta et al., Reference Obrochta, Miyahara, Yokoyama and Crowley2012). (c and d) Laguna Potrok Aike (5022-2CP) with individual magnetic susceptibility (MS) and Ca data points and the respective 200 yr running means as bold lines. (e) East Antarctic Dronning Maud Land δ18O including orange-numbered Antarctic Isotope Maxima (EPICA Community Members, 2006, 2010). Intervals with increased Ca and MS for 5022-2CP are shaded. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ACKNOWLEDGMENTS
This research is supported by the ICDP in the framework of the PASADO (“Potrok Aike Maar Lake Sediment Archive Drilling Project”). Funding for drilling was provided by the ICDP, the German Science Foundation (DFG ZO 102/11-1,2), the Swiss National Funds, the Natural Sciences and Engineering Research Council of Canada, the Swedish Vetenskapsradet, and the University of Bremen.
