Precise timing of MIS 7 sub-stages from the Austrian Alps

Investigating the precise timing of regional-scale climate changes during glacial terminations and the interglacial periods that follow is key to unraveling the mechanisms behind these global climate shifts. Here, we present a high precision time series of climate changes in the Austrian Alps that coincide with the later portion of Termination III (TIII), the entire penultimate interglacial (Marine Isotope Stage (MIS) 7), Termination IIIa (TIIIa), and the penultimate glacial inception (MIS 7/6 transition). Using state-of-the-art mass spectrometry techniques, we 15 have constructed a uranium-series chronology with relative age uncertainties averaging 1.7‰ (2s) for our study period (247 to 191 thousand years before present). Results reveal the onset of warming in the Austrian Alps associated with TIII at 242.5±0.2 ka and the duration of MIS 7e warming between 241.8 to 236.7 (±0.6) ka. An abrupt shift towards higher δ18O values at 216.8 ka marks the onset of regional warming associated with TIIIa. Two periods of high δ18O values (greater than -10‰ VPDB) between 215.9–213.3 and 204.3–197.5 (±0.4) ka coincide 20 with interglacial substages MIS 7c and 7a, respectively. Multiple fluorescent inclusions suggest a partial retreat of the local Alpine glacier during peak obliquity forcings at 214.3 ±0.4 ka. Two newly collected stalagmites from Spannagel Cave (SPA146 & 183) provide high-resolution replications of the latter portion of the MIS 7a/6e transition. The resulting multi-stalagmite record reveals important chronological constraints on climate shifts in the Austrian Alps associated with MIS 7, while offering new insight into the timing of millennial-scale changes in the 25 North Atlantic realm leading up to TIII and TIIIa.


Introduction
Marine Isotope Stage (MIS) 7 (ca. 246-186 thousand years before present [ka], where present is 1950 CE) stands distinctively apart from the last several interglacial cycles. Following the low-amplitude glacial Termination III (TIII), MIS 7e was a relatively weak interglacial that returned to glacial conditions (MIS 7d) within 20 thousand 30 years (ky) (PAGES, 2016). Glacial conditions terminated with a second deglaciation (TIIIa), which gave way to a second interglacial with three distinct sub-stages (MIS 7c,b,a). Although the inception of an interglacial is ultimately paced by astronomical forcing, the precise timing (millennial-scale) of each glacial termination and the strength of the successive interglacial depends on various global climate parameters and feedbacks. Investigating these internal forcings requires the intercomparison of precisely dated climate records in order to identify leads, 35 lags, and synchronicities across different climate zones. The role of forcings that led to the last two glacial terminations and the variability within the last two interglacial periods (MIS 5e and Holocene) have been well studied. Obtaining records from MIS 7 and the penultimate glaciation that contain sufficient resolution and chronological precision to make meaningful regional comparisons, however, has proved challenging. In the North Atlantic realm, marine records show that major meltwater pulse events punctuated a period of rapid sea surface 40 warming associated with TIII and TIIIa (e.g. Channell et al., 2012;Hodell et al., 2008;Martrat et al., 2004Martrat et al., , 2007.
The chronologies of these marine records, however, are frequently dependent on orbital tuning or alignment to global benthic δ 18 O stacks, which feature large uncertainties of ±10 ka during MIS 7 (e.g. LR04 stack; Lisiecki and Raymo, 2005). In continental Europe, cave and lacustrine records indicate abrupt environmental shifts associated with the MIS 7 sub-stages (e.g. Tzedakis et al., 2004;Despart et al., 2006;Roucoux et al., 2008) and TIII (Pérezbetween 1900 and 2011 (Huss, 2012). This high degree of sensitivity renders the Austrian Alps an ideal location to pinpoint the timing of continental temperature variations. Here, we focus on Spannagel cave -a high altitude marble cave located in the central Austrian Alps. Drip waters in this cave are fed by glacial and snowpack melt (Mangini et al., 2005). Past studies demonstrate that Spannagel speleothem δ 18 O represents a robust proxy of local winter temperatures and moisture sources, the latter being dominated by the North Atlantic Ocean (Mangini et al.,

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2005; see section 2.1). Pioneering work by Holzkämper et al. (2005) and Spötl et al. (2007Spötl et al. ( , 2008 presented multiple MIS 7 stalagmites and flowstones deposited in Spannagel cave. Since these studies, new developments in the measurement precision of U and Th isotopes by Cheng et al. (2013) and Craig et al. (2016Craig et al. ( , 2017) allow for ultra-high age precision, with 2σ absolute age uncertainties averaging ±300 years for MIS 7. In addition, two newly discovered stalagmites from Spannagel Cave provide further insight into the penultimate glacial inception (MIS 7/6 transition). The resulting Spannagel δ 18 O record provides a high-precision chronology of abrupt climate change in Europe during MIS 7, while addressing the need for absolute-dated paleorecords from regions that are sensitive to the North Atlantic realm.

Study site
Spannagel Cave ( Fig. 1; 47°04'54"N 11°40'2"E; 2300 to 2530 m a.s.l.) is located above the timberline in the vicinity of a retreating glacier (Hintertux Glacier) close to the main Alpine crest. Previous publications provide details on the setting of this cave (Spötl et al., 2004;Mangini 2007, 2010). A few salient features relevant for this study are summarized here. Spannagel cave developed in a ca. 20 m-thick Upper Jurassic marble (Hochstegen Formation) sandwiched between gneiss bedrock. An interesting feature of Spannagel cave drip waters is that they are rich in U. Cave water U concentrations range from 5 ppb (small cave stream) to 33 ppb (stalactite drip water).

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As a result, calcitic speleothems from this cave are exceptionally high in U concentrations (3 to 399 ppm). The U is most likely sourced from the overlying gneiss which yield significantly higher U concentrations (3.4-12.0 ppm) than the marble (0.3-2.2 ppm).
Spannagel is a well-ventilated cave system with stable air temperatures in the interior of the cave that stay within 80 a narrow range of +1.8 to +2.2°C (measured at several sites using data loggers over a period of about 20 years).
These values are about 2°C higher than the mean annual air temperature at the elevation of the cave, which is a reflection of the small positive thermal anomaly due to the ascending geometry of the cave (chimney effect ;Spötl and Pavuza, 2016). Relative humidity in the cave interior is invariably higher than 96%. Today, the actively retreating Hintertux Glacier terminates ca. 500 m south of Spannagel cave. During the last glacial maximum, the cave was completed covered by as much as 150-250 m of glacial ice (van Husen, 1987). The deposition of speleothems during several cold climate periods suggests that the Hintertux Glacier has remained largely temperate, or warm-based . Under glacial conditions, it is thought that the oxidation of pyrite in the host rock allowed karst dissolution beneath the glacier without the input of soil-derived carbon dioxide .

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Stalagmite SPA121 (Fig. S1) was found in the northern segment of the cave embedded in unconsolidated silty sand which was most likely transported into the cave by high-discharge streams during the last deglaciation. The stalagmite is 19.3 cm tall and was found attached to a platy and angular piece of calcite-cemented gravel. The surface of the sample is covered by a thin layer of medium gray, clay-rich calcite, but the interior preserves mostly 95 transparent, inclusion-poor calcite, showing a striking pattern of cracks. The origin of these cracks is enigmatic but may be related to freezing of the cave or the sediments in which the sample was embedded during peak glacial periods. SPA183 is a 39 cm tall stalagmite collected from the central part of the cave and a slightly higher altitude than SPA 121. SPA183 revealed three distinct growth axes that are marked by variations in color (Fig. S1). SPA146 is a 24 cm tall stalagmite which grew in the same small chamber as SPA183. Both stalagmites were detached from 100 their growth substrate and embedded in sand, but likely transported less than a few meters. SPA146 shows two distinct growth axes (Fig. S1).

Climate setting
The Austrian Alps are intermediately situated between the Westerlies and northerly flowing Mediterranean air 105 masses. Back-trajectory studies show that regional precipitation is predominantly sourced from North Atlantic and Arctic Oceans (~60%) and the Mediterranean Sea (~20%) (Kaiser et al., 2002;Sodemann and Zubler, 2010).
Seasonally, strong westerly flow during winter months transports moisture from the North Atlantic Ocean, whereas infrequent northerly flow from the Mediterranean Sea is more common during summer months (Kaiser et al., 2002;Sodemann and Zubler, 2010). Winter precipitation is stored in Alpine glaciers and snow cover. Glacial and 110 snowpack melt is the dominant source of groundwater recharge in the Alps and the largest supplier to Spannagel dripwaters (Mangini et al., 2005;Spötl and Mangini, 2007). Variations in the δ 18 O of calcite (δ 18 Oc) that precipitates from dripwaters have been shown to reflect variations in winter precipitation δ 18 O (Mangini et al., 2005;Spötl et al., 2006;Mangini, 2002, 2007).
The Austrian Network for Isotopes in Precipitation (ANIP) reveals a tight coupling between the mean annual δ 18 O of regional precipitation and air temperatures (Kaiser et al., 2002) in concordance with other locations across the European Alps (e.g. Schürch et al., 2003;Field, 2010). On sub-annual scales, the mean monthly δ 18 O of ANIP stations show an almost parallel evolution with average air temperatures (Hager and Foelsche, 2015). Additional 120 influences on δ 18 O may include changes in the proportion of seasonal precipitation, which is linked to moisture source. Stable isotope analysis of rainfall collected from the Patscherkofel mountain station (20 km northwest of Hintertux; 2200 m a.s.l.) suggest that δ 18 O precipitation sourced from the North Atlantic Ocean is depleted relative to Mediterranean sources, due to (i) the lower isotopic value of Atlantic Ocean water and (ii) a longer transport pathway (Kaiser et al., 2002). However, due to the high elevation of our study site, orographic effects are likely to 125 smooth any large source-based differences in precipitation δ 18 O.

Methods
Stalagmites SPA121, 146, and 183 were halved and polished. Subsamples for U-Th dating (n=40) were hand drilled 140 along the growth axis of halved stalagmites. Subsample trenches were drilled no larger than 1 mm in width, such that the sampling error (~175 years) remained within the analytical uncertainties (see Results). Target sample weights ranged from 200 to 20 mg in concordance with changes in U concentration. Subsamples were spiked with a mixed 233 U-236 U-229 Th spike similar to that described in Edwards et al. (1987). Procedures for U and Th chemical separation and preparation of reagent solutions follow the methods described in Edwards et al. (1987) and Shen et 145 al. (2002).
U and Th isotopic measurements were made on a Thermo Scientific Neptune Plus MC-ICP-MS following the instrument calibration and the Faraday cup measurement method described in Cheng et al. (2013). In addition, a 10 13 Ohm amplifier was installed within the detection system in order to collect low ion beam intensities (e.g. increments. 946 stable isotope measurements on stalagmite SPA121 were previously published in Spötl et al. (2008). 890 and 583 new stable isotope measurements from stalagmites SPA183 and SPA146, respectively, are reported here. A total of 13 Hendy tests were drilled along individual growth layers from stalagmites SPA146 and SPA 183 (Figs. S4-5). All calcite powders were analyzed using a Gasbench II coupled with a Delta V Plus isotope ratio mass spectrometer. The 1-sigma precision is 0.06 and 0.08 ‰ for δ 13 C and δ 18 O, respectively. Results are 160 reported relative to VPDB.

Results
Resulting U-Th ages and their respective replicates are in stratigraphic order within uncertainties (Table S1). This study focuses on the MIS 7 portion of stalagmite SPA121, which was deposited without interruption between 248.5 and 191.5 (±1) ka (see Spötl et al., 2008 for details on all SPA121 growth phases). New U-Th ages for this 165 stalagmite fall within age uncertainties of the previously published ages (Spötl et al. 2008;Fig. 2). Similar δ 234 Ui values and 232 Th and 238 U concentrations further underscore a high degree of reproducibility between the two SPA121 data sets, which were measured in different laboratories using different instruments (MC-ICP-MS vs. TIMS). New SPA121 ages improve the precision of previously published age uncertainties by an order of magnitude (Fig. 2), from an average of 1.7% to 0.17%.

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The late MIS 7 growth phase of stalagmites 3) ka, respectively. The exact onset of growth is unknown, as both stalagmites show evidence of diagenesis spanning first 2-3 cm of the late MIS 7 growth phase ( Fig. S2; Table S1). Evidence of diagenetic alteration includes a conspicuously white, milky calcite fabric and U-Th ages that are out of stratigraphic order and unable to be replicated. As a result, this study focuses only on the unaltered portion of the late MIS 7 growth phases of stalagmites SPA183 and SPA146 (Fig. S1). Age uncertainties of SPA146 and 183 average 0.09% and 0.16%, respectively. Significantly higher δ 234 Ui values from SPA146 and 183 relative to SPA121 suggest differing drip sources between the first two neighboring stalagmites and SPA121. Growth rates calculated from new SPA121 ages closely agree with previously published data in Spötl et al. (2008) (Fig. 2). The average growth rate is 5.6 180 μm/yr, excluding one period of exceptionally low growth rate (0.8 μm/yr) between 231.1 and 219.6 ±0.6 ka. In contrast, average growth rates of SPA183 and SPA146 are higher (63 and 140 μm/yr, respectively). Differences in growth rates are likely due to differing drip sources. SPA121 stable isotope values used in this study are from Spötl et al. (2008). New stable isotope data from SPA146 185 and 183 is reported in Table S2. The range of SPA121 δ 18 Oc values (-8.1 to -14.7‰) and δ 13 C values (9.7 to 0.8‰) is in agreement with newly measured δ 18 Oc values from SPA146 and SPA183 (Fig. S3). Slight (<1‰) offsets in absolute values between stalagmites are observed, but do not influence the relative variations in stable isotopes which are the focus of this study. Hendy test results (Figs. S4-5; Spötl et al., 2008) show no evidence for significant kinetic fractionation during the deposition of all three stalagmites.
The δ 13 C signature of all three stalagmites is higher relative to modern and Holocene speleothems (-10 to -7‰), reflecting a signal that is buffered by the isotopic composition of the host rock (Spötl et al., 2004). High and commonly positive δ 13 C values indicate no significant input of organic C into the system, thereby indicating an absence of soil and vegetation above the cave throughout the duration of speleothem deposition (Spötl et al., 2008).
Cooler surface temperatures and the absence of soil argue for a significantly larger Hintertux Glacier during MIS 7 relative to today, likely covering a large portion of the cave. The growth of speleothems during cold climate periods of MIS 7 is likely due to the warm-based nature of the glacier, which provides a supply of melt water while 205 preventing the cave from freezing.

Discussion
The new Spannagel δ 18 O record spans the period of Northern Hemisphere warming associated with the later portion of TIII, the five substages of MIS 7, TIIIa, and the MIS 7/6 glacial inception (Fig. 3). Due to our unprecedented 210 age control, we can determine the precise timing of regional changes associated with each MIS 7 sub-stages, as well as the onset of warming in the Austrian Alps associated with TIII and TIIIa (Fig. 4). While terrestrial records cannot directly date changes to the ocean-cryosphere system during a glacial termination, the climatic excursions in high-sensitivity regions, such as the Austrian Alps, provides key temporal constraints on the climate events leading up to and during these transitions.

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On millennial timescales, remarkable similarities are observed between Spannagel δ 18 O and paleorecords that are sensitive to the North Atlantic climate (Fig, 4). These similarities highlight the rapid climatic link between the European Alps and North Atlantic realm, as observed in later interglacial periods (e.g. Holzkämper et al., 2004; striking is the millennial-scale covariance of Spannagel δ 18 O and Chinese Monsoon δ 18 O, which is explained through the following teleconnections: temperature anomalies in the North Atlantic region influence the intensity of heat transport by northern Hadley Cell circulation, which triggers a latitudinal shift in its ascending branch, known as the Intertropical Convergence Zone (ITCZ). Latitudinal shifts in the ITCZ, in turn, influence the Chinese Monsoon strength (see Cheng et al., 2016 for details). Thus, both Chinese and Spannagel speleothems respond to 235 common climate forcings on millennial timescales. Identifying the mechanisms behind North Atlantic-forced excursions in Spannagel δ 18 Oc is challenging due to the complex array of processes that influence Spannagel δ 18 O, but is likely linked to synchronous temperature changes and/or latitudinal shifts in the westerlies. In alignment with previous work, we interpret Spannagel δ 18 Oc as a faithful recorder of millennial-scale changes in the North Atlantic realm during MIS 7.

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For the remainder of this discussion, we will examine the variations in Spannagel δ 18 O associated with MIS 7 in order to provide new temporal constraints on climate changes in the Austrian Alps, as well as new insights into millennial-scale changes in the North Atlantic leading up to TIII and TIIIa.

Termination III
The last four main glacial terminations can be separated into two categories: those that were interrupted by Northern Hemisphere stadial events (TI and TIII) and those that were uninterrupted or minimally interrupted (TII and TIV)  Cheng et al., 2009Cheng et al., , 2016. The Spannagel timing of TIII additionally coincides (within uncertainties) with an abrupt warming of sea surface temperatures (SST) in the North Atlantic (Martrat et al., 2007) and Mediterranean (Fig. 5;Martrat et al., 2004). Combined, these globally distributed records point to a rapid warming in the North Atlantic realm at this time. We interpret the ~3‰ increase in Spannagel δ 18 Oc as an abrupt 275 rise in local winter temperatures. A possible increase in the advection of isotopically enriched Mediterranean moisture to the Austrian Alps may have amplified this warming signal, although the extent to which Mediterraneansourced precipitation influences the δ 18 O signature at our study site is still unclear. Using our high-precision chronology, we assign the onset of warming to 242.5 ±0.3 ka.

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The Spannagel δ 18 O record indicates a 7.6±0.3 ky lag in the onset of regional warming relative to the rise in 65°N summer insolation associated with TIII (Berger, 1978). The observed lag is similar but greater than the 5.1±0.9 ky lag in regional warming relative to TII, as recorded in a speleothem from Hölloch on the northern rim of the Alps (Moseley et al., 2015). A likely explanation for a longer lag-time may be the low obliquity forcings during TIII, resulting in lower-than-average insolation during boreal summers which may have delayed warming in the Alps until near-peak insolation.

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Depleted Spannagel δ 18 O values (<-12‰) from 234.3 to 216.9 ±0.3 ka coincide with MIS 7d (Fig. 3). Depleted Spannagel δ 18 O values during this time can be attributed to cooler local temperatures, although uninterrupted stalagmite deposition indicates that temperatures remained above freezing in this cave .
Maximum cooling in the Alps also coincides with an abrupt weakening of the Chinese Monsoon within ~1 ka uncertainties (Cheng et al., 2009), suggesting Northern Hemisphere-wide cooling. During this time, sea level fell between -18.5 m and -21 m relative to modern levels throughout (Dutton et al., 2009), atmospheric pCO2 dipped below 203 ppmv between 229.6 and 220.9 (±4) ka (Bazin et al., 2013), and North Atlantic SSTs fell to near glacial 310 levels (Martrat et al., 2007). In Europe, δ 13 C records from the Iberian Peninsula (Pérez-Mejías et al., 2017) and pollen records from Albania (Francke et al., 2016) and Greece (Tzedakis et al., 2006) (Dutton et al., 2009) and melt water pulses in the Black Sea 320 (Badertscher et al., 2011) between ~230 to 217 ka support the of the presence of Eurasian ice sheets, which likely contributed to cooler temperatures and/or southerly shifted westerlies over Europe.

Termination IIIa
TIIIa is often referred to an "extra" termination that resulted from the collapse of MIS 7d ice sheets in response to 325 anomalously high insolation. Similar to main glacial terminations, TIIIa is characterized by a rapid rise in North Atlantic SSTs (e.g. Martrat et al., 2007), atmospheric pCO2 (Bazin et al., 2013), global benthic marine δ 18 O (Lisiecki and Raymo, 2005), and an abrupt rise in sea level (e.g. Dutton et al., 2009). However, due to large age uncertainties, the exact timing of TIIIa in marine and ice records remains unclear. To resolve this issue, the timing of TIIIa has been previously determined by precisely dated Chinese stalagmites, which reveal millennial-scale 330 weak monsoon intervals that correspond to meltwater discharge events in the North Atlantic (Cheng et al., 2009(Cheng et al., , 2016. The exact weak monsoon interval corresponding to TIIIa, however, remains a topic of debate. Cheng et al.

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Maximum δ 18 Oc values during MIS 7a-c indicate warmer winter temperatures in the Alps relative to MIS 7e, with MIS 7a the warmest sub-stage. This MIS 7 sub-stage comparison is in agreement with planktonic foraminiferal assemblages on an Iberian Margin sediment core which suggest higher winter temperatures (+1.5°C) during MIS 7a relative to MIS 7e (Desprat et al., 2006). European pollen data indicates that MIS 7c-a had the longest duration, the most diverse and complete forest succession, and the warmest temperatures relative to MIS 7e (Penaud et al., 2008;Tzedakis et al., 2004). Enriched Spannagel δ 18 Oc values during MIS 7a-c may have been further amplified by an increase in northerly advection from the Mediterranean, as suggested by humid conditions in Sardinia (Columbu et al., 2019) and eastern Mediterranean sapropel deposits (e.g. Ziegler et al., 2010), although the extent to which Mediterranean-sourced precipitation influences the δ 18 O signature at our study site is still unclear.
360 Spötl et al. (2008) proposed that four fluorescent inclusions (i.e. dust layers) identified in SPA 121 reflect a partial retreat (or repeated partial retreats) of the Hintertux glacier. Our updated chronology places the timing of the dust layers at 214.3 ±0.4 ka, which coincides with maximum obliquity forcing (Fig. 3;Berger, 1978). This finding supports previous work which argues that changes in obliquity act as a major control on alpine glacier mass balance 365 by influencing the latitudinal distribution of solar radiation (e.g. Huybers et al., 2006). High obliquity and insolation forcing likely drove increased ablation of the Hintertux glacier and warm regional temperatures (as suggested by maximum MIS 7c δ 18 Oc values) at 214.3 ±0.4 ka.

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A drop in Spannagel δ 18 O starting at 197.1 ±0.2 ka marks the end of MIS 7a in the Alps (Fig. 3). We define the Spannagel MIS 7/6 transition to between 197.1-191.4 (±0.3) ka, which coincides with a drop of the Mediterranean Sea level, as shown by Bard et al. (2002). Two newly collected stalagmites from this cave (SPA146 & 183) that grew between 191.8 and 182.3 (±0.6) ka provide additional records of the late MIS 7a/6e transition and early MIS 6e. All three stalagmites show a gradual decreasing trend in δ 18 Oc until approximately 187 ka. A lack of calcite deposition after 182.3 ±0.2 ka suggests unfavorable conditions in Spannagel Cave, possibly related to due to partly cold-base conditions of the glacier above the cave.

Conclusions
The response of climatically sensitive regions to glacial-interglacial cycles and their abrupt transitions provides 380 key insight into the timing of global climate change. In this study, we present the first ever paleo-record of MIS 7 with relative age uncertainties <2‰. Using this chronology, we can determine the precise timing and duration of climate variations in the Austrian Alps in response to the MIS 7 sub-stages and associated glacial terminations.