Biomarker Proxy Records of Arctic Climate Change During the Mid-Pleistocene Transition from Lake El’gygytgyn (Far East Russia)

. The Mid-Pleistocene Transition (MPT) is a widely recognized global climate shift occurring between approximately 1,250 to 700 ka. At this time, Earth's climate underwent a major transition from dominant 40 kyr glacial- 10 interglacial cycles to quasi-100 kyr cycles. The cause of the MPT remains a puzzling aspect of Pleistocene climate. Presently, there are few, if any, continuous MPT records from the Arctic, yet understanding the role and response of the high latitudes to the MPT is required to better evaluate the causes of this climatic shift. Here, we present new continental biomarker records of temperature and vegetation spanning 1,142 to 752 ka from Lake El'gygytgyn (Far East Russia). We reconstruct warm-season temperature variations across the MPT based on branched glycerol dialkyl glycerol tetraethers 15 (brGDGTs) using the MBTʹ 5ME proxy. The new Arctic temperature record does not display an overall cooling trend during the MPT but does exhibit strong glacial-interglacial cyclicity. Spectral analysis demonstrates persistent obliquity and precession pacing over the study interval and reveals substantial sub-orbital temperature variations at ~900 ka during the first “skipped” interglacial. Interestingly, Marine Isotope Stage (MIS) 31, which is widely recognized as a particularly warm interglacial, does not exhibit exceptional warmth in the Lake El'gygytgyn brGDGT record. Instead, we find that MIS 29, 27 20 and 21 were as warm or warmer than MIS 31. In particular, MIS 21 (~870 to 820 ka) stands out as an especially warm and long interglacial in the continental Arctic while MIS 25 is a notably cold interglacial. Throughout the MPT, Lake El'gygytgyn pollen data exhibit a long-term drying trend, with a shift to an increasingly open landscape noted after around 900 ka (Zhao et al., 2018), which is also reflected in our higher plant leaf wax ( n -alkane) distributions. Although the mechanisms driving the MPT remain a matter of debate, our new climate records from the continental Arctic exhibit some 25 similarities to changes noted around the North Pacific region. Overall, the new organic geochemical data from Lake El'gygytgyn contribute to expanding our knowledge of the high-latitude response to the MPT.


Introduction
Since the start of the Industrial Revolution, high-latitude temperatures have increased at about twice the global average rate (Serreze et al., 2009;Davy et al., 2018). Likewise, during past interglacial periods, Arctic temperature reconstructions 30 indicate significant warming events (e.g., de Wet et al., 2016). The importance of studying past Arctic temperature variability is widely recognized (Miller et al., 2010;Melles et al., 2012;Brigham-Grette et al., 2013) yet few long and continuous records exist from the continental Arctic. Lake El'gygytgyn, located in Far East Arctic Russia, is a unique site that escaped continental glaciation and preserves a 3.6 myr-long sedimentary record (Nolan and Brigham-Grette, 2007;Melles et al., 2012). Prior work on Lake El'gygytgyn has revealed exceptional changes during the Plio-Pleistocene. During 35 the Pliocene, cool mixed forest and cool conifer forest dominated the landscape , whereas today the tree line lies ~150 km to the southwest and the lake is surrounded by tundra vegetation . In addition to notable changes between Pliocene and Pleistocene climates Melles et al., 2012), However, at present, relatively few lacustrine records spanning the MPT have been drilled. These include Lake Malawi Cohen et al., 2007), Lake Baikal (Prokopenko et al., 2006), and Lake Ohrid (Wagner et al., 2014;Just et al., 2019); Lake El'gygytgyn is the only such record from the Arctic (Melles et al., 2012;Brigham-Grette et al., 2013). 75 Investigating the cause(s) of the MPT is beyond the scope of this study. However, our new records provide important information regarding high-latitude continental temperature during this critical climate transition. This study also provides new insights on the strength and duration of past interglacials in the continental Arctic. Understanding past Arctic temperature variability is particularly important for placing the current warming into context and for improving models of future climate. 80

Study Location
Lake El'gygytgyn (67.5 °N, 172.1 °E, 492 masl) is located within the Anadyr Highlands of northeast Russia, approximately 150 km south of the Arctic Ocean coast (Fig. 1). It is a crater lake, formed following a meteorite impact at 3.58 Ma (Layer, 2000), that has accumulated a continuous sedimentary record since its formation. The 318 m sedimentary sequence was collected in 2009 (Melles et al., 2012;Brigham-Grette et al., 2013), providing a uniquely long Arctic paleoclimate record 85 including the MPT interval (Melles et al., 2012;Brigham-Grette et al., 2013;Nowaczyk et al., 2013;Zhao et al., 2018). The age model for the Lake El'gygytgyn drill core was previously published by Nowaczyk et al. (2013) and is applied here. This age model is based on iterative tie-point identifications using 1) paleomagnetic reversals, 2) comparison of biogenic silica to the LR04 benthic oxygen isotope stack (Lisiecki and Raymo, 2005), and 3) comparison of total organic carbon (TOC) and magnetic susceptibility to summer insolation (Laskar et al., 2004) and has an age precision of ~500 years relative to the 90 insolation reference curve . During our study interval, paleomagnetic reversals/excursions provide age constraints at 0.780 Ma, 0.991 Ma, 1.0142 Ma, 1.0192 Ma, and 1.075 Ma (Haltia and Nowaczyk, 2014;Nowaczyk et al., 2013).

100
The lake basin morphology and local meteorology are well-characterized by Nolan and Brigham-Grette (2007). The lake is approximately 12 km wide and 175 m deep and resides in an impact crater 18 km in width. There is a single stream outlet, the Enmyvaam River, which drains to the south into the Bering Sea. Approximately 50 small streams, with headwaters located within the impact crater, drain into the lake. Mean annual air temperature is -10.3 °C and summer temperatures (JJA) 105 average 10 °C. In contrast, the lake water never exceeds 4 °C except over shallower regions and in the fringing lagoons which reach 5-6 °C during summer (Nolan and Brigham-Grette, 2007). Over the past 50 years, air temperatures have risen by over 3 °C, driven largely by increasing winter temperatures (Nolan et al., 2013). While positive air temperature anomalies are associated with a strong low-pressure system over the Bering Sea and high pressure over the Beaufort Sea, which advects warm air from the south and east, Nolan et al. (2013) demonstrated that over the observational period, this temperature 110 increase is caused by general warming of the atmosphere, rather than changes in storm tracks. It is somewhat unclear if mean weather patterns at Lake El'gygytgyn are subject to reorganization over longer timescales, such as might be associated with Bering Land Bridge exposure or submergence, or changes in North Pacific Ocean circulation.

Methods
In this study, we analyzed 127 samples from the Lake El'gygytgyn drill core (ICDP Site 5011-1) spanning the interval from 115 1,142 ka to 752 ka, corresponding to depths of 46.77 m to 31.06 m in the composite profile. Additionally, we re-analyzed approximately every third sample from the study of de Wet et al. (2016) (41 samples), who also studied Site 5011-1 in the interval from Marine Isotope Stage (MIS) 33 to MIS 31 (1.11 to 1.05 to Ma), using a newer High-Performance Liquid Chromatography (HPLC) method that separates the 5-and 6-methyl brGDGT isomers . We also include 85 n-alkane samples from another study by de Wet (2017) in our analysis. Overall, the sample spacing averages 2.3 120 kyr for the brGDGTs and 2.7 kyr for the n-alkanes, with each 1 cm interval integrating an average 450 years, thereby allowing sufficient resolution to evaluate orbital-scale climate variability.

Biomarker Analyses
Sediment samples were freeze-dried, homogenized, and extracted using a Dionex accelerated solvent extractor (ASE 200) with a mixture of 9:1 of dichloromethane:methanol (v/v). Known quantities of a synthetic C46 GDGT were added to the total 125 lipid extract (TLE) as an internal standard (Huguet et al., 2006). The TLE was then separated via alumina oxide columns using 9:1 hexane:dichloromethane (v/v) to elute the apolar fraction and 1:1 dichloromethane:methanol (v/v) to elute the polar fraction.

n-alkane analysis
Apolar fractions were analyzed on an Agilent 7890A gas chromatograph-flame ionization detector (GC-FID) to determine 130 concentrations of the n-alkanes. Samples were run in splitless mode with an inlet temperature of 250 °C and inlet flow rate of 26.5 mL min -1 , using hydrogen as the carrier gas. Separation was achieved using a 5% phenyl methyl siloxane column (HP-5; 60 m x 320 μm x 0.25 μm), with a flow rate of 4.6 mL min -1 . The oven temperature program was as follows: 70 °C for two minutes, increasing at 17 °C min -1 to 130 °C, then increasing at 7 °C min -1 to 320 °C, and finally holding at 320 °C for 15 minutes. Compound identification was performed by comparison to a standard mixture of C21-C40 n-alkanes injected 135 during each run and by confirming compound identification for a subset of samples via GC-mass spectrometry, following the methods detailed in Keisling et al. (2017). Concentrations of each n-alkane were determined using an external squalene calibration curve. Here, we examine leaf wax distributions using the Average Chain Length (ACL; Bray and Evans, 1961;Bush and McInerney, 2013), calculated for the terrestrial (C27 to C33) n-alkanes (Eq. 1): (1) ACL = ∑(C n * n)/ ∑(C n ) 140

GDGT analysis
The polar fractions were filtered through a 0.45 μm PTFE filter in 99:1 hexane: isopropanol (v/v). Isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs) and brGDGTs were analyzed using an Agilent 1260 Ultra High-Performance Liquid Chromatograph (UHPLC) coupled to an Agilent 6120 single quadrupole mass selective detector (MSD) and following the methods detailed by Hopmans et al. (2016), which separates the 5-and 6-methyl brGDGT isomers. Briefly, separation is 145 achieved using a silica precolumn with two BEH HILIC columns in series (2.1 x 150 mm, 1.7 μm; Waters®). Samples were eluted using hexane (solvent A) and hexane:isopropanol (9:1 v/v; solvent B) in the following program: 18% solvent B for 25 minutes, a linear increase to solvent B over 25 minutes, then a linear increase to 100% solvent B for 30 minutes. The column temperature was 30°C and the flow rate was 0.2 ml min -1 . Compounds were ionized using atmospheric pressure chemical ionization and the MSD was run in single ion monitoring (SIM) mode for the GDGT core lipids. 150 Lake El'gygytgyn sediments are dominated by brGDGTs with only small concentrations of iGDGTs present (Fig. S4) (Holland et al., 2013;D'Anjou et al., 2013;de Wet et al., 2016;Keisling et al., 2017;. Thus, we use the Methylation of Branched Tetraethers index (Eq. 2), considering only the 5-methyl isomers (MBTʹ5ME), to reconstruct past temperature (De Jonge et al., 2014). 155 ( 2)  We explored the use of several lacustrine brGDGT calibrations to convert MBTʹ5ME values to temperature. The first is based on a suite of lakes in East Africa that span an elevation transect to capture temperature gradients Eq. 3).
This calibration is to mean annual air temperature.
(4) JJA Water Temp = −1.82 + 56.06 * MBT′ 5ME (RMSE = 0.58 °C) Raberg et al. (2021) explored relationships between compound fractional abundances (FAs) within structural groups based on methylation number, methylation position, and cyclization number and environmental parameters using a previously 165 published globally distributed data and adding new sites from the high-latitudes. They recommend using their "Meth" calibration for general use in lake sediments (Eq. 5): Raberg et al., 2021 also provide a "Full" calibration (Eq.6), provided the highest R 2 and lowest RMSE in their data and suggest this calibration may be appropriate to apply at sites with good conductivity or pH control.  We also use a mean annual air temperature calibration from Feng et al. (2019) which was developed using lake sediments 175 and several decades of instrumental climate data from Tiancai Lake, China (Eq. 7): We also apply the global Bayesian calibration for lacustrine brGDGTs, known as BayMBT, using the baymbt_predict() MATLAB function (Martínez-Sosa et al., 2021). Following the approach of Martínez-Sosa et al. (2021), we estimated the 180 prior value for our dataset by calculating a mean temperature using the Russell et al. (2018) calibration; we also used 10°C for the standard deviation.

n-alkanes
Plant leaf waxes (n-alkanes) were present in all samples. However, 58 of the 212 samples analyzed (27%) contained an 185 unresolved complex mixture in the apolar fractions, hampering peak identification without further sample purification. The n-alkanes were not quantified for these samples, resulting in lower sample resolution record for the leaf waxes compared to the GDGTs. For the 153 samples where n-alkanes could be sufficiently resolved, samples contained C20 through C33 nalkanes, with the shorter chain lengths not always present in each sample. The total concentration of odd-numbered nalkanes ranges from 0.6 to 60.8 μg g -1 , with a mean concentration of 4.5 μg g -1 . Odd-numbered chain lengths dominate, 190 particularly the C23 and C27 homologues (Fig. S1), and the carbon preference index (CPI; Eq. S1) over the C25-C33 compounds averages 3.6 (±0.6 s.d.). When considering all n-alkanes from C17 to C33, the ACL averages 26.2 (±0.9 s.d.). The C27-C33 ACL (Eq. 1) varied between 28.5 and 30.1 (Fig. 2a), with a mean of 29.4. The C27-C33 ACL exhibits a clear shift at around 900 ka (Fig. 2, Fig. S2); samples older than 900 ka samples are characterized by lower ACL values whereas samples younger than 900 ka exhibit higher ACL values (Fig. 2b). This shift in ACL values appears to track changes in the pollen-195 derived landscape openness index (Zhao et al., 2018), giving rise to a negative correlation between these metrics (Fig. S3).

GDGTs
iGDGTs are present in all samples analyzed. However, total iGDGT concentrations are low and range from 0.3 ng g -1 to 3916 ng g -1 with a mean concentration of 69 ng g sed -1 (Fig. S4). Nearly all the iGDGTs present are represented by GDGT-0 205 and GDGT-4 (Daniels et al., 2021), with average abundances of 75% and 15% of the total iGDGTs. The high GDGT0/crenarchaeol ratio (average 68.8), as well as the fact that many samples did not contain all of the iGDGTs required to calculate a TEX86 value, precludes the use of the TEX86 paleothermometer (Schouten et al., 2002) at Lake El'gygytgyn.

Spectral Signatures of Temperature Variability
To characterize the frequency of temperature variability, we analyzed spectral signatures of the MBTʹ5ME temperature record using PAST3 software (Hammer et al., 2001). Frequency analysis was not performed on the n-alkane data because of its lower sample resolution. As noted above, the age model for the El'gygytgyn core tunes TOC and magnetic susceptibility to summer insolation, and indeed, orbital frequencies are observed in several El'gygytgyn proxies 240 Francke et al., 2013). Interestingly, however, the relative strength of precession, obliquity, and eccentricity bands appear to differ between proxies. Here, we use the REDFIT package (Schulz and Mudelsee, 2002) in PAST3 (Hammer et al., 2001), which utilizes the Lomb-Scargle method for spectral analysis (Lomb, 1976;Scargle, 1982;Schulz and Mudelsee, 2002), to evaluate the overall periodogram. We use the PAST3 Short-time Fourier transform package to generate the evolutionary periodogram. Over our study interval, the REDFIT periodogram shows a small spectral peak at ~82 kyr/cycle, a peak at ~41 245 kyr/cycles, a dispersed peak in the 23-14 kyr/cycle, as well as a significant peak at ~11 kyr/cycle possibly reflecting halfprecession (Fig. 4a). The 40 kyr obliquity signal is strongest in the earlier part of the temperature record, from MIS 32 to MIS 26, but weakens by ~980 ka, when the relative power of the higher-frequency bands increases (Fig. 4b). Toward the later part of the record, beginning at ~900 ka, we observe increased variability at longer wavelengths, potentially reflecting both 40-and 80-kyr cyclicity (e.g., obliquity and 2x obliquity) in the temperature record. 250

Interpretation of the n-alkane ACL record
The long-chain n-alkanes (n-alkanoic acids, n-alcohols) with 27 to 35 carbon atoms are biomarkers of terrestrial higher plants (Eglinton and Hamilton, 1967). Indeed, long-chain (C27-C33) n-alkanes are predominantly derived from higher terrestrial plants in the El'gygytgyn catchment (Wilkie et al., 2013), similar to observations at other Arctic locations (Daniels 260 et al., 2017;O'Connor et al., 2020). Over the MPT study interval, CPI values average 3.6 indicating that higher plant inputs dominate the n-alkane pool (Fig. S1). Both the distributions and isotopic composition of n-alkanes can provide information on past vegetation change or the response of vegetation to climate (e.g., temperature or precipitation; Meyers, 2003;Castañeda and Schouten, 2011). We did not examine leaf wax deuterium or carbon isotopes in this study, but we observe changes in the n-alkane ACL across the MPT (Fig. 2). Prior studies with independent temperature or aridity reconstructions 265 (e.g., from leaf wax isotopes, lignin phenols, GDGTs or pollen) have reported that ACL increases with increasing aridity (e.g., Liu and Huang, 2005;Schefuß et al., 2003;Peltzer, 1989;Poynter et al., 1989) or increasing temperature (e.g., Castañeda et al., 2009;Zhang et al., 2006;Kawamura et al., 2003;Rommerskirchen et al., 2003;Gagosian and Peltzer, 1986). However, at some locations there is no clear relationship between ACL and temperature or aridity. Furthermore, global correlations are weak hindering a quantitative climate assessment using ACL (Bush and McInerney, 2013). 270 Nonetheless, in the Arctic, higher ACL values have been associated with arid conditions (Andersson et al., 2011). At Lake El'gygytgyn, Keisling et al. (2017) suggested that ACL during the Pliocene is correlated with independent metrics of aridity but is secondarily affected by temperature and vegetation change. As there are available pollen assemblage data from Lake El'gygytgyn spanning the MPT (Zhao et al., 2018), we refine our interpretation of ACL here.
275 Zhao et al. (2018) examined pollen assemblages in the Lake El'gygytgyn drill core spanning 1,091 to 715 ka and found that shrub tundra and cold steppe communities dominate throughout the study interval. The pollen record exhibits a clear response to glacial-interglacial climate forcing with higher percentages of herbaceous taxa (Poaceae, Cyperaceae, and Artemisia) present during glacial periods, reflecting an open landscape (treeless or shrubless) and cold and arid conditions. Conversely, during interglacials increased percentages of tree and shrub pollen are present, with dwarf birch (Betula) and 280 shrub alder (Alnus) being the most common (Zhao et al., 2018). The authors developed a landscape openness index, which is the relative difference between the maximum scores of forest and open biomes, as a qualitative assessment of forest vs. an open landscape (Zhao et al., 2018). This record shows higher (positive) values during interglacials, with especially high values noted during MIS 31 and MIS 25 (Fig. 2a). Throughout the MPT, a shift in the landscape openness index is observed with an increasingly open landscape noted after around 890 ka, reflecting a long-term cooling and drying trend during the 285 MPT (Zhao et al., 2018). We find that the ACL at Lake El'gygytgyn tracks changes in the landscape openness index, with lower values observed prior to ~900 ka and higher values afterwards when the landscape is more open and herbaceous taxa are more prevalent (Fig. 2b). Thus, we interpret ACL variations as representing large-scale vegetation changes around Lake El'gygytgyn, which likely reflect a combination of climate-driven vegetation change coupled with the direct response of plants to moisture availability. 290

Interpretation of the brGDGT record
Prior to interpreting the brGDGT record, the source(s) of the brGDGTs (soil or lacustrine), the seasonality of production, and the choice of temperature calibration need to be considered. The distribution of brGDGTs in our dataset indicates a dominant lacustrine source (Fig. S5) throughout the MPT, in agreement with other previously studied time intervals of the Lake El'gygytgyn record (Holland et al., 2013;D'Anjou et al., 2013;de Wet et al., 2016;Keisling et al., 2017). Moreover, the 295 samples are dominated by 5-methyl brGDGTs, suggesting that the MBTʹ5ME index is most suitable for reconstructing temperatures at Lake El'gygytgyn ( Fig. S4). There are currently several lacustrine MBTʹ5ME calibrations that can be applied to reconstruct past temperature (Dang et al., 2018;Russell et al., 2018;Feng et al., 2019;Zhao et al., 2021;Martínez-Sosa et al., 2021;Raberg et al., 2021). Naturally, the reconstructions based on MBTʹ5ME all show the same temporal structure. The calibration of Dang et al. (2018) is based on alkaline lakes, and so is inappropriate to apply at Lake El'gygytgyn, which has a 300 pH of ~6 (Cremer et al., 2005). Likewise, the calibration of Feng et al. (2019) (Eq. 7) yields unrealistically low temperatures and shows strong deviations from other local and global climate records (Fig. S8) and is deemed not to be applicable here.
The calibration of Russell et al. (2018) reconstructs mean annual air temperature (MAAT) and is based on a suite of tropical African lakes. The resulting MAAT from that calibration dramatically over-estimates current MAAT at El'gygygtyn but results are similar to summer (JJA) reconstructed temperatures based on pollen (Melles et al., 2012). The Zhao et al. (2021)  305 calibration reconstructs summer water temperature and is based on settling particulate material from a southern Greenland lake. It should be noted that the Zhao et al. (2021) calibration is currently the only MBTʹ5ME calibration to water temperature; the other studies calibrate to MAAT or air temperature of months above freezing because in-situ measurements of lake water temperature are often not available. The global "Meth" calibration (Eq. 5) of Raberg et al. (2021) yields values generally similar to those based on MBTʹ5ME (e.g., Russell et al., 2018;Martínez-Sosa et al., 2021) but some features of the data 310 present in other calibrations (and other Lake E'gygytgyn proxies), such as warmth during MIS 21, do not stand out (Fig. S8).
The Raberg et al. (2021) calibrations utilize different subsets of brGDGTs and therefore exhibit some differences compared to calibrations based on MBTʹ5ME (Fig. S8) structure apparent in the MBTʹ5ME record, and in the pollen spectra (for intervals where it exists), we therefore base our interpretation on MBTʹ5ME. We further evaluate the choice of calibration by looking at the record across a well-constrained glacial-interglacial cycle during the MPT.
MIS 31 (1.082-1.062 Ma) is widely recognized as an exceptionally warm interglacial period (e.g., Maiorano et al., 2009;320 Teitler et al., 2015), and there are independent temperature estimates (mean temperature of the warmest month) for Lake El'gygytgyn based on pollen assemblages ( Fig. 5; Melles et al., 2012). Previously, de Wet et al., 2016 applied the MBT/CBT calibration of Sun et al. (2011) to examine MIS 31. They found a good agreement between their reconstructed temperatures and pollen-inferred temperature of the warmest month. While the Greenland lakes brGDGT calibration (Zhao et al., 2021) yields similar temperature estimates to the pollen-based reconstruction across MIS 31 (Melles et al., 2012;de Wet et al., 325 2016), the overall amplitude through the remainder of the study interval is larger. Both the African lakes  and global Bayesian calibrations (Martínez-Sosa et al., 2021) produce similar values and yield comparatively lower MIS 31 temperatures compared to the pollen estimates, as well as an overall lower amplitude of temperature variability (Fig.  5). Given that Lake El'gygytgyn is a deep and cold lake, and today the shallowest parts of the lake do not exceed 5-6 °C during summer (Nolan and Brigham-Grette, 2007), a calibration with an overall lower amplitude is likely more realistic. 330 Therefore, throughout the remainder of this discussion we plot our MBTʹ5ME data using the BayMBT calibration (Martínez-Sosa et al., 2021). While a lacustrine-based brGDGT temperature calibration needs to be applied to Lake El'gygytgyn, it should be noted that our samples display a somewhat different brGDGT distribution compared to other lake datasets (Fig.   S5), potentially reflecting a need for a pan-Arctic brGDGT calibration or a site-specific calibration. We therefore place more emphasis on relative trends in the data (warming and cooling events), which remain robust regardless of the calibration 335 applied. Furthermore, given that several studies now document that brGDGT production in high-latitude lakes peaks during summer, and that both older brGDGT calibrations that were based on the combined methylation and cyclization of brGDGTs, as well as the newer MBTʹ5ME index show the strongest correlation with growing-season temperatures (Pearson et al., 2011;Sun et al., 2011;Shanahan et al., 2013;Zhao et al., 2021;Miller et al., 2018;Martínez-Sosa et al., 2021), we interpret relative temperature changes at Lake El'gygytgyn as reflecting conditions during the ice-free summer growing 340 season.

Climate variability during the MPT
Our new brGDGT data from Lake El'gygytgyn document relative temperature changes, revealing several important aspects of orbital and long-term climate variability throughout the MPT. The MBTʹ5ME record shows that temperatures varied at Milankovitch and sub-Milankovitch time scales, and generally follows changes noted in the global benthic oxygen isotope stack (Lisiecki and Raymo, 2005) and insolation (Fig. 3). This observation agrees with prior studies of Lake El'gygytgyn 355 (e.g., de Wet et al., 2016), although it is evident that the relative importance of obliquity and precession-pacing has varied temporally ( Fig. 3 and 4). Our MBTʹ5ME temperature record supports the observation of Melles et al. (2012) that congruency between peak precession, obliquity, and eccentricity, together with inter-hemispheric teleconnections can generate superinterglacial periods such as MIS 11 and 31. The brGDGT data generally support this interpretation during MIS 31, as well as during the alignment of northern hemisphere summer perihelion and obliquity during the warm MIS 21 (Huybers and 360 Wunsch, 2005), but we note that several other intervals are as warm in the MBTʹ5ME record despite differences in the orbital configurations. Furthermore, we note that the inverse is also true for temperatures at Lake El'gygytgyn whereby congruent minima in summer insolation, obliquity, and relatively low eccentricity resulted in very cold conditions during MIS 28 and MIS 22 (Fig. 3).

365
We observe no long-term trend in the MBTʹ5ME data (Fig. 3), which contrasts with cooling trends observed in the global benthic δ 18 O stack (Lisiecki and Raymo, 2005;Clark et al., 2006;Li et al., 2004), Mg/Ca-derived temperatures of Atlantic deep waters (Sosdian and Rosenthal, 2009), and some sea surface temperature records from the northern high latitudes (e.g. Martínez-Garcia et al., 2010;Lawrence et al., 2009;McClymont et al., 2008). At Lake El'gygytgyn, Francke et al. (2013) report changes in orbital frequencies of grain size distributions before and after the MPT, while other proxies at Lake 370 El'gygytgyn do not exhibit any long term trends across the MPT interval (Wennrich et al., 2016). These other proxies do not directly record temperature, yet appear to agree with the brGDGT data in suggesting relatively stable long-term conditions across the MPT. The lack of MPT cooling at El'gygytgyn is difficult to explain given the expansion of northern hemisphere ice sheets at that time. It could imply that the climate at the study site is not representative of the pan-Arctic region, and indeed, there is considerable spatial variability in climate change across the Arctic Tulenko et al., 375 2020). Alternatively, it may suggest that Arctic cooling was not the critical driver of intensified ice sheet growth, implicating a strong role for the regolith removal hypothesis (Clark and Pollard, 1998;Yehudai et al., 2021) or southern hemisphere (i.e. Antarctic) cooling and ice sheet expansion (Ford and Raymo, 2020). Although we see no overall trend in the brGDGT data, there is a notable increase in leaf wax ACL values reflecting a significant climate-driven ecological change across the MPT (Fig. 2), most likely indicating aridification of the study region. A long-term aridification trend over the past ~1 Myr is 380 similarly noted from the Chinese Loess Plateau (Zhao et al., 2018;Zhou et al., 2018;Wu et al., 2020). At El'gygytgyn, aridity is in part controlled by the amount of moisture being sourced from the nearby high-latitude seas. A sedimentary record from the Northwind Ridge in the Western Arctic Ocean indicates a transition from seasonal sea ice to perennial sea ice around 1 Ma, which could explain the observed trend toward drier conditions (Dipre et al., 2018). The contrast between enhanced sea ice coverage and stable temperatures at El'gygytgyn points to strong geographic variations in climatic cooling 385 across the MPT, particularly between the marine realm and northeast Russia.
A characteristic feature of the MPT is the appearance of longer glacial cycles, expressed in the frequency domain of many palaeoceanographic and continental records as increasing power in the quasi-100 kyr band, although there is no fundamental change in orbital forcing across the MPT. In the Arctic, precession and obliquity are key drivers of warm-season 390 temperatures because of the large changes in peak summer insolation and changes in summer duration. Indeed, spectral analysis of the Lake El'gygytgyn MBTʹ5ME record reveals significant obliquity and precession cycles over the MPT (Fig. 4), with the obliquity signal being the strongest prior to ~1.0 Ma (Fig. 4). Increased variability at longer wavelengths is observed after ~900 ka but at an ~82 kyr cyclicity, which may be twice an obliquity signal rather than eccentricity. However, we note that the interval studied here may be too short to fully evaluate changes in longer orbital frequencies occurring during the 395 MPT. Prior studies of the Lake El'gygytgyn drill core report similar spectral results but with some notable differences in the relative strength of various frequencies. Grain-size data, reflecting climate-dependent (glacial-interglacial) clastic sedimentation processes at Lake El'gygytgyn, also display a strong obliquity cycle throughout the MPT. Yet in contrast to the brGDGT data, the strongest obliquity signature is noted in the younger part of the record from 950 to 670 ka . The grain-size data also exhibits a strong precession cycle from 1100 to 900 ka , in 400 agreement with the brGDGT data. The authors note that changes in grain-size at Lake El'gygytgyn are not directly coupled to changes in global ice volume.
The effect of MPT ice sheet expansion on spectral signatures would naturally be strongest in direct proximity to where the ice sheets develop, namely North America, Greenland, and Fenno-Scandia. In northeast Siberia, however, ice sheets have not 405 been as prominent a feature of the Pleistocene environment , and it is not entirely clear how the 100 kyr ice sheet influence is transferred to Lake El'gygytgyn. Through model simulations, Melles et al. (2012) indicated that temperature variability at Lake El'gygytgyn is decoupled from Greenland Ice Sheet growth/decay. Instead, precession variations here are more likely connected with regional insolation differences or latitudinal climatic teleconnections , which may help explain the lack of brGDGT-inferred cooling across the MPT. 410 Interestingly, the Lake El'gygytgyn brGDGT data exhibit the strongest spectral power at ~10.9-kyr cyclicity (Fig. 4), which could reflect a half-precession signal (Verschuren et al., 2009). The signal is strongest between 850 and 1030 ka, and is apparent, for example, in the structure of the substages of MIS 21. Sub-orbital climate variations have been noted at Lake El'gygytgyn but not discussed in detail (e.g. Wennrich et al., 2016), as most proxies are dominated by the Milankovitch 415 frequencies. Nonetheless, half-precession has been observed during the MPT in the northern high latitudes. Haneda et al. (2020) report half-precession variability in the Kuroshio current of the western North Pacific during MIS 19. Likewise, Ferretti et al. (2010) report that during MIS 21, foraminifera δ 18 O varied strongly at the 10.6 kyr wavelength at Site U1313 ( Fig. 1; this Site is a revisit of DSDP 607) and a number of other sites across the North Atlantic. Their finding supports a previous identification of Milankovitch harmonics in the North Atlantic (Wara et al., 2002). In that region, the isotope signal 420 was driven by variations in temperature and the strength of Atlantic Meridional Overturning Circulation (AMOC).
The presence of half-precession variability in the North Atlantic has been attributed to non-linear feedbacks due to orbital forcing within the North Atlantic region, related to the different timescales of ice sheet dynamics, deep ocean convection, and moisture feedbacks (Wara et al., 2002). It has alternatively been linked to tropical hemi-precession (e.g. Verschuren et 425 al., 2009) (Niebauer, 1988). Considering that Bering Strait climatology can exert a strong influence on temperatures at Lake El'gygytgyn (Nolan et al., 2013), this atmospheric teleconnection may be the most direct explanation for the occurrence of half-precession in the El'gygytgyn MBTʹ5ME record. 435 In addition to potentially influencing sub-orbital climate dynamics at El'gygytgyn, strengthening of Walker circulation between 1.17 Ma and 0.9 Ma has been hypothesized to be a key driver of the mid-Pleistocene transition (McClymont and Rosell-Melé, 2005). Walker cell intensification likely resulted in a westward shift and deepening of the Aleutian low, thereby lowering air and sea surface temperatures over the Bering Sea and potentially contributing to enhanced upwelling 440 and expanded sea ice (Worne et al., 2021). In contrast to the potential half-precession sensitivity of Lake El'gygytgyn MBTʹ5ME to tropical/sub-tropical Pacific dynamics, we do not see a clear effect Walker circulation changes across the MPT in the MBTʹ5ME temperature record presented here, potentially because changing position of the Aleutian low can result in variable temperature responses at El'gygytgyn (Nolan et al., 2013). Furthermore, McClymont and Rosell-Melé (2005) speculate that intensification of Walker circulation increased winter precipitation in the Arctic, contributing to ice sheet 445 expansion. This mechanism contrasts with the vegetation change and moisture reduction across the MPT suggested by pollen and leaf waxes at El'gygytgyn, potentially because of locally expanded sea-ice or lower continental temperatures suppressing evaporation in Lake El'gygytgyn moisture source regions. We note that the half-precession signal in the brGDGT record (Fig. 4) gets stronger between approximately 1.1 and 0.9 Ma, while Walker circulation was intensifying and prior to the first skipped interglacial. Future climate modeling could elucidate how shifts in Walker circulation may have 450 influenced sub-orbital variations in Pacific climate variability, including its influence at Lake El'gygytgyn.
In the following discussion, we take a closer look at glacial-interglacial variability noted in our new Lake El'gygytgyn biomarker records.

MIS 34-26 (1150 to 959 ka) 455
In the interval spanning MIS 34 to 26, we observe a strong correspondence between the MBTʹ5ME temperatures and global benthic foraminifera oxygen isotope ( 18 O) stack (Lisiecki and Raymo, 2005) (Fig. 3). The coldest periods in the MBTʹ5ME record align closely with glacial Facies A in the El'gygytgyn sediment profile (Fig. 3), as well as with biomarker inferred expansions of sea ice beyond the marginal ice zone of the Bering Sea ( Fig. 6; Detlef et al., 2018) indicating cooling across the Bering region during the MIS 32, 30, 28, and 26. While precession and half-precession are apparent in the evolutionary 460 power spectrum (Fig. 4), the temperature was primarily paced at 41 kyr over this interval, as seen in alignment between the smoothed brGDGT temperature record and the obliquity history (Fig. S9). This could reflect the importance of summer duration on Arctic summer temperature or lake water temperature. Alternatively, the 41-kyr pacing may be driven by changes in glacial/interglacial pCO2 variations, for which there is not a well-resolved record over this timespan but it likely tracked global climate conditions at 41 kyr pacing (Berends et al., 2021). 465 During MIS 31, the MBTʹ5ME record shows somewhat surprising results in comparison to other Lake El'gygytgyn proxies (Fig. 3). In the El'gygytgyn record, superinterglacial periods (interglacials that are exceptionally warm and wet) were previously identified based on the presence of Facies C, a sedimentary unit characterized by weakly laminated reddish 475 oxidized sediments, high Si/Ti ratios, and low organic matter content (Melles et al., 2012). Facies C is interpreted as reflecting an oxygenated water column, high aquatic productivity, seasonal lake ice, and generally warm conditions (Melles et al., 2012). Pollen spectra from Facies C intervals typically corroborate this interpretation, exhibiting a temporary appearance of birch, alder or other trees (Melles et al., 2012). The MBTʹ5ME data show a temperature increase of ca. 5-8 °C demonstrated that precipitation is a key driver of Facies C formation. This may imply that MIS 29, 27, and 21 were relatively arid in comparison to MIS 31. The pollen record indicates that cool conifer forest was present around Lake El'gygytgyn during MIS 31 and that higher precipitation characterized this superinterglacial (Zhao et al., 2018;Lozhkin and Anderson, 2013;Melles et al., 2012). Additionally, it is possible that MIS 31 was windier than other interglacials, thereby decreasing 490 water column stability and increasing ventilation and nutrient availability in the surface waters. Indeed, high Si/Ti values, a proxy for diatom productivity, are noted in the Lake El'gygytgyn record during MIS 31 (Melles et al., 2012).
In the interval from MIS 34 to 26, the n-alkane ACL record has the highest resolution during MIS 31. At this time, a shift to lower ACL values is noted ( Fig. 2 and 5), potentially driven by wetter conditions. During the other interglacials in this 495 interval (MIS 33,29,and 27), lower ACL values generally occur during peak interglacial conditions. However, low sample resolution during MIS 27 hampers full evaluation of this relationship and higher variability in ACL values is noted during MIS 33, 29 and 27 compared to MIS 31. In the pollen record, MIS 29 is characterized by Alnus and Betula pollen while MIS 27 is characterized by an increase in larch, dwarf birch and alder pollen (Zhao et al., 2018). Our MBTʹ5ME record differs from the pollen record in that MIS 29 appears warmer compared to MIS 27 whereas in the pollen record, MIS 27 is interpreted as 500 the warmer and wetter of these interglacials (Zhao et al., 2018). The Lake El'gygytgyn pollen record displays a sharp increase in cold steppe pollen at ~975 ka indicating the onset of glacial MIS 26 (Zhao et al., 2018). This dramatic change appears to be reflected in the ACL record with the largest shift in our record initiating at ~963 ka, when ACL values increase significantly (Fig. 2).

MIS 25-22 (959-866 ka) 505
In the Lake El'gygytgyn brGDGT record, MIS 25 is comparatively cooler than other interglacial periods (Fig. 3). Summer insolation during MIS 25 reaches its highest value of the study interval yet reconstructed temperatures are similar to the preceding and following glacial stages when insolation and obliquity were lower. MBTʹ5ME values average ~0.25, or 6°C on the BayMBT calibration. Following a brief increase in n-alkane ACL values at the termination of MIS 26, ACL again decline during the MIS 25 obliquity maximum circa 953 ka, coinciding with the expansion of trees and shrubs in the area as 510 noted in the pollen record (Zhao et al., 2018). Whereas Zhao et al. (2018) infer warm conditions during MIS 25, the cool brGDGT-inferred temperatures suggest that the vegetation change was rather driven by a combination of longer growing seasons and increase in the moisture balance. During MIS 25, the El'gygytgyn Si/Ti ratio is slightly elevated (Fig. 3), suggesting an increase in lake productivity. In the absence of increased summer temperatures, the change in productivity was likely caused by a combination of longer growing season, increased runoff, or increased windiness promoting mixing of the 515 water column. Regionally, the weak MIS 25 warming agrees well with alkenone-derived SSTs from ODP 882 in the North Pacific (Martínez-Garcia et al., 2010), which also exhibits a relatively cool MIS 25. At Site U1343 in the nearby Bering Sea, palaeoceanographic changes are complex. A low abundance of sea ice biomarkers points to ice-free conditions during MIS 25 ( Fig. 6g; Detlef et al., 2018), while high opal accumulation rates (Kim et al., 2014), the presence of ice-marginal diatom species (Worne et al., 2021), and high nutrient upwelling index (Worne et al., 2020) indicate a peak in marginal sea ice 520 conditions, increased wind strength, and a longer sea ice melt season. If these changes in wind strength and seasonality extended over Arctic Asia, it would help explain the limnological and vegetation changes observed at El'gygytygyn.
Numerous MPT studies note a global cooling trend with increasing sea ice extent culminating in an anonymously cool or "skipped" MIS 23 interglacial and a strong glacial period during MIS 22 (Head and Gibbard, 2015 and references therein), in 525 effect giving rise to the first long glacial cycle of the late-Pleistocene. The lack of warming at El'gygytgyn during MIS 25 obscures the nature of the first long glacial cycle from MIS 24-22 that is apparent in the δ 18 O history (Lisiecki and Raymo, 2005;Clark et al., 2006). From MIS 24 to 22, there is a strengthening of sub-orbital temperature variations, with two shortlived warming excursions bracketing a brief cold period at 905 ka despite peaks in both obliquity and northern hemisphere summer insolation (Fig. 3). The evolutionary power spectrum shows that the strength of the 41-kyr climate variability 530 weakens from MIS 25-22 whereas the sub-Milankovitch frequencies become more prominent (Fig. 4), suggesting a breakdown of the climate dynamics that dominated previously. The lower resolution ACL data are in closer agreement with the benthic isotope stack, exhibiting an increasing trend from MIS 25 to 22 and an abrupt decrease at the beginning of MIS 21 (Fig. 3).
Overprinting the increase in ACL values are a series of glacial-interglacial oscillations. For reasons that are not clear, pollen is absent in Lake El'gygytgyn sediments during MIS 23 (Zhao et al., 2018) yet n-alkanes are present. During the peak of MIS 23, ACL values are high, and are higher than those of the previous glacials, suggesting conditions that were either arid or more glacial-like (Fig. 2). Temperatures during MIS 22 were particularly cold and arid, characterized by some of the lowest MBTʹ5ME values and ACL values of the record (Fig. 3). These cool conditions were associated with particularly low 540 productivity, seen in the Si/Ti ratio (Fig. 3). There was a two-phase termination of MIS 22, with an abrupt warming at 880 ka followed by a brief return to cold conditions just prior to the final warming event. The two-phase deglaciation is also apparent in the doublet of Facies A during MIS 22 (Fig. 3). The deglacial warming is approximately synchronous with the increasing obliquity, leading the deglacial decrease in δ 18 O (Fig. 3).

MIS 21-20 (866-790 ka) 545
While MIS 29, 27, and 21 all show MBTʹ5ME values as high as the superinterglacial MIS 31, the most pronounced warm period of the brGDGT record is MIS 21 (Fig. 3). Warming in the MBTʹ5ME starts early in MIS 22 and continues to ~868 ka when peak interglacial conditions are noted. Temperatures increased by at least 4 °C and possibly up to 10 °C at the start of MIS 21 as seen in the BayMBT calibration. The comparatively rapid warming recorded at Lake El'gygytgyn from MIS 22 to MIS 21, agrees with Pacific records ( Fig. 6; Martínez-Garcia et al., 2010;McClymont et al., 2013;Kender et al., 2018;550 Lattaud et al., 2019). From 865-845 ka shrubby vegetation communities, including stone pine, birch and alder, returned to the region, while n-alkane ACL decreased, thereby suggesting warm and wet conditions (Zhao et al., 2018). The reemergence of tree and shrub communities during MIS 21 is also reflected in the loess sediments of northeast China (not shown; Zhou et al., 2018) suggesting a widespread shift to milder conditions. Despite the notable warmth and a slight increase in aquatic productivity seen in the Si/Ti ratios, MIS 21 is not identified as a superinterglacial interval based on the 555 Si/Ti or lithofacies records (Melles et al., 2012).
Interglacial conditions remained relatively warm for approximately 40 kyr during MIS 21-20. The return to glacial conditions at El'gygytgyn differs notably from the global benthic isotope composite (Lisiecki and Raymo, 2005); whereas the δ 18 O data indicate a gradual cooling or gradual development of ice sheets culminating in full glacial conditions around 560 814 ka, the brGDGT data indicate an earlier and more rapid cooling to glacial conditions around 835 ka, which was then followed by relatively stable or even warming climate moving into MIS 20 (Fig. 3). This early cooling is also seen in the pollen record, which shows the appearance of open steppe vegetation from 845-810 ka (Zhao et al., 2018). The apparently cool, open-landscape environment in the latter half of MIS 21 occurred despite increasing obliquity, reminiscent of the scenario across MIS 25. 565

Comparison with North Pacific Marine Records
At present, air temperature anomalies at Lake El'gygytgyn are largely governed by variations in the air masses originating over the sub-Arctic North Pacific, the Bering Sea, and the proximal Arctic Ocean (Nolan and Brigham-Grette, 2007). Based on this, Melles et al. (2012) hypothesized that temperatures during past interglacials were responsive to changes taking place in the North Pacific Ocean. Specifically, the extremely warm and wet superinterglacials at El'gygytgyn were linked to 570 greater stratification and hence higher SSTs in the North Pacific. The increased oceanic stratification, in turn, was hypothesized to be controlled by southern hemisphere processes, namely reduced Antarctic Bottom Water flow into the Pacific basin. Since that study, several new marine SST and upwelling records spanning the MPT have been developed from the North Pacific allowing for a more direct assessment of this proposed mechanism.
The MPT temperature history at El'gygytgyn generally resembles marine conditions at glacial-interglacial timescales (Fig.   6). Biomarker records of sea ice expansion at Site U1343 (Fig. 6f) in the Bering Sea (Detlef et al., 2018) correspond to cool glacial stages in the brGDGT record. Likewise, the reconstructed warmth and rapid onset of MIS 21 are also seen in TEX86 and alkenone-based temperature records from the Sea of Okhotsk (Lattaud et al., 2019) and ODP Site 882 (Martínez-Garcia et al., 2010), while cool conditions during MIS 25 are seen at both El'gygytgyn and at ODP Site 882 (Fig. 6b, d). However, 580 the relationship with North Pacific upwelling appears more complex. The strength of upwelling in the Bering Sea during the MPT was reconstructed using biogenic silica accumulation rates and δ 15 N offsets between Site U1343 in the Bering Sea and Site 1012 in the tropical North Pacific (Stroynowski et al., 2017;Worne et al., 2020). During MIS 25, the cool temperatures recorded at El'gygytgyn ( Fig. 6b) and Site 882 (Fig. 6d) are consistent with a temporary increase in the upwelling index at Site U1343 (Fig. 6e). In contrast, during MIS 31, strengthened upwelling in the Bering Sea corresponds with warm SST at 585 Site 882 and superinterglacial conditions at El'gygytgyn. The rapid transition into the notably warm MIS 21 coincides with warming seen in the nearby marine records (Martínez-Garcia et al., 2010;Lattaud et al., 2019), and is likewise associated with an increase in Bering Sea upwelling ( Fig. 6e; Worne et al., 2020).
A secular decline in upwelling at Site U1343 from MIS 30 to 22 corresponds with declining SSTs and expanding sea ice 590 (Fig. 6e, f). These finding challenge the previous hypothesis of Melles et al. (2012) who suggested that reduced upwelling should cause an increase in temperature in the North Pacific and at El'gygytgyn. To explain this, Worne et al. (2020) suggest that North Pacific cooling and sea ice expansion contributed to a reduction in upwelling via enhanced brine rejection on the Bering shelf and associated expansion of North Pacific Intermediate Waters. Worne et al. (2020) further hypothesize that the reduced upwelling suppresses CO2 transfer from the deep Pacific to the atmosphere, providing an essential feedback to the 595 global climate during MPT. This process was possibly enhanced by sea level decline (Berends et al., 2021;Elderfield et al., 2012) and the initial closure of the Bering Strait during MIS 23 (Kender et al., 2018). This mechanism is supported by North Pacific upwelling and surface water pH changes during the last glacial termination (Gray et al., 2018;Basak et al., 2018).
The Lake El'gygytgyn temperatures exhibit no strong trend associated with the decline in North Pacific upwelling. Rather, temperatures fluctuated, with minima occurring during both MIS 23, the first "skipped interglacial", and again during MIS 600 22 (Fig. 3 and 6). The high-frequency variability during MIS 23-21 approximates the half-precession timescales, potentially indicating a strengthening of tropical influences on the high latitudes during this critical transition period of the MPT.
While reduced upwelling did not drive a persistent increase or decrease in temperatures at Lake El'gygytgyn across MIS 30-22, increased n-alkane ACL values, especially from MIS 26-22, suggest the continental climate was responsive to changes in 605 the North Pacific (Fig. 6g). Vegetation assemblages at Lake El'gygytgyn suggests a progression of arid, open steppe communities (Zhao et al., 2018). Increased sea ice coverage leading up to and during the 900 ka event at MIS 23 (Fig. 6f) could have suppressed moisture transport to Lake El'gygytgyn. Based on estimates of global sea levels during the MPT, Kender et al. (2018) suggest that the Bering Strait was open during both interglacial and glacial periods prior to MIS 24 then open only during interglacials afterwards. It remains unclear whether changes in sea ice volume and North Pacific circulation 610 around MIS 26 prior to the Bering Strait closure were sufficient to suppress moisture transport at Lake El'gygytgyn or if the Bering Strait might have closed during that glacial. High-resolution sea ice reconstructions from the Arctic Ocean are needed to better evaluate moisture source changes across the MPT.

Conclusions
The Lake El'gygytgyn brGDGT reconstruction presented here is the only time-continuous record of Arctic continental 615 temperatures spanning the MPT, thereby providing novel insights into the role and the response of high-latitude climate through the mid-Pleistocene. Our new brGDGT record of relative temperature variability captures glacial-interglacial climate fluctuations at Lake El'gygytgyn although the ability of brGDGTs to reconstruct absolute temperature is less clear and a pan-Arctic or site-specific calibration may be needed. Spectral analysis of the brGDGT record, in comparison with marine records, suggests that obliquity, precession, and possibly half-precession are persistent characteristics of high-latitude 620 climate across the MPT. While some locations indicate an overall cooling trend throughout the MPT, this is not observed at Lake El'gygytgyn. We find that MIS 31, which is widely recognized as a particularly warm interglacial, does not exhibit exceptional warmth in the brGDGT record but instead find that MIS 21 was an especially warm interglacial both at Lake El'gygytgyn and throughout much of the North Pacific region. Throughout the MPT, Lake El'gygytgyn pollen and n-alkane data exhibit a long-term cooling and drying trend, with a shift to an increasingly open landscape noted after around 900 ka 625 (Zhao et al., 2018). The lack of overall MPT cooling in our terrestrial record contradicts upwelling-driven climate trends observed in North Pacific marine proxies. Having a better understanding of history of the Bering Strait's closure will be critical for resolving the differences between terrestrial and marine climate records in the region. Our new data from Lake El'gygytgyn provide a number of constraints for understanding the evolution of Arctic temperature change across the mid-Pleistocene. 630

Data Availability
The biomarker (n-alkane and GDGT) data used in this study is archived at the NOAA National Centers for Environmental Information: https://doi.org/10.25921/z73y-mx49

Acknowledgements
This work was supported by the National Science Foundation (award number 1204087) and a University of Massachusetts 635 Amherst Commonwealth Honors College undergraduate research grant awarded to Kurt Lindberg. We thank Martin Melles, Volker Wennrich, and numerous other international collaborators of the Lake El'gygytgyn Drilling Project. We also thank Anders Noren, Kristina Brady and the staff at LacCore for their assistance and support with numerous large sample requests, as well as John Sweeney and Jeff Salacup at University of Massachusetts for technical support.

Supplement 640
The supplement to this manuscript contains figures S1 through S9 and supporting text.