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The Last Interglacial (129–116 thousand years ago (ka)) represents one of the warmest climate intervals of the past 800,000 years and the most recent time when sea level was metres higher than today. However, the timing and magnitude of the peak warmth varies between reconstructions, and the relative importance of individual sources that contribute to the elevated sea level (mass gain versus seawater expansion) during the Last Interglacial remains uncertain. Here we present the first mean ocean temperature record for this interval from noble gas measurements in ice cores and constrain the thermal expansion contribution to sea level. Mean ocean temperature reached its maximum value of 1.1 ± 0.3 °C warmer-than-modern values at the end of the penultimate deglaciation at 129 ka, which resulted in 0.7 ± 0.3 m of thermosteric sea-level rise relative to present level. However, this maximum in ocean heat content was a transient feature; mean ocean temperature decreased in the first several thousand years of the interglacial and achieved a stable, comparable-to-modern value by ~127 ka. The synchroneity of the peak in mean ocean temperature with proxy records of abrupt transitions in the oceanic and atmospheric circulation suggests that the mean ocean temperature maximum is related to the accumulation of heat in the ocean interior during the preceding period of reduced overturning circulation.

The energy imbalance at the top of the atmosphere determines the temporal evolution of the global climate, and vice versa changes in the climate system can alter the planetary energy fluxes. This interplay is fundamental to our understanding of Earth's heat budget and the climate system. However, even today, the direct measurement of global radiative fluxes is difficult, such that most assessments are based on changes in the total energy content of the climate system. We apply the same approach to estimate the long-term evolution of Earth's radiative imbalance in the past. New measurements of noble gas-derived mean ocean temperature from the European Project for Ice Coring in Antarctica Dome C ice core covering the last 40,000 y, combined with recent results from the West Antarctic Ice Sheet Divide ice core and the sea-level record, allow us to quantitatively reconstruct the history of the climate system energy budget. The temporal derivative of this quantity must be equal to the planetary radiative imbalance. During the deglaciation, a positive imbalance of typically +0.2 W⋅m -2 is maintained for ∼10,000 y, however, with two distinct peaks that reach up to 0.4 W⋅m -2 during times of substantially reduced Atlantic Meridional Overturning Circulation. We conclude that these peaks are related to net changes in ocean heat uptake, likely due to rapid changes in North Atlantic deep-water formation and their impact on the global radiative balance, while changes in cloud coverage, albeit uncertain, may also factor into the picture., Competing Interests: The authors declare no conflict of interest.

Exploitation of the extensive polymetallic deposits of the Andean Altiplano in South America since precolonial times has caused substantial emissions of neurotoxic lead (Pb) into the atmosphere; however, its historical significance compared to recent Pb pollution from leaded gasoline is not yet resolved. We present a comprehensive Pb emission history for the last two millennia for South America, based on a continuous, high-resolution, ice core record from Illimani glacier. Illimani is the highest mountain of the eastern Bolivian Andes and is located at the northeastern margin of the Andean Altiplano. The ice core Pb deposition history revealed enhanced Pb enrichment factors (EFs) due to metallurgical processing for silver production during periods of the Tiwanaku/Wari culture (AD 450-950), the Inca empires (AD 1450-1532), colonial times (AD 1532-1900), and tin production at the beginning of the 20th century. After the 1960s, Pb EFs increased by a factor of 3 compared to the emission level from metal production, which we attribute to gasoline-related Pb emissions. Our results show that anthropogenic Pb pollution levels from road traffic in South America exceed those of any historical metallurgy in the last two millennia, even in regions with exceptional high local metallurgical activity.

Trace element records from glacier and ice sheet archives provide insights into biogeochemical cycles, atmospheric circulation changes, and anthropogenic pollution history. We present the first continuous high-resolution thallium (Tl) record, derived from an accurately dated ice core from tropical South America, and discuss Tl as a tracer for volcanic eruptions. We identify four prominent Tl peaks and propose that they represent signals from the massive explosive eruptions of the "unknown 1258" A.D. volcano, of Kuwae ( approximately 1450 A.D.), Tambora (1815 A.D.), and Krakatoa (1883 A.D.). The highly resolved record was obtained with an improved setup for the continuous analysis of trace elements in ice with inductively coupled plasma sector field mass spectrometry (ICP-SFMS). The new setup allowed for a stronger initial acidification of the meltwater and shorter tubing length, thereby reducing the risk of memory effects and losses of analytes to the capillary walls. With a comparison of the continuous method to the established conventional decontamination and analysis procedure for discrete samples, we demonstrate the accuracy of the continuous method for Tl analyses.

Natural attenuation processes of chlorinated solvents in soils and groundwaters are increasingly considered as options to manage contaminated sites. Under anoxic conditions, reactions with ferrous iron sorbed at iron(hyro)xides may dominate the overall transformation of carbon tetrachloride (CCl4) and other chlorinated aliphatic hydrocarbons. We investigated mechanisms and product formation of CCl4 reduction by Fe(II) sorbed to goethite, which may lead to completely dehalogenated products or to chloroform (CHCl3), a toxic product which is fairly persistent under anoxic conditions. A simultaneous transfer of two electrons and cleavage of two C-Cl bonds of CCl4 would completely circumvent chloroform production. To distinguish between initial one- or two-bond cleavage, 13C-isotope fractionation of CCl4 was studied for reactions with Fe(II)/ goethite (isotopic enrichment factor epsilon = -26.5% percent per thousand) and with model systems for one C-Cl bond cleavage and either single-electron transfer (Fe(II) porphyrin, epsilon = -26.1 percent per thousand) or partial two-electron transfer (polysulfide, epsilon = -22.2 percent per thousand). These epsilon values differ significantlyfrom calculations for simultaneous cleavage of two C-Cl bonds (epsilon approximately equal to -50 percent per thousand), indicating that only one C-Cl bond is broken in the critical first step of the reaction. At pH 7, reduction of CCl4 by Fe(II)/ goethite produced approximately 33% CHCl3, 20% carbon monoxide (CO), and up to 40% formate (HCOO-). Addition of 2-propanol-d8 resulted in 33% CDCl3 and only 4% CO, indicating that both products were generated from trichloromethyl radicals (*CCl3), chloroform by reaction with hydrogen radical donors and CO by an alternative pathway likely to involve surface-bound intermediates. Hydrolysis of CO to HCOO-was surface-catalyzed by goethite butwastoo slow to account for the measured formate concentrations. Chloroform yields slightly increased with pH at constant Fe(II) sorption density, suggesting that pH-dependent surface processes direct product branching ratios. Surface-stabilized intermediates may thus facilitate abiotic mineralization of CCl4, whereas the presence of H radical donors, such as natural organic matter, enhances formation of toxic CHCl3.