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The International Ocean Discovery Program (IODP) Expedition 396 to the mid-Norwegian margin recovered > 1300 m of pristinely preserved, volcanic-ash-rich sediments deposited during the late Paleocene and early Eocene from close to the centre of the North Atlantic Igneous Province (NAIP). Remarkably, many of these cores contain glendonites, pseudomorphs after the purported cold-water mineral ikaite, from sediments dated to the late Paleocene and early Eocene. These time intervals span some of the hottest climates of the Cenozoic, including the Paleocene–Eocene Thermal Maximum (PETM). Global deep-ocean temperatures are not thought to have dropped below 10 ∘C at any point during this time, making the occurrence of supposedly cold-water (near-freezing temperature) glendonite pseudomorphs seemingly paradoxical. This study presents a detailed sedimentological, geochemical, and microscopic study of the IODP Exp. 396 glendonites and presents an updated model for the ikaite-to-calcite transformation for these glendonites. Specifically, we show that early diagenesis of basaltic ashes of the NAIP appear to have chemically promoted ikaite growth in the sediments in this region. Together with existing knowledge of late Paleocene and early Eocene glendonites from Svalbard to the north and early Eocene glendonites from Denmark to the south, these new glendonite finds possibly imply episodic, short-duration, and likely localized cooling in the Nordic Seas region, which may have been directly or indirectly linked to the emplacement of the NAIP.

The carbon isotopic signature inherited from soil and epikarst processes may be modified by degassing and prior calcite precipitation (PCP) before its imprint on speleothem calcite. Despite laboratory demonstration of PCP effects on carbon isotopes and increasingly sophisticated models of the governing processes, to date, there has been limited effort to deconvolve the dual PCP and soil-epikarst components in measured speleothem isotopic time series. In this contribution, we explore the feasibility, advantages, and disadvantages of using trace element ratios and δ⁴⁴Ca to remove the overprinting effect of PCP on measured δ¹³C to infer the temporal variations in the initial δ¹³C of drip water prior to degassing and PCP. In nine examined stalagmites, the most widely utilized PCP indicators Mg/Ca and δ⁴⁴Ca covary as expected. However, Srg/gCa does not show consistent relationships with δ⁴⁴Ca so PCP is not the dominant control on Srg/gCa. From δ⁴⁴Ca and Mg/Ca, our calculation of PCP as f_Ca, the fraction of initial Ca remaining in solution at the time the stalagmite layer is deposited, yields multiple viable solutions depending on the assumed δ⁴⁴Ca fractionation factor and inferred variation in DMg. Uncertainty in the effective fractionation of δ¹³C during degassing and precipitation contributes to uncertainty in the absolute value of estimated initial δ¹³C. Nonetheless, the trends in initial δ¹³C are less sensitive to these uncertainties. In coeval stalagmites from the same cave spanning the 94 to 82 ka interval, trends in calculated initial δ¹³C are more similar than those in measured δ¹³C and reveal a common positive-Anomaly initial δ¹³C during a stadial cooling event. During deglaciations, calculated initial δ¹³C implies a trend of greater respiration rates and higher soil CO₂, although the higher interglacial drip water saturation favors more extensive PCP. Initial δ¹³C can be estimated for active and fossil speleothems from a range of settings, wherever there is confidence that Mg/Ca and/or δ⁴⁴Ca provides a quantitative indication of past changes in PCP. Further study of Mg partitioning in speleothems will improve the robustness of Mg/Ca as a PCP proxy. Climate of the Past, 19 (12) ISSN:1814-9332 ISSN:1814-9324

10Be is produced by the interaction between galactic cosmic rays (GCRs) and solar energetic particles (SEPs) with the Earth's atmospheric constituents. The flux of GCRs is modulated by the varying strength of the magnetic fields of the Earth and the Sun. Measurement of 10Be concentrations from polar ice cores is thus a valuable tool to reconstruct the variations in the geomagnetic field and solar activity levels. The interpretation of 10Be records is, however, complicated by non-production-related effects on the 10Be deposition rate caused by climate-or weather-induced variability. Furthermore, volcanic eruptions have been proposed to lead to short-Term 10Be deposition enhancements. In this study, we test the use of excess meltwater from continuous flow analysis (CFA) to measure 10Be, allowing less time-consuming and more cost-effective sample preparation. We compare two records obtained from CFA and discrete samples from the East Greenland Ice core Project (EGRIP) S6 firn core, reaching back to 1900gCE. We find that the two records agree well and that the 10Be record from CFA samples agrees as well as the discrete samples with other records from Greenland. Furthermore, by subtracting the theoretically expected GCR-induced signal, we investigate the high-frequency variability in the 10Be records from Greenland and Antarctica after 1951gCE, focusing on SEP events and volcanic eruptions. Finally, we use the 10Be records from Greenland and Antarctica to study the 11-year solar cycles, allowing us to assess the suitability of the CFA samples for the reconstruction of solar activity. This result opens new opportunities for the collection of continuous 10Be records with less time-consuming sample preparation, while saving an important portion of the ice cores for other measurements. Climate of the Past, 19 (11) ISSN:1814-9332 ISSN:1814-9324

The Last Glacial Maximum (LGM, from ∼26 to 20 ka BP) was the most recent period with large ice sheets in Eurasia and North America. At that time, global temperatures were 5–7 ∘C lower than today, and sea level ∼125 m lower. LGM simulations are useful to understand earth system dynamics, including climate–ice sheet interactions, and to evaluate and improve the models representing those dynamics. Here, we present two simulations of the Northern Hemisphere ice sheet climate and surface mass balance (SMB) with the Community Earth System Model v2.1 (CESM2.1) using the Community Atmosphere Model v5 (CAM5) with prescribed ice sheets for two time periods that bracket the LGM period: 26 and 21 ka BP. CESM2.1 includes an explicit simulation of snow/firn compaction, albedo, refreezing, and direct coupling of the ice sheet surface energy fluxes with the atmosphere. The simulated mean snow accumulation is lowest for the Greenland and Barents–Kara Sea ice sheets (GrIS, BKIS) and highest for British and Irish (BIIS) and Icelandic (IcIS) ice sheets. Melt rates are negligible for the dry BKIS and GrIS, and relatively large for the BIIS, North American ice sheet complex (NAISC; i.e. Laurentide, Cordilleran, and Innuitian), Scandinavian ice sheet (SIS), and IcIS, and are reduced by almost a third in the colder (lower temperature) 26 ka BP climate compared with 21 ka BP. The SMB is positive for the GrIS, BKIS, SIS, and IcIS during the LGM (26 and 21 ka BP) and negative for the NAISC and BIIS. Relatively wide ablation areas are simulated along the southern (terrestrial), Pacific and Atlantic margins of the NAISC, across the majority of the BIIS, and along the terrestrial southern margin of the SIS. The integrated SMB substantially increases for the NAISC and BIIS in the 26 ka BP climate, but it does not reverse the negative sign. Summer incoming surface solar radiation is largest over the high interior of the NAISC and GrIS, and minimum over the BIIS and southern margin of NAISC. Summer net radiation is maximum over the ablation areas and minimum where the albedo is highest, namely in the interior of the GrIS, northern NAISC, and all of the BKIS. Summer sensible and latent heat fluxes are highest over the ablation areas, positively contributing to melt energy. Refreezing is largest along the equilibrium line altitude for all ice sheets and prevents 40 %–50 % of meltwater entering the ocean. The large simulated melt for the NAISC suggests potential biases in the climate simulation, ice sheet reconstruction, and/or highly non-equilibrated climate and ice sheet at the LGM time.

Repeated carbon isotope excursions and widespread organic-rich shale deposition mark the Middle and Upper Devonian series. Various explanations such as extensive volcanism and land plant evolution have been given for these perturbations and the general sensitivity of the Devonian oceans to the development of anoxia, but their repeated nature suggests that astronomical forcing may have controlled their timing. Here, a cyclostratigraphic study of the Kellwasser Crisis at the Frasnian–Famennian stage boundary (ca. 372 Ma) is carried out. The Kellwasser Crisis was one of the most ecologically impactful of the Devonian perturbations and is ranked among the “Big Five” Phanerozoic mass extinctions. The studied site is the Winsenberg roadcut section in the Rhenish Massif, Germany, which represents a quiet tropical shelf basin setting. Centimetre-scale elemental records, generated by portable X-ray scanning, allow for testing of the hypothesis that a 2.4 Myr eccentricity node preceded the Upper Kellwasser event. The study's results are supportive of this hypothesis. We find enhanced chemical weathering (K2O / Al2O3) during the period leading up to the Upper Kellwasser and a peak in distal detrital input (SiO2 / CaO) and riverine runoff (TiO2 / Al2O3) just prior to the start of the Upper Kellwasser event. We interpret this pattern as the long-term eccentricity minimum facilitating excessive regolith build-up in the absence of strong seasonal contrasts. The Earth's system coming out of this node would have rapidly intensified the hydrological cycle, causing these nutrient-rich regoliths to be eroded and washed away to the oceans, where they resulted in eutrophication and anoxia. An astronomical control on regional climate is observed beyond this single crisis. Wet–dry cycles were paced by 405 kyr eccentricity, with both the Lower and Upper Kellwasser events taking place during comparatively drier times. A precession-sensitive monsoonal climate system prevailed on shorter timescales. Intensification of this monsoonal system following the node may have caused the widespread regolith erosion. We estimate the total duration of the Kellwasser Crisis at ca. 900 kyr, with the individual events lasting for ca. 250 and 100 kyr, respectively. If astronomical control indeed operated via regolith development in monsoonal climates, then land plants may have played an important role. This would not have been through evolutionary steps directly triggering Kellwasser perturbations but by gradually strengthening the climatic response to orbital forcing via soil development – creating soils thick enough to meaningfully respond to orbital forcing – and intensifying the hydrological cycle.

Abstract. Lake Petén Itzá (Guatemala) possesses one of the longest lacustrine sediment records in the northern Neotropics, which enabled study of paleoclimate variability in the region during the last ∼400 000 years. We used geochemical (Ti, Ca/(Ti+Fe) and Mn/Fe) and mineralogical (carbonates, gypsum, quartz, clay) data from sediment core PI-2 to infer past changes in runoff, lake evaporation, organic matter sources and redox conditions in the water column, caused by hydrological changes in the northern Neotropics during Marine Isotope Stages (MISs) 3–2. From 59 to 39 cal ka BP climate conditions were relatively wet, and the lake was marked by higher primary productivity and anoxic bottom waters. This wet environment was interrupted for two periods of possible low water level at 52 and 46 cal ka BP, when our data suggest higher evaporation, high terrestrial organic matter input and persistent oxic conditions. Between 39 and 23 cal ka BP, evaporation and input of terrestrial organic matter increased considerably, lake level declined, and lake bottom waters generally became oxic. These conditions reversed during the Last Glacial Maximum (23.5–18.0 cal ka BP), when runoff and lake productivity increased, and rising lake level caused bottom waters to again become anoxic. Comparison of our hydrologic proxy data with sea surface temperature anomalies between the eastern Pacific and the Caribbean suggests that changes in the intensity of the Caribbean Low-Level Jet (CLLJ) may have influenced long-term changes in runoff during MISs 3–2. Higher intensity of the CLLJ during the onset of MIS 3 and the LGM might have led to greater runoff into the lake, whereas the MIS 3–2 transition experienced a weaker CLLJ and consequently less runoff. A refined, high-resolution age–depth model for the PI-2 sediment core enabled us to identify millennial-scale Greenland interstadials (GIs) 14–2, Greenland stadials (GSs) 14–2 and Heinrich stadials (HSs) 5–1. In general, HSs and GSs were characterized by drier conditions. In contrast to GSs and HSs, GIs were characterized by greater runoff and overall wetter conditions, with the most pronounced GI peaks between 40 and 30 cal ka BP. Whereas GSs 9, 8, 7 and 6 began with abrupt increases in evaporation and ended with gradual increases in humidity, GSs 11 and 10 showed reversed patterns. The Lake Petén Itzá paleohydrology record, along with other regional paleoclimate records, led us to conclude that shifts in the position of the Intertropical Convergence Zone (ITCZ) altered moisture delivery to the lake on millennial timescales. During GSs and HSs, high evaporation from Petén Itzá (dry climate conditions) was associated with a more southerly position of the ITCZ, whereas wetter GIs prevailed during a more northerly ITCZ position. Although abrupt millennial-scale shifts in ITCZ and hydroclimate between GSs/HSs and GIs can be linked to instabilities in the Atlantic Meridional Overturning Circulation (AMOC), longer-term changes were additionally influenced by changes in atmospheric convection linked to modulations of the CLLJ in response to ΔSST between the equatorial Pacific and tropical Atlantic.