Search

Across a 1000‐km stretch of the River Nile, from the 1st Cataract in southern Egypt to the 4th Cataract in Sudan, many hundreds of drystone walls are located within active channels, on seasonally inundated floodplains or in now‐dry Holocene palaeochannel belts. These walls (or river groynes) functioned as flood and flow control structures and are of a type now commonly in use worldwide. In the Nile Valley, the structures have been subject only to localised investigations, and none have been radiometrically dated. Some were built within living memory to trap nutrient‐rich Nile silts for agriculture, a practice already recorded in the early 19th century C.E. However, others situated within ancient palaeochannel belts indicate construction over much longer time frames. In this paper, we map the distribution of these river groynes using remote sensing and drone survey. We then establish their probable functions and a provisional chronology using ethnoarchaeological investigation and the ground survey, excavation and radiometric dating of the structures in northern Sudan, focusing on the Holocene riverine landscape surrounding the pharaonic settlement of Amara West (c. 1300–1000 B.C.E.). Finally, we consider the historical and economic implications of this form of hydraulic engineering in the Nile Valley over the past three millennia.

Magnetic susceptibility (SUS) of the Quaternary long-term mid-latitude Maros fluvial fan (Pannonian Basin) was recorded to understand the stratigraphic features of source proximal fluvial depositional settings. Three fully cored 500 m deep boreholes were sampled with 0.5 m intervals; low field and frequency dependent magnetic susceptibility were measured, and complementary hysteresis and SEM-EDAX investigations were performed on selected samples. Logged SUS data were also used to log correlations established by a comparison of wireline log and laboratory measurements. The time-series analyses of the SUS records reveal the apparent occurrence of the ~41 ka frequency together with the customary ~100 ka cycles. Towards the source-distal sections the intensity of the ~41 ka cycles decreases, while that of the ~100 ka cycles remains strong. Stratigraphic and spectral similarities were observed between fluvial fan and loess SUS records ; however, based on complementary magnetic data, the magnetic phase of the Maros Fan sections is related to the detrital magnetite that originates from the catchment during early postglacial permafrost degradations. The amplification of the ~41 ka cycles revealed can be attributed to the very high SUS values in source proximal settings and to the special stratigraphic feature of the distributive fluvial settings. This comprises the increased avulsion frequency on the fluvial fans in ‘glacial recession periods’, in concert with the ‘early postglacial’ occurrence of the permafrost-related magnetite originated from the catchment. As a local phenomenon, this is significant since it records the obliquity-driven variations of permafrost development in a catchment. However, fluvial and alluvial fans are widespread depositional landforms within the Eurasian Mountains and were possibly the same during the Quaternary deglaciations. Thus, obliquity-driven SUS variations of source-proximal fan deposits attached or adjacent to regions of loess deposition should also be considered when scanning for potential source material of aeolian deposits.

The classic work on the morphology of limestone calcite cements done in the 1960s is extended here by utilising growth zones to reconstruct the growth of cement crystals. Only cement composed of fitted polyhedral monocrystals that form by passive crystallisation of calcite on the walls of liquid‐filled, static pores and fissures is considered. Cement can either be initiated by (1) nucleation, when new crystals start but are not attached to their substrate, or (2) seeding, when new crystals are seamlessly connected to and influenced by substrate crystals. After seeding, epitaxial cement growth starts with many sub‐crystals that coalesce distally, followed by layered mantle growth. Junctions between three intercrystalline boundaries in cement aggregates with one interfacial angel = 180° are of two types: the first, enfacial junctions are caused by a pause in the growth of one crystal and the second is caused by movement of all boundaries due to dissolution of adjacent calcite. Growth zone offsetting at some intercrystalline boundaries is caused by dissolution of calcite at boundaries when permeability values are low. The same width to height ratio of mature aggregate crystals is predicted from the shape of the crystal's growth surfaces; dogtooth calcite forms columnar and nail‐head calcite forms tabular‐shaped crystals. Seeding on different sized crystals causes variations in epitaxial growth rate with faster growth on large crystals resulting in a disorganised cement fabric; the variation in epitaxial growth rate is perpetuated into mantle growth. Echinoderm syntaxial crystals dominate many pore cements due to the large size of their seed ossicles, at the same time, syntaxial crystals form on relatively tiny seeds. Texturally mature crystal aggregates with isopachous fabric are initiated from three different substrate to cement arrangements. Calcite cement zones preserve their original positions allowing the investigation of cement's growth and chemical history.

Over the course of the last decade the concept of the Anthropocene has become widely established within and beyond the geoscientific literature but its boundaries remain undefined. Formal definition of the Anthropocene as a chronostratigraphical series and geochronological epoch following the Holocene, at a fixed horizon and with a precise global start date, has been proposed, but fails to account for the diachronic nature of human impacts on global environmental systems during the late Quaternary. By contrast, defining the Anthropocene as an ongoing geological event more closely reflects the reality of both historical and ongoing human-environment interactions, encapsulating spatial and temporal heterogeneity, as well as diverse social and environmental processes that characterise anthropogenic global changes. Thus, an Anthropocene Event incorporates a substantially wider range of anthropogenic environmental and cultural effects, whilst at the same time applying more readily in different academic contexts than would be the case with a rigidly-defined Anthropocene Series/Epoch.

Sequential analyses of δ13C, δ18O and Δ47 values of calcite and dolomite deposited in millimetre‐sized cavities are reported from the Ronaldsway Member packstones, Isle of Man. The Ronaldsway brachiopods have δ13C values of ca +2.3‰ and δ18O values of ca −7.2‰; carbon is like predicted Carboniferous values, while oxygen values are more negative. The brachiopods show preserved microstructure but have marginal alteration and a streaky cathodoluminescence pattern. Crinoid ossicles have δ13C values of ca +2.3‰ and one with a δ18O value of ca −3.1‰, compatible with Carboniferous marine precipitates; three samples have δ18O values of ca −6.5‰ and are 18O‐depleted. Calcite stages 1 and 2 have δ13C values ca +3.2‰ and δ18O values ca −2.5‰, compatible with Carboniferous sea water. Stage 1 and 2 have non‐luminescent to orange CL zones. Stage 1 and early stage 2 contain red luminescent dolomite micro crystals generated during Mg calcite stabilisation. The Δ47 values for stage 1 and 2 cements indicate temperatures of 86 and 105°C that occurred after the stabilisation of Mg calcite. Stage 3–8 zoned cements preserve their original growth surfaces and their δ13C and δ18O values suggest precipitation during burial and exhumation. The Δ47 values of the brachiopods and crinoids indicate temperatures between 85 and 140°C indicating they were either recrystallised at high temperatures or affected by solid state reordering. To evaluate these alternatives two quantitative models, water–rock reaction and reordering models are performed. The allochems and cements are progressively altered by porewater towards the fluid‐buffered behaviour. The quantitative evaluation of calcite and dolomite solid‐state reordering suggests the elevated clumped isotopic temperatures are produced by interaction with hydrothermal fluids. This study improves understanding by applying previously untried techniques; further Δ47 data and quantifying elemental variations would help further interpretation but the poorly documented post‐depositional history is a drawback.

Sediments deposited by glacial meltwaters (for example, ice‐contact delta deposits) form permeable packages in the subsurface that can act as reservoirs for both water and hydrocarbons. They are also an important source of aggregate for the construction industry. As reservoirs they are challenging to characterize in terms of their structure, flow and storage properties due to their complex depositional history. In this study, ice‐contact deltas of Salpausselkä I and II end moraines in Southern Finland are studied using a combination of geomorphological mapping, sedimentological studies and near surface geophysical methods. Sedimentary logs from isolated outcrops were correlated to ground penetrating radar (GPR) profiles to unravel the internal structure and depositional history of these ice‐contact deltas. Subsequently, electrical resistivity tomography (ERT) and gravity data were analysed to estimate the depth to bedrock and to model porosity distribution within the sediments. Results of the study suggest that the delta deposits have a broad range of porosities (10 to 42%) with lowest values found in the bottomset beds. The most variable porosities are in the subaqueous ice‐contact–fan zone, and consistently high porosities occur in delta foreset/topset facies. Detailed sedimentary logging linked to the GPR data shows heterogeneities such as mud drapes on foresets and kettle holes which are below the resolution of ERT and gravity methods but significantly affect reservoir properties of the deltas. Moreover, oscillation of the ice‐margin may have introduced larger heterogeneities (for example, buried ice marginal ridges, or eskers) into the sedimentary sequence which are atypical for other Gilbert‐type deltas. Finally, subglacially sculpted, highly variable bedrock topography exerts a major control on sediment distribution within the delta making reservoir volume and quality less predictable. This work has implications for present‐day freshwater aquifers and low enthalpy geothermal energy in southern Finland and other deglaciated regions, as well as hydrocarbon exploration of analogous deposits in the subsurface from Pleistocene and pre‐Pleistocene glaciogenic sequences.

The year 2021 marked the bicentenary of the birth of James Croll (1821–1890), the self‐educated son of a crofter‐stonemason, whose life was characterised by a dizzying range of occupations and homes, poor health and financial concerns, and yet he became a pioneer of orbital dynamics and ice age climate change with an impressive record of publication. Drawing upon archival information and recently published observations, this paper explores selected aspects of Croll's biography, his scientific connections and controversies, and that area of his life relevant to Quaternary science. He was a 19th century polymath whose multifaceted contributions have been a catalyst for subsequent systems‐based climate science on the grand scale, including the foundations for the seminal work of Milutin Milankovitch on the rhythms of Quaternary environmental change.

Funder: NZ Federation of Graduate Women, Funder: University of Otago; Id: http://dx.doi.org/10.13039/100008247, The oldest‐known rocks of the eroded Dunedin Volcano (DV), New Zealand, active 16.0–11.0 Ma, were emplaced by penecontemporaneous explosive eruptions and intrusions of magmas with a remarkable range of compositions, including basanite, alkali basalt, trachyte and phonolite. The compositional diversity of magmas emplaced in rapid succession during the earliest development of the volcano signify the existence of a complex magmatic system and highly evolved magmas, typical of those attributed to mature magmatic systems; here it developed in concert with the volcano's surface eruptions. We infer that multiple deep and shallow reservoirs were suddenly tapped as volcanism commenced. DV repeatedly erupted the same range of magmas over 5 Myr, suggests that these distributed reservoirs failed to significantly integrate through millions of years of slow volcano growth.

Palaeoclimatic evidence is necessary to place the current warming and drying trends of the Mediterranean region in a long‐term perspective of pre‐industrial variability. Annually resolved and absolutely dated climate proxies that extend back into medieval times are, however, limited to a few sites only. Here we present a network of long ring width chronologies from Pinus nigra tree‐line sites in northern Corsica (France) that cohere exceptionally well over centuries and support the development of a single high‐elevation pine chronology extending back to 974 CE. We apply various detrending methods to these data to retain high‐to‐low frequency ring width variability and scale the resulting chronologies against instrumental precipitation and drought observations to produce hydroclimate reconstructions for the last millennium. Proxy calibration and transfer are challenged by a lack of high‐elevation meteorological data, however, limiting our understanding of precipitation changes in sub‐alpine tree‐line environments. Our new reconstructions extend beyond existing records and provide evidence for low‐frequency precipitation variability in the central‐western Mediterranean from 974–2016 CE. Comparison with a European scale drought reconstruction network shows that regional predictor chronologies are needed to accurately estimate long‐term hydroclimate variability on Corsica.

Funder: Deutscher Akademischer Austauschdienst, Due to a lack of visible tephras in the Dead Sea record, this unique palaeoenvironmental archive is largely unconnected to the well‐established Mediterranean tephrostratigraphy. Here we present first results of the ongoing search for cryptotephras in the International Continental Drilling Program (ICDP) sediment core from the deep Dead Sea basin. This study focusses on the Lateglacial (~15–11.4 cal. ka BP), when Lake Lisan – the precursor of the Dead Sea – shrank from its glacial highstand to the Holocene low levels. We developed a glass shard separation protocol and counting procedure that is adapted to the extreme salinity and sediment recycling of the Dead Sea. Cryptotephra is abundant in the Dead Sea record (up to ~100 shards cm-3), but often glasses are physically and/or chemically altered. Six glass samples from five tephra horizons reveal a heterogeneous geochemical composition, with mainly rhyolitic and some trachytic glasses potentially sourced from Italian, Aegean and Anatolian volcanoes. Most shards likely originate from the eastern Anatolian volcanic province and can be correlated using major element analyses with tephra deposits from swarm eruptions of the Süphan Volcano ~13 ka BP and with ashes from Nemrut Volcano, presumably the Lake Van V‐16 volcanic layer at ~13.8 ka BP. In addition to glasses that match the TM‐10‐1 from Lago Grande di Monticchio (15 820±790 cal. a BP) tentatively correlated with the St. Angelo Tuff of Ischia, we further identified a cryptotephra with glass analyses which are chemically identical with those of the PhT1 tephra in the Philippon peat record (13.9–10.5 ka BP), and also a compositional match for the glass analyses of the Santorini Cape Riva Tephra (Y‐2 marine tephra, 22 024±642 cal. a BP). These first results demonstrate the great potential of cryptotephrochronology in the Dead Sea record for improving its chronology and connecting the Levantine region to the Mediterranean tephra framework.

Recent research in northern Spain has revealed the disappearance of Neanderthal populations in the Vasco Cantabrian region a few millennia earlier than in eastern and southern Iberia and discovered a short period of overlap with modern humans, at least, in terms of radiocarbon dates. However, the causes of Neanderthal decline understood as a regional and temporal process remain open. Despite the abundance of technological studies, modern?quality chronological dating, and the availability of archaeofaunal and palaeoenvironmental data, there is a lack of consensus about how climatic and environmental conditions could have affected ungulate prey and, therefore, Neanderthal subsistence strategies. In this paper, an analytical summary of the archaeofaunal and taphonomic data available for the Vasco?Cantabrian region, combined with the most recent chronological evidence, present general knowledge about animal biogeography and ecology during the Middle?Upper Palaeolithic transition, and provides an interpretation of the behaviour of both human species in the region. This work reviews the palaeomammal community of animals represented in the record as exploited by human groups in several caves and rock shelters and pointing to continuing lacunae in knowledge. Further research is needed to verify and potentially explain the apparent hominin population gap and the ultimate fate of the Neanderthals This research is funded by the European Research Council under the European Union's Horizon 2020 Research and Innovation Programme (grant agreement No. 818299‐ SUBSILIENCE project (https://www.subsilience.eu) and the Ministerio de Ciencia, Innovación y Universidades (ABRUPT ‐ HAR2017‐84997‐P) to ABMA. ASR's PhD is supported within the SUBSILIENCE project. The authors would like to thank J. García Sanchez (Instituto de Arqueología‐ Merida, CSIC‐Junta de Extramadura) for technical assistance with catchment areas analysis and Figs. 1, 5 and 6. Special thanks to L. Agudo (EvoAdapta‐University of Cantabria) for her technical assistance and G. Terlato for comments on the final version of the manuscript. The authors wish to acknowledge L. Straus for editing the manuscript and for his suggestions to improve this manuscript, as kind and effective as always.

The early stages of community development influence longer-term establishment of species, traits, and ultimately ecosystem function. How this process varies with small- and large-scale abiotic and biotic conditions is poorly studied in microbes. Here we tested how different spatial scales influenced the rate at which taxonomic and functional composition of microbial communities changed over time in lake sediments, and whether these changes occurred synchronously across different environments given the same initial communities. We manipulated the small-scale environment by creating sediments with different terrestrial organic matter (t-OM) quantity and quality, and placing these in two lakes differing in trophic status to vary the large-scale environment. We found that archaeal and bacterial communities, but not fungi, became taxonomically dissimilar over two months despite being derived from the same initial leaf material, primarily because of small-scale environmental conditions. Sediment t-OM quantity consistently explained changes in community composition both temporally within mesocosms and spatially between mesocosms in different lakes. Archaea, bacteria, and fungi also varied by up to 10-times in how quickly they changed, providing among the first evidence in the same study system that they respond differently over time to abiotic and biotic conditions. Finally, functional composition was influenced by both small- and large-scale environmental conditions, with genes involved in t-OM decomposition showing some of the largest changes in abundance after one year. Our study highlights that future changes to both sediments and lake waters can modify how sediment microbial communities develop with consequences for important ecosystem functions like carbon cycling.

It is uncommon in the North Sea basin and northwestern Europe for the ice-marginal glacial successions of the Middle Pleistocene, Anglian (Elsterian) age to be well preserved and not overridden by subsequent glaciations. The existence of extensive and thick (˜20 m) Middle Pleistocene sand and gravel successions in East Anglia, England, provide a unique opportunity to reconstruct and understand the palaeoenvironmental conditions in the Anglian ice-marginal zone, and further across the North Sea basin. This paper uses data from 80 sections in two sand and gravel quarries in East Anglia to provide the first evidence concerning: (i) the character of the ice-marginal processes in the unique, transitional zone between the topographically constrained and unconstrained Anglian ice-sheet margin; (ii) the role of meltwater in the re-shaping of topographically driven preglacial drainage; and (iii) the position and the number of oscillations of the Anglian ice-sheet margin in the form of a sediment-landform assemblage. Moreover the current research adds to the discussion on the presence and extent of the proglacial lake in the North Sea Basin during the Anglian glaciation. The sand and gravel successions in the Anglian ice-marginal zone are primarily reworked proto-Thames sediments deposited by meltwater. At the beginning of the glaciation, the meltwater followed the preglacial (proto-Thames) river course. However, as the ice sheet advanced, it was re-routed, overwhelming and abandoning the old river course and depositing an extensive ice-marginal subaqueous fan. The succession includes evidence for at least two enhanced meltwater release events, as well as indications of glaciolacustrine sedimentation. The character of the described sedimentary settings is discussed in the wider context of the presence of the North Sea Lake.

Marine radiocarbon dates, corrected for ocean-atmosphere reservoir age offsets (R-ages) are widely used to constrain marine chronologies. R-ages also represent the surface boundary condition that links the ocean interior radiocarbon distribution (i.e. ‘radiocarbon ventilation ages’) to the ocean’s large-scale overturning circulation. Understanding how R-ages have varied over time is therefore essential both for accurate dating and for investigations into past ocean circulation/carbon cycle interactions. A number or recent studies have shed light on surface reservoir age changes over the last deglaciation; however a clear picture of global/regional spatiotemporal patterns of variability has yet to emerge. Here we combine new and existing reservoir age estimates to show coherent but distinct regional reservoir age trends in the sub-polar North Atlantic and Southern Ocean. It can be further shown that similar, but lower amplitude changes occurred at mid-latitudes in each hemisphere. An apparent link between regional patterns of reservoir age variability and the ‘thermal bipolar seesaw’ suggests a causal link with changes in ocean circulation, mixed layer depth, and/or sea ice dynamics. A further link to atmospheric CO2 is also apparent, and underlines a potentially dominant role for changes in the ocean’s ‘disequilibrium carbon’ pool, rather than changes in ocean transport. The existence of significant R-age variability over the last deglaciation poses a problem for marine radiocarbon age calibrations. However, its apparent regional consistency also raises the prospect of developing region-specific marine calibration curves for radiocarbon-dating purposes.

An accurate description of the carbonate system in porewaters is valuable in studies involving the degradation of sedimentary organic matter, recrystallization of calcium carbonate minerals, calculations of mineral saturation state and cycling of ions affected by pH. Here we analyze water chemistry of porewater extracted using centrifugation and Rhizon samplers from hemipelagic sediments in the Gulf of Aqaba, Red Sea, and a shallow salt marsh from Norfolk, England. In both study areas, the data is internally consistent for each porewater separation technique, but the measured isotopic composition of the dissolved inorganic carbon (δ13C(DIC)) differs between the two techniques. We performed laboratory experiments that show that both Rhizons and centrifugation are prone to degassing of CO2 enriched with 12C. We suggest that during sampling with Rhizons, air fills the voids left by extracted porewater; combined with the membrane’s design to exclude air, some of the aqueous CO2 diffuses into these air bubbles instead of the sampler. Rhizons produce reliable calcium, strontium, manganese and barium concentration data when soaked in deionized water and then flushed with the sample immediately prior to sampling. However, porewater extractions with Rhizons are less reliable for analyses of pH and δ13C(DIC). Centrifugation produces reliable carbonate chemistry and major element data when tubes are fully filled without headspace and sealed tightly. Working in CO2 low/free atmosphere (e.g., N2 glovebox) enhances the chance of losing CO2 from the sample in both sampling techniques due to increased negative gradient of CO2 between the core and its surrounding.

We perform joint nonlinear inversions of glacial isostatic adjustment (GIA) data, including the following: postglacial decay times in Canada and Scandinavia, the Fennoscandian relaxation spectrum (FRS), late-Holocene differential sea level (DSL) highstands (based on recent compilations of Australian sea level histories), and the rate of change of the degree 2 zonal harmonic of the geopotential, $J_2$. Resolving power analyses demonstrate the following: (1) the FRS constrains mean upper mantle viscosity to be ∼3 × 10$^{20}$ Pa s, (2) postglacial decay time data require the average viscosity in the top ∼1500 km of the mantle to be 10$^{21}$ Pa s, and (3) the $J_2$ datum constrains mean lower mantle viscosity to be ∼5 × 10$^{21}$ Pa s. To reconcile (2) and (3), viscosity must increase to 10$^{22}$-10$^{23}$ Pa s in the deep mantle. Our analysis highlights the importance of accurately correcting the $J_2$ observation for modern glacier melting in order to robustly infer deep mantle viscosity. We also perform a large series of forward calculations to investigate the compatibility of the GIA data sets with a viscosity jump within the lower mantle, as suggested by geodynamic and seismic studies, and conclude that the GIA data may accommodate a sharp jump of 1-2 orders of magnitude in viscosity across a boundary placed in a depth range of 1000-1700 km but does not require such a feature. Finally, we find that no 1-D viscosity profile appears capable of simultaneously reconciling the DSL highstand data and suggest that this discord is likely due to laterally heterogeneous mantle viscosity, an issue we explore in a companion study.

The magmatic plumbing system beneath Askja, a volcano in the central Icelandic highlands, is imaged using local earthquake tomography. We use a catalog of more than 1300 earthquakes widely distributed in location and depth to invert for the $P$ wave velocity ($V_p$) and the $V_p$/$V_s$ ratio. Extensive synthetic tests show that the minimum size of any velocity anomaly recovered by the model is ~4 km in the upper crust (depth < 8 km below sea level (bsl)), increasing to ~10 km in the lower crust at a depth of 20 km bsl. The plumbing system of Askja is revealed as a series of high-$V_p$/$V_s$ ratio bodies situated at discrete depths throughout the crust to depths of over 20 km. We interpret these to be regions of the crust which currently store melt with melt fractions of ~10%. The lower crustal bodies are all seismically active, suggesting that melt is being actively transported in these regions. The main melt storage regions lie beneath Askja volcano, concentrated at depths of 5 km bsl with a smaller region at 9 km bsl. Their total volume is ~100 km$^3$. Using the recorded waveforms, we show that there is also likely to be a small, highly attenuating magmatic body at a shallower depth of about 2 km bsl.

X-ray fluorescence (XRF) core scanning and X-ray computed tomography data were measured every 1 mm to study the structure of Heinrich Event 1 during the last deglaciation at International Ocean Discovery Program Site U1308. Heinrich Layer 1 comprises two distinct layers of ice-rafted detritus (IRD), which are rich in detrital carbonate (DC) and poor in foraminifera. Each DC layer consists of poorly sorted, coarse-grained clasts of IRD embedded in a dense, fine-grained matrix of glacial rock flour that is partially cemented. The radiocarbon ages of foraminifera at the base of the two layers indicate a difference of 1400 $^{14}$C years, suggesting that they are two distinct events, but the calendar ages depend upon assumptions made for surface reservoir ages. The double peak indicates at least two distinct stages of discharge of the ice streams that drained the Laurentide Ice Sheet through Hudson Strait during HE1 or, alternatively, the discharge of two independent ice streams containing detrital carbonate. Heinrich Event 1.1 was the larger of the two events and began at ~16.2 ka (15.5–17.1 ka) when the polar North Atlantic was already cold and Atlantic Meridional Overturning Circulation (AMOC) weakened. The younger peak (H1.2) at ~15.1 ka (14.3 to 15.9 ka) was a weaker event than H1.1 that was accompanied by minor cooling. Our results support a complex history for Heinrich Stadial 1 (HS1) with reduction in AMOC during the early part (~20–16.2 ka) possibly driven by melting of European ice sheets, whereas the Laurentide Ice Sheet assumed a greater role during the latter half (~16.2–14.7 ka).

New crystallization temperatures for four eruptions from the Northern Volcanic Zone of Iceland are determined using olivine-spinel aluminum exchange thermometry. Differences in the olivine crystallization temperatures between these eruptions are consistent with variable extents of cooling during fractional crystallization. However, the crystallization temperatures for Iceland are systematically offset to higher temperatures than equivalent olivine-spinel aluminum exchange crystallization temperatures published for MORB, an effect that cannot be explained by fractional crystallization. The highest observed crystallization temperature in Iceland is 1399 ± 20°C. In order to convert crystallization temperatures to mantle potential temperature, we developed a model of multilithology mantle melting that tracks the thermal evolution of the mantle during isentropic decompression melting. With this model, we explore the controls on the temperature at which primary melts begin to crystallize, as a function of source composition and the depth from which the magmas are derived. Large differences (200°C) in crystallization temperature can be generated by variations in mantle lithology, a magma's inferred depth of origin, and its thermal history. Combining this model with independent constraints on the magma volume flux and the effect of lithological heterogeneity on melt production, restricted regions of potential temperature-lithology space can be identified as consistent with the observed crystallization temperatures. Mantle potential temperature is constrained to be 1480−30+37 °C for Iceland and 1318−32+44 °C for MORB.

Here we present the results of a detailed cryptotephra investigation through the Lateglacial to early Holocene transition, from a new sediment core record obtained from Lake Hämelsee, Germany. Two tephra horizons, the Laacher See Tephra (Eifel Volcanic Field) and the Saksunarvatn Ash (Iceland), have been previously described in this partially varved sediment record, indicating the potential of the location as an important Lateglacial tephrochronological site in northwest Europe. We have identified three further tephra horizons, which we correlate to: the c. 12.1 ka BP Vedde Ash (Iceland), the c. 11 ka BP Ulmener Maar tephra (Eifel Volcanic Field) and the c. 10.8 ka BP Askja-S tephra (Iceland). Three additional cryptotephra deposits have been found (locally named HÄM_T1616, HÄM_T1470 and HÄM_T1456-1455), which cannot be correlated to any known eruption at present. Geochemical analysis of the deposits suggests that these cryptotephras most likely have an Icelandic origin. Our discoveries provide age constraints for the new sediment records from Lake Hämelsee and enable direct stratigraphical correlations to be made with other tephra-bearing sites across Europe. The new tephrostratigraphical record, within a partially varved Lateglacial sediment record, highlights the importance of Lake Hämelsee as a key site within the European tephra lattice.