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<jats:p> Growing season temperatures play a crucial role in controlling treeline elevation at regional to global scales. However, understanding of treeline dynamics in response to long-term changes in temperature is limited. In this study, we analyze pollen, plant macrofossils, and charcoal preserved in organic layers within a 10,400-year-old ice patch and in sediment from a 6000-year-old wetland located above present-day treeline in the Beartooth Mountains, Wyoming, to explore the relationship between Holocene climate variability and shifts in treeline elevation. Pollen data indicate a lower-than-present treeline between 9000 and 6200 cal yr BP during the warm, dry summer and cold winter conditions of the early Holocene. Increases in arboreal pollen at 6200 cal yr BP suggest an upslope treeline expansion when summers became cooler and wetter. A possible hiatus in the wetland record at ca. 4200–3000 cal yr BP suggests increased snow and ice cover at high elevations and a lowering of treeline. Treeline position continued to fluctuate with growing season warming and cooling during the late-Holocene. Periods of high fire activity correspond with times of increased woody cover at high elevations. The two records indicate that climate was an important driver of vegetation and treeline change during the Holocene. Early Holocene treeline was governed by moisture limitations, whereas late-Holocene treeline was sensitive to increases in growing season temperatures. Climate projections for the region suggest warmer temperatures could decrease effective growing season moisture at high elevations resulting in a reduction of treeline elevation. </jats:p>

<jats:p> A speleothem collected from Palco Cave (Puerto Rico) spans the 8.2 ka event, a time interval associated with fluctuations of Atlantic Ocean circulation and possible drying in the Caribbean region. While stalagmite δ<jats:sup>18</jats:sup>O, δ<jats:sup>13</jats:sup>C, and Mg/Ca data do not show a sustained change in mean state over the 8.2 ka event, the proxies provide robust evidence for three abrupt fluctuations toward drier conditions in rapid succession, each lasting less than two decades, occurring at 8.20, 8.14, and 8.02 ka BP. A cave monitoring program at Palco Cave supports the interpretation of the speleothem proxy records. Because changes in the position of the Intertropical Convergence Zone (ITCZ) are directly coupled to sea-surface temperature variations in the North Atlantic, we hypothesize that cold events in the North Atlantic temporarily limited the northward migration of the ITCZ and tropical rain belt in boreal summer during these abrupt drying periods. The speleothem record suggests that the 8.2 ka event was associated with rapid rainfall fluctuations in the northern Caribbean followed by a comparably warm and wet phase after the 8.2 ka event. This enhanced variability during the transitional period of the deglaciation appears to be linked to a fast coupling between interacting oceanic and atmospheric processes. This involves, in particular, the Atlantic Meridional Overturning Circulation in modulating interhemispheric heat transport. </jats:p>

<jats:p> Pollen records are the most important proxy for reconstructing past terrestrial vegetation. While new approaches for improved quantitative interpretation of pollen data have been developed over the last decades, the availability of pollen records remains mostly limited because pollen samples are still analysed manually, which is a time-consuming task and requires extensive training. Here, we present an approach for automated recognition of pollen and spores from lake sediments using deep convolutional neural networks and machine learning. The approach includes two stages. The detector first locates pollen and spores in the sample matrix, and the classifier then classifies these objects. We have trained the approach on two pollen datasets from two lakes in north-eastern Germany. So far, our approach is able to automatically recognise 10 pollen types and Lycopodium spores with high accuracy. As soon as more training data are available, more pollen and spore types can be added. The preparation of training data, the training of the neural networks and their application are accessible via a freely available, browser-based user interface called TOFSI. </jats:p>

<jats:p> The organic matter composition of lake sediments influences important in-lake biogeochemical processes and stores information on environmental changes. Extracting this information is notoriously difficult because of the complexity of the organic matter matrix, which routinely imposes trade-offs between high temporal and analytical detail in the selection of methods of analysis. Here, we demonstrate the potential of diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) for achieving both of these objectives using untreated bulk samples from two Holocene lake-sediment cores from central Sweden. We develop quantitative models for sediment total organic carbon (TOC) with the same predictive abilities as models based on samples diluted with KBr and qualitatively characterize the organic matter using a spectra processing-pipeline combined with principal component analysis. In the qualitative analysis we identified four organic matter sub-fractions and the interpretation of these is supported and further advanced with molecular data from pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Within these organic fractions, compound groups such as aromatics, lignin, aliphatics, proteins and polysaccharides were identified by means of DRIFTS and the analyses and processes outlined here enables rapid and detailed quantitative and qualitative analysis of sediment organic matter. The DRIFTS approach can be used as stand-alone method for OM characterization with high temporal resolution in Holocene sediment records. It may also function as a screening process for more specific analyses of sample subsets, such as when coupled with pyrolysis-GC/MS to further tease apart the OM composition, identify sources and determine degradation status. </jats:p>

<jats:p> The study presents a multi-proxy archive from hypersaline coastal Lake Chokrak in eastern Crimea, a steppe region which is particularly susceptible to moisture fluctuations. XRF, organic carbon and nitrogen, and palynological analyses are used to reconstruct the evolution of the lake and landscape development around it and to provide information about the driving forces behind those changes, such as climate and human impact. The findings show that the basin was an open marine bay between 7640 and 5500 cal. yr BP. After 5500 cal. yr BP, the low rates of sea-level rise led to gradual infilling of the basin. The prevalence of mesic herb-grass steppe around the site, as well as the expansion of broad-leaved trees in the Crimean mountains, indicate relatively warm and humid climate conditions during the Mid-Holocene. Elemental proxies and marine NPPs point to a transition to lagoonal conditions from 4270 cal. yr BP. The progressive shallowing of the basin was accompanied by the spread of more xeric Artemisia-grass steppe, which suggests a shift to arid conditions. The occurrence of cereal pollen from 3660 cal. yr BP may indicate the first signs of cultivation in the study area. The active build-up of a sand barrier from 3000 cal. yr BP led to a gradual separation of the basin from the sea. By 900 cal. yr BP, the fully-formed sand barrier resulted in a transformation of the lagoon into a hypersaline lake. The closing of the lagoon was accompanied by an expansion of halophytic communities around the site. Human-induced vegetation changes became more pronounced with the development of agrarian and pastoral activities around the site from ca. 750 cal. yr BP. </jats:p>

<jats:p> Paleoecological analysis of three lake sediment cores from the Roraima savannas in northern Brazil revealed systems dominated by Poaceae pollen throughout most of the Holocene. A slight increase of palms and woody taxa, probably linked to wetter conditions, is observed during the last 1000 years but is more noticeable after 300 and 150 calibrated years BP (yr BP) respectively. Charcoal was present throughout affirming the importance of fire for the landscape but showed the highest values mostly in the last millenium. Poaceae pollen size spectra varied considerably, showing more variability than the overall pollen record. Despite evidence of some climate change in the Holocene, these savannas were stable systems. The sedimentary records showed gaps in deposition between 10,000 and 7800 yr BP and between 2500 and 1200 yr BP, suggesting dry periods during which lakes most likely dried out or became impermanent and highlighting the sensitivity of these lakes to local water table variation. The establishment of the modern climate in the region is evident after 1000 yr BP. </jats:p>

<jats:p> Fundamental understanding of paleoecological proxies is necessary when attempting to compare, complement, or contrast two or more methods, and lack thereof may lead to erroneous conclusions. This comment addresses three such misunderstandings found in a recently published paper by Paillard et al. regarding soil macrofossil charcoal analysis (SMCA) and its relationship to lacustrine sediment analysis. The aim is to correct some misinformation associated with the following three assertions: (1) Broadleaved tree species produce fewer charcoal fragments than coniferous species. Although coniferous stands are more fire-prone, experimental burning shows that species with denser wood, that is, broadleaves, produce greater amounts of charcoal under similar fire conditions. (2) Preservation of charcoal particles is poor at the referenced study site. Once buried in the mineral soil compartment, charcoal particles remain quite stable. As such, SMCA has revealed Late Pleistocene marginal stands of broadleaved species. (3) Underestimating the importance of range-edge dynamics on the results of SMCA reconstructions. SMCA offers a stand-scale historical reconstruction that has proven well-suited to study peripheral stands and to reflect the heterogeneity of a landscape mosaic. By attempting to reconcile the SMCA (in situ) and lake sediments (ex situ) narratives, Paillard et al. missed one key aspect of comparing complementary proxies: they show different aspects of the past. </jats:p>

<jats:p> In the Mediterranean Sea, organic carbon-rich sapropels have been deposited periodically over the last fifteen million years. Some sapropels are characterized by high contents of the mat-forming, planktonic diatom Pseudosolenia calcar-avis and the planktonic diatom Thalassionema nitzschioides as a result of their mass sinking in autumn (the so-called “fall dump”). The present study shows that fall dump events also occurred in the brackish Baltic Sea around 6300–5800 calibrated years before present (cal yr BP; present = AD 1950). In sediments from the northern Baltic Sea these events are evidenced by high contents of a lipid specific for P. calcar-avis (a C<jats:sub>25:2</jats:sub> highly branched isoprenoid alkene) and corroborated by fossil remains of both P. calcar-avis and T. nitzschioides. A biomarker index based on long-chain alkyl diol lipids indicates that mat-forming Proboscia diatoms were also present. High contents of calanoid resting eggs further suggest that the copepod population was stressed being unable to feed on such large diatoms. The fall dump events occurred during a complete stratification of the water column and euxinic conditions, as reflected by the redox-sensitive trace metal uranium, allowing upward diffusion of nutrients and the growth of rhizosolenid mat-forming diatoms in a deep chlorophyll maximum. Synchronous remains of P. calcar-avis in the central Baltic Sea further suggest that these events occurred on a basin-wide scale. </jats:p>

<jats:p> An understanding of the geomorphological processes instigated by seismic-related catastrophes such as tsunamis is important to our understanding of their potential impacts on both past and present human populations. Due to the relative scarcity of human records of such events in prehistory, it is necessary to rely on the geological record to identify and interpret the environmental changes caused by such catastrophes. A multi-proxy analysis of geological and geomorphological data is used to identify a local signal of a region-wide 15th century palaeotsunami that inundated the SW North Island coast of Aotearoa/New Zealand. A comparison with regional geological and archaeological evidence enriches our understanding of the nature and extent of this event and its impact on human settlement that complements similar findings in other countries. </jats:p>

<jats:p> Temperature variability likely played an important role in determining the spread and productive potential of North America’s key prehispanic agricultural staple, maize. The United States Southwest (SWUS) also served as the gateway for maize to reach portions of North America to the north and east. Existing temperature reconstructions for the SWUS are typically low in spatial or temporal resolution, shallow in time depth, or subject to unknown degrees of insensitivity to low-frequency variability, hindering accurate determination of temperature’s role in agricultural productivity and variability in distribution and success of prehispanic farmers. Here, we develop a model-based modern analog technique (MAT) approach applied to 29 SWUS fossil pollen sites to reconstruct July temperatures from 3000 BC to AD 2000. Temperatures were generally warmer than or similar to those of the modern (1961–1990) period until the first century AD. Our reconstruction also notes rapid warming beginning in the AD 1800s; modern conditions are unprecedented in at least the last five millennia in the SWUS. Temperature minima were reached around 1800 BC, 1000 BC, AD 400 (the global minimum in this series), the mid-to-late AD 900s, and the AD 1500s. Summer temperatures were generally depressed relative to northern hemisphere norms by a dominance of El Niño-like conditions during much of the second millenium BC and the first millenium AD, but somewhat elevated relative to those same norms in other periods, including from about AD 1300 to the present, due to the dominance of La Niña-like conditions. </jats:p>