We provide fundamentals of climate seasonality and break it down into external and internal forcing, and regional and local and modulating factors.
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We present a transdisciplinary overview on how seasonality is recorded in, and extracted from, different palaeonvironmental archives.
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We propose a framework for transparent communication of seasonality-related research across different communities.
Abstract
The role of seasonality is indisputable in climate and ecosystem dynamics. Seasonal temperature and precipitation variability are of vital importance for the availability of food, water, shelter, migration routes, and raw materials. Thus, understanding past climatic and environmental changes at seasonal scale is equally important for unearthing the history and for predicting the future of human societies under global warming scenarios. Alas, in palaeoenvironmental research, the term ‘seasonality change’ is often used liberally without scrutiny or explanation as to which seasonal parameter has changed and how.
Here we provide fundamentals of climate seasonality and break it down into external (insolation changes) and internal (atmospheric CO2 concentration) forcing, and regional and local and modulating factors (continentality, altitude, large-scale atmospheric circulation patterns). Further, we present a brief overview of the archives with potentially annual/seasonal resolution (historical and instrumental records, marine invertebrate growth increments, stalagmites, tree rings, lake sediments, permafrost, cave ice, and ice cores) and discuss archive-specific challenges and opportunities, and how these limit or foster the use of specific archives in archaeological research.
Next, we address the need for adequate data-quality checks, involving both archive-specific nature (e.g., limited sampling resolution or seasonal sampling bias) and analytical uncertainties. To this end, we present a broad spectrum of carefully selected statistical methods which can be applied to analyze annually- and seasonally-resolved time series. We close the manuscript by proposing a framework for transparent communication of seasonality-related research across different communities.
Introduction
Seasonality is a common denominator for several academic disciplines and its accurate reconstruction is highly relevant across both the natural and human sciences. At a basic level, climate seasonality is expressed intuitively as the cyclical changes in temperature and/or rainfall over the course of the year, which in turn determines both the composition and the dynamics of ecosystems. Overall, climate seasonality plays a critical role in influencing the persistence of all living organisms. For example, the seasonal changes in precipitation and temperature affect different components of the climate system (e.g., soil moisture, snow cover, evaporation rates, river flows and lake levels). The changes in these variables lead further to changes in vegetation and ecologic requirements of plants and animals, which in turn influence the type and amount of food available for humans and other organisms. For the majority of multicellular organisms, the diurnal and seasonal cycles are the most important pacemakers of biological functions. For humans, the influence of seasonality affects the biological world they interact with and extends across the cultural domain, including construction of niches, subsistence, religious, and economic activities. Studying past changes in seasonality is of great interest and importance for palaeoclimatology, palaeoecology, anthropology and archaeology, and, last but not least, modern climate science, conservation and phenology, all of which face the uncertain future of global warming (Santer et al., 2018). Palaeoclimatology aims at documenting how seasonal changes affect the climate system through time (Crowley et al., 1986, Denton et al., 2005a), and the amplitude of seasonal changes (Luterbacher et al., 2004, Felis et al., 2004, Ferguson et al., 2011, Veski et al., 2015, Brocas et al., 2018). Palaeoecology deals with the effect of seasonality changes on the ecosystem (Rivals et al., 2018, Manzano et al., 2019), while anthropology and archaeology document the effect of seasonality and changes in seasonality on human evolution, residency, subsistence strategies, and the adaptation of those strategies (the latter two involving human-ecosystem interaction; (Prendergast et al., 2018), and references therein). Recent work by Degroot et al. (2021) emphasized pitfalls of integrating data and knowledge between academic disciplines with different practices and standards of evidence. Increasing scientific interest in what the authors termed ‘history of climate and society’ warrants proposing frameworks which facilitate interdisciplinary research. Thus, this review proposes a framework for addressing past climate seasonality changes.
An opinion piece by Carré and Cheddadi (2017) echoes the seminal work by Rutherford et al. (2005) and outlines the most important, but often overlooked, aspects of seasonality in palaeoenvironmental studies. Firstly, climate is defined not by annual means of temperature or precipitation, but by the annual cycles of these climate variables (see Box 1). Annual mean values, so often extracted from proxy records, while important, do not fully capture past climate variability. Secondly, relatively small changes in natural processes acting on a seasonal timescale are the drivers that foster large climate shifts. Not detailed by Carré and Cheddadi (2017) are the often simplified or overlooked aspects of spatial heterogeneity of environments and human actions, including the seasonality and timing of subsistence activities, which further influence the rhythms of other cultural behavior(s).
Definitions
‘Seasonality’ is widely used in many disciplines of palaeo-research, yet it is lacking a clear definition. In the scientific literature, references to changes in seasonality are as frequent as they are ambiguous. A survey of this literature raises many questions: what does ‘increased’ or ‘decreased seasonality’ actually mean? Can we quantify this change? And is the amplitude all that matters? What about temporal distribution? Does temperature and precipitation always respond symmetrically and harmonically? Pezzulli et al. (2005) highlighted that one should refer to the annual cycle rather than the seasonal cycle since the period is one year, not one season, and we endorse this approach. Here we define key concepts related to seasonality and how they will be used throughout this review.
Annual cycle of temperature – can be symmetric, sinusoidal, and is defined by maxima and minima. Seasonality of temperature refers to an amplitude between maxima and minima. In theory, the annual budget reaches zero, meaning that colder winters are counterbalanced by warmer summers.
Annual cycle of precipitation – is defined by magnitude (amount) and temporal distribution (timing – when: duration – for how long). Seasonality of rainfall should take all three of these components into consideration, which, in case of palaeoenvironmental archives and their limitations in resolution, is rarely feasible. In modern climatology the beginning of the hydrological year differs from the beginning of the calendar year.
Seasonality of temperature and seasonality of rainfall together make climate seasonality.
Annual cycle of human activities (e.g., foraging, farming, migration) – strongly related to natural temperature and precipitation cycles, which influence the growing season and availability of static resources and the movement patterns of mobile resources (see Box 5). The availability and sustainability of these resources influence human subsistence strategies, which in turn inform other types of cultural behavior. Seasonality of an activity refers to its timing and duration.
The two aspects of seasonality reiterated by Carré and Cheddadi (2017), namely: (1) the fact that it defines climate and (2) that the small changes accumulate in large-scale oscillations (e.g., glacial/interglacial cycles), constituting a challenge for scientists working with archives that often lack seasonal resolution and/or are biased towards one season only. Alternatively, archives record seasonal changes but are discrete in nature and represent only snapshots of time rather than a continuous interval. Consequently, regional palaeoclimate syntheses frequently suggest different responses to seasonality changes to account for discrepancies between different archives and proxies covering the same time span, or between data-based reconstructions and climate model output. The classical example comes from the Mediterranean region where Prentice et al. (1992) reconciled glacial lake levels, where high levels suggested increased humidity, with contemporary pollen records that indicate drier conditions, by proposing an increased seasonality in precipitation with wetter winters and drier summers. Yet, the term ‘seasonality’, while so often used by the palaeo-community, lacks formal definition, and the phrase ‘seasonality change’ is often used to refer to a bundle of processes encompassing changes in both the external forcing and internal conditions modulating the local response. The external forcing is prescribed by the orbital parameters (see Box 2).
Orbital influences on annual and diurnal cycles
The diurnal (Earth rotation around its axis, 24 h) and annual (Earth rotation around the Sun, 1 year) cycles can be observed and experienced during a human lifetime. On longer, multi-millennial time scales, these cycles are influenced by changes in Earth's orbital parameters, obliquity, eccentricity, and precession. Changes in orbital parameters have been calculated theoretically (Milankovitch, 1930) and their persistence on Earth's climate has been documented in the geological record (Hays et al. (1976) and more). Fundamentally, seasonal variability is controlled by the amount of incoming solar radiation (insolation), arriving at different latitudes at different angles as Earth orbits the sun. Below we consider four different scenarios to illustrate how changes in orbital parameters influence the annual insolation distribution and the length of day.
1. If the Earth's rotational axis was perpendicular to the orbital plane, the insolation angle for each latitude would be constant throughout the year. Insolation gradients would exist between the latitudes, but there would be no seasonal changes. Daytime would have the same length at each latitude.
2. Increasing the tilt (obliquity) changes the insolation angle during Earth's rotation around the sun. The hemisphere tilted towards the sun experiences warmer temperatures (summer), and longer days. The amplitude of seasonal differences increases with the tilt. The length of the obliquity cycle is ca 42 ka. At the equator, the length of the daytime is roughly constant throughout the year. It gets longer (24 h) towards the ‘summer’ pole and shorter (0 h) towards ‘winter’ pole. Summer and winter are of equal length in both hemispheres.
3. Changing the shape of the orbit (eccentricity) influences the distance of the Earth to the sun and the length of the seasons. Eccentricity of the orbit modulates the effect of the obliquity. The seasons at aphelion are colder (the Earth is further away from the sun) and longer (further away from the sun its gravitational pull is weaker, so the Earth moves slower) than at perihelion. Eccentricity has two cycles, a short one, ca. 100 ka, and a long one, ca. 400 ka. In the presented scenario (corresponding to modern day conditions) the gradient between summer and winter insolation (here, translated into temperature) is steeper in the southern hemisphere (SH) compared to the northern hemisphere (NH).
4. The wobble of Earth's rotational axis (precession) changes the direction of the tilt and determines which hemisphere is tilted towards the sun at perihelion (summer). The same hemisphere will be tilted away from the sun at aphelion (winter). Precession thus determines on which hemisphere the amplitude of annual change in insolation is larger. The overall length of the precessional cycle is ca 23 ka.
While systematic changes in insolation are, next to atmospheric CO concentration, the most important driver for seasonal temperature variations, other factors can modify temperature variations (see Box 3).
The amount of insolation received at any point on the planet is a function of season and latitude. It can be theoretically calculated for the past and the future and broadly translated into relative temperature changes, with flat seasonal gradients in the tropics and steep gradients at the poles (see Box 3).
External and internal forcing, and internal feedbacks
The energy received from the sun per unit area (insolation) is kept in check by Earth's atmospheric CO concentration. Insolation changes (external forcing) are periodic and fixed for a given season and latitude (see Box 2) and as such are predictable. In pre-industrial times CO concentration (internal forcing) varied little between the hemispheres, following the respective vegetation season, and large variations in CO level were global (Royer et al., 2004; Augustin et al., 2004).
At the low latitudes the total amount of the insolation received is larger than that received at the high latitudes and the poles, but the amplitude of annual change is very small. Hence in low latitudes annual cycle is expressed in precipitation changes (wet and dry season). The amplitude of annual insolation change increases with distance from the equator and manifests itself in temperature and daylight duration changes.
Still, the Earth unit area receiving insolation is rarely homogeneous and the surface properties can modulate (dampen, amplify, or delay) the local response. Internal feedbacks are semi-stochastic.
A novel (in a geological sense) element of internal feedback, referred to as anthropogenic climate variability, combines greenhouse gas emission, deforestation and land use change. The sensitivity of a given archive can further influence the palaeoenvironmental record.
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Continentality: a measure of the difference in the annual temperature maxima and minima that occurs over land compared to water. The ocean's capacity for storing heat (thermal inertia) is greater than that of the continents which means it warms more slowly but also cools more slowly than land masses. Further, the upper ocean layer can distribute heat both, vertically and horizontally. By storing heat in summer and releasing it in winter oceans considerably dampen the annual cycle of temperature. In contrast, the continental interiors experience much larger annual temperature differences. The large thermal inertia of the oceans shifts the annual temperature maxima and minima of surface water and coastal regions in relation to temperature over continental interiors. The land-ocean distribution is also important in moderating the insolation-prescribed hemispheric seasonality contrast: under modern day conditions the gradient between summer and winter insolation is steeper in the SH compared to the NH; however, the SH ocean/land ratio counteracts the large temperature gradients. Size and distribution of the continents have also impact on the seasonal precipitation patterns, with continental interior receiving less rainfall than coastal regions.
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Land cover: differences in surface properties represented by vegetation changes (e.g.: forest vs steppe vs bare rock), snow cover, or surface water distribution affect the albedo and the heat capacity of the surface. The effect of these differences on the overlying atmosphere is analogous to the ocean surface temperature anomalies, but on a much smaller spatial scale.
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Altitude: temperature in the troposphere (lowest layer of atmosphere) decreases with increasing altitude. The rate (lapse rate) is approximately 1°C for every 100 m.
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Atmospheric circulation patterns: seasonal variability of precipitation and temperature is modulated by the large-scale atmospheric circulation patterns and by the ocean circulation, operating on interannual to multidecadal time scales (e.g., El Niño-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), the Pacific North American pattern (PNA), the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO), among others). These atmospheric and ocean modes of variability can influence the precipitation and temperature in different ways. For example, NAO exerts a strong influence on the hydroclimate variability of Europe, while the PNA strongly influences the hydroclimate of the U.S.
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Volcanic activity: volcanic eruptions inject large quantities of aerosols into the atmosphere, and stratospheric circulation distributes them across the planet. In general, aerosols have a cooling effect. However, the scale of this effect depends on where (hemisphere and latitude), when (season), how much (the volume), and for how long (single or multiple eruption events) the material was injected.
The internal forcing – atmospheric CO2 concentration – is a global feature, and its changes are relatively well documented for the course of the Pleistocene and Holocene (Zhang et al., 2013, Augustin et al., 2004). The local conditions, however, are inherently heterogenic, and factors like continentality (landmass and ocean distribution), altitude, land cover, and atmospheric circulation patterns and volcanic activity play an important role in modulating insolation- and CO2-prescribed local temperature (see Box 3). Thus, the resulting local expression of climate seasonality varies between sites along the same or similar latitude (see Box 4). Further, the natural archives exposed to seasonal changes might display a bias or an offset in recording the local signal (see Box 4). Last but not least, if the natural archives might be influenced by, or are the direct outcomes of human activity, considering anthropogenic aspects is essential. Depending on the nature of their adaptations, resilience and sustainability, humans developed different strategies to cope with and/or take advantage of seasonal changes (e.g., choosing migratory or stationary lifestyles, hunting and foraging or farming). Importantly, the degrees to which particular strategies are successful are likely to change through time, based on environmental circumstances, population size, and technology (see Box 6). Consequently, the archaeological archives related to human occupation sites constitute a special case, i.e., a confluence of natural changes and developing human adaptations (e.g., Petrie and Bates, 2017).
Combined influence of latitude, continentality, and altitude
Köppen (1936) classification of climate divides climate zones into 5 main groups (tropical - A, dry - B, temperate - C, continental - D and polar - E), based on seasonal temperature and precipitation patterns. This grouping takes into consideration not only latitude but also continentality and altitude. Köppen's climate zones are the best example of differences in amplitude of seasonal change along the same latitude (the theoretical line subjected to the same insolation forcing). We have chosen 7 examples of archaeologically relevant sites from around the globe to illustrate the possible range of local seasonal temperature and precipitation (modern data from https://en.climate-data.org). Tropical and temperate climates are characterized by larger amplitude of precipitation changes, dry and continental climate by larger amplitude of temperature. In the case of the tropical site in the Andes, the altitude is responsible for low temperature values. Note that the plot does not account for potential evapotranspiration.
This review answers the call by Carré and Cheddadi (2017) for a reevaluation of the scientific focus and methodological habits of the scientific community. It is time to scrutinize ‘seasonality changes’ and address individually different components which together produce the climate – and human history records we work with. Considering the breadth of the audience, first we take a step back and take up the issue of climate seasonality (see Box 1) at a fundamental level of external forcing (see Box 2) internal, regional and local changes (see Box 3, Box 4). Further, we summarize how seasonality is reflected in different archives and explore advantages and potential limitations of each archive. The next chapter demonstrates statistical methods useful in extracting and analyzing seasonal information from high-resolution but often irregularly sampled archives. Finally, we suggest a framework for discussing scientific observations in order to avoid confusion and promote transparency in multi- and transdisciplinary research.
Relevance of seasonal bias recognition and adapting sampling strategy
Treating seasonally biased records as representing annual means might lead to flawed interpretations. The opposite is also true: records reflecting annual means should be treated with caution when interpreting seasonal changes in temperature or rainfall.
In our conceptual example here, some archives record the full range of annual temperatures, while others only a portion. The correct recognition of the recorded interval is crucial for further interpretation, regional or global synthesis, and comparison (Bova et al., 2021; de Winter et al., 2021b). Adequate sampling is an additional challenge, in particular when the sedimentation/growth rate of the archive is low. de Winter et al. (2021a) discuss in depth how sampling strategy might influence obtained results and propose a schematic guide for choosing the optimal approach. Depending on archive sensitivity and sampling strategy, the shift of the baseline without change in the amplitude might be inaccurately perceived as an increase or decrease in seasonality of respective parameter. Further, comparing the same proxies (e.g., O) from different archives, or the same, but geographically distant, archive does not guarantee that they record the same season.
The beauty of seasonally resolved archives, whether continuous or discrete, lays in their capacity of recording the baseline of seasonal variation. Deviation from this baseline can inform on frequency and magnitude of events (e.g., floods or droughts), while stepwise change suggests the reorganization of the large-scale atmospheric and/or oceanic circulation (e.g., glacial termination). Anchoring seasonal changes in a wider palaeoenvironmental narrative allows for insight into the complex dynamics of Earth's system's and human response to external climate forcing.
Not all palaeoenvironmental archives have the potential of recording seasonal variability. Of those which can, not all can be dated with annual resolution. Here, we first focus on instrumental and historical data as these have natural and direct connection to the present. Next, we move to archaeological records as an overarching subject discussing relevance of seasonal changes for humanity's past, beyond instrumental and historical reach. This chapter alludes to natural archives which are often found either directly at sites of human occupation or in close vicinity and have the potential to record seasonal changes. The different archives (i.e., marine biogenic carbonates, stalagmites, tree rings, laminated lake sediments, glacier ice, cave ice, and permafrost ground ice) are highlighted in the following chapters.
The element conspicuous by its absence is pollen. When calibrated, pollen records indisputably provide information on temperature and precipitation ranges in physical units (°C and ml) and as such can be related to specific seasons (Chevalier et al., 2020). Applying transfer functions to pollen assemblages is a powerful tool for quantifying past environmental change. Alas, the temporal resolution of this proxy is inherently coupled with the sedimentation rate of the media it resides in and this review addresses seasonally resolved archives rather than proxies.
Section snippets
Seasonality in historical climatology
Historical climatology aims to extend temperature and precipitation data back to pre-industrial periods, bringing together direct and indirect sources on weather and climate (Fig. 1). Various types of records serve as unique functions in historical climate reconstruction by (i) providing precise climate and weather information, from annual to daily resolution, at defined locations for all seasons and (ii) defining their societal impacts, perceptions and reactions. Information derived from
Methodological advances
Direct observations from historical narratives on climate anomalies and weather patterns, as reported in documentary sources come with a variety of drawbacks. First, such accounts are often sporadic or event specific, and possibly contain gaps when considering long time periods (Pfister, 2010). Nash and Adamson (2014) note that the discontinuous nature of many records can cause major issues, but those are balanced by the excellent dating control and high temporal resolution that is available
Seasonality in the human past: relevance, major research questions and methods
Human behaviour has been shaped deeply by seasonality, but understanding these effects requires transdisciplinary approaches drawing from different components of the earth system (e.g., atmosphere, biology) as well as social sciences. This is especially the case when considering past populations. All life histories involve responses to seasonality, including adaptations to cyclical changes in temperature, humidity, rainfall, ocean currents, cloud cover, and wind (Boyce, 1979). In general,
Seasonality in marine invertebrates
As is the case with other proxies, one of the major concerns influencing the ability to assess seasonal patterns of change in marine records, and in turn make inferences about human subsistence and mobility, is the resolution of proxy time series. This includes both sampling resolution (i.e., the amount of time integrated in a single sample and spacing between them) and temporal resolution of the tested profile (often the archaeological record). West et al. (2018) comprehensively summarize the
Seasonality and seasonal bias in speleothems
The manifestation of seasonality in speleothem records critically depends on processes in the atmosphere, the soil and cave environment and during speleothem formation. The seasonal cycle in rainfall distribution and temperature controls the timing and vigor of respiration within the plant community above a cave. This in turn determines the supply of CO2 to the soil water, controlling the dissolution rate of limestone and precipitation of carbonate in the cave (Fairchild and Baker, 2012).
Theoretical background
Tree stems accumulate wood (xylem) via a circumferential layer of meristematic cells underneath their bark named cambium (Fig. 8). Each year, trees act as living-sensors for a limited interval of time, during the vegetative season, when seasonal climatic conditions change and turn metabolic inactivity into a phase of plant growth, in general, and cambial cell division, in particular. Vegetative season ends when seasonal climatic conditions lower tree metabolic processes to minimal maintenance
Varves – seasonally resolved sediment archives
Lakes, estuaries and oceans have the potential to record seasonal climate variability expressed as changes in the productivity, mineral precipitation or supply of material when deposition site is sheltered from currents, wave activity, turbidity flows, or bioturbation (anoxic or suboxic conditions). Annually laminated sediments - varves, are one of the most pronounced manifestations of seasonal changes within sedimentary archives. Per definition a varve is a sequence of layers, deposited in an
Seasonality in continental ice bodies
Continental ice is a powerful climate archive that directly preserves numerous proxies that inform on seasonal changes in atmospheric conditions. Major efforts have been undertaken to utilize continental ice to reconstruct past climate on various spatial and temporal scales. Of particular importance are the ice sheets of Greenland and Antarctica, but also polar and mountain glaciers, and numerous ice cores have been drilled since the middle of the 20th century. More recently, continental ice
Numerical tools for extracting seasonality changes from palaeoenvironmental time series
As the previous sections show, seasonality expresses itself in different fashions in each environmental archive, and is accompanied with individual challenges and limitations. Seasonality reconstruction is concerned with the extraction of prominent features which closely link to the respective notion of seasonality, such as periodicity, seasonal amplitude, timing/duration or complexity (see Box 7) of seasonal patterns (Fig. 15). In order to characterize seasonal variability, quantitative time
Compositional make-up of climate seasonality
Seasonal changes in our environment are periodic and global, happening at the temporal scale of human behavior. This, and the fact that seasonal dynamics are perceptible over human live spans, makes the reconstruction of past changes in seasonality of paramount importance for the study of past human-environment interactions.
The concept of seasonality is rooted in the annual march of the Earth around the Sun, and the nonlinear response of the physical climate system (and human adaptation) to
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Chris Jazwa received financial support for the data collection from the National Science Foundation (grant NSF BCS-1724639). Tobias Braun received financial support from the Deutsche Forschungsgemeinschaft in the context of the DFG project MA4759/11-1 ‘Nonlinear empirical mode analysis of complex systems: Development of general approach and application in climate’. Aurel Perşoiu was supported by a grant of the Romanian Ministry of Education and Research, CNCS - UEFISCDI, project number
Climate seasonality critically influences the functioning and dynamics of ecosystems in continental areas. The ecological importance of winter temperatures on high-latitude vegetation changes has recently been argued to be largely overlooked in comparison to summer temperatures. The Oymyakon region from eastern Siberia, with its strong continentality of extremely cold winters and moderately warm summers, is ideally suited to study the response of past vegetation to seasonal temperature changes based on long ecological time-series. However, few paleorecords are available from this area. The history of regional glacial activity and potential plant refugia since Marine Isotope Stage (MIS) 3 is not well understood. Here, we present geochemical and plant DNA metabarcoding records retrieved from a sediment core from Lake Ulu in the Oymyakon region, which provides detailed information on glacier and vegetation dynamics over the last 43 cal. ka BP. Our results suggest that glacial fluctuations were primarily driven by summer insolation, and Lake Ulu was likely initiated by glacial retreat during MIS 3. The catchment experienced multiple glacial advance/retreat cycles until the Last Glacial Maximum, and the glaciers fully retreated by 20 cal. ka BP. In addition, a tundra-steppe landscape dominated by Dryas, Papaver, Saliceae, and Anthemideae occupied the catchment for most of the time and began to collapse around 19 cal. ka BP following the expansion of trees and shrubs such as Larix, Betula, Alnus, and Vaccinium. Postglacial plant assemblages in the Oymyakon region exhibit a high sensitivity to summer temperature variations, with minimal impact from winter temperatures. This can be explained by the dominance of summer insolation amplitude, extreme continentality, extended plant growing season, and plant genetic adaptation to cold. Notably, our ancient DNA record show the earliest postglacial expansion of larch in eastern Siberia (around 18.6 cal. ka BP), which is likely related to the presence of local refugia. This implies that the Oymyakon region may be one of the earliest sources for larch recolonization and that more research should be implemented to provide insights into larch expansion and migration, and to better predict the future scenarios for Siberian larch forests.
The distribution of brGDGT lipids produced by soil bacteria has been used to reconstruct temperatures in marine and terrestrial settings as far back as the Cretaceous period. However, modern calibrations of this proxy have primarily relied on air rather than in situ soil temperatures, which can differ by more than 10 °C. Furthermore, the influence of other parameters such as temperature seasonality and soil chemistry on brGDGT lipids is not fully understood. We measured brGDGT distributions, in situ soil temperatures, pH, soil water content, and electrical conductivity on soils from the Eastern Canadian Arctic and Iceland. We compiled our results with those of published soil brGDGT studies that also provide in situ soil temperatures and ancilliary measurements and generated global temperature and pH calibrations from the resulting dataset. Soil temperatures outperformed air temperatures in these calibrations, with mean summer soil temperature providing the highest-performing fit among the 10 tested soil temperature parameters. When applied to a loess/paleosol sequence from the Chinese Loess Plateau, these new calibrations produced paleotemperature and paleo-pH histories consistent with the results of previous studies, encouraging the application of our new calibrations on a broader scale. We also detected 7-methyl and IIIa’’ brGDGT isomers in our Eastern Canadian Arctic and Iceland soils, which have been shown in lakes to relate to salinity and anoxia, respectively. While neither correlated with bulk soil properties such as conductivity, soil water content, or pH, these brGDGT isomers did correlate with seasonality and winter soil temperature. We hypothesize that these compounds are generated in winter by bacteria in habitable niches of more saline, sometimes anoxic liquid water in the otherwise frozen soil matrix. Finally, we report the presence of overly branched GDGTs with m/z = 1064 and suggest that these heptamethylated tetraethers should be investigated as a potential tool for improving brGDGT calibrations. Overall, our results expand our understanding of the seasonality of brGDGT production, especially at high latitudes, and provide in situ soil temperature and pH calibrations for global use.
There is a growing literature on the cultural capacities influencing communities’ adaptation to environmental and social change, including the temporal frameworks they draw on for timely action. This paper focuses on seasonal cultures, and how they enable communities on the Coromandel Peninsula to interpret and adapt practical timings to disrupted patterns of seasonal rhythms. The paper develops and applies a conceptual framework of seasonal cultures as perceived rhythmic patterns practiced by communities as cultural repertoires for action, emphasising the ways cultures evolve as patterns are contested and change. This concept steered critical, mixed-method ethnographic study with communities on the peninsula over two years. The research found that Coromandel communities’ cultures make seasonal change visible as long-term shifts and asynchrony between rhythmic patterns, which they linked to climatic change, environmental degradation, colonisation and globalisation, and shifting relations between society and the environment. As seasonal patterns fail to hold, communities deploy a combination of strategies for re-configuring seasonal rhythms through their practices: (i) maintaining established, institutionalised schemas of activity while coping with seasonal variability; (ii) season-proofing activities from environmental rhythms; or (iii) re-learning and recalibrating cultures to mutable configurations of rhythms in a highly modified environment.
Among numerous concerns, restoration ecologists are routinely plagued with the problem of where to implement conservation efforts to best maintain spatial connectivity and population structure. Knowledge about connectivity within a metapopulation could offer valuable insight to address this issue and could help with the allocation of limited resources more effectively. However, direct estimation of dispersal is challenging because species can disperse widely within a landscape. Here, we developed a novel hierarchical Bayesian model to estimate spatial connectivity from occurrence data of an endangered stream fish, Topeka shiner (Notropis topeka). Our goal was to identify dispersal corridors that are centrally connected to the metapopulation that could be beneficial in decision making about future habitat restorations aimed at maintaining population structure. Connectivity modeling is data intensive and resource managers may not have the necessary data requirements; thus, we also examined the usefulness of graph theory (i.e. network centrality) as a proxy for connectivity. Model selection identified an upstream biased asymmetric dispersal pattern for the species. We were able to quantify and map connectivity and identified over 68 km of stream reaches as highly connected to the metapopulation. Probability of occurrence in dispersal corridors (i.e. streams) increased with connectivity and decreased with drainage area, highlighting the importance of conserving dispersal corridors and preferred habitat patches. Restorations in connected locations would provide critical habitat near important dispersal corridors. Betweenness centrality was positively correlated with connectivity and occurrence in restored habitat. Modeling of metapopulation connectivity and its correlation with graph theory demonstrated the usefulness of these techniques to guide conservation actions, especially in countries where data collection efforts are not common and conservation funding is limited.
This investigation into the Little Ice Age (LIA) climate discrepancies explores sea surface temperature (SST) anomalies in the South China Sea (SCS) using monthly resolved coral Sr/Ca proxies. Results show that, during the LIA, while overall summer and annual SSTs were cooler compared to the Medieval Climate Anomaly and the Current Warm Period, winters were unexpectedly warmer. This warming is corroborated by another coral-based study and aligns with terrestrial data from high-latitude regions reporting similar warmer winters during the LIA. The analysis suggests that these higher winter SSTs in the SCS were likely due to a weakened East Asian winter monsoon and stronger El Niño events. The presence of warmer winters in various regions contradicts the traditional view of a consistently cold LIA, indicating complex regional climate variations influenced by atmospheric and oceanic interactions. This work underscores the need to reassess our understanding of the LIA's heterogeneous climate impacts globally.
Paleoclimatic records of sub-annual to interannual resolution are valuable for the study of climatic forcing mechanisms but these records are rare in terrestrial settings. Here, we show that high-resolution stable carbon isotope data within shells of land snails (δ13Cshell) living in a C3 plant-dominated community can be an indicator of growing season precipitation (GSP) amount. We collected modern land snails along two wet-dry climate gradients from northern Iran and the Chinese Loess Plateau (CLP), where the precipitation is mainly distributed in the boreal winter and boreal summer seasons, respectively. Our results indicate a common pattern of δ13Cshell values increasing with aridity, driven by water stress in the local plant community. In contrast, temperature has a weak and inverse effect on δ13Cshell values in these two regions, suggesting that rainfall amount is the primary control on δ13Cshell for this modern comparison. Furthermore, we present a δ13Cshell record covering the middle Holocene to the middle Pleistocene from the CLP. This record suggests that most of the average δ13Cshell values from interglacial paleosol layers are lower than those from the adjacent glacial loess strata, suggesting higher GSP. However, the reconstructed glacial-interglacial difference in GSP is smaller than that estimated by other proxy indicators. This is likely due to the dietary preference of land snails and/or the sparse nature of this dataset, which might not be sufficient to capture the glacial-interglacial history of rainfall on the CLP. Large datasets with good taxonomic control and high stratigraphic resolution will be required to better describe the glacial-interglacial differences.
