Resilience capacity of Araucaria araucana to extreme drought events

Ongoing climate change has induced modification in the frequency and intensity of extreme climatic events, with consequent impact on tree and forest growth resilience. Araucaria araucana is an endangered Patagonian conifer, which provides several ecosystem services to local human societies and plays fundamental ecological roles in natural communities. These woodlands have historically suffered different types of anthropogenic disturbance, such as fire, logging and grazing, nevertheless the species resilience to extreme drought events remains still poorly understood. To fill this gap of knowledge, we applied dendrochronological methods to several A. araucana stands distributed along a steep bioclimatic gradient in order to reconstruct resilience capacity, in term of stem growth resistance and recovery, to three successive extreme spring-early summer droughts which occurred during the 20th century. Results showed an increase in the species recovery along the considered dry spells, whereas no clear trend emerged for resistance, suggesting no cumulative effect of drought upon resilience. Both resistance and recovery presented different values depending on bioclimatic settings, being xeric stands more sensitive to extreme episodes with respect to mesic woodlands, particularly during the more recent drought event when trees growing in drier environments were not able to reach pre-drought stem growth rates. Tree-level characteristics, such as age and growth trends prior to drought, modulated the species resilience, suggesting that future dry spells would possibly induce shifts in population dynamics, and furthermore be detrimental for fast-growing trees. Our analysis highlighted the response of a key Patagonian tree species to extreme drought events, providing bioclimatic-specific useful information for conservation plans of this natural resource.     

沈阳市区古油松年轮宽度年表的建立

以沈阳城区昭陵古油松为样本,建立了古油松标准化年表、差值年表和自回归年表．结果表明,年表与沈阳1月和4月的极端最低气温显著相关;与2月的降水量、年均水汽压分别为显著和极显著相关,年表对4、5、9和10月的水汽压响应较强,且均与自回归年表显著相关．年表与相对湿度的年指标,4、5、6、9、10和11月月指标的相关性较高,其中与年值和5月值分别为显著相关（差值年表除外）和极显著相关．蒸发与油松的生长在全年和绝大部分月份呈负相关,其中5月最明显,而1月的蒸发量与油松年表呈正相关．年表的窄化突变佐证了1700年以来32次历史资料记录的主要旱灾年．沈阳地区的油松生长也受全球或半球尺度温度波动的影响．3种年表对以往太阳黑子的变化和地磁的活动呈现明显负相关,其中与太阳黑子活动存在显著的11年、23年和50年左右的公共周期,与地磁指标在10．5年、20年和45年左右存在共同的周期变化．     

Large-scale,millennial-length temperature reconstructions from tree-rings

Over the past two decades, the dendroclimate community has produced various annually resolved, warm season temperature reconstructions for the extratropical Northern Hemisphere. Here we compare these tree-ring based reconstructions back to 831 CE and present a set of basic metrics to provide guidance for non-specialists on their interpretation and use. We specifically draw attention to (i) the imbalance between (numerous) short and (few) long site chronologies incorporated into the hemispheric means, (ii) the beneficial effects of including maximum latewood density chronologies in the recently published reconstructions, (iii) a decrease in reconstruction covariance prior to 1400 CE, and (iv) the varying amplitudes and trends of reconstructed temperatures over the past 1100 years. Whereas the reconstructions agree on several important features, such as warmth during medieval times and cooler temperatures in the 17th and 19th centuries, they still exhibit substantial differences during 13th and 14th centuries. We caution users who might consider combining the reconstructions through simple averaging that all reconstructions share some of the same underlying tree-ring data, and provide four recommendations to guide future efforts to better understand past millennium temperature variability.     

Microdensitometric records from humid subtropical China show distinct climate signals in earlywood and latewood

Blue intensity (BI) from tree rings is a technique that has been widely explored for temperature reconstruction purposes in middle and high latitudes. However, it is still rather untested at lower latitudes and in drier climates, particularly in subtropical areas. Here, we develop the first series of BI-based tree-ring parameters (earlywood BI, EWBI; latewood BI, LWBI and ΔBI, the difference between LWBI and EWBI) in humid subtropical China from the species Pinus massiniana. Although the BI parameters have weaker inter-series correlations than do ring widths, they are generally better correlated with climate parameters. Our study shows a positive temperature response in the EWBI parameter and negative responses in the LWBI and ΔBI parameters. Interestingly, the correlation pattern is almost the opposite of that observed at high latitudes, where there is a pronounced positive sensitivity of the LWBI/ΔBI/MXBI parameters to temperature.We find the EWBI to be the most robust parameter for reconstruction purposes. The positive March–May average temperature signal of EWBI is stable across frequencies and shows consistent interdecadal variations with other temperature proxy series from the region. The compilation of new tree-ring records using the BI technique will ultimately support our understanding of climate history. For this reason, we encourage similar attempts to push the boundaries of the BI technique even further.     

Development of tree-ring width chronologies and tree-growth response to climate in the mountains surrounding the Issyk-Kul Lake,Central Asia

Three tree-ring width chronologies were developed from 75 Picea schrenkiana trees ranging from low- to high-elevation in the mountains surrounding the Issyk-Kul Lake, Northeast Kyrgyzstan. The reliable chronologies extend back to the mid-18th and late-19th centuries. Spatial correlation analysis indicates that the chronologies for the relatively high-elevation trees contain large-scale climatic signals, while the chronology at relatively low elevation may reflect the local climate variability. The results of the response of tree growth to climate show that these chronologies contain an annual precipitation signal. Furthermore, the influence of temperature indicates mainly moisture stress that is enhanced with rising elevation. The tree-ring records also captured a wetting trend in eastern Central Asia over the past decades. These new tree-ring width chronologies provide reliable proxies of precipitation variability in Central Asia and contribute to the International Tree-Ring Data Bank.     

Pre-1930 unstable relationship between climate and tree-ring width of Pinus taiwanensis hayata in southeastern China

Although it has been widely recognized that tree-ring response to climate drivers may change over recent decades, often due to anthropogenic environment changes, there are fewer reports of such changes in earlier pre-warming periods. In this paper we report on the pre-1930 unstable relationship between climate and tree-ring width (TRW) of Pinus taiwanensis Hayata in southeastern China based on reliable long-term temperature data. TRW of P. taiwanensis is strongly controlled by temperatures in two seasons, previous spring to summer (March to August, mainly June to July) and previous winter to current spring (December to March). However, TRW are insensitive to previous spring to summer temperature between 1890 and 1930. Reduced summer temperature variability, changing regimes of spring-summer temperature and precipitation, and complicated tree physiological processes behind the complex growth-climate relationship are the more likely causes of this phenomenon. This study adds to the body of knowledge that lower climate sensitivity of tree rings is not specific to the most recent decades.     

Investigating the potential for Southern Hemisphere climate reconstruction using stable isotopes in Tasmanian tree rings

Few annually dated stable isotope records exist across Oceania. Stable carbon and oxygen isotope ratios have the potential to enhance climate reconstructions currently reliant on tree ring width chronologies. The purpose of this study is to explore the sources of variability in a stable oxygen isotope chronology derived from A. selaginoides from Mount Read, Tasmania. This high elevation site receives abundant rainfall throughout the year and is ∼130 km from the Global Network of Isotopes in Precipitation (GNIP) site at Cape Grim. We crossdated 10 new tree core samples against an existing ring width chronology (954–2011 CE) and analyzed the δ¹⁸O from the individual rings for the period 1960–2018. Using high resolution (0.25 degrees) climate data and ECMWF ERA5 reanalysis data, we disentangled the effects of local climate and source region on the isotopic signatures recorded in the annual rings. In addition, we used HYSPLIT backward trajectory analysis to characterize the source region of precipitation to Mount Read and whether the source region has influence over the δ¹⁸O_TR series. Median δ¹⁸O_TR (n = 10) is correlated with local temperature and vapor pressure deficit in the early growing season. In addition, spatial correlations reveal that median δ¹⁸O_TR is positively correlated with temperature and negatively correlated with precipitation in the source region. However, measurements of δ¹⁸O_TR exhibit high inter-tree variation, particularly between 1960 and 1990. Our results indicate that this δ¹⁸O_TR proxy may provide additional information about past moisture conditions during the growing season, potentially contributing to more robust reconstructions of the Southern Hemisphere climate dynamics; however, additional sampling may be necessary to resolve inter-tree variation in δ¹⁸O_TR.     

Different climate responses of spruce and pine growth in Northern European Russia

We report new data on tree-ring growth in northern European Russia, a region with a hitherto relatively sparse tree-ring network. We explore its associations with climate variability. Areas, sampling locations and trees were selected for representativeness rather than climate sensitivity. Using tree rings from 651 conifers from six widely dispersed areas we show strong intercorrelation between trees within each major conifer species within and between areas. Regional composite tree-ring series for spruce and pine contain a major fraction of decadal and multidecadal variability. The most likely driver of this common variability is interannual to multidecadal climate variability. Gridded monthly instrumental climate data for the period 1902–2008, particularly mean temperature and total precipitation, were tested as predictors of each local species-specific tree-ring site chronology. The most consistent pattern emerged for spruce at all but the southernmost area. Cool and moist summers the year before growth were consistent drivers of spruce ring growth throughout the period, with no change in recent decades. Self-calibrating Palmer Drought Severity Index for prior summer was also a strong and consistent driver of spruce ring growth. For pine, there was a weaker but similarly stable association between larger rings and warm, moist conditions, in this case in the current summer. These associations were also identified at multidecadal time scales, particularly for spruce. On the other hand, the specific role of moisture variability in determining interannual to multidecadal variability in tree growth in this high latitude region raises questions about the relative vulnerability of spruce and pine there under global warming.     

The potential of generalized additive modelling for the prediction of radial growth of Norway spruce from Central Germany

During the past decades managed forest ecosystems in Central Europe underwent vast changes, induced by extreme climate conditions and occasionally adverse forest management. Tree ring width patterns mirror these changes and thus have been widely examined as environmental archives and reliable empirical data sources in ‘tree growth modelling’. Dendrochronologists often suppose linear co-variation among the covariates, variable independence and homoscedasticity. Conventionally, these assumptions were achieved by eliminating biological age trends (detrending) and removing the autocorrelation from the time series (pre-whitening). Particularly detrending might be biased according to the scientific problem and sometimes inflexible age models. In this study, we tackle these issues and examine the suitability of a flexible Generalized Additive Model (GAM) on recently developed tree ring width time series of 30 Norway spruce stands (Picea abies [L.] H. Karst) from Central Germany.The model was established to simultaneously cope with the mentioned detrending issue, to unravel nonlinear climate-growth relationships and to predict mean ring width time series for spruce stands in the region. Particularly the latter was of primary interest, since recent forest planning relies on static yield tables that often underestimate the actual growth.The model reliably captured the empirical data, indicated by a small Generalized Cross Validation criterion (GCV = 0.045) and a deviance explained of 88.6 %. The flexible additive smoothers accounted for the social status of individual trees, captured low frequency variations of changing growth conditions adequately and displayed a rather flat biological age trend. The radial increment responded positively to summer season precipitation of the current and previous year. Positive temperature responses were found during the early vegetation period, whereas high summer season temperatures negatively affected the radial growth. The seasonal transition from spring to summer in June induced a shift in the climate response of the linear predictor, leading to a distinct negative effect of temperature and a no-role of precipitation on the linear predictor.Most important, utilizing the calibrated GAM for the purely climate-driven prediction of mean ring width time series from five independent spruce sites revealed proper coherencies. Herein, the mean ring width for sites located within the climatic-optimum for spruce growth were more exactly predicted than for sites with adverse spruce growth conditions. In addition, large mean ring widths were systematically underestimated, whereas small mean ring widths were precisely predicted. Overall, we strongly recommend GAMs as a powerful tool for the investigation of nonlinear climate-growth relationships and for the prediction of radial growth in managed forest ecosystems.     

VS-oscilloscope: A new tool to parameterize tree radial growth based on climate conditions

It is generally assumed in dendroecological studies that annual tree-ring growth is adequately determined by a linear function of local or regional precipitation and temperature with a set of coefficients that are temporally invariant. However, various researchers have maintained that tree-ring records are the result of multivariate, often nonlinear biological and physical processes. To describe critical processes linking climate variables with tree-ring formation, the process-based tree-ring Vaganov–Shashkin model (VS-model) was successfully used. However, the VS-model is a complex tool requiring a considerable number of model parameters that should be re-estimated for each forest stand. Here we present a new visual approach of process-based tree-ring model parameterization (the so-called VS-oscilloscope) which allows the simulation of tree-ring growth and can be easily used by researchers and students. The VS-oscilloscope was tested on tree-ring data for two species (Larix gmeliniiand Picea obovata) growing in the permafrost zone of Central Siberia. The parameterization of the VS-model provided highly significant positive correlations (p < 0.0001) between simulated growth curves and original tree-ring chronologies for the period 1950–2009. The model outputs have shown differences in seasonal tree-ring growth between species that were well supported by the field observations. To better understand seasonal tree-ring growth and to verify the VS-model findings, a multi-year natural field study is needed, including seasonal observation of the thermo-hydrological regime of the soil, duration and rate of tracheid development, as well as measurements of their anatomical features.     

Spatial differences in the radial growth responses of black locust (Robinia pseudoacacia Linn.) to climate on the Loess Plateau,China

The black locust (Robinia pseudoacacia Linn.) is the dominant tree species in the “grain for green” project on the Loess Plateau (LP) of China, and brings many ecological benefits to this planted region. However, there are concerns regarding its suitability as a plantation forest species in different regions of the LP. We used a dendroclimatological approach to investigate the radial growth response of black locust to varying climate in two sites on the LP with differing precipitation gradients. We took tree-ring samples from black locust in Yongshou County (in the semi-humid southern LP) and Shenmu County (in the semi-arid northern LP), and developed tree-ring width (TRW) chronologies for each. We performed moving correlation analyses between TRW chronologies and aggregated thermal (maximum temperature (TMX), minimum temperature (TMN), mean temperature (TMP)) and hydroclimatic factors (precipitation, self-calibrated Drought Severity Index (scPDSI), and humidity). The results demonstrated the increased influence of thermal factors during autumn, and the generally decreased influence of hydroclimatic factors on black locust radial growth in Yongshou, compared with the decreasing influence of thermal factors (during all seasons) and increasing influence of hydroclimatic factors (precipitation during summer, scPDSI and humidity during autumn) on black locust radial growth in Shenmu. The results indicated that black locust radial growth might benefit from the current climatic conditions in the southern LP. However, black locust radial growth stressed by water availability in the northern LP, which may reduce its vitality and productivity as climate warms in the future. These results have implications for regional forestry planning and ecological restoration strategies on the LP.     

Prominent role of volcanism in Common Era climate variability and human history

Climate reconstructions for the Common Era are compromised by the paucity of annually-resolved and absolutely-dated proxy records prior to medieval times. Where reconstructions are based on combinations of different climate archive types (of varying spatiotemporal resolution, dating uncertainty, record length and predictive skill), it is challenging to estimate past amplitude ranges, disentangle the relative roles of natural and anthropogenic forcing, or probe deeper interrelationships between climate variability and human history. Here, we compile and analyse updated versions of all the existing summer temperature sensitive tree-ring width chronologies from the Northern Hemisphere that span the entire Common Era. We apply a novel ensemble approach to reconstruct extra-tropical summer temperatures from 1 to 2010 CE, and calculate uncertainties at continental to hemispheric scales. Peak warming in the 280s, 990s and 1020s, when volcanic forcing was low, was comparable to modern conditions until 2010 CE. The lowest June–August temperature anomaly in 536 not only marks the beginning of the coldest decade, but also defines the onset of the Late Antique Little Ice Age (LALIA). While prolonged warmth during Roman and medieval times roughly coincides with the tendency towards societal prosperity across much of the North Atlantic/European sector and East Asia, major episodes of volcanically-forced summer cooling often presaged widespread famines, plague outbreaks and political upheavals. Our study reveals a larger amplitude of spatially synchronized summer temperature variation during the first millennium of the Common Era than previously recognised.     

Tree age and bark thickness as traits linked to frost ring probability on Araucaria araucana trees in northern Patagonia

Frost events may damage the cambium and consequently the newly produced tracheids whose cell walls have not yet completed their lignifications, leading to the formation of frost rings. This study deals with the presence of frost rings in Araucaria araucana trees according to cambial age and bark thickness, under the assumption that these factors may be involved in physical or physiological mechanisms that increase resistance to freezing temperatures that impact the cambial tissue. The study was conducted in northern Patagonia at two sites of contrasting geomorphology, and therefore potentially associated with a differential degree of exposure to extreme cold. Wood plus bark cores were extracted from main stems at two heights from the ground and from each of the four cardinal point directions for 30 individuals per site. A Linear Mixed Model and a Generalized Linear Mixed Model were applied in order to relate the bark thickness and the frequency of frost rings in accordance with the different sampling points on the stem. It was observed that as bark becomes thicker with cambial age, the frequency of frost rings decreases, indicating a possible thermal-induced mechanism of bark protection. Consequently, there is an increase in the presence of frost rings at the younger stages of tree life. Although the mechanisms of cold hardiness in trees can be complex, including aspects of the tree physiology, our data indicated that as tree age increases, the thickness of the bark is higher, resulting in a potential effect of isolation and passive protection against the harmful effects of frosts. This mechanism may be relevant in the ecology, conservation and management of forests faced with extreme variability in future climate and changing scenarios.     

Challenges for growth of beech and co-occurring conifers in a changing climate context

Given recent climatic trends, it is crucial to understand the plasticity and adaptiveness of forest trees in order to evaluate their current and future responses to changing climatic conditions. We investigated inter- and intra-annual xylem growth and its relation to weather factors in European beech (Fagus sylvatica) and a co-occurring conifer, either Norway spruce (Picea abies) or Scots pine (Pinus sylvestris), at five sites with different elevations and climatic conditions, in Spain, Slovenia and the Czech Republic. The selected sites were located in central and marginal parts across the distribution range of the investigated species. The results showed that climate-growth relationships vary between different species at the same site and within the same species at different sites, indicating diverse survival strategies among the observed species but also high plastic capacity within the same species. Moreover, xylogenesis analysis revealed the capacity of trees to adapt the beginning and cessation of wood formation depending on conditions. In case of beech, shortening growth duration in dryer and warmer environments and the opposite in the case of conifers, with which the growing period was extended in such conditions. Consequently, the plasticity capacity of beech was limited due to short growth duration, while conifers (especially pine) proved to be able to compensate climatic constrictions by growing over a longer period and becoming more adapted to survival in drought-prone environments.     

Increasing temperature sensitivity caused by climate warming,evidence from Northeastern China

The “Divergence Problem” in northern forests has been confirmed in a large number of empirical studies, especially in North America and Europe, climate warming having been identified as a cause for reduced sensitivity of recent tree-growth and increased tree mortality. However, according to other studies, tree growth patterns are keeping pace with climate warming. Covariation between rising temperatures and tree growth varies regionally. Therefore, extensive evidence is still needed across more geographic areas around the world. In the present study, we examined the sensitivity of Manchurian ash forest growth, which is one of the dominant species in the mixed coniferous and broad-leaved forests in the area around Changbai Mountain in Northeastern China. Five Manchurian ash tree-ring width chronologies were constructed from sites ranging along the elevational gradients of 750 m, 800 m, 900 m, 1000 m and 1100 m. We analyzed climate-growth relationships using Pearson correlation coefficients between ring-width indices and climate variables in two separate periods (before 1984 and after 1984), because instrumental temperature data have increased sharply after 1984. Along all of the elevational gradients, the sampled Manchurian ash forests show a higher growth rate and more sensitivity to climatic factors due to climate warming since the beginning of the 1984s. Comparatively, the forest growth at low elevation sites has increased faster than that at high elevation sites. If climate warming continues in northeastern China, further continuous and substantial increase in tree growth would substantially raise forest productivity in mixed coniferous and broad-leaved forests.     

Simulated and predicted responses of tree stem radial growth to climate change—A case study in semi-arid north central China

Precise knowledge how tree growth will respond to future climate change is essential for the adapted management of forest ecosystems. By conducting sensitivity tests, tree-ring process-based cambial growth models can provide an innovative way to better understand wood formation under different climate change scenarios. As a case study in semi-arid north central China, we used artificially increased or decreased daily climatic data as input to the Vaganov-Shashkin dynamic growth model to investigate the response of wood formation to climatic change. By calibrating the tree-ring model using daily climate data over the period 1951–2010, we found that 81% of radial growth was driven by soil moisture, while 13% of growth was controlled by temperature. During the main growing season June–August, significant differences in the integral growth rate occurred after changing precipitation by ± 30% or by decreasing temperature by 3.0 °C (p < 0.05). However, increasing temperature showed only modest effects on tree radial growth rate. During the past 60 years, a significant advancement of the starting dates of growth was detected, whereby non-significant variability was found for the ending dates of growth. Contemporaneously, the effect of previous winter temperature (previous December to current January) on cambial growth initiation declined after 1980. Significant differences in the growth onset dates only occurred when temperature was reduced by 4.5 °C or increased by 5.5 °C. Moreover, both the onset and ending dates of growth in the study region were more sensitive to cooling rather than to warming. If temperature will increase by 2°C and precipitation will increase by 30% at the end of this century as predicted by some Earth system models, tree radial growth might increase by 19% in the study region, compared to the average during the period 1952–2010. Consequently, tree stem radial growth is expected to increase under a warming and wetting climatic scenario, but will decrease under drying conditions.     

A Norway spruce tree-ring width chronology for the Common Era from the Central Scandinavian Mountains

Fennoscandia is one of the most prominent regions in the world for dendroclimatological research. Yet, millennium-long tree-ring chronologies in this region have mainly been developed from Scots pine (Pinus sylvestris L.). To explore the possibility of building long-term chronologies using other dominating tree species in the region, this paper presents the first two millennia-long Norway spruce (Picea abies (L.) Karst.) ring-width chronology from Northern Europe. The chronology is composed of living trees and subfossil wood and covers the period from BCE 115 to 2012 CE. A sufficiently replicated and robust chronology is built for the past 360 years back to 1649 CE. Further back in time, the common growth signal is reduced, and hence the reliability of the earlier section of the chronology is lower. The climate calibration results show that the spruce ring-width correlation with June-July mean temperatures over the period 1901–2012 is positive and significant (r = 0.6, p < 0.01) and representing the temperature variability of a spatial domain covering west-central Scandinavia. These results show the ability of Norway spruce to serve as a proxy for paleoclimatic research and the possibility of extending the chronology far back in time in the region, and therefore present an opportunity for carrying out new inter-and intraregional proxy analyses.     

Medieval oak chronology from Klaipėda,Lithuania

Klaipėda is a town in western Lithuania on the Baltic Sea coast. Oak timber was actively used to construct buildings in this town until the 17th century. Archaeological investigations began in Klaipėda’s old town in 1979, but it has not been possible to use oak timbers for dating due to the lack of regional chronologies. The goal of this study was to fill this gap by developing a well-replicated oak chronology from timber collected in Klaipėda between 1979 and 1987. The resulting oak chronology from Klaipėda spans 306 years from 1247 to 1552 and includes 62 oak timber cross-sections. The study suggests that the timber was felled in the same local woodland, and that tree-ring widths series from Klaipėda are temperature-sensitive. The negative pointer years mostly coincide with negative anomalies of reconstructed April‒September temperature from elsewhere in Europe. The constructed chronology was compared with local chronologies (Vilquro, Smarhoń, Gdańsk) and with oak chronologies made from imported oak timber of southeast Baltic origin (Baltic 1–3, Dutch). We assessed whether it is possible to determine the origins of the Baltic 1, Baltic 3 and Dutch oak chronologies compiled from imported timber. Based on available chronologies, it could be hypothesised that Baltic 1 and Dutch chronologies are originated from western Lithuania and Baltic 3 from eastern Lithuania.     

Growth coherency and climate sensitivity of Larix sibirica at the upper treeline in the Russian Altai-Sayan Mountains

Tree-ring research in the Altai-Sayan Mountains so far only considered a limited number of well-replicated site chronologies. The dendroecological and palaeoclimatological potential and limitations of large parts of south-central Russia therefore remain rather unexplored. Here, we present a newly updated network of 13 larch (Larix sibirica Ldb.) tree-ring width (TRW) chronologies from mid to higher elevations along a nearly 1000 km west-to-east transect across the greater Altai-Sayan region. All data were sampled between 2009 and 2014. The corresponding site chronologies cover periods from 440 to 860 years. The highest TRW agreement is found between chronologies ≥2200 m asl, whereas the material from lower elevations reveals overall less synchronized interannual to longer-term growth variability. While fluctuations in average June–July temperature predominantly contribute to the growth at higher elevations, arid air masses from Mongolia mainly affect TRW formation at lower elevations. Our results are indicative for the dendroclimatological potential of the Altai-Sayan Mountains, where both, variation in summer temperature and hydroclimate can be robustly reconstructed back in time. These findings are valid for a huge region in central Asia where reliable meteorological observations are spatially scarce and temporally restricted to the second half of the 20th century. The development of new high-resolution climate reconstruction over several centuries to millennia will further appear beneficial for timely endeavors at the interface of archaeology, climatology and history.     

Early Cretaceous (Valanginian - Hauterivian) calcareous nannofossils and isotopes of the northern hemisphere: proxies for the understanding of Cretaceous climate

From three boreholes (DSDP Site 535; ODP Site 638; BGS borehole 81/43) of the Central Atlantic and the North Sea Basin 379 samples of early Cretaceous age (Valanginian-Hauterivian) were examined. The localities cover a S-N transect of approximately 3000 km stretching from 17°N to 40°N palaeolatitude. The distribution of calcareous nannofossils and fluctuations of the stable isotopes (δ¹³C, δ¹⁸O) have been recorded and were compared with results of recent studies. We differentiate between high nutrient indicators and oligotrophic taxa and propose a four step scheme to characterize the trophic level of the surface water. (1) High abundances of the fertility group (Biscutum constans/Zeugrhabdotus spp.) combined with a high dominance of B. constans and low abundances of Watznaueria barnesae/W. fossacincta represent a high nutrient environment (eutrophic setting). (2) High abundances of the fertility group combined with a high dominance of Zeugrhabdotus spp. and low abundances of W. barnesae/W. fossacincta reflect enhanced nutrient contents of the surface water (mesotrophic setting). (3) Enhanced abundances of the fertility group combined with high abundances of W. barnesae/W. fossacincta indicate slightly increased nutrient contents of the surface water (meso- to oligotrophic setting). (4) Low abundances of the fertility group and high abundances of W. barnesae/W. fossacincta are of low nutrient affinities (oligotrophic setting). Our estimations of seawater palaeotemperatures in combination with literature data show a distinctive trend for the Valanginian to Hauterivian interval. A general decrease of water temperature from the Valanginian to the early Hauterivian is obvious. This decrease of temperature coincides with the southward migration of the high latitudinal cold water species Crucibiscutum salebrosum to lower latitudes. Our findings shed new light on the evolution of the earliest Cretaceous climate, which may be characterized as a warm greenhouse world with interludes of short cooling.