Short-term Climatic Events Research Articles

Holocene climate change in the western part of Taihu Lake region, East China

The Taihu Lake basin is one of the most sensitive areas to climate change in the East Asian summer monsoon (EASM) influenced region. There are significant differences regarding the interpretation of climate history for this area during the Holocene due to a lack of high-resolution paleoclimate records. To better understand this scientific issue, here we provide a new record of Holocene climate change from the western part of Taihu Lake region, East China. A 4-m-depth profile from Gaochun has provided a 13,200-year record of climate change. Holocene climate change was reconstructed for the western part of Taihu Lake area based on various proxy records including pollen, charcoal, degree of humification (DOH), and loss on ignition (LOI) from the Gaochun site. The results indicate that climate during the late Pleistocene (13,200–10,700calyrB.P.) and early Holocene (10,700–8000calyrB.P.) was warm and humid, alternated with several cold and dry intervals. The late Pleistocene and early Holocene was a transitional period from cold-dry to warm-wet climates. The middle Holocene (8000–4400calyrB.P.) was still warm and humid, with a relatively stable climate. The period between 8000 and 5300calyrB.P. was the Holocene climate optimum (HCO). During the late Holocene (4400–0calyrB.P.), the climate was cool but still humid. In addition, climate change at the western part of Taihu Lake during the late Pleistocene and Holocene was unstable, containing several short-term climate events such as Younger Dryas (YD), 8.2ka, and 4.2ka events. These abruptly occurred climate events were indicated by various paleoclimatic proxy indicators, and can be compared with related records from other regions. Our findings suggest that the EASM strength was strongest during the middle Holocene between 8000 and 5300calyrB.P. which can be depicted as the period of HCO.

Lake geochemistry reveals marked environmental change in Southwest China during the Mid Miocene Climatic Optimum

The Mid-Miocene Climatic Optimum (MMCO; ~ 15–17Ma) was one of the short-term climatic warm events that punctuated the Cenozoic long-term cooling trend. Because there are very few terrestrial records of this event, most of our understanding comes from marine cores. In this report, we first present new palaeomagnetic data that revises the dating of our 400m-thick lacustrine section in Wenshan (Yunnan), previously thought to be Late Miocene. These new data suggest an older age, ca. 15.2–16.5Ma, coinciding with the MMCO. We measured δ13C on bulk organic matter (δ13Corg), total organic carbon (TOC), total nitrogen (TN) and C/N ratios at a high sample resolution to: (1) reconstruct the palaeoenvironmental changes in the lake catchment area, and (2) infer mechanisms responsible for these changes. Our results show that all four geochemical parameters demonstrate that a strong environmental change occurred around the middle of the section, shortly after the C5Cn/C5Br geomagnetic reversal and the Early/Middle Miocene boundary at 15.97Ma. We propose that the environmental shift may be due to a combination of a change in climate, which became cooler, together with a change in organic matter cycling within the lake. This study provides a new insight into the MMCO and demonstrates that although the MMCO was generally a warm event, it was also a time of climatic instability and abrupt environmental changes.

Influence of Late Pleistocene and Holocene climate on vegetation distributions in southwest Africa elucidated from sedimentary n-alkanes – Differences between 12°S and 20°S

Global and local climatic forcing, e.g. concentration of atmospheric CO2 or insolation, influence the distribution of C3 and C4 plants in southwest Africa. C4 plants dominate in more arid and warmer areas and are favoured by lower pCO2 levels. Several studies have assessed past and present continental vegetation by the analysis of terrestrial n-alkanes in near-coastal deep sea sediments using single samples or a small number of samples from a given climatic stage. The objectives of this study were to evaluate vegetation changes in southwest Africa with regard to climatic changes during the Late Pleistocene and the Holocene and to elucidate the potential of single sample simplifications. We analysed two sediment cores at high resolution, altogether ca. 240 samples, from the Southeast Atlantic Ocean (20°S and 12°S) covering the time spans of 18 to 1 ka and 56 to 2 ka, respectively. Our results for 20°S showed marginally decreasing C4 plant domination (of ca. 5%) during deglaciation based on average chain length (ACL27–33 values) and carbon isotopic composition of the C31 and C33n-alkanes. Values for single samples from 18 ka and the Holocene overlap and, thus, are not significantly representative of the climatic stages they derive from. In contrast, at 12°S the n-alkane parameters show a clear difference of plant type for the Late Pleistocene (C4 plant domination, 66% C4 on average) and the Holocene (C3 plant domination, 40% C4 on average). During deglaciation vegetation change highly correlates with the increase in pCO2 (r² = 0.91). Short-term climatic events such as Heinrich Stadials or Antarctic warming periods are not reflected by vegetation changes in the catchment area. Instead, smaller vegetation fluctuations during the Late Pleistocene occur in accordance with local variations of insolation.

Holocene Supra-Regional Environmental Changes as Trigger for Major Socio-Cultural Processes in Northeastern Africa and the Sahara

This contribution presents the environmental changes that occurred over the last ten millennia in a vast region of North Africa, encompassing the Central Sahara, the Greater Nile Valley and the Horn of Africa. These areas are of particular significance in the continent because of the early socioeconomic transformations that occurred there. An up-to-date review of palaeoclimate research is proposed with the aim to highlight new theoretical approaches, analytical methods and innovative techniques. Results of recent research on high-resolution and well-dated palaeoenvironmental archives for proxy data have been used to understand the climatic variability at different scales of resolution. We trace the regional changes in Holocene palaeohydrology, mostly regulated by monsoonal precipitation, and their effects on the landscape, and highlight the occurrence of short-term climatic events, arid or humid that may have had disruptive consequences on human communities. The contribution also discusses the cultural dynamics that occurred in those regions because the latter were exploited by hunter/gatherer groups from the onset of the Holocene until historical times. A reconstruction of patterns of human adjustment to climatic variability is here presented, focusing on key processes such as the origin of food production, social complexity and power, and the rise and fall of complex polities and interregional networks.

Deep-sea trace fossil and benthic foraminiferal assemblages across glacial Terminations 1, 2 and 4 at the “Shackleton Site” (IODP Expedition 339, Site U1385)

Numerous studies focused on the transitions between glacial and interglacial periods, the so-called terminations, due to the associated significant reorganizations of the ocean–atmosphere system. However, analyses combining macro- and micropaleontological information are near absent. In this research, an integrative study of trace fossils and benthic foraminiferal assemblages is conducted in order to improve the characterization of Terminations 1, 2 and 4, as revealing the response of the macro-and microbenthic habitats to the involved paleoenvironmental changes. For this purpose, selected cores from Site U1385 (IODP Expedition 339) located off the western Iberian Margin, have been studied. Changes in trace fossils and benthic foraminifera related to both long-term variations at the glacial/interglacial scale, and short-term millennial-scale climatic events. Food and oxygen availability have been identified as the main factors determining variations in the macro- and microbenthic community structure across glacial terminations in the context of changes in water mass distribution and productivity in the NE Atlantic. A deep-sea multi-tiered tracemaker community, consisting of biodeformational structures, Chondrites, ?Nereites, Palaeophycus, Planolites, Thalassinoides, and Zoophycos, suggest generally well-oxygenated bottom and pore-water conditions during interglacial as well as glacial intervals, with punctual decreases in oxygenation. Short-climatic events registered during Terminations 1, 2, and 4 induce a similar response of trace fossil and benthic foraminifera communities to the variable incidence of food and oxygen availability. Termination 1 shows a severe deterioration of oxic conditions and increasing food availability during the YD and HS 1, favoring appearance/dominance of Zoophycos, together with the lowest miliolid and the highest deep infaunal taxa abundances. Short-term climatic events (HS 11, IRE 10.1) associated with Terminations 2 and 4 are characterized by a major incidence of export productivity and accumulation of organic matter respect to depletion oxygenation, especially affecting the microbenthic habitat. Dark sediment intervals of HS 11 and IRE 10.1 are characterized by higher abundances of Zoophycos, together with strong peaks in hBFAR values, significant lows in miliolids and lower abundance of deep infaunal taxa. The presence of Chondrites during IRE 10.1 also indicates the impoverishment in pore-water conditions deep within the sediment.

Sensitivity of vegetation indices and gross primary production of tallgrass prairie to severe drought

Drought affects vegetation photosynthesis and growth. Many studies have used the normalized difference vegetation index (NDVI), which is calculated as the normalized ratio between near infrared and red spectral bands in satellite images, to evaluate the response of vegetation to drought. In this study, we examined the impacts of drought on three vegetation indices (NDVI, enhanced vegetation index, EVI, and land surface water index, LSWI) and CO2 flux from three tallgrass prairie eddy flux tower sites in the U.S. Gross primary production (GPP) was also modeled using a satellite-based Vegetation Photosynthesis Model (VPM), and the modeled GPP (GPPVPM) was compared with the GPP (GPPEC) derived from eddy covariance measurements. Precipitation at two sites in Oklahoma was 30% below the historical mean in both years of the study period (2005–2006), while the site in Illinois did not experience drought in the 2005–2007 study period. The EVI explained the seasonal dynamics of GPP better than did NDVI. The LSWI dropped below zero during severe droughts in the growing season, showing its potential to track drought. The result shows that GPP was more sensitive to drought than were vegetation indices, and EVI and LSWI were more sensitive than NDVI. We developed a modified function (Wscalar), calculated as a function of LSWI, to account for the effect of severe droughts on GPP in VPM. The GPPVPM from the modified VPM accounted for the rapid reduction in GPP during severe droughts and the seasonal dynamics of GPPVPM agreed reasonably well with GPPEC. Our analysis shows that 8-day averaged values (temperature, vapor-pressure deficit) do not reflect the short-term extreme climate events well, suggesting that satellite-based models may need to be run at daily or hourly scales, especially under unfavorable climatic conditions.

Moisture status during a strong El Niño explains a tropical montane cloud forest’s upper limit

Growing evidence suggests short-duration climate events may drive community structure and composition more directly than long-term climate means, particularly at ecotones where taxa are close to their physiological limits. Here we use an empirical habitat model to evaluate the role of microclimate during a strong El Niño in structuring a tropical montane cloud forest's upper limit and composition in Hawai'i. We interpolate climate surfaces, derived from a high-density network of climate stations, to permanent vegetation plots. Climatic predictor variables include (1) total rainfall, (2) mean relative humidity, and (3) mean temperature representing non-El Niño periods and a strong El Niño drought. Habitat models explained species composition within the cloud forest with non-El Niño rainfall; however, the ecotone at the cloud forest's upper limit was modeled with relative humidity during a strong El Niño drought and secondarily with non-El Niño rainfall. This forest ecotone may be particularly responsive to strong, short-duration climate variability because taxa here, particularly the isohydric dominant Metrosideros polymorpha, are near their physiological limits. Overall, this study demonstrates moisture's overarching influence on a tropical montane ecosystem, and suggests that short-term climate events affecting moisture status are particularly relevant at tropical ecotones. This study further suggests that predicting the consequences of climate change here, and perhaps in other tropical montane settings, will rely on the skill and certainty around future climate models of regional rainfall, relative humidity, and El Niño.

Population structure of the purple sea urchin Heliocidaris erythrogramma along a latitudinal gradient in south-west Australia

Sea urchins are key herbivores in many coastal ecosystems. The purple sea urchin, Heliocidaris erythrogramma, is widely distributed across temperate Australia where it exhibits considerable plasticity in feeding behaviour and ecophysiology. In this study we examined H. erythrogramma populations on subtidal reefs along ~4° of latitude in south-west Australia. We used a multi-factorial survey design to assess variability in H. erythrogramma abundances between locations (>200 km part), sites (≥1 km apart) and habitat types (reef flats and slopes). We also examined spatial variability in urchin size, condition (measured by gonad index), and the relative abundances of two co-occurring subspecies. Urchin densities were generally low and did not vary between locations, but did vary between habitat types and amongst sites. Site-level variability in urchin size and condition was also pronounced. The southernmost population comprised smaller individuals and greater relative abundance of the H. e. erythrogramma subspecies, which is abundant on the east coast of Australia. We observed no indication of population-level responses to a recent extreme warming event that impacted the wider ecology of the region, but further investigation into the effects of both gradual warming and short-term climatic events on the ecology of H. erythrogramma and other key herbivores is required.

Vegetation response in the southern Lake Baikal region to abrupt climate events over the past 33 cal kyr

We reconstructed vegetation changes and fire events in the southern Lake Baikal region over the past 33calkyr using a sediment core from the Buguldeika Saddle, southern Lake Baikal, to examine vegetation response to past short-term climate variations. Herbs, such as Artemisia and Asteraceae, were dominant just before the Last Glacial Maximum (LGM). Low pollen accumulation rates (PARs) with little herb pollen indicate sparse herbaceous vegetation cover during the LGM. Picea expanded after the Bølling interval, suggesting expansion of riparian areas, and shrub Alnus rapidly increased during the following Allerød interval. Abies and Pinus subgen. Haploxylon began to increase at the beginning of the Holocene and Pinus subgen. Diploxylon has been highly dominant since the middle Holocene. The vegetation change after the LGM is similar to that from other sites in the Lake Baikal region, but the responses to short-term climate events are pronounced. Fluctuations in herb elements and vegetation cover as indicated by the PAR correspond to the Younger Dryas (YD) cooling event. The decline in the PAR and increase in herb taxa were remarkable at the beginning of the YD, but Larix and Picea trees were distributed in river valleys near Lake Baikal in the latter half. The rapid expansion of Betula and Artemisia following frequent fires in the early Holocene may have been in response to the 8.2calkaBP cooling event. From these findings, the southern Lake Baikal region, including the Selenga River basin, may possibly have been more sensitive than other parts of the region to abrupt climate changes.

Current state of knowledge regarding South America wetlands and their future under global climate change

The exact size of the wetland area of South America is not known but may comprise as much as 20% of the sub-continent, with river floodplains and intermittent interfluvial wetlands as the most prominent types. A few wetland areas have been well studied, whereas little is known about others, including some that are very large. Despite the fact that most South American countries have signed the Ramsar convention, efforts to elaborate basic data have been insufficient, thereby hindering the formulation of a wetland-friendly policy allowing the sustainable management of these areas. Until now, the low population density in many wetland areas has provided a high level of protection; however, the pressure on wetland integrity is increasing, mainly as a result of land reclamation for agriculture and animal ranching, infrastructure building, pollution, mining activities, and the construction of hydroelectric power plants. The Intergovernmental Panel on Climate Change has predicted increasing temperatures, accelerated melting of the glaciers in Patagonia and the Andes, a rise in sea level of 20–60 cm, and an increase in extreme multiannual and short-term climate events (El Niño and La Niña, heavy rains and droughts, heat waves). Precipitation may decrease slightly near the Caribbean coast as well as over large parts of Brazil, Chile, and Patagonia, but increase in Colombia, Ecuador, and Peru, around the equator, and in southeastern South America. Of even greater impact may be a change in rainfall distribution, with precipitation increasing during the rainy season and decreasing during the dry season. There is no doubt that the predicted changes in global climate will strongly affect South American wetlands, mainly those with a low hydrologic buffer capacity. However, for the coming decades, wetland destruction by wetland-unfriendly development planning will by far outweigh the negative impacts of global climate change. South American governments must bear in mind that there are many benefits that wetlands bring about for the landscape and biodiversity as well as for humans. While water availability will be the key problem for the continent’s cities and agroindustries, intact wetlands can play a major role in storing water, buffering river and stream discharges, and recharging subterranean aquifers.

Winter warming pulses affect the development of planted temperate grassland and dwarf-shrub heath communities

Background: Winter conditions are changing considerably due to climate change. Resulting alterations in the frequency of soil freeze–thaw cycles (FTCs) are ecologically important. Aim: We quantified the impact of winter soil-warming pulses on the community structure of temperate plant communities. Methods: The cover of vascular plant species in two vegetation types, each at three diversity levels, was recorded 1 year before to 3 years after an FTC-manipulation that added five additional FTCs. Changes in species abundance patterns (Bray–Curtis similarity) were analysed by linear mixed effect models. Results: Communities exposed to additional FTCs showed less change in their species abundance patterns than the reference plots. Community development in the grassland differed between the FTC-manipulation and the reference plots in the first growing season after the FTC-manipulation, but such effects disappeared over time, whereas the divergence from the reference plots in the dwarf-shrub heath started in the second year after the FTC-manipulation and effects grew over time. Responses to FTCs were related to growth forms: some grasses increased after the FTC-manipulation, whereas the cover of dwarf shrubs was reduced. There was less change in species abundance distributions in the more diverse communities with legumes present. Conclusions: Winter climate change is a critical driver of temperate ecosystems. Short-term climatic events can have long-term implications on the structure of ecosystems. Community composition regulates alterations in the development and competitive balance of plant communities caused by soil warming pulses.

Micro-scale analysis of tree-ring δ 18O and δ 13C on α-cellulose spline reveals high-resolution intra-annual climate variability and tropical cyclone activity

High-resolution analysis of stable carbon and oxygen isotopic composition (δ 13C and δ 18O) of tree-ring α-cellulose has been increasingly in demand for the study of climate seasonality, droughts, and tropical cyclone (hurricane) activity. The peeling–grinding method commonly used for tree-ring α-cellulose preparation is both time-consuming and labor-intensive, and often results in cross-tracheid sampling that masks high-frequency intra-annual climate signals and short-term climate events. Here, we present a new method by extracting tree-ring α-cellulose directly from wholewood spline that omits the peeling–grinding step, and allows for micro-scale δ 13C and δ 18O analysis that facilitates high-resolution seasonal climate study. This ultrasound-assisted non-destructive extraction method has significantly increased the output of tree-ring α-cellulose preparation while retaining the wood cell fabric. Wood cell-wall fibers collected from the α-cellulose spline were directly loaded into pre-weighed silver (or tin) capsules and packed, after oven drying, for stable oxygen (or carbon) isotope analysis. A 3.045-mm annual ring subdivided over 26 sections for δ 13C and δ 18O analysis revealed remarkable intra-annual isotopic variations (1.62‰ for δ 13C, and 5.85‰ for δ 18O). The intra-annual δ 13C values inversely covary with the seasonal precipitation pattern, suggesting moisture regulation on tree-ring δ 13C variations. The high-resolution δ 18O analysis unravels tropical cyclone activities (Hurricanes Frances and Ivan in 2004) that are characterized by an abrupt decline in δ 18O values by 2.6‰ with a “V-shape” pattern in tree-ring latewood. An algorithm is developed for establishing regional patterns of tree-ring seasonal δ 13C and δ 18O for the study of the mean state of climate seasonality and its anomalies.

Hydrological response of an east-Saharan palaeolake (NW Sudan) to early-Holocene climate

We present a palaeolimnological record in western Nubia (NW Sudan, eastern Sahara) isolated from large-scale artesian systems and thus reflecting hydrological changes resulting from local rainfall. Past changes in lake water chemistry and water depth are interpreted by combining results from biological indicators (diatoms, ostracodes and charophytes) with geochemical and stable oxygen and carbon isotope data. Palaeolake Gureinat existed from ~10.9 cal. ka BP until at least 7.1 cal. ka BP and is devided into five lake phases. For the first two millennia the lake was shallow and fluctuating salinity conditions prevailed (500 to 3000 μS/cm). After ~8.7 cal. ka BP, the lake level rose, waters became fresh to oligosaline, and the lake had a minimum depth of at least 8 m. Both transgressive and stable lake phases were punctuated by at least three drops in lake level with complete desiccation at around 10.9 and 8.9 cal. ka BP. The third of these regressions may be coeval with the 8.2 ka climatic event. Sediments younger than 7.1 cal. ka BP representing the mid-Holocene fading of the wet phase were removed by deflation. This record shows that lakes in the eastern Sahara registered short-term climatic events, as long as they were isolated from large-scale artesian groundwater systems. Consequently, even in this continental region, early-Holocene climatic fluctuations affected water levels and salinities of hydrologically isolated lake systems.

Effects of soil freeze–thaw cycles differ between experimental plant communities

Soil freeze–thaw cycles (FTC) influence nutrient cycling, but their consequences for productivity and composition of vegetation are not well investigated. Ongoing global warming will increase the recurrence of FTC in cool-temperate and high-latitude regions. Here, we report on the above- and belowground biomass production as well as the nitrogen nutrition of two common vegetation types, grassland and heath, after more frequent FTC in a controlled field experiment in Central Europe. Furthermore, we analyze the duration of the observed effects. Five FTC were induced by buried heating wires in addition to three naturally occurring FTC during winter 2005/06. More frequent FTC significantly increased aboveground production of experimental grassland early in the following growing season. However, no reaction was found for experimental heath within the first growing season. Biomass production of heath communities dropped significantly and C/N ratio increased in the freeze–thaw treated plots in the second year after the manipulation, whereas production in the grassland communities was no longer affected significantly, except for an increase in C/N ratio. This response can at least partly be explained by changes in nutrient availability, as plant available nitrate increased in the manipulated grassland plots and decreased in the manipulated heath plots. The results show the high ecological importance of climate changes during winter, with the outcomes differing strongly between contrasting vegetation types. Furthermore, we show that short term climatic events can cause long-lasting effects, sometimes emerging in the vegetation only after considerable time lags (here: one growing season). Bodenfrostwechsel (FTC) beeinflussen Nährstoffkreisläufe, aber ihre Bedeutung für die Vegetation ist kaum erforscht. Im Zuge der Globalen Erwärmung werden FTC in kühl-gemäßigten Breiten häufiger auftreten. In dieser Arbeit wird die ober- und unterirdische Biomasseproduktion sowie die Stickstoffernährung von zwei weit verbreiteten Vegetationstypen (Grünland und Zwergstrauchheide) nach vermehrten FTC in einem kontrollierten Freilandexperiment verglichen. Darüber hinaus wird die zeitliche Dauer der beobachteten Effekte evaluiert. Zusätzlich zu drei natürlichen FTC wurden im Winter 2005/06 fünf FTC durch Heizkabel erzeugt. Intensivierung von FTC führte in der folgenden Vegetationsperiode zu signifikant erhöhter oberirdischer Produktion in den Grünlandsystemen. In den Heidesystemen erfolgte keine Reaktion innerhalb des ersten Jahres. Deren Biomasseproduktion verringerte sich allerdings in der zweiten Vegetationsperiode nach der FTC-Manipulation, begleitet von einer Erweiterung im C/N-Verhältnis in der Biomasse. Zu dieser Zeit zeigte sich kein Behandlungseffekt mehr in der Biomasseproduktion der Grünlandsysteme, obwohl deren C/N-Verhätnis signifikant verengt war. Diese Effekte können zumindest teilweise durch Veränderungen in der Nährstoffverfügbarkeit erklärt werden. Die Konzentration von pflanzenverfügbarem Nitrat stieg in den FTC-behandelten Grünlandplots an, während sie in den FTC-behandelten Heideplots abnahm. Die Ergebnisse belegen die hohe ökologische Bedeutung von Klimaveränderungen im Winter, wobei die Folgen sich zwischen den untersuchten Vegetationstypen stark unterschieden. Darüber hinaus konnte gezeigt werden, dass kurze klimatische Ereignisse zu langfristigen Veränderungen in der Vegetation führen können, und dass diese zum Teil erst nach deutlicher Verzögerung auftreten.

Late Holocene monsoonal-climate change inferred from Lakes Ni-no-Megata and San-no-Megata, northeastern Japan

Micro-sedimentological, geochemical data from Lakes Ni-no-Megata and San-no-Megata in northeastern Japan are used to reconstruct environmental and climate changes over the last 2000 years. Comparing these records in two neighboring maar lakes allows reconstruction of centennial scale climate change concerned the East Asian monsoon activities without the influence of human activities. S content and coarse mineral grains records show that long-term climate changes, with one warm/humid interval from AD 1200 to 750, and two cold/dry intervals from AD 1 to 750, AD 1200 to the present. These climate changes have similar trends to Asian monsoon records in China, and could correspond to the Dark Age Cold Period (DACP), the Medieval Warm Period (MWP) and the Little Ice Age (LIA) climate changes in Europe. Moreover, short-term climatic deterioration events occurred in the 6th, 10th and 18th centuries. Particularly the event around the 10th century is well correlated with other paleoclimate proxies in China, Europe and Mesoamerica, suggesting tele-connection by atmospheric circulation through the Northern Hemisphere. However, the record is asynchronous with solar activity. This might suggest the solar forcing has indirectly influenced lake sedimentation associated with Asian monsoon activities in Japan.

Varve counting reveals high resolution radiocarbon reservoir age variations in palaeolake Lisan

AbstractThe laminated lacustrine sediments deposited in the last glacial Lake Lisan represent annual deposits of primary aragonite and silty detritus that reflect the annual supply of bicarbonate‐bearing freshwater to the lake. A varve‐counting curve was constructed for the time interval of ca. 17.4–22 cal. ka BP based on aragonite U/Th, and atmospheric radiocarbon ages of organic debris recovered from the studied section. Radiocarbon in the primary (evaporitic) aragonite comprises both atmospheric and old carbon (reflecting the reservoir age). The aragonite reservoir ages were determined by comparing the aragonite radiocarbon dates to the varve counting curve, and are found to lie in the range 1900–600 a and display a continuous decline. This opens the possibility for high (annual) resolution monitoring of the reservoir age, similar in quality to tree ring counting, during the upper part of Marine Isotope Stage (MIS) 2. Our work also demonstrates that a ‘uniform’ reservoir age correction is inappropriate when determining the chronology of short‐term climate events in lacustrine environments. Copyright © 2009 John Wiley & Sons, Ltd.

Impact of short-term climatic events on latest Pliocene land settings and communities in Central Italy (Upper Valdarno basin)

The Upper Valdarno (Italy) Plio-Pleistocene continental deposits record the latest uniform subtropical humid conditions and the successive climatic signature of glacial/interglacial cycling. The palynological and sedimentological analyses on the Poggio Rosso alluvial plain sediments reveal two major climatic fluctuations correlated to a glacial/interglacial cycle just before the Plio-Pleistocene boundary. The glacial phase, indicated by the expansion of herbs as well as by ephemeral streams associated with calcareous palaeosols, extends from the base of the succession and culminates close to an impressive mammalian bone accumulation dated at 1.87 Ma. The associated increasing aridity had severe consequences on the faunal communities, inducing the migration of open plain dwellers and the trapping of other residents around a few residual shrinking water bodies. A cooperative Pachycrocuta brevirostris clan acted as regulator of game populations debilitated by drought. Previous research indicates that hyenas were also the major bone accumulators. The following moister and warmer interglacial phase ca 1.83 Ma is documented by arboreal taxa and hydromorphic palaeosols in a floodplain crossed by migrating, perennial, sinuous streams. The Climatic Amplitude Method calculated mean annual temperatures around 12.5–14 °C and mean annual precipitation around 800 mm, with a minimum of 400 mm during the glacial phase and 15/16–19 °C and 750–1200 mm during the interglacial phase. The reconstruction reveals higher mean annual temperatures and precipitation during the interglacial as compared to the present-day climate. During the glacial, conditions were similar to today's in all but lower precipitation values were reached during the acme drought phase just above the fossiliferous bed. Poggio Rosso documents the ecological consequences the buildup of the Apennine chain had under the effects of the latest Pliocene global climatic changes. It therefore represents one of the first signals of the patchiness that, since then, gradually increased leading to the eventual habitat fragmentation typical of the late Pleistocene landscapes.

The reef builder gastropod Dendropomapetreaum – A proxy of short and long term climatic events in the Eastern Mediterranean

High-resolution δ 18O and δ 13C records obtained from seven cores were drilled from ledges of the reef builder gastropod Dendropoma petreaum and used to reconstruct variations in the Levantine basin sea surface temperature, hydrology and productivity during the past 500 years. The δ 18O of the aragonite shell of living D . petreaum indicate that skeletal deposition occurs under isotopic equilibrium and faithfully record the temperature and surface water δ 18O during summer and autumn. The mean down core δ 18O record clearly captures global and local climatic events, such as the Little Ice Age (LIA) and the recent warming of surface waters in the Eastern Mediterranean. Comparison to the Western Mediterranean vermetid δ 18O record reveals changes in the freshwater/evaporation budgets of the two basins during cold and warm periods. The Eastern basin had lower surface temperatures and excess evaporation during the LIA and experienced a relatively larger warming and/or a decrease in freshwater/evaporation during the past 70 years. The D . petraeum δ 13C is strongly related to δ 13C of dissolved inorganic carbon and to the primary productivity of the surface water. The mean down core δ 13C record exhibits enrichment during the LIA maximum and a strong depletion trend during the last century. The LIA δ 13C enrichment is attributed to an increase in primary production and high nutrient levels which resulted from increased vertical mixing and upwelling. The last century δ 13C depletion is mostly related to the increased anthropogenic emissions of 13C depleted carbon dioxide and to a certain decrease in primary production. The data indicate that D. petraeum isotopic signatures are unique proxies for last 500 years high-resolution reconstruction of paleo-oceanographic environments in the Mediterranean and potentially in the sub-tropical Atlantic regions.

Climate dynamics in the borderlands of the Aegean Sea during formation of sapropel S1 deduced from a marine pollen record

To unravel the climatic and environmental dynamics in the borderlands of the Aegean Sea during the early and middle Holocene, and notably for the interval of sapropel S1 (S1) formation, we have analysed terrestrial palynomorphs from a marine core in the northern Aegean Sea. The qualitative results were complemented by quantitative pollen-based climate reconstructions. A land–sea correlation was established based on pollen data and sediment lightness measurements from the same core, and previously published benthic foraminifer data from a nearby core.The borderlands of the Aegean Sea underwent a transition from an open vegetation to oak-dominated woodlands between ∼10.4 and ∼9.5kacalBP. A coeval increase in winter precipitation suggests that moisture availability was the main factor controlling Holocene reforestation. The ∼50% higher winter precipitation during S1 formation relative to “pre-sapropelic” conditions suggests a strong contribution from the borderlands of the Aegean Sea to the freshwater surplus during S1 formation. The humid and mild winter conditions during S1 formation were repeatedly punctuated by short-term climatic events that caused a partial deforestation and a reorganisation within the broad-leaved arboreal vegetation. In the marine realm, these events are documented by improved benthic oxygenation. The strongest event represents the regional expression of the 8.2ka cold event and led to an interruption in S1 formation. Except for the interval of S1 formation, the pollen-derived winter temperatures correlate with the smoothed GISP2 K+ series. They support the previously published, marine-based concept that the intensity of the Siberian High strongly controlled the winter climate in the Aegean region. During S1 formation in the Aegean Sea, however, climate conditions in the borderlands were more strongly affected by the monsoonally influenced climate system of the lower latitudes.

炭素循環から考える新生代のグローバル気候変動

The global carbon cycle controls the climate change in the Earth's environment on a geological timescale and is mainly associated with greenhouse effects produced by atmospheric carbon dioxide (CO2) and methane (CH4). This paper reviews the relationship between the global carbon cycle and presumed climate events during the Cenozoic. The global carbon cycle is primarily regulated by the balance between weathering and metamorphism-volcanism. Moreover, the organic carbon subcycle involving oxidative weathering and burial is of secondary importance. The balance of these geochemical processes results in variations of atmospheric CO2. The past climate on a geological time scale is reconstructed by several geochemical and paleontological methods or proxies. For example, sea-surface and deep-water temperature are deduced from oxygen isotope ratio and Mg/Ca ratio of foraminiferal tests. Terrestrial atmospheric temperature is estimated from leaf fossil and paleovegetation. Atmospheric CO2 level is calculated from carbon isotope ratios of phytoplankton and soil carbonate, stomatal density of leaf fossil, boron isotope ratio of foraminiferal test, Ce anomaly, and global carbon cycle modeling. It is important to consider their advantages and disadvantages in order to evaluate the paleoclimate adequately. Next, we discuss climate change based on these proxies. As a general trend, the Cenozoic climate change is characterized by a transition from ice-free to ice-covered conditions across the Eocene/Oligocene boundary. The Earth's surface environment was significantly warmed from the Paleocene to the Eocene by high levels of atmospheric CO2. Thereafter, it gradually cooled towards the present, which is possibly attributed to changes in ocean currents and other marine environments accompanying continental drift. This trend has been punctuated by several short-term climate events. The Paleocene-Eocene Thermal Maximum (PETM) was a remarkable warming event at the Paleocene/Eocene boundary, possibly attributed to the release of methane from hydrates into the atmosphere. Rapid cooling occurred at the Eocene/Oligocene boundary to form extensive continental ice sheets including the Antarctica, which seems to have been caused by atmospheric CO2 and change of oceanographic circulation and marine environment. After a moderate period from the late Oligocene to the early Miocene, there was a transient but significant warming in the middle Miocene. Since then, the Earth's environment has gradually cooled towards the present accompanied by the evolution of glaciations and marine environmental changes but a causal link between cooling and global carbon cycle has recently been pointed out. Although the carbon cycle including atmospheric CO2 and CH4 cannot explain all of the global climate changes in Cenozoic, it has undoubtedly played a dominant role on the Earth's climate.