Showing papers on "Holocene climatic optimum published in 2013"

North-south palaeohydrological contrasts in the central Mediterranean during the Holocene: tentative synthesis and working hypotheses

Abstract: . On the basis of a multi-proxy approach and a strategy combining lacustrine and marine records along a north–south transect, data collected in the central Mediterranean within the framework of a collaborative project have led to reconstruction of high-resolution and well-dated palaeohydrological records and to assessment of their spatial and temporal coherency. Contrasting patterns of palaeohydrological changes have been evidenced in the central Mediterranean: south (north) of around 40° N of latitude, the middle part of the Holocene was characterised by lake-level maxima (minima), during an interval dated to ca. 10 300–4500 cal BP to the south and 9000–4500 cal BP to the north. Available data suggest that these contrasting palaeohydrological patterns operated throughout the Holocene, both on millennial and centennial scales. Regarding precipitation seasonality, maximum humidity in the central Mediterranean during the middle part of the Holocene was characterised by humid winters and dry summers north of ca. 40° N, and humid winters and summers south of ca. 40° N. This may explain an apparent conflict between palaeoclimatic records depending on the proxies used for reconstruction as well as the synchronous expansion of tree species taxa with contrasting climatic requirements. In addition, south of ca. 40° N, the first millennium of the Holocene was characterised by very dry climatic conditions not only in the eastern, but also in the central- and the western Mediterranean zones as reflected by low lake levels and delayed reforestation. These results suggest that, in addition to the influence of the Nile discharge reinforced by the African monsoon, the deposition of Sapropel 1 has been favoured (1) by an increase in winter precipitation in the northern Mediterranean borderlands, and (2) by an increase in winter and summer precipitation in the southern Mediterranean area. The climate reversal following the Holocene climate optimum appears to have been punctuated by two major climate changes around 7500 and 4500 cal BP. In the central Mediterranean, the Holocene palaeohydrological changes developed in response to a combination of orbital, ice-sheet and solar forcing factors. The maximum humidity interval in the south-central Mediterranean started ca. 10 300 cal BP, in correlation with the decline (1) of the possible blocking effects of the North Atlantic anticyclone linked to maximum insolation, and/or (2) of the influence of the remnant ice sheets and fresh water forcing in the North Atlantic Ocean. In the north-central Mediterranean, the lake-level minimum interval began only around 9000 cal BP when the Fennoscandian ice sheet disappeared and a prevailing positive NAO-(North Atlantic Oscillation) type circulation developed in the North Atlantic area. The major palaeohydrological oscillation around 4500–4000 cal BP may be a non-linear response to the gradual decrease in insolation, with additional key seasonal and interhemispheric changes. On a centennial scale, the successive climatic events which punctuated the entire Holocene in the central Mediterranean coincided with cooling events associated with deglacial outbursts in the North Atlantic area and decreases in solar activity during the interval 11 700–7000 cal BP, and to a possible combination of NAO-type circulation and solar forcing since ca. 7000 cal BP onwards. Thus, regarding the centennial-scale climatic oscillations, the Mediterranean Basin appears to have been strongly linked to the North Atlantic area and affected by solar activity over the entire Holocene. In addition to model experiments, a better understanding of forcing factors and past atmospheric circulation patterns behind the Holocene palaeohydrological changes in the Mediterranean area will require further investigation to establish additional high-resolution and well-dated records in selected locations around the Mediterranean Basin and in adjacent regions. Special attention should be paid to greater precision in the reconstruction, on millennial and centennial timescales, of changes in the latitudinal location of the limit between the northern and southern palaeohydrological Mediterranean sectors, depending on (1) the intensity and/or characteristics of climatic periods/oscillations (e.g. Holocene thermal maximum versus Neoglacial, as well as, for instance, the 8.2 ka event versus the 4 ka event or the Little Ice Age); and (2) on varying geographical conditions from the western to the eastern Mediterranean areas (longitudinal gradients). Finally, on the basis of projects using strategically located study sites, there is a need to explore possible influences of other general atmospheric circulation patterns than NAO, such as the East Atlantic–West Russian or North Sea–Caspian patterns, in explaining the apparent complexity of palaeoclimatic (palaeohydrological) Holocene records from the Mediterranean area.

Eocene cooling linked to early flow across the Tasmanian Gateway

Abstract: The warmest global temperatures of the past 85 million years occurred during a prolonged greenhouse episode known as the Early Eocene Climatic Optimum (52–50 Ma). The Early Eocene Climatic Optimum terminated with a long-term cooling trend that culminated in continental-scale glaciation of Antarctica from 34 Ma onward. Whereas early studies attributed the Eocene transition from greenhouse to icehouse climates to the tectonic opening of Southern Ocean gateways, more recent investigations invoked a dominant role of declining atmospheric greenhouse gas concentrations (e.g., CO2). However, the scarcity of field data has prevented empirical evaluation of these hypotheses. We present marine microfossil and organic geochemical records spanning the early-to-middle Eocene transition from the Wilkes Land Margin, East Antarctica. Dinoflagellate biogeography and sea surface temperature paleothermometry reveal that the earliest throughflow of a westbound Antarctic Counter Current began ∼49–50 Ma through a southern opening of the Tasmanian Gateway. This early opening occurs in conjunction with the simultaneous onset of regional surface water and continental cooling (2–4 °C), evidenced by biomarker- and pollen-based paleothermometry. We interpret that the westbound flowing current flow across the Tasmanian Gateway resulted in cooling of Antarctic surface waters and coasts, which was conveyed to global intermediate waters through invigorated deep convection in southern high latitudes. Although atmospheric CO2 forcing alone would provide a more uniform middle Eocene cooling, the opening of the Tasmanian Gateway better explains Southern Ocean surface water and global deep ocean cooling in the apparent absence of (sub-) equatorial cooling.

Middle to late Miocene stepwise climate cooling: Evidence from a high-resolution deep water isotope curve spanning 8 million years

Abstract: [1] We present high-resolution (2–3 kyr) benthic foraminiferal stable isotopes in a continuous, well-preserved sedimentary archive from the West Pacific Ocean (Ocean Drilling Program Site 1146), which track climate evolution in unprecedented resolution over the period 12.9 to 8.4 Ma. We developed an astronomically tuned chronology over this interval and integrated our new records with published isotope data from the same location to reconstruct long-term climate and ocean circulation development between 16.4 and 8.4 Ma. This extended perspective reveals that the long eccentricity (400 kyr) cycle is prominently encoded in the δ13C signal over most of the record, reflecting long-term fluctuations in the carbon cycle. The δ18O signal closely follows variations in short eccentricity (100 kyr) and obliquity (41 kyr). In particular, the obliquity cycle is prominent from ~14.6 to 14.1 Ma and from ~9.8 to 9.2 Ma, when high-amplitude variability in obliquity is congruent with low-amplitude variability in short eccentricity. The δ18O curve is additionally characterized by a series of incremental steps at ~14.6, 13.9, 13.1, 10.6, 9.9, and 9.0 Ma, which we attribute to progressive deep water cooling and/or glaciation episodes following the end of the Miocene climatic optimum. On the basis of δ18O amplitudes, we find that climate variability decreased substantially after ~13 Ma, except for a remarkable warming episode at ~10.8–10.7 Ma at peak insolation during eccentricity maxima (100 and 400 kyr). This transient warming, associated with a massive negative carbon isotope shift, is reminiscent of intense global warming events at eccentricity maxima during the Miocene climatic optimum.

Evolution of the plankton paleome in the Black Sea from the Deglacial to Anthropocene

Abstract: The complex interplay of climate shifts over Eurasia and global sea level changes modulates freshwater and saltwater inputs to the Black Sea. The dynamics of the hydrologic changes from the Late Glacial into the Holocene remain a matter of debate, and information on how these changes affected the ecology of the Black Sea is sparse. Here we used Roche 454 next-generation pyrosequencing of sedimentary 18S rRNA genes to reconstruct the plankton community structure in the Black Sea over the last ca. 11,400 y. We found that 150 of 2,710 species showed a statistically significant response to four environmental stages. Freshwater chlorophytes were the best indicator species for lacustrine conditions (>9.0 ka B.P.), although the copresence of previously unidentified marine taxa indicated that the Black Sea might have been influenced to some extent by the Marmara Sea since at least 9.6 ka calendar (cal) B.P. Dinoflagellates, cercozoa, eustigmatophytes, and haptophytes responded most dramatically to the gradual increase in salinity after the latest marine reconnection and during the warm and moist mid-Holocene climatic optimum. According to paired analysis of deuterium/hydrogen (D/H) isotope ratios in fossil alkenones, salinity increased rapidly with the onset of the dry Subboreal after ∼5.2 ka B.P., leading to an increase in marine fungi and the first occurrence of marine copepods. A gradual succession of dinoflagellates, diatoms, and chrysophytes occurred during the refreshening after ∼2.5 ka cal B.P. with the onset of the cool and wet Subatlantic climate and recent anthropogenic perturbations.

Holocene vegetational and climatic variation in westerly-dominated areas of Central Asia inferred from the Sayram Lake in northern Xinjiang, China

Abstract: Changes in the vegetation and climate of the westerly-dominated areas in Central Asia during the Holocene were interpreted using pollen-assemblages and charcoal data from a 300-cm-long sediment core of the Sayram Lake, northern Xinjiang. Accele-rator Mass Spectrometry (AMS) radiocarbon dating methods were applied to bulk organic matter of the samples. Artemisia spp./Chenopodiaceae ratios and results from principal component analysis were used to infer that the lake basin was dominated by desert vegetation before ca. 9.6 cal. ka BP, which suggests a warm and dry climate in the early Holocene. Desert steppe/steppe expanded during 9.6-5.5 cal. ka BP, indicating a remarkable increase both in the precipitation and temperature during the mid-Holocene. Desert vegetation dominated between 6.5 and 5.5 cal. ka BP, marking an extreme warmer and drier interval. The steppe/meadow steppe recovered, and temperatures decreased from 5.5 cal. ka BP in the late Holocene, as indicated by the increased abundance of Artemisia and the development of meadows. Holocene temperatures and moisture variations in the Sayram Lake areas were similar to those of adjacent areas. This consistency implies that solar radiation was the main driving factor for regional temperature changes, and that the effect of temperature variations was significant on regional changes in humidity. The evolution of climate and environment in the Sayram Lake areas, which were characterized as dry in the early Holocene and relatively humid in the middle-late Holocene, are clearly different from those in monsoonal areas. Dry conditions in the early Holocene in the Sayram Lake areas were closely related to decreased water vapor advection. These conditions were a result of reduced westerly wind speeds and less evaporation upstream, which in turn were caused by seasonal changes in solar radiation superimposed by strong evaporation following warming and drying local climate.

Chinese deserts and sand fields in Last Glacial Maximum and Holocene Optimum

Abstract: The Last Glacial Maximum (LGM, c. 26-16 ka) and the Holocene Optimum (HO, c. 9-5 ka) were characterized by cold-dry and warm-wet climates respectively in the recently geological Earth. How Chinese deserts and sand fields responded to these distinctive climatic changes is still not clear, however. To reconstruct environments of the deserts and sand fields during the LGM and HO is helpful to understand the forcing mechanisms of environment change in this arid region, and to test paleoclimatic modeling results. Through our long-term field and laboratory investigations, 400 optically stimulated luminescence (OSL) ages and more than 100 depositional records in the Chinese deserts and sand fields were obtained; on the basis of these data, we reconstruct spatial distributions of the deserts and sand fields during the LGM and HO. Our results show that the sand fields of Mu Us, Hunshandake, Horqin and Hulun Buir in northern and northeastern China had expanded 25%, 37%, 38% and 270%, respectively, during the LGM; the sand fields of Gonghe in the northeastern Qinghai-Tibetan Plateau had expanded 20%, and the deserts of Badain Jaran, Tengger in central northern China had expanded 39% and 29% separately during the LGM; the deserts of Taklimakan, Gurbantunggut and Kumtag in northwestern China had expanded 10%–20% respectively, compared to their modern areas. On the other hand, all of the sand fields were nearly completely covered by vegetation during the HO; the deserts in northwestern and central northern China were reduced by around 5%–20% in area during this time. Lakes in this arid region were probably expanded during the HO but this conclusion needs more investigation. Compared with the geological distributions of deserts and sand fields, human activity has clearly changed (expanded) the area of active sand dunes at the present time. Our observations show that environmental conditions of Chinese deserts and sand fields are controlled by regional climate together with human activity.

Holocene vegetation and climate changes in the central Mediterranean inferred from a high-resolution marine pollen record (Adriatic Sea)

Abstract: . The high-resolution multiproxy study of the Adriatic marine core MD 90-917 provides new insights to reconstruct vegetation and regional climate changes over the southcentral Mediterranean during the Younger Dryas (YD) and Holocene. Pollen records show the rapid forest colonization of the Italian and Balkan borderlands and the gradual installation of the Mediterranean association during the Holocene. Quantitative estimates based on pollen data provide Holocene precipitations and temperatures in the Adriatic Sea using a multi-method approach. Clay mineral ratios from the same core reflect the relative contributions of riverine (illite and smectite) and eolian (kaolinite) contributions to the site, and thus act as an additional proxy with which to evaluate precipitation changes in the Holocene. Vegetation climate reconstructions show the response to the Preboreal oscillation (PBO), most likely driven by changes in temperature and seasonal precipitation, which is linked to increasing river inputs from Adriatic rivers recorded by increase in clay mineral contribution to marine sediments. Pollen-inferred temperature declines during the early–mid Holocene, then increases during the mid–late Holocene, similar to southwestern Mediterranean climatic patterns during the Holocene. Several short vegetation and climatic events appear in the record, indicating the sensitivity of vegetation in the region to millennial-scale variability. Reconstructed summer precipitation shows a regional maximum (170–200 mm) between 8000 and 7000 similar to the general pattern across southern Europe. Two important shifts in vegetation occur at 7700 cal yr BP (calendar years before present) and between 7500 and 7000 cal yr BP and are correlated with increased river inputs around the Adriatic Basin respectively from the northern (7700 event) and from the central Adriatic borderlands (7500–7000 event). During the mid-Holocene, the wet summers lead to permanent moisture all year resulting in a homogeneous seasonal precipitation regime. After 6000 cal yr BP, summer precipitation decreases towards present-day values while winter precipitation rises regularly showing the setting up of Mediterranean climate conditions. Multiproxy evidence from core MD 90-917 provides a deeper understanding of the role of precipitation and particularly the seasonality of precipitation in mediating vegetation change in the central Mediterranean during the Holocene.

Late Cenozoic continuous aridification in the western Qaidam Basin: evidence from sporopollen records

Abstract: Cenozoic climate changes in inner Asia provide a basis for understanding linkages between global cooling, the Tibetan Plateau uplift, and possibly the development of the East Asian monsoon Based on a compilation of palynological results from the western Qaidam Basin, this study reconstructed a 15-million-year (Ma) record of changing vegetation and paleoclimates spanning the middle Miocene to present (comprising two series: ~ 18–5 Ma and ~ 31–0 Ma, respectively) The thermophilic percentages were highest between 18 and 14 Ma, and decreased after 14 Ma, closely corresponding to the Middle Miocene Climatic Optimum (MMCO) between 18 and 14 Ma and the following global climatic cooling between 14 and 5 Ma At the same time, decreases in the xerophytic and coniferous taxa percentages, and the increasing logarithmic ratio of non-arboreal pollen to arboreal pollen (ln (NAP/AP)), reveal the continuous aridification across both the basin and surrounding mountains Between ~ 31 and 0 Ma, the percentages of the thermophilic, xerophytic and coniferous pollen as well as the ln (NAP/AP) imply further cooling and drying in this region since 31 Ma We argue that these vegetation and climate patterns during the late Cenozoic western Qaidam Basin are primarily a result of the global cooling, with the Tibetan Plateau uplift and East Asian summer monsoon having contributions of lesser importance

Mass-movement and flood-induced deposits in Lake Ledro, southern Alps, Italy: implications for Holocene palaeohydrology and natural hazards

Abstract: High-resolution seismic profiles and sediment cores from Lake Ledro combined with soil and riverbed samples from the lake's catchment area are used to assess the recurrence of natural hazards (earthquakes and flood events) in the southern Italian Alps during the Holocene. Two welldeveloped deltas and a flat central basin are identified on seismic profiles in Lake Ledro. Lake sediments have been finely laminated in the basin since 9000 cal. yr BP and frequently interrupted by two types of sedimentary events (SEs): lightcoloured massive layers and dark-coloured graded beds. Optical analysis (quantitative organic petrography) of the organic matter present in soil, riverbed and lacustrine samples together with lake sediment bulk density and grainsize analysis illustrate that light-coloured layers consist of a mixture of lacustrine sediments and mainly contain algal particles similar to the ones observed in background sediments. Light-coloured layers thicker than 1.5 cm in the main basin of Lake Ledro are synchronous to numerous coeval mass-wasting deposits remoulding the slopes of the basin. They are interpreted as subaquatic mass-movements triggered by historical and pre-historical regional earthquakes dated to AD2005, AD1891, AD1045 and 1260, 2545, 2595, 3350, 3815, 4740, 7190, 9185 and 11 495 cal. yr BP. Darkcoloured SEs develop high-amplitude reflections in front of the deltas and in the deep central basin. These beds are mainly made of terrestrial organic matter (soils and lignocellulosic debris) and are interpreted as resulting from intense hyperpycnal flood event. Mapping and quantifying the amount of soil material accumulated in the Holocene hyperpycnal flood deposits of the sequence allow estimating that the equivalent soil thickness eroded over the catchment area reached up to 5mm during the largest Holocene flood events. Such significant soil erosion is interpreted as resulting from the combination of heavy rainfall and snowmelt. The recurrence of flash flood events during the Holocene was, however, not high enough to affect pedogenesis processes and highlight several wet regional periods during the Holocene. The Holocene period is divided into four phases of environmental evolution. Over the first half of the Holocene, a progressive stabilization of the soils present through the catchment of Lake Ledro was associated with a progressive reforestation of the area and only interrupted during the wet 8.2 event when the soil destabilization was particularly important. Lower soil erosion was recorded during the mid-Holocene climatic optimum (8000-4200 cal. yr BP) and associated with higher algal production. Between 4200 and 3100 cal. yr BP, both wetter climate and human activities within the drainage basin drastically increased soil erosion rates. Finally, from 3100 cal. yr BP to the present-day, data suggest increasing and changing human land use.

Deglacial and Holocene vegetation and climatic changes in the southern Central Mediterranean from a direct land–sea correlation

Abstract: . Despite a large number of studies, the long-term and millennial to centennial-scale climatic variability in the Mediterranean region during the last deglaciation and the Holocene is still debated, including in the southern Central Mediterranean. In this paper, we present a new marine pollen sequence (core MD04-2797CQ) from the Siculo-Tunisian Strait documenting the regional vegetation and climatic changes in the southern Central Mediterranean during the last deglaciation and the Holocene. The MD04-2797CQ marine pollen sequence shows that semi-desert plants dominated the vegetal cover in the southern Central Mediterranean between 18.2 and 12.3 ka cal BP, indicating prevailing dry conditions during the deglaciation, even during the Greenland Interstadial (GI)-1. Across the transition Greenland Stadial (GS)-1 – Holocene, Asteraceae-Poaceae steppe became dominant till 10.1 ka cal BP. This record underlines with no chronological ambiguity that even though temperatures increased, deficiency in moisture availability persisted into the early Holocene. Temperate trees and shrubs with heath underbrush or maquis expanded between 10.1 and 6.6 ka, corresponding to Sapropel 1 (S1) interval, while Mediterranean plants only developed from 6.6 ka onwards. These changes in vegetal cover show that the regional climate in southern Central Mediterranean was wetter during S1 and became drier during the mid- to late Holocene. Wetter conditions during S1 were likely due to increased winter precipitation while summers remained dry. We suggest, in agreement with published modeling experiments, that the early Holocene increased melting of the Laurentide Ice Sheet in conjunction with weak winter insolation played a major role in the development of winter precipitation maxima in the Mediterranean region in controlling the strength and position of the North Atlantic storm track. Finally, our data provide evidence for centennial-scale vegetation and climatic changes in the southern Central Mediterranean. During the wet early Holocene, alkenone-derived cooling episodes are synchronous with herbaceous composition changes that indicate muted changes in precipitation. In contrast, enhanced aridity episodes, as detected by strong reduction in trees and shrubs, are recorded during the mid- to late Holocene. We show that the impact of the Holocene cooling events on the Mediterranean hydroclimate depend on baseline climate states, i.e. insolation and ice sheet extent, shaping the response of the mid-latitude atmospheric circulation.

Holocene lake-level fluctuations in Lakes Keilambete and Gnotuk, southwestern Victoria, Australia

Abstract: Reconstructed Holocene lake-level curves from two saline, hydrologically closed maar crater lakes in southwestern Victoria, Australia, show near synchronous lake-level changes throughout the Holocene. We show that lake levels, reconstructed from sediment particle size and ostracod valve chemistry (δ18O and Sr/Ca) have undergone rapid ( 10 m) fluctuations throughout the Holocene. Finer sampling resolution shows a more sensitive response to Holocene climate than was previously presented for Lake Keilambete. Both maar crater lakes show a short-lived maximum in Holocene lake levels around 7.2 ka. The period of lake filling leading to peak lake levels matches the phase of most effective precipitation (7.4–7.0 ka) reconstructed from a high-resolution speleothem record from northern Tasmania. Water levels declined in both lakes during the mid Holocene, with a more substantive decline after ~5 ka which coincides with the end of the Southern Hemisphere hypsithermal. Water levels continued to oscil...

Reconstruction of drip-water δ 18 O based on calcite oxygen and clumped isotopes of speleothems from Bunker Cave (Germany)

Abstract: . The geochemical signature of many speleothems used for reconstruction of past continental climates is affected by kinetic isotope fractionation. This limits quantitative paleoclimate reconstruction and, in cases where the kinetic fractionation varies with time, also affects relative paleoclimate interpretations. In carbonate archive research, clumped isotope thermometry is typically used as proxy for absolute temperatures. In the case of speleothems, however, clumped isotopes provide a sensitive indicator for disequilibrium effects. The extent of kinetic fractionation co-varies in Δ47 and δ18O so that it can be used to account for disequilibrium in δ18O and to extract the past drip-water composition. Here we apply this approach to stalagmites from Bunker Cave (Germany) and calculate drip-water δ18Ow values for the Eemian, MIS3, and the Holocene, relying on independent temperature estimates and accounting for disequilibrium. Applying the co-variation method to modern calcite precipitates yields drip-water δ18Ow values in agreement with modern cave drip-water δ18Ow of −7.9 ± 0.3‰, despite large and variable disequilibrium effects in both calcite δ18Oc and Δ47. Reconstructed paleo-drip-water δ18Ow values are lower during colder periods (e.g., MIS3: −8.6 ± 0.4‰ and the early Holocene at 11 ka: −9.7 ± 0.2‰) and show higher values during warmer climatic periods (e.g., the Eemian: −7.6 ± 0.2‰ and the Holocene Climatic Optimum: −7.2 ± 0.3‰). This new approach offers a unique possibility for quantitative climate reconstruction including the assessment of past hydrological conditions while accounting for disequilibrium effects.

Mid- to late-Holocene oceanographic variability on the Southeast Greenland shelf

Abstract: A reconstruction of oceanographic variability of the past 5800 years on the southeast Greenland shelf was obtained by analysing a combined marine sediment record based on two cores from the same site. Cores Fox04G/05R were retrieved from a side basin to a cross-shelf trough connecting the 900 m deep Sermilik Fjord with the Irminger Sea in the northwestern North Atlantic. The record was analysed in terms of grain size distribution, XRF and benthic and planktonic foraminiferal content and the chronology was obtained on the basis of 210Pb and 14C dating. The late-Holocene paleoceanographic variations in the record were characterised by a marked influence from the Irminger Current (IC) at the onset of the record at 5800 cal. yr BP and the regional Holocene Climatic Optimum between 5200 and 4200 cal. yr BP. After 3600 cal. yr BP Neoglacial cooling with increased influence of polar waters from the East Greenland Current (EGC) diminished the influence from the IC. Between 1500 and 700 cal. yr BP, the environment...

Global and regional sea surface temperature trends during Marine Isotope Stage 11

Abstract: . The Marine Isotope Stage (MIS) 11 (424–374 ka) was characterized by a protracted deglaciation and an unusually long climatic optimum. It remains unclear to what degree the climate development during this interglacial reflects the unusually weak orbital forcing or greenhouse gas trends. Previously, arguments about the duration and timing of the MIS11 climatic optimum and about the pace of the deglacial warming were based on a small number of key records, which appear to show regional differences. In order to obtain a global signal of climate evolution during MIS11, we compiled a database of 78 sea surface temperature (SST) records from 57 sites spanning MIS11, aligned these individually on the basis of benthic (N = 28) or planktonic (N = 31) stable oxygen isotope curves to a common time frame and subjected 48 of them to an empirical orthogonal function (EOF) analysis. The analysis revealed a high commonality among all records, with the principal SST trend explaining almost 49% of the variability. This trend indicates that on the global scale, the surface ocean underwent rapid deglacial warming during Termination V, in pace with carbon dioxide rise, followed by a broad SST optimum centered at ~410 kyr. The second EOF, which explained ~18% of the variability, revealed the existence of a different SST trend, characterized by a delayed onset of the temperature optimum during MIS11 at ~398 kyr, followed by a prolonged warm period lasting beyond 380 kyr. This trend is most consistently manifested in the mid-latitude North Atlantic and Mediterranean Sea and is here attributed to the strength of the Atlantic meridional overturning circulation. A sensitivity analysis indicates that these results are robust to record selection and to age-model uncertainties of up to 3–6 kyr, but more sensitive to SST seasonal attribution and SST uncertainties >1 °C. In order to validate the CCSM3 (Community Climate System Model, version 3) predictive potential, the annual and seasonal SST anomalies recorded in a total of 74 proxy records were compared with runs for three time slices representing orbital configuration extremes during the peak interglacial of MIS11. The modeled SST anomalies are characterized by a significantly lower variance compared to the reconstructions. Nevertheless, significant correlations between proxy and model data are found in comparisons on the seasonal basis, indicating that the model captures part of the long-term variability induced by astronomical forcing, which appears to have left a detectable signature in SST trends.

Sea level and astronomically induced environmental changes in Middle and Late Eocene sediments from the East Tasman Plateau

Abstract: Eocene sediments drilled at the East Tasman Plateau (ETP) exhibit well-defined cycles, high-resolution magnetic stratigraphy, and environmentally-controlled dinoflagellate and diatom distribution patterns. We derive a cyclostratigraphy from the spectral analysis of high-resolution elemental concentration records (Ca, Fe) for this shallow marine time series spanning the middle to early late Eocene (C16n.2n - C21). Changes in carbonate content, the ratio between Gonyaulacoid and Peridinioid dinocysts, and relative abundance of oligotrophic diatoms serve as proxies for a high-resolution climatic and sea-level history with high values representing high sea-level stands and decreased eutrophy of surface waters. Changing ratios between high latitude dinocysts versus cosmopolitan species provide clues on sea surface temperature trends and water mass exchange. Our results show that the relatively shallow-water middle Eocene environments of the ETP are influenced by orbitally-forced climatic cycles superimposed on third order relative sea-level changes. Changes in the dominance of Milankovitch frequency at ∼38.6 Ma (late Eocene) is related to an initial deepening-step within the Tasmanian Gateway prior to the major deepening during the middle late Eocene (∼35.5 Ma). Decreasing sedimentation rates at 38 Ma and 37.2 Ma reflect winnowing associated with sea-level fall. This episode is followed by renewed transgression. Dinocyst distribution patterns indicate high latitude, probably cool temperate surface water conditions throughout, with the exception of a sudden surge in cosmopolitan species near the base of subchron C18.2r, at ∼41 Ma; this event is tentatively correlated to the Middle Eocene Climatic Optimum.

Holocene climate variability in the northern North Atlantic region: A review of terrestrial and marine evidence.

Abstract: The Holocene epoch, which followed the last major pulse of glaciation (the Younger Dryas) at the end of the last glaciation, encompasses a period before there was any substantial anthropogenic forcing of climate. A synthesis of climatic development during the Holocene (ca. 11,500 cal. yr BP to the present) is based on pollen-based quantitative temperature reconstructions, tree-line variations, chironomids, tree-ring records, speleothem data, glacier variations, and marine records (stable isotopes, species abundance, lithological changes) from the Nordic Seas. The Holocene has been regarded as a period of relatively stable climate, but recent evidence suggests there have been several significant millennial-scale climate fluctuations (larger than the post mid-19th century warming trend) throughout the Holocene. A general climate warming in the first part of the Holocene was punctuated by a few, abrupt climate reversals, centred at 10,000, 9,700, and 8,200 cal. yr BP. The data suggest there was a period of relatively warm conditions in the first half of the Holocene, in many areas warmer than in the 20th century, after which temperatures generally declined. The temperature decline was punctuated by centennial-scale warmer and colder periods with the most recent cold episode (∼AD 1550-1925), including the Little Ice Age, being one of the coldest of the entire Holocene. The kind of data presented here can be used for detecting mechanisms and forcing factors behind the reconstructed climate variations and to study leads and lags in the Earth's climate system.

Diversity dynamics of mammals in relation to tectonic and climatic history: comparison of three Neogene records from North America

Abstract: In modern ecosystems, regions of topographic heterogeneity, when compared with nearby topographically homogeneous regions, support high species densities of mammals and other groups. This biogeographic pattern could be explained by either greater diversification rates or greater accommodation of species in topographically complex regions. In this context, we assess the hypothesis that changes in landscape history have stimulated diversification in mammals. Landscape history includes tectonic and climatic processes that influence topographic complexity at regional scales. We evaluated the influence of changes in topographic complexity and climate on origination and extinction rates of rodents, the most diverse clade of mammals.We compared the Neogene records of rodent diversity for three regions in North America. The Columbia Basin of the Pacific Northwest (Region 1) and the northern Rocky Mountains (Region 2) were tectonically active over much of the Cenozoic and are characterized by high topographic complexity today. The northern Great Plains (Region 3) have been tectonically quiescent, with low relief, throughout the Cenozoic. These three regions have distinctive geologic histories and substantial fossil records. All three regions showed significant changes in diversification and faunal composition over the Neogene. In the montane regions, originations and extinctions peaked at the onset and close, respectively, of the Miocene Climatic Optimum (17–14 Ma), with significant changes in faunal composition accompanying these episodes of diversification. In the Great Plains, rodents showed considerable turnover but infrequent diversification. Peak Neogene diversity in the Great Plains occurred during cooling after the Miocene Climatic Optimum. These histories suggest that climatic changes interacting with increasing topographic complexity intensify macroevolutionary processes. In addition, close tracking of diversity and fossil productivity with the stratigraphic record suggests either large-scale sampling biases or the mutual response of diversity and depositional processes to changes in landscape history.

Terrestrial paleoenvironmental reconstructions indicate transient peak warming during the early Eocene climatic optimum

Abstract: Major changes in climate and ecology occurred during the early Eocene climatic optimum, sometime between 52 and 50 Ma. Recent work suggests that the timing and duration of the event are characterized by different responses in the marine and terrestrial realms, and that traditional causal mechanisms may not adequately explain such differences. We applied high-resolution paleopedology, geochemical analysis, and phytolith biostratigraphy techniques to paleosol suites within the well-described Wind River Formation of western Wyoming, USA. This multiproxy record indicates a short (<1 m.y.) peak period of carbon isotopic enrichment (up to 2‰ higher) and elevated pCO2, high temperatures (up to 8 °C higher), increased precipitation (up to 500 mm yr –1 higher), and shifts in fl oral composition (up to 10%). Terrestrial climatic and ecological changes of this kind during the early Eocene climatic optimum are consistent with changes in contemporaneous records that have been ascribed to high atmospheric pCO2, but a transient peak interval suggests that the cause of high atmospheric pCO2 during the early Eocene was likely not increased volcanism or decreased silicate weathering, which operate on longer timescales. Instead, terrestrial records from across western North America agree that early Eocene climatic optimum changes may have been caused by other sources, such as a combination of increased ventilation of oceanic carbon and increased petroleum generation in sedimentary basins. The climatic and environmental changes exhibited by this and other North American terrestrial records also defi ne a pattern of regional response that is relevant for understanding the impacts of global climate change events.