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1.
Angola Basin and Cape Basin (southeast Atlantic) surface sediments and sediment cores show that maxima in the abundance of taraxerol (relative to other land-derived lipids) covary with maxima in the relative abundance of pollen from the mangrove tree genus Rhizophora and that in the surface sediments offshore maxima in the relative abundance of taraxerol occur at latitudes with abundant coastal mangrove forests. Together with the observation that Rhizophora mangle and Rhizophora racemosa leaves are extraordinarily rich in taraxerol, this strongly indicates that taraxerol can be used as a lipid biomarker for mangrove input to the SE Atlantic. The proxy-environment relations for taraxerol and Rhizophora pollen down-core show that increased taraxerol and Rhizophora pollen abundances occur during transgressions and periods with a humid climate. These environmental changes modify the coastal erosion and sedimentation patterns, enhancing the extent of the mangrove ecosystem and/or the transport of mangrove organic matter offshore. Analyses of mid-Pleistocene sediments show that interruption of the pattern of taraxerol maxima during precession minima occurs almost only during periods of low obliquity. This demonstrates the complex environmental response of the interaction between precession-related humidity cycles and obliquity-related sea-level changes on mangrove input.  相似文献   

2.
This paper presents biostratigraphical and stable isotope data obtained from core CM92–43, which was recovered from the central Adriatic as part of a comprehensive investigation of the palaeoenvironmental history of the basin. The data span the period of the Last Glacial–Holocene (LG–H) transition (ca. 18000 to 8000 GRIP ice-core yr BP). Regional biozones are defined on the basis of characteristic assemblages of planktic Foraminifera, and these are compared with other foraminiferal biostratigraphical schemes from the southern Adriatic and the Tyrrhenian Sea. Variations in relative abundance of selected planktic Foraminifera and in selected pollen types are shown alongside variations in δ18O and δ13C obtained from Globigerina bulloides and relative abundance of Globigerinoides ex. gr. ruber. The data are compared with the GRIP ice-core record and the event stratigraphy scheme based on this record, and it is concluded that the climate forcing mechanisms that controlled climate variations in the North Atlantic region during the LG–H transition also extended their influence into the Mediterranean region over the same period. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

3.
The Palaeogene was the most recent greenhouse period on Earth. Especially for the Late Palaeocene and Early Eocene, several superimposed short-term hyperthermal events have been described, including extremes such as the Palaeocene?CEocene Thermal Maximum. Major faunal and floral turnovers in the marine and terrestrial realms were recorded in association with these events. High-resolution palynological analysis of the early Middle Eocene maar lake sediments at Messel, near Darmstadt, Germany, provides an insight into the dynamics of a climax vegetation during the Middle Eocene greenhouse climate in a time span without significant climatic excursions. Numerical techniques like detrended correspondence analysis and wavelet analysis have been applied to recognize cyclic fluctuations and long-term trends in the vegetation through a time interval of approximately 640?kyr. Based on the numerical zoning of the pollen diagram, three phases in the development of the vegetation may be distinguished. Throughout these phases, the climax vegetation did not change substantially in qualitative composition, but a trend towards noticeably less humid conditions probably in combination with a drop of the water level in the lake may be recognized. A shift in algal population from the freshwater dinoflagellate cyst Messelodinium thielepfeifferae to a dominance of Botryococcus in the uppermost part of the core is interpreted as a response to changes in acidity and nutrient availability within the lake. Time series analyses of pollen assemblages show that variations in the Milankovitch range of eccentricity, obliquity and precession can be distinguished. In addition, fluctuations in the sub-Milankovitch range are indicated. This demonstrates that floral changes during steady depositional conditions in the Middle Eocene of Messel were controlled by orbital forcing.  相似文献   

4.
To understand Holocene climatic development and to determine drivers of climatic changes and climate variability, high-resolution marine proxy records are required from key oceanic locations. However, information on the Holocene climate development from the Southern Hemisphere is still rare and mainly based on terrestrial archives. Here, we present data with a high-resolution of circa 35 years from sediment cores taken east of the Great Australian Bight, where it is possible to determine Southern Ocean Holocene climate and the longer-term trends of the El Niño–Southern Oscillation (ENSO) conditions. For this purpose, we used the oxygen-isotope records of two planktonic foraminifer species Globigerinoides ruber and Globigerina bulloides which inhabit different water masses as well as faunal counts of planktonic foraminifers. After the ocean frontal systems off southern Australia were pushed northward by orbitally-forced insolation changes during the early Holocene, the data indicate increasing ENSO variability during the mid to late Holocene when the fronts shifted polewards again. A strong circa 1550 year cycle is found in the Globigerina bulloides record which reflects the wider Southern Ocean signal with prominent cold phases centred at circa 9.2, 7.3, 5.8, 4.3, 2.7, 1.4 ka BP and, possibly the Little Ice Age, which have global counterparts.  相似文献   

5.
We describe the evolution of climate system dynamics by examining the climate response to changes in obliquity and precession over the last 5.3 Myr. In particular, we examine changes in the shape of glacial cycles and the power of obliquity and precession response in benthic δ18O. When the exponential trend in δ18O variance is removed, its spectral power exhibits strong, proportional responses to amplitude modulations in orbital forcing over most of the Plio–Pleistocene. Precession responses correlate with modulations in forcing for the last 5 Myr, but 41-kyr response is sensitive to obliquity modulation only before 1.4 Myr. Where responses are sensitive to modulations in forcing, we demonstrate that glacial cycles are orbitally forced rather than being self-sustained or paced by orbital changes. The shapes of glacial cycles have several nonlinear properties, which may be indicative of glacial–interglacial differences in climate sensitivity or response time. The “saw-tooth” asymmetry of glacial cycles first appears shortly after the onset of major northern hemisphere glaciation, and the relative duration of interglacial stages decreases at 1.4 Myr. Collectively, trends in the shape of glacial cycles and the sensitivity of δ18O to obliquity and precession are suggestive of major transitions in climate dynamics at approximately 2.5 and 1.4 Myr but show no significant change associated with the appearance of strong 100-kyr cycles during the mid-Pleistocene transition.  相似文献   

6.
《Applied Geochemistry》2003,18(7):997-1009
The δ18O and δD values in the deep confined aquifer beneath the North China Plain which is located at 112°30′E–119°30′E and 34°46′N–40°25′N, reflect differences in paleoclimatic conditions between the Holocene and the late Pleistocene. Groundwater samples whose 14C ages are between 12 and 25 ka B.P have ranges of −9.4 to −11.7‰ for δ18O and −76‰ to −85‰ for δD values. These very negative δ18O and δD values reflect the cold and arid climate in the last glacial period. The temperature estimated in this period is 6–9 °C cooler than that of the present. The entire ranges of δ18O and δD values for samples with 14C dating from 7 ka B.P to present are −7.7‰ to −10.2‰ and −63‰ to −73‰, respectively. The greater δ18O and δD enrichments of these samples indicate a period of relatively humid and warm climate in the Holocene. However, the wide ranges of δ18O (−9.0‰ to −11.1‰) and δD (−66‰ to −80‰) values for samples with 14C age ranging from 12 to 7 ka B.P. imply an unstable climatic condition of rapidly increasing temperature, which marks the transition from the Pleistocene to the Holocene.  相似文献   

7.
The ODP leg 161 Site 976 (Alboran Sea) is a deep-sea section sampled at a water depth of 1108 m in the Western Mediterranean Sea. Pollen analysis provides a vegetation and climate record of the Mid Pleistocene Transition (MPT), roughly one million years ago. The age-model tied to biostratigraphic events was revised by aligning the pollen climate index (PCI) to Mediterranean (KC01b) and global (LR04) oxygen isotope records. The studied time slice spans the interval ～1.09 Ma (MIS 31) to ～0.90 Ma (MIS 23).Across this interval, past phytogeography of nowadays extinct taxa, which were rare, allows a successful application of the modern analogues technique (MAT) to quantitative climate reconstructions for the MPT. Five, long-term, obliquity-related vegetation successions (O1 to O5), and eight short-term, precession-related vegetation successions (P1 to P8) are observed within the studied interval. These vegetation successions, regardless of their duration, show the same pattern: the progressive replacement of temperate trees by mountainous taxa, and then by herbs and steppe maxima. Precession-related successions correspond, therefore, to as dramatic vegetation changes as those driven by obliquity, including a final steppe phase under deteriorated climate conditions.Wavelet analysis of the PCI record shows that the Western Mediterranean experienced a shift at 1.01 Ma from precession-dominated frequencies (1.05–1.01 Ma) to obliquity-dominated frequencies (1.01–0.9 Ma). There is, therefore, an apparent discrepancy between wavelet analysis results and vegetation dynamic analysis (which suggests that obliquity and precession are recorded throughout the entire studied interval). This discrepancy could result from the fact that the PCI record sums, somehow, similar vegetation changes (wet to dry) occurring at different periodicities. Such a complex vegetation dynamics is mathematically rendered through a single parameter (i.e. principal component), which does not successfully catch the subtle combinations of variability occurring at two close periodicities. Furthermore, the pollen-inferred Early Pleistocene vegetation dynamic (and climate) of the Western Mediterranean region does not show a decrease of the obliquity response relative to the precession response at the onset of the MPT.  相似文献   

8.
The late Ediacaran Shuram-Wonoka excursion, with δ~(13)C_(carb) values as low as-12‰(PDB) in marineshelf deposits and spanning up to 10 Myr, is the deepest and most protracted δ~(13)C_(carb) negative anomaly recognised in Earth history. The excursion formed on at least four continents in low(≤32°) palaeolatitudes, and in China is associated with a major phosphogenic event. Global and intrabasinal correlation, magnetostratigraphy, isotope conglomerate tests and further geochemical data are consistent with a primary or syn-depositional origin for the excursion. Continental-margin phosphorites are generated by oceanic upwelling driven by surface winds, and δ~(13)C_(carb) negative anomalies are explicable by oceanic upwelling of 13 C-depleted deep oceanic waters, arguing that a feature common to these exceptional Ediacaran events was unprecedented perturbation of the world ocean. These events occurred during the transition from an alien Proterozoic world marked by low-palaeolatitude glaciation near sea level and strong seasonality to the familiar Phanerozoic Earth with circum-polar glaciation and temperate climate, suggesting that the Shuram-Wonoka excursion is related to this profound change in Earth's climate system. Of various hypotheses for Proterozoic low-palaeolatitude glaciation, only the high obliquity(54°) hypothesis, which posits secular decrease in obliquity to near the present-day value(23.5°) during the Ediacaran, predicts an unparalleled revolution in the Ediacaran world ocean. The obliquity controls the sense of the world ocean's meridional overturning circulation, which today is driven by the sinking of cold, dense water at the poles and upwelling driven by zonal surface winds.When the decreasing obliquity passed the critical value of 54° during the Ediacaran the meridional temperature gradient reversed, with the equator becoming warmer than the poles and Hadley lowlatitude(30°-35°) atmospheric zonal circulation reversing. This reversal of the temperature gradient is unique to the Ediacaran Period and caused reversal of the oceanic meridional overturning circulation,with upwelling of anoxic, 13 C-depleted deep oceanic waters producing a deeply negative and protractedδ~(13)C_(carb) signature on late Ediacaran marine-shelf deposits.  相似文献   

9.
The Marmes Rockshelter archaeological site in southeastern Washington state contains a > 11 kyr stratigraphic record that was excavated in the 1960s but only recently analyzed in detail. We present the results of physical, chemical, and isotopic analyses of archived Marmes sediments from rockshelter, hillslope, and floodplain locations. Multiple lines of evidence including éboulis production, soil chemistry, and δ13C and δ18O signatures in soil organic matter and calcium carbonate suggest that relatively cool, moist conditions 10,600 to 9700 14C yr BP were followed by relatively warm and dry conditions as early as 9000 14C yr BP. Warm and dry conditions extended to the late Holocene, followed by a return to cooler and moister climate. The limited range of δ13C and δ18O values in Marmes paleosols suggests that the magnitude of moisture and temperature shifts was locally buffered in the lower Snake River Canyon but adequate to generate significant changes in sedimentation and soil formation, possibly due to nonlinear geological and pedological processes. These buffered canyon environments were well suited for establishing residential bases associated with foraging and logistical collecting strategies and may have minimized the influence of climate changes in food resource abundance.  相似文献   

10.
We generated a high-resolution SSTMg/Ca record for the surface-dwelling planktonic foraminifera Globigerina bulloides from the core MD99-2346 collected in the Gulf of Lion, and compared it to that obtained using modern analogue techniques applied to fossil foraminiferal assemblages (SSTMAT). The two temperature records display similar patterns during the last 28,000 years but the SSTMg/Ca estimates are several degrees warmer (∼ +4 °C) than SSTMAT. The temperature shift between SSTMg/Ca and SSTMAT remained relatively constant over time. This seems to exclude a bias on the Mg/Ca record associated with salinity or secondary Mg-rich calcite encrustation on the foraminiferal tests during early diagenesis. Therefore, anomalously high Mg/Ca suggests either: (1) the empirical equation for G. bulloides of Elderfield and Ganssen (2000) is incorrect; or (2) there is a specific Mediterranean genotypes of G. bulloides for which a specific Mg/Ca-temperature calibration is needed.  相似文献   

11.
Three planktonic foraminiferal species Globigerina bulloides, Neogloboquadrina pachyderma (d), and Globorotalia inflata collected from core-tops spanning 35° to 65°N in the North Atlantic were used for U/Ca and Mg/Ca and foraminiferal shell weight analyses. Except for U/Ca in G. bulloides calcified under warm conditions (>∼13 °C), U/Ca ratios in all three studied species increase with decreasing latitude and show strong positive correlations with Mg/Ca ratios. A dissolution effect on planktonic U/Ca is suggested by decreased shell weight and U/Ca and Mg/Ca ratios for shells from very deep water depth (>4.4 km) along the latitudinal transect. G. bulloides from down core samples in the North Atlantic show low U/Ca ratios during the last glacial and high ratios during the Holocene, similar to the Mg/Ca evolution trend. In general, our data indicate that the U incorporation into planktonic foraminiferal carbonates is strongly influenced by calcification temperature, although U/Ca in G. bulloides may be affected by seawater carbonate ion concentration under warm conditions and/or other factors.  相似文献   

12.
The δ13C and δ18O values of well-preserved carbonate rhizoliths (CRs) provide detailed insights into changes in the abundance of C3 and C4 plants in response to approximately decadal-scale changes in growing-season climate. We performed stable isotope analyses on 35-40 CRs sampled at 1-cm intervals from an 18-cm-thick paleosol formed in southern Illinois during Wisconsin interstadial 2. Minimum δ13C values show little variation with depth, whereas maximum values vary dramatically, and average values show noticeable variability; maximum δ18O values vary less than the minimum δ18O values. These findings indicate that a diverse and stable C3 flora with a limited number of C4 grass species prevailed during this interval, and suggest that the maximum growing-season temperatures were relatively stable, but minimum growing-season temperatures varied considerably. Two general patterns characterize the relationships between the δ13C and δ18O values obtained from the 1-cm samples. In some cases, low δ13C values correspond to low δ18O values and high δ13C values correspond to high δ18O values, suggesting that cooler growing-season temperatures favored C3 and warmer growing-season temperatures favored C4 plants. In other cases, low δ13C values correspond to high δ18O values, likely suggesting that wetter growing-season conditions were favorable to C3 plants. The high density of well-preserved CRs in this paleosol provides a unique opportunity to study detailed ecological responses to high-resolution variability in growing-season climate.  相似文献   

13.
Pollen data from two sections from a coastal cliff on the western Yamal Peninsula (69°43.27′N, 66°48.80′E) document the environmental history during the Karginsky (Middle Weichselian) interstadial. Low pollen concentrations, high amounts of redeposited pollen, and relatively high presence of Artemisia pollen characterize sediments deposited at about 33,000 14C yr B.P. Grass-sedge plant associations with few other herbs occupied the area during the late Karginsky interstadial. Artemisia pollen may indicate rather xerophytic vegetation and disturbed soils in the area. The dominance of redeposited pollen reflects scarce (disturbed) vegetation cover and low pollen productivity. The climate was relatively cold and dry. Sediments dated to 32,400 14C yr B.P. contain fewer redeposited pollen and concentration of non-redeposited pollen is significantly higher. Pollen contents indicate the dominance of tundra-like grass-sedge vegetation and more humid conditions. Pollen records dated between 30,100 and 25,100 14C yr B.P. also reflect scarce tundra-like vegetation during this interval. The presence of Betula nana and Salix pollen may reflect limited presence of shrub communities. This suggests that the climate was somewhat warmer during the latter part of the interstadial. However, generally the pollen records show that harsh environmental conditions prevailed on the Yamal Peninsula during the Karginsky interstadial.  相似文献   

14.
Pollen data from two sites provide information on the postglacial vegetation and climate history of the Cascade Range. Indian Prairie in the western Cascade Range was colonized by subalpine forests of Pinus, Picea, and Tsuga and open meadows prior to ca. 12,400 14C yr B.P. The treeline lay 500 to 1000 m below its modern elevation and conditions were cooler than at present. From ca. 12,400 to ca. 9950 14C yr B.P. Abies became important and the forest resembled that presently found at middle elevations in the western Cascade Range. The pollen record implies a rise in treeline and warmer conditions than before. From ca. 10,000 to 4000-4500 14C yr B.P., conditions that were warmer and effectively drier than today led to the establishment of a closed forest composed of Pseudotsuga , Abies, and, at lower elevations, Quercus and Corylus . During this period, Gold Lake Bog in the High Cascades was surrounded by closed forest of Pinus and Abies. The early-Holocene pollen assemblages at both Indian Prairie and Gold Lake Bog lack modern analogues, and it is likely that greater-than-present summer radiation fostered unique climatic conditions and vegetation associations at middle and high elevations. In the late Holocene, beginning ca. 4000-4500 14C yr B.P., cooler and more humid conditions prevailed and the modern vegetation was established. A comparison of these sites with others in the Pacific Northwest suggests that major patterns of vegetational change at individual sites were a response to large-scale changes in the climate system that affected the entire region.  相似文献   

15.
The authors discuss Late Pleistocene–Holocene depositional environments in one of the Fuegian Andes valleys on the basis of palynological, geomorphological, and sedimentological analyses from two sites located near the Beagle Channel. The results obtained at these localities reinforce and refine the Late Pleistocene–Holocene climatic pattern previously recorded there. A colder period, associated with the Younger Dryas stadial event, is suggested by low Nothofagus pollen frequency, and communities of grass, low scrub, and shrub heath expanded into the low/middle slopes (10,310 14C yr BP). By ca. 9500 14C yr BP, warmer and drier conditions occurred, as evidenced by the development of open-grown vegetation in the valley floors (pollen zone O-3), followed by the expansion of open Nothofagus woodland (pollen zone O-2) in the middle Holocene. The milder climate subsequently changed, as indicated by the spreading of the closed forest and mire (pollen zone O-1), to more humid and cooler conditions during the last ca. 5000 yr BP.  相似文献   

16.
Increased interest in paleoenvironmental studies is a result of climatic changes occurring at present and predicted for the future. Such studies could be done using the stable isotope compositions (δ2H and δ18O) of kaolins, which provide knowledge on the paleoenvironmental conditions prevailing during the time of kaolinisation. In this study, the stable isotopic compositions of clay-size fraction of kaolins occurring in Cretaceous and Tertiary Formations of the Douala Sub-Basin in Cameroon are presented, with the aim of reconstructing the paleoenvironmental conditions of the Sub-Basin. To achieve this, the clay-size fraction (< 2 μm fraction) of 8 kaolinite-rich samples were analysed for their δ2H and δ18O compositions, and results were reported as part per mil (‰) relative to the SMOW standard. The δ18O values of kaolins found in the Cretaceous–Tertiary Formations of the Douala Sub-Basin varied between +18.2 and +21.0‰, whereas the δ2H values varied between –69 and –53‰. Nine of the eleven samples plotted on the right of the supergene–hypogene line. Five of these nine samples plotted very close to the kaolinite line, which represents the composition of kaolinite in equilibrium with meteoric water at 20 °C; suggesting a supergene weathering origin of these kaolins. The determination of the temperature of kaolinisation yielded mean formation temperatures of 22 ± 2 °C and 27 ± 6 °C for Cretaceous and Tertiary kaolins, respectively. Excluding the two samples falling in the hypogene field, averages of kaolinisation temperatures were 20 and 25 °C during the Cretaceous and Tertiary periods, respectively. These temperatures are slightly below the present mean annual temperature in Douala (27 °C), thereby suggesting that the climate was becoming warmer from the Cretaceous to the Present. Therefore, Douala had a cooler and rainy climate during the Cretaceous, and the climate is gradually becoming hotter and more humid, favouring the refinement of existing kaolins and the kaolinisation of kaolin-forming minerals in the Sub-Basin.  相似文献   

17.
Sedimentological, malacological, and pollen analyses from 14C-dated alluvial sections from the Luján River provide a detailed record of environmental changes during the Holocene in the northeastern Pampas of Argentina. From 11,200 to 9000 14C yr B.P., both sedimentary and biological components suggest that the depositional environment was eutrophic, alkaline, and freshwater to brackish shallow water bodies without significant water circulation. During this time, bioclastic sedimentation was dominant and the shallow water bodies reached maximum development as the climate became more humid, suggesting an increase in precipitation. Short-term fluctuations in climate during the last stage of this interval may have been sufficient to initiate changes in the water bodies, as reduction of the volume alternated with periods of flooding. The beginning of the evolution of shallow swamps in the wide floodplain or huge wetlands was contemporaneous with a sea level lower than the present one. From 9000 and 7000 14C yr B.P., mesotrophic, alkaline, brackish, probably anoxic swamps existed. Between 7000 and 3000 14C yr B.P., anoxic calcareous swamps were formed, with subaerial exposure and development of the Puesto Berrondo Soil (3500-2900 14C yr B.P.). A trend to a reduction of water bodies is recorded from 9000 to ca. 3000 14C yr B.P., with a significant reduction after ca. 7000 14C yr B.P. A shift to subhumid-dry climate after 7000 14C yr B.P. appears to be the main cause. During this time, an additional external forcing toward higher groundwater levels was caused by Holocene marine transgression causing changes in the water bodies levels. The climate became drier during the late Holocene (ca. 3000 yr B.P.), when clastic sedimentation increased, under subhumid-dry conditions. Flood events increased in frequency during this time. From ca. A.D. 1790 to present, the pollen record reflects widespread disturbance of the vegetation during the European settlement.  相似文献   

18.
We conducted paired measurements of the Mg/Ca ratio and δ18O of planktonic foraminifera, Globigerina bulloides, from a sediment core (MD01‐2420) off central Japan in the northwest Pacific, to reconstruct current movements since the Last Glacial Maximum (LGM). These methods make it possible to determine the magnitude and timing of the sea surface temperature (SST) changes and to reconstruct the variations of the past seawater δ18O (δ18Ow) off central Japan. The amplitude of Mg/Ca‐based SST changes between the Holocene and the LGM was about 10°C. The strong resemblance of the SSTs estimated from both methods, Mg/Ca‐based and δ18O‐based, suggests that the SST changes were caused primarily by latitudinal displacement of the Kuroshio–Oyashio currents and no distinct change in the carbonate dissolution of the core. The southward migration of the water mass was 5–6° in latitude at the LGM. The values for regional δ18Ow changes, which were obtained by subtracting the ice volume contribution from the calculated δ18Ow, describe the millennial‐scale water mass migration over the last 30 kyr. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

19.
The Early Jurassic period was characterized by extreme environmental changes, as reflected by major global carbon isotope anomalies and abrupt changes in oxygen isotope and elemental records of marine organisms. Available data suggest an overall warm Early Jurassic climate interrupted by periods of severe cooling, with a climatic optimum during the early Toarcian. Available geochemical studies, however, have mainly focused on the northern margin of the Tethys Ocean, so that the palaeogeographic extent of these environmental perturbations, latitudinal palaeotemperature gradients and climate belt boundaries remain poorly constrained. Here we report the first stable isotope records of brachiopod shells (δ13C and δ18O values) from the Upper Sinemurian-Middle Toarcian interval in the southern margin of the Tethys Ocean (northwest Algeria). These data were used to better constrain the palaeoenvironmental evolution of the North Gondwana margin during the Early Jurassic, which likely played an important role on supra-regional climate. The diagenetic history of the analysed brachiopod shells was monitored using scanning electron microscopy, and elemental (manganese and strontium) compositions. The brachiopod δ13C and δ18O data show very similar trends as those reported for various Tethyan regions, and record negative carbon and oxygen isotope excursions near the Sinemurian–Pliensbachian and Pliensbachian–Toarcian transitions and during the Toarcian oceanic anoxic event (T-OAE). Despite these similarities, the carbon and oxygen isotope records are systematically offset towards more positive δ13C values (average +0.5‰) and more negative δ18O values (−1.0‰) compared to those obtained from sites of higher palaeolatitudes in the northern Tethyan margin. These offsets suggest a spatial heterogeneity in the stable isotope composition of dissolved inorganic carbon in the Early Jurassic Ocean and a marked latitudinal temperature gradient between the southern and northern margins of the Tethys.  相似文献   

20.
This article presents a combined pollen and phytolith record of a 1.70-m sediment core from the wetlands of India Muerta (33° 42′ S, 53° 57′ W) in the lowland Pampa (grasslands) of southeastern Uruguay. Six 14C dates and the pollen and phytolith content of the samples permitted the recognition of four distinct climatic periods between 14,850 14C yr B.P. and the present. The Late Pleistocene period (between ca. 14,810 and ca. 10,000 14C yr B.P.) was characterized by drier and cooler conditions indicated by the presence of a C3-dominated grassland. These conditions prevailed until the onset of the warmer and more humid climate of the Holocene around 9450 14C yr B.P. The early Holocene (between around 10,000 and 6620 14C yr B.P.) was characterized by the establishment of wetlands in the region as evidenced by the formation of black peat, the increase in wetland taxa, and the replacement of C3 Pooideae by C4 Panicoideae grasses. During the mid-Holocene, around 6620 14C yr B.P., began a period of environmental change characterized by drier climatic conditions, which resulted in the expansion of halophytic communities in the flat, low-lying areas of the wetlands of India Muerta. About 4020 14C yr B.P. a massive spike of Amaranthaceae/Chenopodiaceae coupled with a radical drop in wetland species indicates another major and more severe period of dryness. After ca. 4000 14C yr B.P., a decrease of halophytic species indicates the onset of more humid and stable climatic conditions, which characterized the late Holocene.The findings reported in this article substantially improve our knowledge of the late Glacial and Holocene climate and vegetation in the region. The data provide a detailed record of the timing and severity of mid-Holocene environmental changes in southeastern South America. Significantly, the mid-Holocene drying trend coincided with major organizational changes in settlement, subsistence, and technology of the pre-Hispanic populations in the region, which gave rise to early Formative societies. This study also represents the first combined pollen and phytolith record for southeastern South America reinforcing the utility of phytoliths as significant indicators of long-term grassland dynamics.  相似文献   

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