Search

Central Asia is a large-scale source of dust transport, but it also held a prominent changing hydrological system during the Quaternary. A 223 m long sediment core (GN200) was recovered from the Ejina Basin (synonymously Gaxun Nur Basin) in NW China to reconstruct the main modes of water availability in the area during the Quaternary. The core was drilled from the Heihe alluvial fan, one of the world's largest alluvial fans, which covers a part of the Gobi Desert. Grain-size distributions supported by endmember modelling analyses, geochemical–mineralogical compositions (based on XRF and XRD measurements), and bioindicator data (ostracods, gastropods, pollen and non-pollen palynomorphs, and n -alkanes with leaf-wax δD) are used to infer the main transport processes and related environmental changes during the Pleistocene. Magnetostratigraphy supported by radionuclide dating provides the age model. Grain-size endmembers indicate that lake, playa (sheetflood), fluvial, and aeolian dynamics are the major factors influencing sedimentation in the Ejina Basin. Core GN200 reached the pre-Quaternary quartz- and plagioclase-rich "Red Clay" formation and reworked material derived from it in the core bottom. This part is overlain by silt-dominated sediments between 217 and 110 m core depth, which represent a period of lacustrine and playa-lacustrine sedimentation that presumably formed within an endorheic basin. The upper core half between 110 and 0 m is composed of mainly silty to sandy sediments derived from the Heihe that have accumulated in a giant sediment fan until modern time. Apart from the transition from a siltier to a sandier environment with frequent switches between sediment types upcore, the clay mineral fraction is indicative of different environments. Mixed-layer clay minerals (chlorite/smectite) are increased in the basal Red Clay and reworked sediments, smectite is indicative of lacustrine-playa deposits, and increased chlorite content is characteristic of the Heihe river deposits. The sediment succession in core GN200 based on the detrital proxy interpretation demonstrates that lake-playa sedimentation in the Ejina Basin has been disrupted likely due to tectonic events in the southern part of the catchment around 1 Ma. At this time Heihe broke through from the Hexi Corridor through the Heli Shan ridge into the northern Ejina Basin. This initiated the alluvial fan progradation into the Ejina Basin. Presently the sediment bulge repels the diminishing lacustrine environment further north. In this sense, the uplift of the hinterland served as a tipping element that triggered landscape transformation in the northern Tibetan foreland (i.e. the Hexi Corridor) and further on in the adjacent northern intracontinental Ejina Basin. The onset of alluvial fan formation coincides with increased sedimentation rates on the Chinese Loess Plateau, suggesting that the Heihe alluvial fan may have served as a prominent upwind sediment source for it. [ABSTRACT FROM AUTHOR]

Global climate cooled from the early Eocene hothouse (∼52 –50 Ma) to the latest Eocene (∼34 Ma). At the same time, the tectonic evolution of the Southern Ocean was characterized by the opening and deepening of circum-Antarctic gateways, which affected both surface- and deep-ocean circulation. The Tasmanian Gateway played a key role in regulating ocean throughflow between Australia and Antarctica. Southern Ocean surface currents through and around the Tasmanian Gateway have left recognizable tracers in the spatiotemporal distribution of plankton fossils, including organic-walled dinoflagellate cysts. This spatiotemporal distribution depends on both the physicochemical properties of the water masses and the path of surface-ocean currents. The extent to which climate and tectonics have influenced the distribution and composition of surface currents and thus fossil assemblages has, however, remained unclear. In particular, the contribution of climate change to oceanographic changes, superimposed on long-term and gradual changes induced by tectonics, is still poorly understood. To disentangle the effects of tectonism and climate in the southwest Pacific Ocean, we target a climatic deviation from the long-term Eocene cooling trend: the Middle Eocene Climatic Optimum (MECO; ∼40 Ma). This 500 kyr phase of global warming was unrelated to regional tectonism, and thus provides a test case to investigate the ocean's physicochemical response to climate change alone. We reconstruct changes in surface-water circulation and temperature in and around the Tasmanian Gateway during the MECO through new palynological and organic geochemical records from the central Tasmanian Gateway (Ocean Drilling Program Site 1170), the Otway Basin (southeastern Australia), and the Hampden Beach section (New Zealand). Our results confirm that dinocyst communities track specific surface-ocean currents, yet the variability within the communities can be driven by superimposed temperature change. Together with published results from the east of the Tasmanian Gateway, our new results suggest a shift in surface-ocean circulation during the peak of MECO warmth. Simultaneous with high sea-surface temperatures in the Tasmanian Gateway area, pollen assemblages indicate warm temperate rainforests with paratropical elements along the southeastern margin of Australia. Finally, based on new age constraints, we suggest that a regional southeast Australian transgression might have been coincident with the MECO. [ABSTRACT FROM AUTHOR]

Central Asia is a large-scale source of dust transport, but also holds a prominent changing hydrological system during the Quaternary. A 223-m-long sediment core (GN200) was recovered from the Ejina Basin (synonymously Gaxun Nur Basin) in NW China to reconstructing the main transitional modes of water availability in the area during the Quaternary. The core has been drilled from the Heihe alluvial fan, one of the world's largest continental alluvial fan, which covers a part of the Gobi Desert. Grain-size distributions supported by endmember modelling analyses, geochemical-mineralogical compositions (based on XRF and XRD measurements), and bioindicator data (ostracods, gastropods, n-alkanes with leaf-wax δD) are used to infer the main transport processes and related environmental changes during the Pleistocene. Magnetostratigraphy supported by radionuclide dating provides the age model. Grain-size endmembers indicate that lake, playa (sheetflood), fluvial, and aeolian dynamics are the major factors influencing sedimentation in the Ejina Basin. Core GN200 reached the Pre-Quaternary quartz- and plagioclase-rich "Red Clay Formation" and reworked material derived from it in the core bottom. This part is overlain by silt-dominated sediments between 217 and 110 m core depth, which represent a period of lacustrine and playa-lacustrine sedimentation that presumably formed within an endorheic basin. The upper core half between 110 and 0 m is composed of mainly silty to sandy sediments derived from the Heihe River that have accumulated in a giant sediment fan until modern time. Apart from the transition from a siltier to a sandier environment with frequent switches between sediment types upcore, the clay mineral fraction is indicative for different environments. Mixed layer clay minerals (chlorite/smectite) are increased in the basal "Red Clay Formation" and reworked sediments, smectite is indicative for lacustrine-playa deposits, and an increased chlorite content is characteristic of the Heihe river deposits. The sediment succession in core GN200 based on the detrital proxy interpretation demonstrates that lake-playa sedimentation in the Ejina Basin has been disrupted likely due to tectonic events in the southern part of the catchment around 1 Ma BP. At this time Heihe river broke through from the Hexi Corridor through the Heli Shan ridge into the northern Ejina Basin. This initiated the alluvial fan progradation into the Ejina Basin. Presently the sediment bulge repels the diminishing lacustrine environment further north. In this sense, the uplift of the hinterland served as a tipping element that triggered landscape transformation in the Northern Tibetan foreland (i.e., the Hexi Corridor) and further on in the adjacent northern intracontinental Ejina Basin. The onset of alluvial fan formation coincides with increased sedimentation rates on the Chinese loess plateau, suggesting that the Heihe fluvial/alluvial fan may have served as a prominent upwind sediment source for it. [ABSTRACT FROM AUTHOR]

Global climate cooled from the early Eocene hothouse (~52-50 Ma) to the latest Eocene (~34 Ma). At the same time, the tectonic evolution of the Southern Ocean was characterized by the opening and deepening of circum-Antarctic gateways, which affected both surface- and deep-ocean circulation. The Tasman Gateway played a key role in regulating ocean throughflow between Australia and Antarctica. Southern Ocean surface currents through and around the Tasman Gateway have left recognizable tracers in the spatiotemporal distribution of plankton fossils, including organic-walled dinoflagellate cysts. This spatiotemporal distribution depends on physico-chemical properties of the water masses in which these organisms thrived. The degree to which the geographic path of surface currents (primarily controlled by tectonism) or their physico-chemical properties (significantly impacted by climate) have controlled the composition of the fossil assemblages has, however, remained unclear. In fact, it is yet poorly understood to what extent oceanographic response as a whole was dictated by climate change, independent of tectonics-induced oceanographic changes that operate on longer time scales. To disentangle the effects of tectonism and climate in the southwest Pacific Ocean, we target a climatic deviation from the long-term Eocene cooling trend, a 500 thousand year long global warming phase termed the Middle Eocene Climatic Optimum (MECO; ~40 Ma). The MECO warming is unrelated to regional tectonism, and thus provides a test case to investigate the oceans physiochemical response to climate change only. We reconstruct changes in surface-water circulation and temperature in and around the Tasman Gateway during the MECO through new palynological and organic geochemical records from the central Tasman Gateway (Ocean Drilling Program Site 1170), the Otway Basin (southeastern Australia) and the Hampden Section (New Zealand). Our results confirm that dinocyst communities track tectonically driven circulation patterns, yet the variability within these communities can be driven by superimposed temperature change. Together with published results from the east of the Tasman Gateway, our results suggest that as surface-ocean temperatures rose, the East Australian Current extended further southward during the peak of MECO warmth. Simultaneous with high sea-surface temperatures in the Tasman Gateway area, pollen assemblages indicate warm temperate rainforests with paratropical elements along the southeastern margin of Australia. Finally, based on new age constraints we suggest that a regional southeast Australian transgression might have been caused by sea-level rise during MECO. [ABSTRACT FROM AUTHOR]

The late Pliocene/early Pleistocene intensification of Northern Hemisphere glaciation (iNHG) ~ 2:5 million years ago (marine isotope stages, MIS, 100-96) stands out as an important tipping point in Earth's climate history, which strongly influenced oceanographic and climatic patterns including trade wind and upwelling strength in the eastern equatorial Pacific (EEP). The thermocline depth in the EEP, in turn, plays a pivotal role in the Earth's climate system: small changes in its depth associated with short-term climate phenomena such as the El Niño-Southern Oscillation can affect surface-water properties and the ocean-atmosphere exchange. However, thermocline dynamics in the EEP during the iNHG still remain unclear. While numerous studies have suggested a link between a thermocline shoaling in the EEP and Northern Hemisphere ice growth, other studies have indicated a stable thermocline depth during the iNHG; consequently, a causal relationship between thermocline dynamics and ice-sheet growth has been excluded. In light of these contradictory views, we have generated geochemical (planktic foraminiferal δ18O, δ13C and Mg= Ca), sedimentological (sand accumulation rates) and faunal (abundance data of thermocline-dwelling foraminifera) records for Ocean Drilling Program Site 849 located in the central region of the EEP. Our records span the interval from ~ 2:75 to 2.4 Ma (MIS G7-95), which is critical for understanding thermocline dynamics during the final phase of the iNHG. Our new records document a thermocline shoaling from ~ 2:64 to 2.55 Ma (MIS G2-101) and a relatively shallow thermocline from ~ 2:55 Ma onwards (MIS 101-95). This indicates a state change in thermocline depth at Site 849 shortly before the final phase of the iNHG. Ultimately, our data support the hypothesis that (sub-)tropical thermocline shoaling may have contributed to the development of large Northern Hemisphere ice sheets. [ABSTRACT FROM AUTHOR]

Next to atmospheric CO2 concentrations, iceproximal oceanographic conditions are a critical factor for the stability of Antarctic marine-terminating ice sheets. The Oligocene and Miocene epochs (~34-5 Myr ago) were time intervals with atmospheric CO2 concentrations between those of present-day and those expected for the near future. As such, these past analogues may provide insights into ice-sheet volume stability under warmer-than-presentday climates. We present organic-walled dinoflagellate cyst (dinocyst) assemblages from chronostratigraphically wellconstrained Oligocene to mid-Miocene sediments from Integrated Ocean Drilling Program (IODP) Site U1356. Situated offshore the Wilkes Land continental margin, East Antarctica, the sediments from Site U1356 have archived the dynamics of an ice sheet that is today mostly grounded below sea level. We interpret dinocyst assemblages in terms of paleoceanographic change on different timescales, i.e. with regard to both glacial-interglacial and long-term variability. Our record shows that a sea-ice-related dinocyst species, Selenopemphix antarctica, occurs only for the first 1.5 Myr of the early Oligocene, following the onset of full continental glaciation on Antarctica, and after the Mid-Miocene Climatic Optimum. Dinocysts suggest a weaker-than-modern sea-ice season for the remainder of the Oligocene and Miocene. The assemblages generally bear strong similarity to presentday open-ocean, high-nutrient settings north of the sea-ice edge, with episodic dominance of temperate species similar to those found in the present-day subtropical front. Oligotrophic and temperate surface waters prevailed over the site notably during interglacial times, suggesting that the positions of the (subpolar) oceanic frontal systems have varied in concordance with Oligocene-Miocene glacial-interglacial climate variability. [ABSTRACT FROM AUTHOR]

There is growing interest in the scientific community in reconstructing the paleoceanography of the Southern Ocean during the Oligocene--Miocene because these time intervals experienced atmospheric CO2 concentrations with relevance to our future. However, it has remained notoriously difficult to put the sedimentary archives used in these efforts into a temporal framework. This is at least partially due to the fact that the bio-events recorded in organic-walled dinoflagellate cysts (dinocysts), which often represent the only microfossil group preserved, have not yet been calibrated to the international timescale. Here we present dinocyst ranges from Oligocene--Miocene sediments drilled offshore the Wilkes Land continental margin, East Antarctica (Integrated Ocean Drilling Program (IODP) Hole U1356A). In addition, we apply statistical means to test a priori assumptions about whether the recorded taxa were deposited in situ or were reworked from older strata. Moreover, we describe two new dinocyst species, Selenopemphix brinkhuisii sp. nov. and Lejeunecysta adeliensis sp. nov., which are identified as important markers for regional stratigraphic analysis. Finally, we calibrate all identified dinocyst events to the international timescale using independent age control from calcareous nanoplankton and magnetostratigraphy from IODP Hole U1356A, and we propose a provisional dinoflagellate cyst zonation scheme for the Oligocene--Miocene of the Southern Ocean. [ABSTRACT FROM AUTHOR]

The late Pliocene/early Pleistocene intensification of Northern Hemisphere Glaciation (iNHG) ~ 2.5 million years ago (Marine Isotope Stages [MIS] 100-96) stands out as the most recent major tipping point in Earth's climate history. It strongly influenced oceanographic and climatic patterns including trade-wind and upwelling strength in the Eastern Equatorial Pacific (EEP). The thermocline depth in the EEP, in turn, plays a pivotal role in the evolution of short-term climate phenomena such the El Niño-Southern Oscillation, and thus bears important consequences for the Earth's climate system. However, thermocline dynamics in the EEP during to the iNHG have yet remained unclear. While numerous studies have suggested a link between a thermocline shoaling in the EEP and Northern Hemisphere ice growth, other studies have indicated a stable thermocline depth during iNHG, thereby excluding a causal relationship between thermocline dynamics and ice-sheet growth. In light of these contradictory views, we have generated geochemical (planktic foraminiferal δ18O, δ13C and Mg/Ca), sedimentological (sand-accumulation rates) and faunal (abundance data of thermocline-dwelling foraminifera) records for Ocean Drilling Program Site 849 located in the central part of the EEP. Our records span the interval from ~ 2.75 to 2.4 Ma (MIS G7-95), which is critical for understanding thermocline dynamics during the final phase of the iNHG. They document a thermocline shoaling from ~ 2.64 to 2.55 Ma (MIS G2-101) and a relatively shallow thermocline from ~ 2.55 Ma onwards (MIS 101-95). This indicates a state change in EEP thermocline depth shortly before the final phase of iNHG. Ultimately, our data support the hypothesis that (sub-)tropical thermocline shoaling may have contributed to the development of large Northern Hemisphere ice sheets. [ABSTRACT FROM AUTHOR]

Next to atmospheric CO2 concentrations, oceanographic conditions are a critical factor determining the stability of Antarctic marine-terminating ice sheets. The Oligocene and Miocene epochs (~ 34-5 Ma) were time intervals with atmospheric CO2 concentrations between those of present-day and those expected for the near future. As such, these time intervals may bear information to resolve the uncertainties that still exist in the projection of future ice-sheet volume decline. We present organic-walled dinoflagellate cyst (dinocyst) assemblages from chronostratigraphically well-constrained Oligocene to mid-Miocene sediments from Integrated Ocean Drilling Program Expedition (IODP) Site U1356. Situated offshore the Wilkes Land continental margin, East Antarctica, the sediment core has archived past dynamics of an ice sheet that is today mostly grounded below sea level. We interpret dinocyst assemblages in terms of paleoceanographic change on different time scales, i.e., on glacial-interglacial and long-term variability. Sea-ice indicators occur only for the first 1.5 Ma following the full Antarctic continental glaciation during the early Oligocene, and after the Middle Miocene Climatic Optimum. During the remainder of the Oligocene and Miocene dinocysts suggest a weaker-than-modern sea-ice season. The assemblages generally bear strong similarity to present-day open-ocean, high-nutrient settings north of the sea ice edge, with episodic dominance of temperate species similar to the present-day subtropical front. Oligotrophic and temperate surface waters prevailed over the site notably during interglacial time intervals, suggesting that the position of the (subpolar) oceanic frontal systems have varied in concordance with Oligocene-Miocene glacial-interglacial climate variability. [ABSTRACT FROM AUTHOR]

Antarctic palaeoclimate evolution and vegetation history after the formation of a continent-scale cryosphere at the Eocene-Oligocene boundary, 33.9 million years ago, has remained a matter of controversy. In particular, the reconstruction of terrestrial climate and vegetation has been strongly hampered by uncertainties in unambiguously identifying non-reworked as opposed to reworked sporomorphs that have been transported into Antarctic marine sedimentary records by waxing and waning ice sheets. Whereas reworked sporomorph grains over longer non-successive geological timescales are easily identifiable within younger sporomorph assemblages (e.g. Permian sporomorphs in Pliocene sediments), distinguishing non-reworked from reworked material in palynological assemblages over successive geological time periods (e.g. Eocene sporomorphs in Oligocene sediments) has remained problematic. This study presents a new quantitative approach to identifying non-reworked pollen assemblages in marine sediment cores from circum-Antarctic waters. We measured the fluorescence colour signature, including red, green, and blue fluorescence; brightness; intensity; and saturation values of selected pollen and spore taxa from Eocene, Oligocene, and Miocene sediments from the Wilkes Land margin Site U1356 (East Antarctica) recovered during Integrated Ocean Drilling Program (IODP) Expedition 318. Our study identified statistically significant differences in red-fluorescence values of non-reworked sporomorph taxa against age. We conclude that red fluorescence is a reliable parameter for identifying the presence of non-reworked pollen and spores in Antarctic marine sediment records from the circum-Antarctic realm that are influenced by glaciation and extensive reworking. Our study provides a new tool to accurately reconstruct Cenozoic terrestrial climate change on Antarctica using fossil pollen and spores. [ABSTRACT FROM AUTHOR]

Climate evolution of the Mediterranean region during the Holocene exhibits strong spatial and temporal variability. The spatial differentiation and temporal variability, as evident from different climate proxy datasets, has remained notoriously difficult for models to reproduce. In light of this complexity, we examine the previously described evidence for (i) opposing northern and southern precipitation regimes during the Holocene across the Mediterranean basin, and (ii) an east-to-west precipitation gradient or dipole during the early Holocene, from a wet eastern Mediterranean to dry western Mediterranean. Using quantitative climate information from marine and terrestrial pollen archives, we focus on two key time intervals, the early to mid-Holocene (8000 to 6000&cal&yrs&BP) and the late Holocene (4000 to 2000&yrs&BP), in order to test the above mentioned hypotheses on a Mediterranean-wide scale. Palynologically derived climate information is compared with the output of regional-scale climate-model simulations for the same time intervals.

Quantitative pollen-based precipitation estimates were generated along a longitudinal gradient from the Alboran (West) to the Aegean Sea (East); they are derived from terrestrial pollen records from Greece, Italy and Malta as well as from pollen records obtained from marine cores. Because seasonality represents a key parameter in Mediterranean climates, special attention was given to the reconstruction of season-specific climate information, notably summer and winter precipitation. The reconstructed climatic trends corroborate a previously described north-south partition of precipitation regimes during the Holocene. During the early Holocene, relatively wet conditions occurred in the south-central and eastern Mediterranean region, while drier conditions prevailed from 45°&N northwards. These patterns reversed during the late Holocene, with a wetter northern Mediterranean region and drier conditions in the east and south. More sites from the northern part of the Mediterranean basin are needed to further substantiate these observations. With regard to the existence of a west-east precipitation dipole during the Holocene, our pollen-based climate data show that the strength of this dipole is strongly linked to the seasonal parameter reconstructed: Early Holocene summers show a clear east-to-west gradient, with summer precipitation having been highest in the central and eastern Mediterranean and lowest over the western Mediterranean. In contrast, winter precipitation signals are less spatially coherent. A general drying trend occurred from the early to the late Holocene; particularly in the central and eastern Mediterranean. However, summer precipitation in the east remained above modern values, even during the late Holocene interval.

Pollen-inferred precipitation estimates were compared to regional-scale climate modelling simulations based on the HadAM3 GCM coupled to the dynamic HadSM3 and the high-resolution regional HadRM3 models. Climate model outputs and pollen-inferred precipitation estimates show remarkably good overall correspondence, although many simulated patterns are of marginal statistical significance. Nevertheless, models weakly support an east to west division in summer precipitation and there are suggestions that the eastern Mediterranean experienced wetter summer and winter conditions during the early Holocene and wetter summer conditions during the late Holocene. The extent to which summer monsoonal precipitation may have existed in the southern and eastern Mediterranean during the mid-Holocene remains an outstanding question; our model, consistent with other global models, does not suggest an extension of the African monsoon into the Mediterranean. Given the difficulty in modelling future climate change in Southern Europe, more simulations based on high resolution global models and very high resolution regional downscaling, perhaps even including transient simulations, are required to fully understand the patterns of change in winter and summer circulation patterns over the Mediterranean region [ABSTRACT FROM AUTHOR]

The Arctic history is characterized by high complexity with the Eocene Eurekan Orogeny representing one of the most important tectonic events. It resulted from plate re-organisation and opening of North Atlantic Ocean. Related to that, multiple and changing tectonic regimes affected the wider region, causing differentiated deformation in the different parts of the Arctic Ocean margin. Our study focuses on the area of northern Ellesmere Island (Canadian High Arctic), the northernmost land exposures studied so far. We investigate the temporal and spatial exhumation patterns along the western part of the Eurekan orogen in relation to the wider geotectonic settings. For this, we applied low-temprerature thermochronology (apatite fission track and (U-Th-Sm)/He) methods, along with palynological analysis from Paleocene to Early Eocene sedimentary deposits, and we correlate the results with the history of the spreading ridges in North Atlantic, as well as with climatic changes in the Arctic. Our data show that from Palaeocene to Oligocene, significant exhumation occurred along the Canadian Arctic margin. This was episodic, and highly differentiated from exhumation documented for southern Ellesmere Island. This difference may be well explained by considering our study area as part of a mega-transform-fault-system that extends all the way to Svalbard and NE Greenland. The activity of major strike-slip fault segments, according to the changing tectonic regimes, may have been responsible for increased exhumation and intermittent periods of quiescence. Since the Palaeocene, we distinguish four main periods of enhanced exhumation: (i) ~64-59 Ma (pre-Eurekan), related to crustal stretching and basin formation during the Paleocene, contemporaneously to the opening of the Baffin Bay / Labrador Sea; (ii) ~55 to 48 Ma (Eurekan I), related to strike-slip regime with continuation of deposition in the earlier formed Palaeocene basins, and controlled by the initiation of the Norwegian-Greenland Sea opening. Sediment deposition (that started already in the Palaeocene) occurred in a coastal swamp environment under a warm and humid climate that lasted into the Early Eocene; (iii) ~44 to 38 Ma (Eurekan II), again related to strike-slip movements, leading to rapid exhumation, and presumably to topography build-up and the rise of the Eurekan Belt, and inversion of the Palaeocene basins. This period coincides with the deposition of ice-rafted debris off East Greenland, and therefore we suggest the rise of the Eurekan belt was an important trigger for the formation of continental icecap(s) in Greenland during the Eocene and Oligocene. During this stage, collision between Greenland and the eastern Canadian archipelago may have caused slow-down of spreading rates in the Norwegian-Greenland Sea; and (iv) ~34 to 26 Ma (post-Eurekan), related to strike-slip movements, cessation of Labrador Sea/ Baffin Bay spreading, and unification of Greenland with the North American plate. We explain the cessation of rapid exhumation at ~26 Ma with continental separation between Northeast Greenland and Svalbard due to changes of the active spreading centres in the North Atlantic. This in turns caused decoupling of northern Ellesmere Island from strike-slip movements along the De Geer fault zone, eventually leading to the opening of the Fram Strait. [ABSTRACT FROM AUTHOR]