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•Based on a pollen record, we reconstructed the Mid-Miocene vegetation and climate changes in the Wushan Basin.•The pollen assemblages indicate that the MMCO was also a period of ...climatic instability.•The Mi-3b exerted a major impact on Mid-Miocene vegetation, climate and biota across central Asia.•Global climate change was the first order mechanism driving Mid-Miocene vegetation and climate changes in this region.
There remains no detailed record of the Middle Miocene vegetation and climatic changes which occurred in central Asia and their possible driving mechanisms. This is because there is still a lack of high resolution records. Here, we present a sporopollen record from the Wushan Basin on the northeastern Tibetan Plateau, central Asia, spanning the period ∼16.1–13.6Ma. The sporopollen record shows that a dense mixed forest growing in rather warm and humid climatic conditions was affected by a general drying trend during the period ∼16.1–15Ma. It demonstrates that although the climate was generally warm and humid during the Middle Miocene Climate Optimum (MMCO), it was also a time of climatic instability. The dense forest gave way to an open forest in response to a less humid climate between ∼15 and 14.4Ma. Dense mixed forest made a return with an increasingly humid climate during ∼14.4–13.8Ma. This vegetation and climatic succession could be associated with global cooling, or more particularly, a higher temperature rebound set against the background of a long-term cooling trend. A more open forest appearing in response to drier and colder climatic conditions dominated the study area during the ∼13.8–13.6Ma period. This could be compared to the rapid global cooling event Mi-3b. This significant global cooling event exerted a major impact on terrestrial vegetation, climate and biota. Our high resolution sporopollen record demonstrates that global climate changes could have been the first order driving force for the Middle Miocene vegetation and climate changes seen in the Wushan Basin in central continental Asia, with the tectonic uplift of the Tibetan Plateau probably playing a subordinate role.
The Cenozoic sedimentary succession of the Song Hong Basin, offshore North Vietnam is poor in marine microfossils, especially in the early Miocene and Oligocene and subsequently is poorly dated. ...However, it is rich in terrestrially derived pollen, spores, and freshwater algae. Biostratigraphic assemblages from two wells, termed Well A and B, have been evaluated using the approach of sequence biostratigraphy. The Oligocene fluvio-lacustrine succession through the Dinh Cao Formation displays multiple transgressive-regressive cycles comparable with those reported from the Cuu Long Basin to the south. By applying the local Cuu Long Oligocene palynological zonation (CLO zones), these cycles can be correlated directly with the cycles from the Cuu Long Basin, showing that sedimentation began during sequence 19, at about 29 Ma or a little earlier off-structure, over basement. The deepest interval within Well A yielded abundant pollen of the montane tree Alnus, and with the position of the basin within the northern tropics at 20oN, this suggests that in the mid Oligocene, the basin was intermontane, with Alnus widespread in swamps after the lake shallowed, at an altitude of at least 1000 m. The Miocene succession has been interpreted by reference to SEA (‘Southeast Asia’) depositional cycles following Morley et al. (2021). The evaluation suggests a complete early Miocene succession, varying from fluvial with low sedimentation rates for the youngest part of the Dinh Cao and Phong Chau Formations to shallow marine with increased sedimentation rates after 17 Ma through the Phu Cu Formation which correlates to the middle Miocene climatic optimum. There was a significant period of non-deposition from 13.8 to 9.5 Ma in Well A and from 13.5 to 11.8 Ma in well B, corresponding to the middle Miocene unconformity seen in other nearby basins.
•The Song Hong Basin stratigraphic succession is accurately dated for the first time.•Oligocene deposition displays a regular cyclicity, possibly related to 406 kyr climate cycles.•Correlation to the Cuu Long Oligocene basin is achieved using palynological zones.•Abundant Oligocene Alnus pollen shows that initial deposition was intermontane.•There is a well-developed middle Miocene unconformity from 13.5 to 11.8 Ma.
The first step of ice-sheet build-up on Antarctica at the Eocene-Oligocene boundary was followed by a phase of climate instability culminating during the Miocene with a warming event called the ...Middle Miocene Climate Optimum (MMCO), that ended with a marked cooling phase identified as the Middle Miocene Climate Transition (MMCT). While numerous benthic foraminifera δ18O and Mg/Ca data have been used to capture the global climate evolution during this interval of climate and ice-sheet instability, geochemical records from shallow-water carbonates still remain scarce. Yet such records are crucial to capture the diversity of regional environmental responses to global climate changes, and thus to better understand the behavior of our climate system during this critical interval. In this work, we test the potential of a multi-proxy approach (δ18O, clumped isotope (Δ47), strontium isotopes (87Sr/86Sr)) applied to bivalves recovered from the Liguro-Provencal Basin and Rhodano-Provencal basin (Northwestern of the Mediterranean Sea) to reconstruct the evolution of shallow seawater temperature and salinity in this region over the latest Oligocene to Middle Miocene interval (~10 Myrs). Our results highlight a local cooling in the northwestern Mediterranean Sea during the MMCO that contrasts with the warming observed in other regions, with seawater temperatures inferred from Δ47 analyses in the 13–18 °C range. These cool seawater temperatures recorded in the studied region during the MMCO are much cooler than those recorded in the open oceans, but are in agreement with the proliferation of bryozoan observed in the Castillon-Du-Gard area during this interval. Low bivalve δ18O values (−3.24‰ in average) are recorded during this episode are interpreted to reflect enhanced freshwater inputs, lowering local salinity and seawater δ18O. Such enhanced freshwater inputs point to a phase of enhanced hydrological cycle in the studied region, possibly linked to the global mid-Miocene warming event. Bivalve 87Sr/86Sr data mostly fall within the global seawater Sr isotope reference curve, but remain compatible with enhanced freshwater inputs during the Aquitanian and during the Langhian, as these inputs were probably not important enough to induce a large departure from the seawater curve at a regional scale.
•Coupled Δ47 - δ18O - 87Sr/86Sr analyses of bivalve shells decipher temperature and salinity•Cool seawater temperatures in the northwestern Mediterranean Sea during the MMCO•Enhanced hydrological cycle in western Europe during the MMCO
We reconstructed the orbital-scale variability of deep-water mass properties in the equatorial Indian Ocean during the late early to late Miocene (17.9 to 8.2 Ma) using benthic foraminiferal oxygen ...(δ18O) and carbon (δ13C) isotope records, in combination with X-ray fluorescence scanner elemental records at International Ocean Discovery Program (IODP) Site U1443. These records suggest that hyperthermal-like features (negative excursions in δ18O and δ13C coupled to suboxic and acidic conditions in the deep ocean) developed at eccentricity maxima during the Miocene Climatic Optimum (MCO; 16.9 to 14.7 Ma) on an almost ice-free Earth. Elevated Mn and U concentrations indicate that bottom and pore waters in the Indian Ocean became progressively suboxic during the later phase of the MCO. A fundamental change in deep-water circulation occurred after the expansion of the East Antarctic Ice Sheet during the Middle Miocene Climatic Transition (MMCT; 14.7 to ~13.8 Ma). Stepwise increases in δ18O at ~13.8 and ~13.1 Ma concurred with dampening of deep-water δ18O and δ13C variability, as Antarctic ice cover expanded and became more stable. A marked improvement in deep-water oxygenation after ~13.5 Ma coincided with the end of the last δ13C maximum of the Monterey Excursion. Carbonate dissolution in the equatorial Indian Ocean intensified and prolonged episodes of reduced carbonate deposition during the Carbonate Crash interval (~12.6 to 8.6 Ma) were not restricted to eccentricity maxima as during the MCO. Rising Mn and U concentrations after ~9 Ma indicate decreased oxygenation of bottom and pore waters, associated with the onset of the Biogenic Bloom in the Indian Ocean.
•10-Myr Indian Ocean Miocene record of deep water oxygenation/carbonate preservation.•Warming at Miocene Climatic Optimum onset synchronous in Pacific and Indian Oceans.•Miocene Climatic Optimum hyperthermals coupled to acidic and suboxic deep waters.•Marked improvement in Indian Ocean deep water oxygenation after ~13.5 Ma.
The Miocene Climatic Optimum (MCO) provides important insights into how the climate system operates under elevated temperatures and atmospheric CO2 levels. Few western North Atlantic paleotemperature ...or paleoecological records exist from the MCO, despite their importance for understanding both regional and global climate dynamics. Here we present quantitative MCO paleoecological data from the western North Atlantic, specifically from the Baltimore Gas & Electric (BG&E) marine sediment core from southern Maryland, USA. We examine alkenones and planktic foraminifera and document the first sea surface temperature (SST) and productivity estimates for the MCO and the Middle Miocene Climate Transition (MMCT) from the continental shelf. Increased levels of planktic foraminifer species diversity and surface productivity accompany high sea level intervals of the MCO, indicating coastal upwelling. Cooling episodes correlate to unconformities in the BG&E core that reflect sea level lowstands; these and sedimentary cycles tie the record to eccentricity-paced Antarctic ice sheet growth and decay. This dynamic record not only captures the variability in SST, sea level and coastal productivity during the warm MCO and the transition to cooler global temperatures during the MMCT, but it also demonstrates the variability in local conditions within and between intervals of high sea level.
•MCO and MMCT inner shelf records link local responses to global climate.•Global shifts in SST and sea level are recorded in shallow marine sediments.•Coastal upwelling likely characterized the MCO Atlantic Coastal Plain.•Local conditions vary within and between intervals of high sea level.
The evolution and timing of the inception of the South Asian monsoon (SAM) has been one of the most debated climatic mysteries amongst climate workers. The Himalaya–Tibetan plateau (HTP) complex has ...been implicated as the main driver for SAM circulation. However, there are intense debates as to whether there was a critical elevation of the HTP complex that drove the SAM and at what time such a critical elevation was attained. Also, the role of the HTP complex in driving the SAM has been questioned in recent climate model studies. The model simulation suggests a major intensification of the SAM as early as ~30Ma, whereas marine records from the Arabian Sea indicate a major strengthening of the South Asian summer or southwest monsoon winds between 10 and 8Ma. The continental vegetation, on the other hand, captured a major transition from C3 to C4 type during 8–7Ma but whether this change was local or global and whether this transition was driven by monsoonal precipitation is not unanimously accepted. Our new record from Ocean Drilling Program holes located off the Oman margin and on the Owen Ridge, western Arabian Sea shows appearance of monsoon wind proxy planktic foraminifer Globigerina bulloides, a significant increase in total organic carbon, and a negative shift in stable carbon isotope record of benthic foraminifera at ~12.9Ma. These proxies indicate that present day South Asian monsoon wind system began to develop during the late Middle Miocene (~12.9Ma) and summer monsoon was in its full strength in the late Miocene (~7Ma). From 11 to 7Ma, the summer monsoon was weaker when winter monsoon was stronger.
•We present high resolution new proxy record of South Asian monsoon wind strength from Arabian Sea•The study finds new evidence of the onset of the South Asian monsoon wind system similar to the present day•South Asian monsoon initiated/strengthened in the late Middle Miocene
Abstract
Dissolved oxygen (O
2
) is essential for most ocean ecosystems, fuelling organisms’ respiration and facilitating the cycling of carbon and nutrients. Oxygen measurements have been ...interpreted to indicate that the ocean’s oxygen-deficient zones (ODZs) are expanding under global warming
1,2
. However, models provide an unclear picture of future ODZ change in both the near term and the long term
3–6
. The paleoclimate record can help explore the possible range of ODZ changes in warmer-than-modern periods. Here we use foraminifera-bound nitrogen (N) isotopes to show that water-column denitrification in the eastern tropical North Pacific was greatly reduced during the Middle Miocene Climatic Optimum (MMCO) and the Early Eocene Climatic Optimum (EECO). Because denitrification is restricted to oxygen-poor waters, our results indicate that, in these two Cenozoic periods of sustained warmth, ODZs were contracted, not expanded. ODZ contraction may have arisen from a decrease in upwelling-fuelled biological productivity in the tropical Pacific, which would have reduced oxygen demand in the subsurface. Alternatively, invigoration of deep-water ventilation by the Southern Ocean may have weakened the ocean’s ‘biological carbon pump’, which would have increased deep-ocean oxygen. The mechanism at play would have determined whether the ODZ contractions occurred in step with the warming or took centuries or millennia to develop. Thus, although our results from the Cenozoic do not necessarily apply to the near-term future, they might imply that global warming may eventually cause ODZ contraction.
The Middle Miocene was a period of prominent climatic change, marked by the Mid-Miocene Climatic Optimum (MMCO) and the subsequent global cooling due to a decline of the atmospheric CO2 ...concentrations (pCO2). In addition to this, the closure of the Tethyan Seaway driven by the Arab-Eurasia collision also had an important effect on the paleoclimatic changes during this period. In this study, we use the Community Earth System Model 1.2.2 (CESM 1.2.2) to simulate the effects of global cooling (i.e. pCO2 decline) and the closure of the Tethyan Seaway on the North African and South Asian climates. Our results show that the global cooling led to a precipitation decrease over both North Africa and South Asia, whereas the closure of the Tethyan Seaway resulted in a precipitation decrease over North Africa but an increase over South Asia. The opposite effects over North Africa and South Asia are due to an increased moisture transport from North Africa to South Asia induced by stronger summer atmospheric circulation when the Tethyan Seaway is closed. We further show that the reconstructed records of drying conditions over North Africa during the warming period from the late Early Miocene to the early Middle Miocene from previous studies can be partly explained by the narrowing of the Tethyan Seaway and its climatic continuing deterioration due to the subsequent final closure and global cooling. Both are precursory conditions for the formation of the Sahara desert. The stronger South Asian monsoon during the Middle Miocene transient cooling period found in previous studies can be partially attributed to the final closure of the Tethyan Seaway.
•Closure of the Tethyan Seaway led to precipitation increase in South Asia but decrease in North Africa.•Closure of the Tethyan Seaway led to enhanced moisture transport from North Africa to South Asia.•Global cooling led to precipitation decrease in North Africa and South Asia during the MMCT.
The middle Miocene climate optimum (MMCO, 14.3–16.3Ma) was a warm climatic interval that may provide an analog for future climates in the context of ongoing global warming. In continental regions, ...however, reconstruction of the MMCO remains controversial due to the lack of robust proxies from continuous sedimentary sequences. In this study, we investigated the composition and relative abundance of pedogenic iron oxides in the Zhuanglang Red Clay deposits from the western Chinese Loess Plateau, using rock magnetic, color reflectance, and geochemical analyses. Our aim was to assess the climatic sensitivity of iron oxides in order to produce robust proxies for reconstructing regional temperature and moisture relationships during the MMCO. Our results indicate that the MMCO in the study region was characterized by high effective precipitation, relatively high temperature and strong pedogenesis, suggesting a strong East Asian summer monsoon (EASM). The strengthening of the EASM intensity during the MMCO is consistent with records of enhanced terrigenous chemical weathering in the South China Sea, and the occurrence of a relatively warm and humid climate on a global scale, implying the strong coupling between Asian monsoon evolution and global climate change during the middle Miocene. In addition, the magnetic susceptibility record of the Zhuanglang Red Clay demonstrates that eccentricity played the dominant role in controlling the EASM during the MMCO.
•The pedogenic iron oxides were investigated in the ZL Red Clay from 12 to18Ma.•χ, a*, and Fed/FeT are sensitive to precipitation, temperature, and pedogenesis.•Iron oxide proxies indicate strong EASM intensity during the MMCO in this region.•Asian monsoon evolution is linked with global climate change during middle Miocene.
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