Middle Miocene Climate Transition Research Articles

Structure of the eastern Arabian Sea upper water column in the middle Miocene: Implications for the development of the South Asian monsoon

Global atmospheric circulation experienced drastic changes during the Middle Miocene climate transition (MMCT∼14.7–13.0 Ma) possibly related to the glaciation on West-Antarctica. Palaeoceanographic reconstructions have found that upwelling in the western Arabian Sea due to summer South Asian monsoon (SAM) winds likely occurred since ∼14.7 Ma, with fully modern-like monsoonal wind patterns after the end of the MMCT at around ∼13 Ma. Whether the changes in monsoonal circulation since ∼14.7 Ma are also associated with upper ocean hydrographic changes in the eastern Arabian Sea (EAS) is currently not known for the middle Miocene. To this end, the difference in Mg/Ca-based temperatures (ΔT°C) of surface-dwelling with lower mixed layer/thermocline-dwelling and sub-thermocline-dwelling planktic foraminifera was reconstructed to estimate the upper ocean thermal gradient at Site NGHP-01-01 A in the EAS and thus changes in the upper surface water column structure, i.e., mixed layer depth (MLD) and the depth of thermocline (DOT). The Mg/Ca-temperature estimates from the upper mixed layer down to the sub-thermocline show a prominent cooling trend between ∼14.2 Ma and 13.2 Ma. The upper water column reconstructions reveal weaker mixing, a shallower thermocline, and therefore a well stratified upper water column in the EAS after ∼14.2 Ma. We suggest that the weaker mixing and shallower thermocline in the EAS after ∼14.2 Ma are most likely due to an intensification of the summer SAM. A strong salinity stratification and/or the formation of a barrier layer (BL) because of increased SAM rainfall and advection of low salinity water may also have contributed to a weaker mixing and shallower thermocline in the EAS during the investigated time period. The change in the upper ocean hydrography in the EAS after ∼14.2 Ma fits well into the emerging picture of monsoonal-driven upper ocean hydrographic changes in the equatorial and northern Indian Ocean due to an intensification of the summer SAM since ∼14.7 Ma.

Astronomically forced paleoclimate and sea-level changes recorded in the continental margin of the South China Sea over the past ∼23 m.y.

The Earth has witnessed the emergence of continental-sized ice sheets, starting with Antarctica and gradually extending to both hemispheres over the past 40 million years. These ice accumulations have had a dramatic impact on both paleoclimate and sea level, substantially influencing sediment deposition in the continental margins. However, understanding sediment accumulation on an orbital scale in continental margins remains limited because of the scarcity of high-resolution, chronologically constrained sedimentary records. Here, we conducted a highly resolved cyclostratigraphic analysis based on natural gamma radiation (GR) series in depth domain at the continental margin of the South China Sea. We established a 22.8 m.y.-long high-resolution astronomical time scale spanning from the Miocene to the Quaternary by tuning the GR records to the global deep-sea benthic foraminifera carbon isotope curves and the 405 k.y. eccentricity cycles. The m.y.-scale sea-level changes since the Miocene were reconstructed through the sedimentary noise modeling of the 405-k.y.-tuned GR series. These reconstructions aligned with regional and global sea-level changes. The phase correlation between the filtered 1.2 m.y. cycles of sea-level change curves (dynamic noise after orbital tuning and ρ1 median models) from δ13Cbenthic and tuned GR series and the 1.2 m.y. obliquity amplitude modulation cycles revealed a shift from an anti-phase to an in-phase relationship across the middle Miocene climate transition (ca. 13.8 Ma), suggesting extensive expansion of the Antarctic ice sheet played a key role. In addition, a shift from an in-phase to an anti-phase relationship during the late Miocene (ca. 8 Ma and 5.3 Ma), indicating ephemeral expansion of the Arctic ice sheets or the changes in carbon cycle involving the terrestrial and deep ocean carbon reservoirs, might be the primary driver of eustatic changes. Furthermore, obliquity forcing and changes in meridional gradients in insolation that transported poleward flux of heat, moisture, and precipitation increased ice accumulation in both pole ice sheets and nonlinearly transferred high-latitude signals to low-latitude regions. This phenomenon is supported by the observation of strong obliquity signals in low latitude during global climate cooling interval. Our results suggest that m.y.-scale sea-level variations respond to astronomically induced climate change and ice sheet dynamics of both poles. This work contributes a highly resolved low-latitude geological archive to the future reconstruction of paleoclimate evolution on a global scale.

Late Miocene onset of the modern Antarctic Circumpolar Current

The Antarctic Circumpolar Current plays a pivotal role in global climate through its strong influence on the global overturning circulation, ocean heat and CO2 uptake. However, when and how the Antarctic Circumpolar Current reached its modern-like characteristics remains disputed. Here we present neodymium isotope and sortable silt records from sediment cores in the Southwest Pacific and South Indian oceans spanning the past 31 million years. Our data indicate that a circumpolar current like that of today did not exist before the late Miocene cooling. These findings suggest that the emergence of a homogeneous and deep-reaching strong Antarctic Circumpolar Current was not linked solely to the opening and deepening of Southern Ocean Gateways triggering continental-scale Antarctic Ice Sheet expansion during the Eocene–Oligocene Transition (∼34 Ma). Instead, we find that besides tectonic pre-conditioning, the expansion of the Antarctic Ice Sheet and sea ice since the middle Miocene Climate Transition (∼14 Ma) played a crucial role. This led to stronger density contrast and intensified Southern Westerly Winds across the Southern Ocean, establishing a vigorous deep-reaching circumpolar flow and an enhanced global overturning circulation, which amplified the late Cenozoic global cooling.

Goethite in the Red Clay sequences on the western Chinese Loess Plateau and its responses to the middle Miocene Climate Transition

The middle Miocene Climate Transition (MMCT, ca. 14 Ma), with a sharp drop in temperature, is a key transition period in the Earth’s climatic and biotic evolution. However, our knowledge of how arid-monsoon environments were coupled during the middle Miocene Climate transition remains limited, due to the lack of robust proxies from well-dated sedimentary sequences covering the MMCT in monsoon areas. Goethite is an abundant iron oxide in soils, which is sensitive to persistent wet and cold environments. However, aluminium (Al) substitution is ubiquitous in the pedogenic process, and the Al substitution for goethite in the Red Clay sequence is unclear. In this study, we combined the rock magnetic and DRS methods to assess the Al substitution level of goethite and quantify the goethite content in the Zhuanglang (ZL) Red Clay deposits from the western Chinese Loess Plateau. Our results show that Al substitution in the ZL Red Clay sequence is relatively stable, facilitating the quantification of goethite using this approach. Goethite content displays an increasing trend from approximately 0.2% at 18 Ma to 0.5% at 14 Ma, and then gradually decreased towards the late Miocene in the ZL Red Clay deposits. The maximum goethite production at around 14 Ma indicates a cold and wet climate condition during MMCO at this location. Together with other palaeoenvironmental proxies of this sequences and other sedimentary sequences in the Tianshui Basin, we suggest that climate conditions during the MMCT in the monsoonal western CLP were relatively cold and wet, which was different from the cold and dry climate in the central Asian. Global cooling and the uplift of the Tibetan Plateau might be predominantly responsible for the cold and wet climate during the MMCT.

The emergence of modern zoogeographic regions in Asia examined through climate–dental trait association patterns

The complex and contrasted distribution of terrestrial biota in Asia has been linked to active tectonics and dramatic climatic changes during the Neogene. However, the timings of the emergence of these distributional patterns and the underlying climatic and tectonic mechanisms remain disputed. Here, we apply a computational data analysis technique, called redescription mining, to track these spatiotemporal phenomena by studying the associations between the prevailing herbivore dental traits of mammalian communities and climatic conditions during the Neogene. Our results indicate that the modern latitudinal zoogeographic division emerged after the Middle Miocene climatic transition, and that the modern monsoonal zoogeographic pattern emerged during the late Late Miocene. Furthermore, the presence of a montane forest biodiversity hotspot in the Hengduan Mountains alongside Alpine fauna on the Tibetan Plateau suggests that the modern distribution patterns may have already existed since the Pliocene.

Evolution of Oceanography of the Central Northwest Pacific Over the Past 10 Million Years With Focus on Late Miocene Global Cooling

AbstractSince the middle Miocene climatic transition, the Earth's climate has steadily cooled. The late Miocene global cooling (LMGC) and the Northern Hemisphere Glaciation (NHG) were two key cooling events occurring during this time. To better understand the mechanisms underlying these cooling events, changes in radiolarian microfossil assemblages were examined in this study, aiming at the reconstructing of oceanographic changes that have occurred at Ocean Drilling Program site 1208 during the last 10 million years. Sea surface temperatures (SSTs) were reconstructed based on radiolarian species that were extant 0–10 million years ago. Reconstructed SSTs were then compared with previously published alkenone‐based SSTs at site 1208, and it was found that overall, using SSTs based only on extant radiolarian species yielded a correct record for the last 10 million years. However, large discrepancies were observed between radiolarian‐ and alkenone‐based SSTs during LMGC and the NHG. These discrepancies were attributed to the sustained influence of subsurface water (at depths from ∼50 to 100 m) on assemblages of radiolarians during extreme cooling events. Relative abundances of other radiolarian groups indicated that during LMGC, there was a reorganization of the regional oceanography that probably weakened the Pacific meridional overturning circulation, increased the meridional temperature gradient, and caused a southward migration of the subtropical front. Probably, the North Pacific intermediate water expanded southeastward during NHG.

Biotic response of plankton communities to Middle to Late Miocene monsoon wind and nutrient flux changes in the Oman margin upwelling zone

Abstract. Understanding past dynamics of upwelling cells is an important aspect of assessing potential upwelling changes in future climate change scenarios. Our present understanding of nutrient fluxes throughout the world's oceans emphasizes the importance of intermediate waters transporting nutrients from the Antarctic divergence into the middle and lower latitudes. These nutrient-rich waters fuel productivity within wind-driven upwelling cells in all major oceans. One such upwelling system is located along the Oman margin in the western Arabian Sea (WAS). Driven by cross-hemispheric winds, the WAS upwelling zone's intense productivity led to the formation of one of the most extensive oxygen minimum zones known today. In this study covering the Middle to Late Miocene at Ocean Drilling Program (ODP) Site 722, we investigate the inception of upwelling-derived primary productivity. This study presents new plankton assemblage data in the context of existing model- and data-based evidence constraining the tectonic and atmospheric boundary conditions for upwelling in the WAS. With this research, we build upon the original planktonic foraminifer-based research by Dick Kroon in 1991 as part of his research based on the ODP LEG 117. We show that monsoonal winds likely sustained upwelling since the emergence of the Arabian Peninsula after the Miocene Climatic Optimum (MCO) ∼ 14.7 Ma, with fully monsoonal conditions occurring since the end of the Middle Miocene Climatic Transition (MMCT) at ∼ 13 Ma. However, changing nutrient fluxes through Antarctic Intermediate and sub-Antarctic Mode Waters (AAIW/SAMW) were only established after ∼ 12 Ma. Rare occurrences of diatom frustules correspond to the maximum abundances of Reticulofenestra haqii and Reticulofenestra antarctica, indicating higher upwelling-derived nutrient levels. By 11 Ma, diatom abundance increases significantly, leading to alternating diatom blooms and high-nutrient-adapted nannoplankton taxa. These changes in primary producers are also well reflected in geochemical proxies with increasing δ15Norg. values (> 6 ‰) and high organic carbon accumulation. These proxies provide further independent evidence for high productivity and the onset of denitrification simultaneously. Our multi-proxy-based evaluation of Site 722 primary producers provides evidence for a stepwise evolution of Middle to Late Miocene productivity in the western Arabian Sea for the first time. The absence of a clear correlation with existing deep marine climate records suggests that both local wind patterns and intermediate water nutrient changes likely modulated productivity in the western Arabian Sea during the Middle to Late Miocene. Finally, we show that using a multi-proxy record provides novel insights into how plankton responded to changing nutrient conditions through time in a monsoon-wind-driven upwelling zone.

Redox evolution in the subtropical Northwest Pacific across the Middle Miocene Climate Transition

The climate transition during the middle Miocene promoted a series of environmental evolutions. However, the response of deep-sea redox conditions to this climate transition remains unclear. Here, we reconstruct the redox conditions in the subtropical Northwest Pacific during the Middle Miocene Climate Transition (MMCT) using the uranium and thorium data from drill cores at different water depths. Our findings indicate that the oxygenation level in the bathypelagic site (U1505) was weak at the early MMCT (14.6–14.1 Ma), elevated at the middle MMCT (14.1–13.3 Ma), and weak again at the late MMCT (13.3–12.8 Ma). In contrast, the oxygenation level in the abyssopelagic site (U1438) was strong in the early MMCT and weak in the middle and late MMCT. Combined with the changes in local productivity and ocean circulation, we argue that the varying oxygenation evolution trends at different water depths are probably related to the change in water mass structure driven by climate cooling from the early to the middle MMCT. However, local export production may be the main controlling factor during the late MMCT. Moreover, the decrease in deep-sea oxygenation across the MMCT also reflects the deepening of the respired carbon pool in the subtropical Northwest Pacific, which possibly contributed to the decline in atmospheric CO2 at that time.

Late Miocene paleoecology and paleoclimate in the central High Plains of North America reconstructed from paleopedological, ichnological, and stable isotope analyses of the Ogallala Formation in western Kansas, USA

The late Miocene was a critical time in the development of the North American Great Plains marked by paleoclimate-driven biotic change, including faunal turnovers and the spread of C4 dominated grasslands. The large volume of sediment shed from the Rocky Mountain region during this time preserves a record of these transitions, which can be informed by previously undescribed paleosol and trace fossil properties from the Ogallala Formation of the central High Plains. The purpose of this paper is to reconstruct paleoenvironments, paleoclimate, and paleoecology from outcrops of the Ogallala Formation in western Kansas by integrating paleopedology, ichnology, and stable isotope geochemistry. Eleven lithofacies are recognized in the study area, mostly massive to crudely stratified, fine-to coarse-grained sandstone and pebbly gravel with stratigraphically uncommon fine-grained lithofacies restricted to thinly bedded intervals or lens-shaped geometries within the sand-dominated strata. These host five pedotypes: 1) calcic Vertisols developed on overbank fines, 2) Entisols developed on braid bar gravels, 3) Entisols developed on volcaniclastic sediment lenses, 4) calcic Inceptisols developed on coarse sandy channel fills, and 5) calcareous Mollisols developed on fine sandy bedforms and channel fills. We recognize ten ichnogenera within paleosols, including burrows attributed to ants, bees, beetles or hemipterans, vinegarroon-like arthropods, fossorial reptiles, seed caching mammals, and large carnivorous mammals. Organic carbon stable isotopes indicate that the flora consisted entirely of C3 plants, and paleosols and trace-fossil evidence suggest a tree-limited savanna environment with patches of unvegetated soil. Paleoclimate proxies from paleosol and trace fossil properties indicate mean annual air temperatures between 8 °C and 20 °C with seasonal differences of up to 14 °C between mean monthly temperatures of the warmest and coolest months. Mean annual precipitation was likely between 250 mm and 460 mm with a seasonal difference of up to 250 mm between mean monthly precipitation of the driest and wettest months. While hymenopteran tracemakers were active, soil surface temperatures reached at least 30 °C and moisture content was between 10% and 20%. With timing of deposition constrained biostratigraphically to the Barstovian through Hemphillian, our paleoclimate interpretations are most consistent with the period of relative climate stability after Middle Miocene Climate Transition cooling and before Late Miocene Cooling (∼13.8 Ma–7 Ma). The trace-fossil assemblage reveals previously unknown biodiversity among soil arthropods, as well as important trophic connections between the belowground and aboveground components of the terrestrial food web.

Late Cenozoic sea-surface-temperature evolution of the South Atlantic Ocean

Abstract. At present, a strong latitudinal sea-surface-temperature (SST) gradient of ∼ 16 ∘C exists across the Southern Ocean, maintained by the Antarctic Circumpolar Current (ACC) and a set of complex frontal systems. Together with the Antarctic ice masses, this system has formed one of the most important global climate regulators. The timing of the onset of the ACC system, its development towards modern-day strength and the consequences for the latitudinal SST gradient around the southern Atlantic Ocean are still uncertain. Here we present new TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms)-derived SST records from two sites located east of Drake Passage (south-western South Atlantic) to assist in better understanding two critical time intervals of prominent climate transitions during the Cenozoic: the late Eocene–early Oligocene (Ocean Drilling Program, ODP, Site 696) and Middle–Late Miocene (IODP Site U1536) transitions. Our results show temperate conditions (20–11 ∘C) during the first time interval, with a weaker latitudinal SST gradient (∼ 8 ∘C) across the Atlantic sector of the Southern Ocean compared to present day. We ascribe the similarity in SSTs between Sites 696 and 511 in the late Eocene–early Oligocene South Atlantic to a persistent, strong subpolar gyre circulation connecting the sites, which can only exist in the absence of a strong throughflow across the Drake Passage. Surprisingly, the southern South Atlantic record Site 696 shows comparable SSTs (∼ 12–14 ∘C) during both the earliest Oligocene oxygen isotope step (EOIS, ∼ 33.65 Ma) and the Miocene Climatic Optimum (MCO, ∼ 16.5 Ma). Apparently, maximum Oligocene Antarctic ice volume could coexist with warm ice-proximal surface ocean conditions, while at similar ocean temperatures, the Middle Miocene Antarctic ice sheet was likely reduced. Only a few Middle–Late Miocene (discontinuous) high-latitude records exist due to ice advances causing unconformities. Our low-resolution Site U1536 record of southern South Atlantic SSTs cooled to ∼ 5 ∘C during the Middle Miocene Climate Transition (MMCT, 14 Ma), making it the coldest oceanic region in the poorly recorded Antarctic realm and likely the main location for deep-water formation. The already-cold south-western South Atlantic conditions at the MMCT with relatively moderate additional cooling during the Late Miocene contrasts with the profound cooling in the lower latitudes and other sectors of the Southern Ocean due to northward expansion of the Southern Ocean frontal systems.

Timing and tempo of organic carbon burial in the Monterey Formation of the Santa Barbara Basin and relationships with Miocene climate

Understanding the transfer of carbon between Earth's surface reservoirs is necessary for interpreting climate transitions in Earth history and predicting future climate change. Warming associated with the 16.9–14.7 Ma Miocene Climate Optimum and subsequent cooling during the 14.7–13.8 Ma Middle Miocene Climate Transition provide opportunities to study carbon cycle dynamics in the geologically recent past. The Monterey Hypothesis interprets the Middle Miocene Climate Transition cooling as part of a positive feedback in which enhanced organic carbon burial on the eastern Pacific margin drew down atmospheric CO2. This idea has been supported by the correlation of organic-rich deposits in the Monterey Formation in coastal California with the mid-Miocene Monterey Event, a globally-observed positive shift in the δ13C of marine carbonates that may be indicative of elevated burial of δ13C-depleted organic carbon. Here, we use 31 new U-Pb zircon laser ablation inductively coupled plasma mass spectrometry ages and 14 new isotope dilution thermal ionization mass spectrometry ages from volcanic ash beds in the Monterey Formation along the Santa Barbara coast to constrain the timing and tempo of organic carbon mass accumulation in some of the most organic-rich rocks in California. The new age model demonstrates that peaks in organic carbon mass accumulation rate in the Monterey Formation do not coincide with the Monterey Event, and that total organic carbon content in the Miocene Santa Barbara Basin is inversely correlated with sedimentation rate. We propose that changes in organic carbon burial rates in the Monterey Formation were driven by a combination of sea-level change and local tectonically-mediated basin formation, which provided first-order controls on sedimentation rate, and that organic carbon burial in the Monterey Formation is better described as a response to, rather than a driver of, global climate.

Dominant eccentricity cycles in paleoenvironmental variabilities recorded by pelagic sediments in the western Pacific during 15–11 Ma

The middle Miocene Climate Transition (MMCT) was a major event that resulted in a global cooling, providing valuable insights into the transition from global warming to cooling. In this study, we analyze a well-dated sediment core, GC18, to establish potential linkages between sedimentary processes in the western Pacific and global climate changes during the MMCT. The main findings are as follows: (1) Integrating authigenic 10Be/9Be dating, magnetostratigraphy, and astronomical tuning, we determined that core GC18 predominantly deposited during ~11–15 Ma, with a sedimentation rate of 0.5–2 mm/kyr; (2) Chemical scanning identified ten elements, including Al, Si, P, Ca, Sr, Mn, Fe, Ti, Ni, and Ba, which can be clustered into four groups; (3) These four groups serve as proxies for eolian dust (Al and Si), biogenic debris (P, Ca, and Sr), deep-sea redox changes (Mn, Fe, Ti, and Ni), and marine productivity (Ba). Based on these results, we find that all four processes were influenced by eccentricity cycles, indicating the dominant role of orbital forcing during the MMCT. Overall, sedimentary archives in the western Pacific offer valuable evidence of the various climatic processes during the middle Miocene and warrant further investigations.

Terrigenous provenance of late Oligocene–Miocene sediments in the central basin of the South China Sea and its implications for chemical weathering and climate change

The oceanic red beds (ORB) are unique pelagic sediments in the global ocean, providing excellent archives for understanding the chemical weathering and relevant climate change in their source areas. Clay minerals, Nd and Sr isotopes, and major elements of terrigenous components were investigated on the ORB (27.0–10.6 Ma) recovered at International Ocean Discovery Program (IODP) Expedition 367 Site U1499 in the central basin of the South China Sea, to determine the terrigenous provenance and constrain the main factors controlling terrestrial chemical weathering during the late Oligocene–Miocene. The clay mineral assemblage of the ORB is characterized by dominant smectite (52–95%), moderate illite (4–23%), and minor kaolinite (0–19%) and chlorite (0–7%). Nd isotope value (εNd) and 87Sr/86Sr ratio vary from −12.1 to −6.4 and from 0.7122 to 0.7275, respectively. These results display a three-stage temporal variation pattern with the most significant shift in the sedimentary sources at ∼23 Ma. Stage I (27.0–23.0 Ma) sediments were mainly derived from Palawan, whereas Stages II (23.0–14.0 Ma) and III (14.0–10.0 Ma) sediments were derived from South China and Luzon. Simple NdSr isotope mixing model result shows the greater terrigenous contribution of South China from Stage II (49–60%) to Stage III (64–80%), consistent with an increase in illite, chlorite, and kaolinite contents. Chemical index of alteration, K2O/Na2O ratio, Al2O3/TiO2 ratio, smectite/(illite + chlorite) ratio, and illite crystallinity indicate moderate chemical weathering intensity through the late Oligocene to the Miocene. The weathering was influenced by the lithology of parent rocks in Stage I and primarily by the climatic evolution during Stages II and III. The synchronous reduction of various indicators from the ORB reveals the weakening of terrestrial weathering around the South China Sea at ∼14 Ma, induced by the global cooling at the Middle Miocene Climate Transition.

Modeling the effects of global cooling and the Tethyan Seaway closure on North African and South Asian climates during the Middle Miocene Climate Transition

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.

Multispecies Planktonic and Benthic Foraminiferal Stable Isotopes from North Atlantic Subtropical Site 558: Thermocline Intensification During the Mid-Miocene Climate Transition

Abstract We reconstruct the subtropical North Atlantic water column structure during the Miocene Climate Optimum warming (MCO; 17–14.8 Ma) and the Middle Miocene Climate Transition cooling (MMCT; 14.8–12.8 Ma) by analyzing δ18O and δ13C in four species of foraminifera (surface dwellers Dentoglobigerina altispira and Trilobatus quadrilobatus, thermocline dweller Dentoglobigerina venezuelana, and benthic Planulina wuellerstorfi) from Site 558 (37.8°N). At the end of the MCO, δ18O of surface and thermocline dwellers increased by >1‰, suggesting at least 2°C cooling in the upper ocean as ice growth increased global δ18Osw by ∼0.5‰. The difference in δ18O values between thermocline and surface-dwelling species increased during the MMCT, coinciding with the development of a largely permanent East Antarctic Ice Sheet, and persisted into the Late Miocene. We interpret this increase in vertical δ18O gradient as a strengthening of the thermocline due to intensification of subtropical gyre circulation in response to the MMCT cooling.

East Asian summer monsoon evolution recorded by the middle Miocene pelagic reddish clay, South China Sea

The East Asian summer monsoon (EASM) is essential for the livelihoods of billions of people in East and Southeast Asia. Its present features and evolution history of the Quaternary have been well studied, yet how did EASM operate in much older geological times, especially before the late Miocene, is relatively less constrained. Scientific drillings conducted in the South China Sea (SCS) offer excellent opportunities to resolve this issue to a certain extent considering the EASM has been prevailing in this area since at least the earliest Miocene. In this study, mineralogical and stratigraphic studies were carried out on the middle Miocene pelagic reddish clay recovered at International Ocean Discovery Program Site U1433 using clay mineralogical and natural gamma radiation data. Our results show that the smectite content and illite crystallinity of the deep SCS clay were direct consequences of the EASM intensity varying in precession rhythm and modulated by eccentricity cycles. A high-resolution temporal framework constructed with onboard age controls and cyclostratigraphic analyses demonstrates that the pelagic reddish clay from Site U1433 was accumulated from 17.17 to 12.69 Ma. During this period, deep-sea sedimentation of the SCS was controlled by the EASM and influenced by the sea-level change. Importantly, clay mineral is proved to be an efficient terrigenous standard of astronomical tuning and stratigraphic correlation for pelagic sediments. It is also verified that the EASM intensified during the Miocene Climatic Optimum and the following Middle Miocene Climate Transition, and induced higher humidity and seasonality, respectively. Then, the EASM gradually weakened since the expansion of the East Antarctic Ice Sheet.

Middle Miocene paleoenvironmental change and paleoelevation of the Lunpola Basin, Central Tibet

The Tibetan region underwent diachronous differential rises during the Cenozoic era. Its elevation history is critical for understanding the processes of mountain building, and for evaluating its climatic effects on atmospheric circulations. However, knowledge of paleoenvironment and paleoelevation in different geological domains of the Tibetan Plateau is still limited. Here we provide chronology of lacustrine deposits of the Upper Dingqing Formation in the Lunpola Basin in Central Tibet, which yields a middle Miocene age (15–12 Ma). Both stable isotope and palynological records indicate a climatic transition at ∼13.8 Ma marked by a change towards a cooler and drier climate after the Middle Miocene Climate Transition driven by global climatic cooling. Pollen and spore assemblages suggest that the vegetation was dominated by mixed coniferous–broadleaved forest with some grasslands. New palynological elevation reconstructions constrained by the co-existence approach yield a maximum paleo-elevation of 3100–3600 m (average 3350 m) of the Lunpola Basin, which was consistent with a maximum elevation of 3400 m based on floating-leaf plants. Our new data do not support previous views that the Lunpola Basin achieved its current elevation as early as the late Eocene or early Oligocene, but instead we suggest there has been a rise of up to 1200 m since 12 million years ago. Our results bring new light on both the paleoenvironmental reconstructions and the paleo-elevation estimates of the Lunpola Basin, Central Tibet.

Carbon Sequestration of the Middle Miocene Sunda Shelf Facilitated Global Climate Change

AbstractLong‐term organic carbon (OC) burial on continental shelves has been widely recognized for regulating atmospheric CO2 (pCO2) and the global climate. However, quantitatively assessing shelf OC burial's role and process in prominent climate transitions is challenging. Using 367 drilling sites, we evaluated the impact of OC burial on the Sunda Shelf, the world's largest tropical shelf, on the middle Miocene carbon perturbation and climate change. Comparing the Miocene Climatic Optimum (MCO) and the Middle Miocene Climate Transition (MMCT) results demonstrated that OC was buried faster during the MMCT with an increment of 0.07–0.14 × 103 GtC/Myr. This would cause an additional 34.77–69.16 ppm pCO2 sequestration, contributing at least one‐sixth of the global pCO2 reduction. We found OC burial regulated the long‐standing Monterey carbon isotope excursion via negative and positive feedbacks during the MCO and MMCT, respectively. Expanded terrestrial carbon reservoirs and enhanced burial efficiency were key steps in the latter positive feedback.

East Antarctic Ice Sheet variability during the middle Miocene Climate Transition captured in drill cores from the Friis Hills, Transantarctic Mountains

This study describes 16 well-dated, terrestrial glacial sedimentary cycles deposited during astronomically paced climate cycles from the termination of the Miocene Climatic Optimum (MCO) through the middle Miocene Climate Transition (MMCT) (15.1−13.8 Ma) in the Friis Hills, Transantarctic Mountains, Antarctica. Three locations were continuously cored (79% recovery) to a maximum depth of 50.48 m through a succession of interbedded till sheets and fossil-bearing, fluvio-lacustrine sediments. A composite chronostratigraphic framework is presented for the cores based on the previous mapping, a seismic refraction survey that defines basin geometry, and a new, integrated age model based on paleomagnetic stratigraphy that is constrained by radioisotopic 40Ar/39Ar numeric ages on two newly identified silicic tephra. The paleoecologic and sedimentologic characteristics of organic-rich lithologies are relatively consistent up-section, which implies that successively younger interglacial deposits during the MMCT represented broadly similar environmental and climatic conditions. During these interglacials, the Friis Hills hinterland was likely ice-free. Major disconformities in the section suggest a transition to colder climates, and after ca. 14.6 Ma, thicker, more extensive and erosive ice cover occurred across the Friis Hills during glacial episodes. Diamictites in the upper three cycles suggest that climate cooled and became drier after ca. 14.2 Ma. However, cyclical retreat of the ice and a return to warm climate conditions during interglacials continued through ca. 13.9 Ma. These direct records reflect a highly variable East Antarctic Ice Sheet margin but show that the ice margin became progressively more extensive during successive glacial intervals, which is consistent with a cooling trend toward more glacial values in the far-field benthic foraminifera δ18O proxy ice volume and temperature record. Age constraints show that glacial-interglacial variability at the terrestrial margin of the East Antarctic Ice Sheet was primarily paced by astronomical precession (∼23 k.y.) through the onset of the MMCT (15−14.7 Ma). Precession-driven cycles are modulated by short-period (∼100 k.y.) eccentricity cycles. Intervals of maximum eccentricity (high seasonality) coincide with sedimentary cycles comprising thin diamictites and relatively thick interglacial sandstone and mudstone units. Intervals of minimum eccentricity (low seasonality) coincide with sedimentary cycles comprising thick diamictites and relatively thin interglacial sedimentary deposits. Major disconformities in the Friis Hills succession that span more than ∼100 k.y. reflect episodes of expansion of erosive ice across, and well beyond, the Transantarctic Mountains and coincide with nodes in eccentricity (∼400 k.y.). These relationships suggest that during relatively warm intervals in the middle Miocene, the East Antarctic Ice Sheet expanded and contracted over 100 k.y. cycles, while its margins continued to fluctuate at higher (∼23 k.y.) frequency. After 14.5 Ma, obliquity is the dominant frequency in δ18O records, marking a period during which large regions of the Antarctic Ice Sheet grounded in marine environments.

Very high Middle Miocene surface productivity on the U.S. mid-Atlantic shelf amid glacioeustatic sea level variability

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.