Middle Miocene Climate Transition Research Articles

Miocene Climatic Optimum and Middle Miocene Climate Transition: a foraminiferal record from the central Ross Sea, Antarctica

Abstract. The Ross Sea record of the Miocene Climatic Optimum (MCO; ∼ 16.9–14.7 Ma) and the Middle Miocene Climate Transition (MMCT; ∼ 14.7–13.8 Ma) provides critical insights into Antarctic ocean–cryosphere interactions during a time of extreme warmth and subsequent cooling. Here we report on Lower to Middle Miocene foraminiferal assemblages from the International Ocean Discovery Program (IODP) Site U1521 on the outer shelf of the central Ross Sea to identify regional shifts in environmental and water mass conditions and trace continental shelf evolution. We identified seven benthic biofacies clusters, dominated by abundant Globocassidulina subglobosa (a proposed indicator of proto-Circumpolar Deep Water, pCDW), Uvigerina cf. U. fueguina (high productivity and enhanced bottom-water currents), Nonionella spp. (high productivity), or Melonis spp. (high productivity) using a Q-mode cluster analysis to develop preliminary regional paleoenvironmental interpretations. Four unique assemblages, including Globobulimina cf. G. auriculata (high productivity and low oxygen), are also identified. Unit IV (representing the early MCO event) is a short-lived (∼ 80 ka), progradational, clast-poor sandy diamictite, likely deposited during deglaciation; the upper part of Unit IV is transitional with overlying Unit III. Unit IV sediments contain the most persistently abundant and diverse foraminiferal assemblages recovered at U1521 because they are mud-rich and diatom-poor, despite very high sedimentation rates. The benthic assemblages shift between Globocassidulina and Uvigerina dominance, suggesting changes in the pCDW influence relative to productivity and/or current activity. We suggest the abundance of Uvigerina (a shelf-edge proxy) in Unit IV records the northward progradation of the Ross continental shelf at this location during the late Early to Middle Miocene. Unit III (MCO) was deposited in an open-marine setting, evident by the ice-rafted detritus or debris (IRD) clast-free, diatom-rich/diatom-bearing muds. The sporadic nature of foraminiferal abundances in Unit III is likely due to intervals of terrigenous mud alternating with more diatom-rich/diatom-bearing muds. As in Unit IV, the muddier lithologies (higher natural gamma ray (NGR) values) are more likely to preserve calcareous foraminifera, whereas the most diatom-rich sediments (lower NGR values) are more corrosive to carbonate. We interpret the muddier intervals as interglacials with incursions of pCDW, as indicated by increased Globocassidulina subglobosa, and sporadic occurrences of rare warmer-water planktic foraminifera. Collectively, these multiple incursions of warmer-water planktic foraminifera provide evidence for polar amplification in the Ross Sea during the MCO and MMCT. The diatom-rich muds are interpreted as glacials during the MCO with open-marine conditions and higher productivity. The dominance of Globobulimina in the upper part of Unit III corresponds with the carbon maximum of Carbon Maxima 2 (CM2) and low-oxygen conditions in the sediments at ∼ 16.1 Ma. Subsequent glaciation (including Mi2, Miocene Isotope event 2), marine-based ice sheet grounding, and erosion on the shallow shelf are recorded by the widespread Ross Sea Unconformity 4 (RSU4; ∼ 15.95–14.2 Ma) at Site U1521. Unit II (MMCT) likely represents sedimentation in the interval between the RSU4 and the Mi3 (Miocene Isotope event 3) glaciation at ∼ 13.9–13.8 Ma. The benthic biofacies composition of Unit II shows a further increase in neritic taxa, including Elphidium magellanicum and Epistominella vitrea, suggesting continued shoaling of the continental shelf, which facilitated the growth of marine-based ice sheets during the Middle Miocene. Our initial correlation between Site U1521 and the ANtarctic geological DRILLing Project (ANDRILL) site, AND-2A, yields similar environmental interpretations, including peak warm events 3 and 4 during the MCO, supported by the foraminifera and unit lithologies. Suspected glacial intervals during the MCO, including Mi2 at the top of Unit III, correlate well with the reconstructed deep-sea estimates of ice volume changes (seawater δ18Osw record) from the Ocean Drilling Program (ODP) Site 1171 on the South Tasman Rise.

Re-organization of Pacific overturning circulation across the Miocene Climate Optimum

The response of the ocean overturning circulation to global warming remains controversial. Here, we integrate a multiproxy record from International Ocean Discovery Program Site U1490 in the western equatorial Pacific with published data from the Pacific, Southern and Indian Oceans to investigate the evolution of deep water circulation during the Miocene Climate Optimum (MCO) and Middle Miocene Climate Transition (MMCT). We find that the northward export of southern-sourced deep waters was closely tied to high-latitude climate and Antarctic ice cover variations. Global warming during the MCO drove a progressive decrease in carbonate ion concentration and density stratification, shifting the overturning from intermediate to deeper waters. In the western equatorial Pacific, carbonate dissolution was compensated by increased pelagic productivity, resulting in overall elevated carbonate accumulation rates after ~16 Ma. Stepwise global cooling and Antarctic glacial expansion during the MMCT promoted a gradual improvement in carbonate preservation and the initiation of a near-modern Pacific overturning circulation. We infer that changes in the latitudinal thermal gradient and in Southern Ocean zonal wind stress and upper ocean stratification drove radically different modes of deep water formation and overturning across the MCO and MMCT.

Phylogenomic Analysis of Target Enrichment and Transcriptome Data Uncovers Rapid Radiation and Extensive Hybridization in Slipper Orchid Genus Cypripedium L.

Cypripedium is the most widespread and morphologically diverse genus of slipper orchids. Despite several published phylogenies, the topology and monophyly of its infrageneric taxa remained uncertain. Here, we aimed to reconstruct a robust section-level phylogeny of Cypripedium and explore its evolutionary history using target capture data for the first time. We used the orchid-specific bait set Orchidaceae963 in combination with transcriptomic data to reconstruct the phylogeny of Cypripedium based on 913 nuclear loci, covering all 13 sections. Subsequently, we investigated discordance among nuclear and chloroplast trees, estimated divergence times and ancestral ranges, searched for anomaly zones, polytomies, and diversification rate shifts, and identified potential gene (genome) duplication and hybridization events. All sections were recovered as monophyletic, contrary to the two subsections within sect. Cypripedium. The two subclades within this section did not correspond to its subsections but matched the geographic distribution of their species. Additionally, we discovered high levels of discordance in the short backbone branches of the genus and within sect. Cypripedium, which can be attributed to hybridization events detected based on phylogenetic network analyses, and incomplete lineage sorting caused by rapid radiation. Our biogeographic analysis suggested a Neotropical origin of the genus during the Oligocene (~30 Ma), with a lineage of potentially hybrid origin spreading to the Old World in the Early Miocene (~22 Ma). The rapid radiation at the backbone likely occurred in Southeast Asia around the Middle Miocene Climatic Transition (~15-13 Ma), followed by several independent dispersals back to the New World. Moreover, the Pliocene-Quaternary glacial cycles may have contributed to further speciation and reticulate evolution within Cypripedium. Our study provided novel insights into the evolutionary history of Cypripedium based on high-throughput molecular data, shedding light on the dynamics of its distribution and diversity patterns from its origin to the present.

The Neogene of the Vienna Basin – a synthesis

We present a revision of geological and paleontological data of the Vienna Basin (VB). The deposits comprise Miocene terrestrial, lacustrine and marine sediments, corresponding the Ottnangian, Karpatian, Badenian, Sarmatian and Pannonian regional stages (= Burdigalian to Tortonian). New 3D seismics and the analyses of hundreds of core samples resulted in a wealth of information. Absolute ages were published for two tuffs and the lithostratigraphy was largely formalized. The evolution of the paleoenvironments in the VB strongly reflects the impact by global climatic events such as the Miocene Climatic Optimum, the Middle Miocene Climate Transition and the Tortonian Thermal Maximum. Global 3 rd order sea level cycles are reflected in the successions of marine depositional environments. Major drops of the relative sea level caused strong erosion along the basin margins, with up to 300 m paleorelief. The strongest phase of erosion was linked to tectonics. Around the Early/Middle Miocene boundary, Lower Miocene strata became tilted during the Styrian Tectonic Phase. Up to 600-m-deep subaerial canyons incised the VB at that time. A second phase of basin-wide erosion occurred during the Late Miocene when more than 200 m of upper Pannonian strata became removed from large parts of the VB after 9 Ma.

The Central Paratethys Sea—rise and demise of a Miocene European marine biodiversity hotspot

The Miocene Climate Optimum (MCO, ~ 17–14 Ma) was a time of extraordinary marine biodiversity in the Circum-Mediterranean Region. This boom is best recorded in the deposits of the vanished Central Paratethys Sea, which covered large parts of central to southeastern Europe. This sea harbored an extraordinary tropical to subtropical biotic diversity. Here, we present a georeferenced dataset of 859 gastropod species and discuss geodynamics and climate as the main drivers to explain the changes in diversity. The tectonic reorganization around the Early/Middle Miocene boundary resulted in the formation of an archipelago-like landscape and favorable conditions of the MCO allowed the establishment of coral reefs. Both factors increased habitat heterogeneity, which boosted species richness. The subsequent cooling during the Middle Miocene Climate Transition (~ 14–13 Ma) caused a drastic decline in biodiversity of about 67%. Among the most severely hit groups were corallivorous gastropods, reflecting the loss of coral reefs. Deep-water faunas experienced a loss by 57% of the species due to changing patterns in circulation. The low sea level led to a biogeographic fragmentation reflected in higher turnover rates. The largest turnover occurred with the onset of the Sarmatian when bottom water dysoxia eradicated the deep-water fauna whilst surface waters-dwelling planktotrophic species underwent a crisis.

New metatherians from Collon Cura Formation at Cerro Zeballos (Middle Miocene), Chubut province, Argentina

ABSTRACT High latitude metatherians from the Middle Miocene of Patagonia (Argentina and Chile) are only known from medium-sized Sparassodonta, and undescribed taxa collected in several fossil localities of Collon Cura (Argentina) and Río Frías (Chile) Formations, although the latter has several species reported and mostly recognised for the slightly older Santa Cruz Formation. In this contribution we describe five new genera and six new species of metatherians from Collon Cura Formation at Cerro Zeballos, Chubut province (Argentina), based on 11 specimens that notably increased the diversity of Collon Cura Formation. The specimens were assigned to the orders Paucituberculata (Palaeothentoidea), Sparassodonta (Hathliacynidae), and Polydolopimorphia (Argyrolagidae), and are taxa of small size, including the smallest palaeothentoids known to date. Also, the Palaeothentoidea of Cerro Zeballos are amongst the youngest of the superfamily in Patagonia. The new specimens described herein show morphological differences with other metatherian associations already known, probably related to adaptive changes coincident with the Middle Miocene Climatic Transition and the beginning of a faunal turnover.

The Miocene lacustrine carbonate factory of the Ñirihuau Formation, Ñirihuau Basin, North Patagonian Andes, Argentina

AbstractFacies, microfacies and stable isotope analyses of limestone beds in the northernmost Ñirihuau Basin, North Patagonian Andes, Argentina, document and constrain the past hydrological, sedimentological and climate conditions that prevailed during the deposition of a lacustrine system between ca 15 and 13 Ma. This palaeoenvironment is recorded in the middle section of the Ñirihuau Formation, which holds significance because: (1) It was deposited during a transition from an extensional to a compressional tectonic regime; (2) it spans the Middle Miocene Climatic Optimum and the beginning of the Middle Miocene Climatic Transition; and (3) it contains limestone beds interbedded within a 600 m thick interval of mudstones and siltstones, along with intercalated sandstone and volcaniclastic bodies. Two detailed sedimentary logs were surveyed along the Arroyo Las Bayas, at the western and eastern flank of the David Syncline. Limestones from both stratigraphic sections were sampled as well as isolated limestone beds from two other sites. One facies association was defined and interpreted as a perennial lake associated with a deltaic system and dominated by detrital clastic material. It comprises Facies 1 (Marginal lacustrine) and Facies 2 (Lower delta plain); in both, the presence of grainstones and calcimudstones stands out. Through petrography and cathodoluminescence studies of these continental carbonates, nine microfacies were identified: (a) Intraclastic grainstone, (b) Homogeneous calcimudstone, (c) Silty grainstone, (d) Disrupted micrite, (e) Birds eye micrite, (f) Bioclastic mudstone, (g) Calcimudstone with sparse detrital grains, (h) Fenestral micrite, (i) Stromatolitic boundstone. These indicate mainly bio‐induced subaqueous carbonate precipitation and subordinate deposition by tractive flows with short‐distance transport on a littoral lacustrine environment. Most of these microfacies exhibit very early diagenesis (eogenesis) effects. These features, and the geochemistry results, indicate that they were deposited in a palaeolake system under temperate to warm and humid conditions.

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.