Miocene Climatic Optimum Research Articles

How SST during Ice Age to the New Epoch, Last Interglacial to Holocene, and Miocene and Pliocene's affect the intensity of the Asian summer monsoon

Hypothesis from out group is: in warmer climates monsoons dry seasons were drier. Our group aim is to identify the locations that had greater temperatures in the past in order to predict the power of the monsoon in a warmer future. In order to comprehend how monsoons might impact a hot globe, we shall find the relationship between ocean surface temperature and monsoon strength. Essay mainly focus on three major factors of the past to explore the purpose: the historical period, the last interglacial and Holocene, and the Miocene Climatic optimum.

Global warming drove the Mid-Miocene climate humidification in the northern Tibetan Plateau

The Miocene is a key transitional period for the Asian monsoon evolution and establishment of modern-like geomorphic landscapes in the northern Tibetan Plateau. Elucidating the Miocene paleoenvironmental changes and their relationships with climate and tectonics are significant in understanding future hydrological changes in the northern Tibet under a warming climate. In this study, detailed lithological and rock magnetic investigations were conducted on a Miocene fluvial-lacustrine sediment sequence from the Linxia Basin in Northeast Tibet, to understand the paleoenvironmental and palaeohydrological response to the tectonics and global climatic change, especially to the effects of the Middle Miocene Climatic optimum (MMCO). The results demonstrated that strong waterlogged reducing environments during MMCO precluded the occurrences of pedogenesis in the Linxia Basin owing to highly humid climates. As the precipitation decreased significantly before and after MMCO, pedogenesis occurred increasingly owing to temperate and well-drained conditions. A synthesis of the lithological and magnetic variations in Miocene fluvial-lacustrine sediments further indicated that climate humidification prevailed in most areas of the northern Tibetan Plateau during MMCO. These observations demonstrate that global climate change, rather than the tectonic uplift, significantly influenced the Mid-Miocene paleoenvironmental evolution in northern Tibet.

A well-oxygenated eastern tropical Pacific during the warm Miocene.

The oxygen content of the oceans is susceptible to climate change and has declined in recent decades1, with the largest effect in oxygen-deficient zones (ODZs)2, that is, mid-depth ocean regions with oxygen concentrations <5 μmol kg-1 (ref. 3). Earth-system-model simulations of climate warming predict that ODZs will expand until at least 2100. The response on timescales of hundreds to thousands of years, however, remains uncertain3-5. Here we investigate changes in the response of ocean oxygenation during the warmer-than-present Miocene Climatic Optimum (MCO; 17.0-14.8 million years ago (Ma)). Our planktic foraminifera I/Ca and δ15N data, palaeoceanographic proxies sensitive to ODZ extent and intensity, indicate that dissolved-oxygen concentrations in the eastern tropical Pacific (ETP) exceeded 100 µmol kg-1 during the MCO. Paired Mg/Ca-derived temperature data suggest that an ODZ developed in response to an increased west-to-east temperature gradient and shoaling of the ETP thermocline. Our records align with model simulations of data from recent decades to centuries6,7, suggesting that weaker equatorial Pacific trade winds during warm periods may lead to decreased upwellingin the ETP, causing equatorial productivity and subsurface oxygen demand to be less concentrated in the east. These findings shed light on how warm-climate states such as during the MCO may affect ocean oxygenation. If the MCO is considered as a possible analogue for future warming, our findings seem to support models suggesting that the recent deoxygenation trend and expansion of the ETP ODZ may eventually reverse3,4.

Eruption history of the Columbia River Basalt Group constrained by high-precision U-Pb and 40Ar/39Ar geochronology

Large igneous province volcanism of the Columbia River Basalt Group (CRBG) has been suggested to play a causal role in elevated global temperatures and atmospheric carbon dioxide levels of the Miocene Climate Optimum (MCO). However, assessing the connection between volcanism and warming is dependent upon an accurate and precise chronology for the timing and duration of CRBG emplacement. Building on our previous work (Kasbohm and Schoene, 2018), we present fifteen new high-precision ages, using CA-ID-TIMS U-Pb on zircon and multi-collector 40Ar/39Ar on basaltic groundmass, to provide a detailed dual-chronometer timeline for CRBG eruptions. We use both sets of new ages and precise stratigraphic positions of our samples in an integrated Markov Chain Monte Carlo model to calculate average long-term emplacement rates for main-phase CRBG volcanism of 0.2-0.9 km3/a, with a high likelihood of one prominent hiatus of 60-120 kyr duration occurring after main-phase emplacement. We analyzed trace elements and hafnium isotopes of each dated zircon from CRBG interbeds. The compositions are consistent with both Cascades subduction volcanism and evolved syn-CRBG volcanism proximal to the depositional area. Our age model also yields ages for all magnetic field reversals during the main phase of CRBG emplacement, which can be used to improve calibrations of Miocene paleoclimate records. We find that main-phase CRBG emplacement is coincident with the greatest sustained warmth of the MCO in astronomically-tuned records. Our work shows the power of using both U-Pb and 40Ar/39Ar geochronology in an integrated stratigraphic context to assess data reliability and develop the most robust age model possible for large igneous province emplacement.

Middle Miocene fire activity and C4 vegetation expansion in the Barstow Formation, California, USA

Abstract C4 grassland ecosystems expanded across North America between ca. 8 and 3 Ma. Studies of ungulate enamel and environmental indicators from the middle Miocene Barstow Formation of southern California (USA) have demonstrated the presence of C4 vegetation prior to the late Miocene expansion of C4 grasslands. Fire promotes the growth of modern C4 grasslands and may have contributed to the Miocene expansion of C4 vegetation. We analyzed the concentration and accumulation rate (CHAR) of microscopic charred particles from sediment samples spanning the Barstow Formation in order to investigate the relationship between fire activity, canopy cover, and the presence of C4 vegetation. Concentration and CHAR were low throughout the formation then increased dramatically at 13.5 Ma. Enriched values of δ13C from soil organic matter and phytolith counts indicate the presence of C4 grasses and seasonally dry, open-canopy habitats at this time. The spike in concentration coincides with climatic cooling and drying in southern California after the Miocene Climatic Optimum. Increased fire activity may have contributed to habitat opening from forest to woodland and promoted the spread of C4 plants. This is the first charcoal record of fire activity from the middle Miocene of southwestern North America.

Simulation of the climate and ocean circulations in the Middle Miocene Climate Optimum by a coupled model FGOALS-g3

Simulation of the climate and ocean circulations in the Middle Miocene Climate Optimum by a coupled model FGOALS-g3

A tropical soil (Lixisol) identified in the northernmost part of the Mediterranean (Istria, Croatia)

Terra rossa soils on the Istrian peninsula are generally considered polygenetic relict soils. They overlie and mark the youngest subaerial unconformity in the northwestern part of the Adriatic Carbonate Platform and are susceptible to erosional and redepositional processes. Terra rossa soils represent a valuable archive of information that can be used to understand present and past soil formation processes in the context of climate variability and landscape dynamics, especially in low-lying parts of the karst landscape where thicker deposits are typically found. We studied a 3 m thick terra rossa profile in a karst depression on the northernmost part of the southwestern Istrian planation surface and analysed the micromorphology, physical/chemical properties, geochemistry, bulk and clay mineralogy, and composition of heavy and light mineral fractions. The profile shows several signs of colluviation and polygenesis with clay distribution along the profile, clearly indicating lithic discontinuities. Kaolinisation, along with iron oxide formation, is the dominant process in the studied profile. Ferralitisation and lessivage processes were diagnosed throughout the profile, with intense clay translocation occurring despite the extremely stable soil microaggregates. In accordance with the WRB system, the profile is classified as Rhodic Lixisol (Clayic, Aric, Cutanic). This implies that some terra rossa soils, previously classified as e.g. Cambisols or Luvisols, might actually be Lixisols or other (sub-)tropical soils (e.g. Nitisols), in which relict properties are preserved in the present climate. The finding of a Lixisol in the study area indicates the old age of the surface of the karst depression and provides the first data on the burial history of the southwestern Istrian planation surface. We suggest the older Quaternary interglacials, the mid-Piacenzian Warm Period (Pliocene) and the Miocene Climatic Optimum as favourable periods for the formation of the studied soil.

Neoisoptera repeatedly colonised Madagascar after the Middle Miocene climatic optimum

Madagascar is home to many endemic plant and animal species owing to its ancient isolation from other landmasses. This unique fauna includes several lineages of termites, a group of insects known for their key role in organic matter decomposition in many terrestrial ecosystems. How and when termites colonised Madagascar remains unknown. In this study, we used 601 mitochondrial genomes, 93 of which were generated from Malagasy samples, to infer the global historical biogeography of Neoisoptera, a lineage containing more than 80% of described termite species. Our results indicate that Neoisoptera colonised Madagascar between 7 and 10 times independently during the Miocene, between 8.4 and 16.6 Ma (95% HPD: 6.1–19.9 Ma). This timing matches that of the colonization of Australia by Neoisoptera. Furthermore, the taxonomic composition of the Neoisopteran fauna of Madagascar and Australia are strikingly similar, with Madagascar harbouring an additional two lineages absent from Australia. Therefore, akin to Australia, Neoisoptera colonised Madagascar during the global expansion of grasslands, possibly helped by the ecological opportunities arising from the spread of this new biome.

Latest Oligocene to middle Miocene low-latitude calcareous nannofossil biostratigraphy and paleoenvironmental interpretations of stratigraphic well 4, Sinú-San Jacinto onshore basin, northwest Colombia

Calcareous nannofossils are one of the main fossil groups used in the petroleum industry to determine the relative age and correlation of lithofacies identified in gas and oil wells. A limited number of biostratigraphic zonation schemes based on nanoplankton and paleoceanographic reconstructions have been suggested to onshore Colombian Caribbean. Most of them are compiled in several publications for the northern Colombian units (Mejía-Molina et al., 2008, 2010). In this study, a medium to high-resolution biostratigraphy study was carried out on one of the best-known Neogene sedimentary successions in the Sinú-San Jacinto onshore Basin, where the Stratigraphic Well 4 (SW4) was drilled by Ecopetrol, the Colombian state-run oil company. The nannoplankton biostratigraphic zonation proposed here constitutes the first calcareous nanoplankton research to construct a biochronologic scheme for the Neogene of northern Colombia, which has direct applications for oil exploration, global energy matrix and the transition to clean energy in the onshore areas of the Sinú-San Jacinto domain. The section studied has a thickness of ∼269 m and comprises mainly a very thick succession of claystone and mudstone with occasional intercalations of silty sandstones corresponding to the El Carmen Formation. The studied interval is assigned to the nannofossils Zones NP25 (CP19) to NN4 (CN3) of Martini (1971) and Okada and Bukry (1980), spanning from the latest Oligocene (latest Chattian) to middle Miocene (early Langhian), approximately 24 Ma to 15 Ma. Intervals without fossil content were identified in the record and could constitute hiatuses, but the nannofloral events, which define the limits of the biozones, can be identified. Global and regional climatic changes which occurred in the Caribbean region at those times are reflected in important changes in the fossil assemblages and abundance pattern of nanoplankton. These could indicate variability in environmental conditions linked to temperature and increased nutrients within the nannofossil communities in this part of the San Jacinto folded belt (SJFB), northwest Colombia. Middle Miocene Climate Optimum (MMCO) episodes of “blooming” intervals of Sphenolithus were identified, accompanied with a drastic reduction in total nannofossil abundance and CaCO3 content. Since the Caribbean geology is framed in a very complex scenario, the data acquired and presented in this research has a potential application to improve the current stratigraphy in the area, targeted to hydrocarbon exploration and studies related to the global energy matrix and the transition to clean energy. It also strengthens the paleoceanographic reconstructions based on these organisms at low latitudes.

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 &amp;gt;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.

Xenorhinos bhatnagari sp. nov., a new, nasal-emitting trident bat (Rhinonycteridae, Rhinolophoidea) from early Miocene forests in northern Australia.

A new Old World trident bat (Rhinonycteridae) is described from an early Miocene cave deposit in the Riversleigh World Heritage Area, northwestern Queensland, Australia. Living rhinonycterids comprise a small family of insect-eating, nasal-emitting rhinolophoid bats from Africa, Madagascar, Seychelles, the Middle East, and northern Australia. The new fossil species is one of at least 12 rhinonycterid species known from the Oligo-Miocene cave deposits at Riversleigh. We refer the new species to the genus Xenorhinos (Hand, Journal of Vertebrate Paleontology, 18, 430-439, 1998a) because it shares a number of unusual cranial features with the type and only other species of the genus, X. halli, including a broad rostrum, very wide interorbital region, pronounced ventral flexion of the rostrum, very constricted sphenoidal bridge, and, within the nasal fossa, reduced bony division, and relatively well developed turbinals. Xenorhinos species lived in northern Australia during the global Miocene Climatic Optimum, in closed wet forests, unlike the drier habitats that trident bats largely inhabit today. Our phylogenetic analysis suggests that more than one dispersal event gave rise to the Australian rhinonycterid radiation, with two lineages having sister-group relationships with non-Australian taxa.

Corrigendum

Corrigendum

A new Early–Middle Miocene phalangerid (Marsupialia: Phalangeridae) from the Riversleigh World Heritage Area, Boodjamulla (Lawn Hill) National Park, northwestern Queensland

Archerus johntoniae represents a new genus and species of phalangerid possum from the Early to Middle Miocene of the Riversleigh World Heritage Area, northwestern Queensland. The new taxon is the tenth extinct phalangerid species to be described and is the seventh species recognized from the diverse Riversleigh fossil assemblages. A. johntoniae has its closest morphometric affinities (such as relatively short molars, narrow premolars, an anteriorly narrow upper first molar, and posteriorly narrow fourth molars) with trichosurine and stem phalangerids. Morphologically, A. johntoniae has similarities to phalangerines and trichosurines, but is most dissimilar to other fossil phalangerids. The new species is one of the smallest (ca 1.3 kg) phalangerids known and may have occupied Early and Middle Miocene frugivorous/insectivorous niches demarcated from other phalangerids by body size and diet. A. johntoniae is one of a minority of Riversleigh species to have survived a significant decline in marsupial diversity associated with a rapid spike in global temperature at the Middle Miocene Climatic Optimum.Troy Myers [t.myers@unsw.edu.au], Earth and Sustainability Science Research Centre, School of Biological, Earth & Environmental Sciences, UNSW Sydney, NSW 2052, Australia; Kirsten Crosby [Kirsten.crosby@gmail.com], GHD Group Pty Ltd, Level 15, 133 Castlereagh St, Sydney, NSW, 2000 (currently), School of Biological, Earth & Environmental Sciences, UNSW Sydney, NSW 2052, Australia.

The Miocene Climatic Optimum at the interface of epicontinental sea and large continent: A case study from the Middle Miocene of the Eastern Paratethys

The Miocene Climate Optimum (MCO) and the subsequent Miocene Climate Transition (MCT) are important biotic, environmental and geologic events. Here we address whether this holds true for the epicontinental Eastern Paratethys Sea (today's Black – Caspian Sea region). Two Tarkhanian – lower Chokrakian sequences of Middle Miocene age in the Kerch Peninsula were investigated using foraminifera, calcareous nannofossils, molluscs, fish otoliths, spores and pollen, oxygen and carbon stable isotopes and strontium isotope stratigraphy (SIS). Our results show that the marine environment during the Tarkhanian to early Chokrakian in the study area was characterized by open shelf conditions (near upper part of lower sublittoral zone), variable water column stratification and bottom water oxygen levels. Biostratigraphy and new SIS data suggest an age of >15.5 (∼16.0?) – 14.75 Ma for the Tarkhanian, which implies a considerably longer duration (> 0.75 Ma vs. 0.1 Ma) than was previously suggested. The maximum transgression seen in the middle Tarkhanian could be dated to ∼15.5–15.1 Ma and correlates with the highstand of sequence Bur 5/Lan 1 (15.2 Ma) and terminal phase of the MCO. The vegetation indicates a gradual change from subtropical humid (early Tarkhanian) to arid (early Chokrakian) conditions, which reflect the MCT. The climate change in the Eastern Paratethys occurred slightly earlier than in the Central Paratethys, possibly related to the existence of the large flat Eurasian continent.

Increased primary mineral dissolution control on a terrestrial silicate lithium isotope record during the middle Miocene Climate Optimum

Lithium isotopes have great potential to fractionate at subaerial environment, so they are promising proxies for tracing chemical weathering history. However, it remains uncertain how to relate lithium isotopic compositions (δ7Li) in sediments to silicate weathering and past climate in terrestrial setting. To tackle this question, we choose the warm and wet middle Miocene climate optimum (MMCO, 17–14 Ma) with large climate variation amplitude and the following cold and dry middle Miocene climate cooling period (MMCC, 14–11 Ma) with damped climatic variation amplitude, as two climate contrasting intervals to study lithium isotopes and content ([Li]) variations of terrestrial mudrocks. Our study site is located at the eastern Qaidam Basin of the northeastern Tibetan Plateau and the precipitation pattern of this part of the basin is similar to the Chinese Loess Plateau over the two studied intervals. The study site is suitable for clarifying link between climate and δ7Li variations because previous studies have demonstrated no provenance shift over the studied interval and short transport distance from source to the study site suggests little chemical weathering over the transportation process. Both δ7Li and [Li] values show larger amplitude over the MMCO (spanning from −1.2 to 3.2 ‰ and from 32.8 to 93.9 ug/g, respectively) than the MMCC interval (−1.8 to 1.2 ‰ and 41.7–80.8 ug/g, respectively), consistent with general feature of global and Chinese Loess Plateau climate. However, δ7Li data are higher over the MMCO in comparison with the MMCC, contrary to expectation of more lithium isotope fractionation at warm interval. We interpret the more positive δ7Li values over the MMCO using increased ratio of primary mineral dissolution over secondary mineral precipitation. Qaidam mudrock and benthic Li records show complementary changes, suggesting their possible link. Furthermore, past research reveals lack of focused uplift of the northeastern Tibetan Plateau over the studied 18–11 Ma. So we conclude that global climate, rather than Tibetan uplift, played an important role in modulating inland Qaidam Basin climate variations over the middle Miocene and that increased chemical weathering over warm climate can lead to a small magnitude of lithium isotope fractionation. Therefore, interpreting chemical weathering history, and hence climate, using lithium isotope data in terrestrial setting requires considering relative importance of primary mineral dissolution versus secondary mineral precipitation.

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.

First fossil record of Pelekium (Thuidiaceae) from the middle Miocene Zhangpu amber in Asia

ABSTRACT Zhangpu amber is an important source of Miocene bryophytes in China. Zhangpu amber originates from the Middle Miocene Climatic Optimum (MMCO), which is well known for the appearance of Dipterocarpaceae-dominated megathermal forests in East Asia. The humid and warm climate of these forests might have triggered the development of diverse epiphytic bryophytes; however, the Middle Miocene fossils of bryophytes are rare in Asia. Here, we present evidence for Pelekium (Thuidiaceae) in Zhangpu amber. The fossil is characterised by: (i) regularly pinnate ramifications; (ii) deltoid leaves with a single and percurrent Costa; (ⅲ) abundant unbranched and uniseriate paraphyllia covering the stems and branches; (ⅳ) differentiated stem and branch leaves. It represents the first fossil record of Pelekium, showing that the genus already existed in Fujian, South China during the Middle Miocene. Our fossil enriches the Neogene fossil record of bryophytes in East Asia. Extant Pelekium species comprises stenoecious plants that are mainly distributed in tropical areas, often grow on bark in humid forests. The occurrence of fossil Pelekium from Zhangpu amber is in good accordance with the palaeoclimatic reconstruction based on the Climate Leaf Analysis Multivariate Program.

Nature and origin of variations in pelagic carbonate production in the tropical ocean since the mid-Miocene (ODP Site 927)

Abstract. Marine plankton is an important component of the global carbon cycle. Whereas the production and seafloor export of organic carbon produced by the plankton, the biological pump, has received much attention, the long-term variability in plankton calcification, controlling the carbonate counter pump, remains less well understood. However, it has been shown that changes in pelagic calcification (biological compensation) could affect the ocean's buffering capacity and thus regulate global carbon budget on geological timescales. Here we use Neogene pelagic sediments deposited on the Ceara Rise in the tropical Atlantic to characterize the variability in pelagic carbonate production with a focus on warm climates. A re-evaluation of published records of carbonate accumulation at the Ceara Rise reveals a systematic increase in sedimentation rates since the late Miocene, but the carbonate accumulation rate does not show a clear trend. Instead, we observe substantial orbital timescale variability in carbonate accumulation, combined with a trend towards less carbonate on average at sites located below 4 km, likely due to the effect of carbonate dissolution. To evaluate long-term changes against possible orbital-scale variability, we generated new high-resolution records of carbonate accumulation rate at Ocean Drilling Program (ODP) Site 927 across two Quaternary interglacials (MIS 5 and MIS 9), the Pliocene warm period (MIS KM5) and the Miocene Climatic Optimum (MCO). We observe that the highest carbonate accumulation rates occurred during the Pliocene but that each of the studied intervals was characterized by large-magnitude orbital variability. Prominent variations in carbonate accumulation prior to the Quaternary preservation cycles appear to follow Earth obliquity and eccentricity. These results imply that pelagic carbonate accumulation in the tropical ocean, buffered from large temperature changes, varied on orbital timescales. The magnitude of the orbital-scale variability was similar or even higher than the long-term mean differences among the studied intervals. Since preservation can be excluded as a driver of these changes prior to the Quaternary, the observed variations must reflect changes in the export flux of pelagic biogenic carbonate. We conclude that the overall carbonate production by pelagic calcifiers responded to local changes in light, temperature, and nutrients delivered by upwelling, which followed long orbital cycles, as well as to long-term shifts in climate and/or ocean chemistry. The inferred changes on both timescales were sufficiently large such that when extrapolated on a global scale, they could have played a role in the regulation of the carbon cycle and global climate evolution during the transition from the Miocene warm climates into the Quaternary icehouse.

RETRACTED: A tropical soil (Ferralsol) identified in the northernmost part of the Mediterranean (Istria, Croatia)

Terra rossa soils on the Istrian peninsula are generally considered polygenetic relict soils. They overlie and mark the youngest subaerial unconformity in the northwestern part of the Adriatic Carbonate Platform and are susceptible to erosional and redepositional processes. Terra rossa soils represent a valuable archive of information that can be used to understand present and past soil formation processes in the context of climate variability and landscape dynamics, especially in low-lying parts of the karst landscape where thicker deposits are typically found. We studied a 3 m thick terra rossa profile in a karst depression on the northernmost part of the southwestern Istrian planation surface and analysed the micromorphology, physical/chemical properties, geochemistry, bulk and clay mineralogy, and composition of heavy and light mineral fractions. The profile shows several signs of colluviation and polygenesis with clay distribution along the profile, clearly indicating lithic discontinuities. Kaolinization, along with iron oxide formation, is the dominant process in the studied profile. Ferralitization and lessivage processes were diagnosed throughout the profile, with intense clay translocation occurring despite the extremely stable soil microaggregates. In accordance with the WRB system, the profile is classified as Lixic Rhodic Ferralsol (Clayic, Aric). This implies that some terra rossa soils, previously classified as e.g. Cambisols or Luvisols, might actually be Ferralsols or other tropical soils (e.g. Lixisols), in which relict properties are preserved in the present climate. The finding of a Ferralsol in the study area indicates the old age of the surface of the karst depression and provides the first data on the burial history of the southwestern Istrian planation surface. We suggest the older Quaternary interglacials, the mid-Piacenzian Warm Period (Pliocene) and the Miocene Climatic Optimum as favourable periods for the formation of the studied soil.

Enhanced magmatism played a dominant role in triggering the Miocene Climatic Optimum

The Miocene Climatic Optimum represents one of the major warming events during the Cenozoic and was accompanied by a positive stable carbon isotope excursion. Here, we report seawater osmium isotope data from Pacific Ocean sediments to determine if carbon dioxide emissions from coeval magmatism could have been the trigger for this event. Our data reveal a negative osmium isotope excursion from 0.80 to 0.72 between about 17 and 15.8 million years ago that can be explained by enhanced magmatism during this period. Simple mass balance calculations suggest an increase of 22‒45% in non-radiogenic osmium input from the mantle into the ocean during this period. Using osmium isotopic composition as a constraint for global carbon cycle modelling, we find that such an increase in magmatism is capable of elevating atmospheric carbon dioxide concentrations by 65‒140 ppmv and of causing a positive excursion in seawater stable carbon isotopes of 0.4‒0.7‰, consistent with geochemical observations. We conclude that it is conceivable that the enhanced magmatism played a dominant role in causing the Miocene Climatic Optimum.