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.

Reconciling Southern Ocean fronts equatorward migration with minor Antarctic ice volume change during Miocene cooling

Gradual climate cooling and CO2 decline in the Miocene were recently shown not to be associated with major ice volume expansion, challenging a fundamental paradigm in the functioning of the Antarctic cryosphere. Here, we explore Miocene ice-ocean-climate interactions by presenting a multi-proxy reconstruction of subtropical front migration, bottom water temperature and global ice volume change, using dinoflagellate cyst biogeography, benthic foraminiferal clumped isotopes from offshore Tasmania. We report an equatorward frontal migration and strengthening, concurrent with surface and deep ocean cooling but absence of ice volume change in the mid-late-Miocene. To reconcile these counterintuitive findings, we argue based on new ice sheet modelling that the Antarctic ice sheet progressively lowered in height while expanding seawards, to maintain a stable volume. This can be achieved with rigorous intervention in model precipitation regimes on Antarctica and ice-induced ocean cooling and requires rethinking the interactions between ice, ocean and climate.

Palaeobotanical evidence reveals the living conditions of Miocene Lufengpithecus in East Asia

BACKGROUND

Understanding the relationship between human evolution and environmental changes is the key to lifting the veil on human origin. The hypothesis that environmental changes triggered the divergence of humans from apes (ca. 9.3-6.5 million years ago, Ma) has been poorly tested because of limited continuous environmental data from fossil localities. Lufengpithecus (12.5-6.0 Ma) found on the southeastern margin of the Tibetan Plateau (SEMTP) across the ape-human split provides a good chance for testing this hypothesis.

RESULTS

Here, we reconstructed the habitats of L. keiyuanensis (12.5-11.6 Ma) with comprehensive vegetation, climate, and potential food web data by palaeobotanical evidence, together with other multidisciplinary data and partly tested the environment-driven hypothesis by revealing the living conditions of Lufengpithecus.

CONCLUSION

A detailed comparison of hominoids on different continents reveals their behaviour and fate divergence across the ape-human split against the background of global climate change, i.e., the stable living conditions of SEMTP not only provided a so-called 'refuge' for arboreal Lufengpithecus but also acted as a 'double-edged sword', preventing their further evolution while vegetation shifts in East Africa probably stimulated the emergence of human bipedalism, and the intense climatic changes in Europe possibly prevented those hominoids from surviving that time interval. Our findings provide interesting insight into the environmental impacts on the behavioural evolution of hominoids.

Middle Miocene long-term continental temperature change in and out of pace with marine climate records

Reconstructing long-term continental temperature change provides the required counterpart to age equivalent marine records and can reveal how terrestrial and marine temperatures were related during times of extreme climate change such as the Miocene Climatic Optimum (MCO) and the following Middle Miocene Climatic Transition (MMCT). Carbonate clumped isotope temperatures (T(Δ₄₇)) from 17.5 to 14.0 Ma Central European paleosols (Molasse Basin, Switzerland) display a temperature pattern during the MCO that is similar to coeval marine temperature records. Maximum temperatures in the long-term soil T(Δ₄₇) record (at 16.5 and 14.9 Ma) lag maximum ocean bottom water temperatures, lead global ice volume, and mark the initiation of minimum global ice volume phases. The suggested onset of the MMCT, deduced by a marked and rapid decline in Molasse Basin soil temperatures is coeval with cooling reported in high-latitudinal marine records. This is best explained by a change in the seasonal timing of soil carbonate formation that was likely driven by a modification of rainfall seasonality and thus by a major reorganization of mid-latitude atmospheric circulation across Central Europe. In particular, our data suggest a strong climate coupling between the North Atlantic and Central Europe already in the middle Miocene.

Out of the Garden and into the Cooler? A Quaternary Perspective on Deep-Time Paleoecology

Most of what we know about ecology inevitably comes from Late Quaternary ecosystems, particularly those of the past few decades. 20th Century ecosystems are the only ones for which we have direct observational and experimental studies. We can obtain detailed records of ecological dynamics at decade- to century-scales over the past few centuries using historical, permanent-plot, demographic, and paleoecological techniques, but only for a select few ecosystems. Radiocarbon-dated fossil assemblages provide records of ecological changes over the past 25,000 years. Because 14C-dating allows independent dating of the assemblages with precision of 50–500 years, detailed spatiotemporal patterns of change can be outlined, but these are limited to certain kinds of organisms and to regions with a high density of fossil sites. When we go beyond the radiocarbon barrier 30–40,000 years ago, the density of datable terrestrial assemblages decreases sharply, and our views of past ecosystems become murkier.

A tetraconodontine pig from the Upper Miocene of Turkey

Two species of Tetraconodontinae dispersed in the early Late Miocene from Eurasia into Africa. Tetraconodontinae then became the dominant pigs in the African Late Miocene and Pliocene. Although the dispersal to Africa must have come from SW Asia, no Tetraconodontinae were known from the area dating from the time of this dispersal.

In this paper, a tetraconodontine tooth from the Nuri Yamut locality near AlÇitepe in European Turkey is described. In morphology, size and in geographical and stratigraphical position this tooth is close to a form that is ancestral to part of the African Tetraconodontinae. The tooth is assigned to cf. Conohyus giganteus, one of the species that dispersed to Africa.

The dispersals of these Tetraconodontinae seem to be more or less coeval with that of Hipparion and are assumed to be allowed for by a change in global climate.

Biometrical trends in the Tetraconodontinae, a subfamily of pigs

The Tetraconodontinae Lydekker, 1876 are a subfamily of pigs (Suidae) with a great variety of dental adaptations, such as greatly enlarged or reduced premolars, enlarged posterior molars, hypsodonty, enlarged incisors etc. The essence of most of these adaptations can be described very well by simple biometry. Size increase of the last molars has proved to be of importance in African biostratigraphy. However, it appears that members of different lineages have been mixed up.

Tendencies in relative premolar size indicate that the 21 species known can be grouped in three tribes: (1) the Tetraconodontini, with very large or even extremely enlarged premolars and, in one of the lineages, a tendency to enlarge the posterior molars; (2) the Nyanzachoerini new tribe, characterised by moderately enlarged or even reduced premolars and with a tendency to increase the length of the posterior molars by the addition of distal cusps; and (3) the Parachleuastochoerini new tribe, with relatively long but narrower premolars. Both Nyanzachoerini and Tetraconodontini include species that were placed previously inConohyusandNyanzachoerus.

A very long gradually evolving lineage in the Nyanzachoerini, spanning 13 Ma, is documented. In the last 5 Ma of this lineage, M3 length increased at such a rate that a (sub)species can be recognised about every 0·7 Ma. Slightly lesser rates of evolution are found in some other lineages, but still others experienced apparently long periods of stasis or very slow evolution. In a single case there is a period of stasis combined with a sudden evolutionary jump as in the punctuated equilibria model.

In the cases of intercontinental dispersal events combined with cladogenesis, where the ancestor in the continent of origin could be identified, the cause of cladogenesis seems to have been geographical isolation. Two tetraconodont lineages were restricted to Eurasia for about 5 Ma. During the Late Miocene, members of these lineages dispersed repeatedly to Africa. The Vallesian is a period of profound climatic changes which are reflected in changes in biogeography of many taxa. It is assumed that these and later climatic changes enabled the dispersal of Tetraconodontinae to Africa.

Deep drilling into the Chesapeake Bay impact structure

Samples from a 1.76-kilometer-deep corehole drilled near the center of the late Eocene Chesapeake Bay impact structure (Virginia, USA) reveal its geologic, hydrologic, and biologic history. We conducted stratigraphic and petrologic analyses of the cores to elucidate the timing and results of impact-melt creation and distribution, transient-cavity collapse, and ocean-water resurge. Comparison of post-impact sedimentary sequences inside and outside the structure indicates that compaction of the crater fill influenced long-term sedimentation patterns in the mid-Atlantic region. Salty connate water of the target remains in the crater fill today, where it poses a potential threat to the regional groundwater resource. Observed depth variations in microbial abundance indicate a complex history of impact-related thermal sterilization and habitat modification, and subsequent post-impact repopulation.

Environmental hypotheses of hominin evolution

The study of human evolution has long sought to explain major adaptations and trends that led to the origin of Homo sapiens. Environmental scenarios have played a pivotal role in this endeavor. They represent statements or, more commonly, assumptions concerning the adaptive context in which key hominin traits emerged. In many cases, however, these scenarios are based on very little if any data about the past settings in which early hominins lived. Several environmental hypotheses of human evolution are presented in this paper. Explicit test expectations are laid out, and a preliminary assessment of the hypotheses is made by examining the environmental records of Olduvai, Turkana, Olorgesailie, Zhoukoudian, Combe Grenal, and other hominin localities. Habitat-specific hypotheses have prevailed in almost all previous accounts of human adaptive history. The rise of African dry savanna is often cited as the critical event behind the development of terrestrial bipedality, stone toolmaking, and encephalized brains, among other traits. This savanna hypothesis has been countered recently by the woodland/forest hypothesis, which claims that Pliocene hominins had evolved in and were primarily attracted to closed habitats. The ideas that human evolution was fostered by cold habitats in higher latitudes or by seasonal variations in tropical and temperate zones also have their proponents. An alternative view, the variability selection hypothesis, states that large disparities in environmental conditions were responsible for important episodes of adaptive evolution. The resulting adaptations enhanced behavioral versatility and ultimately ecological diversity in the human lineage. Global environmental records for the late Cenozoic and specific records at hominin sites show the following: 1) early human habitats were subject to large-scale remodeling over time; 2) the evidence for environmental instability does not support habitat-specific explanations of key adaptive changes; 3) the range of environmental change over time was more extensive and the tempo far more prolonged than allowed by the seasonality hypothesis; and 4) the variability selection hypothesis is strongly supported by the persistence of hominins through long sequences of environmental remodeling and the origin of important adaptations in periods of wide habitat diversity. Early bipedality, stone transport, diversification of artifact contexts, encephalization, and enhanced cognitive and social functioning all may reflect adaptations to environmental novelty and highly varying selective contexts.