Middle Miocene Climate Transition Research Articles

Climate, cryosphere and carbon cycle controls on Southeast Atlantic orbital-scale carbonate deposition since the Oligocene (30–0 Ma)

Abstract. The evolution of the Cenozoic cryosphere from unipolar to bipolar over the past 30 million years (Myr) is broadly known. Highly resolved records of carbonate (CaCO3) content provide insight into the evolution of regional and global climate, cryosphere, and carbon cycle dynamics. Here, we generate the first Southeast Atlantic CaCO3 content record spanning the last 30 Myr, derived from X-ray fluorescence (XRF) ln(Ca / Fe) data collected at Ocean Drilling Program Site 1264 (Walvis Ridge, SE Atlantic Ocean). We present a comprehensive and continuous depth and age model for the entirety of Site 1264 (∼ 316 m; 30 Myr). This constitutes a key reference framework for future palaeoclimatic and palaeoceanographic studies at this location. We identify three phases with distinctly different orbital controls on Southeast Atlantic CaCO3 deposition, corresponding to major developments in climate, the cryosphere and the carbon cycle: (1) strong ∼ 110 kyr eccentricity pacing prevails during Oligocene–Miocene global warmth (∼ 30–13 Ma), (2) increased eccentricity-modulated precession pacing appears after the middle Miocene Climate Transition (mMCT) (∼ 14–8 Ma), and (3) pervasive obliquity pacing appears in the late Miocene (∼ 7.7–3.3 Ma) following greater importance of high-latitude processes, such as increased glacial activity and high-latitude cooling. The lowest CaCO3 content (92 %–94 %) occurs between 18.5 and 14.5 Ma, potentially reflecting dissolution caused by widespread early Miocene warmth and preceding Antarctic deglaciation across the Miocene Climatic Optimum (∼ 17–14.5 Ma) by 1.5 Myr. The emergence of precession pacing of CaCO3 deposition at Site 1264 after ∼ 14 Ma could signal a reorganisation of surface and/or deep-water circulation in this region following Antarctic reglaciation at the mMCT. The increased sensitivity to precession at Site 1264 between 14 and 13 Ma is associated with an increase in mass accumulation rates (MARs) and reflects increased regional CaCO3 productivity and/or recurrent influxes of cooler, less corrosive deep waters. The highest carbonate content (%CaCO3) and MARs indicate that the late Miocene–early Pliocene Biogenic Bloom (LMBB) occurs between ∼ 7.8 and 3.3 Ma at Site 1264; broadly contemporaneous with the LMBB in the equatorial Pacific Ocean. At Site 1264, the onset of the LMBB roughly coincides with appearance of strong obliquity pacing of %CaCO3, reflecting increased high-latitude forcing. The global expression of the LMBB may reflect increased nutrient input into the global ocean resulting from enhanced aeolian dust and/or glacial/chemical weathering fluxes, due to enhanced glacial activity and increased meridional temperature gradients. Regional variability in the timing and amplitude of the LMBB may be driven by regional differences in cooling, continental aridification and/or changes in ocean circulation in the late Miocene.

Early evidence of continental aridity and open-habitat grasslands in Europe as revealed by the Middle Miocene microflora of the Madrid Basin

In this study we present a new, extensive palaeobotanical and palaeoclimatic record from 85 microfloral assemblages of Middle Miocene (late Langhian–early Serravallian) age recovered from ten localities in the Madrid Basin. A large, shallow, slightly saline lacustrine system developed in the study area, promoting the occurrence of salt-tolerant plant communities on the lake margin. The regional landscape of the basin was characterised by grass-dominated, savannah-like vegetation under a warm (MAT: 13.8–18.8 °C) and semi-arid (MPwet: 151–71 mm, MPdry: 45–4 mm) climatic regime with marked seasonality of temperature and precipitation. These data re-open the debate on the origin of grasslands linked to dry conditions in Europe. Overall increased cooling and aridity through the Miocene led to the onset of grass-dominated habitats at the expense of forests, a process that in eastern Europe has been shown to occur from the Late Miocene. Our results, together with previous palaeobotanical and fossil mammal data from southern and central Spain, strongly indicate that open habitats were already prevalent in the Iberian Peninsula in the Middle Miocene, suggesting an earlier establishment here than in eastern Europe, possibly following a temporal pattern like the Anatolian Peninsula. The taxonomic diversity yielded by the samples from the Madrid Basin contributes to the understanding of the floristic composition of these early open habitats which were dominated by Poaceae, Amaranthaceae and Asteraceae with scattered shrubby and arboreal plants adapted to dry conditions such as Ricinus, Ephedra, Olea, Quercus, Acacia and Pinus. The age of the studied samples, coincident with the Middle Miocene Climatic Transition, indicates that this climatic episode marked an important step towards the establishment of open vegetation.

A dynamic ocean driven by changes in CO2 and Antarctic ice-sheet in the middle Miocene

The middle Miocene climate transition (MMCT), likely triggered by a decrease in atmospheric CO2 concentration and changes in the Earth's orbital configuration, entailed major expansion of the Antarctic ice-sheet and cooling of the global ocean at ~15–13 Ma. By comparing simulations that use boundary conditions representative of 15 and 13 Ma, we assess the response of ocean temperatures and deep ocean circulation to atmospheric CO2 drawdown (from 400 to 200 ppmv) and Antarctic ice-sheet expansion (43 m of sea-level equivalent) during the MMCT by means of the Community Climate System Model version 3. The model simulates a decrease in mean global sea-surface temperature by 1.6 °C across the transition. Individual forcing experiments reveal that surface cooling is fully attributable to the CO2 drop, whereas expansion of the Antarctic ice-sheet tends to cause a slight global warming of the upper ocean. At depth, modelled ocean temperatures decrease by 1.5–2 °C across the MMCT in most regions. While the CO2 effect dominates deep-ocean cooling, about 25–30% is attributable to the expansion of the Antarctic ice-sheet.In the convection regions of the Southern Ocean, the CO2 drawdown causes an increase in salinity through sea ice formation and changes in the precipitation-evaporation balance. This increase in salinity, together with a decrease in surface temperatures, translates into an intensification in Antarctic Bottom Water formation that cools the deep ocean. The contribution of the Antarctic ice-sheet expansion to bottom and deep water cooling occurs through an increase in the surface heat flux from the ocean to the atmosphere, which is linked to intensified cold surface winds blowing off the Antarctic continent.This study suggests that the magnitude of cooling of deep waters across the MMCT inferred from proxy records can be explained by the combined effects of the CO2 decrease and Antarctic ice-sheet expansion. Our surface cooling estimates are generally lower than those indicated by proxies.

Middle Miocene stepwise climate evolution in the Mediterranean region through high-resolution stable isotopes and calcareous plankton records

The middle Miocene is marked by a changeover from a warm climatic period (Miocene Climatic Optimum, ~17-14.7 Ma), to a transitional phase (Middle Miocene Climatic Transition), culminating in a cold stage (Icehouse Mode, ~13.8 Ma). This period is associated with a positive excursion of δ13C (“Monterey Excursion”), showing a series of δ13C maxima (CM events) highlighted by cooling peaks in the δ18O values (Mi events). DSDP Site 372 obtained a high-resolution record of the middle Miocene of the western Mediterranean (Balearic Islands), and represents one of the best marine sedimentary records for the considered time interval. In this study, high-resolution stable isotope and quantitative calcareous plankton records for the Langhian to the early Serravallian time interval are presented. The stable oxygen and carbon isotope records allowed us to recognize and chronologically frame the Mi and CM events falling between 15.82 and 13.04 Ma.Furthermore, by integrating the stable isotope data with those obtained by the quantitative analyses of the calcareous plankton content, the three-folded climatic and environmental evolution was outlined for the considered time interval in the Mediterranean region.During the first stage (Miocene Climatic Optimum = MCO; from 17 up to 14.55 Ma), an initial warm-surface-water, oligotrophic, high salinity and restricted environment is followed by an open marine setting (starting from 15.18 Ma) within still warm-surface-water and oligotrophic conditions. The re-opening of the connections between the Mediterranean Sea and the Indian Ocean would have likely triggered this latter environmental change.From 14.55 to 13.75 Ma (second stage; Middle Miocene Climatic Transition; MMCT) an initial (up to 14.05 Ma) temperate-warm-surface-water and eutrophic environment occurred, followed by the onset of warm-surface-water, oligotrophic and slightly restricted marine conditions.The third stage (Icehouse Mode = IHM; from 13.75 Ma onwards) points to the development of a cold-water, eutrophic, high salinity environment in an open-marine setting.

MIOCENE NERITIC BENTHIC FORAMINIFERAL COMMUNITY DYNAMICS, CALVERT CLIFFS, MARYLAND, USA: SPECIES POOL, PATTERNS AND PROCESSES

ABSTRACTThe presence/absence and abundance of benthic foraminifera in successive discrete beds (Shattuck “zones”) of the Miocene Calvert and Choptank formations, exposed at the Calvert Cliffs, Maryland, USA, allows for investigation of community dynamics over space and time. The stratigraphic distribution of benthic foraminifera is documented and interpreted in the context of sea-level change, sequence stratigraphy, and the previously published distribution of mollusks. Neritic benthic foraminiferal communities of four sea-level cycles over ∼4 million years of the middle Miocene, encompassing the Miocene Climatic Optimum and the succeeding middle Miocene Climate Transition, are dominated by the same abundant species. They differ in the varying abundance of common species that occur throughout most of the studied section and in the different rare species that appear and disappear. Transgressive systems tracts (TSTs) have higher species diversity than highstand systems tracts (HSTs) but much lower density of specimens. In contrast to some previous research, all beds in the studied section are interpreted as being from the inner part of a broad, low gradient shelf and were deposited at water depths of less than ∼50 m. It is suggested that species are recruited from a regional species pool of propagules throughout the duration of TSTs. Recruitment is curtailed during highstands leading to lower diversity in the HSTs.

A ∼12 Myr Miocene Record of East Asian Monsoon Variability From the South China Sea

AbstractThe long‐term evolution of the East Asian Monsoon and the processes controlling its variability under changing climate boundary conditions remain enigmatic. Here, we integrate new and published high‐resolution planktic and benthic foraminiferal isotope data with proxy records for chemical weathering derived from diffuse reflectance spectroscopy at Ocean Drilling Program Site 1146 (South China Sea) to reconstruct the evolution of the summer monsoon between ∼17 and 5 Ma. Our records show that an overall warm and humid tropical climate prevailed over southeastern Asia during the Miocene Climatic Optimum, suggesting northward expansion of the tropical rain belt in response to greenhouse gas forcing. By contrast, monsoon seasonality increased during the middle Miocene Climatic Transition in tandem with Antarctic glacial expansion and global cooling. Substantial weakening of the summer monsoon between ∼12.7 and 10.9 Ma supports that decreased weathering and riverine input of nutrients and alkalinity contributed to carbonate depletion in the deep ocean during the Carbonate Crash. Intensification of monsoonal circulation and strengthening of the biological pump through the late Miocene promoted carbon burial, drawdown of atmospheric CO2, and climate cooling during the Biogenic Bloom. These results underscore the dynamic evolution of the East Asian Monsoon throughout the middle to late Miocene. Variations in local insolation forcing and in Southern Hemisphere ice volume, influencing the latitudinal thermal gradient, evaporation‐moisture budgets, and the strength of the tropical convection, exerted major controls on the development of the monsoon.

Impacts of the Middle Miocene Climatic Transition on deep-water carbonate preservation and oxygenation in the equatorial Indian Ocean

We reconstructed the orbital-scale variability of deep-water mass properties in the equatorial Indian Ocean during the late early to late Miocene (17.9 to 8.2 Ma) using benthic foraminiferal oxygen (δ18O) and carbon (δ13C) isotope records, in combination with X-ray fluorescence scanner elemental records at International Ocean Discovery Program (IODP) Site U1443. These records suggest that hyperthermal-like features (negative excursions in δ18O and δ13C coupled to suboxic and acidic conditions in the deep ocean) developed at eccentricity maxima during the Miocene Climatic Optimum (MCO; 16.9 to 14.7 Ma) on an almost ice-free Earth. Elevated Mn and U concentrations indicate that bottom and pore waters in the Indian Ocean became progressively suboxic during the later phase of the MCO. A fundamental change in deep-water circulation occurred after the expansion of the East Antarctic Ice Sheet during the Middle Miocene Climatic Transition (MMCT; 14.7 to ~13.8 Ma). Stepwise increases in δ18O at ~13.8 and ~13.1 Ma concurred with dampening of deep-water δ18O and δ13C variability, as Antarctic ice cover expanded and became more stable. A marked improvement in deep-water oxygenation after ~13.5 Ma coincided with the end of the last δ13C maximum of the Monterey Excursion. Carbonate dissolution in the equatorial Indian Ocean intensified and prolonged episodes of reduced carbonate deposition during the Carbonate Crash interval (~12.6 to 8.6 Ma) were not restricted to eccentricity maxima as during the MCO. Rising Mn and U concentrations after ~9 Ma indicate decreased oxygenation of bottom and pore waters, associated with the onset of the Biogenic Bloom in the Indian Ocean.

Temperature Gradients Across the Pacific Ocean During the Middle Miocene

AbstractSea surface temperatures (SSTs) of the tropical Pacific Ocean exert powerful controls on regional and global climates. Previous studies have suggested that during warm climate phases, the east‐west temperature gradient collapsed. To date, there has been no high‐resolution reconstruction of sea surface conditions in both the east and west Pacific Ocean during the Miocene Climate Optimum (MCO) and across the middle Miocene climate transition (MMCT); therefore, our understanding of the mean oceanographic state during this major global climatic shift is limited. Here, we present new SST reconstructions for the eastern Pacific Ocean (15.5–13.3 Ma) which show a clear east‐west temperature gradient of ∼4°C during the warmest interval of the Neogene, implying that the oceanographic processes that produce the modern gradient were present and active. There is no shift in the east‐west gradient across the MMCT indicating that the gradient was not impacted by global cooling and ice growth. We find a 2°C sea surface cooling in the eastern equatorial Pacific, that lags the benthic foraminiferal δ18O positive shift by 150 kyr, indicating that tropical temperature did not decrease synchronously with the expansion of the Antarctic ice sheet. Reconstructed variations in the δ18O composition of seawater, determined by combining our Mg/Ca and δ18O records, reveal a freshening in the eastern Pacific Ocean after 13.8 Ma, suggesting changes in the hydrological cycle and in tropical fronts in response to the new icehouse regime.

A new middle Miocene lineage based on taxonomic revision of the large and rare cricetid-rodent genus Lartetomys

Large-sized cricetid rodents (Cricetodotontini) immigrated to Central Europe at the beginning of the Middle Miocene Climate Transition at ca. 15 My and are thus of great value for biostratigraphic and palaeoecologic purposes. An important fossil material belonging to a relatively small species of Mixocricetodon from the North Alpine Foreland Basin is described. It is a rare component in faunas of equivalent age, and the new material confirms the synonymy with the genus Lartetomys. The lineage L. mirabilis-L dehmi is documented in its chronostratigraphic framework, and the origin of the genus is discussed but remains uncertain.

Early and middle Miocene ice sheet dynamics in the Ross Sea: Results from integrated core-log-seismic interpretation

Abstract Oscillations in ice sheet extent during early and middle Miocene are intermittently preserved in the sedimentary record from the Antarctic continental shelf, with widespread erosion occurring during major ice sheet advances, and open marine deposition during times of ice sheet retreat. Data from seismic reflection surveys and drill sites from Deep Sea Drilling Project Leg 28 and International Ocean Discovery Program Expedition 374, located across the present-day middle continental shelf of the central Ross Sea (Antarctica), indicate the presence of expanded early to middle Miocene sedimentary sections. These include the Miocene climate optimum (MCO ca. 17–14.6 Ma) and the middle Miocene climate transition (MMCT ca. 14.6–13.9 Ma). Here, we correlate drill core records, wireline logs and reflection seismic data to elucidate the depositional architecture of the continental shelf and reconstruct the evolution and variability of dynamic ice sheets in the Ross Sea during the Miocene. Drill-site data are used to constrain seismic isopach maps that document the evolution of different ice sheets and ice caps which influenced sedimentary processes in the Ross Sea through the early to middle Miocene. In the early Miocene, periods of localized advance of the ice margin are revealed by the formation of thick sediment wedges prograding into the basins. At this time, morainal bank complexes are distinguished along the basin margins suggesting sediment supply derived from marine-terminating glaciers. During the MCO, biosiliceous-bearing sediments are regionally mapped within the depocenters of the major sedimentary basin across the Ross Sea, indicative of widespread open marine deposition with reduced glacimarine influence. At the MMCT, a distinct erosive surface is interpreted as representing large-scale marine-based ice sheet advance over most of the Ross Sea paleo-continental shelf. The regional mapping of the seismic stratigraphic architecture and its correlation to drilling data indicate a regional transition through the Miocene from growth of ice caps and inland ice sheets with marine-terminating margins, to widespread marine-based ice sheets extending across the outer continental shelf in the Ross Sea.

CO2 and tectonic controls on Antarctic climate and ice-sheet evolution in the mid-Miocene

Antarctic ice sheet and climate evolution during the mid-Miocene has direct relevance for understanding ice sheet (in)stability and the long-term response to elevated atmospheric CO2 in the future. Geologic records reconstruct major fluctuations in the volume and extent of marine and terrestrial ice during the mid-Miocene, revealing a dynamic Antarctic ice-sheet response to past climatic variations. We use an ensemble of climate – ice sheet – vegetation model simulations spanning a range of CO2 concentrations, Transantarctic Mountain uplift scenarios, and glacial/interglacial climatic conditions to identify climate and ice-sheet conditions consistent with Antarctic mid-Miocene terrestrial and marine geological records. We explore climatic variability at both continental and regional scales, focusing specifically on Victoria Land and Wilkes Land Basin regions using a high-resolution nested climate model over these domains. We find that peak warmth during the Miocene Climate Optimum is characterized by a thick terrestrial ice sheet receded from the coastline under high CO2 concentrations. During the Middle Miocene Climate Transition, CO2 episodically dropped below a threshold value for marine-based ice expansion. Comparison of model results with geologic data support ongoing Transantarctic Mountain uplift throughout the mid-Miocene. Modeled ice sheet dynamics over the Wilkes Land Basin were highly sensitive to CO2 concentrations. This work provides a continental-wide context for localized geologic paleoclimate and vegetation records, integrating multiple datasets to reconstruct snapshots of ice sheet and climatic conditions during a pivotal period in Earth's history.

Atmospheric carbon dioxide variations across the middle Miocene climate transition

Abstract. The middle Miocene climate transition ∼ 14 Ma marks a fundamental step towards the current “ice-house” climate, with a ∼ 1 ‰ δ18O increase and a ∼ 1 ‰ transient δ13C rise in the deep ocean, indicating rapid expansion of the East Antarctic Ice Sheet associated with a change in the operation of the global carbon cycle. The variation of atmospheric CO2 across the carbon-cycle perturbation has been intensely debated as proxy records of pCO2 for this time interval are sparse and partly contradictory. Using boron isotopes (δ11B) in planktonic foraminifers from Ocean Drilling Program (ODP) Site 1092 in the South Atlantic, we show that long-term pCO2 varied at 402 kyr periodicity between ∼ 14.3 and 13.2 Ma and follows the global δ13C variation remarkably well. This suggests a close link to precessional insolation forcing modulated by eccentricity, which governs the monsoon and hence weathering intensity, with enhanced weathering and decreasing pCO2 at high eccentricity and vice versa. The ∼ 50 kyr lag of δ13C and pCO2 behind eccentricity in our records may be related to the slow response of weathering to orbital forcing. A pCO2 drop of ∼ 200 µatm before 13.9 Ma may have facilitated the inception of ice-sheet expansion on Antarctica, which accentuated monsoon-driven carbon cycle changes through a major sea-level fall, invigorated deep-water ventilation, and shelf-to-basin shift of carbonate burial. The temporary rise in pCO2 following Antarctic glaciation would have acted as a negative feedback on the progressing glaciation and helped to stabilize the climate system on its way to the late Cenozoic ice-house world.

Tropical Sea Surface Temperatures Following the Middle Miocene Climate Transition From Laser‐Ablation ICP‐MS Analysis of Glassy Foraminifera

AbstractThe mid‐to‐late Miocene is proposed as a key interval in the transition of the Earth's climate state toward that of the modern‐day. However, it remains a poorly understood interval in the evolution of Cenozoic climate, and the sparse proxy‐based climate reconstructions are associated with large uncertainties. In particular, tropical sea surface temperature (SST) estimates largely rely on the unsaturated alkenone Uk37 proxy, which fails to record temperatures higher than 29˚C, the TEX86 proxy which has challenges around its calibration, and Mg/Ca ratios of poorly preserved foraminifera. We reconstruct robust, absolute, SSTs between 13.5 Ma and 9.5 Ma from the South West Indian Ocean (paleolatitude ∼5.5˚S) using laser‐ablation inductively coupled‐plasma mass spectrometer microanalysis of glassy planktic foraminiferal Mg/Ca. Employing this microanalytical technique, and stringent screening criteria, permits the reconstruction of paleotemperatures using foraminifera which although glassy, are contaminated by authigenic coatings. Our absolute estimates of 24–31°C suggest that SST in the tropical Indian Ocean was relatively constant between 13.5 and 9.5 Ma, similar to those reconstructed from the tropics using the Uk37 alkenone proxy. This finding suggests an interval of enhanced polar amplification between 10 and 7.5 Ma, immediately prior to the global late Miocene Cooling.

Rapid radiation of angraecoids (Orchidaceae, Angraecinae) in tropical Africa characterised by multiple karyotypic shifts under major environmental instability

Angraecoid orchids present a remarkable diversity of chromosome numbers, which makes them a highly suitable system for exploring the impact of karyotypic changes on cladogenesis, diversification and morphological differentiation. We compiled an annotated cytotaxonomic checklist for 126 species of Angraecinae, which was utilised to reconstruct chromosomal evolution using a newly-produced, near-comprehensive phylogenetic tree that includes 245 angraecoid taxa. In tandem with this improved phylogenetic framework, using combined Bayesian, maximum likelihood and parsimony approaches on ITS-1 and five plastid markers, we propose a new cladistic nomenclature for the angraecoids, and we estimate a new timeframe for angraecoid radiation based on a secondary calibration, and calculate diversification rates using a Bayesian approach. Coincident divergence dates between clades with identical geographical distributions in the angraecoids and the pantropical orchid genus Bulbophyllum suggest that the same events may have intervened in the dispersal of these two epiphytic groups between Asia, continental Africa, Madagascar and the Neotropics. The major angraecoid lineages probably began to differentiate in the Middle Miocene, and most genera and species emerged respectively around the Late Miocene-Pliocene boundary and the Pleistocene. Ancestral state reconstruction using maximum likelihood estimation revealed an eventful karyotypic history dominated by descending dysploidy. Karyotypic shifts seem to have paralleled cladogenesis in continental tropical Africa, where approximately 90% of the species have descended from at least one inferred transition from n = 17–18 to n = 25 during the Middle Miocene Climatic Transition, followed by some clade-specific descending and ascending dysploidy from the Late Miocene to the Pleistocene. Conversely, detected polyploidy is restricted to a few species lineages mostly originating during the Pleistocene. No increases in net diversification could be related to chromosome number changes, and the apparent net diversification was found to be highest in Madagascar, where karyotypic stasis predominates. Finally, shifts in chromosome number appear to have paralleled the evolution of rostellum structure, leaflessness, and conspicuous changes in floral colour.

Tropical Sea Surface Temperatures following the Middle Miocene Climate Transition from Laser-Ablation ICP-MS analysis of glassy foraminifera

Tropical Sea Surface Temperatures following the Middle Miocene Climate Transition from Laser-Ablation ICP-MS analysis of glassy foraminifera

Reconstruction of oceanographic and environmental conditions in the eastern Mediterranean (Kottafi Hill section, Cyprus Island) during the middle Miocene Climate Transition

The multi-proxy investigation of the deep-marine Kottafi Hill section (KHS), a part of the carbonate system of the Miocene Pakhna Formation, Cyprus, involved such proxies as calcareous nannofossil analysis, measurements of the oxygen and carbon isotope composition of the planktonic foraminifer Orbulina universa, and determination of the pollen and palynomorph contents, revealed the importance of these sedimentary sequences in the assessment of the impact of major global events during the middle Miocene on the regional scale. The KHS spans the 20.89–11.6 Ma time interval, during which eighteen OC-rich siltstone intercalated laminae have been deposited under warm and humid climate at 15.5–11.6 Ma. These layers can be possibly considered as the precursors of sapropelic layers mostly developed in the eastern Mediterranean Basin during Pliocene–Holocene. The global glacial events Mi3a–Mi5, traced by δ18O planktonic foraminifera records in the KHS, represent the stepwise cooling phase during the middle Miocene Climate Transition.

The role of sea-level and climate changes in the assembly of Sri Lankan biodiversity: A perspective from the Miocene Jaffna Limestone

As part of the Western Ghats-Sri Lanka biodiversity hotspot, Sri Lanka harbors a rich and diverse fauna with a considerable number of endemic species, especially in the wet southwestern part of the island. Understanding the origin of this biodiversity requires consideration of the land bridge history between the island and southern Peninsular India. To examine the influence of fluctuating sea levels and climate on island connectivity and isolation during the Miocene, we describe the lithological succession of the Jaffna Limestone in surface outcrops of north(west)ern Sri Lanka and interpret it with respect to palaeoenvironments and sequence stratigraphy. A stack of three marine intervals was identified, which were deposited in seagrass-, coral reef-, and nearshore siliciclastic environments of a land-attached carbonate platform and are separated by caliche and palaeokarst unconformities. They represent high sea-level peaks during the long-term sea-level maximum related to the Middle Miocene Climate Optimum (MMCO). In support of divergence time from Sri Lankan and south Indian species, it is argued that the Palk Isthmus connected Sri Lanka to mainland India prior the MMCO (~17–15 Ma) and after the Middle Miocene Climate Transition (~15–13 Ma), whereas a marine barrier continued to persist during low sea-level stands associated with the Mi2, Mi3a and Mi3 glaciations within this period of time. The early and middle Miocene landbridge connections opened up dispersal opportunities for terrestrial animals. Species migrations via the Palk Isthmus were, however, impacted by a climatic filter, which promoted a high level of ancient endemism on Sri Lanka. Our results indicate that the Sri Lankan sub-center of biodiversity started to evolve in response to increased sea-level and climate variability associated with Antarctic ice dynamics and the initiation of the South Asian monsoon system during the early stage of the current ice house period.

Permanent closure of the Tethyan Seaway in the northwestern Iranian Plateau driven by cyclic sea-level fluctuations in the late Middle Miocene

The opening and closure of marine gateways have been suggested to substantially affect ocean current circulations and global climatic changes. In the Miocene, the closure of the Tethyan Seaway, which connected the proto-Mediterranean Sea with the Indian and Atlantic Oceans, blocked thermohaline exchanges between them and possibly caused a major climate change in the Middle Miocene Climatic Transition. However, the timing, process, and forcing mechanism of the closure of this seaway are controversial. To address this question, we focus on the chronology and sedimentary facies of Miocene strata in the Zagros foreland basin in the Western Iranian Plateau. Based on magnetostratigraphy and biostratigraphy, we establish a chronology of 17 to 10 Ma for the studied Miocene strata and constrain the timing of the permanent closure of the northwestern segment of the Tethyan Seaway to 12.8 Ma. By comparison with global sea-level fluctuations, we suggest that middle Miocene marine transgressions and regressions in the studied region were mainly driven by global climatic change through its control on the growth and decay of ice sheets and thus sea level changes. Our detailed depositional sequence analysis exhibits 100-kyr Earth's eccentricity cycles of marine transgressions and regressions between 15.0 and 12.8 Ma implying they were controlled by astronomical factors. Moreover, the Tethyan Seaway experienced stepwise evolution changing from partially opened seaway, restricted marine connection, intermittent connections, to permanent closure in its northwestern segment during the early and middle Miocene.

Middle Miocene Intensification of South Asian Monsoonal Rainfall

AbstractDuring the middle Miocene, Earth's climate changed from a global warm period (Miocene Climatic Optimum) into a colder one with the expansion of the Antarctic ice sheet. This prominent climate transition was also a period of drastic changes in global atmospheric circulation. The development of the South Asian monsoon is not well understood and mainly derived from proxy records of wind strength. Data for middle Miocene changes in rainfall are virtually non‐existent for India and the Arabian Sea prior to 11 Ma. This study presents planktic foraminiferal trace element (Mg/Ca and Ba/Ca) and stable oxygen isotope records from NGHP‐01 Site 01A off the coast of West India in the Eastern Arabian Sea (EAS) to reconstruct the regional surface hydrography and hydroclimate in the South Asian monsoon (SAM) region during the middle Miocene. The Ba/Ca and local seawater δ18O (δ18Osw) changes reveal a notable gradual increase in SAM rainfall intensity during the middle Miocene. Additionally to this long‐term increase in precipitation, the seawater δ18O is punctuated by a prominent decrease, i.e. freshening, at ~14 Ma contemporary with Antarctic glaciation. This suggests that Southern Ocean Intermediate Waters (SOIW) transmitted Antarctic salinity changes into the Arabian Sea via an “oceanic tunnel” mechanism. The middle Miocene increase in SAM rainfall is consistent with climate model simulations of an overall strengthening Asian monsoon from the Eocene to the middle/late Miocene with a further acceleration after the middle Miocene climate transition.

Initiation of the Western Pacific Warm Pool at the Middle Miocene Climate Transition?

AbstractAcross the middle Miocene, Earth's climate underwent a major cooling and expansion of the Antarctic ice sheet. However, the associated response and development of the tropical climate system is not fully understood, in part because this is influenced by both global climate and also low‐latitude tectonic gateways and paleoceanography. Here we use combined δ18O and Mg/Ca of planktic foraminifera to reconstruct the thermal history and changes in hydrology from the Indo‐Pacific region from 16.5 to 11.5 Ma. During the warmth of the early middle Miocene, our records indicate a dynamic ocean‐atmosphere system in the Indo‐Pacific region, with episodes of saltier and warmer tropical surface waters associated with high pCO2 and retreat of the Antarctic ice sheet. We show that across the Middle Miocene Climate Transition (MMCT) surface ocean temperatures in the Indo‐Pacific cooled by ~2°C, synchronous with the advance of the Antarctic ice sheet. The associated cooling in the Southern Ocean appears to have started earlier and was stronger. Further, we show that western Pacific Ocean warmed and eastern tropical Indian Ocean freshened following the MMCT, likely caused by the constriction of the Indonesian Seaway and reduced connectivity between the Pacific and Indian Oceans following Antarctic glaciation. The MMCT therefore represented a key phase in the evolution of the West Pacific Warm Pool and associated tropical climate dynamics.