Mid-Miocene planktonic foraminifera of the Kerguelen Plateau, Antarctica

Miocene deep-sea sediments from ODP Site 744 (Kerguelen Plateau, southern Indian Ocean) contain abundant and diverse planktonic foraminiferal assemblages. Their analysis led to the identification of the interval between 17.0 and 14.2 Ma as a time of mid-Miocene warmth, which is investigated here in detail. This investigation includes reconstruction of trends in foraminiferal faunal composition and diversity through time, as well as in morphology and coiling direction within Globorotalia praescitula and Globorotalia zealandica plexi. These two large-globorotaliid plexi constitute the most characteristic component of the mid-Miocene foraminiferal faunas at ODP Site 744. Selected benthic (Cibicidoides sp.) and planktonic foraminifera were also analyzed for delta18O and delta13C ratios. Distinctive planktonic assemblages were the basis for identification of three foraminiferal biofacies between 17.0 and 14.2 Ma. The most prominent faunal changes took place between Biofacies 2 and 3 (15.5-15.0 Ma). Six of 11 macroperforate planktonic foraminifera from the >150-µm size fraction occur principally within Biofacies 3. Three other taxa are present throughout the interval analyzed. Moreover, both aforementioned globorotaliid plexi exhibit an increase in morphological diversity between Biofacies 2 and 3. Within the same interval, the G. zealandica plexus shows a switch from random coiling (50% sinistral) to clearly sinistral-dominated coiling. The faunal changes recognized are interpreted as the result of foraminiferal immigrations (increase in faunal diversity) and evolutionary trends (increase in morphological variability and change in coiling mode among the globorotaliid plexi). The stable isotopic results allow paleoenvironmental interpretation of these faunal changes. According to the delta18O values, no significant change in sea-surface temperature occurred between 17.0 and 14.2 Ma. However, the same data suggest an increase in ecological distance between various niches, which is expressed by a rising delta18O gradient recorded between various planktonic taxa upward within the section. This trend suggests niche-space availability as a likely factor responsible for the faunal changes recognized. Changes in the shape and depth of the thermocline, as well as in seasonality and eutrophication are considered as possible causes. Among these an increase in seasonality appears to have been responsible for the increase in species and morphological diversities between 15.5 and 15.0 Ma. The proposed scenario suggests that changes in seasonality may be an important factor driving faunal migrations and evolution. Variable seasonality may also affect the oxygen isotopic record of planktonic foraminiferal taxa.

Stable isotope record of Cibicidoides spp. from early and middle Miocene sediments

The middle Miocene delta18O increase represents a fundamental change in the ocean-atmosphere system which, like late Pleistocene climates, may be related to deepwater circulation patterns. There has been some debate concerning the early to early middle Miocene deepwater circulation patterns. Specifically, recent discussions have focused on the relative roles of Northern Component Water (NCW) production and warm, saline deep water originating in the eastern Tethys. Our time series and time slice reconstructions indicate that NCW and Tethyan outflow water, two relatively warm deepwater masses, were produced from ~20 to 16 Ma. NCW was produced again from 12.5 to 10.5 Ma. Another feature of the early and middle Miocene oceans was the presence of a high delta13C intermediate water mass in the southern hemisphere, which apparently originated in the Southern Ocean. Miocene climates appear to be related directly to deepwater circulation changes. Deep-waters warmed in the early Miocene by ~3°C (?20 to 16 Ma) and cooled by a similar amount during the middle Miocene delta18O increase (14.8 to 12.6 Ma), corresponding to the increase (?20 Ma) and subsequent decrease (~16 Ma) in the production of NCW and Tethyan outflow water. Large (>0.6 per mil), relatively rapid (~0.5 m.y.) delta18O increases in both benthic and planktonic foraminifera (i.e., the Mi zones of Miller et al. (1991a) and Wright and Miller (1992a)) were superimposed in the long-term deepwater temperature changes; they are interpreted as reflecting continental ice growth events. Seven of these m.y. glacial/interglacial cycles have been recognized in the early to middle Miocene. Two of these glacial/interglacial cycles (Mi3 and Mi4) combined with a 2° to 3°C decrease in deepwater temperatures to produce the middle Miocene delta18O shift.

Miocene microflora and palaeoclimatic estimates of the Stetten section (Austria)

Pollen analyses have been proven to possess the possibility to decipher rapid vegetational and climate shifts in Neogene sedimentary records. Herein, a c. 21-kyr-long transgression-regression cycle from the Lower Austrian locality Stetten is analysed in detail to evaluate climatic benchmarks for the early phase of the Middle Miocene Climate Optimum and to estimate the pace of environmental change. Based on the Coexistence Approach, a very clear signal of seasonality can be reconstructed. A warm and wet summer season with c. 204-236 mm precipitation during the wettest month was opposed by a rather dry winter season with precipitation of c. 9-24 mm during the driest month. The mean annual temperature ranged between 15.7 and 20.8 °C, with about 9.6-13.3 °C during the cold season and 24.7-27.9 °C during the warmest month. In contrast, today's climate of this area, with an annual temperature of 9.8 °C and 660 mm rainfall, is characterized by the winter season (mean temperature: -1.4 °C, mean precipitation: 39 mm) and a summer mean temperature of 19.9 °C (mean precipitation: 84 mm). Different modes of environmental shifts shaped the composition of the vegetation. Within few millennia, marshes and salt marshes with abundant Cyperaceae rapidly graded into Taxodiaceae swamps. This quick but gradual process was interrupted by swift marine ingressions which took place on a decadal to centennial scale. The transgression is accompanied by blooms of dinoflagellates and of the green alga Prasinophyta and an increase in Abies and Picea. Afterwards, the retreat of the sea and the progradation of estuarine and wetland settings were a gradual progress again. Despite a clear sedimentological cyclicity, which is related to the 21-kyr precessional forcing, the climate data show little variation. This missing pattern might be due to the buffering of the precessional-related climate signal by the subtropical vegetation. Another explanation could be the method-inherent broad range of climate-parameter estimates that could cover small scale climatic changes.

Miocene stable isotope record of ODP Hole 162-982B

Oxygen and carbon isotope records are presented for the benthic foraminifer Cibicidoides wuellerstorfi from upper middle through lower upper Miocene (11.6-8.2 Ma) sediments recovered at intermediate water depth (1134 m) at Ocean Drilling Program Site 982 on Rockall Plateau. Oxygen isotopic values generally lighter than those for the Holocene indicate significantly warmer intermediate waters and/or less global ice volume during the late middle to early late Miocene than at the present. The most depleted oxygen isotope values occurred at around 10.5 Ma. After this time a long-term increase in d18O suggests a gradual increase in global ice volume and/or cooling of intermediate waters during the late Miocene. Comparison of the intermediate depth benthic foraminiferal carbon isotope record from Site 982 and records from various North Atlantic deep sites shows that intermediate waters were generally better ventilated than deep waters between 11.6 and 9.6 Ma. During this time period, increased ventilation of intermediate waters was linked to cooling or the build up of polar ice caps. The Mi events originally proposed by Miller et al. (1991, doi:10.1029/90JB02015) and Wright and Miller (1992, doi:10.2973/odp.proc.sr.120.193.1992) are difficult to identify with certainty in sediments sampled at high resolution (<10**4 year). Comparison of the high-resolution benthic d18O records from ODP Site 982 with the low-resolution benthic d18O record from Monte Gibliscemi (Mediterranean) show that Mi events, if real, may not be of importance as a stratigraphic tool in upper Miocene sedimentary sequences.

Benthic foraminiferal faunas of Miocene to Holocene sediments of ODP Site 184-1143 from the southern South China Sea

Deep-sea benthic foraminiferal assemblages from Ocean Drilling Program (ODP) Site 1143 located in the southern South China Sea (SCS) were investigated to evaluate the relationship between faunal composition patterns and paleoceanographic changes during the last 6 million years (late Miocene to Holocene). We used multivariate statistics (correspondence analysis) to analyze carbon-flux-related changes in assemblage composition of benthic foraminifers. Additional proxies for carbon flux and deep-water ventilation include delta13C records of epifaunal Cibicidoides wuellerstorfi and infaunal Uvigerina peregrina var. dirupta and Melonis pompilioides, benthic foraminiferal accumulation rates (BFARs), diversity indices, and relative abundances of indicator species. We observe three significant benthic faunal changes in the southern South China Sea during the last 6 million years. Strong fluctuations in BFAR and relative abundance of productivity indicator species between glacial and interglacial stages after the mid-Pleistocene revolution (MPR) at approximately 0.9 Ma, indicating stronger seasonal carbon flux fluctuations, are accompanied by the extinction of such species as Stilostomella spp. Increases in carbon flux indicator species are coupled with an overall decrease in benthic foraminifer diversity around 3.0 Ma in the late Pliocene. This may indicate increasing carbon flux in a period of productivity maximum caused by enhanced offshore upwelling from intensified winter monsoon wind strength.

Grain size distribution and clay mineralogy of upper Miocene sediments of ODP Hole 107-652A

The upper Miocene sedimentary sequence of Site 652, located on the lower continental margin of eastern Sardinia, was cored and logged during Ocean Drilling Program (ODP) Leg 107. Geophysical and geochemical logs from the interval 170-365 m below seafloor (mbsf), as well as various core measurements (CaCO3, grain size, X-ray diffraction), provide a mineralogical-geochemical picture that is interpreted in the framework of the climatic and tectonic evolution of the western Tyrrhenian. The results indicate the presence of short- and long-term mineralogical variations. Short-term variations are represented by calcium-carbonate fluctuations in which the amount of CaCO3 is correlated to the grain size of the sediments; coarser sediments are associated with high carbonate content and abundant detrital material. Long-term variation corresponds to a gross grain-size change in the upper part of the sequence, where predominantly fine-grained sediments may indicate a gradual deepening of the lacustrine basin towards the Pliocene. Regional climatic changes and rift-related tectonism are possible causes of this variability in the sedimentation patterns. The clay association is characterized by chlorite, illite, and smectite as dominant minerals, as well as mixed-layers clays, kaolinite, and palygorskite. Chlorite, mixed-layers clays, and illite increase at the expense of smectite below the pebble zone (335 mbsf). This is indicative of diagenetic processes related to the high geothermal gradient and to the chemistry of the evaporative pore waters, rather than to changes in the depositional environment.

Clay mineralogy of the Cenozoic Pagodroma Group, East Antarctica

The Cenozoic Pagodroma Group in the northern Prince Charles Mountains, East Antarctica, is a glaciomarine succession of fjordal character, comprising four uplifted formations of different ages. The composition of the <2 µm fraction of sediments of the Pagodroma Group was analysed in order to help identify source areas, past weathering conditions and glacial regimes. Both clay and non-clay minerals have been quantified. The assemblage of the upper Oligocene to lower Miocene Mount Johnston Formation is characterised by the dominance of illite and intermediate concentrations of chlorite. Similar to that assemblage is the clay mineral suite of the middle Miocene Fisher Bench Formation, where illite and chlorite together account for 95% of the clay minerals. The middle to upper Miocene Battye Glacier Formation is the only formation with significant and persistent smectite concentrations, although illite is still dominant. The kaolinite concentration is also high and is even higher than that of chlorite. The clay fraction of the upper Pliocene to lower Pleistocene Bardin Bluffs Formation is characterised by maximum kaolinite concentrations and relatively low illite and chlorite concentrations. The bulk of the clay fraction in each formation can be explained by the physical weathering and erosion of a nearby source under glacial conditions. In the case of Mount Johnston Formation and Fisher Bench Formation this source may be situated in the metavolcanic and gneissic rocks of Fisher Massif. The sediments of the Bardin Bluffs Formation indicate a local source within the Amery Oasis, where Proterozoic granitoid rocks and gneisses, and Permo-Triassic fluvial rocks of the Amery Group are exposed. These results suggest a strong local imprint on the glacial sediments as northwards flowing ice eroded the bedrock in these areas. The origin of the clay fraction of the Battye Glacier Formation is a matter of debate. The smectite and kaolinite content most easily can be explained by erosion of sources largely hidden beneath the ice upstream. Less likely, these clay minerals reflect climatic conditions that were much warmer and wetter than today, facilitating chemical weathering.

Helium concentrations of late Miocene sediments

Throughout the history of the Solar System, Earth has been bombarded by interplanetary dust particles (IDPs), which are asteroid and comet fragments of diameter 1-1,000 µm. The IDP flux is believed to be in quasi-steady state: particles created by episodic main belt collisions or cometary fragmentation replace those removed by comminution, dynamical ejection, and planetary or solar impact. Because IDPs are rich in 3He, seafloor sediment 3He concentrations provide a unique means of probing the major events that have affected the IDP flux and its source bodies over geological timescales (Farley et al., 1998, doi:10.1126/science.280.5367.1250; Takayanagi and Ozima, 1987, doi:10.1029/JB092iB12p12531; Farley, 1995, doi:10.1038/376153a0; Kortenkamp and Dermott, 1998, doi:10.1126/science.280.5365.874). Here we report that collisional disruption of the >150-km-diameter asteroid that created the Veritas family 8.3 +/- 0.5 Myr ago (Nesvorny et al., 2003, doi:10.1086/374807) also produced a transient increase in the flux of interplanetary dust-derived 3He. The increase began at 8.2 +/- 0.1 Myr ago, reached a maximum of 4 times pre-event levels, and dissipated over 1.5 Myr. The terrestrial IDP accretion rate was overwhelmingly dominated by Veritas family fragments during the late Miocene. No other event of this magnitude over the past 10**8 yr has been deduced from main belt asteroid orbits. One remarkably similar event is present in the 3He record 35 Myr ago, but its origin by comet shower (Farley et al., 1998, doi:10.1126/science.280.5367.1250) or asteroid collision (Tagle and Claeys, 2004, doi:10.1126/science.1098481) remains uncertain.

Oligocene and lowermost Miocene diatom stratigraphy and datums of ODP Site 199-1220 sediments

Completion of studies on material collected during Ocean Drilling Program Leg 199 at Site 1220 in the equatorial Pacific allows calibration of the ranges of >35 stratigraphically important diatoms to paleomagnetic stratigraphy for the Oligocene and earliest Miocene (~33.5-21.5 Ma). The taxonomy of these taxa is reviewed, and age estimates of their first and last occurrences are compiled. The diatom zonation for the Oligocene and earliest Miocene of the equatorial Pacific is revised and correlated with paleomagnetic stratigraphy. This biostratigraphy is likely to be applicable throughout the low-latitude regions of the world's oceans.

Middle Miocene to Recent radiolarians from the Weddell Sea

Well preserved middle Miocene to Recent radiolarians were recovered from several sites in the Weddell Sea by ODP (Ocean Drilling Program) Leg 113. Low rates of sedimentation, hiatuses, and poor core recovery in some sites are offset by the nearly complete recovery of a late middle Miocene to late Pliocene section at Site 689 on the Maud Rise. Although a hiatus within the latest Miocene exists, this site still provides an excellent reference section for Antarctic biostratigraphy. A detailed radiolarian stratigraphy for the middle Miocene to late Pliocene of Site 689 is given, together with supplemental stratigraphic data from ODP Leg 113 Sites 690, 693, 695, 696, and 697. A refined Antarctic zonation for the middle Miocene to Recent is presented, based on the previous zonations of Hays (1965), Chen (1975), Weaver (1976b), and Keany (1979). The late Miocene radiolarian Acrosphaera australis n. sp. is described and used to define the A. australis zone, ranging from the first appearance of the nominate species to the last appearance of Cycladophora spongothorax (Chen) Lombari and Lazarus 1988. The species Botryopera deflandrei Petrushevskaya 1975 is transferred to Antarctissa deflandrei (Petrushevskaya) n. comb.