Clay mineralogy of late Eocene to early Miocene sediments of McMurdo Sound, Antarctica

The clay mineral assemblages of upper Eocene to lower Miocene sediments recovered at the CIROS-1 and MSSTS-1 drill sites on the McMurdo Sound shelf, Antarctica, were analyzed in order to reconstruct the Cenozoic Antarctic paleoclimate and ice dynamics. The assemblages are dominated by smectite and illite, with minor amounts of chlorite and kaolinite. The highest smectite amounts and best smectite crystallinities occur in the upper Eocene part of CIROS-1, below 425-445 mbsf. They indicate that during their deposition, chemical weathering conditions prevailed on the nearby continent. Large parts of East Antarctica were probably ice-free at that time, but some glaciers reached the sea and contributed to the glaciomarine sedimentation. In contrast, only minor total amounts of smectite are present in Oligocene and younger sediments due to the shift to mainly physical weathering on an ice-covered Antarctic continent. However, relative smectite percentages rise to more than 60% during two late Oligocene intervals (ca. 27.5-26.2 and 25.0-24.5 Ma) and during one early Miocene interval starting at ca. 23.3 Ma. These intervals are characterized by ice masses coming probably from the south, where volcanic rocks acted as a source, as also indicated by the composition of the sand and gravel fractions. During the other intervals, the ice came from the west, where the physical erosion of basement rocks and sedimentary rocks of the Beacon Supergroup in the Transantarctic Mountains provided high illite concentrations. Because the two drill sites are only 4 km apart, their clay mineral records can be correlated. This led to a new interpretation of the Oligocene paleomagnetic data of the MSSTS-1 site and to a more detailed lithostratigraphic correlation of the Miocene parts of the cores.

Tab. 1: Radiocarbon ages of the late Pleistocene and early Holocene of Mecklenburg-Vorpommern and surrounding areas

Several geoscientific projects in the last decade led to a marked increase of radiocarbon dates in Mecklenburg-Vorpommern and in neighbouring areas. The studies were mostly focussed on the genesis of the Baltic Basin and the last termination. In this Paper, a regional collection of 271 radiocarbon dates of the late Pleistocene and early Holocene (ca. 50,000-8,000 14C yr BP) is presented. The dates were calibrated, correlate, and assessed with regard to their credibility. The evaluation of the data is focussed on problems of regional palaeogeography. The age of the last Weichselian deglaciation (deglaciation after the Mecklenburg Advance) is assumed to be around 14,000 14C yr BP through radiocarbon dates from the Pomeranian Bay. This data is ca. 1,000 years older compared to former views. On the other hand, the database allows the dating of late Pleistocene basin sequences from the Baltic coast, This indicates three stratigraphic units for basin areas 0-15 m above sea level - glaciolacustrine sedimentation in the late Pleniglacial, lacustrine and telmatic sedimentation as well as soil formation in the early Lateglacial and Alleroed and aeolian sedimentation in the Younger Dryas. The Younger Dryas in the huge Mecklenburg Bay-Darss Basin NE of Rostock is characterised by lacustrine sedimentation ca. 20 m below sea level ("Baltic Ice Lake"), and by aeolian sedimentation above sea level.

Stable carbon and oxygen isotope ratios of Late Miocene and Early Pliocene foraminifera from the Southwest Pacific

The Late Miocene-Early Pliocene paleoclimatic history has been evaluated for a deep drilled sediment sequence at Deep Sea Drilling Project Site 281 and a shallow water marine sediment sequence at Blind River, New Zealand, both of which lay within the Subantarctic water mass during the Late Miocene. A major, faunally determined, cooling event within the latest Miocene at Site 281 and Blind River coincides with oxygen isotopic changes in benthonic foraminiferal composition at DSDP Site 284 considered by Shackleton and Kennett (1975) to indicate a significant increase in Antarctic ice sheet volume. However, at Site 281 benthonic foraminiferal oxygen isotopic changes do not record such a large increase in Antarctic ice volume. It is possible that the critical interval is within an unsampled section (no recovery) in the latest Miocene. Two benthonic oxygen isotopic events in the Late Miocene (0.5 ? and 1 ? in the light direction) may be useful as time-stratigraphic markers. A permanent, negative, carbon isotopic shift at both Site 281 and Blind River allows precise correlations to be made between the two sections and to other sites in the Pacific region. Close interval sampling below the carbon shift at Site 281 revealed dramatic fluctuations in surface-water temperatures prior to a latest Miocene interval of refrigeration (Kapitean) and a strong pulse of dissolution between 6.6 and 6.2 +/- 0.1 m.y. which may be related to a fundamental geochemical change in the oceans at the time of the carbon shift (6.3-6.2 m.y.). No similar close interval sampling at Blind River was possible because of a lack of outcrop over the critical interval. Paleoclimatic histories from the two sections are very similar. Surface water temperatures and Antarctic ice-cap volume appear to have been relatively stable during the late Middle-early Late Miocene (early-late Tongaporutuan). By 6.4 m.y. cooler conditions prevailed at Site 281. Between 6.3 and 6.2 -+ 0.1 m.y. the carbon isotopic shift occurred followed, within 100,000 yr, by a distinct shallowing of water depths at Blind River. The earliest Pliocene (Opoitian) is marked by increasing surface-water temperatures.

A multiproxy analysis of late MIS 6 to early MIS 5d OSP Site 161-975 sediments

A multiproxy analysis based on planktic foraminiferal abundances, derived SSTs, and stable planktic isotopes measurements together with alkenone abundances and Uk'37 SSTs was performed on late MIS 6 to early MIS 5d sediment recovered from Site 975 (ODP Leg 161) in the South Balearic Islands basin (Western Mediterranean) with emphasis on reconstructing the climate progression of the last interglacial period. A number of abrupt climate changes related to alternative influence of nutrient rich northern and oligotrophic southern water masses were revealed. Heinrich event 11 and cooling events C27, C26, C25, C24, C23, which have been previously described in the North Atlantic, were recognized. However, in comparison to the eastern North Atlantic mid-latitude region, events C27 and C26 at Site 975 seem to be significantly more pronounced. Together with evidence of a two-phase climate optimum with maximum SSTs reached during its later phase, this implies a close similarity in climate dynamics between the Western Mediterranean and the Nordic seas. We propose that postglacial effects in the Nordic seas had an influence on the western Mediterranean climate via atmospheric circulation and that these effects competed with the insolation force.

Late Eocene-early Oligocene radiolarian events and abundances in ODP Leg 199 sediments

During Ocean Drilling Program Leg 199, eight sites (Sites 1215-1222) were cored in the Central Pacific. Late Eocene-early Oligocene thick radiolarian-rich biogenic sediments were collected from Holes 1218A, 1219A, and 1220A. This is the first attempt to calibrate the ages of Paleogene radiolarian events using magnetostratigraphy in this region. A total of 107 species and species groups, which are valuable for stratigraphic correlation, are listed with numeric data and figures. Among these three holes, a total of 77 radiolarian events were recognized and their ages were calibrated by correlation with paleomagnetic events recorded in Hole 1220A.

Late Miocene to Early Pliocene radiolarians from ODP Hole 178-1095B

Early Pliocene to middle late Miocene hemipelagic and distal turbidite sediments from Hole 1095B, near the Antarctic Peninsula, yield moderately abundant, moderately well preserved radiolarian faunas and other biosiliceous material (diatoms, silicoflagellates, and sponge spicules). Preservation characteristics, however, vary strongly even between closely related samples, and there are many intervals of poor preservation. In the 140- to 460-meters below seafloor interval studied, it was possible to identify the following standard Southern Ocean radiolarian zones: Upsilon, Tau, Amphymenium challengerae, Acrosphaera? labrata, Siphonosphaera vesuvius, and upper Acrosphaera australis (total age range ~4-10 Ma). Some normally common radiolarian groups, such as actinommids, are unusually rare in the studied material, and the relative ranges of several individual species, such as Acrosphaera labrata vs. A. australis, appear to be somewhat anomalous. These observations imply that the ranges of taxa in this section may be somewhat diachronous, due to either local ecologic factors and/or the highly variable preservation of the faunas. Thus, the ages of events reported are probably only approximate, although they are still useful for constraining the age of sediments in this section.

Late Cretaceous-Early Neogene oxygen and carbon isotope record of the Indian Ocean

Stable isotopic data of calcareous nannofossil, monogeneric and monospecific planktic and benthic foraminifera from five Indian Ocean DSDP sites (212, 217, 220, 237, and 253), leads to the following paleoclimatic and paleoceanographic conclusions: - The latest Cretaceous oxygen isotopic record implies a cooling (3-4°C) during the Maastrichtian. At the Cretaceous/Tertiary boundary only a minor warming (about 2°C) has been recorded. The parallel delta13C decrease of more than 1? indicates a significant decrease in productivity. - During the latest Paleocene a positive delta13C excursion was detected in Sites 217 and 237. This transient enrichment in delta13C may be due to productivity changes on continents and/or a change in the storage rate of organic matter in marginal basins or shelf areas. - The most striking feature in the oxygen isotopic record is noted at the Early/Middle Eocene transition. The shift towards more positive values (which were probably enhanced to a certain extent by a preceding diagenetic alteration) delineates a dramatic climatic deterioration at high and mid latitudes during the earlier Tertiary. - Near the Eocene/Oligocene boundary a cooling is evident within the latest Eocene interval. During the earliest Oligocene time a hiatus at Sites 217 and 253 partially obscures the climatic record. - Several climatic fluctuations have been noted during the Oligocene: a cooling at the base of Zone NP 23, a warming at the top of Zone NP 23 through NP 24, and a cooling during Zone NP 25. - The Miocene oxygen isotopic record is dominated by changes in surface and bottom water environments during Zone NN5. The decreasing and then increasing delta18O values, together with the subsequent steepening of the vertical delta18O gradient, point towards major climatic instabilities. These events coincide with the Mid-Miocene build-up of Antarctic ice-sheets. During the latest Miocene to the earliest Pliocene the delta18O record of planktic foraminifera indicates a significant warming of the Indian Ocean at mid-latitudes. - The delta13C record during the Oligocene and Miocene reveals several cycles (delta13C enrichments: NP 24, NN2, NN5, NN9, and base NN 11) which are most likely related to changes in storage rates of organic matter and biological productivity due to climatic changes and transgression/regression cycles. In addition, changes in the circulation patterns may also have influenced the carbon isotopic record.

Sedimentation rates and geochemistry of the Atlantic and Indic Ocean of Late Miocene - Early Pliocene

Studies of the late Miocene-early Pliocene biogenic bloom typically have focused on high-productivity areas in the Indian and Pacific Oceans in order to achieve high resolution samples. Thus there is a paucity of information concerning whether the Atlantic Ocean, in general or low-productivity regions in all three basins experienced this bloom. This study measured the phosphorus mass accumulation rate (PMAR). in five cores from low-productivity regions of the Atlantic and Indian Oceans. All cores exhibit a peak in productivity 4-5.5 Ma, coincident with the Indo-Pacific bloom. This suggests that nutrients were not shifted away from low-productivity regions nor out of the Atlantic Ocean. Instead, it appears that the bloom was caused by an overall increase in nutrient flux into the world oceans. Four of the cores record the bloom's PMAR peak as bimodal, indicating a pulsed increase in phosphorus to the oceans. This suggests that there may have been multiple causes of the biogenic bloom.

Stable isotope composition and ice rafted debris in early Late Pliocene sediments from Maud Rise, South Atlantic

We have conducted an integrated study of ice-rafted debris (IRD) and oxygen isotopes (measured on Cibicides, Globigerina bulloides, and Neogloboquadrina pachyderma, using identical samples). We used samples from the early Late Pliocene Gauss Chron from ODP Site 114-704 on the Meteor Rise in the subantarctic South Atlantic. During the early Gauss Chron, the oxygen isotopic ratios are generally up to 0.5?-0.6? less than their respective Holocene values. The lowest values in this record can accommodate a warming of about 2.5°C or a sea-level rise of about 50 m, but not both, and probably result from some warming and a small reduction in global ice volume. Starting with isotope stage MG2 [ 3.23 Ma on the Berggren et al. ( 1985) time scale; 3.38 on the Shackleton et al. ( 1995b) time scale] oxygen-isotopic values generally increase (and oscillate about a Holocene mean). The first significant IRD appears at the same time. There is a subsequent increase in IRD amounts upsection. In order to reach the site, this material must have been transported by large, tabular icebergs derived from Antarctic ice shelves or ice tongues, similar to occasional, large modern icebergs. This combined record suggests strongly that the Antarctic ice sheet was essentially intact; some warming at the drill site is indicated, but not a major reduction in ice-volume on Antarctica.