Sea surface temperatures reconstructed from sediment core U1338

This study presents a new alkenone-derived Sea Surface Temperature (SST) record and d18ONoelaerhabdaceae data of the 2-5 µm carbonate fractions from the IODP site U1338 located in the Eastern Equatorial Pacific (EEP), over the Miocene-Pliocene. Our data and those available from other sites of the same area show the establishment of a cold tongue during the early Pliocene (4.4 - 3.6 Ma). SST and d18ONoelaerhabdaceae time-series indicate periods of significant salinity variations. Comparison with the d18Obenthic curve from sediment cores of the Equatorial Pacific Ocean allow us to distinguish between global changes and local surface salinity variations in the EEP. Ice sheet growth and evaporation-precipitation are then discussed as possible drivers of such changes, as well as the role of Central American and the Indonesian seaway restriction. Our data suggest a shallowing of the thermocline in the EEP, between 6.8 and 6 Ma, and its shoaling between 4.8 and 4.0 Ma, suggesting the appearance of the cold tongue (Steph et al., 2010). The Pliocene climate transition would therefore not be primarily driven by Northern hemisphere glaciation.

Cenozoic global ice volume and temperature simulations over the past 40 million years

Variations in global ice volume and temperature over the Cenozoic era have been investigated with a set of one-dimensional (1-D) ice-sheet models. Simulations include three ice sheets representing glaciation in the Northern Hemisphere, i.e. in Eurasia, North America and Greenland, and two separate ice sheets for Antarctic glaciation. The continental mean Northern Hemisphere surface-air temperature has been derived through an inverse procedure from observed benthic d18O records. These data have yielded a mutually consistent and continuous record of temperature, global ice volume and benthic d18O over the past 35 Ma. The simple 1-D model shows good agreement with a comprehensive 3-D ice-sheet model for the past 3 Ma. On average, differences are only 1.0°C for temperature and 6.2 m for sea level. Most notably, over the 35 Ma period, the reconstructed ice volume-temperature sensitivity shows a transition from a climate controlled by Southern Hemisphere ice sheets to one controlled by Northern Hemisphere ice sheets. Although the transient behaviour is important, equilibrium experiments show that the relationship between temperature and sea level is linear and symmetric, providing limited evidence for hysteresis. Furthermore, the results show a good comparison with other simulations of Antarctic ice volume and observed sea level.

Tab. 1: Proportion of glacial quartz grains contained in the deposits

The Yari-Hotaka Mountain Range is one of the most famous formerly-glaciated areas of Japan. Many glacial landforms remain in three neighbouring U-shaped valleys, named Yarisawa, Yokoo and Migimata. Moraines and outwash terraces can be classified into four groups according to their location and to the amount of glacial quartz grains contained in the deposits. A glaciation is proved for other parts of the Northern Japanese Alps before 100 000 years B.P., but not for the Yari-Hotaka Mountain Range, because the corresponding glacial landforms cannot be found here. The oldest known Ichinomata stage before and after 60 000 years B.P. corresponds to the Yokoo glacial which is proved wirhin the whole Japanese Alps. The three younger stages, Babadaira stage (before 30 000 years B.P.), Yarisawa stage I (about 30000 years B.P.) and Yarisawa stage II (about 15000 years B.P.), belong to the Karasawa glacial. About 10 000 years B.P. the glaciers melted away. At all times the relief-influence was especially important for Ihe mass-balances of Japanese glaciers. Wind-drifted snow from the west-exposed windward slopes to the slopes in eastern (lee) exposition, and a voluminous snow accumulation by avalanches from the high rocky walls onto the glacier surfaces beneath, caused very low situated glaciers as well as low equilibrium-lines. In most cases the snow-lines were situated 100 m or more above the equilibrium-lines. During the Ichinomata stage the snow-line reached an altitude of 2400-2450 m. It rose about 100 m to the Babadaira stage, 300 m to Yarisawa stage I and about 450 m to Yarisawa stage II. At present the snow-line is situated above the Northern Japanese Alps at over 4000 m. Therefore only perennial snow-patches exist. If the snow-line would go down by a few hundred meters, this region would be highly interesting Ifor studies on the beginning of mountain glaciation.

Late Cenozoic stable isotope record of benthic and planktonic foraminifera from the Pacific Ocean

Stable isotopic analyses of Middle Miocene to Quaternary foraminiferal calcite from east equatorial and central north Pacific DSDP cores have provided much new informatlon on the paleoceanography of the Pacific Neogene The history of delta18O change in planktonic foraminifera reflects the changing Isotopic composition and temperature of seawater at the time of test formation. Changes in the isotopic composition of benthonic foraminifera largely reflect changes m the volume of continental ice. Isotopic data from these cores indicates the following sequence of events related to continental glaciation (1) A permanent Antarctic ice sheet developed late in the Middle Miocene (about 13 to 11.5 m.y. ago) (2) The Late Miocene (about 11.5 to 5 m.y. ago) is marked by significant variation in delta18O of about 0.5? throughout, indicating instability of Antarctic ice cap size or bottom-water temperatures (3) The early Pliocene (5 to about 3 m.y. ago) was a time of relative stability in ice volume and bottom-water temperature (4) Growth of permanent Northern Hemisphere ice sheets is referred to have begun about 3 m.y. ago (5) The late Pliocene (3 to about 1.8 m.y. ago) is marked by one major glaciation or bottom-water cooling dated between about 2.1 to 2.3 m.y. (6) There is some evidence that the frequency of glacial-interglacial cycles increased at about 0.9 m.y. There is significant variation in delta13C at these sites but no geochemical interpretation is offered in this paper. The most outstanding feature of delta13C results is a permanent shift of about -0.8? found at about 6.5 m.y. in east equatorial and central north Pacific benthonic foraminifera. This benthonic carbon shift may form a useful marker in deep-sea cores recovering Late Miocene carbonates.

Magnetic properties of maar lake sediments from Otago, New Zealand

Foulden Maar is a highly resolved maar lake deposit from the South Island of New Zealand comprising laminated diatomite punctuated by numerous diatomaceous turbidites. Basaltic clasts found in debris flow deposits at the base of the cored sedimentary sequence yielded two new 40Ar/39Ar dates of 24.51±0.24 Ma and 23.38±0.24 Ma (2sigma). The younger date agrees within error with a previously published 40Ar/39Ar date of 23.17±0.19 Ma from a basaltic dyke adjacent to the maar crater. The diatomite is inferred to have been deposited over several tens of thousands of years in the latest Oligocene/earliest Miocene, and may have overlapped with the period of rapid glaciation and subsequent deglaciation of Antarctica known as the Mi-1 event. Sediment magnetic properties and SEM measurements indicate that the magnetic signal is dominated by pseudo-single domain pyrrhotite. The most likely source of detrital pyrrhotite is schist country rock fragments from the inferred tephra ring created by the phreatomagmatic eruption that formed the maar. Variations in magnetic concentration and lamina thickness indicate a decrease in erosional input and increase in diatom productivity throughout the depositional period, suggesting a long-term (tens of thousands of years) climatic change in New Zealand in the latest Oligocene/earliest Miocene.

(Table 1) Diatom mats from ODP Leg 177 Sites in the Southern Ocean, Atlantic Sector

The interaction between biogenic silica export and burial, paleoceanography, diatom species succession and mats formation was examined based on relative abundances data of Plio/Pleistocene diatoms from six cores recovered during ODP Leg 177 on a transect across the Antarctic Circumpolar Current (ACC) in the Atlantic sector of the Southern Ocean. Fragilariopsis kerguelensis, Actinocyclus ingens and species of the genus Thalassiothrix were the main contributors to the diatom assemblages. Three main steps marked the development of the silica system in the Southern Ocean: Step 1 (at ca. 2.77 Ma), establishment of increased biogenic silica burial in the Antarctic Circumpolar Current area, following the large-scale oceanic reorganization connected to the increased northern hemisphere glaciation; Step 2 (at ca. 1.93 Ma), the Antarctic Polar Front becomes the main biogenic silica sink, diatom mats are widespread, and are also found slightly to the north and south of the APF; Step 3 (at ca. 0.63 Ma), with the strong drop in abundance (and later extinction at 0.38 Ma) of A. ingens and the rise to dominance of F. kerguelensis, the system enters a glacial-interglacial mode, with diatom mats occurring during interglacials at the APF and in the Antarctic Zone, but disappearing north of it.

Contents of water, sulphur, total organic and inorganic carbon, XRF counts of Ti, S and Ca and radiocarbon ages of sediment core Co1008 from Skua Lake, Rauer Group, East Antarctica

Limited information on the East Antarctic Ice Sheet (EAIS) geometry during Marine Isotope Stage 3 (MIS 3; 60-25 ka) restricts our understanding of its behaviour during periods of climate and sea level change. Ice sheet models forced by global parameters suggest an expanded EAIS compared to the Holocene during MIS 3, but field evidence from East Antarctic coastal areas contradicts such modelling, and suggests that the ice sheet margins were no more advanced than at present. Here we present a new lake sediment record, and cosmogenic exposure results from bedrock, which confirm that Rauer Group (eastern Prydz Bay) was ice-free for much of MIS 3. We also refine the likely duration of the Last Glacial Maximum (LGM) glaciation in the region. Lacustrine and marine sediments from Rauer Group indicate the penultimate period of ice retreat predates 50 ka. The lacustrine record indicates a change from warmer/wetter conditions to cooler/drier conditions after ca. 35 ka. Substantive ice sheet re-advance, however, may not have occurred until much closer to 20 ka. Contemporary coastal areas were still connected to the sea during MIS 3, restricting the possible extent of grounded ice in Prydz Bay on the continental shelf. In contrast, relative sea levels (RSL) deduced from field evidence indicate an extra ice load averaging several hundred metres thicker ice across the Bay between 45 and 32 ka. Thus, ice must either have been thicker immediately inland (with a steeper ice profile), or there were additional ice domes on the shallow banks of the outer continental shelf. Further work is required to reconcile the differences between empirical evidence of past ice sheet histories, and the history predicted by ice sheet models from far-field temperature and sea level records.

(Table 1) Luminescence datings from Varchushka and Pyoza rivers, Russia

The Pyoza River area in the Arkhangelsk district exposes sedimentary sequences suitable for study of the interaction between consecutive Valdaian ice sheets in Northern Russia. Lithostratigraphic investigations combined with luminescence dating have revealed new evidence on the Late Pleistocene history of the area. Overlying glacigenic deposits of the Moscowian (Saalian) glaciation marine deposits previously confined to three separate transgression phases have all been connected to the Mikulinian (Eemian) interglacial. Early Valdaian (E. Weichselian) proglacial, lacustrine and fluvial deposits indicate glaciation to the east or north and consequently glacier damming and meltwater run-off in the Pyoza area around 90-110 ka BP. Interstadial conditions with forest-steppe tundra vegetation and lacustrine and fluvial deposition prevailed at the end of the Early Valdaian around 75-95 ka BP. A terrestrial-based glaciation from easterly uplands reached the Pyoza area at the Early to Middle Valdaian transition around 65-75 ka BP and deposited glaciofluvial strata and subglacial till (Yolkino Till). During deglaciation, laterally extensive glaciolacustrine sediments were deposited in ice-dammed lakes in the early Middle Valdaian around 55-75 ka BP. The Barents-Kara Sea ice sheet deposited the Viryuga Till on the lower Pyoza from northerly directions. The ice sheet formed the Pyoza marginal moraines, which can be correlated with the Markhida moraines further east, and proglacial lacustrine deposition persisted in the area during the first part of the Middle Valdaian. Glacio-isostatic uplift caused erosion followed by pedogenesis and the formation of a deflation horizon in the Middle Valdaian. Widely dispersed periglacial river plains were formed during the Late Valdaian around 10-20 ka BP. Thus, the evidence of a terrestrial-based ice sheet from easterly uplands in the Pyoza area suggests that local piedmont glaciers situated in highlands such as the Timan Ridge or the Urals could have developed into larger, regionally confined ice sheets. Two phases of ice damming and development of proglacial lakes occurred during the Early and Middle Valdaian. The region did not experience glaciation during the Late Valdaian.

Firn line elevation of the Dickson Land peninsula

The Dickson Land peninsula is located in central West-Spitsbergen between the NNE branches of Isfjorden. The climatic firn line lying at 500 m causes plateau glaciers with outlet tongues which are characteristic of S-Dickson Land. The distribution of valley glaciers and the variations of the orographic firn line depend on wind direction. In comparing the firn lines established by the methods of LICHTENECKER (1938) and VISSER (1938), to the values calculated by the method of v. HÖFER (1879), differences of up to l07 m are found. These differences may depend on the inclination and distance relationships of the glaciers above and below the real firn lines. During the latest glacial advance, Dickson Land was located on the peripheries of two local glaciation centers. At that time an inland glaciation of West-Spitsbergen did not exist . The formation of a subglacial channel system dates back to the maximum extent of the late glacial phase before 17500 B.P, (+2000/-1375 years). A correlation of postglacial stadia and 14C dated marine terraces (FEYLING-HANSSEN & OLSSON, 1960; FEYLING-HANSSEN, 1965) is possible. Considering isostatic movement and the difference between calculated and real firn lines, a postglacial stadium at about 10400 B. P. can be reconstructed with a firn line lying 265 m above former sea level. On average, the absolute depression below the recent firn line amounted to 246 m. Stagnation at 9650 B.P. coincided with a firn line at 315 m above former sea level and a depression of 173 m. Around 1890 A.D., glacial fluctuations corresponded to a firn line at 415 m (depression: 64 m). To some extent the morphology of the main valleys appears to depend on structure and petrography. Therefore their value as indicators of former glaciations is questionable. The periglacial forms are shown on a large-scale map. At the time of the "Holocene warm interval", between 7000 and 2000 B.P. (FEYLING-HANSSEN, 1955a, 1965), an increase of periglacial activity seems likely. This can be explained by a simultaneous increase in the depth of the active layer in both soil and bedrock.

Palynological counts (dinoflagellate cysts) and foraminiferal geochemistry (d18O, d18C, Mg/Ca) of the Pliocene-Pleistocene transition in the North Atlantic

The position of the North Atlantic Current (NAC) during the intensification of Northern Hemisphere glaciation (iNHG) has been evaluated using dinoflagellate cyst assemblages and foraminiferal geochemistry from a ~260 kyr interval straddling the base of the Quaternary System from two sites: eastern North Atlantic Deep Sea Drilling Project Site 610 in the path of the present NAC and central North Atlantic Integrated Ocean Drilling Program Site U1313 in the subtropical gyre. Stable isotope and foraminiferal Mg/Ca analyses confirm cooling near the marine isotope stage (MIS) G7-G6 transition (2.74 Ma). However, a continued dominance of the dinoflagellate cyst Operculodinium centrocarpum sensu Wall and Dale (1966) indicates an active NAC in the eastern North Atlantic for a further 140 kyr. At MIS 104 (~2.60 Ma), a profound dinoflagellate cyst assemblage turnover indicates NAC shutdown in the eastern North Atlantic, implying elevated atmospheric pressure over the Arctic and a resulting shift in the westerlies that would have driven the NAC. These findings challenge recent suggestions that there was no significant southward shift of the NAC or the Arctic Front during iNHG, and reveal a fundamental climatic reorganization near the base of the Quaternary.

Geochemistry and sedimentology of IODP Hole 307-U1317E

The Plio-Pleistocene intensification of Northern Hemisphere continental ice-sheet development is known to have profoundly affected the global climate system. Evidence for early continental glaciation is preserved in sediments throughout the North Atlantic Ocean, where ice-rafted detritus (IRD) layers attest to the calving of sediment-loaded icebergs from circum-Atlantic ice sheets. So far, Early-Pleistocene IRD deposition has been attributed to the presence of high-latitudinal ice sheets, whereas the existence and extent of ice accumulation in more temperate, mid-latitudinal regions remains enigmatic. Here we present results from the multiproxy provenance analysis of a unique, Pleistocene-Holocene IRD sequence from the Irish NE Atlantic continental margin. There, the Challenger coral carbonate mound (IODP Expedition 307 site U1317) preserved an Early-Pleistocene record of 16 distinctive IRD events, deposited between ca 2.6 and 1.7 Ma. Strong and complex IRD signals are also identified during the mid-Pleistocene climate transition (ca 1.2 to 0.65 Ma) and throughout the Middle-Late Pleistocene interval. Radiogenic isotope source-fingerprinting, in combination with coarse lithic component analysis, indicates a dominant sediment source in the nearby British-Irish Isles, even for the oldest, Early-Pleistocene IRD deposits. Hence, our findings demonstrate, for the first time, repeated and substantial (i.e. marine-terminating) ice accumulation on the British-Irish Isles since the beginning of the Pleistocene. Contemporaneous expansion of both high- and mid-latitudinal ice sheets in the North Atlantic region is therefore implied at the onset of the Pleistocene. Moreover, it suggests the recurrent establishment of (climatically) favourable conditions for ice sheet inception, growth and instability in mid-latitudinal regions, even in the earliest stages of Northern Hemisphere glacial expansion and in an obliquity-driven climate system.