Late Pleistocene oxygen isotope record of biognic silica

We determined the isotopic composition of oxygen in marine diatoms in eight deep-sea cores recovered from the Atlantic sector of the Southern Ocean. The analytical reproducibility and core-to-core consistency of the isotopic signal suggests that diatom delta18O can be used as a new paleocenographic tool to reconstruct past variations in surface water characteristics and to generate 18O -isotope-based stratigraphy for the Southern Ocean. The data indicate that diatom delta18O reflects sea surface temperature and seawater isotopic composition and that diatoms retain their isotopic signal on timescales of a least 430 ka. The delta18O analyses of different diatom assemblages reveal that the isotopic signal is free of species effects and that the common Antarctic species have the same water-opal fractionation. The transition from the last glacial maximum (LGM) to the Holocene is fully recorded in high sedimentation rate cores. An 18O enrichment during the LGM, a post-LGM meltwater spike and an input of meltwater during the late Holocene are the main isotopic features observed in down core records. The origin of this meltwater was very likely melting icebergs and/or continental ice or by melting sea ice that had accumulated snow. The most pronounced meltwater effects are recorded in cores that are associated with the Weddel gyre. Our results provide the basis for extending isotope studies to oceanic regions devoid of carbonate; further, isotopic stratigraphies may be constructed for records and regions where they were previously not possible.

Composition of Holocene lithificates at the slopes of the Untersee Mountain Valley, East Antarctica

It was found out that the lower parts of slopes of the Untersee mountain valley (East Antarctica) were locally covered with lithificates (both carbonate-free and carbonate-poor). They occur in three modes: crusts, films, and impregnates. All of them cover Late Pleistocene moraine material and consist of mixture of lacustrine sedimentary material and filling material of moraines. A mechanism of their genesis is offered.

A Late Pleistocene sea level stack

Late Pleistocene sea level has been reconstructed from ocean sediment core data using a wide variety of proxies and models. However, the accuracy of individual reconstructions is limited by measurement error, local variations in salinity and temperature, and assumptions particular to each technique. Here we present a sea level stack (average) which increases the signal-to-noise ratio of individual reconstructions. Specifically, we perform principal component analysis (PCA) on seven records from 0-430 ka and five records from 0-798 ka. The first principal component, which we use as the stack, describes ~80 % of the variance in the data and is similar using either five or seven records. After scaling the stack based on Holocene and Last Glacial Maximum (LGM) sea level estimates, the stack agrees to within 5 m with isostatically adjusted coral sea level estimates for Marine Isotope Stages 5e and 11 (125 and 400 ka, respectively). When we compare the sea level stack with the d18O of benthic foraminifera, we find that sea level change accounts for about ~40 % of the total orbital-band variance in benthic d18O, compared to a 65 % contribution during the LGM-to-Holocene transition. Additionally, the second and third principal components of our analyses reflect differences between proxy records associated with spatial variations in the d18O of seawater.

Pliocene-Pleistocene stack of globally distributed benthic stable oxygen isotope records

We present a 5.3-Myr stack (the ''LR04'' stack) of benthic d18O records from 57 globally distributed sites aligned by an automated graphic correlation algorithm. This is the first benthic delta18O stack composed of more than three records to extend beyond 850 ka, and we use its improved signal quality to identify 24 new marine isotope stages in the early Pliocene. We also present a new LR04 age model for the Pliocene-Pleistocene derived from tuning the delta18O stack to a simple ice model based on 21 June insolation at 65 N. Stacked sedimentation rates provide additional age model constraints to prevent overtuning. Despite a conservative tuning strategy, the LR04 benthic stack exhibits significant coherency with insolation in the obliquity band throughout the entire 5.3 Myr and in the precession band for more than half of the record. The LR04 stack contains significantly more variance in benthic delta18O than previously published stacks of the late Pleistocene as the result of higher resolution records, a better alignment technique, and a greater percentage of records from the Atlantic. Finally, the relative phases of the stack's 41- and 23-kyr components suggest that the precession component of delta18O from 2.7-1.6 Ma is primarily a deep-water temperature signal and that the phase of d18O precession response changed suddenly at 1.6 Ma.

Atlantic 231Pa/230Th and biogenic opal compilation of the Holocene and the LGM

The strength and geometry of the Atlantic meridional overturning circulation is tightly coupled to climate on glacial-interglacial and millennial timescales, but has proved difficult to reconstruct, particularly for the Last Glacial Maximum. Today, the return flow from the northern North Atlantic to lower latitudes associated with the Atlantic meridional overturning circulation reaches down to approximately 4,000 m. In contrast, during the Last Glacial Maximum this return flow is thought to have occurred primarily at shallower depths. Measurements of sedimentary 231Pa/230Th have been used to reconstruct the strength of circulation in the North Atlantic Ocean, but the effects of biogenic silica on 231Pa/230Th-based estimates remain controversial. Here we use measurements of 231Pa/230Th ratios and biogenic silica in Holocene-aged Atlantic sediments and simulations with a two-dimensional scavenging model to demonstrate that the geometry and strength of the Atlantic meridional overturning circulation are the primary controls of 231Pa/230Th ratios in modern Atlantic sediments. For the glacial maximum, a simulation of Atlantic overturning with a shallow, but vigorous circulation and bulk water transport at around 2,000 m depth best matched observed glacial Atlantic 231Pa/230Th values. We estimate that the transport of intermediate water during the Last Glacial Maximum was at least as strong as deep water transport today.

Sea-surface paleotemperature reconstructions for the middle Miocene-Holocene of ODP Site 133-811 in the Coral Sea off northeast Australia

The derivation of a detailed sea-surface paleotemperature curve for the middle Miocene-Holocene (10-0 Ma) from ODP Site 811 on the Queensland Plateau, northeast Australia, has clarified the role of sea-surface temperature fluctuations as a control on the initiation and development of the extensive carbonate platforms of this region. This curve was derived from isotopic analyses of the planktonic foraminifer Globigerinoides ruber, and converted to temperature using the surface-water paleotemperature equation accounting for variations in global ice volume. The accuracy of these data were confirmed by derivation of paleotemperatures using the water column isotopic gradient (Delta delta18O), corrected for salinity and variations in seafloor water mass temperature. Results indicate that during this period surface-water temperatures were, on average, greater than the minimum required for tropical reef growth (20°C; Veron, 1986), with the exception of the late Miocene and earliest early Pliocene (10-4.9 Ma), when there were repeated intervals of temperatures between 18-20°C. Tropical reef growth on the Queensland Plateau was extensive from the early to early middle Miocene (~21-13 Ma), after which reef development began to decline. A lowstand near 11 Ma probably exposed shallower portions of the plateau; after re-immersion near 7 Ma, the areal extent of reef development was greatly reduced (~ 50%). Paleotemperature data from Site 811 indicate that decreased sea-surface temperatures were likely to have been instrumental in reducing the area of active reef growth on the Queensland Plateau. Reduced reefal growth rates continued until the late Pliocene or Quaternary, despite the increase of average sea-surface paleotemperatures to 22-23°C. Studies on modern corals show that when sea-surface temperatures are below ~24°C, as they were from the late Miocene to the Pleistocene off northeast Australia, corals are stressed and growth rates are greatly reduced. Consequently, when temperatures are in this range, corals have difficulty keeping pace with subsidence and changing environmental factors. In the late Pliocene, sedimentation rates increased due to increases in non-reefal carbonate production and falling sea levels. It was not until the mid-Quaternary (0.6-0.7 Ma) that sea-surface paleotemperatures increased above 24°C as a result of the formation of a western Coral Sea warm water pool. Because of age discrepancies, it is unclear exactly when an effective barrier developed on the central Great Barrier Reef; the formation of the warm water pool was likely to have either assisted the formation of this barrier and/or permitted increased coral growth rates. Fluctuations in sea-surface temperature can account for much of the observed spatial and temporal variations of reef growth and carbonate platform distribution off northeast Australia, and therefore we conclude that paleotemperature variations are a critical control on the development of carbonate platforms, and must be considered an important cause of ancient platform "drowning".

Plio-Pleistocene diatom biostratigraphy from ODP Leg 177, Atlantic sector of the Southern Ocean

Seven sites were drilled during Ocean Drilling Program Leg 177 in the Atlantic sector of the Southern Ocean (SO) on a transect over the Antarctic Circumpolar Current from the Subantarctic to the Antarctic Zone. At four sites sediments were recovered with a Pliocene/Pleistocene sediment package of up to 580 m allowing the refinement of previous diatom zonation concepts. Samples were analyzed on stratigraphic distribution and abundance of diatom species. A refined diatom biozonation tied to the geomagnetic polarity record is proposed. For the middle and late Pleistocene two zonations applicable to the northern and southern area of the SO were constructed, considering different latitudinal distributions of biostratigraphic diatom marker species. The southern zonation for the Pleistocene relies on the occurrence of species of the genus Rouxia, R. leventerae and R. constricta n. sp. as well as on a revised last occurrence datum (LOD) of Actinocyclus ingens (0.38 Ma, late marine isotope stage (MIS) 11). The use of these new stratigraphic marker species refines the temporal resolution for biostratigraphic age assignment to up to 0.1 Myr. In particular the LOD of R. leventerae as an indicator for the MIS 6/5 boundary (Termination II) will improve future dating of carbonate-free Antarctic sediments. These new data were obtained from sediments of Sites 1093 and 1094 (Antarctic Zone). The northern zonation for the middle and late Pleistocene time interval is based on the Pleistocene abundance pattern of Hemidiscus karstenii which was already proposed by previous investigations (e.g. Gersonde and Barcena, 1998). One new species (R. constricta) and two new combinations (Fragilariopsis clementia, Fragilariopsis reinholdii) are proposed in this study.

Clay mineral and heavy metal composition of sea-bed surface sediments and pre-Holocene sediment samples taken during POLARSTERN cruise ANT-XXIII/9

Surface mineralogical compositions and their association to modern processes are well known from the east Atlantic and south-west Indian sectors of the Southern Ocean, but data from the interface of these areas - the Prydz Bay-Kerguelen region - is still missing. The objective of our study was to provide mineralogical data of reference samples from this region and to relate these mineralogical assemblages to hinterland geology, weathering, transport and depositional processes. Clay mineral assemblages were analysed by means of X-ray diffraction technique. Heavy mineral assemblages were determined by counting of gravity-separated grains under a polarizing microscope. Results show that by use of clay mineral assemblages four mineralogical provinces can be subdivided: i) continental shelf, ii) continental slope, iii) deep sea, iv) Kerguelen Plateau. Heavy mineral assemblages in the fine sand fraction are relatively uniform except for samples taken from the East Antarctic shelf. Our findings show that mineralogical studies on sediment cores from the study area have the potential to provide insights into past shifts in ice-supported transport and activity and provenance of different water masses (e.g. Antarctic slope current and deep western boundary current) in the Prydz Bay-Kerguelen region.

Age determinations from Bykovsky Peninsula, Siberia

A wide variety of environmental records is necessary for analysing and understanding the complex Late Quaternary dynamics of permafrost-dominated Arctic landscapes. A NE Siberian periglacial key region was studied in detail using sediment records, remote sensing data, and terrain modelling, all incorporated in a geographical information system (GIS). The study area consists of the Bykovsky Peninsula and the adjacent Khorogor Valley in the Kharaulakh Ridge situated a few kilometres southeast of the Lena Delta. In this study a comprehensive cryolithological database containing information from 176 sites was compiled. The information from these sites is based on the review of previously published borehole data, outcrop profiles, surface samples, and our own field data. These archives cover depositional records of three periods: from Pliocene to Early Pleistocene, the Late Pleistocene and the Holocene. The main sediment sequences on the Bykovsky Peninsula consist of up to 50 m thick ice-rich permafrost deposits (Ice Complex) that were accumulated during the Late Pleistocene. They were formed as a result of nival processes around extensive snowfields in the Kharaulakh Ridge, slope processes in these mountains (such as in the Khorogor Valley), and alluvial/proluvial sedimentation in a flat accumulation plain dominated by polygonal tundra in the mountain foreland (Bykovsky Peninsula). During the early to middle Holocene warming, a general landscape transformation occurred from an extensive Late Pleistocene accumulation plain to a strongly thermokarst-dominated relief dissected by numerous depressions. Thermokarst subsidence had an enormous influence on the periglacial hydrological patterns, the sediment deposition, and on the composition and distribution of habitats. Climate deterioration, lake drainage, and talik refreezing occurred during the middle to late Holocene. The investigated region was reached by the post-glacial sea level rise during the middle Holocene, triggering thermo-abrasion of ice-rich coasts and the marine inundation of thermokarst depressions.

Late Pliocene and Early Pleistocene dinoflagellate cysts and sea surface temperture reconstruction for several IODP and DSDP sites in the North Atlantic

In an attempt to document the palaeoecological affinities of individual extant and extinct dinoflagellate cysts, Late Pliocene and Early Pleistocene dinoflagellate cyst assemblages have been compared with geochemical data from the same samples. Mg/Ca ratios of Globigerina bulloides were measured to estimate the spring-summer sea-surface temperatures from four North Atlantic IODP/DSDP sites. Currently, our Pliocene-Pleistocene database contains 204 dinoflagellate cyst samples calibrated to geochemical data. This palaeo-database is compared with modern North Atlantic and global datasets. The focus lies in the quantitative relationship between Mg/Ca-based (i.e. spring-summer) sea-surface temperature (SSTMg/Ca) and dinoflagellate cyst distributions. In general, extant species are shown to have comparable spring-summer SST ranges in the past and today, demonstrating that our new approach is valid for inferring spring-summer SST ranges for extinct species. For example, Habibacysta tectata represents SSTMg/Ca values between 10° and 15°C when it exceeds 30% of the assemblage, and Invertocysta lacrymosa exceeds 15% when SSTMg/Ca values are between 18.6° and 23.5°C. However, comparing Pliocene and Pleistocene SSTMg/Ca values with present day summer values for the extant Impagidinium pallidum suggests a greater tolerance of higher temperatures in the past. This species occupies more than 5% of the assemblage at SSTMg/Ca values of 11.6-17.9°C in the Pliocene and Pleistocene, whereas present day summer SSTs are around -1.7 to 6.9°C. This observation questions the value of Impagidinium pallidum as reliable indicator of cold waters in older deposits, and may explain its bipolar distribution.