Biomarker proxies on sediment cores in the central Arctic Ocean

Although the permanently to seasonally ice-covered Arctic Ocean is a unique and sensitive component in the Earth's climate system, the knowledge of its long-term climate history remains very limited due to the restricted number of pre-Quaternary sedimentary records. During Polarstern Expedition PS87/2014, we discovered multiple submarine landslides over a distance of >350 km along Lomonosov Ridge. Removal of younger sediments from steep headwalls has led to exhumation of Miocene to early Quaternary sediments close to the seafloor, allowing the retrieval of such old sediments with gravity cores. Multi-proxy biomarker analyses of these gravity cores reveal for the first time that the late Miocene central Arctic Ocean was relatively warm (4-7°C) and ice-free during summer, whereas sea ice occurred during spring and autumn/winter. A comparison of our proxy data with Miocene climate simulations seems to favour relatively high late Miocene atmospheric CO2 concentrations. These new findings from the Arctic region provide new benchmarks for groundtruthing global climate reconstructions and modeling.

Mean snow accumulation rate, and mean methane sulfonate, chloride and nitrate concentration in snow pits and firn cores of Dronning Maud Land

We quantified postdepositional losses of methane sulfonate (MSA-), nitrate, and chloride at the European Project for Ice Coring in Antarctica (EPICA) drilling site in Dronning Maud Land (DML) (75°S, 0°E). Analyses of four intermediate deep firn cores and 13 snow pits were considered. We found that about 26 ± 13% of the once deposited nitrate and typically 51 ± 20% of MSA- were lost, while for chloride, no significant depletion could be observed in firn older than one year. Assuming a first order exponential decay rate, the characteristic e-folding time for MSA- is 6.4 ± 3 years and 19 ± 6 years for nitrate. It turns out that for nitrate and MSA- the typical mean concentrations representative for the last 100 years were reached after 5.4 and 6.5 years, respectively, indicating that beneath a depth of around 1.2-1.4 m postdepositional losses can be neglected. In the area of investigation, only MSA- concentrations and postdepositional losses showed a distinct dependence on snow accumulation rate. Consequently, MSA- concentrations archived at this site should be significantly dependent on the variability of annual snow accumulation, and we recommend a corresponding correction. With a simple approach, we estimated the partial pressure of the free acids MSA, HNO3, and HCl on the basis of Henry's law assuming that ionic impurities of the bulk ice matrix are localized in a quasi-brine layer (QBL). In contrast to measurements, this approach predicts a nearly complete loss of MSA-, NO3 - , and Cl-.

X-ray fluorescence (XRF) Ca intensity of sediment cores MD04-2760 and MD04-2788

Accelerator mass spectrometry (AMS) radiocarbon dating of ostracod and gastropod shells from the southwestern Black Sea cores combined with tephrochronology provides the basis for studying reservoir age changes in the lateglacial Black Sea. The comparison of our data with records from the northwestern Black Sea shows that an apparent reservoir age of ~1450 14C yr found in the glacial is characteristic of a homogenized water column. This apparent reservoir age is most likely due to the hardwater effect. Though data indicate that a reservoir age of ~1450 14C yr may have persisted until the Bølling-Allerød warm period, a comparison with the GISP2 ice-core record suggests a gradual reduction of the reservoir age to ~1000 14C yr, which might have been caused by dilution effects of inflowing meltwater. During the Bølling-Allerød warm period, soil development and increased vegetation cover in the catchment area of the Black Sea could have hampered erosion of carbonate bedrock, and hence diminished contamination by "old" carbon brought to the Black Sea basin by rivers. A further reduction of the reservoir age most probably occurred contemporary to the precipitation of inorganic carbonates triggered by increased phytoplankton activity, and was confined to the upper water column. Intensified deep water formation subsequently enhanced the mixing/convection and renewal of intermediate water. During the Younger Dryas, the age of the upper water column was close to 0 yr, while the intermediate water was ~900 14C yr older. The first inflow of saline Mediterranean water, at ~8300 14C yr BP, shifted the surface water age towards the recent value of ~400 14C yr.

Profiles of diatoms, pollen, XRD, XRF and organic composition of two sediment cores (PG2022 and PG2023) from Lake Kyutyunda in Yakutia, NE Siberia, Russia

Although the climate development over the Holocene in the Northern Hemisphere is well known, palaeolimnological climate reconstructions reveal spatiotemporal variability in northern Eurasia. Here we present a multi-proxy study from north-eastern Siberia combining sediment geochemistry, and diatom and pollen data from lake-sediment cores covering the last 38,000 cal. years. Our results show major changes in pyrite content and fragilarioid diatom species distributions, indicating prolonged seasonal lake-ice cover between ~13,500 and ~8,900 cal. years BP and possibly during the 8,200 cal. years BP cold event. A pollen-based climate reconstruction generated a mean July temperature of 17.8°C during the Holocene Thermal Maximum (HTM) between ~8,900 and ~4,500 cal. years BP. Naviculoid diatoms appear in the late Holocene indicating a shortening of the seasonal ice cover that continues today. Our results reveal a strong correlation between the applied terrestrial and aquatic indicators and natural seasonal climate dynamics in the Holocene. Planktonic diatoms show a strong response to changes in the lake ecosystem due to recent climate warming in the Anthropocene. We assess other palaeolimnological studies to infer the spatiotemporal pattern of the HTM and affirm that the timing of its onset, a difference of up to 3,000 years from north to south, can be well explained by climatic teleconnections. The westerlies brought cold air to this part of Siberia until the Laurentide ice-sheet vanished 7,000 years ago. The apparent delayed ending of the HTM in the central Siberian record can be ascribed to the exceedance of ecological thresholds trailing behind increases in winter temperatures and decreases in contrast in insolation between seasons during the mid to late Holocene as well as lacking differentiation between summer and winter trends in paleolimnological reconstructions.

Stable isotope ratios and foraminiferal abundance of sediment cores from the Western Iberian Margin

Rapid climate changes at the onset of the last deglaciation and during Heinrich Event H4 were studied in detail at IMAGES cores MD95-2039 and MD95-2040 from the Western Iberian margin. A major reorganisation of surface water hydrography, benthic foraminiferal community structure, and deepwater isotopic composition commenced already 540 years before the Last Isotopic Maximum (LIM) at 17.43 cal. ka and within 670 years affected all environments. Changes were initiated by meltwater spill in the Nordic Seas and northern North Atlantic that commenced 100 years before concomitant changes were felt off western Iberia. Benthic foraminiferal associations record the drawdown of deepwater oxygenation during meltwater and subsequent Heinrich Events H1 and H4 with a bloom of dysoxic species. At a water depth of 3380 m, benthic oxygen isotopes depict the influence of brines from sea ice formation during ice-rafting pulses and meltwater spill. The brines conceivably were a source of ventilation and provided oxygen to the deeper water masses. Some if not most of the lower deep water came from the South Atlantic. Benthic foraminiferal assemblages display a multi-centennial, approximately 300-year periodicity of oxygen supply at 2470-m water depth. This pattern suggests a probable influence of atmospheric oscillations on the thermohaline convection with frequencies similar to Holocene climate variations. For Heinrich Events H1 and H4, response times of surface water properties off western Iberia to meltwater injection to the Nordic Seas were extremely short, in the range of a few decades only. The ensuing reduction of deepwater ventilation commenced within 500-600 years after the first onset of meltwater spill. These fast temporal responses lend credence to numerical simulations that indicate ocean-climate responses on similar and even faster time scales.

Distribution of phytolithes in CRP sediment cores from the Ross Sea, Antarctica

Phytoliths (siliceous plant microfossils) have been recovered from Cenozoic sediments (c. 34 to 17 Ma) in the CRP-2/2A and CRP-3 drillholes cored off Cape Roberts, Victoria Land Basin, Antarctica. The phytolith assemblages are sparse, but well-preserved and dominated by spherical forms similar to those of modern trees or shrubs. Rare phytoliths comparable to modern grass forms are also present. However, due to the paucity of phytolith data, any interpretations made are necessarily tentative. The assemblages of CRP-2/2A and the upper c. 250 m of CRP-3 are interpreted as representing a predominantly woody vegetation, including Nothofagus and Libocedrus with local areas of grass in the more exposed locations. A cool climate is interpreted to have prevailed throughout both cores. However, beneath c. 250 metres below sea floor in CRP-3, the dominant woody vegetation is supplemented by pockets of Palmae, ?Proteaceae and 'warm' climate grasses. This association represents vegetation growth in sheltered, moist sites - possibly north-facing mid-slopes or the coastal fringe. It may also represent remnant vegetation that grew in moist, temperate conditions during the Middle to Late Eocene, previously interpreted from the Southern McMurdo Sound erratics and lower part of the CIROS-1 drillhole. The phytolith analysis compares well to the terrestrial palynomorph record from both cores and provides additional independent taxonomic and climatic interpretations.

Radiocarbon ages and diatom abundance in cores GA306-BC4 and GA306-GC4 obtained during Galathea 3 cruise, West Greenland

Diatom assemblages from Holsteinsborg Dyb on the West Greenland shelf were analysed with high temporal resolution for the last 1200 years. A high degree of consistency between changes in frequency of selected diatom species and instrumental data from the same area during the last 70 years confirms the reliability of diatoms (particularly sea-ice species and warm-water species) for the study of palaeoceanographic changes in this area. A general cooling trend with some fluctuations is marked by an increase in sea-ice species throughout the last 1200 years. A relatively warm period with increased influence of Atlantic water masses of the Irminger Current (IC) is found at AD 750-1330, although with some oceanographic variability after AD 1000. A pronounced oceanographic shift occurred at AD 1330, corresponding in time to the transition from the so-called 'Medieval Warm Period' (MWP) to the 'Little Ice Age' (LIA). The LIA cold episode is characterized by three intervals with particularly cold sea-surface conditions at AD 1330-1350, AD 1400-1575 and AD 1660-1710 as a result of variable influence of Polar waters in the area. During the last 70 years, two relatively warm periods and one cold period (the early 1960s to mid-1990s) are indicated by changes in the diatom components. Our study demonstrates that sedimentary records on the West Greenland shelf provide valuable palaeoenvironment data that confirm a linkage between local and large-scale North Atlantic oceanographic and atmospheric oscillations.

Radiocarbon ages and clay minerals in sediment cores obtained during a 2009 Jan Mayen cruise to the Norwegian Sea

The origin of two acoustic sediment units has been studied based on lithological facies, chronology and benthic stable isotope values as well as on foraminifera and clay mineral assemblages in six marine sediment cores from Kveithola, a small trough west of Spitsbergenbanken on the western Barents Sea margin. We have identified four time slices with characteristic sedimentary environments. Before c. 14.2 cal. ka, rhythmically laminated muds indicate extensive sea ice cover in the area. From c. 13.9 to 14.2 cal. ka, muds rich in ice-rafted debris were deposited during the disintegration of grounded ice on Spitsbergenbanken. From c. 10.3 to 13.1 cal. ka, sediments with heterogeneous lithologies suggest a shifting influence of suspension settling and iceberg rafting, probably derived from a decaying Barents Sea Ice Sheet in the inner-fjord and land areas to the north of Kveithola. Holocene deposition was episodic and characterized by the deposition of calcareous sands and shell debris, indicative of strong bottom currents. We speculate that a marked erosional boundary at c. 8.2 cal. ka may have been caused by the Storegga tsunami. Whilst deposition was sparse during the Holocene, Kveithola acted as a sediment trap during the preceding deglaciation. Investigation of the deglacial sediments provides unprecedented details on the dynamics and timing of glacial retreat from Spitsbergenbanken.

Age determination of sediment cores from the Weddell Sea

We report here the results of a study aimed at providing radiometric age control on glacial events in the Weddell Sea during the late Quaternary. Sediment cores from the eastern continental shelf, where the East Antarctic ice sheet was grounded, have recovered glacial-marine sediments resting on tills and the latter deposits predate the isotope stage 2 last glacial maximum. Sediment cores from the continental slope and rise sampled a prominent ice-rafted debris layer, and radiocarbon ages indicate that this ice-rafting event took place prior to 26 000 yr B.P. Thus, the combined data indicate that significant deglaciation of the Weddell Sea continental shelf took place prior to the last glacial maximum. Our data also suggest that the ice masses that border the Weddell Sea are more extensive than they were during the previous glacial minimum.

Age determination of sediment cores from the northern Mid-Atlantic Ridge

We construct age models for a suite of cores from the northeast Atlantic Ocean by means of accelerator mass spectrometer dating of a key core, BOFS 5K, and correlation with the rest of the suite. The effects of bioturbation and foraminiferal species abundance gradients upon the age record are modeled using a simple equation. The degree of bioturbation is estimated by comparing modeled profiles with dispersal of the Vedde Ash layer in core 5K, and we find a mixing depth of roughly 8 cm for sand-sized material. Using this value, we estimate that age offsets between unbioturbated sediment and some foraminifera species after mixing may be up to 2500 years, with lesser effect on fine carbonate (< 10 µm) ages. The bioturbation model illustrates problems associated with the dating of 'instantaneous' events such as ash layers and the 'Heinrich' peaks of ice-rafted detritus. Correlations between core 5K and the other cores from the BOFS suite are made on the basis of similarities in the downcore profiles of oxygen and carbon isotopes, magnetic susceptibility, water and carbonate content, and via marker horizons in X radiographs and ash beds.