Decadal-to-centennial scale variations during the Holocene reconstructed from sediment core D13902

Sea surface temperature (SST), marine productivity, and fluvial input have been reconstructed for the last 11.5 calendar (cal) ka B.P. using a high-resolution study of C37 alkenones, coccolithophores, iron content, and higher plant n-alkanes and n-alkan-1-ols in sedimentary sequences from the inner shelf off the Tagus River Estuary in the Portuguese Margin. The SST record is marked by a continuous decrease from 19C, at 10.5 and 7 ka, to 15C at present. This trend is interrupted by a fall from 18C during the Roman and Medieval Warm Periods to 16C in the Little Ice Age. River input was very low in the early Holocene but increased in the last 3 cal ka B.P. in association with an intensification of agriculture and deforestation and possibly the onset of the North Atlantic Oscillation/Atlantic Multidecadal Oscillation modes of variability. River influence must have reinforced the marine cooling trend relative to the lower amplitude in similar latitude sites of the eastern Atlantic. The total concentration of alkenones reflects river-induced productivity, being low in the early Holocene but increasing as river input became more important. Rapid cooling, of 1-2C occurring in 250 years, is observed at 11.1, 10.6, 8.2, 6.9, and 5.4 cal ka B.P. The estimated age of these events matches the ages of equivalent episodes common in the NE Atlantic- Mediterranean region. This synchronicity reveals a common widespread climate feature, which considering the twentieth century analog between colder SSTs and negative North Atlantic Oscillation (NAO), is likely to reflect periods of strong negative NAO.

Hydrogen isotope compositions of terrestrial plant-wax n-alkanes analysed on a transect of marine surface sediment samples from 1°N to 28°S off southwest Africa

A transect of marine surface sediment samples from 1° N to 28° S off southwest Africa was analysed to verify the application of hydrogen isotope compositions of terrestrial plant-wax n-alkanes preserved in ocean sediments as a proxy for continental hydrological conditions. Conditions on the adjacent continent range from humid evergreen forests to deciduous forests, wood- and shrub land and further to arid grasslands and deserts. The hydrogen isotope values for the dominant n-alkane homologues (C29, C31 and C33) vary from -123 per mil to -141 per mil VSMOW and correlate with the modelled hydrogen isotope composition of mean annual and growing season precipitation of postulated continental source areas (r up to 0.8, p < 0.01). The apparent hydrogen isotope fractionation between alkanes and mean annual precipitation is remarkably uniform (-109 per mil on average, Sigma <= 5 per mil, n = 27). Potentially, effects of aridity on the apparent hydrogen isotope fractionation are concealed by the contribution of different plants (C3 dicotyledons vs C4 grasses). Thus, isotope ratios of leaf wax n-alkanes preserved in ocean margin sediments in these and similar tropical regions may be directly converted to dD ratios of ancient precipitation by employing a constant hydrogen isotope fractionation.

Terrigenous input and marine productivity for the Pacific sector of the Southern Ocean based on lipid biomarkers

In this study, we present a new multiproxy data set of terrigenous input, marine productivity and sea surface temperature (SST) from 52 surface sediment samples collected along E-W transects in the Pacific sector of the Southern Ocean. Allochtonous terrigenous input was characterized by the distribution of plant wax n-alkanes and soil-derived branched glycerol dialkyl glycerol tetraethers (brGDGTs). 230Th-normalized burial rates of both compound groups were highest close to the potential sources in Australia and New Zealand and are strongly related to lithogenic contents, indicating common sources and transport. Detection of both long-chain n-alkanes and brGDGTs at the most remote sites in the open ocean strongly suggests a primarily eolian transport mechanism to at least 110°W, i.e. by prevailing westerly winds. Two independent organic SST proxies were used, the UK'37 based on long-chain alkenones, and the TEX86 based on isoprenoid GDGTs. Both, UK'37 and TEX86 indices show robust relationships with temperature over a temperature range between 0.5 and 20°C, likely implying different seasonal and regional imprints on the temperature signal. While alkenone-based temperature estimates reliably reflect modern SST even at the low temperature end, large temperature residuals are observed for the polar ocean using the TEX86 index. 230Th-normalized burial rates of alkenones are highest close to the Subtropical Front and are positively related to lithogenic fluxes throughout the study area. In contrast, highest isoGDGT burial south of the Antarctic Polar Front is not related with dust flux but may be largely controlled by diatom blooms, and thus high opal fluxes during austral summer.

(Table 1) Lipid biomarkers in surface sediments from the Gulf of Genoa

A series of molecular organic markers were determined in surface sediments from the Gulf of Genoa (Ligurian Sea) in order to evaluate their potential for palaeo-environmental reconstructions. Allochthonous input can be characterized by the distributions of n-C29 and n-C31 alkanes, n-C26 and n-C28 alkanols and branched glycerol dialkyl glycerol tetraethers (GDGTs), whose concentrations are generally highest near the river mouths. In the open basin however, terrestrial n-alkanes and n-alkanols may have an additional, aeolian source. Autochthonous input is represented by crenarchaeol and isoprenoid GDGTs. Their concentrations are highest in the open basin showing the preference of Thaumarchaeota for oligotrophic waters. Indications of a significant degradation of sterols and C37 alkenones exclude these lipids as reliable productivity proxies. Using terrestrial and aquatic lipids as end-members allows estimating the percentage of terrestrial organic matter between 20% and 58% in the coastal area decreasing to 1 to 30% in the deep basin. The spatial distribution of sea surface temperature (SST) estimates using the alkenone-based UK'37 index is very similar to the autumnal (November) mean satellite-based SST distribution. Conversely, TEXH86-derived SST estimates are close to winter SSTs in the coastal area and summer SSTs in the open basin. This pattern reflects presumably a shift in the main production of Thaumarchaeota from the coastal area in winter to the open basin in summer. This study represents a major prerequisite for the future application of lipid biomarkers on sediment cores from the Gulf of Genoa.

Biogeochemical parameters and processes in waters and bottom sediments of the Chukchi Sea

Study of biogeochemical processes in waters and sediments of the Chukchi Sea in August 2004 revealed atypical maxima of biogenic element (N, P, and Si) concentrations and rate of microbial sulfate reduction in the surface layer (0-3 cm) of marine sediments. The C/N/P ratio in organic matter (OM) of this layer does not fit the Redfield-Richards stoichiometric model. Specific features of biogeochemical processes in the sea are likely related to the complex dynamics of water, high primary produc¬tivity (110-1400 mg C/m**2/day), low depth of the basin (<50 m for 60% of the water area), reduced food chain due to low population of zooplankton, high density of zoobenthos (up to 4230 g/m**2), and high activity of microbial processes. Drastic decrease in concentrations of biogenic elements, iodine, total alkalinity, and population of microorganisms beneath the 0-3 cm layer testify to large-scale OM decay at the water-seafloor barrier. Our original experimental data support high annual rate of OM mineralization at the bottom of the Chukchi Sea.

Contents of hydrocarbons in waters and bottom sediments of the southwestern Amur Bay (Sea of Japan) in April 2005

Abundances and compositions of aliphatic hydrocarbons (AHC) and polyaromatic hydrocarbons (PAH) were investigated in water and bottom sediments of the southwestern Amur Bay, Sea of Japan. Water contained 0-129 ?g/l AHC (average 42.2 ?g/l) and 5-85 ng/l PAH (average 18 ng/l). Bottom sediments contained 168-2098 ?g/g AHC and 7.2-1100 ng/g dry mass PAH. It was shown that input of anthropogenic hydrocarbons is better recorded by molecular markers than by distribution of AHC and PAH concentrations. Discovery of elevated hydrocarbon concentrations in the bottom water layer suggests that bottom sediments induced secondary contamination of the water body.

Sea surface temperature reconstruction for the middle Pleistocene from sediment core MD03-2699

Past sea surface water conditions of the western Iberian Margin were reconstructed based on biomarker analyses of a marine deep sea core MD03-2699 from the Estremadura Spur north off Lisbon, providing new insights into orbital and suborbital-scale climate variability between marine isotope stage (MIS) 15 to MIS 9 (580 to 300 ka). We use biomarker-based proxy records such as the alkenone unsaturated index to estimate sea surface temperature (SST), the total alkenone concentration to reconstruct phytoplankton productivity, and terrestrial biomarkers to evaluate the continental input. The results extend the existing biomarker record, namely the SST for the Iberian Margin, back to the sixth climatic cycle (580 ka). A general trend of stable interglacials contrasts with glacial periods and glacial inceptions which are marked by high-frequency variability. Thus, several short-lived climatic coolings were identified by large SST decreases, the occurrence of ice-rafted detritus and high percentages of the tetraunsaturated alkenone C 37:4. Some of these events were extremely cold and similar in their general trends to the well-known Heinrich events of the last glaciation. We identified eight Heinrich-type events between 580 and 300 ka. The general deglaciation pattern detected between MIS 15 and MIS 9 is similar in their general trends to that characterizing the more recent climatic cycles, i.e., marked by two coolings separated by a short warming episode which may reflect the southward, northward, and southward migration of the Polar Front.

(Table 1) Sample descriptions, carbon contents, and lipid yields across the K/T boundary at DSDP Hole 93-605 and at Stevns Klint, Denmark

This reconnaissance study was undertaken to determine whether the mass extinctions and faunal successions that mark the Cretaceous/Tertiary (K/T) boundary left a discernible molecular fossil record in the sediments of this period. Lipid signatures of sediments taken from above and below the K/T boundary were compared in core and outcrop samples taken from two locations: the U.S. east coast continental margin (western Atlantic Ocean, DSDP Site 605) and Stevns Klint, Denmark. Four calcareous sediments taken from above and below the K/T boundary in DSDP Hole 605, Section 605-66-1, revealed changing lipid signatures between above and below that are characterized by a large component of unresolved naphthenic hydrocarbons and a homologous series of n-alkanes ranging from Ci6 to C33. These lipid signatures are attributed to an influx of a terrestrial higher plant component and to bacterial reworking of the sediments under partially anoxic depositional and/or diagenetic conditions. The outcrop samples from Stevns Klint had extremely low concentrations of indigenous lipids. The fish clay at the K/T boundary contained traces of microbial hydrocarbons and fatty acids, whereas the carbonates above and below had only microbial fatty acids and additional terrestrial resin acids. The data from both sites indicate a perturbation in the deposition of lipid compound classes across the K/T boundary.

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.

Results of the pollen analysis of the Dyanushka peat sediments K7/P2

A 415cm thick permafrost peat section from the Verkhoyansk Mountains was radiocarbon-dated and studied using palaeobotanical and sedimentological approaches. Accumulation of organic-rich sediment commenced in a former oxbow lake, detached from a Dyanushka River meander during the Younger Dryas stadial, at ~12.5 kyr BP. Pollen data indicate that larch trees, shrub alder and dwarf birch were abundant in the vegetation at that time. Local presence of larch during the Younger Dryas is documented by well-preserved and radiocarbon-dated needles and cones. The early Holocene pollen assemblages reveal high percentages of Artemisia pollen, suggesting the presence of steppe-like communities around the site, possibly in response to a relatively warm and dry climate ~11.4-11.2 kyr BP. Both pollen and plant macrofossil data demonstrate that larch woods were common in the river valley. Remains of charcoal and pollen of Epilobium indicate fire events and mark a hiatus ~11.0-8.7 kyr BP. Changes in peat properties, C31/C27 alkane ratios and radiocarbon dates suggest that two other hiatuses occurred ~8.2-6.9 and ~6.7-0.6 kyr BP. Prior to 0.6 kyr BP, a major fire destroyed the mire surface. The upper 60 cm of the studied section is composed of aeolian sands modified in the uppermost part by the modern soil formation. For the first time, local growth of larch during the Younger Dryas has been verified in the western foreland of the Verkhoyansk Mountains (~170km south of the Arctic Circle), thus increasing our understanding of the quick reforestation of northern Eurasia by the early Holocene.

Geochemical investigations of sediment profiles in the Norwegian-Greenland Sea

High-, i.e. 15-140-yr-resolution climate records from sediment cores 23071, 23074, and PS2644 from the Nordic Seas were used to recon:;truct changes in the surface and deep water circulation during marine isotope stages 1-5.1, i.e. the last 82 000 yr. From this the causal links between the paleoceanographic signals and the Dansgaard-Oeschger events 1-21 revealed in 0180-ice-core records from Greenland were determined. The stratigraphy of the cores is based on the planktic 0180 curves, the minima of which were directly correlated with the GISP2-0180 record, numerous AMS 14C ages, and some ash layers. The planktic d18O and dl3C curves of all three cores reveal numerous meltwater events, the most pronounced of which were assigned to the Heinrich events 1-6. The meltwater events, among other things also accompanied by cold sea surface temperatures and high IRD concentration, correlate with the stadial phases of the Dansgaard-Oeschger cycles and in the western Iceland Sea also to colder periods or abrupt drops in 0180 within a few longer interstadials. Besides being more numerous, the meltwater events also show isotope values lighter in the Iceland Sea than in the central Norwegian Sea, especially if compared to core 23071. This implies a continuous inflow of relative warm Atlantic water into the Norwegian Sea and a cyclonic circulation regime.