Stable isotope composition of Holocene benthic foraminifers from the Eastern Mediterranean Sea

Temporal and regional changes in paleoproductivity and paleoceanography in the eastern Mediterranean Sea during the past 12 kyr were reconstructed on the basis of the stable oxygen and carbon isotope composition of the epibenthic Planulina ariminensis and the shallow endobenthic Uvigerina mediterranea from three sediment cores of the Aegean Sea and Levantine Basin. The Younger Dryas is characterized by high d18O values, indicating enhanced salinities and low temperatures of deep water masses at all investigated sites. With the onset of the Holocene, d18O records show a continuous decrease towards the onset of sapropel S1 formation, mainly caused by a freshening and warming of surface waters at deep water formation sites. In the middle and late Holocene, the similarity of d18O values from the southern Aegean Sea and Levantine Basin suggests the influence of isotopically identical deep water masses. By contrast, slightly higher d18O values are observed the northern Aegean Sea, which probably point to lower temperatures of North Aegean deep waters. The epifaunal d13C records reveal clear changes in sources and residence times of eastern Mediterranean deep waters associated with period of S1 formation. Available data for the early and late phase of sapropel S1 formation and for the interruption around 8.2 kyr display drops by 0.5 and 1.5 per mil, indicating the slow-down of deep water circulation and enhanced riverine input of isotopically light dissolved inorganic carbon from terrestrial sources into the eastern Mediterranean Sea. The decrease in epifaunal d13C signals is particularly expressed in the southern Aegean Sea and Levantine Basin, while it is less pronounced in the northern Aegean Sea. This points to a strong reduction in deep water exchange rates in the southern areas, but the persistence of local deep water formation in the northern Aegean Sea. The d13C values of U. mediterranea records reveal temporal and regional differences in paleoproductivity during the past 12 kyr, with rather eutrophic and mesotrophic conditions in the North Aegean Sea and southeast Levantine Basin, respectively, while the South Aegean Sea is characterized by rather oligotrophic conditions. After S1 formation, increasing d13C values reflect a progressive decrease in surface water productivity in the eastern Mediterranean Sea during the middle and late Holocene. In the northern Aegean Sea, this time interval is marked by repetitive changes in organic matter fluxes documented by significant fluctuations in the d13C signal of U. mediterranea on millennial- to multi-centennial time scales. These fluctuations can be linked to short-term changes in river runoff driven by northern hemisphere climatic variability.

Seawater carbonate chemistry and coral (Acropora digitifera and Acropora tenuis) algal infection rate, survival and surface area of plyps during experiments, 2009

Ocean acidification, caused by increased atmospheric carbon dioxide (CO2) concentrations, is currently an important environmental problem. It is therefore necessary to investigate the effects of ocean acidification on all life stages of a wide range of marine organisms. However, few studies have examined the effects of increased CO2 on early life stages of organisms, including corals. Using a range of pH values (pH 7.3, 7.6, and 8.0) in manipulative duplicate aquarium experiments, we have evaluated the effects of increased CO2 on early life stages (larval and polyp stages) of Acropora spp. with the aim of estimating CO2 tolerance thresholds at these stages. Larval survival rates did not differ significantly between the reduced pH and control conditions. In contrast, polyp growth and algal infection rates were significantly decreased at reduced pH levels compared to control conditions. These results suggest that future ocean acidification may lead to reduced primary polyp growth and delayed establishment of symbiosis. Stress exposure experiments using longer experimental time scales and lower levels of CO2 concentrations than those used in this study are needed to establish the threshold of CO2 emissions required to sustain coral reef ecosystems.

Epibenthic foraminifera stable isotope and planktic foraminifera Ba/Ca data of sediment core MD03-2664

Deep ocean circulation has been considered relatively stable during interglacial periods, yet little is known about its behavior on submillennial time scales. Using a subcentennially resolved epibenthic foraminiferal d13C record we show that North Atlantic Deep Water (NADW) influence was strong at the onset of the last interglacial period and then interrupted by several prominent, centennial-scale reductions. These NADW transients occurred during periods of increased ice rafting and southward expansions of polar water influence, suggesting that a buoyancy threshold for convective instability was triggered by freshwater and circum-Arctic cryosphere changes. The deep Atlantic chemical changes were similar in magnitude to those associated with glaciations, implying that the canonical view of a relatively stable interglacial circulation may not hold for conditions warmer/fresher than at present.

Deforesting Europe: pollen-inferred Holocene land cover change using the 'pseudobiomization' approach

Maps of continental-scale land cover are utilized by a range of diverse users but whilst a range of products exist that describe present and recent land cover in Europe, there are currently no datasets that describe past variations over long time-scales. User groups with an interest in past land cover include the climate modelling community, socio-ecological historians and earth system scientists. Europe is one of the continents with the longest histories of land conversion from forest to farmland, thus understanding land cover change in this area is globally significant. This study applies the pseudobiomization method (PBM) to 982 pollen records from across Europe, taken from the European Pollen Database (EPD) to produce a first synthesis of pan-European land cover change for the period 9000 BP to present, in contiguous 200 year time intervals. The PBM transforms pollen proportions from each site to one of eight land cover classes (LCCs) that are directly comparable to the CORINE land cover classification. The proportion of LCCs represented in each time window provides a spatially aggregated record of land cover change for temperate and northern Europe, and for a series of case study regions (western France, the western Alps, and the Czech Republic and Slovakia). At the European scale, the impact of Neolithic food producing economies appear to be detectable from 6000 BP through reduction in broad-leaf forests resulting from human land use activities such as forest clearance. Total forest cover at a pan-European scale moved outside the range of previous background variability from 4000 BP onwards. From 2200 BP land cover change intensified, and the broad pattern of land cover for preindustrial Europe was established by 1000 BP. Recognizing the timing of anthropogenic land cover change in Europe will further the understanding of land cover-climate interactions, and the origins of the modern cultural landscape.

Chlorofluorocarbons and noble gas measured on water bottle samples during three POLARSTERN cruises

We use a 27 year long time series of repeated transient tracer observations to investigate the evolution of the ventilation time scales and the related content of anthropogenic carbon (Cant) in deep and bottom water in the Weddell Sea. This time series consists of chlorofluorocarbon (CFC) observations from 1984 to 2008 together with first combined CFC and sulphur hexafluoride (SF6) measurements from 2010/2011 along the Prime Meridian in the Antarctic Ocean and across the Weddell Sea. Applying the Transit Time Distribution (TTD) method we find that all deep water masses in the Weddell Sea have been continually growing older and getting less ventilated during the last 27 years. The decline of the ventilation rate of Weddell Sea Bottom Water (WSBW) and Weddell Sea Deep Water (WSDW) along the Prime Meridian is in the order of 15-21%; the Warm Deep Water (WDW) ventilation rate declined much faster by 33%. About 88-94% of the age increase in WSBW near its source regions (1.8-2.4 years per year) is explained by the age increase of WDW (4.5 years per year). As a consequence of the aging, the Cant increase in the deep and bottom water formed in the Weddell Sea slowed down by 14-21% over the period of observations.

Lucky Strike records of hydrothermal outflow temperatures

We deployed autonomous temperature sensors at black smoker chimneys, cracks, and diffuse flow areas at the Lucky Strike hydrothermal field (Mid-Atlantic Ridge, ~37°17'N) between summer 2009 and summer 2012 and contemporaneously measured tidal pressures and currents as part of the long-term MoMAR experiment to monitor hydrothermal activity. We classify the temperature data according to the hydrogeologic setting of the measurement sites: a high-temperature regime (>190°C) representing discharge of essentially unmixed, primary hydrothermal fluids through chimneys, an intermediate-temperature regime (10-100°C) associated with mixing of primary fluids with cold pore fluids discharging through cracks, and a low-temperature regime (<10°C) associated with a thermal boundary layer forming over bacterial mats associated with diffuse outflow of warm fluids. Temperature records from all the regimes exhibit variations at semi-diurnal tidal periods, and cross-spectral analyses reveal that high-temperature discharge correlates to tidal pressure while low-temperature discharge correlates to tidal currents. Intermediate-temperature discharge exhibits a transitional behavior correlating to both tidal pressure and currents. Episodic perturbations, with transient temperature drops of up to ~150°C, which occur in the high-temperature and intermediate-temperature records, are not observed on multiple probes (including nearby probes at the same site), and they are not correlated with microearthquake activity, indicating that the perturbation mechanism is highly localized at the measurement sites within the hydrothermal structures. The average temperature at a given site may increase or decrease at annual time scales, but the average temperature of the hydrothermal field, as a whole, appears to be stable over our 3 year observation period.

Paleomagnetic of ODP Hole 197-1203A

On the basis of studies of Holocene samples,submarine basaltic glass (SBG) is thought to be an ideal paleointensity recorder because it contains unaltered single domain magnetic inclusions that yield Thellier paleointensity data of exceptional quality. To be useful as a recorder of the long-term geomagnetic field, older SBG must retain these optimal properties. Here, we examine this issue through rock magnetic and transmission electron microscope (TEM) analyses of Cretaceous SBG recovered at Ocean Drilling Program Site 1203 (northwestern Pacific Ocean). These SBG samples have very low natural remanent magnetization intensities (NRM <50 nAm**2/g) and TEM analyses indicate a correspondingly low concentration of crystalline inclusions. Thellier experiments on samples with the strongest NRM intensity (>5*10**-11 Am**2) show a rapid acquisition of thermoremanent magnetization (TRM) with respect to NRM demagnetization. Taken at face value,this behavior implies magnetization in a very weak (617 WT) ambient field. But monitoring of magnetic hysteresis properties during the Thellier experiments (on subsamples of the SBG samples used for paleointensity determinations) indicates systematic variations in values over the same temperature range where the rapid TRM acquisition is observed. A similar change in properties during heating is observed on monitor SBG specimens using low-temperature data: with progressive heatings the Verwey transition becomes more distinct. We suggest that these experimental data record the partial melting and neocrystallization of magnetic grains in SBG during the thermal treatments required by the Thellier method,resulting in paleointensity values biased to low values. We further propose that this process is pronounced in Cretaceous and Jurassic SBG (relative to Holocene SBG) because devitrification on geologic time scales (i.e., tens of millions of years) lowers the transition temperature at which the neocrystallization can commence. Magnetic hysteresis monitoring may provide a straightforward means of detecting the formation of new magnetic inclusions in SBG during Thellier experiments.

Triennial skeletal stable oxygen isotope ratios of Diploria labyrinthiformis

The reconstruction of the climatic history during the past several hundred years requires a sufficient geographical coverage of combined climate proxy series. Especially in order to identify causal connections between the atmosphere and the ocean, inclusion of marine records into composite climate time series is of fundamental importance. We present two skeletal delta18O chronologies of coral skeletons of Diploria labyrinthiformis from Bermuda fore-reef sites covering periods in the nineteenth and twentieth centuries and compare them with instrumental temperature data. Both time series are demonstrated to display sea-surface temperature (SST) variability on inter-annual to decadal time scales. On the basis of a specific modern delta18O vs instrumental SST calibration we reconstruct a time series of SST anomalies between AD 1350 and 1630 covering periods during the Little Ice Age. The application of the coral delta18O vs temperature relationship leads to estimates of past SST variability which are comparable to the magnitude of modern variations. Parallel to delta18O chronologies we present time series of skeletal bulk density. Coral delta18O and skeletal density reveal a strong similarity during Little Ice Age, confirming the reliability of both proxy climate indicators. The past coral records, presented in this study, share features with a previously published climate proxy record from Bermuda and a composite time series of reconstructed Northern Hemisphere summer temperatures. The coral proxy data presented here represent a valuable contribution to elucidate northern Atlantic subtropical climate variation during the past several centuries.

Paleoclimate record combinatio of speleothems and of sediment core GeoB3910-2

A substantial strengthening of the South American monsoon system (SAMS) during Heinrich Stadials (HS) points toward decreased cross-equatorial heat transport as the main driver of monsoonal hydroclimate variability at millennial time-scales. In order to better constrain the exact timing and internal structure of HS1 over tropical South America we assessed two precisely dated speleothem records from central-eastern and northeastern Brazil in combination with two marine records of terrestrial organic and inorganic matter input into the western equatorial Atlantic. During HS1 we recognize at least two events of widespread intensification of the SAMS across the entire region influenced by the South Atlantic Convergence Zone (SACZ) at 16.11-14.69 kyr BP and 18.1-16.66 kyr BP (labeled as HS1a and HS1c, respectively), separated by a dry excursion from 16.66-16.11 kyr BP (HS1b). In view of the spatial structure of precipitation anomalies, the widespread increase of monsoon precipitation over the SACZ domain was termed 'Mega-SACZ'.

Sea surface temperature calculated from UK'37 and IP25 sea-ice biomarker of sediment core SO201-2-85KL

During the past decades, remarkable changes in sea-surface temperature (SST) and sea-ice extent have been observed in the marginal seas of the subarctic Pacific. However, little is known about natural climate variability at millennial time scales far beyond instrumental observations. Geological proxy records, such as those derived from marine sediments, offer a unique opportunity to investigate millennial-scale natural climate variability of the Artic and subarctic environments during past glacial-interglacial cycles. Here we provide reconstructions of sea-ice variability inferred from IP25 (Ice Proxy with 25 carbon atoms) sea-ice biomarker and SST fluctuations based on alkenone unsaturation index (UK'37) of the subarctic Pacific realm between 138 and 70 ka. Warmest sea-surface conditions were found during the early Eemian interglacial (128 to 126 ka), exceeding modern SSTs by ~2 °C. The further North Pacific climate evolu- tion is marked by pronounced oscillations in SST and sea-ice extent on millennial time scales, which correspond remarkably well to short-term temperature oscillations known from Green- land and the North Atlantic. These results imply a common forcing, which seems to be closely coupled to dynamics of the Atlantic meridional overturning circulation. However, immediate propagation of such climate fluctuations far beyond the North Atlantic basin suggests a rapid circumpolar coupling mechanism probably acting through the atmosphere, a prerequisite to explain the apparent synchronicity of remote climatic reorganizations in the subarctic Pacific.

Development of bacterial and archaeal communities in erupted subsurface muds at the Håkon Mosby mud volcano

The microbial oxidation of methane controls the emission of the greenhouse gas methane from the ocean floor. However, some seabed structures such as mud volcanoes have leaky microbial methane filters and can be important sources of methane. We investigated the disturbance and recovery of a methanotrophic mud volcano microbiome (Håkon Mosby mud volcano, 1250 m water depth), to assess time scales of community succession and function in the natural deep-sea environment. We analyzed 10 surface and 5 subsurface sediment samples across HMMV mud flows from most recently discharged subsurface muds towards old consolidated muds as well as one reference site (REF) located approximately 0.5 km outside of the HMMV. Surface samples were obtained in 2003, 2009 and 2010. The surface of the new mud flows at the geographical center was sampled in 2009 and 2010. Around 100 m south of the center, we sampled more consolidated aged muds in 2003 and 2010. Old mud flows were sampled around 300 m southeast and 100 m north of the geographical center in 2003, 2009 and 2010. Surface sediment samples (0-20 cm) were recovered either by TV-guided Multicorer or by push cores using the remotely operated vehicle Quest (Marum, University Bremen). Subsurface sediments of all zones (>2 m below sea floor) were obtained in 2003 by gravity corer. After recovery, sediments were immediately subsampled in a refrigerated container (0°C) and further processed for biogeochemical analyses or preserved at -20°C for later DNA analyses. Our study show that freshly erupted muds hosted heterotrophic deep subsurface communities, which were replaced by surface communities within a few years of exposure. Aerobic methanotrophy was established at the top surface layer within less than a year, followed by anaerobic methanotrophy, sulfate reduction and finally thiotrophy. Our data indicate that it takes decades in cold environments before efficient methanotrophic communities establish to control methane emission. The observed succession provides insights to the response time of complex deep-sea communities to seafloor disturbances.