Northern Hemisphere atmospheric pattern enhancing Eastern Mediterranean Transient-type events during the past 1000 years

High-resolution climate model simulations for the last millennium were used to elucidate the main winter Northern Hemisphere atmospheric pattern during enhanced Eastern Mediterranean Transient (EMT-type) events, a situation in which an additional overturning cell is detected in the Mediterranean at the Aegean Sea. The differential upward heat flux between the Aegean Basin and the Gulf of Lion was taken as a proxy of EMT-type events and correlated with winter mean geopotential height at 500 mbar in the Northern Hemisphere (20-90 degrees N and 100 degrees W-80 degrees E). Correlations revealed a pattern similar to the East Atlantic/Western Russian (EA/WR) mode as the main driver of EMT-type events, with the past 1000 years of EA/WR-like mode simulations being enhanced during insolation minima. Our model results are consistent with alkenone sea surface temperature (SST) reconstructions that documented an increase in the west-east basin gradients during EMT-type events

An abrupt weakening of the subpolar gyre as trigger of Little Ice Age-type episodes

We investigate the mechanism of a decadal-scale weakening shift in the strength of the subpolar gyre (SPG) that is found in one among three last millennium simulations with a state-of-the-art Earth system model. The SPG shift triggers multicentennial anomalies in the North Atlantic climate driven by long-lasting internal feedbacks relating anomalous oceanic and atmospheric circulation, sea ice extent, and upper-ocean salinity in the Labrador Sea. Yet changes throughout or after the shift are not associated with a persistent weakening of the Atlantic Meridional Overturning Circulation or shifts in the North Atlantic Oscillation. The anomalous climate state of the North Atlantic simulated after the shift agrees well with climate reconstructions from within the area, which describe a transition between a stronger and weaker SPG during the relatively warm medieval climate and the cold Little Ice Age respectively. However, model and data differ in the timing of the onset. The simulated SPG shift is caused by a rapid increase in the freshwater export from the Arctic and associated freshening in the upper Labrador Sea. Such freshwater anomaly relates to prominent thickening of the Arctic sea ice, following the cluster of relatively small-magnitude volcanic eruptions by 1600 CE. Sensitivity experiments without volcanic forcing can nonetheless produce similar abrupt events; a necessary causal link between the volcanic cluster and the SPG shift can therefore be excluded. Instead, preconditioning by internal variability explains discrepancies in the timing between the simulated SPG shift and the reconstructed estimates for the Little Ice Age onset

Screening Level of PAHs in Sediment Core from Lake Hongfeng, Southwest China

Using data from a 25-year retrospective of polycyclic aromatic hydrocarbons (PAHs) in sediment core from Lake Hongfeng, Southwest China, their possible sources and potential toxicologic significance were investigated. The total PAH concentrations (16 priority PAHs as proposed by the United States Environmental Protection Agency) in sediments ranged from 2936.1 to 5282.3 ng/g and gradually increased from the analyzed deeper sediments to surface sediments. PAHs were dominated by low molecular-weight components, especially phenanthrene (PHEN) and fluorene (FLU). However, a significantly increased number of high molecular-weight (HMW) PAHs was found in upper segments. The temporal trends of individual PAH species suggest that there may have been a change in energy use from low- to high-temperature combustion, especially after approximately 2001. PAH input to Lake Hongfeng originated mainly from domestic coal combustion and biomass burning, whereas fuel combustion characteristics have also been found in recent years. Sediment-quality assessment implied that potential adverse biologic impact could be a probability for most low-ring PAHs (including naphthalene, acenaphthylene, acenaphthylene, FLU, PHEN, and anthracene). Nevertheless, more concern should be paid to HMW PAHs in the future due to their rapidly increasing trends in upper sediments. Because only one core was analyzed in this study, more work is needed to confirm the sources and toxicity of PAHs in Lake Hongfeng

Discerning natural and anthropogenic organic matter inputs to salt marsh sediments of Ria Formosa lagoon (South Portugal)

Sedimentary organic matter (OM) origin and molecular composition provide useful information to understand carbon cycling in coastal wetlands. Core sediments from threors' Contributionse transects along Ria Formosa lagoon intertidal zone were analysed using analytical pyrolysis (Py-GC/MS) to determine composition, distribution and origin of sedimentary OM. The distribution of alkyl compounds (alkanes, alkanoic acids and alkan-2-ones), polycyclic aromatic hydrocarbons (PAHs), lignin-derived methoxyphenols, linear alkylbenzenes (LABs), steranes and hopanes indicated OM inputs to the intertidal environment from natural-autochthonous and allochthonous-as well as anthropogenic. Several n-alkane geochemical indices used to assess the distribution of main OM sources (terrestrial and marine) in the sediments indicate that algal and aquatic macrophyte derived OM inputs dominated over terrigenous plant sources. The lignin-derived methoxyphenol assemblage, dominated by vinylguaiacol and vinylsyringol derivatives in all sediments, points to large OM contribution from higher plants. The spatial distributions of PAHs (polyaromatic hydrocarbons) showed that most pollution sources were mixed sources including both pyrogenic and petrogenic. Low carbon preference indexes (CPI > 1) for n-alkanes, the presence of UCM (unresolved complex mixture) and the distribution of hopanes (C-29-C-36) and steranes (C-27-C-29) suggested localized petroleum-derived hydrocarbon inputs to the core sediments. Series of LABs were found in most sediment samples also pointing to domestic sewage anthropogenic contributions to the sediment OM.EU Erasmus Mundus Joint Doctorate fellowship (FUECA, University of Cadiz, Spain)EUEuropean Commission [FP7-ENV-2011, 282845, FP7-534 ENV-2012, 308392]MINECO project INTERCARBON [CGL2016-78937-R]info:eu-repo/semantics/publishedVersio

Multispectral analysis of Northern Hemisphere temperature records over the last five millennia

Aiming to describe spatio-temporal climate variability on decadal-to-centennial time scales and longer, we analyzed a data set of 26 proxy records extending back 1,000–5,000 years; all records chosen were calibrated to yield temperatures. The seven irregularly sampled series in the data set were interpolated to a regular grid by optimized methods and then two advanced spectral methods—namely singular-spectrum analysis (SSA) and the continuous wavelet transform—were applied to individual series to separate significant oscillations from the high noise background. This univariate analysis identified several common periods across many of the 26 proxy records: a millennial trend, as well as oscillations of about 100 and 200 years, and a broad peak in the 40–70-year band. To study common NH oscillations, we then applied Multichannel SSA. Temperature variations on time scales longer than 600 years appear in our analysis as a dominant trend component, which shows climate features consistent with the Medieval Warm Period and the Little Ice Age. Statistically significant NH-wide peaks appear at 330, 250 and 110 years, as well as in a broad 50–80-year band. Strong variability centers in several bands are located around the North Atlantic basin and are in phase opposition between Greenland and Western Europe

Relationship of the tetra-unsaturated C-37 alkenone to salinity and temperature: Implications for paleoproxy applications

This study assesses the relationship to salinity and temperature of the levels of the tetra-unsaturated 37-carbon methyl alkenone (C-37:4) in the surface ocean. U-37(K'), a measure of the relative abundances of the C-37:2 and the C-37:3 alkenones, has a well constrained linear relationship to temperature in the open ocean [Prahl and Wakeham, 1987] and is a well-established technique for estimating past sea surface temperatures in the sediments (e. g. [Muller et al., 1998]). Unlike the di- and tri-unsaturated C-37 alkenones, the temperature response of the tetra-unsaturated C-37 alkenone is less certain [Sikes et al., 1997], and recent work has suggested a relationship to salinity instead [Rosell-Mele, 1998; Schulz et al., 2000]. Our study examined 106 surface water and sediment trap samples from the Atlantic, Pacific, and Southern Oceans to assess the relationship of the relative abundance of C-37:4 to temperature and salinity. We also examined the relative unsaturation of C-37:2 and C-37:3 (the parameter U-37(K')) to the same parameters to place the C-37:4 results in context. U-37(K') has a strong correlation to salinity in the Atlantic, but the relationship does not hold worldwide, whereas U-37(K') shows a strong linear relationship to temperatures in all ocean basins as shown in previous calibrations. The salinity response in the Atlantic does not confirm cause and effect and interpretation of the broader data set suggests any correlation is an artifact of the strong correlation of salinity to temperature in this basin implying salinity has no effect on the unsaturation of the C-37:2 or C-37:3 alkenones. The C-37:4 alkenone shows no discernable relationship to temperature or salinity across the several basins, even when correlations are restricted to cooler temperatures where the tetra-unsaturated alkenone would be expected to be present. These results indicate that C-37:4 alkenone levels in the open ocean do not reflect either salinity levels or temperature but respond most strongly to some other environmental variable, perhaps changes in growth rate, light, or nutrient supply as suggested by culture studies

Deltaic and coastal sediments as recorders of Mediterranean regional climate and human impact over the past three millennia

Deltaic and shallow marine sediments represent unique natural archives to study the evolutionof surface coastal ocean water properties as compared to environmental changes in adjacent continents. Seasurface temperatures (SSTs) and higher plant biomarker records were generated from the Rhone and VarRiver deltaic sediments (NW Mediterranean Sea), and three sites in the South Adriatic Sea (Central/EasternMediterranean Sea), spanning all or part of the past three millennia. Because of the high sedimentaccumulation rates at all core sites, we were able to produce time series at decadal time scale. SSTs in the Gulfof Lion and the convection area of the South Adriatic Sea indicate similar cold mean values (around 17 °C) andpronounced cold spells, reflecting strong wind-driven surface water heat loss. However, they differ in therate of postindustrial warming, which is steeper in the Gulf of Lion. The three Adriatic Sea SST records arenotably different reflecting different hydrological influence from nearshore to open sea sites. Thecompositional features of higher plant n-alkanes in the Rhone and Var delta sediments and inferredvegetation types show differences consistent with the latitudinal extension of the drainage basins of bothriver streams. In the Adriatic Sea, both coastal and open sea sediments indicate enhanced land-derivedmaterial over the past 500 years, which is not seen in the NW Mediterranean record. We suggest thatincreased erosion as the result of changes in land use practices is the most likely cause for this tren