Naturally forced multidecadal variability of the Atlantic meridional overturning circulation

The mechanisms by which natural forcing factors alone could drive simulated multidecadal variability in the Atlantic meridional overturning circulation (AMOC) are assessed in an ensemble of climate model simulations. It is shown for a new state-of-the-art general circulation model, HadGEM2-ES, that the most important of these natural forcings, in terms of the multidecadal response of the AMOC, is solar rather than volcanic forcing. AMOC strengthening occurs through a densification of the North Atlantic, driven by anomalous surface freshwater fluxes due to increased evaporation. These are related to persistent North Atlantic atmospheric circulation anomalies, driven by forced changes in the stratosphere, associated with anomalously weak solar irradiance during the late nineteenth and early twentieth centuries. Within a period of approximately 100 years the 11-year smoothed ensemble mean AMOC strengthens by 1.5 Sv and subsequently weakens by 1.9 Sv, representing respectively approximately 3 and 4 standard deviations of the 11-year smoothed control simulation. The solar-induced variability of the AMOC has various relevant climate impacts, such as a northward shift of the intertropical convergence zone, anomalous Amazonian rainfall, and a sustained increase in European temperatures. While this model has only a partial representation of the atmospheric response to solar variability, these results demonstrate the potential for solar variability to have a multidecadal impact on North Atlantic climate.     

History matching for exploring and reducing climate model parameter space using observations and a large perturbed physics ensemble

We apply an established statistical methodology called history matching to constrain the parameter space of a coupled non-flux-adjusted climate model (the third Hadley Centre Climate Model; HadCM3) by using a 10,000-member perturbed physics ensemble and observational metrics. History matching uses emulators (fast statistical representations of climate models that include a measure of uncertainty in the prediction of climate model output) to rule out regions of the parameter space of the climate model that are inconsistent with physical observations given the relevant uncertainties. Our methods rule out about half of the parameter space of the climate model even though we only use a small number of historical observations. We explore 2 dimensional projections of the remaining space and observe a region whose shape mainly depends on parameters controlling cloud processes and one ocean mixing parameter. We find that global mean surface air temperature (SAT) is the dominant constraint of those used, and that the others provide little further constraint after matching to SAT. The Atlantic meridional overturning circulation (AMOC) has a non linear relationship with SAT and is not a good proxy for the meridional heat transport in the unconstrained parameter space, but these relationships are linear in our reduced space. We find that the transient response of the AMOC to idealised CO₂ forcing at 1 and 2 % per year shows a greater average reduction in strength in the constrained parameter space than in the unconstrained space. We test extended ranges of a number of parameters of HadCM3 and discover that no part of the extended ranges can by ruled out using any of our constraints. Constraining parameter space using easy to emulate observational metrics prior to analysis of more complex processes is an important and powerful tool. It can remove complex and irrelevant behaviour in unrealistic parts of parameter space, allowing the processes in question to be more easily studied or emulated, perhaps as a precursor to the application of further relevant constraints.     

Response of a mountain‐foreland fluvial system to an advancing ice sheet,southern Poland

In this paper, fluvial deposits of Middle Pleistocene age in the mountain‐foreland area of southern Poland (Eastern Sudetes and Western Carpathians) are studied in order to document the evolution of fluvial systems during the coldest stages of glacial periods when the Scandinavian Ice Sheet advanced far to the south. The focus is on fluvial response to climate change and glacial impact on river system behaviour. Also considered is the tectonic uplift of the mountain part of river catchments and its potential influence on the style of fluvial sedimentation in the fore‐mountain area. Three drainage basins that were active during the Elsterian and Saalian glaciations are investigated. Facies analyses are carried out on thick successions of braided river deposits covered with till or glaciolacustrine sediments, which result in a reconstruction of the fluvial activity synchronous with the ice‐sheet advance. The results suggest that fluvial activity declined prior to ice‐sheet advance into the fore‐mountain area. This climatically induced change is directly recorded in alluvial successions by upward‐decreasing bed thicknesses and grain sizes. River longitudinal profiles were shortened in front of the advancing ice sheet. The base level of the studied rivers, created by the ice‐sheet margin, rose in parallel with glacial advance. As a result, the successive reaches of rivers (degradational, transitional, aggradational) underwent shortening and moved upstream within the catchments. Moreover, tectonically induced local increases of river slopes may have influenced the depositional processes.     

Integrating X‐ray mapping and microtomography of garnet with thermobarometry to define the P–T evolution of the (near) UHP Międzygórze eclogite,Sudetes, SW Poland

Detailed X‐ray compositional mapping and microtomography have revealed the complex zoning and growth history of garnet in a kyanite‐bearing eclogite. The garnet occurs as clusters of coalesced grains with cores revealing slightly higher Ca and lower Mg than the rims forming the coalescence zones between the grains. Core regions of the garnet host inclusions of omphacite with the highest jadeite, and phengite with the highest Si, similar to values in the cores of omphacite and phengite located in the matrix. Therefore, the core compositions of garnet, omphacite, and phengite have been chosen for the peak pressure estimate. Coupled conventional thermobarometry, average P–T, and phase equilibrium modelling in the NCKFMMnASHT system yields P–T conditions of 26–30 kbar at 800–930°C. Although coesite is not preserved, these P–T conditions partially overlap the coesite stability field, suggesting near ultra‐high–pressure (UHP) conditions during the formation of this eclogite. Therefore, the peak pressure assemblage is suggested to have been garnet–omphacite–kyanite–phengite–coesite/quartz–rutile. Additional lines of evidence for the possible UHP origin of the Mi?dzygórze eclogite are the presence of rod‐shaped inclusions of quartz parallel to the c‐axis in omphacite as well as relatively high values of Ca‐Tschermak and Ca‐Eskola components. Late zoisite, rare diopside–plagioclase symplectites rimming omphacite, and minor phlogopite–plagioclase symplectites replacing phengite formed during retrogression together with later amphibole. These retrograde assemblages lack minerals typical of granulite facies, which suggests simultaneous decompression and cooling during exhumation before the crustal‐scale folding that was responsible for final exhumation of the eclogite.     

Eclogite and garnet pyroxenite from Stor Jougdan,Seve Nappe Complex,Sweden: implications for UHP metamorphism of allochthons in the Scandinavian Caledonides

Ultrahigh‐pressure metamorphism (UHPM) has recently been discovered in far‐travelled allochthons of the Scandinavian Caledonides, including finding of diamond in the Seve Nappe Complex. This UHPM of Late Ordovician age is older and less recognized than that in the Western Gneiss Region of southwestern Norway, which was related to terminal collision between Baltica and Laurentia. Here we report new evidence of UHPM in the Lower Seve Nappe, recorded by eclogite and garnet pyroxenite from the area of Stor Jougdan in northern Jämtland, central Sweden. Peak‐metamorphic assemblage of eclogite, garnet + omphacite + phengite + rutile + coesite? yields P–T conditions of 2.8–4.0 GPa and 750–900 °C, constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMTASH system. The prograde metamorphic evolution of the eclogite is inferred from inclusions of zoisite and amphibole in garnet, which are stable at lower pressure, whereas the retrograde evolution is recorded by formation of diopsidic clinopyroxene + plagioclase symplectites after omphacite, growth of amphibole replacing these symplectites, and of titanite around rutile. In garnet pyroxenite the peak‐metamorphic assemblage consists of garnet + orthopyroxene + clinopyroxene + olivine. P–T conditions of 2.3–3.8 GPa and 810–960 °C have been derived based on the conventional geothermobarometry and thermodynamic modelling in the CFMASH and CFMAS systems. Retrograde evolution has been recognized from replacement of pyroxene and garnet by amphibole. The results show that eclogite was metamorphosed during deep subduction of continental crust, most probably derived from the continental margin of Baltica, whereas the origin and tectonic setting of the garnet pyroxenite is ambiguous. The studied pyroxenite/peridotite of Baltican subcontinental affinity could have been metamorphosed as a part of the subducting plate and exhumed due to the downward extraction of a forearc lithospheric block.     

Multiple drivers of Holocene lake level changes at a lowland lake in northeastern Germany

Many German lakes experienced significant water level declines in recent decades that are not fully understood due to the short observation period. At a typical northeastern German groundwater‐fed lake with a complex basin morphology, an acoustic sub‐bottom profile was analysed together with a transect of five sediment cores, which were correlated using multiple proxies (sediment facies, μ‐XRF, macrofossils, subfossil Cladocera). Shifts in the boundary between sand and mud deposition were controlled by lake level changes, and hence, allowed the quantification of an absolute lake level amplitude of ~8 m for the Holocene. This clearly exceeded observed modern fluctuations of 1.3 m (AD 1973–2010). Past lake level changes were traced continuously using the calcium‐record. During high lake levels, massive organic muds were deposited in the deepest lake basin, whereas lower lake levels isolated the sub‐basins and allowed carbonate deposition. During the beginning of the Holocene (>9700 cal. a BP), lake levels were high, probably due to final melting of permafrost and dead‐ice remains. The establishment of water‐use intensive Pinus forests caused generally low (3–4 m below modern) but fluctuating lake levels (9700–6400 cal. a BP). Afterwards, the lake showed an increasing trend and reached a short‐term highstand at c. 5000 cal. a BP (4 m above modern). At the transition towards a cooler and wetter late Holocene, forests dominated by Quercus and Fagus and initial human impact probably contributed more positively to groundwater recharge. Lake levels remained high between 3800 and 800 cal. a BP, but the lake system was not sensitive enough to record short‐term fluctuations during this period. Lake level changes were recorded again when humans profoundly affected the drainage system, land cover and lake trophy. Hence, local Holocene water level changes reflect feedbacks between catchment and vegetation characteristics and human impact superimposed by climate change at multiple temporal scales.     

A study of fecal coliform sources at a coastal site using colored dissolved organic matter (CDOM) as a water source tracer

Optical properties of colored dissolved organic matter (CDOM) were measured as a tracer of polluted waters in a Southern California surf-zone with consistently high levels of fecal indicator bacteria. Salinity, temperature, fecal coliform, absorbance (200-700nm) and fluorescence (lambda(excitation)=350nm; lambda(emission)=360-650nm) were measured in the creek and surf-zone during a dry and rain event. Fluorescence to absorption ratios for CDOM were used to distinguish water masses, with two distinct CDOM end-members identified as creek (flu/abs=8.7+/-0.8x10(4)) and coastal (flu/abs=2.2+/-0.3x10(4)). Waters containing the same CDOM end-member had highly variable bacterial levels during the dry event, suggesting intermittent sources of bacteria added to a uniform water source, consistent with marine birds. During the rain event, increased levels of the creek end-member and bacteria indicated a second bacteria source from runoff.     

Metal sources to the Baltic clam Macoma balthica (Mollusca: Bivalvia) in the southern Baltic Sea (the Gulf of Gdansk)

Metal concentrations of Cu, Fe, Mn, Ni, Pb and Zn in an infaunal facultative deposit-feeding bivalve, the Baltic clam Macoma balthica, in the Gulf of Gdansk (southern Baltic Sea) were assessed and compared to selected concentrations of metals in the environment. Between October 1996 and September 1997, dissolved and easy extractable (by 1M HCl) metal fractions of total suspended particulate matter (TPM) in the overlying water and of surficial sediments (<63 microm) were measured monthly at five sublittoral sites in the Gulf of Gdansk, and accumulated tissue metal concentrations in M. balthica were determined simultaneously. The study highlights the importance of sediment geochemistry as a factor modifying ambient trace metal bioavailabilities. Surficial sediments appeared to contribute most to the accumulation of Cu and Pb in M. balthica, reflecting the high metal availability in the Gulf. Assimilation of Cu from sediments is controlled by Mn components possibly through an inhibitory effect of Mn oxyhydroxides, while Pb accumulation from sediments depends on the organic content of the sediment. A dual metal uptake pathway, with a suspended particulate-bound fraction and surficial sediments, was apparent for Mn and Zn. Partitioning of Mn in sediments was related to the concentration of labile Fe, with increased levels of Fe tending to inhibit the accumulation of Mn by the clam. Tissue accumulated Zn might have been altered by the clam's internal regulation, making Zn tissue concentrations, to some degree, independent of its environmental level. The principal source of Ni accumulated by the clams exists in the soluble phase.     

Study of litter influence on magnetic susceptibility measurements of urban forest topsoils using the MS2D sensor

Field magnetometry is fast and convenient method used to detect areas contaminated by technogenic magnetic particles and potentially polluted with heavy metals. Frequently, measurements of soil magnetic susceptibility (κ) are carried out with MS2D Bartington sensor, which penetration range equals 10 cm, although 90% of the total signal is detected from a depth of up to 6 cm. Thick uppermost organic soil layers may significantly influence on the measured κ because the penetration range may be not large enough to cover the layers where the most of anthropogenic contaminants are cumulated. The aim of the study was to investigate on how the removal of the litter improves the MS2D measurements of soil pollution. Accordingly, the correlations between κ values measured on the successively removed overlying soil sub-horizons were investigated. Measurements were performed at 15 sites located in different forest in the Upper Silesian Industrial Area, Poland. The results showed that the litter removal enabled the MS2D to measure the κ more related to the anthropogenic pollution, and did not affect the measuring variance.     

Seasonal and spatial changes of metal concentrations in groundwater outflows from porous sediments in the Gryżyna-Grabin Tunnel Valley in western Poland

Groundwater outflows (springs and seeps) from porous sediments on European lowlands play a significant role in the surface water balance. It is therefore important to document these outflows’ water quality and spatial and temporal changes. The present study focused on changes in iron, manganese, and heavy metals in waters of groundwater outflows in an area of low anthropogenic impact—the Gryżyna-Grabin Tunnel Valley in western Poland. Eleven outflows were studied for a period of 2 years. Most of the samples analyzed revealed low element contents and little variation in time and space. Only values of iron and lead were above World Health Organization limits for drinking water. The concentrations of these elements were also variable in time and space. The seasonal changes in iron content are probably due to natural, temperature- and pH-controlled reactions of shallow groundwater with aquifer sediments. On the other hand, the elevated lead content points to regional water contamination.