Changes In Source Region Research Articles

In situ optical and microphysical properties of tropospheric aerosols in the Canadian High Arctic from 2016 to 2019

Since aerosols are an integral part of the Arctic climate system, understanding aerosol radiative properties and the relation of these properties to each other is important for constraining aerosol radiative forcing effects in this remote region where measurements are sparse. In situ measurements of aerosol size distribution, aerosol light scattering and absorption were taken near Eureka (80.05oN, 86.42oW), on Ellesmere Island, in the Canadian High Arctic over three consecutive years to provide insights into radiative properties of Arctic aerosols.During periods of Arctic haze, we find that the single scattering albedo (SSA) at 405 nm is generally higher and more stable than that determined at 870 nm, with values ranging between 0.90–0.99 and 0.79–0.97, respectively. Events with elevated absorption coefficients (Babs) exhibit generally an absorption Ångström exponent (AAE) of around 1 suggesting that black carbon (BC) is the dominant absorbing aerosol for the measurement period. AAE values close to 2 occurring with scattering Ångström exponent (SAE) values near 0 and SAE values below 0 occasionally observed in December indicate a potential contribution from mineral dust aerosols in late fall and early winter. The apparent real and imaginary parts of the complex refractive index at 405 nm have been found to range between 1.6–1.9 and 0.002–0.02, respectively. The low imaginary component indicates very weak intrinsic absorption compared to BC-rich aerosols.Systematic variabilities between different aerosol optical and microphysical properties depend strongly on the given wavelength. SSA at 405 nm shows a strong inverse dependence with Babs, because Babs correlates positively with the imaginary component of the refractive index. On the other hand, SSA at 870 nm correlates with scattering coefficient (Bsca) and not with Babs due to a greater sensitivity to the ambient particle size distribution for 870 nm scattering. Smaller particles with higher SAE that are prevalent during less polluted periods only weakly scatter at 870 nm leading to lower SSA when Bsca is also low.Lastly, FLEXPART back-trajectories show that lower aerosol SSA and higher Babs correspond to air masses which are more influenced by Eurasian and Alaskan regions, including regions known to have important BC emissions. This work emphasizes the important variability in Arctic aerosol optical properties during winter and spring, which is likely due to changes in source regions.

Lake Sediments Reveal Large Variations in Flood Frequency Over the Last 6,500 Years in South-Western Norway

Lake sediments can retain imprints of past floods, enabling reconstructions that span well beyond instrumental time series. Time series covering thousands of years can document the natural range of flood variability, which is critical for understanding the potential causality between changing flood patterns and climate. Here, we analyzed sediments from Lake Sandvinvatnet in southwest Norway. Detailed environmental magnetic analyses of an 830cm-long sediment core covering the last 6,500 years captured decadal scale trends in local flood frequency. Magnetic susceptibility (MS) assessments were carried out both on split cores and individual samples to track variability in sedimentary influx; the ratios of MS measured at 77 K and 293 K (MS ratios) provided information on potential changes in source regions. The results suggested that sediments from the Buerdalen valley dominate the signal in the core, and the amount of ferromagnetic (high MS) carriers increases during flood events. These carriers were assumed to be transported from slope deposits in Buerdalen during rainstorm-triggered flood events. The reconstructed flood frequency, based on sediment layers with ferromagnetic carriers, showed high variability over the past 6,500 years, and the finding was validated by overlapping with known historical floods in the area. We observed periods with a high frequency of extreme floods (4100-3140 cal. yr BP) compared with intervals with a few or no extreme floods (6050-4100 cal. yr BP). Floods in this region are commonly a result of intense rain events during fall and snow and glacial melt during late spring and summer. The systematic frequency changes during the past 6,500 years suggest a certain persistency in the processes that cause floods, where mean trends in summer temperature and precipitation may have played a role.

Detrital zircon U–Pb ages, Hf isotopic constraints, and trace element analysis of Upper Cretaceous–Neogene sedimentary units in the Western Nepal Himalaya: Implications for provenance changes and India–Asia collision

The present study integrates detrital zircon U–Pb–Hf isotopic analysis from 13 sandstone samples from an Upper Cretaceous–Miocene sedimentary sequence in Nepal Himalaya to determine their provenance. These sequences constrain a shifting of provenance from south to north. The U–Pb ages from Upper Cretaceous–Palaeocene strata (Amile Formation) mainly cluster between ~1,860 and 1,400 Ma with a peak at ~1,630 Ma and an absence of grains younger than the Palaeoproterozoic (1,400 Ma) age. The detritus yielded positive detrital zircon ƐHf (t) values (as high as +10). However, the detrital zircon U–Pb ages from Eocene–Miocene sequence cluster at ~500–650, ~700–900, ~1,600–1,850, and ~2,500 Ma, and in addition, they have both positive and negative ƐHf (t) values (+11 to −25). This finding further elucidates that the detritus in the Amile Formation was entirely sourced from India, which changed following the time of the Bhainskati Formation deposition, to a mixture of both Asian and Indian affinities (the Himalayan region). This change in source region marks the possible time of the India–Asia collision during this transition phase, that is, Late Palaeocene–Earliest Eocene.

Tall Tower Vertical Profiles and Diurnal Trends of Ammonia in the Colorado Front Range

AbstractAmmonia (NH3) mixing ratios were measured between the surface and 280 m aboveground level from a moveable carriage at the Boulder Atmospheric Observatory tower in summer 2014 as part of the Front Range Air Pollution and Photochemistry Éxperiment. The campaign median mixing ratio was 3.3 ppb, ranging from below detection limits to 192 ppb. Median vertical profiles show an overall increase in NH3 mixing ratios toward the surface of 6.7 ppb (89%) during the day and 3.9 ppb (141%) at night. In contrast to the overall increasing trend, some individual profiles show decreasing NH3 in the lowest 10 m. This suggests that the local surface is capable of acting as either a source or a sink, depending on the relative amounts of NH3 at the surface, and in advected air parcels. We further use this data set to investigate the variation in diurnal patterns of NH3 as a function of height above the surface. At higher altitudes (100 ± 5 and 280 ± 5 m), NH3 mixing ratios reach a gradual maximum during the day between 9:00 and 16:00 local time, likely driven by changes in source region. At lower altitudes (10 ± 5 m), the daytime maximum begins earlier at about 7:00 local time, followed by a sharper increase at 9:00 local time. At this height we also observe a peak in NH3 mixing ratios during the night, likely driven by the trapping of emitted NH3 within the shallower nocturnal boundary layer.

The Laurentia – West Greenland connection at 1.9 Ga: New insights from the Rinkian fold belt

U-Pb data from the Rinkian fold Belt, western Greenland, provide new constraints on provenance and timing of deposition of the Karrat Group, the emplacement interval of the Prøven Igneous Complex, and the tectonic setting of west Greenland during the interval 2.03–1.83Ga. U-Pb detrital data establish the entire Karrat Group metasedimentary succession as Paleoproterozoic in age, initiated after ca. 2029Ma (Qeqertarssuaq Formation), with deeper water deposition after 1953±31Ma and 1905±20Ma (Nûkavsak Formation). The detrital age profiles highlight a profound change in source region after ca. 1.95Ga that we attribute to thickening, uplift, and exhumation related to collision of a juvenile magmatic arc with the northwest margin of Rae craton at ca. 1.97–1.95Ga (Thelon Orogen). Accordingly, the Karrat Group is viewed as having initiated as an extensional rift basin(s) that received ca. 3.00–2.95Ga detritus from local basement sources, and which evolved into a deeper water foreland-basin succession derived from the north. This foreland basin was intruded by the Prøven Igneous Complex, for which new U-Pb data establish emplacement between 1.90 and 1.87Ga in a within-(Rae) plate setting. Prøven plutonism is coeval with lower-plate mantle magmatism elsewhere across NE Laurentia which may have been triggered by asthenospheric thinning due to plume-induced extension of lower-plate Rae craton at 1.95–1.92Ga. Our data refute a direct link between the Prøven Igneous Complex and the voluminous 1.86–1.845Ga Cumberland Batholith, Baffin Island, long considered its counterpart.

Identifying Changes in Source Regions Impacting Speciated Atmospheric Mercury at a Rural Site in the Eastern United States

AbstractTo investigate the effectiveness of emission reductions on the concentrations of gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM) at a rural site in Maryland (MD08), long-term (2005–14) measurements of speciated atmospheric mercury were analyzed using concentration-weighted trajectory (CWT) analysis. CWT results suggested that the number of major source regions contributing to GEM, GOM, and reactive mercury (RM = GOM + PBM) over the eastern United States and southeastern Canada declined over time. Across much of these regions, source contributions in 2011–14 decreased by up to 20% for GEM, by greater than 60% for GOM, and by 20%–60% for PBM compared to 2006–08, largely because of the decreases in power-plant mercury emissions since 2009. Changes in the spatial distribution of the source regions were also observed over time. Increases in source contributions of GEM after 2011 over the northeastern United States and southeastern Canada were predominantly from emission increases in metal and steel production and forest fires. Source contribution increases in PBM were more widespread, which can be attributed potentially to mercury transformation processes in the air or wood combustion rather than industrial sources.

Impact of Source Region on the δ18O Signal in Snow: A Case Study from Mount Wrangell, Alaska

Abstract The stable isotopic composition of water in ice cores is an important source of information on past climate variability. At its simplest level, the underlying assumption is that there is an empirical relationship between the normalized difference in the concentration for these stable isotopes and a specified local temperature at the ice core site. There are, however, nonlocal processes, such as a change in source region or a change in the atmospheric pathway, which can impact the stable isotope signal, thereby complicating its use as a proxy for temperature. In this paper, the importance of these nonlocal processes are investigated through the analysis of the synoptic-scale circulation during a snowfall event at the summit of Mount Wrangell (62°N, 144°W; 4300 m MSL) in south-central Alaska. During this event there was, over a 1-day period in which the local temperature was approximately constant, a change in δ18O that exceeded half that normally seen to occur in the region between summer and winter. As shall be shown, this arose from a change in the source region, from the subtropical eastern Pacific to northeastern Asia, for the snow that fell on Mount Wrangell during the event.

Deep-sea benthic foraminiferal turnover during the early–middle Eocene transition at Walvis Ridge (SE Atlantic)

The early–middle Eocene transition (EMET, ~51–47Ma) was a pivotal time in Cenozoic global climate development, encompassing the end of the Early Eocene Climate Optimum (EECO) and the start of high-latitude cooling, culminating in the early Oligocene Glacial Maximum (34–33Ma). We studied benthic foraminiferal assemblages and stable isotopes at upper abyssal Site 1263 (paleodepth ~1800m) and lower abyssal Site 1267 (paleodepth ~3500m) on Walvis Ridge (SE Atlantic) across the EMET. Benthic δ18O values at both sites rose from ~49.7Ma on, becoming slightly higher at the shallower site than at the deeper site from ~44Ma on. Benthic δ13C values show a broad maximum at ~49–48Ma. Benthic foraminiferal assemblages at both sites are dominated by calcareous taxa, particularly Nuttallides truempyi, indicative of oligotrophic conditions. Large species of Bulimina, indicative of more food-rich conditions, were less rare at the shallower site, where benthic foraminiferal accumulation rates were generally higher. Specimens intermediate in morphology between N. truempyi (a cosmopolitan warm-water species which died out in the latest Eocene) and its extant, also cosmopolitan, cold-water descendant Nuttallides umbonifera (indicative of corrosive bottom waters), are present, as well as various Epistominella species, indicative of seasonal delivery of phytodetritus to the sea floor, and globally more common after the Eocene–Oligocene transition. These assemblages indicate that deep water environments started a gradual evolution from conditions typical for the warm early Eocene greenhouse to those typical for icehouse conditions by the end of the early Eocene. Characteristic early Eocene species such as Aragonia aragonensis and Quadrimorphina profunda persisted at least until 48Ma (the end of the studied interval) at the deeper site, whereas at the shallower site, they had their last appearances at ~49.7Ma, the time of increase in δ18O values, i.e., high latitude and deep-water cooling. The benthic foraminiferal transition towards assemblages typical for a glaciated world thus started close to the end of the early Eocene in the SE Atlantic, with the middle Eocene already representing a transitional world. The faunal changes in the SE Atlantic began earlier at upper abyssal than at lower abyssal depths. This suggests that a more stratified ocean started to develop in the SE Atlantic at the end of the EECO, possibly due to changes in source regions of intermediate water masses due to polar cooling and/or circulation changes around the Antarctic continent.

Depositional history, tectonics, and detrital zircon geochronology of Ordovician and Devonian strata in southwestern Mongolia

Lower Paleozoic successions of the Gobi-Altai zone of southern Mongolia record an abrupt facies transition from deposition of predominantly fine-grained uppermost Ordovician through lowermost Devonian carbonate and marl facies to deposition of coarse clastic strata of the Lower Devonian Tsakhir Formation. The Tsakhir generally fines upward from alluvial-fan cobble and pebble conglomerate to interbedded coarse- and fine-grained marine siliciclastic and carbonate strata, which were deposited within a tectonically active basin. The marine strata, deposited in a storm-influenced proximal to distal fan delta, include unusual event beds that grade from pebble conglomerate to hummocky cross-stratified grainstone and sandstone. These beds represent sediment emplaced by gravity flows during flood events and reworked by large gravity waves associated with storm events. The interpreted link between flood deposition and storm wave reworking supports a hyperpycnal flow interpretation for these deposits. The sudden facies transition at the base of the formation represents the sedimentological and stratigraphic signature of Early Devonian tectonism in the Gobi-Altai zone. The general upward-fining pattern of the Tsakhir is interpreted as a response to the creation of accommodation space at a greater rate than progradation of the fan delta, in large part due to tectonic subsidence, although some component of eustasy may have been involved. The production of steep relief and deposition of associated volcanics suggest a transition from relatively passive deposition to active tectonics in this region during the Lochkovian to Pragian stages of the Early Devonian. We herein introduce the term “Tsakhir event” for this important tectonic transition. Range-bounding faults for this event are not preserved, but alluvial-fan deposition, the development of unconformities, renewed subsidence, and magmatism throughout the Gobi-Altai zone all suggest syndepositional tectonism. Detrital zircon spectra from both Ordovician and Devonian strata contain Archean to Paleozoic ages. Minor differences between Ordovician and Devonian samples suggest changes in source regions and/or transport paths prior to, and after, the Tsakhir event. The paleoenvironmental setting of the Tsakhir Formation requires short transport distances, and thus the age spectrum of a sample from this formation represents proximal basement rocks of the Shine Jinst region of the Gobi-Altai zone. Basement rocks are not exposed in the Shine Jinst region, but the wide variety of ages in all of the detrital spectra suggest a nearby continental source. Our detrital age spectra contain peaks that coincide with basement ages and magmatic events on the adjacent Mongolian microcontinent and also have strong similarities with recently published spectra of nearby landmasses in Neoproterozoic to Paleozoic paleogeographic reconstructions, namely, Siberia, North China, eastern Gondwana, and Tarim. These similarities extend to spectra of late Neoproterozoic to middle Paleozoic rocks throughout Gondwanaland and also Siberia, illustrating the somewhat limited utility of detrital spectra for determining the tectonic affinities of crustal blocks at this time in Earth history.

Stable isotopic evidence for climate and basin evolution of the Late Cretaceous Songliao basin, China

Because of the need to understand the links and feedbacks of the carbon cycle during times of global greenhouse conditions numerous studies have focused on the Cretaceous climate. Much of what we know about this warm period in Earth's history comes from the study of ocean sediments recovered from both ocean drill cores and marine sediments exposed at the Earth's surface. In contrast, there are few studies of Cretaceous terrestrial sediments. The Songliao basin located in northeast China offers a unique opportunity to understand Cretaceous paleoclimate of terrestrial settings because it contains a nearly complete record of lacustrine sediments deposited throughout the Cretaceous and there is an active drilling program to recover core from this paleolake. We present carbon, oxygen and strontium isotopic data from ostracods collected from two drill cores (SK-1 (N) and SK-1 (S)) that cover a time interval that extends from the Turonian through the Maastrichtian. These data record robust isotopic trends with numerous carbon and oxygen isotope shifts that are both rapid and long-term. We tentatively interpret this record to reflect changes in both global climate and regional basin evolution. In the Turonian and Coniacian Qingshankou Formation we observe several carbon isotope shifts that appear to be correlative to marine isotopic records based upon timing and magnitude of the isotopic changes. We suggest that the carbon isotope record in the Songliao basin reflect the decrease in carbon isotope ratios following the strong positive excursion at the Cenomanian/Turonian boundary, a positive isotope excursion in the late Turonian, and the negative isotope shift that occurs at the Turonian/Coniacian boundary. Upward in the section, however, the marine and Songliao isotopic records diverge as sediment sources shift from the southwest, east and north to more northerly. Strontium isotopes record the change in source region as they increase markedly between the Coniacian/Santonian Yaojia and Santonian/Campanian Nenjiang Formations. As this is the first isotopic record of the Songliao basin we are cautious about many of our interpretations of these isotopic data. Yet it is clear that with tighter age constraints and focused sampling centered on many of the isotope excursions presented here will allow for a deeper understanding of the terrestrial response to changing climate and the hydrologic response to basin evolution.

Greater supply of Patagonian-sourced detritus and transport by the ACC to the Atlantic sector of the Southern Ocean during the last glacial period

Reconstructing past detrital flux and provenance in the Southern Ocean provides information about changes in source regions associated with climate variations and transport pathways. We present a Last Glacial Maximum (LGM) to Holocene comparison of 230Th normalised fluxes combined with sediment provenance data (Pb, Nd and Sr isotopes) from a latitudinal core transect in the eastern Atlantic sector of the Southern Ocean (ODP Leg 177 cores). We compare the radiogenic isotopic composition (IC) of detritus in these cores to that of cores proximal to potential source areas.We observe a well-defined latitudinal Holocene gradient in both detrital flux and provenance of sediment. High detrital fluxes in the north are associated with terrigenous material derived from southern Africa, while low detrital fluxes in the south are associated with supply from southern South America, West Antarctica and the South Sandwich Islands. The data suggest that this well-defined Holocene gradient in detrital flux and sediment provenance is controlled by the flow of the Antarctic Circumpolar Current (ACC) and the position of its frontal zones. The LGM is characterised by 2 to 6 times higher than modern detrital fluxes at most ODP Leg 177 sites. The LGM detrital fluxes do not show a latitudinal trend and suggest a greater supply of glaciogenic detritus sourced from southern South America. Glacial Patagonian outwash sediments (<5μm fraction) were analysed and compared to the bulk compositions of the marine sediments. The Pb IC of the Patagonian sediments is very similar to the glacial IC of sediments in the Scotia Sea and at ~49° S latitude in the eastern Atlantic sector. We propose that the glacial IC of sediments is controlled by increased delivery of Patagonian detritus initially supplied by glaciers and then transported at depth via the ACC.

Seasonal variation of the concentrations of nitrogenous species and their nitrogen isotopic ratios in aerosols at Gosan, Jeju Island: Implications for atmospheric processing and source changes of aerosols

Atmospheric aerosol samples (n = 84) were collected at Gosan site, Jeju Island, South Korea between April 2003 and April 2004 for the measurements of total nitrogen (TN) and its isotopic ratio (δ15N) as well as nitrogen species (NH4+ and NO3−). Measurements were also conducted for remained nitrogen (remained N) and removed nitrogen (removed N) on HCl fume treatment. A pronounced seasonal variation was found in the δ15N of TN, remained N (mostly NH4+), and removed N (mostly NO3−). The highest mean δ15N values of TN (+16.9‰ ± 4.5‰) and remained N (+20.2‰ ± 5.2‰) are detected in summer (June–August) whereas the lowest mean δ15N values (+12.9‰ ± 3.4‰ and +11.3‰ ± 5.1‰, respectively) are in winter (December–February). These trends can partly be explained by an enhanced contribution of 15N‐enriched emissions from agricultural straw burning in China in a harvest season (summer and autumn). The mean δ15N of removed N showed an opposite trend: the lowest (+8.9‰ ± 3.7‰) in warm season (March–August) and the highest (+14.1‰ ± 3.7‰) in cold season (September–February). These results can be explained by changes in source regions and emission strengths of nitrogenous species, as well as difference in secondary aerosol nitrogen formation between warm and cold seasons. Higher ratios of Ca2+/Na+ and the lowest ratios of Na+/(Cl− + NO3−) are associated with lower δ15N values of removed N as a result of less isotopic enrichment (ɛproduct‐reactant) during the reaction between HNO3 and dust particles. This study proposes that 15N/14N ratio can be regarded as process tracer of nitrogenous species in the atmosphere.

Geochemistry of Early–Middle Palaeozoic basalts in the Hodgkinson Province: a key to tectono-magmatic evolution of the Tasman Fold Belt System in northeastern Queensland, Australia

The Palaeozoic Hodgkinson Province in northeastern Queensland, Australia, is host to Late Ordovician to Devonian rock assemblages that contain tholeiitic to calc-alkaline basalts. These basalts occur as massive fault-bounded units interspersed with marine sedimentary rocks and limestones that are metamorphosed to lower greenschist facies in the Ordovician Mulgrave, Silurian Chillagoe and Devonian Hodgkinson formations, respectively. The petrogenetic and Sm–Nd isotope characteristics of these mafic volcanic rocks were investigated to constrain the tectonic setting in which they erupted. Major, trace and rare earth element analyses were carried out on samples from these formations and intrusive dolerites. The mafic rocks can be classified as basalts and basaltic andesites with distinct MORB characteristics. Furthermore, the basalts are characterized by a slight to moderate enrichment in Th and concomitant depletion in Nb, both of which become less pronounced with basalt evolution through time. These features are consistent with decreasing volcanic arc affinity of Silurian and Devonian MORB-type basalts in the Hodgkinson Province. Sm–Nd isotope characteristics of these basalts indicate a change in source region from dominantly sub-continental lithospheric mantle in the Silurian to asthenospheric input in the Devonian. Collectively, the geochemical and isotopic characteristics of the Hodgkinson Province basalts are interpreted to reflect deposition in an evolving back-arc basin setting. The onset of basin extension was initiated in the Silurian. Accelerated basin subsidence occurred throughout the Devonian and was halted by basin inversion in the Late Devonian. Basin evolution was controlled by an eastward stepping subduction zone outboard of the Australian Craton.

Characterization of African dust transported to Puerto Rico by individual particle and size segregated bulk analysis

As part of the Puerto Rico Dust Experiment (PRIDE), airborne and surface dust particle samples from Africa were collected and subjected to bulk elemental and single‐particle analysis. Airborne samples were collected on polycarbonate filters at various altitudes and underwent single‐particle scanning electron microscopy with energy dispersive analysis with X‐rays (EDAX) to derive elemental ratios of key soil elements. Particle chemistry was related to size and morphological characteristics. At the principle surface site, particles were collected on a Davis Rotating Drum (DRUM) cascade impactor strips in eight stages from 0.1 to 12 μm at 4 hour time resolution. These samples were subjected to X‐ray florescence (XRF) to determine bulk elemental composition from Al through Zn. The elemental data showed good correlation between the DRUM and the aircraft samples. Cluster analysis of single‐particle data resulted in 63 statistically significant clusters. Several clusters can be easily related to their parent mineralogical species. However, as dust particles are to a large extent aggregates, most clusters are based on a continuum of varied mineralogical species and cannot be easily categorized. With 60,500 total particles counted from the airborne filters, a statistically significant number of large particles could be analyzed. Estimated mean surface area modal diameter is ∼5 μm, with an average aspect ratio of 1.9. An apparent change in source region is seen in the morphological data and non alumino‐silicate minerals but is not seen in the aluminum to silicon ratio. We suspect homogenization during long‐range transport.

Growth of subcontinental lithosphere: evidence from repeated dike injections in the Balmuccia lherzolite massif, Italian Alps

The Balmuccia alpine lherzolite massif is a fragment of subcontinental lithospheric mantle emplaced into the lower crust 251 Ma ago during the final, extensional phase of the Hercynian orogeny. The Balmuccia massif consists largely of lherzolite, with subordinate harzburgite and dunite, and an array of dike rocks formed in the mantle before crustal emplacement. Dike rocks include websterite and bronzitite of the Cr-diopside suite, spinel clinopyroxenite and spinel-poor websterite of the Al-augite suite, gabbro and gabbronorite of the late gabbro suite, and hornblendite of the hydrous vein suite. The dike rocks display consistent intrusive relationships with one another, such that Cr-diopside suite dikes are always older than dikes and veins of the Al-augite suite, followed by dikes of the late gabbro suite and veins of the hydrous vein suite. Phlogopite (phl) veinlets that formed during interaction with the adjacent crust are the youngest event. There are at least three generations of Cr-diopside suite dikes, as shown by crosscutting relations. Dikes of the Al-augite suite form a polybaric fractionation series from spinel clinopyroxenite to websterite and feldspathic websterite, which crystallized from aluminous alkaline magmas at relatively high pressures. The late gabbro suite of dikes intruded at lower pressures, where plagioclase saturation occurred before significant mafic phase fractionation. Hornblendite veins have distinct compositional and isotopic characteristics, which show that they are not related to either the Al-augite suite or to the late gabbro dike suite. Cr-diopside suite dikes have Nd and Sr isotopic compositions similar to those of the host lherzolite and within the range of compositions defined by ocean–island basalts. The Al-augite dikes and the hornblendite veins have Sr and Nd isotopic compositions similar to those of Cr-diopside suite lherzolite and websterite. The late gabbro dikes have Nd and Sr isotopic compositions similar to mid-ocean ridge basalt (MORB) asthenosphere. Lead isotopic compositions for all of the samples fall in the present-day MORB field on the 208 Pb/ 204 Pb vs. 206 Pb/ 204 Pb diagram but are displaced above this field on the 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb diagram. There is overlap in the data between the Cr-diopside suite and the Al-augite and hydrous vein suites, with the exception that the Cr-diopside websterite dikes have more radiogenic Pb than any of the other samples. In Pb–Pb space as well, the late gabbro suite has the least radiogenic isotopic compositions, reflecting a change in magma source region during uplift. These data show that tectonic thinning of subcontinental lithospheric mantle during extension caused a change in the source regions of mantle-derived magmas from an ocean island basalt (OIB)-like lithosphere to the underlying MORB asthenosphere. They also demonstrate that the upper mantle acquires its heterogeneous isotopic character through several different processes, including in situ radiogenic growth, addition of asthenospheric melts, dike-wall rock ionic exchange, redistribution of the lithospheric dike and vein materials by melting, and in the late stages of emplacement, assimilation of crustal materials.

THERMI ON LESBOS: A CASE STUDY OF CHANGING TRADE PATTERNS

Summary Provenance studies on archaeological metal objects rely on their trace element abundance pattern and lead isotopic composition. A comparison of these features in artifacts from different archaeological sites often requires a comparison of results reported by different laboratories. In this case reasonable conclusions can only be expected if it is assured that the respective laboratories produce compatible results. A comparison of some twenty copper‐based artifacts from Thermi I and II shows this compatibility still to be less than perfect between Oxford and Heidelberg — Mainz. We find the lead isotopic compositions reported by the two laboratories to agree within the stated uncertainties of the abundance ratios of 0.1 percent. The same is also true, to within 50 percent or better, for the contents of As, Sb, Ag, Au and possibly Ni. Serious discrepancies exist, however, for Co, Pb and Zn that we argue to be due to analytical problems at Oxford.Discriminant analysis of the data from all metal‐bearing occupational levels at Thermi reveals a remarkably good correspondence between trace element signature and occupational level which suggests that at the beginning of the third millennium BC intercourse between Lesbians and Aegean metal traders was rare. Lead isotope data show that the change in metal, at least that upon the transition from Towns I + II to Towns III–IV, was indeed due to a change in source region of the metal and not one caused by a change in technology.

The Atmosphere During the Younger Dryas

One of the most dramatic climate change events observed in marine and ice core records is the Younger Dryas, a return to near-glacial conditions that punctuated the last deglaciation. High-resolution, continuous glaciochemical records, newly retrieved from central Greenland, record the chemical composition of the arctic atmosphere at this time. This record shows that both the onset and the termination of the Younger Dryas occurred within 10 to 20 years and that massive, frequent, and short-term (decadal or less) changes in atmospheric composition occurred throughout this event. Changes in atmospheric composition are attributable to changes in the size of the polar atmospheric cell and resultant changes in source regions and to the growth and decay of continental biogenic source regions.

Significance of clay mineral assemblages in the Antarctic Ocean

Typical examples from different morphological and geological settings in the Antarctic Ocean are reviewed in order to discuss the value of clay mineral assemblages for reconstructing the glacial history of Antarctica, the paleoceanographic history of the Antarctic Ocean and the sedimentary processes at the Antarctic continental margin. The significance of clay minerals for paleoenvironmental reconstructions strongly varies with both the position of the sites under investigation and the age of the sediments. In late Mesozoic to Paleogene sediments clay mineral assemblages are sensitive tools for reconstructing climatic conditions. For example, the shift from smectite-dominated assemblages to illite- and chlorite-dominated assemblages in the earliest Oligocene clearly documents the transition from chemical weathering conditions under a warm and humid climate to physical weathering under cooler conditions. Submarine elevations such as Maud Rise and Kerguelen Plateau give the best record for direct paleoclimatic and paleoceanographic studies. At the proximal sites of the continental slope and shelf, as well as in the deep sea, the paleoclimatic information normally is masked by a variety of processes resulting in sediment redistribution. At those sites, in contrast, the clay mineral assemblages bear a wealth of information on different sedimentary processes. After the establishment of a continental East Antarctic ice sheet, physical weathering prevailed. Variations in the clay mineral records predominantly reflect the influence of different sediment sources resulting from different glacial, hydrographic or gravitational transport processes. Because these sedimentation processes are generally linked to climatic variations, the clay mineral assemblages in most of the Neogene and Quaternary sediments provide indirect paleoclimatic information. The processes are best documented in the clay mineral composition in those areas where changes in source regions with distinct petrographic differences are expected and where distances from the source region are low.

Temporal variability of atmospheric lead concentrations and fluxes over the northwestern Mediterranean Sea

Beginning in 1985, daily 24‐hour aerosol samples were collected on 0.4‐pm pore size filters at a coastal location in northwestern Corsica. Total atmospheric deposition (wet + dry) were sampled between February 1985 and October 1987 with a collection period of about 15 days. As indicated by three‐dimentional air mass trajectories, lead aerosol particles collected at this site are primarily derived from European continental source regions. The variability of lead aerosol concentrations on both daily and seasonal time scales is primarily due to the scavenging of lead aerosol particles by rain rather than to changes in source regions. Our results suggest that the ratio between the total atmospheric deposition of Pb and the corresponding mean daily precipitation (mdp) rate is not constant. This ratio reaches a maximum during the Mediterranean summer. We attribute this difference to wet scavenging processes which wash a more loaded atmosphere during the dry season than during the wet season. The precipitation frequency (FP) is a major factor influencing seasonal variability of the total atmospheric deposition of lead over the western Mediterranean. An intermediate value of FP allows sufficient reloading of the atmosphere with long‐range transported Pb aerosol particles as well as efficient scavenging by precipitation events.

Case studies of aerosol size distribution and chemistry during passages of a cold and a warm front

Surface-based measurements of particle size distributions, mass, water content, and the ammonium, sulfate and nitrate ion concentrations of atmospheric aerosols, centered around precipitation events associated with cold and warm frontal passages, are reported. In both cases precipitation scavenges coarse aerosols (diameters > 1.5 μm) considerably more efficiently than it removes fine aerosols (diameters < 1.5 μm). The concentration of the coarse mode recovers to near pre-rain levels 3–4 h after the rain for both cases. The rates of precipitation scavenging on the coarse mode are found to be about 48 and 67% h −1 for the cold and warm front cases, respectively. The corresponding value for the fine mode is about 32% h −1 for the warm front case, which had little wind shift and thus no change of source region, but no estimate is made for the cold front case because of the marked wind shift. An estimate of the water content of the ambient aerosol indicates that, at times, as much as 95% of the aerosol mass may be water. With few exceptions, significant increases in the concentrations of aerosol mass and the above chemical components have been observed at the onset of rain, with the nitrate ion increase being most dramatic. Ammonium sulfate is found almost exclusively in the accumulation mode while the ammonium nitrate content varies between the accumulation mode and the larger particle mode.