Cycle Climate Change Research Articles

Book Review: Wisdom from Russia: ‘Cyclic Climate Changes and Fish Productivity’

Book Review: Wisdom from Russia: ‘Cyclic Climate Changes and Fish Productivity’

Cyclic climate and vegetation change in the late Miocene of Western Bulgaria

A late Miocene paludal to lacustrine sequence from a carbonate basin in NW Bulgaria (Staniantsi Basin) is analysed displaying up to 27 rhythmically bedded sedimentary cycles. In the lower part of the sequence, the cycles consist of alternating autochthonous brown coal and marls containing diverse mollusc assemblages. The upper part of the sequence is characterized by alternating dark to light grey clays and calcareous silts. A palynomorph record comprising 163 samples is analysed by statistical means to reconstruct vegetation changes. The Coexistence Approach is used to calculate quantitative palaeoclimate records for 6 parameters. The studied section displays hierarchical cyclicity patterns. Longer-term cycles possibly related to eccentricity (period ~ 100 kyr) are present in the palynomorph record and show climate changes of warmer/wetter and cooler/drier periods in combination with frequency oscillations of thermophilous elements. Short-term cycles most probably related to precession (period ~ 21.7 kyr) are expressed by alternations of brown coal and marl/shell beds and show cyclic change in peat-forming vegetation related to oscillations of the groundwater level. As a triggering mechanism, wetter/warmer and drier/cooler climate phases related to orbital precession are probable. In addition, sections sampled at high resolution display small scale climate and vegetational variability. As is shown by the analysis ferns were an important component of the peat-forming vegetation, while outside the mire, a wetland vegetation consisting of pioneers and a mixed mesophytic forest with evergreen shrubs existed. An oligo- to mesotrophic slightly alkaline lake became repeatedly established with a diverse mollusc fauna and a dense hydrophytic vegetation with characean meadows. In the upper part of the section, a spreading of herbaceous vegetation is observed, also known from other contemporaneaous palynomorph records in Bulgaria and surrounding areas. The increase of Asteraceae in the upper part of the section, combined with a marked decrease in woody taxa, points to an opening of habitats and a decrease in mean annual precipitation. This trend is paralleled by the mollusc fauna which yields several terrestrial, partly xerophilous taxa.

The Early Aptian of Aralar (northern Spain): stratigraphy, sedimentology, ammonite biozonation, and OAE1

The stratigraphy, sedimentology, ammonite faunas, and geochemistry of the early Aptian of the Aralar Mountains in northern Spain are described. This area contains a 983-m-thick succession of sediments of early Aptian age. The lithological succession evolves from lutites, marls, and calcarenites of the Errenaga Formation to rudist micritic limestones of the Sarastarri Formation, and finally marls, lutites, and sandstones of the Lareo Formation. Late Aptian calcarenites and marly limestones rest on top of the Early Aptian succession. The sedimentology indicates general deposition in a shallow marine environment, corresponding to mixed siliciclastic-carbonate ramps and a shallow-water carbonate platform. Ammonite occurrences allow recognition of the Deshayesites oglanlensis, D. weissi, D. deshayesi, and Dufrenoyia furcata Zones of the Mediterranean Tethyan belt scheme. A transition between the deshayesi and furcata Zones with the co-occurrence of the ammonite genera Deshayesites and Dufrenoyia is described for the first time in the Aralar succession and is currently unique. Black shales have been recognized at the top of the Barremian succession, in the D. deshayesi-D. furcata transition Zones, and in the D. furcata Zone. They correspond to three periods within the OAE1 interval. Geochemical TOC analyses indicate variations referable to cyclical climatic changes caused by orbital forcing. d13C and d18O geochemical signatures in Aralar serve for identifying and dating with greater precision to some of the previously recognised worldwide early Aptian major events: two negative and one positive carbon isotope curve shifts and two warming phases.

Grafting the molecular phylogenetic tree with morphological branches to reconstruct the evolutionary history of the genus Zaprionus (Diptera: Drosophilidae)

A molecular phylogeny for the drosophilid genus Zaprionus was inferred using a mitochondrial (CO-II) and a nuclear (Amyrel) gene using 22 available species. The combined molecular tree does not support the current classification, dubbed phylogenetic, based entirely upon a morphocline of forefemoral ornamentation. For species for which DNA was not available, phylogenetic positioning was only assigned using morphological characters. In order to avoid conflict between DNA and morphology in the combined analyses (supermatrix method), we developed a new method in which few morphological characters were sampled according to an a priori homoplasy assessment on the consensus molecular tree. At each internal node of the tree, a number of synapomorphies was determined, and species with no molecular sequences were grafted thereon. Analogously to tree vocabulary, we called our method ‘morphological grafting’. New species groups and complexes were then defined in the light of our findings. Further, divergence times were estimated under a relaxed molecular clock, and historical biogeography was reconstructed under a maximum likelihood model. Zaprionus appears to be of recent origin in the Oriental region during the Late Miocene (∼10MYA), and colonization of Africa started shortly after (∼7MYA) via the maritime route of the Indian Ocean Islands. Most of the morphological and ecological diversification took place, later, in Western Africa during the Quaternary cyclic climatic changes. Furthermore, some species became recent invaders, with one, Zaprionus indianus, has successfully invaded South and North America during the last decade.

Phylogeography of rusty-necklaced partridge ( Alectoris magna) in northwestern China

Rusty-necklaced partridges ( Alectoris magna) are found in Qinghai, Ningxia and Gansu provinces of northwestern China, distributed in mountainous areas that were heavily affected by cyclical climate changes throughout the Pleistocene. We investigated the potential impact of Pleistocene range shifts on phylogeographic patterns using 458 nucleotides of mitochondrial control-region from 102 individuals sampled from 10 populations across the current range. We found 31 haplotypes defined by 33 polymorphic positions. Phylogenetic analyses revealed two robustly supported clades, one corresponding to the Chaidamu Basin and the other including all individuals from all other populations. At the population level we also found significant evidence of differentiation. The phylogeographic structure of the rusty-necklaced partridge is consistent with the “phylogenetic discontinuity, spatial separation” pattern outlined by Avise et al. [Avise, J.C., Arnold, J., Ball, R.M., Bermingham, E., Lamb, T., Niegel, J.E., Reeb, C.A., Saunders, N.C., 1987. Intraspecific phylogeography: the mitochondrial DNA bridge between population genetics and systematics. Ann. Rev. Ecol. Syst. 18, 489–522].

Cantor set model of eolian dust deposits on desert alluvial fan terraces

Available data show that eolian dust accumulation rates on desert alluvial fan terraces are often inversely proportional to the square root of the terrace age for a given area. This temporal scaling is similar to that observed in fluvial and marine stratigraphic sequences in which a fractal distribution of hiatuses occurs (i.e., a Cantor set). Eolian dust accumulation on alluvial fan terraces is controlled by regional climatic influences on dust deposition (e.g., dessication of pluvial lakes in the early Holocene) and local surface characteristics (e.g., vegetation and pavement coverage) that control dust preservation. In order to interpret the observed scaling in terms of these relative influences I consider two end-member models: (1) a deterministic model in which dust deposition is controlled by cyclical climatic changes (i.e., glacial-interglacial cycles) and in which no erosion occurs, and (2) a stochastic model in which erosion and deposition take place with equal probability and magnitude during each time step (i.e., a bounded random walk). The observed temporal scaling is most consistent with the bounded random walk model, suggesting that long-term eolian dust accumulation is predominantly controlled by episodic changes in vegetation and pavement coverage over time and that eolian dust deposits are strongly punctuated by episodes of erosional reworking at a wide range of temporal scales.

Vegetation response to early Pleistocene climatic cycles in the Lamone valley (Northern Apennines, Italy)

The vegetation response to cyclic climatic changes has been investigated by means of pollen analysis carried out on marine deposits outcropping in the Lamone valley (Northern Italy). The deposits consist of grey–blue silty–clays of the “Argille Azzurre” Formation. Three sedimentary successions, which form the Lamone section, have been analysed. These successions, according to previous studies, have been referred to the first two substages of the lower Pleistocene, the Santernian and the Emilian. Pollen data from sixty-nine samples document vegetation dynamics consisting of a temporal spread of three distinct forest formations: mixed-deciduous forest (with Quercus dominating), Juglandaceae forest (with Carya dominating) and mountain coniferous forest (with Picea dominating). Each forest formation formed a vegetation belt, characterized by distinct temperature and precipitation conditions. The spread of the mixed deciduous forest indicates that the temperature increases whereas the humidity is low. The transition to a Juglandaceae forest indicates the increase of humidity; this phase is characterized by “optimum” of temperature and precipitations. The spread of mountain coniferous forest points out temperature drop and humidity still high. When forest dynamics is interrupted by the spread of open vegetation, indicating decrease of humidity, a complete vegetation cycle occurs. Such cycle suggests that temperature and humidity independently vary, with minimum and maximum of temperature generally preceding humidity extremes. Each vegetation cycle, therefore, reveals the response of vegetation to cyclic climatic changes occurring during the early Pleistocene (glacial/interglacial cycles). Comparison with coeval pollen series from other Italian sites shows wider spread of Picea and minor expansion of open vegetation in Northern Italy with respect to Central and Southern Italy, suggesting the hypothesis of more humid conditions at the North (as also documented by previous works). Taxodium pollen-type is continuously represented in the Lamone pollen record, exhibiting values of ∼ 7% on average excluding Pinus (∼ 4% including pine). Sciadopitys shows a discontinuous presence and very low percentages. Local and peculiar climate conditions may have favoured the long survival of these taxa into the Pleistocene. On the basis of a comparison between the profile of Picea + Tsuga pollen curve (recorded at Lamone) and the δ 18O signal established at Site 607 (North Atlantic), the vegetation cycles documented in the Lamone valley have been tentatively correlated to oxygen isotope stages from 64 to 46. Stages 59 and 51 seem to be partially recorded, whereas, stages 58, 57, 50 and 49 are not recorded.

Grain-size features of a Miocene loess-soil sequence at Qinan: Implications on its origin

In this study, grain-size of 507 bulk samples from the QA-I Miocene loess-soil sequence at Qinan were analyzed, and the grain-size features are compared with those of typical Quaternary loess and soil samples, representative lacustrine and fluvial samples. The results indicate that the grain-size distribution pattern of the Miocene loess is essentially similar to that of Quaternary loess, but greatly differs from the lacustrine and fluvial sediments. Loess layers are regularly coarser than soil layers, indicating cyclical climate changes. Median grain-size along the 253.1 m sequence varies from 6 to 13 μm and the >63 μm fraction represents only 5.3% in maximum, 0.9% in average. Long-term grain-size variations are consistent with the loess accumulation rate at Qinan and with the eolian mass accumulation rate in the North Pacific. These features firmly indicate an eolian origin of the studied sequence, and also reveal a coeval changes between the long-term changes of eolian grain-size and continental aridity in the dust source regions.

THE EVOLUTION OF FEMALE MATING PREFERENCES: DIFFERENTIATION FROM SPECIES WITH PROMISCUOUS MALES CAN PROMOTE SPECIATION

Females of many species are frequently courted by promiscuous males of their own and other closely related species. Such mating interactions may impose strong selection on female mating preferences to favor trait values in conspecific males that allow females to discriminate them from their heterospecific rivals. We explore the consequences of such selection in models of the evolution of female mating preferences when females must interact with heterospecific males from which they are completely postreproductively isolated. Specifically, we allow the values of both the most preferred male trait and the tolerance of females for males that deviate from this most preferred trait to evolve. Also, we consider situations in which females base their mating decisions on multiple male traits and must interact with males of multiple species. Females will rapidly differentiate in preference when they sometimes mistake heterospecific males for suitable mates, and the differentiation of female preference will select for conspecific male traits to differentiate as well. In most circumstances, this differentiation continues indefinitely, but slows substantially once females are differentiated enough to make mistakes rare. Populations of females with broader preference functions (i.e., broader tolerance for males with trait values that deviate from females' most preferred values) will evolve further to differentiate if the shape of the function cannot evolve. Also, the magnitude of separation that evolves is larger and achieved faster when conspecific males have lower relative abundance. The direction of differentiation is also very sensitive to initial conditions if females base their mate choices on multiple male traits. We discuss how these selection pressures on female mate choice may lead to speciation by generating differentiation among populations of a progenitor species that experiences different assemblages of heterospecifics. Opportunities for differentiation increase as the number of traits involved in mate choice increase and as the number of species involved increases. We suggest that this mode of speciation may have been particularly prevalent in response to the cycles of climatic change throughout the Quaternary that forced the assembly and disassembly of entire communities on a continentwide basis.

Phylogeography of sexualHeteronotia binoei(Gekkonidae) in the Australian arid zone: climatic cycling and repetitive hybridization

The biota of much of continental Australia have evolved within the context of gradual aridification of the region over several million years, and more recently of climatic cycling between relatively dry and humid conditions. We performed a phylogeographical study of three sexual chromosome races of the Heteronotia binoei complex of geckos found throughout the Australian arid zone. Two of these three races were involved in two separate hybridization events leading to parthenogenetic lineages (also H. binoei), and the third is widespread and broadly sympatric with the parthenogens. Based on our analyses, the three sexual races diversified approximately 6 million years ago in eastern Australia, during a period of aridification, then each moved west through northern, southern, and central dispersal corridors to occupy their current ranges. In each case, the timing of major phylogeographical inferences corresponds to inferred palaeoclimatic changes in continental Australia. This scenario provides a simple explanation for diversification, secondary contact, and hybridization between the races. However, data presented elsewhere indicate that formation of the parthenogens was considerably more recent than the westward expansion of the hybridizing races, and that multiple hybridization events were geographically and temporally distinct. We suggest that cyclical climate changes may have led to regional range changes that facilitated hybridization between the races, which are not currently known to be in sympatry.

Effects of Quaternary Climatic Change on Speciation in Mammals

An ongoing controversy in evolutionary biology is the extent to which climatic changes drive evolutionary processes. On the one hand are “Red Queen” hypotheses, which maintain that climatic change is less important than biotic interactions in causing evolutionary change. On the other hand are “Court Jester” models, which recognize climatic change as a very important stimulus to speciation. The Quaternary Period (the last 1.8 million years), characterized by multiple climatic changes in the form of glacial–interglacial transitions, offers a fertile testing ground for ascertaining whether cyclical climatic changes that operate at the 100,000-year time scale appreciably influence evolutionary patterns in mammals. Despite the increased potential for isolation of populations that should occur with multiple advances and retreats of glaciers and rearrangement of climatic zones, empirical data suggests that speciation rates were neither appreciably elevated for Quaternary mammals, nor strongly correlated with glacial–interglacial transitions. Abundant evidence attests to population-level changes within the Quaternary, but these did not usually lead to the origin of new species. This suggests that if climatic change does influence speciation rates in mammals, it does so over time scales longer than a typical glacial–interglacial cycle.

Internal stresses in glulam due to moisture gradients in the grain direction

Abstract An experimental study on the internal stress state perpendicular to grain in glued laminated timber induced by moisture variations along the grain is presented. The stress is evaluated by using measured modulus of elasticity and strain. Different types of tests are performed, in which specimens are exposed to artificial climate change when the specimens are in either a wetting or a drying state, and specimens are exposed to a cyclic climate change. Test results from the specimens that were in a drying state show that the tensile stresses near the end of the specimens at some point exceeds the strength of the wood and cracks. In the opposite test, when the specimens are in a moistening phase, the maximum tensile stresses are lower and in the order of 0.2 MPa during the tested period. The cyclic test, which continued during 20 weeks, showed that there were no accumulated effects regarding strain and stress values. In this test, the average maximum tensile and compression stresses were of the order 0.20 and 0.10 MPa, respectively.

Explaining the global biodiversity gradient: energy, area, history and natural selection

Summary Hubbell’s neutral theory of biodiversity is used to investigate the decline in species richness from the tropics to the poles. On this basis, biodiversity should correlate with productivity or climate (there is strong statistical evidence for this), with the latitudinal width of the continents (insufficiently investigated as yet), and with the speciation rate (which may not vary in such a way as to produce a planetary gradient). According to the neutral, model biodiversity will vary with the area of the ‘‘metacommunity’’: it is suggested that at higher latitudes species disperse most readily east–west, within their climatic belt, but that the relatively uniform temperature across the intertropical belt allows isotropic dispersal there. Metacommunities within the tropics may therefore be an order of magnitude larger than those at other latitudes. This could explain the extra bulge in the gradient in the tropics. It is further possible that long-term and cyclical climate change generates a tropic-pole gradient. Niche assembly models will also explain tropical biodiversity, but the enhanced division of habitat may be the result, not the cause, of the species richness. The neutrality–competition debate in ecology closely parallels the neutrality–natural selection debate in evolution and may be equally hard to resolve. & 2004 Elsevier GmbH. All rights reserved.

Internal stresses in the cross-grain direction in glulam induced by climate variations

Abstract Results are presented from an experimental investigation to determine internal stress states perpendicular to grain in glulam induced by moisture variations. The stresses are determined by measuring the released strains before and after cutting. Stress distributions are determined for specimens seasoned in constant humidity, specimens exposed to an artificial single climate change, specimens exposed to cyclic climate change and for specimens exposed to natural climate outdoors under shelter. Results for seasoned specimens show that internal stresses exist in glulam without the presence of moisture gradients. Stresses in specimens with an induced moisture gradient become larger when moistening from a specific climate A to another climate B than when drying from B to A. Tests in an outdoor sheltered climate show large variations in strains and stresses. The tensile stress level in the inner part of the glulam cross section exceeds the characteristic strength of 0.5 MPa during a period of approximately 80 days.

Holocene climate in the Atlantic sector of the Southern Ocean: Controlled by insolation or oceanic circulation?

ABSTRACTThe Holocene climate of the Southern Ocean is not well un-derstood, mainly because of the lack of high-resolution reconstruc-tions of ocean surface properties. Here we present a 12,500-yr-long,decadal-scale record of Holocene sea-surface temperatures and sea-ice presence from the Polar Front of the East Atlantic SouthernOcean. The record shows gradual climate change, with no abruptNeoglacial cooling, and an unprecedented late Holocene warming.The dominant forcing factor appears to be precessional insolation;Northern Hemisphere summer insolation correlates to at least theearly to middle Holocene climate trend. Spectral analysis revealscentennial-scale cyclic climate changes with periods of 1220, 1070,400, and 150 yr. The record shows good correlation to East Ant-arctic ice cores and to climate records from South Georgia andBunger Oasis. However, the record shows out-of-phase behaviorwith regard to climate records from the western Antarctic Penin-sula and the Peru-Chile Current; such behavior hints at a climaticdivide through Patagonia, the Drake Passage, and between Westand East Antarctica.Keywords: Southern Ocean, paleoclimate, diatoms, modern analogtechnique, Holocene, insolation, Antarctic Polar Front.INTRODUCTIONThe Holocene climate variability has received less attention thanthe more dramatic changes during the last glacial period. The Holoceneis generally assumed to be a stable interglacial period when climatechanges in the North Atlantic are transmitted to the rest of the worldthrough a strong thermohaline circulation (Imbrie et al., 1992). Thelarge fluctuations of the last glacial period appear to continue into thisinterglacial, although in a subdued manner (Bond et al., 2001).The Southern Ocean has received even less attention than the highnorthern latitudes; oceanic records are few and coarse. They generallydisplay a warm early Holocene followed by cooling (Labracherie etal., 1989; Pichon et al., 1992; Hodell et al., 2001), similar to NorthAtlantic climate (e.g., Koc¸ and Jansen, 1994). High-resolution recordsare restricted to Antarctic ice cores, which mostly display little climatevariability during the Holocene (Jouzel et al., 2001). An exception isthe Taylor Dome deuterium record, in which the Holocene appears tobe split by an abrupt cooling event ca. 5.5–5 ka (Steig et al., 1998).This cooling appears to fit the late Holocene global cooling trend (Por-ter, 2000) as a nonlinear response from the climate system to insolationchanges (deMenocal et al., 2000; Hodell et al., 2001).High-resolution Holocene records available for Southern Oceanclimate reconstruction are rarely from the Antarctic Circumpolar Cur-rent proper (e.g., Domack et al., 2001; Stenni et al., 2001; Lamy et al.,2002), with the exception of a multiproxy approach to Holocene cli-mate that includes a high-resolution description of lithic fragments(Hodell et al., 2001).We present data from site TN057-17 near the Polar Front in theEast Atlantic Southern Ocean. It is based on quantitative estimates ofsummer sea-surface temperature (SSST) and sea-ice presence (SIP),utilizing the down-core variability of diatom assemblages. The recordshows good correlation to Antarctic ice cores, but a gradual change inphase relationships. During the early Holocene, the record shows anin-phase relationship with the Ross Sea region ice cores; during thelate Holocene, the record shows an in-phase relationship with the EastAntarctic region ice cores.Antarctic Polar FrontThe Antarctic Polar Front (APF, Fig. 1) is an important climateboundary in terms of air-sea fluxes and the heat and salt budgets ofthe oceans. At the surface, the APF is a steep sea-surface temperaturegradient, while the subsurface expression is the southern limit for sub-duction of cold, fresh Antarctic Surface Water below warmer and moresaline Subantarctic Surface Water (Orsi et al., 1995). Globally, the APFcould be the southern limit of synchrony with Northern Hemisphereclimate (Broecker, 1996). The APF is subject to large spring and sum-mer blooms of diatoms (Fischer et al., 2002), and achieves the largestbiogenic sedimentation rates of the diatom ooze belt of the SouthernOcean (Burckle and Cirilli, 1987). It is these high sedimentation ratesof diatomaceous oozes and the sensitivity of diatoms especially to lightand temperature that make sites in the APF region suitable for pro-ducing high-resolution records of climate change.

Varying depositional environments of gypsum successions in the Upper Miocene Eskisehir-Sivrihisar lacustrine basin, NW Turkiye

The basement rocks of Eskisehir-Sivrihisar basin are made of Upper Paleozoic metamorphic rocks and Upper Cretaceous ophiolite complex. These rocks are overlain by Middle Miocene sedimentary rocks the base. Upper Miocene-Pliocene stratigraphic succession consists of basal conglomerates, sandstones, claystones, limestones, dolomites and tuffs are thinly interbedded with cyclic evaporites. The main clay mineral assemblage consists of smectite, illite and sepiolite that are enriched in different percentage in different horizons. The Upper Miocene-Pliocene stratigraphic succession consists of alterations of gypsum, clayey carbonates and unconsolidated very soft dolomite beds, which are conformably underlain by sepiolite-bearing clayey carbonates and disconformably overlain by alluvium and conglomerate series of Pleistocene age. The Upper Miocene unit includes three different facies, based on sedimentary properties of gypsum from the base to the top; (I) laminated gypsum beds are thinly interbedded with dolomites and green clays; (2) gypsum rosettes within thin reddish and greenish clays and carbonates; and (3) scattered gypsum mainly within carbonates. Gypsums had been deposited in somewhat different depositional conditions in the same basin where multi-periodic evaporative episodes, ranging from saline to schizohaline environments, had been involved. During these episodes, under high Eh-pH intervals and high salinity conditions, three different types of gypsum formations had been developed in a meromictic-type lake under the influence of varying paleoclimatologic conditions and pore water chemistry of brines. All these gypsum morphologies respectively reflect the different conditions of environments of deposition in a Miocene playa lake basin: (1) subaqueous environments in playa lakes with short evaporation episodes; (2) marginal swamps with cyclic fluctuation of lake level; and (3) extended closed-lake with evaporatic lake water. These different gypsum morphologies are associated with distinctive sedimentary subfacies, which are also primarily related to cyclic climatic changes, alternating semi-arid and evaporite conditions, fluctuation of lake water level and sulphate input in the chemistry of lake water from runoff and hydrothermal source adjacent to basin.

The effect of moisture gradients on tensile strength perpendicular to grain in glulam

Experimental results are presented concerning tension perpendicular to grain in glulam sections where internal stresses induced by climate changes are present. The results reveal that moisture induced internal stresses may affect the tension capacity both in a positive and a negative way compared to reference specimens, free from moisture gradients. The tension capacity is reduced during moistening from RH 40% up to RH 80% whereas it is increased during drying from RH 80% to 40%. The tension capacity during wetting is only 50% of that observed for drying specimens. This behaviour can be explained by the combination between initial moisture induced stresses and stresses from external loading. The same tendency was observed for specimens exposed to cyclic climate changes, where it was also found that time and numbers of cycles do not affect the tension capacity in any specific direction.

Earlier spring snowmelt in northern Alaska as an indicator of climate change

Predictions of global circulation models (GCMs) that account for increasing concentrations of greenhouse gases and aerosols in the atmosphere show that warming in the Arctic will be amplified in response to the melting of sea ice and snow cover. There is now conclusive evidence that much of the Arctic has warmed in recent decades. Northern Alaska is one region where significant warming has occurred, especially during winter and spring. We investigate how the changing climate of northern Alaska has influenced the annual cycle of snow cover there and in turn, how changes in snow cover perturb the region's surface radiation budget and temperature regime. The focus is on Barrow, Alaska, for which comprehensive data sets exist. A review of earlier studies that documented a trend toward an earlier disappearance of snow in spring is given. Detection and monitoring activities at Barrow are described, and records of snow disappearance from other sites in the Alaskan Arctic are compared. Correlated variations and trends in the date of final snowmelt (melt date) are found by examining several independent time series. Since the mid‐1960s the melt date in northern Alaska has advanced by ∼;8 days. The advance appears to be a consequence of decreased snowfall in winter, followed by warmer spring conditions. These changes in snowfall and temperature are attributed to variations in regional circulation patterns. In recent decades, there has been a higher frequency of northerly airflow during winter that tends to diminish snowfall over northern Alaska. During spring, however, intrusions of warm moist air from the North Pacific have become more common, and these tend to accelerate the ablation of snow on the North Slope of Alaska. One result of an earlier melt date is an increase in the net surface radiation budget. At Barrow, net radiative forcing can exceed 150 W m−2 on a daily basis immediately following the last day of snowmelt, and as a result of an 8‐day advance in this event, we estimate an increase of ∼2 W m−2 on an annual basis. Our results are in general agreement with earlier analyses suggesting that reductions in snow cover over a large portion of the Arctic on an annual basis have contributed to a warming of the Northern Hemisphere (NH). In addition, the terrestrial ecosystems of the region are very sensitive to snow cover variations. There is growing concern that these perturbations are anthropogenically forced and adapting to these environmental changes will have significant social and economic consequences. While observed decreases in NH snow cover are in broad agreement with GCM simulations, our analyses suggest that internal (or natural) shifts in circulation patterns underlie the observed variations. Continued monitoring and further study is needed to determine whether the earlier disappearance of snow cover in spring in northern Alaska is an indicator of greenhouse‐forced global warming or is a manifestation of a more natural, long‐term cycle of climate change.

Geochemical signature and related climatic-oceanographic processes for early Albian black shales: Site 417D, North Atlantic Ocean

A sediment sequence which may contain the early Albian OAE 1b was investigated in a deep-water (3800m) open-ocean environment at Site 417D, western North Atlantic Ocean. Redox cycles, which contain black shale intervals and occur in the early Albian M. gracilis radiolarian Biozone, were studied in order to show processes and climate-associated controlling factors during the deposition of early Albian sediments. The black shale intervals are characterized by the enhanced accumulation and preservation of marine-derived organic matter as determined by Rock-Eval pyrolysis and organic petrology. The presence of laminated sediments, the relationships between organic carbon, iron and total sulfur, pyrite size analysis and trace-metal enrichment indicate the periodic prevalence of anoxic conditions in the pore waters, which may at times have extended to the sediment/water interface. Changes in the mineralogical composition throughout the black shale-dominated interval, i.e., quartz content and clay-mineral assemblage, resulted in the variation of the major-element chemistry and probably reflect cyclic climatic changes in northern Africa combined with flooding of coastal lowlands during an overall transgressive phase in the early Albian. The geochemical signatures observed at different scales demonstrate a genetic link between the climate system on land and processes in the deep ocean during the deposition of black shales in deep-water environments of the western North Atlantic.

Late Quaternary course changes of the Kuroshio Current in the Ryukyu Arc region, northwestern Pacific Ocean

The existence of a land bridge connecting the central-southern Ryukyu Arc to Taiwan during the last glacial stage (oxygen isotope stages 2 to 4) and its postglacial disappearance are demonstrated by variations in the planktic δ 18O values and the frequency of Pulleniatina in 11 piston cores from the Okinawa Trough region. The upward increase in the Pulleniatina group in the cores signals the progressive inflow of the Kuroshio Current into the Okinawa Trough area, because this group is characteristic of the Kuroshio Current, as revealed here by planktic foraminiferal distribution using 56 surface sediment samples collected from the whole area of this region. The Ryukyu–Taiwan land bridge caused eastward deflection of the Kuroshio Current at the southern end of the Ryukyu Arc. A Pulleniatina minimum event in the period from ∼4500 to 3000 yr B.P. is recognized in 10 cores from the Okinawa Trough and one from the northern Ryukyu Trench slope. In contrast to the LGM, the decline of Pulleniatina in this event was not ascribed to cyclic climate changes such as worldwide cooling but to the formation of a barrier between Taiwan and the southern Okinawa Arc. However, this regional event influences largely on the Holocene climate of Japan and its adjacent seas.