Ice-covered Conditions Research Articles

The impact of low ice cover on shoreline recession: A case study from Western Point Pelee, Canada

The western shore of Point Pelee National Park (PPNP) in Ontario, Canada, exhibited high recession rates in the winter of 2005/2006. During the period 1959–2004, the western side of the park generally exhibited accretion, attributed to longer duration of ice coverage during winter and favorable topography and sedimentary conditions in comparison to the eastern side. Compared to the winter of 2003/2004, changes in Lake Erie water levels, winter wind regime and human-made structures and interferences were negligible during 2004–2006 within the Colchester-to-Southeast Shoal littoral cell in western Lake Erie, where PPNP is located. However, the winter of 2005/2006 exhibited overall low ice cover conditions unlike the typical freeze-up conditions in western Lake Erie. The 2006 high recession rates on the western shore of Point Pelee are attributed to the low ice cover conditions in the winter of 2005/2006. While less-than-average ice cover does not in its own cause erosion, in certain areas it increases the opportunity for sediment removal by waves by removing a significant source of protection during winter. The episode illustrates the potential impacts of ice-free and low ice cover winters on the western shore of Point Pelee and shores under similar conditions. Future climate change patterns are expected to result in shorter ice-durations and more frequent ice-free winters in the western basin of Lake Erie, exposing western PPNP to wave action during the winter season and resulting in increased erosion. The increased opportunity for shore erosion will negatively impact ecologically and economically viable spaces in PPNP.

Storm‐induced upwelling of high pCO2 waters onto the continental shelf of the western Arctic Ocean and implications for carbonate mineral saturation states

The carbon system of the western Arctic Ocean is undergoing a rapid transition as sea ice extent and thickness decline. These processes are dynamically forcing the region, with unknown consequences for CO2 fluxes and carbonate mineral saturation states, particularly in the coastal regions where sensitive ecosystems are already under threat from multiple stressors. In October 2011, persistent wind‐driven upwelling occurred in open water along the continental shelf of the Beaufort Sea in the western Arctic Ocean. During this time, cold (<−1.2°C), salty (>32.4) halocline water—supersaturated with respect to atmospheric CO2 (pCO2 > 550 μatm) and undersaturated in aragonite (Ωaragonite < 1.0) was transported onto the Beaufort shelf. A single 10‐day event led to the outgassing of 0.18–0.54 Tg‐C and caused aragonite undersaturations throughout the water column over the shelf. If we assume a conservative estimate of four such upwelling events each year, then the annual flux to the atmosphere would be 0.72–2.16 Tg‐C, which is approximately the total annual sink of CO2 in the Beaufort Sea from primary production. Although a natural process, these upwelling events have likely been exacerbated in recent years by declining sea ice cover and changing atmospheric conditions in the region, and could have significant impacts on regional carbon budgets. As sea ice retreat continues and storms increase in frequency and intensity, further outgassing events and the expansion of waters that are undersaturated in carbonate minerals over the shelf are probable.

The Effects of Climate Change on Harp Seals (Pagophilus groenlandicus)

Harp seals (Pagophilus groenlandicus) have evolved life history strategies to exploit seasonal sea ice as a breeding platform. As such, individuals are prepared to deal with fluctuations in the quantity and quality of ice in their breeding areas. It remains unclear, however, how shifts in climate may affect seal populations. The present study assesses the effects of climate change on harp seals through three linked analyses. First, we tested the effects of short-term climate variability on young-of-the year harp seal mortality using a linear regression of sea ice cover in the Gulf of St. Lawrence against stranding rates of dead harp seals in the region during 1992 to 2010. A similar regression of stranding rates and North Atlantic Oscillation (NAO) index values was also conducted. These analyses revealed negative correlations between both ice cover and NAO conditions and seal mortality, indicating that lighter ice cover and lower NAO values result in higher mortality. A retrospective cross-correlation analysis of NAO conditions and sea ice cover from 1978 to 2011 revealed that NAO-related changes in sea ice may have contributed to the depletion of seals on the east coast of Canada during 1950 to 1972, and to their recovery during 1973 to 2000. This historical retrospective also reveals opposite links between neonatal mortality in harp seals in the Northeast Atlantic and NAO phase. Finally, an assessment of the long-term trends in sea ice cover in the breeding regions of harp seals across the entire North Atlantic during 1979 through 2011 using multiple linear regression models and mixed effects linear regression models revealed that sea ice cover in all harp seal breeding regions has been declining by as much as 6 percent per decade over the time series of available satellite data.

Ice effects on flow distributions within the Athabasca Delta, Canada

AbstractThe Peace‐Athabasca Delta (PAD) is a unique and ecologically sensitive area made up of many interconnected lakes, wetlands, streams and rivers. The hydrology of the Delta is heavily influenced by inflows from the Athabasca River and the complex river ice regime within it. In this study, a detailed one‐dimensional network hydraulic model of the Athabasca River Delta (comprising the southern portion of the PAD) is presented and applied to demonstrate how variable stream‐flow and ice cover conditions affect the distribution of flow among the major channels conveying water to Lake Athabasca and the rest of the Delta.The current channel network model, based on new measurements concentrated at the main channel junctions combined with carefully designed geometry throughout the un‐surveyed reaches, is shown to provide flow allocations to the major channels that are within 3% of the corresponding observations. At the three main flow‐split junctions, steady flow simulations at varying discharge, Lake Athabasca level, and ice thickness were performed to assess their impact on the percentage of flow carried by each channel. Each junction was found to exhibit a unique response arising from its physical geometry and location within the Delta.Transient simulations for the historical period 1960–2007 demonstrate that water demand on the Athabasca River upstream of the PAD may have a significant impact on the availability of fish habitat in winter. Imposing a 20 m3/s water demand increased the total occurrences of a no‐flow condition in the Fletcher Channel, analogous to it freezing to the bed, by 53%. Copyright © 2010 John Wiley & Sons, Ltd.

Seasonal and interannual variability of sedimentation and organic matter distribution in the Buor-Khaya Gulf: the primary recipient of input from Lena River and coastal erosion in the southeast Laptev Sea

Abstract. Climate warming is amplified in the land-sea system of the East Siberian Arctic, which also holds large pools of vulnerable carbon in permafrost. This coastal area is strongly influenced by sediment and carbon transport from both its large rivers and extensive erosion of Pleistocene permafrost along its coastline. This study is investigating the coastal fate of the sediment and organic carbon delivered to the Buor-Khaya Gulf, which is the first recipient of the overwhelming fluvial discharge from the Lena River and is additionally receiving large input from extensive erosion of the coastal ice-complex (permafrost a.k.a. Yedoma; loess soil with high organic carbon content). Both water column suspended particulate matter (SPM) and surface sediments were sampled at about 250 oceanographic stations in the Gulf in this multi-year effort, including one winter campaign, and analyzed for the distribution and sorting of sediment size, organic carbon content, and stable carbon isotope signals. The composition of the surface sediment suggests an overwhelmingly terrestrial contribution from both river and coastal erosion. The objective of this paper is to improve our understanding of the seasonal (i.e., winter vs summer) and interannual variability of these coastal sedimentation processes and the dynamics of organic carbon (OC) distribution in both the water column SPM and the surface sediments of the Buor-Khaya Gulf. Based on data collected during several years in the period 2000–2008, two different sedimentation regimes were revealed for the Buor-Khaya Gulf, the relative importance of each at a given time depend on hydrometeorological conditions, the Lena River water discharge and sea-ice regime: Type 1 erosion-accumulation and Type 2 accumulation. The Type 1 erosion-accumulation sedimentation regime is typical (2000–2006) for the ice-free period of the year (here considered in detail for August 2005). Under such conditions terrigenous sources of SPM and particulate organic carbon (POC) stem predominantly from river discharge, thermal erosion of coastal ice-complex and remobilized bottom sediments. The Type 2 accumulation sedimentation regime develops under ice-covered conditions, and only occasionally during the ice-free period (August 2008). In Type 2 winter, combined terrigenous and marine-biogenic SPM and POC sources are dominating due to relatively low overall terrigenous input (April 2007). In Type 2 summer, river alluvium becomes the major SPM and POC source (August 2008). The water column SPM and POC loadings vary by more than a factor of two between the two regimes. This study underscores the necessity of multi-year investigations to better understand the functioning of the primary recipient of terrestrially expulsed matter in the East Siberian Arctic.

Spring-to-summer changes and regional variability of benthic processes in the western Canadian Arctic

Seasonal dynamics in the activity of Arctic shelf benthos have been the subject of few local studies, and the pronounced among-site variability characterizing their results makes it difficult to upscale and generalize their conclusions. In a regional study encompassing five sites at 100–595 m water depth in the southeastern Beaufort Sea, we found that total pigment concentrations in surficial sediments, used as proxies of general food supply to the benthos, rose significantly after the transition from ice-covered conditions in spring (March–June 2008) to open-water conditions in summer (June–August 2008), whereas sediment Chl a concentrations, typical markers of fresh food input, did not. Macrobenthic biomass (including agglutinated foraminifera >500 μm) varied significantly among sites (1.2–6.4 g C m−2 in spring, 1.1–12.6 g C m−2 in summer), whereas a general spring-to-summer increase was not detected. Benthic carbon remineralisation also ranged significantly among sites (11.9–33.2 mg C m−2 day−1 in spring, 11.6–44.4 mg C m−2 day−1 in summer) and did in addition exhibit a general significant increase from spring-to-summer. Multiple regression analysis suggests that in both spring and summer, sediment Chl a concentration is the prime determinant of benthic carbon remineralisation, but other factors have a significant secondary influence, such as foraminiferan biomass (negative in both seasons), water depth (in spring) and infaunal biomass (in summer). Our findings indicate the importance of the combined and dynamic effects of food supply and benthic community patterns on the carbon remineralisation of the polar shelf benthos in seasonally ice-covered seas.

Transverse Mixing in an Unregulated Northern River

Transverse mixing in an unregulated northern river in Canada was studied in different discharge conditions with and without ice cover. The distribution of cumulative discharge was constructed on the basis of river cross-sectional shapes, and a modified streamtube method was proposed to describe transverse mixing. The modified method only uses raw field data to calibrate the modeled concentration profiles and thus, can produce a reliable mixing coefficient even with relatively low-quality field data. The effects of river discharge and ice cover on the transverse mixing coefficient were examined in a fixed study reach. It was found that the transverse mixing coefficient increased approximately linearly with river discharge (from 84 to 960 m3/s). The dimensionless transverse mixing coefficient, scaled with river shear velocity and river depth, appeared unaffected (within 14%) by the river discharge in both open-water and ice-covered conditions, and its value was 21% smaller in the ice-covered condition than in the open-water condition.

Biodiversity change after climate-induced ice-shelf collapse in the Antarctic

The marine ecosystem on the eastern shelf of the Antarctic Peninsula was surveyed 5 and 12 years after the climate-induced collapse of the Larsen A and B ice shelves. An impoverished benthic fauna was discovered, that included deep-sea species presumed to be remnants from ice-covered conditions. The current structure of various ecosystem components appears to result from extremely different response rates to the change from an oligotrophic sub-ice-shelf ecosystem to a productive shelf ecosystem. Meiobenthic communities remained impoverished only inside the embayments. On local scales, macro- and mega-epibenthic diversity was generally low, with pioneer species and typical Antarctic megabenthic shelf species interspersed. Antarctic Minke whales and seals utilised the Larsen A/B area to feed on presumably newly established krill and pelagic fish biomass. Ecosystem impacts also extended well beyond the zone of ice-shelf collapse, with areas of high benthic disturbance resulting from scour by icebergs discharged from the Larsen embayments.

Long-term trends in water temperature and ice cover in the subalpine lake, Øvre Heimdalsvatn, and nearby lakes and rivers

Long-term data series of ice cover on lakes and river temperatures from the mountain areas of Norway are lacking. The present study analyses the last four decades of ice data from the subalpine lake, Ovre Heimdalsvatn, and water temperature data from its outlet river, Hinogla. These data are compared to water temperature data from three neighbouring, quite different locations, the glacier-fed rivers Leirungsai and Sjoa, and the alpine lake, Bessvatn. The study also examines the air temperature/river temperature relationships, and the air temperature/ice freeze-up and break-up dates. During the months of July, August and September, the water temperature in Hinogla was well correlated to the air temperature, but the correlation was poor in the remaining months due to the ice cover and snow conditions. A significant temperature increase of 2-3°C has been observed in Hinogla in the months August-October since 1984. There were only minor changes in the duration of the ice cover season during the last 40 years, but a delay of 9 days was found in the freeze-up date and a delay of 6 days in the break-up date, although the latter was not significant.

Impact of climate change on the hydrology of St. Lawrence tributaries

Changes in temperature and precipitation projected for the next century will induce important modifications into the hydrological regimes of the St. Lawrence tributaries (Quebec, Canada). The temperature increase anticipated during the winter and spring seasons will affect precipitation phase and consequently the snow/precipitation ratio and the water volume stored into snow cover. The impact on northern river hydrology and geomorphology will be significant. In this study we aim to assess the magnitude of the hydrological alteration associated with climate change; to model the projected temporal shift in the occurrence of winter/spring center-volume date; to assess the sensitivity of the winter/spring center-volume date to changes in climatic variables and to examine the latitudinal component of the projected changes through the use of five watersheds on both shores of the St. Lawrence. The study emphasizes changes in the winter and spring seasons. Projected river discharges for the next century were generated with the hydrological model HSAMI run with six climate series projections. Three General Circulation Models (HadCM3, CSIRO-Mk2 and ECHAM4) and two greenhouse gas emissions scenarios (A2 and B2) were used to create a range of plausible scenarios. The projected daily climate series were produced using the historical data of a reference period (1961–1990) with a perturbation factor equivalent to the monthly mean difference (temperature and precipitation) between a GCM in the future for three 30 year horizons (2010–2039, 2040–2069; 2070–2099) and the reference period. These climate projections represent an uncertainty envelope for the projected hydrologic data. Despite the differences due mainly to the GCM used, most of the hydrological simulations projected an increase in winter discharges and a decrease in spring discharges. The center-volume date is expected to be in advance by 22–34 days depending on the latitude of the watershed. The increase in mean temperature with the simultaneous decrease of the snow/precipitation ratio during the winter and spring period explain a large part of the projected hydrological changes. The latitude of the river governed the timing of occurrence of the maximum change (sooner for tributaries located south) and the duration of the period affected by marked changes in the temporal distribution of discharge (longer time scale for rivers located at higher latitudes). Higher winter discharges are expected to have an important geomorphological impact mostly because they may occur under ice-cover conditions. Lower spring discharges may promote sedimentation into the tributary and at their confluence with the St. Lawrence River. The combined effects of modifications in river hydrology and geomorphological processes will likely impact riparian ecosystems.

The Potential for Using Little Diomede Island as a Platform for Observing Environmental Conditions in Bering Strait

The Pacific waters that enter the Arctic via the Bering Strait exert a major influence on the Arctic Ocean’s stratification, ice cover, and ecosystem. We demonstrate the potential of a shore-based laboratory to monitor the water masses that flow predominantly northward past Little Diomede Island in the center of the Bering Strait into the Arctic Ocean. We determined near-surface water column salinity, inorganic nutrient concentrations, natural fluorescence associated with chlorophyll, and the oxygen isotope composition of seawater, both in summer during the open-water period and in late winter under ice-covered conditions, by pumping ashore water from shallow depths near the island. Additional surveys were undertaken within 5 km of the island to assess the influence of local sources of nutrients. Water mass variability was much greater during the open-water period than under ice-covered conditions, presumably because the relatively immobile ice cover attenuates wind forcing and the decrease in run-off reduces cross-shelf gradients. The mean oxygen isotope composition of the summer (?18O = -1.11‰) and late winter (?18O = -0.98) collections, however, was close to that which has been established for Bering Sea waters in the Pacific-dominated upper halocline of the Arctic Ocean (-1.1‰) particularly considering the higher seasonal flow of runoff in the summer. A comparison with data from shipboard sampling at various locations across the Bering Strait indicates that the oxygen isotope composition of near-surface water sampled at Diomede varies in response to wind-forcing. If the least saline (< 30.5) water near the Alaska coast is excluded, the ?18O values of Diomede and shipboard samples cannot be distinguished statistically. This similarity suggests that the water sampled from the island also reasonably represents the ?18O value of Bering Sea waters that contribute to the upper halocline of the Arctic Ocean. Effects of benthic recycling, human activity, and seabird nesting on nutrient concentrations appeared to be concentrated within ~200 m of the island. Our results are discussed in the practical context of availability of electricity, interested local residents, and a geotechnical study indicating that it is feasible to construct and operate a more permanent undersea water intake system to improve environmental observation capabilities in the Bering Strait region.

History of the Antarctic Peninsula Ice Sheet since the early Pliocene—Evidence from cosmogenic dating of Pliocene lavas on James Ross Island, Antarctica

Knowledge of the thickness, extent and basal thermal conditions of ice cover on Antarctica during past climatic fluctuations is essential if we are to accurately predict the contribution from the Antarctic ice sheets to future global sea level rise. When combined with geomorphological evidence, terrestrial cosmogenic nuclides are a powerful tool for palaeo-ice sheet reconstructions. Here we present results from the first terrestrial cosmogenic nuclide study on James Ross Island, northern Antarctic Peninsula. We measured cosmogenic 3He concentrations in primary flow surface features of Pliocene basalt lavas which are exposed more than 600 m above present-day sea level. Our results suggest that the lava surfaces have been ice-free for no more than 15 kyr since their eruption at 4.69 Ma. This implies that a glacial cover has persisted on James Ross Island since 4.69 Ma, even during interglacial periods, and much (if not all) of the warm Pliocene (5–3 Ma, when average global temperatures were up to 3 °C higher than today). This is consistent with results of recent modelling studies, but contradicts General Circulation Models that suggest an ice-free Antarctic Peninsula during that time. Field observations of striated lava surfaces found adjacent to primary volcanogenic features, such as ropy textures, combined with cosmogenic 3He data suggest that the ice cover was locally wet-based, and relatively thin (not exceeding 45–200 m) for the majority of the past 4.69 Myr. The ice temporarily reached a thickness sufficient to cause some localised erosion in hollows, but this erosion was not widespread. Although the pristinely-preserved lavas found on upstanding tumuli appear never to have been eroded, our exposure age data show that they were covered by ice for most of their history, and therefore could not have been refugia for terrestrial organisms during glacial periods.

3D modelling of ice-covered flows in the vicinity of a deep hole in the East Channel of the Mackenzie Delta, N.W.T.

The discovery of vast quantities of hydrocarbons during exploration of the Mackenzie Delta has led to the detection of numerous anomalous deep holes in various delta channels in the past four decades. The proposed pipeline crossing from the Mackenzie Delta to northern Alberta has renewed interest in studying the stability of these deep holes. The main goal of this research project was to study ice-covered flow velocity and bed shear stress distributions using a previously calibrated computational fluid dynamics (CFD) model for a 30 m deep hole in the East Channel of the Mackenzie Delta to provide an initial assessment of its stability. In this study, a previously developed 3D CFD model using the FLUENT code with the renormalization group theory (RNG) k-ε turbulence closure model with nonequilibrium wall functions was adapted to represent ice-covered flow conditions for the study reach. The numerical model was applied to simulate four flow conditions (500, 720, and 1000 m3/s along with a moving ice cover during a flow rate of 720 m3/s). Results indicated that a single vortex was formed near the inner bend above the hole and deposition may be a key process occurring during ice-covered flow conditions.

The relevance of the sedimentary history of the Grand Conglomerat Formation (Central Africa) to the interpretation of the climate during a major Cryogenian glacial event

The Cryogenian Grand Conglomerat Formation (<765 and >735 Ma in age and with considerable thickness variations up to a maximum thickness of about 950 m) is an association of interbedded glaciogenic, clastic, periglacial and non-glacial deposits topped by the Kakontwe Limestone cap carbonate. These units occur within the Nguba Group of the Katanga Supergroup, which was deposited in a rift basin and subsequently deformed as part of the Pan African Lufilian orogenic belt that separates the Congo and Kalahari cratons. Both, correlation of regional unconformities and facies distribution suggest that the Grand Conglomerat sediments were deposited (during and after eruption of flood basalts) in an asymmetrical rift, with a strongly uplifted southern shoulder, and a graded shelf defining the northern margin. Glaciomarine sedimentary rocks along the southern margin of the Katangan rift are preserved within fan delta conglomerates supplied from an elevated rift shoulder. By contrast, the northern margin of the rift was the site of continental glaciation with cross-bedded, glaciofluvial and marginal marine sandstones and conglomerates, associated with massive tills (glaciogenic diamictites) that pass laterally towards the south into glaciomarine mixtite interlayered with wedges of dolomitic sandstone. The Kakontwe Limestone cap carbonate is present only in the distal parts of the basin. Its absence in proximal regions is considered to reflect very high rates of sedimentation of fine-grained, glaciogenic debris derived from deglaciated source areas. Palaeomagnetic data indicates that the Grand Conglomerat glaciogenic sediments were deposited close to the Equator during the Cryogenian. This low latitude setting coupled with the absence of a topographical trigger would suggest that glaciation was related to global atmospheric cooling. However, the presence of water-borne, glaciogenic, on-shore sediments and offshore sediments derived from floating glaciers suggests that the ocean during this part of the Cryogenian was not completely frozen. Associations of glaciogenic facies with non-glaciogenic sedimentary rocks imply glaciation with interglacial periods and gradual deglaciation, instead of severe conditions of permanent sea ice cover and rapid change to the greenhouse environment.

Effects of freezing, growth, and ice cover on gas transport processes in laboratory seawater experiments

Gas exchange through sea ice is a determining factor in the polar ocean budget of climatically‐active gases. We use SF6 and O2 as conservative gas tracers to observe transport between the water, ice and air during conditions of freezing and partial ice cover in artificial seawater. During ice growth, O2 and SF6, as non‐polar solutes, were rejected from the ice into the underlying water at a faster rate than that observed for salt. Measurements of the gas exchange rate, k, through partial ice cover exceeded that expected from linear scaling between 100% open water (k100%) and complete ice cover: at 15% open water, k was 25% of k100%. These results indicate that the net flux of gas through the ice pack may not scale linearly with open water area, as circulation processes under the ice affect the gas exchange rate.

Microbial Mn(IV) and Fe(III) reduction in northern Barents Sea sediments under different conditions of ice cover and organic carbon deposition

Carbon oxidation rates and pathways were determined in two sediments at latitude 751 and 771N southeast of Svalbard in the northern Barents Sea. Seasonal ice cover restricts primary production to few months a year, which determines the sedimentation rate of organic material to the seafloor. At one station, with seasonally extended ice cover, low organic carbon content and sedimentation rate combined with relatively high concentrations of Mn and Fe(III) oxides favored dissimilatory Fe and Mn reduction (98% of anaerobic carbon oxidation) over sulfate reduction in the top 12 cm of the sediment. In contrast, in a sediment that had not been ice covered for at least 12 months and with more organic carbon and a higher sedimentation rate, sulfate reduction was the most important anaerobic electronaccepting process (480% of anaerobic carbon oxidation). In the upper 3 cm, microbial Fe and sulfate reduction occurred simultaneously, and sulfate reduction dominated at 3–12 cm. Oxygen consumption rates (1.9 and 3.7 mmol m � 2 d � 1 ) and anaerobic CO2 production rates (1.3 and 6.4 mmol m � 2 d � 1 ) of both

The Influence of Ice Cover on Two Lake-Effect Snow Events over Lake Erie

Abstract It is generally understood that extensive regions of significant lake ice cover impact lake-effect (LE) snow storms by decreasing the upward heat and moisture fluxes from the lake surface; however, it is only recently that studies have been conducted to more thoroughly examine this relationship. This study provides the first examination of Great Lakes LE snow storms that developed in association with an extensively ice-covered lake. The LE snow events that occurred downwind of Lake Erie on 12–14 February 2003 and 28–31 January 2004 produced maximum snowfall totals of 43 and 64 cm in western New York state, respectively. The presence of widespread ice cover led these snows to be less anticipated than snowfalls from Lake Ontario, which had limited ice cover. For both events, a variety of ice-cover conditions and meso- and synoptic-scale factors (i) helped support LE snow storm development, (ii) lead to the transitions in LE convective type, and (iii) resulted in noteworthy snowfalls near Lake Erie. Thinner ice cover along with favorable fetch directions during the 2004 event likely aided the development of more significant snowband time periods and the resulting greater snowfall. Although Lake Erie had regions with lower ice concentration during the 2003 event, thicker ice cover was present across a greater area of the lake, fetch directions during lake-effect time periods were positioned over higher ice concentration regions, and snowbands had a shorter duration and impacted the same region to a lesser degree than the 2004 case.

Winter phytoplankton communities in different depths of three mesotrophic lakes (Łęczna-Włodawa Lakeland, Eastern Poland)

AbstractPhytoplankton samples were collected from three mesotrophic lakes: Piaseczno, Rogóźno and Krasne during winter seasons (from January to March). The samples were analyzed for species analysis and abundance of planktonic algae in relation to different depths of water column (0–7 m). Selected water physical-chemical parameters were also measured. Abundance of phytoplankton depended strongly on the thickness of snow and ice cover or mixing conditions. The maximal phytoplankton total number reached about 5 × 106 ind. L−1 beneath the clear ice in the Krasne Lake, minimal numbers were recorded under the thick snow and ice layers in the Piaseczno Lake (2 × 103 ind. L−1). The winter phytoplankton communities were dominated by flagellates principally cryptomonads (Cryptomonas spp. Rhodomonas minuta), euglenophytes (Trachelomonas volvocina, T. volvocinopsis), dinoflagellates (Peridinium bipes, Gymnodinium helveticum) and chrysophytes (Mallomonas elongata, M. akrokomos, Dinobryon sociale) or non-motile small species of blue-green algae (e.g. Rhabdoderma lineare, Limnothrix redekei), diatoms (Stephanodiscus spp., Asterionella formosa), and green algae (e.g. Scenedesmus spp., Monoraphidium spp.). Phytoplankton abundance and structure showed differentiation during the winter season and along the water column as well.

炭素循環から考える新生代のグローバル気候変動

The global carbon cycle controls the climate change in the Earth's environment on a geological timescale and is mainly associated with greenhouse effects produced by atmospheric carbon dioxide (CO2) and methane (CH4). This paper reviews the relationship between the global carbon cycle and presumed climate events during the Cenozoic. The global carbon cycle is primarily regulated by the balance between weathering and metamorphism-volcanism. Moreover, the organic carbon subcycle involving oxidative weathering and burial is of secondary importance. The balance of these geochemical processes results in variations of atmospheric CO2. The past climate on a geological time scale is reconstructed by several geochemical and paleontological methods or proxies. For example, sea-surface and deep-water temperature are deduced from oxygen isotope ratio and Mg/Ca ratio of foraminiferal tests. Terrestrial atmospheric temperature is estimated from leaf fossil and paleovegetation. Atmospheric CO2 level is calculated from carbon isotope ratios of phytoplankton and soil carbonate, stomatal density of leaf fossil, boron isotope ratio of foraminiferal test, Ce anomaly, and global carbon cycle modeling. It is important to consider their advantages and disadvantages in order to evaluate the paleoclimate adequately. Next, we discuss climate change based on these proxies. As a general trend, the Cenozoic climate change is characterized by a transition from ice-free to ice-covered conditions across the Eocene/Oligocene boundary. The Earth's surface environment was significantly warmed from the Paleocene to the Eocene by high levels of atmospheric CO2. Thereafter, it gradually cooled towards the present, which is possibly attributed to changes in ocean currents and other marine environments accompanying continental drift. This trend has been punctuated by several short-term climate events. The Paleocene-Eocene Thermal Maximum (PETM) was a remarkable warming event at the Paleocene/Eocene boundary, possibly attributed to the release of methane from hydrates into the atmosphere. Rapid cooling occurred at the Eocene/Oligocene boundary to form extensive continental ice sheets including the Antarctica, which seems to have been caused by atmospheric CO2 and change of oceanographic circulation and marine environment. After a moderate period from the late Oligocene to the early Miocene, there was a transient but significant warming in the middle Miocene. Since then, the Earth's environment has gradually cooled towards the present accompanied by the evolution of glaciations and marine environmental changes but a causal link between cooling and global carbon cycle has recently been pointed out. Although the carbon cycle including atmospheric CO2 and CH4 cannot explain all of the global climate changes in Cenozoic, it has undoubtedly played a dominant role on the Earth's climate.

Unraveling complex exposure-burial histories of bedrock surfaces under ice sheets by integrating cosmogenic nuclide concentrations with climate proxy records

The production, accumulation, and decay of cosmogenic radionuclides in rock surfaces subjected to episodes of exposure and burial by ice results in nuclide concentrations in present day rock surfaces that can be used to address a variety of questions in glacial geomorphology and Quaternary geology. Of particular importance is the fact that these nuclide concentrations reflect both the timing of initial exposure of the rock surface and the chronology of subsequent exposure, burial, and erosion episodes. For landscapes where geomorphic evidence indicates that little/no erosion occurred, constraining the timing of initial exposure and the number of phases of exposure and burial that a rock surface has been subjected to is possible using multiple cosmogenic radionuclide concentrations combined with proxies for the timing and duration of periods of ice cover, such as ice core or marine isotope records. However, interpretations based on this approach require determination of an appropriate cutoff value to separate the proxy record into ice-free and ice-covered conditions and assessment of the sensitivity of the results to different cutoff values. We have developed a numerical model to evaluate variations in total exposure and burial durations as a function of different proxy records and cutoff values. This program is available at http://www.missouri.edu/~liyk/ClimateProxyCurve.zip. Initial results for sites in West Antarctica and northern Sweden show that the method provides a quick and robust way to derive best-fit cutoff values and chronologies of burial and exposure, and small changes in cutoff values can result in significant shifts in results. The method described here provides new insight into the interpretation and reliability of multiple nuclide samples. This approach also has the potential to provide improved constraints for ice sheet dynamics and landscape evolution, and a means to assess the sensitivity of calculated initial exposure dates to assumptions about ice sheet history.