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1.
Sea-ice and water samples were collected at 14 stations on the shelves and slope regions of the Chukchi and Beaufort Seas during the spring 2002 expedition as part of the Shelf–Basin Interaction Studies. Algal pigment content, particulate organic carbon and nitrogen, and primary productivity were estimated for both habitats based on ice cores, brine collection and water samples from 5-m depth. The pigment content (0.2–304.3 mg pigments m−2) and primary productivity (0.1–23.0 mg C m−3 h−1) of the sea-ice algae significantly exceeded water-column parameters (0.2 and 1.0 mg pigments m−3; <0.1–0.4 mg C m−3 h−1), making sea ice the habitat with the highest food availability for herbivores in early spring in the Chukchi and Beaufort Seas. Stable isotope signatures for ice and water samples did not differ significantly for δ15N, but for δ13C (ice: −25.1‰ to −14.2‰; water: −26.1‰ to −22.4‰). The analysis of nutrient concentrations and the pulse-amplitude-modulated fluorescence signal of ice algae and phytoplankton indicate that nutrients were the prime limiting factor for sea-ice algal productivity. The estimated spring primary production of about 1–2 g C m−2 of sea-ice algae on the shelves requires the use of substantial nutrient reservoirs from the water column.  相似文献   

2.
《Marine Chemistry》2006,98(2-4):210-222
This study presents concentrations of dimethylsulphide (DMS) and its precursor compound dimethylsulphoniopropionate (DMSP) in a variety of sea ice and seawater habitats in the Antarctic Sea Ice Zone (ASIZ) during spring and summer. Sixty-two sea ice cores of pack and fast ice were collected from twenty-seven sites across an area of the eastern ASIZ (64°E to 110°E; and the Antarctic coastline north to 62°S). Concentrations of DMS in 81 sections of sea ice ranged from < 0.3 to 75 nM, with an average of 12 nM. DMSP in 60 whole sea ice cores ranged from 25 to 796 nM and showed a negative relationship with ice thickness (y = 125x− 0.8). Extremely high DMSP concentrations were found in 2 cores of rafted sea ice (2910 and 1110 nM). The relationship of DMSP with ice thickness (excluding rafted ice) suggests that the release of large amounts of DMSP during sea ice melting may occur in discrete areas defined by ice thickness distribution, and may produce ‘hot spots’ of elevated seawater DMS concentration of the order of 100 nM. During early summer across a 500 km transect through melting pack ice, elevated DMS concentrations (range 21–37 nM, mean 31 nM, n = 15) were found in surface seawater. This band of elevated DMS concentration appeared to have been associated with the release of sea ice DMS and DMSP rather than in situ production by an ice edge algal bloom, as chlorophyll a concentrations were relatively low (0.09–0.42 μg l− 1). During fast ice melting in the area of Davis station, Prydz Bay, sea ice DMSP was released mostly as extracellular DMSP, since intracellular DMSP was negligible in both hyposaline brine (5 ppt) and in a melt water lens (4–5 ppt), while extracellular DMSP concentrations were as high as 149 and 54 nM, respectively in these habitats. DMS in a melt water lens was relatively high at 11 nM. During the ice-free summer in the coastal Davis area, DMS concentrations in surface seawater were highest immediately following breakout of the fast ice cover in late December (range 5–14 nM), and then remained at relatively low concentrations through to late February (< 0.3–6 nM). These measurements support the view that the melting of Antarctic sea ice produces elevated seawater DMS due to release of sea ice DMS and DMSP.  相似文献   

3.
《Marine Geology》2001,172(1-2):75-89
We have generated a detailed oxygen isotope time-series from the fossil coral reefs from offshore Barbados. The Barbados coral-based record is a unique paleoceanographic data set with an equivalent sedimentation rate in excess of 600 cm/kyr where not only is the annual signal uniquely preserved but seasonality as well. Oxygen isotope values during the late glacial and LGM (16–20 14C kyrs; 19–24 calendar kyrs) are ∼2.3‰ heavier than corresponding living specimens, and indicate a regional cooling on the order of 4.5°C. There is also an isotopic expression of the Bølling-Pre-Boreal climate oscillation, with values reflecting a cooling during the Younger Dryas. Seasonality, a key diagnostic of the state of the climate system, remained the same or slightly less than present and indicates that the observed coolings were a change in the mean state of the western tropical Atlantic. Pan-tropic cooling during the last glaciation is best explained by a change in the radiative balance of the tropics. Variable tropical sea surface temperatures during climate oscillations such as the Younger Dryas challenge the paradigm that climate change only cascades from variations in North Atlantic deep water production.  相似文献   

4.
The Arctic Ocean has wide shelf areas with extensive biological activity including a high primary productivity and an active microbial loop within the surface sediment. This in combination with brine production during sea ice formation result in the decay products exiting from the shelf into the deep basin typically at a depth of about 150 m and over a wide salinity range centered around S ～33. We present data from the Beringia cruise in 2005 along a section in the Canada Basin from the continental margin north of Alaska towards the north and from the International Siberian Shelf Study in 2008 (ISSS-08) to illustrate the impact of these processes. The water rich in decay products, nutrients and dissolved inorganic carbon (DIC), exits the shelf not only from the Chukchi Sea, as has been shown earlier, but also from the East Siberian Sea. The excess of DIC found in the Canada Basin in a depth range of about 50–250 m amounts to 90±40 g C m?2. If this excess is integrated over the whole Canadian Basin the excess equals 320±140×1012 g C. The high DIC concentration layer also has low pH and consequently a low degree of calcium carbonate saturation, with minimum aragonite values of 60% saturation and calcite values just below saturation. The mean age of the waters in the top 300 m was calculated using the transit time distribution method. By applying a future exponential increase of atmospheric CO2 the invasion of anthropogenic carbon into these waters will result in an under-saturated surface water with respect to aragonite by the year 2050, even without any freshening caused by melting sea ice or increased river discharge.  相似文献   

5.
Surface concentrations and vertical fluxes of particulate organic carbon (POC) were assessed in the Amundsen Gulf (southeastern Beaufort Sea, Arctic Ocean) over the years 2004 to 2006 by using ocean color remote-sensing imagery and sequential sediment traps moored over the ca. 400 m isobath. Environmental conditions (sea ice, wind) and oceanographic variables (temperature, salinity, fluorescence and currents) were investigated to explain the variability of POC data. Annual downward POC fluxes in 2004, 2005 and 2006 cumulated, respectively, to 3.3, 4.2 and 6.0 g C m?2 yr?1 at ～100 m depth, and to 1.3, 2.2 and 3.3 g C m?2 yr?1 at ～210 m depth. The fraction of settling POC attributable to autochthonous processes occurring at or next to ice break-up was estimated to be 75–84% of the 100 m annual fluxes and to be 61–75% of the 210 m fluxes. Over the three ice-reduced seasons, distinct scenarios between ice conditions, surface POC pools and vertical POC export at 100 m were identified: (1) in 2004, despite a normal ice break-up, a weak primary production was measured and low vertical fluxes were collected as old ice moved across the region; (2) in 2005, a lengthened ice-free period allowed an extended season of surface POC production near-shore, while an intermediate increase of vertical fluxes was recorded offshore; and (3) in 2006, a late ice melt gave rise to a pulsed ice edge bloom and to large vertical fluxes also associated with extra ice-flushed material. Linear regressions of vertical POC fluxes against satellite-derived surface POC concentrations suggested that the pelagic POC retention in the upper 100 m of the Amundsen Gulf ranged from ca. 70% to 90% depending on the timing of ice cover melt. Regardless of the inter-annual variability, the estimated fraction of the surface POC reservoir reaching the 210 m water depth was reduced to ～5%. Therefore, as the Arctic Ocean warms up, our results support the expectation that the increasing extent of the seasonal ice zone will promote the POC pathways that benefit pelagic webs rather than benthic communities.  相似文献   

6.
Between November 2001 and March 2002 an Australian/Japanese collaborative study completed six passes of a transect line in the Seasonal-Ice Zone (south of 62°S) along 140°E. Zooplankton samples were collected with a NORPAC net on 22–28 November, and a Continuous Plankton Recorder on 10–15 January, 11–12 February, 19–22 February, 25–26 February, and 10–11 March. Zooplankton densities were lowest on 22–28 November (ave=61 individuals (ind) m−3), when almost the entire transect was covered by sea ice. By 10–15 January sea surface temperature had increased by ∼2 °C across the transect line, and the study area was ice-free. Total zooplankton abundance had increased to maximum levels for the season (ave=1301 ind m−3; max=1979 ind m−3), dominated by a “Peak Community” comprising Oithona similis, Ctenocalanus citer, Clausocalanus laticeps, foraminiferans, Limacina spp., appendicularians, Rhincalanus gigas and large calanoid copepodites (C1–3). Total densities declined on each subsequent transect, returning to an average of 169 ind m−3 on 10–11 March. The seasonal density decline was due to the decline in densities of “Peak Community” taxa, but coincided with the rise of Euphausia superba larvae into the surface waters, increased densities of Salpa thompsoni, and an increased contribution of C4 to adult stages to the populations of Calanoides acutus, Calanus propinquus and Calanus simillimus. The seasonal community succession appeared to be influenced by the low sea ice extent and southward projection of the ACC in this region. The relatively warm ACC waters, together with low krill biomass, favoured high densities of small grazers during the January/February bloom conditions. The persistence of relatively warm surface waters in March and the seasonal decrease in chlorophyll a biomass provided favorable conditions for salps, which were able to penetrate south of the Southern Boundary.  相似文献   

7.
Vertically integrated heat content from the Weddell Sea along 69 °S in vicinity of the Greenwich meridian from the 1977–2001 period is presented. This demonstrates that the east-west variability in the area is lower than the natural variability. The sensitivity on the choice of lower boundary for the heat content integral is discussed in relation to upwelling from the deeper water masses. The cooling around Maud Rise during the 1990's on the order of 20 W/m2 remains significant, and may be explained by increased surface heat fluxes during winter months with lower than normal sea ice concentration.  相似文献   

8.
During two cruises to the Greenland Sea, we studied the abundance and biomass of the sea ice biota in summer and late autumn. The mean calculated biomass of the sympagic community was 0.2 g C m−2 ice. Algae contributed on average 43% to total biomass, followed by bacteria (31%), heterotrophic flagellates (20%), and meiofauna (4%). Diatoms were the main primary producers (60% of total algal biomass), but flagellated cells contributed significantly to the algal biomass. Among the meiofauna, ciliates, nematodes, acoel turbellarians and crustaceans were dominant. Calculated potential ingestion rates of meiofauna (0.6 g C m−2 (120 d)−1) are on the same order of magnitude as annual primary production estimates for Arctic multi-year sea ice. We therefore assume that grazing can control biomass accumulation of primary producers inside the sea ice.  相似文献   

9.
Temperature, salinity and chlorofluorocarbons (CFCs) 11, 12 and 113 were measured on a line of stations along the front of the Ross Ice Shelf in the austral summers of 1984, 1994 and 2000. Water mass distributions were similar each year but with high variability in the cross-sectional areas. CFC concentrations increased and salinity decreased with time throughout the water column. CFC saturation levels in the shelf and surface waters also increased with time and ranged from 43% to 90%. The undersaturation was due to inflow of low-CFC modified Circumpolar Deep Water, gas exchange limited by sea ice cover and isolation of water from the atmosphere beneath the ice shelf. The residence time of dense shelf waters resulting from sea ice formation is less well constrained by the chemical data than is the strong flow into the Ross Ice Shelf cavity. Shelf waters are transformed over about 3.5 years, by net basal melting of the ice shelf, into fresher Ice Shelf Water (ISW), which emerges as a large plume near the central ice front at temperatures below the sea surface freezing point. We estimate an average ISW production rate of 0.86 Sv and an average net basal melt rate of 60 km3/year for the Ross Ice Shelf exceeding a 300 m draft (75% of the ice cavity) during recent decades from box and stream tube models fit to all of the CFC and salinity data. Model fits to the individual data sets suggest ISW production and net basal melt rate variability due to interannual changes on a shorter time scale than our observations. ISW production based on the CFC budget is better constrained than net basal melting based on thermohaline data, with a heat budget yielding a rate of only 20 km3/yr. Reconciling differences between apparent freshwater and temperature changes under the ice shelf involves considerations of mixing, freezing and the flow of meltwater across the ice shelf grounding line.  相似文献   

10.
New productivity measurements using the 15N tracer technique were conducted in the north-eastern (NE) Arabian Sea during six expeditions from 2003 to 2007, mostly in winter. Our results indicate that the NE Arabian Sea has a potential for higher new productivity during blooms. Nitrate uptake by plankton is the highest during late winter. New productivity and f-ratios in the NE Arabian Sea are mainly controlled by hydrodynamic and meteorological parameters such as wind strength, sea surface temperature (SST), mixed layer depth (MLD) and mixed layer nitrate. Deepening of the mixed layer supplies nitrate from below, which supports the observed nitrogen uptake. Higher f-ratios during blooms indicate the strong coupling between surface layers and sub-surface layers. Deepening of mixed layer below 100 m (from its inter-monsoon value between 30 and 40 m) transferred often more than 100 mmol N–NO3 m? 2 into the surface layers from below. The observed winter blooms in the region are supported by such input and are sustained for more than a month. Higher new productivity has been found in late winter, whereas transport of nitrate is maximum in early winter. In general, new production varies progressively during winter. Diurnal cycling of the mixed layer could be the reason for the under utilization of entrained nitrate during early winter. New productivity values and wind strength show significant differences during Feb–Mar 03 and Feb–Mar 04. These differences indicate that the winter cooling and parameters related the biological productivity also vary inter-annually. However, the difference between the new productivity values between Feb–Mar 03 and Feb–Mar 04 is much lower than the difference between Jan 03 and Feb–Mar 03. The results suggest that amplitude of seasonal variation is higher than the inter-annual variation in the region. During spring, Fickian diffusive fluxes of nitrate into the surface layer range from 0.51 to 1.38 mmol N–NO3 m? 2 day? 1, and can account for 67% and 78% of the observed nitrogen uptake in the coastal and open ocean regions, respectively. We document the intra-seasonal and inter-annual variations in new productivity during winter and identify sources of nitrate which support the observed productivity during spring.  相似文献   

11.
Measurements of sea-ice thickness were obtained from drill holes, an ice-based electromagnetic induction instrument (IEM), and a ship-borne electromagnetic induction instrument (SEM) during the early-melt season in the southern Chukchi Sea in 2002 and 2004, and in late summer 2003 at the time of minimum ice extent in the northern Chukchi Sea. An ice roughness criterion was applied to distinguish between level and rough or ridged ice. Ice-thickness modes in the probability density functions (PDFs) derived from drill-hole and IEM measurements agreed well, with modes at 1.5–1.6 and 1.8–1.9 m for all data from level ice. The PDFs derived from SEM measurements show that the primary modes are at 0.1 and 1.1 m in 2003 and 0.7 m in 2004. In 2002 and 2004, significant fractions (between one-third and one-half) of level ice were found to consist of rafted ice segments. Snow depth varied significantly between years, with 2004 data showing more than half the snow cover on level ice to be at or below 0.05 m depth in late spring. Ice growth simulations and examination of ice drift and deformation history indicate that impacts of atmospheric and oceanic warming on level-ice thickness in the region over the past few decades are masked to a large extent by variability in snow depth and the contribution of deformation processes. In comparison with submarine sonar ice-thickness data from previous decades, a reduction in ice thickness by about 0.5–1 m is in part explained by the replacement of multi-year with first-year ice over the Chukchi and Beaufort shelves.  相似文献   

12.
《Marine Geology》2006,225(1-4):223-246
We present detailed bathymetry, remotely operated vehicle (ROV) and submersible observations, and sedimentary and radiocarbon age data from carbonate deposits recovered from two submerged terraces at − 150 m (T1) and − 230 m (T2) off Lanai, Hawaii. The tops of the terraces are veneered by relatively thin (< 5 m) in situ accumulations of coralline algal nodule, coralgal nodule, Halimeda and a derived oolitic facies deposited in intermediate (30–60 m) to deep fore-reef slope settings (60–120 m). The data are used to develop a sedimentary facies model that is consistent with eustatic sea-level variations over the last 30 ka. Both nodule facies on T1 and T2 initiated growth 30–29 ka following a fall in sea level of ∼50 m and increase in bottom currents during the transition from Marine Isotope Stage 3 to 2. The nodules accreted slowly throughout the Last Glacial Maximum when sea-level was relatively stable. Drowning occurred during the early deglaciation (17–16 ka) and was marked by the complete drowning of coralline algal nodules facies on T2 and incipient drowning of coralgal facies on T1. Abrupt sea-level rise during the middle deglaciation, perhaps associated with global meltwater pulse 1A (14–15 ka), finally drowned the coralgal facies on T1, which in turn was overlain by a deep-water Halimeda facies or an oolitic facies derived from upslope. Our data indicates that Lanai has experienced relatively little vertical tectonic movement over the last 30 ka. Using paleobathymetric data derived from the sedimentary facies, age vs. depth relationships, and published sea-level curves, we estimate that Lanai could be either slowly uplifting or subsiding, but at rates < 0.1 m/kyr (uplift) or < 0.4 m/kyr (subsidence) over this 30 kyr period.  相似文献   

13.
It is demonstrated that weakened wind mixing and strengthened water column stratification resulted in the anomalously low sea surface chlorophyll in the northern South China Sea during the 1997–1998 El Niño event. Remotely sensed sea surface temperature, wind and chlorophyll, which were validated by shipboard observations at the SouthEast Asian Time-series Study (SEATS) station (18°N, 116°E) in the northern South China Sea (SCS) provided the basis for this study. During the 1997–1998 winter at the SEATS station, the sea surface temperature was elevated by about 2 °C above the climatological mean, while the wind speed of the northeast monsoon was reduced from a climatological mean of 9.4 to 6.8 m/s. The concentration of surface chlorophyll-a dropped from 0.2 to 0.1 mg/m3. The monthly area-averaged integrated primary production estimated for the northern SCS area (112–119°E, 15–21°N) was reduced by about 40% of the normal winter value. Under the anomalously high sea surface temperature and weak monsoon, the mixed-layer depth would have been reduced from an average of 65 to 45 m and the nutrients in the mixed layer would have been reduced by half, according to observations at the SEATS station in more recent years. During the 1997–1998 El Niño event, the onset of warming in the northern SCS lagged behind that in the eastern equatorial Pacific by about 5 months and lingered for 11 months. This course of change resembled that of the western Pacific warm pool region. However, contrary to the northern SCS, the sea surface chlorophyll was enhanced in the warm pool region during the event, probably mainly because of the uplifted nutricline. Unlike the eastern equatorial Pacific, the dramatic recovery of biological production did not happen in the SCS in the summer of 1998. These distinctive biogeochemical responses reflect fundamental differences between the SCS and the equatorial Pacific in terms of upper water column dynamics.  相似文献   

14.
Deep-sea benthic communities and their structural and functional characteristics are regulated by surface water processes. Our study focused on the impact of changes in water depth and food supplies on small-sized metazoan bottom-fauna (meiobenthos) along a bathymetric transect (1200–5500 m) in the western Fram Strait. The samples were collected every summer season from 2005 to 2009 within the scope of the HAUSGARTEN monitoring program. In comparison to other polar regions, the large inflow of organic matter to the sea floor translates into relatively high meiofaunal densities in this region. Densities along the bathymetric gradient range from approximately 2400 ind. 10 cm-2 at 1200 m to approximately 300 ind. 10 cm-2 at 4000 m. Differences in meiofaunal distribution among sediment layers (i.e., vertical profile) were stronger than among stations (i.e., bathymetric gradient). At all the stations meiofaunal densities and number of taxa were the highest in the surface sediment layer (0–1 cm), and these decreased with increasing sediment depth (down to 4–5 cm). However, the shape of the decreasing pattern differed significantly among stations. Meiofaunal densities and taxonomic richness decreased gradually with increasing sediment depth at the shallower stations with higher food availability. At deeper stations, where the availability of organic matter is generally lower, meiofaunal densities decreased sharply to minor proportions at sediment depths already at 2–3 cm. Nematodes were the most abundant organisms (60–98%) in all the sediment layers. The environmental factors best correlated to the vertical patterns of the meiofaunal community were sediment-bound chloroplastic pigments that indicate phytodetrital matter.  相似文献   

15.
This study investigates a construction-induced sea level rise and tide characteristics change, using a regression analysis to separate the local construction effect such as sea-dike/seawalls and global warming from total sea level change. The study also makes it clear why and how the extreme high water level has risen just after constructions at Mokpo harbor in Korea. As a result of the regression analysis, it is found that the high water level rise for the period of 1960–2006 is ～60 cm, which is summation of four components: ～23 cm for Youngsan River sea-dike (1981), ～15 cm for Youngam seawall (1991), ～8 cm for Geumho seawall (1994), and ～14 cm for gradual rise (due mainly to global warming). Then, a numerical simulation at Mokpo coastal zone is performed to identify each component, and the results support the premise that the tidal amplification caused by constructions is due mainly to the extinguishment of the tidal choking effect at outer Mokpogu. The tidal flat effect makes the amplification greater at spring tide or extraordinary high tide, which would result in the increase of inundation risk at the Mokpo coastal zone. Frequency distribution of observed high water level data shows increasing trend for both maximum value of astronomical tide component (simulated high water level) and meteorological tide component (surge height) after the coastal constructions. A frequency analysis presents that the high water level for 50 year return period, which is often used for design in practice, is 474 cm before the construction, and while that is 553 cm after the construction. Furthermore, design height might steadily be elevated considering future global sea level rise.  相似文献   

16.
A three-dimensional hydrodynamic ocean model coupled to a thermohydrodynamic model for young sea ice is applied to study shallow haline convection in the central Greenland Sea, with an emphasis on sub-mesoscale ice–ocean interactions. Two types of young sea ice are distinguished; i.e., frazil and pancake ice, both acting different on surface heat, salt, and momentum fluxes. Two scenarios are considered: (a) continued frazil-ice production during steady winds, and (b) the same scenario but with the intermittent formation of pancake ice during a short intervening period of low winds. Brine release due to new-ice production creates shallow convection in both cases. Under conditions of continued frazil-ice production, ice streaks develop at the sea surface, finally becoming oriented roughly parallel to the wind. These streaks are the result of convective plumes that induce organized patterns of convergent and divergent surface currents. Frazil-ice is herded into convergence zones where it becomes as thick as 6 m within 24 h. The studies suggest a strong relationship between streak spacing and the penetration depth of convection, given by an aspect ratio in the range of 2–3. After pancake ice has been formed, however, the organized ice streaks vanish, developing into complex patterns of pancake ice. This finding is in agreement with recent field observations in the Greenland Sea Is-Odden ice tongue. With the existence of pancake ice, moreover, the surface-averaged buoyancy flux decreases and is determined from the integral of local sub-mesoscale ice–ocean interactions.  相似文献   

17.
The macrozooplankton and micronekton community of the Lazarev Sea (Southern Ocean) was investigated at 3 depth layers during austral summer, autumn and winter: (1) the surface layer (0–2 m); (2) the epipelagic layer (0–200 m); and (3) the deep layer (0–3000 m). Altogether, 132 species were identified. Species composition changed with depth from a euphausiid-dominated community in the surface layer, via a siphonophore-dominated community in the epipelagic layer, to a chaetognath-dominated community in the deep layer. The surface layer community predominantly changed along gradients of surface water temperature and sea ice parameters, whereas the epipelagic community mainly changed along hydrographical gradients. Although representing only 1% of the depth range of the epipelagic layer, mean per-area macrofauna densities in the surface layer ranged at 8% of corresponding epipelagic densities in summer, 6% in autumn, and 24% in winter. Seasonal shifts of these proportional densities in abundant species indicated different strategies in the use of the surface layer, including both hibernal downward and hibernal upward shift in the vertical distribution, as well as year-round surface layer use by Antarctic krill. These findings imply that the surface layer, especially when it is ice-covered, is an important functional node of the pelagic ecosystem that has been underestimated by conventional depth-integrated sampling in the past. The exposure of this key habitat to climate-driven forces most likely adds to the known susceptibility of Antarctic pelagic ecosystems to temperature rise and changing sea ice conditions.  相似文献   

18.
The macrotidal bay of Marennes-Oléron is the most important French site for shellfish production (oysters and mussels); yet the primary productivity of the phytoplankton compartment in this system is not well known. In this study, photosynthetic parameters were determined using 14C incubations of bottom and surface water samples, during fall, winter and summer (2001–2002), along a north–south transect in the bay. Estimates of primary productivity showed that water column primary production is light-limited in the bay and that a BZpI0 type model can be applied. Spatial differences existed in the bay, with a more productive northern zone and less productive river area. With a water column primary production of 185 g C m−2 yr−1, Marennes-Oléron Bay lies in the mean range for phytoplankton primary production capacity among European and North American estuaries.  相似文献   

19.
Late Holocene paleoceanography and climate variability of the Southeastern Beaufort Sea (Canadian Arctic) have been investigated on the basis of sedimentary cores collected over the Mackenzie Slope. Piston, trigger and box cores were sampled at station 803 in 2004 aboard the CCGS Amundsen at 218 m water depth. The chronology of the piston core is constrained by 4 AMS-14C dates, as the sedimentation rate in the box core is assessed from 210Pb data. We obtain a continuous composite sequence covering the last 4600 years, with a sedimentation rate of ~ 140 cm.kyr? 1. Transfer functions (modern analogue technique) based on dinoflagellate cyst (dinocyst) assemblages were used to reconstruct the evolution of sea-surface conditions over the time period covered by the cores.Palynological data reveal that dinocyst assemblages are dominated by Operculodinium centrocarpum sensu lato (mean of 43.3%) throughout the core, with the accompanying taxa Brigantedinium spp. (19.6%), Islandinium minutum (15.6%) and cysts of Pentapharsodinium dalei (13.7%). Four zones have been established on the basis of dinocyst relative abundances. Dinocyst assemblage zone 1 (D1), from 4600 to 2700 cal years BP, is dominated by O. centrocarpum (mean of 49.0%). In zone D2 (2700–1500 cal years BP), the relative abundances of O. centrocarpum decrease (34.4%) in favour of the opportunistic, heterotrophic taxa Brigantedinium spp. (28.8%) and cysts of Polykrikos sp. var. arctic/quadratus (2.8%). Dinocyst zone D3 (1500–30 cal years BP or 450–1920 AD) is characterised by the high relative abundance of the peridinioid taxa I. minutum (19.9%). The last zone (D4), spanning from 1920 to 2004 AD, is again dominated by O. centrocarpum (44.5%), and shows low relative abundances of Brigantedinium spp. and cf. Echinidinium karaense.Quantitative reconstructions of past sea-surface parameters (August sea-surface temperature: SST, August sea-surface salinity: SSS, and duration of sea-ice cover) indicate relatively stable conditions over the last 4.6 kyr, with episodic cooling events (SST of ~ 1.5 °C below the modern value of 6 °C) that took place between 700 and 1820 AD. We associate the last and the longest of these cooling events (1560–1820 AD) with the Little Ice Age. Reconstructed SSS shows decadal oscillations since 1920 AD that we tentatively associate with the accumulation of freshwater by the Beaufort Gyre and the subsequent Great Salinity Anomalies. Our data suggest that similar salinity anomalies could have occurred ca. 1860 and 1790 AD.Stable isotopic data show a slight increase in δ13C values (from ~?27.1‰ at the base to ~?25.8‰ at the top) over the last 4.6 kyr that we associate with the gradual increase in atmospheric CO2 concentration as recorded by Antarctic ice cores. Variations in the δ15N profile suggest variations in Pacific water influence from 4600 to ~ 1300 cal years BP, associated with centennial scale shifts of the Arctic Oscillation phases.  相似文献   

20.
Hole 887B of the Ocean Drilling Program (ODP) comprises a 44 m (750 kyr) long continuous section recovered from the Patton–Murray Rise, an elevated plateau that is largely isolated from turbidite deposition. The Patton–Murray area is centered under the Alaska Gyre, a region characterized by the domal upwelling of nutrient-rich waters. Marked increases in productivity and rapid settling of biogenic matter are suggested throughout the section by the episodic accumulation of diatomaceous oozes up to ∼1 m thick that are accompanied by barium enrichments. Significant δ13Corg maxima in the major diatomaceous bands suggest that mixed-layer [CO2(aq)] may have been drawn down significantly during some of the productivity events. The episodes of enhanced productivity at Site 887 occur synchronously with short-lived minima in planktonic foram δ18O, suggesting a direct link to low salinity, or less likely, warming, events in the Gulf of Alaska. There is no obvious explanation for the events, but they may be related to seasonal incursions of meltwater from Alaska. We speculate that episodic input of meltwater- or dust-borne iron from Asian or Alaskan sources may have promoted the extraordinary diatom production events recorded in the sedimentary record.  相似文献   

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