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1.
In order to compare radiocarbon dates on marine and terrestrial samples the former have to be corrected for a reservoir age. We present reservoir ages from dating 21 whales collected 1860–1901 and recalculating dates of 23 molluscs collected 1857–1926. Most of the whales were caught along the coast of Norway, but one is from France and one from Iceland. We assume the former mainly lived in the North and equatorial Atlantic and in the Norwegian Sea. Whales feed only on pelagic organisms and will provide the reservoir age for the open ocean surface water. However, they travel long distances and will integrate the reservoir ages of the different water masses along their way. Molluscs (dated from Norway, Spitsbergen and Arctic Canada) are stationary and monitor the sea water passing their dwelling site, but some also take up carbon from particulate food or sediment pore water. Coastal water also often contains some continental carbon. We present two different views on how to analyze and interpret the data. Mangerud recommends to use reservoir ages based on a combination of the whale and mollusc dates, i.e. 380±30 and 360±30 yr relative to Intcal04 and British oak, respectively, and a ΔR value of 20±30 for the surface water in the N-Atlantic and Norwegian Sea. Bondevik and Gulliksen maintain that the reservoir age—and ΔR—along the Norwegian coast is latitude dependant, with ΔR-values increasing from −3±22 in the South to 105±24 at Spitsbergen. Whales, reflecting North Atlantic open ocean surface water have lower ΔR (7±11) than most molluscs.  相似文献   

2.
Eolian and subaqueous landforms composed of gypsum sand provide geomorphic evidence for a wet episode at the termination of glacial climate in southwestern North America. Drying of pluvial Lake Estancia, central New Mexico, occurred after ca. 12,000 14C yr B.P. Thereafter, eolian landforms on the old lake floor, constructed of gypsum sand, were overridden by rising lake water, modified by subaqueous processes, and organized into beach ridges along the lake's eastern shore. Preservation of preexisting eolian landforms in the shallow lake suggests abupt changes in lake level and climate. Available radiocarbon ages suggest that the final highstand recorded by beach ridges may have developed during the Younger Dryas (YD) stade. The beach ridges provide information about lake surface area, which was 45% of the lake area reached during the maximum highstands of the late Pleistocene. A similar proportional response has been reported for YD climate changes outside the North Atlantic region.  相似文献   

3.
In order to quantify the reservoir age in Baltic Sea sediment, one sedimentary sequence from an isostatically isolated basin was subject to high-resolution AMS radiocarbon dates. Diatom analysis confirmed deposition during the Litorina Sea stage and later, in a freshwater lake. Macrofossils from well preserved seeds and other remnants from terrestrial plants were used for AMS datings. It is assumed that these fragile plant remains are not redeposited or affected by internal ages. The ages obtained from the macrofossils range from 6460±125 to 5580±75 14C yr BP. By comparing these radiocarbon ages with those obtained by bulk sediment dates, it was obvious that the bulk samples were affected by reservoir ages, resulting in too old ages. The reservoir ages varied within the sediment column; during the most saline phase, the reservoir age was approximately 750 yr, shortly after the isolation ca 400 yr and in the freshwater lake, the age differences between the two series were neglectable.  相似文献   

4.
A current scenario to explain much of the atmospheric CO2 increase during the Glacial to Holocene climate transition requires the outgassing of a deep, old oceanic CO2 reservoir thought to be located in the Southern Ocean. In this scenario, CO2-rich and 14C-depleted subsurface Antarctic-sourced water, ventilates the thermocline where it is purged to the atmosphere in the equatorial regions, a view that has been met with conflicting results. Using a novel approach (paired surface and deep-dwelling planktonic foraminifer radiocarbon analyses), we document that the equatorial Atlantic thermocline did not see old, 14C-depleted water, which would be characteristic of the proposed isolated deep ocean CO2 reservoir. Data from several studies concur that, during the deglaciation, Antarctic intermediate waters were contributing to Atlantic thermocline waters even more than today, therefore, our observations challenge the current purging hypothesis. Together with other studies, these results suggest that the mechanism responsible for the deglacial CO2 rise cannot invoke contemporary circulation modes and/or thermocline ventilation pathways.  相似文献   

5.
Models of late-glacial environmental change in coastal areas are commonly based on radiocarbon ages on marine shell and basal lake sediments, both of which may be compromised by reservoir effects. The magnitude of the oceanic reservoir age in the inland waters of the Georgia Basin and Puget Lowland of northwestern North America is inferred from radiocarbon ages on shell-wood pairs in Saanich Inlet and previously published estimates. The weighted mean oceanic reservoir correction in the early and mid Holocene is −720±90 yr, slightly smaller than, but not significantly different from, the modern value. The correction in late-glacial time is −950±50 yr. Valley-head sites yield higher reservoir values (−1200±130 yr) immediately after deglaciation. The magnitude of the gyttja reservoir effect is inferred from pairs of bulk gyttja and plant macrofossil ages from four lakes in the region. Incorporation of old carbon into basal gyttja yields ages from bulk samples that are initially about 600 yr too old. The reservoir age declines to less than 100 yr after the first millennium of lake development. When these corrections are accounted for, dates of deglaciation and late-glacial sea-level change in the study area are pushed forward in time by more than 500 yr.  相似文献   

6.
The temporal and spatial extent of Holocene climate change is an area of considerable uncertainty, with solar forcing recently proposed to be the origin of cycles identified in the North Atlantic region. To address these issues we have developed an annually resolved record of changes in Irish bog tree populations over the last 7468 years which, together with radiocarbon‐dated bog and lake‐edge populations, extend the dataset back to ～9000 yr ago. The Irish trees underpin the internationally accepted radiocarbon calibration curve, used to derive a proxy of solar activity, and allow us to test solar forcing of Holocene climate change. Tree populations and age structures provide unambiguous evidence of major shifts in Holocene surface moisture, with a dominant cyclicity of 800 yr, similar to marine cycles in the North Atlantic, indicating significant changes in the latitude and intensity of zonal atmospheric circulation across the region. The cycles, however, are not coherent with changes in solar activity (both being on the same absolute timescale), indicating that Holocene North Atlantic climate variability at the millennial and centennial scale is not driven by a linear response to changes in solar activity. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

7.
The AMS 14C technique has the advantage that small samples of Late Quaternary age can be dated with high accuracy, and that errors due to reservoir effects can be avoided if specifically determined terrestrial micro- and macrofossils are measured. However, to obtain such high-accuracy measurements, it is important how small samples are handled prior to treatment in the radiocarbon laboratories. Here we present a set of 51 AMS 14C measurements, of which 31 dates gave expected ages and 20 dates resulted in anomalously young ages, despite the fact that all samples consisted of clearly identified Late Weichselian terrestrial plant macrofossils. To evaluate possible sources of error, we compared these samples in respect to preparation methods, sample storage and sample weight. Our results show that the long-term storage of wet macrofossil samples appears to have a significant effect on the radiocarbon age obtained, even when the samples are kept cool. Fungi or micro-organisms may easily be incorporated into a sample during preparation and identification, and can easily contribute to the contamination of a sample, if stored cool and wet for several months. © 1998 John Wiley & Sons, Ltd.  相似文献   

8.
The laminated lacustrine sediments deposited in the last glacial Lake Lisan represent annual deposits of primary aragonite and silty detritus that reflect the annual supply of bicarbonate‐bearing freshwater to the lake. A varve‐counting curve was constructed for the time interval of ca. 17.4–22 cal. ka BP based on aragonite U/Th, and atmospheric radiocarbon ages of organic debris recovered from the studied section. Radiocarbon in the primary (evaporitic) aragonite comprises both atmospheric and old carbon (reflecting the reservoir age). The aragonite reservoir ages were determined by comparing the aragonite radiocarbon dates to the varve counting curve, and are found to lie in the range 1900–600 a and display a continuous decline. This opens the possibility for high (annual) resolution monitoring of the reservoir age, similar in quality to tree ring counting, during the upper part of Marine Isotope Stage (MIS) 2. Our work also demonstrates that a ‘uniform’ reservoir age correction is inappropriate when determining the chronology of short‐term climate events in lacustrine environments. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

9.
Oxygen- and carbon-isotopic analyses have been performed on the benthic foraminifer Planulina wuellerstorfi in seven Late Quaternary cores from the Vema Channel-Rio Grande Rise region. The cores are distributed over the water-depth interval of 2340 to 3939 m, which includes the present transition from North Atlantic Deep Water (NADW) to Antarctic Bottom Water (AABW).The carbon-isotopic records in the cores vary as a function of water depth. The shallowest and deepest cores show no significant glacial-interglacial difference in δ13C. Four of the five cores presently located in the NADW have benthic foraminiferal δ13C that is lower during glacial isotopic stages. Based on bathymetric gradients in δ13C, we conclude that, like today, there were two water masses present in the Vema Channel during glacial intervals: a water mass enriched in 13C overlying another water mass depleted in 13C. The largest gradient of change of δ13C with depth, however, occurred at 2.7 km, ～ 1 km shallower than the present position of this gradient.On the basis of paleontologic and sedimentologic evidence, we consider it unlikely that the NADW:AABW transition shallowed to this level. Reduced carbon-isotopic gradients between the deep basins of the North Atlantic and Pacific Oceans during the last glaciation suggest that production of NADW was reduced. Lower production of NADW may have modified the local abyssal circulation pattern in the Vema Channel region.  相似文献   

10.
In this paper we discuss the limitation of radiocarbon dates on the pedogenic calcic nodules formed in situ within the vertisols in the upland region of Coimbatore, Tamil Nadu. The radiocarbon ages were obtained using low-level scintillation counters and the dates range from ∼24 to 31 14C kyrs BP. The ages correlate with the marine isotope stage of Late MIS3. However, since the calcic nodules are pedogenised and formed in an open system, the 14C ages should only be considered as estimates and not absolute ages because of the possibility of open-system behaviour with respect to carbon. Thus, we express caution in the interpretation of these and other radiocarbon ages obtained on pedogenic carbonate nodules. Multiple sub-mm size subsamples could provide more reliable age estimates.  相似文献   

11.
A study of the 140–100 ka interval in core T90-9P from the North Atlantic (45° N, 25° W), based on analysis of oxygen and carbon isotope records from planktonic and benthonic foraminifera, and from the bulk sediment fine fraction facilitates a detailed paleoceanographic reconstruction of the penultimate deglaciation (Termination II), and of the Eemian interglacial (δ18O stage 5e). The first step of Termination II was characterised by low productivity and a mixed water column, which was a remnant of glacial conditions. A 3 ka period of relatively stable conditions, with a stratified water column (‘Termination II pause’), occurred half-way through Termination II, and preceeded a second and more rapid climatic shift. The end of the deglaciation (Eemian maximum, i.e. isotopic event 5.53) initiated the establishment of strong, seasonal, water column stratification. North Atlantic Deep Water (NADW) production remained low during the complete glacial–interglacial transition. After the Eemian maximum, NADW prodution was restored, and bottom waters remained quite stable during the course of the Eemian, while surface waters gradually cooled in the second half of the stage. A short surface water cooling event accompanied by a reduced seasonal water column stratification and nutrient instability occurred at approximately 117 ka BP.  相似文献   

12.
Articulated molluscs, sea urchins and barnacle fragments close to the Vedde Ash Bed in a shallow marine deposit on the west coast of Norway have been 14C dated. The weighted mean of four dates from a sediment slice 8 cm thick centred on the Vedde Ash Bed is 10920 ± 24 14C yr BP. The most accurate 14C age of the Vedde Ash from terrestrial plant macrofossils is 10310 ± 50 yr BP. The difference is the 14C reservoir age for coastal water at the west coast of Norway during the mid‐Younger Dryas and equals 610 ± 55 yr. This is 230 yr older than the reservoir age for the Bølling/Allerød and for the present day in this area. The result supports earlier conclusions of a higher reservoir age for the Younger Dryas in the North Atlantic and Nordic Seas, although our reservoir age of 610 ± 55 yr is a few hundred years younger. This suggests that the 14C reservoir age at Vedde Ash time may increase from coastal water towards the open North Atlantic and Nordic Seas. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

13.
A comparison of the first results of a comprehensive micropaleontological analysis (pollen, spores, organic-walled microfossils, diatoms, ostracods) and radiocarbon ages (AMS14C) from sediment core recovered in the northeastern outer shelf of the Laptev Sea (51 m water depth) revealed a temporal coincidence between terrestrial and marine environmental changes that occurred between 11.2–10.3 cal ka. This interval provided evidence for a landscape transition from grass tundra to shrub tundra and the development of a freshwater estuarine basin with the strong influence of riverine discharge and a minor advection of North Atlantic waters. The establishment of a warmer and wetter climate promoted the expansion of shrub tundra habitats. The interval of 9.5–7.5 cal ka recorded a transition from a shrub tundra environment to forest-tundra vegetation. This interval also revealed a series of short-term temperature fluctuations, when summer temperatures were 3–4°C higher than today. The active advection of North Atlantic waters and the increase in salinity were also recorded by this interval.  相似文献   

14.
The North Atlantic Oscillation (NAO) is one of the modes of climate variability in the North Atlantic region. The atmospheric circulation during the winter season in this region commonly displays a strong meridional (north–south) pressure contrast, with low air pressure (cyclone) centred close to Iceland and high air pressure (anticyclone) near the Azores. This pressure gradient drives the mean surface winds and the mid‐latitude winter storms from west to east across the North Atlantic, bringing mild moist air to northwest Europe. The NAO index is based on the difference of normalised sea‐level pressures (SLP) between Ponta Delgada, Azores and Stykkisholmur, Iceland. The SLP anomalies at these stations are normalised by division of each monthly pressure by the long‐term (1865–1984) standard deviation. Interannual atmospheric climate variability in northwest Europe, especially over Great Britain and western Scandinavia has, during the last decades, been attributed mainly to the NAO, causing variations in the winter weather over the northeast North Atlantic and the adjacent land areas. A comparison between the NAO index and the winter (December–March) precipitation between ad 1864 and 1995 in western Norway shows that these are strongly linked (correlation coefficient 0.77). Variations in the NAO index are also reflected in the mass balance records of glaciers in western Scandinavia. The NAO index is best correlated with mass balance data from maritime glaciers in southern Norway (e.g. Ålfotbreen R2 = 0.51). The record of Holocene (last ca. 11 500 cal. yr) glacier variations of maritime glaciers in western Scandinavia is thus a proxy of pre‐instrumental NAO variations. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

15.
Two sites in the eastern Fram Strait, the Vestnesa Ridge and the Yermak Plateau, have been surveyed and sampled providing a depositional record over the last glacial‐interglacial cycle. The Fram Strait is the only deep‐water connection from the Arctic Ocean to the North Atlantic and contains a marine sediment record of both high latitude thermohaline flow and ice sheet interaction. On the Vestnesa Ridge, the western Svalbard margin, a sediment drift was identified in 1226 m of water. Gravity and multicores from the crest of the drift recovered turbidites and contourites. 14C dating indicates an age range of 8287 to 26 900 years BP (Early Holocene to Late Weichselian). The Yermak Plateau is characterized by slope sediments in 961 m of water. Gravity and multicores recovered contourites and hemipelagites. 14C ages were between 8615 and 46 437 years BP (Early Holocene to mid‐Weichselian). Downcore dinoflagellate cyst analyses from both sites provide a record of changing surface water conditions since the mid‐Weichselian, suggesting variable sea ice extent, productivity and polynyas present even during the Last Glacial Maximum. Four layers of ice‐rafted debris were also identified and correlated within the cores. These events occurred ca at 9, 24 to 25, 26 to 27 and 43 ka, asynchronous with Heinrich layers in the wider north‐east Atlantic and here interpreted as reflecting instability in the Svalbard/Barents Ice sheet and the northward advection of warm Atlantic water during the Late Weichselian. The activity of the ancestral West Spitsbergen Current is interpreted using mean sortable silt records from the cores. On the Vestnesa Ridge drift the modern mass accumulation rate, calculated using excess 210Pb, is 0·076 g cm?2 year?1. On the Yermak Plateau slope the modern mass accumulation rate is 0·053 g cm?2 year?1.  相似文献   

16.
Holocene high-resolution cores from the margin of the Arctic Ocean are rare. Core P189AR-P45 collected in 405-m water depth on the Beaufort Sea slope, west of the Mackenzie River delta (70°33.03′N and 141°52.08′W), is in close vertical proximity to the present-day upper limit of modified Atlantic water. The 5.11-m core spans the interval between ∼6800 and 10,400 14C yr B.P. (with an 800-yr ocean reservoir correction). The sediment is primarily silty clay with an average grain-size of 9 φ. The chronology is constrained by seven radiocarbon dates. The rate of sediment accumulation averaged 1.35 mm/yr. Stable isotopic data (δ18O and δ13C) were obtained on the polar planktonic foraminifera Neogloboquadrina pachyderma (s) and the benthic infaunal species Cassidulina neoteretis. A distinct low-δ18O event is captured in both the benthic and planktonic data at ∼10,000 14C yr B.P.—probably recording the glacial Lake Agassiz outburst flood associated with the North Atlantic preboreal cold event. The benthic foraminifera are dominated in the earliest Holocene by C. neoteretis, a species associated with modified Atlantic water masses. This species decreases toward the core top with a marked environmental reversal occurring ∼7800 14C yr B.P., possibly coincident with the northern hemisphere 8200 cal yr B.P. cold event.  相似文献   

17.
Benthic carbon isotope data indicate that the rate of North Atlantic Deep Water (NADW) formation and the mode of oceanic thermohaline circulation (THC) varied considerably across the transition from the Last Glacial Maximum (LGM) to the Heinrich 1 meltwater event (MWE) and, subsequently, to the Bølling warm period. We simulate changes in the Ocean-atmosphere carbon cycle induced by and linked to these oceanic fluctuations by means of a carbon cycle box model which resolves the major oceanic basins. The output from an ocean general circulation model (OGCM), which is forced by observed or reconstructed boundary conditions at its surface, serves to constrain the physical parameters of the carbon cycle model. The OGCM depicts three modes of Atlantic THC: an interglacial mode with vigorous NADW formation; a glacial mode with active, although weaker (-65%) NADW formation; and an MWE mode characterized by the complete lack of NADW formation. The carbon cycle model is forced from the LGM scenario into the MWE and finally into the Bølling interstadial. The glacial circulation mode accounts for approximately half (i.e., 37Dž µatm, depending on parameterization of biological productivity) of the observed glacial reduction in atmospheric CO2 partial pressure (pCO2). Approximately 70% of this pCO2 decline is linked to changes in sea-surface temperature and salinity. The MWE circulation mode has only a small effect on atmospheric pCO2 (ǃ µatm) but goes along with a massive redistribution of carbon from the Indo-Pacific and Southern oceans to the Atlantic Ocean, which stores 85NJ Gt (gigatons) excess carbon during the MWE. The onset of NADW formation after a meltwater event, has the potential to release 81Lj Gt carbon from the model ocean to the atmosphere, corresponding to an atmospheric pCO2 increase by 38Dž µatm, equivalent to approximately half of the modern, man-made pCO2 load.  相似文献   

18.
Excavations in the late-glacial Presumpscot Formation at Portland, Maine, uncovered tree remains and other terrestrial organics associated with marine invertebrate shells in a landslide deposit. Buds of Populus balsamifera (balsam poplar) occurred with twigs of Picea glauca (white spruce) in the Presumpscot clay. Tree rings in Picea logs indicate that the trees all died during winter dormancy in the same year. Ring widths show patterns of variation indicating responses to environmental changes. Fossil mosses and insects represent a variety of species and wet to dry microsites. The late-glacial environment at the site was similar to that of today's Maine coast. Radiocarbon ages of 14 tree samples are 11,907 ± 31 to 11,650 ± 50 14C yr BP. Wiggle matching of dated tree-ring segments to radiocarbon calibration data sets dates the landslide occurrence at ca. 13,520 + 95/−20 cal yr BP. Ages of shells juxtaposed with the logs are 12,850 ± 65 14C yr BP (Mytilus edulis) and 12,800 ± 55 14C yr BP (Balanus sp.), indicating a marine reservoir age of about 1000 yr. Using this value to correct previously published radiocarbon ages reduces the discrepancy between the Maine deglaciation chronology and the varve-based chronology elsewhere in New England.  相似文献   

19.
Radiocarbon dated lacustrine sequences in Perú show that the chronology of glaciation during the late glacial in the tropical Andes was significantly out-of-phase with the record of climate change in the North Atlantic region. Fluvial incision of glacial-lake deposits in the Cordillera Blanca, central Perú, has exposed a glacial outwash gravel; radiocarbon dates from peat stratigraphically bounding the gravel imply that a glacier advance culminated between 11,280 and 10,990 14C yr B.P.; rapid ice recession followed. Similarly, in southern Perú, ice readvanced between 11,500 and 10,900 14C yr B.P. as shown by a basal radiocarbon date of 10,870 14C yr B.P. from a lake within 1 km of the Quelccaya Ice Cap. By 10,900 14C yr B.P. the ice front had retreated to nearly within its modern limits. Thus, glaciers in central and southern Perú advanced and retreated in near lockstep with one another. The Younger Dryas in the Peruvian Andes was apparently marked by retreating ice fronts in spite of the cool conditions that are inferred from the ∂18O record of Sajama ice. This retreat was apparently driven by reduced precipitation, which is consistent with interpretations of other paleoclimatic indicators from the region and which may have been a nonlinear response to steadily decreasing summer insolation.  相似文献   

20.
Cores from five high alpine basins in the northern San Juan Mountains show several fluctuations in lithology. Typically, peats are interbedded with coarser clastic sediments or else woody peats alternate with fibrous peat. Twenty 14C dates provide radiometric control. Sediment rates averaged about 2.5 cm/100 yr but varied at the different sites between 1.19 and 50 cm/100 yr. Rates were lower during the middle of the Holocene. Basal radiocarbon dates indicate that these high (ca. 3600 m a.s.l.) northeasterly facing cirques were icefree by 9000 BP. There is some evidence in the cores for a short climatic reversal sometime between 8000 and 7000 BP. A major change occurred in the high basins very close to 5000 BP and thereafter there are several intervals of increased clastic sedimentation which may be related to Neoglacial climatic fluctuations. Analysis of a 2.15 m core near Hurricane Basin indicates significant fluctuation of pollen and macrofossils occurred during the 9000 ± year record. The Picea/Pinus ratios are used to delimit changes in the apparent elevation of the site: the ratios indicate that a short drop of “treeline” occurred about 8000 BP and then remained near present level until about ≥1800 BP when the apparent elevation of the site rose. Macrofossils indicate that spruce was present in the Hurricane Basin (and others) at specific periods and confirms the general results of the Picea/Pinus ratios. The San Juan Mountains do not possess a glacial Neoglacial record but the stratigraphy of these high cirque basins can be used to define glacial stades (cf. Jardine, 1972). The interpreted climatic response record on vegetation and sediment flux has both similarities and differences from other records in the western mountains of North America.  相似文献   

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