Accelerator Mass Spectrometer 14C Research Articles

Enhanced sulfidization in a sedimentary turbidite layer from the Nansha Trough in the southern South China Sea

Microbial sulfate reduction occurs in diverse depositional sedimentary environments, especially in fine-grained sedimentary turbidites. However, the extent to which turbidites impact the mechanism of sulfidization and the carbon–sulfur–iron cycle is poorly understood. Here, a sediment gravity core (10GC) with 50-cm thick turbidites was collected from the Nansha Trough of the southwestern South China Sea (SSCS) to unravel the biogeochemistry and sulfidization processes. One accelerator mass spectrometric 14C date in the planktonic foraminifera indicates that the turbidite layer was deposited at or immediately before 16 ka. Using a suite of elemental (Fe, K, Si Al, and Zr) and sediment grain size proxies suggests that the sediments within the turbidites were derived from the Sunda Shelf and the Malay Peninsula. The TOC/TN values (9–20) of the turbidites indicate deposition of terrestrial organic matter with a mixture of C3 and C4 types of vegetation. The relatively negative 13CTIC values (−7.76‰ to 2.09‰) are robust evidence for the accumulation of authigenic carbonates, which was attributed to organoclastic sulfate reduction. The rather positive δ34SCRS (CRS: chromium reducible sulfur) (−37.5‰ to −10.5‰) and high CRS contents of the turbidites compared to the hemipelagic sediments suggest that the enhanced sulfidization occurred due to a high rate of sulfate reduction in the closed system. In contrast to the turbidites, the more negative δ34SCRS values (−50.9‰ to −47.4‰) in the hemipelagic sediments reflect early diagenetic pyritization at the sediment–water interface. This study highlights the importance of turbidite sedimentation on the biogeochemical cycling of sulfur, carbon, and iron. However, further research is needed to better understand and quantify the contribution of marine turbidite sediments to the global ocean C–S–Fe cycle.

Standard operation procedures and performance of the MICADAS radiocarbon laboratory at Alfred Wegener Institute (AWI), Germany

The radiocarbon analysis laboratory at Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) is equipped with an accelerator mass spectrometer (AMS) MICADAS (MIni CArbon Dating System). The laboratory provides routine 14C analyses on bulk organic matter, plant fragments, dissolved and particulate organic matter, individual molecular lipids, and carbonate (micro-) fossils with the aim to foster international research efforts in vulnerable high latitudes. The AWI MICADAS allows AMS 14C analysis on graphite targets as well as on CO2 samples via gas injection into the hybrid ion source. The laboratory thus provides reliable datasets even if only small amounts of sample material are available, a problem often encountered in polar research. Here we describe the standard operation procedures and sample preparation methods employed, and demonstrate the instrument performance and data quality based on repeat analysis of international reference materials.

Holocene vertebrates from a dry cave on Eleuthera Island, Commonwealth of The Bahamas

We report a mid- to late-Holocene, non-cultural vertebrate assemblage from Garden Cave (site EL-229), Eleuthera Island, The Bahamas, with 2450 fossils representing 26 species. The chronology is based on accelerator mass spectrometer (AMS) radiocarbon (14C) dates determined directly on individual bones of the hutia ( Geocapromys ingrahami), an extirpated species of rodent that dominates the bone assemblage at Garden Cave. Four AMS 14C dates from our excavation range from 1340 to 1280 cal. BP (surface of the site) to 4860 to 4830 cal. BP with depth. A hutia bone lying on the surface from elsewhere in the cave dated to 450 to 290 cal. BP, which is roughly the time of European and African contact on Eleuthera. Other extirpated species from Garden Cave are tortoise ( Chelonoidis sp.), rock iguana ( Cyclura sp.), skink ( Mabuya sp.), parrot ( Amazona leucocephala), crow ( Corvus nasicus), and southeastern myotis ( Myotis austroriparius). Each of these species may have survived on Eleuthera until sometime after the initial human occupation of the island (~1000 cal. BP), although we have direct AMS 14C dates for only the hutia. During the time of fossil deposition in Garden Cave, sea levels were approaching that of today, yet land areas were considerably larger than now, connecting Eleuthera to New Providence, and potentially to Exuma as well. Such relatively recent connections are important in explaining past and present distributions of terrestrial plants and animals.

Late Holocene Historical Ecology: The Timing of Vertebrate Extirpation on Crooked Island, Commonwealth of The Bahamas

ABSTRACTWe report eight new accelerator-mass spectrometer (AMS) radiocarbon (14C) dates performed directly on individual bones of extirpated species from Crooked Island, The Bahamas. Three dates from the hutia (Geocapromys ingrahami), recovered from a culturally derived bone assemblage in McKay's Bluff Cave (site CR-5), all broadly overlap from AD 1450 to 1620, which encompasses the time of first European contact with the Lucayan on Crooked Island (AD 1492). Marine fish and hutia dominate the bone assemblage at McKay's Bluff Cave, shedding light on vertebrate consumption by the Lucayans just before their demise. A fourth AMS 14C date on a hutia bone, from a non-cultural surface context in Crossbed Cave (site CR-25), is similar (AD 1465 to 1645) to those from McKay's Bluff Cave. From Pittstown Landing (site CR-14), an open coastal archaeological site, a femur of the Cuban crocodile (Crocodylus rhombifer) yielded an AMS 14C date of AD ∼1050–1250, which is early in the Lucayan cultural sequence. From a humerus in a non-cultural surface context in 1702 Cave (site CR-26), we document survival of the Cuban crocodile on Crooked Island until AD ∼1300–1400, which is several hundred years later than the well-documented extinction of Cuban crocodiles on Abaco in the northern Bahamas. We lack a clear explanation of why Cuban crocodiles likely survived longer on Crooked Island than on a larger Bahamian island such as Abaco. One AMS 14C date on Crooked Island's extinct, undescribed species of tortoise (Chelonoidis sp.) from 1702 Cave is BC 790 to 540 (2740 to 2490 cal BP), which is ∼1500–1700 years prior to human arrival. A second AMS 14C date, on a fibula of this tortoise from McKay's Bluff Cave, is AD 1025 to 1165, thereby demonstrating survival of this extinct species into the period of human occupation.

Homogeneity evaluation of Chinese sugar carbon (CSC) standard for AMS 14C measurement

The Chinese national radiocarbon standard Chinese Sugar Carbon (CSC), or often called the “Chinese Sucrose Charcoal,” was investigated for its homogeneity and its suitability as a radiocarbon standard for Accelerator Mass Spectrometer (AMS). We randomly chose ten new CSC bottles and two opened bottles and measured their carbon isotope compositions. δ13C results obtained by Isotope Ratio Mass Spectrometer (IRMS) showed relatively large variation: from −15.9 to −22.2‰ with an average of −19.0±1.2‰ (n=49), indicating CSC grains in the mg range are not isotopically homogeneous. However, 14C results of CSC, expressed as Fraction Modern (FM), are well constrained to 2.4‰ after fractionation correction to −25‰. Measurements from all 12 bottles gave an average FM value of 1.3533±0.0034 (n=55). This value is about 7‰ higher that the FM value of 1.346±0.002, converted from the originally reported R value of 1.362±0.002 (n=14, with δ13C of −19.32, n=9) [1,2]. The R value of 1.362 is the ratio of 14C activity of CSC to 95% of OXI, not an FM value. The online simultaneous AMS 13C/12C measurement that allows for correction of all isotopic fractionation, including manufacture, graphitization and machine induced fractionation, is the key for obtaining consistent 14C measurements on this standard. Without further homogenization, CSC may not be a good choice of standard for an AMS that does not have the capability of measuring online AMS δ13C. Further study is needed to see if mixing/grinding can reduce the inhomogeneity. Re-measurement is suggested with more laboratories involved in order to get a better defined consensus value.

Late Quaternary mega-lakes fed by the northern and southern river systems of central Australia: Varying moisture sources and increased continental aridity

Optically stimulated and thermoluminescence ages from relict shorelines, along with accelerator mass spectrometer 14C ages from freshwater molluscs reveal a record of variable moisture sources supplied by northern and southern river systems to Lake Mega-Frome in southern central Australia during the late Quaternary. Additional lacustrine, palynological and terrestrial proxies are used to reconstruct a record that extends back to 105ka, confirming that Lakes Mega-Frome and Mega-Eyre were joined to create the largest system of palaeolakes on the Australian continent as recently as 50–47ka. The palaeohydrological record indicates a progressive shift to more arid conditions, with marked drying after 45ka. Subsequently, Lake Mega-Frome has filled independently at 33–31ka and at the termination of the Last Glacial Maximum to volumes some 40 times those of today. Further sequentially declining filling episodes (to volumes 25–10 those of today) occurred immediately prior to the Younger Dryas stadial, in the mid Holocene and during the medieval climatic anomaly. Southern hemisphere summer insolation maxima are a poor predictor of palaeolake-filling episodes. An examination of multiple active moisture sources suggests that palaeolake phases were driven independently of insolation and at times by some combination of enhanced Southern Ocean circulation and strengthened tropical moisture sources.

A multi‐proxy record of changing environments from ca. 30 000 to 9000 cal. a BP: Onepoto maar palaeolake, Auckland, New Zealand

AbstractWe present a high‐resolution record of lacustrine sedimentation spanning ca. 30 000 to 9000 cal. a BP from Onepoto maar, northern North Island, New Zealand. The multi‐proxy record of environmental change is constrained by tephrochronology and accelerator mass spectrometric 14C ages and provides evidence for episodes of rapid environmental change during the Last Glacial Coldest Period (LGCP) and Last Glacial–Interglacial Transition (LGIT) from northern New Zealand. The multi‐proxy palaeoenvironmental record from Onepoto indicates that the LGCP was cold, dry and windy in the Auckland region, with vegetation dominated by herb and grass in a beech forest mosaic between ca. 28 500 and 18 000 cal. a BP. The LGCP was accompanied by more frequent fires and influx of clastic sediment indicating increased erosion during the LGCP, with a mid‐LGCP interstadial identified between ca. 25 000 and 23 000 cal. a BP. Rapid climate amelioration at ca. 18 000 cal. a BP was accompanied by increased terrestrial biomass exemplified by the expansion of lowland podocarp forest, especially Dacrydium cupressinum. Increasing biomass production is reversed briefly by LGIT perturbations which are apparent in many of the proxies that span ca. 14 000–10 500 cal. a BP, suggesting generally increased wetness and higher in situ aquatic plant productivity with reduced terrestrial organic matter and terrigenous detrital influx. Furthermore, conditions at that time were probably warmer and frosts rare based on the increasing importance of Ascarina. The subsequent early Holocene is characterised by podocarp conifer forest and moist mild conditions. Postglacial sea‐level rise breached the crater rim and deposited 36 m of estuarine mud after ca. 9000 cal. a BP. Copyright © 2011 John Wiley & Sons, Ltd.

Middle and late Pleistocene glaciations in the Campo Felice Basin (central Apennines, Italy)

The present paper refers to research conducted in the tectonic-karst depression of Campo Felice in the central Apennines (Italy), in which glacial, alluvial and lacustrine sediments have been preserved. Stratigraphic interpretations of sediments underlying the Campo Felice Plain are based on evidence obtained from nine continuous-core boreholes. The boreholes reach a depth of 120 m and provide evidence of five sedimentation cycles separated by erosion surfaces. Each cycle is interpreted as an initial response to a mainly warm stage, characterized by low-energy alluvial and colluvial deposition, pedogenesis, and limited episodes of marsh formation. In turn, a mainly cold stage follows during which a lake formed, and glaciers developed and expanded, leading to deposition of glacial and fluvioglacial deposits. The chronological framework is established by eleven accelerator mass spectrometer 14C ages and three 39Ar– 40Ar ages on leucites from interbedded tephra layers. These age determinations indicate five glacial advances that respectively occurred during marine oxygen isotope stages 2, 3–4, 6, 10 and 14.

230Th/234U and 14C dating of a lowstand coral reef beneath the insular shelf off Irabu Island, Ryukyus, southwestern Japan

Abstract High‐resolution seismic reflection profiles delineated the distribution of mound‐shaped reflections, which were interpreted as reefs, beneath the insular shelf western off Irabu Island, Ryukyus, southwestern Japan. A sediment core through one of the mounded structures was recovered from the sea floor at a depth of −118.2 m by offshore drilling and was dated by radiometric methods. The lithology and coral fauna of the core indicate that the mounded structure was composed of coral–algal boundstone suggesting a small‐scaled coral reef. High‐precision α‐spectrometric 230Th/234U dating coupled with calibrated accelerator mass spectrometric 14C ages of corals obtained reliable ages of this reef ranging from 22.18 ± 0.63 to 30.47 ± 0.98 ka. This proves that such a submerged reef was formed during the lowstand stage of marine oxygen isotope stages 3–2. The existence of low‐Mg calcite in the aragonitic coral skeleton of 22.18 ± 0.63 ka provides evidence that the reef had once been exposed by lowering of the relative sealevel to at least −126 m during the last glacial maximum in the study area. There is no room for doubt that a coral reef grew during the last glacial period on the shelf off Irabu Island of Ryukyus in the subtropical region of western Pacific.

Sea surface temperature variability during the last glacial–interglacial cycle: assessing the magnitude and pattern of climate change in the North Atlantic

Sea surface temperature and salinity estimates reconstructed using planktonic foraminiferal abundance and δ 18O records from core SU90-03 (40°N, 32°W, 2475 m water depth) reveal large climatic fluctuations linked to major instabilities in Northern Hemisphere ice sheets over the last 150 000 years. Episodes of enhanced ice rafted detritus (IRD) input were accompanied by discrete temperature minima, representing coolings of between 4 and 8°C, and reductions in surface salinity of up to 2.5–3.5‰. Several additional cooling episodes of a similar magnitude were documented during intervals of low IRD input that appear to be synchronous, within the limits of dating, with ice rafting events spatially confined to higher latitudes. Accelerator mass spectrometer 14C dates for Heinrich events (H1 — 14.2 ka, H2 — 21.4 ka, H3 — 26.7 ka, H4 — 34.8 ka, H5 — 47.2 ka) obtained from core SU90-03 agree well with other published age estimates and suggest a contemporaneous pattern of climate change throughout the North Atlantic during the last glacial period. This interpretation is supported by a comparison of IRD and palaeotemperature records from DSDP site 609 and core SU90-03, which clearly shows that the major climatic fluctuations identified at high latitudes were transmitted toward the subtropics. However, 14C dates suggest that ice rafting episodes may be diachronous to some extent. The northward migration of the polar front after the H1 event at 40°N in the mid-Atlantic occurred at 14 ka, approximately 500 years earlier than along the Portuguese margin, where the southerly advection of polar waters persisted within eastern boundary current system.

The Structural and Sedimentological Controls on the Reoccupation of Quaternary Incised Valleys, Belize Southern Lagoon

A grid of high-resolution single-channel seismic data from the Belize southern lagoon has documented the development of several generations of stacked late Pleistocene (400 ka-Holocene) incised-valley fills. A feedback mechanism between tectonics, reefal buildups, and incised-valley location is apparent in the data. Incised valleys are recognized as medium- to high-amplitude reflections that truncate reflections produced by older strata on the seismic data. The Holocene valley fills in the Belize southern lagoon contain three seismic facies units (from bottom to top): (1) a basal unit characterized by moderately high-amplitude, chaotic reflections, or progradational patterns, that downlap onto the valley floor; (2) a middle unit characterized by continuous, horizontal, near-transparent reflections that drape the basal unit and onlap the valley walls; and (3) an upper unit that consists predominantly of transparent to low-amplitude, low-angle downlapping reflections. The middle and upper units are separated by an erosional surface. Based on core evidence and seismic facies analysis, the basal, middle, and upper seismic and sedimentary facies units are interpreted as fluvial and carbonate sands, estuarine muds, and marine sands and muds, respectively. The erosional surface separating the upper and middle units is interpreted to be a ravinement surface that marks the transition from brackish estuarine conditions to open-marine conditions. Faulting decreases and reefal buildups increase upsection. The oldest valleys mimic structural trends and flow through areas currently occupied by extensive carbonate reefs. Rapid, high-amplitude late Pleistocene sea level rises may have led to wides pread reef development, creating a template that subsequent incisions (reoccupation valleys) followed, regardless of structural trends. The intimate relationship among tectonics, sea level change, carbonate reef buildups, and incised-valley reoccupation serves as a model for understanding ancient, tropical mixed-carbonate-siliciclastic systems. Accelerator mass spectrometer 14C dates of sediment core samples (marine gastropods, marine mollusks, barnacles, worm tubes, peat, and oysters) indicate all the sediment cored (1-4 m) in this study is Holocene in age (between 4740 ±60 and 11,230 ±90 14C age). This radiocarbon dating consistently links the erosional surface between the upper and middle units with the intersection of the regional sea level curve (nonmarine/marine boundary).

Life and times of the Bering land bridge

UNDERSTANDING the environment of the Bering land bridge and determining the timing of late Wisconsin inundation are important for several areas of study. These include: (1) the timing of the re-establishment of circulation between Pacific and Atlantic Oceans; (2) the timing of development of a northern biotic refugium and the closing of the bridge to species immigration; (3) Palaeoindian migration routes; and (4) palaeotopographic data for atmospheric general circulation models1. Late Wisconsin palaeobotanical and fossil insect data from the central and northern sectors of the Bering land bridge indicate widespread mesic shrub tundra environments even during the last glacial maximum. Contrary to previous hypotheses, we found no evidence of steppe tundra on the land bridge. New accelerator mass spectrometer 14C dates show much of the land bridge was above sea level and thus available for human and animal migration until 11,000 yr BP. Insect evidence suggests that summer temperatures at that time were substantially warmer than now.

Sedimentological proof and dating of the Early Holocene volcanic eruption of Ulmener Maar (Vulkaneifel, Germany)

A minerogeniclayer occurs in early postglacial organic sediments from five maar lakes (West Eifel Volcanic Field, Germany). The mineralogy and stratigraphic position of this tephra suggests that it is related to the youngest German volcano, Ulmener Maar, nearby. Radiocarbon dating of wood from the base of the Ulmener Maar Tephra at two locations provide ages in agreement with an accelerator mass spectrometer 14C date for the minerogenic layer from sediments of Lake Holzmaar situated 13 km south-west of Ulmener Maar. The mean radiocarbon age is 9 560 years BP. Dating by varve chronology provides an age of 10017 years VT (varve time in years before 1950) or 10 895 years corrected VT. Based on palynology the Ulmener Maar Tephra was deposited at the end of the Preboreal. High values of natural remnant magnetization intensity, typical of pyroclastic material, confirm that this minerogenic layer differs in composition from other clastic deposits of the sedimentary record. Geochemical analyses reveal increased values of total trace elements for the Laacher See Tephra and Ulmener Maar Tephra. An isopach map based on thickness variations of the Ulmener Maar Tephra at five investigated maar lakes indicates that the tephra was mainly transported to the south west.

南九州，姶良カルデラ起源の大隅降下軽石と入戸火砕流中の炭化樹木の加速器質量分析法による｀１４´Ｃ年代

南九州，姶良カルデラ起源の大隅降下軽石と入戸火砕流中の炭化樹木の加速器質量分析法による｀１４´Ｃ年代

Sedimentation processes within channel‐ridge systems, southeastern Weddell Sea, Antarctica

On the continental margin of the southeastern Weddell Sea, Antarctica, several channel‐ridge systems can be traced on the eastern side of the Crary Fan. Swath mapping of the bathymetry reveals three southwest‐northeast trending ridges up to 300 m high with channels on their southeastern side. The structures occur on a terrace of the continental slope in water depths of 2000 – 3300 m. We carried out sedimentological studies on cores from three sites. Two of the studied cores are from ridges, one is from the northwestern part of the terrace. The stratigraphy of the recovered sediments is based on accelerator mass spectrometer 14C determinations, stable oxygen and carbon isotopes analyses and paleomagnetic measurements. The sediments represent a period from the last glacial maximum (LGM) to recent time. They are composed predominantly of terrigenous components. We distinguish four different sedimentary facies and assign them to processes controlling sedimentation. Microlaminated muds and cross‐stratified coarse‐silty sediments originated from contour currents. Bioturbated sediments reflect the increasing influence of hemipelagic sedimentation. Structureless sediments with high contents of ice‐rafted debris characterize slumps. The inferred contour currents shaping the continental slope during the LGM were canalized within the channels and supplied microlaminated mud to the western sedimentary ridges due to deflection to the left induced by the Coriolis force. The lamination of the sediments is attributed to seasonal variations of current velocities. The thermohaline bottom currents were directed to the northeast and hence opposite to the Weddell Gyre. Cross‐stratified coarse‐silty contourites on the ridges are intercalated with the muds and indicate spillover of faster thermohaline flows. Average sedimentation rates on the terrace of the continental slope were unusually high (250 cm/ka) during the LGM, indicating active growth phases of the Crary Fan during glacial intervals. A substantial environmental change at 19.5 – 20 ka is documented in the sediments by a gradual change from lamination to bioturbation. During the recent interglacial, bioturbated sediments were deposited in all parts of the terrace. Because of a reduction of the contour current velocities (4–7 cm/s), the water masses of the Weddell Gyre, supplying fine‐grained sediments from northeast, gain a greater influence on sedimentation on the continental slope. Higher percentages of microfossils indicate enhanced biogenic productivity. Increased iceberg activity is documented by greater amounts of ice‐rafted debris. The interglacial sedimentation rates decrease to a few cm/ka and indicate that the Crary Fan became relatively sediment‐starved during interglacial intervals.

Two episodes of meltwater influx from glacial Lake Agassiz into the Lake Michigan basin and their climatic contrasts

Two episodes of meltwater influx from glacial Lake Agassiz are recorded as prominent sedimentologic, isotopic, magnetic, and faunal signatures in southern Lake Michigan profundal sediments. As a tributary to the main path of eastward Lake Agassiz flow, southern Lake Michigan recorded only the largest, catastrophic discharges. The distinctive Wilmette Bed, a massive gray mud that interrupts laminated red glaciolacustrine clays, marks the first episode, which occurred near the beginning of the Younger Dryas cooling event. The associated discharge may have played a role in the inception or severity of the Younger Dryas event. An oxygen isotope excursion in biogenic carbonate and changes in ostracode assemblages mark the second episode, which appears to have had at least two pulses, dated by accelerator mass spectrometer 14C ages on biogenic carbonate at about 8.9 and 8.6 ka. The second episode occurred during the early Holocene peak in global meltwater discharge and apparently had little wide- spread climatic or oceanographic effect. The contrast between the effects associated with these two episodes of meltwater discharge emphasizes the complexity of the ice sheet-ocean-climate system.

Discordant 14C Ages from Buried Tidal-Marsh Soils in the Cascadia Subduction Zone, Southern Oregon Coast

AbstractPeaty, tidal-marsh soils interbedded with estuarine mud in late Holocene stratigraphic sequences near Coos Bay, Oregon, may have been submerged and buried during great (M > 8) subduction earthquakes, smaller localized earthquakes, or by nontectonic processes. Radiocarbon dating might help distinguish among these alternatives by showing that soils at different sites were submerged at different times along this part of the Cascadia subduction zone. But comparison of conventional 14C ages for different materials from the same buried soils shows that they contain materials that differ in age by many hundreds of years. Errors in calibrated soil ages represent about the same length of time as recurrence times for submergence events (150–500 yr)—this similarity precludes using conventional 14C ages to distinguish buried soils along the southern Oregon coast. Accelerator mass spectrometer 14C ages of carefully selected macrofossils from the tops of peaty soils should provide more precise estimates of the times of submergence events.

14C offsets and apparently non-synchronous δ18O stratigraphies between nannofossil and foraminiferal pelagic carbonates

Multiple stratigraphies of subtropical South Atlantic cores reveal significant stratigraphic discrepancies between the nannofossil and the foraminiferal carbonates. Variations in the stratigraphic signals carried in the nannofossil-dominated <38 μm fraction and foraminifera-dominated >38 μm fraction were measured with detailed stratigraphies of δ 18O, δ 13C, 14C, grain size, percentage carbonate, percentage aragonite, and taxonomic composition across the last deglaciation in INMD box core 111 No. 9. Three other cores (INMD box cores 113 and 115, and V 22–174), also from the shallow flanks of the mid-Atlantic ridge in the South Atlantic (10°–17° S), contain similar stratigraphies indicating that these cores represent regional patterns. The onset of the deglacial δ 18O shift in foraminiferal carbonate occurs 6 to 20 cm deeper than the δ 18O shift in the nannofossil fraction. Nineteen accelerator mass spectrometer 14C dates of various fractions (<38 μm, 38–62 μm, 62–150 μm, 150–250 μm, >355 μm, and G. ruber) from INMD Box 111 show that the components within an individual core slice may differ by up to 4900 14C years. Twelve traditional 14C dates (determined by beta counting) of the >38 and <38 μm fractions from six levels in INMD Box 111 No. 6 confirm the large offsets. The observed isotopic offsets are not explained by an individual process, and suggest that multiple causes have left a lumpy stratigraphic record. However, the variability in the data illustrates the difficulty in accurately measuring the chronology of deglaciation, reopens the question of which components of pelagic sediment best monitor surface water conditions, and complicates the direct interpretation of benthic-planktonic age differences in terms of ocean ventilation changes.

Initiation of Fennoscandian ice-sheet retreat during the last deglaciation

ALTHOUGH the retreat history of the southern margin of the Laurentide ice sheet during the last deglaciation has been well known for several decades1–3 and recently supported by evidence from accelerator mass spectrometer (AMS) radiocarbon dating of meltwater in the Gulf of Mexico4 and the North Atlantic subtropical gyre5, the retreat history of the Fennoscandian ice sheet before 13kyr ago is still poorly documented. From AMS 14C dating and isotopic analysis of a rapidly deposited series of supra-till sediments in the Norwegian Channel (North Sea), we find that the southern margin of the Fennoscandian ice sheet was in retreat by 15 kyr BP (before present), approximately 2,000 yr earlier than previously supposed. Oxygen isotope analyses in the channel sediments confirm earlier evidence from deep Norwegian Sea sediment cores for low salinity due to ice sheet discharge beginning ∼15–14.5 kyr BP (ref. 6). Recent recalibration of the 14C timescale indicates that the onset of deglaciation within the Norwegian Sea occurred during an interval of rapidly increasing summer insolation beginning∼18,000 calendar yr BP (ref. 7), suggesting that ice-sheet retreat may have been caused by increasing insolation acting on a particularly climate-sensitive ice-sheet configuration. Fresh water and icebergs discharged into the Norwegian Sea at this time reached the North Atlantic and may have contributed to a brief interval of extreme oceanic cooling8 and reduced production of North Atlantic Deep Water9,10. These factors may also have helped to briefly reverse the deglaciation trend (Erie Interstade) already underway in North America.

Analysis of insoluble carbonaceous materials from airborne particles collected in pristine regions of colorado

Atmospheric particulate samples collected from remote mountainous locations in Colorado were extracted in sequence with methanol, methylene chloride, acetone and hexane. The resulting insoluble material was investigated using pyrolysis-mass spectrometry (Py-MS) and accelerator mass spectrometric 14C analysis. The pyrolysis produced a complex mixture of aromatic and aliphatic hydrocarbons. The aliphatic hydrocarbons were postulated as being derived from vegetation. The 14C analysis and the GC/MS results suggested that the aromatics occurred from combustion of living materials and also from long-range transport of industrial pollution. Appropriate samples from power plants and controlled forest fires were used to substantiate the conclusions.