A. J. Timothy Jull

Preparation of small samples for 14C accelerator targets by catalytic reduction of CO.

Graphite in various forms has become the standard target for accelerator 14 C dating. Graphite has been made by catalytic graphitization of charcoals (Lowe, 1984). Thin films of graphite have also been produced by thermal cracking (Beukens & Lee, 1981), electric discharge (Andree et al , 1984; Wand, Gillespie & Hedges, 1984). Vogel et al (1984) pointed out the ease of graphite formation on iron from CO 2 and H 2 mixtures at ca 600°C. The deposition reactions of carbon from the CO, H 2 , and CO 2 equilibria are well known (Wagman et al , 1945) and well studied. Formation of graphite from CO 2 was discussed extensively by Boudouard (1902) and Schenck and Zimmerman (1903), and was known to chemists in France in 1851. We have used a related method, where graphite forms away from the iron, by using a higher temperature, and reduction of CO 2 to CO over Zn in the presence of H 2 (Jull et al , 1986) as an alternative to the use of Fe alone. The object of this paper is to point out an even simpler graphite preparation system, which eliminates hydrogen. The decomposition reaction of CO (Boudouard, 1902) takes place according to reaction (1).

Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka

Two atmospheric circulation systems, the mid-latitude Westerlies and the Asian summer monsoon (ASM), play key roles in northern-hemisphere climatic changes. However, the variability of the Westerlies in Asia and their relationship to the ASM remain unclear. Here, we present the longest and highest-resolution drill core from Lake Qinghai on the northeastern Tibetan Plateau (TP), which uniquely records the variability of both the Westerlies and the ASM since 32 ka, reflecting the interplay of these two systems. These records document the anti-phase relationship of the Westerlies and the ASM for both glacial-interglacial and glacial millennial timescales. During the last glaciation, the influence of the Westerlies dominated; prominent dust-rich intervals, correlated with Heinrich events, reflect intensified Westerlies linked to northern high-latitude climate. During the Holocene, the dominant ASM circulation, punctuated by weak events, indicates linkages of the ASM to orbital forcing, North Atlantic abrupt events, and perhaps solar activity changes.

Fire-induced erosion and millennial-scale climate change in northern ponderosa pine forests

Western US ponderosa pine forests have recently suffered extensive stand-replacing fires followed by hillslope erosion and sedimentation. These fires are usually attributed to increased stand density as a result of fire suppression, grazing and other land use, and are often considered uncharacteristic or unprecedented. Tree-ring records from the past 500 years indicate that before Euro-American settlement, frequent, low-severity fires maintained open stands. However, the pre-settlement period between about ad 1500 and ad 1900 was also generally colder than present, raising the possibility that rapid twentieth-century warming promoted recent catastrophic fires. Here we date fire-related sediment deposits in alluvial fans in central Idaho to reconstruct Holocene fire history in xeric ponderosa pine forests and examine links to climate. We find that colder periods experienced frequent low-severity fires, probably fuelled by increased understory growth. Warmer periods experienced severe droughts, stand-replacing fires and large debris-flow events that comprise a large component of long-term erosion and coincide with similar events in sub-alpine forests of Yellowstone National Park. Our results suggest that given the powerful influence of climate, restoration of processes typical of pre-settlement times may be difficult in a warmer future that promotes severe fires.

Fire and alluvial chronology in Yellowstone National Park: Climatic and intrinsic controls on Holocene geomorphic processes

We employed a systemwide approach, a large and robust set of radiocarbon ages, and modern process analogs to interpret the Holocene history of forest fire–related sedimentation and overall alluvial activity in northeastern Yellowstone National Park. Debris-flow and flood events following the 1988 fires provided facies models for interpreting the stratigraphic record of fire-related sedimentation within valley-side alluvial fans of Soda Butte Creek. Fire-related deposits make up approximately 30% of the late Holocene fan alluvium. Fifty 14 C ages on fire-related events cluster within the intervals of 3300–2900, 2600–2400, 2200–1800, and 1400–800 yr B.P. and suggest earlier episodes of large fires and fan aggradation around 7500, 5500, and 4600–4000 yr B.P. A major pulse of fire-related debris-flow activity between 950 and 800 yr B.P. coincided with the height of the widely recognized Medieval Warm Period (ca. a.d. 1050–1200). Instrumental climate records over the last ∼100 yr in Yellowstone imply that the intensity and interannual variability of summer precipitation are greater during warmer periods, enhancing the potential for severe short-term drought, major forest fires, and storm-generated fan deposition. Along lower Soda Butte Creek, fill-cut terrace treads were created by lateral migration of channels and accumulation of overbank sediments ca. 8000 yr B.P. (terrace level T1a), 7000–5600 (T1b), 3100–2600 (T2), 2000–1300 (T3), and post–800 yr B.P. (T4). These periods coincide with overbank sedimentation on Slough Creek and the Lamar River but alternate with intervals of fire-related fan deposition, implying a strong climatic control. Local paleoclimatic data suggest cooler, effectively wetter conditions during terrace tread formation. In warmer, drier intervals, reduced average runoff in axial streams results in meander-belt narrowing; concurrent channel incision may be caused by infrequent large floods. Greater resistance to downcutting, however, allowed fewer terraces to be formed along Slough Creek and the Lamar River. Alluvial systems in northeastern Yellowstone show a clear response to millennial-scale climatic cycles, wherein alluvial fans aggrade and prograde over flood plains in drier periods. Axial streams widen their flood plains and trim back the fans during wetter periods. “Small-scale” climatic fluctuations of the Holocene thus had substantial impact on postglacial landscapes in northeastern Yellowstone.

A new extraction technique and production rate estimate for in situ cosmogenic 14C in quartz

The potential of in situ cosmogenic 14C (in situ 14C) for surficial process studies is widely recognized, yet a reliable means of isolating it has proved difficult to develop. We present here a new method for extracting in situ 14C from quartz that overcomes difficulties encountered with earlier techniques, yielding more reliable production rate estimates. Comparison of 14C thermal release patterns from surficial and deeply shielded quartz samples (Lifton, 1997) demonstrated that contaminant 14C is released at or below 500°C, and that 14C released between 500 and 1500°C is essentially all in situ-produced. The new technique builds on this key result, using resistance heating of samples in the presence of a lithium metaborate (LiBO2) flux, and collection of all evolved carbon as CO2 between 500°C and 1100 to 1200°C. Our improved method has four distinct advantages over other extraction methods: (1) we can identify and quantitatively eliminate atmospheric/organic 14C contamination; (2) we can identify the in situ 14C component unambiguously without assumptions of 14CO/14CO2 production proportions within the rock or equilibria on extraction; (3) in situ 14C is reliably extracted from quartz at lower temperatures and in less time than earlier methods and (4) blank 14C levels are consistently low ((2.3 ± 0.1) × 105 14C atoms (1σ)). Our new extraction procedures should thus enable researchers to use in situ 14C in diverse applications without reservation. We developed our new procedures using samples of wave-cut quartzite benches from the well-dated Bonneville (17.4 ± 0.3 cal ky) and Provo (16.8 ± 0.3 cal ky) shorelines of Pleistocene Lake Bonneville, Utah, and from underlying deeply shielded locations. In situ 14C was extracted from quartz separated from 2 Bonneville shoreline samples (6 aliquots) and 1 Provo shoreline sample (2 aliquots). Results demonstrate that our new procedures can effectively isolate the in situ 14C fraction with replicate analytical precision better than 2% (1σ, n = 5), while remaining consistent with earlier results. This level of precision and accuracy is comparable to or exceeds those currently obtainable with in situ cosmogenic 10Be, 26Al, 3He, 21Ne, and 36Cl. Resulting weighted mean in situ 14C site production rates for the Bonneville and Provo shorelines are 52.9 ± 1.7 (14C atoms/g SiO2)/y and 48.7 ± 2.8 (14C atoms/g SiO2)/y (1σ), respectively—consistent with earlier production rate estimates. Current and previously published in situ 14C site production rate estimates were then scaled to sea level and high geomagnetic latitude using the latitude-altitude scaling models of Lal (1991) and Dunai (2000). Results indicate that both models yield sea level, high-latitude production rates consistent with independent estimates. Our new in situ 14C data yield integrated late Quaternary production rate estimates at sea level and high latitude of 15.1 ± 0.5 (14C atoms/g SiO2)/y using the Lal (1991) model, and 15.8 ± 0.5 (14C atoms/g SiO2)/y with that of Dunai (2000). Until significant uncertainties in these models are addressed, however, we prefer the value from the widely-used Lal (1991) model as our best estimate of the integrated late Quaternary production rate for in situ 14C.

Two-step deglaciation of the southeastern Barents Sea

Marine geologic evidence from the western Barents Sea shelf leaves little doubt that theareawascoveredbygroundediceduringthelastglaciation,butthepatternandtiming of the subsequent deglaciation were not well determined. Here we reconstruct the timing and mechanism of ice-sheet retreat as constrained by seismic stratigraphy and lithostratigraphy and accelerator-mass-spectrometer 14 C dating of foraminifera and mollusc shellsinsedimentboringsfromthesoutheasternBarentsSea, ;1000kmfromthewestern shelf edge and former ice-sheet margin. The deposition of ice-proximal glaciomarine sediments upon till began at or shortly before 12.7 ka, indicating that the ice-sheet retreat— whichmostlikelycommencedalongthewesternmarginoftheBarentsSeashelfat15‐14.5 ka—reached the southeastern part of the shelf in #2 ka. The subsequent accumulation of glaciomarinesedimentstookplaceintwodistinctpulses(;12.7‐12.1kaand ;10.5‐9.4ka) separatedbyanintervalofnondepositionlasting

Radiocarbon reservoir correction ages in the peter the Great Gulf, Sea of Japan, and eastern coast of the Kunashir, Southern Kuriles (northwestern Pacific)

1.5ka.Thetwopulsesofsedimentation were coeval with periods of increasing surface-water and air temperatures in the northern Atlanticregionandacceleratedeustaticsea-levelrise,suggestingthattheremainingretreat of the Barents Shelf ice sheet was paced by these factors.

Report on the first stage of the Iron Age dating project in Israel: Supporting a low chronology

The radiocarbon reservoir age correction values (R) for the Russian Far East are estimated as 370+ or -26 yr for the northwestern Sea of Japan, and 711+ or -46 yr for the southern Kurile Islands.

Late Holocene advance of the Muller Ice Shelf, Antarctic Peninsula: sedimentological, geochemical and palaeontological evidence

The traditional chronology of ancient Israel in the 11th9th centuries BCE was constructed mainly by correlating archaeological phenomena with biblical narratives and with Bible-derived chronology. The chronology of Cyprus and Greece, and hence of points further west, are in turn based on that of the Levant. Thus, a newly proposed chronology, about 75100 yr lower than the conventional one, bears crucial implications not only for biblical history and historiography but also for cultural processes around the Mediterranean. A comprehensive radiocarbon program was initiated to try and resolve this dilemma. It involves several hundreds of measurements from 21 sites in Israel. Creating the extensive databases necessary for the resolution of tight chronological problems typical of historical periods involves issues of quality control, statistical treatment, modeling, and robustness analysis. The results of the first phase of the dating program favor the new, lower chronology.

Radiocarbon in dissolved organic and inorganic carbon from the central North Pacific

Marine sediment cores were obtained from in front of the Muller Ice Shelfin Lallemand Fjord, Antarctic Peninsula in the austral summer of 199&91. Sedimentological and geochemical data from these cores document a warm period that preceded the advance of the Miiller Ice Shelf into Lallemand Fjord. The advance of the ice shelf is inferred from a reduction in the total organiccarboncontent and an increase in well-sorted, aeolian, sand in cores proximal to the present calving line. This sedimentological change is paralleled by a change in the foraminifera1 assemblages within the cores. Advance of the ice shelf is indicated by a shift from assemblages dominated by calcareous benthic and planktonic forms to those dominated by agglutinated forms. A I4C chronology for the cores indicates that the advance of the Miiller Ice Shelf took place c. 400 years ago, coincident with glacier advances in other high southern latitude sites during the onset of the Little Ice Age. Ice core evidence, however, documents this period as one of warmer temperatures for the Antarctic Peninsula. We suggest that the ice shelf advance was linked to the exclusion of circumpolar deep water from the fjord. This contributed to increased mass balance of the ice shelf system by preventing the rapid undermelt that is today associated with warm circumpolar deep water within the fjord. We also document the recent retreat of the calving line of the Miiller Ice Shelf that is apparently in response to a recent (four decade long) warming trend along the western side of the Antarctic Peninsula.