Geochronology and paleoenvironment of pluvial Harper Lake,Mojave Desert,California, USA     

Anna L. Garcia  Jeffrey R. Knott  Shannon A. Mahan  Jordon Bright 《Quaternary Research》2014

Accurate reconstruction of the paleo-Mojave River and pluvial lake (Harper, Manix, Cronese, and Mojave) system of southern California is critical to understanding paleoclimate and the North American polar jet stream position over the last 500 ka. Previous studies inferred a polar jet stream south of 35°N at 18 ka and at ~ 40°N at 17–14 ka. Highstand sediments of Harper Lake, the upstream-most pluvial lake along the Mojave River, have yielded uncalibrated radiocarbon ages ranging from 24,000 to > 30,000 ¹⁴C yr BP. Based on geologic mapping, radiocarbon and optically stimulated luminescence dating, we infer a ~ 45–40 ka age for the Harper Lake highstand sediments. Combining the Harper Lake highstand with other Great Basin pluvial lake/spring and marine climate records, we infer that the North American polar jet stream was south of 35°N about 45–40 ka, but shifted to 40°N by ~ 35 ka. Ostracodes (Limnocythere ceriotuberosa) from Harper Lake highstand sediments are consistent with an alkaline lake environment that received seasonal inflow from the Mojave River, thus confirming the lake was fed by the Mojave River. The ~ 45–40 ka highstand at Harper Lake coincides with a shallowing interval at downstream Lake Manix.  相似文献   

Age and impacts of the caldera-forming Aniakchak II eruption in western Alaska     

J.J. Blackford  R.J. Payne  M.P. Heggen  A. de la Riva Caballero  J. van der Plicht 《Quaternary Research》2014

The mid-Holocene eruption of Aniakchak volcano (Aniakchak II) in southwest Alaska was among the largest eruptions globally in the last 10,000 years (VEI-6). Despite evidence for possible impacts on global climate, the precise age of the eruption is not well-constrained and little is known about regional environmental impacts. A closely spaced sequence of radiocarbon dates at a peatland site over 1000 km from the volcano show that peat accumulation was greatly reduced with a hiatus of approximately 90–120 yr following tephra deposition. During this inferred hiatus no paleoenvironmental data are available but once vegetation returned the flora changed from a Cyperaceae-dominated assemblage to a Poaceae-dominated vegetation cover, suggesting a drier and/or more nutrient-rich ecosystem. Oribatid mites are extremely abundant in the peat at the depth of the ash, and show a longer-term, increasingly wet peat surface across the tephra layer. The radiocarbon sample immediately below the tephra gave a date of 1636–1446 cal yr BC suggesting that the eruption might be younger than previously thought. Our findings suggest that the eruption may have led to a widespread reduction in peatland carbon sequestration and that the impacts on ecosystem functioning were profound and long-lasting.  相似文献   

Advances and limitations in establishing a contiguous high-resolution atmospheric radiocarbon record derived from subfossil kauri tree rings for the interval 60–27 cal kyr BP     

《Quaternary Geochronology》2022

Radiocarbon dating is the most widely applied and reliable dating technique for providing chronological control during Marine Isotope Stage 3 (MIS3; ∼60–27 cal kyr BP). Past variations in the atmospheric concentration of radiocarbon mean a calibration curve is required. IntCal20 and SHCal20 calibration curves covering MIS3 are presently largely based on non-atmospheric records which, in combination with larger radiocarbon (¹⁴C) dating uncertainties, results in significant smoothing and reduced resolution in calibration curve structure. Floating tree ring radiocarbon chronologies that are wiggle-matched to other palaeo records (particularly to Hulu Cave speleothems) have the potential to provide detailed structure to the MIS3 portion of extant calibration curves. New Zealand subfossil kauri (Agathis australis) trees are long-lived and are useful for constructing temporally-floating MIS3 atmospheric radiocarbon datasets. This paper presents extant and emerging data from several important Northland subfossil kauri locations (Omaha, Babylon Coast, Bream Bay, Kai Iwi Lakes, Mangawhai). We show the span of seven floating MIS3 kauri sequences (individual trees and chronologies) from which sequential radiocarbon series covering a total of 7556 years is now in development (representing 23% of the period 60–27 cal kyr BP). We also report radiocarbon dates for an additional 34 ancient kauri from MIS3 that can provide additional coverage. After these floating subfossil wood sequences have been produced, close to 40% of MIS3 (12,420 years) will be covered by contiguous subfossil kauri radiocarbon measurements. Based on our findings, we discuss the prospects and limitations for obtaining a highly resolved and precise atmospheric radiocarbon calibration curve comprehensively covering MIS3 using subfossil kauri.  相似文献   

Wisconsinan Late-glacial environmental change in southern New England: A regional synthesis     

D. M. Peteet  R. Daniels  L. E. Heusser  J. S. Vogel  J. R. Southon  D. E. Nelson 《第四纪科学杂志》1994,9(2):151-154

A synthesis of vegetational and climatic history during the Wisconsinan Late-glacial (13-9 ka BP) is presented for the southern New England region. The interpretations are based on a number of pollen-stratigraphical investigations supported by plant macrofossil data. Chronology is based partly on AMS dates of plant macrofossils. Reference is made briefly to the controversial nature of the evidence for deglaciation in the region. A curve representing mean July temperature changes in the region during the period 13-9 ka BP is provided.  相似文献   

Épisodes majeurs de la podzolisation en forêt de Fontainebleau (France). Essai de synthèse à l'aide du radiocarbone naturel     

Anne-Marie Robin 《Comptes Rendus Geoscience》2005,337(6):599-608

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Upwelling signals in radiocarbon from early 20th-century Peruvian bay scallop (Argopecten purpuratus) shells     

Kevin B. Jones  Gregory W.L. Hodgins  Miguel F. Etayo-Cadavid  C. Fred T. Andrus 《Quaternary Research》2009,72(3):452-456

We quantified Δ¹⁴C, δ¹⁸O, and δ¹³C cycles along ontogeny within four bay scallop (Argopecten purpuratus) shells collected from Callao Bay, Salaverry, and Sechura Bay, Peru following the 1907–1908 non-El Niño years and the 1925–1926 El Niño. Δ¹⁴C and δ¹³C generally covary; Δ¹⁴C and δ¹⁸O vary inversely. Simultaneous decreases in Δ¹⁴C and increases in δ¹⁸O in non-El Niño shells are followed by constant Δ¹⁴C and gradually decreasing δ¹⁸O, which we interpret as evidence for discrete marine upwelling events followed by warming of the initially cold upwelled water. Upwelling changes from El Niño events are detectable with difficulty in mollusk shell Δ¹⁴C.  相似文献   

An analysis of bomb radiocarbon trends in the Pacific     

Amala Mahadevan   《Marine Chemistry》2001,73(3-4)

The post-nuclear time-series curves of Δ¹⁴C from corals at different locations in the surface of the Pacific Ocean show a variation in the shape, amplitude and timing of the peak, with the subtropical records peaking first, followed by the western, and then eastern tropical records with lower maxima. This work takes an in-depth look at the processes that shape the time histories of Δ¹⁴C in surface waters at different locations in the Pacific. A one-dimensional (1-D) model is used to examine whether convection and diffusion can delay the peaking of the Δ¹⁴C time series. Using the three-dimensional (3-D) MIT general circulation model (GCM), the distribution and evolution of Δ¹⁴C is simulated “offline” from 1955 onwards at 1° resolution globally. The GCM is used to tease apart the contribution of various processes, viz. advection, air–sea flux, convection and diffusion, to altering the Δ¹⁴C content of surface waters at different locations in the Pacific. A time history of ¹⁴C column inventories from the model is constructed to examine the role of horizontal advection in supplying tropical locations with ¹⁴C much after the peak atmospheric flux. This model analysis supports the idea of ¹⁴C-rich waters from the subtropics being transported to the western tropics via the subsurface, and then being advected eastward in the equatorial undercurrent and upwelled in the east.  相似文献   

Forecast of the Decadal Average Sunspot Number     

D. M. Volobuev  N. G. Makarenko 《Solar physics》2008,249(1):121-133

The forecast of the decadal average sunspot number (SN) becomes possible with an extension of telescopic observations based on proxy reconstructions using the tree ring radiocarbon data during the Holocene. These decadal numbers (SN_RC) provide a powerful statistic to verify the forecasting methods. Complicated dynamics of long-term solar activity and noise of proxy-based reconstruction make the one-step-ahead forecast challenging for any forecasting method. Here we construct a continuous data set of SN_RC which extends the group sunspot number and the international sunspot number. The known technique of nonlinear forecast, the local linear approximation, is adapted to estimate the coming SN. Both the method and the continuous data set were tested and tuned to obtain the minimum of a normalized average prediction error (E) during the last millennium using several past millennia as a training data set. E=0.58σ _D is achieved to forecast the SN successive differences whose standard deviation is σ _D=7.39 for the period of training. This corresponds to the correlation (r=0.97) between true and forecasted SN. This error is significantly smaller than the prediction error when the surrogate data were used for the training data set, and proves the nonlinearity in the decadal SN. The estimated coming SN is smaller than the previous one.  相似文献   

Kīlauea summit overflows: their ages and distribution in the Puna District, Hawai'i     

David A. Clague  Jonathan T. Hagstrum  Duane E. Champion  Melvin H. Beeson 《Bulletin of Volcanology》1999,61(6):363-381

 The tube-fed pāhoehoe lava flows covering much of the northeast flank of Kīlauea Volcano are named the 'Ailā'au flows. Their eruption age, based on published and six new radiocarbon dates, is approximately AD 1445. The flows have distinctive paleomagnetic directions with steep inclinations (40°–50°) and easterly declinations (0°–10°E). The lava was transported ∼40 km from the vent to the coast in long, large-diameter lava tubes; the longest tube (Kazumura Cave) reaches from near the summit to within several kilometers of the coast near Kaloli Point. The estimated volume of the 'Ailā'au flow field is 5.2±0.8 km³, and the eruption that formed it probably lasted for approximately 50 years. Summit overflows from Kīlauea may have been nearly continuous between approximately AD 1290 and 1470, during which time a series of shields formed at and around the summit. The 'Ailā'au shield was either the youngest or the next to youngest in this series of shields. Site-mean paleomagnetic directions for lava flows underlying the 'Ailā'au flows form only six groups. These older pāhoehoe flows range in age from 2750 to <18,000 BP, and the region was inundated by lava flows only three times in the past 5000 years. The known intervals between eruptive events average ∼1600 years and range from ∼1250 years to >2200 years. Lava flows from most of these summit eruptions also reached the coast, but none appears as extensive as the 'Ailā'au flow field. The chemistry of the melts erupted during each of these summit overflow events is remarkably similar, averaging approximately 6.3 wt.% MgO near the coast and 6.8 wt.% MgO near the summit. The present-day caldera probably formed more recently than the eruption that formed the 'Ailā'au flows (estimated termination ca. AD 1470). The earliest explosive eruptions that formed the Keanakāko'i Ash, which is stratigraphically above the 'Ailā'au flows, cannot be older than this age. Received: 10 October 1998 / Accepted: 12 May 1999  相似文献   

Large 14C age offsets between the fine fraction and coexisting planktonic foraminifera in shallow Caribbean sediments     

《Quaternary Geochronology》2017

Absolute chronologies in paleoceanographic records are often constructed using the ¹⁴C dating of coarse fraction foraminifera (>150 μm). However, due to processes such as changes in sediment sources or abundances, sedimentation rates, bioturbation, reworking, the adsorption of modern carbon, etc., several studies conducted in different environmental settings have shown time-lags between records obtained from various granulometric fractions. In this study, we examined temporal phasing between the coarse foraminifera and fine fractions by studying changes in the abundances of δ¹⁸O, the ¹⁴C ages of the planktonic foraminifera Globigerinoides ruber (G. ruber, 250–350 μm), and the sediment fine fraction (<63 μm) over the last 45 ka in a core obtained from the northern Caribbean Sea. All of the records were found to be in phase during part of the Holocene (at least for the last ≈6 ka). As determined from δ¹⁸O records and ¹⁴C ages, the fine fraction was younger than G. ruber during the Last Deglaciation (of 1.89 ka). The coupling between bioturbation and changes in the fine fraction, and G. ruber abundances, as tested using a numerical model of the bioturbation record within a mixed-layer depth of 8 cm, was sufficient to explain the results. ¹⁴C age discrepancies increased from 5.64 to 8.5 ka during Marine Isotopic Stages (MIS) 2 and 3, respectively. These chronological discrepancies could not be explained by only one process and seemed to result from the interplay between mechanisms: size-differentiated bioturbation (for 1.5 to 2.5 ka), the adsorption of modern atmospheric CO₂ (for 3.04 to 5.92 ka), and variations in sedimentological processes that influenced the fine carbonate fraction. However, even if variations in the mineralogical composition of the fine carbonate fraction were identified using scanning-electron microscopy observations, X-ray diffraction measurements, and geochemical analyses (the mol % MgCO₃ of magnesian calcite and the Sr/Ca ratio of the bulk fine fraction), they can not account for the observed age differences. The results presented for core MD03-2628 extend beyond this case study because they illustrate the need for a detailed characterization of the various size fractions prior to paleoclimate signal interpretations, especially for chronological studies.  相似文献