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1.
An IRSL age of 17.0 ± 2.2 ka (and a “mean age” of ca. 19 ka) reported by Grapes et al. [Grapes, R., Rieser, U., Wang, N. Optical luminescence dating of a loess section containing a critical tephra marker horizon, SW North Island of New Zealand. Quaternary Geochronology 5(2-3), 164–169.] for the Kawakawa/Oruanui tephra, and other ages associated with a loess section in New Zealand are untenable: age data presented are inconsistent, no formal statistical treatments or error determinations were undertaken in age analysis, and the ages proposed are seriously at odds with multiple radiocarbon age determinations on tephra sequences bracketing the Kawakawa/Oruanui tephra and with palaeoenvironmental evidence elsewhere for the time period concerned. We suggest that the bulk polymineral IRSL ages on the tephra and encapsulating loess deposits were underestimated in part because of contamination of the loess by the integration of younger materials during slow deposition and continuous modification by upbuilding pedogenesis. Single-grain luminescence assays may reveal such contamination. A 14C-based age of ca. 27 ± 1 ka cal BP (2σ), reported in 2008, currently remains the best estimate for the age of eruption of the Kawakawa/Oruanui tephra.  相似文献   

2.
Changes in oceanic radiocarbon (14C) reservoir ages through the deglaciation and Holocene can provide important information on ocean circulation as Earth's climate warmed. Here, we present reservoir ages for the western tropical Pacific that span the mid-Holocene transition from less to more frequent El Niño events. Reservoir ages were calculated using paired U–Th and conventional 14C dating of eight individual fossil coral samples from Koil and Muschu Islands, northern coastal Papua New Guinea (PNG). AMS 14C and MC-ICPMS U–Th dating of additional samples from six of the fossil corals were used to confirm the TIMS U–Th and conventional 14C ages. The combined results show average reservoir ages of 185±30 14C yr (n=4) for 7220–5850 yr BP compared to 420 14C yr for a modern coral from Muschu Island. From 5850 to 5420 yr BP reservoir ages increase to modern values. The relatively young reservoir ages from 7220 to 5850 yr BP are best explained by greater influx of well-equilibrated sub-tropical water from the southern branch of the South Equatorial Current (SEC). This is consistent with strengthening trade winds (facilitating air–sea exchange) and a more northerly position of the Intertropical Convergence Zone thought to have occurred at this time. The transition to more modern-like reservoir ages from 5850 to 5420 yr BP suggests modern oceanic circulation patterns were established during this interval. The onset of modern El Niño activity around this time would have served to enhance the intrusion of 14C-depleted equatorial waters via the south equatorial branch of the SEC. Overall, the changes in reservoir age presented here for the western tropical Pacific suggest that Holocene changes in the El Niño–Southern Oscillation state of the tropical Pacific resulted in reorganisation of oceanic circulation in this region.  相似文献   

3.
A new IRSL dataset is presented for the age and setting of a critical Late Glacial Maximum tephra isochron marker. The rhyolitic tephra, known as the Kawakawa Tephra, occurs as a 14 cm thick layer within a 5.9 m thick loess section overlying alluvial gravels in the Rangitikei River valley, SW North Island of New Zealand. Ages range from 21 at the base to 5 ka near the top of the loess and bracket an age of 17.0 ± 2.2 for the tephra. The new IRSL ages are in agreement with published and unpublished luminescence ages from other localities of loess, sand and ash above and below the tephra and of the tephra itself, that indicate an age of ca. 19 ka for the Kawakawa Tephra. This age is considerably younger than the generally accepted 14C 27.1 ka cal yrs BP age of the Kawakawa Tephra and highlights an unresolved discrepancy between the two dating systems.  相似文献   

4.
Dust depositions are critical archives for understanding interior aridification and westerly climatic changes in Central Asia. Accurate and reliable dating of loess is very important for interpreting and correlating environmental records. There remains a disparity between luminescence ages and radiocarbon dating of late Quaternary loess from the Ili Basin in Central Asia. In this study, we establish a closely spaced quartz optically stimulated luminescence (OSL) chronology for the 20.5-m-thick Nilka loess section in the Ili Basin. Based on OSL ages, two intervals of higher mass accumulation rate occurred at 49–43 ka and 24–14 ka. We further compare these OSL ages with 23 accelerator mass spectrometry (AMS) 14C ages of bulk organic matter. The results indicate that the OSL and radiocarbon ages agree well for ages younger than ca. 25 14C cal ka BP. However, beyond 30 cal ka BP, there is no consistent increase in AMS 14C age with depth, while the OSL ages continue to increase. These differences confirm the observation that the AMS 14C ages obtained using conventional acid–base–acid (ABA) pretreatment are severely underestimated in other terrestrial deposits in Central Asia, which could be due to 2–4% modern carbon contamination. However, OSL dating is applicable for constructing an accurate chronology beyond 30 cal ka BP. We suggest caution when interpreting paleoenvironmental changes based on radiocarbon ages older than 25 cal ka BP.  相似文献   

5.
High-resolution Pliocene and Pleistocene sequences exposed on land in New Zealand are some of the few detailed records of widepread marine bioevents and paleoclimatic changes in the Southern Hemisphere. Marine biostratigraphy calibrated in deep-sea cores by paleomagnetic reversals has been the primary basis for the chronology of these sequences. We have determined ages for several tephra beds which now provide an independent numerical age calibration for a well-studied marine and terrestrial section in Wairarapa. By using the isothermal plateau fission track (ITPFT) method on volcanic glass we have overcome the problems of partial track fading and detrital mineral contamination, which hindered earlier studies, to reveal a new chronology extending back to nearly 5 Ma.

Our ages for the Hikawera Tuff (4.91 ± 0.25 Ma) and Spooner Tuff (3.44 ± 0.13 Ma) are consistent with the appearance and disappearance of many early Pliocene foraminiferial species, validating their age calibration in New Zealand. However, some fossil occurrences, including coccoliths, differ temporally by as much as 0.55 Ma, perhaps due to local tectonic-induced recycling.

Four Pleistocene tephra beds (Potaka tephra (1.00 ± 0.03 Ma), Kaukatea tephra (0.87 ± 0.05 Ma), Rangitawa tephra (ca. 0.35 Ma) and Kawakawa tephra (ca. 0.22 Ma)) are now recognised in the Wairarapa sequence via stratigraphic and new geochemical and age data. These beds allow direct correlation to other marine and terrestrial basins, as well as volcanic regions in New Zealand, and will ultimately aid in a regional paleoenvironmental reconstruction where bioevents are absent. The tephra ages indicate that the marine sediment accumulation rates varied from 90 to 250 m/Ma between different sections of the Pliocene and reached ca. 350 m/Ma in the last 2.4 Ma, when the sequence displays pronounced glacioeustatic cyclic deposition. In the terrestrial realm, the oldest loess in New Zealand is now constrained to between 1.00 and 0.87 Ma.  相似文献   


6.
Optically stimulated luminescence (OSL) dating of perennially frozen loess was tested on quartz grains extracted from deposits associated with the late Pleistocene Dawson tephra in western Yukon Territory, Canada. OSL samples were obtained from ice-rich loess bracketing the Dawson tephra, while radiocarbon (14C) samples were collected from the bulk sediments directly underlying the tephra and from a ground-squirrel burrow 2.7 m below the tephra. Here we report the OSL characteristics and ages of the extracted quartz grains, as well as additional radiocarbon ages for samples described in Froese [2002. Age and significance of the late Pleistocene Dawson tephra in eastern Beringia. Quaternary Science Reviews 21, 2137–2142; 2006. Seasonality of the late Pleistocene Dawson tephra and exceptional preservation of a buried riparian surface in central Yukon Territory, Canada. Quaternary Science Reviews 25, 1542–1551]. We refine the time of Dawson tephra deposition to between 25,420±70 and 25,290±80 14C a BP. Bayesian analysis of constraining radiocarbon ages places the deposition of the Dawson tephra at between 30,433 and 30,032 cal a BP. Linear modulation (LM) OSL analysis of multi-grain aliquots of quartz showed that the initial part of the decay curve is dominated by a rapidly bleached (‘fast’) component; these samples, however, had relatively dim continuous wave (CW) OSL signals at the multi-grain aliquot (each composed of 80 grains) and single-grain scales of analysis. The single-aliquot regenerative-dose protocol was applied to multi-grain aliquots and single grains to obtain equivalent dose (De) values for samples collected from below and above the Dawson tephra. The De values were examined graphically and numerically, the latter using the central age, minimum age, and finite mixture models. For multi-grain aliquots, the central age model gave weighted mean De values between 30 and 50 Gy, which greatly underestimated the expected De of 74–81 Gy for both samples studied. Possible reasons for these underestimations are discussed, and a solution proposed based on single-grain analysis. Measurements of single grains produced De values in agreement with the expected De, and yielded OSL ages of 28±5 and 30±4 ka for the samples taken from above and below the Dawson tephra, respectively. Examination of individual grains with differing luminescence behaviors showed that a significant number of the measured quartz grains exhibited anomalous luminescence properties that would have compromised the results obtained from multi-grain aliquots. We therefore recommend analysis of individual grains to overcome the age-shortfall from multi-grain analysis of these and similar samples of quartz.  相似文献   

7.
The arguments presented by Lowe et al. [Lowe, D.J., Wilson, C.J.N., Newham, R.M., Hogg, A.G., 2010. Dating the Kawakawa/Oruanui eruption: comment on “optical luminescence dating of a loess section containing a critical tephra marker horizon, SW North Island of New Zealand” by R. Grapes et al. Quaternary Geochronology 5(4), 493–496] against our IRSL results, which suggested that the widespread Kawakawa tephra (KkT) could be considerably younger than the generally accepted 27.1 ka cal BP age, are unsustainable. We discuss the points raised by Lowe et al., in terms of: 1) Presentation and analysis of luminescence ages (comparison between reporting and error margins of luminescence and 14C ages, statistical treatment of age data); 2) Possible sources of error (“upbuilding pedogenesis” and its affect on U and Th distribution in loess, effect of biotubation, variation of K in loess, single grain luminescence dating of quartz, probability of luminescence age underestimation in dating tephra); 3) Stratigraphic and paleoenvironmental considerations (ages of tephras overlying KkT, timing of the end of Ohakea loess deposition and its distribution; 4) Radiocarbon-based ages of KkT (problems with the currently accepted 14C 27.1 ka cal BP age of KkT). We stress that our study was not to establish a new benchmark age for the KkT, but to open debate about the currently accepted benchmark age of the KkT, which we deem to be erroneous.  相似文献   

8.
Late Pleistocene records of loess deposition are a critical archive for understanding terrestrial paleoenvironment changes in Central Asia. The age of loess is not well known for the deserts regions and surrounding high plateaus in Central Asia. Previous studies have shown that there remains a disparity between ages for loess deposition by luminescence and 14C dating. This study evaluates the potential of optically stimulated luminescence (OSL) to date a loess sequence resting on fluvial sands in the east Ili Basin, Central Asia. The single-aliquot regenerative-dose (SAR) protocol on coarse grain quartz was employed for equivalent dose determinations. The basal fluvial sand returned a secure OSL age, with low overdispersion value in equivalent doses (19 ± 2%) of ca. 36 ka and provides a close, but maximum age estimate (within 5 ka) on the initiation of loess deposition. However, the loess yielded high overdispersion values for equivalent doses and age reversals, coincident with diffuse paleosols; indicating that pedoturbation with loess deposition may be a dominant process. OSL ages between ca. 45 and 14 ka calculated using a maximum age model and OSL ages from other sites in the Basin suggests that the latest major period of loess deposition was between 70 and 10 ka ago. A future hypothesis to test based on these analyses is that there may be three periods of heightened loess deposition at ca. 45, 35 to 19 and 14 ka, when desert source areas to the west were particularly dry.  相似文献   

9.
The Ko-g and Ma-f~j tephras are two key isochronous marker layers in northern Japan, which are from the largest Plinian eruptions of Komagatake volcano (VEI = 5) and Mashu caldera (VEI = 6), respectively. Despite extensive radiocarbon studies associated with the two tephras, individual calibrated results show considerable variations and thus accurate ages of these important eruptions remain controversial. Bayesian statistical approaches to calibrating radiocarbon determinations have proven successful in increasing accuracy and sometimes precision for dating tephras, which is achieved through the incorporation of additional stratigraphic information and the combination of evidence from multiple records. Here we use Bayesian approaches to analyse the proximal and distal information associated with the two tephra markers. Through establishing phase and deposition models, we have taken into account all of the currently available stratigraphic and chronological information. The cross-referencing of phase models with the deposition model allows the refinement of eruption ages and the deposition model itself. Using this we are able to provide the most robust current age estimates for the two tephra layers. The Ko-g and Ma-f~j tephras are hereby dated to 6657-6505 (95.4%; 6586±40, μ±σ) cal yr BP, and 7670-7395 (95.4%; 7532±72, μ±σ) cal yr BP, respectively. These updated age determinations underpin the reported East Asian Holocene tephrostratigraphic framework, and allow sites where the tephra layers are present to be dated more precisely and accurately. Our results encourage further applications of Bayesian modelling techniques in the volcanically active East Asian region.  相似文献   

10.
This study presents results from 20 optically stimulated luminescence (OSL) ages from one of the world's largest beach ridge plains, the Jerup beach ridge plain at the base of the Skagen Spit in the northernmost part of Jutland, Denmark. The OSL ages were obtained using quartz and a SAR protocol, and used to establish a chronology for the beach ridge plain and for the underlying coastal plain. The accuracy of the chronology is tested both by laboratory tests (recuperation, recycling ratio and dose recovery) and by comparison with independent age controls, e.g. previously reported radiocarbon dates, map sources, anecdotal evidence and settlement names. It is concluded that the OSL signals are internally consistent, and that the derived OSL ages are in good agreement with a large range of independent age controls. The ridge plain is shown to cover a time span from 1000 to 2700 years ago; this chronology is more detailed and precise than those previously available, and gives an average beach ridge formation rate of 15 yr/ridge and an average lateral migration rate of 2.0 m/yr. This study adds to the growing knowledge that OSL dating has a large potential for establishing detailed and precise chronologies in coastal marine sediments, including beach ridges.  相似文献   

11.
A 1075 cm long core (Lz1120) was recovered in the south-eastern part of the Lake Ohrid (Republics of Macedonia and Albania) and sampled for identification of tephra layers. Magnetic susceptibility investigations show rather high magnetic values throughout the core, with peaks unrelated to the occurrence of tephra layers but instead to the relative abundance of detrital magnetic minerals in the sediment. Naked-eye inspection of the core allowed us to identify of two tephra layers, at 896–897 cm and 1070–1075 cm. Laboratory inspection of the grain-size fraction > 125 μm allowed for the identification of a third cryptotephra at 310–315 cm. Major element analyses on glass shards of the tephra layers at 896–897 cm and 1070–1075 cm show a trachytic composition, and indicate a correlation with the regionally dispersed Y-3 and Y-5 tephra layers, dated at ca 30 and 39 cal ka BP. The cryptotephra at 310–315 cm has a mugearitic–benmoreitic composition, and was correlated with the FL eruption of Mt. Etna, dated at 3370 ± 70 cal yr BP. These ages are in agreement with five 14C AMS measurements carried out on plant remains and macrofossils from the lake sediments at different depths along the core.  相似文献   

12.
Optically stimulated luminescence (OSL) dating was applied to glacial and loess deposits in the north flank of the Terskey-Alatoo Range, Kyrgyz Republic, to elucidate the glacier chronology of the central Asian mountains during the Last Glacial. Moraines in five parts of study area were classified into four stages (Terskey Stages I–IV) based on their geographical position and elevation, and their moraine rock weathering. According to this classification, the oldest moraines (Terskey Stage I) were at 2100–2250 m a.s.l. and the second-oldest moraines (Terskey Stage II) were at 2400–2700 m a.s.l. Quartz samples from moraines of these two stages were used for OSL dating. The OSL ages of the quartz samples indicate that glacier expansion in the Terskey Stage II occurred between 21 and 29 ka BP.  相似文献   

13.
Similar to the loess in the Chinese Loess Plateau (CLP), the loess deposits in the Ili basin of Central Asia arid area play an important role in understanding the climate and environmental changes. However, in contrast to the intensively investigated loess deposits in the CLP, the Ili loess is still insufficiently known and poorly understood. The geochronology study of the Ili loess remains controversial. In order to examine the potential of optically stimulated luminescence (OSL) dating for the Ili loess, we carry out a combined luminescence and radiocarbon dating study on a 6.9 m loess section in the south margin of the Ili basin. Polymineral fine grains were investigated by post infrared (IR) OSL using a Multiple-Aliquot Regenerative-dose (MAR) protocol. Radiocarbon dating of organic carbon were carried in a 3 Megavolt (MV) multi-element Accelerator Mass Spectrometry (AMS). The results indicate that the OSL ages are in agreement with the observed stratigraphy in the field, which is well correlated with that of the CLP, but the AMS 14C ages are much younger than the OSL and assumed stratigraphical ages. Thus, the OSL dating technique may provide an absolute chronology in this loess section. Further methodological approaches and more samples analysis will lead to the improvement of this chronology for high-resolution paleoclimatic interpretation.  相似文献   

14.
The loess deposits surrounding the high mountainous regions of Central Asia play an important role in understanding environmental changes in Eurasia on orbital and sub-orbital timescales. However, problems with dating Central Asian loess have limited the interpretation of climatic and environmental data, especially on sub-orbital timescales. We selected a controversial loess section, Zeketai (ZKT, with a thickness of 23 m), in the Yili basin in Xinjiang Province in western China, to establish a detailed and systematic Optically Stimulated Luminescence (OSL) chronology. Quartz grains of 38–63 μm were isolated from 15 samples and the single-aliquot regenerative-dose (SAR) protocol was employed for De determination. OSL ages are in stratigraphic order and range from 13.8 ± 1 to 72 ± 6 ka, suggesting continuous loess accumulation during the last glaciation. We compared these dating results with that of the previously published fine-grain sized quartz (4–11 μm) using simplified multiple aliquot regenerative-dose (SMAR) protocol, and with the previous published radiocarbon dating (14C) ages on snail shells. With the exception of three young samples from the upper 6 m of the section, the SMAR dating results are basically consistent with the results using the SAR protocol. Both the SMAR and SAR OSL ages are consistently older than the 14C ages, and the radiocarbon date results should be used with caution since they appear to have been underestimated.  相似文献   

15.
This study reports on the first investigation into the potential of quartz luminescence dating to establish formation ages of ferruginous duricrust deposits (ironstones) of the Xingu River in Eastern Amazonia, Brazil. The studied ironstones comprise sand and gravel cemented by goethite (FeO(OH)), occurring as sandstones and conglomerates in the riverbed of the Xingu River, a major tributary of the Amazon River. The Xingu ironstones have a cavernous morphology and give origin to particular habitat for benthic biota in an area that hosts the largest rapids in Amazonia. So far, the Xingu ironstones have uncertain formation ages and their sedimentary origin is still poorly understood. In this way, seven samples of ironstones distributed along the lower Xingu River were collected for optically stimulated luminescence (OSL) dating of their detrital quartz sand grains. Additionally, the organic content of some samples was dated by radiocarbon (14C) for comparison with quartz OSL ages. The luminescence ages of the sand-sized quartz grains extracted from the ironstones were obtained from medium (100–300 grains) and small (10–20 grains) aliquots using the single aliquot regenerative-dose (SAR) protocol. Equivalent doses (De) distributions have varied overdispersion (OD) both for medium size aliquots (OD = 19–58 %) and small size aliquots (OD = 29–76 %). No significant trend was observed between De and aliquot size. The studied ironstones grow over the riverbed, but stay below or above water throughout the year due to the seasonal water level variation of the Xingu River. However, the effect of water saturation in dose rates is reduced due to relatively low porosity of ironstones. Water saturated dose rates (dry sample dose rates) range from 2.70 ± 0.21 (2.79 ± 0.22) Gy/ka to 12.34 ± 0.97 (13.26 ± 1.12) Gy/ka, which are relatively high when compared to values reported for Brazilian sandy sediments elsewhere (∼1 Gy/ka). Samples with high overdispersion (>40 %) are mainly attributed to mixing of grains trapped in different time periods by goethite cementation. The obtained OSL ages for water saturated (dry) samples range from 3.4 ± 0.3 (3.3 ± 0.4) ka to 59.6 ± 6.0 (58.1 ± 6.4) ka, using De determined from medium size aliquots and dose response curves fitted by an exponential plus linear function. Radiocarbon ages of the bulk organic matter extracted from selected ironstone varied from ca. 4 cal ka BP to ca. 23 cal ka BP. Significant differences were observed between OSL and radiocarbon ages, suggesting asynchronous trapping of organic matter and detrital quartz within the ironstone matrix. These late Pleistocene to Holocene ages indicate that ironstones of the Xingu River result from an active surface geochemical system able to precipitate goethite and cement detrital sediments under transport. The obtained ages and differences between OSL and radiocarbon ages point out that the ironstones have multiphase and spatially heterogeneous growth across the Xingu riverbed. Our results also expand the application of luminescence dating to different sedimentary deposits.  相似文献   

16.
Similar to the loess deposits on the Chinese Loess Plateau (CLP), a reliable chronology plays also an important role in revisiting past climate and environment changes recorded by the loess in Central Asia. Previously, a few luminescence and 14C ages, mainly covering the last glacial, were obtained for several loess sections in Central Asia, which are often controversial. Until now, there is still a lack of reliable high-resolution chronologies covering the late Pleistocene, especially the Holocene. Here, the fine-grained (4–11 μm) quartz single-aliquot regenerative-dose (SAR) optically stimulated luminescence (OSL) dating protocol is used to establish a detailed chronology of the uppermost part (∼5 m) of Talede loess in the Ili Basin Central Asia. Conventional tests of the SAR protocol and the general luminescence characteristics indicate that this protocol is suitable for dating the Talede loess. Finally, 15 closely-spaced quartz OSL ages, ranging from 0.72 ± 0.05 to 28.9 ± 2.12 ka and with no reversals, were obtained. Consistent with previous studies in the Ili Basin, two of the three 14C ages show discrepancy when compared with quartz OSL ages for the Talede loess section. The constructed OSL chronology of the Talede loess reveals that the dust accumulation is rapid during marine isotope stage (MIS) 2, especially during the Last Glacial Maximum (LGM), and is also continuous during the Holocene, with the dust accumulation rate exhibiting a relatively low level and an increasing trend. Comparison of the dust accumulation at Talede with that at other sites in the Ili Basin indicates that the rapid dust accumulation during the LGM is not universal, and the slow dust accumulation during the Holocene is probably true for the entire Ili Basin. Comparison of different grain sized quartz OSL ages from Central Asian loess show characteristics of both consistency and inconsistency, which needs to be further investigated.  相似文献   

17.
Fossil oyster reefs are indicators of past sea levels, and their formation is usually dated by means of radiocarbon. However, radiocarbon dating of the shells from coastal areas may be complicated by the varying sources of carbon. Here we applied optical dating methods to date the samples from above and below a fossil oyster bed in a section on the coast of Bohai Bay, China. The optical ages of the sediments were used to constrain the oyster bed. Single-aliquot regenerative-dose procedures using the OSL signal from fine grain quartz, the IRSL and post-IR OSL signals from polymineral fine grains were employed to determine equivalent dose (De). The behaviors of the different luminescence signals from quartz and polymineral grains during De measurements were examined. The results showed that the quartz OSL signal is more reliable than the polymineral IRSL and post-IR OSL signals with respect to dating for these coastal samples. The optical ages indicated that the oyster reef formed between ca. 6.2 and 5.0 ka.  相似文献   

18.
The selection and pre-treatment of reliable organic fractions for radiocarbon age determination is fundamental to the development of accurate chronologies. Sampling from tropical lakes is particularly challenging given the adverse preservation conditions and diagenesis in these environments. Our research is the first to examine and quantify the differences between radiocarbon ages from different carbon fractions and pretreatment protocols from tropical lake sediments. Six different organic fractions (bulk organics, pollen concentrate, cellulose, stable polycyclic aromatic carbon (SPAC), macrocharcoal >250 μm and microcharcoal >63 μm) were compared at six different depths along a 1.72 m long core extracted from Sanamere Lagoon, Cape York Peninsula, northern Australia. Acid-base-acid (ABA), modified ABA (30% hydrogen peroxide + ABA), 2chlorOx (a novel cellulose pre-treatment method) and hydrogen pyrolysis (hypy) were used to pre-treat the organic fractions. The oldest date is ∼31,300 calibrated years before present (cal yr BP) and the youngest is ∼2800 cal yr BP, spanning ∼28,500 years. The smallest offset between the minimum and the maximum age for different fractions and across pretreatment methods at a given depth was found to be 832 years (between SPAC and pollen) and the largest ∼16,750 years (between pollen concentrate and SPAC). The SPAC fractions pre-treated with hypy yielded older ages compared to all other fractions in most cases, while bulk organics yielded consistently younger ages. The magnitude and consistency of the offsets and the physical and chemical properties of the tested organic fractions suggest that SPAC is the most reliable fraction to date in tropical lake sediments and that hypy successfully removes exogenous carbon contamination.  相似文献   

19.
Tephra dispersed during the Millennium eruption (ME), Changbaishan Volcano, NE China provides one of the key stratigraphic links between Asia and Greenland for the synchronization of palaeoenvironmental records. However, controversy surrounds proximal-distal tephra correlations because (a) the proposed proximal correlatives of the distal ME tephra (i.e. B–Tm) lack an unequivocal chronostratigraphic context, and (b) the ME tephra deposits have not been chemically characterized for a full spectrum of element using grain-specific techniques. Here we present grain-specific glass chemistry, including for the first time, single grain trace element data, for a composite proximal sequence and a distal tephra from Lake Kushu, northern Japan (ca. 1100 km away from Changbaishan). We demonstrate a robust proximal-distal correlation and that the Kushu tephra is chemically associated with the ME/B–Tm. We propose that three of the proximal pyroclastic fall units were erupted as part of the ME. The radiocarbon chronology of the Kushu sedimentary record has been utilised to generate a Bayesian age-depth model, providing an age for the Kushu tephra which is consistent with high resolution ages determined for the eruption and therefore supports our geochemical correlation. Two further Bayesian age-depth models were independently constructed each incorporating one of two ice-core derived ages for the B–Tm tephra, providing Bayesian modelled ages of 933–949 and 944–947 cal AD (95.4%) for the Kushu tephra. The high resolution ice-core tephra ages imported into the deposition models help test and ultimately constrain the radiocarbon chronology in this interval of the Lake Kushu sedimentary record. The observed geochemical diversity between proximal and distal ME tephra deposits clearly evidences the interaction of two compositionally distinct magma batches during this caldera forming eruption.  相似文献   

20.
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