Age-depth Curve Research Articles

Simulating carbon accumulation and loss in the central Congo peatlands.

Peatlands of the central Congo Basin have accumulated carbon over millennia. They currently store some 29 billion tonnes of carbon in peat. However, our understanding of the controls on peat carbon accumulation and loss and the vulnerability of this stored carbon to climate change is in its infancy. Here we present a new model of tropical peatland development, DigiBog_Congo, that we use to simulate peat carbon accumulation and loss in a rain-fed interfluvial peatland that began forming ~20,000 calendar years Before Present (cal. yr BP, where 'present' is 1950 CE). Overall, the simulated age-depth curve is in good agreement with palaeoenvironmental reconstructions derived from a peat core at the same location as our model simulation. We find two key controls on long-term peat accumulation: water at the peat surface (surface wetness) and the very slow anoxic decay of recalcitrant material. Our main simulation shows that between the Late Glacial and early Holocene there were several multidecadal periods where net peat and carbon gain alternated with net loss. Later, a climatic dry phase beginning ~5200 cal. yr BP caused the peatland to become a long-term carbon source from ~3975 to 900 cal. yr BP. Peat as old as ~7000 cal. yr BP was decomposed before the peatland's surface became wetter again, suggesting that changes in rainfall alone were sufficient to cause a catastrophic loss of peat carbon lasting thousands of years. During this time, 6.4 m of the column of peat was lost, resulting in 57% of the simulated carbon stock being released. Our study provides an approach to understanding the future impact of climate change and potential land-use change on this vulnerable store of carbon.

TOWARDS THE UNDERSTANDING OF THE PRESENT-DAY HUMAN IMPACT ON PEATLAND DEPOSITS FORMED SINCE THE LATE GLACIAL: A “RETROSPECTIVE” AGE–DEPTH MODEL OF THE GREL RAISED BOG (POLISH INNER CARPATHIANS)

ABSTRACTDuring the last 60 years, the thickness of the Grel peatland deposits (Polish Inner Carpathians), was decreased twice: from about 7.4 m in 1962, to 3.9 m in 2019. Pollen analyses of peat deposits performed in 1962 (Koperowa 1962) and 2019 (this study) showed that the peatland deposits have accumulated since the Oldest Dryas Stadial up to the present day. Comparative analysis of both palynological profiles was carried out, which exhibits a high consistency of the percentage of pollen of several taxa for the 10 horizons occurring in both profiles. Based on the age–depth curve made for the new profile, the modeled age for the above-mentioned compliance levels was obtained and based on this age, a “retrospective” age–depth model was elaborated for the Koperowa’s profile. The analysis of both profiles showed that the compaction of sediments did not occur proportionally in the entire sequence. The conducted research indicates that contemporary human activity may significantly affect the whole length of peatland profiles, not only their upper parts, as previously was considered. The reconstruction of the palaeoenvironment (palaeoclimate) based on multiproxy analysis of the peatland deposits is capable to precisely document this phenomenon.

High‐resolution upper Pliocene to Pleistocene calcareous nannofossil biostratigraphy in Ocean Drilling Program Hole 1146A in the South China Sea

AbstractWe established a high‐resolution calcareous nannofossil biostratigraphy for the late Pliocene–Pleistocene by analyzing a 242 m‐thick, continuous sedimentary succession from Ocean Drilling Program Site 1146, Hole A, in the South China Sea (SCS). A total of 14 calcareous nannofossil datums were detected in the SCS succession. They are, in descending order: first occurrence (FO) of Emiliania huxleyi, last occurrence (LO) of Pseudoemiliania lacunosa, LO of Reticulofenestra asanoi, FO of Gephyrocapsa parallela, FO of R. asanoi, LO of large Gephyrocapsa spp., FO of large G. spp., FO of Gephyrocapsa oceanica, FO of Gephyrocapsa caribbeanica, LO of Calcidiscus macintyrei, LO of Discoaster brouweri, LO of Discoaster pentaradiatus, LO of Discoaster surculus, and LO of Discoaster tamalis. The FO of E. huxleyi was not precisely detected due to poor preservation and dissolution of nannofossils in the underlying strata. We refined the previous calcareous nannofossil biostratigraphy in the SCS by identifying Gephyrocapsa species and four evolutionary extinction events of the genus Discoaster. The proposed calcareous nannofossil biostratigraphy correlates with those reported in other terrestrial and marine areas/sites and global benthic foraminiferal δ18O records. The age–depth curves based on nannofossil biostratigraphy indicate a significant increase in the sedimentation rates at the LO of R. asanoi (0.91–0.85 Ma). The timing of this increase corresponds to reef expansion in the Ryukyu Islands linked to a stepwise increase in Kuroshio Current intensity. This timing is broadly coeval with a sea surface temperature increase of ∼2 °C in the northwestern Pacific due to expansion of the Western Pacific Warm Pool towards the north and south subtropical regions. This can be explained by increased weathering and erosion of terrestrial areas in glacial periods and increased rainfall causing higher sediment transport in interglacial periods, which were both linked to Middle Pleistocene Transition‐related climatic changes.

Geothermal heat flux from measured temperature profiles in deep ice boreholes in Antarctica

Abstract. The temperature at the Antarctic Ice Sheet bed and the temperature gradient in subglacial rocks have been directly measured only a few times, although extensive thermodynamic modeling has been used to estimate the geothermal heat flux (GHF) under the ice sheet. During the last 5 decades, deep ice-core drilling projects at six sites – Byrd, WAIS Divide, Dome C, Kohnen, Dome F, and Vostok – have succeeded in reaching or nearly reaching the bed at inland locations in Antarctica. When temperature profiles in these boreholes and steady-state heat flow modeling are combined with estimates of vertical velocity, the heat flow at the ice-sheet base is translated to a geothermal heat flux of 57.9 ± 6.4 mW m−2 at Dome C, 78.9 ± 5.0 mW m−2 at Dome F, and 86.9 ± 16.6 mW m−2 at Kohnen, all higher than the predicted values at these sites. This warm base under the East Antarctic Ice Sheet (EAIS) could be caused by radiogenic heat effects or hydrothermal circulation not accounted for by the models. The GHF at the base of the ice sheet at Vostok has a negative value of −3.6 ± 5.3 mW m−2, indicating that water from Lake Vostok is freezing onto the ice-sheet base. Correlation analyses between modeled and measured depth–age scales at the EAIS sites indicate that all of them can be adequately approximated by a steady-state model. Horizontal velocities and their variation over ice-age cycles are much greater for the West Antarctic Ice Sheet than for the interior EAIS sites; a steady-state model cannot precisely describe the temperature distribution here. Even if the correlation factors for the best fitting age–depth curve are only moderate for the West Antarctic sites, the GHF values estimated here of 88.4 ± 7.6 mW m−2 at Byrd and 113.3 ± 16.9 mW m−2 at WAIS Divide can be used as references before more precise estimates are made on the subject.

New evidence on the spatial-temporal distribution of superlobes in the Yellow River Delta Complex

Unraveling the spatial-temporal distribution of delta superlobes precisely is particularly important and challenging in the Yellow River Delta Complex (YRDC). Previous research on this issue can be dated back to the early 1990s, when the paleoenvironmental reconstructions mainly hinged on the historical documents and geomorphologic analysis of well-developed cheniers from the west coast of Bohai Bay, and only few drilled cores with scant radiocarbon dates were available. To build a refined model of the Holocene stratigraphic architecture and reconstruct the sedimentary evolution of the YRDC, detailed sedimentological investigation of 12 boreholes, including 59 new AMS 14C radiocarbon dates, was combined with other published 25 cores and 174 AMS 14C dates therein. Age-depth curves were grouped by stratigraphic positions and facies characteristics of these 233 radiocarbon dated samples across different sectors of the YRDC, exhibiting consistent patterns that can be used to identify distinct phases of delta superlobe activities. Eight new superlobes with relatively accurate dating spans were, therefore, proposed in the YRDC. Our results demonstrate that very high aggradation rates almost invariably correlate with the onset of deltaic deposition. Age-depth patterns, thus, may be very useful to elucidate the complex sedimentary evolution of delta systems with multiple lobes.

Coastal landforms and accumulation of mangrove peat increase carbon sequestration and storage

Given their relatively small area, mangroves and their organic sediments are of disproportionate importance to global carbon sequestration and carbon storage. Peat deposition and preservation allows some mangroves to accrete vertically and keep pace with sea-level rise by growing on their own root remains. In this study we show that mangroves in desert inlets in the coasts of the Baja California have been accumulating root peat for nearly 2,000 y and harbor a belowground carbon content of 900-34,00 Mg C/ha, with an average value of 1,130 (± 128) Mg C/ha, and a belowground carbon accumulation similar to that found under some of the tallest tropical mangroves in the Mexican Pacific coast. The depth-age curve for the mangrove sediments of Baja California indicates that sea level in the peninsula has been rising at a mean rate of 0.70 mm/y (± 0.07) during the last 17 centuries, a value similar to the rates of sea-level rise estimated for the Caribbean during a comparable period. By accreting on their own accumulated peat, these desert mangroves store large amounts of carbon in their sediments. We estimate that mangroves and halophyte scrubs in Mexico's arid northwest, with less than 1% of the terrestrial area, store in their belowground sediments around 28% of the total belowground carbon pool of the whole region.

Semi-Empirical Oceanic Depth–Age Relationship Inferred from Bathymetric Curve

In this paper, we report on a preliminary investigation into a semi-empirical method for derivation of depth–age relationship for oceanic lithosphere. The global 30-arcsecond bathymetry data from the General Bathymetric Charts of the Oceans (GEBCO) were corrected for (1) sediment thickness using the Total Sediment Thickness of the World’s Oceans and Marginal Seas and (2) isostasy. The corrected bathymetry was processed to obtain the empirical bathymetric curve, the solution computed with 50 m elevation bin. Subsequently, the data-based curve was approximated with the optimal polynomial model. By combining the model with a formula for derivative of area with respect to age, we obtained the approximate differential equation for depth–age relationship. We solved the equation numerically. The solution was compared with (1) depth–age relationships derived empirically using the combination of the corrected GEBCO bathymetry with digital isochrons of the oceans, (2) Parsons Sclater Model (PSM) and (3) Global Depth Heatflow model (GDH1). In the new depth–age curve, three sections with specific relationships of ocean depth versus age of the crust are identified: (1) moderate increase in depth from 2500 to 5900 m for lithospheric ages 0–118 Ma, (2) more pronounced increase in depth from 5900 to 6700 m for the lithosphere 118–147 Ma old, (3) stabilization of ocean depth at 6700–6760 m for the lithosphere older than 147 Ma. The fit to empirical data as well as PSM and GDH1 models is good for the first section, but rather imperfect for the other two. Reasons for mismatches are complex and probably different for dissimilar sections of the curve.

OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula

Optically stimulated luminescence (OSL) dating was performed on Late Quaternary deltaic sequences from a 55-m-long core sampled from the Nakdong River estuary, Korea. OSL ages obtained from chemically separated fine (4–11 μm) and coarse (90–212 μm) quartz grains ranged from 29.4 ± 2.6 to 0.4 ± 0.04 ka, revealing clear consistency between the grain-size fractions. The De values from the standardized growth curve (SGC) are consistent with those from the single-aliquot regenerative-dose (SAR) procedure, which suggests that the SGC is valid for the Nakdong deltaic sediments. The 14C ages of shells and wood fragments ranged from 11 to 2.9 ka, demonstrating reasonable agreement with the OSL ages, within the error range. However, the limited number and random sampling interval of the 14C age data (10 ages) result in a simple linear and exponential trend in the depth–age curve. In contrast, OSL ages obtained by high-resolution sampling show down-section variations in the depth–age curve, indicating the occurrence of rapid changes in sedimentation rate. It is suggested that the high-sampling-resolution OSL ages provide a more realistic and detailed depth–age curve and sedimentation rate. The Nakdong deltaic sediments were divided into five units based on sedimentation rate. The lowest (unit 5) shows a break in sedimentation between the last glacial maximum (LGM) and the Holocene. The sedimentation rate increased in units 4 and 3, presumably corresponding to the early to middle Holocene sea level rise and high stand. Unit 2 shows a gradually decreasing sedimentation rate following the regression of the shoreline, until about 2 ka. The progradation of the Nakdong River delta resulted in the rapid accumulation of unit 1 during the last 2000 years.

Constraining the age and origin of the seamount province in the Northeast Indian Ocean using geophysical techniques

The breakup of western margin of Australia from Greater India started around 155 Ma and progressed southwards. After the separation, the interceding intraplate region experienced large volumes of submarine volcanism, extending over 100 Myrs. The Christmas Island Seamount Province (CHRISP, as it has been dubbed) lies south of the Java-Sunda Trench, and contains numerous submerged volcanic seamounts, and two sub-aerially exposed island groups—Cocos (Keeling) Islands, and Christmas Island. While recent geochronological investigations have shed light on the diverse eruption ages of the volcanics of this region, some islands/seamounts have demonstrated protracted volcanic histories, and it is not clear how the volcanic loading, tectonic subsidence, and subsequent emergence history of the islands relates to these discrete volcanic episodes. This study utilises a number of geophysical techniques to determine the crustal structure, loading and subsidence history, and last sub-aerial exposure age for the CHRISP. The study shows that flexural and subsidence modelling are reliable techniques in constraining the age of the seamounts when geochronological techniques are not possible. Utilising regional gravity signatures, we model the crustal structure underneath the Cocos (Keeling) Island, and constrain the thickness of the limestone cover between 900 and 2,100 m. Using age-depth subsidence curves for oceanic lithosphere the time since these seamounts were exposed above sea-level was determined, and a trend in exposure ages that youngs towards the west is observed. Two episodes of volcanism have been recorded at Christmas Island and they are of different origin. The younger phase in the Pliocene is a manifestation of flexure induced cracks produced in the lithosphere as it rides the subduction fore-bulge, whereas a low velocity seismic zone rising from the lower mantle, and tectonic reorganization, may be associated with the older Eocene volcanic phase, as well as much of the rest of the province. Our modelling also supports the existence of an older, undated volcanic core to Christmas Island, based on the loading ages from flexural modelling.

Vegetation Response to Climate Change and Human Impacts in the Usambara Mountains

East and West Usambara Mountain blocks are unique based on three characteristics. Firstly, they are connected blocks; secondly, they have an oceanic-influenced climate; and thirdly, the rain seasons are not easily discernible due to their close proximity to the Indian Ocean and Equator. Sediment cores were collected from peat bogs in Derema (DRM) and Mbomole (MBML) in East Usambara and from Madumu (DUMU) in West Usambara. The multiproxy record provides an understanding on climate and vegetation changes during the last 5000 years. DRM and MBML cores result in radiocarbon ages and age-depth curve which showed hiatus at 20 cm and 61 cm and huge inversion for DUMU core at 57 cm. Period 5000–4000 14C yr BP for DUMU core revealed increased Montane forest indicative of relatively moist conditions. Periods 3000–2000 and 2000–1000 14C yr BP, DUMU core demonstrated increased submontane and lowland forests. Period 1000–200 14C yr BP, DUMU core signified increased coprophilous fungi while DRM and MBML cores signified fluctuating herbaceous pollen spectra (wet-dry episodes). Period 200 14C yr BP to present, all cores demonstrated stable recovery of forest types especially dominance of submontane forests. Abundant coprophilous fungi indicated increased human impacts including forest fires, cultivation, and grazing.

Bio-magnetochronology for the upper Campanian - Maastrichtian from the Gubbio area, Italy: new results from the Contessa Highway and Bottaccione sections

A new bio-chronostratigraphic framework is presented for the upper Campanian – Maastrichtian pelagic sediments of the Gubbio area (Bottaccione and Contessa Highway sections, Italy). New planktonic foraminiferal (FO of P. hantkeninoides), calcareous nannofossil (FO of M. prinsii, base acme of M. murus) and magnetostratigraphic data are provided and integrated to construct an age-depth curve based on the recent astronomical calibration of the late Campanian – Maastrichtian magnetic polarity time scale (Husson, D., Galbrun, B., Laskar, J., Hinnov, L. A., Thibault, N., Gardin, S., Locklair, R. E., 2011. Astronomical calibration of the Maastrichtian, Late Cretaceous. Earth and Planetary Science Letters 305, 328–340). All bio-horizons recorded in both sections fall on or are very close to the Line of Correlation (LOC), which testifies to the reliability of biochronologic studies in the Umbria-Marche basin. The proposed age model allows to estimate the sedimentation rates and the ages of calcareous nannofossil and planktonic foraminiferal bio-horizons in both sections. Also, it provides an excellent late Campanian – Maastrichtian reference for sections in the Umbria-Marche area and the entire Tethys. Due to the presence of a ~ 425 kyr-long hiatus in Chron C31n of the Bottaccione section, the Contessa Highway section constitutes a more complete late Campanian – Maastrichtian reference for bio-magnetochronology in the Tethyan realm.

Radiocarbon analysis of halophilic microbial lipids from an Australian salt lake

Assigning accurate dates to hypersaline sediments opens important terrestrial records of local and regional paleoecologies and paleoclimatology. However, as of yet no conventional method of dating hypersaline systems has been widely adopted. Biomarker, mineralogical, and radiocarbon analyses of sediments and organic extracts from a shallow (13cm) core from a hypersaline playa, Lake Tyrrell, southeastern Australia, produce a coherent age-depth curve beginning with modern microbial mats and extending to ~7500calyrBP. These analyses are furthermore used to identify and constrain the timing of the most recent change in hydrological regime at Lake Tyrrell, a shift from a clay deposit to the precipitation of evaporitic sands occurring at some time between ~4500 and 7000yr. These analyses show the potential for widespread dating of hypersaline systems integrating the biomarker approach, reinforce the value of the radiocarbon content of biomarkers in understanding the flow of carbon in modern ecologies, and validate the temporal dimension of data provided by biomarkers when dating late Quaternary sediments.

New pollen, sedimentary, and radiocarbon records from the Marquesas Islands, East Polynesia: Implications for archaeological and palaeoclimate studies

Three sediment cores, from one upland and two lowland sites on Nuku Hiva, Marquesas Islands, provide a c. 750-year record of palaeo-environmental change on the island. Accelerator mass spectrometry (AMS) dating of pollen concentrates from the base of each core indicates that the three widely separated marshes developed c. AD 1200 and 1350, after the known period of human colonisation and establishment. Detailed analyses (pollen, sediments, and charcoal) and additional dating of the upland core from Tōvi’i Plateau (810 m) allow for identification of four chronozones. The core sediment data and age-depth curve suggest an alternation of wet-dry-wet conditions over the c. 750-year period. The pollen spectra, in contrast, are fairly stable, with ferns dominating but arborescent species also present. The micro-charcoal evidence points to regional burning and long-distance transport until c. AD 1640, after which localised burning may be indicated. Among the more notable changes is a major increase in pollen and spore deposition after c. AD 1640, a trend most evident in the pollen concentration diagram. Overall, the data suggest rapid sedimentation in the fourteenth century AD, followed by drier and/or more settled conditions until the mid-seventeenth century, and finally wetter conditions after c. AD 1640. The latter in particular is consistent with emerging regional evidence for warm-wet conditions in the central eastern Pacific during the seventeenth century, the height of the Northern Hemisphere ‘Little Ice Age’. The charcoal record also provides insights into human activities on the island, suggesting burning in the lowlands from the fourteenth century AD, probably in conjunction with forest clearance preparatory to tree and root crop cultivation.

Free‐shape 14C age–depth modelling of an intensively dated modern peat profile

AbstractThe paper presents a new algorithm for finding the best‐fit age–depth model of a peat profile dated with 14C. This algorithm of free‐shape modelling is useful when the age–depth curve is wiggly and difficult to approximate by a simple mathematical function. The performance of the free‐shape algorithm is illustrated with the example of the MA2 peat profile from Mauntschas mire, using 29 radiocarbon dates spanning the last 1300 a, and where the age–depth curve revealed a relatively complicated shape.The best‐fit age–depth model searched for is a curve maximising the ‘Fit quality’, defined as a combination of fit of 14C dates to the radiocarbon calibration curve, general smoothness of the age–depth line, and similarity of relative changes in the modelled sediment accumulation rates to those suggested by independent data. The algorithm of free‐shape age–depth modelling searched for a reasonable equilibrium or compromise between the conditions above. Uncertainty of the free‐shape model was assessed with a Monte Carlo Markov chain built from large number of age–depth lines accepted with probabilities dependent on the ‘fit quality’ defined as above.Parallel fluctuations in concentrations of most pollen taxa in MA2 strongly suggest a complicated shape of the age–depth curve, which is supported in the upper part of the profile by the record of spheroidal carbonaceous particles, and in some levels also by the 14C dates. A modification of total pollen concentrations was therefore used as a quantitative indicator of relative changes of the peat accumulation rate. The use of pollen concentrations improved the fit of the 14C dates in several cases.Application of the final age–depth model to the pollen data resulted in a diagram of pollen accumulation which allows interpretation in ecological terms, whereas age–depth models that did not use pollen concentrations led to problematic and confusing results. The obtained age–depth model is therefore accepted as a good approximation of the actual conditions. Copyright © 2009 John Wiley & Sons, Ltd.

ボーリングコア解析と｀１４´Ｃ年代測定にもとづく木曽川デルタの形成プロセス

The depositional process of the Latest Pleistocene to Holocene shallow-marine and fluvial successions is investigated using five 14C-dated borehole cores from the Kiso River delta, central Japan. The delta succession provides high-quality records of the transgression and regression of an enclosed delta system and reveals the development of fluvial-dominated coastal lowlands. Based on a facies analysis, the cored sediments are classified into five sedimentary units: A) basal gravel, B) fluvial and intertidal sand and silt, C) transgressive lag deposit, prodelta mud, and sandy silt, D) delta-front-slope sandy silt and sand, and delta-front-platform sand, and, E) delta-plain and fluvial sand and silt. Detailed age-depth curves of these cores are reconstructed based on 108 AMS 14C ages including 83 new 14C ages. These curves show patterns similar to those of temporal variations in accumulation rates, and are divided into four sections (I to IV) from the base to the top: I mainly consists of unit B with rapid accumulation (7.3-21.4mm/yr); II mainly consists of unit C with moderate accumulation (2.4-2.8mm/yr); III mainly consists of unit D with rapid accumulation (6.7-17.8mm/yr); and, IV consists of unit D and E with moderate accumulation (1.3-3.9mm/yr). The section boundaries of I/II and II/III, which coincide with the unit boundaries of B/C and C/D, reflect the inundation of a flood plain by seawater, developing a bay environment, and the subsequent progradation of the delta–front slope, respectively. The ages of these boundaries constrain the estimation of the expanding rate of the bay and progradation rates of the delta. The expanding rate of the bay is estimated at 10m/yr during the period 10200-7900 cal yrs BP, and the progradation rates of the delta are 3-4m/yr (6500-4100 cal yrs BP), 5m/yr (4100-1300 cal yrs BP), and 10m/yr (1300 cal yrs BP to the present). These age data indicate that the transition from transgression to regression occurred between 7800 and 7300 cal yrs BP. The geological cross-section along the Kiso River along with the isochrones constructed by over a hundred 14C dates indicates the following depositional history of the Kiso River delta: (1) from 10000 to 7280 cal yrs BP (K-Ah horizon) —During the expansion of the bay, onlap of unit C on unit B is visible, and unit C overlapped all the core sites just before the K-Ah fall, and (2) from 7280 cal yrs BP to present—Isochrones younger than 6500 cal yrs BP are similar to each other and cross the unit boundaries of C/D and D/E. This demonstrates that units C–E are contemporaneous heterotopic facies formed in response to delta progradation after the K-Ah fall.

Physical setting and tephrochronology of the summit caldera ice record at Mount Moulton, West Antarctica

A site on the shoulder of the ice-fi lled summit crater of Mount Moulton, located at an elevation of 2800 m in West Antarctica, exhibits a 400-m-long section of exposed blue ice and intercalated tephra layers. A total of 48 tephra layers are observed. Many are thick (up to 10 cm) and coarse (pumice up to 3 cm). Detailed dip measurements and global positioning system (GPS) mapping reveals a simple geometry of parallel tephra layers all exhibiting consistent dips. The simple stratigraphy of the tephra layers suggests that although the local ice is thinned, based on the boudinaged appearance of the thicker tephra layers, it is otherwise undeformed. Local measured ice motion and ablation rates are similar, averaging around 3 cm/yr for the 4-yr measurement period. Most of the Mount Moulton tephra layers are trachytic, and are derived from Mount Berlin, a still thermally active volcano located ~30 km away to the west. Eight of the tephra layers at the Mount Moulton site have been directly dated, using 40 Ar/ 39 Ar geochronology of potassic feldspar phenocrysts, to (in stratigraphic order from top to base of the blue ice section) 10.5 ± 2.5; 24.7 ± 1.5; 92.1 ± 0.9; 104.9 ± 0.6; 118.1 ± 1.3; 135.6 ± 0.9; 225.7 ± 11.6; and 495.6 ± 9.7 ka. The depth-age curve for the Mount Moulton site, based on these radioisotopic ages, is qualitatively similar to that of the Siple Dome ice core, also in West Antarctica. Although not part of the West Antarctic ice sheet, the Mount Moulton site certainly contains some of the oldest known ice in West Antarctica, and provides a long and detailed climate record.

Chronology building using objective identification of annual signals in trace element profiles of stalagmites

Determination of annual lamination provides important additional constraints to radiometric dates on speleothems, both for dating the duration of specific growth intervals and optimizing growth models. In the absence of visible laminae, however, speleothem age models are reliant upon curve fitting through discretely dated points and are therefore inherently more uncertain than annual chronologies from laminae. Given that the impact of seasonality on speleothems is expected to be strong enough to generate an annual pulse in trace element chemistry regardless of whether or not visible or fluorescent growth laminae are visible, we demonstrate the potential for deriving high-resolution stalagmite chronologies from non-laminated samples using annual chemical variations in stalagmites from two Alpine caves (Obir, Austria and Ernesto, NE Italy). Trace element data were obtained by ion microprobe analyses for H, P, Mg, Na, Sr and Ba and the annual signal was sought using spectral and wavelet analysis. An automated chemical peak-counting software tool was developed in MATLAB ©. It counts significant annual peaks using criteria of minimum amplitude in relation to the local standard deviation of signal variation and minimum separation between peaks determined by the thickness of the preceding layers. Verification of the tool using visibly laminated samples suggests the software is a reliable and accurate method of chronology building, with hit ratios greater than 0.93 and less than 0.75% false alarm occurrences. Used in conjunction with other dating methods such as radiocarbon, U–Th and sulphur peak dating, the automated chemical laminae chronology-building approach provides a more meaningful alternative to simple age-depth curve fitting for non-laminated samples.

CHRONOSTRATIGRAPHIC FRAMEWORK FOR UPPER CAMPANIAN-MAASTRICHTIAN SEDIMENTS ON THE BLAKE NOSE (SUBTROPICAL NORTH ATLANTIC)

A new chronostratigraphic framework is presented for upper Campanian-Maastrichtian pelagic sediments cored at DSDP/ODP Sites 390A/1049, 1050, and 1052, which were drilled across a 1300-m depth transect on the Blake Nose (subtropical western Atlantic Ocean). The planktonic foraminiferal zonation is based on standard global biozones for this interval, but rare and sporadic occurrence of two zonal biomarkers, Gansserina gansseri and Globotruncana aegyptiaca , precludes reliable identification of the nominate zones for those species. The Pseudoguembelina palpebra Partial-range Zone is defined as a means of subdividing this upper Campanian biostratigraphic interval. Planktonic foraminiferal, calcareous nannofossil and paleomagnetic datums are integrated to construct reliable age-depth curves for each of the Blake Nose drill sites. These age models are largely confirmed by Sr-isotopic data and suggest significant unconformities at various positions within the sections. Compilation of stable isotope datasets for this time interval from the Blake Nose sites reveals no significant shifts in δ 18O and δ 13C at the time of the inoceramid bivalve extinction (now dated as 68.5–68.7 Ma in the Blake Nose sections) and no correlation with proposed Campanian-Maastrichtian glacial intervals. However, benthic warming associated with Deccan volcanism during the late Maastrichtian is supported.

Drawing the Optimal Depth-Age Curve on the Basis of Calibrated Radiocarbon Dates

Drawing the Optimal Depth-Age Curve on the Basis of Calibrated Radiocarbon Dates The radiocarbon determination of age has the form of a complicated probability density function. In some cases however it is possible to exploit it in a precise way, in drawing the depth-age curve when a stratigraphic sequence of 14C ages is available. It is also possible to use this function in drawing the depth-age curve by hand. The necessary additional constraint on the depth-age curve adopted here is the simplicity of its shape, namely the low curvature.

Glacial-interglacial changes in moisture balance and the impact on vegetation in the southern hemisphere tropical Andes (Bolivia/Peru)

A palynological investigation of the last glacial-interglacial cycle in the southern hemisphere tropical Andes reveals changes in the moisture balance as the main driver in vegetation change. Thirty accelerator mass spectrometry radiocarbon dates, biostratigraphy and tephra correlation reveal that a 119 m sediment core recovered from the Huiñaimarca sub-basin of Lake Titicaca (16.0° to 17.5° S, 68.5° to 70° W; 3810 masl) contains sediments covering > 151,000 years. Correlation of aridity indicators with precessional variations in insolation is used to fine tune the structure of the age-depth curve within this period. Variations in Isoëtes concentration (above/below 10,000 grains/cm 3) identify the extent of shallow water environments. Examination of another palaeolimnological indicator ( Pediastrum) and consideration of the bathymetry of the Huiñaimarca sub-basin allow the reconstruction of lake-level fluctuations. These data indicate five wet/dry cycles between c. 151,000 and 14,200 cal yr BP. High stands are suggested during the transition into (c. 134,000 cal yr BP), and out of (c. 114,000 and 92,000 cal yr BP), the last interglacial, and during full glacial conditions (c. 70,000 and 45,000 cal yr BP). These cycles are superimposed on a general trend of deepening lake levels through the glacial period. This interpretation is supported by correlation with sediments from Salar de Uyuni (20°S, 68°W; 3653 masl). The youngest wet episode is concurrent with palaeolake Minchin (c. 45,000 cal yr BP), with further evidence for an additional wet period commencing c. 28,000 cal yr BP, concomitant with palaeolake Tauca. The timing of lake level fluctuations is also supported by palaeoshoreline reconstructions from the Uyuni-Poopó region. However, our data do not suggest a major peak in lake level in Huiñaimarca during the Ouki lake cycle (c. 120,000–98,000 cal yr BP) as inferred from U–Th ages obtained from palaeoshorelines around Lago Poopó. The most extreme dry event occurs during the last interglacial period and resulted in a sedimentary hiatus tentatively dated to c. 121,000–129,000 cal yr BP. The observed wet/dry cycles are shown to have a marked and rapid impact on the vegetation. The aridity of the last interglacial promoted a community dominated by Chenopodiaceae/Amaranthacae, with no modern Andean analogue. Polylepis/ Acaena pollen is also shown to fluctuate markedly (0–20%), particularly during the transitions into, and out of, the last interglacial. It is probable that this pollen taxon is primarily representative of the high altitude arboreal genus Polylepis, which is a key component of highly biodiverse Andean woodlands today. Rapid fluctuations indicate the sensitivity of this ecosystem to natural environmental pressure and potential vulnerability to future human impact and climate change. The 100,000 year (eccentricity) solar cycle is shown to be the major controlling factor in moisture balance and vegetation over the last glacial-interglacial cycle. However, significant fluctuations in moisture balance are also evident on timescales considerably shorter than the full glacial-interglacial cycle. We have linked these to precessional (21,000 year) forcing. Nevertheless, precise independent dating during the full glacial cycle is required to confirm the importance of this forcing mechanism.