Fluvio–sedimentary response of the monsoon-fed Indian rivers to Late Pleistocene–Holocene changes in monsoon strength: reconstruction based on existing 14C dates

The primary objective of the present study is to identify major phases of alluviation in the Indian region since the abrupt Deglacial intensification of the monsoon (∼15 cal ka BP) on the basis of analysis of 68 radiocarbon dates from two major hydro-geomorphic regions of India: the Central Ganga Basin (CGB) and the Deccan Peninsula (DP). The recognition of main phases of alluviation and incision has been achieved by evaluating the temporal distribution and clustering of the radiocarbon dates from alluvial sequences. The clusters were detected on the basis of the interpretation of the summed probability distribution plots derived by using OxCal version 4.0.1 and CALPAL (version May 2006) software packages.The summed probability plots reveal that periods of alluviation in the CGB, represented by three clusters (13.9–12.3, 11.9–11.2 and 9.8–9.0 cal ka BP) occur roughly before the onset of Early Holocene monsoon optimum phase. Two other clusters occur in the intervals 3.6–2.8 and 1.1–0.9 cal ka BP. The peak monsoon period generally lacks clusters of radiocarbon dates implying fluvial erosion and channel incision. This period also shows clustering of radiocarbon dates of the abandoned channels. In comparison, ¹⁴C dates from DP alluvial units form clusters at 16.4–14, 12.8–11.2, 10.8–8.9, 8.1–6.7 and 5.1–3.9 cal ka BP, indicating an association with the Deglacial–Early Holocene humid phase. Alluviation in the DP appears to have continued, more or less, uninterrupted till the middle of the Holocene epoch. The beginning and end of the discernible gap in the radiocarbon dates of CGB (9.0–3.6 cal ka) broadly corresponds with the two well-established short-term events of the Holocene, 8.2 and 4.2 ka cal BP. In comparison, the prominent gap of DP radiocarbon dates (3.9–2.1 ka cal BP) approximately begins with the 4.2 ka cal BP short-term event (onset of aridity) and ends with the 2.0 ka cal BP enhanced monsoon event.Notwithstanding the inter-regional differences in the fluvio–sedimentary response in the India region, the clusters of radiocarbon dates indicate that the century to millennium scale variations in fluvial activity in the Indian subcontinent were intimately linked to long-term fluctuations in the monsoon strength during the Late Quaternary.     

Initiation of the Mekong River delta at 8 ka: evidence from the sedimentary succession in the Cambodian lowland

Modern deltas are understood to have initiated around 7.5–9 ka in response to the deceleration of sea-level rise. This episode of delta initiation is closely related to the last deglacial meltwater events and eustatic sea-level rises. The initial stage of the Mekong River delta, one of the world's largest deltas, is well recorded in Cambodian lowland sediments. This paper integrates analyses of sedimentary facies, diatom assemblages, and radiocarbon dates for three drill cores from the lowland to demonstrate Holocene sedimentary evolution in relation to sea-level changes. The cores are characterized by a tripartite succession: (1) aggrading flood plain to natural levee and tidal–fluvial channel during the postglacial sea-level rise (10–8.4 ka); (2) aggrading to prograding tidal flats and mangrove forests around and after the maximum flooding of the sea (8.4–6.3 ka); and (3) a prograding fluvial system on the delta plain (6.3 ka to the present). The maximum flooding of the sea occurred at 8.0 ± 0.1 ka, 2000 years before the mid-Holocene sea-level highstand, and tidal flats penetrated up to 20–50 km southeast of Phnom Penh after a period of abrupt ～5 m sea-level rise at 8.5–8.4 ka. The delta progradation then initiated as a result of the sea-level stillstand at around 8–7.5 ka. Another rapid sea-level rise at 7.5–7 ka allowed thick mangrove peat to be widely deposited in the Cambodian lowland, and the peat accumulation endured until 6.3 ka. Since 6.3 ka, a fluvial system has characterized the delta plain, and the fluvial sediment discharge has contributed to rapid delta progradation. The uppermost part of the sedimentary succession, composed of flood plain to natural-levee sediments, reveals a sudden increase in sediment accumulation over the past 600–1000 years. This increase might reflect an increase in the sediment yield due to human activities in the upper to middle reaches of the Mekong, as with other Asian rivers.     

Cosmogenic 3He age estimates of Plio-Pleistocene alluvial-fan surfaces in the Lower Colorado River Corridor,Arizona, USA

Plio-Pleistocene deposits of the Lower Colorado River (LCR) and tributary alluvial fans emanating from the Black Mountains near Golden Shores, Arizona record six cycles of Late Cenozoic aggradation and incision of the LCR and its adjacent alluvial fans. Cosmogenic ³He (³He_c) ages of basalt boulders on fan terraces yield age ranges of: 3.3–2.2 Ma, 2.2–1.1 Ma, 1.1 Ma to 110 ka, < 350 ka, < 150 ka, and < 63 ka. T1 and Q1 fans are especially significant, because they overlie Bullhead Alluvium, i.e. the first alluvial deposit of the LCR since its inception ca. 4.2 Ma. ³He_c data suggest that the LCR began downcutting into the Bullhead Alluvium as early as 3.3 Ma and as late as 2.2 Ma. Younger Q2a to Q4 fans very broadly correlate in number and age with alluvial terraces elsewhere in the southwestern USA. Large uncertainties in ³He_c ages preclude a temporal link between the genesis of the Black Mountain fans and specific climate transitions. Fan-terrace morphology and the absence of significant Plio-Quaternary faulting in the area, however, indicate regional, episodic increases in sediment supply, and that climate change has possibly played a role in Late Cenozoic piedmont and valley-floor aggradation in the LCR valley.     

Late Pleistocene vegetation change in Korea and its possible link to East Asian monsoon and Dansgaard–Oeschger (D–O) cycles

Late Pleistocene carbon isotope (δ¹³C) records from a paleolithic sedimentary sequence collected from Baeki, Hongcheon, central Korea, show long-term changes with superimposed short-term isotopic excursions. The δ¹³C value of the sedimentary organic matter, a proxy for past vegetation change, varied from ? 26‰ to ? 23‰ for the period between 30 and 90 ka, with a long-term variation similar to insolation changes. High-amplitude (? 1‰ to approximately ? 1.5‰) fluctuations superimposed on the long-term changes in the δ¹³C values decreased during stronger summer monsoon intervals but increased during the weakened summer monsoon. This millennial-scale pattern is generally similar to Greenland Dansgaard–Oeschger (D–O) cycles. The possible connection between the Hongcheon area, Korea and high latitudes may be explained by atmospheric circulation changing in response to the D–O oscillations in the Northern Hemisphere.     

The subfossil tree deposits from the Garonne Valley and their implications on Holocene alluvial plain dynamics

Subfossil tree trunks deposits are common in large rivers, but their status as a source for dating alluvial sequences and palaeoenvironmental studies is still discussed. Particularly their origin and the process(es) of deposition as well as a possible remobilization were pointed as a limit to their use to document river alluvial changes. In this work we report the discovery of the largest subfossil trunks deposits in the Garonne valley. These new data are compared to the previous ones. A set of 17 tree trunks and more than 300 smaller wood fragments were collected. The xylologic study shows the prevalence of Quercus and a single occurrence of Ulmus. These two hardwood species are commonly associated with riparian forest. The ¹⁴C dating carried out on seven trunks and a single branch of Quercus on the outermost identified growth rings, indicates age ranging from 8400–8000 cal. BP for the oldest fragment (bough) to 4300–4000 cal. BP for the most recent tree trunk. Radiocarbon ages of the trunks are aggregated into two main periods: 5300–5600 cal. BP (four trunks) and 4300–4000 cal. BP (three trunks). The radiocarbon (charcoal) dating of the top of the alluvial sequence overlaying the trunks gives an age between 1965–1820 and 1570–1810 cal. BP, i.e. between the 2nd and the 5th c. AD. In addition, the discovery of two unpublished subfossil tree trunks deposits in Finhan are reported (six trunks). At the light of these results, we discuss previously proposed models for the Garonne floodplain building.     

Preliminary optical and radiocarbon age determinations for Upper Pleistocene alluvial sediments in the southern Anti Atlas Mountains,MoroccoDatation des sédiments alluviaux du Pléistocène supérieur du versant sud de l'Anti-Atlas,Maroc

This paper presents the first results of a ¹⁴C and optical luminescence dating project on extensive alluvial silt formations in the southern Anti Atlas Mountains in southern Morocco. The silts comprise three members: basal silts, an inter-silt fluvial gravel member and a top silt member and are incised by the rivers. Top silt dates are: 11 340–11 940, 12 670–13 130 cal. yr BP, and 10.6±0.7 ka (OSL). OSL dates for the inter-silt gravels are 29.6±2.0 ka and for the bottom silts is 45.0±3.7 ka. The sediments correlate with Soltanian stratigraphies known elsewhere in Morocco. To cite this article: M. Thorp et al., C. R. Geoscience 334 (2002) 903–908.     

The Plio–Pleistocene evolution of extensional tectonics in northern Tuscany,as constrained by new gravimetric data from the Montecarlo Basin (lower Arno Valley,Italy)

A detailed gravimetric study has been integrated with the most recent stratigraphic data in the area comprised between the Arno river and the foothills of the Northern Apennines, in northern Tuscany (central Italy). A Plio–Pleistocene basin lies in this area; its sedimentary succession can be subdivided from the bottom, in five allostratigraphic units: (1) Lower–Middle Pliocene shallow marine deposits; (2) Late Pliocene (?)–Early Pleistocene fluvio-lacustrine deposits; (3) late–Early Pleistocene–Middle Pleistocene alluvial to fluvial red conglomerates (Montecarlo Formation); (4) Middle Pleistocene alluvial to fluvial red conglomerates (Cerbaie and Casa Poggio ai Lecci Formations); (5) alluvial to fluvial deposits of Late Pleistocene age. The Bouguer anomaly map displays a strong minimum in the northeastern sector of the basin, and a gentle gradient from west to east. The map of the horizontal gradients permits to recognise three major fault zones, two of which along the southwestern and northeastern margins of the basin, and one along the southeastern edge of the Pisani Mountains. A 2.5D gravimetric modelling along a SW–NE section across the basin displays a thick wedge of sediments of density 2.25 g/cm³ (about 1700 m in the depocenter) overlying a layer of density 2.55 g/cm³, 1000 m thick, which rests on a basement of 2.72 g/cm³. The most of the sediment wedge is here referred to Upper Pliocene (?)–Lower Pleistocene, because borehole data show Pliocene marine deposits thinning northward close to the southern margin of the area. The layer below is referred to Ligurids and upper Tuscan Nappe units; the densest layer is interpreted as composed of Triassic evaporites, quartzites and Palaeozoic basement. According to Carmignani low-angle extensional tectonics began between Serravallian and early Messinian, thinning the Apennine nappe stack. At the end of Middle Pliocene, syn-rift deposition ceased in the Viareggio Basin (west of the investigated area) as demonstrated by Argnani and co-workers, and high-angle extensional tectonics migrated eastward up to the Monte Albano Ridge. A syn-rift continental sedimentary wedge developed in Late Pliocene–Early Pleistocene, until its hanging wall block was dismembered, during late Early Pleistocene, by NE-dipping faults, causing the uplift of its western portion (the Pisani Mountains). This breakup caused exhumation and erosion of Triassic units whose clastics where shed into the surrounding palaeo-Arno Valley in alluvial–fluvial deposits unconformably overlying the Lower Pleistocene syn-rift deposits. In the late Pleistocene SW–NE-trending fault systems created the steep southeastern edge of the Pisani Mountains and the resulting throw is recorded in Middle Pleistocene deposits across the present Arno Valley. This tectonic phase probably continues at present, offshore Livorno, as evidenced by the epicentres of earthquakes.     

15,000 Years of vegetation change in the Bonneville basin: the Blue Lake pollen record

This paper contributes to the emerging picture of late Pleistocene and Holocene environmental change in the Bonneville basin, western North America, through analysis of pollen and sediments from the Blue Lake marsh system, a major wetland area located on the western margin of the Great Salt Lake desert. Analyses of data obtained from the upper 4 m of the Blue Lake core suggest that during the latest Pleistocene, when Lake Bonneville covered the Blue Lake site, pine and sagebrush dominated terrestrial plant communities. These steppe-woodland taxa declined in abundance after ～12 cal ka BP. Wetland plant communities developed at or nearby Blue Lake by ～11.9 cal ka BP and bulrush-dominated marshes were established no later than 10.8 cal ka BP. The Blue Lake wetlands largely desiccated during a dry and warm early middle Holocene ～8.3–6.5 cal ka BP. Climatic amelioration starting ～6.5 cal ka BP is marked principally by a local return of marshes at the expense of playa and grass meadow communities, and a regional increase in sagebrush relative to other dryland shrubs. Singleleaf pinyon pine migrated into the nearby Goshute Mountains after ～8 cal ka BP. Late Holocene fluctuations include cool intervals from ～4.4 to 3.4 and ～2.7 to 1.5 cal ka BP and warmer conditions from 3.4 to 2.7 cal BP and after 1.5 cal ka BP.     

Radiocarbon dating from 40 to 60 ka BP at Border Cave,South Africa

We present 21 radiocarbon dates on 19 charcoal samples from the sedimentary sequence preserved in Border Cave, South Africa. The background radiocarbon activity for charcoal from the cave was determined to be 0.050±0.018 percent modern carbon, from the analysis of a radiocarbon-dead sample from unit 5WA. Radiocarbon ages for individual samples ranged from 25.2 to >58.2 ka BP.The error-weighted mean ages for successively older strata are 38.5+0.85/−0.95 ka BP for unit 1WA, 50.2+1.1/−1.0 ka BP for units 2BS.LR.A and 2BS.LR.B, 56.5+2.7/−2.0 ka BP for unit 2BS.LR.C and 59.2+3.4/−2.4 ka BP for unit 2WA. This radiocarbon chronology is consistent with independent chronologies derived from electron spin resonance and amino acid racemization dating. The results therefore provide further evidence that radiocarbon dating of charcoal by the ABOX-SC technique can yield reliable radiocarbon ages beyond 40 ka BP. They also imply that Border Cave 5, a modern human mandible, predates >58.2 ka BP and that the Middle Stone Age (Mode 3)—Later Stone Age (Mode 5) transition of Border Cave was largely effected between ∼56.5 and ∼41.6 ka ago.     

Was the Younger Dryas (Loch Lomond Stadial) icefield on Rannoch Moor,western Scotland,deglaciated as early as c.12.5 cal ka BP?

The concept that Rannoch Moor, the centre of the Younger Dryas (YD), West Highland Icefield, was deglaciated as early as 12.5 cal ka BP is discussed in the light of radiocarbon dates and varve sequences from outlet glaciers of this icefield, and climate change during the YD. The maximum positions of three YD glaciers were reached after 11.6–11.8 cal ka BP (Lomond), and after 11.8–11.9 cal ka BP (Spean and Treig) indicating that ice remained on Rannoch Moor until long after c.12.5 cal ka BP, and possibly until the YD/Holocene transition at c.11.7 cal ka BP. Further, the Spean glacier dammed a proglacial lake in Lochaber for at least 495 varve years over a period that included the deposition of the Vedde Ash (c.12.1 cal ka BP) and a late YD ash layer (c. 11.7–11.2 cal ka BP), a thesis at variance with supposed early YD deglaciation. Recent examination of this issue using ¹⁰Be exposure age determinations from Rannoch Moor is equivocal. In view of the presence of hard water algae at the sampling site on Rannoch Moor it is recommended that the ‘early’ ¹⁴C dates from Rannoch Moor need to be further reassessed using chronological constraints provided by dated microtephra, and a collaborative radiocarbon dating programme.     

Paleoseismic investigation along Nalagarh Thrust: Evidence of Late Pleistocene earthquake in Pinjaur Dun,Northwestern sub-Himalaya

The present article is the first time reporting of a paleoearthquake that occurred during Late Pleistocene time along the Nalagarh Thrust (NT) in the Pinjaur Dun in northwestern sub-Himalaya. Using CORONA satellite photographs, multi-spectral IRS satellite data, and aerial photographs, a prominent active fault has been identified at Nalagarh in Pinjaur Dun. This fault in the alluvial fan is located very close to the NT which borders the topographic front of the Tertiary rocks against Quaternary deposits. A trench excavation survey was carried out at Nalagarh for detailed paleoseismic studies across this thrust fault. Displacing all the lithologic units of the fan sequence, the fault plane has an average dip of 30° due ENE and a vertical displacement of 1.6 m and slip of ～2.5 m along the fault. The lithological units, consisting of alternating sand and gravel, show back tilting and asymmetrical tight folding. Based on Optically Stimulated Luminescence (OSL) ages, the oldest litho-unit in the trench is 85.83 ± 7.2 ka and the youngest is 67.05 ± 8.4 ka. The OSL age of the sample collected from the easterly exposure of the fault shows an age of 20 ka. The faulting and associated induced deformation features suggest occurrence of a Late Pleistocene large magnitude earthquake along NT in the Nalagarh region of the Pinjaur Dun following the deposition of the Quaternary sedimentary units. The Late Pleistocene fault substantiates the seismic potential of Pinjaur Dun and calls for more exhaustive study of paleoearthquakes in this fast developing industrial belt and highly populous mountainous region.     

A palaeoseismological and geoarchaeological investigation of the Eliki fault,Gulf of Corinth,Greece

Palaeoseismological and morphotectonic analyses enable us to define a 400-m-wide actively deformed zone associated with the active Eliki normal fault, central Greece, bounded on the south by a second-order fault and on the north by a composite and prominent fault scarp. This scarp is further analysed by trenching. Based on colluvium stratigraphy, displacement of distinct horizons and deposition of sedimentary layers, three faulting events have been identified along four fault strands affecting unconsolidated sediments in the trench. The two younger events, with throws of 0.93 and 1.37 m, respectively, the third event, with a throw of 0.44 m, and the penultimate 373 BC event suggest a variable seismic history.The entire alluvial plain of the Kerynitis and Vouraikos rivers, which cross the Eliki fault, has subsided at a rate of 1.4 mm/year, resulting in the burial of the Late Hellenistic–Roman occupation horizons under 3 m of fluvial and colluvial sediments in places.Extension in the broader area is accommodated by the seismically active Eliki and Egion faults. Structural and palaeoseismological analysis of those two faults indicates that they accommodate 1.5 mm/year, or about 10% of the geodetically estimated extension of up to 13 mm/year.     

Late Quaternary seismic stratigraphy and geological development of the south Vøring margin,Norwegian Sea

High-resolution seismic data and sediment cores show that an up to 280 m thick sedimentary sequence has been deposited on the south Vøring margin, off mid-Norway, the last ca 250 ka. The sedimentary succession has been divided into six seismic units, dominated by hemipelagic sediments. Five wedge-shaped massive sequences, of marine isotope stages 8, 6 and 2, interfinger the hemipelagic deposits on the upper slope. The wedge-shaped sequences represent glacigenic debris flows that have been fed by till transported to the shelf edge by grounded ice sheets during maximum glaciations. The hemipelagic units show well-defined depocentres, of various thicknesses, on the upper continental slope. Seismic facies interpretation indicates that the sediment distribution locally has been controlled by currents. Commonly, the hemipelagic units are characterised by parallel and continuous reflectors. However, the second youngest unit identified, deposited between 15.7 and 15.0 ¹⁴C ka BP, is acoustic transparent. We suggest that this unit has been sourced by along-slope transported meltwater plume deposits, released during the initial stage of the last deglaciation of the Norwegian Channel. The hemipelagic sedimentation rates have varied considerably throughout the studied time period. Until ca 21 ¹⁴C ka BP the rates did not exceed 1.4 m/kyr, whereas during the Last Glacial Maximum the rates increased and reached values of about 36 m/kyr before decreasing again at ca 15 ¹⁴C ka BP. Observation of iceberg scourings, of MIS 8 age, about 800 m below the present day sea level, suggest that the south Vøring margin has subsided by a rate of 1.2 m/kyr in the Late Quaternary.     

Stratigraphic architecture and quaternary evolution of the Val d'Agri intermontane basin (Southern Apennines,Italy)

The sedimentary record of the Val d'Agri basin is of great importance for understanding the Quaternary tectonic activity and climatic variability in the Southern Apennines. Changes in tectonic controls, sediment supply and climatic input have been identified. The interval from ～ 56 to ～ 43 ka was associated with asymmetric subsidence restricted to the north-eastern actively faulted margin of the basin and development of axial braided river and transverse alluvial fan systems. Short-lasting Mediterranean-type pedogenesis between ～ 43 and ～ 32 ka (MIS Stage 3) coexisted with progradation–aggradation of the southern alluvial fan deposits and southwards tilting of the basin floor. Aggradation ended with consumption of accommodation space after 32 ka. During a subsequent stage of decline of vegetation cover, possibly as a consequence of climatic cooling (probably MIS Stage 2), active progradation of alluvial fans occurred. Breakthrough of the basin threshold and entrenchment of the drainage network must therefore be attributed to a latest Pleistocene to Holocene age. The first stages of basin opening and fill, predating ～ 56 ka have only been inferred by stratigraphic considerations: the earliest lacustrine sedimentation should be middle Pleistocene or older in age. The following south-eastward basin widening allowed progradation of alluvial fan systems, which completely filled the lacustrine area (tentatively late middle Pleistocene). Pedogenesis in “Mediterranean-like” climate conditions caused the final development of a highly mature fersiallitic paleosol at the top of the fan surfaces, in areas of morpho-tectonic stability, plausibly during MIS Stage 5. The study results demonstrate the potential of applying a multidisciplinary approach in an intermontane continental settings marked by a relative rapid and constant tectonic subsidence and a high rate of sediment supply during the Pleistocene glacial–interglacial cycles.     

Determination of slip rate by optical dating of fluvial deposits from the Wangsan fault,SE Korea

The time-integrated slip rate in fault zones can be determined if the deformed deposits are reliably dated. Here, we report optically stimulated luminescence (OSL) ages of Late Pleistocene fluvial deposits cut by the Wangsan fault, southeastern Korea, which displaces a hanging wall block of about 28 m. Five sandy samples of the deformed Quaternary deposits were dated by quartz OSL using the single aliquot regenerative-dose (SAR) protocol. Three samples taken from the footwall block show stratigraphically consistent OSL ages of 54±7, 76±5 and 90±6 ka, from top to bottom. Two samples collected from the same layer in the hanging wall block show reproducible OSL ages of 81±5 and 82±5 ka, which are also in good agreement with the stratigraphic relationships. Our OSL ages yield an average sedimentation rate of the Quaternary deposits as around 0.04 mm a⁻¹, and a minimum value of time-integrated slip rate as 0.52 mm a⁻¹. This minimum slip rate is considerably higher than those reported earlier for Quaternary faults in southeastern Korea. The youngest OSL age (54±7 ka) constrains the maximum value of the recurrence interval of the fault movement.     

The Late Quaternary sediment successions of Llangorse Lake,south Wales

The last British-Irish Ice Sheet (BIIS) created a landscape with many sedimentary basins that preserve archives of paleoenvironmental and paleoclimatic change during the Last Glacial-Interglacial Transition (LGIT; ~ 18-8 ka BP). The typical lithostratigraphic succession of these archives is composed of minerogenic/allogenic sediments formed during cold climatic conditions and organic-rich/authigenic sediments during warmer climates. This paper presents a multi-core lithostratigraphy compiled from the extant lake and surrounding basin at Llangorse Lake, south Wales, a basin lying within the southernmost limits of the last BIIS. This lake contains one of the longest continuous terrestrial sediment successions in the UK. Uncertainty previously existed concerning the presence and distribution of sediments at the site related to the Windermere Interstadial (~ 14.7 to ~ 12.9 ka BP) and Loch Lomond Stadial (~ 12.9 to 11.7 ka BP). A new borehole survey demonstrates that LGIT-age sediments are present at the site with nekron mud (gyttja), corresponding to the Lateglacial Interstadial, deposited in the deeper part of the lake waters and that these deposits are equivalent in age to marl deposits found at shallower depths at the margins of the basin. These deposits are associated with warmer conditions experienced during the Windermere Interstadial and Holocene, whilst minerogenic-rich sediments were deposited during the colder climatic conditions of the Dimlington Stadial and the Loch Lomond Stadial with rangefinder radiocarbon dates confirming this attribution. A model of lake level changes shows that drainage of the Dimlington Stadial glacial lake caused the largest fall, but there was also a further, smaller lake level fall at the end of the Windermere Interstadial and/or the start of the Loch Lomond Stadial, before the level rose in the early Holocene. The lithostratigraphic results presented here form the framework for further paleoenvironmental and paleoclimatic research at Llangorse Lake.     

Carbon isotope stratigraphy using carbonate cements in the Triassic Sherwood Sandstone Group: Corrib Field,west of Ireland

Carbon stable isotopes from carbonate minerals (mainly dolomite) from six wells from the Lower Triassic Sherwood Sandstones of the Corrib Gas Field, Slyne Basin, west of Ireland, allow stratigraphic correlation. The results also provide information on palaeoenvironmental change during the deposition of these continental redbed sedimentary rocks. The Triassic reservoir rocks have been buried to > 4000 m and heated to > 165 °C and now contain methane-rich gas. Although the oxygen isotopic signal has been at least partially reset during burial and heating, a primary carbon isotopic signal appears to have survived diagenesis. The carbon isotope ratio varies from − 3.2‰ to + 2.1‰. All six wells show similar stratigraphic changes when all the carbon isotope data are plotted relative to a major playa horizon. δ¹³C increases from about − 3‰ at the base of the Sherwood to about + 2‰ 170 m above the base. δ¹³C then decreases to about − 2‰ for the next 70 m and remains steady for the following 50 m. The top 20 m of the Sherwood contains carbonate with a δ¹³C values decreasing to about − 3‰. The occurrence of a stratigraphically-correlatable carbon isotope pattern implies that the primary evolution signal has been preserved. The change in δ¹³C correlates with indicators of aridity and biological stress such that the highest δ¹³C values are in sedimentary rocks deposited in a playa lake (arid times); these rocks contain the greatest quantity of dolomite cement. Conversely, the lowest δ¹³C values correspond to sedimentary rocks deposited from well-developed rivers (relatively humid times) from the lowest quantity of dolomite cement. The same carbon isotope evolution has been found in another well in the Slyne basin and in Belgium, suggesting that the palaeoenvironmental isotope signal in the Triassic sedimentary rocks of the Corrib Field may have a regional significance.     

Tropical glacier fluctuations in the Cordillera Blanca,Peru between 12.5 and 7.6 ka from cosmogenic 10Be dating

We report cosmogenic surface exposure ¹⁰Be ages of 21 boulders on moraines in the Jeullesh and Tuco Valleys, Cordillera Blanca, Peru (～10°S at altitudes above 4200 m). Ages are based on the sea-level at high-latitude reference production rate and scaling system of Lifton et al. (2005. Addressing solar modulation and long-term uncertainties in scaling secondary cosmic rays for in situ cosmogenic nuclide applications. Earth and Planetary Science Letters 239, 140–161) in the CRONUS-Earth online calculator of Balco et al. (2008. A complete and easily accessible means of calculating surface exposure ages or erosion rates from ¹⁰Be and ²⁶Al measurements. Quaternary Geochronology 3, 174–195). Using the Lifton system, large outer lateral moraines in the Jeullesh Valley have a ¹⁰Be exposure age of 12.4 ka, inside of which are smaller moraine systems dated to 10.8, 9.7 and 7.6 ka. Large outer lateral moraines in the Tuco Valley have a ¹⁰Be exposure age of 12.5 ka, with inner moraines dated to 11.3 and 10.7 ka. Collectively, these data indicate that glacier recession from the Last Glacial Maximum (LGM) in the Cordillera Blanca was punctuated by three to four stillstands or minor advances during the period 12.5–7.6 ka, spanning the Younger Dryas Chronozone (YDC; ～12.9–11.6 ka) and the cold event identified in Greenland ice cores and many other parts of the world at 8.2 ka. The inferred fluctuations of tropical glaciers at these times, well after their withdrawal from the LGM, indicate an increase in precipitation or a decrease in temperature in this region. Although palaeoenvironmental records show regional and temporal variability, comparison with proxy records (lacustrine sediments and ice cores) indicate that regionally this was a cold, dry period so we ascribe these glacier advances to reduced atmospheric temperature rather than increased precipitation.     

Cool,wet conditions late in the Younger Dryas in semi-arid New Mexico

A thick alluvial sequence in central New Mexico contains the Scholle wet meadow deposit that traces upstream to a paleospring. The wet meadow sediments contain an abundant fauna of twenty-one species of freshwater and terrestrial mollusks and ten species of ostracodes. The mollusks and ostracodes are indicative of a local high alluvial water table with spring-supported perennial flow but without standing water. Pollen analysis documents shrub grassland vegetation with sedges, willow, and alder in a riparian community. Stable carbon isotopes from the wet meadow sediments have δ¹³C values ranging from ? 22.8 to ? 23.3‰, indicating that 80% of the organic carbon in the sediment is derived from C₃ species. The wet meadow deposit is AMS dated 10,400 to 9700 ¹⁴C yr BP, corresponding to 12,300 to 11,100 cal yr BP and overlapping in time with the Younger Dryas event (YD). The wet meadow became active about 500 yr after the beginning of the YD and persisted 400 yr after the YD ended. The Scholle wet meadow is the only record of perennial flow and high water table conditions in the Abo Arroyo drainage basin during the past 13 ka.     

Lake ecosystem dynamics and links to climate change inferred from a stable isotope and organic palaeorecord from a mountain lake in southwestern China (ca. 22.6–10.5 cal ka BP)

A detailed understanding of long-term climatic and environmental change in southwestern China is hampered by a lack of long-term regional palaeorecords. Organic analysis (%TOC, %TN, C/N ratios and δ¹³C values) of a sediment sequence from Lake Shudu, Yunnan Province (ca. 22.6–10.5 cal ka BP) indicates generally low aquatic palaeoproductivity rates over millennial timescales in response to cold, dry climatic conditions. However, the record is punctuated by two marked phases of increased aquatic productivity from ca. 17.7 to 17.1 cal ka BP and from ca. 11.9 to 10.5 cal ka BP. We hypothesise that these shifts reflect a marked, stepwise lacustrine response to Asian summer monsoon strengthening during the last deglaciation.