Quantifying the seasonal variations in fluvial and eolian sources of terrigenous material to Cariaco Basin,Venezuela

The varved sediments that accumulate in the Cariaco Basin provide a detailed archive of the region’s climatic history, including a record of the quantity of fluvial and wind-transported material. In this study, we examine the sedimentological characteristics (clay mineralogy and grain size) of both surface sediments and sinking lithogenic material collected from sediment trap samples over a three-year period from 1997 to 2000. Data from biweekly sediment trap samples show a tri-modal particle size distribution, with prominent peaks at 2, 22 and 80 μm, indicating sediment contributions from both eolian and fluvial sources. The clay mineralogy of the water column samples collected from 1997 to 1999 also shows distinctive characteristics of eolian and fluvial material. An examination of surface sediment samples from the Cariaco Basin indicates that the Unare River is the main source of riverine sediments to the eastern sub-basin. By combining these sedimentological proxies, we estimate that ～10% of the terrigenous material delivered to the Cariaco Basin is eolian, while ～90% is fluvial. This represents an annual dust accumulation rate of ～0.59 mg/cm²/yr. Since aerosols are closely linked to climate variability, the ability to quantify paleo-dust fluxes using sedimentological characteristics will be a useful tool for future paleoclimate studies looking at sub-Saharan aridity and latitudinal migration of the Intertropical Convergence Zone.     

Late Quaternary climate change record from two long sediment cores from Guaymas Basin,Gulf of California

Modern Guaymas Basin (Gulf of California, Mexico) is a region of high diatom productivity where exceptional preservation factors maintain biannually alternating sediment deposition as annual varves. New sediment cores from Guaymas Basin (MD02‐2512 and MD02‐2515) present the opportunity to construct climate records from below the last glacial period. A low‐resolution age model has been constructed from oxygen isotope analysis, correlation with other dated short piston cores from Guaymas Basin and an estimate of sedimentation rate. MD02‐2512 from eastern Guaymas Basin has an age range from the Holocene to late marine isotope stage 6 (MIS 6); MD02‐2515 from western Guaymas Basin has an age range from ～8000 to 40 000 yr. Shipboard analyses of colour reflectance, magnetic susceptibility and sediment density are combined with continuous X‐ray fluorescence scans to reconstruct a picture of glacial climate in the Gulf of California. Eastern Guaymas Basin is affected by glacial sea level fall, which results in a drastic change in productivity rates and sediment type. The laminated record of MIS 5 allows comparison with the Holocene, showing a similarity of sedimentation patterns during deglaciation and a series of very rapid variations just prior to the last glaciation. In western Guaymas Basin there are a series of Younger Dryas‐like events during the glacial, typified by low productivity and high terrigenous input. Long‐term climate and productivity changes appear to be caused by the southward displacement of the Subtropical High pressure zone. Copyright © 2005 John Wiley & Sons, Ltd.     

Climate change in lowland Central America during the late deglacial and early Holocene

The transition from arid glacial to moist early Holocene conditions represented a profound change in northern lowland Neotropical climate. Here we report a detailed record of changes in moisture availability during the latter part of this transition (～11 250 to 7500 cal. yr BP) inferred from sediment cores retrieved in Lake Petén Itzá, northern Guatemala. Pollen assemblages demonstrate that a mesic forest had been largely established by ～11 250 cal. yr BP, but sediment properties indicate that lake level was more than 35 m below modern stage. From 11 250 to 10 350 cal. yr BP, during the Preboreal period, lithologic changes in sediments from deep‐water cores (>50 m below modern water level) indicate several wet–dry cycles that suggest distinct changes in effective moisture. Four dry events (designated PBE1‐4) occurred centred at 11 200, 10 900, 10 700 and 10 400 cal. yr BP and correlate with similar variability observed in the Cariaco Basin titanium record and glacial meltwater pulses into the Gulf of Mexico. After 10 350 cal. yr BP, multiple sediment proxies suggest a shift to a more persistently moist early Holocene climate. Comparison of results from Lake Petén Itzá with other records from the circum‐Caribbean demonstrates a coherent climate response during the entire span of our record. Furthermore, lowland Neotropical climate during the late deglacial and early Holocene period appears to be tightly linked to climate change in the high‐latitude North Atlantic. We speculate that the observed changes in lowland Neotropical precipitation were related to the intensity of the annual cycle and associated displacements in the mean latitudinal position of the Intertropical Convergence Zone and Azores–Bermuda high‐pressure system. This mechanism operated on millennial‐to‐submillennial timescales and may have responded to changes in solar radiation, glacial meltwater, North Atlantic sea ice, and the Atlantic meridional overturning circulation (MOC). Copyright © 2005 John Wiley & Sons, Ltd.     

A lacustrine record from Lop Nur,Xinjiang, China: Implications for paleoclimate change during Late Pleistocene

Climate variability during the Late Pleistocene is studied from the proxies in core CK-2 drilled from the Luobei Depression (91°03′E, 40°47′N), Lop Nur in the eastern Tarim Basin, Xinjiang, China. Geophysical and geochemical properties, including magnetic susceptibility, granularity, chroma, carbonate content, loss on ignition and trace elements, have been determined to reconstruct the environmental evolution of the area during 32–9 ka BP. The chronology is established by uranium–thorium disequilibrium dating techniques.Our data suggest four paleoclimate stages, indicating glacial variations between cold–humid and warm–arid environments. A period of extreme humidity occurred during 31,900–19,200 yr BP is attributed the last glacial maximum (LGM). The period was followed by a warm–arid episode during 19,200–13,500 yr BP. Then a cold–humid interval during 13,500–12,700 yr BP may correspond to another cooling phases at high latitudes of the Northern Hemisphere. The last stage from 12,700 to 9000 yr BP has a trend that the climate turned warm and arid. The Lop Nur region is characterized by particularly humid stadials and arid interstadials. The climate variability in Lop Nur was constrained by global climate change because it is correlated with Dansgaard–Oeschger and Heinrich events, which were observed at the northern high latitudes. The synchroneity of the palaeoclimatic events suggested that cold air activity at the northern high latitudes was the most important factor that influenced the climate evolution in the Lop Nur region. A probable mechanism that involves the migration of westerly winds is proposed to interpret this synchroneity.     

Sediment geochemical records of productivity and oxygen depletion along the margin of western North America during the past 60,000 years: teleconnections with Greenland Ice and the Cariaco Basin

Many sediment records from the margins of the Californias (Alta and Baja) collected in water depths between 60 and 1200 m contain anoxic intervals (laminated sediments) that can be correlated with interstadial intervals as defined by the oxygen-isotope composition of Greenland ice (Dansgaard–Oeschger, D–O, cycles). These intervals include all or parts of Oxygen Isotope Stage 3 (OIS3; 60–24 cal ka), the Bölling/Alleröd warm interval (B/A; 15–13 cal ka), and the Holocene. This study uses organic carbon (Corg) and trace-element proxies for anoxia and productivity, namely elevated concentrations and accumulation rates of molybdenum and cadmium, in these laminated sediments to suggest that productivity may be more important than ventilation in producing changes in bottom-water oxygen (BWO) conditions on open, highly productive continental margins. The main conclusion from these proxies is that during the last glacial interval (LGI; 24–15 cal ka) and the Younger Dryas cold interval (YD; 13–11.6 cal ka) productivity was lower and BWO levels were higher than during OIS3, the B/A, and the Holocene on all margins of the Californias. The Corg and trace-element profiles in the LGI–B/A–Holocene transition in the Cariaco Basin on the margin of northern Venezuela are remarkably similar to those in the transition on the northern California margin. Correlation between D–O cycles in Greenland ice with gray-scale measurements in varved sediments in the Cariaco Basin also is well established. Synchronous climate-driven changes as recorded in the sediments on the margins of the Californias, sediments from the Cariaco Basin, and in the GISP-2 Greenland ice core support the hypothesis that changes in atmospheric dynamics played a major role in abrupt climate change during the last 60 ka. Millennial-scale cycles in productivity and oxygen depletion on the margins of the Californias demonstrate that the California Current System was poised at a threshold whereby perturbations of atmospheric circulation produced rapid changes in circulation in the eastern North Pacific Ocean. It is likely that the Pacific and Atlantic Oceans were linked through the atmosphere. Warmer air temperatures during interstadials would have strengthened Hadley and Walker circulations, which, in turn, would have strengthened the subtropical high pressure systems in both the North Pacific and the North Atlantic, producing increased rainfall over the Cariaco Basin and increased upwelling along the margins of the Californias.     

Neotropical vegetation response to rapid climate changes during the last glacial period: Palynological evidence from the Cariaco Basin

We present new palynological information from the anoxic Cariaco Basin, off Venezuela, that provides insight into the response of northernmost South American vegetation to rapid climate changes between 68 and 28 ka, specifically during North Atlantic Heinrich events (HEs) and Dansgaard/Oeschger cycles. We defined three different vegetation modes: (1) an interstadial mode characterized by the highest pollen concentration and the maximum extension of semi-deciduous and evergreen forests; (2) a stadial mode characterized by increases of salt marshes, herbs, and montane forests; and (3) a Heinrich event mode characterized by the lowest pollen concentrations, abrupt increases of salt marshes, and decreased forest abundance. Similarly, indices of C4/C3 plants show increases during stadials with clear peaks during the onset of HEs, though grasslands did not become dominant during these periods. We alternatively propose that these expansions of C4 plants are associated with the expansion of coastal salt marshes. Our vegetation record suggests the prevalence of humid conditions during interstadials, dry and cold conditions during stadials, and dry and cold conditions together with changes in sea level during HEs. This new palynological evidence supports previous interpretations that main environmental changes in northernmost South America were driven by latitudinal displacements of the Intertropical Convergence Zone and sea-level changes.     

Holocene sedimentation in the Skagerrak interpreted from chirp sonar and core data

High‐resolution chirp sonar profiling in the northeastern Skagerrak shows acoustically stratified sediments draping a rough‐surfaced substratum. A 32 metre long sediment core retrieved from the survey area encompasses the entire Holocene and latest Pleistocene. The uppermost seismo‐acoustic units in the chirp profiles represent Holocene marine sediments. The lowermost unit is interpreted as ice‐proximal glacial‐marine sediments rapidly deposited during the last deglaciation. The end of ice‐proximal sedimentation is marked by a strong reflector, interpreted to have been formed during latest Pleistocene time as a consequence of rapid ice retreat and drastically lowered sedimentation rate. The subsequent distal glacial‐marine sediments were deposited with initially high sedimentation rates caused by an isostatic rebound‐associated sea‐level fall. Based on correlation between the core and the chirp sonar profiles using measured sediment physical properties and AMS ¹⁴C dating, we propose a revised position for the Pleistocene/Holocene boundary in the seismo‐acoustic stratigraphy of the investigated area. Copyright © 2005 John Wiley & Sons, Ltd.     

Character of vegetational and environmental changes in southern Europe during the last glacial period; evidence from Lesvos Island,Greece

This paper presents high-resolution results of palynological and sedimentological analyses undertaken on two sediment cores from the Megali Limni (ML) basin, an area characterised by serpentine soils, in the southeastern part of Lesvos Island, Greece. Six tephra horizons and multiple radiocarbon dates provide independent controls towards the development of a chronological framework. The composite pollen record spans the period from 22 to 62 thousand years ago (ka) BP and shows a number of oscillations between steppe, forest-steppe and forest, in concert with North Atlantic millennial-scale variability. Vegetation during the late Marine Isotope Stage (MIS) 4 was grassland/steppe, indicating cold and arid conditions, while sediment composition suggests increased erosion rates. Arboreal populations (dominated by Pinus and to a lesser extent deciduous Quercus) expanded during MIS 3 interstadials, suggesting increases in precipitation and temperature. Within the course of the longer interstadials, changes in vegetation composition point to a trend towards increased aridity and sometimes decreasing winter temperatures. During intervening stadials, vegetation was composed mainly of Artemisia, Chenopodiaceae and Gramineae, indicating reversals to arid and cold conditions, with most extreme conditions recorded during stadials corresponding to Heinrich Events. During the course of MIS 3, the basin was progressively infilled with sediments. Only a small portion of MIS 2 is represented in the sequence, showing a short-lived expansion of arboreal populations. Comparisons with other pollen sequences from southern Europe underscore the important role of Pinus throughout the last glacial period, a reflection of the serpentine soils of the Megali Limni area, where Pinus brutia dominates today.     

Sea-level and tectonic control of middle to late Pleistocene turbidite systems in Santa Monica Basin, offshore California

Small turbidite systems offshore from southern California provide an opportunity to track sediment from river source through the turbidity‐current initiation process to ultimate deposition, and to evaluate the impact of changing sea level and tectonics. The Santa Monica Basin is almost a closed system for terrigenous sediment input, and is supplied principally from the Santa Clara River. The Hueneme fan is supplied directly by the river, whereas the smaller Mugu and Dume fans are nourished by southward longshore drift. This study of the Late Quaternary turbidite fill of the Santa Monica Basin uses a dense grid of high‐resolution seismic‐reflection profiles tied to new radiocarbon ages for Ocean Drilling Program (ODP) Site 1015 back to 32 ka. Over the last glacial cycle, sedimentation rates in the distal part of Santa Monica Basin averaged 2–3 mm yr^?1, with increases at times of extreme relative sea‐level lowstand. Coarser‐grained mid‐fan lobes prograded into the basin from the Hueneme, Mugu and Dume fans at times of rapid sea‐level fall. These pulses of coarse‐grained sediment resulted from river channel incision and delta cannibalization. During the extreme lowstand of the last glacial maximum, sediment delivery was concentrated on the Hueneme Fan, with mean depositional rates of up to 13 mm yr^?1 on the mid‐ and upper fan. During the marine isotope stage (MIS) 2 transgression, enhanced rates of sedimentation of > 4 mm yr^?1 occurred on the Mugu and Dume fans, as a result of distributary switching and southward littoral drift providing nourishment to these fan systems. Longer‐term sediment delivery to Santa Monica Basin was controlled by tectonics. Prior to MIS 10, the Anacapa ridge blocked the southward discharge of the Santa Clara River into the Santa Monica Basin. The pattern and distribution of turbidite sedimentation was strongly controlled by sea level through the rate of supply of coarse sediment and the style of initiation of turbidity currents. These two factors appear to have been more important than the absolute position of sea level.     

30 000 years of productivity and salinity variations in the late Quaternary Cariaco Basin revealed by dinoflagellate cysts

Dinoflagellate cysts and other palynomorphs were studied from ODP Hole 1002C in the Cariaco Basin over the past 30 000 years. The assemblage shifts between a dominance of heterotrophic dinoflagellate cysts (mainly Brigantedinium spp., Lejeunecysta spp., Selenopemphix nephroides and Stelladinium reidii ) and autotrophic dinoflagellate cysts (mainly Spiniferites ramosus , Lingulodinium machaerophorum and Operculodinium centrocarpum ). These assemblage shifts are associated with stronger upwelling during stadials and stronger river influx during interstadials. Increases in productivity caused by enhanced upwelling are coupled to improved preservation and vice versa. More stratified water is indicated by higher abundances of L. machaerophorum and succeeds Heinrich events. The average process length of L. machaerophorum can be used to track changes in salinity, since this shows a similar pattern as the δ¹⁸O_SW (paired Mg/Ca −δ¹⁸O) reconstruction. During the last glacial, conditions were more saline than during the current interglacial. On a millennial scale, changes in salinity are opposite to open ocean salinities and the hydrological proxies, which can be explained by a modulation of the signal by stratification, isolation of the Basin or advection of freshwater masses. These results highlight both generalities and particularities of the palaeoecological record of this tropical semi-enclosed basin.     

Paläoklima und paläoozeanische Verhältnisse im SW-Pazifik während der letzten 6 Millionen Jahre (DSDP - Site 594, Chatham Rücken, östlich Neuseeland)

Detailed sedimentological investigations were performed on sediments from DSDP-Site 594 (Chatham Rise, east of New Zealand) in order to reconstruct the evolution of paleoclimate and paleoceanographic conditions in the Southwest Pacific during the last 6 million years. The results can be summarized as follows:

1. High accumulation rates of biogenic opal and carbonate and the dominance of smectites in the clay fraction suggest increased oceanic productivity and an equable dominantly humid climate during the late Miocene.
2. During Pliocene times, decreasing contents of smectites and increasing feldspar/quartz ratios point to an aridification in the source area of the terrigenous sediments, culmunating near 2.5 Ma. At that time, accumulation rates of terrigenous components distinctly increased probably caused by increased sediment supply due to intensified atmospheric and oceanic circulation, lowered sea level, and decreased vegetation cover.
3. A hiatus (1.45 to 0.73 Ma) suggests intensified intermediate-water circulation.
4. Major glacial/interglacial cycles characterize the upper 0.73 Ma. During glacial times, oceanic productivity and terrigenous sediment supply was distinctly increased because of intensified atmospheric and oceanic circulations and lowered sea level, whereas during interglacials productivity and terrigenous sediment supply were reduced.
5. An increased content of amphibols in the sediments of Site 594 indicates increased volcanic activities during the last 4.25 Ma.

     

Tropical salt marsh succession as sea-level indicator during Heinrich events

Centennial–millennial dynamics of tropical salt marsh vegetation are documented in the pollen record from marine core MD03-2622, Cariaco Basin, Venezuela, which spans the glacial period between 63 and 29 ka. Five rapid and abrupt expansions of salt marsh vegetation are linked with North Atlantic Heinrich events (HEs). Within each event, a recurrent pattern – starting with species of Chenopodiaceae, followed by grasses, and subsequently by Cyperaceae species – suggests a successional process that is determined by the close relationship between sea-level and community dynamics. The salt tolerant Chenopodiaceae, at the base of each sequence, indicate hypersaline intertidal environments, which were most likely promoted by extremely dry atmospheric conditions. Rapid sea-level rise characterizes the onset of HE stadials, causing erosion of marsh sediments, and continued recruitment of pioneer species (Chenopodiaceae), which are the only ones capable of tolerating the rapid rate of disturbance. Once, as sea-level drops or as rise decelerates, marsh plants are able to trap and stabilize sediments, favouring the establishment of more competitive species (graminoids). The increment of marsh height as a result of autochthonous sediment accumulation reduces the extent of hypersaline environments, and allows the establishment of mesohaline species. These results add to the scarce knowledge on tropical salt marsh ecosystems, and provide independent paleoclimatic evidence on sea-level changes occurring simultaneously with Antarctica climate variations.     

季节性海冰驱动的冰期北大西洋“电容器”效应

第四纪冰期的千年尺度气候突变事件——Dansgaard-Oeschger Event (D-O事件),一直是古气候学领域关注的重点。近年来,数值模拟的研究发现,北大西洋副极地地区年际-年代际气候变率的振幅在D-O事件中的冰阶冷期远大于间冰阶暖期,这一现象为理解该区域海温代理指标的气候学意义提供了重要参考价值,但其动力机制尚不清晰。本文利用海气耦合气候模型(COSMOS),通过模拟氧同位素(MIS)3阶段的一个典型D-O事件过程,探讨了冰阶冷期北大西洋气候变率的放大机制。结果显示,北大西洋副极地海域的季节性海冰通过调控海气间热量交换,影响当地气候变率的幅度。冰阶期,热带暖水向北输送导致海洋次表层逐渐升温,削弱了表层-次表层海水的密度层结,有利于次表层暖水上涌,促进海冰融化及海表温度升高。这将激发出海平面气压的负异常,引起气旋式风切变,并通过Ekman抽吸作用加速表层-次表层海水的垂直混合,进一步促进次表层暖水的上涌。这一正反馈机制造成海洋次表层热量的迅速释放,海表温度快速升高。当次表层热量释放结束后,海表将无暖水补充,导致海表温度下降,海冰增多。该过程激发的海表气压正异常(即反气旋式风切变)将抑制垂直混合发生,促进次表层热量积累,为下一次放热过程提供条件。在间冰阶暖期,随着北大西洋季节性海冰消失,海气间热交换不再受海冰变化影响,海洋次表层与大气间的热交换始终处于准平衡态,气候变率的振幅显著下降。本研究结果显示,北大西洋季节性海冰的存在可以调控海洋次表层热量积累-释放的过程,产生“电容器”效应,这对理解冰期年际-年代际气候变率放大现象有重要启示意义。     

An OSL‐dated sediment sequence at Idre,west‐central Sweden,indicates ice‐free conditions in MIS 3

In central and northern Sweden, glacial sediments and landforms, formed during Early and Middle Weichselian stadials and their transition into interstadials, are often preserved in spite of having been overridden by later glacial advances. This study presents an OSL‐dated glacial stratigraphy from Idre in west‐central Sweden, expanding the area in which Middle Weichselian ice‐free conditions have been identified. Three sedimentary units were identified, with the lowermost unit consisting of glaciolacustrine sand, deposited in a stagnant water‐body. Nine OSL samples gave ages ranging from 54 to 41 ka, suggesting deposition during a deglacial phase in MIS 3. Normal faults and silt veins, formed after deposition, indicate that the area was ice‐free for a prolonged period, enabling the melting of buried stagnant ice. Above an erosional unconformity is a sediment unit characterized by gravels and sands deposited in a proximal braided‐river environment. OSL ages range from 180 to 41 ka, indicating poor sediment bleaching during deposition. We thus consider them to give a maximum age of the sedimentation, indicating deposition at or after 41 ka. The uppermost unit consists of a stacked succession of subglacial traction tills and glaciotectonite beds, representing the Late Weichselian glaciation of the area, probably during the inception phase with a wet‐based glacier regime. At the last deglaciation of the area there was extensive meltwater erosion, eroding all sedimentary units and forming a landscape with terraces and channels, and erosional remnants of the uppermost diamict as free‐standing hummocks.     

The tail of the Storegga Slide: insights from the geochemistry and sedimentology of the Norwegian Basin deposits

Deposits within the floor of the Norwegian Basin were sampled to characterize the deposition from the Storegga Slide, the largest known Holocene‐aged continental margin slope failure complex. A 29 to 67 cm thick veneer of variable‐coloured, finely layered Holocene sediment caps a homogeneous, extremely well‐sorted, poorly consolidated, very fine‐grained, grey‐coloured sediment section that is >20 m thick on the basin floor. This homogeneous unit is interpreted to represent the uppermost deposits generated by a gravity flow associated with the last major Storegga Slide event. Sediments analogous to the inferred source material of the slide deposits were collected from upslope on the Norwegian Margin. Sediments sampled within the basin are distinguishable from the purported source sediments, suggesting that size sorting has significantly altered this material along its flow path. Moreover, the very fine grain size (3·1 ± 0·3 μm) suggests that the >20 m thick homogeneous unit which was sampled settled from suspension after the turbulent flow was over. Although the turbulent phase of the gravity flow that moved material out into the basin may have been brief (days), significantly more time (years) is required for turbid sediments to settle and dewater and for the new sea floor to be colonized with a normal benthonic fauna. Pore water sulphate concentrations within the uppermost 20 m of the event deposit are higher than those normally found in sea water. Apparently the impact of microbial sulphate reduction over the last ca 8·1 cal ka bp since the re‐deposition of these sediments has not been adequate to regenerate a typical sulphate gradient of decreasing concentration with sub‐bottom depth. This observation suggests low rates of microbial reactions, which may be attributed to the refractory carbon composition in these re‐deposited sediments.     

Middle Weichselian glacial event in the central part of the Scandinavian Ice Sheet recorded in the Hitura pit, Ostrobothnia, Finland

The Hitura open pit exposes a sedimentary sequence up to 50 m thick representing Late Saalian to Holocene glacial and non-glacial sediments. The sequence was investigated using sedimentological methods, OSL-dating and pollen and diatom analyses to reconstruct the Middle Weichselian (MWG) glacial event in the central part of the Scandinavian Ice Sheet (SIS). The results indicate that the sediment succession represents two entire glacial advance and retreat cycles. The lowermost deposits are Late Saalian esker and delta sediments overlain by sediments that correlate with the early Eemian lacustrine phase. Remnants of the Eemian soil post-dating the lacustrine phase were also observed. The area was ice-free during the entire Early Weichselian (EWG). The first glacial advance recorded in the sediments is related to the MWG. It started 79 kyr ago, deformed underlying sediments and deposited an immature till, including large detached sediment pods containing remains of organic material, soils and fluvial sediments representing allochthonous material from EWG ice-free stadials and interstadials. The glacial deposits are conformably overlain by glaciolacustrine and littoral accumulations, indicating MWG deglaciation between 62 and 55 kyr ago. Based on the fabric measurements from the till unit overlying the MWG sediments, ice advance during the Late Weichselian (LWG) was initially from the west and later from a north-northwesterly direction. The Hitura strata provide the first dating of the MWG deglaciation (55 to 62 kyr ago) from central parts of the SIS. It can be considered as a key site for studying the growth and decay of SIS during the poorly known early parts of the glaciation.     

The effect of geomorphological setting on Holocene lake sediment variability,northern Swedish Lapland

Comparison of catchment geomorphology and lithostratigraphical analysis of sediments in two small neighbouring alpine lakes show that the minerogenic influx into the lakes has varied significantly during the Holocene, despite similarities in environmental setting. One lake contains a homogeneous organic‐rich sediment sequence whereas the sediment of the other lake is laminated and has a higher minerogenic content. X‐ray radiographs are used to visualise lithostratigraphical structures and provide high‐resolution density data. We find that moderate differences in geomorphology and process activity in the lakeshore region around alpine lakes can significantly affect the lake sediment composition. Minerogenic sediment accumulation rates vary strongly over time, owing to different depositional processes, which complicate temporal reconstructions. We also find that non‐glacial processes deposit minerogenic sediment layers with similar characteristics (high density, low organic content) as layers interpreted as having a glaciofluvial origin. This has implications for palaeoclimate studies based on proglacial lacustrine sediment. Our results indicate that erosion of surface sediments in the catchments characterised the early Holocene. A low and constant minerogenic inflow indicates that stable environmental conditions (with little fluvial erosion) were established in the catchments during the middle Holocene. The variability in sediment composition increased again in the late Holocene, possibly as a result of short‐term climate fluctuations superimposed on a general climate deterioration trend. Copyright © 2003 John Wiley & Sons, Ltd.     

Sedimentary evolution and environmental history of Lake Van (Turkey) over the past 600 000 years

The lithostratigraphic framework of Lake Van, eastern Turkey, has been systematically analysed to document the sedimentary evolution and the environmental history of the lake during the past ca 600 000 years. The lithostratigraphy and chemostratigraphy of a 219 m long drill core from Lake Van serve to separate global climate oscillations from local factors caused by tectonic and volcanic activity. An age model was established based on the climatostratigraphic alignment of chemical and lithological signatures, validated by ⁴⁰Ar/³⁹Ar ages. The drilled sequence consists of ca 76% lacustrine carbonaceous clayey silt, ca 2% fluvial deposits, ca 17% volcaniclastic deposits and 5% gaps. Six lacustrine lithotypes were separated from the fluvial and event deposits, such as volcaniclastics (ca 300 layers) and graded beds (ca 375 layers), and their depositional environments are documented. These lithotypes are: (i) graded beds frequently intercalated with varved clayey silts reflecting rising lake levels during the terminations; (ii) varved clayey silts reflecting strong seasonality and an intralake oxic–anoxic boundary, for example, lake‐level highstands during interglacials/interstadials; (iii) CaCO₃‐rich banded sediments which are representative of a lowering of the oxic–anoxic boundary, for example, lake level decreases during glacial inceptions; (iv) CaCO₃‐poor banded and mottled clayey silts reflecting an oxic–anoxic boundary close to the sediment–water interface, for example, lake‐level lowstands during glacials/stadials; (v) diatomaceous muds were deposited during the early beginning of the lake as a fresh water system; and (vi) fluvial sands and gravels indicating the initial flooding of the lake basin. The recurrence of lithologies (i) to (iv) follows the past five glacial/interglacial cycles. A 20 m thick disturbed unit reflects an interval of major tectonic activity in Lake Van at ca 414 ka bp . Although local environmental processes such as tectonic and volcanic activity influenced sedimentation, the lithostratigraphic pattern and organic matter content clearly reflect past global climate changes, making Lake Van an outstanding terrestrial archive of unprecedented sensitivity for the reconstruction of the regional climate over the last 600 000 years.     

Ecological variations in diatom assemblages in the Paleo-Kathmandu Lake linked with global and Indian monsoon climate changes for the last 600,000 years

Variations in fossil diatom assemblages and their relationship with global and Indian monsoon climate changes for the last 600,000 yr were investigated using a core of ancient lake (Paleo-Kathmandu Lake) sediments drilled at the Kathmandu Basin, Nepal Himalaya. Chronological scales of the core were constructed by tuning pollen wet and dry index records to the SPECMAP δ¹⁸O stack record. Examinations of biogenic silica contents and fossil diatom assemblages revealed that variations in productivity and compositions of diatom assemblages were closely linked with global and Indian monsoon climate changes on glacial and interglacial time scales. When summer monsoonal rainfall increased during interglacials (interstadials), diatom productivity increased because of increased inputs of terrestrial nutrients into the lake. When summer monsoonal rainfall reduced and/or winter monsoonal aridification enhanced during glacials (stadials), productivity of the diatoms decreased and lake-level falling brought about changes in compositions of diatom assemblages. Monospecific assemblages by unique Cyclotella kathmanduensis and Puncticulata versiformis appeared during about 590 to 390 ka. This might be attributed to evolutionary fine-tuning of diatom assemblages to specific lake environmental conditions. Additionally, low-amplitude precessional variations in monsoon climate and less lake-level changes may have also allowed both species to dominate over the long periods.     

Climate variability and glacial processes in eastern Iceland during the past 700 years based on varved lake sediments

Striberger, J., Björck, S., Ingólfsson, Ó., Kjær, K. H., Snowball, I. & Uvo, C. B. 2010: Climate variability and glacial processes in eastern Iceland during the past 700 years based on varved lake sediments. Boreas, 10.1111/j.1502‐3885.2010.00153.x. ISSN 0300‐9483. Properties of varved sediments from Lake Lögurinn in eastern Iceland and their link to climate and glacial processes of Eyjabakkajökull, an outlet glacier of the Vatnajökull icecap, were examined. A varve chronology, which covers the period AD 1262–2005, was constructed from visual observations, high‐resolution images, X‐ray density and geochemical properties determined from X‐radiography and X‐ray fluorescence scanning. Independent dating provided by ¹³⁷Cs analysis and eight historical tephras verify the varve chronology. The thickness of dark‐coloured seasonal laminae, formed mainly of coarser suspended matter from the non‐glacial river Grímsá, is positively correlated (r=0.70) with winter precipitation, and our 743‐year‐long varve series indicates that precipitation was higher and more varied during the later part of the Little Ice Age. Light‐coloured laminae thickness, controlled mainly by the amount of finer suspended matter from the glacial river Jökulsáí Fljótsdal, increased significantly during the AD 1972 surge of Eyjabakkajökull. As a consequence of the surge, the ice‐dammed Lake Háöldulón formed and recurrently drained and delivered significant amounts of rock flour to Lake Lögurinn. Based on these observations, and the recurring cyclic pattern of periods of thicker light‐coloured laminae in the sediment record, we suggest that Eyjabakkajökull has surged repeatedly during the past 743 years, but with an increased frequency during the later part of the Little Ice Age.