mid-Holocene Climatic Optimum Research Articles

Diatom evidence for Holocene paleoclimatic change in the South Scotia Sea, West Antarctica

Diatom data of the core sediment from a deep basin in the South Scotia Sea, West Antarctica provide high-resolution information on changes in oceanographic processes and paleoclimate during the late Quaternary. Three main climatic changes can be distinguished in diatom assemblages: Last Glacial Maximu (LGM), mid-Holocene climatic optimum and Neoglacial cold event. Diatom assemblages have been deposited in a variable sea ice condition over the last 25,000 yr in response to the climate change. During the LGM to early-Holocene (23,370–8,300 yr BP), the core site might be influenced by increased dense sea-ice cover, which could reduce biogenic flux from the surface water, depositing relatively increased amount of sea ice-related diatoms (Actinocyclas actinochilus, Eucampia antarctica, Fragilariopsis curta, andFragilariopsis cylindrus). During these periods, it is likely that the increased sea ice cover between the Weddell and Scotia seas would have severely preventedChaetoceros resting spores in the Weddell Sea ice margin from being laterally advected to the Scotia Sea, resulting in the reduction ofC. resting spore abundance in the sediment. Afterward, a warm period followed from 8,300 to 2,400 yr BP in the mid Holocene when open water assemblages (Rhizosolenia styliformis andThalassiosira antarctica (warn)) were deposited. Significant dilution of the number ofFragilariopsis kerguelensis indicates the opening of communication between the Weddell and Scotia seas allowing lateral advection ofC. resting spores from the Weddell Sea to the Scotia Sea. A colder condition (Neoglacial cooling) then resumed since <2,400 yr BP in the late Holocene supported by other paleoclimatic records in the Antarctic Peninsula. The assemblage is characterized either by the increase of sea ice-related diatoms (A. actinochilus andF. cylindrus) or by the decrease of open water taxa (R. styliformis andT. antarctica (warm)) compared to that in the mid-Holocene optimum. The loose sea-ice assemblage was, however, different from dense sea-ice assemblage deposited in the LGM. Similarity of abundance ofChaetoceros between the Neoglacial and the mid-Holocene implies that the loose sea ice condition was not able to sufficiently restrict the lateral advection ofC. resting spore to the Scotia Sea from the Weddell Sea.

Biomarker records, organic carbon accumulation, and river discharge in the Holocene southern Kara Sea (Arctic Ocean)

Within the Russian–German research project on “Siberian River Run-off (SIRRO)” dealing with freshwater discharge and its influence on biological, geochemical, and geological processes in the Kara Sea, sedimentological and organic-geochemical investigations were carried out on two well-dated sediment cores from the Yenisei Estuary area. The main goal of this study was to quantify terrigenous organic carbon accumulation based on biomarker and bulk accumulation rate data, and its relationship to Yenisei river discharge and climate change through Holocene times. The biomarker data in both cores clearly indicate the predominance of terrigenous organic matter, reaching 70–100 and 50–80% of total organic carbon within and directly north of the estuary, respectively. During the last ca. 9 cal ka b.p. represented in the studied sediment section, siliciclastic sediment and (terrigenous) organic carbon input was strongly influenced by postglacial sea-level rise and climate-related changes in river discharge. The mid-Holocene Climatic Optimum is documented by maximum river discharge between 8.2 and 7.3 cal ka b.p. During the last 2,000 years, river discharge probably decreased, and accumulation of both terrigenous and marine organic carbon increased due to enhanced coagulation of fine-grained material.

Palynological evidence for Holocene environmental change in the Changjiang (Yangtze River) Delta, China

Holocene vegetation changes in response to climate fluctuations and human impacts are reviewed on the basis of pollen analyses from borehole cores taken from the Changjiang (Yangtze River) delta, China, and other previously reported data. During the earliest Holocene (10,930−9000 cal yr BP), the climate was warm and wet, allowing thermophilous hardwoods to occupy mid- and low-elevations surrounding a palaeo-Changjiang estuary. The climate became gradually cooler, and cool-tolerant conifers, grasses and ferns became dominant until 7600 cal yr BP, when the estuary became a delta. A mid-Holocene climatic optimum occurred between 7600 and 4800 cal yr BP, when evergreen and broadleaved deciduous trees flourished at mid- and low-elevations surrounding the delta front-prodelta. After this time, climate became cooler again until 1340 cal yr BP. During this period, evergreen and broadleaved deciduous trees were replaced by conifers and grasses inhabiting the inter/subtidal flat-delta front. This development of conifer–grassland vegetation was shortly interrupted between 3860 and 3200 cal yr BP when thermophilous tree cover increased and open vegetation with scattered conifers was reduced. Since 1340 cal yr BP, the vegetation has been similar to that at present under warm, wet conditions. Human impacts are recognized by the first appearance of Fagopyrum and a sudden increase in herb pollen at 4500 and 1340 cal yr BP, respectively.

Morphological variability of Brachidontes Swainson (Bivalvia, Mytilidae) in the marine Quaternary of Argentina (SW Atlantic)

Morphometric analyses of abundant shells of Brachidontes from 27 localities (19 littoral deposits of Late Quaternary age and eight modern sites) in Argentina, where Brachidontes rodriguezi d'Orb., 1846, Brachidontes darwinianus d'Orb., 1846 and Brachidontes purpuratus Lamk., 1819 have been reported, show implications for palaeoenvironmental reconstructions. Only shell characters have been generally used for taxonomic discrimination of these taxa (i.e., general shape, size, external ornamentation, colour). Morphometric variation ranges from elongate, comparatively small shells (R), typical of coarse sandy substrates and warm or warm-temperate water masses in typically marine and polyhaline environments (dominant in the mid-Holocene), to bigger, more strongly ribbed and more globose shells (P), characteristic of pebbly or rocky substrates, and shallow, cold water masses in typically open marine habitats (dominant in the Magellanean province at present). Traditional and geometric morphometric techniques performed on large amounts of fossil and modern shells help to objectively define the morphometric variation of Brachidontes in the area. The coordinates for the shell outline (165 specimens) and five linear dimensions (485 specimens) were used for morphometric analyses of shell specimens of the three main morphs (D = darwinianus; R = rodriguezi; P = purpuratus) identified within the samples collected, for two main reasons: 1) They represent the most common features used for taxonomic discrimination at the species level; 2) comparative analysis of outer ornamentation and colour could produce deceiving results due to the varying preservation grades between fossil and modern shells. Results both by Relative Warps and Principal Component techniques show no difference among shell morphs, implying that B. darwinianus and B. rodriguezi (= B. solisianus d'Orb. = B. exustus Linn.) might have to be synonymized with B. purpuratus (P), which dominates in colder waters at higher latitudes. The morphotypes are extremes within a wide morphological range and can be linked together in a chain of transitional forms. They respond to different habitats (ecomorphs) controlled by temperature (geographical variation), substrate, energy and water-depth, and by salinity, and by competitive strategies for space (availability of food resources), all influencing growth. The morphometric variation of organisms in fossil strata could represent a valid tool to reconstruct palaeoenvironmental changes that occurred during high sea-level stands since ca. 300 ka (Pleistocene, MOIS 9) to ca. 7.6–4.5 ka (mid-Holocene Climatic Optimum, transgressive maximum of MOIS1) and up to present.

북극해 스발바드 군도 피오르드에서 일어난 홀로세의 빙해양 퇴적작용과 고기후적 의미

Analyses of sedimentological and geochemical parameters from two radiocarbon-dated sediment cores (JM98-845-PC and JM98-818-PC) retrieved from the central part of Isfjorden, Svalbard, in the Arctic Sea, reveal detailed paleoclimatic and paleoceanographic histories over the last 15,000 radiocarbon years. The overconsolidated diamicton at the base of core JM98-845-PC is supposed to be a basal till deposited beneath pounding glacier that had advanced during the LGM (Last Glacial Maximum). Deglaciation of the fjord commenced after the glacial maximum, marked by the deposition of interlaminated sand and mud in the ice-proximal zone by subglacial meltwater discharge, and prevailed between 13,700 and 10,800 yr B.P. with enriched-terrigenous organic materials. A return to colder conditions occurred at around 10,800 yr B.P. with a drop in TOC content, which is probably coincident with the Younger Dryas event in the North Atlantic region. At this time, an abrupt decrease of TOC content as well as an increase in C/N ratio suggests enhanced terrigenous input due to the glacial readvance. A climatic optimum is recognized between 8,395 and 2,442 yr B.P., coinciding with 'a mid-Holocene climatic optimum' in Northern Hemisphere sites (e.g., the Laurentide Ice sheet). During this time, as the sea ice receded from the fjord, enhanced primary productivity occurred in open marine conditions, resulting in the deposition of organic-enriched pebbly mud with evidence of TOC maxima and C/N ratio minima in sediments. Fast ice also disappeared from the coast, providing the maximum of IRD (ice-rafted debris) input. Around 2,442 yr B.p. (the onset of Neoglacial), pebbly mud, characterized by a decrease in TOC content, reflects the formation of more extensive sea ice and fast ice, which might cause decreased primary productivity in the surface water, as evidenced by a decrease in TOC content. Our results provide evidence of climatic change on the Svalbard fjords that helps to refine the existence and timing of late Pleistocene and Holocene millennial-scale climatic events in the Northern Hemisphere.

Holocene climatic and environmental changes inferred from midge records (Diptera: Chironomidae, Chaoboridae, Ceratopogonidae) at Lake Berkut, southern Kola Peninsula, Russia

A radiocarbon-dated sediment sequence from Lake Berkut in the southern part of the Kola Peninsula, northwest Russia, was investigated by means of midge analysis in order to reconstruct the Holocene climatic and environmental history of the region. Past mean July air temperatures at the study site and hypolimnetic oxygen contents of the lake water were inferred from chironomid-based transfer functions. The early Holocene ( c. 10 100-8400 cal. yr BP) is characterized by summer temperatures and hypolimnetic oxygen concentrations broadly similar to present-day conditions. The midge records give evidence of a lake-level fall at c. 7000 cal. yr BP, resulting in weakened thermal stratification of the lake water, and improvement of the hypolimnetic oxygen conditions. After c. 4000 cal. yr BP midge assemblages suggest a lake-level rise related to increased effective moisture. A secondary mid-Holocene climatic optimum with inferred mean July air temperatures of c. 12.1° C was recorded between 6000 and 4400 cal. yr BP, followed by minimum values (c. 11.3°C) between 3500 and 1500 cal. yr BP, accompanied by the onset of natural acidification of the lake. The late-Holocene midge assemblages suggest weakly acidic conditions, low hypolimnetic oxygen contents and a general warming trend during the last c. 1500 years, although paludification and natural acidification may have influenced the temperature reconstruction at this stage. However, the modern chironomid-inferred mean July air temperature (12.5°C) is consistent with meteorological data from a nearby station. In general, the midge records reflect relatively warm and moist conditions at Lake Berkut before c. 7000 cal. yr BP, a relatively warm and dry climate at c. 7000-4000 cal. yr BP, and a shift to cooler and wetter climatic conditions in the region after c. 4000 cal. yr BP.

A 6000-year high-resolution climatic record from a stalagmite in Xiangshui Cave, Guilin, China

Middle-to late-Holocene palaeoclimate change has been reconstructed at high resolution by the analysis of the carbon and oxygen isotopes from a thermal ionization mass spectrometric (TIMS) U/Th dated stalagmite from Xiangshui Cave, near Guilin, Guangxi Province, China. The carbon and oxygen isotopic records from the stalagmite suggest that changes in the Asian monsoon since the middle Holocene (6000 BP) can be divided into two periods: (1) an interval from 6000 to 3800 BP when a strong East Asian summer monsoon gradually weakened and climate was relatively warm and humid; (2) a cool period from 3800 to 373 BP when the East Asian summer monsoon was relatively weak and the winter monsoon was probably relatively strong. This cooler interval was interspersed with a number of short warm periods. A This interpretation is largely based upon the general increase in 6180 values of the stalagmite between 6000 and 3800 BP and shifts in 6180 about a relatively heavy mean value between 3800 and 373 BP. The 6000 to 3800 BP trend is probably associated with decrease in precipitation and temperature subsequent to the mid-Holocene climatic optimum.

The Holocene evolution and palaeosalinity history of Beall Lake, Windmill Islands (East Antarctica) using an expanded diatom-based weighted averaging model

A mid-Holocene climate optimum is inferred from a palaeosalinity reconstruction of a closed saline lake (Beall Lake) from the Windmill Islands, East Antarctica using an expanded diatom salinity weighted averaging (WA) regression and calibration model.The addition of 14 lakes and ponds from the Windmill Islands, East Antarctica, to an existing weighted averaging regression and calibration palaeosalinity model of 33 lakes from the Vestfold Hills, East Antarctica expands the number of taxa and lakes and the range of salinity in the existing model and improves the model's predictive ability. This improved model was used to infer Holocene changes in lake water salinity in Beall Lake, Windmill Islands.Six changes in diatom-inferred salinity in Beall Lake are put into broad chronological context based on three radiocarbon dates: as the East Antarctic Ice Sheet (EAIS) retreated from the Windmill Islands during the early Holocene (∼9000–8130 corr. yr BP), Beall Lake formed as a melt water-fed freshwater lake, which gradually became more saline as marine influence increased from ∼8000 corr. yr BP. Between ∼8000 and 4800 corr. yr BP, the diatom assemblage included planktonic marine taxa such as Chaetoceros spp. and cryophilic taxa such as Fragilariopsis cylindrus, which indicate favourable summer growth conditions. A mid-Holocene warm period produced a climate that was warmer and more humid with increased precipitation and snow accumulation. This is reflected in the Beall Lake core as a reduction in the salinity of the lake diatom assemblage from ∼4800–4600 corr. yr BP. Holocene isostatic uplift rates in the Windmill Islands vary from 5–6 m/1000 yr. By applying this uplift rate, it is calculated that the bedrock would have risen above sea level by ∼4000 yr BP.The Beall Lake core diatom assemblage from ∼4600–2900 corr. yr BP includes both marine cryophilic and planktonic taxa together with freshwater benthic and planktonic lacustrine taxa. This mix of species indicates the emergence of the lake from the sea around ∼4600 corr. yr BP. From ∼2800 corr. yr BP, retreat of the ice margin led to increasing melt water inputs and associated freshening of the lake basin until ∼1900 corr. yr BP. The lake basin had no oceanic influence by this time, allowing a terrestrial freshwater flora to establish and thrive for the next ∼1000 yr.At ∼1850 corr. yr BP, a sudden and rapid salinity change is evident in Beall Lake. A late Holocene warm period between 2000 and 1000 yr BP has been observed in ice core records from Law Dome (an ice cap abutting the Windmill Islands to the east and north). It is therefore inferred that, at ∼1850 corr. yr BP, summer temperatures within the Beall Lake catchment area were much higher than present summer temperatures.The climate optimum identified in the Beall Lake core ∼4800 yr BP confirms mid-Holocene warming of the Windmill Islands and suggests a synchronous mid-Holocene climate optimum occurred across coastal East Antarctica. In addition, the abrupt climate change inferred at ∼1850 yr BP suggests that higher resolution sampling of sediment cores from coastal East Antarctic limnological oases will provide more evidence of rapid climate change events over coastal East Antarctica in future.

Lateglacial and Holocene environmental changes in Ganga plain, Northern India

Stable isotope, elemental geochemistry and pollen analysis of a sediment profile from Sanai lake, Central Ganga Plain, were used to document climatic oscillations during the last ∼15,000 yr in Indian subcontinent. Micaceous sandy sediments at the base of the profile indicate an active channel and humid conditions before ∼15,000 14C yr BP. The channel was abandoned during comparatively arid conditions (15,000–13,000 14C yr BP) and converted into a swamp. Enhanced humidity around 13,000 14C yr BP led to submergence of marshes and establishment of a large lake. A dry phase is also identified around an estimated age of 11,500–10,500 14C yr BP which might correspond to the Younger Dryas event witnessed globally. The Early to Mid-Holocene climatic optimum (∼10,000–5800 14C yr BP) is characterised by a lake environment. Occurence of warmth loving aquatic plants, lower δ 18O values of gastropod aragonite, heavier δ 13C org values indicate a sensitive response of the lake ecology to the climatic warming and increased rainfall. This is followed by an arid event during 5000–2000 14C yr BP and from ∼1700 14C yr BP there is evidence of climatic amelioration. The sequence and magnitude of the millenium-scale climatic oscillations documented along the lake profile correlate well with records from other parts of the Indian subcontinent indicating that the recorded changes are an expression of broad scale, probably global and climatic change.

On the Holocene sea-level highstand along the Yangtze Delta and Ningshao Plain, East China

The temporal-spatial distribution of Neolithic sites shows a long period of absence of human settlements during the early Holocene on the Yangtze Delta, and the area has not been colonized until 7000 aBP, corresponding to the onset of the Majiabang culture. Besides the regions east to the sandbar of Maqiao Site in Shanghai, foraminifera are not found in Neolithic cultural layers since 7000 aBP, indicating a widespread transgression between 10000 and 7000 aBP. Evidence of radiocarbon-dated cultural relics, foraminifera and pollen records from the Hemudu Site suggests that the Hemudu culture emerged and developed during the mid-Holocene Climatic Optimum. Therefore, the hypothesis of a mid-Holocene sea-level highstand along the Yangtze Delta and Ningshao Plain from 7000 to 5000 aBP is an open debate. New evidence from archaeological excavations and bio-stratigraphy suggests that, prior to 7000 aBP, a high sea level occurred along the Ningshao Plain. Subsequently, the sea level was lowered between 7000 and 5000 aBP, and the area suffered from frequent floods from 5000 to 3900 aBP.

Holocene palynology of Whitehaven Swamp, Whitsunday Island, Queensland, and implications for the regional archaeological record

Palynological study of Whitehaven Swamp, Whitsunday Island, provides the first Holocene palaeoenvironmental record for the Whitsunday region on the central Queensland coast. Sediment stratigraphy and radiocarbon dating indicate continuous freshwater swamp conditions since around 7000 radiocarbon years Before Present (BP). Pollen and charcoal analyses provide local and regional vegetation and fire histories for the site and surrounding area. Varying representation of swamp elements, particularly Leptocarpus and Cyperaceae, provides evidence for phases of permanent and ephemeral swamp conditions. The regional vegetation record is dominated by rainforest, sclerophyll and beach strand elements. Strongest rainforest representation occurs around the mid-Holocene, while sclerophyll elements increase from the late Holocene to present. Charcoal analyses indicate that fire has been a constant component of the Whitsunday environment throughout the period represented. Negative correlation between high charcoal and Leptocarpus pollen concentrations suggests a strong local component to the charcoal record and a history of on-site burning during ephemeral swamp phases. The vegetation reconstruction suggests moister than present conditions at Whitehaven between approximately 7000 to 4500 BP. This complies with claims for a mid-Holocene climatic optimum based on pollen records from the Atherton Tableland to the north, but contrasts with suggested mid-Holocene aridity based on a surrogate lake water level record from Fraser Island to the south. Comparisons with the regional archaeological record provide no evidence for direct links between major environmental change and archaeologically identified cultural change. In particular, claims for late Holocene population intensification are not matched by changes in the charcoal record. This may suggest that widespread vegetation burning was not a predominant feature of hunter–gatherer strategies that were focused towards marine resources, and/or that human-induced fire regimes were already well entrenched prior to intensification.

A continuous record of Holocene eolian activity in West Greenland

Eolian landforms are widespread alongside proglacial valley-sandurs in West Greenland and comprise low-relief sand sheets, climbing dunes, and upland loess. Sedimentary facies mainly reflect distance to outwash-source zones and the influence of vegetation cover. The sediments show stratification types typical for poorly to moderately vegetated sand-sheets, alternately laminated silt/peat sequences, and unstratified loess. Twenty-five accelerator mass spectrometry 14C dates provide the basis for the chronostratigraphy of the inland eolian deposits. 14C dates from interstratified sand-sheets suggest that the bulk of eolian sands were deposited prior to 3400 cal yr B.P. and after 550 cal yr B.P. This two-phase formation for the inland dunes most likely reflects local changes in proglacial floodplain development and meltwater rerouting associated with a significant recession of the Greenland ice sheet during the mid Holocene climate optimum. Subsequent floodplain regeneration and renewed sand-sheet formation after 550 cal yr B.P. followed when the ice margin readvanced to its present position. In contrast, atmospheric deposition of regionally derived silt in upland peat mires has been continuous since at least 4750 cal yr B.P. Silt influx data demonstrate a strongly episodic history of the intensity of eolian activity over the past five millennia, which tentatively reflects alternating periods of (winter) aridity associated with the variable incursion of maritime air masses over the interior ice-free areas of West Greenland.

Climate Change in Northern Africa: The Past is Not the Future

By using a climate system model of intermediate complexity, we have simulated long-term natural climate changes occurring over the last 9000 years. The paleo-simulations in which the model is driven by orbital forcing only, i.e., by changes in insolation caused by changes in the Earth's orbit, are compared with sensitivity simulations in which various scenarios of increasing atmospheric CO2 concentration are prescribed. Focussing on climate and vegetation change in northern Africa, we recapture the strong greening of the Sahara in the early and mid-Holocene (some 9000–6000 years ago), and we show that some expansion of grasslandinto the Sahara is theoretically possible, if the atmospheric CO2 concentration increases well above pre-industrial values and if vegetation growth is not disturbed. Depending on the rate of CO2 increase, vegetation migration into the Sahara can be rapid, up to 1/10th of the Saharan area per decade, but could not exceed a coverage of 45%. In ourmodel, vegetation expansion into today's Sahara is triggered by an increase in summer precipitation which is amplified by a positive feedback between vegetation and precipitation. This is valid for simulations with orbital forcing and greenhouse-gas forcing. However, we argue that the mid-Holocene climate optimum some 9000 to 6000 years ago with its marked reduction of deserts in northern Africa is not a direct analogue for future greenhouse-gas induced climate change, as previously hypothesized. Not only does the global pattern of climate change differ between the mid-Holocene model experiments and the greenhouse-gas sensitivity experiments, but the relative role of mechanisms which lead to a reduction of the Sahara also changes. Moreover, the amplitude of simulated vegetation cover changes in northern Africa is less than is estimated for mid-Holocene climate.

Oxygen-isotope (δ18O) evidence of Holocene hydrological changes at Signy Island, maritime Antarctica

A record of Holocene hydrological changes has been produced from variations in oxygen-isotope composition (δ18O) preserved in freshwater lake sediments from maritime Antarctica. Small amounts (&lt;5%) of authigenic carbonate have been extracted from a non-marl sediment core from Sombre Lake, Signy Island (60°43'S, 45°38'W). Oxygen-isotope and particle-size analysis provide a sensitive record of hydrological events in the lake arising from local and regional climate phenomena. The climate affects δ18O through snowpack volume and glacier activity in the catchment, lakewater temperatures, the input versus evaporation balance and the duration of seasonal lake ice cover. The most depleted (negative) δ18O values are associated with influxes of meltwater at times of glacier advance or retreat. Enriched (positive) δ18O values occur during more arid, warmer conditions with longer periods of open water in summer. This isotope record can be used to determine century-scale to decadal variability in air circulation and moisture origin. Strong similarities with other Holo cene proxy records from the Weddell Sea and Antarctic Peninsula Region are apparent, including the mid-Holocene climate optimum followed by the Neoglacial and, most recently, late twentieth-century climatic warming. The oxygen-isotope record from Sombre Lake illustrates the importance of remote islands in contribu ting to our understanding of teleconnections in atmospheric and oceanographic circulation, sea-ice extent, air temperatures and precipitation in the Southern Ocean.

Sediment core from beneath the Amery Ice Shelf, East Antarctica, suggests mid-Holocene ice-shelf retreat

The stability of floating ice shelves is an important indicator of ocean circulation and ice-shelf mass balance. A sub–ice-shelf sediment core collected during the Austral summer of 2000–2001 from site AM02 (69°42.8′S, 72°38.4′E) on the Amery Ice Shelf, East Antarctica, contains a full and continuous record of glacial retreat. The AM02 core site is ∼80 km south of the floating ice shelf edge and contains a 0.5-m-thick Holocene surface layer of siliceous mud and diatom ooze of marine origin. Core data are supportive of sub–ice-shelf circulation models that predict the landward flow of oceanic water, and prove that the landward transport of hemipelagic sediments occurs beneath floating ice shelves over distances of at least ∼80 km. An increase in sea-ice–associated diatom deposition in the upper part of the Holocene suggests that a major retreat of the Amery Ice Shelf to at least 80 km landward of its present location may have occurred during the mid-Holocene climatic optimum.

Rock-magnetic carriers of century-scale susceptibility cycles in glacial–marine sediments from the Palmer Deep, Antarctic Peninsula

A 4200 year long magnetic susceptibility profile from the Palmer Deep records century- and millennial-scale variability in paleoenvironmental conditions along the western Antarctic Peninsula. The susceptibility profile shows regularly spaced highs and lows over the last 3360 years B.P. then drops to uniformly low values. These features are formed by the variable dilution of terrigenous material with biogenic sediments and shifts in magnetic mineralogy. High-susceptibility intervals are characterized by multi-domain (MD) pure magnetite. Low-susceptibility intervals are characterized by pseudo-single-domain (PSD) magnetite and titanomagnetite. Prior to 3360 years B.P., the magnetic mineral assemblage is dominated by fine PSD titanium-rich titanomagnetite and superparamagnetic particles, suggesting a change in sediment provenance and/or sediment transport processes. The absence of MD magnetite suggests a cessation in locally derived ice-rated debris. This horizon represents the transition from the mid-Holocene climatic optimum to the Neoglacial period, marked by an increase in glacial sedimentation.

Sharp Decrease in Summer Monsoon Strength 4000–3500 cal yr B.P. in the Central Higher Himalaya of India Based on Pollen Evidence from Alpine Peat

Age-constrained pollen data and magnetic susceptibility of an alpine peat profile from the Garhwal Higher Himalaya display a continuous record of climate and monsoon trends for the past 7800 yr. About 7800 cal yr B.P., dominance of evergreen oak (Quercus semecarpifolia), alder (Alnus), and grasses in the pollen record reflect a cold, wet climate with moderate monsoon precipitation. From 7800 to 5000 cal yr B.P., vegetation was progressively dominated by conifers, indicating ameliorated climate with a stronger monsoon. A warm, humid climate, with highest monsoon intensity, from 6000–4500 cal yr B.P. represents the mid-Holocene climatic optimum. Between 4000 and 3500 cal yr B.P., the abundance of conifers sharply decreased, with the greatest increase in evergreen oak. This trend suggests progressive cooling, with a decrease in the monsoon to its minimum about 3500 cal yr B.P. Two relatively minor cold/dry events at ca. 3000 and 2000 cal yr B.P. marked step-wise strengthening of the monsoon until ca. 1000 cal yr B.P. After a cold/dry episode that culminated ca. 800 cal yr B.P., the monsoon again strengthened and continued until today. A sharp decrease in temperature and rainfall at 4000–3500 cal yr B.P. represents the weakest monsoon event of the Holocene record. This cold/dry event correlates with proxy data from other localities of the Indian subcontinent, Arabian Sea, and western Tibet.

From Pleniglacial to Holocene: a 14C chronostratigraphy of environmental changes in the Konya Plain, Turkey

In the endoreic, semi-arid Konya basin on the central Anatolian plateaux, long-term hydrological evolution has left various landforms and lacustrine deposits reflecting the regional climatic evolution, as well as human influence on the local environments. This paper presents results from a cooperative programme grouping several institutes from Turkey and France, on lacustrine, marshy and aeolian sediment sequences of Upper Pleistocene and Holocene age. The detailed study of environmental evolution is based on the reconstruction as well as on the characterization of the extension and contraction phases of wetlands occupying the lowest parts of the Konya plain. A soil and a marsh layer are 14C dated ca. 28,000–25,000 yr bp. Three phases of Pleniglacial (from ca. 22,000 to 17,000 yr bp) high lake levels are distinguished. Complementary OSL dates on aeolian dunes confirm the occurrence of two drought periods: the first occurs around the start of the Late Glacial, the second after the Mid-Holocene climatic optimum, the latter being ‘in phase’ with a similar drought in other Eastern Mediterranean regions. After 17,000 yr bp, no lacustrine phase reached as high a level as the Pleniglacial lake. During the Late Glacial, a shallow freshwater lacustrine phase is identified from >12,500 to 11,000 yr bp. The Late Glacial to Holocene transition corresponds to a general absence of deposits and dateable material, thus suggesting a period of drought, to which no aeolian features have so far been related. The Holocene environmental evolution shows a period of marsh and shallow lake extansion from 6000 to 5500 yr bp; this wetter period is interrupted by the second drought (ca. 5500 yrs bp) as indicated by aeolian dune activity. During the Late Holocene, a renewal of marshes, as well as soil development on slopes, can be interpreted either as climatic changes or as impacts of human use of water and soil resources during prehistoric and historic times.

Pulleniatina obliquiloculata as a paleoceanographic indicator in the southern Okinawa trough during the last 20,000 years

Pulleniatina obliquiloculata, a tropical species of planktonic foraminifera, is indicative of the Kuroshio current and sensitive to winter sea surface temperature for the late Quaternary in the Okinawa Trough. Its relative abundance fluctuations are significant and correlatable between three gravity cores (cores 255, 170, 253) raised from the southern Okinawa Trough. Four major changes in its abundances with paleoceanographic significance have been recognized during the last 20,000 years: an abrupt increase at the Pleistocene/Holocene boundary, a short-term decrease indicative of a ‘Younger Dryas’-type climate reversal at about 11.4-9.6 ka B.P., the P. obliquiloculata maximum zone (around 7-6 ka B.P.) corresponding to the mid-Holocene climate optimum and the P. obliquiloculata minimum zone (around 4-2 ka B.P.) correlated probably to the Neoglacial cooling. The widespread occurrence of these events in the western Pacific and the correlated variations of its abundance between sea areas suggest that P. obliquiloculata is well promising as a paleoceanographic and climatic monitor for high-resolution reconstruction and sea-land correlation.

Lake Qinghai, China: closed-basin like levels and the oxygen isotope record for ostracoda since the latest Pleistocene

In 1985 a Sino-Swiss limnogeological expedition to China's largest inland lake recovered seismic profiles and piston cores to help calibrate palaeoclimatic models of monsoon and insolation fluctuations over the northern Tibet-Qinghai Plateau since the Last Glacial Maximum. Lake Qinghai, a perennial lake with an area of 4278 km 2 (1985) at 3194 m above sea level, receives drainage from 30,000 km 2 of catchment near the northwest corner of the Plateau. The 26.5 m deep terminal lake (1985), with brackish, alkaline, magnesium-sulphate rich waters (14 g 1 −1 TDS), has not overflowed since the last Glacial, but its levels have responded to climatic and hydrologic changes. Seismic-reflection profiles for the sub-bottom sediment structures revealed a complex topography with channels, relict terraces, tectonic tilting and deformation in deposits buried under a 4–6 m packet of mainly flat-bedded sediments from which core samples were recovered. The lowermost cored material (≈14,500 yr B.P.) comprises mainly water-lain loess deposited in a smaller flood-plain lake of up a few metres depth under cold arid climatic conditions. AMS radiocarbon dating indicates average sedimentation rates for latest Pleistocene and Holocene deposits in central basin regions of ≈0.3–0.5 mm yr −1. The δ 18O values of fossil ostracod valves provide a record of changes in the isotopic chemistry of the lake waters that can be related to the precipitation: evaporation budgets which have controlled the palaeo-levels for Lake Qinghai. The precipitation/evaporation budget for the Lake Qinghai region is largely determined by the Asian monsoon system. The Lake Qinghai sediment record shows that there was no large influx of glacoer melt-waters into the lake basin during the post-Glacial. The regional climate remained arid until at least ≈ 12,500 yr B.P., when an incipient monsoon was possibly initiated. River-transported silts first reached the basin centre at ≈11,800 yr B.P., when the lake depth may have increased to ≈ 5 m. Possibly a monsoonal weakening for several centuries after ≈ 10,800 yr B.P. (Europe, Younger Dryas) caused a further period of aridity. At ≈ 10,200 yr B.P. the basin began filling towards modern lake dimensions in response to a permanent strengthening of the Asian monsoon. Substantial lake-level rises probably characterised the periods ≈ 10,200-9800, 9500-8500, 8300-7200 yr B.P. and through perhaps a century at about 6800 yr B.P. Decreasing levels probably characterised the intervening intervals. Short-term lake-level fluctuations through the mid-Holocene climatic optimum were minimal, indicating a very stable climatic / hydrologic regime. An isotopic steady-state for the lake waters since 3000 ± 500 yr B.P. obscures the climatic / hydrologic regime dependent δ 18O gradient, but there is little stratigraphic evidence to suggest that the lake level has been significantly below its present level since that time. Three negative δ 18O excursions for the lake waters may correspond to known glacier advances on the Plateau at ≈3000, 2000 and 1500 yr B.P. during China's neoglacial.