Late Holocene Baltic Sea outflow changes reconstructed using C37:4 content from marine cores

The Baltic Sea is an intra‐continental brackish water body. Low saline surface water, the so‐called Baltic outflow current, exits the Baltic Sea through the Kattegat into the Skagerrak. Ingressions of saline oxygen‐rich bottom water enter the Baltic Sea basins via the narrow and shallow Kattegat and are of great importance for the ecological and ventilation state of the Baltic Sea. Over recent decades, progress has been made in studying Holocene changes in saline water inflow. However, reconstructions of past variations in Baltic Sea outflow changes are sparse and hampered because of the lack of suitable proxies. Here, we used the relative proportion of tetra‐unsaturated C₃₇ ketones (C_37:4 %) in long‐chain alkenones produced by coccolithophorids as a proxy for outflowing Baltic Sea water in the Skagerrak. To evaluate the applicability of the proxy, we compared the biomarker results with grain‐size records from the Kattegat and Mecklenburg Bay in addition to previously published salinity reconstructions from the Kattegat over the last 5000 years. All Skagerrak records showed an increase in C_37:4 % that is accompanied by enhanced bottom water currents in the Kattegat and western Baltic Sea over the past 3500 cal. a BP, indicating an increase in Baltic Sea outflow. This probably reflects higher precipitation in the Baltic Sea catchment area owing to a re‐organization of North Atlantic atmospheric circulation with an increased influence of wintertime Westerlies over the Baltic catchment from the mid‐ to the late Holocene.     

Meltwater Discharge to the Skagerrak–Kattegat from the Baltic Ice Lake during the Younger Dryas Interval

Diatom data from the Skagerrak–Kattegat show that large amounts of meltwater were discharged into the Kattegat–Skagerrak from the Baltic Ice Lake during the Younger Dryas interval. Strong meltwater discharge greatly freshened surface-water salinity in the Kattegat and areas along the Swedish west coast and possibly changed the directions of sea-surface salinity gradients from north–south to east–west or northwest–southeast. It resulted in a markedly stratified water column in salinity in the Kattegat, which complicates the environmental interpretation based on different types of microfossils. The meltwater influence on the large area of the Skagerrak during the Younger Dryas was, however, restricted along the Norwegian coast where it flowed into the Norwegian Sea.     

Process length variation in cysts of the dinoflagellate Protoceratium reticulatum,from surface sediments of the Baltic–Kattegat–Skagerrak estuarine system: a regional salinity proxy

Mertens, K. N., Dale, B., Ellegaard, M., Jansson, I.‐M., Godhe, A., Kremp, A. & Louwye, S. 2010: Process length variation in cysts of the dinoflagellate Protoceratium reticulatum, from surface sediments of the Baltic–Kattegat–Skagerrak estuarine system: a regional salinity proxy. Boreas, 10.1111/j.1502‐3885.2010.00193.x. ISSN 0300‐9483. Results are presented from a regional comparison of average process length variation in cysts of Protoceratium reticulatum and Lingulodinium polyedrum, extracted from surface sediments in the Skagerrak–Kattegat–Baltic estuarine system, with the environmental variables of seawater temperature and salinity. Although too few cysts of Lingulodinium polyedrum were recovered from the sediments to make reliable correlations, cysts of Protoceratium reticulatum were well represented, and average process length was correlated significantly with both salinity and temperature. Owing to dominant summer surface production, and regional covariation between salinity and density, we propose the use of the significant correlation with summer sea surface salinity (SSS_summer) by the equation SSS_summer=3.16 × average process length ?0.84 (R²=0.8). Application of this equation down‐core in Limfjord (northern Denmark) shows its usefulness as a regional palaeosalinity proxy.     

A diatom‐based reconstruction of summer sea‐surface salinity in the Southern Okinawa Trough,East China Sea,over the last millennium

A high‐resolution diatom record from site MD05‐2908 in the Southern Okinawa Trough, East China Sea, reveals pronounced multidecadal‐ to centennial‐scale palaeoceanographic changes throughout the last millennium. Summer sea‐surface salinity (SSS) was reconstructed using a weighted averaging partial least squares diatom‐based training set. The reconstructed SSS shows slightly decreasing values during the period AD 905–1930 with considerable fluctuations superimposed on this general trend. Relatively high‐salinity conditions during the interval AD 905–1450 probably suggest a low flood frequency in north‐eastern Taiwan. Furthermore, the high SSS values are associated with a strong and stable influence of the Kuroshio Current on the Southern Okinawa Trough during the Medieval Climate Anomaly. The period AD 1450–1930 is characterized by three low‐salinity intervals (AD 1450–1500, AD 1625–1725 and AD 1770–1880) separated by periods of relatively high salinity. The low SSS intervals indicate increased freshwater discharge into the Southern Okinawa Trough during the Little Ice Age, probably as a result of higher flood frequencies in north‐eastern Taiwan. Spectral and wavelet analyses suggest that this pattern was linked to multidecadal variations in summer SSS, presumably associated with the Pacific Decadal Oscillation. Copyright © 2012 John Wiley & Sons, Ltd.     

Diatom‐based reconstruction of summer sea‐surface salinity in the South China Sea over the last 15 000 years

We present a new reconstruction of summer sea‐surface salinity (SSS) over the past 15 000 years based on a diatom record from piston core 17940, located on the northern slope of the South China Sea (SCS). The reconstructed diatom‐based summer SSS values for the modern period are in accord with instrumental observations of summer SSS in the area. Here, the modern summer SSS is primarily controlled by river runoff, in particular from the Pearl River. The reconstruction presented in this study shows that the summer SSS varied between 33.3 and 34.2 psu over the past 15 000 years. The long‐term summer SSS trend closely followed the trend of the orbitally controlled solar insolation at 20°N, suggesting that orbital forcing was the dominant driver of changes in summer SSS in this area. Comparisons to speleothem δ¹⁸O data and studies of surface hydrography in the region suggest that changes in solar insolation affected the summer SSS through changes in the East Asian Monsoon and sea‐level changes associated with the last deglaciation. Univariate spectral analyses indicate that centennial‐scale oscillatory variations in summer SSS were superimposed on the long‐term trend. During the deglacial period (c. 12 000–9000 cal. a BP), the dominant periodicity was centred around 230–250 years, whereas a ～350‐year oscillation dominated in the period 2200–4500 cal. a BP. The balance of evidence suggests that these centennial‐scale changes in summer SSS may have been driven by solar‐induced changes in the East Asian Monsoon, but further evidence is needed to firmly establish this relationship.     

Oceanography in northwestern Europe during the last interglacial from intrashell δO ranges in Littorina littorea gastropods

Coastal sea-surface temperature (SST) and sea-surface salinity (SSS), including seasonality, in northwest (NW) Europe during the early phase of the Eemian interglacial ca. 125 ka ago were reconstructed from Littorina littorea (common periwinkle) gastropods. The results were based on intra-annual δ¹⁸O analyses in recent and fossil shells, mainly originating from the sea of Kattegat (Sweden) and the English Channel (United Kingdom), and confined to intertidal settings. The Eemian L. littorea shells indicated annual SSTs in the range 8–18°C for the English Channel and 8–26°C for Kattegat. All specimens from the Eemian sites experienced summer SSTs of ca. 1–3°C above recent conditions. The estimated winter SST in the English Channel during the Eemian was comparable to modern measurements of ca. 8°C. However, the Kattegat region displayed Eemian winter SST approximately 8°C warmer than today, and similar to conditions in the western English Channel. The recent-fossil isotope analogue approach indicated high SSS above 35 practical salinity units (psu) for a channel south of England in full contact with the North Atlantic Ocean during the last interglacial. In addition, the Kattegat shells indicated a SSS of ca. 29 psu, which points out a North Sea affinity for this region during the Eemian.     

Contrasting patterns of precipitation seasonality during the Holocene in the south‐ and north‐central Mediterranean

Pollen‐based quantitative estimates of seasonal precipitation from Lake Pergusa and lake‐level data from Lake Preola in Sicily (southern Italy) allow three successive periods to be distinguished within the Holocene: an early Holocene period before ca. 9800 cal a BP with rather dry climate conditions in winter and summer, a mid‐Holocene period between ca. 9800 and 4500 cal a BP with maximum winter and summer wetness, and a late Holocene period after 4500 cal a BP with declining winter and summer wetness. This evolution observed in the south‐central Mediterranean shows strong similarities to that recognized in the eastern Mediterranean. But, it contrasts with that reconstructed in north‐central Italy, where the mid‐Holocene appears to be characterized by a winter (summer) precipitation maximum (minimum), while the late Holocene coincided with a decrease (increase) in winter (summer) precipitation. Maximum precipitation at ca. 10 000–4500 cal a BP may have resulted from (i) increased local convection in response to a Holocene insolation maximum at 10 000 cal a BP and then (ii) the gradual weakening of the Hadley cell activity, which allowed the winter rainy westerlies to reach the Mediterranean area more frequently. After 4500 cal a BP, changes in precipitation seasonality may reflect non‐linear responses to orbitally driven insolation decrease in addition to seasonal and inter‐hemispheric changes of insolation. Copyright © 2011 John Wiley & Sons, Ltd.     

Early Last Interglacial palaeoenvironments in the western Baltic Sea: benthic foraminiferal stable isotopes and diatom‐based sea‐surface salinity

Knudsen, K. L., Jiang, H., Kristensen, P., Gibbard, P. L. & Haila, H. 2011: Early Last Interglacial palaeoenvironments in the western Baltic Sea: benthic foraminiferal stable isotopes and diatom‐based sea‐surface salinity. Boreas, 10.1111/j.1502‐3885.2011.00206.x. ISSN 0300‐9483. Stable isotopes from benthic foraminifera, combined with diatom assemblage analysis and diatom‐based sea‐surface salinity reconstructions, are used for the interpretation of changes in bottom‐ and surface‐water conditions through the early Eemian at Ristinge Klint in the western Baltic Sea. Correlation of the sediments with the Eemian Stage is based on a previously published pollen analysis that indicates that they represent pollen zones E2–E5 and span ～3400 years. An initial brackish‐water phase, initiated c. 300 years after the beginning of the interglacial, is characterized by a rapid increase in sea‐surface and sea‐bottom salinity, followed by a major increase at c. 650 years, which is related to the opening of the Danish Straits to the western Baltic. The diatoms allow estimation of the maximum sea‐surface salinity in the time interval of c. 650–1250 years. After that, slightly reduced salinity is estimated for the interval of c. 1250–2600 years (with minimum values at c. 1600–2200 years). This may be related to a period of high precipitation/humidity and thus increased freshwater run‐off from land. Together with a continuous increase in the water depth, this may have contributed to the gradual development of a stratified water column after c. 1600 years. The stratification was, however, particularly pronounced between c. 2600 and 3400 years, a period with particularly high sea‐surface temperature, as well as bottom‐water salinity, and thus a maximum influence of Atlantic water masses. The freshwater run‐off from land may have been reduced as a result of particularly high summer temperatures during the climatic optimum.     

Past occurrences of hypoxia in the Baltic Sea and the role of climate variability,environmental change and human impact

The hypoxic zone in the Baltic Sea has increased in area about four times since 1960 and widespread oxygen deficiency has severely reduced macro benthic communities below the halocline in the Baltic Proper and the Gulf of Finland, which in turn has affected food chain dynamics, fish habitats and fisheries in the entire Baltic Sea. The cause of increased hypoxia is believed to be enhanced eutrophication through increased anthropogenic input of nutrients, such as nitrogen and phosphorus. However, the spatial variability of hypoxia on long time-scales is poorly known: and so are the driving mechanisms. We review the occurrence of hypoxia in modern time (last c. 50 years), modern historical time (AD 1950–1800) and during the more distant past (the last c. 10 000 years) and explore the role of climate variability, environmental change and human impact. We present a compilation of proxy records of hypoxia (laminated sediments) based on long sediment cores from the Baltic Sea. The cumulated results show that the deeper depressions of the Baltic Sea have experienced intermittent hypoxia during most of the Holocene and that regular laminations started to form c. 8500–7800 cal. yr BP ago, in association with the formation of a permanent halocline at the transition between the Early Littorina Sea and the Littorina Sea s. str. Laminated sediments were deposited during three main periods (i.e. between c. 8000–4000, 2000–800 cal. yr BP and subsequent to AD 1800) which overlap the Holocene Thermal Maximum (c. 9000–5000 cal. yr BP), the Medieval Warm Period (c. AD 750–1200) and the modern historical period (AD 1800 to present) and coincide with intervals of high surface salinity (at least during the Littorina s. str.) and high total organic carbon content. This study implies that there may be a correlation between climate variability in the past and the state of the marine environment, where milder and dryer periods with less freshwater run-off correspond to increased salinities and higher accumulation of organic carbon resulting in amplified hypoxia and enlarged distribution of laminated sediments. We suggest that hydrology changes in the drainage area on long time-scales have, as well as the inflow of saltier North Sea waters, controlled the deep oxic conditions in the Baltic Sea and that such changes have followed the general Holocene climate development in Northwest Europe. Increased hypoxia during the Medieval Warm Period also correlates with large-scale changes in land use that occurred in much of the Baltic Sea watershed during the early-medieval expansion. We suggest that hypoxia during this period in the Baltic Sea was not only caused by climate, but increased human impact was most likely an additional trigger. Large areas of the Baltic Sea have experienced intermittent hypoxic from at least AD 1900 with laminated sediments present in the Gotland Basin in the Baltic Proper since then and up to present time. This period coincides with the industrial revolution in Northwestern Europe which started around AD 1850, when population grew, cutting of drainage ditches intensified, and agricultural and forest industry expanded extensively.     

Mid Holocene origin of the sea-surface salinity low in the subarctic North Pacific

IMAGES core MD01-2416 (51°N, 168°E) provides the first centennial-scale multiproxy record of Holocene variation in North Pacific sea-surface temperature (SST), salinity, and biogenic productivity. Our results reveal a gradual decrease in subarctic SST by 3–5 °C from 11.1 to 4.2 ka and a stepwise long-term decrease in sea surface salinity (SSS) by 2–3 p.s.u. Early Holocene SSS were as high as in the modern subtropical Pacific. The steep halocline and stratification that is characteristic of the present-day subarctic North Pacific surface ocean is a fairly recent feature, developed as a product of mid-Holocene environmental change. High SSS matched a salient productivity maximum of biogenic opal during Bølling-to-Early Holocene times, reaching levels similar to those observed during preglacial times in the warm mid-Pliocene prior to 2.73 Ma. Similar productivity spikes marked every preceding glacial termination of the last 800 ka, indicating recurrent short-term events of mid-Pliocene-style intense upwelling of nutrient-rich Pacific Deepwater in the Pleistocene. Such events led to a repeated exposure of CO₂-rich deepwater at the ocean surface facilitating a transient CO₂ release to the atmosphere, but the timing and duration of these events repudiate a long-term influence of the subarctic North Pacific on global atmospheric CO₂ concentration.     

Late Weichselian-Holocene foraminiferal stratigraphy and palaeohydrogrphic changes in the Gothenburg area, southwestern Sweden

Marine clay from two cores (50 and 36m deep) from Gothenburg, southwestern Sweden, have been analysed using different stratigraphic methods. Foraminiferal stratigraphy complemented withe lithostratigraphy, pollen and mollusc analyses show an environmental succession from arctic conditions with water depths up to 100m during Late Welchselian time, to a boreal shallow water environment in early Holocene time. A comparison of the foraminiferal faunas with those from corresponding investigations from southern Bohuslän, NW of Gothenburg, shows a similar development in the two areas. The sudden environmental change around the Pleistocene/Holocene boundary (10,000 years B.P.) along the Swadish west coast is attributed to changes of the hydrographic patterns; a general shift of the circulation pattern of the Skagerrak/ Kattegat at that time and or a large supply of fresh water flowing into the area from the Lake Vanern basin.     

Holocene climate dynamics in Latvia,eastern Baltic region: a pollen‐based summer temperature reconstruction and regional comparison

Heikkilä, M. & Seppä, H. 2010: Holocene climate dynamics in Latvia, eastern Baltic region: a pollen‐based summer temperature reconstruction and regional comparison. Boreas, Vol. 39, pp. 705–719. 10.1111/j.1502‐3885.2010.00164.x. ISSN 0300‐9483. A pollen‐based summer temperature (T_summer) reconstruction reveals the Holocene climate history in southeastern Latvia and contributes to the limited understanding of past climate behaviour in the eastern sector of northern Europe. Notably, steady climate warming of the early Holocene was interrupted c. 8350–8150 cal. yr BP by the well‐known 8.2 ka cold event, recorded as a decrease of 0.9 to 1.8 °C in T_summer. During the Holocene Thermal Maximum, c. 8000–4000 cal. yr BP, the reconstructed summer temperature was ～2.5–3.5 °C higher than the modern reconstructed value, and subsequently declined towards present‐day values. Comparison of the current reconstruction with other pollen‐based reconstructions in northern Europe shows that the 8.2 ka event is particularly clearly reflected in the Baltic region, possibly as a result of distinct climatic and ecological gradients and the sensitivity of the vegetation growth pattern to seasonal temperature change. The new reconstruction also reveals that the Holocene Thermal Maximum was warmer in Latvia than in central Europe and Fennoscandia. In fact, a gradient of increasing positive temperature anomalies is detected from northernmost Fennoscandia towards the south and from the Atlantic coast in Norway towards the continental East European Plain. The dynamics of the temperate broadleaved tree species Tilia and Quercus in Latvia and adjacent northern Europe during the mid‐Holocene give complementary information on the multifaceted climatic and environmental changes in the region.     

Holocene environmental history of the Ångermanälven Estuary,northern Baltic Sea

The Baltic Sea has experienced a complex geological history, with notable swings in salinity driven by changes to its connection with the Atlantic and glacio‐isostatic rebound. Sediments obtained during International Ocean Drilling Program Expedition 347 allow the study of the effects of these changes on the ecology of the Baltic in high resolution through the Holocene in areas where continuous records had not always been available. Sites M0061 and M0062, drilled in the Ångermanälven Estuary (northern Baltic Sea), contain records of Holocene‐aged sediments and microfossils. Here we present detailed records of palaeoecological and palaeoenvironmental changes to the Ångermanälven Estuary inferred from diatom, palynomorph and organic‐geochemical data. Based on diatom assemblages, the record is divided into four zones that comprise the Ancylus Lake, Littorina Sea, Post‐Littorina Sea and Recent Baltic Sea stages. The Ancylus Lake phase is initially characterized as oligotrophic, with the majority of primary productivity in the upper water column. This transition to a eutrophic state continues into the Initial Littorina Sea stage. The Initial Littorina Sea stage contains the most marine phase recorded here, as well as low surface water temperatures. These conditions end before the Littorina Sea stage, which is marked by a return to oligotrophic conditions and warmer waters of the Holocene Thermal Maximum. Glacio‐isostatic rebound leads to a shallowing of the water column, allowing for increased benthic primary productivity and stratification of the water column. The Medieval Climate Anomaly is also identified within Post‐Littorina Sea sediments. Modern Baltic sediments and evidence of human‐induced eutrophication are seen. Human influence upon the Baltic Sea begins c. 1700 cal. a BP and becomes more intense c. 215 cal. a BP.     

Out-of-phase evolution between summer and winter East Asian monsoons during the Holocene as recorded by Chinese loess deposits

We analyzed climate proxies from loessic-soil sections of the southern Chinese Loess Plateau. The early Holocene paleosol, S0, is 3.2 m thick and contains six sub-soil units. Co-eval soils from the central Loess Plateau are thinner (~ 1 m). Consequently higher-resolution stratigraphic analyses can be made on our new sections and provide more insight into Holocene temporal variation of the East Asian monsoon. Both summer and winter monsoon evolution signals are recorded in the same sections, enabling the study of phase relationships between the signals. Our analyses consist of (i) measurements of magnetic properties sensitive to the production of fine-grained magnetic minerals which reflect precipitation intensity and summer monsoon strength; and (ii) grain-size analyses which reflect winter monsoon strength. Our results indicate that the Holocene precipitation maximum occurred in the mid-Holocene, ~ 7.8–3.5 cal ka BP, with an arid interval at 6.3–5.3 cal ka BP. The winter monsoon intensity declined to a minimum during 5.0–3.4 cal ka BP. These results suggest that the East Asian summer and winter monsoons were out of phase during the Holocene, possibly due to their different sensitivities to ice and snow coverage at high latitudes and to sea-surface temperature at low latitudes.     

An evaluation of the diatom response to Late Quaternary environmental change in two lakes in the Konya Basin,Turkey, by comparison with stable isotope data

In palaeolimnological studies of closed-basin lakes, diatoms provide an excellent source of palaeoclimate data owing to their sensitivity to salinity and lake-level change. One of the best ways of assessing the relationships between diatom species response, salinity and lake-level change is comparison with other proxy indicators, such as stable isotopes. The sensitivity of different lake systems to climate change (changes in the ratio of precipitation to evapotranspiration) depends to a large extent on the character of the basin hydrology. Here, we compare evidence for Late Quaternary palaeohydrological change in two lakes which have contrasting groundwater hydrology, located in close proximity to each other in the Konya Basin, southern central Turkey, and whose sediment core chronologies show considerable overlap. Süleymanhacı gölü has alternated between being hydrologically closed and open during its history, and the diatom record shows clear evidence for changes in palaeoconductivity and lake level which correspond well to inferred changes in evaporative concentration during the Late Pleistocene and part of the Holocene. In contrast, the diatom record of the karstic spring-fed pool, Pinarbaşi, indicates that it remained fresh throughout the Late Pleistocene; it is subject to major aquifer flow and behaves effectively as an open system. In this case the complacency of the diatom record is important in allowing inferences on temperature effects to be drawn from relatively subtle shifts in stable isotope values.     

A multiproxy record of Holocene environmental changes in the central Kola Peninsula,northwest Russia

A sediment core from Chuna Lake (Kola Peninsula, northwest Russia) was studied for pollen, diatoms and sediment chemistry in order to infer post‐glacial environmental changes and to investigate responses of the lake ecosystem to these changes. The past pH and dissolved organic carbon (DOC) of the lake were inferred using diatom‐based transfer functions. Between 9000 and 4200 cal. yr BP, slow natural acidification and major changes in the diatom flora occurred in Chuna Lake. These correlated with changes in regional pollen, the arrival of trees in the catchment, changes in erosion, sediment organic content and DOC. During the past 4200 yr diatom‐based proxies showed no clear response to changes in vegetation and erosion, as autochthonous ecological processes became more important than external climate influences during the late Holocene. The pollen stratigraphy reflects the major climate patterns of the central Kola Peninsula during the Holocene, i.e. a climate optimum between 9000 and 5400/5000 cal. yr BP when climate was warm and dry, and gradual climate cooling and an increase in moisture during the past 5400/5000 yr. This agrees with the occurrence of the north–south humidity gradient in Fennoscandia during the Holocene. Copyright © 2002 John Wiley & Sons, Ltd.     

渭河盆地末次冰盛期以来沉积环境变化研究

研究过去气候快速变化能为当前极端气候分析和未来环境预测提供自然背景理解。亚洲季风在北半球乃至全球的第四纪气候变化中扮演着重要角色,其演化是全球气候变化背景下的典型区域响应。然而,不同地质载体及不同指标所记录的亚洲冬、夏季风变化存在着较大差异,产生差异的原因及受到的动力机制是值得深入研究的科学问题。渭河盆地位于黄土高原和古三门湖沉积交叠的区域,是研究第四纪亚洲季风演化的理想场所。在盆地西南部西安市户县和长安县获取了两个黄土沉积钻孔,户县ZZC孔长4 m,长安县XFC孔长3 m,两孔的年代均超过25 ka。通过两钻孔的粒度和元素地球化学等代用指标研究,对比分析不同指标对气候变化的敏感度差异,反演了末次冰盛期(LGM)以来的区域沉积环境变化,并尝试探讨该时期发生的气候突变事件及反映的季风强度变化。结果表明,两钻孔的平均粒径从LGM到中全新世逐渐变细,中全新世之后少许变粗,空间上表现出一致性,总体反映了末次冰盛期以来的冬季风强度演化;Ca/Ti反映了与季风降水相关的淋溶强度,从LGM到全新世暖期夏季风逐渐减弱,并记录了若干次气候快速变化。粒度和元素比值变化表明,渭河盆地沉积良好地记录了末次冰盛期至全新世的大幅冷干-暖湿波动及若干次持续时间较短的快速水文变化事件,主要是受到太阳辐射和冰量等因素调控的影响。由于渭河盆地有上千米的新生代沉积,未来开展高分辨率研究有望揭示不同时间尺度季风变化特征及其与区域和全球变化的联系。     

Subrecent changes in the foraminiferal distribution in the Kattegat and the Skagerrak, Scandinavia: anthropogenic influence and natural causes

Foraminifera from surface samples in the Kattegat and the Skagerrak, northwestern Europe, have been analysed to determine the modern foraminiferal distribution. A total of five foraminiferal assemblages are distinguished. These are the Elphidium excavatum, Cassidulina laevigata, Bulimina marginata, Cibicides lobatulus and Trochammina sp. assemblages. Only the first three are found over large areas and these are correlated to either depth, organic carbon content or grain size. At each station a short core was studied to determine whether changes have occurred in the assemblages during the last few hundred years. In some areas no such variations were found, but several of the cores from the Skagerrak and all cores from the Kattegat document changes within this period. The fluctuations in the Skagerrak may be attributed to natural causes, such as species migrations or re-deposition. In the Kattegat a change from a Hyalinea balthica assemblage to the modern B. marginata assemblage always occurs at approximately the same core depth, which presumably represents the biological mixing depth. This change is presumably due to anthropogenic influences, which have caused oxygen depletion in the bottom waters of the Kattegat during the last few decades.     

Wadi Bakht revisited: Holocene climate change and prehistoric occupation in the Gilf Kebir region of the Eastern Sahara,SW Egypt

Geoarchaeological and chronological evidence from the remote Gilf Kebir Plateau in southwest Egypt suggests a new model for the influence of early and mid‐Holocene precipitation regimes on land‐use strategies of prehistoric settlers in what is now the center of the largest hyperarid area on earth. We hypothesize that the quantitatively higher, daytime, monsoon summer rainfall characteristic of the early Holocene (9300–5400 ¹⁴C yr B.P./8400–4300 yr B.C.) resulted in less grass growth on the plateau compared to the winter rains that presumably fell in the cool nights during the terminal phase of the Holocene pluvial (5400–4500 yr B.P./4300–3300 yr B.C.). The unparalleled climatic transition at 5400 yr B.P. (4300 yr B.C.) caused a fundamental environmental change that resulted in different patterns of human behavior, economy, and land use in the canyon‐like valleys and on the plains surrounding the plateau. The model emphasizes the crucial impact of seasonal rainfall distribution on cultural landscapes in arid regions and the lower significance of annual precipitation rates, with implications for future numeric climate models. It also serves as an example of how past climate changes have affected human societies. © 2004 Wiley Periodicals, Inc.     

Holocene precipitation and temperature variations in the East Asian monsoonal margin from pollen data from Hulun Lake in northeastern Inner Mongolia,China

Wen, R. L., Xiao, J. L., Chang, Z. G., Zhai, D. Y., Xu, Q. H., Li, Y. C. & Itoh, S. 2009: Holocene precipitation and temperature variations in the East Asian monsoonal margin from pollen data from Hulun Lake in northeastern Inner Mongolia, China. Boreas, 10.1111/j.1502‐3885.2009.00125.x. ISSN 0300‐9483. Quantitative palaeoclimatic reconstruction with the weighted averaging partial least squares method was applied to the pollen profile from Hulun Lake in northeastern Inner Mongolia. The data provide a detailed history of variations in precipitation and temperature over the northeastern margin of the East Asian summer monsoon during the Holocene. A warm and dry climate prevailed over the lake region until c. 8000 cal. BP. During the period c. 8000–4400 cal. BP, precipitation increased markedly and temperature gradually declined. The interval between c. 4400 and 3350 cal. BP was marked by extremely dry and relatively cold conditions. Precipitation recovered from c. 3350 to 1000 cal. BP, with temperatures rising c. 3350–2050 cal. BP and dropping c. 2050–1000 cal. BP. During the last 500 years, the climate of the lake region displayed a general trend of warming and wetting. While Holocene temperature variations in the mid‐high latitude monsoonal margin were controlled by changes in summer solar radiation in the Northern Hemisphere, they could also be related to the strength of the East Asian summer monsoon. The lack of precipitation during the early Holocene could be attributed to the weakened summer monsoon resulting from the existence of remnant ice sheets in the Northern Hemisphere. Changes in the monsoonal precipitation during the middle to late Holocene would have been associated with the ocean–atmosphere interacting processes occurring in the western tropical Pacific.