Sedimentology and paleontology on core Lz1104 from Lake Hjort, Greenland

Two sediment cores of 70 and 252 cm length were recovered from Hjort Sø, a small lake on Store Koldewey, Northeast Greenland, and studied with a multidisciplinary approach in order to reconstruct the local environmental history and to test the relevance of proxies for paleoenvironmental information. The basal sediments from the longer core are dominated by clastic matter, which was likely deposited during deglaciation of the lake basin. These clastic sediments are overlain by gyttja, which is also present throughout the shorter core. AMS radiocarbon dating was conducted on plant macrofossils of 11 samples from the gyttja in both cores. A reliable chronology was established for both cores, which dated the onset of organic accumulation at 9,500 cal. year BP. The Holocene temperature development, with an early to mid Holocene thermal maximum, is best reflected in the grain-size composition. Nutrient availability was apparently low during the early Holocene and led to low productivity in the lake and its vicinity. From ca. 7,000 cal. year BP, productivity in the lake increased significantly, probably induced by external nutrient input from goose excrements. From this time, micro- and macrofossil remains reflect relatively well the climate history of East Greenland, with a cooling during the middle Holocene, the medieval warming, and the Little Ice Age. The amount of organic matter in the sequence seems to be more affected by lake ice cover or by nutrient supply from the catchment than by temperature changes. The record from Hjort Sø thus reveals the difficulties in interpreting sedimentary records from high arctic regions.

(Table 2) Radiocarbon ages of lake sediments from Boresø, East Greenland

This chapter provides a review of proxy data from a variety of natural archives sampled in the Wollaston Forland region, central Northeast Greenland. The data are used to describe long-term environmental and climatic changes. The focus is on reconstructing the Holocene conditions particularly in the Zackenberg area. In addition, this chapter provides an overview of the archaeological evidence for prehistoric occupation of the region. The Zackenberg area has been covered by the Greenland Ice Sheet several times during the Quaternary. At the Last Glacial Maximum (LGM, about 22,000 years BP), temperatures were much lower than at present, and only very hardy organisms may have survived in the region, even if ice-free areas existed. Marked warming at around 11,700 years BP led to ice recession, and the Zackenberg area was deglaciated in the early Holocene, prior to 10,100 years BP. Rapid early Holocene land emergence was replaced by a slight transgression in the late Holocene. During the Holocene, summer solar insolation decreased in the north. Following deglaciation of the region, summer temperatures probably peaked in the early to mid-Holocene, as indicated by the occurrence of a southern beetle species. However, the timing for the onset of the Holocene thermal maximum is rather poorly constrained because of delayed immigration of key plant species. During the thermal maximum, the mean July temperature was at least 2-3°C higher than at present. Evidence for declining summer temperatures is seen at around 5500, 4500 and 3500 years BP. The cooling culminated during the Little Ice Age that peaked about 100-200 years ago. The first plants that immigrated to the region were herbs and mosses. The first dwarf shrubs arrived in Northeast Greenland prior to 10,400 years BP, and dwarf birch arrived around 8800 years BP. The first people arrived about 4500 years BP, but the region was depopulated several times before the last people disappeared some time after 1823 AD, perhaps as a consequence of poor hunting conditions during the peak of the Little Ice Age.

Temperature measured in permafrost borehole at ICDP site 5011-3 in El'gygytgyn Crater, 2008-2011

This study focuses on the temperature field observed in boreholes drilled as part of interdisciplinary scientific campaign targeting the El'gygytgyn Crater Lake in NE Russia. Temperature data are available from two sites: the lake borehole 5011-1 located near the center of the lake reaching 400 m depth, and the land borehole 5011-3 at the rim of the lake, with a depth of 140 m. Constraints on permafrost depth and past climate changes are derived from numerical simulation of the thermal regime associated with the lake-related talik structure. The thermal properties of the subsurface needed for these simulations are based on laboratory measurements of representative cores from the quaternary sediments and the underlying impact-affected rock, complemented by further information from geophysical logs and data from published literature. The temperature observations in the lake borehole 5011-1 are dominated by thermal perturbations related to the drilling process, and thus only give reliable values for the lowermost value in the borehole. Undisturbed temperature data recorded over more than two years are available in the 140 m deep land-based borehole 5011-3. The analysis of these observations allows determination of not only the recent mean annual ground surface temperature, but also the ground surface temperature history, though with large uncertainties. Although the depth of this borehole is by far too insufficient for a complete reconstruction of past temperatures back to the Last Glacial Maximum, it still affects the thermal regime, and thus permafrost depth. This effect is constrained by numerical modeling: assuming that the lake borehole observations are hardly influenced by the past changes in surface air temperature, an estimate of steady-state conditions is possible, leading to a meaningful value of 14 ± 5 K for the post-glacial warming. The strong curvature of the temperature data in shallower depths around 60 m can be explained by a comparatively large amplitude of the Little Ice Age (up to 4 K), with low temperatures prevailing far into the 20th century. Other mechanisms, like varying porosity, may also have an influence on the temperature profile, however, our modeling studies imply a major contribution from recent climate changes.

Mid-to Late Holocene flood frequency in the northeastern Alps as recorded in varved sediments of Lake Mondsee

Annually laminated (varved) lake sediments with intercalated detrital layers resulting from sedimentary input by runoff events are ideal archives to establish precisely dated records of past extreme runoff events. In this study, the mid- to late Holocene varved sediments of Lake Mondsee (Upper Austria) were analysed by combining sedimentological, geophysical and geochemical methods. This approach allows to distinguish two types of detrital layers related to different types of extreme runoff events (floods and debris flows) and to detect changes in flood activity during the last 7100 years. In total, 271 flood and 47 debris flow layers, deposited during spring and summer, were identified, which cluster in 18 main flood episodes (FE 1-18) with durations of 30-50 years each. These main flood periods occurred during the Late Neolithic (7100-7050 vyr BP and 6470-4450 vyr BP), the late Bronze Age and the early Iron Age (3300-3250 and 2800-2750 vyr BP), the late Iron Age (2050-2000 vyr BP), throughout the Dark Ages Cold Period (1500-1200 vyr BP), and at the end of the Medieval Warm Period and the Little Ice Age (810-430 vyr BP). Summer flood episodes in Lake Mondsee are generally more abundant during the last 1500 years, often coinciding with major advances of alpine glaciers. Prior to 1500 vyr BP, spring/summer floods and debris flows are generally less frequent, indicating a lower number of intense rainfall events that triggered erosion. In comparison with the increase of late Holocene flood activity in western and northwestern (NW) Europe, commencing already as early as 2800 yr BP, the hydro-meteorological shift in the Lake Mondsee region occurred much later. These time lags in the onset of increased hydrological activity might be either due to regional differences in atmospheric circulation pattern or to the sensitivity of the individual flood archives. The Lake Mondsee sediments represent the first precisely dated and several millennia long summer flood record for the northeastern (NE) Alps, a key region at the climatic boundary of Atlantic, Mediterranean and East European air masses aiding a better understanding of regional and seasonal peculiarities of flood occurrence under changing climate conditions.

Relative sea-level history of Marguerite Bay, Antarctic Peninsula derived from optically stimulated luminescence-dated beach cobbles

Calmette Bay within Marguerite Bay along the western side of the Antarctic Peninsula contains one of the most continuous flights of raised beaches described to date in Antarctica. Raised beaches extend to 40.8 m above sea level (masl) and are thought to reflect glacial isostatic adjustment due to the retreat of the Antarctic Peninsula Ice Sheet. Using optically stimulated luminescence (OSL), we dated quartz extracts from cobble surfaces buried in raised beaches at Calmette Bay. The beaches are separated into upper and lower beaches based on OSL ages, geomorphology, and sedimentary fabric. The two sets of beaches are separated by a prominent scarp. One of our OSL ages from the upper beaches dates to 9.3 thousand years ago (ka; as of 1950) consistent with previous extrapolation of sea-level data and the time of ice retreat from inner Marguerite Bay. However, four of the seven ages from the upper beaches date to the timing of glaciation. We interpret these ages to represent reworking of beaches deposited prior to the Last Glacial Maximum (LGM) by advancing and retreating LGM ice. Ages from the lower beaches record relative sea-level fall due to Holocene glacial-isostatic adjustment. We suggest a Holocene marine limit of 21.7 masl with an age of 5.5-7.3 ka based on OSL ages from Calmette Bay and other sea-level constraints in the area. A marine limit at 21.7 masl implies half as much relative sea-level change in Marguerite Bay during the Holocene as suggested by previous sea-level reconstructions. No evidence for a relative sea-level signature of neoglacial events, such as a decrease followed by an increase in RSL fall due to ice advance and retreat associated with the Little Ice Age, is found within Marguerite Bay indicating either: (1) no significant neoglacial advances occurred within Marguerite Bay; (2) rheological heterogeneity allows part of the Antarctic Peninsula (i.e. the South Shetland Islands) to respond to rapid ice mass changes while other regions are incapable of responding to short-lived ice advances; or (3) the magnitude of neoglacial events within Marguerite Bay is too small to resolve through relative sea-level reconstructions. Although the application of reconstructing sea-level histories using OSL-dated raised beach deposits provides a better understanding of the timing and nature of relative sea-level change in Marguerite Bay, we highlight possible problems associated with using raised beaches as sea-level indices due to post-depositional reworking by storm waves.

Bulk sediment characteristics and geomicrobiology of sediment core 5022-1J from Laguna Potrok Aike, southern Patagonia

Living microorganisms inhabit every environment of the biosphere but only in the last decades their importance governing biochemical cycles in deep sediments has been widely recognized. Most investigations have been accomplished in the marine realm whereas there is a clear paucity of comparable studies in lacustrine sediments. One of the main challenges is to define geomicrobiological proxies that can be used to identify different microbial signals in the sediments. Laguna Potrok Aike, a maar lake located in Southeastern Patagonia, has an annually not stratifying cold water column with temperatures ranging between 4 and 10 °C, and most probably an anoxic water/sediment interface. These unusual features make it a peculiar and interesting site for geomicrobiological studies. Living microbial activity within the sediments was inspected by the first time in a sedimentary core retrieved during an ICDP-sponsored drilling operation. The main goals to study this cold subsaline environment were to characterize the living microbial consortium; to detect early diagenetic signals triggered by active microbes; and to investigate plausible links between climate and microbial populations. Results from a meter long gravity core suggest that microbial activity in lacustrine sediments can be sustained deeper than previously thought due to their adaptation to both changing temperature and oxygen availability. A multi-proxy study of the same core allowed defining past water column conditions and further microbial reworking of the organic fraction within the sediments. Methane content shows a gradual increase with depth as a result of the fermentation of methylated substrates, first methanogenic pathway to take place in the shallow subsurface of freshwater and subsaline environments. Statistical analyses of DGGE microbial diversity profiles indicate four clusters for Bacteria reflecting layered communities linked to the oxidant type whereas three clusters characterize Archaea communities that can be linked to both denitrifiers and methanogens. Independent sedimentary and biological proxies suggest that organic matter production and/or preservation have been lower during the Medieval Climate Anomaly (MCA) coinciding with a low microbial colonization of the sediments. Conversely, a reversed trend with higher organic matter content and substantial microbial activity characterizes the sediments deposited during the Little Ice Age (LIA). Thus, the initial sediments deposited during distinctive time intervals under contrasting environmental conditions have to be taken into account to understand their impact on the development of microbial communities throughout the sediments and their further imprint on early diagenetic signals.

Radiocarbon ages, grain sizes, paleomagentic properties and XRF data of Lake Nam Co sediments, Tibetan Plateau

In September 2008 several cores (68 cm-115 cm length) (water depth: 93 m) were retrieved from Lake Nam Co (southern-central Tibetan Plateau; 4718 m a.s.l.). This study focuses on the interpretation of high-resolution (partly 0.2 cm) data from three gravity cores and the upper part of a 10.4 m long piston core, i.e., the past 4000 cal BP in terms of lake level changes, hydrological variations in the catchment area and consequently variations in monsoon strength. A wide spectrum of sedimentological, geochemical and mineralogical investigations was carried out. Results are presented for XRF core-scans, grain size distribution, XRD-measurements and SEM-image analyses. These data are complemented by an age-depth model using 210Pb and 137Cs analyses as well as eleven AMS-14C-ages. This model is supported by excellent agreement between secular variations determined on one of the gravity cores to geomagnetic field models. This is a significant improvement of the chronology as most catchments of lacustrine systems on the Tibetan Plateau contain carbonates resulting in an unknown reservoir effect for radiocarbon dates. The good correlation of our record to the geomagnetic field models confirms our age-depth model and indicates only insignificant changes in the reservoir effect throughout the last 4 ka. High (summer-) monsoonal activity, i.e. moist environmental conditions, was detected in our record between approximately 4000 and 1950 cal BP as well as between 1480 and 1200 cal BP. Accordingly, lower monsoon activity prevails in periods between the two intervals and thereafter. This pattern shows a good correlation to the variability of the Indian Ocean Summer Monsoon (IOSM) as recorded in a peat bog ~1000 km in NE direction from Lake Nam Co. This is the first time that such a supra regional homogenous monsoon activity is shown on the Tibetan Plateau and beyond. Finally our data show a significant lake level rise after the Little Ice Age (LIA) in Lake Nam Co which is suggested to be linked to glacier melting in consequence of rising temperatures occurring on the whole Tibetan Plateau during this time.

Multi-proxy records of Two-Yurts Lake

Within the scope of Russian-German palaeoenvironmental research, Two-Yurts Lake (TYL, Dvuh-Yurtochnoe in Russian) was chosen as the main scientific target area to decipher Holocene climate variability on Kamchatka. The 5x2 km large and 26 m deep lake is of proglacial origin and situated on the eastern flank of Sredinny Ridge at the northwestern end of the Central Kamchatka Valley, outside the direct influence of active volcanism. Here, we present results of a multi-proxy study on sediment cores, spanning about the last 7000 years. The general tenor of the TYL record is an increase in continentality and winter snow cover in conjunction with a decrease in temperature, humidity, and biological productivity after 5000-4500 cal yrs BP, inferred from pollen and diatom data and the isotopic composition of organic carbon. The TYL proxy data also show that the late Holocene was punctuated by two colder spells, roughly between 4500 and 3500 cal yrs BP and between 1000 and 200 cal yrs BP, as local expressions of the Neoglacial and Little Ice Age, respectively. These environmental changes can be regarded as direct and indirect responses to climate change, as also demonstrated by other records in the regional terrestrial and marine realm. Long-term climate deterioration was driven by decreasing insolation, while the short-term climate excursions are best explained by local climatic processes. The latter affect the configuration of atmospheric pressure systems that control the sources as well as the temperature and moisture of air masses reaching Kamchatka.

Sedimentology, geochemistry and ostracodes from Lake Ohrid

Ancient Lake Ohrid, located in the southern Balkan Peninsula in Macedonia and Albania is characterized by a high degree of endemism and it is considered to be the oldest lake in Europe. But its exact age (between one and ten million years) and also its origin are so far not known. To unravel these uncertainties an ICDP (International Continental Scientific Drilling Program) drilling project (Scientific Collaboration On Past Speciation Conditions in Ohrid (SCOPSCO)), started in April 2013. In addition to the investigations about the age and origin, other paleolimnological studies, e.g., the reconstruction of past climate and of past lake level changes, should be performed with the drilled cores. Used proxies in such paleolimnological studies are, e.g., ostracodes because they respond sensitively to environmental changes but an accurate knowledge of their preferences and tolerances to specific environmental conditions is necessary for this purpose. So far, this knowledge about the, mostly endemic, Ohrid ostracodes was limited. Thus, within the framework of this thesis, ostracodes and a multiplicity of environmental data were collected in Lake Ohrid and its adjacent waters during four field campaigns. In a total of 47 ostracode species could be detected in the entire study area and 32 of them were found alive in Lake Ohrid. Multivariate statistic identified that water depth, salinity, conductivity, pH, and dissolved oxygen were the main determining factors for ostracode distribution in the entire study area. In Lake Ohrid, the distribution was mainly controlled by water depth, water temperature, and pH. Some ostracodes were identified as strong indicator species for important environmental variables, e.g., water temperature and water depth. A distinctive feature of Lake Ohrid was the finding of the ostracode genus Amnicythere whose species normally inhabit oligo-(meso-)haline waters and this could point to a marine origin of the lake. So far, the specialized endemic ostracodes show the highest abundances and the greatest spatial distribution in Lake Ohrid but during the sampling eight widespread species were found for the first time in the lake. They inhabited mainly the northern part of the lake, where two cities are located and industry and agriculture play a major role, and they were limited to water depths above 50 m and this could be an evidence for an increasing anthropogenic pressure because widespread ostracode species often replace endemic species. To unravel the human impact on Lake Ohrid during the last decades short sediment cores were taken and the multi-proxy study indicated that the lake productivity between the early 1920s and the late 1980s was relatively low. Diatom assemblages indicate a rising productivity in the southern part of Lake Ohrid since the mid 1970s and geochemical proxies and ostracodes point to an increasing productivity since the late 1980s in the southern and in the northern part. A slight increase in the productivity continued until 2009. Noticeable is the fact that since the early 1990s, the increasing productivity and the increasing concentrations of heavy metals correspond to a decreasing number of ostracodes in the northern part of Lake Ohrid. Perhaps, this indicates that living conditions in this lake part became less favorable for the mostly endemic ostracode species. Furthermore, the sediment samples from the cores show relatively high concentrations of arsenic, iron, and nickel. Fluctuations in ostracode assemblages from three longer sediment cores, the longest spans approximately 136 ka, taken in Lake Ohrid, correspond to fluctuations in the productivity, in the carbonate content, of the lake level, and of climate changes. Between the marine isotope stage (MIS) 6 and MIS 2 the number of ostracode valves is very low or the valves were completely absent. This corresponds to a low lake productivity, a low carbonate content, and a low lake level. At the onset of the Holocene, the number of valves increased markedly and this correlates with an increased productivity and carbonate content and a warmer climate. But during the Little Ice Age (LIA), the number of valves dropped again and species which prefer warmer waters disappeared completely. This drop corresponds also to a low productivity. After the LIA, the number of species increased again but since 1895 AD a strong and abrupt decrease is visible. A reason for this could be an increase in the heavy metal concentrations.

(Table S1) TEX86 and BIT record from sediment core PBBC-1

A molecular organic geochemical proxy (TEX86) for sea surface temperature (SST) is compared with a foraminifera-based SST proxy (Mg/Ca) in a decadal-resolution marine sedimentary record spanning the last 1000 years from the Gulf of Mexico. We assess the relative strengths of the organic and inorganic paleoceanographic techniques for reconstructing high-resolution SST variability during recent climate events, including the Little Ice Age (LIA) and the Medieval Warm Period (MWP). SST estimates based on the molecular organic proxy TEX86 show a similar magnitude and pattern of SST variability to foraminiferal Mg/Ca-SST estimates but with some important differences. For instance, both proxies show a cooling (1°C-2°C) of Gulf of Mexico SSTs during the LIA. During the MWP, however, Mg/Ca-SSTs are similar to near-modern SSTs, while TEX86 indicates SSTs that were cooler than modern. Using the respective SST calibrations for each proxy results in TEX86-SST estimates that are 2°C-4°C warmer than Mg/Ca-SST throughout the 1000 year record. We interpret the TEX86-SST as a summer-weighted SST signal from the upper mixed layer, whereas the Mg/Ca-SST better reflects the mean annual SST. Downcore differences in the SST estimates between the two proxies (DeltaT = TEX86 - Mg/Ca) are interpreted in the context of varying seasonality and/or changing water column temperature gradients.

(Table 1) Mean annual ground temperatures and description of permafrost boreholes in Russia

The results of the International Permafrost Association's International Polar Year Thermal State of Permafrost (TSP) project are presented based on field measurements from Russia during the IPY years (2007-09) and collected historical data. Most ground temperatures measured in existing and new boreholes show a substantial warming during the last 20 to 30 years. The magnitude of the warming varied with location, but was typically from 0.5°C to 2°C at the depth of zero annual amplitude. Thawing of Little Ice Age permafrost is ongoing at many locations. There are some indications that the late Holocene permafrost has begun to thaw at some undisturbed locations in northeastern Europe and northwest Siberia. Thawing of permafrost is most noticeable within the discontinuous permafrost domain. However, permafrost in Russia is also starting to thaw at some limited locations in the continuous permafrost zone. As a result, a northward displacement of the boundary between continuous and discontinuous permafrost zones was observed. This data set will serve as a baseline against which to measure changes of near-surface permafrost temperatures and permafrost boundaries, to validate climate model scenarios, and for temperature reanalysis.

Biogeochemical, sedimentological and isotopic sediment record of Lake Dojran

A Late Glacial to Holocene sediment sequence (Co1260, 717 cm) from Lake Dojran, located at the boarder of the F.Y.R. of Macedonia and Greece, has been investigated to provide information on climate variability in the Balkan region. A robust age-model was established from 13 radiocarbon ages, and indicates that the base of the sequence was deposited at ca. 12 500 cal yr BP, when the lake-level was low. Variations in sedimentological (H2O, TOC, CaCO3, TS, TOC/TN, TOC/TS, grain-size, XRF, d18Ocarb, d13Ccarb, d13Corg) data were linked to hydro-acoustic data and indicate that warmer and more humid climate conditions characterised the remaining period of the Younger Dryas until the beginning of the Holocene. The Holocene exhibits significant environmental variations, including the 8.2 and 4.2 ka cooling events, the Medieval Warm Period and the Little Ice Age. Human induced erosion processes in the catchment of Lake Dojran intensified after 2800 cal yr BP.

X-ray fluorescent (XRF) ratios and diatom groups of sediment core GeoB13813-4

The continental shelf adjacent to the Río de la Plata (RdlP) exhibits extremely complex hydrographic and ecological characteristics which are of great socioeconomic importance. Since the long-term environmental variations related to the atmospheric (wind fields), hydrologic (freshwater plume), and oceanographic (currents and fronts) regimes are little known, the aim of this study is to reconstruct the changes in the terrigenous input into the inner continental shelf during the late Holocene period (associated with the RdlP sediment discharge) and to unravel the climatic forcing mechanisms behind them. To achieve this, we retrieved a 10 m long sediment core from the RdlP mud depocenter at 57 m water depth (GeoB 13813-4). The radiocarbon age control indicated an extremely high sedimentation rate of 0.8 cm per year, encompassing the past 1200 years (AD 750-2000). We used element ratios (Ti / Ca, Fe / Ca, Ti / Al, Fe / K) as regional proxies for the fluvial input signal and the variations in relative abundance of salinity-indicative diatom groups (freshwater versus marine-brackish) to assess the variability in terrigenous freshwater and sediment discharges. Ti / Ca, Fe / Ca, Ti / Al, Fe / K and the freshwater diatom group showed the lowest values between AD 850 and 1300, while the highest values occurred between AD 1300 and 1850. The variations in the sedimentary record can be attributed to the Medieval Climatic Anomaly (MCA) and the Little Ice Age (LIA), both of which had a significant impact on rainfall and wind patterns over the region. During the MCA, a weakening of the South American summer monsoon system (SAMS) and the South Atlantic Convergence Zone (SACZ), could explain the lowest element ratios (indicative of a lower terrigenous input) and a marine-dominated diatom record, both indicative of a reduced RdlP freshwater plume. In contrast, during the LIA, a strengthening of SAMS and SACZ may have led to an expansion of the RdlP river plume to the far north, as indicated by higher element ratios and a marked freshwater diatom signal. Furthermore, a possible multidecadal oscillation probably associated with Atlantic Multidecadal Oscillation (AMO) since AD 1300 reflects the variability in both the SAMS and SACZ systems.

Decadal-to-centennial scale variations during the Holocene reconstructed from sediment core D13902

Sea surface temperature (SST), marine productivity, and fluvial input have been reconstructed for the last 11.5 calendar (cal) ka B.P. using a high-resolution study of C37 alkenones, coccolithophores, iron content, and higher plant n-alkanes and n-alkan-1-ols in sedimentary sequences from the inner shelf off the Tagus River Estuary in the Portuguese Margin. The SST record is marked by a continuous decrease from 19C, at 10.5 and 7 ka, to 15C at present. This trend is interrupted by a fall from 18C during the Roman and Medieval Warm Periods to 16C in the Little Ice Age. River input was very low in the early Holocene but increased in the last 3 cal ka B.P. in association with an intensification of agriculture and deforestation and possibly the onset of the North Atlantic Oscillation/Atlantic Multidecadal Oscillation modes of variability. River influence must have reinforced the marine cooling trend relative to the lower amplitude in similar latitude sites of the eastern Atlantic. The total concentration of alkenones reflects river-induced productivity, being low in the early Holocene but increasing as river input became more important. Rapid cooling, of 1-2C occurring in 250 years, is observed at 11.1, 10.6, 8.2, 6.9, and 5.4 cal ka B.P. The estimated age of these events matches the ages of equivalent episodes common in the NE Atlantic- Mediterranean region. This synchronicity reveals a common widespread climate feature, which considering the twentieth century analog between colder SSTs and negative North Atlantic Oscillation (NAO), is likely to reflect periods of strong negative NAO.

Sea surface temperature and primary productivity reconstruction of sediment core SO130-275KL from the northeastern Arabian Sea during the late Holocene

Variability in the oceanic environment of the Arabian Sea region is strongly influenced by the seasonal monsoon cycle of alternating wind directions. Prominent and well studied is the summer monsoon, but much less is known about late Holocene changes in winter monsoon strength with winds from the northeast that drive convective mixing and high surface ocean productivity in the northeastern Arabian Sea. To establish a high-resolution record of winter monsoon variability for the late Holocene, we analyzed alkenone-derived sea surface temperature (SST) variations and proxies of primary productivity (organic carbon and d15N) in a well-laminated sediment core from the Pakistan continental margin. Weak winter monsoon intensities off Pakistan are indicated from 400 B.C. to 250 A.D. by reduced productivity and relatively high SST. At about 250 A.D., the intensity of the winter monsoon increased off Pakistan as indicated by a trend to lower SST. We infer that monsoon conditions were relatively unstable from ~500 to 1300 A.D., because primary production and SST were highly variable. Declining SST and elevated biological production from 1400 to 1900 A.D. suggest invigorated convective winter mixing by strengthening winter monsoon circulation, most likely a regional expression of colder climate conditions during the Little Ice Age on the Northern Hemisphere. The comparison of winter monsoon intensity with records of summer monsoon intensity suggests that an inverse relationship between summer and winter monsoon strength exists in the Asian monsoon system during the late Holocene, effected by shifts in the Intertropical Convergence Zone.