Palaeoclimatic Studies Research Articles

Integrated stratigraphy of a continental Pliensbachian–Toarcian Boundary (Lower Jurassic) section at Taskomirsay, Leontiev Graben, southwest Kazakhstan

Abstract During the Early Jurassic, major palaeoclimatic changes, associated with large carbon-cycle perturbations, occurred at the Pliensbachian–Toarcian boundary. Although the detailed marine palaeoclimatic record of this time interval and its impact on the marine biota is well recorded, much less is known about the continental realm. The current study documents new palynological and high-resolution carbon isotope data measured on bulk organic matter from the continental Lower Jurassic section of Taskomirsay in Kazakhstan, Central Asia. Both datasets allow a transition zone between the Pliensbachian and Toarcian to be identified. In addition, the spore and pollen distribution suggests a warming trend from the Pliensbachian to the Toarcian, most probably associated with a shift in floristic associations from the Siberian to the Sino-European palaeofloristic provinces, as recorded elsewhere in Central Asia during the Early Jurassic. The Taskomirsay section is thus of primary interest for palaeoclimatic studies as it is one of the very few well-dated continental section worldwide that records the Pliensbachian–Toarcian palaeoclimatic changes. Supplementary material: Table of δ 13 C org data, Taskomirsay section, Karatau Graben, Kazakhstan is available at https://doi.org/10.6084/m9.figshare.c.3495645

A sediment-based reconstruction of Caribbean effective precipitation during the ‘Little Ice Age’ from Freshwater Pond, Barbuda

Contemporary climate dynamics of the circum-Caribbean region are characterised by significant precipitation variability on interannual and interdecadal timescales controlled primarily by El Niño Southern Oscillation (ENSO) and the Atlantic Multidecadal Oscillation (AMO). However, our understanding of pre-industrial climate variability in the region is hampered by the sparse geographic distribution of palaeoclimate archives. Here, we present a high-resolution reconstruction of effective precipitation for Barbuda since the mid-16th century, based on biostratigraphic and stable isotope analyses of fossil ostracods and gastropods recovered from lake sediment cores from Freshwater Pond, the only freshwater lake on the island. We interpret episodic fluctuations in shell accumulation in the sediment record to represent changes in the balance between precipitation and evaporation during the ‘Little Ice Age’ (LIA; ~1400–1850 CE) and Industrial (1850–present) periods. Comparisons between indices of reconstructed ENSO and AMO variability, the abundance of the freshwater gastropod Pyrgophorus parvulus and the δ18O records from ostracod calcite suggest that the relative influence of ENSO and AMO on long-term rainfall patterns in Barbuda has changed over the last 400 years. Our findings are in agreement with other high-resolution palaeoclimate studies that suggest that long-term changes in effective precipitation during the LIA were much more variable, temporally and spatially, than previously suggested.

Foraminifera eco-biostratigraphy of the southern Evoikos outer shelf, central Aegean Sea, during MIS 5 to present

The South Evoikos Basin is a marginal basin in the Aegean Sea which receives little terrigenous supply and its sedimentation is dominated by hemipelagic processes. Late Quaternary benthic and planktonic foraminifera from core PAG-155 are investigated in order to understand their response to the glacial-interglacial cycles in this region. The quantitative analysis of planktonic foraminifera, coupled with accelerator mass spectrometry (14C-AMS) radiocarbon date measurements, provide an integrated chrono-stratigraphic time framework over the last 90ka (time interval between late Marine Isotopic Stages 5 and 1; MIS5-MIS1). The temporary appearance and disappearance as well as several abundance peaks in the quantitative distribution of selected climate-sensitive planktonic species allowed the identification of several eco-bioevents, useful to accurately mark the boundaries of the eco-biozones widely recognized in the Mediterranean records and used for large-scale correlations. The established bio-ecozonation scheme allows a detailed palaecological reconstruction for the late Pleistocene archive in the central Aegean, and furthermore provides a notable contribution for palaeoclimatic studies, facilitating intercorrelations between various oceanographic basins. The quantitative analyses of benthic foraminifera identify four distinct assemblages, namely Biofacies: Elphidium spp., Haynesina spp. Biofacies, characterized by neritic species, dominated during the transition from MIS 5 to MIS 4; Cassidulina laevigata/carinata Biofacies dominated till 42ka (transgressive trend from MIS 4 to MIS 3); Bulimina gibba Biofacies dominated from 42ka to 9.5ka (extensive regression MIS 3,2 through lowstand and early transgression; beginning of MIS 1); Bulimina marginata, Uvigerina spp. Biofacies dominated from 9.5ka to the present (late transgression through early highstand; MIS 1)., This study showed that the South Evoikos Basin which is characterized by its critical depths and connections to the open sea, and its small volume water masses that nourished foraminiferal assemblages, accurately records 5th−4th order sea level and climatic fluctuations. Especially, the basin's limited communication with the open ocean implies that any climatic signals will be recorded in an amplified fashion, and therefore this heightened sensitivity to the effects of climate variability further underlies the prominent role of such marginal basins in the understanding of the global climatic evolution.

GlaRe, a GIS tool to reconstruct the 3D surface of palaeoglaciers

Glacier reconstructions are widely used in palaeoclimatic studies and this paper presents a new semi-automated method for generating glacier reconstructions: GlaRe, is a toolbox coded in Python and operating in ArcGIS. This toolbox provides tools to generate the ice thickness from the bed topography along a palaeoglacier flowline applying the standard flow law for ice, and generates the 3D surface of the palaeoglacier using multiple interpolation methods. The toolbox performance has been evaluated using two extant glaciers, an icefield and a cirque/valley glacier from which the subglacial topography is known, using the basic reconstruction routine in GlaRe. Results in terms of ice surface, ice extent and equilibrium line altitude show excellent agreement that confirms the robustness of this procedure in the reconstruction of palaeoglaciers from glacial landforms such as frontal moraines.

Palaeoclimatic studies of Lake Chad Basin and its implication on lake level fluctuation using isotope geochemistry

Palaeoclimatic studies of Lake Chad Basin and its implication on lake level fluctuation using isotope geochemistry

Spatial heterogeneity of sources of branched tetraethers in shelf systems: The geochemistry of tetraethers in the Berau River delta (Kalimantan, Indonesia)

The bulk organic matter composition (total organic carbon (TOC) content and δ13CTOC) and composition of isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGT) in surface sediments from 43 stations in the Berau River delta (east Kalimantan, Indonesia), including two coast-shelf transects and stations within the river mouth, were examined to reveal the spatial heterogeneity in these parameters in order to assess the impact of a tropical river loaded with suspended matter on the sedimentary organic matter in the shelf system. The high-resolution study showed that, despite the extensive transport of eroded soil material by the river to the sea, terrestrial organic matter and brGDGTs are only deposited on a relatively small part of the shelf. The concentrations of brGDGTs are highest (up to 120μgg−1 TOC) in sediments deposited in and close to the mouth of the Berau River and their distribution indicates that they represent a mixture of soil-derived and river in-situ produced brGDGTs. Crenarchaeol concentrations reach 700μgg−1 TOC in sediments deposited on the outer shelf due to Thaumarchaeotal production in shelf waters. This results in a strong gradient (0.93–0.03) in the BIT index, with high values in the river mouth and low values on the shelf. The decline in the BIT index is caused by both decreasing concentrations of the brGDGTs and increasing concentrations of crenarchaeol. The BIT index shows a highly significant but non-linear relationship with δ13CTOC. On the shelf, in the area not under the direct influence of the Berau River, cyclic brGDGTs become relatively dominant, most probably due to in-situ production in the alkaline pore waters of the surface sediments. The spatial heterogeneity of sources of brGDGTs on the Berau shelf complicates the use of brGDGTs as temperature proxies. Application of the global soil calibration to sedimentary mixtures of brGDGTs in the river-influenced area of the shelf results in a severe underestimation of mean annual air temperature (MAT) by 6°C. This is due to the mixed origin of the brGDGTs, which are not only derived from soil erosion but, likely, also from riverine production, as has been observed for other river systems.Comparison of the Berau shelf other shelf systems indicates that in-situ production of brGDGTs in shelf sediments is a widespread phenomenon that is especially pronounced at water depths of ca. 50–300m. It is hypothesized that this is so because benthic in-situ production of heterotrophic brGDGT-producing bacteria is fueled by the higher delivery of fresh organic matter to these sediments as the consequence of higher primary productivity in shelf waters and a decreased mineralization due to the relatively short settling times of particles on the shelf. For palaeoclimatic studies of marine shelf sediments the application of brGDGTs as proxies is severely complicated by the heterogeneity of sources of brGDGTs. Comparison of the brGDGT composition of soils with those of shelf sediments may assist in deciding if sedimentary brGDGTs are predominantly derived from soil erosion. Several methods to do so are discussed.

Grain-size distribution of Pleistocene loess deposits in northern Iran and its palaeoclimatic implications

Abstract The loess deposits in northern Iran are located in a key region connected to the European and central Asian loess belts. However, the lack of previous detailed sedimentological and palaeoclimatic studies of the Pleistocene loess in the region limits our understanding of the nature of ancient aeolian processes and loess history in the mid-latitudes of Euro-Asia as a whole. Here, we present the results of grain-size analyses of the Pleistocene loess from the so-called Iranian Loess Plateau (ILP) in northern Iran. Our results reveal that the grain-size distribution of the deposits is characterized by trimodal and bimodal distributions, comprising a dominant well-sorted coarse dust component (ca. 7–75 μm), a small poorly-sorted fine dust component (ca. 2–7 μm), and a minor pedogenic clay component (

87Sr/86Sr as a quantitative geochemical proxy for 14C reservoir age in dynamic, brackish waters: Assessing applicability and quantifying uncertainties

AbstractAccurate geochronologies are crucial for reconstructing the sensitivity of brackish and estuarine environments to dynamic external impacts of the past. Radiocarbon (14C) dating is commonly used for palaeoclimate studies, but its application in brackish environments is severely limited by an inability to quantify spatiotemporal variations in 14C reservoir age, or R(t), due to dynamic interplay between river runoff and marine water. Additionally, old carbon effects and species‐specific behavioral processes also influence 14C ages. Using the world's largest brackish water body (the estuarine Baltic Sea) as a test bed, combined with a comprehensive approach that objectively excludes both old carbon (using GIS) and species‐specific 14C effects, we demonstrate the use of 87Sr/86Sr ratios for quantifying R(t) in ubiquitous mollusc shell material, leading to almost an order of magnitude increase in Baltic Sea 14C geochronological precision over the current state of the art. We propose that similar proxy methods can be developed for other brackish water bodies worldwide.

Geology and archaeology: submerged landscapes of the continental shelf: an introduction

Sea-level change has influenced human populations globally since prehistoric times. Even in early phases of cultural development, human populations were faced with marine regression and transgression associated with the glacial–interglacial climatic cycle, amplified by glacio-isostatic adjustments in some regions. Global marine regression during the last glaciation changed the palaeogeography of the continental shelf, converting former marine environments to attractive terrestrial habitats for prehistoric human occupation, and adding an extensive new increment of land, in the case of Europe amounting to an additional 40% of the existing land area, and on a global scale to some 20 million km of additional territory that is now submerged. These areas of the shelf were used as hunting and gathering areas, and as pathways of dispersal between regions and between continents, until they were resubmerged by the post-glacial marine transgression. They also most probably witnessed the earliest developments in seafaring, marine exploitation and permanent settlements. Based on modern marine research technologies and the integration of large databases, proxy data are now becoming available for the reconstruction of these submerged Quaternary landscapes. Concerted efforts are also now being devoted to the search for prehistoric archaeological sites and artefacts on the seabed, often in collaboration with marine scientists. This search has been stimulated by the increasing amount of material that has demonstrably survived inundation, often with excellent preservation of organic remains, by closer collaboration with offshore industry, and by the growing realization of the importance of these submerged data for understanding human prehistoric developments during periods of rapidly changing climate and environment. Moreover, these new research trends are not simply being driven by an archaeological need for scientific and technological input from other disciplines, but by collaborations involving genuine mutual benefit, in which all partners have something to gain. Archaeological problems often pose new questions about geological change, stimulating new techniques of observation, new technologies and new investigations, which in their turn can offer new data, often at higher resolution and with better dates, in relation to geological and environmental issues such as sea-level change and palaeoclimatic variability on the continental shelf. The expansion of early human populations to occupy new territory and new continents is one of the great narratives of human evolution. It is currently a theme of wide interest and topicality, and has received significant impetus and publicity from the new science of palaeogenetics. Together with ongoing discoveries of new fossil and archaeological material, and new studies of palaeoclimate and palaeoenvironment, the field itself is rapidly expanding, leading to new discoveries and new controversies (Grine et al. 2009; Gamble 2013; Smith & Ahern 2013). All the current indications are that the human species originated in Africa, with at least two major episodes of dispersal. The earliest, some time after about 2 myr ago, led to the expansion of Homo ergaster/erectus populations across southern Europe and Asia, extending from Britain in the west to China and Indonesia in the east and south. The second involved our own species, Homo sapiens (Anatomically Modern Humans or AMH), and took place some time after about 200 ka. This resulted in the replacement of earlier hominin populations in Europe and Asia, and expansion further afield: into New Guinea and Australia, certainly involving sea crossings over distances of at least 50 km, by about 50 ka; to the higher latitudes of northern Eurasia, with entry into the Americas by at least 15 ka; and into new territory exposed by the melting of the northern hemisphere ice sheets less than 10 kyr ago. Anthropological and archaeological investigations of these processes are directly connected

Late Pleistocene glacial–interglacial shell-size–isotope variability in planktonic foraminifera as a function of local hydrography

Abstract. So-called "vital effects" are a collective term for a suite of physiologically and metabolically induced variability in oxygen (δ18O) and carbon (δ13C) isotope ratios of planktonic foraminifer shells that hamper precise quantitative reconstruction of past ocean parameters. Correction for potential isotopic offsets from equilibrium or the expected value is paramount, as too is the ability to define a comparable life stage for each species that allows for direct comparison. Past research has focused upon finding a specific size range for individual species in lieu of other identifiable features, thus allowing ocean parameters from a particular constant (i.e. a specific depth or season) to be reconstructed. Single-shell isotope analysis of fossil shells from a mid-latitude North Atlantic Ocean piston core covering Termination III (200 to 250 ka) highlight the advantage of using a dynamic size range, i.e. utilising measurements from multiple narrow sieve size fractions spanning a large range of total body sizes, in studies of palaeoclimate. Using this methodology, we show that isotopic offsets between specimens in successive size fractions of Globorotalia inflata and Globorotalia truncatulinoides are not constant over time, contrary to previous findings. For δ18O in smaller-sized globorotalids (212–250 μm) it is suggested that the offset from other size fractions may reflect a shallower habitat in an early ontogenetic stage. A reduction in the difference between small and large specimens of G. inflata between insolation minima and maxima is interpreted to relate to a prolonged period of reduced water column stratification. For the shallow-dwelling species Globigerina bulloides, no size–isotope difference between size fractions is observed, and the variability in the oxygen isotopic values is shown to correlate well with the seasonal insolation patterns. As such, patterns in oxygen isotope variability of fossil populations may be used to reconstruct past seasonality changes.

Stable nitrogen isotope analysis of the Upper Devonian palynomorph, Tasmanites

Abstract Bulk samples are commonly used in stable (N) isotope studies to infer climate changes. However, due to uncertainties in the composition of bulk samples, the inconsistency of diagenetic alteration and the variation of dominant oceanic features, such as N-fixation and denitrification, the isolation of specific sedimentary N fractions has become a crucial step to understanding the nitrogen cycle. This paper describes the extraction techniques and the stable nitrogen isotope analysis (on a nano-EA-IRMS) of the palynomorph, Tasmanites spp. (80–200 μm) found in Upper Devonian (Famennian) black shales from Kentucky, USA. Tasmanites is a widely occurring and long-ranging prasinophyte (unicellular green algae) genus. Palynomorphs, such as Tasmanites, are highly resistant to most forms of decay. It is therefore suggested that the δ15N signature of this palynomorph may be a useful, unaltered indicator of baseline isotopic change in future palaeoclimate studies. The greatly reduced sample size accepted by the nano-EA-IRMS makes this technique a viable complement to bulk sediment analyses. The results, using the nano-EA-IRMS, demonstrate that the δ15N signature of the bulk organic samples differ significantly from that of the isolated Tasmanites spp. Three distinct isotopic relationships were observed from the three different sample localities. These include parallel changes in the δ15N values of bulk samples and extracted Tasmanites spp., variation in the δ15N of bulk samples, with no corresponding change recorded in Tasmanites spp., and opposing δ15N changes recorded between the bulk samples and the extracted Tasmanites spp. This inconsistent isotopic variation further emphasises the complexity of the nitrogen cycle. Possible explanations for the observed variation include changes in the rate of N-fixation and denitrification due to climate change, incomplete nitrification, and variation in the bulk organic content.

Depositional system and palaeoclimatic interpretations of Middle to Late Pleistocene travertines: Kocabaş, Denizli, south‐west Turkey

AbstractTravertine deposits in western Turkey are very well‐exposed in the area of Kocabaş, in the eastern part of the Denizli Basin. The palaeoclimatic significance of these travertines is discussed using U/Th dates, stable isotope data and palynological evidence. The Kocabaş travertine occurrences are characterized by successions of depositional terraces associated with palaeosols and karstic features. The travertines have been classified into eight lithotypes and one erosional horizon, namely: laminated, coated bubble, reed, paper‐thin raft, intraclasts, micritic travertine with gastropods, extra‐formational pebbles and a palaeosol layer. The analysed travertines mostly formed between 181 ka and 80 ka (Middle to Late Pleistocene) during a series of climatic changes including glacial and interglacial intervals; their δ13C and δ18O values indicate that the depositional waters were mainly of basinal thermal origin, occasionally mixed with surficial meteoric water. Palynological results obtained from the palaeosols showed an abundance of non‐arboreal percentage and xerophytic plants (Oleaceae and Quercus evergreen type) indicating that a drought occurred. Marine Isotope Stage 6 is represented by grassland species but Marine Isotope Stage 5 is represented by Pinaceae–Pinus and Abies, Quercus and Oleaceae. Uranium/thorium analyses of the Kocabaş travertines show that deposition began in Marine Isotope Stage 6 (glacial) and continued to Marine Isotope Stage 4 (glacial), but mostly occurred in Marine Isotope Stage 5 (interglacial). The travertine deposition continued to ca 80 ka in the south‐west of the study area, in one particular depression depositional system. Palaeoenvironmental indicators suggest that the travertine depositional evolution was probably controlled by fault‐related movements that influenced groundwater flow. Good correlation of the stable isotope values and dates of deposition of the travertines and palynological data of palaeosols in the Kocabaş travertines serve as a starting point for further palaeoclimate studies in south‐west Turkey. Additionally, the study can be compared with other regional palaeoclimate archives.

Exponentially decreasing tooth growth rate in horse teeth: implications for isotopic analyses

A range of archaeological and palaeoclimatic studies use isotopic analyses of ungulate hypsodont tooth enamel. Such studies commonly assume a constant growth rate, though this has not been fully tested. Here, we use stable isotope analyses of sequential enamel samples to study horse tooth growth. We fit the data using models corresponding to constant and exponentially decreasing rates of growth, and compare the results to metrical data showing the geometry and timing of apposition. The results indicate enamel apposition and maturation advances at an exponentially decreasing rate. An understanding of this variable growth rate is crucial for interpreting isotopic data from equid teeth.

Dependence of the cyclization of branched tetraethers on soil moisture in alkaline soils from arid–subhumid China: implications for palaeorainfall reconstructions on the Chinese Loess Plateau

Abstract. The use of branched glycerol dialkyl glycerol tetraethers (bGDGTs) in loess–palaeosol sequences (LPSs) has shown promises in continental palaeotemperature reconstructions. Thus far, however, little is known about the effect of soil moisture on their distributions in the water-limited Chinese Loess Plateau (CLP). In this study, the relationships between environmental variables and the cyclization of branched tetraethers (CBT) were investigated in arid–subhumid China using 97 surface soils in the CLP and its vicinity, as well as 78 soils with pH > 7 which have been previously published. We find that CBT correlates best with soil water content (SWC) or mean annual precipitation (MAP) for the overall data set. This indicates that CBT is mainly controlled by soil moisture instead of soil pH in alkaline soils from arid–subhumid regions, where water availability is a limiting factor for the producers of bGDGTs. Therefore, we suggest that CBT can potentially be used as a palaeorainfall proxy on the alkaline CLP. According to the preliminary CBT–MAP relationship for modern CLP soils (CBT = −0.0021 × MAP + 1.7, n = 37, r = −0.93), palaeorainfall history was reconstructed from three LPSs (Yuanbao, Lantian, and Mangshan) with published bGDGT data spanning the past 70 ka. The CBT-derived MAP records of the three sites consistently show precession-driven variation resembling the monsoon record based on speleothem δ18O, supporting CBT as a reasonable proxy for palaeorainfall reconstruction in LPS. The direct application of CBT as a palaeorainfall proxy in corroboration with the bGDGT-based temperature proxy may enable us to further assess the temperature/hydrological association for palaeoclimate studies on the CLP.

Northern Russian chironomid-based modern summer temperature data set and inference models

West and East Siberian data sets and 55 new sites were merged based on the high taxonomic similarity, and the strong relationship between mean July air temperature and the distribution of chironomid taxa in both data sets compared with other environmental parameters. Multivariate statistical analysis of chironomid and environmental data from the combined data set consisting of 268 lakes, located in northern Russia, suggests that mean July air temperature explains the greatest amount of variance in chironomid distribution compared with other measured variables (latitude, longitude, altitude, water depth, lake surface area, pH, conductivity, mean January air temperature, mean July air temperature, and continentality). We established two robust inference models to reconstruct mean summer air temperatures from subfossil chironomids based on ecological and geographical approaches. The North Russian 2-component WA-PLS model (RMSEPJack=1.35°C, rJack2=0.87) can be recommended for application in palaeoclimatic studies in northern Russia. Based on distinctive chironomid fauna and climatic regimes of Kamchatka the Far East 2-component WAPLS model (RMSEPJack=1.3°C, rJack2=0.81) has potentially better applicability in Kamchatka.

Cross calibration between XRF and ICP-MS for high spatial resolution analysis of ombrotrophic peat cores for palaeoclimatic studies.

Ombrotrophic peatlands are remarkable repositories of high-quality climatic signals because their only source of nutrients is precipitation. Although several analytical techniques are available for analysing inorganic components in peat samples, they generally provide only low-resolution data sets. Here we present a new analytical approach for producing high-resolution data on main and trace elements from ombrotrophic peat cores. Analyses were carried out on a 7-m-long peat core collected from Danta di Cadore, North-Eastern Italy (46° 34' 16″ N, 12° 29' 58″ E). Ca, Ti, Cr, Fe, Cu, Zn, Ga, Sr, Y, Cd, Ba and Pb were detected at a resolution of 2.5mm with a non-destructive X-ray fluorescence core scanner (XRF-CS). Calibration and quantification of the XRF-CS intensities was obtained using collision reaction cell inductively coupled plasma quadruple mass spectrometry (CRC-ICP-QMS). CRC-ICP-QMS measurements were carried out on discrete samples at a resolution of 1cm, after dissolution of 150-mg aliquots with 9ml HNO3 and 1ml HF at 220°C in a microwave system. We compare qualitative XRF-CS and quantitative CRC-ICP-MS data and, however the several sources of variability of the data, develop a robust statistical approach to determine the R (2) and the coefficient of a simple regression model together with confidence intervals. Perfect positive correlations were estimated for Cd, Cr, Pb, Sr, Ti and Zn; high positive correlations for Ba (0.8954), Y (0.7378), Fe (0.7349) and Cu (0.7028); while moderate positive correlations for Ga (0.5951) and Ca (0.5435). With our results, we demonstrate that XRF scanning techniques can be used, together with other well-established geochemical techniques (such as ICP-MS), to produce high-resolution (up to 2.5mm) quantitative data from ombrotrophic peat bog cores.

Seasonal and interannual variations in the nitrogen cycle in the Arabian Sea

Abstract. The Arabian Sea plays an important role in the marine nitrogen cycle because of its pronounced mid-water oxygen minimum zone (OMZ) in which bio-available nitrate (NO3−) is reduced to dinitrogen gas (N2). As the nitrogen cycle can respond fast to climate-induced changes in productivity and circulation, the Arabian Sea sediments are an important palaeoclimatic archive. In order to understand seasonal and interannual variations in the nitrogen cycle, nutrient data were obtained from the literature published prior to 1993, evaluated, and compared with data measured during five expeditions carried out in the framework of the Joint Global Ocean Flux Study (JGOFS) in the Arabian Sea in 1995 and during a research cruise of RV Meteor in 2007. The data comparison showed that the area characterized by a pronounced secondary nitrite maximum (SNM) was by 63% larger in 1995 than a similarly determined estimate based on pre-JGOFS data. This area, referred to as the core of the denitrifying zone, showed strong seasonal and interannual variations driven by the monsoon. During the SW monsoon, the SNM retreated eastward due to the inflow of oxygen-enriched Indian Ocean Central Water (ICW). During the NE monsoon, the SNM expanded westward because of the reversal of the current regime. On an interannual timescale, a weaker SW monsoon decreased the inflow of ICW from the equatorial Indian Ocean and increased the accumulation of denitrification tracers by extending the residence time of water in the SNM. This is supported by palaeoclimatic studies showing an enhanced preservation of accumulative denitrification tracers in marine sediments in conjunction with a weakening of the SW monsoon during the late Holocene.

ENSO and monsoon variability during the past 1.5 kyr as reflected in sediments from Lake Kalimpaa, Central Sulawesi (Indonesia)

The climate of Sulawesi is driven by the monsoon system as well as the El Niño-Southern Oscillation (ENSO). Until now, mechanisms and long-term variations of these complex interacting climate processes have been poorly understood. This paper uses a sediment record from Lake Kalimpaa to investigate long-term rainfall trends of the past ~1500 years. Granulometric and geochemical data provide indications for an increasingly wetter climate (higher rainfall intensities and/or mean rainfall) on centennial to millennial time scales from approximately ad 560 to the 20th century. Highest rainfall intensities probably occurred at the end of the ‘Little Ice Age’ (LIA). The trend towards wetter conditions during this time could also be detected in other palaeoclimatic studies from the region. A plausible explanation for these observations is the southward displacement of the Intertropical Convergence Zone (ITCZ) associated with changes in monsoon dynamics. However, comparison of the results with other proxy and model data indicates that the long-term rainfall variability in Central Sulawesi is also determined by variations in ENSO. During the 20th century, the climate signal in the Kalimpaa record is superimposed by human impact. Moreover, the data suggest that two periods of disturbance occurred within the lake catchment between about ad 1090–1190 and ad 1450–1620. Comparison with fire frequency derived from macro-charcoal analysis indicates that these events were caused by forest fires which likely took place during periods of drought. Broadly simultaneous drought periods have been detected in records from East Java suggesting a regional drought occurrence affecting at least East Java and Sulawesi.

Holocene vegetation and fire dynamics in central-eastern Brazil: Molecular records from the Pau de Fruta peatland

Tropical climate dynamics play a major role in the global climate system. Precipitation patterns in the Serra do Espinhaço Meridional (Minas Gerais, Brazil) are influenced by both the South Atlantic Convergence Zone (SACZ) and the Inter Tropical Convergence Zone (ITCZ). This makes the area important for the interpretation of climate dynamics. The composition of the Holocene Pau de Fruta peat deposit was analysed using pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS). Molecular records agreed well with δ13C and δ15N stable isotopes, C content and mineral content, and pollen records. Changes in peatland plant species composition and fire occurrence indicated several wet and dry periods for the Pau de Fruta peatland during the Holocene; from the beginning of the Holocene to 7400calyr BP (phase I; dry and cold), from 7400 to 4200calyr BP (phase II; wet and warm), from 4200 to 1000calyr BP (phase III; dry and warm), from 1000 to 400calyrBP (phase IV; wet and warm) and from 400calyr BP to the present (V). The hydrological shifts showed a chronology comparable with that described in other palaeoclimatic studies for central-eastern Brazil, indicating that local conditions agree with regional ones. Analytical Py of the peat deposit provided a multi-proxy record via the identification of specific markers for both peatland plants (vegetation) and charred material (fire), whereas the general chemistry provided information on the degree of decomposition of the organic matter. Molecular chemistry of the peat core sampled at high resolution provided insight into peatland C dynamics, which is important for the interpretation of C sequestration in tropical peatlands. Molecular aspects accurately reflected local environmental change. The general agreement of Py data with pollen and inorganic chemistry supports the use of the molecular approach, because it provides local proxy indicators for peatlands, avoiding a mixture of local and regional signals.

Protracted active surge phase in the high arctic (svalbard): dynamic development and morphological impact

Glacial surges in valbard are protracted and characterized by individual dynamic evolution, in contrast to many other areas, which calls for a subdivision of the classic two‐phased surge cycle. A dominating part of the ice masses seem to have a surge potential and this represents a considerable challenge for palaeoclimatic studies. Glaciological and geological models therefore need to be coupled. The issue is discussed with Fridtjovbreen glacier as an example. This ice mass is one of few glaciers studied throughout a surge cycle. It was active for 12 years (1991–2002) and represents the most protracted surge documented. The maximum advance rate was 4.2 m day−1, its maximum extent was reached after seven years, its run‐out distance was 4 km, and the relocated ice filled 5 km2 of the fjord. Intense subglacial thrusting occurred during various stages, including part of the ice‐front retreat, as shown by sub‐bottom profiling data from 2002. A six‐stage model is presented and processes are discussed with emphasis on the ice‐front retreat with transition to the quiescent phase. Although the surge mechanism itself is unrelated to climate, climatic conditions obviously play a major role in the course of a surge. During the surge, the ice mass made a dramatic impression in the landscape, but 10 years after the maximum extent, there is little onshore evidence of the event.