Paleoclimate Archives Research Articles

Middle–late Holocene Caribbean aridity inferred from foraminifera and elemental data in sediment cores from two Cuban lagoons

Abstract Coastal lagoons are rarely used as paleoclimate archives because of their complex geomorphic histories, which can be affected by both climate and sea-level change. Combined foraminiferal and XRF element analysis of sediment cores from Punta de Cartas and Playa Bailen, Cuba, isolated the effects of climate change (wet vs dry) on the lagoon environments. Foraminiferal assemblages from both Punta de Cartas and Playa Bailen show increasing diversity over the past 4000 yr BP, with a prominent increase at ~ 1400–1100 yr BP. Assemblages were initially dominated by Ammonia spp. (e.g., Ammonia tepida ) and Elphidium spp. (e.g., Elphidium excavatum ), indicating brackish conditions, but increased miliolid species (e.g., Triloculina spp. Quinqueloculina spp.), indicate a shift to more marine conditions up-core. Correspondingly, terrigenous input to the lagoon (Fe, Ti, Ti/Ca and K) declined over the past 4000 yr BP with a flexion at ~ 1200–1100 yr BP that is likely a consequence of decreasing precipitation. Fe, Ti and K have been used as proxies for detrital erosion and transport rates in tropical and sub-tropical basins, with greater input during wet periods, but have rarely been applied to shallow lagoon systems. Coincident changes in the XRF and foraminiferal data indicate decreased freshwater input to the lagoon and support an inference for the onset of drier climate conditions. Similar temporal patterns in the foraminifera and XRF records from the two lagoons, which are ~ 10 km apart, suggest a regional climate influence, with increasingly arid conditions developing since the middle-Holocene (4 kyr BP). A pronounced drying over the last ~ 1200 years agrees with other climate records from the Caribbean.

Statistical paleoclimate reconstructions via Markov random fields

Understanding centennial scale climate variability requires data sets that are accurate, long, continuous and of broad spatial coverage. Since instrumental measurements are generally only available after 1850, temperature fields must be reconstructed using paleoclimate archives, known as proxies. Various climate field reconstructions (CFR) methods have been proposed to relate past temperature to such proxy networks. In this work, we propose a new CFR method, called GraphEM, based on Gaussian Markov random fields embedded within an EM algorithm. Gaussian Markov random fields provide a natural and flexible framework for modeling high-dimensional spatial fields. At the same time, they provide the parameter reduction necessary for obtaining precise and well-conditioned estimates of the covariance structure, even in the sample-starved setting common in paleoclimate applications. In this paper, we propose and compare the performance of different methods to estimate the graphical structure of climate fields, and demonstrate how the GraphEM algorithm can be used to reconstruct past climate variations. The performance of GraphEM is compared to the widely used CFR method RegEM with regularization via truncated total least squares, using synthetic data. Our results show that GraphEM can yield significant improvements, with uniform gains over space, and far better risk properties. We demonstrate that the spatial structure of temperature fields can be well estimated by graphs where each neighbor is only connected to a few geographically close neighbors, and that the increase in performance is directly related to recovering the underlying sparsity in the covariance of the spatial field. Our work demonstrates how significant improvements can be made in climate reconstruction methods by better modeling the covariance structure of the climate field.

The role of the Gulf Stream in European climate.

The Gulf Stream carries the warm, poleward return flow of the wind-driven North Atlantic subtropical gyre and the Atlantic Meridional Overturning Circulation. This northward flow drives a significant meridional heat transport. Various lines of evidence suggest that Gulf Stream heat transport profoundly influences the climate of the entire Northern Hemisphere and, thus, Europe's climate on timescales of decades and longer. The Gulf Stream's influence is mediated through feedback processes between the ocean, atmosphere, and cryosphere. This review synthesizes paleoclimate archives, model simulations, and the instrumental record, which collectively suggest that decadal and longer-scale variability of the Gulf Stream's heat transport manifests in changes in European temperature, precipitation, and storminess. Given that anthropogenic climate change is projected to weaken the Atlantic Meridional Overturning Circulation, associated changes in European climate are expected. However, large uncertainty in the magnitude of the anticipated weakening undermines the predictability of the future climate in Europe.

Early Cenozoic tropical climate: report from the Tanzania Onshore Paleogene Integrated Coring (TOPIC) workshop

Abstract. We are currently developing a proposal for a new International Continental Scientific Drilling Program (ICDP) project to recover a stratigraphic and paleoclimatic record from the full succession of Eocene hemipelagic sediments that are now exposed on land in southern Tanzania. Funding for a workshop was provided by ICDP, and the project was advertised in the normal way. A group of about 30 delegates assembled in Dar-es-Salaam for 3 intensive days of discussion, project development, and proposal writing. The event was hosted by the Tanzania Petroleum Development Corporation (TPDC) and was attended by several geologists, geochemists, geophysicists, and micropaleontologists from TPDC and the University of Dar-es-Salaam. International delegates were from Canada, Germany, India, Ireland, Italy, the Netherlands, United Kingdom, and United States (and we also have project partners from Australia, Belgium, and Sweden who were not able to attend). Some of the scientists are veterans of previous scientific drilling in the area, but over half are new on the scene, mostly having been attracted by Tanzania's reputation for world-class paleoclimate archives. Here we outline the broad aims of the proposed drilling and give a flavor of the discussions and the way our proposal developed during the workshop. A video of the workshop with an introduction to the scientific goals and interviews of many of the participants is available at http://vimeo.com/107911777.

Alpine lacustrine varved record reveals summer temperature as main control of glacier fluctuations over the past 2250 years

Glacier fluctuations are a key indicator of changing climate. Their reconstruction beyond historical times unravels glacier variability and its forcing factors on long time scales, which can considerably improve our understanding of the climate–glacier relationship. Here, we present a 2250-year-long reconstruction of particle-mass accumulation rates recorded in the lacustrine sediments of Lake Trüebsee (Central Swiss Alps) that are directly related to glacier extent, thus reflecting a continuous record of fluctuations of the upstream-located Titlis Glacier. Mass accumulation rate values show strong centennial to multi-centennial fluctuations and reveal 12 well-pronounced periods of enhanced values corresponding to times of maximum extent of the neighboring Lower Grindelwald Glacier. This result supports previous studies of proglacial lake sediments that documented high mass accumulation rate values during glacier advances. The strong variability in the Lake Trüebsee mass accumulation rate record thus represents a highly sensitive paleoclimatic archive, which mirrors rapid and pronounced feedbacks of Titlis Glacier to climatic changes over the past 2250 years. The comparison of our data with independent paleo-temperature reconstructions from tree rings suggests that variations in mean summer temperature were the primary driving factor of fluctuations of Titlis Glacier. Also, advances of Titlis Glacier occurred during the grand solar minima (Dalton, Maunder, Spörer, Wolf) of the last millennium. This relation of glacier extent with summer temperature reveals strong evidence that the mass balance of this Alpine glacier is primarily controlled by the intensity of glacier melting during summer.

Time-scale and astronomical forcing of Serbian loess–paleosol sequences

The loess in Vojvodina region in Northern Serbia is regarded as one of the thickest and most complete paleoclimate archives in Europe. Recent studies showed that loess in Vojvodina spans the last million years. Based on the interprofile correlation between two most important loess–paleosol sequences Stari Slankamen and Titel Loess Plateau a synthetic profile was formed regarded as Stratotype Record of Serbian loess (SRSL). The synthetic profile has a total thickness of 62m and is comprised of 10 loess layers intercalated with nine pedocomplexes. A new astronomically tuned age model is proposed based on correlating peak magnetic susceptibility (χ) responses with the timing of June perihelia. This target curve is derived from analysis of regional climate proxy responses during the last interglacial and independently dated last glacial–interglacial cycle of Serbian loess–paleosol sequence. The use of a precession index target as the only tuning target presents the requirement of minimal tuning approach, which means that only the precession frequencies of the record will be manipulated.Spectral and wavelet analyses of tuned magnetic susceptibility record reveal the presence of frequencies corresponding to 139kyr, 94kyr, 65kyr and small amplitude cycles of 43 and 23kyr in χ record. Spectral analysis of frequency dependent magnetic susceptibility (χfd) has spectral content concentrated around 100kyr and 77kyr, while higher frequency cycles are below significance level. There is a strong 245kyr peak in spectral results of both χ and χfd records that needs additional attention.Around 800kyr in SRSLχ time scale and between 900kyr and 600kyr in SRSLχfd series the 100kyr cycles become dominant, which could be identified as the period of Mid-Pleistocene Transition. This is the first record of this paleoclimate transition in the Pannonian basin. The results presented in this study highlight the potential of Serbian loess–paleosol sequences for investigation of detailed and long-term climate reconstruction over continental Eurasian extent.

Bivalve shells: ultra high-resolution paleoclimate archives

Bivalve shells: ultra high-resolution paleoclimate archives

Environmental change across a terrestrial Cretaceous-Paleogene boundary section in eastern Montana, USA, constrained by carbonate clumped isotope paleothermometry

The Cretaceous-Paleogene (K-Pg) mass extinction has been attributed to the impact of a large bolide at the end of the Cretaceous Period, although other potential causes have also been proposed, most notably climate change caused by Deccan Traps (India) flood volcanism. Reconstructing paleoclimate, particularly in terrestrial settings, has been hindered by a lack of reliable proxies. The recent development of carbonate clumped isotope paleothermometry has contributed to temperature reconstructions using geochemical proxies in terrestrial settings. We employ this method, along with new stratigraphic constraints, in the Hell Creek (Cretaceous) and overlying Fort Union (Paleogene) Formations (Montana, USA) to examine changes in temperature leading to and across the K-Pg boundary. We demonstrate that well-preserved ca. 66 Ma aragonitic bivalves serve as suitable paleoclimate archives. Although there are limitations in the stratigraphic availability of fossil bivalves for clumped isotope analysis, we record an apparent 8 °C decrease in summer temperatures over the last 300 k.y. of the Cretaceous that corresponds broadly with patterns observed in other paleotemperature proxies. This observed decrease plausibly could be explained by an absolute temperature decrease or by other environmental stresses on the organisms, but in either case suggests changing living conditions over the interval. Previously documented declines in vertebrate and invertebrate biodiversity occur over the same stratigraphic interval at this study location. These results are consistent with published models of the end-Cretaceous mass extinction in which destabilized ecosystems become more susceptible to an abrupt event like a bolide impact.

Hindcasting the continuum of Dansgaard–Oeschger variability: mechanisms, patterns and timing

Abstract. Millennial-scale variability associated with Dansgaard–Oeschger events is arguably one of the most puzzling climate phenomena ever discovered in paleoclimate archives. Here, we set out to elucidate the underlying dynamics by conducting a transient global hindcast simulation with a 3-D intermediate complexity earth system model covering the period 50 to 30 ka BP. The model is forced by time-varying external boundary conditions (greenhouse gases, orbital forcing, and ice-sheet orography and albedo) and anomalous North Atlantic freshwater fluxes, which mimic the effects of changing northern hemispheric ice volume on millennial timescales. Together these forcings generate a realistic global climate trajectory, as demonstrated by an extensive model/paleo data comparison. Our results are consistent with the idea that variations in ice-sheet calving and subsequent changes of the Atlantic Meridional Overturning Circulation were the main drivers for the continuum of glacial millennial-scale variability seen in paleorecords across the globe.

Natural pedogenic pathway of iron oxides

The exact formation pathway of iron oxides via pedogenesis has long been a contentious topic in geological and environmental studies [1]. Pedogenesis is crucial to accurately interpret the environmental significances of iron oxides in paleoclimatic archives, for example, the Chinese Loess/Paleosol sequences [2]. Several models have been developed but all lack direct constraints from natural records [2]. Hu et al. adopted a novel approach by integrating rock magnetism, dynamic dissolution and diffuse reflectance spectroscopy techniques [3]. They successfully separated and quantified major iron oxides of different origins (detrital and pedogenic) from a loess– paleosol–loesstransition sequence of the classic Luochuan Loess section in Shaanxi, China. Their major scientific contribution is 2-fold. First, they characterized, in detail, the pedogenic and detrital (or aeolian) hematite in loess/paleosols. Pedogenic hematite is nano-sized, preferentially dissolved in a weak reductive environment and, with a greater content of Al, can be easily demagnetized by the alternating field (AF) (remanence coercivity=∼130 mT). In contrast, the detrital counterpart is micro-sized, more resistant to dissolution, is almost stoichiometric and is resistant more to the AF demagnetization (remanence coercivity = ∼1 T). Second, based on the separation of iron oxides of different origins, the researchers quantified and described the relationships between different iron oxides. The results indicated that superparamagnetic (SP) pedogenic maghemite was Figure 1. Identification of pedogenic (Ped) and detrital (Det) hematite (Hm) in the Chinese loess/paleosol sequence. a) Schematic representation of Luochuan section; b) The depth variation of low field susceptibility, χ ; c) Hematite dissolution behavior in a typical loess sample; d) Isothermal remanent magnetization (IRM) decomposition of a typical loess sample, indicating the presence of detrital hematite with the central remanence coercivity around 1. 5 T. The stars defines the end of the first step where Ped–Hm is significantly dissolved, but the Det–Hm remains almost untouched; e) Hematite dissolution behavior in a typical paleosol sample; f) IRM decomposition of a typical paleosol sample, indicating the presence of both detrital and pedogenic hematitewith central remanence coercivity around 1 T and 130 mT, respectively. The black and color curves indicate the original and the dissolved samples, respectively; g) Relationship between the concentration indicator and the remanence indicator of pedogenic hematite, indicating the initial production of pedogenic hematite is in the SD region. The dashed line represents the case when SP hematites are largely produced initially. The solid line indicates the linear trend corresponding to the case that the initial production is in an SD state. (Revised from Hu et al. [3]).

MIS 7 interglacial sea-surface temperature and salinity reconstructions from a southwestern subtropical Pacific coral

We generated a 5.5-yr snapshot of biweekly-to-monthly resolved time series of carbon and oxygen isotope composition (δ13C and δ18O) and Sr/Ca and Mg/Ca from annually banded aragonite skeleton of a ~197ka pristine Porites coral collected at Niue Island (19°00′S, 169°50′W) in the southwestern subtropical Pacific Ocean. This report is the first of a high-resolution coral-based paleoclimate archive during the Marine Isotope Stage (MIS) 7 interglacial. Statistical results suggest that annual averages of sea-surface temperature (SST) and salinity (SSS) at ~197ka were not significantly different from and ~1.2 higher than at present, respectively. Monthly mean variations showed increased SSS at ~197ka that was higher (1.4–1.9 relative to today) in the austral summer than in the austral winter. Monthly SST and SSS anomalies at ~197ka indicated smaller amplitudes by ~0.3°C (11%) and ~0.3 (24%) relative to the present, possibly suggesting less influence of interannual climate variability around Niue. Our results, taken together with other climate proxy records, imply seasonal and interannual modulation of thermal and hydrological conditions, different from today, in the southwestern subtropical Pacific Ocean associated with the Western Pacific Warm Pool and the South Pacific Convergence Zone variability during the MIS 7 interglacial.

The coralline red alga Lithophyllum kotschyanum f. affine as proxy of climate variability in the Yemen coast, Gulf of Aden (NW Indian Ocean)

Recent investigations have shown the potential of red coralline algae as paleoclimatic archive. A previously unexplored subfamily of coralline algae, the Lithophylloideae, was investigated from the Gulf of Aden (Balhaf, Yemen). Seasonal changes in Mg/Ca, Li/Ca and Ba/Ca composition of Lithophyllum kotschyanum f. affine were investigated by Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). For the first time in coralline algae, the Li/Ca composition was analyzed and showed a highly significant and positive correlation with Mg/Ca and SST. Monthly algal Mg/Ca and Li/Ca variations indicate a positive correlation with sea surface temperature (SST), and sea surface salinity (SSS), although low growth rates decrease the resolution of the algal record. Albeit no or weak positive correlation between monthly algal Ba/Ca and local SST was found, fluctuations in Ba/Ca suggest the seasonal influence of nutrient-rich deep waters introduced by upwelling, and record an increase of sedimentation at the sampling site likely due to an intensified land use in the area. The Mg/Ca age model shows an average algal extension rate of 1.15mmyr−1, and reveals multiple intra-annual banding (previously unreported in the genus Lithophyllum) together with carposporangia formation in late February-early March, when temperature begins to increase. The concentration of MgCO3 in the thallus of L. kotschyanum f. affine is 20mol% (1SE), confirming that within the genus, the species sampled in warmer regions contain higher mol% MgCO3. The concentrations of LiCO3 and BaCO3 are 8μmol% (0.7SE) and 0.5μmol% (0.03SE), respectively. Despite the limitations from low-growth rate and species-specific vital effect, coralline algae confirm their utility in climate and oceanographic reconstruction.

Cosmogenic 3He production rate in the high tropical Andes (3800 m, 20°S): Implications for the local last glacial maximum

To improve both precision and accuracy of cosmic-ray exposure dating methods, there is a crucial need of calibration sites constraining the production rate of cosmogenic isotopes. This is particularly true in the high tropical area, where existing scaling models present significant discrepancies. This study presents a new calibration site for cosmogenic 3He, located at ∼3800 m in the tropical Altiplano (19.9°S, 67.6°W), on the southern flank of the Tunupa volcano, in the vicinity of the Salar de Uyuni. It consists in a fluvio-glacial outwash that is stratigraphically bracketed by two successive lacustrine shorelines. These shorelines are well-dated by 14C and U-series dating (n=57), allowing determination of the age of the boulder field at the surface of the delta, at 15.3±0.5 ka (1σ). Eleven andesitic boulders were sampled on this fluvio-glacial surface and the cosmogenic 3He contents of their pyroxene phenocrysts and amphiboles was analyzed. The nucleogenic contribution from 6Li capture is well-constrained by determining the (U–Th–Sm)/4He age of these rocks. This correction is minimal (<2%) and does not represent a significant source of uncertainty. Cosmogenic 3He (3Hec) concentrations are characterized by a very high reproducibility: 10 samples overlap within 1σ analytical error. This suggests that pre- or post-deposition processes have minimal impact on the 3Hec data. Furthermore, there is no correlation between the Li and the 3Hec content of the mineral, indicating that 3Hec production from cosmogenic thermal neutron is here negligible.Combined with the absolute dating of the delta, these 3Hec data yield a local production rate of 999±38 (1σ) at g−1 yr−1, at 3800 m and 19.89°S. After scaling with the time dependent scaling scheme of (Stone, 2000), this result yields a rate of 121±5 (1σ) at g−1 yr−1 at high latitude and sea level. This new calibration is the highest among the existing global dataset. It will thus permit the establishment of new robust glacial chronologies in the high Tropics, with an uncertainty lower than 5% at 1σ. Such precision may have important implications in paleoclimatology, notably because it will allow comparison with other well-dated paleoclimatic archives. Including this new data, the updated global 3Hec production rate is 122±15 atg−1yr−1, using the time-dependent scaling of Stone (2000).The new site-specific 3Hec production rate is used here to refine glacier fluctuations on Cerro Tunupa, confirming that the local last glacial maximum was synchronous with the Lake Tauca highstand (15.5 ka). Data also suggest that a dramatic glacial retreat occurred at about 15 ka, few hundred years before the Lake Tauca regression, synchronously with the onset of the Bolling-Allerod.

Paleomagnetism and biostratigraphy of sediments from Southern Ocean ODP Site 744 (southern Kerguelen Plateau): Implications for early-to-middle Miocene climate in Antarctica

New paleomagnetic results from lower-to-middle Miocene samples from Ocean Drilling Program (ODP) Holes 744A and 744B, cored during ODP Leg 119 on the southern Kerguelen Plateau (Indian Ocean sector; Southern Ocean), provide a chronostratigraphic framework for an existing and under-utilized paleoclimate archive during a key period of Antarctic climate and ice sheet evolution. Site 744 is strategically positioned for high-latitude paleoceanographic and paleoclimatic studies because it lies within the southern domain of the Antarctic Circumpolar Current (ACC) and in proximity to the large and active Lambert Glacier-Amery Ice Shelf drainage system of the East Antarctic Ice Sheet. Magnetostratigraphic results were reported previously for this site, but technical difficulties and limited sampling prevented confident correlation of the magnetic polarity record with the geomagnetic polarity timescale. Our results, which are constrained by new semi-quantitative analyses of diatom assemblages and radiolarian first and last appearance events that are evaluated within a regional Southern Ocean biostratigraphic dataset through Constrained Optimization (CONOP) model runs, permit significant refinement of previous age models for the lower-to-middle Miocene sequence recovered at Site 744 (spanning the interval from ~21 to 13.7Ma). An extended record of sediment accumulation, with average sedimentation rates of ~0.7–0.9cm/kyr, is interrupted by a series of hiatuses in the middle Miocene. These disruptions in sediment supply, or erosional events, could mark a local response of north–south fluctuations in the location and/or strength of the Antarctic Circumpolar Current during transient glacial events within the Mid-Miocene Climate Optimum (MMCO; ~17 to 14.45Ma). With the enhanced age control provided by this study, combined with a refined chronostratigraphy for the underlying upper Eocene to Oligocene strata, Site 744 becomes a good candidate for future high-resolution stable isotope and microfossil paleoecological work, which will further elucidate the late Paleogene and early Neogene paleoenvironmental history of the Southern Ocean.

High regional climate sensitivity over continental China constrained by glacial-recent changes in temperature and the hydrological cycle

The East Asian monsoon is one of Earth's most significant climatic phenomena, and numerous paleoclimate archives have revealed that it exhibits variations on orbital and suborbital time scales. Quantitative constraints on the climate changes associated with these past variations are limited, yet are needed to constrain sensitivity of the region to changes in greenhouse gas levels. Here, we show central China is a region that experienced a much larger temperature change since the Last Glacial Maximum than typically simulated by climate models. We applied clumped isotope thermometry to carbonates from the central Chinese Loess Plateau to reconstruct temperature and water isotope shifts from the Last Glacial Maximum to present. We find a summertime temperature change of 6-7 °C that is reproduced by climate model simulations presented here. Proxy data reveal evidence for a shift to lighter isotopic composition of meteoric waters in glacial times, which is also captured by our model. Analysis of model outputs suggests that glacial cooling over continental China is significantly amplified by the influence of stationary waves, which, in turn, are enhanced by continental ice sheets. These results not only support high regional climate sensitivity in Central China but highlight the fundamental role of planetary-scale atmospheric dynamics in the sensitivity of regional climates to continental glaciation, changing greenhouse gas levels, and insolation.

East Asian Summer Monsoon variations in the past 12.5 ka: High-resolution δ18O record from a precisely dated aragonite stalagmite in central China

Due to possible aragonite to calcite transformation resulting in alteration in isotopic signatures and a bias in age-dating, aragonite speleothems are often excluded from paleoclimatic archives. However, aragonite stalagmites contain ppm-level uranium content, making them much easier to date, achieving higher age-dating precisions than calcite stalagmites. In this regard, provided aragonite-to-calcite transformation did not occur, aragonite stalagmites are potentially well suited for Holocene climate research, given their climate proxies can be placed into a better constrained chronological framework. In this paper, we present high-precision U/Th dates and O isotopic time series for a 82cm long, continuous growth aragonite stalagmite, LH2, from Lianhua Cave, Hunan Province, China, and discuss East Asian Summer Monsoon (EASM) variability for the last 12.5ka BP (before present). The U/Th-dated δ18O sequence with a mean 16-year resolution and the growth rate pattern of LH2 show that EASM experienced a strengthening stage, a strong stage and a weakening stage during the last 12.5ka. During the YD (12.5–11.5ka BP), heavy δ18O values and low growth rate indicate a weak monsoon period. During the Preboreal (from ∼11.5 to 10.6ka BP), δ18O values decreased dramatically (∼1.94‰) reflecting abrupt strengthening of the monsoon. From 10.6 to 4.2ka BP, the record is characterized by the lightest δ18O values and high growth rates, suggesting a strong monsoon period. The summer monsoon weakened substantially after 4.2ka BP, as inferred from gradually increasing δ18O values and decreasing growth rate. Overall, the intensity of the EASM is regulated by summer insolation at 30°N during the last 12.5ka. Although oxygen isotope fractionation is different between aragonite–H2O and calcite–H2O because of Rayleigh Fractionation Law, the overall temporal pattern of δ18O values from aragonite stalagmite LH2 is concordant with other high-resolution Holocene calcite stalagmite records from South China. The comparison among these records shows that the Holocene Optimum was synchronous across Asian continental region influenced by the EASM and Indian Summer Monsoon (ISM), lasting from ∼10.6 to 4.2ka BP, as reflected by consistently lighter δ18O values among all these records regardless of their latitudinal difference. These records do not support previously reported asynchronism between EASM and ISM. In details, the long-term δ18O trend in LH2 is punctuated by a number of centennial fluctuations. For instance, two weak monsoon events occurred at 9232±57yr BP and 8137±21yr BP, correlating in time with cooling events in Greenland ice cores. The latest weak monsoon event centered at 302±8yr BP which is related to the Little Ice Age (LIA). In addition, the monsoon intensity derived from our record also shows a strong connection with latitudinal migration of the Inter-Tropical Convergence Zone (ITCZ) as recorded in the Cariaco Basin sediments. Spectral analysis of δ18O values shows that significant peaks match with solar periodicities of 208yr (de Vries cycle), 86yr (Gleissberg cycle) or related to Δ14C production suggesting shorter-term monsoon variations are forced by solar radiation. Overall, our study suggests that the δ18O record in the aragonite stalagmite is highly consistent with those derived from calcite stalagmites, suggesting that aragonite stalagmites are suitable for palaeoclimate reconstruction, especially for the Holocene period, as aragonite–calcite transformation has not occurred.

Hydroclimatic variations over the last millennium in eastern Finland disentangled by fossil Cladocera

Future climate warming is expected to alter atmospheric moisture balance and therefore paleoclimatic archives of past moisture and hydrological patterns may provide understanding of the magnitude, speed, and ecosystem responses to the regionally altered climatic regimes. The present study aimed at disentangling millennial variation in hydroclimatic parameters, including summer effective moisture (lake levels) and ice-free season by using sedimentary cladoceran fossils. We used a site-specific cladoceran-based inference model to reconstruct lake level fluctuations in a 1500-yr sediment record from Lake Pieni-Kauro, eastern Finland in the context of previously established paleoclimatic data of summer temperature and precipitation. In addition, we applied cladoceran planktonic/littoral ratio, mean body size, and abundance of fossil ephippia as supplementary proxies for paleoclimatic interpretations. The results showed that lake level was lower during the pre- and early Medieval Climate Anomaly (MCA, around 500-1000 AD) than at present and started to rise toward the culmination during the Little Ice Age (LIA, around 1600–1700AD) when the lake level was higher than at present. Pelagic cladoceran body size was larger and abundance of ephippia increased during the LIA suggesting significant reduction in the length of the ice-free season and lower water temperatures. Comparison of the lake level reconstruction with the previous paleoclimatic data suggested that effective moisture has had a long-term negative relationship with temperature and positive relationship with summer precipitation in eastern Finland. The results indicated that the pre- and early MCA was warm and dry (long ice-free season, low summer effective moisture) and the culmination of the LIA was cool and moist (reduced ice-free season, increased summer effective moisture). Unraveling the seasonality of the multicentury hydroclimatic patterns in eastern Finland may offer important insights in linking the large-scale atmospheric circulation patterns to seasonal climate events.

The climate of the Holocene and its landscape and biotic impacts

The Holocene Epoch has abundant paleoclimatic archives at relatively high temporal and spatial resolution, which have helped to reveal the patterns of natural climate variation during the present interglacial period and the impacts of that variation on landscapes and biota. This article presents a personal review of some interesting insights that have emerged from analysis of Holocene paleoclimatic records from continental archives at orbital to multidecadal scales. These include how the increased density of sites in Asia, South America and Africa have revealed unforeseen spatial patterns of variation in the dynamics of the monsoon systems at orbital scales and a better characterisation of the magnitude of multidecadal and centennial variation in various parts of the globe. Among interglacial periods, the Holocene is unique as the period in which more complex human civilisations and agriculture developed, and many recent studies have evolved our understanding of the nature of the human impact relative to natural dynamics prior to large-scale population expansion.This paper is part of a Thematic Cluster in honor of the late Professor Bert Bolin for his outstanding contributions to climate science.

Moisture fluctuations reconstructed from a compilation of paleoclimate archives in the Iberian Peninsula: is there a common pattern during the Medieval Climate Anomaly?

Moisture fluctuations reconstructed from a compilation of paleoclimate archives in the Iberian Peninsula: is there a common pattern during the Medieval Climate Anomaly?

Varved sediments of Lago Plomo (46°59 S / 72°52 W) as a paleoclimate archive AD 1534 – 2009

Varved sediments of Lago Plomo (46°59 S / 72°52 W) as a paleoclimate archive AD 1534 – 2009