Medieval Warm Period Research Articles

Sustainable Indigenous Fishing in the Pre-Contact Caribbean: Evidence and Critical Considerations from Carriacou, Grenada

Multiple studies reveal pre-1492 anthropogenic impacts on Caribbean fisheries that are consistent with overfishing, including changes in targeted prey, shifts in marine habitats exploited, and decreases in the average body size of taxa. At the Indigenous Caribbean village of Sabazan (AD 400–1400) on Carriacou, Lesser Antilles, post-AD 800 declines in fishing, increased mollusk collection, and changes in resource patch emphasis accord with the archaeological correlates of resource depression predicted by foraging theory models from behavioral ecology. Here, I apply foraging theory logic and abundance indices incorporating body size and fish habitat to test the predictions of expanded diet breadth, declining prey body size, and shifts to more distant fishing patches that are typically associated with overfishing. Results uphold a significant decrease in overall fishing, which may be due to habitat change associated with the Medieval Warm Period. Indices of fish size and resource patch use do not meet foraging theory expectations for resource depression, however. Instead, they suggest an absence of resource depression in the Sabazan fishery and at least 600 years of sustainable fishing. I review similar findings for other Caribbean archaeological sites with either negative evidence for fisheries’ declines or quantitatively demonstrated sustainable fishing. These sites collectively serve as a critical reminder of the heterogeneous trajectories of Indigenous social–ecological systems in the pre-contact Caribbean and the need for meta-level analyses of the region’s ancient fisheries. I discuss the application of the sustainability concept in archaeological studies of fishing and conclude that a more critical, explicit approach to defining and measuring sustainability in ancient fisheries is needed.

Holocene vegetation and hydroclimate changes in the Kansk forest steppe, Yenisei River Basin, East Siberia

Our study focuses on the Holocene hydroclimate long‐term trends in southern East Siberia and places them in the context of the adjacent areas of the Siberian taiga in the north and the arid central Asia in the south. We present a new record from the eutrophic mire, in the forest‐steppe region, covering the last 9000 years. Our multi‐proxy approach includes physiochemical (ash content, grain size, micro‐charcoal, peat decomposition) and biological indicators (pollen, non‐pollen palynomorphs, plant macrofossils, testate amoebae, molluscs). The record indicates increased humidity during the Holocene Thermal Maximum at 8150–7400 cal. a BP, and drier conditions with evidence for fires at 7400–5100 cal. a BP. Next, wetter conditions occurred at the coring site at 5100–1400 cal. a BP based on evidence of fluvial influence, which is consistent with other records in the wider region. A short‐term dry episode occurred at 1400–1300 cal. a BP, followed by a wet Medieval Warm Period (1300–650 cal. a BP) and Little Ice Age (450–300 cal. a BP) and a moderately wet environment from 300 cal. a BP to the present day. These results are consistent with the previously published palaeoenvironmental reconstructions from the steppe and forest‐steppe regions in the south of East and West Siberia and taken together reveal a common trend towards wetter conditions with a maximum wetness between 4500 and 3000 cal. a BP. This differs from the taiga region further north and from the Altai‐Sayan Mountains and Kazakhstan Hills further south, which show the trend from dry to wet and then to drier conditions in the Holocene, and from the Trans‐Baikal region where the Holocene trend is to drier conditions. This study supports the predominant influence of the North Atlantic on the regional palaeoclimate changes in the forest‐steppe region of East Siberia during the Holocene.

The Last 1200 Years of Rainfall/Runoff Variability along the Central Mexico Pacific Coast Associated with the North American Monsoon

This study presents new evidence for long-term variability in the late Holocene North American Monsoon (NAM), Pacific coast of Mexico. We have carried out a rock magnetic study on two deep-sea sediment cores from the Pacific coast Pescadero Basin. The magnetic intensities estimate total magnetic material and are a proxy for total clastic sediment. Ratios of magnetic intensities estimate the grain size of magnetic material. The rock magnetic data show a decimeter scale, multi-decadal oscillation with fourteen cycles (A-N) over the last 1200 years. These oscillations reflect alternating intervals of stronger/coarser magnetic/clastic flux to the coastal ocean and intervals of weaker/finer magnetic flux. We think these variations are caused by variations in long-term dominance of the NAM; summer (wet) monsoons produce rainy conditions (with runoff) while winter (dry) monsoons produce significant offshore winds, increased upwelling/biological productivity. We can correlate our variability to two other published studies southeast of Pescadero Basin, coastal lake sediments in Laguna de Juanacatlan and a Juxtlahuaca Cave stalagmite. Both of these studies estimate local rainfall. We see evidence of the same pattern of multi-decadal rainfall-runoff variability in these records as we see in Pescadero Basin, which is synchronous to within ±25 years over the last 1200 years. The multi-dacadal pattern of hydrologic variability in all three records varies in cycle duration from ~90-years wet/dry cycles in the Little Ice Age (1400–1850 AD) to ~60-years cycles in the Medieval Climate Optimum (1100–1400 AD). This variability in cycle duration suggests some chaotic nature to the regional NAM climate pattern or some long-term non-linear forcing (PDO?).

Indian summer monsoon variability over last 2000 years inferred from sediment magnetic characteristics in Lugu Lake, southwest China

Knowledge of climatic change during the past 2000 years in southwestern China is important for understanding the evolution of the Indian summer monsoon (ISM). For this study, we conducted systematic analysis of magnetic minerals in sediments from Lugu Lake in northwestern Yunnan Province. Magnetic measurements suggest that magnetite dominated the studied magnetic mineral assemblages. Both natural origins and anthropogenic activities are considered to be the sources of the magnetic minerals based on the correlations of χ vs. χfd% and χARM/χ. The magnetic minerals of natural origins can be attributed to terrestrial detrital input rather than biogenic sources. Combined with diatom and geochemical records, magnetic parameters including χ, χfd%, and S-ratio can be used as sensitive indicators of ISM change, and suggest that two significant dry periods occurred in the Medieval Warm Period (MWP, 800–1300 CE) and Roman Warm Period (RWP, 0–400 CE) and two relatively wet intervals in the Dark Ages Cold Period (DACP, 400–800 CE) and Little Ice Age (LIA, 1300–1850 CE). Significant shifts of magnetic parameters in Lugu Lake samples revealed sensitivity to precipitation-controlled catchment erosion. The climatic patterns shown by the magnetic characteristics were generally similar to those in extensive areas dominated by the ISM. We speculate that rainfall caused by the ISM during the last 2000 years may be related to the variations of El Niño/Southern Oscillation (ENSO) and the mean position migration of the ITCZ.

Late Holocene climate variation on the northern Tibetan Plateau inferred from Lake Ayakum

Understanding late Holocene climate variation on the northern Tibetan Plateau is essential for understanding regional climate dynamics, especially the potential response to mid-latitude westerlies. A continuous lacustrine sediment record covering the past 1500 years was recovered from Lake Ayakum on the northern margin of the Tibetan Plateau, in which a typical cold-phase mineral (mirabilite) was detected. Assisted by the simulation results in the Pitzer model, the preservation of mirabilite in the sediment core as well as its relative proportion was interpreted in terms of past temperature variations, in which a high mirabilite content corresponded to a relatively cold environment. Grain size analysis and subsequent end-member modeling results revealed potential correlations with regional moisture conditions, that more coarse materials were transported by increased river discharge under relatively wet conditions. The results showed that cold and humid conditions dominated the Dark Age Cold Period and Little Ice Age, while warm and dry conditions dominated the Medieval Warm Period. Following the increase in regional temperature, more coarse materials were deposited during the last 200 years owing to probable increased fluvial transport supplied by glacier meltwater. The overall temperature and moisture variations were broadly consistent with records from western China and central Asia, indicating that the northern Tibetan Plateau was predominately influenced by the mid-latitude westerlies during the late Holocene. In addition, the temperature change showed a close correlation with solar irradiance, pointing to a potential response to solar activity. On the other hand, the moisture change responded sensitively to the North Atlantic Oscillation and the migration of mid-latitude westerlies.

Modern and sub-fossil corals suggest reduced temperature variability in the eastern pole of the Indian Ocean Dipole during the medieval climate anomaly

We present two 40 year records of monthly coral Sr/Ca ratios from the eastern pole of the Indian Ocean Dipole. A modern coral covers the period from 1968 to 2007. A sub-fossil coral derives from the medieval climate anomaly (MCA) and spans 1100–1140 ad. The modern coral records SST variability in the eastern pole of the Indian Ocean Dipole. A strong correlation is also found between coral Sr/Ca and the IOD index. The correlation with ENSO is asymmetric: the coral shows a moderate correlation with El Niño and a weak correlation with La Niña. The modern coral shows large interannual variability. Extreme IOD events cause cooling > 3 °C (1994, 1997) or ~ 2 °C (2006). In total, the modern coral indicates 32 warm/cool events, with 16 cool and 16 warm events. The MCA coral shows 24 warm/cool events, with 14 cool and 10 warm events. Only one cool event could be comparable to the positive Indian Ocean Dipole in 2006. The seasonal cycle of the MCA coral is reduced (< 50% of to the modern) and the skewness of the Sr/Ca data is lower. This suggests a deeper thermocline in the eastern Indian Ocean associated with a La Niña-like mean state in the Indo-Pacific during the MCA.

Central eastern China hydrological changes and ENSO-like variability over the past 1800 yr

Abstract Variations in East Asian summer monsoon (EASM) precipitation impact agriculture, water resources, electricity generation, and economic development in the densely populated region of central eastern China. However, uncertainties remain in the history and driving mechanisms of precipitation changes in this region over the past two millennia. We present an 1800 yr multiproxy reconstruction of monsoon precipitation based on hydroclimate variations inferred from Lake Nvshan sediments in the Jiang-Huai region of central eastern China. We find that rainfall in the Jiang-Huai region was higher during the Little Ice Age (LIA) than during the Medieval Warm Period (MWP), and that these changes were modulated by the mean state of the tropical Pacific Ocean on centennial time scales. We infer that an El Niño–like (La Niña–like) mean state caused a strengthening and westward displacement (weakening and eastward displacement) of the Western Pacific Subtropical High (WPSH) and a weakening (strengthening) of the EASM, causing more (less) rainfall in the Jiang-Huai region during the LIA (MWP). These hydroclimate changes are likely to have been a response to changes in effective radiative forcing.

Heavy mineral record from the east China sea inner shelf: Implications for provenance and climate changes over the past 1500 years

The heavy mineral composition and sensitive grain size changes from core SS4 retrieved from the mud wedge on the East China Sea inner shelf have been studied for better understanding provenance variations in the sediment and climate changes during the last ~1500 yr. High dolomite content and occurrence of ubiquitous polysynthetic mica indicate that the sediments in core SS4 were mainly derived from the Yangtze River, while the relatively high epidote content represent the contribution of small local rivers from Zhejiang and Fujian provinces. The substantial decrease in the epidote/hornblende ratio since ~730 yr BP manifests a decline in the relative contribution of sediment from the small local rivers to the core. We propose that the changes in drought/flood conditions in response to the East Asian winter monsoon (EAWM) and human activities might have been mainly responsible for changes in sedimentary characteristics. During 1400–730 yr BP, the core sediments were characterized by relatively finer sensitive grain size and high epidote/hornblende ratio, due to a possibly weaker EAWM (i.e., less Yangtze River-derived sediment transported by the southward coast current). This time interval coincides with the Sui-Tang Warm Period (1400–1130 yr BP) in China and the Medieval Warm Period (1000–730 yr BP) on a broader regional scale. The prolonged warm periods were, however, interrupted by three short cold phases at 1350–1280 yr BP, 1130–1000 yr BP and 870–750 yr BP, characterized by coarse sensitive grain size and decreased epidote/hornblende ratio. Between 360 and 150 yr BP, the sensitive grain size significantly coarsened, which corresponded to the Little Ice Age. Our new findings suggest that sedimentation processes of mud wedge formation are responsive to natural environmental change and human activity.

Biogeochemical evidence for environmental and vegetation changes in peatlands from the middle Yangtze river catchment during the medieval warm period and little ice Age

The reconstruction of past climates and vegetation can provide valuable information for understanding the mechanisms of the variability of the East Asian summer monsoon in eastern China. In this study, organic geochemical evidence from the compositions of sedimentary leaf-wax stable isotopes and n-alkane parameters investigates the changes in vegetation and climate over the last 1200 years in the Xiyaohu peatland, Jiangxi Province, southeast China. Combined with temperature records, three climatic periods are presented: (a) a warm and humid period with an increase in C4 plants from 900 to 1450 AD, which coincides with the Medieval Warm Period (MWP); (b) a cool and dry period with the expansion of C3 plants from 1450 to 1800 AD, coinciding with the Little Ice Age (LIA); and (c) the Present Warm Period (PWP) from 1800 AD until the present, with warm and wet conditions. The sub-stages within the MWP and LIA intervals are also presented. The earlier MWP stage (900–1125 AD) was drier than the latter one (1125–1450 AD), and the earlier LIA stage (1450–1650 AD) was drier than the late LIA (1650–1800 AD). Increased solar irradiance and enhanced El Niño activities are related to the warm and humid climate during the MWP and PWP, whereas reduced solar irradiance and La Niña activities correspond to the cool and dry climate during the LIA. The present results provide insights into paleoclimatic changes in eastern monsoonal China and provide an understanding of centennial-scale climatic fluctuations and their driving factors.

Review of speleothem records of the late Holocene: Indian summer monsoon variability & interplay between the solar and oceanic forcing

Indian summer monsoon (ISM) variability in the late Holocene (last 4000 years) is of great interest as the arid phase of ISM and cultivation of kharif (rainy season) crops started in this period. The present study aims to revisit the available speleothem records to study ISM rainfall variations and possible controlling factors in different Indian regions viz., core monsoon zone (CMZ), peninsular, the Himalaya, and the Bay of Bengal in the last 4000 years. The late Holocene encompasses notable global warm periods viz., Medieval Climate Anomaly, Roman Warm Period (RWP) & Minoan Warm Period, and cold periods viz., Dark Age cold period and Little Ice Age (LIA). ISM rainfall declines until the middle of RWP in all the regions. The decline is linked with the southward migration of the Intertropical convergence zone caused by the northern Hemisphere summer insolation. Though the Solar insolation follows the same trend, all the records show large variability with the disappearance of the declining trend around the middle of RWP. This change coincided with the reversal in the north Atlantic Sea surface temperature (SST) trend, northern hemisphere temperature, and abrupt changes in the southeastern Indian Ocean SST. This period also witnessed high El-Nino Southern Oscillation activities. This synergy of ISM variations in the characterized zones falls apart during the pre-LIA with a slight increasing (decreasing) trend in CMZ and Bay of Bengal (peninsular and Himalayan). The spatially variable response of ISM rainfall in recent centuries might be caused by the interaction between internal variability and external forcing and also by anthropogenic-induced warming and aerosol forcing.

Rocky Mountain subalpine forests now burning more than any time in recent millennia

The 2020 fire season punctuated a decades-long trend of increased fire activity across the western United States, nearly doubling the total area burned in the central Rocky Mountains since 1984. Understanding the causes and implications of such extreme fire seasons, particularly in subalpine forests that have historically burned infrequently, requires a long-term perspective not afforded by observational records. We place 21st century fire activity in subalpine forests in the context of climate and fire history spanning the past 2,000 y using a unique network of 20 paleofire records. Largely because of extensive burning in 2020, the 21st century fire rotation period is now 117 y, reflecting nearly double the average rate of burning over the past 2,000 y. More strikingly, contemporary rates of burning are now 22% higher than the maximum rate reconstructed over the past two millennia, during the early Medieval Climate Anomaly (MCA) (770 to 870 Common Era), when Northern Hemisphere temperatures were ∼0.3 °C above the 20th century average. The 2020 fire season thus exemplifies how extreme events are demarcating newly emerging fire regimes as climate warms. With 21st century temperatures now surpassing those during the MCA, fire activity in Rocky Mountain subalpine forests is exceeding the range of variability that shaped these ecosystems for millennia.

Formation history and material budget of holocene shelf mud depocenters in the Gulf of Cadiz

Mud depocenters (MDCs) are common elements on modern continental shelves and act as a major shallow-marine sink for fluviogenic material. These most proximal depocenters, thus, play a major role in material cycling and carbon availability on global and regional scales, though individual formation history, dependence on external forcing mechanisms, and material composition makes each of them a unique case. This study establishes a chronostratigraphic framework and deciphers the depositional dynamics for the two main MDCs on the continental shelf in the eastern Gulf of Cadiz, as a prime example, with the goal to calculate a regional sediment and carbon budget. Based on the analysis of 2040 km of subbottom profiles and 18 sediment cores, the fine-grained depocenters began to grow during maximum flooding around 6.5 cal ka BP. Sedimentation rates ranged between 2 and 35 cm/ka until 2.7 cal ka BP and increased significantly around the Roman Warm Period (30–200 cm/ka), caused by regional humidification as well as mining and agricultural activities. After 1.0 cal ka BP, sedimentation rates rose further (20–3000 cm/ka), due to land clearing in coincidence with erosion-favoring aridity during the Islamic period and the Medieval Climate Anomaly. Unprecedented sediment accumulation started with the Industrial Era. The total sediment volume of the two MDCs is 5.80 km3 with a dry mass of 12,971 Mt. 85 Mt of organic matter and 3637 Mt of carbonate make this depocenter an important shallow-marine sink, with a total of 521 Mt carbon as a significant player in the regional terrestrial-marine carbon cycle.

Controls on rare earth elements distribution from Kerala coast, southwest India over the past 2000 years

Climate changes on the Southwest coast of India have essential impacts on social and economic development, and the ecosystem of Southern India. Here, we present the last 2000 cal yr AD rare earth elemental records of the sediment core from the Cherai, Kerala coast, southwest India. The light rare earth elements (LREE) were the most abundant fraction in the core sediment, followed by medium rare earth elements (MREE) and heavy rare earth elements (HREE). Total REE (ΣREE) showed a significant correlation with fine fractions and Y, Al, Ti, Fe and total organic carbon content of the sediments, suggesting an intrinsic relationship between REE, total organic carbon and fine grain-size fraction. The LREE and MREE were enriched over the MREE due to their high adsorption with the fine fraction. The lack of correlation between terrigenous elements and low Y/Ho elemental ratio suggests a minor shift in the source materials. High ΣREE values during warm (Medieval Warm Period/ Medieval Climate Anomaly (MWP/MCA) and cold Little Ice Age (LIA) periods indicate a high influx of lithogenous materials during these intervals. The negative δCe values except MCA suggest a suboxic condition and the δEu values suggest feldspar-rich source rock material derived from moderately weathered sediments. It also indicates that the REE variations are controlled by factors like river supply, biological productivity and redox condition. A close link between monsoonal precipitation and REE records existed on the southwest coast of India. During the Dark Age Cold Period (DACP)/MWP transition, a sudden increase in monsoon is documented and the monsoon precipitation was further decreased during LIA and Current Warming Period. The Cherai core REE records synchronize with the episodes of centennial-scale warm/cool events and essentially follow the northern hemispheric climate cycles.

Palaeoecological changes from 580 to 1220 CE from the Indo-Burma region: A biotic assessment from the Barak Valley of Assam, northeast India

The pollen and diatom analyses of soil samples from a 80-cm deep sedimentary core from the Barak valley of Assam provides an explicit understanding of vegetation, climatic and ecological change in the Indo-Burma region from 580 CE (1370 cal. years BP) to 1220 CE (730 cal. years BP). The impact of the Medieval Warm Period (MWP) has been well documented from palynological records. Based on changes in vegetation succession, the Mean Annual Temperature (MAT) and Mean Annual Precipitation (MAP) during four pollen phases CP (1–4) were calculated using the coexistence approach. Between 580 and 760 CE (pre-MWP), the occurrence of major riparian tree pollen taxa like Barringtonia, Duabanga and Sapotaceae along with scanty diatom occurrence indicates a warm and relatively less humid climate attributable to a weak Southwest monsoon with low seasonality. This changed to a strikingly enhanced seasonality from 760 to 940 CE (beginning of MWP), indicating the establishment of the dense forest around the lake under increased warm and humid climatic conditions. During the interval from 940 to 1220 CE (MWP peak), the average values of maximum MAT and MAP were the highest recorded (≈31 °C and ≈2250 mm, respectively). From 1220 CE onwards (post MWP), the decline in arboreal pollen coincides with an exponential rise in cereal pollen clumps and shrubby pollen like Melastoma, Clerodendrum and Justicia adhatoda along with the presence of anthropogenic indicator diatom taxa like Ulnaria ulna and Gomphonema indicating increased landscape changes due to human impact under a relatively less warm and humid climate. In many parts of the world, regional data coverage pertinent to the MCA warming has now reached a point which allows compiling palaeoclimate maps for well-defined time intervals. In this direction, future studies need to address the major climatic data gaps from the Indian sub-continent especially during the last millennia.

The link between climate change and biodiversity of lacustrine inhabitants and terrestrial plant communities of the Uvs Nuur Basin (Mongolia) during the last three millennia

The paper is focused on changes in biodiversity, the environment, and human activity in the Uvs Nuur Basin during the last three millennia based on biological and geochemical proxies from the lake Bayan Nuur. Regions with high biodiversity and relatively low anthropogenic pressures are typically the most vulnerable to both climate change and human activities. One such area is the Uvs Nuur Basin located on the north of the Great Lake Depression of Mongolia. The main objective of this study is to assess changes in the past biodiversity of the lake’s microflora and microfauna, and surrounding vegetation biodiversity in the Uvs Nuur Basin, and to determine the main drivers of diversity change. Based on the analysis of pollen and chironomids we conclude that the most humid and afforested phase was between 1400 and 1800 CE. We assume that the Little Ice Age in the Uvs Nuur Basin was humid with mean annual precipitation ca. 305 mm/year and mean July temperature about 13°C. Conversely, the warmest and most arid period was between 650 and 1350 CE with mean annual precipitation ca. 280 mm/year and mean July temperature of about 16°C, attributed to the Medieval Warm Period. The biodiversity of terrestrial plants, chironomids, and Cladocera positive react to changes in annual precipitation and July temperature, whereas diatoms do not correlate directly to the climatic factors. The diversity and the evenness of plants are strongly correlated with the change in the leading biomes. The calculated species turnover suggests no significant changes in plant and Cladocera taxa composition, but significant changes in diatom and chironomid communities. This may be explained by the instability of lake ecology due to the fluctuation of the salinity and acidity of the water. An additional aim was to assess if dung fungi in lacustrine sediments reflect changes in human population density around the lake. We found that neither historical sources of human presence nor the influx of coprophilous fungi are correlated with the inferred climate changes. Coprophilous fungi can be used as individual or additional sources of assessment for the peopling and human-related herbivore density including overgrazing of the studied area.

Holocene Spatiotemporal Redox Variations in the Southern Baltic Sea

Low oxygen conditions in the modern Baltic Sea are exacerbated by human activities; however, anoxic conditions also prevailed naturally over the Holocene. Few studies have characterized the specific paleoredox conditions (manganous, ferruginous, euxinic) and their frequency in southern Baltic sub-basins during these ancient events. Here, we apply a suite of isotope systems (Fe, Mo, S) and associated elemental proxies (e.g., Fe speciation, Mn) to specifically define water column redox regimes through the Baltic Holocene in a sill-proximal to sill-distal transect (Lille Belt, Bornholm Basin, Landsort Deep) using samples collected during the Integrated Ocean Drilling Program Expedition 347. At the sill-proximal Lille Belt, there is evidence for anoxic manganous/ferruginous conditions for most of the cored interval following the transition from the Ancylus Lake to Littorina Sea but with no clear excursion to more reducing or euxinic conditions associated with the Holocene Thermal Maximum (HTM) or Medieval Climate Anomaly (MCA) events. At the sill-distal southern sub-basin, Bornholm Basin, a combination of Fe speciation, pore water Fe, and solid phase Mo concentration and isotope data point to manganous/ferruginous conditions during the Ancylus Lake-to-Littorina Sea transition and HTM but with only brief excursions to intermittently or weakly euxinic conditions during this interval. At the western Baltic Proper sub-basin, Landsort Deep, new Fe and S isotope data bolster previous Mo isotope records and Fe speciation evidence for two distinct anoxic periods but also suggest that sulfide accumulation beyond transient levels was largely restricted to the sediment-water interface. Ultimately, the combined data from all three locations indicate that Fe enrichments typically indicative of euxinia may be best explained by Fe deposition as oxides following events likely analogous to the periodic incursions of oxygenated North Sea waters observed today, with subsequent pyrite formation in sulfidic pore waters. Additionally, the Mo isotope data from multiple Baltic Sea southern basins argue against restricted and widespread euxinic conditions, as has been demonstrated in the Baltic Proper and Bothnian Sea during the HTM or MCA. Instead, similar to today, each past Baltic anoxic event is characterized by redox conditions that become progressively more reducing with increasing distance from the sill.

Chronology and paleoclimatic implications of lacustrine sediments at Inexpressible Island, Ross Sea, Antarctica

Lacustrine sediments from ice-free areas of Antarctica record both paleoecological and paleoclimatic information. Four sediment profiles (IIL1, IIL3, IIL4 and IIL9) were collected at Inexpressible Island, Ross Sea, to establish a robust late-Holocene chronology using accelerator mass spectrometry (AMS) 14C dating and geochemical and lithological analyses. The IIL1 and IIL4 sediments were strongly affected by penguin guano, and their bottom (oldest) ages were dated to 1659 and 4820 yr BP, respectively, using a Mixed Marine SoHem mode. By contrast, the organic matter of IIL3 and IIL9 sediments were predominantly sourced from aquatic microbial mats with the bottom ages of these two cores at 3179 and 2945 yr BP, respectively, based on a SHCal13 mode. The mass accumulation rates of the four sediment profiles inferred from this chronology showed peaks during ~1400–800 yr BP, corresponding to greater mean grain size and higher sand fraction ratios in the IIL3 and IIL9 profiles, suggesting a strengthened hydrodynamic effect in this period. Our results indicate a relatively warm period occurred in the study area, in accordance with an ‘optimum’ warming in the Ross Sea region. From a regional view, this warm period was also consolidated with climatic records from the western Ross Sea, most likely corresponding to a well-recognized climate perturbation known as the Medieval Climate Anomaly (MCA) in many parts of the world.

Monsoonal climatic reconstruction from Central India during the last ca. 3600 cal yr: signatures of global climatic events, based on lacustrine sediment pollen records

Pollen analyses of a 90 cm deep lacustrine sediment profile from the Mahasamund District of Chhattisgarh State, central India demonstrated that between ca. 3600 and 2500 cal yr BP (Neoglacial climate interval), open forest vegetation occupied the landscape in the region under a dry climate, probably indicating reduced monsoon (the Indian Summer Monsoon: ISM) rainfall. Further, the open forest vegetation was succeeded by a mixed tropical deciduous forest between ca. 2500 and 1650 cal yr BP (∼550 BC to AD ∼300) under a warm and humid climate with increased monsoon rainfall. The warming period and an amelioration of climate (strong ISM condition) is comparable to the Roman Warm Period (RWP), recorded globally between 2500–1600 cal yr BP (∼550 BC to AD ∼350), and also known as the ‘Golden Age of India’ owing to increased agricultural output, trade commerce and socio-economic prosperity in India. Subsequently, between ca. 1650 and 950 cal yr BP (AD ∼300–1000), the existing mixed tropical deciduous forest transformed into an open mixed tropical deciduous forest under a warm and relatively less humid climate with reduced monsoon rainfall, correlatable with the Dark Ages Cold Period (DACP). Finally, between ca. 950 cal yr BP to the present (AD 1000 onwards), mixed tropical deciduous forest again came into existence and occupied the landscape around the study area under a warm and relatively more humid climate with intensification of the monsoon rainfall, corresponds with the Medieval Warm Period (MWP), which is known between AD 740 and 1150 worldwide.

The middle to Late Holocene environment on the Iturup Island (Kurils, North Western Pacific)

The Kuril Islands stretch southwest from Kamchatka, Russia, to Hokkaido, Japan and separate the Sea of Okhotsk from the northern Pacific Ocean. A series of transgressions and regressions linked to variations in climatically affected global ice volume are among the most important drivers of Holocene environmental changes in the region. Despite a long research history, reconstructions of the Holocene palaeoenvironment are sparse with inconsistent interpretations, arising from insufficient dating control, different temporal resolutions, and specific local geographical features, such as high tectonic activity and the isolated nature of the area. We have investigated a 550 cm lake sediment section from Iturup Island, the largest among the Kuril Islands. The 6600 year old sediment section was studied using sedimentological, geochemical, chironomid, diatom, and pollen analyses to reconstruct environmental and climatic changes and sea level fluctuations (transgression – regression stages). During the warm late phase of the Middle Holocene (6600–4400 cal BP) an open bay or lagoon with shallow overgrown littorals existed at the sampling site. The cooling between 5600 and 4400 cal BP can be correlated with Neoglacial cooling. The cool period between 4200 and 3200 cal BP was a transition towards the formation of a freshwater lagoon and can be related to a decline of the Japan Late Jomon transgression (Sakaguchi, 1983). Between 3200 and 2800 cal BP the lagoon separated from the marine environment in response to a further sea level decrease during the Japan Latest Jomon cold stage and regression. The following increase in the share of broad-leaved pollen indicated a slight warming (Yayoi transition stage) that was interrupted by a short-term cooling spell between 1500 and 1400 cal BP (cold Japan Kofun stage). The period between ca 1100 and 800 cal BP can be related to the European Medieval Climate Anomaly (MCA) or relatively dry Japan Nara-Heian-Kamakura warm stage. The Little Ice Age cooling and Edo regression were evident after ca 800 cal BP. Modern warming however is not well seen in the investigated core.

A multi-proxy reconstruction of the late Holocene climate evolution in the Kapsabet Swamp, Kenya (East Africa)

Multi-proxy high-resolution records relating to climate and dominant vegetation cover have been obtained from a peat deposit retrieved from the Kapsabet swamp in western Kenya. The 4-m long peat sequence provided a continuous sedimentation record spanning ca. 3023 cal yr BP to the present and is representative of the late Holocene changes in the relatively high-altitude part of the Lake Victoria catchment. Paleoenvironmental factors influencing peat formation and organic matter (OM) source inputs in Kapsabet were reconstructed based on total organic carbon, carbon accumulation rate, δ13C, δ15N, C/N ratio, and specific biomarker-based n-alkane ratios. The Kapsabet peat sequence was divided into five stages based on different climatic conditions: Stage 1 (3023–1670 cal yr BP) represents a progression from cool dry to wet conditions. Stage 2 (1670–1187 cal yr BP) is a relatively warm and wet period with increased productivity and high OM input. Stage 3 (1187–625 cal yr BP) represents gradual warming coeval to the late Holocene Medieval Warm Period. Stage 4 (625–188 cal yr BP) is a cool and wet period with high variability in precipitation and hydrological conditions representing the Little Ice Age. Stage 5 (188 cal yr BP to present) represents a relatively cool and wet period that coincides with the expansion of agriculture, particularly in the lowlands. The proxies indicate a progressive change from a forested landscape to an open woodland coeval to a decline in terrigenous inputs and the advent of more wet conditions. The climate on the highlands was less variable than in the lowlands, which underwent several periods of drought and intermittent wet conditions. The changes in the catchment coincided with the expansion of agriculture and land clearance marking increased human activities in the lowlands. Overall, the Kapsabet peat sequence tracks the regional climatic changes in East Africa and marks a promising sedimentary archive for palaeoclimate reconstruction from a region with a paucity of palaeoenvironmental and palaeovegetation histories.