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Past forest fire events and fire frequencies are reconstructed with sediment–charcoal records at lake catchment spatial scales. Few quantitative palaeofire analyses exist in tropical montane forests, where fire return intervals are long (decadal and centennial scales) because of the infrequency of fire weather and fuel conditions. Fire return intervals are a key characteristic of fire regimes and changing fire frequencies rapidly alter land cover compositions and vegetation structure. Charcoal records from small lakes with relatively small catchments covered with dense forest provide an opportunity to reconstruct low‐frequency, high‐severity fires through a time series decomposition approach to identify charcoal peaks above a varying background rate as a proxy for palaeofire events. The sediment core from Rumuiku wetland on Mount Kenya, equatorial eastern Africa, accumulated a nearly linear age–depth model and provided a high temporal resolution (10 years cm–1) sieved charcoal count record (>125 µm). Pollen analysis showed a significant change in montane forest assemblage occurred at 21 200 cal a bp from a montane forest with abundant Podocarpus and Juniperus to a forest with more abundant Hagenia. This change in forest altered the vegetation composition and structure with concomitant changes to the fire regime. Forest biomass in the Hagenia forests decreased and it is likely that fire activity qualitatively changed toward lower intensity and lower severity fires. The quantitative fire event reconstruction focuses on the interval from 27 000 to 16 500 cal a bp and the older montane forest that experienced higher severity fires from 27 000 to 21 200 cal a bp, which reconstructed a temporally heterogeneous fire regime with fire return intervals that ranged from 30–430 years and a mean of 120 years (median 160 years) in the catchment. These are the first estimates of fire return intervals of mountain forests in eastern Africa. We then explore the potential for further comparative research and incremental research contributions to improve quantitative and qualitative palaeofire research in tropical forest ecosystems. We discuss the potential to use these types of data for characterizing variables of fire regimes prior to ostensibly significant modification by anthropogenic activity as well as during the recent past as human land use pressures increased within Afromontane forests. [ABSTRACT FROM AUTHOR]

Isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs) are commonly preserved molecular biomarkers of archaea whose distributions can be used to reconstruct past temperature, and possibly, methane and nitrogen cycling. To date, iGDGT systematics have not been widely investigated in Arctic lacustrine environments. Here, we analyze iGDGTs in sediments of Lake El'gygytgyn, located in the Russian Arctic, to reconstruct the paleoenvironmental conditions from the Pliocene to today using TEX86 and other indices. The TEX86‐inferred temperature history shows a long‐term warming trend, in stark contrast to other Arctic records and other proxies from Lake El'gygytgyn, suggesting that non‐temperature factors obfuscate the use of TEX86 at this site. Other GDGT‐based indices including the BIT Index, ΔRing Index, Methane Index and the GDGT‐0/crenarchaeol ratio suggest that TEX86 is strongly influenced by archaeal community changes. The most significant community shifts are observed c. 2.4 Ma and record an increase in Euryarchaeota production and/or a decrease in Thaumarchaeota production, which was driven by the establishment of permafrost and perennial lake ice during the early Pleistocene. Overall, this study demonstrates an important interpretative framework for iGDGTs in lacustrine systems and describes variations in Arctic climate and lake biogeochemistry over timescales of thousands to millions of years. [ABSTRACT FROM AUTHOR]

The Mau Forest Complex is Kenya's largest fragment of Afromontane forest, providing critical ecosystem services, and has been subject to intense land use changes since colonial times. It forms the upper catchment of rivers that drain into major drainage networks, thus supporting the livelihoods of millions of Kenyans and providing important wildlife areas. We present the results of a sedimentological and palynological analysis of a Late Pleistocene–Holocene sediment record of Afromontane forest change from Nyabuiyabui wetland in the Eastern Mau Forest, a highland region that has received limited geological characterization and palaeoecological study. Sedimentology, pollen, charcoal, X‐ray fluorescence and radiocarbon data record environmental and ecosystem change over the last ~16 000 cal a bp. The pollen record suggests Afromontane forests characterized the end of the Late Pleistocene to the Holocene with dominant taxa changing from Apodytes, Celtis, Dracaena, Hagenia and Podocarpus to Cordia, Croton, Ficus, Juniperus and Olea. The Late Holocene is characterized by a more open Afromontane forest with increased grass and herbaceous cover. Continuous Poaceae, Cyperaceae and Juncaceae vegetation currently cover the wetland and the water level has been decreasing over the recent past. Intensive agroforestry since the 1920s has reduced Afromontane forest cover as introduced taxa have increased (Pinus, Cupressus and Eucalyptus). [ABSTRACT FROM AUTHOR]

The oxygen isotopic composition of Stenomelania gastropod shells was investigated to reconstruct Holocene palaeoclimate change at Lake Kutubu in the southern highlands of Papua New Guinea. Oxygen isotope (δ18O) values recorded in aquatic gastropod shells change according to ambient water δ18O values and temperature. The gastropod shells appear to form in oxygen isotopic equilibrium with the surrounding water and record a shift in average shell oxygen isotopic composition through time, probably as a result of warmer/wetter conditions at ca. 600–900 and 5900–6200 cal a bp. Shorter term fluctuations in oxygen isotope values were also identified and may relate to changes in the intensity or source of rainfall. Further δ18O analyses of gastropod shells or other carbonate proxies found in the Lake Kutubu sediments are warranted. © 2020 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Radiocarbon dating of archaeological carbonates from seven cultural stages of Dholavira, Great Rann of Kachchh (GRK), the largest excavated Harappan settlement in India, suggests the beginning of occupation at ~5500 years BP (pre‐Harappan), and continuation until ~3800 years BP (early part of the Late Harappan period). The settlement rapidly expanded under favourable monsoonal climate conditions when architectural elements such as the Citadel, Bailey, Lower and Middle Town were added between the Early and mid‐Mature Harappan periods. Abundant local mangroves grew around the GRK sustaining prolific populations of the edible gastropod Terebralia palustris. Oxygen isotope (δ18O) sclerochronology of Early Harappan gastropod shell suggests seasonal mixing of some depleted (δ18O ~ −12‰) river water in summer/monsoon months (through ancient Saraswati and/or Indus distributary channels) with seawater that periodically inundated the GRK. Evaporation from this semi‐enclosed water body during the non‐monsoon months enriched the δ18O of water/shell carbonates. The humid fluvial landscape possibly changed due to a catastrophic drought driving the final collapse of the settlement of Dholavira exactly at the onset of the Meghalayan (Late Holocene) stage (~4300–4100 years BP). Indeed, Dholavira presents a classic case for understanding how climate change can increase future drought risk as predicted by the IPCC working group. Copyright © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

ABSTRACT: Reconstructing the deglacial history of palaeo‐glaciers provides vital information on retreat processes, information which can inform predictions of the future behaviour of many of the world's glaciers. On this basis, this paper presents 170 Schmidt Hammer exposure ages from moraine boulders and glacially sculpted bedrock to reveal the post‐Last Glacial Maximum (LGM) history of the Wicklow Mountains, Ireland. These data suggest that large ice masses survived for 4–7 ka after retreat of the Irish Sea Ice Stream and were sustained by summit ice fields until ∼16.6 ka. Post‐LGM retreat was driven by climate and involved numerous short‐term ice front oscillations (≤1 ka), with widespread moraine deposition during Heinrich Stadial 1. In contrast, marked asynchroneity in the timing of Younger Dryas deglaciation is closely linked to snow redistribution which demonstrates the sensitivity of small cirque glaciers (≤1 km2) to local topography. This result has important implications for palaeoclimate reconstructions as cirque glacier dynamics may be (at least partly) decoupled from climate. This is further complicated by post‐depositional processes which can result in moraine ages (e.g. 10Be) which post‐date retreat. Future palaeoclimate studies should prioritize cirques where snow‐contributing areas are small and where post‐depositional disturbance of moraines is limited. [ABSTRACT FROM AUTHOR]

ABSTRACT In the past two decades, much has been learned about the late Quaternary climate history of the Atacama Desert with some details still unclear about the seasonality, timing and extent of wet and dry phases. Modern climate studies reveal that, far from exhibiting a unique pattern, seasonal precipitation originates from many sources and mechanisms. For the last 16 ka, we attempt to sort out these complexities in pollen records from four fossil rodent midden series spanning 22°-25°S in northern Chile. Widespread wet conditions prevailed during the late Pleistocene and early Holocene, particularly between 13 and 9 ka, evidenced by <400 m lowering of pollen zones (plant communities) compared to today. Regional differences in the timing and magnitude of this displacement may be related to the prevailing source (tropical/extra-tropical) or mode (NNW/SE) of tropical precipitation through time. Wet conditions persisted well into the early Holocene, lasting ∼1-1.5 ka longer than previously suggested. The pollen record suggests extreme drying ∼8 ka, possibly associated with a northward shift of the Inter Tropical Convergence Zone, tracking minimum insolation values at subtropical latitudes during the austral summer. The establishment of conditions similar to today happened ∼4 ka. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

ABSTRACT A paucity of empirical non-marine data means that uncertainty surrounds the impact of climate change on terrestrial ecosystems in tropical regions beyond the last glacial period. The sedimentary fill of the Bosumtwi impact crater (Ghana) provides the longest continuous Quaternary terrestrial archive of environmental change in West Africa, spanning the last ∼1.08 million years. Here we explore the drivers of change in ecosystem and climate in tropical West Africa for the past ∼540 000 years using pollen analysis and the nitrogen isotope composition of bulk organic matter preserved in sediments from Lake Bosumtwi. Variations in grass pollen abundance (0−99%) indicate transitions between grassland and forest. Coeval variations in the nitrogen isotopic composition of organic matter indicate that intervals of grassland expansion coincided with minimum lake levels and low regional moisture availability. The observed changes responded to orbitally paced global climate variations on both glacial-interglacial and shorter timescales. Importantly, the magnitude of ecosystem change revealed by our data exceeds that previously determined from marine records, demonstrating for the first time the high sensitivity of tropical lowland ecosystems to Quaternary climate change. [ABSTRACT FROM AUTHOR]

ABSTRACT Branched glycerol dialkyl glycerol tetraethers (brGDGTs) were abundant in surface sediments of freshwater lakes and in catchment soils at altitudes from 10 to 2020 m in New Zealand. Significant differences in brGDGT compositions between lake sediments and soils indicate sources from separate microbial habitats. An expanded modern calibration dataset comprising 33 lakes has enabled a revised calibration function for determining past mean annual air temperature (MAT) from brGDGTs in New Zealand lake sediments: MAT (°C) = −31.664 × MBTm + 16.252 ( n = 30). The function uses a modified methylation index of branched tetraethers (MBTm) that incorporates brGDGT III in the numerator to overcome the lower correlation found between our larger dataset and the unmodified MBT which had been used for previous calibrations. Calibrations combining the cyclization index of branched tetraethers (CBT) and the MBTm or using only certain brGDGTs are possible but have limitations. The revised function shows slightly higher correlation with MAT ( R2 = 0.75) than previous calibrations, which were based on nine sites. The refined calibration function is applied to a ∼16 000-year lake sediment sequence from northern South Island, New Zealand, and yields temperature reconstructions that are consistent with independently derived climate trends from the same sequence. [ABSTRACT FROM AUTHOR]

ABSTRACT Stable oxygen and carbon isotope investigations were conducted on shells of the freshwater gastropod Valvata piscinalis from palaeolake deposits of Holsteinian age (Marine Isotope Stage 11) from eastern Poland. The isotope record of this species was used for the first time as an archive of environmental conditions for selected lakes between the protocratic stage and the post-optimum period of the interglacial. The lowest δ18O and δ13C values of −8.1 and −12.1‰, respectively, were recorded in the Picea-Alnus zone at Ortel Królewski palaeolake. The sparse vegetation and low productivity of that time are reflected in the relatively low δ13C values. The first significant increase of temperature was recorded in the Taxus zone (i.e. an increase in δ18O). The higher temperature influenced the expansion of macrophytes, which stimulated enhanced photosynthesis. Similar mean δ18O values in the Taxus and Pinus-Larix zones imply no distinct temperature change. V. piscinalis populations from Ossówka-Hrud and Roskosz palaeolakes record variations in lake productivity during the climatic optimum of the interglacial ( Carpinus-Abies zone). The δ18O values of that time are relatively high (up to −4.5‰) and reveal no significant differences between the lakes. Decreases of the oxygen isotope values in the Szymanowo lake reflect post-optimum cooling. [ABSTRACT FROM AUTHOR]

ABSTRACT This study investigates hydrological responses to climatic shifts using sediment flux data derived from two dated palaeolake records in south-east Arabia. Flux values are generally low during the early Holocene humid period (EHHP) (∼9.0-6.4k cal a BP) although several short-lived pulses of increased detrital input are recorded, the most prominent of which is dated between ∼8.3 and 7.9k cal a BP. The EHHP is separated from the mid-Holocene humid period (MHHP) (∼5.0-4.3k cal a BP) by a phase of increased sediment flux and aridity, which began between ∼6.4 and 5.9k cal a BP and peaked between ∼5.2 and 5.0k cal a BP. The termination of the MHHP is marked by a phase of high detrital sediment flux between ∼4.3 and 3.9k cal a BP. While long-term shifts in climate are most likely linked to changes in the summer position of the Intertropical Convergence Zone and associated Indian and African monsoon systems, it is noted that the abrupt, short-term phases of aridity observed in both records are coeval with intervals of rapid climate change globally, which triggered non-linear, widespread landscape reconfigurations throughout south-east Arabia. [ABSTRACT FROM AUTHOR]

ABSTRACT Multi-proxy analyses on sediment cores from two closed basin lakes in the Serra Sul de Carajás were used to reconstruct environmental changes in south-eastern Amazonia during the late Quaternary. The sediment record between 65 000 and 40 000 cal a BP (older dates with high level of uncertainty) is dominated by organic sediments intercalated with detritus (e.g. Al and Ti), indicating fluctuating lake levels related to changes in precipitation. A period of lower precipitation occurred from 40 000 to 35 000 cal a BP. Between ∼35 000 and 20 000 cal a BP, the dominance of detrital sediments suggests wetter and warmer conditions. However, iron enrichment in a siderite-rich layer associated with the end of this period coincides with the Last Glacial Maximum, suggesting diagenetic changes in the profundal portion of the lakes and indicating a transition from oxidizing to reducing conditions. Subsequently, the period between ∼20 000 and 15 000 cal a BP was marked by a transition from drier to wetter climate, resulting in higher energy flows (e.g. Zr) and increased erosion of the basins. Another drier period was observed between ∼7000 and 3000 cal a BP (low detrital input and high Ca/Ti), when the lake levels were very low and eutrophication was high. [ABSTRACT FROM AUTHOR]

We present 30 10Be dates on three moraines formed by small glaciers in the southern tropical Andes of Peru to determine the timing of the local Last Glacial Maximum (LGM) and subsequent deglaciation. Two terminal moraines indicate that the local LGM ended at 23.5±0.5 and 21.2±0.8 ka, or well before the end of the global LGM (20-19 ka). A recessional moraine perched at the base of a steep valley wall has a 10Be age of 19.8±0.3 ka, suggesting that glaciers had largely retreated from the area during the earliest stages of the last termination and before the onset of the deglacial CO2 rise and increase in tropical sea surface temperatures. These ages generally agree with other tropical glacial chronologies and reflect a broad synchroneity between low- and high-latitude deglaciation at the LGM, but the trigger for early deglaciation of the tropics remains unclear. [ABSTRACT FROM AUTHOR]

ABSTRACT Over the last 50 years, there have been significant developments in the range and sophistication of the chronological tools now available to Quaternary scientists. Notable milestones include the introduction of new methods (e.g. optically stimulated luminescence, cosmogenic radionuclide dating, ice-layer counting, molecular 'clocks') and the refinement of established techniques (e.g. accelerator mass spectrometry 14C dating, 14C calibration, high-precision uranium-series dating, argon-argon dating, cryptotephra analysis). Developing new techniques and methodologies for measuring Quaternary time is not an end in itself, however: it is stimulated by a parallel need, which is a deeper understanding of the mode, pattern and rates of environmental processes, and how different processes inter-connect. Here we review some of the important limitations that continue to constrain our ability to provide coherent chronologies for Quaternary environmental reconstructions at both millennial and sub-millennial timescales. We focus our discussion on two unique stratigraphic templates: first, the marine oxygen isotope sequence that spans the full Quaternary period; and, second, the Greenland ice-core record, which provides a basis for dating environmental changes through the last glacial cycle. [ABSTRACT FROM AUTHOR]

ABSTRACT We examine the Atlantic maritime vegetation history from Praia Rei Cortiço, Portugal, to understand natural vegetation patterns and infer climatic conditions during the transition from the last interglacial to early glacial periods. Optically stimulated luminescence dating of quartz grains from sediments surrounding the 1.3-m-thick organic deposit date the section to within 120-89 ka. Pollen data were used to reconstruct the vegetation history and determine a range of possible past climates using an ensemble of seven modern analogue approaches. A five-part environmental sequence transitioned from heath-pine-dominated communities, deciduous hardwood forests and pine forests to heathlands surrounded by steppe-like environments. Mixed temperate and Mediterranean forest elements existed during the last interglacial period at Praia Rei Cortiço, suggesting a proximal limit to these floras. Abundant Fagus pollen (>6%) suggests local beech populations existed within maritime forests of the Iberian Atlantic margin, representing a greater distribution of this genus during interglacial periods than has previously been recognized. Climate reconstructions indicate that variations in moisture availability may have been a major driver of the observed environmental changes. [ABSTRACT FROM AUTHOR]