Showing papers on "Holocene climatic optimum published in 2022"

Northern Hemisphere vegetation change drives a Holocene thermal maximum

Abstract: The Holocene thermal maximum, a period of global warmth evident in early to mid-Holocene proxy reconstructions, is controversial. Most model simulations of the Holocene have not reproduced this warming, leading to a disagreement known as the Holocene Temperature Conundrum. Pollen records document the expansion of vegetation in the early and mid-Holocene African Sahara and Northern Hemisphere mid- and high latitudes, which has been overlooked in previous modeling studies. Here, we use time slice simulations of the Community Earth System Model to assess the impact of Northern Hemisphere vegetation change on Holocene annual mean temperatures. Our simulations indicate that expansion of Northern Hemisphere vegetation 9000 and 6000 years ago warms Earth’s surface by ~0.8° and 0.7°C, respectively, producing a better match with proxy-based reconstructions. Our results suggest that vegetation change is critical for modeling Holocene temperature evolution and highlight its role in driving a mid-Holocene temperature maximum.

Pollen Data as a Temperature Indicator in the Late Holocene: A Review of Results on Regional, Continental and Global Scales

Abstract: The temperature history of the Holocene is intensely debated, with both proxy seasonality effects and climate model sensitivity being proposed as possible reasons for the “Holocene temperature conundrum”. However, possible human impacts on terrestrial proxies are often not adequately considered, especially for the late Holocene. Based on a review of pollen-based and pollen-dominated temperature records on regional, continental and global scales, we propose that pollen may be a poor indicator of temperature change in late Holocene, mainly as a result of the impacts of intensified human activities on terrestrial vegetation. Thus, we suggest that possible human impacts on terrestrial Holocene temperature records should be carefully evaluated; and also that more Holocene temperature records are obtained from areas without significant direct human impacts, or that proxies that are insensitive to human impacts are used for late Holocene climatic reconstruction.

Evidence for a Relatively Warm Mid‐to Late Holocene on the Southeastern Tibetan Plateau

Abstract: The Holocene temperature discrepancy between paleoclimate reconstructions and climate model simulations—known as the Holocene temperature conundrum—calls for new high-quality Holocene temperature records at high elevations. Here, we present a quantitative Holocene mean annual air temperature record based on a site-specific branched glycerol dialkyl glycerol tetraethers calibration from a small remote alpine lake on the southeastern Tibetan Plateau. The record reveals a temperature history comprising a relatively cool early Holocene (before 7 ka) followed by a warmer mid- to late-Holocene (after 7 ka), which was likely linked to increasing local annual insolation and greenhouse gases. Three cold events punctuated the general warming trend ca. 10.4 ka, 3.7 ka, and 1.7 ka BP, and correspond closely in time to ice rafting events in the North Atlantic, and to episodes of volcanism and/or unusual solar activity. The entire Holocene temperatures are cooler than the previously identified anthropogenic warming from 1990–2015 AD.

The Molecular Composition of Humic Acids in Permafrost Peats in the European Arctic as Paleorecord of the Environmental Conditions of the Holocene

Abstract: The purpose of our research is focused on examination of the transformation regularities of molecular composition of humic acids (HAs) in the hummocky frozen peatlands of the European Arctic as a marker of climatic changes in the Holocene, and assessment of the stabilization of soil organic matter under the conditions of modern climatic warming. Histosols located in the two subzones of the European Arctic served as the research subjects. This territory is actively used for reindeer breeding, which is a vital agricultural branch in the Far North of the Russian Federation. The data obtained reveal the main trends in the formation of HAs from Arctic peatlands under different environmental conditions. Modern peat sediments (top layers) in the middle and late Holocene period formed out of bryophyte residues and contained HAs with long-chain carbohydrate and paraffin structures in their composition. These structures enlarged the dynamic radii of HA molecules, and, thus, caused high average molecular weight values. The more favorable climatic conditions of the early Holocene (the Atlantic optimum) defined the botanical composition of peat, which was dominated by tree and sedge communities with high contents of lignin components and, as a consequence, a larger share of aromatic fragments, characterized by thermo-biodynamic resistance in HAs of horizons in the lower and central profile parts. The molecules of HAs are an archive of paleoclimatic records. The Subboreal and Subatlantic climatic conditions determined the specifics of vegetation precursors and, as a result, the molecular structure of HAs in seasonally thawed layers, with a predominance of long-chain aliphatic fragments. The conversion of HAs from Histosols led to an increase in the proportion of carbon in branched and short-chain paraffinic structures with their subsequent cyclization and aromatization. The results of this process are most clearly manifested in layers formed during the Holocene I and II climatic optima. Higher biologically active temperatures of the seasonally thawed layer of soils at bare spots (without vegetation) determined the accumulation of thermodynamically more stable HA molecules with a high content of aromatic fragments. This contributed to both the stabilization of the SOM and the conservation of peatlands in general.

Reconstructing Holocene temperatures in time and space using paleoclimate data assimilation

Abstract: Abstract. Paleoclimatic records provide valuable information about Holocene climate, revealing aspects of climate variability for a multitude of sites around the world. However, such data also possess limitations. Proxy networks are spatially uneven, seasonally biased, uncertain in time, and present a variety of challenges when used in concert to illustrate the complex variations of past climate. Paleoclimatic data assimilation provides one approach to reconstructing past climate that can account for the diverse nature of proxy records while maintaining the physics-based covariance structures simulated by climate models. Here, we use paleoclimate data assimilation to create a spatially complete reconstruction of temperature over the past 12 000 years using proxy data from the Temperature 12k database and output from transient climate model simulations. Following the last glacial period, the reconstruction shows Holocene temperatures warming to a peak near 6400 years ago followed by a slow cooling toward the present day, supporting a mid-Holocene which is at least as warm as the preindustrial. Sensitivity tests show that if proxies have an overlooked summer bias, some apparent mid-Holocene warmth could actually represent summer trends rather than annual mean trends. Regardless, the potential effects of proxy seasonal biases are insufficient to align the reconstructed global mean temperature with the warming trends seen in transient model simulations.

Antarctic environmental change and ice sheet evolution through the Miocene to Pliocene – a perspective from the Ross Sea and George V to Wilkes Land Coasts

Abstract: The Miocene to Pliocene (Neogene) occurred between 23.04 and 2.58 million years ago and includes intervals of peak global warmth where Earth’s average surface temperature was up to 8℃ warmer than present. Major cooling steps also occurred, across which Antarctica’s ice sheets advanced to the continental shelf for the first time and sea ice expanded across the Southern Ocean. Knowledge of Antarctic environmental change and ice sheet variability through this dynamic period in Earth history has advanced over the past 15 years. Major field and ship-based efforts to obtain new geological information have been completed and significant advances in numerical modelling approaches have occurred. Integration of ice proximal data and coupled climate-ice sheet model outputs with high-resolution reconstructions of ice volume and temperature variability from deep sea δ18O records now offer detailed insight into thresholds and tipping points in Earth’s climate system. Here we review paleoenvironmental data through key episodes in the evolution of Neogene climate to include the Miocene Climatic Optimum (MCO), Middle Miocene Climate Transition (MMCT), Tortonian Thermal Maximum (TTM), Late Miocene Cooling (LMC), and Pliocene Warm Period (PWP). This review shows that Antarctica’s climate and ice sheets remained dynamic throughout the Neogene. Given the analogous nature of warm episodes in the Miocene and Pliocene to future projections, the environmental reconstructions presented in this chapter offer a stark warning about the potential future of the AIS if warming continues at its current rate. If average global surface warming above pre-industrial values exceeds 2℃, a threshold will be crossed and AIS instabilities would likely be irreversible on multi-century timescales.

Holocene Aeolian Activity Recorded by Mountain Paleosols, Gonghe Basin, Northeast Qinghai-Tibet Plateau

Abstract: The Gonghe Basin (GHB) on the northeastern Qinghai-Tibet Plateau (NE-QTP) is sensitive to climatic change due to the interplay of the Asian summer monsoon and the westerlies. Extensive aeolian sediments in the basin represent important archives of regional environmental evolution. However, the paleosol development timing is still not clear because of limited number of optically stimulated luminescence (OSL) sampling and dating, which restricts our understanding of past aeolian activities during the Holocene in GHB. In this study, a loess-paleosol section, Najiao (NJ), from the southeastern margin of GHB was investigated. Eighteen OSL samples were obtained from the 400-cm section in order to construct a high-resolution chronological framework. Paleoenvironmental proxies including grain size distribution (GS), magnetic susceptibility (MS), total organic carbon (TOC), and geochemical elements were measured to reconstruct the Holocene aeolian activity. Results show a successive accumulation from Early to Middle Holocene at NJ section, but a c. 3 ka sedimentary hiatus is found between c. 5 and 1.5 ka. Paleosol ages are constrained by high-resolution OSL ages which are from c. 7–5 ka. Consistent with previous studies, strong aeolian activities occurred in GHB during the Early Holocene (c. 13–9 ka), indicating dry climate conditions. Initiation of pedogenesis was at c. 9 ka, and the intensified soil development and lowest aeolian activity were between c. 7 and 5 ka. The increased sand content after c. 1.5 ka indicates enhanced human activities in the interior of GHB in the Late Holocene.

Regional pollen-based Holocene temperature and precipitation patterns depart from the Northern Hemisphere mean trends

Abstract: Abstract. A mismatch between model- and proxy-based Holocene climate change, known as the Holocene conundrum, may partially originate from the poor spatial coverage of climate reconstructions in, for example, Asia, limiting the number of grid-cells for model-data comparisons. Here we investigate hemispheric, latitudinal, and regional mean time-series as well as anomaly maps of pollen-based reconstructions of mean annual temperature, mean July temperature, and annual precipitation from 1676 records in the Northern Hemisphere extratropics. Temperature trends show strong latitudinal patterns and differ between (sub-)continents. While the circum-Atlantic regions in Europe and eastern North America show a pronounced mid-Holocene temperature maximum, western North America shows only weak changes and Asia mostly a continuous Holocene temperature increase but with strong latitudinal differences. Likewise, precipitation trends show certain regional peculiarities such as the pronounced mid-Holocene optimum between 30 and 40° N in Asia and Holocene increasing trends in Europe and western North America which can all be linked with Holocene changes of the regional circulation pattern linked to temperature change. Given a background of strong regional heterogeneity, we conclude that the calculation of global or hemispheric means which initiated the Holocene conundrum debate should focus more on understanding the spatio-temporal patterns and their regional drivers.

Multiproxy investigation of the last 2,000 years BP marine paleoenvironmental record along the western Spitsbergen margin

Abstract: ABSTRACT A reconstruction of the last 2,000 years BP of environmental and oceanographic changes on the western margin of Spitsbergen was performed using a multidisciplinary approach including the fossil assemblages of diatoms, planktic and benthic foraminifera and calcareous nannofossils and the use of geochemistry (X-ray fluorescence spectroscopy, X-ray diffraction). We identified two warm periods (2,000–1,600 years BP and 1,300–700 years BP) that were associated with the Roman Warm Period and the Medieval Warm Period that alternate with colder oceanic conditions and sea ice coverage occurred during the Dark Ages (1,600–1,300 years BP) and the beginning of the Little Ice Age. During the Medieval Warm Period the occurrence of ice-rafted debris and Aulocoseira spp., a specific diatom genus commonly associated with continental freshwater, suggests significant runoff of meltwaters from local glaciers.

Holocene Millennial‐Scale Precipitation Variations in the Asian Summer Monsoon Margin of Northwest China and Their Relation to Migrations of Monsoon Northern Boundary via Oxygen Isotope Analysis of Calcareous Root Tubes in Deserts

Abstract: The Asian summer monsoon (ASM) margin of northwest China is a critical region for studying Asian monsoon variability during the Holocene. However, the pattern of precipitation variations and migration of the ASM northern boundary in arid regions during the Holocene remains unclear. In this study, we analyzed oxygen isotopes of 70 calcareous root tube (CRT) samples from the desert hinterlands in the ASM margin of northwest China, including the Badain Jaran, Tengger, Ulan Buh, and Kubuqi deserts. The CRT samples were analyzed to reconstruct millennial-scale precipitation variation patterns during the Holocene and further explored the location of the ASM northwestern boundary during the middle Holocene. Our results demonstrated that the δ18O values of CRTs during the late Holocene (4.0–1.0 cal kyr BP) were all less negative than those during the middle Holocene (8.0–4.0 cal kyr BP), which revealed that the precipitation decreased from the middle Holocene to the late Holocene. Moreover, an increase in δ18O values from the southeast to northwest revealed that the precipitation decreased. We argued that the Holocene millennial-scale precipitation changes in this region were influenced by the migrations in the ASM northern boundary. During the middle Holocene, due to the strong summer solar radiation and reduced ice sheets, the ASM northern boundary in the arid region advanced by at least 250 km to the northwest, which resulted in increased precipitation. During the late Holocene, the ASM northern boundary retreated southeastward, resulting in decreased precipitation.

Simulation of Holocene soil erosion and sediment deposition processes in the Yellow River basin during the Holocene

Abstract: The Yellow River Basin is ideal for studying fluvial landscape evolution under climate change and human activities. Quantitative reconstruction of soil erosion, sediment transport, and delta deposition in the basin enhances understanding of the dynamic mechanisms of fluvial surface processes. This study reconstructs the spatial and temporal distribution of precipitation and vegetation cover based on geological profile records, paleoclimatic datasets, and historical documentation in the Yellow River basin during the Holocene. A landscape evolution model (Landlab) is introduced to simulate soil erosion and sediment deposition processes. Modeling results show that soil erosion intensity in the middle reaches of the Yellow River was weak during the early and middle Holocene. Since the late Holocene, soil erosion has increased due to vegetation degradation, as the climate became drier, but with more fluctuations. Since 2.0 ka (1 ka = 1,000 calibrated a B.P.), and especially since 1.0 ka, soil erosion in the middle reaches of the Yellow River increased rapidly due to strong human activity, with the intensity-six times higher than the average level before the late Holocene. Average sediment deposition thickness in the lower reach of the Yellow River (North China Plain) was about 2.4 m, and sediment volume was about 3.6 × 1011 m3. The deposition rate and soil erosion intensity showed synchronous changes during the Holocene. Since 2.0 ka, the sediment load and deposition rate have increased rapidly, to 6–10 times higher than in the early and middle Holocene. On this basis, the effects of climate change and human activities on the evolution of the fluvial landscape are discussed.

Climate and ice conditions of East Siberian Sea during Holocene: Reconstructions based on sedimentary geochemical multiproxy

Abstract: Reconstruction of ice conditions and climate of the East Siberian Sea during the Late Middle Holocene was implemented for the first time using a transfer function method based on marine sedimentation proxies. Transfer functions were developed based on a hydrometeorological time series of changes in mean annual air temperature (ΔT10) and the ice-free period (IF10) for the last 70–90 years and a geochemical time series of bottom sediments accumulated during this time, which were obtained using a submillimeter scanning of the core in the X-ray fluorescence analysis of synchrotron radiation. Geochemical time series of Holocene deposits were obtained from X-ray fluorescence analysis of the core in a step of 1–2 cm. Reconstruction of ice conditions and average annual air temperature revealed a decreasing trend of ΔT10 and IF10, which started from the middle Holocene up to the Little Ice Age due to an orbitally determined decrease in insolation. A synchronous periodicity of changes in ice cover and air temperature was revealed and approximated by periods of 1600 years for IF10 and 1740 years for ΔT10, which are comparable to Bond’s climatic cycles. These results indicates the telecommunication of the Atlantic processes, climate of the East Siberian Sea, and predominance of the cyclonic type of atmospheric circulation during the Holocene.

The Holocene hypsithermal in the Australian region

Abstract: Close examination of key and well-dated Holocene sites, both on land and at sea in the Australian region indicate that at the very beginning of the Holocene, as a result of strong westerlies, there must have been a continuous positive Southern Annular Mode (SAM). Following from that, the entire region switched to a negative SAM scenario and, during that time, the westerlies must have retreated further south. Afterwards, a period of time spaning ∼8200 to ∼5500 years ago temperatures were higher than today. We refer to it as the Holocene Hyspithermal . Coincident to this period, lake levels and postulated rainfall were extraordinarily high and vegetation spectra in places very different compared to today. The extent of this period varies by a few centuries between sites, but this may result from the level of resolution and also appears to be controlled by latitude. There is also clear indication that the influence of the westerlies was reduced over Australia during those two and a half millennia. Nevertheless, air temperatures recognised in Antarctic ice cores are at the opposite to those recognised in Australia. In addition, during the Australian Holocene Hypsithermal, CO 2 levels were at their lowest in Antarctic ice cores. Climatic conditions then progressively deteriorated everywhere a bit after ∼6000 years BP until recent times as ENSO signals with alternating El Niño and La Niña conditions across the entire Pacific region as already described by Perner et al. (2018) based on the same cores studied here. Brief mention is also made to the presence of humans in SE Australia during the Holocene. It seems that human activities changed well after the period of high temperatures and rainfall, with more sedentary activities along the major rivers, with an enhancement of food production in organized settings suggestive of villages.

Holocene climatic record of Ladakh, Trans-Himalaya

Abstract: A composite and complete picture of the Holocene climatic variation and forcings is presented using the sediment archive and multiproxy analysis. Five prominent arid phases mark the Holocene period in Ladakh (∼10,800–10,000; ∼8800–8600; a longer arid phase at ∼5200–2600 with increasing aridity toward the top part; ∼1700–1500 and ∼500 cal yr BP) intervened by comparatively warmer conditions in between. Two sources of precipitation (westerlies and ISM) are responsible for governing the hydroclimate of the region. The westerly dominates in the beginning of Holocene while the mid-Holocene sees the advent of ISM to the region. Westerlies further take over from mid-Holocene to ∼3200 cal yr BP and from this period onward the ISM again seem to dominate the hydroclimate of the region. The lake records and other published records from the region show that this change in the moisture source has occurred several times in the Holocene—7200, 5200, and 2600 cal yr BP although the westerly has been a dominant precipitation source in the Ladakh region through most of Holocene.