mid-Holocene Optimum Research Articles

The Possible Stimulation of the Mid-Holocene Period’s Initial Hydrological Recession on the Development of Neolithic Cultures along the Margin of the East Asian Summer Monsoon

A better understanding of past East Asian summer monsoonal (EASM) variations, which play a key role in the development of the largely rain-watered agriculture in China, could contribute to better appraising potential impacts on EASM with regard to global climate change. However, our knowledge of the relationship between mid-Holocene hydrological recession and the development of Neolithic culture is limited due to a lack of joint studies and a compilation of spatiotemporal data, especially on the episode of ~6–5 ka from the mid-Holocene Optimum (HO) along the peripheral realm of the EASM. Here, we suggest that the hydrological recession between ~6–5 ka, on the basis of lithology and geochemical element analysis, occurred not only in the Horqin sandy land, but also in other fluvial-lacustrine, stalagmitic, loess, and aeolian records across the whole monsoon-influenced boundary belt. These records indicated varied, more or less synchronous, and coherent moisture changes, yet with not entirely consistent onsets, durations, and degrees. We attributed this spatiotemporal complexity to the orbit-induced weakening of summer solar insolation, and the interactions of the Asian monsoon (AM) and westerlies, as well as topography and regional vegetation factors. Furthermore, the mid-Holocene initial hydroclimatic recession during ~6–5 ka within the thresholds of an eco-environment bearing a capacity system, might have facilitated the development of mid–late Neolithic culture and stimulated the north and northwest expansion and integration of region-specific Neolithic culture.

Comparison of climate change from Cenozoic surface uplift and glacial-interglacial episodes in the Himalaya-Tibet region: Insights from a regional climate model and proxy data

Abstract Assessing paleo-climatic changes and the underlying driving mechanisms are an essential (and often poorly understood) first-step for understanding if natural variability in Earth's climate system from tectonic processes and orbital forcing could produce observed changes in surface processes. In this study, we take this first step of evaluating climate change in the Tibetan Plateau region for different distinct climate states. We do this using a high-resolution regional climate model parameterized for the Cenozoic rise of the Plateau and prominent Quaternary glacial and interglacial episodes. The main objective is to delimit the range of climate variability due to important natural drivers in the region by comparing climate changes during the main Cenozoic uplift period with climate anomalies during the last glacial maximum and the mid-Holocene optimum. This helps to interpret environmental changes documented by proxy data and to benchmark man-made climate changes expected during the 21st century. The innovative aspects of this study pertain to the use of a consistent high-resolution modeling framework and a multivariate statistical assessment of climate types and their shift during the various paleo-climatic episodes. Reduced plateau elevation leads to regionally differentiated patterns of higher temperature and lower precipitation amounts on the plateau itself, whereas surrounding regions are subject to colder conditions. In particular, Central Asia receives much more precipitation prior to the uplift, mainly due to a shift of the stationary wave train over Eurasia. Cluster analysis indicates that the continental-desert type climate, which is widespread over Central Asia today, appears with the Tibetan Plateau reaching 50% of its present-day elevation. The mid-Holocene is characterized by slightly colder temperatures, and the last glacial maximum by considerably colder conditions over most of central and southern Asia. Precipitation anomalies during these episodes are less pronounced and spatially heterogeneous over the Tibetan Plateau. The simulated changes are in good agreement with available paleo-climatic reconstructions from proxy data. The present-day climate classification is only slightly sensitive to the changed boundary conditions in the Quaternary. However, it is shown that in some regions of the Tibetan Plateau the climate anomalies during the Quaternary have been as strong as the changes occurring during the uplift period.

Early-Holocene monsoon instability and climatic optimum recorded by Chinese stalagmites

The timing and duration of the Holocene East Asian summer monsoon (EASM) maximum and the interpretation of Chinese stalagmite δ18O records have long been disputed. Notably, interpretations of Holocene EASM variations are frequently based on a single record or study area and are often contradictory. In this study, we conducted stable isotope analyses of four Holocene stalagmites from Chongqing, southwest China. The results reveal differences in the timing of the Holocene EASM maximum and to try to resolve the inconsistency we analyzed and statistically integrated a total of 16 Holocene stalagmite records from 14 caves in the EASM region. The resulting synthesized Holocene stalagmite δ18O (δ18Osyn) record is in agreement with other EASM records and confirms that stalagmite δ18O records are a valid indicator of EASM intensity, rather of local precipitation amount. The δ18Osyn record shows that the EASM intensified rapidly from the onset of the early Holocene; notably, however, there were distinct EASM oscillations in the early Holocene, consisting of three abrupt millennial-scale events. This indicates that, contrary to several previous interpretations, the early Holocene EASM was unstable. Subsequently, during 8–6 kyr BP, the EASM was relatively stable and strong, with the strongest monsoon occurring during 8–7 kyr BP. This evidence of a stable and strong mid-Holocene EASM in eastern China is in accord with the classical view of a mid-Holocene Optimum in China.

Distribution of Trace Elements in Holocene Loess-paleosol Sequence and Environmental Change in Lower Reaches of The Yellow River

Based on field investigation, samples in loess-paleosol sequence profile were collected systematically at Longshan site in Shangdong province in lower reaches of The Yellow River. Trace elements(Mn, Cu, Ba, Ni, Pb, Rb, Sr, Ti, V and Zn) were measured and analyzed systematically by X-Ray and compared with climate substitution index(grain size, Rb/Sr, Ba/Sr). The results as follows:(1) According to the significant correlation between the content of Ti, Ba, Cu, Ni, V, Zn, Rb, Pb, Sr in the LS loess-soil sequence and the index of soil strength (soil grain size, Rb/Sr, Ba/Sr), it can be concluded that the migration and change of trace elements in Holocene period were mainly affected by the environmental evolution of this period. Mn is insensitive to climate change in this profile. The content of Pb in the surface layer of soil is the highest, which may be caused by human activities. (2) The contents of Ti, Ba, Cu, Ni, V, Zn and Rb in the paleosol( S0 ) were accumulated accompanied by the clay(< 5μm) increase while the coarse silt(10∼50μm) reduced, suggesting the intensified pedogenic modification to the accumulated dust when it was humid climate. It implied that the climate was warm and humid during the mid-Holocene optimum. (3) Content of Ti, Ba, Cu, Ni, V, Zn, Rb and clay(< 5μm)in loess layer (Lt, L1, L0) was lower and coarse silt (10-50μm) content was higher when compared with the paleosol layer (S0), suggesting that the intensity of soil formation was weak and aeolian sand activity was frequent. It further indicated that the paleoenvironment was cold and dry with frequent dust storms during the periods of loess formation.

Phytolith-based environmental reconstruction from an altitudinal gradient in Mpumalanga, South Africa, 10,600 BP–present

Studying vegetation change across biome boundaries provides insight into vegetation resilience. In this study, shifts in grassland composition are reconstructed from sediments in three wetland sites across altitudinal gradient from 2128 to 897 m.a.s.l., representing a gradient from the grassland biome to the grassland/savanna boundary in the Mpumalanga region, north-eastern South Africa. Phytolith records from Verloren Valei (dated from 10,600 BP), Graskop (dated from 6500 BP) and Versailles (dated from 4500 BP) are used to reconstruct shifts in grassland composition and vegetation change. Phytolith morphotypes are used to construct environmental indices that are correlated with pollen main ecological groups, charcoal and δ13C and C/N ratio. The results are compared to available regional paleoclimate data. Both Verloren Valei and Graskop have been dominated by grassland, but Versailles show a stronger influence of bushveld/savanna pollen. Phytolith data suggest that grassland composition was stable at Versailles and Graskop, but grassland at Verloren Valei has changed significantly over time. The early Holocene was dominated by a Pooideae/Chloridoideae C3 and C4 grassland, probably a remnant of the earlier Pleistocene cool-dry conditions. After 8500 BP grassland composition changed gradually to a Chloridoideae and Panicoidea dominated C4 grassland BP, and finally a moist Cyperaceae and Panicoidea dominated C3/C4 grassland after 4000 BP. This shift possibly occurs as a delayed response to the warmer and wetter conditions of the mid Holocene optimum at this high altitude site. The results suggest that the grassland/savanna boundary has remained stable over time, indicating considerable resilience of grasslands to climate change. This resilience may be related to the turnover of species within the grassland biome, as indicated by shifts between 8500 and 4000 BP at Verloren Valei.

Holocene human adaptation in the Namib Desert: A model based on the concept of Holling's loop

Climatic amelioration during the mid-Holocene Optimum is associated with hunter-gatherer occupation of remote sites in the Namib Desert. Subsequent changes in late Holocene site distribution suggest there were alternative responses to increasing aridity during this period. Abandonment or episodic occupation is evident in some areas, while others show an emphasis on mountain refugia and resource anomalies. Specialized coping strategies developed during this time allowed a broad re-occupation of the desert when conditions improved briefly during the Medieval Warm Epoch. Successive patterns of settlement and subsistence in the Namib Desert Holocene sequence exemplify the four moments of Holling's adaptive cycle.

High-resolution palynological record of Holocene climatic and oceanographic changes in the northern South China Sea

For the first time, palynological records of terrestrial palynomorphs and dinoflagellate cysts are investigated in a sediment core from the northern South China Sea (SCS) covering the last 12,500years. Both terrestrial and marine palynomorph records show strong signals of the sea-level change during the studied interval. The highest herb pollen content was associated with extensive grasslands on the exposed shelf at the low sea-level stand during the Younger Dryas and early Holocene. The increase in fern spores and decrease in concentrations of dinoflagellate cysts and terrestrial palynomorphs was observed during the sea-level rise interval from 12,500 to ~6800 (or 6000) calyr BP. Then, the sea level became stabilized and consistently low dinoflagellate cyst abundances and high abundances of fern spores were recorded.A high abundance of Impagidinium in the period ~12,000–10,400calyr BP possibly resulted from increased input of western Philippine Sea waters into the SCS and the branching of the Kuroshio Current. A short-term decrease of Impagidinium at ~11,700–11,000calyr BP corresponding to the MWP-1B event might be associated with input of the East China Sea waters through the Taiwan Strait.The relationship between the sedimentation rates and the concentrations of terrestrial palynomorphs indicates a water-dominant transport for pollen and spore dispersal prior to ~6300calyr BP, whereas wind transport became more prominent thereafter. The timing of this change corresponds to the highest sea-level stand at ~6800–6000calyr BP, when the present oceanographic setting was formed. The mid-Holocene Optimum can be seen by the highest abundance of subtropical-tropical broad-leaved arboreal pollen and by the highest abundances of Dapsilidinium pastielsii. Three strengthened winter monsoon intervals at ~5500calyr BP, 4000calyr BP, and 2500calyr BP are reflected by increases in Pinus pollen content after the present oceanographic condition formed.

Palaeoenvironmental reconstruction based on charophytes and sedimentology: Can the mid-Holocene Optimum be recognised in western Argentina?

We sought to recognise to which extent the mid-Holocene Optimum can be identified in the Río Jarilla region of western Argentina, situated at the boundaries of the South American Arid Diagonal. The palaeoenvironmental conditions of the Arco del Desaguadero Formation, formed by a sedimentary sequence of late Pleistocene to middle Holocene age, were reconstructed based on charophyte assemblages and sedimentology. Three fluvio-lacustrine cycles were recognised with evaporitic layers at the top of each, reflecting the high temperatures that characterise the Holocene. For the second cycle, three 14C dates were obtained, ranging from ca. 11 to 7ka BP which, together with sedimentology and the palaeontological record allowed us to infer a warm and semiarid period assigned to the mid-Holocene Climatic Optimum. The gyrogonites found in the sediments belong to Chara cf. contraria, Chara cf. papillosa, Chara halina and Chara hornemannii, species related to extant species from Argentina. The presence of C. hornemannii imbedded in sedimentary rocks (evaporites) reflects higher temperatures in the past, although further research is needed to establish the significance of this species for reconstructing Quaternary climate.

A technique for generating consistent ice sheet initial conditions for coupled ice sheet/climate models

Abstract. A transient technique for generating ice sheet preindustrial initial conditions for long-term coupled ice sheet/climate model simulations is developed and demonstrated over the Greenland ice sheet using the Community Earth System Model (CESM). End-member paleoclimate simulations of the last glacial maximum, mid-Holocene optimum and the preindustrial are combined using weighting provided by ice core data time series to derive continuous energy-balance-model-derived surface mass balance and surface temperature fields, which are subsequently used to force a long transient ice sheet model simulation of the last glacial cycle, ending at the preindustrial. The procedure accounts for the evolution of climate through the last glacial period and converges to a simulated preindustrial ice sheet that is geometrically and thermodynamically consistent with the preindustrial CESM state, yet contains a transient memory of past climate. The preindustrial state generated using this technique notably improves upon the standard equilibrium spin-up technique, relative to observations and other model studies, although in the demonstration we present here, large biases remain due primarily to climate model forcing biases. Ultimately, the method we describe provides a clear template for generating initial conditions for ice sheets within a fully coupled climate model framework that allows for the effects of past climate history to be self-consistently included in long-term simulations of the fully coupled ice sheet/climate system.

Basin-wide Holocene environmental changes in the marginal area of the Asian monsoon, northwest China

In arid regions, because of spatial variability, using single climate records is difficult to reconstruct the past climate change for the drainage basins. Holocene environmental records were collected from the upper, middle and lower regions of the Shiyang River drainage basin in the marginal area of the Asian monsoon (northwest China). The main objective of this paper was to compare the records from the terminal lake and the middle and upper reaches of the basin to study the basin-wide environmental changes. During the early Holocene the vegetation was sparse, and the effective moisture was relatively low in the basin. The Holocene Climatic Optimum started between 7.0 and 8.0 cal ka BP, during which the lake level reached the highest level in the terminal lake; the vegetation density and the effective moisture reached the highest level during the Holocene in the drainage basin. From 4.7 cal ka BP the terminal lake began to shrink, while the vegetation density decreased dramatically. In the middle and upper regions of the drainage, the effective moisture began to decrease since 3.5 cal ka BP, and the arid tendency was earlier in the terminal lake than it was in the middle and upper regions of the drainage basin. During the early Holocene the relatively arid environment was affected by the gradually intensifying East Asian monsoon and the dry westerly winds. The mid-Holocene Optimum benefited from the intensive East Asian monsoon and the humid westerly winds. Then, the East Asian monsoon retreated since the late-Holocene. In the basin the arid tendency may be related to the retracting of the East Asian monsoon. However, the intensifying acidification after 1.5 cal ka BP may be correlated to the increasing dryness of the westerly winds.

Genetic and historic evidence for climate-driven population fragmentation in a top cetacean predator: the harbour porpoises in European water

Recent climate change has triggered profound reorganization in northeast Atlantic ecosystems, with substantial impact on the distribution of marine assemblages from plankton to fishes. However, assessing the repercussions on apex marine predators remains a challenging issue, especially for pelagic species. In this study, we use Bayesian coalescent modelling of microsatellite variation to track the population demographic history of one of the smallest temperate cetaceans, the harbour porpoise (Phocoena phocoena) in European waters. Combining genetic inferences with palaeo-oceanographic and historical records provides strong evidence that populations of harbour porpoises have responded markedly to the recent climate-driven reorganization in the eastern North Atlantic food web. This response includes the isolation of porpoises in Iberian waters from those further north only approximately 300 years ago with a predominant northward migration, contemporaneous with the warming trend underway since the ‘Little Ice Age’ period and with the ongoing retreat of cold-water fishes from the Bay of Biscay. The extinction or exodus of harbour porpoises from the Mediterranean Sea (leaving an isolated relict population in the Black Sea) has lacked a coherent explanation. The present results suggest that the fragmentation of harbour distribution range in the Mediterranean Sea was triggered during the warm ‘Mid-Holocene Optimum’ period (approx. 5000 years ago), by the end of the post-glacial nutrient-rich ‘Sapropel’ conditions that prevailed before that time.

Holocene trends in the foraminifer record from the Norwegian Sea and the North Atlantic Ocean

Abstract. The early to mid-Holocene thermal optimum is a well-known feature in a wide variety of paleoclimate archives from the Northern Hemisphere. Reconstructed summer temperature anomalies from across northern Europe show a clear maximum around 6000 years before present (6 ka). For the marine realm, Holocene trends in sea-surface temperature reconstructions for the North Atlantic and Norwegian Sea do not exhibit a consistent pattern of early to mid-Holocene warmth. Sea-surface temperature records based on alkenones and diatoms generally show the existence of a warm early to mid-Holocene optimum. In contrast, several foraminifer and radiolarian based temperature records from the North Atlantic and Norwegian Sea show a cool mid-Holocene anomaly and a trend towards warmer temperatures in the late Holocene. In this paper, we revisit the foraminifer record from the Vøring Plateau in the Norwegian Sea. We also compare this record with published foraminifer based temperature reconstructions from the North Atlantic and with modelled (CCSM3) upper ocean temperatures. Model results indicate that while the seasonal summer warming of the sea-surface was stronger during the mid-Holocene, sub-surface depths experienced a cooling. This hydrographic setting can explain the discrepancies between the Holocene trends exhibited by phytoplankton and zooplankton based temperature proxy records.

Diatom evidence for Holocene paleoclimatic change in the South Scotia Sea, West Antarctica

Diatom data of the core sediment from a deep basin in the South Scotia Sea, West Antarctica provide high-resolution information on changes in oceanographic processes and paleoclimate during the late Quaternary. Three main climatic changes can be distinguished in diatom assemblages: Last Glacial Maximu (LGM), mid-Holocene climatic optimum and Neoglacial cold event. Diatom assemblages have been deposited in a variable sea ice condition over the last 25,000 yr in response to the climate change. During the LGM to early-Holocene (23,370–8,300 yr BP), the core site might be influenced by increased dense sea-ice cover, which could reduce biogenic flux from the surface water, depositing relatively increased amount of sea ice-related diatoms (Actinocyclas actinochilus, Eucampia antarctica, Fragilariopsis curta, andFragilariopsis cylindrus). During these periods, it is likely that the increased sea ice cover between the Weddell and Scotia seas would have severely preventedChaetoceros resting spores in the Weddell Sea ice margin from being laterally advected to the Scotia Sea, resulting in the reduction ofC. resting spore abundance in the sediment. Afterward, a warm period followed from 8,300 to 2,400 yr BP in the mid Holocene when open water assemblages (Rhizosolenia styliformis andThalassiosira antarctica (warn)) were deposited. Significant dilution of the number ofFragilariopsis kerguelensis indicates the opening of communication between the Weddell and Scotia seas allowing lateral advection ofC. resting spores from the Weddell Sea to the Scotia Sea. A colder condition (Neoglacial cooling) then resumed since <2,400 yr BP in the late Holocene supported by other paleoclimatic records in the Antarctic Peninsula. The assemblage is characterized either by the increase of sea ice-related diatoms (A. actinochilus andF. cylindrus) or by the decrease of open water taxa (R. styliformis andT. antarctica (warm)) compared to that in the mid-Holocene optimum. The loose sea-ice assemblage was, however, different from dense sea-ice assemblage deposited in the LGM. Similarity of abundance ofChaetoceros between the Neoglacial and the mid-Holocene implies that the loose sea ice condition was not able to sufficiently restrict the lateral advection ofC. resting spore to the Scotia Sea from the Weddell Sea.

Holocene climate fluctuations in the Yangtze delta of eastern China and the Neolithic response

Six major pollen-spore assemblage zones can be identified showing alternate warm and cool climate fluctuations on a millennial timescale. The early-Holocene optimum at c. 8000-7800 radiocarbon years BP, is marked by a large proportion of evergreen broad-leaf species in Zone I, suggesting the warmest/hottest climate setting of the Holocene. The mid-Holocene optimum ran from c. 7500 to 5000 BP (Zone III), during which many subtropical warmand wet-tolerant species represented by Castanopsis and Cyclobalanopsis glauca occurred, indicating the megathermal trend. The temperature then declined upcore in Zone IV accompanied by a high proportion of Pinus and Cupressaceae. A cooling event occurred at c. 4200 BP, as evidenced by Fagus and Potamogeton in the upper section of Zone IV. Pollen assemblages in Zones V-VI indicate a generally cooler temperature in the late Holocene with a high proportion of Gramineae, indicative of rice cultivation. Onset of the mid-Holocene optimum was associated with sediment deposition of the Holocene delta construction. This promoted the migration of Neolithic settlements onto the delta plain from highlands west of the study area. The cooling event recognized at c. 4200-4000 BP may have triggered the cultural ‘break up’ caused by the climatic transition from early warm-humid to a later cool-wet setting, and lake expansion caused by heavy precipitation.

Early–Mid Holocene climatic variations in Tasmania, Australia: multi-proxy records in a stalagmite from Lynds Cave

Mass spectrometric uranium-series dating and C–O isotopic analysis of a stalagmite from Lynds Cave, northern Tasmania, Australia provide a high-resolution record of regional climate change between 5100 and 9200 yr before present (BP). Combined δ18O, δ13C, growth rate, initial 234U/238U and physical property (color, transparency and porosity) records allow recognition of seven climatic stages: Stage I (>9080 yr BP) – a relatively dry period at the beginning of stalagmite growth evidenced by elevated 234U/238U; Stage II (9080–8600 yr BP) – a period of unstable climate characterized by high-frequency variability in temperature and bio-productivity; Stage III (8600–8000 yr BP) – a period of stable and moderate precipitation and stable and high bio-productivity, with a continuously rising temperature; Stage IV (8000–7400 yr BP) – the warmest period with high evaporation and low effective precipitation (rainfall less evaporation); Stage V (7400–7000 yr BP) – the wettest period with highest stalagmite growth and enhanced but unstable bio-productivity; Stage VI (7000–6600 yr BP) – a period with a significantly reduced precipitation and bio-productivity without noticeable change in temperature; Stage VII (6600–5100 yr BP) – a period of lowest temperature and precipitation marking a significant climatic deterioration. Overall, the records suggest that the warmest climate occurred between 8000 and 7400 yr BP, followed by a wettest period between 7400 and 7000 yr BP. These are broadly correlated with the so-called ‘Mid Holocene optimum’ previously proposed using pollen and lake level records. However, the timing and resolution of the speleothem record from Lynds Cave are significantly higher than in both the pollen and lake level records. This allows us to correlate the abrupt change in physical property, δ18O, δ13C, growth rate, and initial 234U/238U of the stalagmite at ∼8000 yr BP with a global climatic event at Early–Mid Holocene transition.