mid-Holocene Climate Research Articles

Simulations of Mid-Holocene Climate Using an Atmospheric General Circulation Model

The authors describe a first paleoclimatological application of the Canadian Centre for Climate Modelling and Analysis atmospheric general circulation model (AGCM) to simulate the climate state 6000 calendar years before present (6 kyr BP). Climate reconstructions for this period are performed with both fixed SSTs and with the AGCM coupled to mixed layer ocean and thermodynamic sea‐ice modules. The most important difference between this epoch and the present involves the increased surface heating and cooling of the continental land masses in the Northern Hemisphere during summer and winter, respectively, which are a consequence of the modified orbital configuration. A comparison of a fixed SST experiment with a calculated SST experiment, incorporating a thermodynamic representation of oceanic response, is performed to assess the impact on the mid-Holocene climate. The results are also contrasted with those obtained on the basis of proxy climate reconstructions during this mid-Holocene optimum period. Of interest in this calculated SST experiment is the impact on the seasonal cycle of sea‐ice distribution due to the increased insolation at high latitudes during Northern Hemisphere summer. Also important is the fact that the mixed layer ocean in the simulation is found to further enhance the monsoon circulation beyond the enhancement found to occur due to the influence of modified orbital forcing alone. This increased response is found to be a consequence of the sensitivity of tropical SST to the amplification of the seasonal cycle due to the change in insolation forcing that was characteristic of the midHolocene period.

The climate and biomes of Europe at 6000 yr BP : comparison of model simulations and pollen-based reconstructions

14C-dated pollen and lake-level data from Europe are used to assess the spatial patterns of climate change between 6000 yr BP and present, as simulated by the NCAR CCM1 (National Center for Atmospheric Research, Community Climate Model, version 1) in response to the change in the Earth’s orbital parameters during this perod. First, reconstructed 6000 yr BP values of bioclimate variables obtained from pollen and lake-level data with the constrained-analogue technique are compared with simulated values. Then a 6000 yr BP biome map obtained from pollen data with an objective biome reconstruction (biomization) technique is compared with BIOME model results derived from the same simulation. Data and simulations agree in some features: warmer-than-present growing seasons in N and C Europe allowed forests to extend further north and to higher elevations than today, and warmer winters in C and E Europe prevented boreal conifers from spreading west. More generally, however, the agreement is poor. Predominantly deciduous forest types in Fennoscandia imply warmer winters than the model allows. The model fails to simulate winters cold enough, or summers wet enough, to allow temperate deciduous forests their former extended distribution in S Europe, and it incorrectly simulates a much expanded area of steppe vegetation in SE Europe. Similar errors have also been noted in numerous 6000 yr BP simulations with prescribed modern sea surface temperatures. These errors are evidently not resolved by the inclusion of interactive sea-surface conditions in the CCM1. Accurate representation of mid-Holocene climates in Europe may require the inclusion of dynamical ocean–atmosphere and/or vegetation–atmosphere interactions that most palaeoclimate model simulations have so far disregarded.

Assessing the impact of volcanic activity on mid-Holocene climate in Ireland: the need for replicate data

Analyses of pollen, tephra, mineral input and degree of peat humification from three neighbouring raised peat profiles at Corlea, central Ireland, covering the period of the deposition of a tephra layer dated to just before 2290 cal. BC, and thought to represent Hekla-4 (2310±20 BC), are used to show the problems of relying on data from a single profile when invoking relationships between volcanic activity, climate and ecosys-tem response. While there appears to be a strong correlation between tephra deposition and flooding of the bog surface in one profile, with a short-lived increase in the rate of peat accumulation, comparison with the other two records suggests that peat had already begun a trend to a less humified condition before tephra deposition, and that evidence of local bog surface flooding was neither consistent nor synchronous.

Pulleniatina obliquiloculata as a paleoceanographic indicator in the southern Okinawa trough during the last 20,000 years

Pulleniatina obliquiloculata, a tropical species of planktonic foraminifera, is indicative of the Kuroshio current and sensitive to winter sea surface temperature for the late Quaternary in the Okinawa Trough. Its relative abundance fluctuations are significant and correlatable between three gravity cores (cores 255, 170, 253) raised from the southern Okinawa Trough. Four major changes in its abundances with paleoceanographic significance have been recognized during the last 20,000 years: an abrupt increase at the Pleistocene/Holocene boundary, a short-term decrease indicative of a ‘Younger Dryas’-type climate reversal at about 11.4-9.6 ka B.P., the P. obliquiloculata maximum zone (around 7-6 ka B.P.) corresponding to the mid-Holocene climate optimum and the P. obliquiloculata minimum zone (around 4-2 ka B.P.) correlated probably to the Neoglacial cooling. The widespread occurrence of these events in the western Pacific and the correlated variations of its abundance between sea areas suggest that P. obliquiloculata is well promising as a paleoceanographic and climatic monitor for high-resolution reconstruction and sea-land correlation.

Mid-Holocene Climate and Culture Change in the Atacama Desert, Northern Chile

Twenty archaeological campsites intercalated between more than 30 debris flows caused by heavy rainfall events between 6200 and 310014C yr B.P. have recently been discovered at Quebrada Puripica in the Atacama Desert of northern Chile. This record provides detailed information about extreme, short-lived climatic events during the hyperarid mid-Holocene period. For the first time, we found evidence of continuous human occupation in this area, filling the regional hiatus in the Atacama basin (“Silencio Arqueologico”) between 8000 and 480014C yr B.P. The transformation of Early Archaic hunters into the complex Late Archaic cultural tradition was an adaptive process. During this time, the site was a local ecological refuge with abundant resources in a generally hostile environment.

Abrupt Early to Mid-Holocene Climatic Transition Registered at the Equator and the Poles

Paleoclimatic records from equatorial East Africa, Antarctica, and Greenland reveal that atmospheric circulation changed abruptly at the early to mid-Holocene transition to full postglacial conditions. A climatic reorganization occurred at all three sites between 8200 and 7800 years ago that lasted 200 years or less and appears to have been related to abrupt transitions in both marine and terrestrial records around the world.

Influences of Holocene climate and water levels on vegetation dynamics of a lakeside wetland

Sediment lithology, pollen, and plant-macrofossil data from the Paynter Site, southern Ontario, revealed three wetland developmental stages during the past 11 000 years: (i) a Carex, Eupatorium, and Eleocharis dominated marsh with some Larix, Abies, and Picea (ca. 11 000–8300 cal years BP); (ii) a Verbena hastata and Mentha arvensis marsh (ca. 8300–7460 cal years BP); and (iii) a white cedar (Thuja occidentalis) swamp (7460 cal years BP to present). There were no significant successional changes in the initial diverse marsh (stage 1) for about 2700 years; its high taxon richness was maintained by fluctuating water levels. The succession from marsh to swamp at 7460 cal years BP was caused by mid-Holocene warm and dry climate, which corresponded with cedar expansion elsewhere in Ontario. The swamp peat record was interrupted at ca. 6400 cal years BP by declining water levels, culminating in a dry period, as indicated by a sandy layer, rare macrofossils, and a low sediment-accumulation rate (0.012 cm/year). The reappearance of cedar swamp macrofossils since 3200 cal years BP corresponded with the recovery of water levels owing to a more humid late Holocene climate and flooding from isostatic tilt. Human disturbance such as damming and logging caused the development of historical cedar and thicket swamps with abundant Alnus rugosa and weedy taxa. This sequence of wetland development did not match the present vegetational gradients from Carex–Typha herb marsh, through Myrica–Decodon shrub marsh and Alnus–Fraxinus thicket, to Thuja swamp. At this site, the wetland development was mainly influenced by allogenic factors such as water levels and climate rather than autogenic factors. Keywords: wetland succession, climate change, hydrological change, Holocene, paleoecology, disturbance, space-for-time substitution, plant macrofossils, Thuja expansion, Ontario.

Feedbacks between climate and boreal forests during the Holocene epoch

PREVIOUS studies1–5 have demonstrated that the predictions of global climate models are highly sensitive to large changes in vegetation cover, such as the complete removal of tropical or boreal forests. Although these studies have illustrated the potential effects of massive deforestation on the climate system, vegetation changes of this scale are very unlikely to occur. Investigating past environments may better illustrate the possible interactions between climate and vegetation cover. For example, palaeobotanical evidence indicates that 6,000 years ago boreal forests extended north of the modern treeline6, apparently in response to high-latitude warming resulting from variations in the Earth's orbit7,8. The expanded boreal forests, which took the place of tundra, must also have affected climate by significantly reducing the surface albedo5. Here we use a global climate model to examine the relative effects of orbitally-induced insolation variations and of the northward extension of boreal forests on the mid-Holocene climate. Orbital variations alone warm the high latitudes by 2 °C or more in summer, autumn and winter. The subsequent northward extension of boreal forests gives rise to an additional warming of approximately 4 °C in spring and about 1 °C in the other seasons. This suggests that large positive feedbacks between climate and boreal forests may have taken place in the recent geological past.

Mid-holocene climates and environments in China

This paper focuses mainly on the climatic and environmental variations 5 to 3 ka. B.P. with particular reference to one stable warmer and wetter millennium (7.2-6 ka B.P.). Some large-scale centennial warming was accompanied by an increase in precipitation owing to the expansion of the monsoon circulation, e.g. rapid warming during the period from 8.5 to 8.3 ka B.P. and was reflected in the high level of some inland lakes and the sudden expansion of vegetation in pollen diagrams. The growth of the Neolithic culture with agriculture and settlement in the present semi-arid area of Northwest China was undoubtedly related to the dramatic warming and wetting before 8 ka B.P. According to the proxy data of palynological studies, the deviation of annual mean temperature from today's about 7-6 ka B.P. was roughly estimated at about 1°C in South China, 2°C in the Changjiang (Yangtze) Valley, 3°C in North China and Northeast China. The strongest warming by 4–5°C was recorded in Qinghai-Xizang (Tibet) Plateau. The winter temperature rise was much greater than the annual average temperature. In the eastern half of China, the vegetation zones are mainly parallel to the latitude. During the climax of the Holocene warm period, the northern limit of the tropical monsoonal rain forest shifted less than 1° of latitude northward, the subtropical broad-leaved evergreen forest about 1° of latitude, the north subtropical deciduous and evergreen broad-leaved mixed forest about 3° of latitude in the coastal area but only 1–1.5° of latitude in mid-West China and the warm temperate deciduous forest moved 4° of latitude northward, greatly expanding its distribution. Further north, the cool temperate boreal forest withdrew from its southern boundary by about 2° of latitude. Global climatic warming in the mid-Holocene resulted in a sea-level rise. During the period from 6 to 5 ka BP. the sea-level was about 1–3 m higher than the present level. Large area of coastal lowland was submerged by sea water and frequency of occurrence of storm surges also increased during the high sea-level period.

Palynology and palaeoclimatic implications of two Holocene sequences from southwestern Australia

Results from two palynological investigations of Holocene lake and swamp deposits from southwestern Western Australia are presented. These are used to assess the validity of the existing palaeoclimatic evidence for Holocene climates, which is contradictory. The Boggy Lake sequence from the far southwest of Western Australia shows that, while vegetation changes have taken place over the last ca. 4500 years, these cannot be attributed to climatic change. Palaeoecological interpretations of earlier work at the site are shown to have an insecure basis. The Loch McNess Swamp sequence from further north shows only minor vegetation changes since ca. 9000 yr B.P. which cannot be attributed with certainty to climatic causes. The investigations do not support earlier claims of Mid-Holocene climates wetter than present or of an extensive arid phase in the Mid- to Late-Holocene.

A High-Resolution Record of Holocene Climate Change in Speleothem Calcite from Cold Water Cave, Northeast Iowa

High-precision uranium-thorium mass spectrometric chronology and (18)O-(13)C isotopic analysis of speleothem calcite from Cold Water Cave in northeast Iowa have been used to chart mid-Holocene climate change. Significant shifts in dagger(18)O and dagger(13)C isotopic values coincide with well-documented Holocene vegetation changes. Temperature estimates based on (18)O/(16)O ratios suggest that the climate warmed rapidly by about 3 degrees C at 5900 years before present and then cooled by 4 degrees C at 3600 years before present. Initiation of a gradual increase in dagger(13)C at 5900 years before present suggests that turnover of the forest soil biomass was slow and that equilibrium with prairie vegetation was not attained by 3600 years before present.

A continuous record of Holocene eolian activity and vegetation change at Lake Ann, east-central Minnesota

Wind-blown silt and sand in the lacustrine sediments of Lake Ann in east-central Minnesota provide a continuous record of Holocene eolian activity of the adjacent Lake Ann dune field. Results of grain size, loss-on-ignition, and especially magnetic susceptibility determinations indicate the variable quantities of eolian material present in the lacustrine sediments. Comparison of this eolian component with the vegetation history determined by pollen analysis indicates that dunes were apparently not present during Late Wisconsin (late glacial) and early Holocene times when trees were common on the Anoka Sand Plain. Dune development began about 8000 yr B.P. when arboreal vegetation was seriously reduced by dry conditions. The middle Holocene history is subdivided into three distinct episodes, each with a similar sequence of events: (1) drought and reduced vegetation cover, (2) increased eolian flux, (3) increased precipitation and tree cover, and (4) slow decline in eolian erosion. Maximum eolian flux during these episodes occurs at ca. 7400, 5800, and 4900 yr B.P. The mid-Holocene climate at Lake Ann is similar to that inferred for other Midwest sites, such as Elk Lake in northwestern Minnesota, where intervals of dryness or strong winds occur when eolian activity is high at Lake Ann. Dune activity in east-central Minnesota appears to have been primarily initiated by severe droughts of relatively brief duration during the general aridity of the mid-Holocene interval.

Thermally-anomalous Holocene molluscan assemblages from coastal Peru: evidence for paleographic, not climatic change

Holocene marine mollusks, many in situ, are found in great numbers on anemergent coastal plain north of the Rio Santa, on the north-central coast of Peru. Samples of the mollusks were collected, identified, and grouped into assemblages of recurring species. The surficial sedimentology and geomorphology of the coastal plain were studied to determine the paleoecological significance of the molluscan shells, many of which belong to species now restricted to a biogeographically transitional zone north of Bayovar, Peru, 200 km north of the study of the area. Radiocarbon dates, mostly obtained from shells in natural deposits and middens, were used to track the Holocene development of the coastline. The warm-water mollusks lived within a shallow lagoon of about 29 km 2 between 4300–6500 yr B.P. Free tidal exchange with the Pacific Ocean was impeded by partially-drowned beach ridges within the lagoon and by a barrier-forming beach ridge that joined the mainland with an offshore island. Contemporaneus, in situ, cold-water faunas were discovered at the mouth of the lagoon and in washover deposits landward of the barrier beach ridge. The association of contemporaneous and adjacent assemblages of warm-water and cold-water mollusks shows that the thermally-anomalous molluscan assemblage (TAMA) at Santa is a consequence of local coastal paleographyy, not regional or global mid-Holocene climate change.

Mid-Holocene climate in Northern Minnesota

Study of Holocene ostracodes and diatoms from Elk Lake, in North-Central Minnesota, indicates that the local climate of the mid-Holocene can be subdivided into three intervals. Throughout interval 1 (ca. 7800 to 6700 yr B.P.), climate was colder and much drier than today. During intervals 2 and 3 (ca. 6700 to 4000 yr B.P.) average mean-annual air temperatures approached the modern mean (3.7°C), but warm summers persisted throughout interval 2, whereas during interval 3 warm summers fell into discrete episodes. Furthermore, average mean-annual precipitation was about 85 and 90% of modern during intervals 2 and 3, respectively. Transition times between the principal intervals were less than 50 yr. The expected effects of a retreating Laurentide Ice Sheet that initially maintained a winter-style circulation, followed by transitional climate states, and finally a near-modern circulation pattern may explain these local climatic events.