Showing papers on "Holocene climatic optimum published in 1990"

A record of Holocene summer climate from a Canadian high-Arctic ice core

Abstract: MELT layers in ice cores are formed by melting on the snow-pack surface, which produces water that percolates down and refreezes in the colder snow layers below. Melt layers are distinguished by their 'bubble' texture. Because more ice forms in the snow pack in warm summers than in cool summers, the changing concentration with depth of melt layers in snow pits1,2 and ice cores3–5 is an indicator of past summer climate. Here we report on studies of melt layers in two Canadian high-Arctic surface-to-bedrock ice cores. Although changes in elevation of the drill site owing to isostatic depression and uplift may contribute up to 40% of the total climatic signal recorded in the melt layers, we interpret their varying concentrations in terms of changing summer temperatures over the past 10,000 years. The warmest summers occurred 8–10 kyr ago and the coldest only 150 years ago. The summers over the past 100 years have been the warmest for more than 1,000 years, but are still not as warm as those of the early Holocene. The melt-layer record is in broad agreement with geological records of glacier retreat during the Holocene6–8, indicating that these data also contain information of regional significance.

Holocene Paleoecology of the Boreal Forest and Great Lakes-St. Lawrence Forest in Northern Ontario

Abstract: This paper presents pollen and macrofossil stratigraphies derived from sediment cores taken near the deepest parts of Nina Lake (46°36' N, 81°30' W), Jack Lake (47°19' N, 81°46' W), and Lake Six (48°24' N, 81°19' W), which are situated along a transect across the ecotone between the boreal forest and the Great Lakes—St. Lawrence forest in northern Ontario. Paleoecological data from this region of steep climatic gradient and varied landforms provide sensitive records of postglacial vegetational and climatic changes. These data are used to evaluate questions concerning the occurrence of late—glacial vegetation communities without modern analog. Hypsithermal ecotonal movements, and individualistic species response to climatic change and soil development. The early postglacial boreal forest that colonized the Canadian Shield upland after ca. 10 000 BP was dominated by white spruce with little or no black spruce, and probably contained more oak, elm, poplar, and heliophytic herbs and shrubs than its modern counterpart. Its lack of modern analog is probably due to the widespread occurrence of fresh, unleached soil in a newly deglaciated landscape. Spruce declined and was replaced by jack pine after ca. 9000 BP as the climate continued to warm. The boreal forest was enriched floristically by the successive immigration of species such as Myrica. Alnus crispa, and A. rugosa. Boreal forest was transformed into Great Lakes—St. Lawrence forest ca. 7400 yr ago when white pine, beech, and hemlock immigrated to Nina Lake. Species of the Great Lakes—St. Lawrence forest responded individualistically to Hypsithermal climatic changes. White pine populations continued to spread northward during ca. 7000—3000 BP, causing the boreal forest/Great Lakes—St. Lawrence forest ecotone to advance ca. 140 km north of its present position, only constrained by the physiographic boundary between the Clay Belt and the Canadian Shield upland. During 6000—4500 BP northern white cedar proliferated in the Clay Belt lowlands due to a warmer and drier climate. The regional water table was lowered, permitting Thuja populations that were otherwise restricted to the margins of swamps and open peatlands to spread to the center. This vegetation response was landform selective, being more pronounced in the Clay Belt than on the Canadian Shield upland. The predominance of calcareous substrates and wetland habitats in the Clay Belt was favorable to the expansion of Thuja under a suitable climate. Neoglacial cooling decimated the populations of northern white cedar in the Clay Belt and those of white pine in the whole region. Spruce, jack pine, and balsam fir increased over the last 4000 yr. The ecotone retreated from near Lake Six after 3000 BP, reaching Jack Lake at ca. 2600 BP, and was stabilized in its modern position during the last millennium.

Recent Japanese Research on Relative Sea Level Changes in the Holocene and Related Problems

Abstract: This paper reviews studies conducted between 1980 and 1988 on relative sea level changes and coastal evolution during the Holocene in Japan. The Japanese Working Group of IGCP Project 200, on “late Quaternary sea level changes, ” compiled the two-volume “Atlas of Sea Level Records in the late Quarternary in Japan” in 1987, which included materials related to this topic, based on papers published since 1980. The group also compiled the “Middle Holocene Shoreline Map of Japan (1:200, 000), which demonstrated the location of the middle Holocene shoreline with numerous data on height and radiocarbon age representing the sea level of that stage, and with 15 insets, considered to be typical examples of various types of study. Numbers of papers by year in terms of research field and study area are summarized in Figs. 1 and 2. Several review papers on sea level study have been also published in the last several years, in addition to local studies.Relative sea level curves published in the past 10 years are shown in Fig. 3. The curve patterns show noticeable local or regional differences, reflecting tectonic factors with a different amount and character in each area. Some areas characterized by a rather late culmination age of the postglacial transgression contrast with most of the Japanese coast, which has a culmination age of ca. 6, 000 to 6, 500y.B.P. Two minor fluctuations of Holocene sea level which were pointed out by OTA et al. (1982), have been recognized in several areas; a eustatic origin for such fluctuation is most likely, judging by the nearly coincident occurrence of climatic fluctuation revealed by pollen analyses, molluscan assemblage analyses and submarine core data.The following topics are discussed in particular detail in this paper: 1) Progress of excavation on the Holocene lowland and coral reefs, in order to obtain systematic samples for identification of marine limits and samples for analyses of various fossils and for dating. 2) Holocene marine terrace study with special reference to coseismic uplift and volcanic activity. 3) Identification of the former sea level on the rocky coast. Barnacles and tube worms (Pomatoleios kraussii) as sea level indicators are discussed, including problems with accuracy of radiocarbon dating. 4) Problems concerning the recognition and accuracy of former shorelines in the large alluvial plain. 5) The significance of small drowned valleys as a suitable field for the reconstruction of sea level change. 6) Climatic fluctuation during the Holocene, with relation to sea level fluctuation. 7) Increased overseas studies on Holocene sea level change by Japanese scientists.

Holocene environmental change in South Georgia: evidence from lake sediments

Abstract: The Holocene environmental history of South Georgia is important because of the island's location in the Southern Westerlies in an oceanic zone of the world devoid of high resolution terrestrial records. This is the first attempt to interpret a palaeoenvironmental record from lake sediments in South Georgia. It is based on a wide variety of analyses undertaken on cores from two lakes. Both are in the same, unglaciated, drainage basin, but one is at 80 m above sea-level and near the altitudinal limit for vegetation growth, whereas the other is at 25 m and within the zone of continuous vegetation cover. Results from both lakes indicate shifts of vegetation boundaries, which, together with evidence for changing biotic productivity within the lakes themselves, are interpreted as indicating climatic changes. Radiocarbon dates on the main changes identify a climatic optimum, beginning before 5620 ± 290 14C yr BP, and ending at around 4815 ± 330 14C yr BP, when conditions in the upper part of the catchment were more conducive to plant growth than they are today. The record obtained from the lower lake was shorter, but indicates two periods of harsher climate relative to the present since 4000 yr BP. This interpretation of the lake evidence agrees with other dated evidence of environmental change from peat sections, glacial stratigraphy and geomorphology in South Georgia. Together the work allows an overall reconstruction of environmental change in the Holocene.

Microlithic sites and their paleoenvironmental setting, Southeast India; a reevaluation

Abstract: A reevaluation is presented of the distribution, age, and paleoenvironmental context of the microlithic sites (Tinnevelly Ten Industry) found on Quaternary coastal sediments in southeast India. Microliths were found to occur widely on the red teri sands; as extensive surface scatters and in concentrated ‘sites’. the extent and number of sites is far greater than hitherto known. the occupation/use of the area occurred within the period between late last glacial and mid-Holocene, and it had certainly ceased by 4000 years B.P. Occupation appears to post-date the aeolian deposition of the teri sands, and to be contemporaneous with the deep weathering of the sands during the early-mid Holocene climatic optimum. During this moist phase the sands were stable and densely vegetated, and relatively high (+ 2−3 m) mid-Holocene sea levels led to the formation of lagoons seaward of the tori sands along much of the coastline. the available stratigraphic evidence and radiocarbon dates suggest that this microlithic industry, with geometrical elements, is older than hitherto believed (Zeuner and Allchin, 1956), but not as old as recent suggestions for a similar industry from Sri Lanka (Singhvi et al., 1986).

Dark Colored Organic Soil Formation and Environmental Changes Since Latest Pleistocene Time in the Upper Qingshui - he Basin, the Loess Plateau, China

Abstract: The dark colored organic soils represented by A horizon of Dark loessial soils, called “Heilutu”, are extensively distributed on the Loess Plateau, China. They show that the Plateau was formerly covered with dense vegetation. On the other hand, in the upper drainage basin of the Qingshui-he River, which flows into the Huang-he at the western part of the central Loess Plateau, Holocene river terraces are widely developed and their deposits consist of gravel bed, peat layers and loessial flood loam bed in ascending order. The peat layers intercalated in the terrace deposits indicate that a peatland was widely expanded along the Qingshui-he during a period of Holocene time. The authors examine the stratigraphical relationship between the peat layers of Holocene terraces and the dark colored organic soils distributed on mountain slopes, and discuss the environmental changes based on the changes in facies of terrace and slope deposits as well as 14C dates of peat and soils.The upper drainage basin of the Qingshui-he, whose altitude ranges from 1, 500 to 2, 900 m above sea level, is situated in a semi-arid region with annual precipitation of 400 to 650 mm, and a part of the basin is occupied by a present peatland with small area. The peat layers of the present peatland are divided into the following four layers; the lower, the middle, the upper and the uppermost, by intervening clay layers. The four layers are different in color, thickness and in content of organic matter and clay. The middle peat layer is the most developed with dark color and rich in organic matter. Three peat layers except the uppermost layer are recognized in the Holocene terrace deposits. The dark colored soils on the mountain slopes and the Pleistocene terraces are stratigraphically correlated to the middle and upper layers of the peat deposits.The 14C dates show 11, 800±200 B. P. (TK-826) for the lower part of the lower peat layer, 7, 980±80 B. P. (TK-827) for the lower part of the middle peat layer and 5, 040±70B. P. (TK-828) for the upper part of the upper peat layer at Kaicheng of 1, 930 m in altitude. The lowest part of dark colored organic horizon of Heilutu distributed on the mountain slope at Mt. Yunwu-shan (2, 148 m a.s.1.) shows 6, 280±70 B. P. (TK-829). The dark colored organic horizon are proved to have been formed mainly during the period from 8, 000 B. P. to 5, 000 B. P., meaning that the Loess Plateau was covered with dense vegetation during the Hypsithermal. The uppermost horizon of loessial flood loam covering over the Holocene terrace consists of thin and light colored humus, indicating that the formation of organic soils with weak activity was continued after 5, 000 B. P.The changes in facies of terrace and mountain slope deposits, and in vegetation density suggest the environmental changes as follows;The gravel bed of the lowest layer of Holocene terrace deposits, forming alluvial fans at mountain foot, indicates that the stream was powerful, suggesting that heavy rainfalls occurred frequently before 12, 000 B. P. During the period from 12, 000 B. P. to 8, 000 B. P., the periglacial debris layers were deposited on the mountain slopes, and the clayey deposits without gravel along the river courses. The former shows the cold and wet condition with sparse vegetation, and the latter the small precipitation. Therefore, the cold and dry climate but the wet soil condition are inferred.During the period from 8, 000 B. P. to 5, 000 B. P., the dark colored organic soils were developed even in the present arid zone with annual precipitation less than 350 mm, indicating that it was more humid than the present. The periglacial action on high mountains ceased under dense vegetation cover, indicating that it was warmer.

CHANGES OF ATMOSPHERIC METHANE CONCENTRATION PARALLEL TO CLIMATIC CHANGES (Abstract)

Abstract: Measurements on ice-core samples showed that atmospheric methane concentration changed with the large climatic cycles during the last two glaciations (Stauffer and others, 1988; Raynaud and others, 1988). The methane concentration is lower in cold periods and higher in warm periods. In this paper we discuss the results of CH 4 measurements of samples from periods of minor climatic change, like the climatic optimum 8000 years B.P. and the Younger Dryas period about 10000 to II 000 years B.P .. The data are interpreted in terms of the present understanding o f methane sources and sinks.

Working Group 5: Impact of Paleoclimatic Proxy Data on Climate Modeling

Abstract: During the past 18 years, since the start of the CLIMAP project (Climate, Long Range Investigation, Mapping and Prediction), a great wealth of paleoclimatic proxy data have been obtained. These proxy data have been employed as boundary conditions for the simulation of past states of the ocean such as that of the Last Glacial Maximum (LGM) 18 thousand years before the present (kyBP), the Climatic Optimum 9ky BP, and the penultimate interglacial 120ky BP. These proxy data include sea surface temperatures, winds, and the position of the North Atlantic polar front as deduced by the CLIMAP and COHMAP (Cooperative Holocene Mapping Project; Ruddiman and Wright, 1987) projects, and by various European groups working in Bordeaux, Cambridge, Kiel and Paris.