Variations In Effective Moisture Research Articles

Moisture evolution in Central Asia since 26 ka: Insights from a Kyrgyz loess section, Western Tian Shan

Central Asia (CA) is located in the hinterland of the Eurasian continent, and faces an extreme risk of continued aridification under the current global warming trend. To predict future climate change in CA, paleoclimate studies provide essential references. However, the mechanisms underlying climate changes in CA remain relatively poorly known. Here, we investigate moisture variations in CA, as recorded in a 6-m loess section in the Western Tian Shan, Kyrgyzstan, which has accumulated since ∼26 ka based on the radiometric AMS 14C dating. We reconstructed the evolution of the wind regime and moisture conditions in the Western Tian Shan with a modified grain size index (GSIm) and a new L∗ proxy (ΔL∗). While the frequency-dependent magnetic susceptibility (χfd), redness (a∗) and yellowness (b∗) did not provide reliable paleoclimatic indicators, due to weak pedogenesis and other controlling factors. The GSIm records demonstrated a precipitation maximum in summer before ∼15 ka, shifting to a maximum in spring after ∼15 ka, probably associated with large-scale modulation of the latitudinal climatic zones and atmospheric circulation in response to retreat of the Northern Hemisphere ice sheets. Based on the ΔL∗ records, we place emphases on the important role of temperature-modulated evaporation in moisture evolution across CA; however, this has only influenced moisture evolution in spring and summer, with little effect on winter moisture conditions. Moreover, the ΔL∗ records may also indicate that atmospheric humidity can partly affect the long-term effective moisture variations.

Increasing effective moisture during the Holocene in the semiarid regions of the Yili Basin, Central Asia: Evidence from loess sections

The evolution of Holocene moisture in semiarid and arid regions of Central Asia is key to understanding changes in current and future water resources, and has been a hotly debated topic in the past few decades. Central Asian loess deposits have shown potential as records of past moisture changes, particularly during the Holocene. Here, the variations in effective moisture during the Holocene in the semiarid regions of the Yili Basin, Central Asia, are obtained from three loess sections. Reliable chronologies were constructed using 26 quartz optically stimulated luminescence (OSL) ages. The strengthening pedogenesis, steady increases in magnetic susceptibility, and continuous decreases in mean grain size in the three loess sections all suggest that effective moisture continuously increased throughout the Holocene, indicating a dry early Holocene, a moderately humid middle Holocene, and a wet late Holocene. Rainfall carried by the Asian summer monsoon and the Westerlies contributes to the moisture changes in Central Asia. However, considering the dominance of evaporation over rainfall in controlling aridity-humidity shifts in this region, the continuous weakening of evaporation caused by a decrease in Northern Hemisphere summer insolation and associated atmospheric temperatures is primarily responsible for the wetting trend throughout the Holocene in the semiarid regions of the Yili Basin.

Late Holocene Records of Changing Moisture Regime from Wetlands in Southwestern Nova Scotia, Canada: Implications for Wetland Conservation and Restoration

An understanding of the morphological stability and succession of open water and wetland ecosystems in Nova Scotia is a priority for informing the conservation management of critical habitats for a complex of nationally listed, rare, disjunct wetland species. Baltzer Bog and Big Meadow Bog in southwestern Nova Scotia contain stratigraphic records of late Holocene moisture variability. Baltzer Bog is a shrub bog that formed in an elevated, enclosed kettle basin. Excavated sections exposed by peat mining revealed 2 distinct wood-rich horizons that are located above a well-developed soil and wood horizon that yielded a radiocarbon-dated age of 3260 cal. BP from an upright stump. The overlying wood-rich horizons were dated at 1640 and 1045 cal. BP and were overlain by Sphagnum species transitions indicative of increasing wetness. At Big Meadow Bog, a thin wood mat in Sphagnum at 90 cm depth was dated at 1760 cal. BP. These records are broadly correlative with pollen and stratigraphic data from Pleasant River Fen in central Nova Scotia that indicate periods of high and low productivity and a fluctuating water table from 1950 cal. BP until present. Though other high-resolution paleoclimate records from the region indicate that the late Holocene was a time of increasing precipitation and cooler air temperatures, these wetland records demonstrate that in Nova Scotia this time period was characterized by rapid variations in effective moisture and that significant and sustained dry periods likely occurred. This record of late Holocene moisture variability and its influence on habitat structure serves to better establish the potential for long-term residency of threatened and endangered species at wetland sites.

Hydroclimatic changes over the past 900 years documented by the sediments of Tiewaike Lake, Altai Mountains, Northwestern China

The reconstruction of variations in effective moisture over the past millennium are important for assessing potential future climatic trends in environmentally-sensitive arid Central Asia (ACA), especially in the context of ongoing global warming. However, high-resolution time series reflecting past moisture variability in the region are scarce, thus preventing a full understanding of the hydroclimatic evolution. Here we present the results of multi-proxy analysis of a sediment core spanning the past 900 years from Tiewaike Lake in the Altai Mountains, northern Xinjiang. The climatic proxies measured include grain size, organic matter and carbonate content, C/N ratio, and the stable carbon isotope composition of bulk organic matter. The results show that over the past 900 years the climatic evolution of the study area can be divided into distinct hydroclimatic phases: the arid Medieval Climate Anomaly (MCA, ∼1090–1430 AD), the humid Little Ice Age (LIA, 1430–1940 AD), and the arid Current Warm Period (CWP, 1940–2012 AD). During the LIA, the water level at Tiewaike Lake was overall higher than during the other climatic intervals; in addition, the LIA climate was unstable and characterized by six intervals with the occurrence of heavy precipitation events on a multi-decadal timescale. A humid, unstable climate during the LIA is also suggested by climatic records from adjacent regions. Our results support the view that over the past millennium, on a multi-centennial timescale, the moisture variations in ACA were generally out-of-phase with those in the region affected by the Asian summer monsoon. The humid, unstable LIA climate in ACA may have been associated with changes in the North Atlantic Oscillation (NAO) index and/or with variations in solar irradiance.

Palynological evidence of the geoecological belts dynamics from Eastern Cordillera of NW Argentina (23° S) during the Pre-Last Glacial Maximum

This paper presents the first palynological record for the dynamics of geoecological belts of the Eastern Cordillera in northwestern Argentina prior to the Last Glacial Maximum (LGM). The study was carried out on a 5-m deep sedimentary core raised at Laguna Blanca (23°09’S, 65°12’W; 4,256 m a.s.l.), province of Jujuy. The geochronology was based on three 14C dating by AMS method. Results show the sensitive response of the geoecological belts to temperature and effective moisture variations from ca. 29,000 to ca. 25,000 cal. years BP. The High Andean belt probably suffered an altitudinal descent forced by intense cold climate conditions ca. 29,000 cal. years BP, and may have reached a comparable distribution to the present one on 26,300 cal. years BP, favored by a milder climate. The Puna belt did not suffer great changes, since it remained with low representation in the pollen spectrum. High percentages of tree species from the Yungas ca. 29,000 cal. years BP are probably a response to the intensification of anabatic winds ascending the slope of the Sub-Andean Range towards the Eastern Cordillera. These results represent an important contribution to the paleoenvironmental discussion of the Late Pleistocene for the northwestern Argentina, where the available information on vegetation history is scarce.

Carbon and oxygen isotopic records from Lake Tuosu over the last 120 years in the Qaidam Basin, Northwestern China: The implications for paleoenvironmental reconstruction

Isotopic compositions of total organic carbon (TOC) and authigenic carbonate in lakes have been widely used to reconstruct paleoclimatic changes and the depositional environments of lake sediments. However, since these proxies are often controlled by multiple environmental factors, detailed examinations of modern environmental processes is necessary before further applying them into paleoclimatic studies, especially in arid/semi-arid northwestern China. Here we generate High-resolution multi-proxy sedimentary records from Lake Tuosu, a hydrologically closed, saline and alkaline lake located at the north margin of the Qaidam Basin, through analysis of carbon isotope of TOC, and δ18O and δ13C values of ostracods over the last 120years. Together with the meteorological data (precipitation and temperature), lake area record, and other tree-ring evidence, we examine how these sedimentary indices respond to changes in hydrologic balance and climate at interannual to decadal timescales. We found that sedimentary δ13Corg values resemble the variation of lake areas of Lake Tuosu over the last 40years, suggesting that δ13Corg values would be an ideal indicator of lake area/level fluctuations and thus effective moisture variations (precipitation vs. evaporation). However, ostracod δ18O, which was previously used as proxies of effective precipitation, is not well correlated with δ13Corg values in Lake Tuosu. Therefore, the changes of ostracod δ18O values cannot be straightforwardly explained as the effective precipitation. Instead, the isotopic composition of carbonate would be additionally controlled by other factors including isotopic compositions of input water and drainage pattern.

Late and postglacial vegetation and fire history in Southern Patagonia and Tierra del Fuego

Two 17,000-yr-old peat bog records from low-elevation sites in Tierra del Fuego (Harberton, 54°54′S) and southern Patagonia (Río Rubens, 52°4′48″S) and one c. 14,000 cal yr BP record from the upper treeline in Tierra del Fuego (Paso Garibaldi, 54°43′S) were analyzed for pollen, charcoal, and plant macrofossils to reconstruct changes in regional and local vegetation, fire frequency, and bog hydrology, respectively. Past environmental changes in both lowland records and in the upper treeline record are interpreted in terms of variations in effective moisture. During the late-glacial period, effective moisture changes at both low and high-elevation sites were interpreted from comparable shifts between mesic herb-rich grasslands and arid Empetrum heath or shrub–grassland with abundant disturbance indicators. The late-glacial effective moisture changes were primarily driven by temperature changes. During the early Holocene, expansion of open Nothofagus woodlands in the lowlands in present-day areas of dense forest was related to a marked precipitation increase. However, precipitation must have remained highly variable with century-long periods of increased summer drought, as evidenced by repeated intervals when bogs dried out and fire frequency was high. Up-slope shifts of the Nothofagus forest — Andean tundra ecotone at 11,000 and 9000 cal yr BP also appear to reflect precipitation increases. Precipitation variability, however, must not have affected the upper treeline environments as no fires were recorded and the present-day Nothofagus forest had become established after 9000 cal yr BP. Upper treeline apparently was below present from 8000 to 7000 and 2500 to c. 400 cal yr BP. During those times low-elevation environments did not register change which suggests that the upper treeline may have been affected by lower temperatures. After c. 5000 cal yr BP fires became rare in the lowlands, suggesting a shift to an equable precipitation regime with only minor intervals of summer drought. No simultaneous change was recorded at the upper treeline. Thus, for the late-glacial and early Holocene the upper and lower treeline environments apparently responded similarly to primarily moisture changes. Only during the mid- and late Holocene environmental changes at high and low elevations differed, suggesting responses to different climate signals, precipitation in the lowlands and temperature at high elevation.

Moisture changes over the last millennium in arid central Asia: a review, synthesis and comparison with monsoon region

There is a strong chance that 20th century warming will cause differences in precipitation distribution, hydrological cycle and effective moisture changes over the globe. Arid central Asia (ACA), a unique dry-land area whose atmospheric circulation is dominated today by the westerlies, is one of the specific regions that are likely to be strongly impacted by global warming. An understanding of past variations in effective moisture in such regions is an important prerequisite for the prediction of future hydrological change. Here we evaluate spatial and temporal patterns of effective moisture variations documented by different proxies from 17 records in ACA, and synthesize a decadal-resolution moisture curve for ACA over the past millennium, using 5 of the 17 records selected on the basis of reliable chronologies and robust proxies. The high- and low-resolution data all show that, over the past millennium, ACA has been characterized by a relatively dry Medieval Warm Period (MWP; the period from ∼1000 to 1350 AD), a wet Little Ice Age (LIA; from ∼1500 to 1850 AD) and increasing moisture during recent decades. As a whole, the LIA in the ACA was not only relatively humid but also had high precipitation. Over the past millennium, the multi-centennial moisture changes in ACA show a generally inverse relationship with the temperature changes in the Northern Hemisphere, China, and western central Asia. The effective moisture history in ACA also shows an out-of-phase relationship with that in monsoon Asia (especially during the LIA). We propose that the humid LIA in ACA, possibly extending to Mediterranean Sea and Western Europe, may have resulted from increased precipitation due to more frequent mid-latitude cyclone activities as a result of the strengthening and equator-ward shift of the westerly jet stream, and the predominantly negative North Atlantic Oscillation conditions, coupled with a decrease in evapotranspiration caused by the cooling at that time.

Regional and local controls on postglacial vegetation and fire in the Siskiyou Mountains, northern California, USA

The Siskiyou Mountains of northwestern California and southwestern Oregon are a floristic hotspot, and the high diversity of conifers there likely results from a combination of geological, ecological, climatological and historical factors. To evaluate how past climate variability has influenced the composition, structure and fire regime of the Siskiyou forests, pollen, charcoal, and lithological evidence was examined from two lakes along a moisture gradient to reconstruct the vegetation, fire and climate history. The late-glacial period was characterized by subalpine parkland and infrequent fire at both sites. During the late-glacial/Early Holocene transition period, subalpine parkland was replaced by a closed forest of Pinus, Cupressaceae, Abies and Pseudotsuga and more frequent fires a 1000 years earlier at the wetter site, and it is likely that reduced Pacific Ocean upwelling created warmer drier conditions at the coast. In the Early Holocene, Pinus, Cupressaceae were less abundant and fire less frequent at the coastal site during a period of increased coastal upwelling and fog production. In the Late Holocene, Abies, Pseudotsuga, Pinus, and Quercus vaccinifolia increased in the forest at both sites suggesting a widespread response to cooling. Fewer fires at the wetter site may account for the abundance of Picea breweriana within the last 1000 years. The comparison of the two records implies that large-scale controls in climate during the last 14,000 cal yr BP have resulted in major changes in vegetation and fire regime. Asynchrony in the ecosystem response of wetter and drier sites arises from small-scale spatial variations in effective moisture and temperature resulting from topographically-influenced microclimates and coastal-to-inland climate gradients.

Holocene solifluction, climate variation and fire in a subarctic landscape at Pippokangas, Finnish Lapland, based on radiocarbon‐dated buried charcoal

AbstractA large number of radiocarbon dates from charcoal layers buried beneath stacked solifluction lobes at Pippokangas, in the northern boreal zone of Finnish Lapland, are used to reconstruct a Holocene history of solifluction. Although the site is surrounded by Scots pine forest, the solifluction lobes occur on the lower slopes of a kettle hole, the microclimate of which prevents the growth of trees. Samples from the upslope end of charcoal layers have enabled the recognition of four synchronous phases of solifluction lobe initiation: 7400–6700, 4200–3400, 2600–2100 and 1500–500 cal. yr BP. Rates of lobe advance are shown to be lobe‐dependent and age‐dependent: initially, average rates were commonly 0.14–0.19 cm yr−1, later falling to 0.02–0.07 cm yr−1 or less as the lobes approached the bottom of the slope. The absence of charcoal prior to 8000 cal. yr BP, together with single IRSL and TL dates, indicate a relatively stable early Holocene landscape. The onset of solifluction around 7400 cal. yr BP. appears to have followed the immigration of pine around the site, which increased the frequency of forest fires. Phases of solifluction activity seem to have been triggered by millennial‐scale variations in effective moisture (the climatic hypothesis), rather than episodic burning of the surface vegetation cover (the geoecological hypothesis), although climate may also have affected fire frequency and severity. Copyright © 2005 John Wiley & Sons, Ltd.

Climatic and landscape controls of the boreal forest fire regime: Holocene records from Alaska

Summary The response of ecosystems to past and future climatic change is difficult to understand due to the uncertainties in the direction and magnitude of changes and the relative importance of interactions between climate and local factors. In boreal ecosystems such interactions may dictate the response to climatic change, but the interaction of climate, vegetation composition and the fire regime remains poorly understood. Sediment cores from lakes in south‐central Alaska were analysed for lithology, macrofossils, pollen and charcoal to investigate the relationships between moisture availability, species composition and mean fire return intervals (MFI). Macrofossil and lithological evidence suggests that variations in effective moisture occurred over the past 7000 years and that the regional climate has been wetter during the past c. 3800 years than before. Boreal forests existed in the region throughout the past 7000 years. Picea glauca and Picea mariana were the prevalent forest species around Chokasna Lake, whereas P. glauca and hardwood species (e.g. Betula) co‐dominated the landscape around Moose Lake. Picea mariana replaced P. glauca as the dominant Picea species around Chokasna Lake at c. 2000 bp. MFI was > 500 years before 3800 bp, except from 5800 to 5000 bp at Moose Lake and 5400 to 4550 bp at Chokasna Lake, when values were c. 200 years. MFI decreased to c. 200 years during the late‐Holocene at both sites and to c. 150 years after 2000 bp at Chokasna Lake. At both sites, fires occurred more frequently under wetter climatic conditions. Our results therefore support other recent studies demonstrating that warmer/drier climatic conditions do not necessarily induce greater fire importance. A combination of increased ignition by lightning strikes and seasonal‐moisture variability probably resulted in more frequent fires under wetter conditions.

Reconstruction of hydrological changes and response to effective moisture variations from North-Central USA lake sediments

Ratios of the stable isotopes of oxygen and carbon in benthic ostracodes and marl from cores taken from two lakes in north-central Minnesota reflect Holocene hydrological and vegetation changes. Oxygen isotopes show that Williams and Shingobee Lakes, located in the same watershed but with different positions along a hydrologic gradient, were connected before 9.8 ka as part of a larger lake, Lake Willobee. From 9.8–7.7 ka, the level of Lake Willobee fell as a result of glacial retreat and increasing evaporation, leaving small separated basins. Further decreases in lake level after 7.7 ka due to increasing aridity triggered the inflow of ground water in Williams Lake at about 7 ka, and in Shingobee Lake at about 5 ka. After 4 ka effective moisture increased. The carbon-isotope record reflects changes in vegetation with higher δ 13C values during the prairie period (7.7–4 ka) and lower values during preceding and succeeding forest periods. The differences in timing of hydrological events show that the biotic and geochemical response of the lakes to climate variations is mediated by their hydrologic systems. The response may be strongly spatially heterogeneous and can result in contrasting information from geochemical and biotic proxies from the same paleorecord.