Late Holocene Moisture Research Articles

OSL age and stratigraphy of the Strauss sand sheet in New Mexico, USA

The Strauss sand sheet occurs in south-central New Mexico, USA, and northern Chihuahua, Mexico, covering an area of about 4740km2. Its chronology is determined by 19 OSL ages. The sand sheet formed primarily during three phases of eolian deflation and deposition, each phase with a separate sand source and under different climatic and environmental circumstances. The first phase of eolian sedimentation occurred 45 to 15ka with the deposition of unit 1. The sand source for the first phase was beach-related features along the eastern shoreline of pluvial Lake Palomas in Mexico. The glacial-age climate was cool, wet, and windy because of the southern path of the jet stream at that time. After 15ka, with the onset of warmer conditions of the Bølling–Allerød, the shutting down of the Palomas sand source, and wet conditions of the Younger Dryas, the sand sheet stabilized with weak soil development in unit 1. By 11ka, the climate shifted to Holocene drying conditions and the second phase of sand accumulation began, forming unit 2; the sand source was the local deflation of the previously deposited unit 1 sand. The sand sheet stabilized again by 1.9ka with slightly wetter late Holocene climate; a weak soil formed in unit 2 sand. About A.D. 1500 and extending to about A.D. 1850 or later, an A horizon formed on the sand sheet, probably in response to a desert grassland vegetation during the period of wet climate of the Little Ice Age. In an anthropogenic third phase of eolian activity, after A.D. 1850, the vegetation was likely disturbed by overgrazing; and the unit 2 and A horizon (unit 3) sands were deflated, resulting in the deposition of a thin layer of massive eolian sand (unit 4) across the sand sheet. By about A.D. 1900 mesquite shrubs had increased in abundance; and deflated sand, largely from unit 2, began to accumulate around the shrubs, forming coppice dunes (unit 5). Mesquite coppice dunes continued to increase in number and volume during the twentieth century and at present dominate most of the sand sheet. This third phase of eolian deflation–deposition is ongoing today.

LATE GLACIAL AND HOLOCENE ENVIRONMENTAL CHANGE INFERRED FROM THE PÁRAMO OF CAJANUMA IN THE PODOCARPUS NATIONAL PARK, SOUTHERN ECUADOR

To reconstruct the environmental history including vegetation, fire and climate dynamics, from the Cajanuma valley area ( 3285 m elevation) in the Podocarpus National Park, southern Ecuador , we address the following major research question: (1) How did the mountain vegetation developed during the late Glacial and Holocene? (2) Did fire played an important control on the vegetation change and was it natural or of anthropogenic origin?. Palaeoenvironmental changes were investigated using multiple proxies such as pollen, spores, charcoal analyses and radiocarbon dating. Pollen data indicated that during the late Glacial and transition to the early Holocene (ca. 16 000–10 500 cal yr BP) herb paramo was the main vegetation type around the study area, while subparamo and mountain rainforest were scarcely represented. The early and mid-Holocene (ca. 10 500 to 5600 cal yr BP) is marked by high abundance of paramo during the early Holocene followed by a slight expansion of mountain forest during the mid-Holocene. During the mid- to late Holocene (ca. 5600–1200 cal yr BP) there is a significant presence of paramo and subparamo while Lower Mountain Forest decreased substantially, although, Upper Mountain Forest remained relatively stable during this period. The late Holocene, from ca. 1200 cal yr BP to present, was characterized by paramo; however, mountain forest and subparamo presented significantly abundance compared to the previous periods. Fires became frequent since the late Holocene. The marked increased local and regional fire intensity during the wetter late Holocene strongly suggests that were of anthropogenic origin. During the late Glacial and early Holocene, the upper forest line was located at low elevations; but shifted slightly upslope to higher elevations during the mid-Holocene.

Mid- to Late-Holocene Australian–Indonesian summer monsoon variability

The Australian–Indonesian monsoon has a governing influence on the agricultural practices and livelihood in the highly populated islands of Indonesia. However, little is known about the factors that have influenced past monsoon activity in southern Indonesia. Here, we present a ∼6000 years high-resolution record of Australian-Indonesian summer monsoon (AISM) rainfall variations based on bulk sediment element analysis in a sediment archive retrieved offshore northwest Sumba Island (Indonesia). The record suggests lower riverine detrital supply and hence weaker AISM rainfall between 6000 yr BP and ∼3000 yr BP compared to the Late Holocene. We find a distinct shift in terrigenous sediment supply at around 2800 yr BP indicating a reorganization of the AISM from a drier Mid Holocene to a wetter Late Holocene in southern Indonesia. The abrupt increase in rainfall at around 2800 yr BP coincides with a grand solar minimum. An increase in southern Indonesian rainfall in response to a solar minimum is consistent with climate model simulations that provide a possible explanation of the underlying mechanism responsible for the monsoonal shift. We conclude that variations in solar activity play a significant role in monsoonal rainfall variability at multi-decadal and longer timescales. The combined effect of orbital and solar forcing explains important details in the temporal evolution of AISM rainfall during the last 6000 years. By contrast, we find neither evidence for volcanic forcing of AISM variability nor for a control by long-term variations in the El Niño-Southern Oscillation (ENSO).

A mineralogical and organic geochemical overview of the effects of Holocene changes in Amazon River flow on three floodplain lakes

A synthesis of the impacts of the Amazon River hydrological changes on the sedimentation process of organic matter (OM) in three different floodplain lakes (Santa Ninha, Maracá, and Comprido lakes) is presented in this study. Today the Santa Ninha and Maracá lakes are directly and permanently connected with the main channel of the Amazon River, in contrast to Comprido Lake, which is indirectly and periodically influenced by the Amazon River due to its greater distance from the main channel. All three lake sediment records showed a reduced river inflow due to dry climatic conditions during the early and middle Holocene followed by an increased fluvial input during the wetter late Holocene. In Santa Ninha and Maraca Lakes, the reduced river inflow period was recorded by sediments with a low abundance of smectite (on average ~20wt.%), a clay mineral mainly transported by the fluvial system, high total organic carbon (TOC) contents (on average ~8.2wt.%), and a predominantly acidic soil OM input evidenced by high concentrations (on average 180μggTOC−1) of branched glycerol dialkyl glycerol tetraethers (GDGT). During the late Holocene, a higher smectite abundance (on average ~43 wt.%) and a lower TOC content (on average ~1.4 wt.%) pointed to greater dilution by riverine lithogenic matter. This change was accompanied by a proportional increase in the aquatically produced crenarchaeol, suggesting a higher lake water level. In Comprido Lake, a sedimentation gap occurred during the early and middle Holocene. The wetter late Holocene, since 3000calyears BP, was characterized by high TOC values (on average ~9wt.%) and a sharp increase in soil OM input as recorded by an increase in branched GDGT concentrations (on average ~81μggTOC−1), but the smectite content was low (on average ~14%). This combination suggests that the soil OM input to Comprido Lake from the local catchment area became dominant during the wet-climate late Holocene due to the large distance of the lake from the Amazon River main channel. Consequently, our study shows that the sedimentation processes of OM in Amazonian floodplain lakes were strongly influenced by variations in the hydrodynamic regime of the Amazon River during the Holocene. However, the impacts of the variations on the three floodplain lakes were different depending on the distance of each lake from the main channel of the Amazon River.

How different proxies record precipitation variability over southeastern South America

Detrending natural and anthropogenic components of climate variability is arguably an issue of utmost importance to society. To accomplish this issue, one must rely on a comprehensive understanding of the natural variability of the climate system on a regional level. Here we explore how different proxies (e.g., stalagmite oxygen isotopic composition, pollen percentages, bulk sediment elemental ratios) record Holocene precipitation variability over southeastern South America. We found a general good agreement between the different records both on orbital and centennial time-scales. Dry mid Holocene, and wet late Holocene, Younger Dryas and a period between ~9.4 and 8.12 cal kyr BP seem to be pervasive features. Moreover, we show that proxy-specific sensitivity can greatly improve past precipitation reconstructions.

Late-Holocene moisture balance inferred from diatom and lake sediment records in western Mongolia

The Valley of the Great Lakes in western Mongolia is a unique ecosystem comprising a wide variety of terrestrial and aquatic habitats. Relatively little is known about the long-term climate history of the area or the relative impact of changes in climate and land use in the region. Tree-ring records have established an understanding of regional temperature fluctuations; however, the few records of moisture balance from Mongolia offer little insight into potential changes in the precipitation-to-evaporation balance in the region with warming temperatures. Modern and paleoclimatic records from across arid Central Asia show substantial temporal and spatial variability in the correlation between temperature and moisture availability and climate models are inconsistent in predictions for future changes with continued warming. In order to refine the understanding of moisture balance in the region, we developed six diatom-inferred records of lake salinity between 200 and 2000 years in length, demonstrating a negative correlation between temperature and effective moisture (warm-dry and cool-wet conditions) over most of this time period. In addition to climate warming, Mongolian grasslands have seen substantial human impacts over the last several decades. Recent core records show evidence of eutrophication and must be interpreted carefully to understand the individual and combined impacts of climate and land use on sedimentary records and the potential for human—environment interactions to confound inferences about climate changes.

The pattern of Holocene climate change in the arid central Asia: a case study based on lakes

The palaeoclimate records from lakes at the arid central Asian region (ACA) climatically dominated by the Westerly circulation shows synchronous and coherent moisture changes during the Holocene. Therefore, the sediment records from lakes in ACA with reliable chronologies and robust proxies were selected to reconstruct moisture histories based on a five-class ordinal wetness index with assigned scores from the driest to wettest periods at individual sites for 200-year time slices. There are 8 lakes suitable for the synthesis. The results show that ACA as a whole experienced a dry early Holocene, a wetter (less dry) early to mid-Holocene, and a moderately wet late Holocene, which is different from Holocene monsoon history recently recovered by documents such as speleothem. We propose that the pattern of Holocene effective-moisture evolution in the westerly dominated ACA was mainly determined by North Atlantic sea-surface temperatures (SSTs) and high-latitude air temperatures that affect the availability, amount and transport of water vapor.

Holocene moisture evolution in arid central Asia and its out-of-phase relationship with Asian monsoon history

We synthesize palaeoclimate records from the mid-latitude arid Asian region dominated today by the Westerlies (“arid central Asia” (ACA)) to evaluate spatial and temporal patterns of moisture changes during the Holocene. Sediment records from 11 lakes with reliable chronologies and robust proxies were selected to reconstruct moisture histories based on a five-class ordinal wetness index with assigned scores from the driest to wettest periods at individual sites for 200-year time slices. The proxies used in these records include pollen and diatom assemblages, sediment lithology, lake levels, and geochemistry (mainly isotope) data. The results of our synthesis show that ACA as a whole experienced synchronous and coherent moisture changes during the Holocene, namely a dry early Holocene, a wetter (less dry) early to mid-Holocene, and a moderately wet late Holocene. During the early Holocene most of the lakes experienced very low water levels and even dried out before ca 8 ka (1 ka=1000 cal a BP). Hence the effective-moisture history in ACA is out-of-phase with that in monsoonal Asia as documented by numerous palaeoclimate records. In monsoonal Asia, a strong summer monsoon and humid climate characterized the early Holocene, and a weakened summer monsoon and drier climate prevailed during the late Holocene, which were mainly controlled by changes in low-latitude summer insolation. In contrast, we propose that the pattern of Holocene effective-moisture evolution in the westerly dominated ACA was mainly determined by North Atlantic sea-surface temperatures (SSTs) and high-latitude air temperatures that affect the availability, amount and transport of water vapor. Also, topography of the Tibetan Plateau and adjacent Asian highlands could have contributed to the intensification of dry climate in ACA during the early Holocene, as a result of strengthening the subsidence of dry air masses, associated with stronger uplift motion on the plateau by intense heating under a stronger summer insolation. Summer insolation might have played a key role in directly controlling moisture conditions in ACA but only after the northern hemisphere ice-sheets had disappeared in the mid- and late Holocene.

Middle-to late-Holocene moisture changes in the desert of northwest Namibia derived from fossil hyrax dung pollen

New pollen results and radiocarbon dating from fossil hyrax middens derived from the edge of the northern Namib Desert address the shortage of continental palaeobotanical evidence in arid Namibia by providing evidence for the environmental conditions during the mid to late Holocene in the region. The results obtained reflect long-term stability in the area throughout most of the sequence. Higher than modern moisture availability is suggested between c. 6 and 1 ka BP by the increased abundance of Poaceae, Cyperaceae or Chenopodiaceae, which respond rapidly by flowering after modest quantities of rainfall. Around 1 ka and recently arid conditions seem to have prevailed, with a decrease in Poaceae and an increase in Acanthaceae. Other palaeoecological evidence from the local and regional surroundings that focused on fluvial deposits, marine palynology and mineralogy confirms this mid-Holocene increase in more effective rainfall.

Late Holocene moisture balance variability in the southwest Yukon Territory, Canada

Analyses of sediment cores from Marcella Lake, a small, hydrologically closed lake in the semi-arid southwest Yukon, provides effective moisture information for the last ∼4500 years at century-scale resolution. Water chemistry and oxygen isotope analyses from lakes and precipitation in the region indicate that Marcella Lake is currently enriched in 18O by summer evaporation. Past lake water values are inferred from oxygen isotope analyses of sedimentary endogenic carbonate in the form of algal Charophyte stem encrustations. A record of the δ 18O composition of mean annual precipitation at Jellybean Lake, a nearby evaporation-insensitive system, provides data of simultaneous δ 18O variations related to decade-to-century scale shifts in Aleutian Low intensity/position. The difference between the two isotope records, Δδ, represents 18O-enrichment in Marcella Lake water caused by summer effective moisture conditions. Results indicate increased effective moisture between ∼3000 and 1200 cal BP and two marked shifts toward increased aridity at ∼1200 and between 300 and 200 cal BP. These prominent late Holocene changes in effective moisture occurred simultaneously with changes in Aleutian Low circulation patterns over the Gulf of Alaska indicated by Jellybean Lake. The reconstructed climate patterns are consistent with the topographically controlled climatic heterogeneity observed in the coastal mountains and interior valleys of the region today.

A high-resolution record of late-Holocene moisture variability from a Michigan raised bog, USA

We reconstructed the late-Holocene surface-moisture history and vegetation dynamics of a raised bog in Michigan using testate amoebae, peat humification, pollen, stomata and plant macrofossils. Our primary objective was to compare bog palaeohydrology with the water-level history of Lake Michigan, and to regional and local vegetation changes. Hydrologic histories inferred from testate amoebae and humification show similar trends, and correspond with records of past water-level variability in Lake Michigan. The bog clearly shows effective-moisture increases during the Algoma highstand (∼3200–2300 cal. BP) and a later unnamed highstand (∼1900–1300 cal. BP). Some higher-frequency fluctuations are also similar. The good correspondence indicates that bog hydrology and the water levels of Lake Michigan have been driven by changes in large-scale atmospheric circulation patterns for at least the past 3500 years. Climate during the Algoma highstand may have been spatiotemporally complex, with increasing moisture occurring several hundred years earlier in the northern portion of the western Great Lakes basin. Locally, the expansion and contraction of Picea populations on the bog surface was probably directly related to moisture conditions. The well-documented decline in Fagus populations in the central Great Lakes region after 1000 cal. BP is contemporaneous with a major shift towards drier conditions, suggesting that relative dryness caused the decline.

Contributions of La Niña and El Niño to middle Holocene drought and late Holocene moisture in the American Southwest

Eolian landforms in Estancia basin, central New Mexico, record two episodes of extreme drought and low groundwater levels during the middle Holocene (7000-5400 1 4 C yr B.P.), followed by a rise in the water table throughthe late Holocene. Blowouts and associated lunettes formed when water levels fell below the desiccated floor of pluvial Lake Estancia, allowing widespread deflation. Elevation of the water table in the basin is regulated by a balance between evaporation from playas that occupy the blowouts and recharge of aquifers in the adjacent Manzano Mountains. Isotopic analyses of modern precipitation and groundwater reveal that recharge originates primarily as winter moisture, which is amplified during El Ninos and diminished during La Ninas. Thus, changes in the elevation of a reconstructed Holocene water table may reflect systematic changes in El Nino-Southern Oscillation (ENSO) circulation.

Holocene environmental changes in the Central Amazon Basin inferred from Lago Calado (Brazil)

Holocene environments have been reconstructed by sedimentological, mineralogical geochemical and high resolution pollen analysis of a 1190 cm-long core from Lago Calado near Manaus in Central Amazonia. The sediment core, dated by eight AMS-radiocarbon dates, has a basal age of 8330±50 14 C yr bp and documents the landscape development from a river valley before 8280 14C yr bp to a lake environment. This change is related to the rise of the Central Amazonian water level, controlled by the postglacial sea-level rise. Pollen analytical results show the formation of a local Mauritia palm-swamp along the river margin between 8280 and 7700 14C yr bp. At that time, highly diverse terra firme (unflooded upland) Amazon rainforest and poorly developed várzea/igapó (inundated) forests along the river characterized the vegetation. The expansion of várzea/igapó forests and a strong presence of aquatic plants started at 7700 14C yr bp, reflecting the rise of the Amazonian water level and the formation of the Lago Calado. The occurrence of abundant Poaceae and Cyperaceae pollen in the lake sediments since 7770 14C yr bp, suggest both the local colonization of unflooded mud banks around the lake margin and the formation of flooding meadows. Open exposed unflooded mud areas, periodically colonized by herbs, are related to seasonal Amazonian high and low water stands. In respect to the abundance of Poaceae pollen during the early and mid Holocene, these open areas must have been large, suggesting brief annual high and extensive annual low Amazonian water stands. After 4070 14C yr bp the proportion of várzea/igapó forest areas was larger than before. Herbs were less abundant, probably due to the smaller area of unflooded mud banks. The Amazonian water levels must have been higher and the period of the annual high water stands was probably longer than before. Since 2080 14C yr bp, the largest proportion of várzea/igapo forests is recorded, reflecting the highest Amazonian water level. Herbs were rare, suggesting that open unflooded mud areas around the lake were relatively small. This may perhaps also indicate that the seasonal period of the Amazonian high water level was the longest since 2080 14C yr bp. The increase of Cecropia, Poaceae and Alternanthera during the last 150 yr, suggests stronger human impact on the local vegetation. Changes to higher Amazonian water levels are related to the rise of the Atlantic sea-level. Relatively short annual high water levels during the early Holocene and relatively long annual high water levels since 4070 and especially since 2080 14C yr bp, can be interpreted as a climate change from a drier early Holocene (with lower annual rainfall rates and longer dry periods) until 4070 14C yr bp to a wetter late Holocene.

Holocene changes in seasonal precipitation highlighted by fire incidence in eastern Canada

Postglacial fire history has been reconstructed for eastern Canada from charcoal-influx anomalies from 30 sites taken from a lacustrine charcoal database. The reconstruction exhibits coherent patterns of fire occurrence in space and time. The early Holocene is characterised by high fire incidence. There is a major change to much lower occurrence slightly after 8 ka BP. A return to more fire appears after 3 ka BP. This sequence does not fit with the hydro-climatic reconstruction deduced from lake level reconstructions for northeastern North America, which indicates a dry early and mid-Holocene, and a wet late-Holocene. Fire occurrence however closely matches summer relative humidity inferred from δ18O. The differences between fire frequency and lake level history, are due to changes in the seasonality of precipitation and drought frequency. Lake levels are essentially controlled by winter precipitation while summer precipitation controls fire occurrence. The early Holocene before 8–7.5 ka BP experienced dry summers due to higher solar radiation and dry adiabatic winds from the residual Laurentide Ice Sheet. The middle Holocene was dominated by wet summers due to stability of the Atlantic air mass over eastern Canada. After 2.5 ka BP, summers became drier, albeit not as fire-conducive as during the early Holocene. Late-Holocene summers conducive to fire are explained by more frequent incursions of dry Cool Pacific or Cold Arctic air masses over eastern Canada.

Late Quaternary vegetation, climate and fire history from the tropical mountain region of Morro de Itapeva, SE Brazil

Late Quaternary paleoenvironments have been interpreted from a 35,000 yr B.P. pollen and charcoal record from Morro de Itapeva (22°47′S, 45°32′W, 1850 m a.s.l.) in the SE Brazilian highlands. Changes in taxa composition and elevational shifts in high elevation grassland (campos de altitude), Araucaria forest, cloud forest and Atlantic rain forest, all of which can be found in the study area today, indicate climatic changes in the past. Expanded high elevation grasslands, absence of Araucaria forest, cloud forest and Atlantic rain forest during the recorded last glacial period (ca. 35,000–17,000 yr B.P.) suggest a climate markedly cooler and drier than today. Development of a Sphagnum peat bog, rare presence of Araucaria forest taxa and existence of a narrow cloud forest and rain forest belt at lower elevations during the late glacial period (17,000-ca. 10,000 yr B.P.) indicate a change to a cool and somewhat moister climates. During the early Holocene cloud forest developed close to the study site, reflecting a warm and moist climate on the east facing slopes, but a drier climate on the highland plateau, evidenced by the rare presence of Araucaria and Podocarpus. Only during the late Holocene moisture increases in the highlands, suggested by the higher frequency of Araucaria and Podocarpus. Fires were more frequent during the last glacial than during the Holocene.

Oxygen and carbon isotope trends and sedimentological evolution of a meromictic and saline lacustrine system: the Holocene Medicine Lake basin, North American Great Plains, USA

Abstract The succession of sedimentary subfacies in the Medicine Lake basin (South Dakota, USA) documents numerous changes in total dissolved solids (TDS), lake level, and the nature and stability of meromixis during the Holocene. The lake formed about 10.6 ka B.P. when coarse clastics and massive fine-grained muds were deposited in a well-mixed freshwater system. Subsequently the lake became saline and meromictic, depositing a finely laminated unit composed of variegated and gray muds, organic facies, gypsum and aragonite laminae. Phototrophic bacterial plate communities (BPC) at the chemocline were significant producers of pelagic organic matter. Relatively high lake levels (at least 10 m higher than today) but saline and meromictic conditions predominated through the early to mid Holocene. At the end of this phase, lake level dropped and stable meromixis was disrupted. Sands and massive or banded muds were deposited in sublittoral areas and benthic microbial communities (BMC) spread over the lake, replacing BPCs as the predominant microbial producers. When lake level rose again, massive and laminated units were deposited under less saline conditions. Another pronounced episode of low lake level and high salinity followed. The most recent sediments are sapropels, massive or banded muds, and littoral sands. The δ18O record from authigenic aragonite shows more positive values during the arid mid-Holocene interval, with an abrupt increase at the end of this period, followed by generally more negative values during the cooler and wetter late Holocene. The small range of δ18O values (3%.) suggests that the lake was well buffered against high-frequency climatic cycles, probably as a result of groundwater modulation. We postulate that meromixis and the type of photosynthetic microbial communities dominating the basinal region of the lake system—either BPCs or BMCs—exerted major controls on the carbon cycle, and consequently, on the development of isotopic trends linking δ13C and δ18O values. More negative δ13C values during the early and mid Holocene are interpreted as a consequence of the predominance of 12C-enriched microbial carbon from BPCs. Examples of both covariant and inverse covariant isotope trends between δ13C and δ18O values occur in the Medicine Lake record. Inverse covariant trends characterize long periods of stable meromixis when BPCs predominate over BMCs. We hypothesize that well-developed water stratification caused by greater influx of relatively 18O-depleted freshwater induces higher productivity at the chemocline and, consequently, more positive δ13C values in the precipitated carbonates. Unstable phases of meromixis or holomictic conditions are characterized by a predominance of BMCs or other biogenic producers over BPCs, and a number of other factors combine to promote covariant trends. The Medicine Lake record is consistent with the paleoclimatic interpretation for the northern Great Plains of a more arid early and middle Holocene followed by an abrupt transition to a relatively cooler and wetter late Holocene. The sedimentary succesion in Medicine Lake documents a much more fluctuating and abrupt climate behavior than suggested by the vegetation paleorecord and suggest a late Holocene climate punctuated by arid episodes.