Onset Of Wetter Conditions Research Articles

A Chironomid Record of Early-Middle Holocene Environmental Evolution in the Darhad Basin, Northern Mongolia.

Simple SummaryThe Holocene humidity evolution presents spatial heterogeneity in the Mongolian Plateau where multiple circulation systems converged. Inconsistent records of regional environmental evolution affect accurate assessment of lake ecosystems. Chironomid larvae are one of the most abundant benthic invertebrates in lakes. The taxa composition of their functional groups is largely determined by aquatic habitats. Therefore, analyzing the changes of their functional traits can effectively indicate the hydrological dynamics in the past. In this study, we analyzed the early-middle Holocene (9.0–4.5 cal kyr BP) subfossil chironomid assemblages of a sedimentary sequence from the Darhad Basin in northern Mongolia. At 9.0 cal kyr BP, the community structure suddenly changed from littoral taxa to sublittoral/profundal taxa, reflecting an environmental transition from a river or shallow lake condition to a deep lake environment, which lasted until 4.5 cal kyr BP. Those hydrological patterns are consistent with the humidity evolution in the Westerlies dominated region, except that the onset of wetness occurred one thousand years earlier when comparing our results with previous ones, which may be related to the melting of regional glaciers and permafrost caused by increased summer solar insolation.Under the influence of various circulation systems, the Holocene humidity conditions on the Mongolian Plateau are spatially heterogeneous and the underlying mechanism is still ambiguous. The complexity of climate change may affect the accuracy of assessing lake ecosystem evolution. In this study, based on the precise chronology, a chironomid assemblage sequence from the Darhad Basin in northern Mongolia is analyzed to elucidate the hydroclimate variation during the early-middle Holocene. The results show that the chironomid communities changed suddenly from littoral taxa to sublittoral/profundal taxa at about 9 cal kyr BP, reflecting an environmental transition from a river or shallow lake condition to a deep lake environment. Thereafter, most parts of the paleolake remained at a relatively high level until 4.5 cal kyr BP. This hydrological pattern resembles the typical humidity variations in the Westerlies affected regions, except that the onset of wetter conditions occurred one thousand years earlier as reflected in our results. The melting of glaciers and permafrost in the basin resulting from the early increased summer solar insolation could be a feasible explanation for these time advances.

Zaraa Uul: An archaeological record of Pleistocene-Holocene palaeoecology in the Gobi Desert.

Environmentally-based archaeological research at Zaraa Uul, including zooarchaeology, phytolith analysis, and radiocarbon dating, is the first of its kind in Mongolia and presents critical new insight on the relationship between periods of occupational intensity and climatic amelioration from the earliest anatomically modern humans to the adoption of pastoralism. The palaeoenvironmental and faunal record of Zaraa Uul show that Early-Middle Holocene hydrology and species distributions were distinct from all other periods of human occupation. Holocene hunter-gatherers inhabited an ecosystem characterized by extensive marshes, riparian shrub and arboreal vegetation along the hill slopes and drainages. The exploitation of species associated with riparian and wetland settings supports the hypothesis of, but suggests an earlier timing for, oasis-based logistical foraging during the Early-Middle Holocene of arid Northeast Asia. The onset of wetter conditions at 8500 cal BP agrees with other regional studies, but multiple lines of evidence present the first integrated field- and laboratory-based record of human-environment relationships in arid East Asia during the Holocene Climatic Optimum. We compare it to Late Pleistocene climatic amelioration, and highlight specific responses of the hydrological, vegetative and faunal communities to climate change in arid Northeast Asia.

Vegetation changes and plant wax biomarkers from an ombrotrophic bog define hydroclimate trends and human-environment interactions during the Holocene in northern Norway

Holocene climate records from northern Europe improve our understanding of important North Atlantic ocean and atmospheric circulation systems to long-term insolation-driven changes, as well as more rapid forcing and feedback mechanisms. Here we assess Holocene climate and environmental changes in northern Norway based on the analysis of pollen, non-pollen palynomorphs, plant macrofossils, and plant wax biomarkers from a high latitude ombrotrophic bog. We define the extent and thickness of Hollabåttjønnen Bog (0.16 km2), which is located 10 km north of Tromsø. Several cores were analyzed, including a 5.16-m core that spans the last 9.5 cal ka BP. Vegetation changes from several sites were reconstructed and the distribution and hydrogen isotopic composition (δD) of n-alkanes (C21–C33) were analyzed. Our data show several distinct climate intervals that primarily indicate changes in bog surface moisture. In the early Holocene (c. 9.5–7.7 cal ka BP), wetter conditions are defined by the presence of wetland sedges and grasses, higher concentrations of mid-chain length n-alkanes, and a similarity in δD values among homologs. A dry mid-Holocene (c. 7.7–3.8 cal ka BP) is inferred from the presence of a heath shrubland, low peat accumulations rates, and significant differences between δD values of mid- and long-chain length n-alkanes. The late Holocene (c. 3.8 cal ka BP-present) is marked by the onset of wetter conditions, lateral bog expansion, and an increase in sedges and grasses. The Hollabåttjønnen Bog record is also significant because its margins were an important location for human settlement. We correlate early Holocene environmental conditions with changes in Stone Age structures recently excavated, and we identify the occurrence of coprophilous fungi, such as Sporormiella and Sordaria, likely associated with reindeer grazing activity beginning c. 1 cal ka BP. This site therefore provides important regional paleoclimate information as well as context for evaluating local prehistoric human-environment interactions.

Sediment biomarkers elucidate the Holocene ontogeny of a shallow lake.

We carried out geochemical analyses on a sediment core from Lake Harris, Florida (USA) to identify sources of organic matter to the sediment throughout the Holocene, and relate changes in those sources to shifts in past climate and environmental conditions. We hypothesized that the sources of organic matter changed in response to regional hydrologic shifts following de-glaciation, and to human population expansion in the state during the 20th century. Hydroclimate shifts in Florida were related to: 1) a steady rise in relative sea level and the fresh water table that began in the early Holocene, 2) wetland formation and expansion ca. 5,000 cal yrs BP, and 3) the onset of the modern El Niño (ENSO) cycle ~3,000 cal yrs BP. Stratigraphic changes in sediment variables from Lake Harris reflect each of these hydroclimate periods. Early in the Holocene, Lake Harris was a marsh-like system in a relatively dry, open-prairie environment. Organic sediments deposited at that time were derived largely from terrestrial sources, as inferred from high TOC/TN ratios, a dominance of longer-chain of n-alkanes (n-C29-31), relatively negative organic carbon isotope values (δ13CTOC), and low biogenic silica concentrations. In the middle Holocene, a positive shift in δ13CTOC coincided with the onset of wetter conditions in Florida. Submerged macrophyte biomarkers (n-C21-23) dominated, and during that period bulk organic carbon isotope values were most similar to δ13C values of mid-chain-length n-alkanes. In the late Holocene, δ13CTOC values declined, CaCO3 levels decreased to trace amounts, organic carbon concentrations increased and diatom biogenic silica concentrations increased from 10 to 120 mg g-1. Around 2,900 cal yrs BP, the effects of ENSO intensified and many Florida lakes deepened to their current limnetic state. Concentrations of algal and cyanobacterial biomarkers in the Lake Harris core increased by orders of magnitude after about AD 1940, in response to human-induced eutrophication, an inference supported by values of δ15N that fluctuate around zero.

Hydrology and climatology at Laguna La Gaiba, lowland Bolivia: complex responses to climatic forcings over the last 25 000 years

ABSTRACTDiatom, geochemical and isotopic data provide a record of environmental change in Laguna La Gaiba, lowland Bolivia (17°45′S, 57°35′W), over the last ca. 25 000 years. High‐resolution diatom analysis around the Last Glacial–Interglacial Transition provides new insights into this period of change. The full and late glacial lake was generally quite shallow, but with evidence of periodic flooding. At about 13 100 cal a BP, just before the start of the Younger Dryas chronozone, the diatoms indicate shallower water conditions, but there is a marked change at about 12 200 cal a BP indicating the onset of a period of high variability, with rising water levels punctuated by periodic drying. From ca. 11 800 to 10 000 cal a BP, stable, deeper water conditions persisted. There is evidence for drying in the early to middle Holocene, but not as pronounced as that reported from elsewhere in the southern hemisphere tropics of South America. This was followed by the onset of wetter conditions in the late Holocene consistent with insolation forcing. Conditions very similar to present were established about 2100 cal a BP. A complex response to both insolation forcing and millennial‐scale events originating in the North Atlantic is noted.

Lake Malawi's response to “megadrought” terminations: Sedimentary records of flooding, weathering and erosion

Sediment records from the northern basin of Lake Malawi provide a means of evaluating the lake basin's response to climate change over the past 75 ky, notably to increased precipitation at the terminations of droughts. Transitions from drier to wetter conditions provide an opportunity to evaluate the system's response to climate shifts. Upon termination of drought episodes at 62 and 72 ka, enhanced precipitation and an associated increase in streampower led to enhanced physical erosion and landscapes were flooded by rising lake waters. These processes appear to have left their mark in the sedimentary record, bringing about a spike of deposition of organic matter (probably of terrestrial origin) at times of increased rainfall. This was immediately followed by a period of deposition of chemically-weathered material that had been retained on the landscape during arid times and mobilized in response to increased precipitation. After this altered material was removed (perhaps a thousand years after the transition to wetter conditions), fresher material, richer in soluble elements including nutrients, was exposed to chemical weathering, leading to substantial diatom blooms. The lag between the onset of wetter conditions and the diatom blooms is inconsistent with significant storage of bioavailable silica in soils in this system. However, biological cycling of silica, including formation and dissolution of phytoliths, may have played a role in mobilization of the silica necessary for the diatom productivity.

Seasonal water relations of Lyginia barbata (Southern rush) in relation to root xylem development and summer dormancy of root apices

*Periods of dormancy in shallow roots allow perennial monocotyledons to establish deep root systems, but we know little about patterns of xylem maturation, water-transport capacities and associated economies in water use of growing and dormant roots. *Xylem development, anatomy, conductance and in situ cellular [K] and [Cl] were investigated in roots of field-grown Lyginia barbata (Restionaceae) in Mediterranean southwestern Australia. Parallel studies of gas exchange, culm relative water loss and soil water content were conducted. *Stomatal conductance and photosynthesis decreased during summer drought as soil profiles dried, but rates recovered when dormant roots became active with the onset of wetter conditions. Anatomical studies identified sites of close juxtaposition of phloem and xylem in dormant and growing roots. Ion data and dye tracing showed mature late metaxylem of growing roots was located >or= 100 mm from the tip, but at only <or= 10 mm for dormant roots. Dormant roots remained hydrated in dry soils (0.001-0.005 g g(-1)). *Effective regulation of growth and water-conserving/obtaining properties permits the survival of shallow roots of L. barbata during summer drought and may represent important strategies for establishing deeper perennial root systems in other monocotyledonous plants adapted to seasonally dry habitats.

Holocene landscape evolution along the Andean piedmont, Bolivian Chaco

As part of the Gran Chaco, the Andean piedmont in Eastern Bolivia is characterized by a highly seasonal climate and large rainfall variability. It receives sediments from small drainage basins in the Subandean ranges. Therefore it should constitute a suitable sedimentary and geomorphic archive for the detection of past environmental changes. This study reconstructs Holocene landscape evolution along the Andean piedmont in the Bolivian Chaco by combining large-scale geomorphological analysis and the investigation of sedimentary sequences. Fluvial aggradation and channel avulsions are the dominant processes of piedmont construction, and were particularly pronounced during the mid-Holocene. This is interpreted to reflect a generally active landscape characterized by reduced forest cover as result of increased aridity. Widespread soil formation and landscape stability in all investigated catchments start after 4 cal ka BP indicating the onset of wetter conditions and successively denser forest cover in the Bolivian Chaco. Renewed sedimentation during the last millennium could imply a shift to more arid environmental conditions, probably in relation to larger-scale climatic phenomena such as the Little Ice Age (LIA). Particularly during the last centuries the fluvial system of the study area has undergone the most profound changes of the entire Holocene with sedimentation of very coarse sediments and subsequent incision. This reflects important changes both in discharge and sediment supply, and might be the result of either shorter-term climate changes or increasing human impact.

What is the impact of Holocene climatic changes on human societies? Analysis of West African Neolithic populations dietary customs

This work explores the evolution of the subsistence economy and strategies of West African Neolithic populations in relation to Holocene climatic changes, focusing on the dynamic cultural responses of Neolithic people to the desiccation of their environment. These climatic changes are diachronic within the whole of northern Africa, but followed the same general trend: onset of wet conditions during early Holocene followed by a progressive desiccation proceeding by successive short phases of aridity. Population dynamics are strongly related to this climatic context, and the archaeozoological evidence indicates that the human response to the onset of arid climate conditions is a diversification of their dietary resources, maintaining hunting and fishing when possible, and adopting cattle breeding towards the end of Holocene. Only at the Neolithic/Historic transition is the drastic reduction of wild fauna associated with significant demographic and climatic stress, leading to a profound re-structuring of diet through the intensification of pastoral activities.

High resolution tropical climate record for the last 6,000 years

Planktonic foraminiferal oxygen isotope records from the Cariaco Basin, offshore Venezuela were used to construct high‐resolution tropical Atlantic climate records for the mid to late Holocene. Our results indicate that major increases in δ18O due both to increases in salinity and decreases in sea surface temperature occurred at least six times over the last 6,000 years (∼6,000–5,000, 4,500–4,200, 3,800–3,200, 3,000–2,800, 2,200–2,000, and 1200–800 cal yrs. B.P.). The initial δ18O increase centered at 5,500 years ago is coincident with the development of arid conditions in the Caribbean region and the end of the “African humid period” indicating a global drying of the northern tropics at this time. Synchronous with the aridification of the Caribbean region are the onset of wetter conditions in the South American Altiplano and the reoccupation of this region by humans. This combination of contrasting climate conditions is attributed to a southward displacement of the Inter‐Tropical Convergence Zone (ITCZ) that would have resulted in decreased precipitation and increased trade wind intensity in the Caribbean region and increased rainfall over the Altiplano.

Holocene hydrological reconstructions from stable isotopes and paleolimnology, Cordillera Real, Bolivia

Multiproxy analyses of sediment cores from Lago Taypi Chaka Kkota (LTCK) Cordillera Real, Bolivia, provide a record of drier conditions following late Pleistocene deglaciation culminating in pronounced aridity between 6.2 and 2.3 ka B.P. Today LTCK is a glacial-fed lake that is relatively insensitive to changes in P–E because it is largely buffered from dry season draw-down through the year-round supply of glacial meltwater. This was not the case during the middle to late Holocene when glaciers were absent from the watershed. Lake-water δ 18 O values inferred from δ 18 O analysis of sediment cellulose range from −12.9 to −5.3‰ and average −8.7‰ between 6.2 and 2.3 ka B.P. Modern lake-water δ 18 O from LTCK averages −14.8‰ which is compatible with the δ 18 O lw value of −14.3‰ for the surface sediment cellulose. Analyses of δ 18 O from modern surface waters in 23 lakes that span the range from glacial-fed to closed basin vary from −16.6 to −2.5‰. This approximates the magnitude of the down-core shift in δ 18 O lw values in LTCK during the middle to late Holocene from −12.9 to −5.3‰. Strong paleohydrologic change during the middle Holocene is also evident in diatom assemblages that consist of shallow-water, non-glacial periphytic taxa and bulk organic δ 13 C and δ 15 N that show increases likely resulting from degradation of lacustrine organic matter periodically exposed to subaerial conditions. Neoglaciation began after 2.3 ka B.P. as indicated by changes in the composition of the sediments, lower δ 18 O values, and a return to diatom assemblages characteristic of the glacial sediments that formed during the Late Pleistocene. Collectively, these data indicate that the past 2.3 ka B.P. have been the wettest interval during the Holocene. Millennial-scale shifts in the paleohydrologic record of LTCK during the early to middle Holocene conform to other regional paleoclimatic time-series, including Lake Titicaca and Nevado Sajama, and may be driven by insolation and resultant changes in atmospheric circulation and moisture supply. In contrast, an apparent 1200-year lag in the onset of wetter conditions at LTCK (2.3 ka B.P.) compared to Lake Titicaca (3.5 ka B.P.) provides evidence for variable sub-regional hydrologic response to climate change during the middle to late Holocene.

Implications of late Quaternary mammalian fauna from Lukenya Hill (south-central Kenya) for paleoenvironmental change and faunal extinctions

Late Quaternary paleoenvironmental data for East Africa are derived primarily from montane sources and thus we know little about the changing composition of East African savannas. Four archaeological sites at Lukenya Hill in the savanna of the Athi-Kapiti Plains of Kenya that date to the last 40,000 yr preserve a large mammalian fauna. The prehistoric hunters concentrated on migratory ungulates and virtually ignored the resident inselberg ungulates throughout the occupation. Faunas of the last glacial maximum are dominated by an extinct small alcelaphine antelope. Arid-adapted ungulates are present that are regionally absent historically, and Pelorovis is present as well. The small alcelaphine and arid-adapted ungulates are absent in the Holocene deposits. This suggests that there was an expansion of dry savannas during the last glacial maximum. The last glacial maximum aridity, combined with a lack of pastoral-set fires, would have resulted in a vegetative mosaic distinct from the present. Dry woody growth and dry and/or tall grass, all of which are poor forage for ungulates, would have been common where seasonally moist short grasslands are presently extant. These conditions favored the large-bodied, highly hypsodont species in Africa that became extinct with the onset of wet conditions during the early Holocene.