The Kawakawa/Oruanui tephra (KOT) is a key chronostratigraphic marker in terrestrial and marine deposits of the New Zealand (NZ) sector of the southwest Pacific. Erupted early during the Last Glacial ...Maximum (LGM), the wide distribution of the KOT enables inter-regional alignment of proxy records and facilitates comparison between NZ climatic variations and those from well-dated records elsewhere. We present 22 new radiocarbon ages for the KOT from sites and materials considered optimal for dating, and apply Bayesian statistical methods via OxCal4.1.7 that incorporate stratigraphic information to develop a new age probability model for KOT. The revised calibrated age, ±2 standard deviations, for the eruption of the KOT is 25,360 ± 160 cal yr BP. The age revision provides a basis for refining marine reservoir ages for the LGM in the southwest Pacific.
► Presents a new age of 25.4 cal ka for Kawakawa/Oruanui tephra. ► Applies Bayesian statistical modelling to quantify and reduce uncertainties. ► Refines age of critical datum for LGM climate comparisons in the Southwest Pacific.
Branched isoalkyl glycerol dialkyl glycerol tetraethers (GDGTs) of bacterial origin have been found in high abundance in both modern and glacial sediments of New Zealand South Island freshwater lakes ...covering a wide range of altitude (101–2000
m). Like isoprenoid GDGTs of archaeal origin, they provide excellent potential for temperature assessment. For this study, their distribution patterns (MBT, methylation ratio and CBT, cyclisation ratio of branched GDGTs) have been successfully used to develop an initial temperature calibration for the study area and to provide preliminary (palaeo)environmental interpretations. MBT data from modern lake sediments correlate well with measured annual air temperature (
R
2 0.74), enabling a regional calibration for reconstructing palaeotemperatures for fossil samples. MBT-derived palaeotemperatures for Alpine Lake, calibrated against mean annual temperature, were determined for the Last Glacial during an early cold phase (between 29,000 and 26,000
years BP) and for later less cold phases (between 23,000 and 18,000
years BP). Compared with the modern temperature regime, the MBT data indicate a decrease of ca. 5.6 and 2.8
°C respectively, during this time. Modern and past MBT-derived temperatures correlate with chironomid-based temperature reconstructions in the area. Archaeal GDGTs, commonly used for the TEX
86 index, are abundant in fossil sediments (Alpine Lake) but scarce in modern sediments, hindering a new local calibration for this palaeotemperature proxy.
Humans have altered natural patterns of fire for millennia, but the impact of human-set fires is thought to have been slight in wet closed-canopy forests. In the South Island of New Zealand, ...Polynesians (Māori), who arrived 700–800 calibrated years (cal y) ago, and then Europeans, who settled ∼150 cal y ago, used fire as a tool for forest clearance, but the structure and environmental consequences of these fires are poorly understood. High-resolution charcoal and pollen records from 16 lakes were analyzed to reconstruct the fire and vegetation history of the last 1,000 y. Diatom, chironomid, and element concentration data were examined to identify disturbance-related limnobiotic and biogeochemical changes within burned watersheds. At most sites, several high-severity fire events occurred within the first two centuries of Māori arrival and were often accompanied by a transformation in vegetation, slope stability, and lake chemistry. Proxies of past climate suggest that human activity alone, rather than unusually dry or warm conditions, was responsible for this increased fire activity. The transformation of scrub to grassland by Europeans in the mid-19th century triggered further, sometimes severe, watershed change, through additional fires, erosion, and the introduction of nonnative plant species. Alteration of natural disturbance regimes had lasting impacts, primarily because native forests had little or no previous history of fire and little resilience to the severity of burning. Anthropogenic burning in New Zealand highlights the vulnerability of closed-canopy forests to novel disturbance regimes and suggests that similar settings may be less resilient to climate-induced changes in the future.
Analyses of pollen and plant macrofossils from a new core spanning the past 60,000 years from Lake Tulane, Florida show a strong antiphase relationship in temperature between Florida and the North ...Atlantic region. During the Pleistocene, oak-scrub and prairie phases were coeval with long, intense Dansgaard–Oeschger interstadials (warm periods) that initiated Bond cycles. Pine phases were coeval with the North Atlantic long stadials (cold periods) that ended Bond cycles and were terminated by Heinrich events. Lake levels were higher during pine phases, and climate was wetter. However, climate in Florida was also warmer during these phases, which were cold periods in the North Atlantic. Perhaps diminution of thermohaline circulation before and during Heinrich events reduced northward heat transport and retained warmth in the subtropical Atlantic and Gulf of Mexico.
A comprehensively
14C AMS dated pollen and chironomid record from Boundary Stream Tarn provides the first chironomid-derived temperature reconstruction to quantify temperature change during ...Lateglacial times (17,500–10,000
cal
yr
BP) in the Southern Alps, New Zealand. The records indicate a ca 1000-year disruption to the Lateglacial warming trend and an overall cooling consistent with the Antarctic Cold Reversal (ACR). The main interval of chironomid-inferred summer temperature depression (∼2–3
°C) lasted about 700 years during the ACR. Following this cooling event, both proxies indicate a warming step to temperatures slightly cooler than present during the Younger Dryas chronozone (12,900–11,500
cal
yr
BP). These results highlight a direct linkage between Antarctica and mid-latitude terrestrial climate systems and the largely asynchronous nature of the interhemispheric climate system during the last glacial transition. The greater magnitude of temperature changes shown by the chironomid record is attributed to the response of the proxies to differences in seasonal climate with chironomids reflecting summer temperature and vegetation more strongly controlled by duration of winter or by minimum temperatures. These differences imply stronger seasonality at times during the Lateglacial, which may explain some of the variability between other paleoclimate records from New Zealand and have wider implications for understanding differences between proxy records for abrupt climate change.
Paleohydrology studies at Mathews Pond and Whitehead Lake in northern Maine revealed synchronous changes in lake levels from about 12,000 ^sup 14^C yrs BP to the present. We analyzed gross sediment ...structure, organic and carbonate content, mineral grain size, and macrofossils of six cores from each of the two lakes, and obtained 72 radiocarbon dates. Interpretation of this paleo-environmental data suggests that the late-glacial and Younger Dryas climate was dry, and lake levels were low. Early Holocene lake levels were considerably higher but declined for an interval from about 8000 to 7200 ^sup 14^C yrs BP. Sediment of both lakes contains evidence of a dry period at 7400 ^sup 14^C yrs BP (8200 cal yr). Lake levels of both sites declined abruptly about 4800 ^sup 14^C yrs BP and remained low until 3000 ^sup 14^C yrs BP. Modern lake levels were achieved only within the past 600 years. The west-to-east, time-transgressive nature of lake-level changes from several sites across northeastern North America suggests periodic changes in atmospheric circulation patterns as a driving force behind observed moisture balance changes.PUBLICATION ABSTRACT
Geological evidence for modeled Younger Dryas ice expansion in northern Maine is assessed in conjunction with temperature and precipitation estimates from chironomids and pollen, and plant ...macrofossil and lake-level analyses from lake sediment. Pollen and chironomid temperature and precipitation transfer-function estimates for the Allerød warming period indicate colder winters, precipitation levels half that of modern times, and summer temperatures near modern levels. The combination of cold winters and low precipitation prevented forest establishment in northern Maine along the Maine/New Brunswick border. While winter temperatures and precipitation remained stable, summer temperatures decreased as much as 7.5 °C during the Younger Dryas stadial, forcing a shift from shrub-dominated to sedge-dominated tundra. Summer and winter temperatures, as well as annual precipitation, increased rapidly at the Holocene onset.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The development of quantitative temperature reconstructions in regions of paleoclimate interest is an important step for providing reliable temperature estimates in that region. Fossil chironomid ...assemblages have been studied in Patagonia showing great promise for reconstructing paleotemperatures; however there is still a lack of robust temperature inference models in that area.
To contribute to the understanding of climate change, a transfer function using chironomids preserved in 46 lakes in Chile and Argentina was developed. The best performing model to infer the mean air temperature of the warmest month was a 3-component WA-PLS model with a coefficient of correlation (r2jack) of 0.56, a root mean square error of prediction (RMSEP) of 1.69°C and a maximum bias of 2.07°C. This model was applied to the chironomids preserved in the sediment of the Huelmo mire (41°31′ S, 73°00′ W), in the lake district of northwestern Patagonia. The reconstruction showed several cold spells (one at 13,200 to 13,000calyrBP and a cooling trend between 12,600 and 11,500calyrBP) associated with the Younger Dryas and/or Huelmo–Mascardi Cold Reversal (HMCR). Our findings support climate models proposing fast acting inter-hemispheric coupling mechanisms including the recently proposed bipolar atmospheric and/or bipolar ocean teleconnections rather than a bipolar see-saw model.
•A chironomid-based transfer function to reconstruct the temperature was developed.•This transfer function was applied to a lateglacial sequence from Huelmo mire in Patagonia.•The reconstruction showed colder spells during the YD, ACR and HMCR chronozones.•The reconstruction confirms interhemispheric climate coupling mechanisms.
A palaeohydrological study of Mansell Pond, a small (4 ha), closed-basin lake in central Maine, revealed distinct changes in lake levels throughout the Holocene. We examined the texture, organic ...content and macrofossils of eight cores and obtained 29 radiocarbon dates on transitions between sedimentary units. The water level fell between about 8000 14 C yr BP and 6000 14 C yr BP, and remained low until after 5000 14 C yr BP. The water level may have begun rising as early as 4750 14 C yr BP, with the most rapid increase occurring from 3225 14 C yr BP to 2780 14 C yr BP. This interpretation varies at some points from an earlier assessment of Mansell Pond's Holocene lake levels that was based on pollen and charcoal evidence from a single core. Records of Holocene lake levels from eastern Canada, the Lake Ontario region and southern New England are inconsistent with each other, suggesting that the influence of dominant air masses has been highly localized in this region
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, OILJ, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Many ecosystem processes that influence Earth system feedbacks - vegetation growth, water and nutrient cycling, disturbance regimes - are strongly influenced by multidecadal- to millennial-scale ...climate variations that cannot be directly observed. Paleoclimate records provide information about these variations, forming the basis of our understanding and modeling of them. Fossil pollen records are abundant in the NE US, but cannot simultaneously provide information about paleoclimate and past vegetation in a modeling context because this leads to circular logic. If pollen data are used to constrain past vegetation changes, then the remaining paleoclimate archives in the northeastern US (NE US) are quite limited. Nonetheless, a growing number of diverse reconstructions have been developed but have not yet been examined together. Here we conduct a systematic review, assessment, and comparison of paleotemperature and paleohydrological proxies from the NE US for the last 3000 years. Regional temperature reconstructions (primarily summer) show a long-term cooling trend (1000BCE - 1700CE) consistent with hemispheric-scale reconstructions, while hydroclimate data show gradually wetter conditions through the present day. Multiple proxies suggest that a prolonged, widespread drought occurred between 550 and 750CE. Dry conditions are also evident during the Medieval Climate Anomaly, which was warmer and drier than the Little Ice Age and drier than today. There is some evidence for an acceleration of the longer-term wetting trend in the NE US during the past century; coupled with an abrupt shift from decreasing to increasing temperatures in the past century, these changes could have wide-ranging implications for species distributions, ecosystem dynamics, and extreme weather events. More work is needed to gather paleoclimate data in the NE US to make inter-proxy comparisons and to improve estimates of uncertainty in reconstructions.
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