A record of Lateglacial and early Holocene environmental and ecological change from southwestern Connecticut,USA

Analyses of a sediment core from Highstead Swamp in southwestern Connecticut, USA, reveal Lateglacial and early Holocene ecological and hydrological changes. Lateglacial pollen assemblages are dominated by Picea and Pinus subg. Pinus, and the onset of the Younger Dryas (YD) cold interval is evidenced by higher abundance of Abies and Alnus viridis subsp. crispa. As climate warmed at the end of the YD, Picea and Abies declined and Pinus strobus became the dominant upland tree species. A shift from lacustrine sediment to organic peat at the YD–Holocene boundary suggests that the lake that existed in the basin during the Lateglacial interval developed into a swamp in response to reduced effective moisture. A change in wetland vegetation from Myrica gale to Alnus incana subsp. rugosa and Sphagnum is consistent with this interpretation of environmental changes at the beginning of the Holocene. Copyright © 2009 John Wiley & Sons, Ltd.     

A late-glacial transition from Picea glauca to Picea mariana in southern New England

Picea is an important taxon in late-glacial pollen records from eastern North America, but little is known about which species of Picea were present. We apply a recently developed palynological method for discriminating the three Picea species in eastern North America to three records from New England. Picea glauca was dominant at  14,500–14,000 cal yr BP, followed by a transition to Picea mariana between  14,000 and 13,500 cal yr BP. Comparison of the pollen data with hydrogen isotope data shows clearly that this transition began before the beginning of the Younger Dryas Chronozone. The ecological changes of the late-glacial interval were not a simple oscillation in the position of a single species' range, but rather major changes in vegetation structure and composition occurring during an interval of variations in several environmental factors, including climate, edaphic conditions, and atmospheric CO₂ levels.     

The postglacial history of three Picea species in New England, USA

Given the difficulty of separating the three Picea species—P. glauca, P. mariana, and P. rubens (white, black, and red spruce)—in the pollen record, little is known about their unique histories in eastern North America following deglaciation. Here we report the first use of a classification tree analysis (CART) to distinguish pollen grains of these species. It was successfully applied to fossil pollen from eight sites in Maine and one in Massachusetts. We focused on the late glacial/early Holocene (14,000 to 8000 cal yr B.P.) and the late Holocene (1400 cal yr B.P. to present)—the two key periods since deglaciation when Picea has been abundant in the region. The result shows a shift from a Picea forest of P. glauca and P. mariana in the late glacial to a forest of P. rubens and P. mariana in the late Holocene. The small number of P. rubens grains identified from the late glacial/early Holocene samples (<5%) suggests that that species was either absent or rare at most of the sites. The occurrence and distribution of the three species do not reveal any geographic or temporal trend during late glacial time, but the data suggest that they were distributed in local patches on the landscape. The results of this study indicate that the recent population expansion of Picea (1000 to 500 cal yr B.P.) was likely the first time since deglaciation that P. rubens was abundant in the region.     

A Holocene pollen record of persistent droughts from Pyramid Lake, Nevada, USA

Pollen and algae microfossils preserved in sediments from Pyramid Lake, Nevada, provide evidence for periods of persistent drought during the Holocene age. We analyzed one hundred nineteen 1-cm-thick samples for pollen and algae from a set of cores that span the past 7630 years. The early middle Holocene, 7600 to 6300 cal yr B.P., was found to be the driest period, although it included one short but intense wet phase. We suggest that Lake Tahoe was below its rim for most of this period, greatly reducing the volume and depth of Pyramid Lake. Middle Holocene aridity eased between 5000 and 3500 cal yr B.P. and climate became variable with distinct wet and dry phases. Lake Tahoe probably spilled intermittently during this time. No core was recovered that represented the period between 3500 and 2600 cal yr B.P. The past 2500 years appear to have had recurrent persistent droughts. The timing and magnitude of droughts identified in the pollen record compares favorably with previously published δ¹⁸O data from Pyramid Lake. The timing of these droughts also agrees with the ages of submerged rooted stumps in the Eastern Sierra Nevada and woodrat midden data from central Nevada. Prolonged drought episodes appear to correspond with the timing of ice drift minima (solar maxima) identified from North Atlantic marine sediments, suggesting that changes in solar irradiance may be a possible mechanism influencing century-scale drought in the western Great Basin.     

Multi-proxy record of postglacial environmental change, south-central Melville Island, Northwest Territories, Canada

A sediment core from Lake BC01 (75°10.945′N, 111°55.181′W, 225 m asl) on south-central Melville Island, NWT, Canada, provides the first continuous postglacial environmental record for the region. Fossil pollen results indicate that the postglacial landscape was dominated by Poaceae and Salix, typical of a High Arctic plant community, whereas the Arctic herb Oxyria underwent a gradual increase during the late Holocene. Pollen-based climate reconstructions suggests the presence of a cold and dry period ~12,000 cal yr BP, possibly representing the Younger Dryas, followed by warmer and wetter conditions from 11,000 to 5000 cal yr BP, likely reflective of the Holocene Thermal Maximum. The climate then underwent a gradual cooling and drying from 5000 cal yr BP to the present, suggesting a late Holocene neoglacial cooling. Diatom preservation was poor prior to 5000 cal yr BP, when conditions were warmest, suggesting that diatom dissolution may in part be climatically controlled. Diatom concentrations were highest ~4500 cal yr BP but then decreased substantially by 3500 cal yr BP and remained low before recovering slightly in the 20th century. An abrupt warming occurred during the past 70 yr at the site, although the magnitude of this warming did not exceed that of the early Holocene.     

A 7000-year record of environmental change,including early farming impact,based on lake-sediment geochemistry and pollen data from County Sligo,western Ireland

Detailed, chronologically tightly constrained, lake-sediment-based geochemical and pollen records have enabled local changes in soil erosion, woodland cover and composition, and prehistoric farming impact to be reconstructed in considerable detail. The profile opens shortly after 7800 BC when tall canopy trees were well-established and presumably in equilibrium with their environment. A distinct perturbation that involved an increase in pine and birch, a decrease in oak and a minor opening-up of the woodland is regarded as the local expression of the 8.2 ka climate anomaly. Lack of response in the geochemical erosional indicators is interpreted as evidence for drier conditions. A short-lived, over-compensation in climate recovery followed the 8.2 ka event. Neolithic farming impact is clearly expressed in both the pollen and geochemical data. Both datasets indicate that Neolithic impact was concentrated in the early Neolithic (3715–3440 BC). In the interval 3000–2700 BC there appears to have been a break in farming activity. The pollen data suggest substantially increased farming impact (both arable and pastoral) in the Bronze Age, with maximum farming and woodland clearances taking place in the late Bronze Age (1155–935 BC). These developments are poorly expressed in the geochemical record, possibly due to within-lake changes.     

A 40,000-yr record of environmental change from Burial Lake in Northwest Alaska

Burial Lake in northwest Alaska records changes in water level and regional vegetation since ∼ 39,000 cal yr BP based on terrestrial macrofossil AMS radiocarbon dates. A sedimentary unconformity is dated between 34,800 and 23,200 cal yr BP. During all or some of this period there was a hiatus in deposition indicating a major drop in lake level and deflation of lacustrine sediments. MIS 3 vegetation was herb-shrub tundra; more xeric graminoid-herb tundra developed after 23,200 cal yr BP. The tundra gradually became more mesic after 17,000 cal yr BP. Expansions of Salix then Betula, at 15,000 and 14,000 cal yr BP, respectively, are coincident with a major rise in lake level marked by increasing fine-grained sediment and higher organic matter content. Several sites in the region display disrupted sedimentation and probable hiatuses during the last glacial maximum (LGM); together regional data indicate an arid interval prior to and during the LGM and continued low moisture levels until ∼ 15,000 cal yr BP. AMS ¹⁴C dates from Burial Lake are approximately synchronous with AMS ¹⁴C dates reported for the Betula expansion at nearby sites and sites across northern Alaska, but 1000-2000 yr younger than bulk-sediment dates.     

Middle Pleistocene climate and habitat change at Zhoukoudian, China, from the carbon and oxygen isotopic record from herbivore tooth enamel

The Pleistocene deposits at Zhoukoudian, often referred to as the “Peking Man” site, contain dental remains from a diverse group of herbivores, including Equus sanmeniensis, Cervus elaphus, Cervus nippon, Megaloceros pachyosteus, Sus lydekkeri, and Dicerorhinus choukoutienensis. The carbon and oxygen isotopic compositions of structural carbonate within the enamel of these teeth are used to reconstruct the paleodiet and paleoenvironment of the mammals. The δ¹³C values of enamel from Zhoukoudian range from −2.3‰ to −13.0‰, indicating that these mammals consumed between 25% and 100% C₃ plants. The presence of significant amounts of C₄ plants in the diets of some herbivore species indicates that at the onset of the Middle Pleistocene local habitats included mixed C₃/C₄ vegetation. By approximately 470,000 yr ago, C₃ plants dominated the diets of herbivores studied, suggesting that the abundance of C₄ flora had decreased in the area. For all deer analyzed in this study, the values of δ¹³C and δ¹⁸O decrease substantially from about 720,000 to 470,000 yr ago. This trend may be due to a strengthening of the winter monsoon during the Middle Pleistocene.     

Development of the mixed conifer forest in northern New Mexico and its relationship to Holocene environmental change

Chihuahueños Bog (2925 m) in the Jemez Mountains of northern New Mexico contains one of the few records of late-glacial and postglacial development of the mixed conifer forest in southwestern North America. The Chihuahueños Bog record extends to over 15,000 cal yr BP. An Artemisia steppe, then an open Picea woodland grew around a small pond until ca. 11,700 cal yr BP when Pinus ponderosa became established. C/N ratios, δ¹³C and δ¹⁵N values indicate both terrestrial and aquatic organic matter was incorporated into the sediment. Higher percentages of aquatic algae and elevated C/N ratios indicate higher lake levels at the opening of the Holocene, but a wetland developed subsequently as climate warmed. From ca. 8500 to 6400 cal yr BP the pond desiccated in what must have been the driest period of the Holocene there. C/N ratios declined to their lowest Holocene levels, indicating intense decomposition in the sediment. Wetter conditions returned after 6400 cal yr BP, with conversion of the site to a sedge bog as groundwater levels rose. Higher charcoal influx rates after 6400 cal yr BP probably result from greater biomass production rates. Only minor shifts in the overstory species occurred during the Holocene, suggesting that mixed conifer forest dominated throughout the record.     

Use of CART analysis to differentiate pollen of red pine (Pinus resinosa) and jack pine (P. banksiana) in New England

The identification of fossil pollen at the generic rather than species level is hampering progress in understanding the biogeography and dynamics of paleo-vegetation. We used CART analysis to facilitate the differentiation of fossil pollen of Pinus banksiana and Pinusresinosa, which are morphologically similar and nearly always combined in paleoecological studies. The CART model, using four of the ten morphological traits measured, exhibited a high level of correct identification for pollen of each of the species and shows promise as a tool for increasing the detail of paleoecological records and inferences.     

Human-ecosystem interactions in relation to Holocene environmental change in Port Joli Harbour,southwestern Nova Scotia,Canada

A high-resolution pollen record from Path Lake in Port Joli Harbour, Nova Scotia, Canada, provides a paleo-ecological perspective on Holocene climate and vegetation variability within the context of local archaeological research. Pollen assemblages in the early Holocene reflect a post-glacial forest dominated by Pinus, Tsuga, Betula and Quercus. During this time, a lower frequency of radiocarbon dated cultural material suggests lower human settlement intensity. Shallow water aquatic (Isoetes) and wetland (Alnus, Sphagnum) taxa increased after 3400 cal yr BP in response to a transition towards wetter climatic conditions. Culturally significant periods, where settlement intensity increased in the Maritimes and Maine, coincide with maximum values of reconstructed total annual precipitation, suggesting that environmental conditions may have influenced prehistoric human activity. European settlement, after 350 cal yr BP, was marked by a rise in Ambrosia. The impact of anthropogenic fire disturbances on the landscape was evidenced by peak charcoal accumulations after European settlement.     

Geochronology and geochemistry of Palaeozoic intrusive rocks in the Rockvale region,southern New England Orogen,New South Wales

Palaeozoic intrusive rocks of the New England Batholith from the Rockvale district in the southern New England Orogen form three distinct associations: (i) the Carboniferous Rockvale Adamellite, a member of the Hillgrove Suite of deformed S‐type granitoids; (ii) a small I‐type igneous complex on the northwestern margin of the Rockvale Adamellite: several members of this complex have similar chemical compositions to the most mafic members of the Moonbi Suite of New England Batholith I‐types; and (iii) a suite of dyke rocks ranging in composition from calc‐alkaline lamprophyre through hornblende and biotite porphyrite to aplite. Ion‐microprobe U‐Pb zircon analysis indicates intrusion of the Rockvale Adamellite at 303 ±3 Ma (weighted mean ²⁰⁶Pb/²³⁸U age; 95% confidence limits). Preliminary investigation of zircon inheritance within the Rockvale Adamellite is consistent with chemical and isotopic indications of derivation of New England Batholith S‐type granitoids from a relatively juvenile protolith. Deformation of the Rockvale Adamellite occurred after complete crystallization of the pluton and prior to emplacement of dykes and I‐type intrusives. K‐Ar biotite and hornblende ages show broadly synchronous intrusion of I‐type magmas and lamprophyre dykes at ca 255 Ma, indicating that mantle magmatism associated with lamprophyres was contemporaneous with the crustal production of I‐type melts. Chemical similarities between the most mafic Moonbi Suite members and calc‐alkaline lamprophyres may also indicate a direct mantle contribution to some I‐type magmas.     

A diverse Rancholabrean vertebrate microfauna from southern California includes the first fossil record of ensatina (Ensatina eschscholtzii: Plethodontidae)

Analysis of late Pleistocene fossils recovered from near the Huntington Beach, California (USA), pier (site LACM 7679) has revealed a diverse fauna dating to approximately 40 ¹⁴C ka BP. Extinct megafauna (three genera) are present; however, a microfauna including three genera of fish, five genera of amphibians, twelve genera of reptiles, two genera of birds, and ten genera of small mammals dominates the assemblage in terms of diversity. Additional identification of seven genera of non-marine mollusks and various macro- and microscopic plant remains including grasses, three families of herbs, and seven genera of trees provides a wealth of information concerning the past ecology of what is currently a coastal dune field complex. During the Rancholabrean Period, the LACM 7679 locality was approximately 10 km inland from the Pleistocene coastline and contained lush riparian zones interspersed with coastal sage scrub, a few trees, and grasslands teeming with a variety of small and large animals.     

Terminal Pleistocene change in mammal communities in southeastern Washington State,USA

Small mammal communities in western North America experienced declines in taxonomic richness across the late Pleistocene to Holocene transition (PHT), a recent natural global warming event. One community also experienced a decline in evenness and others replaced one species with a congener. Variability in response of small mammal communities to PHT warming is apparent. At the presently arid and xeric Marmes site in the Columbia Basin of southeastern Washington State, megafauna were absent by about 13,000 cal yr BP, evenness of small mammals declined about 11,700 cal yr BP and again about 11,400 cal yr BP whereas richness declined about 11,400 cal BP. Regional faunal turnover was, however, minimal among small-bodied taxa. Local mammal communities are depauperate as a result of megafaunal extinctions and subsequent decreases in small-mammal richness and evenness. The latter chronologically corresponds with a decrease in primary productivity driven by increasing warmth and aridity. More faunas must be studied in order to fully document the range of variability in the responses of mammalian communities to PHT warming. Documentation of patterns in those responses will facilitate understanding and enhance predictive accuracy with respect to responses of mammalian communities to modern global warming.     

A re-evaluation of Ophiomorpha burrows in the Wealden Group (Lower Cretaceous) of southern England

Attribution of burrows in the Wealden Group of southern England to Ophiomorpha is rejected. The burrows are essentially cylindrical, unlined and with a meniscate fill. Any outer knobbly appearance is due to diagenetic poikilotopic cementation or to differential weathering of a mudchip-sand fill. The variable nature of meniscate fill reflects passage of the producer through the thin-bedded, alternating sand-mud sediments or along sand-mud interfaces. The burrows are assigned to Beaconites, though, since the identity of this ichnotaxon has been questioned, reference is also made to Taenidium. Two ichnoassociations are recognized: (1) a Beaconites antarcticus-Scoyenia (or Taenidium-Scoyenia) association (Weald Clay) of marginal lacustrine situation with fluvial input, and (2) a Beaconites barretti-Planolites (or Taenidium-Planolites ) association of the fluvial (lacustrine delta) of the Lee Ness Sandstone (Ashdown Formation). The Wealden burrows offer no inherent indications of palaeosalinity, and inferences made on supposed occurrences of Ophiomorpha in the Wealden Group must be reassessed. Other occurrences of Ophiomorpha in non-marine facies are questioned.     

Medieval Warming, Little Ice Age, and European impact on the environment during the last millennium in the lower Hudson Valley, New York, USA

Establishing natural climate variability becomes particularly important in large urban areas in anticipation of droughts. We present a well-dated bi-decadal record of vegetation, climate, land use, and fire frequency from a tidal marsh in the Hudson River Estuary. The classic Medieval Warm Period is evident through striking increases in charcoal and Pinus dominance from 800–1300 A.D., paralleling paleorecords southward along the Atlantic seaboard. Higher inputs of inorganic sediment during this interval suggest increased watershed erosion during drought conditions. The presence of the Little Ice Age ensues with increases in Picea and Tsuga, coupled with increasing organic percentages due to cooler, moister conditions. European impact is manifested by a decline in arboreal pollen due to land clearance, increased weedy plant cover (i.e., Ambrosia, Plantago, and Rumex), and an increase in inorganic particles to the watershed.     

An 8000-year record of vegetation, climate, and human disturbance from the Sierra de Apaneca, El Salvador

An ∼8000-cal-yr stratigraphic record of vegetation change from the Sierra de Apaneca, El Salvador, documents a mid-Holocene warm phase, followed by late Holocene cooling. Pollen evidence reveals that during the mid-Holocene (∼8000-5500 cal yr B.P.) lowland tropical plant taxa were growing at elevations ∼200-250 m higher than at present, suggesting conditions about 1.0°C warmer than those prevailing today. Cloud forest genera (Liquidambar, Juglans, Alnus, Ulmus) were also more abundant in the mid-Holocene, indicating greater cloud cover during the dry season. A gradual cooling and drying trend began by ∼5500 cal yr B.P., culminating in the modern forest composition by ∼3500 cal yr B.P. A rise in pollen from weedy plant taxa associated with agriculture occurred ∼5000 cal yr B.P., and pollen from Zea first appeared in the record at ∼4440 cal yr B.P. Human impacts on local vegetation remained high throughout the late Holocene, but decreased abruptly following the Tierra Blanca Joven (TBJ) eruption of Volcán Ilopango at ∼1520 cal yr B.P. The past 1500 years are marked by higher lake levels and periodic depositions of exogenous inorganic sediments, perhaps indicating increased climatic variability.     

Reconstructing the migration patterns of late Pleistocene mammals from northern Florida, USA

We used analyses of the strontium isotope (⁸⁷Sr/⁸⁶Sr) ratios of tooth enamel to reconstruct the migration patterns of fossil mammals collected along the Aucilla River in northern Florida. Specimens date to the late-glacial period and before the last glacial maximum (pre-LGM). Deer and tapir displayed low ⁸⁷Sr/⁸⁶Sr ratios that were similar to the ratios of Florida environments, which suggest that these taxa did not migrate long distance outside of the Florida region. Mastodons, mammoths, and equids all displayed a wide range of ⁸⁷Sr/⁸⁶Sr ratios. Some individuals in each taxon displayed low ⁸⁷Sr/⁸⁶Sr ratios that suggest they ranged locally, while other animals had high ⁸⁷Sr/⁸⁶Sr ratios that suggest they migrated long distances (> 150 km) outside of the Florida region. Mastodons were the only taxa from this region that provided enough well-dated specimens to compare changes in migration patterns over time. Pre-LGM mastodons displayed significantly lower ⁸⁷Sr/⁸⁶Sr ratios than late-glacial mastodons, which suggests that late-glacial mastodons from Florida migrated longer distances than their earlier counterparts. This change in movement patterns reflects temporal changes in regional vegetation patterns.     

Paleoenvironmental proxy records from Lake Hovsgol, Mongolia, and a synthesis of Holocene climate change in the Lake Baikal watershed

Here we discuss paleoenvironmental evolution in the Baikal region during the Holocene using new records of aquatic (diatom) and terrestrial vegetation changes from Hovsgol, Mongolia's largest and deepest lake. We reconcile previous contradictory Baikal timescales by constraining reservoir corrections of AMS dates on bulk sedimentary organic carbon. Synthesis of the Holocene records in the Baikal watershed reveals a northward progression in landscape/vegetation changes and an anti-phase behavior of diatom and biogenic silica proxies in neighboring rift lakes. In Lake Baikal, these proxies appear to be responsive to annual temperature increases after 6 ka, whereas in Lake Hovsgol they respond to higher precipitation/runoff from 11 to 7 ka. Unlike around Lake Baikal, warmer summers between 6 and 3.5 ka resulted in the decline, not expansion, of forest vegetation around Lake Hovsgol, apparently as a result of higher soil temperatures and lower moisture availability. The regional climatic proxy data are consistent with a series of 500-yr time slice Holocene GCM simulations for continental Eurasia. Our results allow reevaluation of the concepts of ‘the Holocene optimum’ and a ‘maximum of the Asian summer monsoon’, as applied to paleoclimate records from continental Asia.     

Wisconsinan Late-glacial environmental change in southern New England: A regional synthesis

A synthesis of vegetational and climatic history during the Wisconsinan Late-glacial (13-9 ka BP) is presented for the southern New England region. The interpretations are based on a number of pollen-stratigraphical investigations supported by plant macrofossil data. Chronology is based partly on AMS dates of plant macrofossils. Reference is made briefly to the controversial nature of the evidence for deglaciation in the region. A curve representing mean July temperature changes in the region during the period 13-9 ka BP is provided.