sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

applications of organic geochemistry to paleolimnological reconstructions a summary of examples from the laurentian great lakes the organic matter content of lake sediments contains information that helps to reconstruct past environmental conditions evaluate histories of climate change and assess impacts of humans on local ecosystems the elemental isotopic and molecular compositions of organic matter buried in sediment provide evidence of the biota that have lived in a lake and its catchment area and they serve as proxies of organic matter delivery and accumulation sedimentary records from the north american great lakes provide examples of applications of organic geochemistry to paleolimnological reconstructions the records of these lakes date from retreat of the laurentide ice sheet around 12 ka include the mid holocene hypsithermal and show consequences of recent human changes low corg ntotal ratios indicate that most of the sediment organic matter in the great lakes is from algal production yet changes in biomarker molecule compositions also show that varying amounts of land plant organic matter have been delivered to the lakes elevated algal productivity that accompanies nutrient enrichments of lake waters is recorded as excursions to less negative 13c values in the organic matter of sediments that were deposited in the 1960s and 1970s increased organic carbon mass accumulation rates mirror the isotopic excursions in most parts of the great lakes accumulations of petroleum residues and pyrogenic polycyclic aromatic hydrocarbons in sediments identify fluvial and eolian delivery of organic matter components to different parts of the great lakes emerging applications of multiple and novel organic geochemical proxies to paleolimnological reconstructions are promising yet some potentially important measurements remain underutilized

organic geochemical proxies of paleoceanographic paleolimnologic and paleoclimatic processes abstract organic matter constitutes a minor fraction of marine and freshwater sediments yet its important contribution to the sedimentary record can be used to reconstruct marine and continental paleoenvironments the organic matter content of sediments is the residue of past biota the amounts and types of organic matter present in sediments consequently reflect environmental conditions that impacted ecosystems at different past times general sources of the organic matter are inferred from bulk properties such as elemental compositions carbon and nitrogen stable isotope ratios rock eval pyrolysis data and organic petrography details of organic matter origins are refined by analyses of biomarker molecular compositions source changes are proxies for fluctuations in sea level oceanic surface currents and continental climates algal paleoproductivity rates are indicated by organic carbon mass accumulation rates and carbon and nitrogen stable isotopic compositions these parameters record past availability of nutrients and therefore are proxies of surface mixing in the oceans and amount of land runoff to lakes sea surface paleotemperatures are recorded by the number of carbon carbon double bonds in lipid biomarkers produced by marine algae larger proportions of the double bonds are proxies for the cooler surface waters that accompanied periods of global glaciation and intervals of enhanced upwelling the 13c and d values of plant organic matter record past concentrations of carbon dioxide in the atmosphere and changes in delivery of atmospheric moisture respectively diagenesis which causes the concentration and composition of organic matter in sedimentary settings to differ from those of the original biologically synthesized materials can bias organic geochemical paleoenvironmental records the magnitude of this potential source of misinformation must always be considered and evaluated comparison of multiple organic geochemical proxies of past conditions helps to compensate for the effects of diagenetic alterations and thereby to improve interpretations of paleoenvironmental change

the influence of organic matter in organogenic dolomitization we hypothesize that the decomposition of organic debris derived from plants planktonic animals and algae by sulfate reducing bacteria and specifically by the enzymatic degradation of protein is vital to organogenic dolomitization the role of this degradation is to increase both alkalinity and ph of the pore waters providing the necessary solution and surface chemistries for dolomitization to occur ammonia derived by the enzymatic breakdown of proteins is the only simple base available to supply the ph conditions necessary for dolomitization protein is abundant in organic matter derived from a marine source because of the large enthalpies of hydration of mg super 2 and ca super 2 ions we propose that dolomitization proceeds with the transfer of neutral ion pairs caco 3 and mgco 3 to and from the dolomite surface within a replacement zone for this reason dolomitization will occur only when the concentration of the co 3 super 2 ion is high and in large excess to the hco 3 super ion in solution and at the carbonate crystal surface these conditions will be met when the ph and or ionic strength is high and the carbonate alkalinity is also high

sediment yields and sources in a welsh upland lake catchment during the past 800 years estimates of suspended sediment yield over the past 800 years have been derived from a whole basin study of lake sediments in llyn geirionydd north wales magnetic and physical sediment properties were used to correlate a suite of cores to a master chronology based on 210pb and 14c dates the sources of sediment within the catchment were identified by comparing the magnetic and heavy metal properties of sediments soils and stream sediments mean suspended sediment yields range from 6 to 18 t km 2 yr 1 with high yields in the periods 1765 1830 and 1903 1985 corresponding to mining activities in the catchment the impact of earlier deforestation agricultural expansion or climatic change on sediment yields is low although there is evidence that agricultural activities increased levels of peak stream discharge afforestation in the 20th century does not appear to have significantly increased sediment yield sediment sources have remained fairly constant through time they appear to be widespread and dominated by stream channel sides rather than point sources except during the mining phases when spoil material has dominated the sediment load sediment loads to the lake today are still dominated by sediment derived from unvegetated spoil heaps

a 14 000 year sediment yield record from western germany based on annually laminated lake sediments the reconstruction of sediment yield data for the catchment of lake holzmaar 205 8 ha in the eifel mountains western germany is based on high resolution geochemical and physical analyses of an absolutely dated sediment record this technique provides flux rates of allochthonous minerogenic deposition for the last 13 840 years the pleistocene holocene transition is characterized by instable soils and vegetation caused by adjustment of the catchment to interglacial climatic conditions these instabilities are the reason for elevated mean sediment yields in the catchment of 16 t km 2 yr 1 until 9800 cal bp during most of the holocene erosive processes are insignificant and sediment yields are very low 1 5 t km 2 yr 1 since humans cleared the forests for agriculture and early industrial purposes the natural environment has been disturbed and sediment yields increased to mean values of 14 t km 2 yr 1 during the last 2750 years comparisons of reconstructions from the eifel area with similar data from southern sweden and with measured data from lakes reservoirs and monitoring sites suggest that quantitative reconstructions based on lake holzmaar sediments may be regarded as reliable despite uncertainties related to the methods of reconstruction applied

geomagnetic and mineral magnetic characterization of the anthropocene geomagnetic and mineral magnetic data provide geological indices that are both independent of human impact i e geomagnetic and respond to human induced environmental impact i e mineral magnetic we provide the first discussion of such magnetic events for help in defining the anthropocene within the holocene a potential geomagnetic marker for the anthropocene is the low dipole latitude at c 2700 cal a bp which is associated with distinct palaeosecular variation features in northerly mid to high latitude sites mineral magnetic records from lake and marine sediments identify major deforestation and soil delivery events from catchment systems in many parts of the world during the last 4000 years in europe clusters of these events occur around both 2600 cal a bp and ad 1100 the former coinciding with a low in geomagnetic field dipole latitude and peak intensity mineral magnetic records in peats and lake sediments can reflect particulate pollution from fossil fuel burning the expansion of major coal burning began c ad 1800 in western europe and eastern north america but around ad 1900 this expanded due to more widely distributed coal use and this event is the most clear mineral magnetic marker for the base of the anthropocene

forest disturbance in the central north island new zealand following the 1850 bp taupo eruption the 1850 bp taupo eruption covered c 30 000 km2 of the central north island with airfall deposits and c 20 000 km2 with ignimbrite this paper reviews pollen and charcoal analyses of lake and peat sediment cores from sites at various distances and directions from the taupo vent to establish the effects of this eruption on the surrounding forests forests within range of the ignimbrite were destroyed and forests located up to 170 km east of the vent suffered variable degrees of damage from ashfall stands of pteridium esculentum and other seral taxa flourished immediately after the eruption fires occurred during the eruption and continued for several decades after the degree and nature of vegetation disturbance above the taupo tephra varied according to the thickness of ashfall local topographical features and probably the vigour of the forest revegetation was complete within 200 years of the eruption even at sites overwhelmed by the taupo ignimbrite post eruption forests were similar to those exi

middle and late holocene paleotemperatures reconstructed from oxygen isotopes and gdgts of sediments from lake pupuke new zealand abstract quantitative holocene temperature reconstructions for southern hemisphere terrestrial environments are still restricted to few records and multi proxy methods are rarely applied to the same system here we test the applicability and comparability of temperature reconstructions inferred from different methodological approaches we combined sedimentary cellulose and diatom oxygen isotope data from lake pupuke north island new zealand to derive a record of lake water temperatures this record is complemented with mean annual air temperatures derived from the branched gdgt index mbt measured on the same sedimentary record the datasets of both reconstructions show little temperature variation for the period 1320 7110 cal bp the exceptions are a warmer period culminating between 1700 and 1600 cal bp reflected in both proxies and centennial scale intervals with cooler water temperatures at 3380 and 2750 cal bp recorded in the oxygen isotope based temperature reconstruction but not in the mbt inferred air temperature estimates we suggest that variations in lake circulation are the likely reason for deviations between temperature reconstructed from different proxies

environmental changes and human impact as recorded in a sediment sequence offshore from a viking age town birka southeastern sweden the viking age town of birka which existed between ad 750 and 975 on the island of bjorko in the malar archipelago of the baltic sea is known as the oldest centre used for large scale international trading in sweden birka had a strong defence town rampart hillfort and a water palisade studies of lithology and microand macroremains in a sedimentary sequence accumulated offshore from birka outside the water palisade revealed a stratigraphically distinct refuse stratum referred to as the birka layer we argue that the bulk material of the birka layer emanates from human activities in the town e g latrine cleanings cuttings of shore vegetation cleaning of ditches and storage areas and domestic animal dung organic remains from vegetation and animals deposited during activities on the ice offshore from the town e g winter markets may also have contributed to the composition the material constituting the birka layer started to accumulate after the construction of the protecting water pali

influence of sediment source and type on the magnetic susceptibility of fiord and shelf deposits baffin island and baffin bay n w t volume and mass magnetic susceptibility are measured on recent and late quaternary sediments from the fiords and shelf of eastern baffin island baffin bay and the northern labrador sea major changes in hinterland bedrock geology are associated with order of magnitude changes in magnetic susceptibility the fiords that head into the proterozoic foxe fold belt have mass magnetic susceptibilities ms of 2 4 10 7 m3 kg compared with 25 60 10 7 m3 kg in sediments derived from archean gneisses and granites another low ms region is the middle reach of frobisher bay ms studies in cambridge mcbeth and itirbilung fiords included analyses of surface samples and samples from lehigh 3 m and piston 11 m cores lehigh cores had higher ms values on average than the piston cores suggesting a coarsening in texture or an increase in the flux of magnetic minerals during the last 0 3 3 ka an analysis of the ms logs of 17 piston cores indicated that six broad patterns of ms can be recognized based on a

late pleistocene and holocene paleoenvironments of chalco lake central mexico abstract in order to establish paleoenvironmental conditions during the late quaternary four cores from the basin of mexico central mexico were drilled in chalco lake located in the southeastern part of the basin the upper 8 m of two parallel cores were studied using paleomagnetic loss on ignition pollen and diatom analyses based on 11 14c ages the analyzed record spans the last 19 000 14c yr b p volcanic activity has affected microfossil abundances both directly and indirectly resulting in absence or reduction of pollen and diatom assemblages important volcanic activity took place between 19 000 and 15 000 yr b p when the lake was a shallow alkaline marsh and an increase of grassland pollen suggests a dry cold climate during this interval abrupt environmental changes with increasing moisture occurred from 15 000 until 12 500 yr b p the lake level increased and the pollen indicates wetter conditions the highest lake level is registered from 12 500 to ca 9000 yr b p the end of the pleistocene is characterized by an increase in humidity from 9000 until ca 3000 yr b p chalco lake was a saline marsh and the pollen record indicates warmer conditions after 3000 yr b p the lake level increased and human disturbance dominates the lacustrine record

abrupt environmental oscillations during the early weichselian recorded at lago grande di monticchio southern italy abstract this study presents multi proxy data from a new core from lago grande di monticchio italy extending back to about 100 000 calendar years bp the time scale of this record is independent from deep sea and ice core chronologies and has been established from a combination of varve counts and high resolution sedimentation rate calculations based on microscopic sediment investigation this paper focuses on environmental variability between 100 000 and 65 000 calendar years bp several abrupt cold periods have been recognised and correlated to other terrestrial as well as to ice core and deep sea records the longest of the monticchio cold phases corresponds to marine isotopic stage 5b and lasted from ca 87 000 to 84 000 calendar years bp two shorter events about 1000 years long occurred during the transition from interstadial to full glacial conditions between 79 000 and 75 000 calendar years bp the increased frequency of cold events during this time interval probably indicates a higher degree of climatic instability in periods of major changes dating discrepancies between monticchio grip and deep sea chronologies range between a few hundred to a few thousand years however these differences in dating are not constant but vary significantly for different sections of the records nevertheless the good match of environmental fluctuations at monticchio with atmospheric variability at the greenland summit as well as variations in the oceanic circulation pattern suggests a close link of the mediterranean to the atlantic climate regime

using rat gnawed seeds to independently date the arrival of pacific rats and humans in new zealand the pacific rat rattus exulans was transported throughout the south pacific with voyaging humans thus the earliest dated evidence of pacific rat can be used to infer first human contact until recently it was considered that rats arrived in new zealand with humans in the thirteenth century ad however controversial radiocarbon dates on pacific rat bones now suggest that rats reached the remote islands of new zealand with people c ad 50 150 these dates are anomalous because they imply human contact with new zealand more than 1000 years before any archaeological evidence for human presence and precede settlement of tropical eastern polynesia the ancestral homeland of maori the first new zealanders the early rat bone dates are controversial for other technical reasons which have been debated in the literature here distinctive rat gnawed seed cases preserved in sediments are used as a proxy to independently date the arrival of the pacific rat and humans in new zealand this method effectively b

a new interpolation method to measure delta evolution and sediment flux application to the late holocene coastal plain of the argens river in the western mediterranean abstract rapid environmental changes along the mediterranean coasts influenced the sedimentary dynamics shoreline position and human settlements in deltaic areas over the last millennia an innovative and multiproxy approach using geostatistical modelling was developed to estimate geomorphic evolution and sediment fluxes in deltaic areas while palaeoecological conditions and environmental changes were assessed from geochemistry and cluster analyses of molluscan fauna this method was applied to the coastal plain of the argens river in southern france depositional environments of prodelta delta front river channel floodplain marsh and abandoned channel were identified from the study of facies associations the late holocene sediment flux in the argens river bayhead delta ranged from 15 800 2300 to 52 000 8500 m3 yr 1 the highest rate of sediment deposition between 2500 and 2000 cal yr bp was associated with increased river flooding in southeastern france a general decrease in subaqueous sediment flux over the past 2500 years was primarily controlled by a decrease in accommodation space and delta slope from 500 to 0 cal yr bp a sharp increase in subaerial sediment flux probably due to hydroclimatic change was contemporaneous with a period of high flood frequency during the little ice age a southward shift of the north atlantic westerlies combined with either a negative phase of the east atlantic west russian pattern or a negative phase of the east atlantic pattern coupled to a northward migration of the intertropical convergence zone could have led to increased fluvial activity in the lower argens valley during the late holocene

records of southern and central baltic sea eutrophication in δ 13 c and δ 15 n of sedimentary organic matter abstract the baltic sea catchment area has seen considerable agricultural and industrial development over the last century but observational evidence of eutrophication is ambiguous we examined organic carbon accumulation rates and the stable isotope signature of 13 c and 15 n in organic matter in four 210 pb dated short sediment cores from the southern and central baltic sea to search for sedimentary records of increased nutrient input and biological production organic carbon accumulation rates vary little through time in the cores from the oder estuary and the arkona basin but increase significantly in the bornholm basin and in the gotland deep records over the last 40 years 15 n in the sediments show distinct increases over the last 80 50 years in the oder estuary arkona basin and bornholm basin with the greatest increase near the mouth of river oder the gotland deep record shows increasing 15 n values from 1900 to 1975 and decreased 15 n values during the last 20 years the 15 n records are interpreted to trace the effects of eutrophication in nearshore and basinal sediments over the last 100 years and the magnitude of 15 n increase may be a measure for its extent the switch to lower 15 n values in the gotland basin sediment core after initially increasing values coincides with increasing abundance of summer blooms of nitrogen fixing cyanobacteria as observed in monitoring programs 13 c toc values of organic matter increase in the oder estuary over the last 80 years and over the last 30 years in the bornholm basin and gotland basin the tendency to heavier 13 c indicates decreasing isotope fractionation during massive plankton blooms that this effect is not well documented in the sediments of the arkona basin may be due to enhanced degradation processes in the permanently mixed water column a comparison of organic carbon concentrations and 13 c toc and 15 n from early litorina stage around 7 000 years bp and modern sediments from the gotland sea suggests that the trophic state of the baltic sea was comparable to that of the modern environment productivity was nitrate based in spring and based on nitrogen fixation in summer much higher organic carbon concentrations and 13 c enrichment in modern sediments are indications of enhanced productivity compared to early litorina times

documentation of the physical properties of annually laminated varved sediments at a sub annual to decadal resolution for environmental interpretation this paper describes a method to digitize high resolution physical properties of annually laminated lake sediments we modified a water acetone epoxy exchange method in order to preserve all the fine structural details that varved sequences provide by applying sediment microstratigraphical investigations x ray radiography sem and semi automatic image analysis we were able to establish detailed information about varve composition and structure mineral magnetic measurements were also applied for comparison by comparing all available physical data at high resolution it will be possible to identify environmental changes that has occurred in the lake drainage basin

tephras and new zealand archaeology establishing an accurate date for earliest polynesian settlement in new zealand is essential for understanding patterns of settlement and associated enviromental impacts and the processes and rates of cultural change in eastern polynesia tephra deposits from five volcanic centres together with exotic sea rafted pumice provide isochronous constraints on the timing of earliest settlement and human impacts in northern new zealand a local basaltic tephra from rangitoto island auckland and locally distributed andesitic tephras from egmont volcano directly date human occupation toc ad 1400 1450 distal andesitic tephras tufa trig formation from mt ruapehu tongariro volcanic centre help constrain the timing of earliest anthropogenic deforestation signals in hawke s bay sea rafted loisels pumice s although of uncertain stratigraphic reliability in places overlies cultural remains that can be no younger thanc ad 1350 along the east coast north island the regionally extensive rhyolitic kaharoa tephra which erupted from okataina volcano between c ad 1300 1390 is the critical settlement layer datum for dating prehistory in the north island no human artefacts are recorded beneath it and the earliest inferred environmental impacts by humans are dated to c ad 1280 just prior to its deposition this maximum date matches the earliest radiocarbon dates derived for both settlement and human impacts from archaeological and natural sites c ad 1250 and implies that the onset of deforestation was essentially contemporaneous with initial settlement the widespread rhyolitic taupo tephra which erupted from taupo volcanoc ad 200 provides an isochronous benchmark well before earliest settlement the tephra may coincide approximately with a putative earlier transient contact in new zealand based on pacific rat bone rattus exulans dates more precise calendrical dates on the tephras via dendrochronology or ice core records or other dating methods would help refine assessment of the timing of earliest settlement while extending the distributional range of critical tephra layers through application of crypto tephra analysis could lead to a greater understanding of settlement patterns

palaeoenvironmental changes in southern patagonia during the last millennium recorded in lake sediments from laguna azul argentina marked environmental changes in the southern patagonian steppe during the last 1100 years are detected by a multi proxy study of radiocarbon dated sediment cores from the crater lake laguna azul 52805vs 69835vw a prominent shift in carbon isotope records occurred between ad 1670 and ad 1890 induced by a change to cooler climate conditions with a concurrent lake level rise a second perturbation of the lake ecosystem started with a fire event around ad 1830 the fire event triggered increased soil erosion initiating a change of the diatom assemblages this shift in diatom assemblages may have been enhanced by shrinkage of littoral habitats and higher nutrient supply in the course of permanent european settlement at the end of the 19th century the introduction of neophytes by european sheep farmers is confirmed by the permanent occurrence of rumex pollen in the sediment record since the beginning of the 20th century d 2005 elsevier b v all rights reserved

multi proxy evidence of late holocene human induced environmental changes at lake pupuke auckland new zealand abstract new zealand was the last major landmass in the world to be colonised by people despite the short time depth of human presence in the country there is ongoing debate about the date of earliest arrival of people that resulted in the emergence of two contrasting colonisation hypotheses the early hypothesis and the short prehistory to decide between both hypotheses we employed a range of multi proxy investigations geochemistry stable isotopes and mineral magnetism on a lacustrine sequence from lake pupuke a maar lake in the city of auckland and an area potentially among the first places in new zealand to have been colonised by early people the environmental history reconstructed from the multi proxy evidence identified a clear lack of catchment disturbance from c 2000 b p until several decades before the eruption of the rangitoto volcano the nature and abruptness of disturbance unambiguously point to anthropogenic forcings and are likely to mark the onset of prehistoric human colonisation at the site linear interpolation between independently dated tephrochronostratigraphic marker beds present in the sediments allowed to date this event to c 610 cal yr b p this date is in compliance with the short prehistory for this region

thin tephra layers in peat revealed by x radiography x radiography has been found to provide a rapid economical non destructive method of locating very thin fine grained tephra of icelandic origin in scottish peats tephra layers form time parallel marker horizons or isochrones that are of great archaeological significance because they cover large areas offer precise chronological control for associated material and occur at times of significant cultural and environmental changes not all of the discrete deposits of recent volcanic fallout in scotland are sufficiently dense to show clearly on x radiographs but those which do provide particularly clear easily located marker horizons with many applications in palaeoenvironmental studies the tephra layer in caithness produced by fallout from a c 4000 by eruption of hekla provides a particularly good example of the use of x radiography to locate volcanic ash the method also reveals other structures of archaeological and palaeoenvironmental significance butler 1992 journal of archaeological science 19 151 161

holocene river dynamics in northland new zealand the influence of valley floor confinement on floodplain development abstract valley floor mapping sedimentology and 14c dating have been used to reconstruct the fluvial history at eight floodplain sites spread throughout northland a region removed from the main areas of tectonic and volcanic activity in new zealand we present a probability based record of holocene river behaviour for northland using 14c dated holocene fluvial deposits and compare this with independent palaeoclimate proxy records from the north island holocene floodplain evolution and fluvial behaviour have been conditioned by the degree of valley floor confinement in the most and least confined valley settings holocene floodplain evolution has involved the development of a single floodplain surface at partly confined sites the river terrace and floodplain geomorphology are more complex region wide progressive floodplain alluviation through the mid to late holocene and a period of increased river activity between 3500 and 2800 cal ybp in response to climatically driven increases in sediment supply was followed by a period of valley floor incision and terrace formation beginning after 1900 cal ybp in partly confined valley settings this was followed by the aggradation of a lower holocene floodplain surface with rapid rates of vertical accretion in response to post settlement catchment disturbance the results of this study indicate that valley floor confinement has played a major role in controlling northland holocene river floodplain development producing a continuum of floodplain and river terrace landforms in response to climatically and anthropogenically driven variations in sediment flux

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

magnetic properties of lake sediments from lake chalco central mexico and their palaeoenvironmental implications lake chalco 99 0 w 19 5 n in the basin of mexico was formed during the pleistocene after the emplacement of the chichinautzin volcanic field that closed the former drainage system the lake sediment record has been influenced by a number of factors including glacial interglacial cycles local volcanism erosion of soils and anthropogenic disturbances the magnetic properties of the lake sediments and the associated tephra layers of the last 16500 yr have been studied it is found that the magnetic properties of the lake chalco sediments are very distinctive magnetic concentration varies by a factor of 1000 and magnetic stability also varies over an extremely wide range the predominant magnetic mineral is titanomagnetite in addition to an imperfect antiferromagnetic phase possibly goethite an unusually large range of coercivities is found in certain of the tephras down core variations in magnetic properties closely follow climatic environmental changes previously established by other proxy methods the late pleistocene and late holocene lake sediments display a higher concentration of magnetic minerals than the early middle holocene sediments in the non volcanic sediments fluctuations in the magnetic concentration reflect changes in both the intensity of erosion as represented by ferrimagnetic and paramagnetic minerals and by the maturing of soils as represented by geothite copyright 2000 john wiley sons ltd

paleoenvironmental reconstruction of the semi arid chaco region of argentina based on multiproxy lake records over the last six hundred years abstract in this paper we analyze the paleoclimatic and paleoenvironmental evolution of laguna yema in semi arid chaco region of argentina over the past six hundred years high resolution multiproxy studies of lake sediments utilize analyses of lithology mineralogy geochemistry palynology and are constrained by radiocarbon and gamma spectrometry dating laguna yema sediments were mainly composed of well stratified fine sediments silts and clays with variable proportions of quartz clays illite and feldspar microcline and albite twelve light and heavy geochemical elements were registered most elements al si k ti fe rb ba and br are associated with illite and albite different material transport processes related to the changes in aridity and humidity of the basin were identified using the main mineralogical origins of geochemical elements palynological records indicate cycles of contraction and expansion of the lake with an increase in concentration of alternanthera aquatica during wet periods expansion of lake and an increase in ambrosia poaceae and fern spores during dry periods contraction of lake these changes are linked to fluctuations in moisture conditions in the subandean mountains and semi arid chaco regions in response to interactions between the south american monsoon system sams and the south american low level jet sallj which send warm and humid air to northern argentina in a regional context the laguna yema records are in accordance with the analyses of the temporal and spatial pattern of moisture distribution for the last six centuries

lacustrine responses to tephra deposition examples from mexico one of the major problems with palaeoclimate investigations in volcanic regions is that tephra inputs to lakes can cause changes in proxies analogous to those of climate forcing we review the range of impacts thought to be associated with tephra deposition distinguishing between direct effects on lake ecosystems and indirect changes to catchment nutrient cycles to achieve better understanding of these complex responses we have used high resolution diatom analysis from around 17 tephra layers in three mexican lakes a positive response to the tephra inputs has been identified for over half of these layers the most common response was for diatom concentrations to increase after tephra deposition also in plankton dominated systems fragilaria spp replaced stephanodiscus spp and aulacoseira spp an increase in the supply of silica to the lakes through their catchments is probably the cause of the diatom changes in the lakes studied direct effects of the tephra inputs to these lakes can be excluded as they would not generate changes of sufficient longevity the impact of these tephras which lasted for several decades was insufficient to perturb the underlying long term climate forcing of these lake systems c 2004 elsevier ltd all rights reserved

extracting a primary holocene crytoptephra record from pupuke maar sediments auckland new zealand post depositional reworking of volcanic glass shards is a common problem in tephrochronological studies distinguishing between primary and reworked tephra is challenging particularly in cryptotephra investigations that depend on identifying peaks of glass shard concentrations in sediment sequences rather than discrete macroscopic tephra layers here we investigated macro and cryptotephra in sediments of lake pupuke new zealand to identify primary from reworked tephra and thus refine the record of tephra fall events in the region over the last ca 9 cal ka geochemical fingerprinting stratigraphic correlation 14c ages and x ray fluorescence and magnetic susceptibility core scanning were employed to filter out reworked tephra and thus establish a new holocene tephrochronological framework for the region five primary macrotephra layers and 18 primary cryptotephra were identified confidence levels are attached to the newly identified cryptotephra and geochemical characterization of well dated rhyolite tephra was used to establish an associated age model our results suggest that holocene tephra fall from both local and distal volcanoes affected auckland more frequently than previously documented with a possible average frequency of at least once every 424 years furthermore the study demonstrates the importance of identifying the extent of post depositional reworking to establish the veracity of the cryptotephra record

characteristics of sedimentary varve chronologies a review abstract we constructed a varve data base vdb to study the worldwide distribution of varved sediment records and the fidelity of their varve chronologies clusters of lakes with varved sediments exist in central north america in the canadian arctic and in northern and central europe among the 108 sites included in the vdb varved records are typically 200 500 cm long and cover a period of 1000 2000 years their varve chronologies are often based on counting of annual layers from fresh sediment surfaces and photographs or from epoxy embedded sediment blocks and thin sections the vdb indicates that chronological errors associated with counting sedimentary varves fall generally between 1 and 3 but only 57 of published records are providing quantitative error estimations for their chronologies with a careful documentation of varves and by applying radiometric dating methods to surface sediments as well as using historical events as time markers there is the potential of having centennial long varved records with a very precise age depth control however it is unrealistic to expect errors significantly below 1 from several millennia long sections we found no indication that varve chronologies would have been substantially more accurate and precise in some parts of the world than in others or statistically dependent on varve thickness or temporal extension of a varved sequence according to the vdb close to 90 of the published varve chronologies have been cross checked with some independent dating methods

environmental magnetism past present and future environmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport deposition or transformation of magnetic grains is influenced by environmental processes in the atmosphere hydrosphere and lithosphere the first explicit description of environmental magnetism as a distinct field was in 1980 since that time environmental magnetism has become a broad field that is finding application in an ever increasing array of scientific disciplines in this review of the present state of environmental magnetic studies we divide the field into three broad but arbitrary categories the first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments this category includes the correlation of sediment cores using magnetic susceptibility measurements studies of geomagnetic field behavior the analysis of depositional and postdepositional mechanical processes that affect sediments and the examination of magnetic parameters that might represent proxies for paleoclimatic variation the second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment these provenance investigations include studies of changes in catchment derived sediment in lakes fluctuations in contributions from terrigenous aeolian and glaciogenic components in deep sea sediments and the origin of atmospheric particulates the final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments including pedogenesis authigenetic diagenetic formation of ferrimagnetic phases dissolution of magnetic minerals due to reductive diagenesis and contributions of biomagnetism to sedimentary magnetism because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques it is likely that the scope of environmental magnetism will continue to expand rapidly environmental magnetism is capable of providing important data for studies of global environmental change climatic processes and the impact of humans on the environment all of which are major research initiatives in the international scientific community these factors suggest that environmental magnetism has a bright and diverse future

annually laminated lake sediments and the study of quaternary environmental changes a review abstract annually laminated lake sediments non glacial varves have in recent years been discovered in increasing abundance especially since the development of in situ freezer samplers so far the majority of examples have been found in three main areas eastern north america northern and central europe and east and central africa a much more widespread distribution seems highly probable however the formation of annual laminations in freshwater lakes is governed by a number of factors the most important of which appears to be morphometry seasonality of sediment supply which may be controlled by physical biological or even cultural processes in the lake or in its catchment is also clearly significant four main types of laminations here termed ferrogenic calcareous biogenic and clastic have so far been recorded although it is quite clear that others exist special field and laboratory techniques are needed in order that the particular properties of varved sediments may be fully exploited annually laminated sediments have so far been applied to the investigation of a number of aspects of the quaternary these include the study of a geochronology b climatic change c vegetation history especially the role of fire in forest ecosystems d the calculation of rates of sediment influx to lakes and of rates of erosion from their catchments and e the monitoring of processes in the modern environment such as heavy metal pollution eutrophication and acid rain annually laminated lake sediments combine an internal independent chronology with the preservation of discrete visible increments of seasonal and annual sedimentation given this unique property and the likelihood that many more examples will be discovered the prospects for further applications of studies of these sediments to the investigation of the quaternary seem very good indeed

continental climate response to orbital forcing from biogenic silica records in lake baikal changes in insolation caused by periodic changes in the earth s orbital parameters provide the primary forcing for global ice ages1 6 but it is not clear to what extent the climates in continental interiors are controlled directly by regional variations in insolation and to what extent they are driven instead by the highly nonlinear response of the oceans and ice sheets here we investigate this question using the record of biogenic silica in lake baikal as a proxy for climate change in this high latitude mid continental region we find a good correlation between this record and that of marine oxygen isotopes4 over the past 250 kyr the baikal record exhibits both a strongly nonlinear component manifested in a 100 kyr periodicity and weaker direct insolation components manifested in the 41 kyr obliquity and 23 and 19 kyr precession orbital cycles these results show that even though extreme continental climates such as this are influenced directly by insolation variations they are dominated by the nonlinear rhythm of the oceans and ice sheets

the use of mineral magnetic analyses in identifying middle and late holocene agriculture a study of peat profiles in smaland southern sweden a transect of profiles from the bratamossen bog near the village of axlarp in the province of smaland southern sweden was investigated with respect to magnetic properties human impact pollen indicators and loss on ignition the bog is situated close to an area with clearance cairns an indication of abandoned arable fields there are also graves from the bronze age and the early iron age close to the bog a variety of analytical methods were used to reconstruct the land use history of this area since c 6000 14 c years bp while pollen analysis was used to detect changes in vegetation the magnetic analyses reflected how the soils in the landscape were affected by these changes the pollen analysis showed that the area became deforested and that forest vegetation was succeeded by pasture and arable land magnetic analyses indicated soil erosion as a consequence of agricultural activity the conventional technique of determining the minerogenic content by loss on ignition yielded no valuable information

relationship of environmental changes in central sri lanka to possible prehistoric land use and climate changes abstract a radiocarbon dated pollen analysed peat sequence from the horton plains 2000 m a s l in central sri lanka together with physical and chemical parameters organic carbon mineral magnetics carbon isotopes and phytoliths indicates major environmental changes during the last 24 000 years the results suggest that a mobile life form i e a hunter forage culture predominated in an open landscape associated with xerophytic vegetation e g chenopodium spp at 17 5 ka bp incipient management of cereal plants and slash and burn techniques seem to have prevailed between 17 5 and 13 ka bp which was indigenous and associated with grazing evidence of systematic cereal cultivation in the form of oat and barley pollen grains is found from the late pleistocene 13 ka bp this is the earliest evidence of farming activities noted in sri lanka as well as in south asia after 13 ka bp cereal cultivation was associated with an increase in humidity with a later abrupt increase in aridity agricultural land use decreased from 8 to 3 6 ka bp when the area appears to have been almost deserted after a severe middle holocene arid phase i e 5 4 3 6 ka bp the agricultural activity with a limited extension was again initiated by 2 9 ka bp during the next 900 years cultivation ceased allowing the upper montane rain forest to dominate between 0 2 and 0 15 ka bp new phases of agricultural activities were undertaken and potato cultivation took place lately between 1950 and 1969 ad

cortex a shipboard xrf scanner for element analyses in split sediment cores abstract the cortex corescanner texel is a non destructive xrf system for the logging of split sediment cores it is a seaworthy instrument that remains active during heavy weather conditions the scanner produces reliable counts for k to sr highly correlated with concentrations measured with atomic absorption spectrometry aas the option to scan al si and elements heavier than sr is in preparation a core section of 1 m can be scanned at 2 cm intervals within an hour a maximum resolution of 1 mm can be achieved at present each region and sedimentary environment requires separate calibration in the nearby future the generator settings will be standardized which may result in the development of a standard calibration procedure because of the high resolution of its semi quantitative records the cortex is a useful tool for detailed time series analysis inter core correlation and shipboard analysis

radiocarbon age of the kaharoa tephra a key marker for late holocene stratigraphy and archaeology in new zealand the kaharoa eruption the most recent rhyolitic volcanic event in new zealand resulted in the deposition of the compositionally distinctive kaharoa tephra over at least 30 000 km2 of northern and eastern north island the tephra forms an isochronous marker bed for various late holocene stratigraphic and palaeoen vironmental studies but is particularly important for evaluating the chronology of new zealand s notably short prehistory because it was erupted within the last millennium close to the time of first polynesian colonization we derive a definitive radiocarbon age previously ambiguous for the kaharoa tephra of 665615 bp using cluster analysis of 22 radiocarbon ages relevant to the kaharoa eruptive episode the error weighted mean age obtained on unscreened ages minus outliers is supported by statistically identical ages obtained from three sets of screened ages selected to minimize the effects of inbuilt age or contamination based on the intercepts method and 1 sigma range of northern hemisph

the palaeoenvironments of coastal lagoons in the southern baltic sea i the application of sedimentary corg n ratios as source indicators of organic matter abstract c n ratios of organic matter sources in coastal lagoons and their drainage areas in the southern baltic sea region are investigated to determine the origin of organic matter i e terrigenous or marine in the sediments of the lagoons these ratios are compared with 13c values in the area terrestrial plants in the study area show c n ratios between 10 and 85 whereas aquatic macrophytes have values in the range of 6 to 44 peat and shore vegetation have c n ratios between 15 and 16 and plankton shows values close to 6 in conjunction with 13c values the c n ratios in the sediments allow the reconstruction of the holocene palaeoenvironments and palaeoecology in the lagoons oder estuary and greifswalder bodden distinct stages in the development of the water bodies resulting from sea level changes in the region can be derived post glacial lake stages with sandy sedimentation lacustrine phases with high autochthonous productivity terrestrial stages with peat formation sedimentation as a result of marine transgression and brackish sedimentation after the formation of sand spits and barrier islands the application of corg ntot ratios instead of corg norg does not have any significant influence on the interpretation of the data and may thus be used in the coastal sediments of the southern baltic sea after having applied the test procedures presented in this study

production sedimentation and isotopic composition of organic matter in lake ontario organic matter and its carbon and nitrogen isotopic composition were measured in sequential sediment trap and core samples from the rochester basin of lake ontario to evaluate their usefulness in reconstructing historic changes in lake productivity the greatest flux of organic matter from the epilimnion occurred during late summer and coincided with whiting events indicating that calcite precipitation is an effective mechanism for sedimenting organic matter carbon isotopes of organic matter were low prior to the onset of stratification increased to maximum values in late summer and then decreased following fall overturn this pattern is controlled mainly by the timing of stratification and primary productivity which preferentially removes 12co2 from the epilimnion the physiological effect of decreased carbon isotopic fractionation with decreasing supplies of co2 aq may have also contributed to increased 13corgc nitrogen isotopes showed a seasonal pattern opposite to that of carbon whereby 15n values were low during the summer stratified period and high for the remainder of the year seasonal variability in 15norgn probably reflects changes in the source of sedimented organic particles which is dominated by isotopically depleted phytodetritus during the stratified period and isotopically enriched organic matter from heterotrophic or detrital sources during the mixed period a comparison of organic carbon accumulation rates and 13corgc between sediment cores collected in 1987 and 1993 1994 confirms earlier predictions that diagenetic processes reduce the mass accumulation of organic carbon in the zone of oxic pore waters but will not change the 13corgc values all cores analyzed for 13corgc display the reproducible pattern of a progressive increase in the 19oos peaking in the early to mid 1970s and then decreasing to the present this pattern matches the historical trends of phosphorus loading to the basin suggesting that 13c of organic carbon is a reliable proxy for paleoproductivity and responds to spring phosphorus supplies in the water column the 15n of sedimentary organic matter increased linearly from 1840 to 1960 at a rate of 0 3ppt per decade and remained relatively constant thereafter except for an increase in the upper few centimeters of sediment the increase in 15norgn reflects a combination of factors including early forest clearance by europeans increased sewering by municipalities after 1940 and increased nitrate utilization as productivity increased in the lower great lakes increased rates of denitrification in the central basin of upstream lake erie from the 1930s to the early 1970s may have also contributed to the rise in 15norgn values

the net water circulation in the far northern great barrier reef abstract the 500 km long far northern great barrier reef fngbr is a semi enclosed sea with little connectivity with the rest of the gbr further south its mean circulation is controlled by the prevailing southeasterly northwestward wind that generates a wind driven mean longshore flow that is enabled by an inflow of oceanic water in the south from an examination of single point mid depth current meter data at ten sites over one year it appears that there is no net current during calm weather which implies that the northward north queensland coastal current in the adjoining coral sea does not intrude in the fngbr only about 20 40 of the wind driven longshore transport continues northward to exit the fngbr through torres strait because of blockage by reefs shoals and islands and by the tidal friction effect in shallow waters the remaining 60 80 of the flow appears to be deflected seaward to the coral sea in the north an observation that appears to be supported by oceanographic modelling this situation differs from that in the central and southern great barrier reef where the southward flowing east australian current intrudes on the shelf and generates a net current even during calm weather

natural and human induced environmental changes preserved in a holocene sediment sequence from the etoliko lagoon greece new evidence from geochemical proxies abstract a key feature of greece is the large amount of historical and archaeological records the sedimentary record of the etoliko lagoon aetolia western greece offers an ideal opportunity to study human environment interaction and to disentangle natural and anthropogenic imprints in the sedimentary record by applying an interdisciplinary approach of combining geoscientific methods xrf loi grain size analysis with archaeological and historical records the 8 8 m long sedimentary sequence eto1c reveals the palaeoenvironmental history of the lagoon and its catchment since 11 670 cal bp with a thorough chronology based on 14 c age depth modelling including varve counting different evolutionary stages were put in a chronological context these stages include a lake period 11 670 8310 cal bp followed by a period of sporadic saltwater intrusion 8310 1350 cal bp as a result of continuing transgression phases of limnic predominance associated with freshwater inflow of episodically activated distributaries around 5230 cal bp still occurred by 1350 cal bp ongoing sea level rise had connected the lagoons of etoliko and messolonghi and freshwater influence had ceased with the onset of settlement activity in the late helladic 1700 1100 cal bc humans took advantage of the prevailing environmental landscape a sudden increase in coarse sedimentation correlates with the history of human occupation with its peak of prosperity from the late helladic until the end of the hellenistic period 30 cal bc

dating initial maori environmental impact in new zealand abstract over the last decade there has been an intense debate as to whether new zealand prehistory is long 1500 yr short

the late quaternary vegetational and climatic history of easter island easter island occupies an exceptionally isolated position in the south pacific ocean it is entirely volcanic and is famous for its giant statues late quaternary sediments have been investigated in three craters rano raraku rano aroi and rano kao giving a continuous record over the past 30 ka pollen records indicate that the island was formerly forested palynological and sedimentological evidence suggests a late pleistocene climate slightly cooler and drier than the present deforestation by people occurred mainly between 1200 and 800 yr bp this may have led to an ecological disaster and to the decline of the megalithic civilization the depauperation of the present native flora owes more to human activity than to isolation

preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis abstract the carbon and nitrogen isotope composition of organic matter has been widely used to trace biogeochemical processes in marine and lacustrine environments in order to reconstruct past environmental changes from sedimentary organic matter it is crucial to consider potential alteration of the primary isotopic signal by bacterial degradation in the water column and during early diagenesis in the sediments in a series of oxic and anoxic incubation experiments we examined the fate of organic matter and the alteration of its carbon and nitrogen isotopic composition during microbial degradation the decomposition rates determined with a double exponential decay model show that the more reactive fraction of organic matter degrades at similar rates under oxic and anoxic conditions however under oxic conditions the proportion of organic matter resistent to degradation is much lower than under anoxic conditions within three months of incubation the 13c of bulk organic matter decreased by 1 6 with respect to the initial value the depletion can be attributed to the selective preservation of 13c depleted organic compounds during anoxic decay the 15n values continuously decreased to about 3 below the initial value the decrease probably results from bacterial growth adding 15n depleted biomass to the residual material in the oxic experiment 15n values increased by more then 3 before decreasing to a value indistinguishable from the initial isotopic composition the dissimilarity between oxic and anoxic conditions may be attributed to differences in the type timing and degree of microbial activity and preferential degradation in agreement with the anoxic incubation experiments sediments from eutrophic lake lugano are on average depleted in 13c 1 5 and 15n 1 2 with respect to sinking particulate organic matter collected during a long term sediment trap study

a 750 year fire history based on lake sediment records in central yellowstone national park usa a 750 year fire history was reconstructed for the central plateau of yellowstone national park from the deep water sediments of five lakes the charcoal record from a large lake provided a chronology of regional fires data from four small lakes were used to study local and extralocal fires the co occurrence of abundant charcoal and high magnetic susceptibility values at the same stratigraphic level was used as evidence of a local catchment fire and a charcoal peak without high magnetic susceptibility was ascribed to an extralocal fire or a local fire without a related erosion event the fire history was compared with the dendrochronologic fire record for the last 450 years and the close agreement provided the justification to extend the chronology back in time large areas of the region burned in ad 1988 c 1700 c 1560 and c 1440 from c 1220 to 1440 and c 1700 to 1987 intermediate to small areas burned the near absence of fires in the twentieth century prior to the large fires of 1988 is evi

lacustrine varve formation through time abstract studies using sediment traps in lakes reveal that the seasonal flux of sediment regulates both the composition and timing of deposition of materials that reach the bottoms of lakes if the bottom waters of a lake are partly or totally anoxic the seasonally deposited materials are preserved as annual groupings of laminae varves common components that form individual laminae consist of allochthonous clastic material derived from the drainage basin precipitated carbonate minerals diatom frustules iron rich and manganese rich flocs autochthonous organic detritus and autochthonous and allochthonous materials resuspended from the bottom the style of varving has changed over geologic time reflecting changes in biologic evolution and types of materials available precipitated iron rich laminations were common in the middle precambrian graded sets of clastic organic laminations persisted through the precambrian prior to the evolution of bioturbating benthic organisms glaciolacustrine varves appear to have retained their distinctive character through time carbonate rich varves occurred sporadically in the precambrian and phanerozoic with the exception of diatoms major components of modern lacustrine varves were present through the paleozoic and mesozoic and yet varves are rare in strata of these ages and may have accumulated in marine to brackish water environments diatoms were introduced into lacustrine systems in early tertiary time and are common components of varves from then on diatom laminae combined with a greater chance for geologic preservation of younger lake deposits have increased the number of geologically young occurrences of varved sediments however seasonal associations of modern varve components and the processes they represent are present in ancient deposits and provide clues to the interpretation of ancient environments

fingerprints and age models for widespread new zealand tephra marker beds erupted since 30 000 years ago a framework for nz intimate abstract the role of tephras in the nz intimate project is a critical one because most high resolution palaeoclimatic records are linked and dated by one or more tephra layers in this review first we document eruptive distributional and compositional fingerprinting data both mineralogical and geochemical for 22 key marker tephras erupted since 30 000 years ago to facilitate their identification and correlation we include new glass compositional data the selected marker tephras comprise 10 from taupo and nine from okataina volcanoes rhyolitic one from tuhua volcano peralkaline rhyolitic and one each from tongariro and egmont volcanoes andesitic second we use four approaches to develop 2 age models for the tephras youngest to oldest 1 calendar ages for kaharoa and taupo y were obtained by wiggle matching log derived tree ring sequences dated by 14c 2 whakaipo v was dated using an age depth model from peat 3 14 tephras in the montane kaipo peat sequence waimihia s unit k whakatane tuhua mamaku rotoma opepe e poronui c karapiti b okupata konini waiohau rotorua rerewhakaaitu were dated by simultaneously wiggle matching stratigraphic position and 51 independent 14c age points against intcal04 using bayesian probability methods via both oxcal and bpeat and 4 the five oldest tephras erupted before ca 18 000 cal yr bp were dated by calibrating limited numbers of 14c ages using intcal04 okareka or comparison curves of the expanded cariaco basin sequence te rere kawakawa oruanui poihipi okaia kawakawa oruanui tephra the most widely distributed marker tephra was erupted probably ca 27 097 957 cal yr bp potential dating approaches for the older tephras include their identification in antarctic ice cores if present or annually laminated sediments for which robust calendar age models have been constructed high precision ams 14c dating on appropriate material from environmentally stable sites systematic luminescence dating or new radiometric techniques e g u th he if suitable minerals are available and errors markedly reduced further application of bayesian age modelling to stratigraphic sequences of 14c ages possibly augmented with luminescence ages may help refine age models for pre holocene tephras with the largest errors finally we discuss the critical role these marker tephras play in the ongoing construction of an event stratigraphy for the new zealand region which is a key objective of australasian and southern hemisphere intimate projects