what is diatomite different types of biogenic remains ranging from siliceous algae to carbonate precipitates accumulate in the sediments of lakes and other aquatic ecosystems unicellular algae called diatoms which form a siliceous test or frustule are an ecologically and biogeochemically important group of organisms in aquatic environments and are often preserved in lake or marine sediments when diatoms accumulate in large numbers in sediments the fossilized remains can form diatomite in sedimentological literature diatomite is defined as a friable light coloured sedimentary rock with a diatom content of at least 50 however in the quaternary science literature diatomite is commonly used as a description of a sediment type that contains a large quantity of diatom frustules without a precise description of diatom abundance here we pose the question what is diatomite what quantity of diatoms define a sediment as diatomite is it an uncompacted sediment or a compacted sediment we provide a short overview of prior practices and suggest that sediment with more than 50 of sediment weight comprised of diatom sio2 and having high 70 porosity is diatomaceous ooze if unconsolidated and diatomite if consolidated greater burial depth and higher temperatures result in porosity loss and recrystallization into porcelanite chert and pure quartz less

uplift of quaternary shorelines in eastern patagonia darwin revisited during his journey on the beagle darwin observed the uniformity in the elevation of coastal eastern patagonia along more than 2000 km more than one century later the sequences of quaternary shorelines of eastern patagonia have been described and their deposits dated but not yet interpreted in terms of geodynamics consequently we i mapped the repartition of the quaternary coastal sequences in argentinean patagonia ii secured accurate altitudes of shoreline angles associated with erosional morphologies i e marine terraces and notches iii took into account previous chrono stratigraphical interpretations in order to calculate mean uplift rates since 440 ka mis 11 and proposed age ranges for the higher and older features up to 180 m and iv focused on the last interglacial maximum terrace mis 5e as the best constrained marine terrace in terms of age and altitude in order to use it as a tectonic benchmark to quantify uplift rates along the entire passive margin of eastern south america our results show that the eastern patagonia uplift is constant through time and twice the uplift of the rest of the south american margin we suggest that the enhanced uplift along the eastern patagonian coast that interested darwin during his journey around south america on the beagle could originate from the subduction of the chile ridge and the associated dynamic uplift

fluids in high to ultrahigh temperature metamorphism along collisional sutures record from fluid inclusions abstract petrographic studies and microthermometric investigations on fluid inclusions associated with high to ultrahigh temperature metamorphic rocks in three major precambrian suture zones on the globe demonstrate the dominant occurrence of co 2 rich fluids these rocks form part of hot orogens developed along collisional plate boundaries the sapphirine quartz bearing mg al rich rock from the palghat cauvery suture zone a trace of the cambrian gondwana suture zone in southern india preserves evidence for a prograde high pressure event and subsequent peak ultrahigh temperature metamorphism along a clockwise path and contains abundant co 2 rich inclusions in corundum garnet and sapphirine most of the fluid inclusions are either primary or secondary and preserve low density co 2 rich fluids 0 569 0 807 g cm 3 similar low density co 2 rich fluid inclusions 0 853 0 953 g cm 3 are also present in pelitic granulites from the limpopo complex of southern africa a neoarchean granulite facies orogen formed by continent continent collision in contrast the garnet orthopyroxene granulite from tonagh island in the neoarchean napier complex in east antarctica contains very high density primary 1 095 1 129 g cm 3 and secondary 0 960 1 179 g cm 3 carbonic inclusions in garnet and quartz the calculated isochores for the fluid inclusions from the palghat cauvery suture zone and the limpopo complex yield significantly lower pressure estimates than those predicted from peak metamorphic conditions we interpret this as a result of significant density decrease due to rapid decompression along a clockwise p t trajectory in contrast the estimated isochores for primary inclusions in garnet orthopyroxene granulites from the napier complex are consistent with the peak p t conditions estimated from mineral phase equilibria for the tonagh island rocks suggesting that most of the fluid inclusions in these rocks did not undergo any marked effect of volume change and density decrease the contrasting fluid densities among the localities investigated in this study are probably related to the nature of the p t trajectory the tonagh island rocks had a near isochoric exhumation history whereas the metamorphic orogens in the other two sutures witnessed rapid decompression our results suggest that whereas the composition of the syn metamorphic fluids are preserved in all cases density reversal occurs within inclusions as a function of the tectonic history and exhumation style

high pressure and ultrahigh temperature granulite facies metamorphism of precambrian high grade terranes case study of the limpopo complex

geochemistry of holocene cryptotephras from the north iceland shelf md99 2269 intercalibration with radiocarbon and palaeomagnetic chronostratigraphies tephra grain counting morphology and geochemistry in the 150 1000 m size fraction were used to identify 22 individual tephra horizons and one visible tephra layer saksunarvatn from postglacial sediment of the north iceland shelf core md99 2269 66 37 53 n 20 51 16 w water depth 365 m length 2533 cm from the reykjafjardarall basin is constrained at the base by the vedde tephra c 12 000 cal yr bp geochemical analyses and stratigraphical position of the 22 tephra horizons were used to identify the following cryptotephras kol 1372 ad 1372 hekla 1104 ad 1104 snaefellsnes 1 c 1790 cal yr bp hekla 3 c 2970 cal yr bp hekla 4 c 4230 cal yr bp tv 5 c 6860 cal yr bp and suduroy c 8070 cal yr bp a tephra of the same geochemistry as kol 1372 was found near the base of the core and is here named kol3 2269 11 000 cal yr bp eleven additional cryptotephras were geochemically typed to their respective icelandic volcanic centres of origin and their ages constrained four teph

a new high resolution holocene tephra stratigraphy in eastern iceland improving the icelandic and north atlantic tephrochronology a new and improved holocene tephra stratigraphy and tephrochronological framework for eastern and northern iceland is presented investigations of a sediment sequence from lake logurinn have revealed a comprehensive tephra record spanning the last 10 200 years a total of 157 tephra layers have been identified whereof 149 tephra layers have been correlated to its source volcanic system using geochemistry stratigraphy and age fifteen layers have chemical composition of two affinities that possibly represent two very closely spaced eruptions thus these 157 tephra layers are believed to represent 172 explosive eruptions nineteen tephra marker layers have been identified in the lake logurinn record g1922 a1875 v1477 v1410 h1636 k1625 o1362 g1354 k1262 v874 hrafnkatla sn 1 grakolla hy h3 h4 ho ll1755 and reitsvik 8 tephra markers new potential tephra markers are the silicic askja l 9400 cal bp the low titanium basalt layers ll 1774 10 150 cal bp and ll 1755 9990 cal bp assigned to veidivotn bardarbunga and the tephra layers ll 1527 8 7850 cal bp ll 911 2 2370 cal bp ll 908 4 2350 cal bp ll 781 9 1930 cal bp ll 644 4 1480 cal bp not yet correlated to a source volcanic system a silicic tephra marker layer reitsvik 8 correlated to the fosen tephra in norway has been identified in lake logurinn the lake logurinn tephra record has been connected and integrated with the icelandic terrestrial tephrochronology and stratigraphy through 102 tephra layers the marine tephra stratigraphy through 39 layers and overseas through 9 tephra layers this record is the first high resolution tephra stratigraphical and chronological framework for the holocene in eastern iceland as well as the most detailed and continuous record and has considerable potential to serve as a key section or a stratotype for the holocene in eastern iceland and the north atlantic

a precisely dated lake level rise marked by diatomite formation in northeastern ireland the intercorrelation of palaeoclimate events from various studies is often hindered by a lack of precise chronological control tephra isochrons can overcome this problem by providing direct site linkages this paper outlines a study of holocene peat and diatomite deposits that accumulated within the floodplain of lough neagh northern ireland the icelandic hekla 4 tephra has been identified at the base of diatomite deposits at a number of sites and provides firm dating evidence for a widespread flooding event in the area at ca 2300 bc the evidence is consistent with other studies in ireland and elsewhere for increased wetness at this time the results demonstrate that the terrestrial deposits around lough neagh contain an important record of holocene lake level change dendrochronological evidence from the lough neagh area provides additional information about lake level fluctuations over the past two millennia copyright 2004 john wiley sons ltd

river response to rapid holocene environmental change evidence and explanation in british catchments macklin m g jones a f lewin j 2010 river response to rapid holocene environmental change evidence and explanation in british catchments quaternary science reviews 29 13 14 1555 1576 sponsorship welsh assembly government and the higher education funding council for wales for funding this study through its support of the centre for catchment and coastal research at aberystwyth university

role of fluids in the exhumation of the southern marginal zone of the limpopo complex south africa abstract the limpopo complex lc of southern africa displays structural metamorphic and geochronological evidence for a complex exhumation controlled evolutionary history that followed after burial and attainment of peak metamorphic conditions p 9 kbar t 900 c in the root zone central zone cz of the orogen before 2 72 ga the southern marginal zone smz of the lc in addition provides evidence for thrust controlled emplacement of a hot granulite facies nappe onto the adjacent low grade granite greenstone terrane of the northern kaapvaal craton nkvc in the interval 2 69 2 62 ga a tectono metamorphic scenario in which initial near vertical channeling exhumation at 2 69 ga was transferred into horizontal channeling thrusting at 2 67 2 64 ga is interpreted to have commenced with in situ partial melting of crustal rocks in the root zone of the orogen cz this resulted in formation of melt weakened crust migmatized gneisses that with continuous shortening led to vertical transfer of lower crustal material accompanied by fluid fluxed decompression melting that resulted in production of large volumes of leucocratic granitic material this material detached to form buoyant diapirs that accompanied the emplacement at 2 65 ga of mega sheath folds at the middle crustal level throughout the entire cz of the lc this early stage of near vertical exhumation is reflected i by the fact that linear elements of sheath folds and shear zones plunge consistently at steep angles to the sw throughout the entire cz and ii by decompression cooling dc segments of p t paths constructed for metapelitic rocks dc paths in the far northern part of the smz that are associated with the steep north dipping portion of the hout river shear zone hrsz traversed from p 8 5 kbar t 850 c to p 6 5 kbar t 720 c early near vertical exhumation transferred into horizontal channel flow that controlled emplacement at the middle crustal level of the granulite facies nappe smz horizontal channeling thrusting is recorded in metapelitic granulite by near isobaric cooling p 6 kbar t 700 550 c segments of earlier dc p t paths that are associated with the shallow north dipping portion of the hrsz devolitilization of underthrusted greenschist due to thrusting hot granulite over cool greenschist released large volumes of high temperature aqueous fluids from the underthrusted rocks that constantly infiltrated the overriding granulite during southwards migration of the nappe immiscible co 2 rich and brine fluids thus derived persistently infiltrated into and reacted with cooling granulite and triggered the main pulse of anatexis at p 7 5 kbar t 750 c similar fluids established a regional retrograde isograd and associated zone of 4500 km 2 retrograde hydrated granulite at p 6 kbar t 600 630 c located in the hanging wall of the shallow north dipping hrsz that bounds the smz in the south shear zone hosted metasomatic alteration including formation of lode gold deposits occurred at t 600 630 c p 6 kbar in the zone of rehydrated granulite and at higher p t conditions in the granulite zone the granulite facies smz of the lc thus preserves clear evidence for a direct link between persistent fluid flux and fluid rock interaction as an integral component of a conveyor like tectonic mechanism driven by vertical tectonics that operated from the core region of the orogen cz this mechanism explains the important link between the thrust controlled emplacement of the smz onto the nkvc and deep crustal fluid flow

coastal barriers northern gulf last eustatic cycle genetic categories and development contrasts a review abstract a large body of chronologically well constrained and detailed quaternary data accumulated in the last decades from the coastal shelf region of the northern gulf of mexico consequently a preliminary synthesis of major aspects of the late quaternary barrier and mainland coastal development on the northern gulf of mexico is timely several major factors account for the striking differences between barrier size and development styles in the three northern gulf coast regions barriers are nearly continuous in the nw where they dominate by the greatest dimensions and continuity sand supply from shelf and mainland sources continued at variable rates throughout six stages of the last eustatic cycle lec differences in antecedent topography in fluvial runoff volumes and relative sea level rise including rapid overstepping flooding events were critical in the regional distinctions leaving fewer and smaller relict fluvial delta behind drowned shore and nearshore landforms on the shelf only two large fluvial systems impacted the ne the nw shelf and nearshore received abundant sediment supply from the mississippi and lesser streams indirectly by ravinement erosion of relict landforms and landward directed cross shelf sediment transfer sands originated mainly in relict deltas fluvial tidal channel and inlet fill as well as from submerged shore ridge remnants on the continental shelf shelf margin deltas represented major secondary sources barrier formation categories and their respective importance in specific areas represent critical aspects of coastal development the authorship of various formation concepts e g penck gilbert mcgee haage ganong and keilhack often is still miscredited or remains unrecognized in the literature recognition of the stratigraphic and sedimentological diagnostic characteristics of the basic genetic barrier categories plays a key role in testing the validity of barrier evolution models closely knit process form relationships prevail between hydrodynamic sea level change storm tidal current overwash sedimentary shoal and island aggradation rollover and eolian accumulation processes and the resulting landform morphology the most detailed accounts that deal with the development of the alabama louisiana and apalachicola barrier chains are particularly instructive almost all modern gulf barriers with identifiable genetic background formed by shoal aggradation between 5 5 and 2 0 ka during a marked deceleration in sea level rise concurrently a sizable mainland strand plain complex extended the ne coastal plain tide and storm driven sediment transport to the large mobile bay ebb delta the focal role of composite east dauphin island in establishing the ms la island chain and the island blocking role of mississippi delta advance were prime factors in a complex barrier history migration paradoxically may also occur by transgressive rollover concurrently with regressive landward progradation of island strand plains stream discharge and sand reworked from relict lithosomes on the shelf were insufficient in compensating for wave and tide related sediment loss gulf levels stayed below the present throughout the mid and late holocene

deep crystal response to continental collision the limpopo belt of southern africa the limpopo belt is an example of an archean 2700 ma continental collisional zone in which the thickened crust 70 km has virtually attained equilibrium with the adjacent non thickened plates by a process of upward movement and lateral spreading of the underthrust high grade metamorphosed plates

sapphirine quartz corona around magnesian xmg 0 58 staurolite from the palghat cauvery suture zone southern india evidence for high pressure and ultrahigh temperature metamorphism within the gondwana suture abstract we report the first finding of equilibrium sapphirine quartz assemblage from the palghat cauvery suture zone pcsz in southern india providing unequivocal evidence for extreme crustal metamorphism at ultrahigh temperature uht conditions associated with the collisional assembly of the gondwana supercontinent in the late neoproterozoic cambrian the sapphirine and quartz occur as coronas around mg rich x mg 0 58 staurolite within poikiloblastic garnet in an mg al rich rock suggesting the progress of the prograde dehydration reaction staurolite garnet sapphirine quartz h 2 o although the occurrence of mg rich staurolite was previously noted from some localities in the pcsz no sapphirine quartz direct association has yet been reported the available experimental studies on mg rich staurolite indicate that the mineral is stable at p 15 kbar and suggest that the complex texture that we report here might correspond to a peak high pressure regime prior to the uht event the symplectic sapphirine quartz around staurolite probably implies decompression from p 15 kbar toward the stability of sapphirine quartz at temperatures of c 1000 c along a clockwise p t path the prograde high pressure metamorphism and following uht event correlate with the subduction collision tectonics associated with the final stage of amalgamation of gondwana supercontinent

tectonic lineaments of pliocene and quaternary shorelines northeast gulf coast ridges scarps and level plains in the coastal area from mississippi to the florida panhandle have been interpreted as a succession of littoral and lagoonal landforms evolved during pre sangamon pleistocene high stands of sea level new sediment and fossil information refutes this view only a single late pleistocene transgressive regressive sequence has been documented in surface exposures and drill cores tectonism reflected in hitherto unrecognized structural lineaments was probably responsible for coastal escarpments formed in three periods stream segments and segments of late pleistocene and holocene shorelines commonly parallel the lineaments the reason for the conspicuous absence of clearly recognizable pre sangamon littoral and estuarine deposits and landforms probably lies largely in the erosional history of the region related to slow regional uplifts

multi proxy evidence for holocene lake level and salinity changes at an loch mor a coastal lake on the aran islands western ireland abstract curves for holocene lake levels and salinity changes are presented for an loch mor a small oligohaline lake on the aran islands off the west coast of ireland based on palaeoecological investigations of a 12 m long lake sediment core new insights are also provided into holocene sea level change in the galway bay region particular emphasis has been placed on the ostracod fauna both past and present salinity and lake level changes were reconstructed from the fossil ostracod assemblages based on the known tolerances of individual species and on the assemblages as a whole additional evidence was provided by other proxies including strontium isotope ratios derived from ostracod shells and other carbonates plant macrofossil and pollen analyses and sedimentological changes the early holocene pre boreal i e 11 5 10 ka was characterised by low lake levels and slightly elevated salinity values probably the result of high evapotranspiration and low precipitation rather than elevated sea levels early holocene plant and animal migration to the island does not seem to have been impeded but relative sea levels were not necessarily so low below 40 m a s l that landbridges were present to the mainland between ca 10 and 8 5 ka relatively high lake levels prevailed at 8 3 and 7 5 ka minor fluctuations lowering of the lake level occurred that are assumed to relate to early holocene abrupt events beginning at 7 05 ka lake levels declined sharply a general trend towards rising lake levels started at ca 6 4 ka and accelerated at ca 5 6 ka as runoff increased as a result of neolithic clearances at ca 4 8 ka lake levels began to rise once again probably in response to changes in rainfall and or evapotranspiration and runoff lower lake levels during the first half of the 1st millennium ad were probably a response to decreased runoff as a result of a drier climate coupled with regeneration of woody vegetation the sharpest rise in both lake levels and salinity began during the ninth century ad which is attributed to a rapid rise in relative sea level

multiple tectonothermal events in the granulite blocks of southern india revealed from epma dating implications on the history of supercontinents abstract we report age data on zircon monazite uraninite and huttonite from a suite of 29 samples covering four major granulite blocks in southern india using an electron microprobe technique the rocks analysed in this study cover all of the major lithounits in these terrains and include garnet bearing and garnet free charnockites garnet biotite gneisses khondalites calc silicate rocks and a suite of orthogneisses biotite gneiss biotite hornblende gneiss two pink metagranites representing the magmatic phase were also analysed zircons from the madras block yield well defined isochrons at 2 5 2 6 ga core to rim analyses of single zircon grains show age zoning with 2 6 2 9 ga igneous cores mantled by 2 4 2 5 ga rims detrital zircons show age up to ca 3 2 ga monazites in this block have cores and rims with 2 5 2 3 ga a suite of 19 samples from the madurai block brings out the multiple tectonothermal events in this terrain zircons from an orthogneiss yield well defined isochrons at 1 7 0 1 ga 0 82 0 05 ga and 0 58 0 04 ga from core inner rim and rim portions respectively zircon grains in other rocks preserve either core or secondary growth ages at 0 8 1 0 ga zircons in a pink metagranite from this block show sharply defined isochrons of 0 68 0 03 ga for the core and 0 57 0 01 ga for the secondary portion a late pan african overprint is observed throughout this block with zircon rims monazite uraninite and huttonite yielding age values in the range of 0 45 0 60 ga zircons from both the trivandrum and nagercoil blocks show a major tectonothermal event at 0 55 ga with faint indications of previous tectonothermal events during 0 8 1 0 and 1 7 2 0 ga monazite data from both the trivandrum and nagercoil blocks are essentially similar to those from the madurai block except for presence of relic monazite in the former our study confirms the notion that the palghat cauvery shear zone marks the major terrain boundary between an archean craton in the north and proterozoic terrains in the south it also strengthens the view of paleoproterozoic accretion and pan african reworking of blocks south of this shear zone the ages of production accretion and reworking in the terrains of southern india yield important information for the histories of columbia 1 8 1 5 ga rodinia 1 1 1 0 ga and gondwana 0 6 0 5 ga supercontinents the southern indian terrains formed part of a worldwide network of orogenic belts that is centered around 1 8 ga and outlines the configuration of columbia accretion of terrains in the madurai block to archean rocks north of the palghat cauvery shear zone at this time is consistent with the ages suggested to be the oldest metamorphic event in the eastern ghats orogen of eastern india and the rayner belt of coastal east antarctica our data confirm earlier evidence that all of southernmost india underwent resetting of isotopic systems during the final accretion of gondwana at the time of the pan african orogeny 0 5 ga the possibility that the trivandrum and nagercoil terrains accreted to the madurai block after 0 7 ga suggests that this pan african zone may also have involved accretion and closure of ocean basins if that happened then this zone may be the long sought suture between east and west gondwana

geochronological problems related to polymetamorphism in the limpopo complex south africa abstract the integration of new and published geochronologic data with structural magmatic anatectic and pressure temperature p t process information allow the recognition of high grade polymetamorphic granulites and associated high grade shear zones in the central zone cz of the limpopo high grade terrain in south africa together these two important features reflect a major high grade d 3 m 3 event at 2 02 ga that overprinted the 2 63 ga high grade neoarchaean d 2 m 2 event characterized by sw plunging sheath folds these major d 2 m 2 folds developed before 2 63 ga based on u pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high grade gneissic fabric the d 3 m 3 shear event is accurately dated by u pb monazite 2017 1 2 8 ma and pbsl garnet 2023 11 ma age data obtained from syntectonic anatectic material and from sheared metapelitic gneisses that were completely reworked during the high grade shear event the shear event was preceded by isobaric heating p 6 kbar and t 670 780 c which resulted in the widespread formation of polymetamorphic granulites many efforts to date high grade gneisses from the cz using pbsl garnet dating resulted in a large spread of ages 2 0 2 6 ga that reflect the polymetamorphic nature of these complexly deformed high grade rocks

defining the potential source region of volcanic ash in northwest europe during the mid to late holocene abstract far travelled volcanic ash fine grained tephra clouds pose economic and societal risks largely through their potential impact on the aviation industry a long view of ash clouds extending over northwest europe assembled from the study and geochemical characterisation of cryptotephra i e tephra invisible to the naked eye in sedimentary records can help inform the understanding of the frequency of such events and the nature of eruptions that generate them here we review the records of cryptotephra deposition in northwest europe during the mid to late holocene the last eight millennia to gain insights into the sources of volcanic ash reaching this region we consider almost 500 published and unpublished datasets for cryptotephras recorded in sedimentary sequences ranging from ireland to latvia and on the basis of their major element geochemical composition we identify 90 unique ash horizons and their likely provenance the majority 72 of the cryptotephras derive from icelandic eruptions but a large proportion 24 originates from other volcanic regions including alaska the cascades kamchatka mexico the azores and possibly the mediterranean although moderate magnitude eruptions in iceland may potentially disperse ash clouds over europe it appears that only large magnitude events in more distant regions have generally had this capacity and then only when suitable meteorological conditions prevailed nevertheless the cryptotephra records demonstrate that ash clouds from most volcanic regions capable of producing large explosive eruptions in the northern mid to high latitudes particularly those to the west of europe as well as in tropical north america have the potential to extend to northwest europe volcanic ash forecasts for this region need therefore to take into account possible risks posed not only by icelandic eruptions but also by any large magnitude eruptions in north america or the north pacific area

abrupt climatic changes and an unstable transition into a late holocene thermal decline a multiproxy lacustrine record from southern sweden the transition from a middle holocene relatively warm and stable climate to a cooler and unstable late holocene climate is reconstructed using sediments from lake igelsjon south central sweden this multiproxy study illustrates local regional and global scale responses to climatic change by focusing on a previously identified abrupt hydrological shift to cooler and or wetter conditions around 4000 cal yr bp the results suggest that between ca 4600 and ca 3400 cal yr bp the environment around and within the lake responded in two major well defined steps the first between 4450 and 4350 cal yr bp and the second between 4000 and 3800 cal yr bp a series of rapid fluctuations of short duration were superimposed on the general cooling trend with the most severe aquatic response peaking at ca 3800 cal yr bp pollen percentage and influx values show forest composition and pollen productivity changes and a distinct decline in total and corylus pollen influx in the period of 4000 3500 cal yr bp stomatal based reconstruction of atmospheric co2 concentration produced a tenuous decrease with a minimum between 3650 and 3500 cal yr bp copyright 2005 john wiley sons ltd

low frequency and high frequency changes in temperature and effective humidity during the holocene in south central sweden implications for atmospheric and oceanic forcings of climate an integrated use of independent palaeoclimatological proxy techniques that reflect different components of the climate system provides a potential key for functional analysis of past climate changes here we report a 10 000 year quantitative record of annual mean temperature tann based on pollen climate transfer functions and pollen stratigraphical data from lake flarken south central sweden the pollen based temperature reconstruction is compared with a reconstruction of effective humidity as reflected by a 18o record obtained on stratigraphy of lacustrine carbonates from lake igelsjon c 10 km from lake flarken which gives evidence of pronounced changes in effective humidity the relatively low tann and high effective humidity as reflected by a low evaporation inflow ratio suggest a maritime early holocene climate 10 000 8 300 cal year bp seemingly incompatible with the highly seasonal solar insolation configuration we argue that the maritime climate was due to the stronger than present zonal flow enhanced by the high early holocene sea surface temperatures in the north atlantic the maritime climate mode was disrupted by the abrupt cold event at 8 200 cal year bp followed at 8 000 cal year bp by a stable holocene thermal maximum the latter was characterized by tann values about 2 5 c higher than at present and markedly dry conditions indicative of stable summer time anti cyclonic circulation possibly corresponding with modern blocking anticyclonic conditions the last 4 300 year period is characterized by an increasingly cold moist and unstable climate the results demonstrate the value of combining two independent palaeoclimatic proxies in enhancing the reliability generality and interpretability of the palaeoclimatic results further methodological refinements especially in resolving past seasonal climatic contrasts are needed to better understand the role of different forcing factors in driving millennial scale climate dynamics

beach ridges definitions and significance abstract beach ridges frequent components of quaternary coastal plains and other coastal landforms have been cited as indicators of the positions of ancient seashores and associated sea levels numerous authors utilized the term beach ridge for active and relict usually wave built supratidal and or intertidal forms wind built ridges have been only occasionally included in the definition the term was applied also to submerged landward shifting eventually stranded bars a consistent redefinition of the term is highly desirable beach ridges should include all relict strandplain ridges whether dominated by wave swash built or by eolian lithosomes all active ridge like shore features regardless of dimensions morphology and origin are excluded because of the resistance of coarse clastic ridges to wave and wind erosion swash built gravel or coarse shell storm ridges may build several meters above the level of high tide swash built high berms even on pure sandy beaches exceed the highest tides during episodes of wind induced record water levels frequently but not always burying underlying low relief berm ridges of berm lithosomes sequences of relatively steep multiple foredunes are commonly named beach ridge plains the narrow subparallel relict foredunes that form these strandplains presently are designated as eolian beach ridges beach ridges thus are defined as relict semiparallel multiple wave and wind built landforms that originated in the inter and supratidal zones until separated from the shoreline by progradation sandy pebbly or shell enriched backshore berm ridges behind an active foreshore should not be considered beach ridges strandplain progradation is either continuous or with the inclusion of subtidal cat s eye ponds discontinuous contrary to claims transgressive cheniers do not represent true cheniers alone within their overall progradational context cheniers a special category of beach ridges bracketed by subtidal intertidal mudflats may be transgressive or regressive in character landward driven transgressive ridges should be designated beach ridges only after they are stabilized on intertidal flats when recognizable between clearly identifiable intertidal and overlying eolian intervals the horizontal interface between these lithosomes in beach ridges may help the reconstruction of ancient tide lake levels diagnostic sedimentary textures structures and fossils however often may be unavailable in the deposits along with various types of elevated terraces composed of raised marine deposits and certain coastal landforms of erosional origin that occur worldwide beach ridges of clearly proven wave built origin may also serve as indicators of ancient higher than present sea levels

carbonic metamorphism at ultrahigh temperatures evidence from north china craton abstract the north china craton ncc constitutes one of the fundamental precambrian nuclei of asia the paleoproterozoic khondalite belt along the northern margin of the ncc shows evidence for extreme crustal metamorphism at temperatures exceeding 1000 c and pressures of 10 12 kbar with the formation of diagnostic ultrahigh temperature uht mineral assemblages including sapphirine quartz high alumina orthopyroxene sillimanite and high temperature mesoperthites here we record for the first time the common occurrence of co2 rich fluid inclusions in peak uht minerals in the mg al granulites of ncc and trace the fluid evolution history from microthermometric and laser raman spectroscopic studies we show that the syn metamorphic fluid in the uht rocks was pure co2 while very high density carbonic fluids are preserved in some cases corresponding to entrapment at peak conditions many of the inclusions underwent density modification during the rapid and near isothermal exhumation of the rocks following peak uht metamorphism and initial cooling at depth the present study area within the northern margin of the ncc defines a collisional orogenic belt where continental collision and deep subduction were possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusions of paleoproterozoic age that are seen emplaced within the granulite facies supracrustals the thermal anomaly displayed by the uht granulites may thus correlate with asthenospheric upwelling and emplacement of mafic magmas possibly associated with plume activity which provided the heat and co2 input to achieve extreme crustal metamorphism and generation of dry granulite facies assemblages our study is the first report of syn metamorphic co2 rich fluids in uht granulites from a paleoproterozoic ca 1 92 ga terrane and provide evidence for carbonic metamorphism at ultrahigh temperatures as north china craton joined the columbia supercontinent amalgam

fluid assisted granulite metamorphism a continental journey abstract lower crustal granulites which constitute the base of all continents belong to two series high pressure granulites generated by crustal thickening subduction and ultra high temperature granulites associated with crustal extension fluid inclusions and metasomatic features indicate that the latter were metamorphosed in the presence of low water activity fluids high density co2 and brines which have invaded the lower crust at peak metamorphic conditions fluid assisted granulite metamorphism high pressure and ultra high temperature granulites commonly occur along elongated paired belts they were formed from the early proterozoic onwards during a small number of active periods lasting a few hundreds of m y these periods were separated from each other by longer periods of stability each period ended with the formation of a supercontinent whose amalgamation coincided with low to medium pressure ultra high temperature granulite metamorphism immediately before continental break up it is proposed that large quantities of mantle derived co2 stored in the lower crust at the final stage of supercontinent amalgamation are released into the hydro and atmosphere during breakup of the supercontinent fluid assisted granulite metamorphism therefore appears to be an important mechanism for transferring deep mantle fluids towards the earth s surface possible consequences were for example the sudden end of proterozoic glaciations as well as the post cambrian explosion of life

sequence morphodynamics at an emergent barrier island middle atlantic coast of north america abstract the southern delmarva peninsula is located along the middle atlantic coastal plain of the united states the axial highland of the peninsula formed in four stages of pleistocene spit progradation the landward shoreline of the peninsula is on the chesapeake bay the seaside shoreline of the peninsula is on the atlantic ocean the coast of the peninsula is composed of five landscape sections described as a headland a left hand spit a right hand spit a wave dominated barrier island and tide dominated barrier islands fisherman island is a barrier island located at the southern end of the southern delmarva peninsula the landscape features on fisherman island do not illustrate a direct linkage to 1 the sediment dispersion from the delaware headland or 2 the influence of local antecedent topography the island has a bipolar progradational history that is normal to the axis of the southerly sediment dispersion pattern from the delmarva headlands during the late holocene sea level rise flooded the low elevation land at the distal end of the southern delmarva peninsula the submerged area formed a shallow platform in the entrance to the chesapeake bay two sediment dispersion tracts affected the development of this area on the ocean side of the peninsula sediment moved southward along the lower shoreface to the chesapeake bay entrance on the west side of the peninsula southerly moving bay currents also dispersed sediment to the entrance of the bay the two tracts converged on the northern side of the bay entrance forming a broad sand shoal wave diffraction and refraction around the margins of the shoal swept sediment into linear sand bars that migrated back toward the peninsula by the middle of the 19th century the fusion of sand bars on the shoal surface produced a permanent nucleus for island development wave refraction caused wave crests to wrap around the island core producing separate easterly and westerly components of shore aggradation the westerly aggradational history is recorded in closely spaced sets of beach ridges the easterly aggradational history is recorded in broadly spaced hammocks