biogeochemical processes and geotechnical applications progress opportunities and challenges consideration of soil as a living ecosystem offers the potential for innovative and sustainable solutions to geotechnical problems this is a new paradigm for many in geotechnical engineering realising the potential of this paradigm requires a multidisciplinary approach that embraces biology and geochemistry to develop techniques for beneficial ground modification this paper assesses the progress opportunities and challenges in this emerging field biomediated geochemical processes which consist of a geochemical reaction regulated by subsurface microbiology currently being explored include mineral precipitation gas generation biofilm formation and biopolymer generation for each of these processes subsurface microbial processes are employed to create an environment conducive to the desired geochemical reactions among the minerals organic matter pore fluids and gases that constitute soil geotechnical applications currently being explored include cementation of sands to enhance bearing capacity and liquefaction resistance sequestration of carbon soil erosion control groundwater flow control and remediation of soil and groundwater impacted by metals and radionuclides challenges in biomediated ground modification include upscaling processes from the laboratory to the field in situ monitoring of reactions reaction products and properties developing integrated biogeochemical and geotechnical models management of treatment by products establishing the durability and longevity reversibility of the process and education of engineers and researchers

direct evolution of w rich brines from crystallizing melt within the mariktikan granite pluton west transbaikalia uneconomic tungsten mineralization associated with the mariktikan granite pluton of the transbaikalian igneous province eastern siberia is confined to a marginal part of the pluton referred to as the andreyevsky body this is composed of rocks similar to those of the main pluton but is likely to be an autonomous although kindred body on the basis of cross cutting relationships between rock varieties and their textural features the crystallization history is subdivided into three stages of unequal duration melt and fluid inclusions related to each of them have been studied using microthermometric and micro analytical procedures including a technique for atomic emission spectroscopy of individual fluid inclusions opened by a laser microprobe during the main crystallization stage 1045 1012 c more than 70 of the parental magma is believed to have crystallized at the level studied resulting in the formation of a crystalline framework rigid enough for the appearance of fractures within which some portion of the intercrystalline melt accumulated giving rise to thin aplite veins almost complete crystallization of intercrystalline and fracture hosted melt occurred during the late stage 1012 990 c whereas during the final stage only small bunches of schlieric sometimes miarolitic pegmatite were formed from a few pockets of residual melt 990 917 c in spite of the low water content of the melt about 1 wt fluid separation took place from the onset of crystallization during the main crystallization stage the exsolved fluid divided into two immiscible phases co2 rich gas and salt rich liquid brine however it was homogeneous thereafter major ore components of the brine mn fe w displayed different behaviour as crystallization progressed w concentration was below 0 1 wt at the main stage attained 1 wt during the late stage and increased to 1 8 wt at the final stage comparison of these data with calculations carried out using estimated parameters for the parental magma enables us to infer that obtained concentration values are reasonable for the special case studied here however they are unlikely to be attainable if the magma is h2o enriched and lacks an anomalously high w content in the andreyevsky body of the mariktikan pluton the following features of the parental magma made possible the generation of w rich solutions 1 high liquidus temperature 1045 c and elevated cl content c 0 15 wt that resulted in enlargement of the fluid melt distribution coefficient c 10 2 low water content c 1 and elevated w content c 0 001 wt that provided a relatively high w h2o ratio within the system at the same time because of the low water content of the magma the total mass of w bearing solutions has proved to be insufficient for the production of large scale mineralization a reduced w content of the great bulk of the exsolved brine may have had an unfavourable effect as well

a sm nd isotopic study of the south harris igneous complex the outer hebrides sm nd geochronology may be used to bracket the age of metamorphism in rocks which are difficult to date by other methods by coupling whole rock sm nd analyses of the principal members of the south harris igneous complex with sm nd mineral isochrons on two anothositic gabbros the age of granulite facios metamorphism has been defined whole rock analyses of three pairs of closely spaced samples of the anorthosite give consistent ages averaging 2 18 0 06 gyr but in general the data from the anorthosite do not define an isochron as a result of variable contamination of the evolving magma chamber whole rock data on the tonalite indicate that it is younger than 2 06 gyr its mean tchur age is 1 86 0 05 gyr garnet pyroxene amphibole plagioclase mineral isochrons on two anorthosite samples give identical 1 87 0 04 gyr ages which date cooling after the high pressure granulite facies metamorphism together with the tonalite whole rock data this defines the age of that metamorphism and confirms dearnley s original assignment of an early laxfordian age

proposal for a terrane based nomenclature for the lewisian gneiss complex of nw scotland the current nomenclature for the lewisian gneiss complex has evolved from lithological and structural correlations made prior to any dating initial flawed geochronological studies gave some names an apparent chronological standing but as work advanced fitting events into a coherent regional framework became increasingly difficult modern dating studies have shown that the lewisian gneiss complex was progressively assembled from disparate blocks of archaean continental crust and juvenile proterozoic arcs which satisfy the definition of terranes each terrane had its own separate accretionary and metamorphic history followed by a common history once juxtaposed against other terranes based on a new compilation of modern geochronology allied to the many detailed structural and metamorphic studies this paper proposes a new systematic terminology for the lewisian gneiss complex that is more applicable to this new tectonic framework

new light on the construction evolution and correlation of the langavat belt lewisian complex outer hebrides scotland field petrographic and geochronological evidence for an early proterozoic imbricate zone the south harris complex is a domain of largely palaeoproterozoic rocks within the late archaean tonalite trondhjemite granodiorite ttg dominated lewisian gneisses of the outer hebrides nw scotland the complex is distinguished by a high proportion of metasedimentary rocks and distinctive meta igneous units in part representing the remnants of a continental volcanic arc the langavat belt defines the ne border zone of the south harris complex separating the latter from the archaean gneisses further ne and has been repeatedly interpreted as a discrete supracrustal unit however detailed mapping and petrographic analysis reveals that up to 60 of the belt may be composed of highly deformed felsic orthogneiss the metasedimentary rocks that are present have a late archaean zircon provenance signature implying that they are younger than and possibly derived from the ttg gneisses to the ne previous work on zircon populations in the leverburgh metasediments towards the southern flank of the south harris complex indicates very different provenance and we repudiate correlation of the two sequences the disposition of units of metasediment amphibolite and felsic orthogneiss is not a primary pre tectonic feature and is very unlikely to be the result of folding although direct evidence in the form of early fabrics has been wiped out by later penetrative ductile shearing and metamorphic annealing we propose that the langavat belt was assembled as a zone of thrust imbrication during early proterozoic contraction after imbrication ductile shearing occurred in two distinct periods separated by intrusion of granite pegmatites dated at c 1660 ma

sm nd dating and cooling history of scourian granulites sutherland the scourian gneisses of the lewisian complex of north west scotland have been the subject of many geochronological investigations sm nd whole rock measurements suggest that the protoliths of the lewisian complex differentiated 2 92 0 05 gyr ago from an approximately chrondritic mantle1 rb sr and pb pb whole rock ages between 2 8 and 2 6 gyr reflect subsequent depletion in rb and u refs 2 3 which has generally been associated with granulite facies metamorphism zircons which are thought to have crystallized during this metamorphism have also given u pb ages of 2 66 gyr ref 4 this granulite facies metamorphism involving pressures in excess of 10 kbar and peak temperatures estimated between 820 50 c ref 5 and 1 250 c refs 6 7 affects a terrain including supracrustal rocks7 and thus implies a major tectonic event we report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in sm nd ratios garnet especially is strongly enriched in sm ref 8 data for three garnetiferous basic gneiss assemblages suggest that closure of the sm nd system occurred significantly later than the peak of the metamorphism

the different diffusive transport behaviours of some metals in layers of peru basin surface sediment abstract diffusion experiments with a duration of 90 days were carried out with radio tracers in an oxic and a suboxic sediment layer from two deep sea sediment cores diffusion coefficients breakthrough and time lag values were determined and the results checked by modelling only small differences of diffusive transport were observed for na and cs during the four experiments na behaved as a perfect conservative tracer and the somewhat stronger uptake of cs in one sample was caused by preferential absorption by illite that was enriched in this sample the significant differences between the four experiments for the diffusion parameters of mn zn co and cd correlate with the differing chemical compositions of the sediment layers higher mn oxide concentrations in the sample led to slower diffusion through the sediment sample and to significant sorption of the diffusing heavy metals this result confirms the dominating role of mn oxide phases in controlling the diffusive exchange of heavy metals between deep sea sediment and bottom water sediment resuspension of the oxic surface layer such as during potential ferromanganese nodule mining in the deep sea would be followed by increased diffusive flux of dissolved heavy metals especially mn from suboxic sediment layers to the new disturbed sediment bottom water interface according to our modelling the mn flux from the oxic sediment surface of 0 3 mg m 2 yr 1 in the undisturbed state would increase to up to several mg m 2 yr 1 within the first few weeks after disturbance depending on the time necessary until a stable oxic and mn oxide rich layer is again re established a total mn2 release of 54 mg m 2 was calculated if the original state is reached within 100 yr in contrast to only 3 5 mg m 2 for 5 yr of re establishment the fluxes of the other heavy metals would be smaller corresponding to their lower pore water concentrations in the suboxic layer compared to mn

geotechnical and sedimentological investigations of deep sea sediments from a manganese nodule field of the peru basin abstract deep sea mining of manganese nodules will significantly modify the surface layer of deep sea surface sediments are mechanically disturbed and a cloud of sediment particles suspended and re deposited assessment of sedimentological and soil mechanical characteristics of undisturbed top layer sediments from a manganese nodule field of the peru basin are necessary to provide baseline data for soil mechanical estimations and for the physical and numerical modelling of environmental impacts of future ocean mining activities this will assist the international seabed authority to make decisions on how to formulate the mining code and will increase our understanding of sedimentation processes in the peru basin the deployment of a new sampling device maxicorer guaranteed undisturbed sediment samples in which soil mechanics related parameters were measured including sediment density water content shear strength and penetration force data evaluation show a close relationship between the soil mechanical parameters and various sedimentological properties such as grain size distribution mineral composition and micro structure from the results it can be concluded that in the worst case a deep sea nodule mining vehicle would sink about 15 20 cm into the sea bed

geochronological studies of proterozoic crustal evolution in the northern outer hebrides abstract new data on the proterozoic thermal and tectonic crustal history of the lewisian complex have been determined using sm nd and 40 ar 39 ar isotopic analyses igneous assemblages from two mafic ultramafic masses indicate crystallization at 2 54 and 2 14 ga from magmas with initial nd isotopic composition lower than predicted depleted mantle at those times sm nd isotope systematics t dm 2 45 ga in two samples of metasediment from the leverburgh belt imply that they contain a significant post archaean component and cannot therefore be part of an archaean stratigraphic sequence sm nd mineral isotope data on three samples confirm the proterozoic age of high pressure granulite facies metamorphism in the south harris igneous complex the most precise estimate is 1 827 0 016 ga on a metagabbro which was intruded after deformation of the host anorthosite under granulite facies conditions localized granulite facies assemblages in younger basic bodies further north are significantly younger at 1 63 ga whole rock sm nd data on two samples from the 1 7 ga granite injection complex of south harris indicate nd between 11 5 and 13 7 at the time of intrusion providing a strong indication that they were produced by remelting of pre existing crust similar to the exposed grey gneisses which had values close to 12 at that time 40 ar 39 ar data from 13 hornblendes generally show complex argon release patterns with indications of excess argon in most samples flat sections of the spectra range in age between 1 63 and 1 78 ga previous rb sr data on biotite showed a distinct break in age pattern across the langavat belt there is no indication of such a break in the hornblende data and it is inferred that despite variations in the timing of peak metamorphism the whole of the northern outer hebrides cooled through hornblende closure temperatures in the interval 1 7 1 6 ga

tectono metamorphic events in the north atlantic region in the palaeoproterozoic from the view point of high grade metamorphic rocks in the lewisian complex south harris nw scotland abstract the tectono thermal history of the lewisian complex in south harris south harris complex was inferred from its geologic and metamorphic characteristics the lithological assemblages and geochemical features of the complex suggest that its precursory rocks were composed of the subduction related accretionary complex formed in the palaeo convergent margin the complex has suffered the ultra high temperature uht metamorphism that was contemporaneous with the igneous activity to make the south harris igneous complex shic and the subsequent continent continent collisional activity a similar complex recording the geological processes of the subduction the uht metamorphism and the collision has been recognized in the lapland kola belt and new quebec in the palaeoproterozoic this suggests an assembly of micro continents to form the palaeoproterozoic supercontinent in the north atlantic region

importance of different types of marine particles for the scavenging of heavy metals in the deep sea bottom water abstract in experiments of 7 days duration using voltammetric and radiotracer measurement techniques the role of different particle types in the sorption of dissolved metal species in a disturbed deep sea bottom seawater system were investigated resuspension of oxic to suboxic surface sediment into the bottom water in the deep sea either by natural events or industrial activities like mn nodule mining has been shown to be followed quickly by scavenging of dissolved heavy metals e g released from interstitial water on the resuspended particles compared to other deep sea particles like clay minerals calcite and apatite mn and fe oxides and oxyhydroxides were found to be by far the most important phases in scavenging many dissolved heavy metals only pb was sorbed strongly on all particles used with highest affinity to carbonate fluorapatite caesium was significantly scavenged only by clay minerals like illite the sorption experiments support a simple electrostatic model hydrated cations and labile cationic chloro complexes in seawater like mn2 mncl co2 ni2 cu2 zn2 ba2 and pbcl are preferentially adsorbed or ion exchanged on the negatively charged surfaces of mn oxides in contrast oxyanions and neutrally or negatively charged complexes like hvo42 moo42 haso42 uo2 co3 22 and pbco30 associate with neutral to slightly positive amphoteric fe oxyhydroxide particles metals forming strong chloro complexes in seawater like cd cdcl20 are less readily sorbed by oxides than others a comparison of the results of voltammetric and radiotracer techniques revealed that after fast sorption within the first hour isotopic exchange dominated reactions on mno2 rich particles in the following days this was especially pronounced for mn and co which are bound to the mn oxide surface via a redox transformation

heavy metal distributions in peru basin surface sediments in relation to historic present and disturbed redox environments abstract heavy metal distributions in deep sea surface sediments and pore water profiles from five areas in the peru basin were investigated with respect to the redox environment and diagenetic processes in these areas the 10 20 cm thick mn oxide rich and minor metal rich top layer is underlain by an increase in dissolved mn and ni concentrations resulting from the reduction of the mno 2 phase below the oxic zone the mobilised associated metals like co zn and cu are partly immobilised by sorption on clay organic or fe compounds in the post oxic environment enrichment of dissolved cu zn ni co pb cd fe and v within the upper 1 5 cm of the oxic zone can be attributed to the degradation of organic matter in a core from one area at around 22 25 cm depth striking enrichments of these metals in dissolved and solid forms were observed offset distributions between oxygen penetration and mn reduction and the thickness of the mn oxide rich layer indicate fluctuations of the mn redox boundary on a short term time scale within the objectives of the german atesepp research programme the effect of an industrial impact such as manganese nodule mining on the heavy metal cycle in the surface sediment was considered if the oxic surface were to be removed or disturbed oxygen would penetrate deep into the formerly suboxic sediment and precipitate mn 2 and metals like ni and co which are preferably scavenged by mno 2 the solid enrichments of cd v and other metals formed in post oxic environments would move downward with the new redox boundary until a new equilibrium between oxygen diffusion and consumption is reached

disturbed 40ar39ar spectra from hornblendes thermal loss or contamination abstract accurately weighed aliquots of the 500 ma old interlaboratory standard hornblende mmhb 1 were mixed with known quantities of a 100 ma old himalayan biotite and subjected to normal 40 ar 39 ar incremental heating analysis the addition of the biotite created monotonically increasing apparent ages similar to profiles interpreted as showing ar diffusive loss in the literature the release of ar from biotite dominates the low temperature portion of the spectra and lowers the observed age lower ca k ratios in the first increments are consistent with contamination but do not distinguish the composition of the contaminant phase a large number of published amphibole diffusive loss profiles may in fact be due to contamination by biotite or other phases which release ar at low extraction temperatures

u pb baddeleyite ages for the scourie dyke swarm scotland evidence for two distinct intrusion events precise and accurate radiometric ages for continental mafic dyke swarms are a prerequisite for global correlation of mafic magmatic events calibration of apparent polar wander paths and deciphering mechanisms of dyke emplacement precambrian dyke swarms such as the scourie dyke swarm in north west scotland are invaluable time markers so that a precise and accurate knowledge of their emplacement age is critical when unravelling the complex geological evolution of many terrains precise dating is often difficult however because magma interaction with country rock and subsequent metamorphic events can severely perturb some isotopic systems recent advances in u pb geochronology1 3 combined with the discovery that some mafic dykes contain trace amounts of uranium bearing minerals such as baddeleyite zro2 and or zircon have made it possible to obtain u pb ages for these rocks with a precision typically on the order of 1 2 myr4 5 some of the first detailed u pb studies of precambrian mafic dyke swarms5 6 have shown that large volumes of mafic magma 100 000 km3 were emplaced into the continental crust in surprisingly short periods of time 2 myr here we report the results from a u pb study of three members of the scourie dyke swarm and find at least two periods of dyke emplacement at 2 418 and 1 992 myr bp we speculate on a potential global correlation of early proterozoic mafic magmatism and hence on the origin of dyke swarms

compositional changes of surface sediments and variability of manganese nodules in the peru basin abstract two types of manganese nodules were observed in the peru basin large botryoidal nodules in basins and small ellipsoidal nodules on slope positions the sediment in areas with large botryoidal nodules contains a thinner and weaker oxidation zone than the sediment under small ellipsoidal nodules indicating that diagenetic processes in the sediment which supply manganese nodules with metals for their growth are stronger in sediments on which large botryoidal nodules grow organic matter which activates remobilization of metals occurs mostly in the form of refractory lipidic compounds in the inner capsule of radiolaria this material needs bacterial degradation to act as a reducing agent easily oxidizable organic components could not be found in the sediments other changes in sediment composition do not have a link to manganese nodule growth biogenous components radiolarians organogenic barite and apatite increase towards the equatorial high productivity zone authigenous clay minerals nontronite as well as montmorillonite with high fe 3 incorporation on positions of ochtaedral al increase with distance from the continent the assessment of environmental impacts will have to take into account the regional differences in sediment composition and the small scale variability of manganese nodules

the behaviour of deep sea sediments under the impact of nodule mining processes abstract the complex processes of the suspension resettling and consolidation behaviour of deep sea sediments caused by deep ocean mining were investigated on board the german rv sonne and in the laboratory using sediment from the uppermost semi liquid layer of the peru basin on board tests were accomplished directly with undisturbed sediment samples recovered by the maxicorer a sediment sampling device especially developed for such studies the impact was simulated by the use of a propeller generated water jet the results were used to prepare a substitute for the semi liquid top layer for large scale laboratory tests to simulate the sea floor conditions to be expected during manganese nodule mining in line with the design of most nodule miners these tests were performed with the use of water jets the amounts of displaced and suspended sediment were determined as was the resettling behaviour the tests revealed that a mean value of 16 of the sediment was suspended by the water jets the remainder was remoulded and laterally displaced for a short distance but not suspended into the water column the results of the on board tests were confirmed by the laboratory tests and have great significance in the prediction of sediment clouds generated by ocean mining activities additionally effects on the resettling behaviour and formation of aggregates due to temperature and particle concentration were investigated it was demonstrated that high particle concentration and high temperature within the suspension accelerate the aggregation process and thus lead to enhanced resettlement of sediment particles finally the fabric of the resettled sediment was evaluated by its void ratio determined by the consolidation process it is estimated that it will take at least 20 years for the sediment to regain its original density

mobility of metals over the redox boundary in peru basin sediments abstract downcore variations and phase distribution of metals in the quaternary sediments from six areas of the peru basin were investigated the sediments are characterised by a 5 20 cm thick oxic layer at the water sediment interface formed under the influence of dissolved oxygen in the bottom water in deeper sediment sections suboxic conditions are found due to the degradation of organic matter variations in the downcore abundance of metals e g fe mn co ni cu zn v mo cr were determined by bulk chemical methods a significant accumulation of manganese is observed at the depth of the manganese redox boundary caused by diagenetic remobilisation under suboxic conditions and reprecipitation under oxic conditions to investigate their phase distributions partitioning of elements in selected samples over the sediment column was carried out in sequential leaching experiments a comparison between the surface layer and deeper sediment layers shows a distinct change in the element distribution between operationally defined host phases carbonates oxy hydroxides detrital components above the redox boundary at the sediment surface transition metals are enriched in the reducible mn oxide fraction when the horizontal position of the mn redox boundary moves upward influenced by naturally occurring enhancement of surface water productivity these elements are mobilised and available to the manganese nodule accretion process if the redox boundary reaches the level of growth of the nodules in contrast to the naturally induced mobilisation of metals a technical impact e g mining of manganese nodules would be a rapid event occurring under completely different redox conditions metals would be released into the bottom water an oxic environment where scavenging of dissolved metals and resedimentation takes place in relatively short periods of time presumably a long distance transport of contaminants will not happen

evaluation of the environmental consequences of polymetallic nodule mining based on the results of the tusch research association abstract realising a need for increased general knowledge of the deep sea for environmental impact assessments related to the permanent storage of waste products and mining of metal resources the german bundesministerium fur bildung und forschung has funded targeted research in the deep sea for more than 10 years the research was carried out in an area in the southeast pacific ocean close to and within a german mining claim to match the interests of german deep sea polymetallic nodule mining enterprises and the developing mining code of the preparatory commission for the international seabed authority and the international tribunal for the law of the sea the tusch abbreviation for tiefsee umweltschutz deep sea environmental protection research association with members from various university and governmental institutions was part of the atesepp effects of technical interventions into the ecosystem of the deep sea in the southeast pacific ocean project between 1996 and 1998 geotechnical sedimentological geochemical hydrographic numerical modelling and ecological studies relevant to environmental impact assessment studies of polymetallic nodule mining were undertaken since general oceanographic knowledge of the deep sea is rather limited these various projects also have increased our general understanding of this region this paper describes the potential impacts of mining processes on the seafloor and the near bottom water layer as well as on bathyal and abyssal pelagic zones that will receive processed water sediment and abraded nodule fines tailings discharged after nodule transport to the mining vessel at the ocean surface the tusch research association defined various recommendations to keep the unavoidable impacts to a minimum such as limited penetration of the mining system into the sediment confining intensive resedimentation to the area behind the miner minimising transport of sediment to the ocean surface and discharging all tailings at great depths ideally close to the seafloor the recommendations are not new but the present studies have improved them from precautionary to scientifically based statements

bioturbation coefficients of deep sea sediments from the peru basin determined by gamma spectrometry of 210pbexc abstract in the investigation of the impact of deep sea mining on the ecosystem the radioactive disequilibrium between 210pb and 226ra was studied in 16 surface sediment cores from the peru basin to obtain a quantitative measure of bioturbation gamma spectrometry was used as a fast non destructive simultaneous assay of 210pb and 226ra buried manganese nodules and geochemical anomalies affect the depth profiles of these two radioisotopes the calculated bioturbation coefficients show a high lateral variability this made it impossible to detect a significant difference in bioturbation between two different study areas

short paper evidence for a grenville event in the lewisian of the northern outer hebrides northern britain lay between two areas of major tectonic activity in the late proterozoic but evidence for geological events of this age within britain are sparse twenty five new rb sr biotite ages from a 50 km traverse through the northern outer hebrides show that a block bounded in the south by the langavat shear zone was generally uniformly affected by an event close to 1 1 ga consideration of the age pattern on either side of the langavat belt suggests that there were kilometre scale displacements on this shear zone at that time involving uplift and erosion of the northern outer hebrides as a whole it is thus possible that the northern outer hebrides acted as a source area for both laxfordian and grenville detritus found in the torridonian

a nd isotope investigation of sediments related to crustal development in the british isles the times of original fractionation of the sm and nd component of clastic sediments from a mantle source crustal residence age have been estimated from sm nd model ages calculated relative to a depleted mantle evolution in this way the provenance and evolution of selected precambrian and phanerozoic sediments and metasediments from the british isles have been examined whereas some archaean and early proterozoic sediments have sm nd model ages that are close to their stratigraphic age the phanerozoic sediments analysed have model ages as much as 2 0 ga in excess of their stratigraphic age a more detailed study of lower palaeozoic sediments deposited on the northern margin of the iapetus ocean provides evidence for a marked change of provenance in the ordovician after the deposition of the dalradian supergroup a component with comparatively high143nd 144nd and sm nd ratio presumably basaltic is present in the sediments throughout the accretionary prism crustal residence age estimates average about 1 5 ga for both these lower palaeozoic sediments and modern pelagic clays and collectively fail to provide any evidence for significant continental growth during the phanerozoic