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Chronostratigraphic carbon isotope record of the Lesser Himalaya
Abstract
Early investigators1,2 noticed that the isotope age curve for sedimentary carbon contains a negative displacement in δ13C at the Precambrian/Cambrian (PC/C) boundary, about 570 million years ago 3, which may have accompanied the profound biotic changes4 from non-skeletal organisms to those with mineralized skeletons. Whereas the certainty of the δ13C shift across the elusive boundary has recently been confirmed from a number of localities5-8, its interpretation remains largely controversial. It has been attributed to variations in oceanic biomass fertility and concomitant fluxes of organic matter burial6-8, changing rates of ocean ventilation9, onset of biomineralization10 and extraterrestrial causes5. Here we report the results of a carbon isotope study from a hitherto unexplored section of marine sediments in the Lesser Himalaya, India, which documents significant isotope variations across the PC/C boundary. These results also provide important details towards resolving the relative timing of the δ 13C shifts and the biotic changes. Evaluation of our data rules out post-depositional isotope exchange as a major cause of the recorded δ13C variations and favours changes in the isotope chemistry of the ocean surface at the close of the Precambrian.
... The Krol-Tal succession of the Lesser Himalaya archives fossils of small shelly fauna, acritarchs, and trilobites (e.g., Hughes et al., 2005;Tiwari, 1999). The presence of the Pc-C boundary in this succession has been established on the basis of chemo-, chrono-, and bio-stratigraphic records (Aharon et al., 1987;Bhargava et al., 1998;Singh et al., 1999). Further, sequence and carbon isotopic stratigraphy of the Krol Group has also been correlated with the Dengying and Doushantuo Formations in South China (Aharon et al., 1987;Jiang et al., 2003;Kaufman et al., 2006). ...
... The presence of the Pc-C boundary in this succession has been established on the basis of chemo-, chrono-, and bio-stratigraphic records (Aharon et al., 1987;Bhargava et al., 1998;Singh et al., 1999). Further, sequence and carbon isotopic stratigraphy of the Krol Group has also been correlated with the Dengying and Doushantuo Formations in South China (Aharon et al., 1987;Jiang et al., 2003;Kaufman et al., 2006). Existing geochemical (Banerjee et al., 1997;Mazumdar & Banerjee, 2001;Mazumdar et al., 1999) and mercury isotopic (Liu et al., 2021) records of the Lesser Himalayan shales have provided evidence for transitional suboxic-anoxic conditions, and the influence of continental erosion and upwelling processes on these varying redox states during the Pc-C transition. ...
... This succession is characterized by assemblages of small shelly faunal genera, such as Anabarites, Protohertzina, and Maldeotaia (Bhatt et al., 1985;Brasier & Singh, 1987). Chemo-(carbon and oxygen isotopes), bio-(acritarchs, small shelly fauna, trilobites, and brachiopods) and chrono-(535 ± 11 Myr; Singh et al., 1999) stratigraphic evidence mark the Pc-C boundary near the Krol-Tal transition (Aharon et al., 1987;Azmi, 1983;Jiang et al., 2003). An older Rb-Sr age of 626 ± 13 Myr (Sharma et al., 1991) for the shales has also been reported for the Lower Tal, which may have been influenced by sediment provenance age. ...
The Precambrian‐Cambrian (Pc‐C) boundary marks significant biological, atmospheric, and oceanic changes. These changes include extinction of the Ediacaran fauna, initiation of complex lifeforms, and oxygenation of the atmosphere and oceans. In this contribution, elemental and Mo‐S isotopic compositions of organic‐rich shales overlying the Pc‐C boundary from the Tal Formation, Lesser Himalaya, have been investigated. These datasets are used to reconstruct past oceanic redox state and sulfate concentrations. The principal component analysis of the elemental dataset identifies six major factors, with factors associated with organic matter and sulfide phases accounting for about half of the total variance. Iron speciation and Mo/U data suggest that the shales were deposited in anoxic and ferruginous deep water conditions. The δ⁹⁸Mo data (1.5 ± 0.2‰) and their mass balance calculations indicate that the areal extent of sulfidic waters and pyrite burial rates were about 2–4 times higher during the Pc‐C transition than in the modern ocean. The pyrite‐δ³⁴S values (3.6–8.3‰) for the Tal shales are isotopically heavier compared to modern‐day sedimentary pyrites (∼−21‰). Calculations involving earlier‐reported δ³⁴S values for early Cambrian seawater and our measured pyrite‐δ³⁴S data estimate the seawater sulfate concentration (8 ± 3 mM) during their deposition. This sulfate value for the Tal basin is higher than that reported for the late Neoproterozoic ocean (<5 mM), attributable to increasing oxygen availability and continental supply during this time. The observed basinal conditions and high terrestrial input may have influenced metazoan diversification.
... Malla Johar area of the Tethyan Himalaya has thick carbonate sequences that were deposited between the late Neoproterozoic and Cambrian hence, constitute a fine representative of biogeochemical changes and events during this important period. Carbon and oxygen stable isotope studies in the Himalayas have been carried out earlier in the Krol-Tal succession in the Lesser Himalayas (Aharon et al., 1987;Banerjee et al., 1997;Kaufman et al., 2006). Secular global trends of the carbon isotope variations in Cambrian and Proterozoic rocks of India, Pakistan and Mangolia have been discussed by Banerjee (1990). ...
... The Tethyan successions in the north-western Himalaya are exposed in four distinct basins namely Kashmir, Zanskar, Spiti and Malla Johar in Kumaun Himalaya. Sedimentation in these basins presumably started around the Neoproterozoic (Aharon et al., 1987). The latest Precambrian-early Cambrian deposit comprises of clastic sedimentary succession. ...
... Kumar and Pandey (2010) had already shown the possible link between three depositional realms, which are lower Cambrian of Tethys Himalaya, Tal unit of Lesser Himalaya and Nagaur Group of the Marwar Supergroup including Salt Range, Pakistan. Therefore, it appears that the Garbyang Formation of the Tethys Himalaya, the Krol-Tal succession of the Lesser Himalaya and the Marwar Supergroup of western India are part of the extensive carbonate platform which developed in North-West India and extended westwards into the Arabian Peninsula (Aharon et al., 1987). In these regions latest Precambrian shows development of evaporite deposit. ...
The carbonate rich deposits of the Garbyang Formation, Tethys Himalaya in Malla Johar area, Uttarakhand, India possibly hosts the latest Precambrian early Cambrian sediments based on earlier studies of trace fossils. To further investigate this issue, fifty-four samples of carbonate were collected and analysed for δ13C-carb and δ18O-carb. The results are significant in absence of any chemostratigraphic dataset of the Garbyang Formation till date. The δ13Ccarb varies from -5.8‰ to 1.0‰. But, δ18O-carb show anomalously low values between -10‰ and -33‰ that reflects significant meteoric water diagenesis. The influence of meteoric water diagenesis on the carbon isotopic ratios is apparently minimal as δ13C-carb stratigraphy of the Garbyang Formation closely
follows the established global δ13C-carb curve for near Precambrian-Cambrian time frame. Signature of the probable Precambrian-Cambrian (Pc-C) boundary has been identified in the lower part of the formation (Garbyang B). Various types of burrows and trace fossils of the Cambrian affinity have been reported from succession overlying the probable Pc-C boundary.
... The similarity in Cambrian ages between the LHF and GHC suggests they may be part of a shared depositional environment, with the LHF as proximal and the GHC as distal (Brookfield, 1993;Parish & Hodges, 1996;Corfield & Searle, 2000;Myrow et al., 2003;Myrow et al., 2006;Long et al., 2011). Alternatively, the GHC may have been a distinct basement unit separating the LHF from the Tethyan metasediments (Saxena, 1971;Aharon et al., 1987) or an exotic terrain involved in a pre-Himalayan collision (DeCelles et al., 2000;Gehrels et al., 2003). ...
The Himalayan orogen exposes assemblages from low-grade Indian shelf sediments of the Tethyan Formation to eclogite and ultra-high-pressure rocks from the suture zone between the Indian craton and Asian subcontinent. Barrovian-grade pelites in the Himalayan core comprise the Greater Himalayan Crystallines and Lesser Himalayan formations. These units are separated by the Main Central Thrust (MCT), which accommodated a significant amount of convergence. We describe and apply isopleth thermobarometry and high-resolution pressure-temperature (P-T) path modeling to decipher the metamorphic history of garnet-bearing rocks collected across the MCT in central Nepal. Results are compared to with previous reports of conventional rim P-T conditions and P-T paths that used Gibb's method on the same data and assemblages. Predictions of the paths on garnet zoning are also presented for the high-resolution P-T path modeling and Gibb's method using the program TheriaG. Although the approaches yield different absolute conditions and P-T path shapes, all are consistent with the development of the MCT shear zone due to the imbrication of distinct rock packages. Greater Himalayan Crystalline garnets experienced higher grade conditions, making extracting its P-T conditions and paths challenging. Lesser Himalayan garnets appear to behave as closed systems and are ideally suited for thermodynamic approaches.
... The present Ediacaran fossil bearing horizon is characterised by δC 13 values that vary from +1‰ to +6‰ PDB (Aharon et al.,1987;Kumar & Tewari,1995;Bhattacharya et al., 1996;Kaufman et al., 2006). Similar isotopic signatures have also been described from the Blue Flower Formation, northwestern Canada Schwarzrand Subgroup of Namibia, Rodda Group strata equivalent to Ediacara Member in Australia, Dengying Formation in China and Khatyspyt Formation in Siberia (Narbonne et al., 1994). ...
Ediacaran multicellular biota, viz. medusoids - Kimberella cf. quadrata, Beltanella cf. gilesi, Cyclomedusa davidi, Conomedusites lobatus, Tirasiana sp., Medusinites asteroides, Sekwia cf. excentrica, Irridinitus sp. and Beltanelliformis cf. brunsae; frondoids - Charniodiscus cf. arboreus, Pteridinium cf. simplex and Zolotytsia biserialis; annelid – Dickinsonia sp.; ichnofossils - Bilinichnus sp. and metaphytic algae- cf. Proterotaenia montana, has been recorded from the Kauriyala Formation (Upper Krol) of the Krol Group, Lesser Himalaya India. The underlying Jarashi Formation (Middle Krol) has yielded frondoid forms - Pteridinium carolinaense and Charniodiscus cf. arboreus and trace fossil - Harlaniella sp. whereas the Mahi Formation (Lower Krol) has yielded medusoid - Nimbia cf. occlusa. This biota is generally cosmopolitan in nature except Dickinsonia which is restricted to Protogondwana. The Ediacaran biota is preserved at the interface of arenite / siltstone and shale which show ripple marks, rhythmic and lenticular bedding at places suggestive of tidal flat environment. The present biota is comparable with Ediacaran multicellular biota of Ediacaran (Terminal Neoproterozoic) Period known from Australia, Canada and Russia. The fossiliferous horizons are characterised by dC13 values that vary from +1% to +6% PDB. Similar isotopic signatures have also been described from other Ediacaran fossil bearing horizons from northwestern Canada, Namibia, Australia, China and north Siberia.
... Radiometrically constrained carbonate-rich successions in Namibia Linnemann et al., 2019) and Oman (Amthor et al., 2003;Burns and Matter, 1993) have played a vital role in late Ediacaran chemostratigraphy and paleobiology, but in both cases, carbonates stop near the Ediacaran-Cambrian boundary, as do skeletal and most soft-bodied Ediacaran macrofossils. Similarly, thick carbonates of Krol succession in sub-Himalayan India have a well-documented chemostratigraphy but while basal phosphatic shales of the overlying Tal Formation provide a point source of biostratigraphic information, there are otherwise few fossils or carbonates in this succession until Cambrian Series 2 and later (Aharon et al., 1987;Hughes, 2016;Landing and Geyer, 2020). Among the best records of basal Cambrian C-isotope stratigraphy is that of the Anti-Atlas in Morocco (Maloof et al., 2010(Maloof et al., , 2005, but these chemostratigraphically informative carbonates do not contain animal fossils. ...
Mixed siliciclastic-carbonate rocks of the Kahar and overlying Soltanieh formations from Chopoghlu, in the Soltanieh Mountains, northern Iran, preserve an expanded stratigraphic record of Ediacaran to lower Cambrian life and environments. To date, regional placement of the Ediacaran-Cambrian boundary has been based on biostratigraphy, with contrasting views of its stratigraphic position. Here we present a high-resolution carbon isotope chemostratigraphic record to provide an independent assessment of correlation to other successions worldwide. Facies analysis indicates that Kahar siliciclastic sediments accumulated in fluvial to tidal environments, with thin microbial carbonates in tidally influenced facies. The overlying Soltanieh Formation records peritidal carbonate deposition followed by significant deepening and deposition of interlaminated mudstones and thin sandstones. δ¹³C values for Kahar carbonates, with a maximum age of about 550 Ma, lie within a narrow range of 0 to -2‰, save for a moderate positive excursion near the top of the formation. Above this, carbonates of the Lower Dolomite Member, Soltanieh Formation, show consistently negative δ¹³C values. The overlying Middle Dolomite, Middle Shale and basal part of the Upper Dolomite members feature high amplitude δ¹³C variations, which rise to values as high as 4‰ towards the top. Considering the integrated chemostratigraphic and paleontological records, Chopoghlu deposition appears to have begun during the late Ediacaran Period. The stratigraphically variable δ¹³C record of Soltanieh carbonates closely resembles lowermost Cambrian successions elsewhere, supporting placement of the E-C boundary near the top of the Lower Dolomite Member (LDM) and a Terreneuvian age for overlying Soltanieh beds. This is consistent with the presence of Protohertzina sp. and Hyolithellus sp. in or just above uppermost LDM carbonates and the lowermost occurrence of Treptichnus pedum above this. Together, bio- and chemostratigraphy help to integrate the Chopoghlu section into the global record of biospheric change as the Phanerozoic Eon began.
... GSSP of Precambrian and Cambrian (Pc-C) was established in Newfoundland, Canada (Landing 1994). In India, once the Krol-Tal succession was considered a good candidate for the Pc-C boundary section but detailed studies brought out the limitations in considering this as a continuous section (Aharon et al. 1987;Jiang et al. 2003;Flügel and Singh 2003;Singh et al. 2019a, b, c Planolites-Palaeophycus, Cruziana problematica, Diplichnites, Cochlichnus anguineus, Bergaueria perata and Psammichnites gigas were established from the sandstone member of the Koti Dhaman Formation (Tal Group) in the Nigalidhar Syncline of the Lesser Himalaya and their palaeoenvironmental and palaeogeographical significance were discussed (Singh et al. 2019a, b, c). Earlier studies recorded trace fossils and Small Shelly Fossils from Mussoorie and Garhwal synclines (Banerjee and Narain 1976;Singh and Rai 1983;Azmi 1983;Azmi and Joshi 1983;Azmi and Pancholi 1983). ...
Significance of Precambrian and early Cambrian palaeobiological remains recorded over the last few years on the Indian subcontinent are discussed in the present review. The importance of these remains is assessed in the backdrop of new reports from different parts of the world. Archaean, Proterozoic and early Cambrian successions of India have revealed some of the oldest rock fragments of the world, most primitive eukaryotic remains, bizarre Ediacaran forms and typical early Cambrian fossils. For the sake of brevity and easy comprehension, palaeobiological remains are grouped under Archaean, Proterozoic, Ediacaran and early Cambrian time-frames. New evidence of life forms, documented from the sedimentary basins exposed in peninsular and extra-peninsular region of the country, have challenged the various paradigms, viz. evolutionary, biostratigraphic status of different lithounits and age of the encompassing sedimentary successions. Newly published geochronological and geochemical results are also considered in this review to understand the developments in these fields and their significance in the evolution of hydrosphere, biosphere, and moderating processes over the geological timescale. Recent Indian endeavors in the field of Astrobiology and inputs provided in the areas of astrobiological studies are also discussed.
... a) The crystalline axis model suggests that the Lesser and Tethyan Himalayan Sequences were deposited simultaneously in different basins which were later separated by the Greater Himalayan Sequence (GHS) (Saxena, 1971). Given the younger detrital zircon ages of the GHS in comparison to other Himalayan units, along with their paleontological, lithological, and sedimentological relations, as well as the absence of any suture zone rocks in the Main Central Thrust (MCT) zone, this model is no longer tenable (Aharon et al., 1987;Parrish and Hodges, 1996;Myrow et al., 2010). b) The single margin https://doi.org/10.1016/j.jseaes.2020.104288 ...
The Jurassic-Cretaceous (Upper Tethyan) flysch well exposed in the Thakkhola Section, Nepal, was deposited prior to the continent-continent collision between the Indian and Eurasian plates. This Tethyan Himalayan Sequence (THS) has preserved the extreme northern passive margin of the Indian plate with its direct deposition onto the eroded Precambrian rocks in the lowest part of this relatively continuous section. The clastic rocks of the THS, as they are useful to constrain the paleogeography and paleotectonics of the Himalayan Orogen. In this study, we report the petrography, detrital zircon U-Pb isotopic ages, and zircon trace element data for sandstone samples from the Upper part of a Triassic stratigraphic section. The mineral modal composition data indicates that these quartz-rich units were derived from recycled orogen. This is supported that the youngest zircons analyzed can be dated to ~400 Ma that predates the depositional age by hundreds of millions of years. A total of 85% of the zircon ages are < 1,500 Ma and the oldest dated zircon is ~3300 Ma. What these sections share in terms of detrital zircon signals are peaks at pre-Rodinian, a Grenvillian, a Gondwanan, and nothing either younger, except for the weak presence or total absence, of a Cretaceous zircon age signal. More importantly, these results drive us to surmise that either sedimentary or crystalline, in the Indian plate that has provided the unusual mix of 475–675 Ma and 825–950 Ma age peaks, with the latter associated with an Eastern African super deltaic fan.
Ediacaran to Early Cambrian sequences are well preserved in several synclines within the Lesser Himalaya. We have conducted multi-proxy geochemical and stable isotopic studies on carbonaceous sediments of the basal Tal Group to decipher redox conditions of Earliest Cambrian Ocean. Organic matter is immature, predominantly aliphatic and likely derived from marine phytoplankton. The S/C(organic) ratios above 0.4; strong enrichment of redox sensitive trace elements V, Mo, U and Cd; higher DOPT, U/Th, V/Sc, Mo/Sc and V/(V+Ni) ratios; V/Al-U/Al-Mo/Al systematics and U-Mo enrichment factors indicate anoxic to euxinic conditions of deposition. Large variations (5-7 ‰) in 13C likely reflect spatial heterogeneity or vertical gradient in DIC compositions of the Early Cambrian Ocean, and possibly captures the part of Basal Cambrian negative Carbon isotope Excursion (BACE). Finely disseminated texture of pyrite framboids with sizes mostly below 10 µm suggest sulfide reduction within the water column in an open system. Isotopically heavy 34S values (+4.5 – +22.8 ‰) overlap with the 34Ssulfate of contemporary Fortunian ocean (+10–+40‰) suggesting sulfidic conditions and small size of the sulfate reservoir. Whereas, the REE+Y proxies of the same set of samples show oxidised seawater like patterns, with strong HREE enrichment, positive Y and La anomalies and negative Ce anomaly (avg. Ce/Ce* = 0.78), indicating REE uptake by phytoplankton from an oxidised euphotic surface zone. The combined geochemical and isotopic data suggest strongly stratified ocean with sulfidic bottom water and oxidised surface water. Episodic shoaling of anoxic deep water onto the surface possibly caused the extinction of benthic Ediacaran fauna, and the surface oxidised zone likely provided the niche for further metazoan radiation.
Carbonate units deposited during the Precambrian-Cambrian transition records a unique δ ¹³ C profile that is useful for chemostratigraphic correlation. However, the Precambrian-Cambrian boundary is currently defined within siliciclastic units where δ ¹³ C data are not available. The mixed siliciclastic-carbonate succession from the southern Great Basin, USA, records the appropriate fossils in the siliciclastic strata interbedded with carbonate strata that record the appropriate shifts in δ ¹³ C to facilitate correlation between the lithologic end-members. Ultimately, the integrated dataset demonstrates that vertical burrowing and the onset of widespread biomineralization was essentially synchronous.
This chapter provides summarized geological and geophysical account of the youngest and highest mountain belt to elucidate the role of the continental lithospheric subduction of the Indian Plate in the evolution of the Himalaya during the Cenozoic.
Dolomites are more common in Precambrian than in Phanerozoic rocks and, from a petrographic-isotopic study of the middle to upper Proterozoic Beck Spring Dolomite of eastern California, it is suggested that Precambrian dolomites are different from those of the Phanerozoic. Petrographically, all fabric details of Precambrian dolomites studied are preserved, just as if they were limestones, and isotopically, they show the same trend as limestones: increasingly negative delta O-18 and to a lesser extent increasingly light delta C-13, from depositional components, through early fibrous dolomite cements to later dolomite spar cements. The data suggest that in Precambrian time dolomite was the principal carbonate mineral precipitated from seawater and during diagenesis, and this implies that Precambrian seawater was different from that of the Phanerozoic. -Author
The core of the former atoll on Niue Island, South Pacific, is extensively dolomitized. A detailed stable isotope study reveals that the dolomites are uniformly enriched in ¹â¸O and ¹³C(delta ¹â¸O = 2.8 per thousand +/- 0.5; delta ¹³C = 2.0 per thousand +/- 0.3) relative to their carbonate precursor. The Sr isotope ratios are equally constant with depth yielding â¸â·Sr/â¸â¶Sr values between 0.70912 and 0.70916. The observed isotopic compositions point to sea water as the dominant dolomitizing fluid. The Sr age of the dolomites is younger than the biostratigraphic age (Mid to Late Miocene) placing the dolomitization event at Plio-Pleistocene time. A model of sea water convection is proposed for atoll dolomitization on the basis of a thermal gradient between the atoll and the ambient ocean water. Sea water is drawn through the atoll margin and transferred upward by convective flow delivering Mg to the sites of dolomite precipitation. Transported with the sea water are volcano-derived metals (Fe, Cu, Zn, Mn) as evidenced by the chemical gradient in the dolomite unit. It is suggested that the thermal convection model can find general application in atolls and other carbonate platforms containing dolomite where thermal instability exists. 70 references.
Strontium isotopic measurements were made on Late Proterozoic carbonates from West African Craton. Comparison of samples with acceptable trace element patterns with coeval data from southern Africa and with the published Australian results suggests that the ratio of the Late Proterozoic sea water evolved in the following manner about 0.7075 at 1000 ± 50 Ma, 0.7056 to 0.7074 at 900 ± 50 Ma, 0.7068 to 0.7091(0.7106) at 800 ± 50 Ma, 0.7074 to 0.7077 at 700 ± 50 Ma, and 0.7076 to 0.7089(0.7096) at 600 ± 50 Ma ago. The variations are comparable in magnitude and frequency to those described previously for the Phanerozoic. Strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of Sr into Late Proterozoic oceans was of comparable isotopic composition to its present day counterpart (∼0.711). Consequently, the non-radiogenic value at ∼900 ± 50 Ma ago signifies a large flux of “mantle” strontium into the ocean at this time. Because the present time resolution is only about 75 ± 25Ma, additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed Late Proterozoic sea water curve.
Dolomites from Middle Devonian carbonate formations in Northern Canada have been analyzed for their mineralogical and isotopic composition and trace amount of sodium impurities. Petrographic and geochemical evidence allows the distinction of three major groups: (1) supratidal and early diagenetic dolomites with δ18O values between −1 and −4 per mil (PDB), more than 5 mol % CaCO3 in excess and up to 700 ppm Na in impurities, (2) late diagenetic dolomites with δ18O values ranging from −7 to −12.5 per mil (PDB), less than 5 mol % CaCO3 in excess and less than 200 ppm Na in impurities, and (3) hydrothermal dolomites with δ18O values between −6 and −11 per mil (PDB), less than 2 mol % CaCO3 in excess and less than 200 ppm Na in impurities.Since these dolomites are closely associated with one another and possess distinct geochemical and mineralogical characteristics the preservation or quasi-preservation of primary compositions had to be assumed. A comparison of the supratidal dolomites with analogous recent dolomites indicates that the Middle Devonian ocean water had a lower18O content than modern oceans.
Strontium isotopic measurements were made on Late Proterozoic carbonates from West African Craton. Comparison of samples with acceptable trace element patterns with coeval data from southern Africa and with the published Australian results suggests that the 87 Sr/ 86 Sr ratio of the Late Proterozoic sea water evolved in the following manner about 0.7075 at 1000 ± 50 Ma, 0.7056 to 0.7074 at 900 ± 50 Ma, 0.7068 to 0.7091(0.7106) at 800 ± 50 Ma, 0.7074 to 0.7077 at 700 ± 50 Ma, and 0.7076 to 0.7089(0.7096) at 600 ± 50 Ma ago. The variations are comparable in magnitude and frequency to those described previously for the Phanerozoic. Strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of Sr into Late Proterozoic oceans was of comparable isotopic composition to its present day counterpart (~0.711). Consequently, the non-radiogenic 87 Sr/ 86 Sr value at ~900 ± 50 Ma ago signifies a large flux of "mantle" strontium into the ocean at this time. Because the present time resolution is only about 75 ± 25Ma, additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed Late Proterozoic sea water curve.
Two basic geochronologic systems are employed to expand the contributions of regional stratigraphy to total Earth history. The first developed was that based on the fossil content of rocks, which records the irreversible trend of biologic evolution; this system, biochronology, has been and will remain the prime basis for correlations and subdivisions of the Phanerozoic. The second system, developed somewhat later, is based on decay of radioactive elements contained in rocks; this system, radiochronology, has particular application to the (generally) unfossiliferous Precambrian and in the future, as at present, will doubtless be the principal means of interregional correlation and time subdivision. Only for the later Precambrian is a biochronologic framework likely to be erected, and it is important that this should be done in harmony with a parallel effort in radiometric geochronology.
Giant clams (Tridacna gigas) associated with the raised coral reefs in New Guinea yield a d13C record spanning the last 105yr which indicates a conspicuous 13C depletion during the interglacials and a pronounced 13C enrichment during the late ice age interstadials. This implies a coupling between climatic changes and the carbon cycle. The synchronous fertility changes in distinct compartments of the marine biota probably reflect a widespread feature of the ice age ocean and are attributed to contemporary variations in the nutrient chemistry of the ocean.-from Author
Clustering of sedimentary phosphate deposits around the
Precambrian-Cambrian boundary is related to a global phosphogenic event.
This event probably resulted from a complex interplay of factors and had
a profound effect on the biota, with high concentrations of phosphorus
in the photic zone producing an increase in the total shallow marine
biomass, faunal diversification, the precipitation of skeletal
structures and explosive evolution.