Chemostratigraphy

The data presented here comprise Ryazanian–Valanginian carbon isotope ratios analyzed from fossil wood and belemnites from the shallow marine Boyarka River succession in Siberia. Additional belemnite carbon isotope ratios from the Izhma River succession (also Ryazanian–Valanginian) in Russia are also presented. The wood-derived and belemnite-derived isotope ratios are considered to primarily reflct changes in the terrestrial and marine carbon isotope reservoirs respectively. The δ13Ccarb and δ13Cwood records reveal a distinct mid-Valanginian positive carbon isotope excursion, with the initiation occurring near the Boreal Russian michalskii–polyptychus zone boundary, which is broadly time-equivalent Tethyan campylotoxus–verrucosum boundary. The Ryazanian–Valanginian δ13Ccarb values fluctuate between c. –1 and +1.5‰ but reach a maximum of +4.1‰ in the Late Valanginian, whilst the δ13Cwood values fluctuate between c.–27 and –23.5‰ and reach a Late Valanginian maximum of –21.2‰. The excursion maximum in the Boreal Russian bidichotomus zones corresponds with the peak of the Tethyan marine carbonate excursion in the verrucosum–eperegrinus zones, the peak of a marine carbonate excursion recorded in the Argentinean atherstoni Zone and also with the peak of a terrestrial organic carbon isotope excursion in the Crimean trinodosum–ecallidiscus ammonite zones. The synchroneity of the positive carbon isotope event between the marine and terrestrial records and between the northern and southern hemispheres and Tethys, clearly indicates a strong coupling of the ocean-atmosphere system at this time and also confirms that this was a global event, which would have affected the total exchangeable carbon reservoir.

The major states, in which Earth's climate operates, i.e., icehouse, greenhouse and hothouse, are epochs of tens of
millions of years. These states set long-termboundary conditions that need to be considered for climate and sea level interpretations. This paper summarizes the conceptual models for hydrological cycling derived from the characteristics of these three climate states. While glacio-eustatic forcing of sea-level changes under icehouse climate conditions is fairly well understood, the drivers of eustatic sea-level fluctuations under greenhouse conditions remain enigmatic. This lack of understanding may be related to incoherencies in the current ideas about the impact of accelerated hydrological cycling on sea level under greenhouse climate conditions.
As an example for a greenhouse climate, we review evidences that link proxies for climate and sea level for the intensely studied, but controversially discussed, mid-Cretaceous  sea-level history. Based on sequence stratigraphy and a recently published high-precision timescale, we demonstrate that the late Middle Turonian Pewsey δ13C isotope maximum represents a major transgression, not a regression as previously stated, which conflicts with the interpretation of a co-occurring δ18O maximum to reflect a short glacial episode. This contradiction can be solved by the concept, presented here, that dominance of aquifer-eustasy characterized sea-level forcing during the Turonian greenhouse climate, despite a possible, though contentious, sporadic presence of minor ice sheets. The effects of temperature and ice volume both lead to a pronounced δ18Ocarb maximum during glacio-eustatic regressions. In contrast, the opposing effects of temperature and groundwater volume on oxygen-isotope fractionation lead to a δ18Ocarb maximum during aquifer-eustatic transgressions. We suggest that, throughout Earth history, both aquifer-eustatic and glacio-eustatic forcing formed a combined sea-level response, with dominance of aquifer-eustasy being typical for the greenhouse climate mode. During the icehouse mode, aquifer-eustasy apparently remains active as a background process, but is outpaced by the glacio-eustatic effect.

ABSTRACT In this chapter we report results of some of the latest cutting-edge research on traditional (C, O, S, N, Sr) isotope stratigraphy and some newer isotope systems (e.g. B, Ca, Mo, Fe, Cr, N), and elemental or elemental ratio (e.g. V, Mo, Hg, Rb/K, V/Cr, Zr/Ti, Mg/Ca, Mn/Sr, I/Ca) stratigraphy. The latter, in parallel with isotope stratigraphy, could be used as potential tracer for glacial events, buildup of volcanic gases (e.g. CO2) during glaciations, role of volcanism in mass extinction, salinity variation, redox state of the ocean and atmosphere and provenance, among other applications. Moreover, this chapter reviews studies that are broad in both space and time, highlighting chemical events from the Archean to the Cenozoic. Special attention is given to Paleoproterozoic and Neoproterozoic isotope chemostratigraphy and problems including the reliability of the use of carbon isotopes for Neoproterozoic stratigraphic correlation. Some attention is also devoted to the early Paleozoic, K–T boundary (KTB) and Paleogene climatic and biogeochemical changes in the light of isotope chemostratigraphic studies.

Elemental and isotope chemostratigraphies are used as tracers for glacial events, buildup of volcanic gases during glaciations (e.g., CO2), role of volcanism in mass extinction, salinity variation, redox state of the ocean and atmosphere, and provenance, among other applications. The use of isotope systems (C, O, S, N, Sr, Nd, Os), nontraditional stable isotope systems (e.g., Ca, Mg, B, Mo, Fe, Cr, Li), and elemental composition or elemental ratio (e.g., V, Ir, Mo, P, Ni, Cu, Hg, Rb/K, V/Cr, Zr/Ti, Li/Ca, B/Ca, Mg/Ca, I/Ca, Sr/Ca, Mn/Sr, Mo/Al, U/Mo,
Th/U) in chemostratigraphy, especially across major chronological boundaries, are reviewed in this chapter.
Furthermore, it is discussed what validates chemostratigraphy as a formal stratigraphic method.

Climate variability is driven by a complex interplay of global-scale processes and our understanding of them depends on sufficient temporal resolution of the geologic records and their precise inter-regional correlation, which in most cases cannot be btained with biostratigraphic methods alone. Chemostratigraphic correlation based on bulk sediment carbon isotopes is increasingly used to facilitate high-resolution correlation over large distances, but complications arise from a multitude of possible influences from local differences in biological, diagenetic and physico-chemical factors on individual δ13C records that can mask the global signal. To better assess the global versus local contribution in a δ13C record it is necessary to compare numerous isotopic records on a global scale. As a contribution to this objective, this paper reviews bulk sediment δ13Ccarb records from the Late Cretaceous in order to identify differences and similarities in secular δ13C trends that help establish a global reference δ13C record for this period. The study presents a global-scale comparison of twenty δ13C records from sections representing various palaeo-latitudes in both hemispheres and different oceanic settings from the Boreal, Tethys, Western Interior, Indian Ocean and Pacific Ocean, and with various diagenetic overprinting. The isotopic patterns are correlated based on independent dating with biostratigraphic and paleomagnetic data and reveal good agreement of the major isotope events despite offsets in absolute δ13C values and variation in amplitude between the sites. These differences reflect the varying local influences e.g. from depositional settings, bottom water age and diagenetic history, whereas the concordant patterns in δ13C shifts might represent δ13C fluctuations in the global seawater dissolved inorganic carbon. The latter is modulated by variations in organic matter burial relative to re-mineralization, in the global-scale formation of authigenic carbonate, and in partitioning of carbon between organic carbon and carbonate sinks. These variations are mainly controlled by changes in climate and eustasy. Additionally, some globally synchronous shifts in the bulk δ13Ccarb records could result from parallel variation in the contribution of authigenic carbonate to the sediment. Formation of these cements through biologically mediated early diagenetic processes is related to availability of oxygen and organic material and, thus, can be globally synchronized by fluctuations in eustasy, atmospheric and oceanic oxygen levels or in large-scale oceanic circulation. Because the influence of early diagenetic cements on the bulk δ13Ccarb signal can, but need not be synchronized, chemostratigraphy should not be used as a stand-alone method for trans-continental correlation, and especially minor isotopic shifts have to be interpreted with utmost care. Nevertheless, the observed consistency of the δ13C correlations confirms global scale applicability of bulk sediment δ13C chemostratigraphy for the Late Cretaceous, including sediments that underwent lithification and burial diagenesis such as the sediments from the Himalayan and Alpine sections. Limitations arise from increased uncertainties (1) in sediments with very low carbonate content, (2) from larger δ13C variability in sediments from very shallow marine environments, (3) from unrecognized hiatuses or strong changes in sedimentation rates, and (4) in sections with short stratigraphic coverage or with few biostratigraphic marker horizons.

The combination of chemostratigraphy with biostratigraphy and magnetostratigraphy substantially increases the precision and temporal resolution of inter-regional correlations and helps overcome problems that arise from differences in biostratigraphic schemes, facies or provincialism of key fossils. By using an iterative approach to stepwise increase precision of the correlations, isochroneity of first and last occurrences of marker species versus chemostratigraphy is tested, which helps to improve biostratigraphic zonations, to assess zonal boundary ages and to identify useful criteria for defining Late Cretaceous stage boundaries, many of which are still not formally defined. The presented correlations indicate a consistent position for most planktic foraminifer zonal boundaries relative to corresponding isotope shifts during the mid-Cretaceous sea-level high, whereas diachroneity appears to be more pronounced during the Late Campanian and Maastrichtian global sea-level fall. A similar pattern is observed for trans-continental consistency in the δ13C shifts. Graphic correlation of isotopic shifts, magnetostratigraphic and biostratigraphic events among the compared sections is used to detect hiatuses or relative changes sediment accumulation rates and visualizes consistency or offsets of individual biostratigraphic markers relative to chemo- and magnetostratigraphy. Finally, an attempt of a global average δ13C stack is presented for the Turonian through Maastrichtian.

The Archean‐Proterozoic boundary is placed at 2500 Ma and marks possibly the most dramatic change in Earth’s history. The Great Oxidation Event (GOE) took place between 2.45 and 2.32 Ga as part of an Archean‐Proterozoic transition rather than sharp boundary and postdates the currently established boundary. Apart from geological proxies of atmospheric oxygenation, such as banded iron formation (BIF) abundance and paleosol mineralogy, isotope chemostratigraphy provides the most powerful tool for studying the GOE and establishing the boundary. Sulfur isotopes, and especially mass‐independent fractionation of sulfur, are the best studied proxy, indicating the formation of an ozone layer at ca. 2.33 Ga. Cr and Mo isotopes are more sensitive indicators of surface environment oxygenation, reflecting the redox state of surface seawater. We discuss three different proposals for the placement of the Archean‐Proterozoic boundary and a corresponding Global Stratotype Section and Point: (i) to keep it at 2500 Ma, aided by prominent BIF units, Mo abundance, and Mo isotopes; (ii) to place it at the base of the second Huronian glaciation (ca. 2.35–2.40 Ga), thought to represent a “snowball” event; and (iii) to use the termination of the mass‐independent fractionation of sulfur and the
increase in the δ34S amplitude of sulfides as the main criteria.

Petrophysical (gamma-ray spectrometry, magnetic susceptibility) and geochemical (X-ray fluorescence spectrometry and inorganic carbon isotope geochemistry) methods are used for stratigraphic correlations and palaeoenvironmental interpretations of the Devonian-Carboniferous boundary interval in the northern part of the Rheinisches Schiefergebirge, Germany. Sections at Oese, Oberrödinghausen and Drewer were studied in the stratigraphic range from the Famennian Upper expansa Zone to the Tournaisian Lower crenulata Zone. The Famennian Wocklum Limestone reveals distinctive cyclicity in the limestone and shale arrangement, whose primary origin has been supported by correlation with K/Al ratio changes. Lower-order K/Al cyclicity follow bundles of alternating layers. On the other hand, Zr/Al cyclicity is rather independent on facies and K/Al and duration of Zr/Al cycles was estimated to be 370 ka, which is close to 405 ka eccentricity. Correlations of the curve patterns of Computed Gamma Ray, U/ThGRS, bulk magnetic susceptibility, K/Al, Zr/Al, S, and Mn proxies have been employed for regional correlation. Our dataset confirms previous palaeoenvironmental interpretations of the Hangenberg Events and additionally provides information on gradual decrease in a bottom oxygenation just prior the events. Peaks in redox and palaeoproductivity proxies (U/Th, Cu+Ni+Zn+Pb) occur in the Hangenberg Black Shale in all studied sections. Although sedimentation of the Hangenberg Black Shale took place during the deepening step, the enhanced palaeoproductivity and hypoxia to anoxia were the main cause of its deposition. An interregional correlation of the Devonian-Carboniferous intervals from the northern Rhenish Massif, Moravian Karst and Carnic Alps was carried out and underlines the potential for further chronostratigraphic definitions.

The Averno 2 eruption (3,700 ± 50 a B.P.) was an explosive low-magnitude event characterized by magmatic and phreatomagmatic explosions, generating mainly fall and surge beds, respectively. It occurred in the Western sector of the Campi Flegrei caldera (Campanian Region, South Italy) at the intersection of two active fault systems, oriented NE and NW. The morphologically complex crater area, largely filled by the Averno lake, resulted from vent activation and migration along the NE-trending fault system. The eruption generated a complex sequence of pyroclastic deposits, including pumice fall deposits in the lower portion, and prevailing surge beds in the intermediate-upper portion. The pyroclastic sequence has been studied through stratigraphical, morphostructural and petrological investigations, and subdivided into three members named A through C. Member A was emplaced during the first phase of the eruption mainly by magmatic explosions which generated columns reaching a maximum height of 10 km. During this phase the eruption reached its climax with a mass discharge rate of 3.2 106 kg/s. Intense fracturing and fault activation favored entry of a significant amount of water into the system, which produced explosions driven by variably efficient water-magma interaction. These explosions generated wet to dry surge deposits that emplaced Member B and C, respectively. Isopachs and isopleths maps, as well as areal distribution of ballistic fragments and facies variation of surge deposits allow definition of four vents that opened along a NE oriented, 2 km long fissure. The total volume of magma extruded during the eruption has been estimated at about 0.07 km3 (DRE). The erupted products range in composition from initial, weakly peralkaline alkali-trachyte, to last-emplaced alkali-trachyte. Isotopic data and modeling suggest that mixing occurred during the Averno 2 eruption between a more evolved, less radiogenic stored magma, and a less evolved, more radiogenic magma that entered the shallow reservoir to trigger the eruption. The early phases of the eruption, during which the vent migrated from SW to the center of the present lake, were fed by the more evolved, uppermost magma, while the following phases extruded the less evolved, lowermost magma. Integration of the geological and petrological results suggests that the Averno 2 complex eruption was fed from a dyke-shaped shallow reservoir intruded into the NE-SW fault system bordering to the west the La Starza resurgent block, within the caldera floor.

New stable carbon and oxygen isotope data from an Upper Cretaceous section in Tibet are presented, and compared to carbon isotope records from England, Italy, and Germany. Together with a stratigraphic reinterpretation of published carbon isotope data from a nearby section in Tibet, our data can surprisingly well be correlated with the European sections. This indicates that, similar to the distinct positive carbon isotope excursion at the Cenomanian-Turonian boundary, also the broad positive carbon isotope shift in the middle-late Coniacian and early Santonian reflects a major perturbation of the carbon cycle on a global scale, even though organic-rich sediments related to the OAE3 appear to be mainly restricted to the Atlantic Ocean and adjacent basins. The data further show that, apart from the broad Coniacian- Santonian carbon isotope excursion, also isotopic shifts on a smaller scale in the Turonian and Coniacian, such as the Round Down, Pewsey, and Hitchwood Events, can be correlated over both hemispheres. This demonstrates that the development of global oceanic anoxic conditions and associated burial of large amounts of organic carbon do not constitute a prerequisite for globally reflected carbon isotopic shifts. The data from Tibet support the concept of a relation between main carbon isotope excursions and major sea-level variations. Cyclic fluctuations of geochemical and lithological parameters are likely to be orbitally driven. These cycles appear to be preferably reflected in the sediments during periods of lower or variable sea-level, whereas the ocean-atmosphere system seems to have operated in a different mode during long phases of high, stable sea-level, as during the Coniacian-Santonian OAE3.

The Kačák Event in the Middle Devonian (Eifelian-Givetian (E-G) boundary) is a period of apparent global anoxia coincident with widespread deposition of black shale in hemipelagic, pelagic, and some neritic facies. Conodont biostratigraphy in the North American Appalachian Basin has proven to be problematic in precisely demarcating the E-G boundary. In this study, we show that the E-G boundary may be defined more accurately through isotope stratigraphy (δ13C) in conjunction with a conodont faunal change across this boundary, identified as the Kačák-otomari Event. The Canadian Hamilton Group outcropping in Hungry Hollow, Ontario, is a 22 m sedimentary succession spanning the Middle Devonian. Conodont biostratigraphy for this section makes it difficult to define the E-G boundary, but the otomari Event can be detected. High-resolution isotopic analysis of bulk sedimentary carbonate and organic matter for this succession records a significant negative δ13C excursion (δ13Ccarb = up to 2‰; δ13Corg = ~3.0‰) that is synchronous with total organic carbon (TOC) values up to 12.5%. We identify this negative δ13C excursion as a result of marine anoxia associated with the Kačák-otomari Event and suggest that the excursion is a global event driven by a source of isotopically light carbon, followed by a productivity event, similar to Mesozoic oceanic anoxic events. Such similarities between Devonian and Mesozoic oceanic anoxic events may become more evident with increased high-resolution isotopic and geochemical investigations of Devonian successions.

The Permo-Carboniferous Unayzah Group is generally lacking in high resolution biostratigraphic control and fails to produce a stratigraphic correlation using lithostratigraphy, due to its large similarities with sandstones encountered in both the Devonian and the Silurian sections. This study strives to propose correlation schemes for the Permo-Carboniferous Unayzah Group in central Saudi Arabia and to define the Unayzah Group and basal Khuff clastics (BKC) boundaries based on chemostratigraphic analysis. were subjected to geochemical analysis. A correlation scheme was developed based on specific changes in elemental ratios dealing with glaciogenic, fluvial, eolian and coarse-grained alluvial sediments. These changes occurred in the following key element ratios: zirconium/niobium (Zr/Nb), niobium/ura-nium (Nb/U), (rubidium + cesium)/lanthanum ((Rb+Cs)/La), aluminum/(calcium + magnesium + potassium + sodium) (Al/ (Ca+Mg+K+Na)), (zirconium*hafnium)/(niobium*tantalum) ((Zr*Hf)/(Nb*Ta)), (zirconium*hafnium)/niobium ((Zr*Hf)/ Nb) and zirconium/(niobium*tantalum) (Zr/(Nb*Ta)). This study shows that the Ghazal, Jawb, Wudayhi and Tinat members are well characterized chemostratigraphically, being associated with distinct chemozones, and so facilitate correlation in the subsurface.

Current chemostratigraphical studies of the Jurassic System primarily involve the use of one sedimentary component (marine organic carbon), one divalent transition metal substituted in carbonate (manganese), and two isotopic tracers: strontium-isotope ratios (87Sr/86Sr) and carbon-isotope ratios (δ13Ccarb and δ13Corg) in carbonate and in organic matter. Other parameters such as Mg/Ca and Sr/Ca ratios in calcite, oxygen-isotope ratios (ä18O) in carbonate, sulphur-isotope ratios (δ34S) in carbonate-hosted sulphate, nitrogen- isotope ratios (δ15Norg) in organic matter, osmium-isotope ratios (187Os/188Os) in black shales and neodymium-isotope ratios (143Nd/144Nd) in various mineral phases are also useful but at present give poor resolution because the database is incomplete or compromised by various factors. Stratigraphical patterns in total organic carbon (TOC) can be of either local or regional significance, depending on the lateral extent of the former nutrient-rich and productive water mass. Divalent manganese follows a similar pattern, being concentrated, most probably as a very early diagenetic phase, only in oxygen-depleted waters that typically underlie zones of elevated organic productivity. Shifts in Mg/Ca and Sr/Ca ratios on the time scale of ammonite subzones seem largely to reflect temperature changes. Strontium-isotope ratios from pristine skeletal calcite provide a global signal; δ13C values from carbonates with minimal diagenetic overprint potentially do the same, although small spatial differences in palaeo-water-mass composition may have been locally significant. Oxygen-isotope determinations on carbonate rocks and fossils generally yield values that are too scattered to be stratigraphically useful, because they reflect palaeotemperature, the evaporation–precipitation balance in sea water and the impact of any diagenesis involving an aqueous phase. Nitrogen-isotope ratios in organic matter reflect the chemistry of ancient water masses as affected by nitrate utilization and denitrification, and the stratigraphical pattern of this parameter is more likely to correlate only on a regional basis. Neodymium- isotope ratios in sea water are also water mass dependent and greatly affected by regional sources and oceanic current systems. Preliminary data on sulphur-isotope ratios in carbonates and osmium-isotope ratios in organic-rich shales, both potentially offering global correlation, indicate that these tracers may be valuable, although the records at present are not sufficiently well established to allow high-resolution regional correlation. In all cases, biostratigraphically well-dated reference sections, against which the relevant geochemical data have been calibrated, are required in the first instance. To date, studies on the stratigraphical distribution of organic carbon have been principally carried out in both northern (Boreal) and southern (Tethyan) Europe; carbon- isotope stratigraphy has been undertaken primarily, but not exclusively, on bulk pelagic sediments from the Alpine–Mediterranean or Tethyan domain; and strontium-isotope stratigraphy has been undertaken largely on calcitic skeletal material (belemnites and oysters) from northern and southern Europe. In many sections, including those containing ammonites, multi-parameter chemostratigraphy can give resolution that exceeds that attainable by classic biostratigraphical means. Strontium-isotope ratios in skeletal calcite are a particularly powerful tool for illustrating changes in sedimentary rate and revealing gaps in the stratigraphical record.

The mass extinction of the olenellid trilobites occurred around the Cambrian Series 2–Series 3 boundary. Like many other crises, it coincided with a negative carbon isotope excursion but the associated palaeoenvironmental changes remain unclear. To investigate the causal mechanism for this event, we report facies changes, pyrite framboid petrography and carbon isotope values from Cambrian Series 2–Series 3 (traditionally Early–Middle Cambrian) boundary strata of the Carrara Formation (Death Valley region, California) and Pioche Formation (Nevada). These data reveal regionally changing water depths from high-energy, nearshore facies (oolitic grainstone) to more offshore silty marl and finer-grained carbonate mudstone. In the Carrara Formation, the series boundary occurs within a deepening succession, transitioning from high-energy, nearshore facies (oolitic grainstone and oncolitic packstone) to offshore marl, the latter of which contains pyrite framboid populations indicative of low-oxygen (dysoxic) depositional conditions. Intermittent dysoxia persisted below sub-wave base settings throughout the early and middle Cambrian, but did not intensify at the time of extinction, arguing against anoxia as a primary cause in the olenellid trilobite extinction. Within both field areas, the extinction interval coincided with a minimum in δ 13 C carb values, which we interpret as the regional manifestation of the Redlichiid-Olenellid Extinction Carbon isotope Excursion (ROECE). The Series 2–Series 3 boundary is reported to closely coincide with a large-amplitude sea-level fall that produced the Sauk I/II sequence boundary, but the placement of the Series 2–Series 3 boundary within a transgressive interval of the Carrara Formation shows that this is not the case.The main sequence boundary in the succession occurs much lower in the succession (at the top of the Zabriskie Quartzite) and therefore precedes the extinction of the olenellids and ROECE.

We present new, detailed carbon-isotope records for bulk carbonate, total organic carbon (TOC) and phytane from three key sections spanning the Cenomanian-Turonian boundary interval (Eastbourne, England; Gubbio, Italy; Tarfaya, Morocco), with the purpose of establishing a common chemostratigraphic framework for Oceanic Anoxic Event (OAE) 2. Isotope curves from all localities are characterized by a positive carbon-isotope excursion of c. 4‰

Here we present initial 187Os/188Os (Osi) values integrated with δ13Corg for the first Paleozoic section — the Ordovician/Silurian boundary GSSP at Dob's Linn, Scotland. Our 187Os/188Os data tracks major changes in climate that occurred during the Late Ordovician (Hirnantian glaciation), which coincides with the second largest known mass extinction. During the complanatus and early anceps Biozones Osi values increase from 0.28–1.08. This provides evidence for a period of increased silicate weathering of radiogenic continental crust, likely from the Caledonian Orogen. This increase in weathering was likely the driving mechanism for the drawdown in atmospheric CO2 and global cooling that resulted in the onset of the Hirnantian Glaciation. A decrease to less radiogenic Osi occurs at the base Hirnantian extraordinarius Biozone and coincides with the trend to more positive δ13Corg values that mark the onset of the Hirnantian Glaciation. The trend in Osi during this interval is ascribed to Hirnantian ice cover and reduced chemical weathering rates cutting the supply of radiogenic material to the Iapetus Ocean. The reduction in silicate weathering enabled atmospheric CO2 to return back to greenhouse levels, causing rapid deglaciation during the mid persculptus Biozone. This period is marked by an abrupt increase in Osi values from 0.6 to 1.08 over 19 cm of stratigraphy and coincides with the deglacial limb of the δ13Corg profile. We interpret the Osi data to reflect the leaching of exposed radiogenic 187Os/188Os bearing glacial deposits and increased weathering of radiogenic 187Os/188Os silicate terrane during the deglaciation. Previous workers have identified the Hirnantian glaciation primarily through δ13C stratigraphy. However, our Os isotope data indicate that an initial mechanism (i.e. increased silicate weathering) was the driving mechanism behind the Hirnantian Glaciation and subsequent mass extinction. Thus, by coupling Osi and δ13Corg proxies we provide the most direct evidence for the initiation and cessation of the Hirnantian glaciation. Furthermore, this study demonstrates the first use of 187Os/188Os chemostratigraphy for the Paleozoic as a proxy for reconstructing the Earth's climate system, particularly palaeoceanography.

Oceanic Anoxic Event 2 (OAE2), spanning the Cenomanian‐Turonian boundary (CTB), represents one of the largest perturbations in the global carbon cycle in the last 100 Myr. The δ13Ccarb, δ13Corg, and δ18O chemostratigraphy of a black shale–bearing CTB succession in the Vocontian Basin of France is described and correlated at high resolution to the European CTB reference section at Eastbourne, England, and to successions in Germany, the equatorial and midlatitude proto‐North Atlantic, and the U.S. Western Interior Seaway (WIS). Δ13C (offset between δ13Ccarb and δ13Corg) is shown to be a good pCO2 proxy that is consistent with pCO2 records obtained using biomarker δ13C data from Atlantic black shales and leaf stomata data from WIS sections. Boreal chalk δ18O records show sea surface temperature (SST) changes that closely follow the Δ13C pCO2 proxy and confirm TEX86 results from deep ocean sites. Rising pCO2 and SST during the Late Cenomanian is attributed to volcanic degassing; pCO2 and SST maxima occurred at the onset of black shale deposition, followed by falling pCO2 and cooling due to carbon sequestration by marine organic productivity and preservation, and increased silicate weathering. A marked pCO2 minimum (∼25% fall) occurred with a SST minimum (Plenus Cold Event) showing >4°C of cooling in ∼40 kyr. Renewed increases in pCO2, SST, and δ13C during latest Cenomanian black shale deposition suggest that a continuing volcanogenic CO2 flux overrode further drawdown effects. Maximum pCO2 and SST followed the end of OAE2, associated with a falling nutrient supply during the Early Turonian eustatic highstand.

A study of the SE extreme of the Nico Pérez Terrane is presented, which allowed the identification of a number of lithotectonic blocks bounded by reverse and transcurrent faults. They are grouped together in the Carapé Tectonic Slab (CTS) , which is a displaced block of the Nico Perez Terrane. The CTS is made up of 5 different units: ( a) Marco de los Reyes Formation, composed of pure limestone, BIF , mica schists, amphibolites and gneisses, of Neoproterozoic age;  (b) Mataojo Formation, comprising dolomite marble, metasandstone, mica schists, amphibolites and gneisses, with a depositional age between 1.8 and 1.5 Ga, (c) Eden Formation, composed of para-amphibolites, paragneisses ,mica schists and rare pyroxenites of unknown age; (d) Carapé Granitic Complex, which comprises all intrusive granitoids, mainly in the north of the CTS; and (e) El Renegado Granite, representing a granite nappe overthrust onto the other units, with a U-Pb crystallization age of 1754±7. It is demonstrated on the basis of U-Pb ages of detrital zircons that the CTS is an integral part of the Nico Pérez Terrane. The CTS is the result of block  juxtaposition which can hardly be achieved with a single collisional event; the youngest tectonic event is representad by the tangential collision generating the Sierra Ballena Shear Zone (530 Ma) . The CTS is an important piece of the Nico Pérez Terrane for the paleogeographic reconstruction of the Río de la Plata Craton.

Increasing evidence shows that Mesoproterozoic rocks are widespread in the Río de la Plata Craton. Carbon and strontium isotope analyses were carried out for three different, carbonate-bearing successions in the southern Nico Pérez Terrane. The Parque UTE Group is erected, comprising (from base to top) the mainly volcanogenic Canada Espinillo Formation, the dolomitic Mina Valencia Formation and the mixed carbonate-siliciclastic Cerro del Mástil Formation. A d13C curve was obtained for carbonates of the Parque UTE Group, which is characterized by a plateau at +1 to +1.6‰ V-PDB, bracketed between two negative excursions (−1.8‰ V-PDB at the base and −3.3‰ V-PDB at the top). These values are consistent with a Mesoproterozoic depositional age for the unit, as indicated by U-Pb ages of synsedimentary volcanics and gabbros of 1429 ± 21 and 1492 ± 4 Ma, respectively. The Mataojo Formation and the Marcos de los Reyes Formation were previously thought to be of similar age. The chemostratigraphic data, however, show that there is a hiatus of at least 700 Myr between these units, and that neither is correlative of the Parque UTE Group. Dolomitic marbles of the Mataojo Formation yielded a d13C curve that varies within a narrow range around 0‰ V-PDB (−0.6 to +0.4‰). This, along with the youngest U-Pb detrital zircon ages of 1802 Ma constrains deposition between 1.8 and 1.5 Ga (late Palaeoproterozoic-early Mesoproterozoic). In contrast with the latter unit, the Marco de los Reyes Formation yielded d13C values defining moderate to high-amplitude d13C excursions between +4.4‰ V-PDB and −3.2‰ V-PDB. Corresponding 87Sr/86Sr values range between 0.7070 and 0.7080. Both C and Sr chemostratigraphy point to a later Neoproterozoic, possibly Ediacaran age for the Marco de los Reyes Formation. It is here proposed that modest 13C seawater enrichment and low-amplitude d13C excursions, following a long period of isotopic invariance around 0‰ V-PDB, began already around 1.50-1.45 Ga. Thereafter, the amplitude of d13C excursions increased gradually until the late Neoproterozoic. One important conclusion emerging from the data presented in this paper is that Mesoproterozoic rocks are common in the Río de la Plata Craton, and probably fringe it at both its western and eastern side. This suggests a rather central position of the craton within Rodinia, as also suggested by other lines of evidence, such as detrital zircon ages and a widespread thermal overprint at 1.25 Ga.