Pronounced Negative Excursion Research Articles

Warming, acidification, and calcification feedback during the first hyperthermal of the Cenozoic—The Latest Danian Event

Abstract The Latest Danian Event (LDE; ca. 62.15 Ma) is a major double-spiked eccentricity-driven transient warming event and carbon cycle perturbation (hyperthermal) in the early Paleocene, which has received significantly less attention compared to the larger events of the late Paleocene–early Eocene. A better understanding of the nature of the LDE may broaden our understanding of hyperthermals more generally and improve our knowledge of Earth system responses to extreme climate states. We present planktic and benthic foraminiferal Mg/Ca and B/Ca records that shed new light on changes in South Atlantic temperature and carbonate chemistry during the LDE. Our planktic Mg/Ca record reveals a pulsed increase in sea-surface temperature of at least ~1.5 °C during the older carbon isotope excursion, and ~0.5 °C during the younger isotope excursion. We observe drops in planktic and benthic B/Ca, synchronous with pronounced negative excursions in benthic δ13C, which suggest a shift in the carbonate system toward more acidic, dissolved inorganic carbon–rich conditions, in both the surface and deep ocean. Conditions remained more acidic following the LDE, which we suggest may be linked to an enhanced ocean alkalinity sink due to changes in the makeup of planktic calcifiers, hinting at a novel feedback between calcifier ecology and ocean-atmosphere CO2.

Paleoenvironmental implications of Sr and Nd isotopes variability over the past 48 ka from the southern Sea of Japan

Millennial scale variations of terrigenous provenance in marine realm are closely related to regional environment and climate changes. Therefore, a wealth of information of past environment and climate can be constrained via fingerprinting sediment provenance. The Sea of Japan is a unique marginal sea in the North Pacific due to its high sill and distinct thermohaline circulation. The modern hydrography in the Sea of Japan is mainly affected by the East Asian Monsoon and Tsushima Warm Current, one branch of the Kuroshio Current. The Sea of Japan communicates with neighboring seas through four shallow and narrow straits, indicating great effects of global eustatic sea level change on its environment over glacial-interglacial cycles. Here we examine the terrigenous provenance in fine-grained fraction (<63 μm) of core KCES1, located near one end of the Tsushima Strait of the Sea of Japan over the last 48 ka, using radiogenic isotopes of strontium (Sr) and neodymium (Nd). Our data suggest that the terrigenous provenance in core KCES1 was mainly derived from the Yangtze River after 7 ka and a mixture of Yangtze and Yellow Rivers during the last glacial and deglacial periods. Notably, pronounced negative excursions of εNd values at HS1 were attributed to minor additions of unradiogenic Nd contribution from China-Korea cratonic hinterland. A binary mixing model further reveals that >85% terrigenous material is derived from the Yangtze and Yellow Rivers over the last 48 ka. Moreover, abrupt variations in sediment provenance occurred at ~18 ka and ~ 7 ka, which coincide with variations in oceanic surface circulation and deep ventilation recorded in the Sea of Japan. We suggest that paleo-Tsushima Warm Current invaded into the Sea of Japan with reopening of the Tsushima Strait at HS1 and the Tsushima Warm Current substantially entered the Sea of Japan after 7 ka due to intensified Kuroshio Current and rising eustatic sea level. The inflow of Tsushima Warm Current gives rise to a range of changes in surface hydrography, deep ventilation, ecological communities and productivity and sediment texture. The combination of fluxes of paleo-rivers and the intensity of Kuroshio Current, which are closely tied to the eustatic sea level and the East Asian Monsoon, plays a key role in controlling the variations in sediment provenance in the Ulleung Basin. Our study provides unique insight into the tight coupling between changes in sediment provenance and oceanic environment over the last 48 ka in the Sea of Japan.

Climate and structure of the 8.2 ka event reconstructed from three speleothems from Germany

The most pronounced climate anomaly of the Holocene was the 8.2 ka cooling event. We present new 230Th/U-ages as well as high-resolution stable isotope and trace element data from three stalagmitesfrom two different cave systems in Germany, which provide important information about the structure and climate variability of the 8.2 ka event in central Europe. In all three speleothems, the 8.2 ka event is clearly recorded as a pronounced negative excursion of the {\delta}18O values and can be divided into a 'whole event' and a 'central event'. All stalagmites show a similar structure of the event with a short negative excursion prior to the 'central event', which marks the beginning of the 'whole event'. The timing and duration of the 8.2.ka event are different for the individual records, which may, however, be related to dating uncertainties. Whereas stalagmite Bu4 from Bunker Cave also shows a negative anomaly in the {\delta}13C values and Mg content during the event, the two speleothems from the Herbstlabyrinth cave system do not show distinct peaks in the other proxies. This may suggest that the speleothem {\delta}18O values recorded in the three stalagmites do not primarily reflect climate change at the cave site, but rather large-scale changes in the North Atlantic. This is supported by comparison with climate modelling data, which suggest that the negative peak in the speleothem {\delta}18O values is mainly due to lower {\delta}18O values of precipitation above the cave and that temperature only played a minor role. Alternatively, the other proxies may not be as sensitive as {\delta}18O values to record this centennial-scale cooling event. This may particularly be the case for speleothem {\delta}13C values as suggested by comparison with a climate modelling study simulating vegetation changes in Europe during the 8.2 ka event. ...

Formation of microbial organic carbonates during the Late Jurassic from the Northern Tethys (Amu Darya Basin, Uzbekistan): Implications for Jurassic anoxic events

Abstract The Late Jurassic was a period of major global carbon cycle perturbations with episodes of anoxia leading to regional accumulation of organic matter in sediments worldwide. The Tubiegatan section (SW Gissar Mountains, Uzbekistan) located in the Northern Tethys, shows atypical organic-rich limestone and marl deposits (up to 6% of total organic carbon) marked by pronounced negative excursions of δ13Ccarb (amplitude of ca. 12‰) and δ13Corg (amplitude of ca. 4‰) recorded during the Middle Oxfordian (Transversarium Zone). A transdisciplinary approach including sedimentology, palynofacies characterization, mineralogy, organic and inorganic geochemistry was carried out to elucidate the origin of these organic-rich deposits. Highest TOC are measured in nodular limestones, and lowest δ13Ccarb values in thinly laminated facies consisting in alternances of infra-millimeter-thick organic and carbonate laminae. In the latter, the presence of organic-carbonate peloids and of possible remnants of exopolymeric substances associated with clay indicate that these structures are probably mineralized laminated benthic microbial mats (i.e., stromatolites). Rock-Eval pyrolysis coupled to palynofacies analyses point to a dominant altered marine organic matter of probable algal/microbial origin, with subordinate continental phytoclasts inputs in the upper part of the organic-rich interval. Trace elements (U/Th, V/Cr and Mo/Al ratios) indicate two anoxic episodes coinciding with the highest TOC, punctuated by dysoxic periods. Such O2-depleted conditions have allowed the preservation and probably the development of anaerobic microbial communities in the microbial mats. In these latter, sulfate reduction probably had a significant contribution to the production of carbonates, which would explain the precipitation of pyrite and the relatively low δ13Ccarb values. The progressive decrease then disappearance of kaolinite from the base of the organic-rich interval, is interpreted as a progressive aridification of the Amu Darya Basin during the Transversarium Zone, culminating with the progradation of a large-scale gypsum sabkha overlying the organic deposits. Overall, the organic-rich deposits could record the onset of the disconnection of the Amu Darya Basin from the open sea to the south, induced by compression and subsequent uplifts in the Afghan and Central Iranian blocks. The elevated evaporation, coupled with the presence of hydrological barriers (such as coral reefs) could have led to the formation of local to regional anoxic conditions in the Amu Darya Basin. Similar microbial organic accumulations are recently known throughout the Tethys (e.g., Arabian Plate, Western Europe) and from other oceans (e.g., Central Atlantic, Pacific) during the Late Jurassic, suggesting common controlling factors. The increase of organic matter storage worldwide coupled with potential methane release could have in turn induced major perturbations of the carbon cycle during the Oxfordian-Kimmeridgian interval. The relatively shallow anoxia model proposed in this study contrasts with the well-known organic carbon-rich pelagic models proposed for the Jurassic anoxia (e.g., Toarcian, Kimmeridgian) and Cretaceous OAEs.

Multiple early Eocene carbon isotope excursions associated with environmental changes in the Dieppe-Hampshire Basin (NW Europe)

The early Eocene experienced a series of short-lived global warming events, known as hyperthermals, associated with negative carbon isotope excursions (CIE). The Paleocene-Eocene Thermal Maximum (PETM or ETM-1) and Eocene Thermal Maximum 2 (ETM-2) are the two main events of this Epoch, both marked by massive sea-floor carbonate dissolution. Their timing, amplitude and impacts are rather well documented, but CIEs with lower amplitudes also associated with carbonate dissolution are still poorly studied (e.g. events E1 to H1), especially in the terrestrial realm where hiatus/disconformities and various sedimentary rates in a single succession may complicate the assignation to global isotopic events. Here we present a new high-resolution multi-proxy study on the terrestrial, lagoonal and shallow marine late Paleocene-early Eocene succession from two sites of the Cap d’Ailly area in the Dieppe-Hampshire Basin (Normandy, France). Carbon isotope data (δ13C) on bulk organic matter and higher-plant derived n-alkanes, and K-Ar ages on authigenic glauconite were determined to provide a stratigraphic framework. Palynofacies, distribution and hydrogen isotope values (δ2H) of higher-plant derived n-alkanes allowed us to unravel paleoenvironmental and paleoclimatic changes. In coastal sediments of the Cap d’Ailly area, δ13C values revealed two main negative CIEs, from base to top CIE1 and CIE2, and 3 less pronounced negative excursions older than the NP11 nannofossil biozone. While the CIE1 is clearly linked with the PETM initiation, the CIE2 could either correspond to 1) a second excursion within the PETM interval caused by strong local environmental changes or 2) a global carbon isotopic event that occurred between the PETM and ETM-2. Paleoenvironmental data indicated that both main CIEs were associated with dramatic changes such as eutrophication, algal and/or dinoflagellate blooms along with paleohydrological variations and an increase in seasonality. They revealed that the intervals immediately below these CIEs are also marked by environmental and climatic changes. Thus, this study shows either 1) a PETM marked by at least two distinct intervals of strong environmental and climatic changes or 2) at least one “minor” CIE: E1, E2, F or G, was associated with strong environmental and climatic changes similar to those that occurred during the PETM.

Timing the termination of the Doushantuo negative carbon isotope excursion: evidence from U-Pb ages from the Dengying and Liuchapo formations, South China

The Doushantuo negative carbon isotope excursion (DOUNCE) is the largest known marine inorganic carbon isotope anomaly. The origin of this pronounced negative excursion is still an enigmatic issue that attracts geologists. Time constraints on the excursion are the critical information that would provide insight into its genesis. In previous decades, the timing of its termination has been constrained by the widely cited zircon U-Pb age of 550.5 ± 0.8 Ma for the tuff at the top of the Miaohe Member at the Jiuqunao section in the Yangtze Gorges area, South China. However, results of recent studies indicate that the reliability of this time constraint needs to be re-evaluated. Here, a geochronological study was carried out using two K-bentonites from Fanglong in South China. A K-bentonite in the lower Dengying Formation yielded a U-Pb age of 557 ± 3 Ma, while a K-bentonite in the basal Liuchapo Formation yielded an age of 550 ± 3 Ma. Based on regional correlations between the Ediacaran successions in South China, the age (557 ± 3 Ma) for the K-bentonite in the lower Dengying Formation may serve as a second critical timing constraint for the ending of the DOUNCE. Combined with available estimates of the DOUNCE duration, our new data indicate that the DOUNCE has a maximum onset age ∼570 Ma.

Lower Aptian ammonite and carbon isotope stratigraphy in the eastern Prebetic Domain (Betic Cordillera, southeastern Spain)

Major global palaeobiologic and palaeoenvironmental changes occurred during the Early Aptian. Precise dating and timing of the different events is crucial to determine possible cause-effect relationships between them. In this regard, the combination of biostratigraphic and chemostratigraphic data can provide a very useful tool for time control. So far attempts to correlate the Lower Aptian carbon isotope record and the ammonite zonation yielded contradictory conclusions. In this paper, we present the results of an integrated analysis of the ammonite stratigraphic distribution and highresolution carbon isotope profiles from Lower Aptian sections of the eastern Prebetic Domain (Betic Cordillera, southeastern Spain). We recognized, in ascending order, the Deshayesites oglanlensis, Deshayesites forbesi, Deshayesites deshayesi, and Dufrenoyia furcata Zones. This succession is the same as that recently identified in the eastern Iberian Chain, and it closely correlates with both standard Mediterranean and Boreal zonations. The carbon isotope record displays the trends globally recognized for the Early Aptian, with two long positive shifts separated by a pronounced negative excursion. Calibration of this isotopic record with the ammonite zonation shows that the age of OAE 1a, which corresponds to the negative excursion and subsequent positive shift, is constrained to the middle/upper part of the Deshayesites forbesi Zone.

Holocene climate variability in Central Germany and a potential link to the polar North Atlantic: A replicated record from three coeval speleothems

Here, we present high-resolution trace element and stable isotope records from three coeval Holocene stalagmites from the Herbstlabyrinth cave system, Central Germany. All stalagmites were precisely dated using MC-ICPMS 230Th/U-dating. One stalagmite started to grow at 13.62 ± 0.13 ka BP, covering the late Glacial; the other two speleothems started to grow at 11.13 ± 0.08 and 10.26 ± 0.08 ka BP, respectively. The combined record covers the entire Holocene. The interpretation of the different climate proxies is supported by data from a detailed cave monitoring programme. Cold conditions during the Younger Dryas are reflected by intermittent stalagmite growth at the Herbstlabyrinth. The δ18O records are in general agreement with the NGRIP δ18O record on millennial time scales indicating that speleothem δ18O values at the Herbstlabyrinth reflect large-scale climate variability in the North Atlantic area. The 8.2 ka event is clearly visible as a pronounced negative excursion in the δ18O records. In all other proxies, it is not reflected as a major excursion. Correlation and principal component analysis enable us to disentangle the various processes affecting the stable isotope and trace element signals. Phases with higher P, Ba and U concentrations and more negative δ13C values are interpreted as reflecting more productive vegetation above the cave. The negative correlation of Mg with P, Ba and U and the positive correlation with δ13C indicate more recharge during phases of more productive vegetation, probably because of increased rainfall. The majority of the observed phases of reduced vegetation productivity and drier climate coincide with cooler periods in the polar North Atlantic as reflected by a higher abundance of hematite-stained grains (i.e. the Bond events), suggesting a close relationship between terrestrial climate in Central Europe and the polar North Atlantic.

Sedimentology, biostratigraphy and event stratigraphy of the Early Aptian Oceanic Anoxic Event (OAE1A) in the Apulia Carbonate Platform Margin – Ionian Basin System (Gargano Promontory, southern Italy)

The stratigraphic and biotic signatures of severe environmental changes across the late Early Aptian Oceanic Anoxic Event (OAE1a) in central Tethyan settings have been investigated in the almost undeformed Apulia Carbonate Platform Margin-Ionian Basin system (ACPM-IB) exposed in the Gargano Promontory (southeastern Italy). The nature of the observed Lower Aptian biofacies shifts within the investigated carbonate system are analyzed using an integrated biostratigraphic approach, based on ammonite-calibrated orbitolinids and caprinids coupled with the published δ13C and δ18O record of coeval pelagic sections of the Ionian basin (Coppitella, eastern Gargano, and Paliambela, northwestern Greece). Detailed field analysis of facies and tracing out of key biosedimentary and chemostratigraphic markers of the Lower Aptian have elucidated the evolution of the platform margin and allowed integrated correlations across the Gargano margin to basin transition.It is shown here that the global-scale environmental changes leading to the OAE1a event (i.e., fluctuations of surface-ocean temperature, available trophic resources and, above all, pCO2) are matched consistently by significant shifts of the biotic associations inhabiting the ACPM. The onset of greenhouse, mesotrophic conditions in the surface ocean undergoing an increasing acidification by CO2 excess favored the rapid spread of calcite shelled, filter feeding, eurytopic opportunist organisms (chondrodontids, ostreids, chaetetids sponges, along with minor bryozoans) as well as echinoids and orbitolinids (foramol productivity mode); this change is recorded by the deposition of a 4–6 m Crisis Interval (CI) that sharply overlies the “Urgonian”-type, rudist-rich platform margin complex (Montagna degli Angeli Limestones) formed from predominantly aragonite shelled, stenotopic organisms (mostly caprinid rudists, with minor corals and Dasycladales and Bryopsidales green algae) (chlorozoan productivity mode). The CI heralded the incipient drowning of the ACPM which occurred immediately after the deposition of a hothouse “out of balance” brachiopod (Orbirhynchia nadiae)-cyanobacteria association (microbial productivity mode). The biostratigraphically constrained CI and the related drowning of the ACPM have been physically correlated with specific chemostratigraphic segments of the δ13Ccarb curve that is available for the facing, proximal Ionian Basin. The early (earliest ?) Late Bedoulian, greenhouse CI (early D. deshayesi – early P. cormyi zones) was deposited during the late C2 interval, whereas the early Late Bedoulian, hothouse “out-of-balance” guild and the ACPM drowning (early deshayesi zone) accompanied the subsequent negative excursion and culmination, respectively, at the very base of the Chemostratigraphic Selli Level (CSL) (latest C2 and C3 interval). These correlations permit regional to global interpretations of historical patterns and explanatory paleoeanographic and paleoecologic hypotheses; furthermore, they suggest that the Apulia shallow-water ecosystem reacted to the environmental disruptions linked with the OAE1a sooner than that of the facing open ocean. The onset of the drowning event was synchronous with the hothouse-induced bloom of the “out of balance” brachiopod-cyanobacteria association that occurred simultaneously with the pronounced negative excursion of the δ13Ccarb curve. This suggests a cause-and-effect relationship with the sudden environmental perturbations linked with the injection into the atmosphere-hydrosphere system of 13C-depleted CO2. The numerical age model of the major biotic and stratigraphic events at the Lower Aptian ACPM, along with their inferred genetic processes call for different timings and causal mechanisms associated with platform demise in northern and central Tethyan settings.

Mid-Ludfordian coeval carbon isotope, natural gamma ray and magnetic susceptibility excursions in the Mielnik IG-1 borehole (Eastern Poland)—Dustiness as a possible link between global climate and the Silurian carbon isotope record

The graptolite-bearing upper Silurian succession of the Mielnik IG-1 borehole (eastern Poland) represents a periplatform setting on a neritic carbonate platform located on the palaeocontinent of Baltica and records widely recorded positive mid-Ludfordian Carbon Isotope Excursion (CIE). The initial increase of δ13C values occurs in the uppermost part of the Neocucullograptus kozlowskii Zone whereas pronounced increase and maximal values (+6.74‰) are noted in the N. kozlowskii–Pseudomonoclimacis latilobus interzone (Pristiograptus dubius bloom interval). The thickness–time relation in the section indicates that the main, initial shift in carbon isotope ratio (of 2.7‰) occurred over a very short time, estimated to be around 4000years, and is coeval with a rapid initiation of deposition of rock-forming, silt-sized detrital dolomite and quartz. The CIE decline is interpreted as steeper than in inner shelf sections, and is coincident with a flooding event in the Ps. latilobus–Slovinograptus balticus Zone; whereas the end of the excursion is coeval with the next flooding surface marked by the Ps. Latilobus–Uncinatograptus acer graptolite assemblage. Combined SGR (Spectral Gamma Ray), facies, and faunal records during the CIE suggest prolonged sea-level lowstand conditions. The abundant detrital material (‘dolomite siltstone facies’), occurring exclusively in the CIE interval, is noted in microfacies as well as in the geophysics record—as pronounced negative excursions in total natural gamma ray and magnetic susceptibility, both resulting from dilution by diamagnetic and low-radioactivity minerals. The perfectly sorted, vertically equally-sized silt material contains unique, constant and low diversified grain assemblage (euhedral dolomite, quartz, minute ooids)—suggesting its provenance exclusively from the shallower and/or emerged parts of the carbonate platform interior. The spot findings of graptolites and nautiloids along with delicate parallel lamination of sediments indicates relatively deep environment, with absence of traction processes. These features, along with the lack of dolomite detritus in accompanying redeposited carbonates, suggest its derivation by wind. With regard to the earlier noted presence of aeolian detritus in the middle Ludfordian, we suggest dustiness as a plausible driving force of the CIE. In the hypothesis proposed, bioavailable Fe-bearing dust inflow in concert with antiestuarine circulation on shelfs, caused nutrient (N:P ratio) disturbances, resulting in methanogenesis in surface layer of the seas. The surface layer methanogenesis should resulting in bathymetric gradient of isotope fractionation, what is consistent with the CIE record. Coincidence of sea-level lowstand, low-latitude aridity, and gustiness of the global climate, are in accordance with the hypothetical short-lived glaciation in mid-Ludfordian time.

Astronomical forcing and chronology of the early Toarcian (Early Jurassic) oceanic anoxic event in Yorkshire, UK

During the early Toarcian (∼183 Ma ago), a high rate of organic carbon burial globally over a brief interval of time has led to the recognition of a major oceanic anoxic event (OAE). A pronounced negative excursion in the carbon‐isotope composition of marine organic matter, marine carbonate and terrestrial plant material is a key feature of this event but the precise timescale and cause(s) of this isotopic anomaly are debated. Associated with the negative carbon‐isotope excursion is evidence for a coeval rise in seawater palaeotemperature, an increase in continental weathering rates, and the mass extinction of marine invertebrate species. The early Toarcian OAE provides evidence for the Earth's response during rapid climate change, and critical to our understanding of the event is a high‐resolution timescale that allows us to quantify the rates, duration and lead/lag times of environmental processes. In this study, we present 2743 new high‐resolution organic carbon, sulphur and carbonate concentration data from samples of well‐preserved organic‐rich mudrocks spanning the early Toarcian OAE in Yorkshire, UK. We have used these data to document the geochemical changes and significantly extend and refine the astronomical timescale across this event. Our detailed analysis of the relationship between astronomical forcing and carbon isotope changes in both Yorkshire and a section from Peniche, Portugal, indicates that astronomical forcing paced the timing of major shifts in δ13C and hence climate in both sections. Our analyses also demonstrate that there was a marked increase in the relative strength of astronomical forcing recorded at the onset of the OAE, and that the recorded nature of astronomical forcing changed during the event. Both the Yorkshire and Peniche cyclostratigraphies suggest that one astronomical forcing parameter paced environmental change through the δ13C event, and that this parameter was obliquity or precession.

Evolution of the ocean's “biological pump”

Earth history is punctuated by a huge variety of transitions and perturbations in climate and global biogeochemical cycles. These may be linked to major extinctions or evolutionary innovations, and may exhibit evidence for greenhouse warming and CO2 release and hence potentially hold direct future-relevant information (1) or may be associated with ice ages. Arguably, no event is more enigmatic or has been more keenly debated than the occurrence of extreme glaciation during the Neoproterozoic (1,000–542 Ma) (2), when, in two separate episodes, the global ocean potentially attained complete sea-ice cover to create a “snowball Earth” (3). One of the main barriers to a full understanding of these intense glacial episodes has been in identifying the trigger; as nothing comparable occurs at any time in the preceding approximately 1,500 Ma or afterward during the Phanerozoic. The mechanism for the initial cooling must also be consistent with a pronounced negative excursion recorded in the carbon isotopic composition (i.e., δ13C) of marine carbonates (2). This latter criterion is particularly challenging because, although the oxidation of reduced, biological forms of carbon such as organic matter and hydrate methane are highly depleted in 13C and hence are able to drive ocean and atmosphere δ13C negative, the release of greenhouse gases such as CO2 (and CH4) will tend to prevent, not cause, a deep ice age (4). In PNAS, Tziperman and colleagues (5) square the circle between cooling [and a lower partial pressure of CO2 (pCO2) in the atmosphere] and negative-trending δ13C by recognizing that alternative pathways for oxidizing organic matter would have been important and that carbon cycling in the ocean may have undergone profound changes around this time.

The significance of an Early Jurassic (Toarcian) carbon-isotope excursion in Haida Gwaii (Queen Charlotte Islands), British Columbia, Canada

During the Early Toarcian there was a significant disruption in the short-term active carbon reservoir as revealed by carbon-isotope records, which show a broad positive shift that is interrupted by a large 5–7‰ negative excursion (δ 13C org). Carbon-isotope excursion co-occurs with the deposition of organic-rich shales in many areas. This perturbation in carbon isotopes is thought to be indicative of severe climate change and marine anoxia. The two leading hypotheses as to the cause of this event invoke either global or regional controls. Here we present carbon-isotope data from Haida Gwaii, British Columbia, Canada showing a significant perturbation within a temporally constrained Early Toarcian succession that was deposited in the northeastern paleo-Pacific Ocean. These data reinforce the concept that the short-term active carbon reservoir was affected globally, and assist with the correlation of ammonite zonal schemes between western North America and Europe. The δ 13C org data show a broad positive shift that is interrupted by a sharp and pronounced negative excursion of 7‰ (8.5‰ in δ 13C wood) in the Early Toarcian Kanense Zone. This negative excursion also coincides with increasing total organic carbon (TOC) from ~ 0.4% to ~ 1.2%. These data suggest that the Early Toarcian carbon-isotope perturbation was indeed global and imprinted itself on all active global reservoirs of the exchangeable carbon cycle (deep marine, shallow marine, atmospheric).

The Triassic isotope record

AbstractA variety of stable isotope measurements have been found useful in studying processes of environmental change. Measurements of δ13C, δ18O, δ34S and 87Sr/86Sr all can provide information about the conditions of the water column in which sediment deposition occurred, but the most widely applied of these is δ13C. The carbon isotope record for the Triassic System is a complex one; a pronounced negative excursion begins below the base of the Triassic System and continues into the basal Triassic. The succeeding 4 to 6 Ma Lower Triassic interval is marked by isotopic instability, with positive and negative excursions, continuing through the basal Middle Triassic. In contrast to the Lower Triassic, most of the Middle and Upper Triassic display relative isotopic stability, with rising values of δ13C likely reflecting environmental recovery and increasing storage of organic carbon in terrestrial environments. The uppermost Triassic is marked by a pronounced negative excursion near the system boundary that has been linked to significant biotic turnover. The causes of the various excursions remain under investigation, particularly those at the system boundaries, with outgassing during volcanic activity, changes in productivity, ocean anoxia, and seafloor methane releases all suggested as mechanisms both for perturbing the global carbon cycle and for forcing biotic extinction.

Expression of the Early Toarcian negative carbon-isotope excursion in separated carbonate microfractions (Jurassic, Paris Basin)

The causes of the pronounced negative excursion in carbon-isotope values that was recorded during the Early Toarcian Oceanic Anoxic Event (T-OAE) are still under debate, particularly with regard to the local versus global pattern of the excursion, and the extent to which recorded signals are under a diagenetic control. In this study we employ a novel microseparation technique in order to investigate the isotopic and mineralogical characteristics of different size fractions of the carbonate content from a Toarcian section recovered from the Sancerre–Couy borehole, southern Paris Basin. Beyond the recognition of a − 6‰ δ 13C excursion in the bulk carbonate content, our data also demonstrate that biogenic particles (such as coccoliths) and inorganic grains precipitated as early diagenetic phases (including dolomite) both record the excursion with the same magnitude. Although several black shales occur through the Paris Basin Toarcian section, it is only that associated with the onset of the OAE that coincides with a large negative carbon-isotope excursion. Taken together these observations indicate that during this event, the entire water column was characterized by homogeneous carbon-isotope values; such a pattern is incompatible with the idea that the negative excursion was generated simply through the upwelling of bottom waters enriched in re-mineralized organic carbon (cf. “the Küspert model”), since this would have required a strong vertical gradient in the water column. Additionally, the Paris Basin data show that the decrease in carbonate δ 13C values during the OAE occurred in several discrete steps (each of some − 2‰), as has previously been found for organic carbon substrates in other European sections. The stepped nature of the isotopic profile, which is part of a stratigraphic signature previously ascribed to Milankovitch forcing, is compatible with regular pulsed input of light carbon into the whole atmosphere–ocean system from a climatically sensitive source such as gas hydrate, or from thermal methanogenesis of organic-rich sediments in the Karoo–Ferrar large igneous province. Contrasts in the amplitude of the negative carbon-isotope excursion on a regional scale remain an important unexplained aspect of the Toarcian record.

Late Aptian to late Albian sea-level fluctuations constrained by geochemical and biological evidence (Nahr Umr Formation, Oman)

ABSTRACT The Aptian to Albian interval of the vast Arabian Platform was studied in northern Oman (Nahr Umr Formation) for its paleoceanographic and sea-level record. The sections indicate seven transgressive-regressive cycles overprinted by small-scale sea-level fluctuations. The peak of regression lead to emergence and exposure of the sea floor and overprinted marine hardground surfaces. The subaerial overprint of marine hardgrounds is difficult to recognize in the field. However, four independent lines of evidence document emergence. These are: (1) Pronounced negative excursions in carbon and partly in oxygen isotope composition beneath hardbottoms. They are attributed to the influx of light, soil-derived carbon and light oxygen from meteoric water. (2) Sparite-filled rudists beneath hardbottoms contain primary brackish-water fluid inclusions of very low salinity interpreted as cement precipitation in the phreatic marine/meteoric mixing zone or pollution of meteoric freshwater by residual salt. (3) Symbiont-bearing and thus light-dependent orbitolinids display small, conical morphotypes in the shallow, well illuminated water prior to exposure and hardbottom formation. Large, flat morphotypes are characteristic of the transgressive shales above those surfaces and document deeper, less illuminated waters. (4) The presence of mycorrhiza (Microcodium). The exposure surfaces overlie facies-incomplete shallowing-upward successions or subtidal sediments, but unequivocal evidence for erosion before or during emergence is lacking. Correlation of transgressive-regressive events in Oman with other sea-level curves suggests the record of spasmodic regional tectonism combined with a strong eustatic component.