Cool episodes in the Late Cretaceous - exploring the effects of physical forcing on Antarctic snow accumulation

Until recently it was assumed that the major modern ice sheets on Antarctica became established around the Eocene-Oligocene boundary about 34 Ma ago. But new evidence (e.g. Miller et al., 2008) indicates that continental ice may have been present much earlier, some of it probably even since the greenhouse times of the Late Cretaceous. Deep sea drilling data suggest changes in sea-level during the Late Cretaceous that could have been caused by the melting and freezing of vast ice sheets on Antarctica. Using a GCM approach to test the whether it would be possible to generate the described high-amplitude sealevel falls is one additional way to test this vigorously discussed issue. As shown above, our numerical approach indicates the possibility of a substantial Antarctic glaciation by changing the physical boundary conditions, eccentricity, pCO2, and elevation within reasonable Late Cretaceous ranges. Our simulations suggest that simulated snowfall and consecutive ice formation on Antarctica might yield sufficient volumes to account for the documented rapid, low-amplitude Cretaceous sea-level fluctuations. Based on cautious assumptions and possible errors the model results show that ice build-up could take place in realistic time spans and in accordance with the proxy records. Thus, the possibility of an Antarctic ice shield build-up large enough to drive sea level fluctuations on the order of tens of meters within 20,000-220,000 years is supported. The initial snow accumulation and following growth of Antarctic ice-sheets in the Cretaceous can be attributed to changes in southern hemisphere summer insolation due to reduced orbital eccentricity. Alternatively and/or additionally, declining atmospheric CO2 values caused further coolin

The Gargasian (Middle Aptian) of La Marcouline section at Cassis-La Bédoule (SE France): Stable isotope record and orbital cyclicity

International audienceBulk rock stable isotope analysis of La Marcouline section (Cassis-La Bédoule area, SE France) revealed a general trend of decreasing δ13C and δ18O values from the bottom to the top of the section. The decrease in δ13C values reflects a global trend in Middle Aptian times, namely a return to pre-excursion values of δ13C values following a major, positive excursion in the Early Aptian, which is a reflection of the Oceanic Anoxic Event (OAE) 1a. Power spectra of the bedding rhythms suggest that precessional and long eccentricity cycles of the Milankovitch band controlled the deposition of marl-limestone bundles. This can be interpreted as the result of a forcing by insolation at low latitudes which resulted in a monsoon-controlled precipitation pattern, that in turn induced the deposition of clay-rich beds. Conversely, limestone beds were formed in periods of dryer climate. Higher sea-surface productivity during wet periods may have been caused by an increase in continental runoff and a consequent enhancement in the delivery of nutrients to epicontinental basins. An orbital cyclicity in the sedimentary patterns in La Marcouline section provides the opportunity to calibrate the duration of the well-established G. ferreolensis foraminiferal Zone with that of orbital chronology. The G. ferreolensis Zone at Cassis-La Bédoule spans 33 precessional cycles and its duration is thus estimated to be approximately 760 ka. This length of time is significantly shorter than the estimates of published timescales for this zone and thus may be an argument for proposing that the Gargasian substage is significantly shorter than its currently accepted range

Drilling of Early Cretaceaous Oceanic Anoxic Event 1a in southern France

The massive concentration of black, laminated, organic carbon-rich shales at certain time periods during the Cretaceous period (∼140–65 Ma) led to the concept of Oceanic Anoxic Events (OAEs). These events are characterized by unusually enhanced preservation of organic matteracross environments ranging from the deep oceans to shelf seas. Enhanced productivity of siliceous and organic-walled primary producers and/or strongly dysaerobic or anoxic conditions in all major oceans were both suggested as likely causes (Meyer and Kump, 2008). Fundamental chemical and biological changes in the world ocean must have been associated with these events

Biogeochemical effects of volcanic degassing on the oxygen-state of the oceans during the Cenomanian/Turonian Anoxic Event 2

ABSTRACT FINAL ID: PP11A-1769 Cretaceous anoxic events may have been triggered by massive volcanic CO2 degassing as large igneous provinces (LIPs) were emplaced on the seafloor. Here, we present a comprehensive modeling study to decipher the marine biogeochemical consequences of enhanced volcanic CO2 emissions. A biogeochemical box model has been developed for transient model runs with time-dependent volcanic CO2 forcing. The box model considers continental weathering processes, marine export production, degradation processes in the water column, the rain of particles to the seafloor, benthic fluxes of dissolved species across the seabed, and burial of particulates in marine sediments. The ocean is represented by twenty-seven boxes. To estimate horizontal and vertical fluxes between boxes, a coupled ocean–atmosphere general circulation model (AOGCM) is run to derive the circulation patterns of the global ocean under Late Cretaceous boundary conditions. The AOGCM modeling predicts a strong thermohaline circulation and intense ventilation in the Late Cretaceous oceans under high pCO2 values. With an appropriate choice of parameter values such as the continental input of phosphorus, the model produces ocean anoxia at low to mid latitudes and changes in marine δ13C that are consistent with geological data such as the well established δ13C curve. The spread of anoxia is supported by an increase in riverine phosphorus fluxes under high pCO2 and a decrease in phosphorus burial efficiency in marine sediments under low oxygen conditions in ambient bottom waters. Here, we suggest that an additional mechanism might contribute to anoxia, an increase in the C:P ratio of marine plankton which is induced by high pCO2 values. According to our AOGCM model results, an intensively ventilated Cretaceous ocean turns anoxic only if the C:P ratio of marine organic particles exported into the deep ocean is allowed to increase under high pCO2 conditions. Being aware of the uncertainties such as diagenesis, this modeling study implies that potential changes in Redfield ratios might be a strong feedback mechanism to attain ocean anoxia via enhanced CO2 emissions. The formation of C-enriched marine organic matter may also explain the frequent occurrence of global anoxia during other geological periods characterized by high pCO2 values

Le Gargasien (Aptien moyen) de la coupe de La Marcouline à Cassis-La Bédoule (SE France) : enregistrement des isotopes stables et cyclicité orbitale

Une analyse sur roche totale des isotopes stables de la coupe de la carrière de La Marcouline (secteur de Cassis-La Bédoule, SE de la France) a révélé une tendance générale à la décroissance des valeurs de δ13C et δ18O depuis la base jusqu'au sommet de la coupe. La décroissance des valeurs de δ13C reflète une tendance globale durant l'Aptien moyen, à savoir un retour à des valeurs inférieures à celles de l'excursion positive observée à l'Aptien inférieur, qui correspond à l'événement anoxique océanique OAE1a. L'analyse spectrale des rythmes de stratification suggère que les cycles de précession et les cycles longs d'excentricité de type MILANKOVITCH ont contrôlé le dépôt des couplets marne-calcaire. Ceci peut s'interpréter comme le résultat d'un forçage solaire aux basses latitudes qui a engendré un mode de précipitations contrôlé par la mousson, induisant le dépôt de couches riches en argile. A l'inverse, les niveaux calcaires se sont déposés durant les périodes de climat plus sec. La plus forte productivité des eaux de surface durant les périodes humides peut avoir été causée par un accroissement des apports venant du continent conduisant à une augmentation des éléments nutritifs au sein des bassins épicontinentaux. La détection d'une cyclicité orbitale dans les rythmes sédimentaires de la coupe de La Marcouline fournit l'occasion de calibrer avec la chronologie orbitale la durée de la zone bien caractérisée par l'espèce de foraminifère planctonique G. ferreolensis. Dans le secteur de Cassis-La Bédoule cette zone s'étage sur 33 cycles précessionels et sa durée peut ainsi être estimée à environ 760.000 ans. Cet intervalle de temps est sensiblement plus bref que les estimations figurant dans certaines échelles de temps publiées jusqu'à présent pour ladite zone. Cette nouvelle donnée constitue donc un argument qui autorise à proposer, pour le sous-étage Gargasien, une durée plus brève que celle qui est admise jusqu'ici.Bulk rock stable isotope analysis of La Marcouline section (Cassis-La Bédoule area, SE France) revealed a general trend of decreasing δ13C and δ18O values from the bottom to the top of the section. The decrease in δ13C values reflects a global trend in Middle Aptian times, namely a return to pre-excursion values of δ13C values following a major, positive excursion in the Early Aptian, which is a reflection of the Oceanic Anoxic Event (OAE) 1a. Power spectra of the bedding rhythms suggest that precessional and long eccentricity cycles of the MILANKOVITCH band controlled the deposition of marl-limestone bundles. This can be interpreted as the result of a forcing by insolation at low latitudes which resulted in a monsoon-controlled precipitation pattern, that in turn induced the deposition of clay-rich beds. Conversely, limestone beds were formed in periods of dryer climate. Higher sea-surface productivity during wet periods may have been be caused by an increase in continental runoff and a consequent enhancement in the delivery of nutrients to epicontinental basins. An orbital cyclicity in the sedimentary patterns in La Marcouline section provides the opportunity to calibrate the duration of the well-established G. ferreolensis foraminiferal Zone with that of orbital chronology. The G. ferreolensis Zone at Cassis-La Bédoule spans 33 precessional cycles and its duration is thus estimated to be approximately 760 ka. This length of time is significantly shorter than the estimates of published timescales for this zone and thus may be an argument for proposing that the Gargasian substage is significantly shorter than its currently accepted range

Evidence for Indonesian Throughflow slowdown during Heinrich events 3-5

We present sea surface and upper thermocline temperature records (60-100 year temporal resolution) spanning marine isotope stage 3 (similar to 24-62 ka B. P.) from International Marine Global Change Study core MD01-2378 (121 degrees 47.27'E and 13 degrees 04.95'S; 1783 m water depth) located in the outflow area of the Indonesian Throughflow within the Timor Sea. Stable isotopes and Mg/Ca of the near-surface-dwelling planktonic foraminifer Globigerinoides ruber (white) and the upper thermocline-dwelling Pulleniatina obliquiloculata reveal rapid changes in the thermal structure of the upper ocean during Heinrich events. Thermocline warming and increased delta O-18(seawater) (P. obliquiloculata record) during Heinrich events 3, 4, and 5 reflect weakening of the relatively cool and fresh thermocline flow and reduced export of less saline water from the North Pacific and Indonesian Seas to the tropical Indian Ocean. Three main factors influenced Indonesian Throughflow variability during marine isotope stage 3: (1) global slowdown in thermohaline circulation during Heinrich events triggered by Northern Hemisphere cooling, (2) increased freshwater export from the Java Sea into the Indonesian Throughflow controlled by rising sea level from similar to 60 to 47 ka, and (3) insolation-related changes in the Australasian monsoon with associated migration of hydrological fronts between Indian Ocean- and Indonesian Throughflow-derived water masses at similar to 46-40 ka

On- and offline detection of structural breaks in thermal spraying processes

The detection of structural changes in time series from industrial processes monitoring is of great interest. We investigate the limits and possibilities of two recently developed methods, one for the off -line detection of changed volatilities [Wied et al., 2011] and one for the on-line detection of changes in the course of the time series [Borowski and Fried, 2011]. The investigation is carried out on several time series from thermal spraying processes. The processes are deliberately manipulated to produce structural changes at known time points

Oxygen minimum zone-type biogeochemical cycling in the Cenomanian-Turonian Proto-North Atlantic across Oceanic Anoxic Event 2

Highlights • We present a 5 myr record of biogeochemical cycling in a Cretaceous upwelling area. • A novel quantitative approach for the evaluation of Fe speciation proxies was applied. • Ferruginous proxy signature reflects intense chemical weathering rather than anoxia. • Water column redox conditions evolved from oxic to nitrogenous to euxinic before OAE2. • Smaller seawater nitrate inventory facilitated sedimentary H2S release and euxinia. Abstract Oceanic Anoxic Events (OAEs) in Earth's history are regarded as analogues for current and future ocean deoxygenation, potentially providing information on its pacing and internal dynamics. In order to predict the Earth system's response to changes in greenhouse gas concentrations and radiative forcing, a sound understanding of how biogeochemical cycling differs in modern and ancient marine environments is required. Here, we report proxy records for iron (Fe), sulfur and nitrogen cycling in the Tarfaya upwelling system in the Cretaceous Proto-North Atlantic before, during and after OAE2 (∼93 Ma). We apply a novel quantitative approach to sedimentary Fe speciation, which takes into account the influence of terrigenous weathering and sedimentation as well as authigenic Fe (non-terrigenous, precipitated onsite) rain rates on Fe-based paleo-redox proxies. Generally elevated ratios of reactive Fe (i.e., bound to oxide, carbonate and sulfide minerals) to total Fe (FeHR/FeT) throughout the 5 million year record are attributed to transport-limited chemical weathering under greenhouse climate conditions. Trace metal and nitrogen isotope systematics indicate a step-wise transition from oxic to nitrogenous to euxinic conditions over several million years prior to OAE2. Taking into consideration the low terrigenous sedimentation rates in the Tarfaya Basin, we demonstrate that highly elevated FeHR/FeT from the mid-Cenomanian through OAE2 were generated with a relatively small flux of additional authigenic Fe. Evaluation of mass accumulation rates of reactive Fe in conjunction with the extent of pyritization of reactive Fe reveals that authigenic Fe and sulfide precipitation rates in the Tarfaya Basin were similar to those in modern upwelling systems. Because of a smaller seawater nitrate inventory, however, chemolithoautotrophic sulfide oxidation with nitrate was less efficient in preventing hydrogen sulfide release into the water column. As terrigenous weathering and sediment flux determine how much authigenic Fe is required to generate an anoxic euxinic or ferruginous proxy signature, we emphasize that both have to be taken into account when interpreting Fe-based paleo-redox proxies