[Show abstract][Hide abstract] ABSTRACT: Based on the lithology assemblage, microfacies, and physical and chemical proxies, we reconstruct the relative sea-level curve of late Turonian-early Coniacian in Tethyan Himalaya. Spectral analysis indicates that fourth order sea-level changes were linked to the astronomically stable 405-kyr eccentricity cycle. By comparing with classic global sea-level curves, we suggest that late Turonian-early Coniacian sea-level changes in the southeastern Tethyan margin were controlled by eustasy. Two rapid and significant regressions during late Turonian, which are recorded in different continents, may be interpreted as the result of continental ice expansion, giving some support to the notion that ephemeral polar ice sheets existed even in the super-greenhouse world.
[Show abstract][Hide abstract] ABSTRACT: During the Middle Miocene, Earth’s climate transitioned from a relatively warm phase (Miocene climatic optimum) to a colder mode with reestablishment of permanent ice sheets on Antarctica, thus marking a fundamental step in Cenozoic cooling. Carbon sequestration and atmospheric CO2 drawdown through increased terrestrial and/or marine productivity have been proposed as the main drivers of this fundamental transition. We integrate high-resolution
(1–3 k.y.) benthic stable isotope data with X-ray fl uorescence scanner–
derived biogenic silica and carbonate accumulation estimates
in an exceptionally well preserved sedimentary archive, recovered
at Integrated Ocean Drilling Program Site U1338, to reconstruct
eastern equatorial Pacifi c productivity variations and to investigate
temporal links between high- and low-latitude climate change over
the interval 16–13 Ma. Our records show that the climatic optimum
(16.8–14.7 Ma) was characterized by high-amplitude climate variations,
marked by intense perturbations of the carbon cycle. Episodes
of peak warmth at (Southern Hemisphere) insolation maxima coincided with transient shoaling of the carbonate compensation depth and enhanced carbonate dissolution in the deep ocean. A switch to obliquity-paced climate variability after 14.7 Ma concurred with a general improvement in carbonate preservation and the onset of stepwise global cooling, culminating with extensive ice growth over Antarctica ca. 13.8 Ma. We fi nd that two massive increases in opal accumulation ca. 14.0 and ca. 13.8 Ma occurred just before and during the final and most prominent cooling step, supporting the hypothesis that enhanced siliceous productivity in the eastern equatorial Pacific contributed to CO2 drawdown.
[Show abstract][Hide abstract] ABSTRACT: The petrography, heavy mineral analysis, major element geochemical compositions and mineral chemistry of Early Cretaceous to Miocene-Pliocene rocks, and recent sediments of the Tarfaya basin, SW Morocco, have been studied to reveal their depositional tectonic setting, weathering history, and provenance. Bulk sediment compositional and mineral chemical data suggest that these rocks were derived from heterogeneous sources in the Reguibat Shield (West African Craton) including the Mauritanides and the western Anti-Atlas, which likely form the basement in this area. The Early Cretaceous sandstones are subarkosic in composition, while the Miocene-Pliocene sandstones and the recent sediments from Wadis are generally carbonate-rich feldspathic or lithic arenites, which is also reflected in their major element geochemical compositions. The studied samples are characterized by moderate SiO2 contents and variable abundances of Al2O3, K2O, Na2O, and ferromagnesian elements. Binary tectonic discrimination diagrams demonstrate that most samples can be characterized as passive continental marginal deposits. Al2O3/Na2O ratios indicate more intense chemical weathering during the Early Cretaceous and a variable intensity of weathering during the Late Cretaceous, Early Eocene, Oligocene-Early Miocene, Miocene-Pliocene and recent times. Moreover, weathered marls of the Late Cretaceous and Miocene-Pliocene horizons also exhibit relatively low but variable intensity of chemical weathering. Our results indicate that siliciclastics of the Early Cretaceous were primarily derived from the Reguibat Shield and the Mauritanides, in the SW of the basin, whereas those of the Miocene-Pliocene had varying sources that probably included western Anti-Atlas (NE part of the basin) in addition to the Reguibat Shield and the Mauritanides.
International Journal of Earth Sciences 12/2013; 103(1). · 2.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: During the Middle Miocene, Earth’s climate transitioned from a relatively warm phase (Miocene climatic optimum) to a colder mode with reestablishment of permanent ice sheets on Antarctica, thus marking a fundamental step in Cenozoic cooling. Carbon sequestration and atmospheric CO2 drawdown through increased terrestrial and/or marine productivity have been proposed as the main drivers of this fundamental transition. We integrate high-resolution (1–3 k.y.) benthic stable isotope data with X-ray fluorescence scanner–
derived biogenic silica and carbonate accumulation estimates in an exceptionally well preserved sedimentary archive, recovered at Integrated Ocean Drilling Program Site U1338, to reconstruct eastern equatorial Pacific productivity variations and to investigate temporal links between high- and low-latitude climate change over the interval 16–13 Ma. Our records show that the climatic optimum (16.8–14.7 Ma) was characterized by high-amplitude climate variations, marked by intense perturbations of the carbon cycle. Episodes of peak warmth at (Southern Hemisphere) insolation maxima coincided with transient shoaling of the carbonate compensation depth and enhanced carbonate dissolution in the deep ocean. A switch to obliquity-paced climate variability after 14.7 Ma concurred with a general improvement in carbonate preservation and the onset of stepwise global cooling, culminating with extensive ice growth over Antarctica ca. 13.8 Ma. We find that two massive increases in opal accumulation ca. 14.0 and ca. 13.8 Ma occurred just before and during the final and most prominent cooling step, supporting the hypothesis that enhanced siliceous productivity in the eastern equatorial Pacific
contributed to CO2 drawdown.
[Show abstract][Hide abstract] ABSTRACT:  We present high-resolution (2–3 kyr) benthic foraminiferal stable isotopes in a continuous, well-preserved sedimentary archive from the West Pacific Ocean (Ocean Drilling Program Site 1146), which track climate evolution in unprecedented resolution over the period 12.9 to 8.4 Ma. We developed an astronomically tuned chronology over this interval and integrated our new records with published isotope data from the same location to reconstruct long-term climate and ocean circulation development between 16.4 and 8.4 Ma. This extended perspective reveals that the long eccentricity (400 kyr) cycle is prominently encoded in the δ13C signal over most of the record, reflecting long-term fluctuations in the carbon cycle. The δ18O signal closely follows variations in short eccentricity (100 kyr) and obliquity (41 kyr). In particular, the obliquity cycle is prominent from ~14.6 to 14.1 Ma and from ~9.8 to 9.2 Ma, when high-amplitude variability in obliquity is congruent with low-amplitude variability in short eccentricity. The δ18O curve is additionally characterized by a series of incremental steps at ~14.6, 13.9, 13.1, 10.6, 9.9, and 9.0 Ma, which we attribute to progressive deep water cooling and/or glaciation episodes following the end of the Miocene climatic optimum. On the basis of δ18O amplitudes, we find that climate variability decreased substantially after ~13 Ma, except for a remarkable warming episode at ~10.8–10.7 Ma at peak insolation during eccentricity maxima (100 and 400 kyr). This transient warming, associated with a massive negative carbon isotope shift, is reminiscent of intense global warming events at eccentricity maxima during the Miocene climatic optimum.
[Show abstract][Hide abstract] ABSTRACT: The Turonian to Santonian organic-rich successions deposited in the
continuously subsiding Tarfaya Atlantic coastal basin (SW Morocco) allow
detailed reconstruction of depositional environments and correlation to
eustatic sea level changes. We present high-resolution X-ray
fluorescence (XRF) scanning and natural gamma-ray (NGR) records from a
newly drilled sedimentary core Tarfaya SN°2 (27° 57´
43.1´´N, 12° 48´ 37.0´´W), which
recovered a continuous sedimentary succession from a middle to outer
shelf environment. In the latest Turonian, the late Coniacian, and the
middle and latest Santonian, high NGR and Al with low Mn and Ca content
indicate pronounced dysoxic horizons that reflect impingement of the
oxygen minimum zone on the shelf during sea level highstands. In
contrast, lower NGR and Al with higher Mn and Ca values indicate high
detrital carbonate content and more oxic conditions related to
regressive events in the late Turonian, early to middle Conacian and
early Santonian. Exceptionally high sedimentation rates (>10cm/kyr)
characterize the late Turonian, and spectral analyses of XRF and NGR
data reveal that sedimentation was mostly controlled by obliquity and
precession, suggesting an overriding glacioeustatic control. However,
the response to orbital forcing weakened during the latest Turonian,
when sedimentation rates declined markedly to ~2 cm/kyr. We will extend
this study to three newly drilled cores nearby that recovered sediment
sequences from the late Albian to late Turonian and from the late
Santonian to Campanian in order to retrace the complete Late Cretaceous
depositional history of the Tarfaya Basin and to develop a
high-resolution carbon isotope stratigraphy allowing correlation to
records from other continental margins. Key words: Late
Cretaceous, Tarfaya Basin, XRF scanning, natural gamma-ray, oxygen
minimum zone, sea level, orbital forcing.
[Show abstract][Hide abstract] ABSTRACT: We integrate micropaleontological and geochemical records (benthic stable isotopes, neodymium isotopes, benthic foraminiferal abundances and XRF-scanner derived elemental data) from well-dated Pacific Ocean successions (15–12.7 Ma) to monitor circulation changes during the middle Miocene transition into a colder climate mode with permanent Antarctic ice cover. Together with previously published records, our results show improvement in deep water ventilation and strengthening of the meridional overturning circulation following major ice expansion at ∼13.9 Ma. Neodymium isotope data reveal, however, that the provenance of intermediate and deep water masses did not change markedly between 15 and 12.7 Ma. We attribute the increased δ13C gradient between Pacific deep and intermediate water masses between ∼13.6 and 12.7 Ma to more vigorous entrainment of Pacific Central Water into the wind-driven ocean circulation due to enhanced production of intermediate and deep waters in the Southern Ocean. Prominent 100 kyr ventilation cycles after 13.9 Ma reveal that the deep Pacific remained poorly ventilated during warmer intervals at high eccentricity, whereas colder periods (low eccentricity) were characterized by a more vigorous meridional overturning circulation with enhanced carbonate preservation. The long-term δ13C decline in Pacific intermediate and deep water sites between 13.5 and 12.7 Ma reflects a global trend, probably related to a re-adjustment response of the global carbon cycle following the last 400 kyr carbon maximum (CM6) of the “Monterey Excursion”.
[Show abstract][Hide abstract] ABSTRACT: Lithological evidence, benthic foraminiferal census counts, and X-ray fluorescence (XRF) scanner-derived elemental data were integrated with planktonic foraminiferal biostratigraphy and bulk carbonate stable isotopes to retrace the Turonian to early Campanian paleoenvironmental evolution and sea-level history of the Tarfaya Atlantic coastal basin (SW Morocco). The lower Turonian is characterized by laminated organic-rich deposits, which contain impoverished benthic foraminiferal assemblages, reflecting impingement of the oxygen minimum zone on the shelf during a sea-level highstand. This highstand level is correlated to the global transgressive pulse above the sequence boundary Tu1. The appearance of low-oxygen tolerant benthic foraminiferal assemblages dominated by Gavelinella sp. in the middle to upper Turonian indicates an improvement in bottom water oxygenation, probably linked to offshore retraction of the oxygen minimum zone during a regressive phase. This interval is marked by major regressive events expressed by a series of erosional truncations associated with the prominent sequence boundaries Tu3 and/or Tu4. Dysoxic–anoxic conditions recorded in the upper Santonian of the Tarfaya Basin coincide with the eustatic sea-level rise prior to Sa3 sequence boundary. The lower Campanian transgression, only recorded in the southern part of the Tarfaya Basin, coincided with substantial deepening, enhanced accumulation of fine-grained clay-rich hemipelagic sediments and improved oxygenation at the seafloor (highest diversity and abundance of benthic foraminiferal assemblages). Stable isotope data from bulk carbonates are tentatively correlated to the English Chalk carbon isotope reference curve, in particular the Hitch Wood Event in the upper Turonian, the Navigation Event in the lower Coniacian, the Horseshoe Bay Event in the Santonian and the Santonian/Campanian Boundary Event.
Cretaceous Research 01/2013; 45:288–305. · 1.63 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present trace element compositions, rare earth elements (REEs) and radiogenic Nd-Sr isotope analyses of Cretaceous to recent sediments of the Tarfaya basin, SW Morocco, in order to identify tectonic setting, source rocks composition and sediments provenance. The results suggest that the sediments originate from heterogeneous source areas of the Reguibat Shield and the Mauritanides (West African Craton), as well as the western Anti-Atlas, which probably form the basement in this area. For interpreting the analyzed trace element results, we assume that elemental ratios such as La/Sc, Th/Sc, Cr/Th, Th/Co, La/Co and Eu/Eu∗ in the detrital silicate fraction of the sedimentary rocks behaved as a closed system during transport and cementation, which is justified by the consistency of all obtained results. The La/Y-Sc/Cr binary and La-Th-Sc ternary relationships suggest that the Tarfaya basin sediments were deposited in a passive margin setting. The trace element ratios of La/Sc, Th/Sc, Cr/Th and Th/Co indicate a felsic source. Moreover, chondrite-normalized REE patterns with light rare earth elements (LREE) enrichment, a flat heavy rare earth elements (HREE) and negative Eu anomalies can also be attributed to a felsic source for the Tarfaya basin sediments. The Nd isotope model ages (TDM=2.0 to 2.2 Ga) of the Early Cretaceous sediments suggest that sediments were derived from the Eburnean terrain (Reguibat Shield). On the other hand, Late Cretaceous to Miocene-Pliocene sediments show younger model ages (TDM=1.8 Ga, on average) indicating an origin from both the Reguibat Shield and the western Anti-Atlas. In contrast, the southernmost studied Sebkha Aridal section (Oligocene to Miocene-Pliocene) yields older provenance ages (TDM=2.5 to 2.6 Ga) indicating that these sediments were dominantly derived from the Archean terrain of the Reguibat Shield.
Journal of African Earth Sciences 01/2013; · 1.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The global carbon cycle during the mid-Cretaceous (˜125-88 million
years ago, Ma) experienced numerous major perturbations linked to
increased organic carbon burial under widespread, possibly basin-scale
oxygen deficiency and episodes of euxinia (anoxic and
H2S-containing). The largest of these episodes, the
Cenomanian-Turonian boundary event (ca. 93.5 Ma), or oceanic anoxic
event (OAE) 2, was marked by pervasive deposition of organic-rich,
laminated black shales in deep waters and in some cases across
continental shelves. This deposition is recorded in a pronounced
positive carbon isotope excursion seen ubiquitously in carbonates and
organic matter. Enrichments of redox-sensitive, often bioessential trace
metals, including Fe and Mo, indicate major shifts in their
biogeochemical cycles under reducing conditions that may be linked to
changes in primary production. Iron enrichments and bulk Fe isotope
compositions track the sources and sinks of Fe in the proto-North
Atlantic at seven localities marked by diverse depositional conditions.
Included are an ancestral mid-ocean ridge and euxinic, intermittently
euxinic, and oxic settings across varying paleodepths throughout the
basin. These data yield evidence for a reactive Fe shuttle that likely
delivered Fe from the shallow shelf to the deep ocean basin, as well as
(1) hydrothermal sources enhanced by accelerated seafloor spreading or
emplacement of large igneous province(s) and (2) local-scale Fe
remobilization within the sediment column. This study, the first to
explore Fe cycling and enrichment patterns on an ocean scale using iron
isotope data, demonstrates the complex processes operating on this scale
that can mask simple source-sink relationships. The data imply that the
proto-North Atlantic received elevated Fe inputs from several sources
(e.g., hydrothermal, shuttle and detrital inputs) and that the redox
state of the basin was not exclusively euxinic, suggesting previously
unknown heterogeneity in depositional conditions and biogeochemical
cycling within those settings during OAE-2.
[Show abstract][Hide abstract] ABSTRACT: The question whether large scale glaciations on Antarctica were possible
in a late Mesozoic greenhouse climate such as the Late Cretaceous is an
intriguing one. The most recent years have provided an increasing number
of studies investigating the growth and decay of paleo-continental ice
sheets on Antarctica possibly large enough to affect sea level. Since
the outcome of these studies doesn't provide a basis for a conclusive
decision we have performed a number of model runs using an Atmospheric
General Circulation Model (AGCM) to test whether large volumes of snow
might have accumulated even under Late Cretaceous greenhouse conditions.
By varying orbital parameters as well as topography, and atmospheric
CO2 concentrations our models indicate 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 years. This
is supported under the assumption of pCO2 levels < 800
ppm, low insolation, and elevated topography. The growth of a major
Antarctic ice sheet would be possible on reasonable time scales. To
accumulate about half the present day snow/ice volume which is required
to explain the documented shifts in oxygen isotopes our model results
suggest a time span between 20,000 and 80,000 years for these ice
volumes to accumulate.
[Show abstract][Hide abstract] ABSTRACT: Cretaceous anoxic events may have been triggered by massive volcanic CO 2 degassing as large igneous provinces (LIPs) were emplaced on the seaoor. Here, we present a comprehensive modeling study to decipher the marine biogeochemical consequences of enhanced volcanic CO 2 emissions. A biogeochemical box model has been developed for transient model runs with time-dependent volcanic CO 2 forcing. The box model considers continental weathering processes, marine export production, degradation processes in the water column, the rain of particles to the seaoor, benthic uxes 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 uxes 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 pCO 2 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 δ 13 C that are consistent with geological data such as the well established δ 13 C curve. The spread of anoxia is supported by an increase in riverine phosphorus uxes under high pCO 2 and a decrease in phosphorus burial efciency 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 pCO 2 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 pCO 2 conditions. Being aware of the uncertainties such as diagenesis, this modeling study implies that potential changes in Redeld ratios might be a strong feedback mechanism to attain ocean anoxia via enhanced CO 2 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 pCO 2 values.
[Show abstract][Hide abstract] ABSTRACT: A palynological analysis of a marine sediment core in the southern Philippines, provides a detailed regional vegetation and
climate history for the West Pacific Warm Pool (WPWP) since the Last Glacial Maximum (LGM). Chronology was determined by a
detailed oxygen isotope record. A higher representation of pollen from tropical upper montane rainforest during the LGM indicate
that this forest type moved down along elevation, probabaly due to the lowered temperature. During the last deglaciation and
the early Holocene, mangroves were more expanded and tropical mid and upper montane rainforests were restricted, suggesting
a rising sea-level and temperature increase. Herbaceous pollen and pteridophyte spore records indicate a much drier condition
during the LGM than the Holocene. Mangrove development is controlled by conditions at the river mouth influenced by river
discharge. Pteridophyte spores are abundant in wet conditions and are mainly transported by rivers. During the mid-Holocene,
the reduction in mangrove pollen and pteridophyte spore appears to be a result of climate change: mainly decrease in river
discharge. This may have been affected by the decreasing intensity of the Southeast Asian Monsoon, and the increasing frequency
and intensity of warm ENSO events, El Niño, in this region.
KeywordsLast Glacial Maximum–tropical rainforests–palynological analyses–sea-level change–river discharge
Chinese Science Bulletin 01/2011; 56(22):2359-2365. · 1.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present sea surface, upper thermocline, and benthic delta O-18 data, as well as temperature and paleoproductivity proxy data, from the International Marine Global Change Study Program (IMAGES) Core MD06-3067 (6 degrees 31' N, 126 degrees 30' E, 1575 m water depth), located in the western equatorial Pacific Ocean within the flow path of the Mindanao Current. Our records reveal considerable glacial-interglacial and suborbital variability in the Mindanao Dome upwelling over the last 160 kyr. Dome activity generally intensified during glacial intervals resulting in cooler thermocline waters, whereas it substantially declined during interglacials, in particular in the early Holocene and early marine oxygen isotope stage (MIS) 5e, when upwelling waters did not reach the thermocline. During MIS 3 and MIS 2, enhanced surface productivity together with remarkably low SST and low upper ocean thermal contrast provide evidence for episodic glacial upwelling to the surface, whereas transient surface warming marks periodic collapses of the Mindanao Dome upwelling during Heinrich events. We attribute the high variability during MIS 3 and MIS 2 to changes in the El Nino-Southern Oscillation state that affected boreal winter monsoonal winds and upper ocean circulation. Glacial upwelling intensified when a strong cyclonic gyre became established, whereas El Nino-like conditions during Heinrich events tended to suppress the cyclonic circulation, reducing Ekman transport. Thus, our findings demonstrate that variations in the Mindanao Dome upwelling are closely linked to the position and intensity of the tropical convection and also reflect far-field influences from the high latitudes.
[Show abstract][Hide abstract] ABSTRACT: On the basis of the radiocarbon (14C) plateau-tuning method, a new age model for Timor Sea Core MD01-2378 was established. It revealed a precise centennial-scale phasing of climate events in the ocean, cryo-, and atmosphere during the last deglacial and provides important new insights into causal linkages controlling events of global climate change. At Site MD01-2378, reservoir ages of surface waters dropped from 1600 yr prior to 20 cal ka to 250-500 yr after 18.8 cal ka. This evidence is crucial for generating a high-resolution age model for deglacial events in the Indo-Pacific Warm Pool. Sea-surface temperatures (SST) started to change near 18.8 cal ka, that is ~ 500 yr after the start of, presumably northern hemispheric, deglacial melt and sea level rise as shown by the benthic foraminiferal oxygen isotope ratio (delta18O). However, the SST rise occurred 500-1000 yr prior to the onset of deglacial Antarctic warming and the first major rise in atmospheric carbon dioxide at about 18 ka. The increase in SST may partly reflect reduced seasonal upwelling of cold subsurface waters along the eastern margin of the Indian Ocean, which is reflected by a doubling of the thermal gradient between the sea surface and the thermocline, a halving of chlorin productivity from 19 to 18.5 cal ka, and in particular, by the strong decrease in surface water reservoir ages. Two significant increases in deglacial Timor Sea surface salinities from 19 to 18.5 and 15.5 to 14.5 cal ka, may partly reflect the deglacial increase in the distance of local river mouths, partly an inter-hemispheric millennial-scale see-saw in tropical monsoon intensity, possibly linked to a deglacial increase in the dominance of Pacific El Niño regimes over Heinrich stadial 1.
[Show abstract][Hide abstract] ABSTRACT: The Gyangze basin, located in southern Tibet, is one of the representative areas for Cretaceous Oceanic Red Beds (CORBs). We investigated and inter-correlated several new CORB outcrops in the western and southeastern part of the Gyangze area in addition to the previously described Chuangde section. The CORBs in the Gyangze basin mainly consist of shales, thin-bedded marls, re-sedimented limestones and clasts supported conglomerates-breccias. Planktonic foraminifers in the carbonate rocks within CORBs belong to the Globotruncana ventricosa and Globotruncanita calcarata zones, indicative of middle Campanian age. The facies assemblages in the different sections indicate that the depositional environments of CORBs range from outer base-of-slope apron to basin zones. Within carbonates, we observed microfacies types characteristic of basin, open deep shelf margin or toe-of-slope environments. Gravity transport resulted from the steepening of the sea floor in the Gyangze basin owing to subduction during the Campanian. Relatively oxic bottom water conditions prevailed below the toe-of-slope environment in the Gyangze basin during CORB deposition. However, the presence of gray clasts within a multicoloured matrix suggests less oxic conditions at shallower water depths within the basin.
[Show abstract][Hide abstract] ABSTRACT: Taxonomic consistency is the basic prerequisite for any foraminiferal study. In particular, interpretation of planktonic foraminiferal geochemical data requires consistent selection of monospecific tests, since different species are adapted to different ecological niches and hence different calcification depths. Recording stable isotope signals and temperatures of ambient seawater during calcification, species-specific planktonic foraminiferal oxygen isotope values (delta18O) and Mg/Ca ratios reflect environmental conditions at different depth levels of the upper water column, which makes them suitable for paleoceanographic and climate reconstructions. However, since slight morphological differences may reflect different life habitats, the geochemical composition of a foraminiferal sample is highly dependent on the selection of morphologically alike specimens used for analysis. In order to exemplify the impact of unintended mixing of slightly varying species on delta18O values and Mg/Ca ratios, this study investigates morphological characteristics and geochemical signatures of Globorotalia cultrata (d'Orbigny, 1839) and Globorotalia menardii (Parker, Jones & Brady, 1865). Both species are often assembled as 'G. menardii' group or referred to as synonyms and are commonly suggested to represent seasonal thermocline habitats. In general, both nonspinose species precipitate circular to oval, lobulate, and low trochospiral tests showing 5-6 chambers in the final whorl. The perforate chambers meet at limbate sutures, which are straight on the umbilical side and curved on the spiral side. Tests are rimmed with an imperforate keel. The umbilical extra-umbilical aperture is furnished with an imperforate lip. In contrast, tests of G. cultrata differ from G. menardii in surface and keel. The surface of G. cultrata is smooth and shiny, at shallow sites transparent, and shows only few subconical pustules of sizes
[Show abstract][Hide abstract] ABSTRACT: 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 sea-level 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 cooling.
[Show abstract][Hide abstract] ABSTRACT: A new model of the global atmosphere-ocean-continent-mantle system was set-up to investigate the triggering of the Oceanic Anoxic Event OAE2 through volcanic degassing processes at large igneous provinces (LIPs). The model simulates the changes in oceanic dissolved oxygen, phosphate, and carbon and the evolution of atmospheric pCO2 values under mid-Cretaceous boundary conditions. It considers the effects of pCO2 on element ratios in marine plankton (C : P) and includes new parameterizations for phosphorus and carbon burial at the seafloor based on modern observations. Independent isotopic and chemical time-series of ocean and atmosphere change over OAE2 are applied to evaluate the model results. The model results support the hypothesis that OAE2 was triggered by massive CO2 emissions at LIPs. According to the model, the phosphorus weathering flux into the ocean and the C : P ratio in marine plankton were enhanced by the rise in surface temperature and atmosphere pCO2 caused by mantle degassing. Marine export production and oxygen consumption in intermediate and deep water masses increased in response to the expansion of the dissolved phosphate inventory of the ocean and the change in plankton element ratios. The spread of anoxic conditions in bottom waters -induced by enhanced carbon export and respiration- was further amplified by the oxygen-dependent burial of phosphorus in marine sediments in a positive feedback loop. The modeling implies that enhanced CO2 emissions favor the spread of low-oxygen conditions also in modern oceans.