Κυριακή, 14 Απριλίου 2019

Palaeogeography, Palaeoclimatology, Palaeoecology

Paleoceanographic changes across the Latest Danian Event in the South Atlantic Ocean and planktic foraminiferal response

Publication date: 1 July 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 525

Author(s): Sofie Jehle, André Bornemann, Anna Friederike Lägel, Arne Deprez, Robert P. Speijer

Abstract

A number of short warming events occurred during Paleocene and Eocene, of which the "Paleocene-Eocene Thermal Maximum" (PETM, 56 Ma) is the most severe and most investigated event. The less known "Latest Danian Event" (LDE) at 62.2 Ma represents a 200 ky-lasting warming phase, superimposed on a long-term cooling trend after the Early Paleocene. South Atlantic ODP Site 1262 data, covering ~1 myr, indicate a warming of the entire water column by 1.5–2.6 °C, accompanying a prominent negative carbon isotope excursion (~0.9–1.1‰) and a long-term re-organization of the planktic foraminiferal fauna associated with the LDE. This study unravels a different paleoceanographic evolution of the upper ocean structure compared to results from Pacific ODP Site 1210. Unlike the Pacific, the Atlantic site lacks an apparent change of stratification as well as an overall dominance of thermocline dwelling planktic foraminifera species and a low abundance of surface dwelling photosymbiotic foraminifera. Within the LDE, indications for a slightly enhanced stratification of the upper water column and transient warming were indicated when surface dwelling planktic foraminifera became temporarily more abundant. The long-term evolution in planktic foraminifera with the disappearance of Praemurica at the LDE onset and a contemporaneous rise in Morozovella is similar to the trends reported from Shatsky Rise ODP Site 1210.



Aquatic biomarkers record Pleistocene environmental changes at Paleolake Olduvai, Tanzania

Publication date: 15 June 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 524

Author(s): Devon E. Colcord, Andrea M. Shilling, Katherine H. Freeman, Jackson K. Njau, Ian G. Stanistreet, Harald Stollhofen, Kathy D. Schick, Nicholas Toth, Simon C. Brassell

Abstract

Several hypotheses invoke climatic variability as a driving force for hominin evolution. Thus, high-resolution records of climate and environmental variability from anthropologically significant locations can help test these hypotheses. Sedimentary sequences recovered by the Olduvai Gorge Coring Project (OGCP) help evaluate climatic and environmental changes at Olduvai Gorge, Tanzania through the analyses of various biogeochemical proxies. The stratigraphic sequence of OGCP Core 2A can be correlated with horizons associated with hominins and is chronologically constrained by distinctive dated horizons, such as the Bed I Basalt and Tuff IB. The lacustrine interval from 76.6 to 86.9 m depth is ideally suited for high-resolution analyses of biogeochemical proxies as it is rich in organic carbon (>1%TOC). The hydrogen isotopic composition of nC31 in this interval of OGCP Core 2A records the effects of precession-driven wet-dry cycles on the terrestrial environment that led to alternations between woodland and grassland ecosystems, comparable to those documented by previous investigations of outcrop analogues at Olduvai. Here, we examine stratigraphic variations in the abundance of biomarkers (C28 steradienes, C17 and C23 n-alkanes, C27 and C28 A-norsteranes, fern-8-ene, and chromans) derived from aquatic organisms (algae, cyanobacteria, sponges, macrophytes, etc.) to determine the response of Paleolake Olduvai to climate variability. In general, these aquatic biomarkers reflect the productivity of the lake environment and exhibit the same precession-driven wet-dry cycles recorded by terrestrial biogeochemical signatures. However, they also provide evidence of abrupt (<~300 yr) changes in lake level and corresponding aquatic communities superimposed on the longer-term Milankovitch cycles. Thus, evidence for climatic variability is manifested through the pacing and intensity of changes in both terrestrial and aquatic ecosystems, but the different rates of the responses on land and in the aquatic environment potentially had a complex influence on water and food resources that were important factors for hominin habitation and evolution.



High-elevation shrub-ring δ18O on the northern slope of the central Himalayas records summer (May–July) temperatures

Publication date: 15 June 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 524

Author(s): Ru Huang, Haifeng Zhu, Eryuan Liang, Jussi Grießinger, Binod Dawadi, Achim Bräuning

Abstract

The potential of shrub-ring oxygen isotopes (SRO) as a paleoclimatic proxy is not well demonstrated. In this study we explored the climate signals embedded in an SRO chronology from Saga, on the northern slope of the central Himalayas by correlation analysis with climate variables and comparison with tree-ring and/or ice-core δ18O records. The Saga SRO had significant and positive correlations with May–July temperatures in both local (r = 0.43) and moisture source areas (r = 0.49) during the period 1955–2015 (CE), while no significant correlations were found with hydroclimatic variables. It is also in good agreement with two tree-ring δ18O series from the southern slope of the central Himalayas, the source of moisture to the site. The temperature signals are further verified by an ice core δ18O record from the Tanggula Mountains on the central Tibetan Plateau. However, it shows little similarity with the nearest Himalayan ice core δ18O record from Dasuopu, demonstrating temperature signals of different seasons. This study suggests that SRO could be used as an acceptable climate proxy, like tree-ring and ice-core δ18O, to broaden the spatial extent of the paleoclimatic records network. However, we should be cautious when combining different proxies with climate signals of different seasons in paleoclimate studies.



Natural assemblages of the conodont Clarkina in lowermost Triassic deep-sea black claystone from northeastern Japan, with probable soft-tissue impressions

Publication date: 15 June 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 524

Author(s): Satoshi Takahashi, Satoshi Yamakita, Noritoshi Suzuki

Abstract

We report the first discovery of Lower Triassic Clarkina assemblages: four natural conodont assemblages from Lower Triassic pelagic black claystones of the North Kitakami Belt in northeastern Japan (Akkamori section). The fossils were obtained from the 2.5-m horizon level above the black claystone base, which is assigned to the end-Permian mass-extinction event. This horizon has been dated to the earliest Triassic (Griesbachian) by the occurrence of Hindeodus parvus, which is the index species for the base of the Triassic, in the same and subjacent horizons. These four fossil assemblages include a paired segminiplanate-formed P1 element, which was identified as the genus Clarkina, and have fully or partially preserved the original components of conodont elements. The most complete assemblage among them includes 15 distinctive elements, namely S0 and pairs of M, S1, S2, S3, S4, P1, and P2. It is noteworthy that these fossil assemblages preserve probable impressions of 'eyes,' which were replaced by aggregations of silicate, phosphate, and sulphide minerals. The occurrence of several sets of fossils that retain the original positioning of the conodonts' elemental apparatuses, as well as the original presence of soft tissue, may be attributed to the process by which the conodonts' bodies were transported to the deep seafloor, and by which the activity of agents of decomposition was inhibited in near-abiotic sediments under anoxic conditions in the pelagic deep sea during the earliest Triassic.



Frost record in tree rings linked to atmospheric circulation in northern Patagonia

Publication date: 15 June 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 524

Author(s): Martín A. Hadad, Julieta Arco Molina, Fidel A. Roig Juñent, Mariano M. Amoroso, Gabriela Müller, Diego Araneo, Jacques C. Tardif

Abstract

Cold air incursions over subtropical South America are the precursor conditions for frost, a recognized extreme thermal weather event affecting plant growth damaging agricultural production over the whole Argentinian territory. Given that sub-freezing temperatures occurring during the active growing season may harm the cambium tissues and their daughter cells, frost injury can be recorded in annual tree rings in the form of anatomical anomalies. For this study, six forest sites of Araucaria araucana, a multi-centennial tree species, from NW Patagonia were considered. From 321 trees, 1374 frost injuries in the growth rings, named frost rings, were identified and their calendar dating allowed the development of a regional chronology of frost rings covering the period AD 1256 to AD 1993 (738 yrs.). This represents the longest record at present of extreme low thermal events for Patagonia. Frost injuries were mostly restricted to the middle section of the growth rings, suggesting the incidence of late spring frosts. Moreover, frost rings were observed mainly in the juvenile portions (<50 yrs. of age and <6 cm in stem diameter) of the tree stem, indicating that young trees display a greater sensitivity to frost events. Large-scale geographical freezing events were recorded in the years AD 1889, 1916, 1941 and 1948. The atmospheric genesis of these events was sustained by a trough at 500 hPa approaching from the west toward the continent, along with an approach of the Pacific anticyclone at 1000 hPa which invaded the southern part of South America on the day of the frost event. At the hemispheric scale, it was found that atmospheric circulation patterns related to La Niña were associated with the regional frost ring record in A. araucana. The regional frost ring record from A. araucana is proposed as a regional proxy of sub-freezing temperatures in paleoclimate reconstruction and as a background for models about the future behavior of the climate in scenarios of change.



Stratigraphic and environmental control on marine benthic community change through the early Toarcian extinction event (Iberian Range, Spain)

Publication date: 15 June 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 524

Author(s): Silvia Danise, Marie-Emilie Clémence, Gregory D. Price, Daniel P. Murphy, Juan J. Gómez, Richard J. Twitchett

Abstract

In the Early Jurassic (~183 Ma ago) global warming and associated environmental changes coincided with an extinction event in the marine realm (early Toarcian extinction event). Anoxia was previously considered to have been the main cause of extinction, but extinctions also occur at localities that remained oxygenated throughout the event, suggesting that other factors, such as temperature, may have played a major role. To test this hypothesis, we integrated quantitative analyses of benthic macro-invertebrates with high-resolution geochemical proxies on the bulk rock (TOC, δ13C, δ18O) and on belemnites and brachiopod shells (δ13C, δ18O) from two sections from the Iberian Range, Spain, with no black shale deposition. The sections are orientated SE-NW along an onshore-offshore gradient deepening to the north. The dominant benthic groups, bivalves and brachiopods, show a different response to the extinction: brachiopods go through a complete species-level turnover, while many bivalve species range through the event. In the shallower section, changes in richness and evenness correlate with TOC (Total Organic Carbon), suggesting that variations in nutrient input from runoff, and the possible local onset of low-redox conditions (TOC > 4 wt%), controlled faunal diversity. In contrast, at the deeper section, community change correlates with changes in δ18O, indicating that temperature variations might have influenced faunal change. Different stratigraphic patterns of extinction occur between the two localities, with last-occurrences clustering at the maximum flooding surface in the shallower section, and at the transgressive surface in the deeper one. The observed differences between the two localities highlight the important role of local sedimentary and stratigraphic processes in controlling the shape of the geochemical and fossil record, and the need for studying multiple sections along onshore-offshore gradients in order to extrapolate regional and global patterns.



The relation of a coastal environment to early diagenetic clinoptilolite (zeolite) formation - New data from the Late Cretaceous European Basin

Publication date: 15 June 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 524

Author(s): Agata Jurkowska, Marcin Barski, Elżbieta Worobiec

Abstract

Clinoptilolite, an authigenic mineral of the zeolite group, is commonly considered as a product of rhyolitic volcanic glass transformation during diagenesis of carbonate pelagic/hemipelagic sediments of Cretaceous-Paleogene age. Another, subordinate way of clinoptilolite formation that has been suggested in the literature is its precipitation during diagenetic transformation of carbonate sediments enriched with biogenic silica. The observations concerning transformation of the latter into zeolite are scarce and mostly based on Cretaceous pelagic, hemipelagic and shallow marine deposits, and post-Cretaceous sediments in which there was no volcanic material. The mechanism of diagenetic clinoptilolite formation in volcanic-free sediments, as well as the role of the depositional setting in its precipitation, is not well understood. In carbonates of the NW part of the European Basin (west side of the Łysogóry-Dobrogea Archipelago) studied here, the clinoptilolite is associated with opal-CT (from the dissolution of sponge spicules) and a detrital clays are documented for the first time in a Cretaceous coastal environment. The carbonates were deposited in a coastal environment which was under the influence of weak currents delivering sponge spicules (biogenic opal) from the offshore zone and the terrestrial input was transported by rivers from a neighboring island. The lush plant vegetation, with numerous pteridophytes and conifers, within a warm (sub-tropical or tropical) and humid climate enhanced the terrestrial weathering of clays and feldspars. This unique combination of elevated silica concentrations (from sponge spicules dissolution) and aluminum availability (from terrestrial weathering of feldspars in a subtropical climate) during the early diagenesis combined with alkalinity triggered by organic matter decomposition, enhanced the clinoptilolite formation a few centimeters below the seabed. The investigation demonstrates a novel route by which pore-fluids enriched with silica and aluminum initiate clinoptilolite formation during the very early stage of diagenesis, a few centimeters below the seafloor. The clinoptilolite distribution in Late Cretaceous silica rich deposits (with no volcanic input) of the European Basin is always associated with opal-CT, from sponge spicules and elevated terrestrial input (clays). Taking into account the distinctive Campanian-Maastrichtian sedimentation in the epicontinental European Basin, composed almost exclusively of pure pelagic carbonates, the depositional settings in which the silica and aluminum were available in pore waters was restricted to the part of the basin influenced by terrestrial input. This leads to the conclusion that clinoptilolite formation in volcanic-free sediment is an indicator of not only enhanced sponge silica availability but also the close proximity of the source area.



Palaeoceanographic reconstruction of surface-ocean changes in the southern Norwegian Sea for the last ~130,000 years based on diatoms and with comparison to foraminiferal records

Publication date: 15 June 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 524

Author(s): Ulrike Hoff, Tine L. Rasmussen, Hanno Meyer, Nalân Koç, Jesper Hansen

Abstract

Fossil marine diatom assemblages in a sediment core from the central northern Faroe slope in the Norwegian Sea were used to reconstruct palaeoceanographic changes in the surface water mixed layer from the last ~130,000 years (Marine Isotope Stage (MIS) 6/5 transition to MIS 1 (including the Eemian and Holocene interglacials) and to compare the results with previously published results on planktic foraminifera representing the subsurface conditions of the thermocline. Diatom floras from MIS 5 of the Nordic seas have rarely been studied in detail before and never the entire period from pre-Eemian to present. The composition of diatom species together with maxima in absolute abundance of diatoms, indicate two periods of warmer sea surface temperatures correlating with the Eemian and Holocene interglacials, respectively. The Eemian differs from the Holocene in that the Iceland-Faroe Front never developed, suggesting the cold East Icelandic Current (originating from the East Greenland Current) running north of Iceland was reduced or more mixed with Atlantic water than during the Holocene and that the surface temperature and salinity gradients were weaker. The northern Faroe slope was in the early Weichselian of MIS 5d–5a mainly influenced by weaker inflow of Atlantic water and stronger influence of the East Icelandic Current and by seasonal sea-ice cover. During the later part of the Weichselian (MIS 4–MIS 2) cold conditions prevailed with extensive sea-ice cover except during the warmer interstadials. The diatom floras were more sensitive to climate changes than the planktic foraminifera and indicate longer periods with warm surface conditions, and increased influence of the warm Atlantic surface water from the Faroe Current in MIS 5, suggesting a shallower and much steeper thermocline during the transitional periods compared to today. The diatoms are good indicators for Atlantic water inflow and possible convection in the Nordic seas, not merely reflecting orbital variations in insolation as previously suggested.



Conodont biostratigraphy and magnetic susceptibility of Upper Devonian Chattanooga Shale, eastern United States: Evidence for episodic deposition and disconformities

Publication date: 15 June 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 524

Author(s): D. Jeffrey Over, Emily Hauf, Jenelle Wallace, Josephine Chiarello, Jin-Si Over, Geoffrey J. Gilleaudeau, Yi Song, Thomas J. Algeo

Abstract

Recognition of stratigraphic hiatuses in fine-grained siliciclastic sedimentary rocks can be challenging but is feasible using high-resolution biostratigraphic and chemostratigraphic data within a regional correlation framework. In this case study of the Upper Devonian Chattanooga Shale in the Dupont GHS drillcore from the western margin of the Nashville Dome, the Upper Devonian Chattanooga Shale comprises several depositional units separated by intraformational hiatuses. These features are developed within a 13.3 m interval consisting of 4.2 m of Frasnian strata and 9.1 m of Famennian strata that unconformably overlie the Sellersburg Formation. Three Frasnian, seven Famennian, and one Tournaisian conodont biozones are recognized. The Frasnian-Famennian boundary is on a disconformity. To the northeast in the southern Illinois Basin, the New Albany Shale in the BCC drillcore consists of 35 m of Givetian and Frasnian strata and 30 m of Famennian strata that conformably overlie the Sellersburg Formation. One Givetian, four Frasnian, and three of the four lowest Famennian conodont zones are recognized. The Frasnian-Famennian boundary is conformable and constrained to within a 5 cm interval. Bulk magnetic susceptibility (MS) shows a long-term increase through most of the Frasnian. Shorter-term MS trends were observed in association with depositional pulses linked to global sea-level rises and highstand system tracts, characterized by total organic carbon (TOC) maxima — eight trends were resolved in the DGHS core, and six in the BCC. The high-frequency shifts in δMS likely represent Milankovitch-band sea-level and depositional cycles at a scale that cannot be resolved based on the condensed and irregular nature of the depositional packages. Gamma ray counts in the DGHS peak in association with the eight depositional cycles, with a TOC peak at the base of each cycle.



Terrestrial and marginal-marine record of the mid-Cretaceous Oceanic Anoxic Event 2 (OAE 2): High-resolution framework, carbon isotopes, CO2 and sea-level change

Publication date: 15 June 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 524

Author(s): Jiří Laurin, Richard S. Barclay, Bradley B. Sageman, Robin R. Dawson, Mark Pagani, Mark Schmitz, Jeffrey Eaton, Francesca A. McInerney, Jennifer C. McElwain

Abstract

Oceanic Anoxic Event 2 (OAE2; c. 94.5–93.9 Ma) offers insight into the mechanisms of past climate change linked to organic productivity and carbon sequestration. It has been studied extensively, but the vast majority of data come from marine records, thus providing an incomplete view of past climate dynamics. Here we integrate new high-resolution data and published records on depositional environments, the carbon-isotope composition of bulk organic carbon (δ13Corg) and plant cuticles (δ13Ccut), and stomatal-index values, a proxy for pCO2, in well-preserved terrestrial through marginal-marine archives of the initial phase of OAE2. The study area is located on the western margin of the Western Interior Seaway (southwestern Utah). Age constraints are based on a new U-Pb bentonite age and correlation to an orbitally calibrated interval of the Bridge Creek Limestone. n-Alkane abundance suggests predominance of terrestrial contributions to bulk organic carbon for most samples. Despite similarities between carbon-isotope variations and transgressive-regressive shoreline movements, it is argued that δ13Corg and δ13Ccut are not strongly affected by local variables. A series of negative, ~2‰ carbon-isotope excursions is identified and attributed to changes in the size and isotopic value of the atmospheric CO2 reservoir. The temporal spacing of these anomalies (80–120 kyr) is consistent with changes in insolation modulated by orbital eccentricity. A systematic, phase-shifted relationship between the negative carbon-isotope excursions and transgressive increments further suggests a link between carbon-cycle perturbations and meter-scale sea-level change on the 100-kyr time scale. A conceptual model involving insolation-controlled aquifer charge/discharge and biomass burial/degradation in the monsoonal belt is proposed. The framework presented here is available to facilitate further research on the interplay of terrestrial and oceanic carbon reservoirs during OAE2.



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