Τετάρτη, 15 Μαΐου 2019

Palaeogeography, Palaeoclimatology, Palaeoecology

Are light-dark coupled laminae in lacustrine shale seasonally controlled? A case study using astronomical tuning from 42.2 to 45.4 Ma in the Dongying Depression, Bohai Bay Basin, eastern China

Publication date: 15 August 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528

Author(s): Ke Zhao, Xuebin Du, Yongchao Lu, Shipeng Xiong, Yong Wang

Abstract

As a typical sedimentary structure in fine-grained rocks, laminae are widely distributed in shales and mudstones in sedimentary basins. The Shahejie Formation (42.2–45.4 Ma) formed during the Eocene of the Paleogene in the Dongying Depression of China is recognized as a typical area to study lacustrine shale. According to the composition of different lamina, four types of couplets are identified, including carbonate-clay couplets, carbonate-organic couplets, clay-organic couplets and carbonate-clay-organic triplets. All couplets are composed of light and dark layers. A combination of core images, microscopic observations, mineral compositions, geochemical data, carbon and oxygen isotopes, and strontium isotopes verifies that the laminae are primarily developed in a saline and anoxic, or even euxinic environment, with a high organic matter (OM) flux. Through astronomical cycle analysis of the natural gamma ray (GR) curve from sample site well NY1, the data suggest that the lacustrine shale laminae are formed annually with an average duration of 1.34 yr. The formation of laminae is affected by the season, which is closely related to the seasonal growth and death of algae. During the period of algae growth, a large number of light-colored carbonate laminae are deposited, whereas when the algae dies, organic matter accumulates at the bottom of lakes and forms dark organic-rich laminae. The interbedded and lenticular laminae, which are subdivided by layering characteristics, are primarily affected by diagenesis in postdepositional processes. Study of the laminae is helpful in understanding the formation process of lacustrine shale, and it provides invaluable sources of information for paleoclimate reconstruction.



Ecophenotypic shape variability within Astarte (Class: Bivalvia) from the Pliocene of the Atlantic Coastal Plain, U.S.A.: A study using geometric morphometrics

Publication date: 15 August 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528

Author(s): R. Philip Roberson, Michelle M. Casey

Abstract

The Atlantic Coastal Plain has long been recognized as a natural laboratory useful for testing hypotheses surrounding the environmental and ecological effects on marine fauna. To conduct these studies in a rigorous manner, a reliable taxonomy must be established for the organisms within this physiographic province. In this study, we focus on the bivalve genus Astarte, which is commonly found in this region, and has many formally recognized species. We test between two competing hypotheses for the unusually high species richness of this genus: 1) taxonomy – the presence of discrete morphotypes represent valid species within this genus; and, 2) ecology – the many recognized 'species' in this genus in fact represent ecophenotypic variants of a smaller number of species, and thus richness is inflated. We test between these two hypotheses using a geometric morphometric analysis of landmarks placed on 646 shells, representing six different taxa from the Pliocene of the Atlantic Coastal Plain. We performed Procrustes transformation and Principle Components Analysis (PCA) on landmark data, allometric residuals, and outline harmonics to fully understand the variability of morphologies present. All PCA results show large amounts of overlap between all species. It is likely some of these species are valid taxonomic groups within the genus but should be classified as sub-species instead of separate species. These results provide strong support for our 'ecology' hypothesis, and suggests the genus Astarte needs revision. Future studies incorporating detailed sedimentological characteristics or ecological information (e.g., drilling frequency) will further elucidate palaeoecological and palaeoenvironmental drivers of ecophenotypic variation present in this genus. Further, the accurate identification of fossil species has important implications for our ability to reconstruct biodiversity trends in relation to the emergence of the Central American Isthmus and subsequent changes in ocean circulation.



Anthropogenic ocean warming and acidification recorded by Sr/Ca, Li/Mg, δ11B and B/Ca in Porites coral from the Kimberley region of northwestern Australia

Publication date: 15 August 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528

Author(s): Xuefei Chen, Juan Pablo D'Olivo, Gangjian Wei, Malcolm McCulloch

Abstract

The impact of climate changes on corals living in naturally extreme environments is poorly understood but crucial to longer-term sustainability of coral reefs. Here we report century-long temperature (Sr/Ca and Li/Mg) and calcifying fluid (CF) carbonate chemistry (δ11B and B/Ca) records for a long-lived (1919 to 2016) Porites coral from the high thermally variable Kimberley region of northwestern Australia. We investigate how increasing temperatures and ocean acidification are manifested in the carbonate chemistry of coral's CF and impacts of climate change on calcification. Using Sr/Ca and Li/Mg multiproxy we show that annual temperature in the nearshore Kimberley exhibited a gradual increase (0.009 ± 0.003 °C/yr) from the 1920s onward. However for the most recent years (2000–2015) more rapid summer warming (0.05 ± 0.01 °C/yr) are registered, indicative of intensified warming. Despite that, we find no significant trend for calcification rate of this coral over the past century, as well as 'normal' seasonal variability in coral's CF carbonate chemistry. Importantly, the coral's ability to concentrate inorganic carbon seems to be affected by recent warming, with reduced DICcf observed during 2008 to 2015, while the minimally-affected pHcf acts to compensate the decreases of DICcf with the calcification rate showing only slight decrease. Additionally, we also find that ocean acidification has clearly led to the long-term reduction in the pH of the CF.



The mid-Homerian (Silurian) biotic crisis in offshore settings of the Prague Synform, Czech Republic: Integration of the graptolite fossil record with conodonts, shelly fauna and carbon isotope data

Publication date: 15 August 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528

Author(s): Štěpán Manda, Petr Štorch, Jiří Frýda, Ladislav Slavík, Zuzana Tasáryová

Abstract

The middle Homerian biotic crisis resulted in the almost complete eradication of graptolites. The shale-dominated Kosov Quarry section, central Bohemia, preserves the most complete graptolite record across the crisis in peri-Gondwanan Europe. The pre-extinction graptolite assemblage of the upper lundgreni Biozone, composed of ten species vanished in three extinction phases recognized in an interval 1.6 m thick. The crisis commenced with the increasing dominance of generalist taxa and subsequent extinction of several abundant species including Cyrtograptus lundgreni. The second phase coincided with the extinctions of genera Cyrtograptus and Testograptus, whereas long-ranging, generalist monograptids prevailed before they became extinct as the crisis culminated in its third phase in the flemingii Biozone. The lower part of the overlying parvus Biozone contains only Pristiograptus parvus, which became abundant in the upper part of the biozone, together with incoming Gothograptus nassa. The recovery interval is marked by a moderate diversification of monograptids and retiolitids. The extinction did not affect the diversity of pelagic cephalopods although their abundance was reduced. A bivalve-dominated benthic fauna disappeared throughout the extinction interval and re-appeared not earlier than in the latest Homerian. It was temporarily replaced by a time-specific fauna of anachronistic trilobites and brachiopods. The extinction interval coincided with sea-level fall, indicated by limestone slump-beds in a generally shaly succession. The post-extinction interval corresponds with a lowstand systems tract with deposition of condensed shelly limestone and burrowed shale. A positive carbon isotope excursion started in the flemingii Biozone and δ13C values increased up to the lower parvus Biozone above which the plateau of the first peak started. The beginning of the graptolite extinction predated the early phase of the late Homerian carbon isotope excursion. The terminal phase of the extinction, nevertheless coincided with the onset of the carbon isotope excursion and change in benthic fauna.



An Oligocene microthermal forest dominated by Nothofagus in Sierra Baguales, Chilean Patagonia: Response to global cooling and tectonic events

Publication date: 15 August 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528

Author(s): Néstor M. Gutiérrez, Juan Pablo Pino, Jacobus P. Le Roux, Viviana Pedroza, José Luis Oyarzun, Luis Felipe Hinojosa

Abstract

A large fossil leaf assemblage (>3700 specimens) is reported from the Oligocene Río Leona Formation in the Sierra Baguales of Chilean Patagonia. The association comprises 29 species but is dominated by Nothofagusgenera, which constitutes 65% of specimens. The collection can be classified as a Mixed Palaeoflora of the Austral-Antarctic association. Stratigraphic analysis of the assemblage indicates a decrease in species diversity and richness over time, which was accompanied by species turnover. Quantitative studies of foliar morphology (CLAMP, Leaf Margin Analysis) indicates cool-to-cold and dry climatic conditions, with a Mean Annual Temperature of 9.2 °C, a relatively high seasonality in temperature and precipitation, and a Mean Annual Precipitation of 931 mm. Nothofagus only became dominant in southeastern Patagonia during the Rupelian, coinciding with a marked global cooling period linked to the initiation of glaciation in Antarctica about 34 Ma. The decrease in precipitation following this event is attributed to the development of a rain shadow to the east of the rising Southern Patagonian Andes, which must have been of the order of 1000 m or more for topographic climate forcing to take effect. This contrasts with the rain shadow development east of the Andes at lower latitudes, which was mainly manifested after the middle Miocene.



Reconstructing a high paleolatitude Mesozoic paleoenvironment from a truncated and deeply buried regolith, Norwegian North Sea

Publication date: 15 August 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 528

Author(s): Lars Riber, Steven G. Driese, Gary E. Stinchcomb, Henning Dypvik, Ronald Sørlie

Abstract

The paleoregolith in this study was unconformably overlain by lower Cretaceous (Valanginian) marine deposits of the Åsgard Formation and buried below two kilometers of sediments, on the Utsira High, Norwegian North Sea. The upwards intensification of chemical alteration, manifested in the replacement of feldspars by clays, leaching of base cations and reduced mechanical rock strength, in addition to the presence of terrestrial biotitic features, suggested weathering during subaerial exposure of the Utsira High during the Mesozoic. The paleoregolith can be divided into three weathering facies: altered coherent rock, saprock and saprolite, whereas the absence of true soil horizons suggested erosional truncation of the regolith before burial. Possible remnants of the eroded soil were identified as pedogenic mud aggregates in the overlying Åsgard Formation, bearing similar compositional characteristics as the saprolite.

Clay mineralogy, bulk geochemical composition, micromorphology and depth of biotic features were used to reconstruct the paleoenvironmental conditions that prevailed during the formation of the regolith in the Mesozoic North Sea. Our results point to a regolith formation under periodically water-restricted conditions, promoting the co-existence of interstratified illite-smectite (beidellite) and kaolinite, deep presence of endomycorrhizae (the fungal extension of the root structure) and precipitation of iron oxide nodules. Application of recently developed geochemical climofunctions indicated that the regolith formed in a mesic (10–12 °C) and subhumid (1157–1406 mm/yr) environment, and under strongly acidic soil conditions (pH 5.2–5.6).



Soft-sediment deformation below mammoth tracks at White Sands National Monument (New Mexico) with implications for biomechanical inferences from tracks

Publication date: 1 August 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 527

Author(s): Matthew R. Bennett, David Bustos, Matteo Belvedere, Patrick Martinez, Sally C. Reynolds, Tommy Urban

Abstract

Implicit in any biomechanical analysis of tracks (footprints), whatever the animal, is the assumption that depth distribution within the track reflects the applied plantar pressure in some way. Here we describe sub-track deformation structures produced by Proboscidea (probably Mammuthus columbi) at White Sands National Monument (WHSA) in New Mexico. Patterns of sub-surface deformation are consistent with the plantar pressure data for modern Proboscidea, but do not reflect track morphology. Our work cautions against overinterpreting track topology of any large animal, including extinct animals such as sauropods, in terms of their biomechanics unless the subsurface stratigraphy and associated variation in shear strength is known.



Formation and evolution of the Ediacaran to Lower Cambrian black shales in the Yangtze Platform, South China

Publication date: 1 August 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 527

Author(s): Xinyan Fang, Liangliang Wu, Ansong Geng, Qian Deng

Abstract

Black shales in the Ediacaran Doushantuo Formation (predominantly from Member II, IV and equivalent strata) and the Lower Cambrian Niutitang Formation (or equivalents) occur widely in the Yangtze Platform, South China. These black shales could provide sufficient hydrocarbons for the petroleum system in this region. However, biomarker parameters have proven invalid in the assessment of petroleum resources because of the high thermal maturity. Therefore, it is necessary to study the characteristics and formation mechanisms of these two black shales. This study analyzed redox-sensitive elements, total organic carbon (TOC) contents, and carbon isotopic compositions of organic matter in four continuous sedimentary successions including the two investigated shales in the Yangtze Platform (namely Jiulongwan on the inner shelf, Songlin in an intra-shelf lagoon, Dongkanshang on the upper slope, and Fengtan in the basin). Combined with other previously reported five sections, the redox conditions of sedimentary waters and marine paleoproductivity of the Ediacaran to Lower Cambrian black shales in the Yangtze Platform were discussed and compared. The results showed that the redox conditions of the Early Cambrian in the study area were similar to those of the Ediacaran Doushantuo period, which were oxic-suboxic at surface water, and anoxic in the deep with the occurrence of euxinic conditions. However, the euxinic conditions in the deep waters were distributed more widely in the Early Cambrian than in the Ediacaran period. The source of organic matter in the Doushantuo Formation was dominated by algae in the shallow-water platform and the shelf-margin areas, while chemoautotrophic/methanotrophic biomass may also have contributed in the intra-shelf lagoon and deep-water basin areas. In contrast, the organic matter of the Niutitang Formation was mainly contributed by the chemoautotrophic/methanotrophic biomass, with a minor contribution by photosynthetic algae and cyanobacteria. During the Doushantuo period, the input of terrigenous clastic materials was low, resulting in the enrichment of organic matter and formation of organic-rich black shale in the Yangtze Platform over a long geological time period. Compared to the Doushantuo period, the shorter sedimentation time and the higher deposition rate during the Early Cambrian produced a thicker and higher-quality Niutitang Formation source rock widely distributed in the Yangtze Platform.



Evaluating the effects of morphology and orientation on feeding in atrypide brachiopods using 3-D printed models

Publication date: 1 August 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 527

Author(s): Rylan V. Dievert, Kristina M. Barclay, Darrin J. Molinaro, Lindsey R. Leighton

Abstract

The brachiopod order Atrypida was one of the most diverse and abundant clades of marine organisms across North America during the Silurian and Devonian. Atrypide brachiopods were active sessile suspension feeders that used their lophophores to capture food particles from the water. Within the subfamily Variatrypinae, there are two end-member morphotypes associated with either high or low energy environments. It has further been suggested shape played an important role in enhancing feeding efficiency. The first morphotype is shield-shaped, weakly convex, and associated with low-energy systems. The second morphotype is globose and associated with high-energy systems. Furthermore, both pedicle-attached and liberosessile life orientations have been suggested for atrypides, based on both fossil material and biomechanical experiments. Any change in orientation could radically affect feeding efficiency. Our study uses biomechanical experiments on morphologically accurate 3-D printed atrypide models to evaluate the effects of shape and orientation on feeding current strength in the shield-shaped and globose morphotypes. We used 3-D scanning and printing to construct two morphologically accurate gaping models of the end-member shapes. The models included simulated body tissue, a flexible scale lophophore and valves of accurate thickness. A recirculating flume tank and dye streams were used to visualize ambient flow within the models in multiple pedicle-attached and pedicle-absent orientations to flow. During the trials the shield shape produced better ambient flow than the globose shape in pedicle-attached orientations. Further, pedicle-attached orientations produced better ambient flow than pedicle-absent orientations. Our results suggest that, shield-shaped atrypides benefited from enhanced ambient flow, while globose atrypides would have relied more on active pumping when in a pedicle attached orientation. As well, the results suggest a pedicle was required to orient atrypides for efficient feeding.



Pleistocene climatic instability drove the historical distribution of forest islands in the northeastern Brazilian Atlantic Forest

Publication date: 1 August 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 527

Author(s): Mario Henrique Barros Silveira, Rilquer Mascarenhas, Domingos Cardoso, Henrique Batalha-Filho

Abstract

To investigate the past spatial dynamics of Atlantic Forest (AF) 'island-like' enclaves in northeastern Brazil, we employed ecological niche modeling (ENM) using a set of georeferenced records of 13 woody plant species. ENMs were constructed using the Ensemble forecasting approach and were projected into past climatic conditions for the last interglacial period (LIG, 120 kyr), last glacial maximum (LGM, 21 kyr), and the Middle Holocene (MH, 6 kyr). Our results suggest an expansion of wetter forests during the LGM into areas currently covered by the Caatinga seasonally dry woodlands, with recent retraction to the current distribution. Central AF islands located south of the São Francisco River underwent a different history compared to Northern AF islands north of this river: the former were mostly connected to coastal AF since the LIG (with a very recent separation), whereas the latter presented a more dynamic historical distribution. Results reveal contrasting spatiotemporal histories of forest instability and isolation across the various enclaves, supporting three main biogeographic hypotheses: i) moderate connectivity with coastal AF and a recent population bottleneck in the Araripe and Pernambuco/Paraíba enclaves, ii) low connectivity to, and long-term isolation from, other enclaves, as well as recent population bottleneck, in the northernmost North Ceará enclaves; and iii) high connectivity with coastal AF and recent population expansion in the southernmost Chapada Diamantina enclaves. Future comparative phylogeography studies will largely aid in assessing the herein proposed biogeographic scenarios during the highly dynamic recent history of the AF enclaves.



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