Σάββατο, 16 Μαρτίου 2019

Palaeogeography, Palaeoclimatology, Palaeoecology

Aberrations in the infrabasal circlet of the cladid crinoid genus Cupulocrinus (Echinodermata) and implications for the origin of flexible crinoids

Publication date: 15 May 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 522

Author(s): M.E. Peter

Abstract

Flexible crinoids (subclass Flexibilia) likely originated from the cladid crinoid genus Cupulocrinus or one of its immediate ancestors in the Middle to Late Ordovician. A remarkably constant and clade-defining character of flexible crinoids is the possession of three plates in the infrabasal circlet of the calyx. This character was a significant morphological modification for the origin of the flexible clade, resulting from the reduction of the number of infrabasal plates from five plates in the ancestral cupulocrinid.

Of 615 museum specimens of Cupulocrinus sp. for which the number of infrabasal plates could be determined, 21 specimens, or 3.4%, displayed a deviation from the normal five infrabasals. Of the aberrant specimens, fifteen have four infrabasal plates, and six have six infrabasal plates. An additional aberrant specimen has five infrabasals, with one significantly reduced in size. Although the number of infrabasals is typically thought to be constant within a species, this trait appears to have been variable in the immediate ancestor of flexible crinoids, in the time period just before the number of infrabasals became fixed at three for the Flexibilia. This paper documents the range of aberrations within the infrabasal circlet of Cupulocrinus and considers the implications for the origin of the flexible crinoids.

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Sedimentary pyrite framboid size-frequency distributions: A meta-analysis

Publication date: 15 May 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 522

Author(s): David Rickard

Abstract

Framboids, microscopic sub-spheroidal aggregates of pyrite microcrystals, are found in sediments of all ages and framboid size-frequency distributions are widely used to determine the oxygenation states of paleo-waters. Sedimentary framboid populations display unimodal log-normal size distributions as a consequence of the multiplicative central limit theorem in probability theory. The application of additive statistics to framboid populations and their consequent characterization in terms of arithmetic means and standard deviations is wrong because it predicts a subset of framboids with negative diameters.

A meta–analysis of 377 sets of measurements of the diameters of 48,063 pyrite framboids from 104 sediment and sedimentary rock locations shows that the geometric mean diameter of sedimentary framboids is 6.2 μm and the geometric standard deviation is 1.5. Ninety-five percent of all sedimentary framboids have geometric mean diameters between 2.9 and 13.9 μm. The geometric mean diameter of modern syngenetic framboids formed within euxinic water columns is 4.7 μm and that of diagenetic framboids formed within sediments is 6.7 μm. The estimated measurement error is ±10%.

Framboid mean diameters can be used to help discriminate the oxygenation state of paleo-waters. A minimum number of 30 measurements is required and ideally ≥100 measurements are recommended. The conflicting evidence reported in the literature for the application of framboid size-frequency measurements results mainly from the intrinsic nature of statistical analyses: there is always a finite chance that a particular framboid size-frequency distribution is a result of either syngenetic or diagenetic processes. Including the systematic stereological error in framboid size measurements, the geometric mean size range for syngenetic framboids is 2.9–10.9 μm and that for diagenetic framboids 3.1–20.9 μm. This suggests a significant overlap in geometric mean diameters of framboids from euxinic and non-euxinic environments between ~3 and ~11 μm. Geometric mean framboid diameters within this size range are not robust proxies for paleo-water oxygenation conditions. The origin of the smaller sizes of syngenetic compared with diagenetic framboids appears to be related to the limited time available for framboid crystal growth within the water column. The statistical uncertainty in assigning particular framboid size-frequency distributions to defined oxygenation conditions results mainly from (a) some diagenetic framboids forming over a limited time, thereby also producing smaller framboids and (b) mixtures of syngenetic and diagenetic framboid populations through unavoidable time-averaging of samples. The absence of any significant differences in the sizes of modern and ancient framboids indicates that framboids do not continue to grow over extended time periods: once formed they remain the same over geologic time periods and only change through infilling, overgrowth and recrystallization. The result is that ancient framboids may sample the contemporary paleoenvironment in the sedimentary system in which they are found.



Combining machine learning algorithms and geometric morphometrics: A study of carnivore tooth marks

Publication date: 15 May 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 522

Author(s): Lloyd A. Courtenay, José Yravedra, Rosa Huguet, Julia Aramendi, Miguel Ángel Maté-González, Diego González-Aguilera, Mari Carmen Arriaza

Abstract

Since the 1980s an intense scientific debate has revolved around the hunting capacities of early hominin populations and the behavioral patterns of carnivores sharing the same ecosystem, and thus competing for the same resources. This debate, commonly known as the hunter-scavenger debate, fostered the emergence of a new research line into the Bone Surface Modifications (BSMs) produced by both taphonomic agents. Throughout the following 20 years, multiple studies concerning the action of carnivores have been developed, with a particular focus on the oldest archaeological sites in East Africa. Recent technological advances applied to taphonomy have provided new insight into carnivore BSMs. A newly developed part of this work relies on Geometric Morphometrics (GMM) studies aimed at discerning carnivore agency through the morphologic characterization of tooth scores and pits. GMM studies have produced promising results, however methodological limitations are still present. This paper presents the first combined application of Machine Learning (ML) algorithms and GMM to the analysis of carnivore tooth marks, generating classification rates of 100% between carnivore species in some cases.



A robust vegetation-based elevation transfer method for reconstructing Arctic polygon mire palaeo-microtopography

Publication date: 15 May 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 522

Author(s): Annette Teltewskoi, Dierk Michaelis, Lutz Schirrmeister, Hans Joosten, Ulf Schiefelbein, Michael Manthey

Abstract

The reconstruction of past environments by means of macrofossil and pollen analysis is commonly based on the modern ecological preferences of the taxa that may have produced these fossils. Here we present a modelling approach, in which we use modern vegetation–surface height relationships to quantify past surface heights in an Arctic ice-wedge polygon mire. Vegetation composition and ground surface height (GSH) were assessed in a polygon mire near Kytalyk (Northeastern Siberia). Cluster analysis revealed five plant communities, which are clearly separated with respect to ground surface height, frost surface height and coverages of open water and vegetation. Based on the composition of modern vegetation we constructed two sets of potential fossil types (plant macrofossils and pollen), an extensive one and a more restricted one to reflect different conditions of preservation and recognisability. We applied Canonical Correspondence Analysis to model the relationships between potential fossil types and measured GSH. Both models show a strong relationship between modelled and measured GSH values and a high accuracy in prediction. Finally, we used the models to predict GSH values for Holocene peat samples and found a fair correspondence with expert-based multi-proxy reconstruction of wetness conditions, even though only a minor part of the encountered fossils were represented in the GSH models, illustrating the robustness of the approach. Our approach can be used to reconstruct palaeoenvironmental conditions in a more objective way and can serve as a template for further palaeoecological studies.

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The evolution and control of detrital sediment provenance in the middle and northern Okinawa Trough since the last deglaciation: Evidence from Sr and Nd isotopes

Publication date: 15 May 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 522

Author(s): Fuqing Jiang, Zhifang Xiong, Martin Frank, Xuebo Yin, Anchun Li

Abstract

The Okinawa Trough (OT) is a large sink of sediments supplied by the East Asian continent. Identifying the provenance of the OT sediments is key to reconstructing the temporal and spatial variations of the terrigenous supply to this area and is important for understanding the impact of paleoclimatic and paleoceanographic variability on the sediment supply to this marginal sea over the late Quaternary. In this contribution, we show that radiogenic strontium (Sr) and neodymium (Nd) isotopes allow to efficiently distinguish Yellow and Yangtze/Taiwan River detrital sediments, and can be used to reconstruct distinct changes in the provenance of the detrital fraction of marine sediments from the middle and northern OT since the last deglaciation. The Sr and Nd isotope signatures are compared to those of the potential sediment sources, namely the Yellow and Yangtze Rivers, the Taiwan orogen, and volcanic material from the OT and nearby islands, and the relative contributions of these sources are reconstructed. The Sr and Nd isotope compositions of the detrital fraction in the two sediment cores recovered from the middle and northern OT show that the sediments mainly originated from the Yangtze River between 18 and 10.5 ka, which was caused by low sea level and a widely developed channel system on the continental shelf. During the period between 10.5 and 7.0 ka, the rising sea level resulted in elevated Yangtze and Yellow Rivers sediment input into the OT. Simultaneously, large-scale volcanic activity also contributed significant amounts of material to the OT. During the last 7.0 ka, besides important contributions from the Yellow River, the intensification of the Kuroshio Current resulted in increased delivery of sediment from Taiwan to the OT.



The Permian-Triassic transition in ocean island setting: Environmental disturbances and new high-resolution carbon-isotope record from the Qiangtang Basin, NW China

Publication date: 15 May 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 522

Author(s): Xiugen Fu, Jian Wang, Chunyan Song, Zhongwei Wang, Shengqiang Zeng, Dong Wang

Abstract

The Permian-Triassic boundary (PTB) events such as carbon-isotope excursion, volcanism, and environmental disturbances are not well understood in an ocean island setting. Here, we develop a new case study for the Permian-Triassic transition in the Qiangtang Basin, Tibet, by combining existing biostratigraphy, new carbon-isotope data, sedimentological data, UPb zircon age, mineralogical and geochemical data. These new data defined the Permian-Triassic transition age as about 252.3 ± 0.9 Ma, corresponding to the lowermost sea level. A long-term negative carbonate carbon isotope trend is present at the same stratigraphic level in different depositional environments, strongly suggesting its global nature. However, our new ocean island setting is characterized by an abrupt shift in carbon isotope values across the level, which is different from many chemostrtigraphic studies that show a gradual shift in carbon isotope values across the event horizon. The most likely explanation for the abrupt shift in δ13C values is a sedimentary hiatus at this level in the study section. Three-stage climatic models through the PTB in the ocean island setting are identified: Stage 1 is characterized by a warm and humid climate with moderate chemical weathering; while a hot and humid climate with intense chemical weathering dominates stage 2; the climate in stage 3 is a relatively hot and arid climatic condition with weak chemical weathering.



Phylogenetic community paleoecology of one of the earliest complex crinoid faunas (Brechin Lagerstätte, Ordovician)

Publication date: 1 May 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 521

Author(s): Selina R. Cole, David F. Wright, William I. Ausich

Abstract

Integrating phylogenetic biology with paleoecology can provide a valuable context for understanding patterns of community structure and niche partitioning in ancient ecosystems. However, the lack of robust phylogenies for many fossil taxa precludes studies of this nature, particularly among marine invertebrates. Fossil Crinoidea (Echinodermata) comprise an ideal model system for phylogenetic community paleoecology for three reasons: (1) they preserve anatomical features that directly relate to feeding ecology, (2) assemblages of well-preserved specimens represent "ecological snapshots" in time, and (3) recent advances in resolving the crinoid tree of life have produced high-resolution phylogenies for Ordovician lineages. Here, we apply multivariate and phylogenetic comparative methods to investigate patterns of paleocommunity structure, niche partitioning, and ecomorphospace occupation in one of the earliest known complex crinoid paleocommunities, the Brechin Lagerstätte (Upper Ordovician, Katian). Results indicate niche differences among species were determined primarily by characters related to filtration fan morphology. Filtration fan density and body size distributions support phylogenetic niche conservatism, but traits related to the size of the feeding area are more labile and exhibit greater divergence than expected among closely related species. Finally, we compare changes in the shape and phylogenetic structure of niche distributions between the Brechin Lagerstätte and the Edwardsville crinoid fauna, a well-studied Mississippian (Viséan) paleocommunity, to examine patterns of community change across the Early to Middle Paleozoic Crinoid Macroevolutionary Faunas.



Paleoenvironmental changes during the late Albian oceanic anoxic event 1d: An example from the Capacho Formation, southwestern Venezuela

Publication date: 1 May 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 521

Author(s): María-Emilia Rodríguez-Cuicas, Jean-Carlos Montero-Serrano, Grony Garbán

Abstract

The late Albian–early Cenomanian oceanic anoxic event 1d (OAE-1d; ~103 to 99.5 Ma) represents a global interval of enhanced organic carbon burial due to widespread oxygen deficiency in the water column and/or increased primary productivity. The biostratigraphy and geochemistry of organic-rich sediments in the La Grita Member (Capacho Formation) in southwestern Venezuela were studied to document the paleoenvironmental conditions that governed the deposition of this succession during OAE-1d. Carbon-isotope (δ13Ccarb and δ13CCorg) chemostratigraphy and biostratigraphic constraints show that the La Grita Member spans the late Aptian period (Rotalipora appenninica Zone), and that OAE-1d is well recorded in this succession. A prominent increase in total organic carbon (TOC) values (up to 10%) is clearly recorded through the onset of OAE-1d, coinciding with a prominent change in overall redox-sensitive proxies suggesting that the La Grita Member sediments accumulated under anoxic (and possibly euxinic) bottom-water conditions in a partially restricted basin. The detrital proxies suggest that the greenhouse climate prevailing during OAE-1d induced significant acceleration of the hydrologic cycle and an increase in continental chemical weathering rates. Overall, the carbon and oxygen isotope records, TOC contents, as well as the detrital proxies show cyclic variation during the late Albian stage. These variations support the hypothesis that orbital forcing likely also modulated the equatorial monsoonal activity during the OAE-1d.



Monsoon-influenced variations in plankton community structure and upper-water column stratification in the western Bay of Bengal during the past 80 ky

Publication date: 1 May 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 521

Author(s): Yuki Ota, Junichiro Kuroda, Asuka Yamaguchi, Atsushi Suzuki, Daisuke Araoka, Toyoho Ishimura, NGHP Expedition 02 JAMSTEC Science Team, Hodaka Kawahata

Abstract

To enhance our understanding of the forcing factors of marine biological community structure in the Bay of Bengal, we studied proxies for biogenic carbonate and silicate production in the western Bay of Bengal and compared them with proxies for upper-ocean stratification and strength of the Indian summer monsoon (ISM). Specifically, we investigated the record in a sediment core from the western Bay of Bengal that extended to approximately 80 ky before the present (BP). The records of Globigerinoides ruber sensu stricto δ18O minus Neogloboquadrina dutertrei δ18O (Δδ18Or–d) were used to investigate changes in upper-ocean stratification. We reconstructed variations in mass accumulation rates (MARs) of CaCO3 and biogenic silica (BSi) to discuss plankton community structure. Greater difference between Globigerinoides ruber sensu stricto δ18O and Neogloboquadrina dutertrei δ18O during the marine isotope stage (MIS) 1 and 5a indicate intervals with less saline and more stratified conditions. In contrast, the relatively small difference between both planktic foraminiferal δ18O during MIS 2 and 4 indicates diminished stratification during these periods. Elevated carbonate production during MIS 2 and 4, demonstrated by the increase in CaCO3 MAR, could have been driven by lower freshwater influx to the western Bay of Bengal resulting from lower ISM precipitation, and thus the increased mixing and upwelling of deep nitrate into the photic zone. The relative increase in BSi MAR during periods of high ISM precipitation and decreased salinity of surface waters suggests a limited recovery in biological siliceous production under interglacial conditions, possibly due to enhanced fluvial SiO4 delivery to the ocean. Fluctuations of Δδ18Or–d during MIS 3 indicate that freshwater stratification was moderate and insufficient to restrict upwelling. This might have induced moderate biogenic silica and carbonate productions during this period.



Recovery of benthic communities following the Toarcian oceanic anoxic event in the Cleveland Basin, UK

Publication date: 1 May 2019

Source: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 521

Author(s): Bryony A. Caswell, Stephanie J. Dawn

Abstract

During the Toarcian oceanic anoxic event (OAE) considerable environmental changes occurred that were associated with global warming, perturbations to the C-cycle and ocean deoxygenation which resulted in a mass extinction of marine fauna. Recovery of the biota after the event was protracted and has to date undergone limited study. However, understanding the patterns and processes of recovery are critical to anticipating ecosystem responses to the environmental changes predicted for the near future. Results showed that increases in benthic diversity, and the re-establishment of the Toarcian infauna was gradual and followed the changing redox conditions. Pioneering infauna, such as Dacryomya ovum that dominated the seafloor after the event in the Cleveland Basin, Yorkshire, UK, can modify the physico-chemical environment and thus facilitate ecological succession after disturbance. The length of D. ovum increased >8 mm throughout the bifrons Zone and these body-size changes were linked with total organic carbon (TOC) content suggesting a link to primary productivity, although only at intermediate levels of deoxygenation. Major changes in the phytoplankton, and so food supply, seem to have driven changes in bivalve body size, across trophic guilds, both during and after the event in Yorkshire, and on the mid to lower shelf in Spain and France, respectively. Primary productivity collapse seems then to have been a major driver of biotic change throughout the Toarcian event, as it was during the Permian–Triassic, Triassic/Jurassic and Cretaceous/Tertiary mass extinctions. Further investigation of both the palaeontological and geochemical changes that occurred within early successional Toarcian infaunal communities are required to more fully understand the pattern of recovery after the OAE.



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