Ageli, Mariam K.Hamilton, Paul B.Bramburger, Andrew J.Weidman, R. Paul...
11页
查看更多>>摘要：Many large, ancient lakes support primarily planktonic-driven food webs. In contrast, the lowest trophic levels of the Malili Lakes of Sulawesi Island, Indonesia are dominated by benthic diatom communities composed mainly of endemic species. Centric diatoms are not observed in the current diatom assemblage and pennate pelagic species are rare. Using two deep drill-cores from Lake Towuti, we investigated whether the lake has always been dominated by benthic primary production. Despite the ultra-oligotrophic conditions observed in the lake today, we observed state changes characterized by productive planktonic communities and less productive, benthic-dominated diatom assemblages. Two periods of planktonic production, each lasting approximately 50 kyr, were dominated by a complex mixture of Aulacoseira spp., with valve densities > 1.0 x 10(9) valves/g with a maximum biovolume of 3.5 x 10(12) mu m(3)/g. A similar planktonic assemblage was observed at much lower abundances (< 107valves/g) in the deeper sediments (> 1 Ma), during the earliest stages of lake formation. Two periods of increased benthic primary production of approximately ~11 kyr in duration, originating from the littoral zone of the lake, were also observed, one between the two planktonic phases and one above the last planktonic maxima. The benthic periods were dominated by Cymbopleura spp., with valve densities of approximately 2.0 x 10(8) valves/g and a maximum biovolume of 1.2 x 10(11) mu m(3)/g. Nonmetric multidimensional scaling analysis of sediment chemistry revealed that the benthic and pelagic states clustered with distinct chemical environments. The benthic phases were associated with well-mixed conditions in the lake whereas the planktonic phases were related to rapidly reoccurring water column mixing followed by intense stratification that generate nutrient recycling events. We conclude that lake mixing and nutrient cycling regimes regulated, in part, the switch between benthic and pelagic states in Lake Towuti.
Kim, SunghanLee, Min KyungShin, Ji YoungYoo, Kyu-Cheul...
10页
查看更多>>摘要：Magnetic susceptibility (MS) is routinely used as a proxy for terrigenous material input to Antarctic continental margin sediments. Variations in terrigenous input on glacial-interglacial timescales in this setting are related to a range of environmental changes; including to the cryosphere, atmosphere, and hydrosphere. However, the exact environmental factors and depositional processes that control MS values in sedimentary cores from the Antarctic continental margin remains unresolved. Here, we explore in detail sedimentary physical characteristics and MS values of core BS17-GC01 collected from Bellingshausen Sea continental rise, to understand how depositional process have varied during the late Quaternary. We found that input of terrigenous material increased markedly during Marine Isotope Stages (MIS) 2 and 4, compared to deposition during the adjacent interstadial and Holocene times. Variation in the bulk MS values were associated with changes in grain size. Size specific mass normalized MS measurements show the fine fraction (<16 mu m) has the highest MS values, followed by the coarse silt fraction (16-63 mu m). However, because coarse silt is the predominant grain size, the increased bulk MS during MIS 2 and 4 are mainly driven by changes in this fraction. We propose that the coarse silt with high bulk MS values observed in glacial sediments were transported to the site as iceberg rafted debris, originating from the Bellingshausen Sea shelf region. In contrast, grains >1 mm - a commonly applied proxy for glacial transport in this setting - were observed during MIS 1 and 3 only. We conclude these contrasting signatures of iceberg rafted debris in this Bellingshausen Sea core arise from changes in the to the erosive dynamics of the adjacent ice shelf and continental shelf on glacial/interglacial timescales.
查看更多>>摘要：Tropical peatlands are an important carbon reservoir; however, they are vulnerable to climate change, fire events, and human disturbances, and may become a significant carbon source if degraded. In this paper, we investigated the late Holocene record of vegetation change, peatland evolution, and C accumulation based on analysis of a sediment core through a Mauritia-flexuosa palm dominated swamp (locally called "aguajal ") in the Madre de Dios region in southern Peru to better understand tropical peatland dynamics. Sedimentation commenced within shallow water, ponded on an impervious substrate, perhaps within an abandoned river channel or oxbow lake (1380-820 cal yr BP) located adjacent to a steep escarpment with terra firme (upland) rainforest above and lateral to floodplain forests. This was followed by the development of marsh (820-640 cal yr BP). A closed-canopy Mauritia flexuosa-dominated palm swamp was subsequently established (640300calyr BP) which later changed into an open canopy and mixed Mauritia-Hedyosmum palm swamp (300 cal yr BP to present). Two major changes in peat and C accumulation rates were observed: (1) onset of peat and C accumulation at 820 cal yr BP and (2) decrease of peat and C accumulation after 520 cal yr BP. Comparisons of our results with palaeoecological records from aguajales in northern Peru suggest that the dynamics were different in this region. The peat and C accumulation rates in our study core in southern Peru were ca. 4 mm yr(-1) and ca. 200 g m(-2) yr(-1) faster, similar to a few sites in northern Peru. Aguajales in Madre de Dios region in southern Peru are an important carbon sink. Analyzing the dynamics of aguajales which are a kind of typical peatlands in Amazonia and comparing among different sites allows exploration and prediction of regional peatlands' development under the potential future impact of natural changes.
查看更多>>摘要：Paleoenvironmental reconstruction of the Mediterranean Basin at the end of the Messinian Salinity Crisis is contentious. One section that records this final phase (Stage 3) is the Pollenzo Section in the Piedmont Basin (NW Italy). Here, we present new stratigraphic, sedimentological, petrographic, micropaleontological (ostracods, calcareous nannofossils, foraminifera, dinoflagellates) and geochemical (Sr-87/Sr-86 ratios) data from the Cassano Spinola Conglomerates (CSC) and interpret the paleoenvironment of this northernmost tip of the Mediterranean Basin. The CSC comprise three depositional units: members A and C, which were deposited subaqueously, and the intervening member B, which is continental. The CSC is topped by a ~ 50 cm-thick black layer, which is directly overlain by the open marine Argille Azzurre Formation of early Zanclean age. Our investigation reveals that member A is largely barren of autochtonous microfossils, except for an almost monospecific ostracod assemblage of Cyprideis torosa at the top, which indicates shallow-water (< 30 m) conditions. Paratethyan ostracods and, possibly, taxa of calcareous nannofossils adapted to low-salinity water first occur in member C. Sr-87/Sr-86 ratios measured on ostracod valves from the member A/B transition (0.708871-0.708870) and member C (0.708834-0.708746) are lower than the coeval Messinian seawater values (~0.709024) and the Sr-87/Sr-86 ratios of a hypothetical lake filling Piedmont (> 0.7090) estimated by means of the present-day Sr-87/Sr-86 signature of the Po river, the main drainage system of Northern Italy that receives the weathering products (including ions) of the Alps and Apennines. These values are likely to reflect the mixing of local high Sr-87/Sr-86 river water with low Sr-87/Sr-86 from the Mediterranean, which at the time was dominated by inputs from Eastern Paratethys, circum-Mediterranean rivers and Atlantic Ocean. Our results suggest that, at times during the final stage of the Messinian Salinity Crisis, the Piedmont Basin was hydrologically connected with the main Medi-terranean Basin. At regional scale, this implies that the water level in the Mediterranean Basin was relatively high.
查看更多>>摘要：The Permian-Triassic Mass Extinction (PTME) is the greatest biodiversity crisis in Earth history and while the marine crisis is increasingly well constrained, the timing and cause(s) of terrestrial losses remain poorly understood. There have been suggestions that the End-Permian Terrestrial Collapse (EPTC) pre-dated, was synchronous with or post-dated the marine crisis, or even occurred asynchronously in different regions. We address these conflicting interpretations through a detailed geochemical study of a terrestrial sequence in the Liujiang Coalfield on the North China Plate (NCP) in which we apply zircon U-Pb dating of tuffaceous claystone, kerogen identification, and analysis of organic carbon isotopic composition (delta C-13(org)), total organic carbon (TOC), continental weathering (via the chemical index of alteration; CIA) and Ni concentrations. Our study constrains the Permian-Triassic boundary (PTB) near the base of bed 20 in our sequence at approximately 251.9 +/- 1.1 Ma, immediately above a Ni anomaly also known from other terrestrial sequences and the marine PTME. Organic carbon isotope chemostratigraphy together with evidence for algal blooms and the presence of mudstone clasts suggests that the onset of the EPTC in the NCP was synchronous with the crisis in low latitudes (e.g., South China), but was about 310 kyr later than the EPTC in higher southerly latitudes (e.g., Australia). The EPTC predates the marine PTME. Kerogen macerals suggest that a phase of increased wildfire was sustained from the onset of the EPTC in the NCP until the marine PTME interval, implicating wildfire as a major driver of the EPTC (at least in low latitudes) that, in turn, had devastating consequences for the marine realm.
查看更多>>摘要：The climatic cooling that began in the late middle Eocene and culminated in the Eocene-Oligocene transition meant major changes in Palaeotheriidae (Perissodactyla, Mammalia) biodiversity in Europe and could have caused the appearance of new dietary strategies. This work is the first to study the spatiotemporal response of one palaeotheriid genus (Plagiolophus) to the Eocene environmental and ecological changes using three dietary proxies: hypsodonty, mesowear and dental microwear texture analysis (DMTA). DMTA is applied for the first time to palaeotheriids. The high diversity and wide chrono-spatial distribution of Plagiolophus make it possible to evaluate spatiotemporal environmental variations, including palaeodiet. We study five Plagiolophus samples from late middle Eocene to early Oligocene from western Iberia to central Europe in order to (1) infer lifetime palaeodiet; (2) infer short-term palaeodiet; and (3) test temporal and spatial trends. All samples present an exclusion of abrasive foodstuff given low hypsodonty index; dominance of browsing given the low mesowear score; and browsing of tough foliage and exclusion of hard items in their diet given DMTA information. These factors point to Plagiolophus as a highly selective feeder that fed on plants with the same features irrespective of chronology or location: tough foliage (leaves from monocots or dicots) avoiding lignified or hard materials, without seasonal variations.
Fielding, Christopher R.Frank, Tracy D.Savatic, KatarinaMays, Chris...
30页
查看更多>>摘要：The upper part of the upper Permian succession in the Bowen Basin of Queensland, NE Australia, was investigated to ascertain the timeline and character of environmental changes in this high southern palaeolatitudinal setting leading up to the End-Permian Extinction (EPE). The study focused on (in ascending order) the Peawaddy Formation, Black Alley Shale, and Bandanna Formation, and laterally correlative units. In the western Bowen Basin, the base of the Peawaddy Formation (257 Ma) records the onset of thrust loading and volcanic activity associated with the Hunter-Bowen contractional orogeny. The Peawaddy Formation comprises a series of coarsening-upward, terrigenous clastic intervals interpreted as the product of repeated progradation of deltas into shallow, open marine environments. The overlying Black Alley Shale also comprises multiple deltaic coarsening-upward cycles, which accumulated in stressed, restricted marine environments. The uppermost Bandanna Formation and equivalents formed in extensive coastal plain to estuarine environments. All three formations accumulated under the influence of explosive volcanic activity from the emerging continental volcanic arc to the east of the foreland basin. Volcanism peaked during deposition of the Black Alley Shale around the Wuchiapingian-Changhsingian transition. Abundant dispersed gravel and glendonites (calcite pseudomorphs after ikaite) indicate that the Peawaddy Formation formed under the influence of cold conditions and possible glacial ice (P4 Glaciation; Wuchiapingian Stage). Direct evidence of cold conditions ends at the top of the Peawaddy Formation (254.5 Ma); however, Chemical Index of Alteration (CIA) data suggest that surface conditions remained cold through the accumulation of the Black Alley Shale, and the lower Bandanna until c. 253 Ma, before gradually rising through the upper Bandanna Formation. The end of P4 glaciation is also characterized by a major spike in the abundance of marine acritarchs (Micrhystridium evansii Acme Zone), reflecting the development of a regional restricted basin of elevated nutrient concentrations but reduced salinity. In contrast to this short interval of stressed marine conditions, the fossil floras indicate remarkably consistent terrestrial ecosystems throughout the late Lopingian until the EPE. The terrestrial EPE is recorded by a distinctive, laminated mudrock bed ('Marker Mudstone') that records a palynological 'dead zone' above the uppermost coal seam or equivalent root-penetrated horizon followed by spikes in non-marine algal abundance. Overall, the time interval 257-252 Ma represented by the studied succession does not record a simple monotonic change in palaeoenvironmental conditions, but rather a series of intermittent stepwise changes towards warmer, and more environmentally stressed conditions leading up to the EPE in eastern Australia.
查看更多>>摘要：Varved clays from a Late Pleistocene proglacial lake in the Pas & x2c7;amine sections (Lithuania) contain diverse organic matter of different origin, i.e. aquatic and allogenous macerals of the groups of liptinite, vitrinite and inertinite (their average contributions are 55%, 13.5% and 31.5%, respectively). The contents of TOC and TN vary from 0.51% to 1.01% and from 0.06% to 0.13%, respectively. Particulate organic matter could constitute the beginning of food chain for invertebrates represented by the trace fossils Cochlichnus anguineus, Glaciichnium liebegastensis, Gordia carickensis, Gordia isp., Helminthoidichnites isp., and Warvichnium ulbrichi. The accumulation of organic matter was higher during summers, as suggested by co-occurrence of TOC and quartz supplied to the lake mostly during ice melting. River inflow during humid seasons fed the lake and facilitated proliferation of algae, whereas, aeolian transportation of mineral (e.g., fine-grained quartz) and organic detritus (semifusinite) can be related to dry periods and the wildfires in vegetated areas. In spite of common thesis that Quaternary proglacial lakes were biological deserts, the Late Pleistocene proglacial lake is characterized by a food chain which starts at least from algae living in the lake and particulate organic matter transported from glaciated and non-glaciated, vegetated areas.
查看更多>>摘要：Middle Devonian (Eifelian/Givetian transition) brachiopod-hosted sclerobiont assemblage from Gondwana (Morocco, eastern Anti-Atlas, Mader Basin) has been studied for the first time. The analysis of hundreds of brachiopod shells revealed at least 26 sclerobiont taxa, making the studied palaeoecommunity one of the most diverse Middle Devonian sclerobiont assemblage reported to date. The palaeocommunity is dominated by encrusting organisms, in particular bryozoans, foraminifers, ascodictyids and microconchids, whereas bioeroders are represented by Clionolithes and Oichnus ichnospecies. As supported by microfacies, fossil preservation and the presence of encrusting Rothpletzella algae, the sclerobionts developed in a calm, euphotic, offshore marine environment situated below normal wave base. Prevalence of articulated shells without signs of either extensive mineral staining, or taphonomic alteration resulting from hydrodynamic processes, as well as preservation of some delicate encrusters (Vinella, Rothpletzella), indicate that the palaeocommunity did not undergo extensive time-averaging. The hosts were colonized syn vivo, as evidenced by the epibiont growth near the commissure margin, the presence of shell malformations and even embedded auloporid corals and cornulitid tubeworms. Competition for space among sclerobionts was apparently limited, as suggested by the very low number of overgrowth interactions, indicating that the shelly substrate available for colonization was abundant. The presence of the same dominant groups of sclerobionts on different host taxa indicates a lack of species-selectivity by these colonizing sclerobionts. It also suggests that the sclerobiont species pool in the environment was generally similar throughout the deposition of the sampled rock interval. As testified by the host brachiopod and bryozoan species, the sclerobiont assemblage from this locality of northern Gondwana had strong affinities with those from the southern margin of Laurussia, most likely reflecting strong connectivity between these regions provided by the South Equatorial - North Gondwana current system during the late Eifelian Kacak transgressive episode.
查看更多>>摘要：Middle Miocene (-18-12 Ma) mammalian faunas of the North American Great Plains contained a higher diversity of presumed browsers than any modern biome. This has been attributed to greater primary productivity, which commonly corresponds with densely vegetated, forested habitats today. However, several lines of evidence suggest that open biomes, which support low browser diversity today, dominated the middle Miocene. To gain a better understanding of middle Miocene habitats on the northern Great Plains, we generated a large dataset of stable carbon isotope values (delta C-13) from a wide variety of fossil ungulates. Tooth enamel was sampled from four local faunas in Nebraska of late Barstovian age (14.8-12.5 Ma). Paleoenvironmental interpretations were made using a predictive model based on delta C-13 values in vegetation and mammalian herbivores in modern biomes. Resulting mean delta C-13 fossil values for local faunas plot in the upper range expected for C-3 vegetation, indicating open biomes. Most mean values for genera plot in the range expected for open canopy habitats with only two genera in the closed canopy range. Of all the taxa, only the grazing horse, Neohipparion, plots in the range for likely C-4 consumption. Neohipparion has the highest mean delta C-13 value of any taxon, with one individual plotting in the range for unequivocal C-4 consumption and others in the mixed C-4/C-3 or C-3 water-stressed range. Neohipparion appears to be the earliest known taxon that included C-4 vegetation in its dietary niche. The range in carbon isotope values suggests browsers, mixed-feeders, and grazers all consumed vegetation in mostly open areas. Our model implies a fairly dry, open biome with limited riparian habitat. This interpretation is generally consistent with previous interpretations of an open savanna-like biome. A high diversity of middle Miocene browsers and other taxa appear to have lived in an open biome with no modern analog.
NSTL
Elsevier