9000 years of chironomid assemblage dynamics in an Alpine lake: long-term trends, sensitivity to disturbance, and resilience of the fauna

Subfossil chironomid analysis was applied to a sediment core from Sägistalsee, a small lake at present-day tree-line elevation in the Swiss Alps. During the whole 9000-year stratigraphy the chironomid fauna was dominated by taxa typical of alpine lakes. Major faunistic trends were caused by changes in accumulation rates of three taxa, namely Procladius, Stictochironomus, and Tanytarsus lugens-type. In the early Holocene Procladius was the dominant taxon. In younger samples, Stictochironomus tended to have as high or higher abundances and both taxa showed an increase in accumulation rates. A possible cause of this succession is the decrease of lake-water depth due to infilling of the lake basin and changes in associated limnological parameters. The immigration of Picea (spruce) at ca. 6500 cal. ¹⁴C yrs BP and the resulting denser woodlands in the lake's catchment may have promoted this trend. During three phases, from ca. 70–1450, 1900–2350, and 3500–3950 cal. BP, remains of Procladius, Stictochironomus, and Tanytarsus lugens-type are absent from the lake sediment, whereas other typical lake taxa and stream chironomids show no change in accumulation rate. Together with sediment chemistry data, this suggests that increased oxygen deficits in the lake's bottom water during these intervals caused the elimination of chironomids living in the deepest part of the lake. All three periods coincide with increased human activity in the catchment, as deduced from palaeobotanical evidence. Therefore, enhanced nutrient loading of the lake due to the presence of humans and their livestock in the catchment is the most likely cause of the increased anoxia. The chironomid fauna reacted the same way to intensive pasturing during the last ca. 1500 years as to Bronze Age clear-cutting and more moderate pasturing during the Bronze, Iron, and Roman Ages, suggesting that alpine lake ecosystems can be extremely sensitive to human activity in the catchment. On the other hand, the chironomid assemblages show a considerable amount of resilience to human disturbance, as the chironomid fauna reverted to the pre-impact stage after the first two periods of human activity. In recent years, even though pasturing decreased again, the chironomid fauna has only partly recovered. This is possibly due to other human-induced changes in the lake ecosystem, e.g., the stocking of the lake with fish. The chironomid stratigraphy is difficult to interpret climatologically as the strongest changes in chironomid-inferred temperatures coincide with periods of intensive human activity in the catchment.     

European ammonoid diversity questions the spreading of anoxia as primary cause for the Cenomanian/Turonian (Late Cretaceous) mass extinction

Ammonoid diversity patterns show that the spreading of oceanic anoxia is not the initial and major kill mechanism for the Cenomanian/Turonian mass extinction as usually suggested. In the Anglo-Paris Basin and the Vocontian Basin, the drop of ammonoid species richness starts around the middle/late Cenomanian boundary, i.e. 0.75 myr before the occurrence of anoxic deepwater sediments. The stepwise extinction of first heteromorphs and then acanthoceratids is incompatible with the rise of the oxygen minimum zone. Moreover, shelf environments of these basins remained well oxygenated during the Cenomanian/Turonian boundary interval. Thus, we stress that other causative mechanisms initiated the ammonoid extinction even if anoxia subsequently participated in the demise of marine ecosystems. Editorial handling: M.J. Benton & J.-P. Billon-Bruyat     

The aftermath of the Carnian carbonate platform demise: a basinal perspective (Dolomites, Southern Alps)

In the Dolomites of northernmost Italy the carbonate‐platform growth came to a standstill late in the Early Carnian (Late Triassic). The response to this shutdown of shallow‐water carbonate production in the interplatform basins is largely unknown because erosion has removed most of the soft basinal sediments, giving rise to today's scenic landscape of the Dolomites. Mapping in the central part of the Dolomites and newly available core material has recently revealed a well‐preserved succession of basinal rocks within the Heiligkreuz Hospiz Basin (ital. Ospizio di Santa Croce Basin). In this paper, the regional depositional nature of arrested carbonate platform production is reconstructed by tracing its sedimentological record across the slope and into the basin. The uppermost St. Cassian Formation, the time‐equivalent basinal rocks to the prograding carbonate platforms, is overlain by the Heiligkreuz Formation, whose basal succession was deposited in a restricted and oxygen‐depleted environment immediately post‐dating the platform demise. The succession consists mainly of mudrocks, marlstones, and peloidal packstones, with abundant low‐diversity ostracod and pelecypod fauna and early diagenetic dolomite. C and O isotope values of the basal Heiligkreuz Formation, post‐dating platform demise, average + 2·4 and ? 2·4‰, respectively, and largely overlap the isotopic composition of St. Cassian carbonates. A shift toward slightly lower δ¹³C values in the Heiligkreuz Formation may reflect incorporation of isotopically depleted C released during bacterial sulphate reduction in the Heiligkreuz sediments. Sedimentological, palaeobiological and geochemical indices suggest that near‐normal marine conditions persisted long after the shutdown of shallow water carbonate‐platform growth, although there are clear indications of severely reduced oxygen levels in the restricted Heiligkreuz Hospiz interplatform basin. The Early Carnian platform demise induced a distinct switch in the locus of carbonate production from the shallow‐water platform and slope to the basin floor and a decrease in the availability of dissolved oxygen in the basinal waters. It is inferred that anoxia extended at least temporarily to the top of the carbonate slope, as indicated by the onlap of normal‐marine mounds by dark marlstones of the basal Heiligkreuz Formation.     

Nitrogen isotope chemostratigraphy across the Permian–Triassic boundary at Chaotian,Sichuan, South China

Nitrogen isotopic compositions of upper Permian to lowermost Triassic rocks were analyzed at Chaotian in northern Sichuan, South China, in order to clarify changes in the oceanic nitrogen cycle around the Permian–Triassic boundary (P–TB) including the entire Changhsingian (Late Late Permian) prior to the extinction. The analyzed ca. 40 m thick interval across the P–TB at Chaotian consists of three stratigraphic units: the upper Wujiaping Formation, the Dalong Formation, and the lowermost Feixianguan Formation, in ascending order. The upper Wujiaping Formation, ca. 10 m thick, is mainly composed of dark gray limestone with diverse shallow-marine fossils such as calcareous algae and brachiopods, deposited on the shallow shelf. In contrast, the overlying Dalong Formation, ca. 25 m thick, is mainly composed of thinly bedded black mudstone and siliceous mudstone containing abundant radiolarians, deposited on the relatively deep slope/basin. Absence of bioturbation, substantially high total organic carbon contents (up to 15%), and abundant occurrence of pyrite framboids in the main part of the Dalong Formation indicate deposition under anoxic condition. The lowermost Feixianguan Formation, ca. 5 m thick, is composed of thinly bedded gray marl and micritic limestone with minor fossils such as ammonoids and conodonts, deposited on the relatively shallow slope. δ¹⁵N_TN values are in positive values around +1 to +2‰ in the upper Wujiaping Formation implying denitrification and/or anammox in the ocean. δ¹⁵N_TN values gradually decrease to −1‰ in the lower Dalong Formation and are consistently low (around 0‰) in the middle Dalong to lowermost Feixianguan Formation. No clear δ¹⁵N_TN shift is recognized across the extinction horizon. The consistently low δ¹⁵N_TN values suggest the enhanced nitrogen fixation in the ocean during the Changhsingian at Chaotian. Composite profiles based on previous and the present studies demonstrate the substantial δ¹⁵N variation on a global scale in the late Permian to earliest Triassic; a systematic δ¹⁵N difference by low and high latitudes is particularly clarified. Although the enhanced nitrogen fixation throughout the Changhsingian at Chaotian was likely a regional event in northwestern South China, the composite δ¹⁵N profiles imply that the sea area in which fixed nitrogen is depleted has gradually developed worldwide in the Changhsingian, possibly acting as a prolonged stress to shallow-marine biota.     

Micropaleontological evidence for redox changes in the OAE3 interval of the US Western Interior: Global vs. local processes

The Coniacian-Santonian interval has been proposed as the youngest of the Cretaceous ocean anoxic events (OAE3), but this designation has long been debated. OAE3 is associated with a long-lasting (∼3 myr) succession of black shales from the central and South Atlantic, Caribbean region, and the North American Western Interior; in the Western Interior it is characterized by an abrupt increase in total organic carbon (TOC) and corresponding trace metal indicators for anoxia. However, the modern concept of OAEs is predicated on detection of global carbon cycle perturbations as recorded by substantial carbon isotope excursions (CIE), and the protracted Coniacian-Santonian black shale interval does not have a large CIE. A more conservative definition of OAE3 might limit the event to the modest positive carbon isotope excursion restricted to the upper Coniacian Scaphites depressus Ammonite Zone. Trace metal proxies suggest that oxygen levels abruptly declined prior to the onset of this CIE in the Western Interior Sea (WIS), but it is unknown whether regional anoxic conditions were confined to sediments/pore waters, or how anoxia may have affected the biota. In an effort to characterize the oxygenation history of the WIS and to better understand the nature of the hypothesized OAE3, we present micropaleontological evidence of declining oxygen in bottom waters prior to the event using benthic foraminifera, which are sensitive to dissolved oxygen. Changes in benthic foraminiferal abundances suggest a decline in oxygen at least 1-myr prior to the CIE (including a nadir immediately below the start of the excursion), improving bottom water oxygen during the CIE, and re-establishment of persistent anoxia following the isotope excursion. Anoxia endured for nearly 3 myr in the central seaway, showing some signs of recovery toward the top of the Niobrara Formation. Our findings suggest that declining oxygen concentrations in the seaway eventually reached a tipping point, after which dissolved oxygen quickly dropped to zero.The late Coniacian CIE is an exception to the trend of declining oxygen in the WIS, and part of a larger pattern in the oxygenation history of the Niobrara Formation which suggests that it does not adhere to standard black shale models. Transgressive periods, including the Fort Hays Limestone and the lower limestone unit of the Smoky Hill Shale (which corresponds to the CIE) are relatively oxic, while periods of highstand (i.e., most of the Smoky Hill) correspond to deteriorating oxygen conditions. This contrasts with the standard black shale model for sea level and oxygen, where transgressions typically correlate with maximum TOC enrichment, interpreted to result from both sediment condensation and oxygen deficiency. The association of global carbon burial/anoxia (as indicated by carbon isotopes) with a regional increase in oxygen and decrease in organic matter preservation is reminiscent of the Cenomanian-Turonian Greenhorn Limestone, which contains OAE2. In both cases, the facies are not typical black shales, but instead have appreciable carbonate content. Western Interior redox trends support the rejection of the original concept of a protracted Coniacian-Santonian OAE3 because it is not a distinct “event.” Increasing local oxygen during the late Coniacian CIE also argues against a narrower OAE designation for this event, because the excursion can't be tied to anoxia here or anywhere else it has been described. Nevertheless, the Late Coniacian Event (as we prefer to call this CIE) still represents an important perturbation of the global carbon cycle. This is emblematic of the shift away from widespread, discrete anoxic events during the ongoing paleoceanographic reorganization of the Late Cretaceous, even as large carbon cycle perturbations continued.     

Role of cyanobacteria in Corg-rich deposits: an example from the Indidura Formation (Cenomanian-Turonian), northeastern Mexico

We present evidence of accumulation of calcareous cyanobacterial “microspheroids” as predominant components of the Cenomanian-Turonian Indidura Formation of northeastern Mexico. The unit at Parras de la Fuente includes a sequence of limestones and marls with well-defined light-dark rhythms at the decimetric to millimetric scale, in which CaCO₃ and total organic carbon vary between 43–78% and 0.3–3.6%, respectively.A distinctive feature of the section is the presence of abundant millimeter-scale microlaminae arranged in nearly even-parallel white and dark gray “varve like” dual lamination less than 3 mm thick, in which the darker units contain scattered planktonic foraminifera and radiolaria, whereas the lighter microlaminae are dominated by calcitic microspheroids (20–40 μm). The white laminae are evidently the result of recurring cycles of calcareous cyanobacterial blooms, possibly associated with fluvial dilution of surface waters.The organic carbon-rich laminated marlstones and laminated biocalcilutites of the Indidura Formation document paleoceanographic conditions favorable to unusual cyanobacterial productivity cycles that were also characterized by strong dysoxic/anoxic bottom conditions.     

High-resolution carbon isotope changes in the Permian–Triassic boundary interval, Chongqing, South China; implications for control and growth of earliest Triassic microbialites

High-resolution δ¹³C_CARB analysis of the Permian–Triassic boundary (PTB) interval at the Laolongdong section, Beibei, near the city of Chongqing, south China, encompasses the latest Permian and earliest Triassic major facies changes in the South China Block (SCB). Microbialites form a distinctive unit in the lowermost 190 cm above the top of the Changhsing Formation (latest Permian) at Laolongdong, comparable to a range of earliest Triassic sites in low latitudes in the Tethyan area. The data show that declining values of δ¹³C_CARB, well-known globally, began at the base of the microbialite. High positive values (+3 to 4 ppt) of δ¹³C_CARB in the Late Permian are interpreted to indicate storage of ¹²C in the deep waters of a stratified ocean, that was released during ocean overturn in the earliest Triassic, contributing to the distinctive fall in isotope values; this interpretation has been stated by other authors and is followed here. The δ¹³C_CARB curve shows fluctuations within the microbialite unit, which are not reflected in the microbialite structure. Comparisons between microbialite branches and adjacent micritic sediment show little difference in δ¹³C_CARB, demonstrating that the microbialite grew in equilibrium with surrounding seawater. The Early Triassic microbialites are interpreted to be a response to upwelling of bicarbonate-rich poorly oxygenated water in low latitudes of Tethys Ocean, consistent with current ocean models for the PTB interval. However, the decline of δ¹³C_CARB may be due to a combination of processes, including productivity collapse resulting from mass extinction, return of deep water to ocean surface, oxidation of methane released from methane hydrate destabilisation, and atmospheric deterioration. Nevertheless, build-up of bicarbonate-rich anoxic deep waters may be expected as a result of the partial isolation of Tethys, due to continental geography; release of bicarbonate-rich deep water, by ocean upwelling, in the earliest Triassic may have been an inevitable consequence of this combination of circumstances.     

9000 years of geochemical evolution of lithogenic major and trace elements in the sediment of an alpine lake – the role of climate, vegetation, and land-use history

A 9000cal. year record of geochemistry was analysed in a sediment core obtained from a Swiss alpine hard-water lake (1937 ma.s.l.) that is located at the present-day tree-line. Geochemical stratigraphies are compared to changes in mineralogy, grain-size, pollen, and macrofossil records. This allows the reconstruction of the effects of changes in vegetation and of 3500 years of land-use in the catchment area on sediment geochemistry. Using principal component analysis, two major geochemical groups are distinguished: (i) Changes in concentrations of Rb, Ti, Zr, Fe, As, and Pb are closely related to corresponding changes in the concentrations of quartz and clay. They are thus considered to represent the silicate fraction which shows an increase from the oldest to the youngest core section. (ii) In contrast, Ca and Sr concentrations are positively correlated with changes in silt, sand, and calcite. They are therefore considered to represent the carbonate fraction which gradually decreased. Based on constrained cluster analysis, the core is divided into two major zones. The oldest zone (A; 9000–6400 cal.BP) is characterised by high concentrations of detrital carbonates. The more open catchment vegetation at that time promoted the physical weathering of these carbonates. The second major zone (B, 6400 cal.BP–1996 AD) is divided into four subsections with boundaries at ca. 3500, 2400, and 160cal. BP. The lower part of this zone, B1, is characterized by a gradual decrease in the carbonate-silt fraction and a pronounced increase in the silicate-clay fraction. This is concurrent with the expansion of Picea in the catchment area, which probably stabilized the soil. The middle part, B2 and B3 (3500–160cal. BP), comprises pronounced fluctuations in all elements, especially Ca, Sr, Mn, and Rb, but also in clay and silt. These changes are related to varying intensities of alpine farming. In the same section, Mn/Fe ratios are highly variable, suggesting changes in the mixing regime of the lake with phases of anoxic bottom water. The uppermost section, B4 (since 160cal. BP), is characterized by a steep decline in the silicate fraction and an increase in Ca and Sr. Despite the decrease in the silicate fraction, Pb increases, due to elevated atmospheric input resulting from early metal pollution, are masked by the high natural variability. Generally, changes in vegetation, which correspond to climate changes in the early Holocene and to human activities since ca. 3700cal. BP, are the controlling factor for variations in the geochemical composition of the sediment of Sägistalsee.     

Laminated–bioturbated cycles in Maastrichtian chalk of the North Sea: oxygenation fluctuations within the Milankovitch frequency band

The Maastrichtian chalk of the southern Central Graben, Danish North Sea, is a homogeneous pure white coccolithic chalk mudstone deposited in a deep epeiric shelf sea, which covered large parts of northern Europe. The sediment displays a pronounced cyclicity marked by decimetre‐thick bioturbated beds alternating with slightly thinner non‐bioturbated, mainly laminated beds. The laminated half‐cycles consist of alternating millimetre‐thick, graded, high‐porosity laminae and non‐graded, low‐porosity laminae. The cyclicity has been interpreted previously as caused by periods of slow background sedimentation and bioturbation interrupted by periods of rapid deposition of laminated beds, with the latter reflecting random and local resedimentation processes. Based on textural and structural analysis, the millimetre‐scale, non‐graded laminae are interpreted as having been deposited directly from pelagic rain of pelleted coccoliths representing the primary production. The graded laminae were deposited from small‐volume, low‐density turbidity currents and suspension clouds. The sedimentation rates of the cyclical chalk are similar to those known elsewhere, and the lamination is interpreted as having been preserved from destruction through bioturbation by anoxic conditions at the seafloor. Bioturbated–laminated cycles are thus formed by slow sedimentation during alternating seafloor redox conditions probably on a Milankovitch scale. A direct implication of this interpretation is that the cycles are areally widespread, probably extending throughout the southern Central Graben area and may be useful for correlation and high‐resolution cyclostratigraphy in the chalk fields of the Danish North sea. If the laminated half‐cycles represent a few rapid resedimentation events, with a high sedimentation rate as suggested by most workers, then the sediment would not be truly cyclic, but would represent event sedimentation within a pelagic background represented by the bioturbated beds. In this case, the cycles would have very limited potential for correlation.     

Sedex brine expulsions to Paleozoic basins may have changed global marine 87Sr/86Sr values,triggered anoxia,and initiated mass extinctions

Sedimentary-exhalative (sedex) ore deposits were formed by discharge of metal-rich brines into ancient ocean basins. Chemical, isotopic, and geologic data from several Paleozoic sedex districts suggest that the brine discharges also supplied enormous quantities of radiogenic Sr and biolimiting nutrients to the oceans. Seven middle Paleozoic sedex events appear to coincide with short-duration positive excursions (“spikes”) in the global marine Sr-isotope record that are not explained by current oceanic models. These strong temporal correlations, combined with mass balance evidence and oceanographic modeling, suggest the flux of radiogenic Sr-rich sedex brines may have been sufficient to cause these prominent spikes. If these sedex hydrothermal events are recorded in the secular record, then the ⁸⁷Sr/⁸⁶Sr record may provide a unique tool for ore genesis studies and for assessing the mineral potential of sedimentary basins of different ages.The apex of these enigmatic ⁸⁷Sr/⁸⁶Sr spikes correlate with global δ¹³C and δ¹⁸O spikes, periods of global anoxia, deposition of metal-rich black shales, deposition of ironstones, climate change, metal-induced malformation (teratology) of marine organisms and significant mass extinctions. While the relationships among these phenomena remain poorly understood and diverse models for these events have been proposed, most invoke an increased flux of biolimiting nutrients resulting in ocean eutrophication. Evidence that the flux of key biolimiting nutrients and metals contained in sedex brines may have been equivalent to or surpass that of the total modern riverine flux to the ocean suggests that these sedex brine exhalations may have triggered global chemical and biological events.