| 1. |
- Coney, Louise, et al.
(författare)
-
Geochemical and mineralogical investigation of the Permian-Triassic boundary in the Continental Realm of the Southern Karoo Basin, South Africa
- 2007
-
Ingår i: Palaeoworld. - : Elsevier BV. - 1875-5887 .- 1871-174X. ; 16, s. 67-104
-
Tidskriftsartikel (refereegranskat)abstract
- Mineralogical and geochemical studies of the non-marine Permian–Triassic (P–Tr) boundary across two stratigraphically wellconstrained sections (Commando Drift Dam and Wapadsberg, Eastern Cape Province) in the southern Karoo Basin, South Africa, have been undertaken to provide further input on the cause of this mass extinction event, and so has a sedimentological and geochemical evaluation of a third P–Tr boundary section at Injusiti (Kwazulu-Natal) in the eastern Karoo Basin, South Africa. The Commando Drift Dam section has been constrained by previous palaeontological and palaeomagnetic work, with a palaeomagnetic reversal positioned 5.3m above the palaeontological P–Tr boundary. The Wapadsberg section has been constrained palaeontologically. All these P–Tr sections studied here mostly comprise mudstones, together with siltstones, sandstones, and in the southern Karoo Basin, carbonate nodular horizons. A change in colour of the mudstones from green-grey to red-brown takes place at the palaeontologically defined boundary. Variations in the major and trace element abundance profiles are usually limited to the carbonate nodular horizons, besides the overall effects of weathering. Bulk carbon isotopic studies of the Commando Drift Dam section revealed a negative δ13Cbulk excursion (background values of −15 to −20‰, with total extent of excursion to −24.9‰) 2 cm below the palaeontological boundary, followed by a gradual recovery and then another decrease in values leading towards the palaeomagnetic boundary. Above this boundary recovery to less negative δ13Cbulk values (ca. −18‰) occurs. The organic carbon record from the Commando Drift Dam (southern Karoo Basin) and Injusiti (eastern Karoo Basin) oscillates between −26.1 and −28.9‰, which is comparable to previous studies of different sections in the southern Karoo Basin. The magnitude of both the bulk and organic carbon isotopic variation can be interpreted to indicate a number of inputs (due to the fluctuating values) of organic carbon. The carbon isotope data for carbonates in the Injusiti section are different from the results on carbonates from other studies, but more work to expand this dataset is necessary. The palynological studies on the Commando Drift Dam section reveal the presence of a low diversity flora composed principally of bryophytes, lycophytes, and gymnosperms. These forms, including several Late Permian key-species, are traces of the surviving plants enduring after the major extinction-pulse. The presence of fungal palynomorphs and dearth of pollen/spores related to photosynthetic plants some metres above the palaeontological P–Tr boundary demonstrate similarities to the pattern of floral extinction at the Cretaceous–Palaeogene (K–Pg) boundary. The timing of vertebrate extinctions in the Karoo Basin has so far not been determined, but the 252.5 Ma age for a single zircon crystal from Commando Drift Dam dated here gives a maximum constraint on the age of the event bed, which is in agreement with the accepted age of the boundary. No evidence for impact-produced microdeformation features were found in quartz grains from either the Wapadsberg or Commando Drift Dam sections. Also, siderophile element data (including platinum group element (PGE)concentrations) do not support the possible presence of a meteoritic component at the boundary. Thus, a link between impact and P–Tr extinction is not indicated by the results of this study.
|
|
| 2. |
- Karcher, Denis B., et al.
(författare)
-
Nitrogen eutrophication particularly promotes turf algae in coral reefs of the central Red Sea
- 2020
-
Ingår i: PeerJ. - : PeerJ. - 2167-8359. ; 8
-
Tidskriftsartikel (refereegranskat)abstract
- While various sources increasingly release nutrients to the Red Sea, knowledge about their effects on benthic coral reef communities is scarce. Here, we provide the first comparative assessment of the response of all major benthic groups (hard and soft corals, turf algae and reef sands-together accounting for 80% of the benthic reef community) to in-situ eutrophication in a central Red Sea coral reef. For 8 weeks, dissolved inorganic nitrogen (DIN) concentrations were experimentally increased 3-fold above environmental background concentrations around natural benthic reef communities using a slow release fertilizer with 15% total nitrogen (N) content. We investigated which major functional groups took up the available N, and how this changed organic carbon (C-org) and N contents using elemental and stable isotope measurements. Findings revealed that hard corals (in their tissue), soft corals and turf algae incorporated fertilizer N as indicated by significant increases in delta N-15 by 8%, 27% and 28%, respectively. Among the investigated groups, C-org content significantly increased in sediments (+24%) and in turf algae (+33%). Altogether, this suggests that among the benthic organisms only turf algae were limited by N availability and thus benefited most from N addition. Thereby, based on higher C-org content, turf algae potentially gained competitive advantage over, for example, hard corals. Local management should, thus, particularly address DIN eutrophication by coastal development and consider the role of turf algae as potential bioindicator for eutrophication.
|
|
| 3. |
- Roth, Florian, et al.
(författare)
-
High rates of carbon and dinitrogen fixation suggest a critical role of benthic pioneer communities in the energy and nutrient dynamics of coral reefs
- 2020
-
Ingår i: Functional Ecology. - : Wiley. - 0269-8463 .- 1365-2435. ; 34:9, s. 1991-2004
-
Tidskriftsartikel (refereegranskat)abstract
- 1. Following coral mortality in tropical reefs, pioneer communities dominated by filamentous and crustose algae efficiently colonize substrates previously occupied by coral tissue. This phenomenon is particularly common after mass coral mortality following prolonged bleaching events associated with marine heatwaves.2. Pioneer communities play an important role for the biological succession and reorganization of reefs after disturbance. However, their significance for critical ecosystem functions previously mediated by corals, such as the efficient cycling of carbon (C) and nitrogen (N) within the reef, remains uncertain.3. We used 96 carbonate tiles to simulate the occurrence of bare substrates after disturbance in a coral reef of the central Red Sea. We measured rates of C and dinitrogen (N-2) fixation of pioneer communities on these tiles monthly over an entire year. Coupled with elemental and stable isotope analyses, these measurements provide insights into macronutrient acquisition, export and the influence of seasonality.4. Pioneer communities exhibited high rates of C and N(2)fixation within 4-8 weeks after the introduction of experimental bare substrates. Ranging from 13 to 25 mu mol C cm(-2) day(-1)and 8 to 54 nmol N cm(-2) day(-1), respectively, C and N(2)fixation rates were comparable to reported values for established Red Sea coral reefs. This similarity indicates that pioneer communities may quickly compensate for the loss of benthic productivity by corals. Notably, between 40% and 85% of fixed organic C was exported into the environment, constituting a vital source of energy for the coral reef food web.5. Our findings suggest that benthic pioneer communities may play a crucial, yet overlooked role in the C and N dynamics of oligotrophic coral reefs by contributing to the input of new C and N after coral mortality. While not substituting other critical ecosystem functions provided by corals (e.g. structural habitat complexity and coastal protection), pioneer communities likely contribute to maintaining coral reef nutrient cycling through the accumulation of biomass and import of macronutrients following coral loss.
|
|
| 4. |
- Rädecker, Nils, et al.
(författare)
-
Heat stress reduces the contribution of diazotrophs to coral holobiont nitrogen cycling
- 2022
-
Ingår i: The ISME Journal. - : Springer Science and Business Media LLC. - 1751-7362 .- 1751-7370. ; 16, s. 1110-1118
-
Tidskriftsartikel (refereegranskat)abstract
- Efficient nutrient cycling in the coral-algal symbiosis requires constant but limited nitrogen availability. Coral-associated diazotrophs, i.e., prokaryotes capable of fixing dinitrogen, may thus support productivity in a stable coral-algal symbiosis but could contribute to its breakdown when overstimulated. However, the effects of environmental conditions on diazotroph communities and their interaction with other members of the coral holobiont remain poorly understood. Here we assessed the effects of heat stress on diazotroph diversity and their contribution to holobiont nutrient cycling in the reef-building coral Stylophora pistillata from the central Red Sea. In a stable symbiotic state, we found that nitrogen fixation by coral-associated diazotrophs constitutes a source of nitrogen to the algal symbionts. Heat stress caused an increase in nitrogen fixation concomitant with a change in diazotroph communities. Yet, this additional fixed nitrogen was not assimilated by the coral tissue or the algal symbionts. We conclude that although diazotrophs may support coral holobiont functioning under low nitrogen availability, altered nutrient cycling during heat stress abates the dependence of the coral host and its algal symbionts on diazotroph-derived nitrogen. Consequently, the role of nitrogen fixation in the coral holobiont is strongly dependent on its nutritional status and varies dynamically with environmental conditions.
|
|
