stable carbon isotopes
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2022 ◽  
Vol 8 ◽  
Leila Chapron ◽  
Verena Schoepf ◽  
Stephen J. Levas ◽  
Matthew D. Aschaffenburg ◽  
Mark E. Warner ◽  

Coral reefs are among the most diverse and complex ecosystems in the world that provide important ecological and economical services. Increases in sea surface temperature linked to global climate change threatens these ecosystems by inducing coral bleaching. However, it is not fully known if natural intra- or inter-annual physiological variability is linked to bleaching resilience or recovery capacity of corals. Here, we monitored the coral physiology of three common Caribbean species (Porites divaricata, Porites astreoides, Orbicella faveolata) at six time points over 2 years by measuring the following traits: calcification, biomass, lipids, proteins, carbohydrates, chlorophyll a, algal endosymbiont density, stable carbon isotopes of the host and endosymbiotic algae, and the stable carbon and oxygen isotopes of the skeleton. The overall physiological profile of all three species varied over time and that of P. divaricata was consistently different from the two other coral species. Porites divaricata had higher energy reserves coupled with higher contributions of heterotrophically derived carbon to host tissues than both P. astreoides and O. faveolata. Consistently higher overall energy reserves and heterotrophic contributions to tissues appear to buffer against environmental stress, including bleaching events. Thus, natural physiological variability among coral species appears to be a stronger predictor of coral bleaching resilience than intra- or inter-annual physiological variability within a coral species.

The Holocene ◽  
2021 ◽  
pp. 095968362110665
Prabhakaran Ramya Bala ◽  
Sarath Pullyottum Kavil ◽  
Ichiro Tayasu ◽  
Chikage Yoshimizu ◽  
Kaustubh Thirumalai ◽  

Peat deposits (>50 ka) in the montane Nilgiris (Western Ghats, India), have been central to the reconstruction of late Quaternary paleoclimate using paleovegetation changes in the forest-grassland vegetation mosaic that coexist here. However, it is well-known that short-term disturbances can also cause vegetation switches when multiple stable vegetation states exist. We studied paleovegetation changes within the alternative stable states framework using stable carbon isotopes (relative abundance of C3-C4 vegetation) on the cellulose fraction from two high-resolution radiocarbon-dated peat cores ~170 m apart in the Sandynallah valley: Core 1 closer to the hillslope (32,000 years old) and Core 2 from the centre of the valley (45,000 years old). Core 1 is located in an ecotone showing shola-sedgeland dynamics with vegetation switching at c.22 ka from shola (possibly due to fire) to a prolonged unstable state until 13 ka sustained by low waterlogging. Following a hiatus c.13 ka, sedgeland dominates, with a shift into shola at 3.75 ka driven by increasing aridity. Core 2 shows a stable sedgeland mixed C3-C4 composition responding to temperature, enriched in C3-vegetation in the last glacial with C4-dominance beginning c.18.5 ka, indicative of deglacial warming. The distinctive vegetation states at corresponding times in Cores 1 and 2 within the same valley, responding independently to disturbances and climate, respectively, is the first paleo-record from an alternative stable states landscape in the montane tropics. Thus, short-term disturbances and site attributes need to be accounted for before ascribing vegetation change to changing climate in such vegetation mosaics.

2021 ◽  
Vol 18 (24) ◽  
pp. 6533-6546
Ralf Conrad ◽  
Pengfei Liu ◽  
Peter Claus

Abstract. Acetate is an important intermediate during the degradation of organic matter in anoxic flooded soils and sediments. Acetate is disproportionated to CH4 and CO2 by methanogenic or is oxidized to CO2 by sulfate-reducing microorganisms. These reactions result in carbon isotope fractionation, depending on the microbial species and their particular carbon metabolism. To learn more about the magnitude of the isotopic enrichment factors (ε) involved, acetate conversion to CH4 and CO2 was measured in anoxic paddy soils from Vercelli (Italy) and the International Rice Research Institute (IRRI, the Philippines) and in anoxic lake sediments from the northeastern and the southwestern basins of Lake Fuchskuhle (Germany). Acetate consumption was measured using samples of paddy soil or lake sediment suspended in water or in phosphate buffer (pH 7.0), both in the absence and presence of sulfate (gypsum), and of methyl fluoride (CH3F), an inhibitor of aceticlastic methanogenesis. Under methanogenic conditions, values of εac for acetate consumption were always in a range of −21 ‰ to −17 ‰ but higher in the lake sediment from the southwestern basin (−11 ‰). Under sulfidogenic conditions εac values tended to be slightly lower (−26 ‰ to −19 ‰), especially when aceticlastic methanogenesis was inhibited. Again, εac in the lake sediment of the southwestern basin was higher (−18 ‰ to −14 ‰). Determination of εCH4 from the accumulation of 13C in CH4 resulted in much lower values (−37 ‰ to −27 ‰) than from the depletion of 13C in acetate (−21 ‰ to −17 ‰ ), especially when acetate degradation was measured in buffer suspensions. The microbial communities were characterized by sequencing the bacterial 16S rRNA (ribosomal ribonucleic acid) genes as well as the methanogenic mcrA and sulfidogenic dsrB genes. The microbial communities were quite different between lake sediments and paddy soils but were similar in the sediments of the two lake basins and in the soils from Vercelli and the IRRI, and they were similar after preincubation without and with addition of sulfate (gypsum). The different microbial compositions could hardly serve for the prediction of the magnitude of enrichment factors.

2021 ◽  
Vol 17 (5) ◽  
pp. 2165-2177
Caroline Welte ◽  
Jens Fohlmeister ◽  
Melina Wertnik ◽  
Lukas Wacker ◽  
Bodo Hattendorf ◽  

Abstract. Rapid and continuous analysis of radiocarbon (14C) concentration in carbonate samples at spatial resolution down to 100 µm has been made possible with the new LA-AMS (laser ablation accelerator mass spectrometry) technique. This novel approach can provide radiocarbon data at a spatial resolution similar to that of stable carbon (C) isotope measurements by isotope ratio mass spectrometry of micromilled samples and, thus, can help to interpret δ13C signatures, which otherwise are difficult to understand due to numerous processes contributing to changes in the C-isotope ratio. In this work, we analyzed δ13C and 14C on the Holocene stalagmite SPA 127 from the high-alpine Spannagel Cave (Austria). Both proxies respond in a complex manner to climate variability. Combined stable carbon and radiocarbon profiles allow three growth periods characterized by different δ13C signatures to be identified: (i) the period 8.5 to 8.0 ka is characterized by relatively low δ13C values with small variability combined with a comparably high radiocarbon reservoir effect (expressed as dead carbon fraction, dcf) of around 60 %. This points towards C contributions of host rock dissolution and/or from an “old” organic matter (OM) reservoir in the karst potentially mobilized due to the warm climatic conditions of the early Holocene. (ii) Between 8 and 3.8 ka there was a strong variability in δ13C with values ranging from −8 ‰ to +1 ‰ and a generally lower dcf. The δ13C variability is most likely caused by changes in C exchange between cave air CO2 and dissolved inorganic carbon in drip water in the cave, which are induced by reduced drip rates as derived from reduced stalagmite growth rates. Additionally, the lower dcf indicates that the OM reservoir contributed less to stalagmite growth in this period possibly as a result of reduced meteoric precipitation or because it was exhausted. (iii) In the youngest section between 3.8 and 2.4 ka, comparably stable and low δ13C values, combined with an increasing dcf reaching up to 50 % again, hint towards a contribution of an aged OM reservoir in the karst. This study reveals the potential of combining high-resolution 14C profiles in speleothems with δ13C records in order to disentangle climate-related C dynamics in karst systems.

Aleksandar I. Goranov ◽  
Morgan F. Schaller ◽  
Jonathan A. Long ◽  
David C. Podgorski ◽  
Sasha Wagner

2021 ◽  
Vol 9 (9) ◽  
pp. 992
Yinan Mao ◽  
Qingyun Ma ◽  
Jiaxin Lin ◽  
Ye Chen ◽  
Qiang Shu

In this study, total organic carbon (TOC), total nitrogen (TN), and stable carbon isotopes (δ13C) were measured in surface intertidal saltmarsh and bare tidal flat sediments along the Rudong coast. The distribution and sources of organic carbon were examined under different depositional environments based on C/N ratios and a two-terminal mixing model. The results showed that the average TOC content of the vegetated saltmarsh sediments, bare tidal flat areas near vegetation (BF1), and bare tidal flat areas far from vegetation (BF2) were 4.05, 2.72, and 1.22 mg/g, respectively. The mean δ13C value within the vegetated saltmarsh area was −22.37‰, and the C/N ratio was 9.3; the corresponding values in the BF1 area were −23.27‰ and 7.95, respectively; and in the BF2 area, the corresponding values were −21.91‰ and 5.36, respectively. These C/N ratios reflect an increasing marine contribution with distance from the vegetated zone. Combined with the two-terminal mixing model, the organic carbon in the vegetated saltmarsh sediments was dominated by terrestrial sources, while the bare tidal flat sediments were more influenced by marine sources, and the bare tidal flat sediments nearer to the vegetated zone (BF1) were influenced by a combination of vegetation, marine sources, and other terrestrial factors.

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